Lithographic apparatus and device manufacturing method.

BACKGROUND

Field of the Invention

The present invention relates to a level sensor, a lithographic apparatus and a method for determining a height map of a substrate for use in a lithographic process.

Description of the Related Art

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

The surface of a substrate on which a pattern should be projected is usually not completely flat. Moreover substrates can show thickness variations of several microns. This flatness and thickness variations of the substrate surface may result in incorrect projection of the pattern, for instance due to focus errors or imaging errors.

To correct for unflatness and thickness variations of a wafer, it has been proposed to provide a level sensor, preferably integrated in the lithographic apparatus. Such level sensor may be used to determine a height map of a substrate, before a pattern is projected on the substrate. This height map may subsequently be used to correct the position of the substrate during projection of the pattern on the substrate.

In a known embodiment a multiple spot level sensor using optical triangulation is provided to determine a height map of the substrate. The substrate is moved with respect to the level sensor to determine a height level at different measurement location across the surface of the substrate to obtain a height map of the substrate.

A drawback of known level sensors is that the measurement range of the level sensor wherein reliable measurement results can be obtained is relatively small, typically in a height range of about 1 pm. This measurement range is smaller than the normal variations in flatness and height of a substrate. Therefore, closed-loop height control is required during measurement of the height level at different measurement locations on the substrate to maintain the respective measurement locations within the reliable measurement range of the level sensor. In this way the small height range in which the level sensor provides accurate results, is optimally used. However, the requirement of closed-loop height control limits the measurement speed of the level sensor, since the substrate has to be continuously repositioned to maintain the subsequent measurement locations within the reliable measurement range of the level sensor.

Moreover, the requirement of closed-loop height control makes simultaneous height measurement at multiple measurement location practically difficult, if not impossible, since the multiple measurement locations cannot be arranged simultaneously within the same limited measurement range of the level sensor.

It is possible to use linearization techniques to increase the measurement range of a level sensor. Also, other types of level sensors and level sensing techniques having a larger measurement range may be proposed to obviate the need for closed-loop height control of the substrate. However, increasing the measurement range may introduce another drawback in height measurement of the substrate. For measurement of a height level of a substrate, typically a measurement beam is used which is reflected on the substrate. When no closed-loop height control is used, the focus height of the level sensor is adjusted to the expected or mean substrate height, i.e. the height of a perfect flat and even substrate. Due to the variations in thickness and flatness of the substrate, this expected or mean height level does not correspond at all measurement locations with the actual substrate height.

In case the height difference between the expected or mean substrate height and the actual substrate height increases, the focus of the measurement beam with respect to the substrate surface may become increasingly important for a correct measurement.

SUMMARY

It is desirable to provide an alternative level sensor for use in a lithographic apparatus which level sensor is preferably less sensitive for effects due to focus height of the measurement beam of the level sensor.

A level sensor having a measurement beam which is out of focus at the measurement location with respect to the local substrate height may become more sensitive for tilt of the substrate. Therefore, it is further desirable to provide a level sensor being less sensitive for tilt of the substrate.

According to an embodiment of the invention, there is provided a level sensor configured to measure a height level of a substrate comprising:

- a projection unit to project a measurement beam on the substrate,

- a detection unit to receive the measurement beam after reflection on the substrate,

- a processing unit to calculate a height level on the basis of the reflected measurement beam received by the detection unit, wherein the level sensor further comprises a tilt measuring device, wherein the tilt measuring device is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate with respect to a nominal plane.

According to an embodiment of the invention, there is provided a lithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the lithographic apparatus comprises a level sensor configured to measure a height level of a substrate on the substrate table, comprising:

- a projection unit to project a measurement beam on the substrate,

- a detection unit to receive the measurement beam after reflection on the substrate,

- a processing unit to calculate a height level on the basis of the reflected measurement beam received by the detection unit, wherein the level sensor further comprises a tilt measuring device, wherein the tilt measuring device is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate with respect to a nominal plane.

According to an embodiment of the invention, there is provided a method for determining a height map of a substrate for use in a lithographic process comprising the steps of:

- providing a level sensor configured to measure a height level of a substrate comprising:

- a projection unit to project a measurement beam on the substrate,

- a detection unit to receive the measurement beam after reflection on the substrate,

- a processing unit to calculate a height level on the basis of the reflected measurement beam received by the detection unit, wherein the level sensor further comprises a tilt measuring device, wherein the tilt measuring device is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate with respect to a nominal plane

- providing a substrate,

- measuring the height of the substrate at a measurement location by projecting a measurement beam on the substrate by the projection unit, receiving the reflected beam by said detection unit, and determining a height level in the processing unit,

- repeating the measuring step for multiple measurement locations on the substrate, and

- determining a height map of the substrate on the basis of the height levels, wherein the method further comprises determining a tilt of the substrate by the tilt measuring device, and wherein the step of determining a height level comprises correcting the height level on the basis of the determined tilt of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

Figure 2 depicts a side view of an embodiment of a level sensor according to the invention;

Figure 3 depicts a schematic view of a projection grating (left side) and resulting image (right side);

Figure 4a depicts a side view of an embodiment of a detection grating;

Figure 4b depicts a detail of the detection grating of Figure 4a;

Figure 5 depicts a top view of an embodiment of a level sensor having multiple measurement locations;

Figure 6 depicts a side view of an embodiment of a level sensor comprising a tilt measuring device;

Figure 7 depicts a top view of a first embodiment of a beam delivery system for a multiple measurement location level sensor; and

Figure 8, 9 and 10 depict a top view, a cross section and a side view of a second embodiment of a beam delivery system for a multiple measurement location level sensor.

DETAILED DESCRIPTION

Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a mask support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The apparatus also includes a substrate table (e.g. a wafer table) WT or substrate support constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters. The apparatus further includes a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The mask support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The mask support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The mask support structure may be a frame or a table, for example, which may be fixed or movable as required. The mask support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or substrate supports (and/or two or more mask tables or mask supports). In such “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system and the substrate during exposure.

Referring to figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or substrate support may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the mask table MT or mask support and the substrate table WT or substrate support are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT or substrate support is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT or mask support and the substrate table WT or substrate support are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT or substrate support relative to the mask table MT or mask support may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the mask table MT or mask support is kept essentially stationary holding a programmable patterning device, and the substrate table WT or substrate support is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or substrate support or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

In figure 1 the possible location of a level sensor 1 according to the invention in a lithographic apparatus is shown. The substrate support WT and substrate W supported thereon are shown in dashed lines in a measurement location of the substrate. In this measurement location a height level of the substrate W may be determined.

Level sensor

Figure 2 shows a level sensor generally indicated by reference numeral 1. The level sensor 1 is configured to determine a height map of a substrate 2. This height map may be used to correct the position of a substrate during projection of a pattern on the substrate 2. The level sensor may be arranged in a stand-alone device, but is preferably integrated in a lithographic apparatus as shown in Figure 1.

The level sensor 1 comprises a projection unit 3, a detection unit 4, and a processing unit 5. The projection unit 3 comprises a light source 6 and a projection grating 7. The light source 6 may be any suitable light source. It is preferably a broadband light source but a polarized or non-polarized laser beam can also be used. The light source 6 provides a measurement beam which is directed to the projection grating 7.

The projection grating 7 comprises a periodical grating, i.e. a pattern having a periodical structure resulting in a measurement beam having a periodical structure in light intensity. The left side of Figure 3 shows an example of such projection grating having a periodical structure. The measurement beam with periodical light intensity is directed towards a measurement location 8 via an optical reflector 9. The substrate is located at this measurement location 8. Further optical elements may be provided to guide the measurement beam towards the substrate 2. At the measurement location 8 the measurement beam is reflected on the substrate 2 and runs via a second optical reflector 10, and possibly further optical elements to the detection unit 4. The detection unit 4 comprises a detection grating 11 and three detectors 12a, 12b, 12c.

The detection grating 11 comprises a periodical structure as shown in Figure 4. This periodical structure comprises for each period an array of three segments 13a, 13b, 13c. The top surface of each segment within the array of segments has a different angle with respect to the angle of incidence of the measurement beam 14. As a result, the measurement beam is split by the three segments in three measurement beam parts 14a, 14b, 14c, each directed to one of the three detectors 12a, 12b, 12c, for instance photodiodes or other elements capable of measuring light intensities.

Since the structure of the segments of the arrays of segments is periodic, each respective segment 13a, 13b, 13c per array of segments directs the part of the measurement beam received by that segment towards the associated detector 14a, 14b, and 14c. Thus all first segments 13a direct light of the measurement beam towards the first detector 14a, second segments 13b of the periodic structure towards the second detector 14b, and third segments 13c of the periodic structure towards the third detector 14c.

The measured light intensities are received by a processing unit 5, wherein on the basis of the light intensities received by the different detectors, a height level of the substrate 2 can be deducted as now will be explained for an exemplary embodiment.

Again referring to Figure 3, the periodical structure shown at the left side of Figure 3 is configured from rhombic shapes having a length L of about 30 pm and a width W of about 4 pm. Due to the small NA of the imaging optics used in the level sensor, the periodicity in the width direction of the projection grating is not resolved, while the periodicity in the length direction L of the projection grating 7 is resolved. It is remarked that in an alternative embodiment the periodicity may also be resolved in the width direction.

At the right side of Figure 3, the resulting projection image of this periodical structure on the substrate 2 is shown. The image of Figure 3 shows that the projection image has a periodicity in the length direction of the projection grating 7. This image is reflected on the top surface of the substrate 2 towards the detection unit 4. The intensity distribution received by the detection grating 11 of the detection unit 4 can be approximated by a sinusoidal intensity distribution, wherein the intensity is dependent on the length direction variable x and the shift s of the image caused by the wafer height.

= A + Bcos(y + 5)

In this equation the pitch of the sinusoidal variation is chosen equal to 2π. The shift of the image s is determined by the wafer height. This shift s has to be determined to calculate the height of the substrate at the respective measurement location 8, while taking into account the parameters A and B which are also unknown and variable.

Figure 4 shows for illustration an aerial image Al of the sinusoidal intensity distribution above the three segments 13a, 13b, and 13c. Each of the segments 13a, 13b, 13c receives another part of the intensity distribution. Due to the different angles of the segments 13a, 13b, 13c each part of the intensity distribution is guided to the respective one of the detectors 12a, 12b, 12c. The light intensities received by the detectors 12a, 12b, 12c are guided to the processing unit 5 to determine the height level of the substrate at the measurement location 8.

The light intensities Dl, D2 and D3, received by each of the detectors 12a, 12b, 12c can be described by the following relations:

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From these three equations with 3 unknown variables two quadrature signals can be derived.

/ \ 2ZX — I), — /9, ^BcosM= svi

<img file="NL2006131A_D0002.tif" />

These two quadrature signals allow to find s for any value of s so there are no linearity errors and no dead-zones with zero sensitivity for substrate height changes. As a result, the level sensor 1 is suitable to determine the height of the substrate in a relative large height range of more than +/- 5 pm, or even +/- 10 qm. Therefore, the need of closed-loop height control during measuring of the height level is obviated.

In an alternative embodiment, the detection grating may comprise four or more segments for each period of the measurement beam. In the embodiment shown in Figure 4, the segments 13a, 13b, 13c each have the same length Is. In an alternative embodiment, the segments may have different lengths, as long as the length of the complete array of segments corresponds to a period of the image of the measurement beam projected on the detection grating 11.

The angles of the top surfaces of the segments are about -15°, 0° and 15° with respect to the main plane of the detection grating 11. Any other suitable angle may also be applied for each of the segments. The difference between the angles should be large enough to split the measurement beam in three distinguishable beam parts that can be directed to the three distinct detectors 12a, 12b, 12c, so that intensity differences between the three beam parts can be determined.

Since the measurement of a height level of the substrate 2 can be measured open-loop, and in a relatively large height range, the level sensor 1 is suitable for measuring simultaneously the height level at multiple measurement locations 8. To locate the measurement location 8 of the level sensor 1 at different positions on the substrate 2 different methods may be applied.

In one embodiment, the substrate 2 may be moved along the level sensor 1 in a scanning movement. Since no closed loop height control is required, this movement can be made at a constant speed therewith avoiding the need of accelerations of the substrate support supporting the substrate 2 during determining a height level of a substrate. In an alternative embodiment, the level sensor 1 may be moved over the substrate 2 while the substrate is stationary. In another embodiment both the level sensor 1 and the substrate may be moved to obtain an optimum path to move the measurement locations over the surface of the substrate.

In an advantageous embodiment, the level sensor may be provided with movable light guiding means configured to direct the measurement beam at different locations on the substrate without moving the complete level sensor and/or substrate. With such level sensor at least the height of a part of a substrate, for instance a line of measurement locations, may be measured without movement of the complete level sensor and/or substrate being required. In such embodiment less movements or less complex movements of the substrate with respect to the level sensor are necessary to obtain a height map of a complete substrate.

Figure 5 shows another embodiment of a level sensor 101. In the level sensor 101 of Figure 5, the same or similar features of the embodiment of Figure 2 are indicated with the same reference numerals.

The level sensor 101 is configured to measure a height of a substrate at different measurement locations 108 spaced on a line crossing the width of the substrate 2. For each measurement location 108, the level sensor 101 comprises the components of the level sensor shown in Figure 2. Thus each measurement location 108 is associated with a projection unit comprising a light source 6 and a projection grating 7, and a detection unit comprising a detection grating 11 and a number of detectors (not shown). Furthermore, optical elements 9 and 10 are provided to guide the measurement beam from the projection unit to the measurement location and, after reflection on a substrate 2, from the measurement location to the detection unit.

In the shown embodiment, for each measurement location 108 one light source 6, one detection grating 11 and a set of three or more detectors is provided. There is one projection grating 7 and one set of optical elements 9 and 10 for all the measurement locations. The projection grating 7 and the optical elements extend for this reason along the measurement locations 108.

In alternative embodiments, one detection grating 11 may be provided to be used for all measurement locations 8. Also one or more parts, for instance projection grating, detection grating etc. may be provided for multiple, but not all measurement locations 8.

The level sensor shown in Figure 5 is configured to measure the height of a substrate at twelve measurement locations 8. The full width of the substrate can be covered by this line of measurement locations 8. When desired more or less measurement locations 8 may be provided. The measurement locations 8 may also be arranged in other configurations than along one line as shown in the embodiment of Figure 5.

The construction of the level sensor 1 as shown in Figure 2 is in particular suitable to be used in a multiple measurement location level sensor since the level sensor is capable of measuring the height of the substrate 2 surface in a relative large height range. Typically, this larger height range for measurement is larger than normal variations in thickness and flatness of a substrate. This larger measurement range obviates the requirement of closed-loop height control as present in prior art level sensors. As a result, the level sensor of Figure 5 can be used to measure the substrate height simultaneously at multiple measurement locations 8 without the requirement of correcting the position of the substrate due to height differences between the heights of the substrate at the different measurement locations.

The multiple measurement locations 8 may be moved along the substrate to obtain height information of a large number of locations on the substrate 2. This movement may be obtained by movement of the level sensor 101, movement of the substrate 2 and/or movement of light guiding elements of the level sensor 101, for instance movement of the optical elements 9 and 10. Also a combination of these movements may be used to obtain a height map of the substrate 2 to be used for correctional movements of the substrate during the actual lithographic process.

Tilt measuring device

Figure 2 shows a level sensor 1 capable of measuring the height of a substrate with relative high accuracy. The level sensor may provide a reliable measurement within a height range of +/5pm or even +/- 10 pm or possibly even larger. However, when the image of the projection grating becomes defocused relative to the detection grating, the level sensor may become sensitive for tilt of the substrate about the x-axis. Although this effect is small, the tilt of the substrate 2 may be measured and taken into account when the height map of a substrate is determined. The correction of the determined height by taking the tilt of the substrate into account may improve the accuracy of the height map even further. An example of a level sensor capable of measuring the tilt of the substrate, will now be discussed.

Figure 6 shows the level sensor of Figure 2, further comprising a tilt measuring device 20. The tilt measuring device 20 is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate about the x-axis (Rx) with respect to a nominal plane 21. The tilt measuring device 20 is connected to the processing unit 5 to feed the tilt signal to the processing unit 5 in order to correct, when needed, the height level measured by the level sensor 1.

In the level sensor 1, the measurement beam reflected on the substrate 2 is received by the optical clement 10 which is a partial mirror which reflects the major part of the measurement beam, for example 80% of the light intensity towards the detection unit 4.

The rest of the measurement beam is transmitted through the partial mirror and used to detect the tilt of the substrate in the tilt measuring device 20. This rest of the measurement beam is indicated in Figure 6 as tilt measurement beam 22.

The tilt measurement beam 22 is received by a beam splitting device 23. The beam splitting device 23 is configured to divide the tilt measurement beam in two parts each part directed to one of two tilt detectors 24, 25. The beam splitting device 23 comprises two wedges placed against each other. The part of the tilt measurement beam received by the lower wedge is guided to the tilt detector 24, and the part of the tilt measurement beam received by the upper wedge is guided to the tilt detector 25. The two wedges of the beam splitting device 23 are spaced in a tilt sensitive direction.

Comparison of a ratio between relative amounts of light received by the two tilt detectors 24, 25 is representative for the tilt of the substrate. For instance, when the substrate is tilted as indicated by the arrow T in Figure 6, the part of the tilt measurement beam 22 received by the upper wedge will increase, while the part received by the lower wedge will decrease. The differences between the amounts of light received by the two tilt detectors 24, 25 can be normalized to determine small intensity differences between the light received by the tilt detectors 24, 25. This comparison of the amount of lights received by the tilt detectors 24, 25 results in detection of small tilt variations of the substrate 2.

The tilt measuring device of Figure 6 is only an example of a tilt sensor that can be used to measure the tilt of the substrate to correct the height determined by the tilt measurement device 20. Any other tilt measuring device 20 capable of measuring the tilt of a substrate may also be applied. Generally it will be desirable to measure the tilt about an axis perpendicular to the plane in which the measurement beam of the level sensor runs. In the shown embodiment, the measurement beam runs in the y-z plane; thus the tilt to be measured is tilt of the substrate about the x-axis (Rx).

The tilt measuring device 20 as shown in Figure 6 can be constructed very compact, and is therefore suitable to be applied in a level sensor system for multiple measurement locations as shown in Figure 5. In such level sensor for each measurement location of multiple measurement locations, a tilt measuring device 20 may be provided to measure for each measurement location 8 a tilt variable of the substrate so that for each measurement location 8 the height determined by the level sensor 101 may be corrected for a tilt of the substrate 2.

Measurement beam delivery system

In Figure 5, a top view of a level sensor 101 is shown configured to measure a height level of a substrate 2 at multiple measurement locations 8. This level sensor 101 provides the possibility to measure the height of the substrate 2 simultaneously at the different measurement locations 8. This level sensor further provides the possibility for open loop scanning of the measurement locations along the surface of the substrate 2. When desired, the tilt of the substrate 2 may be measured by one or more tilt measuring devices to correct the height measured by the level sensor 1 to correct the measured height for any effect due to a tilt of the substrate 2.

Although this level sensor provides possibilities for fast and accurate determination of a height map of a substrate 2, the level sensor as shown in Figure 5 requires substantial space above the substrate 2. Such space may not always be available above the substrate in a lithographic apparatus.

To use the space available in a lithographic apparatus more efficiently, figure 7 shows an alternative beam delivery method to provide measurement beams at multiple measurement locations at the substrate which multiple measurement locations 8 are arranged on a line. The substrate 2 is arranged in a measurement position, i.e. at least one of the measurement locations 8 of the level sensor is located somewhere on the substrate 2 so that height measurement of the substrate 2 can be carried out.

For each measurement location 8, a light source 6, a projection grating 7, a detection grating and three detectors (not shown) are provided. These components of the level sensor as such correspond to the components used in the level sensor shown in Figures 2 and 5. However, the light source 6 and the projection grating 7 are provided at one side next to and the substrate 2, while the detection grating 11 and the detectors are provided at the other side next to the substrate

2. The different components may be located at different heights with respect to the substrate 2, but in the shown embodiment all components are arranged at a height level higher than the substrate 2.

The advantage of this arrangement is that no components are arranged above the substrate 2, and all components at one side of the substrate 2 are arranged relative close to each other resulting in a relative small space required for all components in a lithographic apparatus.

It is remarked that the term “next to” is used to indicate a location that when projected perpendicular on the main plane of the substrate lies outside the substrate. The terms “above” or “below” indicate a location that when projected perpendicular on the main plane of the substrate lies within the substrate.

Figures 8, 9 and 10 show a further embodiment in which the space required for the measurement beams of the level sensor can be further reduced. In the embodiment of Figure 8, 9 and 10, reflection bars 30, 31 are provided above the substrate 2. The reflection bars 30, 31 are each provided with an oblique reflective surface 30a, 31a (see Figure 9).

The angles of the oblique surfaces 30a, 31a are preferably equal with respect to the horizontal plane and are directed towards the surface of the substrate. The angles are chosen such that a measurement beam received by the reflection bar 30 in a substantial horizontal plane is reflected towards a measurement location 8 on the substrate 2. At the measurement location 8 the measurement beam is reflected back towards the reflection bar 31. The reflected beam is received on the reflection bar 31 at substantially the same height as where it was reflected on the reflection bar 30. From the reflection bar 31 the measurement beam is guided in a substantially horizontal direction to the detection unit 4.

In the detection unit 4 further optical elements may be provided to detect signals representative for the height of the substrate 2. These signals may be used to determine the height of the substrate 2 at the measurement location in a processing unit which may be arranged in the detection unit 4 or at any other suitable location. The components of the level sensor may correspond to the components of the above described embodiments, but any other suitable level sensor device may also be used.

The sensor components are depicted by a projection unit 3 and a detection unit 4. The projection unit 4 is configured to provide multiple measurement beams to measure a height level at different measurement locations 8 on the substrate 2. All measurement beams are provided on substantially the same height and directed to the reflection bar 30. All reflected measurement beams are received at substantially the same height by the reflection beam 31, and directed towards the detection unit 4.

The advantage of the measurement beam delivery device of the embodiment shown in Figures 8, 9, and 10 is that the measurement beam can be kept at a relative low height level with respect to the substrate 2, while at the same time the projection unit 3 and the detection unit 4 are arranged next to the substrate where more space may be available for the provision of these units 3, 4.

In the embodiment shown in Figures 8, 9, and 10 only three measurement locations 8 are shown. In practice, many more measurement locations 8, for instance over 50 measurement locations, may be located at a line or in any other suitable configuration. Furthermore, the substrate 2 may be moved to move the measurement locations 8 over the substrate surface to determine the height level of the substrate over the complete surface of the substrate to determine a height map of the substrate. The movement may be realized by movements of the level sensor and/or movements of the substrate or by any other suitable method.

It is remarked that the projection unit 3 and/or detection unit 4 of the embodiments of Figures 7, 8, 9, and 10 may comprise compensation devices to compensate, when necessary, for any difference in measurement beam path length between the different measurement beams.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below:

1. A level sensor configured to measure a height level of a substrate comprising:

- a projection unit to project a measurement beam on the substrate,

- a detection unit to receive the measurement beam after reflection on the substrate,

- a processing unit to calculate a height level on the basis of the reflected measurement beam received by the detection unit, wherein the level sensor further comprises a tilt measuring device, wherein the tilt measuring device is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate with respect to a nominal plane.

2. The level sensor of clause 1, wherein the tilt measuring device is connected to the processing unit to feed the tilt signal to the processing unit in order to correct the height level measured by the measuring device on the basis of the tilt of the substrate.

3. The level sensor of clause 1, wherein the tilt measuring device comprises at least two lilt detectors to receive the reflected measurement beam, wherein the at least two tilt detectors are spaced in a tilt sensitive direction, and wherein a comparison of a ratio between relative amounts of light received by the two tilt detectors is representative for the tilt of the substrate.

4. The level sensor of clause 3, wherein the processing unit is configured to make the comparison.

5. The level sensor of clause 1, wherein the tilt measuring device comprises a beam splitting device to split the measurement beam in two parts each directed to one of two tilt detectors, wherein a comparison of a ratio between relative amounts of light received by the two tilt detectors is representative for the tilt of the substrate.

6. The level sensor of clause 5, wherein the processing unit is configured to make the comparison.

7. The level sensor of clause 1, wherein the level sensor comprises a partially reflective mirror, wherein the partially reflective mirror is arranged to receive the reflected measurement beam and to split the reflected measurement beam in a first part to be received by the detection unit and a second part to be received by the tilt measuring device.

8. The level sensor of clause 7, wherein the partially reflective mirror is more reflective than transmissive.

9. The level sensor of clause 1, wherein the projection unit comprises a light source to provide a measurement beam, and a projection grating arranged to receive the measurement beam and to impart the measurement beam with a periodic light intensity, and wherein the detection unit comprises a detection grating arranged to receive the reflected measurement beam, and at least one detector arranged to receive the measurement beam.

10. The level sensor of clause 1, wherein the tilt measuring device is configured to determine a tilt of the substrate about an axis perpendicular to the plane in which the measurement beam runs.

11. The level sensor of clause 1, wherein the measurement beam runs in an y-z plane and the tilt measurement device is configured to determine a tilt of the substrate about the x-axis (Rx).

12. A lithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the lithographic apparatus comprises a level sensor configured to measure a height level of a substrate on the substrate table, comprising:

- a projection unit to project a measurement beam on the substrate,

- a detection unit to receive the measurement beam after reflection on the substrate,

- a processing unit to calculate a height level on the basis of the reflected measurement beam received by the detection unit, wherein the level sensor further comprises a tilt measuring device, wherein the tilt measuring device is arranged to receive at least partially the reflected measurement beam, and configured to provide a tilt signal representative for a tilt of the substrate with respect to a nominal plane.

13. The lithographic apparatus of clause 12, wherein the tilt measuring device is connected to the processing unit to feed the tilt signal to the processing unit in order to correct the height level measured by the measuring device on the basis of the tilt of the substrate.

14. A method to determine a height map of a substrate comprising the steps of:

- providing a level sensor as mentioned in clause 1,

- providing a substrate,

- measuring the height of the substrate at a measurement location by projecting a measurement beam on the substrate by the projection unit, receiving the reflected beam by said detection unit, and determining a height level in the processing unit,

- repeating the measuring step for multiple measurement locations on the substrate, and

- determining a height map of the substrate on the basis of the height levels, wherein the method further comprises determining a tilt of the substrate by the tilt measuring device, and wherein the step of determining a height level comprises correcting the height level on the basis of the determined tilt of the substrate.