Method and apparatus of operating a scanning probe microscope

The present invention provides a scanning probe microscope (SPM), including an atomic force microscope (AFM), more specifically, an atomic force microscope operating mode that provides high speed, low chip sample interaction forces, and force control at high resolution. set to target.

For example, a scanning probe microscope (SPM) such as an atomic force microscope (AFM) is a device that usually uses a probe having a chip and interacts the chip with a sample surface with a low force to characterize the surface down to the atomic dimension. .. Generally, the probe is introduced on the surface of the sample to detect changes in the characteristics of the sample. By providing relative scanning motion between the chip and the sample, surface-specific data can be acquired over a particular region of the sample and a corresponding sample map can be generated.

A typical atomic force microscopy system is schematically shown in FIG. The atomic force microscope 10 uses a probe device 12 including a probe 12 equipped with a cantilever 15. The scanner 24 produces a relative motion between the probe 12 and the sample 22 while the probe-sample interaction is being measured. In this way, an image of the sample or other measurements can be obtained. The scanner 24 generally consists of one or more actuators that generate motions in three directions (XYZ) that are orthogonal to each other. The scanner 24 is often a single integrated unit that includes one or more actuators that move either the sample or the probe in all three axes, such as a cylindrical piezoelectric actuator. Separately, the scanner can be a conceptual or physical combination of multiple separate actuators. Some atomic force microscopes separate the scanner into multiple components, such as an XY actuator that moves the sample and a separate Z actuator that moves the probe. Therefore, the instrument can generate relative motion between the probe and the sample while measuring the surface shape of the sample or some other property (see, eg, Patent Documents 1, 2 and 3).

In particular, the scanner 24 often includes a laminated piezoelectric element (often referred to herein as a "piezostack") or a cylindrical piezoelectric element used to generate relative motion between the measurement probe and the sample surface. .. A laminated piezoelectric element is a device that moves in one or more directions based on a voltage applied to electrodes arranged on a stack. The laminated piezoelectric element is often used in combination with mechanical bending that acts to induce, suppress, and amplify the motion of the laminated piezoelectric element. Further bending is used to increase the stiffness of the actuator in one or more axial directions (see, eg, Patent Document 4). Actuators can be coupled to probes, samples, or both. Most typically, the actuator assembly is provided in the form of an XY actuator that drives the probe or sample horizontally or in an XY plane and a Z actuator that moves the probe or sample vertically or in the Z direction.

In a typical configuration, the probe 17 is often coupled to a vibrating actuator or drive 16 used to vibrate the probe 17 at or near the resonant frequency of the cantilever 15. Another configuration measures flexion, twisting, or other properties of the cantilever 15. The probe 17 is often a microfabricated cantilever with an integrated insert.

Generally, the electronic signal is an AC signal source 18 under the control of a scanning probe microscope controller 20 to cause the actuator 16 (or another scanner 24) to vibrate the probe 12. Applies from. The probe-sample interaction is usually controlled by the controller 20 via feedback. In particular, the actuator 16 can be coupled to the scanner 24, but the probe 12 can be formed integrally with the cantilever 15 of the probe 12 as part of a self-actuating cantilever / probe.

The selective probe 12 often vibrates and comes into contact with the sample 22 when the sample characteristics are monitored by detecting a change in one or more characteristics of the probe 12's vibration as described above. In this regard, the deflection detector 25 is typically used to direct the light beam towards the dorsal side of the probe 12, after which the light beam is reflected towards a detector 26, such as a four-quadrant optical detector. The deflection detector is often an optical lever system (see, eg, Patent Document 1), but can be some other deflection detector, such as a strain gauge, capacitance, and the like. The detection light source of the device 25 is usually a laser, often a visible or infrared laser diode. The detection beam can also be generated by, for example, a He-Ne or other laser source, a high brightness diode (SLD), an LED, an optical fiber, or any other light source that can be focused on a small spot. Since the rays can transition across the detector 26, the appropriate signal is processed by the signal processing block 28 (eg to measure the RMS deflection of probe 12). The interaction signal (eg, deflection) is then transmitted to the controller 20, which processes the signal to determine the vibration change of the probe 12. Generally, the controller 20 determines the error at the block 30, and then typically of the probe 12 to maintain a relatively constant interaction (or deflection of the lever 15) between the tip and the sample. A control signal (for example, by using the PI gain control block 32) is generated in order to maintain the setting value peculiar to vibration. The control signal is generally amplified by the high voltage amplifier 34 before driving the scanner 24, for example. For example, the control device 20 is often used to maintain the vibration amplitude at a set value, As, in order to secure a generally constant force between the chip and the sample. Also, the set value phase or frequency can be used. The control device 20 is also generally referred to as feedback. In this case, the control effort is to maintain a constant target value (feedback set value) defined by the set value.

A controller 20 that receives data collected from the controller and manipulates the data obtained during scanning to perform data manipulation operations such as point selection, curve matching, and distance determination operations, and a separate control. Workstation 40 is also provided in at least one of the devices or systems of articulated or stand-alone controllers. The workstation stores the information obtained in memory, uses it for further calculations, and at least one of its displays on an appropriate monitor, and its transmission to another computer or device, either wired or wireless. It is possible to do at least one of them. The storage device may include, for example, any computer-readable data storage medium including, but not limited to, computer RAM, hard disk, network storage device, flash memory, or CD ROM.

Atomic force microscopes can be designed to operate in a variety of modes, including contact and vibration modes. The action is to move at least one of the sample and the probe assembly up and down relative to the surface of the sample in response to the deflection of the cantilever of the probe assembly as the sample is scanned over its surface. It is achieved by that. Scanning typically occurs in the "xy" plane, which is generally at least parallel to the surface of the sample, and vertical motion occurs in the "z" direction, which is perpendicular to the xy plane. Of note, the term "generally parallel" is used because many samples have roughness, curvature and slope that deviate from the plane. Thus, the data associated with this vertical motion can be stored and then used, for example, to construct an image of the sample surface corresponding to the measured sample properties of the surface shape. TappingMode In one practical mode of atomic force microscopy operation known as Atomic Force Microscope (TappingMode is the trademark of the current transferee), the chip is at or near the resonant frequency of the probe's associated cantilever. Or, it vibrates due to its harmonics. Feedback Lou By controlling the distance between the chip and the sample (the control distance between the probe and the sample), the device usually uses this vibration to minimize the "tracking force" or force resulting from the chip / sample interaction. Attempts to keep the amplitude constant. Another feedback configuration keeps the phase or vibration frequency constant. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.

Regardless of their mode of operation, atomic force microscopes use piezoelectric scanners, optical lever deflection detectors, and very small cantilever manufactured using photolithography techniques in air, liquid, or vacuum. It is possible to obtain resolutions down to the atomic level for a wide variety of insulating or conductive surfaces. Due to their resolution and versatility, atomic force microscopes are important measuring devices in many different fields, from semiconductor manufacturing to biological research. Notably, the acronyms "scanning probe microscope" and certain types of scanning probe microscopes are used herein to refer to either a microscope device or, for example, a related technique of "atomic force microscopy". It can be used.

As with most measuring devices, atomic force microscopy often requires a trade-off between resolution and acquisition speed. That is, some atomic force microscopes currently available can scan the surface at resolutions below the angstrom. These scanners can scan only a relatively small sample area, but even then they can only scan at relatively low scanning speeds. Conventional commercial atomic force microscopes typically require a total scanning time of several minutes to handle an area of several microns with high resolution (eg 512 x 512 pixels) and low tracking power. The practical limit of atomic force microscope scanning speed is the maximum speed at which an atomic force microscope can be scanned while maintaining a sufficiently low tracking force that does not or causes minimal damage to at least one of the chip and the sample. The result. Significant progress has been made in this area where scanning probe microscopes have achieved high resolution video scanning speeds for small samples and small scanning sizes.

Nonetheless, improvements have been desired given the current state of constraints associated with known operating modes, including both Tapping Mode atomic force microscopy and contact modes. Again, in contact mode, lateral scanning of the chip provides a large force between the chip and the sample that can interfere with both the chip and the sample. In addition, when imaging biological samples and soft samples such as polymers, the surface can be destroyed, making the measurement useless, or at least severely deformed, thereby significantly reducing resolution. Of note, "imaging" typically provides a relative scanning motion between the sample and the probe, and the sample and probe interact accordingly, thereby scanning the probe at multiple points on the sample surface. As used herein to indicate that microscopic data is obtained.

TappingMode Atomic Force Microscope is a lower force technique and is the most widely used atomic force microscope operating mode for mapping the sample surface of particularly delicate samples. The typical force of a chip on a sample is about a few nN to a few tens of nN. Again, the shear force is minimized by vibrating the tip rather than dragging it. That said, the TappingMode atomic force microscope has the drawback that it is difficult to control the normal force acting on the sample surface. Users typically try to select a setting that has little variation from the free air deflection / amplitude of the probe in order to minimize chip-sample interaction for the best reproduction of the sample profile. The dilemma, especially for soft tissue, is that if the imaging power is too low, the chip will not track the sample correctly (ie, maintain interaction with the sample during scanning), while if it is too high, the sample will be damaged / deformed. However, it can result in an image that does not accurately reflect the surface shape. Overall, the better this force can be controlled (ie, the lower it can be kept), the less likely it is that at least one of the sample and the chip will be damaged, thus improving resolution. Is possible.

An overview of chip-sample forces in each of these modes provides clues to understanding each constraint. When the probe interacts with the surface by a Tapping Mode atomic force microscope or Jumping Mode (see, eg, Patent Documents 5, 2, 6; all of which are incorporated herein by reference). , The tip periodically touches the surface. Figure 2A illustrates the physical process within one period "T" of chip motion. FIG. 2A shows the chip trajectory with respect to the sample surface position. Figure 2B shows the corresponding interaction forces of the chip orbits at different positions at the same time. At peak position Amax, the chip is farthest from the sample surface and therefore does not interact with the sample. The tip continues to descend toward the horizontal axis (zero tip-distance between samples), so short-field van der Waals) force, Fa_vdw, and van der Waals attraction causes the tip to snap into contact with the sample. After touching the sample, the chip remains in a state of repulsive interaction for the time domain δT. During this time, the tip is in continuous contact with the sample. A position below zero indicates that the chip may have deformed the sample, indicating that position below the sample surface.

When the tip leaves the surface after δT, the attractive force shows the maximum adsorption force Fa_max just before the meniscus breaks and disappears, as it can develop capillary meniscus. The chip then enters the non-interacting region and continues to the maximum detachment position.

In the free interaction zone, when the probe is farther from the surface, the interaction force is zero or close enough to form a baseline, as shown in FIG. 2B. In FIG. 2B, the force above the horizontal axis is the repulsive force, while the force points below the horizontal axis represent the net attractive or attractive force. The maximum repulsive force Fr_max usually corresponds to the minimum or minimum chip position or distance interval with respect to the sample surface.

In the previously known modes disclosed in the Tapping Mode Atomic Force Microscope and the Jumping Mode Atomic Force Microscope, the amplitude Amax or RMS of the chip vibration amplitude is used as a feedback control parameter. An example of such a feedback controller is shown in FIG.

In conventional control, typically performed using gain control feedback loops, positioning actuators and cantilever response detection components (eg, quadrant optical detectors), atomic force microscopes detect probe deflection or chip-surface interactions. The RMS signal corresponding to the cantilever (ie probe) movement is used as an index of, and the feedback loop is used to maintain the constant or RMS deflection.

Yet another major limitation of conventional atomic force microscopes is their inability to obtain quantitative mechanical property information at the same time as high resolution imaging. Atomic force microscopy has mainly focused on imaging surface shapes. Little progress has been made in the acquisition of quantitative mechanical mapping, including the work of elasticity, plasticity, and adsorption.

Moreover, the TappingMode control uses the amplitude or phase of the measured deflection signal to control the chip-surface interaction using feedback. In particular, both amplitude and phase are average characteristics of probe / chip oscillations with at least one interaction period. More specifically, the mean is related to probe / sample interactions that occur at all positions in the chip orbit (Figure 2). Therefore, there is no possibility that the control feedback is substantially based on the instantaneous chip-sample interaction. It should be noted that the instantaneous interaction in this case refers to the interaction at any point (eg, within 2 microseconds) in Figure 2B (further described below). ).

Furthermore, it is important to note that the Tapping Mode atomic force microscope was developed to overcome the known sticky state that occurs when the probe comes into contact with the sample intermittently. When the probe touches the sample, capillary force can tend to capture the chip and prevent it from being released. The amplitude of the probe oscillation in TappingMode drops to zero, which causes feedback oscillation. The problem is a probe with specific stiffness, typically 10 N / m (Newton / meter) to 60 N / m, while operating the Tapping Mode atomic force microscope with vibration amplitudes greater than about 10 nm between peaks. Was overcome when using Tapping Mode by using a nominal value of 40 N / m. When the probe touches the surface under these conditions, the kinetic energy of the tapping probe is converted into static elastic energy sufficient to overcome the capillary force, thus ensuring a steady amplitude in each period. One drawback of this mode is that the kinetic energy stored in the probe is also proportional to the cantilever spring constant. When using a lower spring constant cantilever, for example 1 N / m, the tappingMode can be used to measure many materials because the cantilever cannot overcome the capillary attraction by using its own resonant vibrational energy. It is impossible. Therefore, most Tapping Mode applications are only possible when a rigid cantilever generally known in the art is used as the lever.

In pulse force mode or another mode in which a scanning probe microscope known as PFM (see, eg, Patent Documents 7 and 8) is operated, the amplitude of the probe's vibration is such that the tips touch and separate during each cycle. It will be adjusted. In this mode, control is provided by monitoring the chip-sample interaction force. The control operates on the basis of another common measurement made in the field of atomic force microscopy, namely the properties associated with the force curve, to measure material properties at a particular location. Force measurements can be mapped across the sample to create images that are common and known as force-volume images.

By analyzing the shape of the force-distance curve in pulse force mode and using that data to control the force acting between the chip and the sample, the amount of data obtained can be obtained from other scanning probe microscopes. It decreases compared to the operation mode. It is important that the pulse force mode generally needs to operate with Fr_i (described below) or peak pulse force that substantially exceeds adsorption-induced deflection as well as coupling-induced deflection. As a result, a high repulsive force is required as a control reference value. Such high forces can damage the sample or chip and thus prevent the acquisition of high resolution images. Moreover, the pulse force mode has been performed to image soft samples, especially with respect to operating speed and resolution constraints, but is not so widespread for all types of atomic force microscopy applications. Not adopted. In addition, imaging in a fluid environment presents a further challenge for pulse force modes, as viscous forces in the fluid produce large deflections, even when the cantilever probe is not interacting with the sample.

More specifically, the main reasons why the imaging rate is limited in the standard pulse force mode atomic force microscope are illustrated in FIG. 2C. FIG. 2C is a graph of chip-sample interaction force vs. time. The interaction force is plotted as a snap contact at "A" and the point repulsive force (sample on the chip) begins at "B". The peak repulsive force occurs approximately at "C" because the adsorption force pulls the chip to approximately point "D" where the chip separates from the sample. Point E represents the deflection peak of the cantilever probe when the tip leaves the sample. Both points C and E appear as peaks in the deflection signal. The C value should exceed E to ensure that the feedback properly controls the chip-sample interaction. In yet another constraint in pulse force mode, a particular ringdown period (probe oscillation period at its resonant frequency) is required before the baseline force required to continue the scan can be determined. .. This is However, it waits for the cantilever ringdown (a free attenuation process as in Tapping Mode ), which limits the modulation frequency and thus the scanning speed. More specifically, the modulation frequency is significantly less than the probe resonance frequency (eg, more than one-fifth less than the probe resonance frequency).

In addition to the above problems, the relatively complex and versatile atomic force microscope setup and operation is particularly unfamiliar with at least one of the immature atomic force microscope operators and scientists or an engineer who is not familiar with complex instruments. It can be time consuming and cumbersome for you. For example, setup and operating parameter values generally depend on factors such as the type of sample material, including whether it is hard or soft, conductive or non-conductive, organic, synthetic or biological in nature.

In other measurement techniques, such as scanning electron microscopy (SEM), the sample can be easily attached to the instrument and good quality images can be obtained by users with little education or expertise. Is. However, atomic force microscopy is often a preferred technique given the ability to make various types of measurements, including multidimensional surface shapes and mechanical properties (such as elasticity). Nevertheless, atomic force microscopy most often requires tools and expertise in the measurements to be made. In this regard, the user needs to locate the desired location and introduce the probe tip into the sample (by moving either the sample or the probe). Next, when the measurement scan is initiated, the user needs to ensure that the chip tracks the sample by maintaining a generally stable feedback loop.

Moreover, once the measurements have been made, it is often difficult to interpret the obtained data. In general, these can be time-consuming tasks that require the knowledge and experience of a physicist or electronic engineer, but most often result from relying on human judgment, with accompanying limitations. Since atomic force microscopy has the potential for widespread applicability, it is important that atomic force microscopy can be beneficial if it does not rely too much on the practitioner's ability to perform. Biologists and materials science experts would more widely adopt atomic force microscopes, given their ability to obtain incompatible material property measurements, including sample maps, for example, if easier to use. In this regard, at least one of the atomic force microscopy and the method of operation both a) maintains feedback stability while making measurements and preparing for measurements, and b) interprets the data obtained. Ease of use will be facilitated if the challenges associated with can be minimized or eliminated.

To address these issues, the basic challenges presented by atomic force microscopy and their currently preferred modes of operation were considered. First, the adjustment of the controller has important implications for maintaining stability in the known atomic force microscopy modes. In most current commercial systems, the user must control both the set value and the gain I (integral) and P (proportional). With respect to the set value, the control depends on the mode. In contact mode, the instrument attempts to maintain a relatively direct constant contact force between the tip and the sample. However, in the most widely used atomic force microscope operating modes, i.e. the vibration mode or Tapping Mode atomic force microscope described above, control of set values (tapping amplitude or phase) is complex. This is because, almost basically, there is no simple relationship between the set value and the tip-sample force. The same set value changes can indicate either high or low chip-sample interaction forces, cantilever dynamics (such as basic resonant frequencies), for imaging in different environments (eg, fluid-to-atmosphere), etc. , Has a big impact.

Stable and optimal feedback also requires the application of appropriate gain. In general, feedback can be unstable at high gains and can reduce tracking capabilities at low gains. The P and I gains are generally adjusted by the user through trial and error to ensure that the feedback remains stable, while also providing sufficient tracking capability. To. However, in the TappingMode atomic force microscope, feedback kinetics are highly affected by the settings. That is, the same gain may exhibit different feedback stability under different amplitude settings. The process of gain optimization is particularly complex because gains do not work independently.

Stable feedback also requires the application of appropriate gain if vibration deviations from the set are detected. The gain must be adjusted to return the vibration to the set value. The P and I gains are generally adjusted by the user through trial and error to ensure that the feedback remains stable. Moreover, gains do not work independently, which complicates the task in particular.

Solutions have been proposed in response to the desire in the instrument field to own an atomic force microscopy system in which less skilled users participate and maintain stable feedback. However, each solution has significant limitations.

"On automating atomi" c force microscopes: An adaptive control approach) "Rifai and Youc In ef-Toumi), as well as "Fast contact-mode atomic force microscopy on biological specimen by model-based. In Schitter et al., Titled "control," higher-order or model-based controllers are preferred over standard P / I controllers. Such controls are difficult to design and are inherently incomplete. It is important that such controllers require information about system dynamics prior to operation. These controls can be useful when operating an atomic force microscope in contact mode, but as suggested above, given that system dynamics change with fluctuations in set values, interatomic forces. It is generally difficult for force microscopes to work when operated in Tapping Mode .

Standard P / I controllers are used in Astrom and Hagglund, but the tuning required for stable operation is automated. Astrom and Hagglund use simple regulators with specifications for phase and amplitude margins. In this method, the target system is most commonly a large plant with a slow time response. In particular, the response time scale is usually minutes to hours. This feature is essentially the opposite of the atomic force microscopy system, which has a high response Q (low energy dissipation) with a response time of a few milliseconds. That is, the automatic tuning of the controller (using a simple regulator with slow response time) as taught by Astrom and Hagglund would not be useful for most atomic force microscopy applications.

In another system disclosed by Rice et al. (Patent Document 9), the system works to detect the manifestation of instability and then makes corrections. However, the period between the onset of instability and the onset of uncontrollable instability (ie, instability of a magnitude that requires stopping and restarting the measurement process) is so short that the measurement process should be stopped before it should be stopped. Control is difficult to implement. As will be understood in the art, historical phenomena are primarily reliable when the system is unable to respond quickly enough. Moreover, in this solution, the system makes a determination based on the measured vibration. If the allowable noise amplitude is defined and exceeds that amplitude, the system adjusts the gain. One major problem concerns the fact that noise amplitudes are very complicated, especially when operating atomic force microscopes in Tapping Mode and when measuring certain types of samples. TappingMode ( In an atomic force microscope, vibration is a non-linear representation of the interaction force between a chip and a sample. Thus, for example, control of tapping amplitude provides indirect control of chip-sample interaction forces. This indirect control of the interaction force is susceptible to the action of variables such as vibration damping and system vibration, including the origin of the piezoelectric actuator itself and the mechanical components of the atomic force microscope. These are the TappingMode dynamics that make the development of robust control algorithms extremely difficult, especially when imaging can occur in a variety of environments.

As a result, the system does not require user input to determine, but its ability to interpret measured vibrations and modulate control is limited when the system is becoming unstable. Again, in the TappingMode atomic force microscope, system dynamics depend on both the set value (eg amplitude or phase) and the gain, thus providing a great deal of ability to develop control algorithms that can adapt to instability. Make it complicated.

In short, attempts have been made to automatically adjust the gain with an atomic force microscope, but this method has not proven to be particularly effective. Known methods deal with both sample surface geometries that can have unpredictable adverse effects on any attempt to maintain stability by gain adjustment, as well as operating parameters such as set values, actuator history phenomena and chip geometries. You may not be able to. As a result, automatic gain adjustment has little effect.

Again, this is not surprising given the many scanning parameters that must be considered during atomic force microscope setup and operation, as well as the parameters that can be required to be adjusted during atomic force microscope operation. For example, the user may need to adjust scan control parameters such as set values, scan speed, proportional gain, integral gain, drive frequency, drive amplitude and scan control parameters such as other parameters. With great care, considerable experience, and sometimes without a little luck, cantilever or sample damage can occur, poor or unusable results can be obtained, and everything seems to be working well. In such a case, the inefficiency in operation can become so large that the scanning time is far from the optimum, so that there is a particular problem in high processing application such as in the semiconductor industry.

Currently, poor performance and unacceptable data can result if the value of any one of several manually selected control parameters is not optimal or is not within its reasonable optimal range. obtain. Moreover, the relatively complex interdependence that exists between specific atomic force microscope parameters often involves trial and error to set up operating procedures, even for highly experienced atomic force microscope operators.

In performing an atomic force microscope setup, the values of some control parameters must be set along with different operating modes and the gain of the feedback loop in other cases where such a gain setup is required. Setups include, for example, scan size, pixels / lines, number of scan lines, scan speed, chip scan speed, digital / analog (D / A) resolution, Z center position, ie Z center voltage or Z piezoelectric actuator operating range center, chip wear. It must be configured with consideration for parameters such as control and minimization of sample damage.

If an atomic force microscope is set up to operate in a vibration mode, such as Tapping Mode , the setup must include selecting vibration-related amplitudes and settings. Moreover, the initial values of the integrated gain (I gain) and the proportional gain (P gain) are also set manually. The choice of gain value is that the gain value is usually less than one of the characteristics of the vibration mode used, sample surface shape, sample hardness and roughness. It can also be awkward because it depends on factors such as one or any other mechanical properties and the medium on which the sample is located, as well as other factors. For example, if the gain setting is too low, the system response tends to be relatively slow, which can cause the chip to stop tracking the sample surface. If the gain setting is too high, the feedback loop can initiate backfeeding with vibration or itself. This can add significant noise to the sample image that is undesirably produced.

In addition, the gain setup is fine at first, but can only be inappropriate later if some other factor, such as surface shape, changes. For example, if the sample is relatively coarse, the gain should generally be set higher in order to image such highly characteristic surface shapes, and any increase in feedback vibration noise obtained will result. acceptable. If the sample is relatively smooth or flat, the gain should be set lower to minimize noise. By keeping the noise low at low gain, better resolution of the flat region is obtained, which allows the atomic force microscope to better image its finer details. However, as will be understood in the art, excess noise can adversely affect the imaging along the flatter areas of the sample. The initial high gain setting will eventually be too high when the sample flattens. Conversely, the initial low gain setting often interferes with the imaging of more advanced features of the sample that produces the image. Such more advanced features are either deformations or omissions.

These setup considerations are even more problematic when operating in Tapping Mode , as the most effective gain generally depends on cantilever kinetics. Since cantilever dynamics is a function of free atmosphere tapping amplitude and set value, gain tuning is very difficult, especially for inexperienced users. In fact, factors such as cantilever kinetics and Z-actuator response speed can cause such problems in the default and gain settings, so operators try and error until the sample image begins to look good. I often rely on it.

Unfortunately, trial and error can last for a long time, as one can affect the other. For example, when the set value is lowered, the gain can be set higher and vice versa. However, lower gain allows lower settings to be used, which can generally increase the cantilever response, but also increase the error generation rate. This can unnecessarily blur or deform the image produced during scanning.

Eventually, the operator sets some initial parameter values, gains and settings, and then manually adjusts each value one by one until feedback vibration occurs and then recedes. This process can manage to work for experienced atomic force microscope operators, but it is very often inefficient, time consuming, and unsatisfactory. Furthermore, nothing addresses the dynamic nature of atomic force microscopy imaging, either by sneaking in certain settings during operation, by observing the image, etc., and further reversing and adjusting. Operators are often required to rescan those parts of a poorly imaged sample with parameter values. Again, this process can be extremely slow.

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<p num="0046"><nplcit num="1"><text>Oliver WC and Pharr GM 2004 Measurement of Hardness and Elastic Modulus by Instrumented Indentation: Advances in Understanding and Refinements to Methodology), J. Mater. Res. 19, March 20, 2004.</text></nplcit><nplcit num="2"><text>"Theoretical Investigation of the Distance Dependence of Capillary and Van der Waals forces in Scanning Force Microscopy", Stifter er al.), Physical Review B, Vol.62 No.20, November 15, 2000.</text></nplcit></p>

<p num="0047"> As a result, the scanning probe microscopy field is preferably easy to use and mechanically image a variety of samples that can minimize the force generated by the chip-sample interaction while maintaining fast imaging rates. We needed what could be considered a "point and shoot" solution for property measurements.</p>

<p num="0048"> A preferred embodiment is the design of a control scheme that minimizes the need for experienced and experienced users. Peak Force Tapping (PFT) Mode (PFT Mode and Peak Force Tapping Mode) Take advantage of the new atomic force microscope operating mode known as Veeco Instruments Inc.), Plainview, a trademark of New York). The peak force tapping mode (PFT mode, peak force tap mode) essentially eliminates the need for the user to tune the gain during imaging. Moreover, the peak force tapping mode makes the use of atomic force microscopes even easier by providing the ability to automatically control operating parameters such as set values, Z limits and scanning speeds.</p><p num="0049"> Basically, a preferred embodiment is intended for an atomic force microscope that limits the need for a skilled user, and the chip is substantially relative to the sample surface to allow the chip to interact with the sample and then move away from the sample. This is achieved by using a peak force tapping mode that functions to move the sample vertically. The feedback circuit uses the instantaneous interaction force at any interaction point (eg, substantially orthogonal to the sample surface), preferably by using the maximum repulsive force. This new mode of operation takes advantage of the instantaneous response of the probe through chip-sample interaction, using a feedback loop to maintain steady-state interactions and control chip tracking on the sample (as in prior art). Without having to wait for the ringdown, the technology measures the baseline or zero force reference value and returns the chip to the surface almost instantaneously). By moving the chip perpendicular to the sample surface, this mode removes at least substantially the frictional forces during raster scanning or other relative probe sample motion in the XY plane of the Tapping Mode atomic force microscope. Share the benefits. Furthermore this The implementation of the mode minimizes parasitic bonds (at least 3 digits) so that much more sensitive force control can be achieved than pulse force mode and Tapping Mode atomic force microscopy. By doing so, the lowest force imaging (using alternating forces) known in atomic force microscopy techniques is realized and directly controlled, so atomic force microscopy is typical of Tapping Mode interatomic forces. Allows you to provide improved high-resolution images that exceed the Tapping Mode atomic force microscope at speeds above the force microscope speed (Tapping Mode band is below 1 kHz).</p><p num="0050"> The added benefit of peak force tapping mode is that each vertical motion period produces a force curve, or multiple force curves at each pixel, which allows simultaneous acquisition and mapping of height and mechanical property data. To. Therefore, this method generates and analyzes individual force curves, then measures and controls the atomic force microscope based on the corresponding peak interaction forces in each chip tapping case on the sample, and the imaging rate. Is higher than the Tapping Mode imaging speed.</p><p num="0051"> A method of operating a scanning probe microscope according to the first feature of the present invention includes generating a relative motion between a probe and a sample and detecting the motion of the probe. The method recovers probe-sample interactions that are substantially independent of parasitic probe deflection (ie, parasitic cantilever motion) from the detected probe motion. </p><p num="0052"> In another feature of the invention, the method of operation of the scanning probe microscope maintains a maximum repulsive probe-sample interaction force of about 10 pN or less during each periodic motion period of the chip that is substantially perpendicular to the sample. Including producing an image while. Such interaction forces can be directly controlled and accurately calibrated.</p><p num="0053"> According to another feature of the present invention, the method of operation of a scanning probe microscope is less than 5 nN without user intervention while maintaining image resolution better than 5 nm regardless of the environment including air, gas, fluid and vacuum. Includes generating an image for at least 1 hour with a peak force of.</p><p num="0054"> In another feature of the invention, a method of operating a scanning probe microscope comprises generating at least one force-distance curve for each imaging pixel. Force-distance curves are used to generate accurate measurements of one or more of the van der Waals adsorption, elasticity, chip-sample interaction adsorption operations, such as plasticity such as hardness and viscoelasticity. It is possible.</p><p num="0055"> The peak force tapping method of operating a scanning probe microscope according to another feature of the present invention allows the ability to map mechanical properties in the range of approximately 0.01 N / m to 1000 N / m (approximately 10 kPa to 100 GPa). Includes the use of a cantilever with a spring constant comparable to. The range of this applicable cantilever is generally several orders of magnitude wider than the cantilever applicable to the Contact Mode atomic force microscope (0.01 to 1 N / m) and the Tapping Mode atomic force microscope (1 N / m to 40 N / m).</p><p num="0056"> Scanning probe microscopes constructed according to the present invention can be used to scan various types of samples, including patterned wafers, biological samples in air and fluids, polymers, thin films, and data storage components.</p><p num="0057"> According to a further feature of the present invention, a method of operating a scanning probe microscope includes interacting a probe tip with a sample, and then terminating the interaction to result in attenuation of probe vibration. The method then repeats the interaction and detects probe motion before the ringdown of damping probe oscillations is substantially complete.</p><p num="0058"> In another feature of the present invention, the method of operating a scanning probe microscope (scanning probe microscope) includes generating a relative motion between the probe and the sample and then detecting the motion of the probe. .. Further methods include recovering a near-instantaneous force between the tip and the sample from the detected probe motion. The method also preferably automatically controls the generation stage to maintain the feedback settings.</p><p num="0059"> In another feature of the invention, the control loop controls the interaction force at a predetermined synchronization distance. The synchronization distance is defined as the period from the beginning of the modulation phase to the time corresponding to the point chosen to control the feedback. The instantaneous force generated at this point is used as a feedback control parameter and is usually selected as the point at which the peak repulsive force occurs.</p><p num="0060"> Again, the tapping mode is complicated by a) indirect force control and b) cantilever resonance kinetics in multiple harmonic mode. Another major drawback is that neither the amplitude nor the phase of the probe oscillation during data acquisition has a monotonous relationship with the chip-sample interaction force. As a result of these complexities, subjective judgment must be used in the feedback optimization process to obtain the desired image, with minimal interaction and the most stabilized (most robust) feedback. It often means that the user must be an atomic force microscope expert in order to obtain high quality images. Synchronized peak force control (peak force tapping mode) of a preferred embodiment eliminates the complexity of cantilever dynamics, as well as the complexity induced by cantilever resonance and its harmonics. The peak force tapping mode uses the interaction force directly as a feedback control parameter for the first time. Even in contact mode atomic force microscopy, constant drift of cantilever deflection due to heat or other system factors makes accurate force control impossible. In peak force tapping mode, the system reestablishes non-interaction baselines by moving the probe farther from the sample during each interaction phase. This process allows the interaction force to be accurately determined each time the probe comes into contact with the sample. Direct force control and removal of complexity with cantilever dynamics have monotonized the criteria needed to obtain the highest quality images. As a result, control loop automation can be performed by designing the appropriate computer program. In order to optimize the feedback performance, for example, the subjective judgment of a skilled user based on the past experience of imaging a similar sample is also excluded.</p><p num="0061"> In yet another feature of the invention, the automatic control step comprises automatically determining the minimum interaction force required for control based on the noise background of the system. This is the minimum interaction force that can be used as a set value in the control feedback loop.</p><p num="0062"> In yet another feature of the invention, the automatic control step determines feedback instability within a 5-chip-sample interaction period (eg, 2.5 ms), about 100 times faster than a skilled visual judgment. including.</p><p num="0063"> In yet another feature of the invention, the automatic control step comprises automatically controlling the gain of the corresponding feedback loop. In yet another feature of the invention, the method includes automatic Z-limit control, and preferably automatic scanning speed control.</p><p num="0064"> These and other features and advantages of the present invention will become apparent to those skilled in the art from the detailed description below and the accompanying drawings. However, as a matter of course, although preferred embodiments of the present invention are shown, detailed description and specific examples are given by way of example, not by limitation. Many changes and improvements can be made within the scope of the invention without departing from the spirit of the invention, and the invention includes all such improvements.</p><p num="0065"> Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings. Similar reference numbers throughout the drawing represent similar parts.</p>

<figref num="1">Block diagram of a conventional atomic force microscope that is properly classified as "prior art".</figref><figref num="2A">Graph of chip-sample distance vs. time in vibrating atomic force microscopy mode.</figref><figref num="2B">Graph of interaction force vs. time in vibrating atomic force microscope mode.</figref><figref num="2C">Graph of scanning probe microscope force curve illustrating probe sample interaction, "ring down" and second probe sample interaction diagram.</figref><figref num="3">A force vs. time graph exemplifying the determination of the instantaneous force of feedback control according to a preferred embodiment.</figref><figref num="4A">Approximate graph illustrating probe deflection vs. time exemplifying chip sample interaction forces that are cyclically modulated by parasitic vibrations in this system.</figref><figref num="4B">Schematic of cantilever probe response vs. time with hydrodynamic (fluid) background vibrations only from parasitic sources.</figref><figref num="4C">A graph of deflection error vs. time after subtraction of hydrodynamic background vibrations.</figref><figref num="5A">Time graph of deflection response before background subtraction.</figref><figref num="5B">Time graph of subtraction background.</figref><figref num="5C">Hydrodynamic background Time graph of deflection error after vibration subtraction.</figref><figref num="6A">FIG. 6 is a schematic diagram of force vs. time illustrating the baseline averaging method of a preferred embodiment.</figref><figref num="6B">Graph of distance vs. time between chip and sample.</figref><figref num="6C">Graph of cantilever deflection vs. time.</figref><figref num="7">A schematic graph of force vs. time exemplifying prior art that averages forces over a period to detect chip sample interactions.</figref><figref num="8A">Approximate force vs. time curve illustrating gate average repulsive force control according to a preferred embodiment.</figref><figref num="8B">Input synchronization signal diagram transmitted by force response by chip-sample interaction to realize gate average repulsive force control according to a preferred embodiment.</figref><figref num="9A">Schematic of a series of force curves used in synchronous averaging according to a preferred embodiment.</figref><figref num="9B">The graph illustrating the sync signal transmitted with the deflection applied in the force curve of FIG. 9A.</figref><figref num="9C">The graph which illustrates the force curve signal after some synchronous averaging cycles of FIG. 9A.</figref><figref num="10">1 Schematic block diagram of an atomic force microscope capable of operating in peak force tapping mode according to an embodiment.</figref><figref num="11">A flowchart illustrating a method according to a preferred embodiment.</figref><figref num="12A">A schematic graph of a force curve exemplifying system settings and measured deflection.</figref><figref num="12B">1 Schematic of the feedback error generated according to the prior art method of controlling atomic force microscopy operation by starting with force after the end of the modulation cycle.</figref><figref num="12C">Schematic of a feedback error similar to FIG. 12B according to a preferred embodiment of the present invention.</figref><figref num="13">A flowchart illustrating a method according to a preferred embodiment illustrating the deflection background subtraction.</figref><figref num="14">A flowchart illustrating cantilever deflection background subtraction using a lock-in amplifier according to a preferred embodiment.</figref><figref num="15">A flowchart illustrating the deflection background subtraction in a normal engagement process.</figref><figref num="16">A flowchart illustrating the deflection background subtraction in the sewing engagement process.</figref><figref num="17">A force vs. time graph illustrating baseline calculation according to a preferred embodiment.</figref><figref num="18">A force-to-time graph exemplifying the algorithm used to determine the instantaneous interaction force.</figref><figref num="19">A flowchart illustrating an instantaneous force control imaging method.</figref><figref num="20A">The graph which illustrates the force vs. time when the instantaneous force control imaging according to a preferred embodiment is used.</figref><figref num="20B">The graph which illustrates the z position when the instantaneous force control imaging according to a preferred embodiment is used.</figref><figref num="21A">Tapping Mode Atomic Force Microscope image.</figref><figref num="21B">An atomic force microscope image illustrating deep trench measurement using an instantaneous force control mode according to a preferred embodiment.</figref><figref num="22A">A graph of force-to-chip-sample distance exemplifying a small-amplitude repulsive force mode (SARF) according to a preferred embodiment.</figref><figref num="22B">A graph illustrating force vs. time for SARF mode.</figref><figref num="23A">A graph of force-to-chip-sample distance exemplifying a small amplitude attractive mode (SAAF) according to a preferred embodiment.</figref><figref num="23B">A graph illustrating force vs. time for SAAF mode.</figref><figref num="24A">A schematic graph of feedback tracking signal vs. scanning position showing sample profile and corresponding tracking signal (height) during atomic force microscopy imaging, exemplifying the difference between stable and unstable feedback.</figref><figref num="24B">A schematic graph of the feedback error signal corresponding to the height signal in Figure 24A.</figref><figref num="25">A schematic graph of spectral amplitude vs. frequency illustrating a feedback signal spectrum used to detect feedback loop instability according to a preferred embodiment.</figref><figref num="26A">A series of schematic graphs exemplifying parachute detection showing that the chip-sample interaction force is approximately baseline during a parachute event.</figref><figref num="26B">A series of schematic graphs exemplifying parachute detection showing that the chip-sample interaction force is approximately baseline during a parachute event.</figref><figref num="26C">A series of schematic graphs exemplifying parachute detection showing that the chip-sample interaction force is approximately baseline during a parachute event.</figref><figref num="26D">A series of schematic graphs exemplifying parachute detection showing that the chip-sample interaction force is approximately baseline during a parachute event.</figref><figref num="27">Schematic diagram of an atomic force microscope according to a preferred embodiment, exemplifying gain control in a feedback loop.</figref><figref num="28">The schematic diagram of the vibration detection algorithm of FIG. 27.</figref><figref num="29A">Schematic of the data resampled and processed by the vibration detection algorithm of FIG. 28.</figref><figref num="29B">Schematic of the data resampled and processed by the vibration detection algorithm of FIG. 28.</figref><figref num="29C">Schematic of the data resampled and processed by the vibration detection algorithm of FIG. 28.</figref><figref num="29D">Schematic of the data resampled and processed by the vibration detection algorithm of FIG. 28.</figref><figref num="30">The figure which illustrates the execution of the preferred embodiment which operates an atomic force microscope in a peak force tapping mode.</figref><figref num="31">Flowchart of scanning speed control algorithm used in peak force tapping mode.</figref><figref num="32A">A schematic graph of chip-sample interaction forces where the scanning speed can be substantially optimized.</figref><figref num="32B">Approximate graph of chip-sample interaction force when scan speed is not substantially optimized.</figref><figref num="33">The figure which illustrates the Z limit control method according to a preferred embodiment.</figref><figref num="34">Schematic chip-sample interaction force diagram illustrating chip radius monitoring using the techniques of preferred embodiments.</figref>

In a preferred embodiment, the interatomic force used to monitor the interaction force between the probe (chip) and the sample and to control the distance between the chip and the sample with very low force without compromising scanning speed. The peak force tapping (PFT) mode of force microscope operation is targeted. The techniques described herein provide high resolution by keeping the probe tip-sample force low, while achieving essentially real-time property mapping of the sample surface. The preferred embodiment is inherently stable, thus facilitating long-term force control while maintaining the ability to acquire reliable data (improved resolution). Moreover, unlike conventional Tapping Mode atomic force microscopes, no tuning is required, so the atomic force microscope setup is faster and easier than in other atomic force microscope modes. Important ideas for driving peak force tapping modes are illustrated and described herein graphically.

In practice, there were three major problems to be solved before atomic force microscopy could be performed using instantaneous interaction forces. These problems were 1) adjustment of the flexure background by coupling; 2) baseline determination; and 3) instantaneous force determination as defined herein.

In FIG. 2A, the modulation period that approaches and separates the probe from the sample (eg, by using a drive that periodically modulates the distance between the probe and the sample) is represented by the period T. The zero position (horizontal axis) represents the surface, while the vertical axis is the distance interval. If the distance between the probe and the sample crosses the horizontal zero line, the chip comes into direct contact with the sample, as represented by the region δT (time zone of chip-sample contact). The interaction forces corresponding to this region are plotted in Figure 2B.

In FIGS. 2A and 2B, Amax is the maximum distance between the sample and the tip of the chip; Fa_vdw is the van der Waals adsorption force; Fa_max is the capillary interaction and adsorption between the chip and the sample surface. It is the maximum adsorption force depending on the work. Both the repulsive force and the attractive force are calculated with respect to the baseline as shown in FIG. 2B. It should be noted that the force referred to here is the resultant force acting on the entire chip, which is usually pyramidal. In fact, only the apex is in the repulsion zone, but the resultant force is still attractive. In this case, even if the resultant force at this point is an attractive force, the feedback can still use the vertex repulsion interaction force at a predetermined synchronization position (defined as described below) for the feedback. .. This is determined by the apex repulsion interaction resulting from Pauli repulsion and ion repulsion between the atom at the very apex of the probe and the atom or molecule of the sample, so the highest imaging resolution with the least interaction force. Provides the benefit of working with.

It is important to distinguish between cantilever deflection and tip-sample interaction forces. Cantilever deflection is used to measure chip-sample interaction forces, but not all deflections represent chip-sample interaction forces. That is, the parasitic force contributes to the deflection of the cantilever. For example, as shown in Figure 2C, the cantilever deflection is plotted as a function of time and the figure represents the actual deflection data. The vibration after point "D" is due to the cantilever free resonance that decays over time. This resonant deflection is not due to chip surface interaction, but is believed to be contributed by parasitic deflection (usually corresponding to parasitic cantilever or probe motion). Point E represents the highest point of deflection where the tip does not interact with the sample. The "flat" portion of the data also has a more gradual deflection variation, usually due to the mechanical coupling of parasitic forces, if the tip does not interact with the sample. Such coupling is a modulation actuator It can be for itself and for either the cantilever response due to braking force from air or fluid. Similarly, it results from laser interference. These parasitic effects will be further illustrated in subsequent figures.

In known force control systems, control is based on the maximum force generated over a period of time. Therefore, the repulsive force must be higher than any parasitic force that is distinguished from the parasitic force and that contributes to the deflection of the true tip-sample interaction traditionally used by feedback loops. This force differentiating requirement required relatively high imaging power that could damage at least one of the chip and the sample, thereby preventing the system from obtaining high resolution.

In a preferred embodiment, the RMS or constant deflection is replaced by the instantaneous interaction force Fr_i determined according to FIG. 3, and the controller set value is in Equation 1 below. δFr = Fr_i-F (baseline) ... (1) F (baseline) is the interaction force when the probe is not in contact with the sample. Its interaction force should be zero. In an atomic force microscope, force is usually expressed by cantilever deflection. In this case, F (baseline) corresponds to the cantilever deflection when the tip does not interact with the surface. Fr_i is the interaction force when the chip comes into close contact with the surface. The synchronization algorithm is used to align the start time of each drive phase so that the region δT (FIGS. 2A-2B) can match the repulsive force and its maximum Fr_max. The time from the start of the period to the expression of Fr_max is the synchronization time, which is accurately determined and controlled (see further below). Sync time distance (Sync Distance) can be determined by measuring the phase delay between the deflection response and the modulation drive signal. Once the sync time distance is determined (if the probe is fixed in the xy direction), the same sync time distance is used throughout the xy raster scan position. During imaging, feedback works to keep Fr_i substantially constant, but the Fr_i value is determined by the sync time distance. It should be noted that the synchronization time distance can also be generalized as the distance from the start of the modulation phase to the moment of interaction.

The synchronization distance or synchronization time distance can be precisely controlled. For example, when the synchronization distance is 48 μsec (μs) and the chip vibration period T is 100 μsec, the interaction force generated at 48 μsec is used as a feedback control parameter. The feedback loop will attempt to maintain the instantaneous interaction force Fr_i (i = 48 μs) 48 μs after the start of the period. In a more general application, the interaction force of any point within the interaction region δT can be used for feedback. δT can also extend beyond the marked region in FIG. 2B to include Fa_vdw (van der Waals attraction region) and Fa_max (capillary adsorption region). Capillary adsorption regions can also be adsorption interactions due to binding forces induced by functional probes and special adhesives on the sample.

To achieve accurate baseline measurements, multiple deflection data points are collected when the chip is not in contact with the sample and are used to generate the average baseline level. Again, the non-interaction region (maximum separation / maximum distance) can be determined by the synchronous time distance, as this region should be approximately half the period of the modulation period after the peak force position. .. The synchronous time distance also determines the feedback force operating point, and the actual force is measured by δFr. δFr is either positive or negative.

Due to the adverse effects of drift (eg heat) on the deflection signal, the response force Fr_i can fluctuate over time. The relative force δFr (for baseline determination) more accurately reflects the chip-surface interaction and is preferably used for feedback control instead of Fr_i. This relative value eliminates the harmful effects of system drift on cantilever deflection. To do.

δFr also represents a force that can be controlled by a feedback loop so that δFr remains constant over time at various positions as the chip scans the sample. In FIGS. 4A-4C, the cantilever response is a mixture of chip-surface interaction forces and background binding when interacting with the sample surface. Such a response is outlined in Figure 4A as "original". The actual chip-sample interaction force is only in the Fr_i portion (shown in Figure 4C) and is buried in the background of the parasitic cantilever or probe motion. By subtracting the background from the original data (for example, probe motion due to both interaction and parasitic forces), it is possible to obtain the magnitude of the interaction force. The background illustrated in FIG. 4B may be due to at least one of the mechanical coupling of resonances from the atomic force microscopy system and the cantilever response to its environmental media such as air and fluids. The background can also be triggered by laser interference as the cantilever moves with respect to the sample. A common feature of the background is that the cantilever deflection, which indicates periodic changes, resembles the tip trajectory, even when the tip does not interact with the sample. Subtraction of successful background experimental data is shown in FIGS. 5A-5C.

More specifically, FIG. 5A shows a schematic of the original probe deflection vs. time. As mentioned above, probe deflection is highly influenced by sources of parasites that can be used to control chip-sample interactions. As illustrated, these periodic parasitic deflections are represented herein by, for example, a low frequency signal referred to herein as, for example, "hydrodynamic background" or, in more general terms, parasitic forces. The contribution of these parasitic forces (including fluid force, drag force and air, off-axis motion, laser interference and any other periodic motion that occurs when the probe is not interacting with the sample) to probe deflection is large. Since the actual chip-sample interaction force to be used as the control signal in the preferred embodiment is superimposed on the parasitic background signal (Fig. 5B), it can be a task to detect the actual chip-sample interaction force. That is, the minimum controllable force is determined by the background contribution to probe deflection (shown in Figure 5A as the minimum controllable force [old]-from less than about 1000 micronewtons (μN) to less than 10 piconewtons (pN). range). Especially from beginning to end, there is a noise signal "N" with low amplitude for both the parasitic contribution to the deflection and the contribution to the deflection due to the chip-sample interaction force.

Moving on to FIGS. 5B and 5C, one important idea of this preferred embodiment is to subtract the parasitic background signal (FIG. 5B) from the deflection signal as described above. This lowers the minimum controllable force. The background signal is determined by sufficiently increasing the chip-sample distance to the control distance to prevent the probe from interacting with the sample (ie, only the parasitic force contributes to the detection deflection of the probe). Will be done. The control distance is generally greater than 100 nm and may be relatively small, but ideally the long-term interaction force does not contribute to probe deflection. As shown in FIG. 5C, the contribution of the tip-sample interaction force to the deflection after subtracting the parasitic background provides a deflection signal with a clear peak associated with the chip-sample interaction. In particular, aperiodic noise can always be present, so in this case, the minimum controllable force as shown in FIG. 5C is determined (minimum controllable force [new]). For a very soft cantilever with a spring constant of 0.01 N / m and a cantilever length of 100 μm, this force can be about 1 pN.

The minimum controllable force available when performing parasitic background subtraction is greatly reduced (eg, 3 digits), which allows the preferred embodiment to control the distance between the chip and the sample, thus allowing the probe-sample interaction force. Is shown to be reduced to the pN range. How this subtraction can be performed in hardware is further described below with respect to FIG.

In FIG. 10, "Z" is the vertical between the chip and the sample surface, which is generally referred to as the chip position. Indicates the direction perpendicular to the sample surface indicating the position. Overall, detecting such a small force and using such a force as a control parameter in a scanning probe microscope feedback loop uses what is referred to herein as "instantaneous force control". Thus, the scanning probe microscope operation according to the present invention is the ability to enable imaging of the sample. Instantaneous force control with real-time force detection provides improved control, thus improving image resolution and minimizing the possibility of sample damage. In this context, real-time or instantaneous force detection, eg, the variability of essentially each point exemplified in FIG. 3, is detected by a preferred embodiment and further controls the operation of the scanning probe microscope instantaneously. Implying that it can be used. That is, the variation acting on the probe due to the probe-sample interaction during each interaction cycle between the probe and the sample [or during each period of distance spacing modulation between the two, i.e., during each cycle of force curve modulation]. Forces can be detected and used by atomic force microscopy to further image the sample in real time. This instantaneous force control is used to provide atomic force microscopy control at any point of interaction that can be one modulation period of the distance between the probe and the sample. Feedback delay is greatly reduced because control is provided before the end of any future modulation period (before the next approach). This will be further shown in relation to FIGS. 12A, 12B and 12C.

Yet another benefit of peak force tapping control is that the control does not need to be operated near the cantilever resonance frequency. Such an operation allows for instantaneous interaction control, as the cantilever delay due to the transient resonant response can be substantially eliminated.

Next, moving to FIG. 6, the preferred embodiment also allows the atomic force microscope to operate at high speed by performing baseline averaging of the force curve to quickly extract the zero force point, and further delays the system almost in time. Allow the probe to interact with the sample without. In contrast to the prior art represented by Figure 2C, the modulation frequency of this atomic force microscope is set to stabilize the imaging system until the probe "ring down" is complete (after the chip jumps off the sample surface). The probe vibration is attenuated to about 1 / e) and is not limited by the requirement to wait for the system to reconstruct the probe-sample interaction. The time required for phosphorus down is determined by cantilever dynamics, which is proportional to Q / f. In this case, Q is the quality factor of the cantilever and f is the cantilever resonance frequency (typically tens of milliseconds for cantilever customarily used for stabilization). In a preferred embodiment as shown in FIG. 6, the cantilever resonance frequency of several cycles after ringdown determines the zero force point (ie, the baseline position at the time of device installation) in essentially real time and is illustrated in FIG. 2C. The system is averaged to allow the probe to interact with the sample much faster than the system. In fact, robust estimation of the zero point (baseline) can also be achieved by averaging only one cycle of cantilever resonance frequencies after ringdown. As a result, the modulation frequency can be significantly increased without compromising system stability. What's more, the additional benefit of running faster is, of course, reducing the effects of noise in the system.

Very soft cantilever (spring constant 0.01N / m to 0.3N / m) is commonly used for very sensitive force detection measurements. These levers have a lower resonant frequency and a very long ringdown time. More importantly, the adsorption-induced vibrations (breaking out of contact) are much stronger, as shown in Figure 6C. In Figure 6C, the flexure response of the soft cantilever is plotted as a function of time. The chip trajectory is also plotted as a positional reference (Fig. 6B). As shown in the figure, the parasitic vibration of the cantilever is far superior to the interaction force, which basically makes it uncontrollable. Prior to the present invention, the user had to wait for the time required for the vibration to disappear so that Fr_i could only reach its maximum value in order to obtain stable feedback control. Will. The more sensitive the cantilever, the longer it will take to wait for the ringdown. In a preferred embodiment of the present invention, the baseline is determined by separating the interacting region and the non-interacting region by synchronous alignment at the closest position between the probe and the sample. The region corresponding to the "interaction region" is locked through the synchronization marker and the reference trigger signal at the start of each cycle. Any deflection point in this region can be used as a feedback parameter for steady-state interaction control. All deflection data outside the interaction zone is averaged to a constant and used as the baseline for calculating ΔFr in FIG. The combination of baseline detection and synchronous control allows the relative force δF to be instantly and accurately determined and controlled. Such control allows Fr_i to be well below the parasitic deflection, as illustrated in Figure 6C.

Steady state also means a constant maximum or minimum force, or a combination of interaction force curve features in probe / sample relative motion for each period. Another major advantage of this technique is the ability to determine baselines with high-amplitude vibration data. Since the resonance frequency of the cantilever is known, in another embodiment the average can be determined in the non-interaction region by analyzing the cantilever resonance frequency period of an integral multiple. Since the integer period averaging can efficiently remove the vibration deflection data, it is possible to obtain a constant baseline.

In particular, the cantilever resonance frequency can also be determined by known techniques such as frequency sweeping and thermal tuning. Moving on to FIGS. 7 and 8A and 8B, preferred embodiments also use what is referred to herein as "gate average repulsive force control". FIG. 7 schematically shows probe deflection including a series of interaction periods after atomic force microscopy operation. The advanced control technique, which uses force as a control parameter, obtains an RMS value to be compared with the force set value by averaging the resultant force over the chip-sample interaction over the entire cycle. As will be understood in the art, the forces exemplified by the force curve are complex. Both the repulsive force and the attractive force act on the probe tip during the cycle as described above. Force sensitivity and imaging resolution are almost always compromised by including, for example, the attractive part (CD in Figure 2C) that tends to offset the repulsive force.

Moving on to FIGS. 8A and 8B, gate average repulsion control is illustrated. In this embodiment, the system sync signal as shown in FIG. 8B is intended to "gate control" the repulsive force portion of the force curve (BC in FIG. 2C) by excluding the attractive portion of the force curve (deflection). (Illustrated by the shaded portion "A" of the curve) used. By controlling the distance between the chip and the sample based on the repulsive part of the force curve, the force sensitivity and imaging resolution are increased to reduce the harmful effects of the attractive part of the curve (ie, the interaction attraction is Lower resolution because it is a long-term interaction force and therefore a sense interaction over a much larger area). Moreover, the gate functions to eliminate noise when performing gate averaging. Again, the synchronization signal is timed so that only the repulsive force region can be used. Such operation is guaranteed by using the gate in a predetermined synchronous position as illustrated and described in connection with FIG.

Further according to the above, as shown in FIGS. 9A and 9B, synchronous averaging can also be used to further improve the signal-to-noise ratio, thus ultimately providing control at near zero power points. Is possible. Other Chip-Similar to the sample deflection diagram, FIG. 9A shows some deflection periods of the probe as the chip interacts with the sample. As already mentioned, noise signals are always present when making these types of scanning probe / atomic force microscopy measurements. Synchronous averaging of deflection is performed by combining the deflection signal with, for example, the corresponding synchronization signal as shown in FIG. 9B. As a result, the effect of noise is greatly reduced according to Equation 2 below.

(D1 + D2 + D3 + D4 + DN) / N ... (2) In the equation, Di represents the data in the i-th period. An averaged signal with an improved signal-to-noise ratio that is a multiple of N is shown in Figure 9C. This reduces the minimum controllable force (a narrow lock-in bandwidth can be used).

Next, moving to FIG. 10, the atomic force microscope 100 capable of operating in the peak force tapping mode includes a probe 102 having a cantilever 104 mounted on the probe holder 108 and supporting the tip 106. In this case, the distance between the chip and the sample is modulated by an actuator 112 (eg, an XYZ cylindrical piezoelectric element) coupled to the probe holder 108. However, of course, a preferred embodiment is applicable to an atomic force microscope instrument that modulates the distance between the chip and the sample by moving the sample in the Z direction.

During operation, probe deflection is measured by directing a ray "L" to the back of the probe and reflecting it towards a detector 114, such as a four-quadrant optical detector. The deflection signal is then transmitted to the analog-to-digital converter 103. The digitized signal is used to keep the chip-sample force low while operating the atomic force microscope at high speed.

In the embodiment shown in FIG. 10, the probe deflection without chip-sample interaction is transmitted to the background generator 105. The background generator will generate a periodic waveform corresponding to the background signal if the chip and sample are not interacting. The amplitude and phase of this waveform are determined by a lock-in amplifier, and its input is the background signal DDS (Direct Digital). It can be generated by Synthesis) Function Generator). This waveform can also be generated by using a synchronous signal to synchronously average the background of multiple cycles. The comparator circuit 120 processes the entire deflection signal by subtracting the background signal to generate a signal that represents the chip-sample interaction force independent of the parasitic background (FIGS. 4C and 5C). (It should be noted that analog or digital circuitry can be described, but of course the operation can be performed on any conventional analog or digital circuitry, but preferred embodiments are those of the present invention. Use the FPGA architecture to implement). This signal is then supplied through a digital filter 122 that processes the post-subtraction deflection error to limit the processing ringdown vibration of the lever to the number of selected cycles. The filtered signal is transmitted to the synchronous averaging circuit 123 to further increase the signal-to-noise ratio. The baseline is determined from the baseline averaging circuit 124 by averaging the data in the non-interaction region utilizing synchronization. The comparator circuit 125 processes the entire deflection signal by subtracting the baseline signal to generate a signal that represents the chip-sample interaction force without cantilever DC drift. This signal is further transmitted to the force detector 126.

The synchronous time-distance calculator 135 determines the phase shift between the deflection and the Z-modulated direct digital synthesizer (block 127), which provides drive and synchronous control in a time-delayed manner. The peak force or repulsive force gate position generator 129 utilizes a synchronization marker and a synchronization time distance to generate a timed signal for the force detector 126. The force detector 126 analyzes the output of adder circuit 125 by identifying either the peak repulsive force or the averaged repulsive force in the gate region illustrated in FIG. 8A. Again, by operating the force detector 126 in this way, force control can be triggered at the selected part of the force curve (eg, the repulsive force region), and the action of the attractive force between the sample and the chip. Higher sensitivity is obtained by reducing. Moreover, the signal-to-noise ratio is improved by excluding noise from the gate of detector 126. The gate repulsive force is then compared to the appropriate set value (block 128) and an error signal is generated and transmitted to the control block (eg PI controller 130). Next, the control signal becomes analog It is converted (converter 132), the sync signal is converted to analog by the converter 136, and then transmitted to the adder circuit 134 to be coupled with the sync signal from block 127. The output of adder circuit 134 is then applied to the Z-piezoelectric actuator 112 to actuate the z-position (in this case, the probe) to maintain an essentially steady-state interaction between the chip and the sample. Corresponding operating methods are described in more detail below in connection with FIG.

Moving to FIG. 11, the operation method 300 of the atomic force microscope according to the peak force tapping mode is shown. After setup and initialization block 302 (no tuning required), the probe vibrates and engages the sample. Preferably, at block 304 (generation stage), a relative XY motion between the probe and the sample begins (scanning, interaction stage).

The motion of the probe is then detected (detection stage); in particular the probe deflection is detected and transmitted to the transducer for further processing. At block 306, the method then functions to recover the probe-sample interaction as described above (recovery step), preferably using either lock-in amplification, or more preferably synchronous averaging of deflection. Perform hydrodynamic background subtraction. After filtering the output in block 308 (eg, selecting the number of ringdown cycles to process), the method preferably detects force by using the repulsion region of the force curve in block 310 (peak force detection / Gate averaging). At block 312, the force is then compared to a set value force set according to the user's desired interaction force. The Z actuator is a control signal used to generate an image of the sample and responds to the control signal at block 316 to adjust the distance between the chip and the sample and maintain the set value force.

Moving from FIG. 12A to FIG. 12C, a diagram of the ability of a preferred embodiment to provide instantaneous force feedback is shown. In FIG. 12A, some schematic force vs. time curves are shown with different peak repulsive forces. In particular, the interactions Q and S exceed the threshold force defined by the set value, while the interaction R exemplifies a peak repulsive force below the set value. The feedback error is illustrated in the prior art force feedback system as shown in Figure 12B. More specifically, when the repulsive force exceeds the set value, the delay "d" is shown before mapping the peak repulsive force at X of the first interaction. This is also the case for the interaction displayed as S, where no feedback error is built until some time after the repulsive force begins to exceed the set value.

Conversely, as shown in FIG. 12C, due to the characteristics of the peak force tapping mode described above, preferably in combination with synchronous averaging, including parasitic background subtraction, baseline averaging and gate averaging, and repulsive force control. Given that there is less feedback delay, the response to any force greater than the set value is detected essentially instantly. By being able to quickly identify forces above the set value, the force corresponding to the chip-sample interaction can be minimized, providing significant advantages for atomic force microscopy operation at high speed and high resolution. Further, this is especially true for undulating samples where sample surface changes can limit at least one of response time and resolution.

[algorithm] Two schemes have been developed, as shown in FIGS. 13 and 14, to ensure accurate background subtraction.

In FIG. 13, the algorithm 400 for subtracting the cantilever deflection background (parasitic contribution to deflection) is shown. Blocks 402 and 404 ensure that the chip is sufficiently far from the sample (eg, 30 nm) to prevent the presence of repulsive impulse interactions on the surface according to the user's choice after setup. Block 406 contains several partial stages. Atomic force microscopy system flexes cantilever with multiple cycles The data is sampled and then digitized into multiple segments, each segment having a period T. The atomic force microscopy method aligns each segment of the data to the beginning of period T and then averages the data. Method 400 then uses the averaged segment data as the background for period T. Block 408 functions, for example, by using an FPGA processor to subtract the background from block 406 from the measurement data at each period T. Block 408 uses background correction data for feedback.

In FIG. 14, another algorithm 500 for subtracting background deflection is shown. Blocks 502 and 504, which calculate the lift and lift the tip with z-feedback off, are used to ensure that the tip is not interacting with the sample. Block 506 uses a lock-in amplifier for the drive signal that moves the cantilever probe as a reference signal, and uses cantilever deflection data as a lock-in input. In block 508, the amplitude and phase data obtained from the lock-in is used to construct a sinusoidal signal, which is adjusted until the deflection is constant (within the noise limit) to further subtract the deflection data. Used. Real-time subtraction is performed at block 510. Once sufficient subtraction is obtained (determined with constant deflection if the chip is not interacting with the surface), the atomic force microscope can use background correction data for feedback in block 512.

The background calculated according to FIGS. 13 and 14 varies substantially as the probe approaches the sample surface. Such fluctuations are due to fluid forces that are a function of the distance between the probe and the sample surface. Such variability can also serve as an indicator of the proximity of the probe to the sample before it actually interacts with the sample. With this knowledge, the electric engagement can proceed at high speed until a predetermined background value is obtained. However, a later engagement step can then be performed.

Background subtraction is also performed during the engagement of the sample surface with the probe, as shown in FIGS. 15 and 16. The difference between the two engagement methods is that the "normal" engagement 600 in FIG. 15 uses a step motor only to drive the probe towards the sample to detect the sample surface. However, FIG. 16 shows a "sewing" engagement in which the probe is driven by a Z piezoelectric actuator at each motor step as Method 700 searches for the sample surface. First referring to FIG. 15, Method 600 first steps the motor in block 602 to reduce the distance between the chip and the sample, for example according to a fixation step of 0.1 nm to about 3 microns. When feedback control is turned on (force detection according to the technique), the feedback loop controls the actuator in block 604 to move the chip towards the sample in this case (delivery stage). At block 606, the algorithm determines if the surface has been detected (ie, if the threshold set force has been reached). If not, the background subtraction operation as described above related to FIG. 5 is performed at block 602 before further stepping the motor. If so, the feedback is released and the lift is calculated by calculating the z-movement between the peak force and the negative maximum adsorption force position by adding a specific margin (eg 10 nm) to the tip. Can be lifted in block 610 (eg to minimize the possibility of destruction). Then, in block 612, the background subtraction operation is executed, and in block 614, the feedback control according to the present technique is reinitialized.

In FIG. 16, blocks 708,712,714 and 716 correspond directly to blocks 606,610,612 and 614 of Algorithm 600 in FIG. However, prior to detecting the surface, for example sewing engagements known in the art are used to lift the tip in block 702 and then step the motor in block 704. Go down. In this case, the lift is 1.5 times that of the motor step. The lift amount can be selected by the user based on the type of sample and the like. Feedback is then turned on in block 706 to detect force according to this technique. If no surface is detected, Algorithm 700 performs background subtraction in block 710 (similar to block 608) and then another lift in block 702. Once the surface is detected, the scanning probe microscope is capable of imaging the sample in block 716.

FIG. 17 illustrates the actual situation of chip-sample interaction and provides a supplement to the above description related to FIG. The actual chip-sample interaction occurs only in the vicinity of the synchronous time-distance marker. In the free interaction region, there is residual autonomous vibration of the cantilever due to the elimination of the suction force (also known as ringdown). Such vibrations cause baseline variability, giving the same δFr variability shown in FIG. Such fluctuations can result in controller noise. To minimize baseline variation, the data displayed as within the "baseline average" area is averaged to a single constant represented by the dotted line. This constant data is used as a baseline in calculating δFr in each feedback period. The area of "baseline average" can vary depending on the data quality. The region should be smaller than the synchronization time distance to avoid averaging the actual chip-sample interactions that occur at approximately synchronization time distances.

The instantaneous interaction force can be determined by using the force δFr calculated by Equation 1. In the equation, Fr_i can be an instantaneous value at a synchronous time distance. As illustrated in FIG. 18, the synchronization time distance can also be a value determined by the gate average (see also FIGS. 7 and 8A / 8B). The gate averaging scheme uses the deflection value in the time domain δt and averages all the data points in this time domain. By doing so, the signal-to-noise ratio can be substantially improved. Fr_i acts as a set value in feedback control. Fr_i can vary from a value that yields a negative δFr to a value that yields a positive high δFr. A high positive number of δFr means that the repulsive interaction with the sample is stronger.

FIG. 19 illustrates procedure 800 of instantaneous force control used in peak force tapping (PFT) imaging. In block 802, the actuator vibrates the probe or sample to generate relative motion, for example with an amplitude in the range of 0.1 nm to 3 μm between peaks. At this time, the chip is not touching the sample, so the baseline and background can be determined in blocks 804 and 806. Once the background is determined, the background is also subtracted from the detected deflection at block 806 to ensure that the minimum detectable force is as small as possible. Block 808 functions to allow the probe to interact with the sample by engagement, as detailed in FIGS. 15 and 16. As the sample interacts with the probe, the deflection data for period T is sampled in block 810 and further digitized to analyze the synchronous time distance (FIG. 18), instantaneous force Fr_i and relative force δFr. Baselines and backgrounds can be re-examined in this block according to FIG.

Feedback is then used to keep δFr and Fr_i at their default values in block 812. The XY scanner is also one or more machines in which block 814 is effective in rearranging the probe with respect to the sample and ultimately exhibits surface profile images and, for example, elasticity, adsorption, and energy dissipation. It is effective for generating a target image.

In FIG. 20, the time-resolved measurement curve in FIG. 20A is transformed into the real-space data in FIG. 20B. More specifically, FIG. 20A is a plot of the interaction force as a function of time in one modulation phase. Figure 20B shows the function of the chip-sample distance in one modulation phase. It is an interactive force. The elastic properties of a material are customarily used at the upper part of the slope using, for example, the Oliver-Pharr model, or another contact mechanical model (see segment DE in Figure 20B, segment CDE). , Illustrating short-range repulsion interactions) can be calculated. (See, for example, Non-Patent Document 1). The van der Waals attraction can be determined from the approach curves (segment BC in FIGS. 20A and 20B), but capillary adsorption that occurs as the tip leaves the sample can also be calculated. (For example, Non-Patent Document 2). By moving the chip in the xy-plane and repeating these measurements, sample properties such as elasticity, van der Waals adsorption and capillary adsorption (segment EF corresponds to attractive and capillary forces) can be obtained on the sample surface. It is possible to image the entire area or a part thereof. Furthermore, the hardness of the sample can also be imaged from the difference between the approach curve and the recovery (disengagement) curve.

FIG. 20B represents two types of data: direct measurement data and derived data. Directly measured data are parameters such as interactions that are measured instantaneously within each period, for example. Derived data is data calculated from any part of the curve within each interaction period. Such data can be a deformation calculated by the penetration depth from point C to point D in FIG. 20B. Another embodiment is the dissipative energy defined by the region surrounded by the approach curve (BCD) and the withdrawal curve (DEFG). Yet another embodiment is the adsorption force calculated by the difference between B and F in FIG. 20B. Any derived data can be used as feedback control parameters. For example, if deformation is selected as the feedback parameter, the control loop in FIG. 1 will generate an image based on constant deformation instead of constant peak force. Any other derived data can serve the same purpose in the feedback loop.

One important application of instantaneous force control imaging is in deep trench measurements. When a TappingMode atomic force microscope is used to image deep trenches (the most difficult to image trenches with an aspect ratio of about 3: 1 or greater and a width of less than 100 nm, generally 10 nm to 100 nm). , The strong attractive force on the side wall causes the amplitude change, which causes an erroneous measurement of the trench depth. By using the direct repulsive force as feedback, the feedback responds only to z-change when the chip is in contact with the sample. As a result, force-controlled feedback can measure deep trenches much more reliably than the Tapping Mode atomic force microscope. 21A and 21B provide a demonstration of this measurement. The measurement uses the same probe and sample at the same sample position. The instantaneous force control feedback loop was able to measure the actual trench depth by the tip reaching the bottom of the trench (Fig. 21B). On the other hand, the TappingMode atomic force microscope moved the tip early, resulting in very shallow depth measurements and no trench bottom measurements (Fig. 21A).

Finally, further features of the invention are described with reference to FIGS. 22A / 22B and 23A / 23B. In FIGS. 22A and 22B, the atomic force microscope shows that the chip-sample interaction always stays in the small amplitude repulsive force mode, that is, the repulsive force region several nanometers away from the surface. It is operated to modulate Z with a sufficiently small amplitude (eg less than a nanometer) to ensure that it is. This is achieved by using either the force difference between peaks (Fa-Fb, corresponding to Z modulation between peaks) or the amplitude output of the lock-in amplifier as feedback. Topeak. The feedback parameter is proportional to the repulsive force gradient if the amplitude is small enough and the force gradient is linear. In this case, the feedback is only sensitive to short-range chemical bonding forces, forces corresponding to atomic resolution. As a result, this technique is ideal for high resolution imaging.

In FIGS. 23A and 23B, an arrangement similar to that shown in FIG. 22A / FIG. 22B is shown, but the attractive portion of the force curve is used (Small Amplitude Attractive Force Mode). In this case, the system modulates Z with a sufficiently small amplitude to ensure that the chip-sample interaction is always in the attractive region. Again, given that the feedback parameters are proportional to the attractive gradient when the amplitude is small enough so that the force gradient can be linear, a simple peak-to-peak force difference (Fa-Fb), or Any of the amplitude outputs of the lock-in amplifier can be used as feedback. This technique is the least destructive to the sample because the tip does not come into contact with the sample. Compared to the Small Amplitude Repulsive Force Mode, the feedback polarity is reversed.

[Advantage-Peak force tapping mode] The main benefits of peak force tapping mode atomic force microscopy operation are: 1. Improved imaging stability. 2. Higher resolution with less chip or sample damage. 3. Higher tracking bandwidth or higher imaging speed. 4. Direct physical quantity measurement ability. 5. Reliable fluid imaging. 6. Ability to choose from a wide range of cantilever types to adapt to different samples and applications. 7. Easy to use.

In short, the benefits of peak force tapping mode atomic force microscopy operation are innumerable. The benefits listed above are from an application perspective. These benefits are a sign of progress in the mechanism of operation.

Improved imaging stability: Assuming inherently stable long-term force control, drift-free sample imaging is a simultaneous presence of height, stiffness, adsorption, elasticity and plasticity at Tapping Mode speeds. It can be obtained with the measurement. Since the technique is unaffected by DC drift (peak force tapping mode produces its own criteria every few hundred microseconds), steady-state operation is obtained without a skilled opera. This allows continuous imaging for hours or even days without substantially compromising image integrity. The benefits of imaging stability are particularly useful for in-process measurements such as crystal growth and monitoring of polymer phase changes that can take minutes or even hours.

Higher resolution with less chip or sample damage: Peak force tapping mode that combines low average tracking force with the de facto elimination of lateral force on the chip when compared to existing modes of atomic force microscopy operation. The low-power, high-speed imaging provided by provides significant advances in high-speed imaging across a wide variety of samples. For example, single molecule elasticity can be measured, as does narrow DNA samples in fluid (eg DNA with a width of 2 nm). By comparison, when imaging DNA in fluid, the TappingMode atomic force microscope has a resolution that is at least 2 nm lower. Moreover, measurement of DNA stiffness in fluid is a challenge with the Tapping Mode (trademark) atomic force microscope. This is because atomic force microscopes do not have the ability to quantify properties and can only provide primarily relative mechanical property measurements (eg, by examining contrast in phase images). By this technique, it is possible to obtain property measurements down to the molecular level.

Compared to TappingMode , peak force tapping modes have higher resolution (eg less than 10 nm, more preferably less than about 1 nm laterally) and less chip-sample force (ie at least one of the chip and sample). Data can be acquired with less damage to. This technique provides a significant speed improvement over other known force feedback techniques, and it does not require the use of small levers. To do so. In fact, rather large levers (&gt; 60 μm in length) peak, as the lever response allows for a bandwidth (&gt; 10 kHz) that far exceeds the bandwidth obtained when using so-called small cantilever. It is possible to operate with subresonance in force tapping mode.

Yet another benefit of this preferred embodiment is that a force curve is generated at every pixel so that the image can provide more information than a typical TappingMode atomic force microscope image. At any pixel, the user can get quantitative information about stiffness, adsorption, elasticity, plasticity, etc. Moreover, again, the distance between the baseline chip and the sample is calibrated at every pixel, so drift is minimized so that significant improvements in productivity and image reliability can be achieved.

Higher tracking bandwidth or higher imaging speed: Especially peak force tapping images can be generated with operating bandwidth greater than 2kHz. The conventional tapping mode bandwidth is about 1 kHz, mainly due to slow cantilever dynamics (slow response of cantilever amplitude to changes in tip-sample distance).

Direct mechanical property measurement capability: The disclosed embodiments independently measure elasticity, adsorption, energy dissipation, and the like. All of these factors contribute to the phase of cantilever vibration. Therefore, the phase channel is used in the TappingMode atomic force microscope to represent mechanical property information, but ambiguity remains in the interpretation of the measured phase. The peak force tapping mode eliminates the phase interpretation problem by providing a direct mechanical property measurement.

Ability to choose from a wide variety of samples and various cantilever types to adapt to application: The peak force tapping mode is insensitive to cantilever kinetics as the measured peak force is not limited by cantilever kinetics. This allows for high speed imaging in vacuum, air and fluids.

Generally, Tapping Mode atomic force microscopes require the cantilever to have a spring constant greater than 0.3 N / m, whereas peak force tapping modes require the cantilever to have a spring constant as low as 0.01 N / m. It can be used. Again, this is due to the fact that the peak force tapping mode does not depend on the vibrational energy stored in the cantilever to overcome the capillary adsorption force. Since the technique uses an external actuating element (preferably in a feedback circuit that is initiated by peak force), the mechanism for overcoming capillary force is much stronger than in Tapping Mode . In TappingMode , the static elastic energy of the cantilever itself (supplied by the kinetic energy of the vibrating probe) pulls the chip away from the sample in overcoming capillary forces. As a result, there are practically no restrictions on the spring constant of the cantilever for stable operation in the presence of the capillary layer. Therefore, the peak force tapping mode enables stable tapping control operation by using a cantilever having a spring constant as low as 0.01 N / m.

The peak force tapping mode allows the use of 0.01 N / m to 1000 N / m cantilever in one mode of atomic force microscopy operation. This allows for high resolution mechanical property mapping of a wide variety of materials (10 kPa ~ 100 GPa in elastic modules) with a single instrument.

Reliable fluid imaging: The fact that the peak force tapping mode does not have to operate at the resonant frequency of the probe offers many advantages when imaging in fluid. Due to the various parasitic coupling forces in the fluid, cantilever tuning is a Tapping Mode flow. It is a difficult stage to obtain a body image successfully. The peak force tapping mode completely eliminates the need to tune the cantilever (baseline averaging, background subtraction, etc.). In addition, the ability to select cantilever from a force control range and a very wide range of spring constants offers enormous potential for imaging control for biological sample imaging.

Ease of use: Chip destruction is virtually eliminated given, more essentially, instantaneous force feedback. Also, since the deflection is kinically corrected, tuning is generally not required and a fast and agile setup can be achieved by virtually any user.

This peak force tapping mode under consideration provides very low force imaging (ie, instantaneous force control) that provides very high resolution by using real-time characteristic mapping. Force control is inherently stable (essentially no drift) for a long enough period to image the sample with minimal user intervention or no intervention. The system does not require tuning (baseline averaging and hydrodynamic background correction), allowing for faster and simpler setups. Moreover, precise control over the force basically eliminates chip destruction, but the technique / system essentially eliminates the lateral force exerted on the sample surface. The system is also insensitive to cantilever kinetics by not having to wait for the probe to ring down before the probe and sample interact. And, as mentioned above, various cantilever are available for users to obtain simultaneous measurements of height, stiffness, adsorption, elasticity and plasticity at TappingMode atomic force microscope speed (&gt; 2kHz). Due to these features, this peak force tapping mode makes it possible to image a sample with a width of, for example, 2 nm in a fluid, and similarly improve the measurement of mechanical properties such as single-molecule elasticity. ..

[Peak power tapping mode ease of use] A preferred embodiment of the present invention uses a peak force tapping mode to give an inexperienced user the ability to generate high quality images with similar quality to a skilled user. For example, when the chip interacts with the sample (representing a complex relationship to the chip-sample force), the Tapping Mode operates by controlling the chip-sample interaction based on the set amplitude or phase deviation of the probe vibration. In contrast to the atomic force microscope, the peak force tapping mode controls the chip-sample interaction based on the chip-sample interaction force at each point along the probe modulation period. Direct control of this interaction simplifies control and allows preferred embodiments to minimize the action of complicating variables, including the dynamics of the cantilever and other mechanical components, including actuators, and thus stability. To be able to maintain.

FIG. 24A shows a schematic graph 1000 of the sample profile (height) 1002 including the ascending region 1004 and the decreasing region 1006. Superimposed on this profile 1002 is a tracking signal or image 1008 obtained by an atomic force microscope. Stable feedback is maintained as the scan continues in the specified direction. Stable feedback refers to self-excitation, a feedback loop that, despite the input, does not tend to produce a oscillating output. However, at point "X", the feedback begins to become unstable and the image begins to appear more noisy. Unstable feedback can be more stable by lowering the feedback gain (at the cost of reduced imaging bandwidth, or imaging speed, etc.). FIG. 24B is an error signal corresponding to the superimposed tracking signal 1008. It is important that both the unstable feedback height signal and the error signal are more noisy than those of the stable feedback. This phenomenon will be used in the automatic gain scheduling apparatus and method of the present invention described below.

FIG. 25 conceptually illustrates the feedback instability detection used by the preferred embodiment using a plot of the feedback height or the amplitude spectrum of the error signal. Signals The spectrum is shown in both stable feedback 1010 and unstable feedback 1012. Feedback instability can be measured quantitatively based on one or more of several criteria. Some examples of these criteria are: 1. Spectral amplitude at a specific frequency (f0). The frequency f0 is determined by using system identification when the feedback is unstable or by observing the spectrum of the feedback signal. 2. RMS error of the height signal or error signal. 3. Standard deviation of the height or error signal.

Moving from FIG. 26A to FIG. 26D, a diagram of the chip-sample force when the chip is no longer in contact with the sample (also known as a "parachute") is shown. Similar to FIG. 24A, FIG. 26A illustrates schematic diagram 1020 showing sample profile 1022 and an atomic force microscope tracking (height) signal 1024 superimposed on it. In this case, in the area labeled "A", the chip will not come into contact with the sample surface during image scanning, and the control system will attempt to return the chip to the sample surface (generally either a probe or a sample). Parachute (by moving). In Figure 26B, on a downhill slope (eg 1026 in Figure 26A), the error signal (difference between the measuring tip-sample interaction force and the set value) is negative and moves the tip towards the sample in the control system. Let me try. In the flat region (1032), the error is zero as the chip is traversing the surface without correction. On the uphill slope (1030), the error is positive and the control system uses this information to move the chip away from the sample. However, in the parachute region "A" (corresponding to the downslope 1028 of the sample), the error first indicates the downslope, but the feedback cannot keep up with the fast downslope, so when the chip-sample interaction is zero. , The chip stops tracking the surface (see Figure 26C).

During the parachute, the chip-sample interaction force is not related to the distance between the chip-sample. Therefore, feedback stability is compromised during parachuting. It is important that parachute events be detected, as gain scheduling is not possible during the parachute period. A method for detecting a parachute using the peak force tapping mode will be described below.

Figure 26D, which shows the spike in chip-sample interaction force data, illustrates a force curve that corresponds to the region of chip-sample interaction (in this case, feedback correction is required). A magnified view of the individual chip-sample interaction force curves is shown in Figure 20A. The interaction force is the attractive force region BC (snap contact-Van der Waals force), the repulsive force region CDE when the chip interacts with the surface and continues its vibration cycle, and when the chip tries to move away from the surface. It can be characterized by the adsorption region EF of, followed by the point F where the chips separate. For example, one advantage of peak force tapping mode over Tapping Mode is that any point on the interaction force curve can be used by the controller to track the surface, as described in detail above. (Without waiting for a ringdown before driving another modulation period). In the case of a parachute chip, the parachute can be detected in currently preferred embodiments by one or more of the following criteria: 1. The peak force / adsorption force or the force between peaks during the vibration period is less than the threshold value. 2. Feedback error Indicates that the signal is between the two thresholds and the peak force is close to zero. 3. Feedback Error At least one of the standard deviation and spectral amplitude of the signal at a particular frequency (or multiple frequencies) is below the threshold, indicating that the feedback loop is open.

An atomic force microscope 1100 capable of operating in peak force tapping mode to minimize the techniques required for the operation of the atomic force microscope is shown schematicly in FIG. 27. Atomic force microscope 1100 includes probe 1102 with cantilever 1104 supporting chip 1106 Mu. The probe 1102 is mounted on the probe holder 1108. In this case, the probe holder 1108 is coupled to the actuator 1110. Actuators 1110 (eg, piezoelectric actuators) can move tip 1106 of probe 1102 in the "Z" direction (perpendicular to the sample surface). When probe 1102 interacts with sample 1109, its deflection is monitored by a deflection detection scheme 1112 that includes a light source 1114 (eg, a laser diode) that directs the ray "L" to the dorsal side of the lever 1104. Lever 1104 reflects a ray "L" toward a detector 1116 (eg, a quadrant optical detector). The detector transmits a signal indicating deflection to ADC1118. After the analog deflection signal is converted to a digital signal by the ADC block 1118, the resulting signal is transmitted to the peak force tapping mode force detection block 1120. The detected force signal (determined according to the above-mentioned device and method for extracting the chip-sample interaction force one by one) is transmitted to the comparison circuit 1122. Preferably, the peak force is compared to the force set value and the error signal is sent to PI controller 1124. The PI control device 1124 outputs a control signal transmitted to the Z scanning DAC block 1126 which converts a digital signal into an analog signal. The signal is further applied to the Z piezoelectric actuator 1110 to control the distance between the chip and the sample. Since the above components form a feedback loop, the interaction force between the chip 1106 and the sample 1109 is adjusted according to the force setting value.

During operation, the Z scan control signals output by the DAC1126 and optimized by the gain control circuit 1123 (feedback control circuit) are the output of the Z offset DAC1136 (more described below) and the Z-modulated DDS (direct digital synthesis). Device) Combined with vibration drive in peak force tapping mode provided by 1138 and adder circuit 1139.

Gains are automatically tuned using gain control circuit 1123 to facilitate stability and thus minimize the need for skilled users. The control signal from the PI controller 1124 used to control the Z piezoelectric actuator 1110 is also transmitted to block 1128, preferably resampling the height data at a position corresponding to the peak force (see block 1120). The vibration detection algorithm 1130 is then used to determine if there is vibration in the height data, that is, if there is any manifestation of instability. If the system tries to vibrate and becomes unstable, high frequency noise will be detected. The method by which Algorithm 1130 determines the amount of noise is described in more detail below in connection with FIG. 28. The vibration detection algorithm 1130 outputs a short signal indicating the magnitude of instability called "noise" only in this chapter. Such instability makes itself look like noise, which is due to a feedback loop. However, it should not be confused with noise in other parts of the system when feedback is off. This noise signal is compared with the noise immunity margin in adder circuit 1132. The noise immunity margin is a predetermined parameter associated with the product and can be further optimized after the start of imaging according to the sample roughness information obtained during the scan. For example, noise immunity can be reduced if the sample is determined to be very flat. If the error output of circuit 1132 exceeds a predetermined margin, the gain controller 1134 will gradually (eg, 5%, for example, 5%) until the magnitude of the unstable signal from, for example, the vibration detection algorithm 1130 is less than the noise immunity margin. Each iteration) Determines the appropriate gain control signal to adjust the gain of controller 1124 by reducing the I and P gains. That is, the gain at each imaging position can be optimized to ensure system stability (optimization stage).

When this automated gain scheduling is active, the need for a skilled user to tune the gain during atomic force microscopy operation is eliminated. One of the decisive factors in feedback gain automation adjustment is the ability to quickly and accurately determine the onset of instability during scanning. Instability is gain control in this judgment It is often complicated by unknown surface shapes that can be misinterpreted when inducing noise in the device. Moving on to FIG. 28, the algorithm 1140 that executes the vibration detection algorithm (vibration detection block) 1130 of FIG. 27 is described in more detail. Height information is calibrated by any atomic force microscope system, but height information is independent of any system-specific parameters such as scanner Z range and cantilever deflection sensitivity. Used to determine the level of instability vibration. The noise immunity margin (block 1155) is defined as the permissible magnitude of instability-induced noise. If this margin is detected with a height signal, such a margin provides the absolute value of noise allowed in the feedback system. For example, if the noise immunity margin is 1 nm, any instability output from block 1146 or 1148 is considered acceptable. At a sample height of 100 nm (range), such a margin corresponds to a signal-to-noise ratio of 100 in the image. However, for flat samples with waveforms less than 1 nm, the noise immunity margin can be greater than the sample height signal. In such situations, the noise immunity margin should be reduced to 0.1 nm in order to obtain a reasonably good image (S / N = 10). This margin can be automatically adjusted based on the roughness of the sample. Height data obtained during atomic force microscopy operation reflects both sample surface geometry and system oscillations. Algorithm 1140 generally functions to filter and remove the sample topology to determine if it is noisy enough to indicate the manifestation of instability. It should be noted that during the scan, the sample topology usually does not change significantly at adjacent pixels. By calculating the height difference between three adjacent points, for example, the sample topology can be largely filtered out. This is shown using the following equation.

The heights of three consecutive pixels around position x0, namely H (x0-Δx), H (x0), H (x0 + Δx), are assumed as follows. H (x0 + Δx) = H (x0) + dH / dx (x = x0) Δx + d²H / dx²(x = x0) Δx²+ d³H / dx³(x = x0) Δx³+ d⁴H / dx⁴(x = x0) Δx⁴+・・・(1) 。

H (x0-Δx) = H (x0) -d H / dx (x = x0) Δx + d²H / dx²(x = x0) Δx²-d³H / dx³(x = x0) Δx³+ d⁴H / dx⁴(x = x0) Δx⁴+・・・ (2) 。

Adding Equation 1 to Equation 2 H (x0 + Δx) + H (x0-Δx) = 2H (x0) + 2d²H / dx²(x = x0) Δx²+ 2d⁴H / dx⁴(x = x0) Δx⁴+・・・(3) 。

Therefore {H (x0 + Δx) + H (x0-Δx)-2H (x0)} / 2 = d²H / dx²(x = x0) Δx²+ d⁴H / dx⁴(x = x0) Δx⁴+・・・(4) 。

Due to the small position change Δx, the height difference becomes small. In this regard, with reference to FIG. 29, one embodiment of the height control signal output by PI controller 1124 (FIG. 27) is indicated by A. In this case, the feedback loop is stable between t1 and t2. The feedback loop begins to oscillate from t2 to t5. Looking back at FIG. 28, the height data is resampled at block 1142. Resampling in such a situation preferably means extracting height data points at the peak force positions of at least three adjacent force curves. At block 1144, the difference in height between selected data points or pixels is determined. For example, if 3 points are selected, the calculation becomes Equation 5.

H Diff (i) = {H (i-1) + H (i + 1) -2 * H (i)} / 2 ... (5). The result of this operation is shown in Figure 29C. The topology data is mostly removed by filtering and only a small amount remains, but the vibration data between t2 and t5 is basically unchanged. Absolute value of this difference, as shown in Figure 29D | H Diff (i) | indicates the degree of stability of feedback at a specific time. With reference to FIG. 28, this is done at block 1146. This stage essentially acts like a vibration detector. Then, in block 1148, a moving average can be determined. By determining the moving average of the altitude difference calculated over a relatively long period of time, a baseline indicating the degree of stability of the feedback loop is established. The determination of the moving average is required only for samples whose topology exhibits significant topology changes that cannot be filtered out in a given sample used for altitude difference calculations. Such samples include, for example, a silicon grid with steep steps. In such cases, abrupt changes in topology cause large spikes in the altitude difference output data. Since these spikes are generally short-lived, comparing them to the moving average of the altitude difference data by the motion shown in Figure 28, block 1149 will eliminate those spikes altogether. On the other hand, in the presence of vibration, the problematic vibration noise generally lasts much longer than the topology change, so the relevant altitude difference data tends to resemble the previous moving average data, thus essentially. Be overlooked.

Continuing method 1140, in block 1149, if the absolute value of the difference obtained in block 1146 is less than a multiple of the moving average, for example four times the moving average calculated in block 1148, the vibration detection algorithm 1140. The output of is | H Diff (i) |. If the absolute value of the difference is greater than a multiple of it, then the output of Algorithm 1140 is a moving average. This amount of RMS value is then determined in block 1150. This is the value compared to the "noise immunity margin" by the adder circuit 1152 described above in connection with FIG. 27. Finally, gain control feedback (gain increase / decrease) is determined in block 1154 and transmitted to PI controller 1124 based on the error output of circuit 1132. The gain is increased if the output of vibration detection algorithm 1130 is lower than the noise immunity margin 1155. The gain is reduced if the output of vibration detection algorithm 1130 is higher than the noise immunity margin 1155.

A detailed execution of the atomic force microscopy operation using the peak force tapping mode is illustrated in FIG. In order to utilize the peak force tapping mode and make the instrument easier to use, it is also referred to as the above-mentioned automatic gain scheduling control (also referred to as "autopilot" or "atomic force microscope autopilot" (automatic control stage) in the present specification). Is executed as follows. The user defines the desired scan size in block 1502. An engagement routine is then initiated at block 1504 to bring the tip into contact with the sample. The atomic force microscopy system then determines if "autopilot" is on at block 1506. If not on, this routine ends (block 1530) and the atomic force microscope works by using opera-controlled feedback without automatic gain control (some skilled users monitor their measurements). You may prefer to manually adjust the gain and settings with). When the autopilot is on, the operating parameters are initialized in block 1508 with factory-defined default values, such as DSP in block 1510. Block 1512 indicates that the autopilot function is performed on the DSP.

When the parameters are initialized, the scan size is set to a small value in block 1514. A small scan (eg 10 nm) is performed with a low gain that determines the initial peak force set value and a gain that provides a set value reference. In most cases, chip lifetime and sample completeness are achieved by minimizing the peak force of chip-sample interactions in all atomic force microscope imaging. Improvements will be brought about. The system can determine the minimum set value based on the information of the baseline noise in the system. For example, if the chip is not interacting with the sample and the force detection noise is 100pN, the set value can be set at 300pN, allowing a sufficient signal-to-noise ratio for feedback control. At block 1516, engagement is confirmed, and at block 1518, the system modulates these in an attempt to optimize the initial gain and settings. Optimization is an iterative process that includes:

1. Determining system background noise by lifting the chip to prevent chip-sample interactions; 2. Determining a setting that is usually three times higher than the peak force noise background determined in step 1; 3. Increase the gain until the noise is approximately equal to the noise immunity margin (eg, iteratively at a given stage).

Once the gain and force settings have been determined in block 1520 with a small scan size, the system restores the user-input scan size in block 1522 and initiates an atomic force microscope operation to obtain further sample data. ..

At block 1524, the system determines if the algorithm is adjusting the gain or set value. If neither the gain nor the set value is adjusted by the algorithm, the default gain / set value is restored in block 1526. The system then enters a monitoring loop (monitoring mode) at block 1528. The monitoring mode determines whether the vibration exceeds the threshold. If exceeded, the gain can be adjusted (decreased). If not, the gain can be increased for better tracking. Surveillance mode also functions to detect parachute events. If the parachute event is detected as described above, the set value may be increased to optimize performance. The increase in the set value is preferably performed in increments of 5% each time (optionally confirm steps 1-3 above outlined). The above continues until the scan of the user-defined sample scan size is completed.

It is important to point out that the relative position between the probe and the sample represents two different concepts. Since the tip is moving periodically, the position of the probe at any moment is referred to as the probe position. The average position during one exercise period is the average probe position. For example, if the probe moves sinusoidally at an angular frequency of "w" and an amplitude of "a", the probe position at any moment is a * sin (wt). However, since the average in one sine period is zero, the average position of the probe is zero.

The Z position controlled by the feedback loop provides control of the average position (control stage). That is, the feedback control described above can maintain substantially the same peak interaction force (predetermined instantaneous force) in each modulation phase of probe vibration / chip-sample interaction. This method automatically determines the set value related to the peak interaction force based on the noise background, and also automatically determines the feedback gain according to the magnitude of the instability vibration. By doing so, the atomic force microscope can be used by an inexperienced user to obtain an image with self-optimized gain and set value.

The feedback characteristics in peak force tapping mode as opposed to tapping mode are quite different. In most atomic force microscope control schemes, the feedback loop is performed by using calculus gain control, or simply the P / I feedback loop. Feedback is driven by the difference between the default value (set point) and the current peak force value. This difference is also called an error signal, as mentioned earlier. P / I feedback The loop is a linear compensator. The P / I feedback loop has the most predictable behavior when the error signal to be corrected to use the feedback also fluctuates linearly due to the chip-sample interaction. The peak force error is inherently linear because such error increases linearly due to chip-sample interaction. The linearity of this error is an important factor in obtaining automatic tuning of P / I gain (also called gain optimization) with long-term robustness over various samples.

The gain optimizations described in FIGS. 28 and 30 serve to increase the gain to the highest bandwidth for the fastest feedback response and thus faster imaging. Set value optimization means the set value required for the process described in FIGS. 28 and 30 to minimize the interaction peak force, and thus for surface tracking.

The scanning speed optimizing means automatically adjusts the scanning speed so that the scanning speed allows a set value that functions with a predetermined margin (peak force error) while obtaining the highest possible scanning speed. For example, if the set value margin is 10 nN, the automatic gain adjustment and set value adjustment will work on any optimization value with a peak force below 10 nN. If the set value adjustment at 10nN is insufficient to maintain stability, the automatic control in FIG. 30 will reduce the scanning speed to ensure that the maximum peak force error is within 10nN. ..

The scan speed can be automatically adjusted for optimization using the peak force tapping mode, as shown in FIG. In FIG. 31, a flowchart of the scanning control algorithm 1600 is shown. In this case, the atomic force microscope is operating in peak force tapping mode at block 1602, which includes continuous monitoring of peak forces in each chip-sample interaction cycle (induction phase). At block 1604, method 1600 determines if the peak force is greater than a predetermined threshold. For example, the threshold can correspond to measurements larger than 8 volts. If the peak force is greater than the predetermined threshold, the scan speed adjustment signal is transmitted to the scanner in block 1608 to reduce the scan speed by an appropriate amount. If the peak force is not greater than the default threshold, the method determines in block 1606 whether the background change is greater than a particular threshold (eg, 25 volts). If larger, the scan speed is reduced in block 1608. If not large, the current scanning speed is maintained at block 1610 (maintenance phase). This optimum scan speed control can be optimized for any pixel when operating in peak force tapping mode. Therefore, the peak force tapping mode provides an ideal balance for acquiring high quality images with the shortest capture time. With reference to FIGS. 32A and 32B as examples to further illustrate block 1606, FIG. 32A illustrates flat background regions on either side of the chip-sample interaction force cycle. In Figure 32B, the background is affected by changes in the sample surface shape-the chip is probably stuck in the sample and may not be able to track the surface. In this case, this background change is used to identify and slow down the scan.

The peak force tapping mode also enables automatic Z-limit control , further facilitating the ease of use of this atomic force microscope. The Z limit parameter defines the dynamic range of the Z piezoelectric actuator. The probe is preferably placed in the center of this range. The larger Z limit allows imaging of samples with large surface shape variations, but at the same time reduces bit resolution. For certain flat samples, the Z limit needs to be adjusted to obtain a high resolution surface profile image. So far, Z limit adjustments have been based on user experience. In the peak force tapping mode, the control of the Z limit parameter is automated. In this regard, moving on to FIG. 33, after method 1700 has started operating in peak force tapping mode at block 1702 (Z limit is set to allow maximum Z range), method 1700 blocks. Defined by the user in 1704 Capture the complete frame of one of the sample surfaces corresponding to the scanned area. The RMS height of the frame is then calculated in block 1706. If the RMS height is less than a threshold (eg 10 nm) as determined in block 1708, the Z limit is adjusted in block 1710. For example, on a flat sample that meets the threshold, the Z limit can be reduced to a certain value, eg 2 microns, and the frame can be rescanned. This can be repeated until the user is satisfied with the image and proceeds to 1712. The adjusted Z limit is preferably maintained until the user changes the scan area.

In addition to automation, peak force tapping modes are useful for ensuring good imaging and maximizing the ability to obtain mechanical property measurements of a sample at any scanning position (eg, pixels) of the sample. For example, the peak force tapping mode can be used to perform chip radius monitoring. One major obstacle to obtaining high quality images is that it is difficult for the user to detect when a sharp probe tip has been injured. Changes in the physical structure of the chip due to contamination (sample or environmental substances adhere to the chip, which often occurs, for example, when imaging in fluid or oily samples). Injury can be caused by at least one of (part of the chip is crushed or worn). The injured tip can be identified by scrutinizing the force curve obtained at the sample location. FIG. 34 illustrates one part of the force curve showing the tone of the tip. In FIG. 34, schematic graph 1801 represents a chip trajectory. This trajectory can be part of a sinusoidal signal and any shape defined using a scanner control signal. The van der Waals attraction near the sample is plotted as segment AB in the approximately graph 1802, where 1802-1 represents the non-interaction zero force baseline. The slope of this segment is determined by using the tip radius. The larger tip radius will move point A to the left in response to the early development of van der Waals forces. By analyzing segment AB, the chip radius can be estimated and it can be determined whether the chip is still sharp. In particular, the slope of region AB provides an indicator of tip artifacts (dotted lines schematically illustrate the response in the presence of artifacts). Since one or more force curves in peak force tapping mode are generated at every pixel, chip force monitoring can occur virtually instantaneously during the scan. Therefore, the test force curve tries to interpret the imaging and identify whether the chip is damaged or not. Rather than obtaining, an atomic force microscope operating in peak force tapping mode can automatically identify such a condition at any scanning position (eg, every few hundred microseconds). Once identified, the scan can be interrupted, and the user is notified, thereby preventing further acquisition of useless data and allowing the user to replace the injured chip. To do.

Another indicator of tip tone is contamination. Such contamination is determined by analyzing the shaded area "w" in schematic graph 1803 in FIG. Pollution is known as the work of adsorption. The action of adsorption is greater if the chip is contaminated with water, or another substance that can form a meniscus when the chip retracts from the surface. The greater the adsorption activity, the more annoying pollution it represents. Since a force curve is acquired for each pixel, chip tone with respect to contamination can also be continuously monitored.

If the chip is functionalized by a chemical bond with a particular chemical compound, for example polyethylene glycol (PEG) or dendron, the action of adsorption is deliberately introduced. In this case, when the chemical compound interacts with a molecular site that exhibits a particular interaction, for example forming a bond with polyethylene glycol (PEG) or dendron, only the functionalized chip produces a significant adsorption action. To do. By monitoring this interaction, the adsorption map can be a chemical or biochemical recognition map.

Electrical, optical, magnetic or thermal perturbations or excitations synchronized to contact point D in schematic graph 1802 of FIG. 34 can also be applied. Synchronous detection of current, voltage, thermal properties, magnetic or spectroscopic reactions provides a substantial signal-to-noise improvement as point D represents control over near-field sample interaction (or near-field interaction). It is possible.

[Advantages-Peak force tapping mode and ease of use] In short, the peak force tapping mode provides some operational advantages that allow an inexperienced user to operate an atomic force microscope. Considering ease of use, some imaging factors must be considered to minimize the need for skilled users. First, the stability of the feedback must be maintained, and with the above-mentioned automatic gain tuning / scheduling enabled by the peak force tapping mode, stability is achieved without the need for a skilled person to manually adjust the gain. .. Next, in order to obtain a good image, the atomic force microscope must track the sample surface. By controlling based on the instantaneous tip-sample interaction force, the set value force can be selected for optimal tracking with minimal error. In addition, the scanning speed and automatic Z limit control as described above minimize the need for an expert when operating the atomic force microscope without interfering with the imaging speed or the ability to acquire high quality images. Work for.

In contrast to known vibrating atomic force microscope operating modes such as Tapping Mode , peak force tapping modes operate in completely different mechanical states. The vibration mode setting is usually the amplitude or phase of vibration, which is a parameter that has a very complex relationship with the interaction and force between the chip and the sample. As described herein, the peak force tapping mode takes into account the tip vibration at each point as the tip interacts with the sample surface and also uses the corresponding force information in its feedback scheme. This allows the preferred embodiment to operate without user controlled feedback and no user adjustments are required during imaging (automatic minimization of error signals). The peak force tapping mode also provides intermittent contact (and its perceived benefits) with the sample by entrainment (requires only a simple pre-imaging procedure-Figure 30), and also allows untuned setup. .. As a result, an immature person can image at a certain resolution (eg 1nN) or less and at a certain speed (eg 1 / 2Hz, 256 pixels) or higher without the need for tuning.

Moreover, by providing a force curve at every pixel, the user can obtain deterministic data (eg, adsorption) at a reasonable speed and a particular resolution, which can be done during imaging. .. All this is possible by direct feedback with a force (chip-sample) that allows a response based on a single interaction between the chip and the sample (representing a linear transfer function-what is a known vibration mode? The exact opposite).

In particular, all of the above concepts can be used in electrical contexts as well (eg STM) so that the instrument feeds back in current. Also, due to the complexity of the feedback, the data obtained in conventional vibration modes generally requires complex indirect interpretations. The peak force tapping mode allows for a direct interpretation of the data, given that it is based on the force curve rather than the "envelope" based tapping.

Another benefit of operating in peak force tapping mode includes the ability to image a particular sample more effectively. For example, in semiconductor applications, atomic force microscopes cannot reliably image narrow trenches, so users who want to perform such measurements are often forced to choose instruments other than atomic force microscopes. However, in peak force tapping mode, the peak interaction force is used as a direct force feedback and the tip is any The contact with the sample in the force curve allows reliable measurement of high aspect ratio sample characteristics.

Furthermore, the peak force tapping mode is independent of parameter drift. For example, the Tapping Mode atomic force microscope free amplitude can change during imaging due to either drive amplitude drift in air or fluid cell drive efficiency drift in liquid, so changes in chip / sample force can be seen. It can result in loss of chip / sample interaction. Such drift prevents the TappingMode atomic force microscope from performing long-term stable imaging. Users can image for less than an hour (including at night) in peak force tapping mode, especially when using conventional vibrating atomic force microscopy modes in liquid environments. ..

Overall, there is non-interference of the cantilever response to environmental conditions in peak force tapping mode. Vacuum (fluid) and atmospheric imaging can be achieved without affecting the setup, making the instrument very easy to use. The vibration frequency can be set regardless of any cantilever resonance-which greatly simplifies its use in fluids. In particular, the known intermittent contact mode requires operation in resonance, while the peak force tapping mode preferably operates in subresonance. This is also due to the control capability based on the very small instantaneous (not average) force (about 1 μN to 1 pN). As a result, atomic force microscopes also provide faster feedback if the cantilever Q is inadequate at subresonance (the transfer function is independent of the energy stored in the cantilever at resonance). It is possible to operate. Finally, the peak force tapping mode also allows the use of cantilever with a spring constant of less than 1-10 N / m, as described above.

The best modes conceivable by the inventor performing the present invention are disclosed above, but the practice of the above invention is not limited thereto. It will be clear that various additions, modifications and rearrangements of the invention can be made without departing from the spirit and scope of the underlying concept of the invention.