Probe detection system

The present invention relates to the field of probe microscopy technology, and more particularly to detection systems used to monitor the position of a microscope probe in the context of a sample surface.

The underlying principle of a scanning probe microscope (SPM) is to perform a mechanical scan of the sample surface with a nanometer-scale probe tip to generate an image of the sample. The features in the image result from varying interactions between the tip and the sample.

A specific example of SPM is an atomic force microscope (AFM). Atomic force microscopy observes the force interaction between the sample and the sharp tip of the probe. A typical AFM probe comprises a very small cantilever that is fixed to a support at the base and has a tip at the opposite (free) end. When the probe tip is brought closer to the sample, an interaction force is generated between the sample and the tip. If the tip is moving, for example vibrating, the interaction force will change that movement in some way. When the tip is stationary, the force displaces the tip with respect to the sample surface.

During the scanning process, the strength of the interaction between the tip and the sample surface changes as the properties of the surface below the probe tip change. A 3-axis high resolution scanning device typically generates relative motion between the sample and the probe to drive either the sample and / or the probe support. The effect of the interacting force on either the position and / or movement of the probe tip is monitored during the scan. In standard AFM operation, the strength of the interaction force is kept constant. That is, the effect of the interaction force on the probe is observed, and if there is a change, the feedback system operates to adjust the separation of the sample and the probe base in response to the change, and the observed parameter is set to a predetermined value. That is, it returns to the set point. Data related to this adjustment (conventionally vertical or "z" movement) is collected and used to construct an image of the sample over a region of the sample surface.

As described above, the adjustment performed to separate the base and the sample is conventionally performed in the vertical direction, that is, the z direction, and the sample surface is scanned in the x and y planes. Although this convention is followed herein, it should be borne in mind that the microscope can be configured so that the sample is not placed in the horizontal plane and therefore the feedback adjustment is not in the vertical direction. That is, the term "vertical" should be understood simply as being perpendicular to the sample surface, or equivalently perpendicular to the surface on which the sample surface is scanned. Conventionally, this direction is called the z direction as shown by the Cartesian axis shown in FIG.

The interpretation of the image formed by the AFM depends to some extent on the nature of the surface under investigation. In general, the shape of the surface contributes most to the image, and the height of the sample is closely followed by the probe during scanning, but other properties such as hydrophobicity, hydrophilicity, and viscoelasticity of the surface may also contribute. .. Probes and microscopes can also be configured to measure other sample properties such as magnetic and electric fields through appropriate interaction forces.

The AFM can be designed to operate in various imaging modes. In contact mode, the probe remains in virtually constant contact with the sample. In dynamic mode, the probe is oscillated and makes no or intermittent contact with the sample.

If a stationary tip is used for surface examination, the position of the tip during scanning is monitored using the effect of the interaction force on the curvature or deflection of the cantilever. As the interaction force between the tip and the surface changes, the tip is either pulled towards the surface or pushed away from the surface. The movement of this tip is transmitted to the cantilever portion of the probe, which bends or bends over its length accordingly. The AFM of this configuration is set to measure the deflection of the cantilever with a sensitive detector such as an optical lever system or other deflection detector, as is well known in the art. Deflection in this context refers to the inclination of the upper surface of the probe, which is commonly used to obtain instructions for cantilever curvature.

Alternatively, the probe can be vibrated vertically at or near one of the resonant frequencies of the probe. Fluctuations in the interaction between the sample and the probe affect probe motion. In detail, the amplitude, phase, and resonant frequency of this vibration can be monitored and the probe-sample separation is adjusted to maintain a constant average interaction.

AFMs can be used to obtain atomic scale images of a wide variety of samples in different environments (air, liquid, or vacuum) regardless of their mode of operation. Generally, AFMs use piezoelectric actuators, light deflection detectors, and very small cantilever manufactured using silicon manufacturing techniques. Due to its high resolution and versatility, AFM has been applied in various fields such as industrial inspection, semiconductor manufacturing, biological research, materials science, and nanolithography.

The terms "probe," "tip," and "cantilever" are often used synonymously in the art, but these terms need more rigorous definitions for the purposes of the present invention. A "tip" (or "probe tip" if more contextually appropriate) refers to a often conical or pyramidal three-dimensional structure located at the free end of a cantilever beam. The tip tapers toward the point of interaction closest to the surface under investigation. The cantilever is the beam itself excluding the tip, the tip is supported at one end, and the other end is held by the microscope device. The cantilever and tip are collectively called a probe.

FIG. 1 shows the basic components of the conventional AFM10. The AFM 10 according to the prior art is provided with a movable base 12, on which the sample 14 investigated by the probe 16 is placed. The probe 16 includes a beam portion 18 and a tip 20 of the cantilever, and the tip 20 tapers toward the micropoint 20a and is located toward one end of the beam portion 18 of the cantilever. The other end of the beam portion 18 of the cantilever is fixed to the support base 22. The z-direction positioning system 24 is connected to the base 12, and the z-direction positioning system 24 is a piezoelectric drive capable of operating so as to move the base 12 toward the probe 16 and away from the probe 16 (z direction). Prepare your body. An additional drive (not shown) is connected to the support 22, or both, and acts to provide relative motion between the sample 14 and the probe 16 on the sample plane (x, y). It is possible. The light source 26 is configured to emit a beam L, and the beam L is guided to the upper surface (rear portion) 18b of the beam portion 18 of the cantilever at the position where the tip 20 is attached. The light reflected from the rear 18b of the cantilever propagates to a position sensitive detector (PSD), which is generally a split photodiode 28, and a feedback signal is generated. The output of the detector 28 is connected to the z-direction positioning system 24 via the feedback controller 30.

The feedback signal from the PSD can be processed to extract quantities such as probe deflection, amplitude, phase, or other parameters. For brevity, this prior art AFM is described as operating in contact mode using feedback based on probe deflection.

The probe 16 is generally made of silicon or silicon nitride (in the case of AFM). Normally, the cantilever 18 has a length of about 100 to 200 micrometers, a width of about 20 to 50 micrometers, and a thickness of about 0.5 micrometers, but of course, this size can be changed according to the application. The shape can also be changed, but is usually rectangular or triangular, with the tip 20 at the apex of the triangle in the latter case. The tip 20 is typically 5 micrometers at its base, 3-10 micrometers in height, and a radius of curvature at the end of 10-20 nanometers. At the time of use, the micropoint 20a at the end of the tip 20 is directed toward the sample.

When taking an image of a sample, the prior art AFM10 operates as follows. Using the z-direction positioning system 24, the tip 20 is first moved towards the sample 14 until the cantilever 18 bends to a predetermined level. For example, the degree of deflection of the predetermined cantilever 18 shown by the contour P1 of the probe in FIG. 1 is the setting point of the feedback controller 30.

The deflection of the cantilever 18 is monitored using the light beam L and the detector 28. The detector 28 is divided into independent detector regions A and B along its length. The output signal from the detector is the difference between the intensity of the light illuminating the area A and the intensity of the light illuminating the area B. Therefore, the cantilever deflection instruction can be obtained from the strength difference output from the detector. In the detector 28, when the cantilever 18 is curved to the position P1 (set point), the light beam L incident on the rear portion 18b of the cantilever is reflected along the path D1 so as to be incident on the regions A and B substantially equally. Be placed. That is, l_Al_B 0, l_AAnd l_BRepresents the intensity of light illuminating areas A and B, respectively. l_AAnd l_BThe exact value of can be adjusted and that value is used to identify the location of the deflection set point.

The tip 20 is then scanned on the surface of sample 14 and typically follows a raster pattern. When the tip 20 encounters a raised portion of the surface, the tip 20 tracking the surface is moved further upwards. Thereby, the probe 16 increases the deflection to, for example, position P2. When the probe 16 is in this position, the angle of incidence between the light beam L and the surface defined by the rear 18b of the cantilever changes. The light beam L is therefore reflected along another path D2 and is therefore more completely incident in region A than in region B of the detector. That is, the difference in the intensity of the light incident on the two parts of the detector 28 l_Al_BHowever, it has changed from the previous value (at the setting point). Therefore, it can be understood that the value of the strength difference gives an instruction on the deflection of the cantilever, and importantly, an instruction on how much the cantilever bends from the set point. The feedback controller 30 adjusts the vertical position of the probe support 22 away from the sample 14 and sets the deflection signal received from the detector 28 to its set point (l in this example)._Al_BIt is set to return to 0). The probe 20 is therefore maintained in the orientation shown as P1.

On the contrary, when the tip 20 encounters a surface portion whose height is one step lower, the eccentric force of the beam portion 18 of the cantilever resulting from the curvature of the set point pushes the tip 20 downward. Therefore, the probe 16 reduces deflection and orients as indicated by P0. In this orientation, the angle of incidence of the light beam L on the rear portion 18b of the cantilever is such that the beam L is reflected along the path D0. Therefore, the area B is more completely illuminated than the area A of the detector. The feedback controller 30 adjusts the vertical position of the probe support 22 again and moves it toward the sample 14 to maintain the deflection signal at the set point. Therefore, the probe 20 is maintained in the orientation shown as P1.

In this way, the feedback of the microscopy system ensures that the deflection of the probe 16 determined by the tilt angle of the rear 18b of the cantilever above the tip remains substantially constant throughout the scanning process. Thereby, it is guaranteed that the interaction force between the probe tip 20 and the sample 14, which acts to pull the tip toward the surface or push it back away from the surface, is also substantially constant. As the scan progresses, the vertical position of the support set by the z-direction positioning system is measured to provide indication of the height of the sample surface. The image is constructed using the adjustments made by the z-direction positioning system 24 during the scanning process.

Recent advances in probe microscopy technology have led to the development of much faster scanning techniques with shorter data acquisition times. However, in this new generation of microscopes, as described in PCT patent applications WO 02/063386 and WO 2004/005844, it is becoming increasingly clear that the constraints imposed by the AFM components themselves limit image acquisition time. ..

With reference to the capabilities of AFM according to the prior art described above, three distinct areas of AFM operation can be distinguished as the scanning speed is increased. In the above-mentioned prior art AFM, the tip 20 is continuously moved on the surface of the sample to cover a set number of pixels per second. The deflection of the cantilever is monitored and the feedback system continuously adjusts the vertical separation between the probe 20 and the sample 14 to keep the probe 16 in the preset curvature direction P1. The speed at which a change in deflection is detected and then the vertical separation can be adjusted in response to that change basically sets an upper bound on the scanning speed of the first (slowest) region. In order for accurate information to be collected, the interaction between the probe and the sample must be constant across all pixel positions. That is, the separation of the probe and the sample takes less time than it takes for the probe to collect the image data corresponding to one pixel position, i.e., before the raster scan moves the probe to a new pixel position. Must be adjusted by the feedback system.

However, when the scan speed is increased, the AFM enters a second region where the feedback system does not have enough time to return the probe to the set deflection within one pixel region. As a result, the feedback system lags behind the data collection system, and the height information (obtained from the vertical adjustment) is the true height of the probe tip at the pixel position (and thus of the sample). Height) will not be reflected. The pixel rate must be reduced by increasing the size of each pixel (in which case the image resolution is reduced) or by slowing down the scanning speed (resulting in increased data acquisition time).

To reduce the effects of this limitation, current research is focused on increasing the speed of the feedback system, i.e., substantially expanding the scope of the first area. One technique uses two actuators to provide cantilever movement in the z direction. The first actuator drives the probe assembly, resulting in a limited feedback response. However, there is provided a second actuator that is integral with the probe and thus can be operated to move only the probe. The range of motion provided by this secondary actuator is narrower than the range of motion provided by the first actuator, but its response time is shorter because it is used to affect only the probe. In this way, the initial adjustment of the tip height can be done via a high speed actuator, with a slower and wider adjustment later. However, using a nested feedback system with such an integrated secondary actuator has only limited success. This is partly due to the difficulty of interpreting the response of the feedback system and therefore determining the height of the tip. Without this information, the surface shape (or other sample properties) cannot be estimated accurately.

An alternative method is to operate the AFM in a mode that allows some variation in the deflection of the cantilever. In this mode, the deflection of the cantilever is measured and the data associated with that deflection is collected and used to construct the image. Therefore, operation in this mode requires improved measurement techniques to extract useful information from probe deflection. As described in more detail below, the prior art detection system shown in FIG. 1 can be adapted to operate in this mode, but such adaptation can result in loss of measurement information. This is unsatisfactory for many applications.

As the scanning speed is further increased, the prior art AFM enters a third region that is completely unsuitable for operation. Beyond a certain scanning speed, it is possible to cause transient movement of the cantilever 18. That is, in the process of scanning, if the tip encounters, for example, a higher region on the surface of the sample, the tip 20 supplies an impact that can cause transient movement of the beam portion of the cantilever. The presence of this transient motion as the probe tracks the contours of the surface causes the probe to orient at any given surface position and thus limits the information available from monitoring probe deflection. Any form of deflection is independent of probe height. Regarding this, J. P. Howard-Knight and J.M. K. Hobbs, "Video rate atomic force microscopic force sensing low stiffness, low resonance frequency cantilever", Ap. Phys. Lett 93 It is described in 104101 (2008) and will be described in more detail below.

Then, referring again to the prior art system of FIG. 1, as the tip 20 of the probe tracks the surface of the sample 14, the vertical position of the tip 20 at each point on the scan line gives a direct indication of the shape of the sample 14. It is clear to provide. Therefore, strictly speaking, measuring the height of the tip point 20a above a reference level, eg, the surface of the table 12, gives a true indication of the height of the sample.

In the first scanning speed region, the prior art system of FIG. 1 simply monitors the difference between the intensity of the light incident on the region A and the intensity of the light incident on the region B. No measurements are made specifically for deflection. The prior art AFM is a nulling system in that it is only required to keep the deflection constant. The probe height information is obtained from the drive that operates the feedback system to ensure constant deflection.

Moving on to the second scanning speed region, it is clear that the prior art system can be used to measure the difference between the intensity of the light incident on the region A and the intensity of the light incident on the region B. That is, the AFM can be operated not as a zeroing system but as a system that directly extracts data (intensity difference) as a result of sample investigation. Such data is, of course, useful as long as information about the height of the probe above the reference point (eg, the surface of the table) can be extracted from the measured deflection data.

By measuring the intensity difference between the two parts of the detector, it is clear that the direction of the light reflected from the rear 18b of the cantilever can be indicated. The direction of reflection is determined by the angle at which the rear 18b of the cantilever is tilted. For small angles, the change in tilt is equal to the angle at which the cantilever beam 18 is rotated, and is therefore proportional to the change in height at the tip 20. Positions P0, P1, and P2 shown in FIG. 1 are greatly exaggerated to improve clarity, and in many situations the cantilever beam 18 does not bend significantly and the small angles described above. Note that the rough estimate is sufficient. It is also clear that the point at which the deflection is measured is the position on the rear portion 18b of the beam portion of the cantilever, not the position at the tip point 20a. These two points are spaced apart by an amount equal to the vertical height of the tip 20. For small deflections, this can be assumed to be constant during the scan.

Notwithstanding the above estimates, the measurement of the height of the tip 20 through deflection is prejudiced in its support in that it is relatively easy to carry out. In the first scanning region, it is not necessary to quantify the variation in probe deflection. In the zeroing system, the deflection of the probe is kept constant. Also in the second scanning region, the operation of the scanning probe microscope is conventionally under the condition that the above assumption is valid.

Although it seems universal to rely on measuring probe height through probe deflection, an example of direct height measurement is US Pat. No. 5,144,150 (Mitutoyo). , European Patent No. 1892727 (Mitutoyo), and European Patent No. 2017599 (Mitutoyo). All microscopes described in these documents utilize interferometers to extract information about the height of the probe.

U.S. Pat. No. 5,144,150 describes using a contact probe to measure the shape of curved surfaces such as spherical lenses and Fresnel lenses, and the shape of semiconductor patterning. A problem in measuring such a surface is that the curved cantilever beam, for example when the probe tracks a particularly high area of the surface, can put unacceptable pressure on the sample surface. The solution is to ensure that the curvature of the cantilever beam remains substantially constant as the sample surface is scanned. This is achieved by connecting the base of the cantilever to a so-called optical probe. The optical probe focuses on the rear of the beam of the cantilever above the tip. When the tip moves relative to the base (ie, the beam bends), the tip moves away from the focal point of the optical probe. The optical probe is then physically moved to bring the tip back into focus and keep the tip and optical probe separated. This so-called "autofocus system" ensures that the cantilever returns to its original curved configuration through the connection between the optical probe and the base of the cantilever. In other words, an optical feedback system is used to keep the cantilever in a substantially constant deflection. To this extent, this configuration resembles a prior art AFM system operating in the constant interaction mode described above.

Unlike the constant-interaction AFM, which simply extracts the z-position of the probe from the vertical distance required to move the beam of the cantilever and maintain the deflection position, the microscope of US Pat. No. 5,144,150 The speed at the rear of the tip is directly measured using interference measurement. From the velocity information, the height of the probe at a specific point during scanning can be estimated. The velocity of the probe is measured by the interference of the laser beam reflected from the rear of the tip with the reference beam. The beam undergoes a Doppler frequency shift when reflected from a moving probe. By using the interference measurement in this way, it is possible to obtain an indication of the height of the probe that does not depend on the deflection of the probe, or an indication of the height of the beam portion of the related cantilever.

However, the configuration described in US Pat. No. 5,144,150 does not overcome any limitation imposed on scanning speed. This configuration can only be operated in the first area. The feedback system that maintains the deflection of the cantilever cannot react instantly to changes in the deflection, which limits the scanning speed and the time it takes to collect the pixels of the image is greater than the response time of the autofocus feedback system. Will also be longer. It is noteworthy that this prior art system comprises a second feedback system that ensures that the laser beam of the interference measurement system is reflected from the rear of the probe and returns along its incident path regardless of the tilt of the probe. Deserves. That is, the scanning speed is limited by the system response time of the slower of these two systems.

Two applications by Mitutoyo, European Patent No. 1892727 and European Patent No. 2017599, relate to improving the accuracy of operating probe microscopes and thus the resolution of images. This is achieved by using an interference measurement to extract a direct measurement of the height of the probe relative to a fixed reference point. In one embodiment, the beam reflected from the rear of the probe interferes with the reference beam and the path difference is extracted. This overcomes the problem of motion errors that occur in the scanning device. The z-direction motion of the probe tip arises purely from the shape of the sample surface. The probe is curved when a scanning device error occurs, but it does not affect the direct height measurement.

The system described in European Patent No. 1892727 does not use any feedback system. That is, no adjustments are made to prevent excessive curvature of the probe, and therefore the system is limited to scanning very flat surfaces.

European Patent No. 2017599 describes a device that is an extension of the device described in the previous Mitutoyo application for which feedback is implemented. This feedback system is simplified compared to the feedback AFM prior art described above in that it uses a single sensor to detect both the contact force (feedback signal) and displacement (height) of the probe. .. During scanning, a small vertical high frequency vibration (dither) is applied to the probe. The amplitude of this vibration provides information about the strength of the interaction force. Therefore, the direct height signal measured by the interferometer contains a low frequency component corresponding to the variation in the height of the sample, which is superimposed on the high frequency component generated from the dither. During signal processing, a filter is used to separate the two components. The height of the base of the probe is adjusted to keep the amplitude of the high frequency components constant. However, this configuration cannot overcome any of the limitations imposed on scanning speed. Accurate data results should only be obtained if the feedback system has sufficient time to respond to changes in surface height, i.e., if the dither amplitude is kept constant between image data collection points. Can be done. Data cannot be collected at higher rates. This is because, firstly, it is necessary to monitor the vibration over multiple cycles (5-10 times in this example) to measure the amplitude, and secondly, the z-direction drive system This is because it is necessary to give time to adjust the probe position in order to restore the amplitude to the set value. Therefore, this device can also be operated only in the first scanning region.

In summary, prior art probe microscopes are either unsuitable for operation outside the first (slow) scanning region identified above, or their application is limited to relatively flat samples. Is.

The deficiencies associated with prior art probe microscopes are even more pronounced given the additional problems encountered when attempting to scan at very high speeds in the third scanning region, i.e., fast enough to cause transient motion of the cantilever. Such a situation occurs, for example, when the cantilever's response to an impact given when it encounters a higher region of the sample surface exceeds the frequency of the cantilever's first constrained resonance. In such a situation, the tilt of the cantilever at the tip is probably not only different in magnitude from the tilt of its steady state (setting point), but may also be in the opposite direction. Transient motion changes the angle of the cantilever over time and position. This problem will be specifically described with reference to FIG.

Note that there are two possible mechanical states of the cantilever, whether the tip is constrained by contact with the sample surface or free. The resonance conditions of the cantilever in these states are referred to as "constrained resonance" and "unconstrained resonance", respectively.

FIGS. 2a, b, c, and d show various situations in which the probe tip 20 receives an impact from the surface of the sample 14. Similar to the above, the probe 16 includes a tip 20 connected to a beam portion 18 of the cantilever. Point 20a of the probe closest to the sample and rear 18b of the cantilever used to measure deflection are shown. The steady-state nominal position of the rear 18b of the cantilever is indicated by the dotted line 40. "Nominal" means the position where the rear 18b should be retained if feedback was acting. That is, it corresponds to the direction P1 of the probe in FIG. The incident light beam L is shown on the right side of each figure, and the expected reflection direction 42 for the probe at the nominal position is shown by a dotted line on the left side. That is, the dotted line 42 corresponds to D1 in FIG.

In FIGS. 2a and 2b, the position of the sample surface is indicated by the solid line 44. The position 44 on the surface is substantially a depression, and to make this example easier to understand, the tip 20 follows the surface and is pulled downward from the nominal position. FIG. 2a shows a situation in which the impact received from the sample (causing a downward movement) takes longer than the settling time of the cantilever 18 and is relatively slow. That is, the transient motion decays within the time it takes for the probe to react to the outer shape of the surface. When the tip 20 descends and comes into contact with the surface 44, the beam portion 18 of the cantilever is curved downward, and the rear portion 18b of the cantilever is tilted toward the left side in the drawing. Therefore, the light beam L for measurement is reflected from the rear portion 18b of the cantilever along the path 46. Therefore, as shown by the arrow 48, the beam emission path is detected in a state of being deviated in the counterclockwise direction. For comparison, FIG. 2b shows an example of the momentary position of the probe 16 when the impact received from the sample 14 has a duration shorter than the settling time of the cantilever 18. As before, the tip 20 is pulled downwards to surface level 44, but the transient vibration of the cantilever 18 means that ripples are observed over the length of the cantilever. Therefore, the instantaneous positional relationship of the probe may be as shown in FIG. 2b. Therefore, the rear portion 18b of the cantilever swings around the contact point 20a of the probe with the sample when the ripple propagates. The momentary position where the rear portion 18b of the cantilever is tilted to the right is shown in FIG. 2b. At this moment, the light beam L is reflected along the path 50. Therefore, the emission path of the beam is observed to be shifted clockwise as shown by the arrow 52.

Thus, it can be seen that momentary measurements of probe deflection, despite tracking the same surface position, produce the opposite results in the contrasting situations shown in FIGS. 2a and 2b. In FIGS. 2c and 2d, the position of the sample surface is raised as shown by the solid line 54. Therefore, the surface position 54 represents a peak, and the tip 20 that follows the surface is lifted upward from the nominal position. FIG. 2c shows a relatively slow situation in which the impact received from the sample causing this movement takes longer than the settling time of the cantilever 18. That is, the transient motion decays within the time it takes for the probe to react to the outer shape of the surface. When the tip 20 rises with the surface 54, the beam portion 18 of the cantilever bends upward and the rear portion 18b of the cantilever is tilted toward the right side of the drawing. Therefore, the light beam L for investigation is reflected from the rear portion 18b of the cantilever along the path 56. Therefore, the emission path of the beam is observed to be shifted clockwise as shown by the arrow 58. Similar to FIG. 2b, FIG. 2d shows an example of the momentary position of the probe 16 when the impact received from the sample 14 has a duration shorter than the settling time of the cantilever 18. Similar to FIG. 2c, the tip 20a of the probe is lifted with the surface level 44, but the transient vibration of the cantilever 18 means that ripples are observed as the transient mode propagates over the length of the cantilever. That is, the rear portion 18b of the cantilever swings around the contact point 20a of the probe with the surface. The instantaneous positional relationship is as shown in the figure, for example. In the example of FIG. 2d, the rear 18b of the cantilever is tilted to the left. At this moment, the light beam L is reflected along the path 60. Therefore, the emission path of the beam is observed to be shifted counterclockwise as shown by the arrow 62.

Again, when transient motion is triggered, that is, when the speed at which the probe scans the surface is such that the probe reacts to surface features within a time frame shorter than the cantilever settling time. Is found to produce contrasting results with measurements at the same surface position 54.

The limitation that defines the start of the third velocity region is that the pixel collection period (the average time it takes to image the surface portion corresponding to one pixel in the image) is shorter than the cantilever settling time. be able to.

It should be understood that the position of the probe 16 shown in FIGS. 2b and 2d is instantaneous. The transient mode propagates along the beam 18 of the cantilever and, if not attenuated, is observed as the swing of the rear 18 of the cantilever and the resulting short variation in the direction of the reflected beam. For example, if the transient mode is damped by using the cantilever beam with a low Q factor, the initial tilt movement of the rear 18b will settle faster.

PCT patent application WO2005 / 0008679 describes a hybrid system with a higher ability to measure the shape of non-uniform sample surfaces. An improved mechanical system that tracks the tip on the sample surface is implemented in combination with an electronic feedback mechanism. The electronic feedback mechanism is used to adjust the vertical separation of the probe base and the sample support, albeit at a time greater than the pixel acquisition rate. Generally, these feedback adjustments are made over a period of time across multiple scan lines, within which the variation in height of the tip on the sample surface must be measured. It is assumed that the sample surface is sufficiently flat at the scale of one scan line and the detection system is not too restrictive. From the measured height variation, an image of surface shape variation can be formed along each scan line and an electronic feedback signal is used to adjust each scan line to a reference level. This improved mechanical tracking system is needed to keep the tip in better contact with the surface when it encounters irregularities. This makes it possible to collect more accurate height information within the time required for each scan line. However, although this prior art device can operate in the second speed range, it does not consider the transient mode of the cantilever. Better tracking is provided, which delays the start of the third region, but such microscopes are still unable to obtain accurate results when operated within the third region.

<p num="0048"> An object of the present invention is to be able to extract information about the height of the probe tip as the probe tip scans the sample surface at a speed that exceeds the limits set by the feedback system to keep the feedback parameters at a predetermined value. It is to provide a detection system for use with an operating probe microscope. In particular, it is an object of the present invention to provide a detection system capable of extracting useful information while the probe is undergoing transient motion.</p>

<p num="0049"> Accordingly, the present invention provides a detection system for use with a scanning probe microscope, which is a beam that illuminates a probe with a cantilever that has a base end and a free end and the free end supports a sharp tip. The beam illuminates the upper surface of the probe near the tip of the probe, and the reflected light deflects the upper surface of the probe. The first component from which the instructions are obtained and the second component sent to a height detection system configured to extract information about the position of the probe's top surface with respect to the reference point from the second component. , Contains two components.</p><p num="0050"> This configuration has the advantage that the detection system can be used to form an image of the sample surface over all three operating regions. In the context of the present invention, "deflection" should not be construed as being limited to the meaning apparent in the prior art described herein. "Deflection" should be construed to include measurements of the orientation or tilt angle of the top surface of the probe (similar to the prior art), or any other angle derived from the orientation of the cantilever. It can also include measurements derived from the vertical separation between the free end and the base end of the cantilever (or any other position on the length of the cantilever). Simply put, the "deflection" of a cantilever should be understood as some measurement of the curved shape of the cantilever as a result of the interaction between the probe and the sample. By monitoring the deflection of the top surface of the probe, feedback can be used to keep the average interaction force between the probe tip and the sample constant. This has the effect of limiting the stress that can be applied to the sample and / or probe as a result of the application of excessively high interaction forces. On the other hand, the path difference measured by the height detection system is a direct measurement of the height of the probe. Regardless of the deflection of the cantilever, this measurement result is a true indication of the probe height with respect to the position set by the reference point. Since this measurement is independent of deflection, it is not affected by the response time of the feedback system, and possibly the presence of transient motion, which can dynamically alter the deflection of the cantilever. It does not affect the height of the cantilever.</p><p num="0051"> As a result of the height measurement being independent of deflection, it is not necessary to have knowledge of the vertical (z) height of the probe base. In a prior art AFM system, the height of the base is measured in order to extract the data that forms the image. In the AFM according to the invention, the image is formed directly from the output of the interferometer. Therefore, in the prior art, the input signal to the drive system that controls the height of the base in the vertical (z) direction must be calibrated against the actual height achieved, whereas in the present invention. There is no need for such calibration.</p><p num="0052"> Ideally, the height detection system comprises an interferometer configured to detect the path difference between the second component of the reflected light and the height reference beam. Such an interferometer can produce accurate measurements of this optical path length, and thus the relative height of the probe. Preferably, the interferometer is a homodyne interferometer that includes means for generating a pair of quadrature interferograms, which can be subjected to accurate interferometric fringe counting techniques across multiple interferometers. This improves the accuracy of the height detection system and expands the range of detectable cantilever movement. Quadrature interferograms can be generated on beam splitters with a phase splitting coating.</p><p num="0053"> The first component can also be sent to the interferometer, which is defined in length by the first component of the reflected light and its position on the cantilever away from the free end, preferably the position of the base end. It is further configured to detect the optical path difference from the deflection reference beam propagating along the optical path, thereby providing information about the tilt of the cantilever. That is, in this example, the deflection measurement is obtained from the vertical height difference between the free end of the cantilever and the second position along the length of the cantilever, which is substantially the same as that of the cantilever. It is a measured value of the average slope. In this embodiment, an interferometer can be used to extract both height and deflection information, thereby reducing the number of components required.</p><p num="0054"> Alternatively, the first component may be delivered to the deflection detector, which is configured to provide an indication of the deflection of the upper surface of the probe. Therefore, a deflection signal can be obtained using any deflection detector known in the prior art.</p><p num="0055"> Specifically, the deflection detector may be a second interferometer, which is the first component of the reflected light and a position on the cantilever away from the free end, preferably at the base end. It is configured to detect the optical path difference from the deflection reference beam propagating along the optical path whose length is defined by the position. Alternatively, the deflection detector may be a split photodiode, in which the first component of the reflected light is in two parts of the diode with a relative intensity determined by the angle of orientation of the top surface of the probe. Oriented to be incidental.</p><p num="0056"> The system may also include a beam splitter configured to split the reflected light into the first and second components referenced above. The probe preferably comprises a tip attached near the free end of the cantilever, the cantilever is supported at the base end, and the top surface of the probe illuminated by the beam is above the tip.</p><p num="0057"> The probe may also include an actuator that can move to move the tip of the probe. In one embodiment, movement of the probe tip can be achieved by varying the angle of orientation of the top surface of the probe within the region of the probe tip. The actuator is configured to drive a much smaller load than any drive that is configured to move vertically through the probe assembly and / or support. For this reason, it offers the possibility of a faster feedback response. Conventional nested feedback systems also utilize such integrated actuators, but have the disadvantage of making the interpretation of the feedback loop and therefore the determination of probe height somewhat more complicated. This embodiment of the present invention avoids this inconvenience because the height of the probe is directly measured regardless of where the feedback actuator is driven.</p><p num="0058"> In a second aspect, the invention provides a scanning probe microscope that images a sample according to the interaction between the sample and the probe, the microscope causing relative motion between the probe and the surface of the sample. It comprises a driving means configured to provide and a probe detection system described herein.</p><p num="0059"> The driving means is preferably relative to an XY scanning device configured to provide relative motion between the probe and the sample surface in a plane substantially parallel to the sample surface and in a direction orthogonal to the sample surface. It comprises a Z-direction drive body configured to provide a functional motion. In such a configuration, the indication of the deflection of the upper surface of the probe obtained from the probe detection system can be added to the feedback system incorporating the Z-direction drive, which allows the Z-direction drive to deflect the probe. It is configured to return to the set level. The Z-direction drive may include a base drive that is configured to move the base of the probe and / or an actuator that is integral with the probe and is configured to move the tip of the probe.</p><p num="0060"> Unlike prior art AFMs, this feedback system operates longer than the time required for the XY scanning device to move the probe between image pixels, adversely affecting the quality of the images recorded by the microscope of the present invention. Does not affect.</p><p num="0061"> The XY scanning device preferably comprises a resonator configured to vibrate the probe and support, or the sample and support, at or near the resonant frequency of the probe or sample support.</p><p num="0062"> In a third aspect, the invention provides a scanning probe microscope that images a sample according to the interaction between the sample and the probe, the microscope causing relative motion between the probe and the surface of the sample. It comprises a driving means configured to provide and a probe detection system configured to provide indication of the position of the probe with respect to a reference point in a direction substantially orthogonal to the sample surface. The probe comprises a cantilever with a base end and a free end, the free end supporting a sharp tip, The driving means comprises an XY scanning device configured to provide relative motion between the probe and the sample surface in a plane substantially parallel to the sample surface, and the scanning device is a probe or sample and a support. Is configured to provide relative motion in a direction substantially orthogonal to the sample surface with at least one resonator configured to vibrate at or near the resonant frequency of the probe or sample support. Equipped with a Z-direction drive body A probe detection system is a light source that produces a beam that illuminates the probe and a condensing means that condenses the light reflected from the probe, the beam illuminating the upper surface of the probe near the probe tip. And a height detection system configured to extract information about the position of the probe's upper surface with respect to a reference point from the light reflected from the probe.</p><p num="0063"> The height detection system ideally comprises an interferometer configured to detect the path difference between the light reflected from the probe and the height reference beam. In a fourth aspect, the invention provides a method of detecting light reflected from the top surface of a scanning probe, which directs the light beam to a point directly above the probe tip on the top surface and tops it. It involves a step of condensing the light reflected from the surface and directing the light to a height detector configured to form an image showing the height of the probe tip above the reference level.</p><p num="0064"> Ideally, the method is preceded by the step of identifying a point directly above the probe tip on the top surface. This step is preferably (A) A step of guiding light onto the upper surface, (B) A step of changing the angle of the upper surface by vibrating the probe around the tip of the probe. (C) Step of monitoring the output of the height detector (D) A step of changing the position of a point on the upper surface where light is incident, and (E) Includes a step of repeating steps (c) and (d) until the variation seen in the output of the height detector as a result of tilting the probe is minimized.</p><p num="0065"> This provides a convenient technique for minimizing the effect of transient motion on image quality. The probe preferably comprises a cantilever that is held at the base end and supports the tip near the free end or free end, and the top surface of the probe illuminated by the beam is near the free end or free end of the cantilever.</p><p num="0066"> Alternatively, the above step of identifying a point on the surface may (A) A step of guiding light onto the upper surface of the cantilever, (B) A step of vertically moving the base of the cantilever while maintaining the tip in a fixed position to change the angle of orientation of the cantilever. (C) Steps to monitor the output of the height detector, (D) A step of changing the position of a point on the upper surface where light is incident, and (E) Steps (c) and (d) are repeated until the variation seen in the output of the height detector as a result of moving the base of the probe is minimized. including.</p><p num="0067"> In embodiments where the feedback signal is obtained from the direction of the probe, the feedback signal is adapted to adjust the height of the base so that the probe is returned in a preset orientation, followed by a cantilever. The step of vertically moving the base of the cantilever can be accomplished by varying a preset orientation.</p><p num="0068"> Alternatively, the above step of identifying a point on the top surface may (A) A step of guiding light onto the upper surface, (B) Displace the probe tip by a known distance, preferably scanning the probe on a sample with known height variation. (C) Steps to monitor the output of the height detector, (D) A step of changing the position of a point on the upper surface where light is incident, and (E) It can include repeating steps (c) and (d) until the output of the height detector matches the expected output for a known displacement.</p><p num="0069"> Alternatively, the above step (A) A step of guiding light onto the upper surface, (B) The step of scanning the probe at a speed sufficient to cause transient motion of the probe, (C) Steps to monitor the output of the height detector, (D) A step of changing the position of a point on the upper surface where light is incident, and (E) A step of repeating steps (c) and (d) can be included until the variation seen in the output of the height detector as a result of the transient motion is minimized.</p><p num="0070"> In a fifth aspect, the invention provides a method of collecting data using a scanning probe microscope, which method is: (A) A step of moving a probe having a base end and a free end and having a cantilever with a free end supporting a sharp tip in the vicinity of the sample surface. (B) A step of guiding the light beam to a point directly above the tip on the upper surface of the probe, (C) A feedback signal obtained by analysis of the first component of the focused light, including the step of scanning the probe on the sample surface while condensing and analyzing the light reflected from the top surface of the probe. In response, the Z-direction drive acts to drive the base of the probe vertically, and the first component of the focused light gives an indication of the deflection of the top surface of the probe and is focused. The second component of light is configured to detect the path difference between the second component and the height reference beam, forming an image showing the height of the probe tip above the reference level. The response to the feedback signal is sent to the interferometer for a longer time than the time required for the plurality of image pixels to be collected.</p><p num="0071"> Next, an embodiment of the present invention will be described as a mere example with reference to the accompanying drawings.</p>

<figref num="1">FIG. 6 is a schematic view of the components of a prior art atomic force microscope showing a typical deflection detection system.</figref><figref num="2">FIG. 2A is a diagram of an example of the orientation of the tip and the cantilever that can be taken when imaging at a scanning frequency below the threshold at which the transient motion of the cantilever is observed. FIG. 2B is a diagram of an example of the orientation of the tip and the cantilever that can be taken when imaging at a scanning frequency above the threshold at which the transient motion of the cantilever is observed. FIG. 2 (c) is a diagram of an example of the orientation of the tip and the cantilever that can be taken when imaging at a scanning frequency below the threshold at which the transient motion of the cantilever is observed. FIG. 2D is a diagram of an example of the orientation of the tip and the cantilever that can be taken when imaging at a scanning frequency above the threshold at which the transient motion of the cantilever is observed.</figref><figref num="3">It is the schematic of the component of the atomic force microscope which has the probe detection system by this invention.</figref><figref num="4">It is the schematic of the probe detection system according to this invention incorporated in the high-speed scanning probe microscope.</figref><figref num="5">It is the schematic of the height detection system using the interference measurement according to this invention.</figref><figref num="6">It is a schematic diagram which shows the influence which changed the point on the rear part of a cantilever whose height is measured.</figref><figref num="7">FIG. 6 is a schematic representation of an apparatus used to align the detection system of the present invention in preparation for performing a scan of the sample surface.</figref>

With reference to FIG. 3, the implementation of the AFM utilizing the first embodiment of the detector constructed according to one aspect of the present invention is schematically shown, represented by 70 in its entirety. Elements common to the prior art AFM described above with reference to FIG. 1 are designated by the same reference numerals. Therefore, the AFM device in the figure comprises a movable pedestal 12 whose surface is adapted to receive the sample 14 examined by the probe 16. The probe 16 includes a beam portion 18 and a tip 20 of the cantilever, and the tip 20 tapers toward the point 20a and is arranged toward one end of the beam portion 18 of the cantilever. The other end of the beam portion 18 of the cantilever is supported by the support base 22.

One or more drive motors (72, not shown) are used to scan sample 14 (with pedestal 12) and / or probe 16 relative to each other in three dimensions in the x, y, and z directions. Drive as you can. According to convention in the art, the z-axis of the Cartesian coordinate system is understood to be the axis orthogonal to the plane occupied by the sample 14. That is, the strength of the interaction force between the probe 16 and the sample 14 depends on both the xy position of the tip 20 on the sample 14 (the pixel in which the tip 20 is imaged) and the height of the tip 20 on the sample. ..

Among various drive motors, the z-direction positioning system 72 is the most promising for the purpose of the present invention. The z-direction positioning system 72 includes a piezoelectric drive body capable of moving the tip 20 in a direction toward and away from the sample 14 (z-direction). In this embodiment, the z-direction positioning system is connected to the probe support 22. Alternatively, it may be connected to the sample table 12 (shown in FIG. 1), or may be provided with a non-piezoelectric drive mechanism such as an audio coil or a thermal bimorph actuator. A sample 14 and tip 20 on the sample surface (x, y) such that additional motors are connected to the support 22, or both, and the tip 20 scans over the sample 14 in a raster or other manner. It can act to provide relative movement between.

In one embodiment, the z-direction positioning system can include both a pedestal (or pedestal) drive as described above and an additional actuator integrated with the probe itself. This integrated actuator can be used to drive the deflection of the probe with a faster feedback response than is possible with the assembly drive.

The probe 16 is a low-mass AFM probe, and an interaction force is generated between the tip 20 and the sample surface during scanning. The probe detection mechanism 74 will be described in more detail below, but is configured to give instructions for both vertical (z) displacement and deflection (tilt) of the rear point 18b of the cantilever above the tip 20. Data on vertical displacement is analyzed and output to a display (not shown). Information about the tilt / deflection of the rear portion 18b of the cantilever is input to the feedback controller 30, which is connected to the drive mechanism of the z-direction positioning system 72.

Note that for convenience, the expression "vertical" applies to the z-direction movement of the microscope system. From the above commentary, it is clear that this is not intended to limit the situation in which the z-axis of the microscope is aligned vertically. The microscope can be arranged in any convenient orientation according to the purpose.

The probe 16 is generally made of silicon (in the case of AFM) or silicon nitride and is formed in the shape and size as described above in the context of prior art. When taking an image of a sample, the AFM 70 operates as follows. Using the z-direction positioning system 72 and an additional drive, the sample 14 is first brought into contact with the tip 20 at the start of raster (x, y) scanning. Conventionally, in AFM terminology, the tip 20 is described as being in contact with the sample 14 when the atomic interaction force is in the repulsive region. When the probe 16 comes into contact with the sample surface, the tip 20 is therefore pulled upward. The base of the cantilever 18 on the side away from the tip is fixed in place by the support 22 so that the cantilever 18 bends or bends upwards. As in the prior art and as described above in connection with the prior art, the support base 22 is lowered and the probe 16 is moved towards the sample 14 until the curvature of the cantilever reaches a predetermined level. This predetermined level becomes a setting point of the feedback controller 30.

As the scanning progresses, the tip 20 moves up and down as the height of the sample surface fluctuates. As the tip moves, the degree of tilt changes, thereby changing the signal supplied to the feedback controller 30 and adjusting the z-direction positioning system 72. The following considerations apply when the base of the probe is adjusted in response to feedback. When the tip moves on the sample at a velocity v, the tip tracks the sample surface and therefore a rate f proportional to v._trackEncounter fluctuations in surface height. A transient motion of decay time or settling time τ is triggered in the cantilever. The response time of the feedback system is the time it takes for the feedback system to detect changes in the feedback parameters and adjust the height of the probe base accordingly.

Three scan speed regions can be identified. If the feedback system response time is shorter than the time interval between data collection points, the feedback system adjusts the probe height while the probe is collecting data on surface height at one image point. Have enough time for. That is, for all image points, the probe can be considered to be in a constant deflection, and therefore the interaction force between the probe and the sample can also be considered constant. This is the first velocity region, in which prior art systems using feedback can extract accurate surface information. However, the apparatus of the present invention differs from the prior art in that information about the height of the probe is directly obtained by the detection system 74. If the response time of the feedback system is longer than the time interval between data collection points, the feedback only works to maintain a constant average deflection. In this second region, the tip velocity is such that surface variability is tracked more frequently than can be accommodated by the response time of the feedback system, so such variability is probe height detection. Must be measured directly by the system. The feedback signal is used to maintain a constant mean deflection, which acts to prevent the probe from experiencing the intensity of extreme interactions that can cause damage to the probe, sample, or both. .. In the third scan region, the cantilever settling time is longer than the time interval between data acquisition points, and the transient motion caused within the cantilever is not attenuated by the time the probe moves to image the subsequent sample region. However, the height detection system of the present invention can extract the measured value of the height of the probe even in the presence of such movement. As a result, if the tip is kept in contact with the sample, the height of the sample can be indicated.

Broad definitions are given to these regions, and the initiation of each region clearly depends on the specific operating conditions, as well as the parameters of the microscope and the sample under investigation. Next, the operation of the detection system 74 according to the present invention will be described in more detail. A light source (not shown) emits a laser beam 76, which is focused by an objective lens 78 on the rear 18b of the cantilever. The reflected lights 80a, b, and c are collected by the lens 78 and guided to the unpolarized beam splitter 82. As can be seen from FIG. 3, the inclination of the cantilever 18b affects the light reflection angle. If the probe is maintained at the set feedback position, the reflected beam follows path 80b. On the other hand, when the rear portion 18b of the cantilever is tilted to the left (with respect to the figure), the reflected beam 80a is deflected counterclockwise, and the tilt to the right deflects the reflected beam 80c clockwise. When the rear 18b of the cantilever is located at or near the focal point of the lens 78, the variation in the angle of the reflected beam is converted into a lateral displacement after passing through the lens. That is, when the cantilever is tilted to the left, the reflected beam is laterally displaced to the left, and when it is tilted to the right, it is displaced to the right with respect to the set feedback position.

The beam splitter 82 is configured to reflect substantially half 84a, b, c of the incident light by 90 degrees and to transmit the other half 86a, b, c. The transmitted components 86a, b, and c are input to the interferometer 88. The operation of this interferometer will be described in more detail below with reference to FIG. In summary, the reflected beams 86a, b, c interfere with the reference beam reflected from the upper surface of the pedestal 12. Alternatively, another fixed point having a known relationship with the table surface may be used. This provides an indication of the path difference between the two beams and, therefore, the height of the rear 18b of the cantilever above the pedestal surface. This measured height variation is extracted to form an image.

The components 84a, b, and c reflected from the beam splitter 82 are focused on the deflection detector 28 by the lens 90. As in the prior art, the detector 28 is divided into independent detector regions A and B over its length. The output signals generated from these regions are input to the differential amplifier 92, which outputs a signal equal to the difference between the two channels. The detector 28 is aligned with the optical characteristics of the focusing so that when the rear 18b of the cantilever is tilted to the set deflection position, the output from the differential amplifier also becomes the set point. That is, the reflected lights 80b and 84b are dispersed in channels A and B so that the difference between the channel outputs becomes a set value. The left tilt of the rear 18b of the cantilever means that the signal from the detector 28 increases in channel B, leading to a decrease in the output from the differential amplifier 92. Conversely, a tilt to the right means that channel A receives an increase in signal, resulting in an increase in output at the differential amplifier 92. The feedback controller 30 operates the z-direction positioning system 72 so as to maintain the signal received from the differential amplifier 92 at its set point.

The true height of the probe on the surface is measured by an interference measurement height detection system, regardless of the feedback signal, or equivalently, independent of cantilever deflection. Therefore, a feedback system is used to ensure that the average deflection is maintained at a constant level.

It is important that the light 76 of the detection system is focused on the rear 18b of the cantilever. For this reason, the z-direction positioning system is also connected to the objective lens 78 so that when the probe 16 is raised or lowered, the lens 78 is raised or lowered by an equal amount. In an alternative embodiment, the objective lens 78 is selected to have a greater depth of focus than the range of expected movement from the tip. Therefore, it is not necessary to adjust the position of the objective lens with respect to the tip 20.

The present invention will be described in the context of FIG. 3, which shows an atomic force microscope, but the detection system can be used with all scanning probe microscopes in which it is important to accurately determine the height of the probe tip. Please note that you can see.

In a further embodiment, the (x, y direction) scanning drive mechanism (not shown) and the z direction positioning system 72 can be replaced with any suitable drive means, not necessarily piezoelectric. These drivers can be connected to the sample base 12, the probe support base 22, or a combination of both.

The deflection detector 28 described above may be replaced by an alternative known means of measuring the deflection of the cantilever 18. For example, the cantilever can be formed with an integrated piezoelectric resistance sensor. A height detection system similar to the system used to measure cantilever height may also be used to measure deflection. That is, an interferometer is used to measure the height of the tip 20 relative to the height of the cantilever base.

The light source used in this embodiment is a laser light source, but alternative light sources are also available. For detection by interferometry, the light must be coherent to the extent required by the interferometer specifications.

The transient motion of the cantilever imposes a fundamental limit on the speed at which feedback can be performed. The cantilever must be able to settle within the time it takes for the feedback system to measure and adjust the deflection. Otherwise, the transient motion will generate inaccurate deflection information. The various methods of measuring deflection impose slightly different limits on the feedback control system accordingly. When measuring deflection using the cantilever angle, the response time of the feedback control system must be greater than the constrained cantilever settling time. However, when using a height detection system (such as the interferometer referred to above), the response time of the feedback control system is limited only by the unrestricted resonance frequency of the cantilever.

An alternative AFM apparatus particularly suitable for use with the detection system of the present invention is shown in FIG. In FIG. 4, one or both of the (x, y) scanning drive mechanisms are replaced by the resonator 94. The resonator 94 is a vibration drive set to vibrate the sample table at its resonance frequency or in the vicinity of the resonance frequency. Such resonant scanning microscopes provide very fast and stable scanning capabilities, which can well exceed the threshold limits when operating in a constant interaction mode. Alternatively, the resonator 94 may be set to vibrate the probe assembly at or near the resonant frequency.

With reference to FIG. 5, the optical components of the detection system 74 are shown in more detail. Similar to the figure above, common components are given similar reference numerals. The light from the laser light source 100 is split into an incident beam 76 and a reference beam 104 by a second beam splitter 102. The incident beam 76 is focused on the rear portion 18b of the cantilever by the objective lens 78. After being reflected by the cantilever, the beam 80 is split by the first beam splitter 82. As mentioned earlier, the first component 84 is guided to the deflection detectors 90, 28, 92 and the second component 86 is guided to the interferometer 88.

Inside the interferometer, the beam 86 reflected from the cantilever 18b is split by the beam splitter 106. The reference beam 104 is guided to a properly placed retroreflector 108 and then to a beam splitter 106. The retroreflector 108 is aligned to provide a fixed optical path length with respect to the vertical (z) orientation of the sample. The beam splitter 106 has an energy-absorbing coating and splits both the incident beam 86 and the reference beam 104 to generate first and second interferograms with a relative phase shift equal to approximately 90 °. To do. The two interferograms are detected by the first photodetector 112 and the second photodetector 114, respectively.

Ideally, the photodetector signal is a complementary sine and cosine signal with a 90 ° phase difference. Moreover, those signals should have no DC offset, have equal amplitude, and depend only on the cantilever position x and the laser wavelength λ. To determine the resulting error in which the two photodetector signals are not perfectly harmonic with equal amplitude and orthogonal phase, and apply corrections to that error, using known methods, varying the optical path difference. Monitor the outputs of the photodetectors 112 and 114. Similarly, the level of DC offset is also corrected by methods known in the art.

This photodetector signal is suitable for use with conventional interferometer reversible interferometric fringe counters and interferometer subdividing devices, which can be provided as dedicated hardware or programmed computers. Using an orthogonal phase interference fringe counter, the displacement of the cantilever position can be measured with an accuracy of λ / 8, that is, 66 nm for light with a wavelength of 532 nm. Known interference fringe subdivision techniques based on the inverse tangent of a signal make it possible to improve accuracy to the nanometer scale or less.

Interference measurement methods for extracting path differences between two coherent beams are well known in the art and therefore will not be described in more detail. In the above embodiment, the reference beam is configured to have an optical path length fixed with respect to the z-direction position of the sample. Thus, as described above, the reference beam can be reflected from the surface of the table on which the sample is placed, or from a retroreflector whose position is associated with the position of the table. Alternatively, the relationship between the reflector and the z position of the sample does not have to be fixed. In such an embodiment, the reference beam can be reflected from a fixed point, which has a known (but fluctuating) relationship with the z-position of the sample. Therefore, the height of the tip is estimated from the optical path difference measured by the interference measurement and the z-direction position of the sample with respect to the fixed point.

The interferometer described herein is an example of a homodyne system. The particular system described herein provides some advantages to the present application. By using two quadrature interferograms, it is possible to measure the displacement of multiple interference fringes and thus the cantilever over a wider displacement range. By using a phase-shifting coating on the beam splitter 106, the interferometer is unaffected by the polarization effect resulting from, for example, a change in polarization as the light beam is reflected from the cantilever. Examples of interferometers based on these principles are described in US Pat. No. 6,678,056. Alternative interferometer systems capable of measuring changes in optical path length may also be used with the present invention. A suitable homodyne polarization interferometer is described in European Patent No. 1892727 referenced above, and a suitable heterodyne interferometer is described in US Pat. No. 5,144,150 referenced above.

It is clear that the AFM can operate in the second scan speed region. The height information extracted by the detection system represents the true height of the probe, and thus the true height of the shape of the sample surface. It does not depend on the position of the probe base relative to the tip, i.e., deflection. Therefore, fluctuations in deflection centered on the average value are allowed during scanning, but this is not related to height measurements by interference measurements.

However, when moving to the third scan speed region, additional steps must be included in preparing the AFM of the present invention for operation. It has already been shown with reference to FIG. 2 that when a transient motion is triggered by the cantilever, the motion manifests itself as a swing of the rear 18b of the cantilever. To avoid such effects appearing as artifacts in height images, it is important that the exploratory laser beam 76 of the detection system focuses on its oscillating nodes. That is, it is a position above the point 20a at the tip. This can be empirically achieved by monitoring the output image and adjusting the detection position of the incident beam 76 until the periodic fluctuations that overlap the image are minimized or even disappear. That is, the probes and interferometers must be set to the correct sequence before or during the extraction of data at scan rates within the third region using the device 70.

The second reason why the position where the investigation beam is incident at the rear of the laser beam is important will be described with reference to FIG. FIG. 6 shows the incident positions of the three possible laser beams at the rear 18b of the cantilever. In the situation (a), the probe 16 is in a certain direction, and in the situation (b), the probe 16 is in the second direction following the step on the sample surface. When the laser beam is aligned with the position A to the right of the probe tip 20, it is clear that the measured height change A2-A1 is greater than the step height because the deflection is at a shallower angle in situation (b). Is. Conversely, when the beam is aligned with position C, the measured change C2-C1 is less than the true step height. A true measurement of step height is only obtained if the laser beam is aligned exactly above position B, the closest contact point between the tip 20 and the sample 14. In other words, it is important to accurately match the interferometer to the probe shape.

The true height problem and the effect of transient motion are basically overcome by aligning the incident beam just above the tip point, so both can be addressed in the same way. Note that the true height issue applies to both the second and third scan speed regions. This is because this problem arises as a result of allowing the tilt / deflection of the cantilever to fluctuate as the height information is extracted by the interference measurement. Therefore, in order to obtain the most accurate image information, the step of matching the height detection system with the shape of the probe must be performed in the scanning speed region where the tilt / deflection of the cantilever is allowed to fluctuate.

As mentioned above, alignment can be empirically achieved at high scan speeds before or during the scan in which the transient motion is triggered. Alternatively, the alignment system shown in FIG. 7 can be used. In this system, the beam exciter 120 is set to cause vibration of the beam 18 of the cantilever. The rear 18b of the cantilever oscillates or oscillates accordingly, the effect of which can be observed on the light reflected from the rear surface 18b as periodic signal fluctuations in the deflection detection and height detection system. In the embodiment of the figure, the control system 122 is configured to monitor the output of the height detection system 88. If periodic fluctuations in this output signal are observed with the vibration of the cantilever, the control system 122 signals to one or more drive bodies 124, which causes the detection point (rear 18b of the cantilever). (Point where light is incident on) moves laterally. The adjustment of the detection point is continued until the influence of the swing in the height detection signal is minimized.

In the embodiment of FIG. 7, the drive body 124 is connected to the height detection system 88 and therefore, as indicated by arrow 126, the alignment of the light incident on the rear 18b of the cantilever is moved so that the detection points are offset. It is possible to operate as it does. Alternatively, the one or more drive bodies 124 are connected to the probe itself and are configured to move the probe laterally according to the instructions of the control system 122 to shift the detection points as described above. That is, the detection point is adjusted by moving the probe or by moving the alignment of the light incident on the probe.

The vibration of the cantilever 18, and thus the vibration of the rear portion 18b, is generated by a known beam vibrating device 120, such as a piezoelectric transducer. Alternatively, the cantilever 18 can be adapted to react to an electric or magnetic field. In that case, the vibration can be generated using an oscillating electric or magnetic field in which the cantilever 18 reacts mechanically.

An alternative method of setting the detection point is to change the height of the cantilever base while monitoring the height detection signal. In that case, the detection point on the rear 18b of the cantilever can be adjusted to minimize variations in the height detection signal that result in a change in the height of the cantilever base. The height of the base can be adjusted by various mechanisms, but one of the advantageous techniques is to change the feedback setting point. Thereby, the vertical separation between the cantilever base and the sample changes until a new set point value is achieved. This method has the advantage of allowing control of the minimum and maximum forces applied between the sample and the tip.

A further alternative is to shift the tip of the cantilever by a known vertical distance by scanning a sample with known height variation, such as a standard step sample. The position of the detection point on the rear 18b of the cantilever is then adjusted until the height detection signal shows the proper displacement or height variation.

In general, the position of the rear 18b of the cantilever is empirically preferably located just above the tip point 20a rather than geometrically from the probe itself. The reason is that with current probe manufacturing methods, there is always some uncertainty in the position of the tip. Of course, once the tip position for a particular probe has been placed by empirical techniques such as those described above, it avoids the need to record that position and then repeat empirical decisions when using the same probe. be able to.

This feature of the present invention is a resonant scanning type as described in WO 2004/005844, in which at least one element of raster scanning is rapidly performed by resonantly vibrating the probe (or equivalent sample and table). Especially suitable for use in microscopes. Such scanning speeds are very likely to cause transient motion of the cantilever, which obscures the images collected by prior art detection systems.