Substrate height measurement

Abstract

A substrate height measuring apparatus comprising a cantilever extending from a body and having a generally spherical tip provided at a distal end, wherein the generally spherical tip and the cantilever have a conductive coating; a modulation system configured to apply a periodic position modulation to a substrate table; a voltage source configured to apply a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; an interferometric detection system configured to measure movement of the cantilever whilst the position modulation and the voltage modulation are being applied; a lock-in detection system configured to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and a processor configured to provide a substrate height measurement based upon the output signals.

Description

SUBSTRATE HEIGHT MEASUREMENTCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 23186104.8 which was filed on 18 July, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a substrate height measuring apparatus, and to a method of measuring the height of a substrate.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask also referred to as a reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A lithographic substrate is not perfectly flat. Instead, different points across a surface of the substrate may be at different heights, and so lithographic radiation focused on the surface of the substrate at a first point (at a first height) may not be in focus at a second point (at a second height). To maintain focus on the substrate surface, the height of the surface of the substrate is measured and corresponding corrections are made.

[0006] The height of the surface of the substrate may be measured using a height measuring optical sensor. However, it is known that light (used by height measuring optical sensors to measure the height of the surface of the substrate) may penetrate partially into the substrate surface instead of being reflected solely by the substrate surface, resulting in a measurement error.

[0007] It may be desirable to provide a substrate height measurement apparatus that overcome a disadvantage associated with the prior art.SUMMARY

[0008] According to a first aspect of the invention, there is provided a substrate height measuring apparatus comprising a cantilever extending from a body and having a generally spherical tip provided at a distal end, wherein the generally spherical tip and the cantilever have a conductive coating; a modulation system configured to apply a periodic position modulation to a substrate table; a voltage source configured to apply a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; an interferometric detection system configured to measure movement of the cantilever whilst the position modulation and the voltage modulation are being applied; a lock-in detection system configured to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and a processor configured to provide a substrate height measurement based upon the output signals.

[0009] Advantageously, the substrate height measuring apparatus provides height measurements that do not suffer from measurement error arising from partial penetration of the substrate by light.

[0010] The interferometric detection system may comprise a laser, optical fiber, an optical interferometer and an optical detector.

[0011] The optical fiber may have an end which is above the distal end of the cantilever.

[0012] The optical fiber may be received in a groove provided in the body from which the cantilever extends.

[0013] The body from which the cantilever extends may have a conductive coating.

[0014] The cantilever may be one of a plurality of cantilevers which extend from the body.

[0015] An interferometric detection system and a lock-in detection system may be provided for each cantilever.

[0016] A multiplexing arrangement using the same interferometric detection system and lock-in detection system may be provided for multiple cantilevers.

[0017] The voltage source may be provided within the lock-in detection system.

[0018] According to a second aspect of the invention, there is provided a lithographic apparatus comprising an illumination system and a projection system, wherein the lithographic apparatus further comprises the substrate height measuring apparatus according to the first aspect.

[0019] According to a third aspect of the invention, there is provided a method of measuring a substrate height using a measuring apparatus comprising a cantilever extending from a body and having a generally spherical tip provided at a distal end, the generally spherical tip and the cantilever having a conductive coating; the method comprising applying a periodic position modulation to a substrate table supporting a substrate; applying a DC voltage and a periodic voltage modulation to the cantilever andgenerally spherical tip; measuring movement of the cantilever whilst the position modulation and the voltage modulation are being applied; using a lock-in detection system to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and using the output signals to determine a substrate height measurement.

[0020] The method may further comprise adjusting the DC voltage to minimise the output signal indicative of electrostatic force acting upon the cantilever.

[0021] The method may further comprise performing step changes of the distance between the generally spherical tip and the substrate W, and obtaining the output signals for each distance.

[0022] A plurality of cantilevers with generally spherical tips may be provided. Height measurements may be obtained from the plurality of cantilevers with generally spherical tips.

[0023] The periodic position modulation and the periodic voltage modulation may both have a frequency of less than 300Hz.

[0024] Features of different aspects of the invention may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus which includes substrate height measurement apparatus according to an embodiment of the disclosure;Figure 2 depicts part of the substrate height measurement apparatus of Figure 1; and Figure 3 depicts the substrate height measurement apparatus.DETAILED DESCRIPTION

[0026] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0027] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0028] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and 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., comprising one or more dies) of the substrate W.

[0029] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0030] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / 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” PS.

[0031] The lithographic apparatus LA may 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 PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0032] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such a “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation for a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W. For example, preparation for a subsequent exposure of a substrate may comprise measuring height across an upper surface of the substrate. In a dual stage lithographic apparatus, a region within the lithographic apparatus where this measurement is performed may be referred to as a measurement location of the lithographic apparatus.

[0033] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. 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 aidof the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0034] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0035] The lithographic apparatus LA includes a height measurement apparatus S according to an embodiment of the disclosure. The height measurement apparatus S measures height of an upper surface of the substrate W, thereby allowing a height map to be generated for the substrate. The height map is a measurement of the height of the substrate as a function of position on the substrate. The height measurement apparatus may for example be located in a measurement location of a dual stage lithographic apparatus.

[0036] The height map generated for the substrate may also be referred to as a map of the topography of the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focused position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the height measurement apparatus performs measurements at a fixed location, with the substrate being moved beneath the height measurement apparatus.

[0037] Part of the height measurement apparatus S is schematically depicted in cross-section in Figure 2. The height measurement apparatus comprises a cantilever 2 which extends from a body 4. A generally spherical protrusion 6 extends from beneath a distal end of the cantilever 2. The generally spherical protrusion may be referred to as a generally spherical tip 6. This is not intended to imply that the protrusion is a perfect sphere. The term “generally spherical tip” includes any generally sphericalshape, and includes a shape which has a generally spherical lower portion but a flattened upper portion (e.g. as depicted in Figure 2).

[0038] An optical fiber 8 extends through the body 4 such that an end 10 of the fiber is located over the distal end of the cantilever 2. The end 10 of the fiber 8 may be a cleaved end. An opposite end (not depicted) of the optical fiber 8 is connected to an interferometric detection system (described further below in connection with Figure 3). The cantilever 2, body 4 and generally spherical tip 6 are coated with gold (or a different conductive material). In other embodiments, the body 4 is not coated with a conductive material, but an electrical connection via a conductor is provided to the cantilever 2 and generally spherical tip 6.

[0039] The cantilever 2 may for example have a length of at least 2mm, e.g. up to 3mm. The cantilever 2 may for example have a rectangular or square cross section. The cantilever 2 may for example have a width of around 200 microns, and may have a height of around 200 microns. The cantilever 2 may for example have a resonant frequency of at least 200 Hz, e.g. up to 300 Hz.

[0040] The generally spherical tip 6 may for example have a diameter of 50 microns or more. The generally spherical tip 6 may for example have a diameter of up to 200 microns. The generally spherical tip 6 may have a larger surface area than for example a pointed tip of a conventional atomic force microscope (AFM). Advantageously, the larger surface area of the generally spherical tip 6 provides measurements that average out an area beneath the tip (compared with a measurement provided by a pointed tip). Such averaged area measurements are useful for lithography because they can provide a height map which is more suited to projection image adjustments that can be performed by a lithographic apparatus. The averaged area measurements also allow a full height map to be generated more quickly (compared with using a pointed tip).

[0041] The generally spherical tip 6 does not come into contact with the substrate W when height measurements are being performed. Instead there is a separation distance d between the generally spherical tip 6 and the substrate W. The separation distance d may be at least 10 microns. The separation distance may for example be up to 100 microns. Advantageously, this relatively large separation minimizes a risk of accidental collision of the generally spherical tip 6 on the substrate W (which would damage the substrate and possibly also the generally spherical tip). In addition, the relatively large separation is such that Casimir forces (which scale as an inverse cube with respect to distance) are sufficiently small that they do not significantly affect the accuracy of height measurements obtained using embodiments of the invention.

[0042] The height measurement apparatus S is located above a lithographic substrate W. In use, the substrate W is moved in a horizontal direction with respect to the height measurement apparatus S. This is achieved by moving the substrate table WT (see Figure 1). Although depicted as having a smooth upper surface, in practice the substrate W will have an upper surface which varies in height. Embodiments of the invention measure the height of the upper surface of the substrate W.

[0043] A modulation is applied to the z-direction position of the substrate table WT. The modulation may for example have an amplitude of hundreds of nanometers (e.g. at least lOOnm, e.g. up to 500nm). The modulation may have a frequency below the resonant frequency of the cantilever, e.g. below 300Hz. Hydrodynamic force arising from the modulation of the substrate table WT acts on the generally spherical tip 6 and causes movement of the cantilever 2. Motion of the substrate table WT and substrate W causes air movements which exert force on the generally spherical tip and thereby cause bending of the cantilever. This hydrodynamic force induced bending has a fixed relationship with respect to the distance d between the generally spherical tip 6 and the substrate W. Therefore, by measuring the bending of the cantilever 2 during modulation of the substrate table WT, the hydrodynamic force can be measured and hence the distance between the generally spherical tip 6 and the substrate W can be determined. This allows a height map to be generated for the substrate W. The movement of the cantilever is measured using laser light emitted from the end 10 of the optical fiber 8, reflected from the cantilever 2 and coupled back into the optical fiber 8. The measurement is performed by an interferometric detection system as explained below.

[0044] Figure 3 schematically depicts the height measurement apparatus S, including the part depicted in Figure 2. In the embodiment depicted in Figure 3, six cantilevers 2 extend from the body 4 (only four cantilevers are visible). A generally spherical tip 6 extends from a bottom distal surface of each cantilever 2. Six optical fibers 8 extend through the body 4. Each optical fiber has an end which is located above a distal end of a respective cantilever 2. Other embodiments may have a different number of cantilevers, and generally spherical tips. In general, a single cantilever, generally spherical tip and optical fiber may be provided, or alternatively multiple cantilevers, associated generally spherical tips and associated optical fibers may be provided. Having multiple cantilevers, etc. allows multiple height measurements to be performed in parallel.

[0045] A laser 12 is used to emit a laser beam into an input optical fiber 14. The input optical fiber 14 connects via a fiber coupler 16 to one of the optical fibers 8 that provides laser light through the body 4. An output optical fiber 18 is connected to the fiber coupler 16. The output optical fiber 18 carries light reflected from a cantilever 2 to an optical detector 20 (e.g. a photodiode). The output optical fiber 18 may be an opposite end of the same optical fiber 8 that provides laser light to the cantilever 2 (this may depend upon a configuration of the fiber coupler 16). Other optical fiber configurations may be used, for example using a fiber beam splitter or a fiber circulator. In general, a configuration which provides an interferometric distance measurement for the cantilever 2 is desired (e.g. using interference between the input laser light and reflected laser light).

[0046] A lock-in amplifier 22 receives an output from the optical detector 20. The lock-in amplifier 22 includes a signal generator which generates a sinusoidal modulation frequency. The modulation frequency may for example be below 300 Hz. The modulation frequency is output from the lock-in amplifier 22 to an actuator 24 which modulates the z-direction position of the substrate table WT. The actuator 24 may include a sensing system that may for example comprise an opticalinterferometer. As noted above, a modulation is applied to the z-direction position of the substrate table WT. In an alternative embodiment, the z-direction modulation may be applied by a separate substrate table position controller (not depicted) which is driven by a signal generator that does not form part of the lock-in amplifier. The lock-in amplifier may receive as an input the modulation that is applied.

[0047] The laser 12, optical fibers 8, 14, 18, fiber coupler 16 optical detector 20 and lock-in amplifier 22 are an example of an interferometric detection system. The fiber coupler 16 acts as an optical interferometer. Other optical interferometers may be used. Other interferometric detection systems may be used.

[0048] A separate interferometric detection system (not depicted) may be provided for each of the optical fibers 8 which pass through the body 4. In such a configuration, if multiple lock-in amplifiers are used, some of the lock-in amplifier may receive as an input the sinusoidal modulation frequency provided from the signal generator of one of the lock-in amplifiers (which may be considered to be a master lock-in amplifier).

[0049] Alternatively, a single interferometric detection system may be used, with multiplexing being used to pass light through the optical fibers in sequence, and the lock-in amplifier 22 providing output values accordingly.

[0050] An embodiment of a method of operation of the height measurement apparatus S is now described in more detail. As explained above, a z-direction modulation is applied to the substrate table WT. The lock-in amplifier 22 locks in on this modulation frequency and provides an output signal indicative of the movement of the cantilever 2 (using the output from the optical detector 20). The substrate table WT is moved downwards (i.e. in the -z-direction) in a series of steps, for example with intervals of 10 microns. This movement is separate from the z-direction modulation. The substrate W may for example have a distance d from the generally spherical tip 6 of 50 microns, 60 microns, 70 microns then 80 microns. Other distances, other step sizes and other numbers of steps may be used. The substrate table WT may be stepped upwards instead of downwards. In general, step changes of the distance between the generally spherical tip and the substrate W are performed. For each distance d the signal output from the lock-in amplifier is recorded. Due to hydrodynamic force, the signal output from the lock-in amplifier has a fixed relationship with the height of the substrate. The fixed relationship may be previously determined for example using calibration measurements or using an analytical model. A fit to the output signals from the lock-in amplifier can be used to determine the height of the substrate.

[0051] The measurement may be repeated for other positions on the substrate W, thereby allowing a height map to be generated.

[0052] Each cantilever may be calibrated to determine a relationship between the hydrodynamic force experienced by that cantilever and the height of a surface. The calibration may be performed using a substrate having a known height map. The result of the calibration may be stored in a memory (not depicted). The result of the calibration may be used when measuring a height of a productionsubstrate during operation of the lithographic apparatus. Each cantilever may have a slightly different length, e.g. with a tolerance of up to 3 microns. This means that each cantilever may have experience a slightly different hydrodynamic force in response to the same distance d and z-direction modulation. The calibration will account for this difference.

[0053] Embodiments of the invention may rely upon hydrodynamic force to provide height measurements. The hydrodynamic force is relatively large due to the generally spherical tip 6 having a relatively large diameter (e.g. at least 50 microns). Other forces may also act upon the generally spherical tip 6, the forces being Casimir force and electrostatic force. As noted above, due to the distance between the generally spherical tip 6 and the substrate surface being at least 10 microns, the Casimir force is negligible. Electrostatic force may be non-negligible. Electrostatic force is force arising from an electrical potential difference between the generally spherical tip 6 and the substrate W. Electrostatic force is inversely proportional to the distance between the generally spherical tip 6 and the substrate W, and is proportional to the square of the potential difference between the generally spherical tip and the substrate. Although the substrate W and the generally spherical tip 6 may both be nominally at ground, there may still be a potential difference between them (the substrate surface is not conducting and so may retain electrical charge). Therefore, some electrostatic force will be present during height measurements which use an embodiment of the invention.

[0054] Hydrodynamic force is inversely proportional to the distance between the generally spherical tip 6 and the substrate W, and is proportional to the velocity of the generally spherical tip.

[0055] The differences between electrostatic force and hydrodynamic force are such that when the Sin(z) z-direction modulation is applied to the substrate table WT, the electrostatic force has a Sin(z) dependency and the hydrodynamic force has a Cos(z) dependency. The lock-in amplifier 22 provides two signals from two output channels. A first signal has a zero phase with respect to the Sin(z) z- direction modulation, and a second signal has a 90° (TC / 2) phase with respect to the Sin(z) z-direction modulation. The first signal indicates electrostatic force, and the second signal indicates hydrodynamic force. This allows the hydrodynamic force to be distinguished from the electrostatic force.

[0056] In practice, there may be some cross-talk between the signals provided from the lock-in amplifier 22, such that the hydrodynamic force output includes a contribution due to electrostatic force. Embodiments of the invention may measure the electrostatic force, thereby allowing adjustment of the hydrodynamic force measurement to obtain a more accurate substrate height measurement.

[0057] A voltage source 26 is used to apply a voltage to the cantilever 2 and generally spherical tip 6. If the body 4 is has a conductive outer surface, then the voltage source may be electrically connected to the body. If the body 4 does not have a conductive outer surface, then the voltage source may be electrically connected to the cantilever. The voltage source provides a voltage which is a combination of a fixed voltage (DC) and a sinusoidal modulation (AC). In other words, the voltage is a DC voltage with an AC ripple. The frequency of the voltage modulation is different to the frequency of the z-direction modulation. The frequency of the voltage modulation may be below the resonantfrequency of the cantilever 2 (e.g. below 300 Hz). In an embodiment, the voltage source 26 may be within the lock-in amplifier 22.

[0058] The modulation of the voltage causes modulated electrostatic force. The electrostatic force is modulated because the potential difference between the generally spherical tip 6 and the substrate W is modulated. The modulation frequency of the electrostatic force corresponds with the voltage modulation frequency. The lock-in amplifier 22 receives as an input the voltage modulation and uses the voltage modulation to lock onto an output signal from the optical detector 20 which has the same frequency. The output signal from the lock-in amplifier provides a measurement of the electrostatic force that is experienced by the cantilever. A feedback loop may be used to adjust the DC voltage to minimise the output signal from the lock-in amplifier 22. This minimizes the electrostatic force that is being exerted on the cantilever 2. Beneficially, minimizing the electrostatic force avoids a possibility that electrostatic force saturates the hydrodynamic force measurement. Minimizing the electrostatic force improves the accuracy of the hydrodynamic force measurement provided from the lock-in amplifier. Some residual electrostatic force may remain. Processing of the first and second lock-in amplifier output signals may account for this. For example, a height measurement may be obtained using the hydrodynamic force measurement. A cantilever deflection caused by residual electrostatic force may be determined (e.g. using Hooke’s law and electrostatic force equations). This cantilever deflection may be subtracted from the height measurement obtained using the hydrodynamic force measurement. This allows for a correction of the substrate height measurement obtained using hydrodynamic force measurement. The feedback loop used to adjust the DC voltage may comprise dedicated electronics and / or may use a processor. A processor may be used to correct the substrate height measurement.

[0059] During the height measurement, the z-direction modulation and the voltage modulation are applied simultaneously. The z-direction modulation and the voltage modulation have different frequencies, so that the resulting cantilever modulations can be distinguished from each other.

[0060] As noted above, the generally spherical tip on a cantilever may for example have a diameter of 50 microns or more. A smaller diameter generally spherical tip may be used. However, when a smaller generally spherical tip is used, the cantilever comes closer to the substrate and will experience forces that introduce an error into the measurement. The generally spherical tip 6 may for example have a diameter of up to 200 microns. The generally spherical tip 6 may have a larger diameter. However, a larger diameter will provide less spatial resolution. In addition, significant capacitance will be generated and this may introduce an error into the measurement.

[0061] The cantilevers 2 may be formed using lithography. Alternatively, the cantilevers may be made separately and glued to the body 4. The generally spherical tips 6 may be beads. The generally spherical tips 6 may be glued to the cantilevers 2.

[0062] In the depicted embodiment the optical fibers 8 pass through the body 4. In an alternative embodiment, the optical fibers 8 may run along grooves provided on an outer face of the body 4. Thisalternative embodiment may be easier to fabricate, because it may be easier to place the optical fibers 8 into grooves than to push the optical fibers through holes in the body 4.

[0063] Embodiments of the invention apply a modulation to the substrate table WT (and thus substrate W), and then measure hydrodynamic force using lock-in amplification. Advantageously, because a modulation is applied and then used as part of the measurement, the measurement may be relatively insensitive to environmental noise.

[0064] A plurality of height measurement apparatus S according to an embodiment may be provided.These may measure the height of different areas of the substrate W in parallel.

[0065] A height measurement apparatus S according to an embodiment of the invention may be provided in a metrology tool (i.e. external to a lithographic apparatus).

[0066] While the methods described herein have been described in relation to a substrate W that has, or will be, exposed to lithographic radiation (i.e. the radiation beam B), as will be clear to the skilled person, the methods (and corresponding apparatus) may beneficially be adapted for use with substrates that are not exposed to lithographic radiation.

[0067] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0068] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0069] 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, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0070] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others.Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0071] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. 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 claims set out below. Other aspects of the invention are set out as in the following numbered clauses:1. A substrate height measuring apparatus comprising: a cantilever extending from a body and having a generally spherical tip provided at a distal end, wherein the generally spherical tip and the cantilever have a conductive coating; a modulation system configured to apply a periodic position modulation to a substrate table; a voltage source configured to apply a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; an interferometric detection system configured to measure movement of the cantilever whilst the position modulation and the voltage modulation are being applied; a lock-in detection system configured to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and a processor configured to provide a substrate height measurement based upon the output signals.2. The substrate height measuring apparatus of clause 1, wherein the interferometric detection system comprises a laser, optical fiber, an optical interferometer and an optical detector.3. The substrate height measuring apparatus of clause 2, wherein the optical fiber has an end which is above the distal end of the cantilever.4. The substrate height measuring apparatus of clause 2 or clause 3, wherein the optical fiber is received in a groove provided in the body from which the cantilever extends.5. The substrate height measuring apparatus of any preceding clause, wherein the body from which the cantilever extends has a conductive coating.6. The substrate height measuring apparatus of any preceding clause, wherein the cantilever has a length of 2mm or more.7. The substrate height measuring apparatus of any preceding clause, wherein the cantilever is one of a plurality of cantilevers which extend from the body.8. The substrate height measuring apparatus of clause 7, wherein an interferometric detection system and a lock-in detection system are provided for each cantilever.9. The substrate height measuring apparatus of clause 7, wherein a multiplexing arrangement using the same interferometric detection system and lock-in detection system is provided for multiple cantilevers.10. The substrate height measuring apparatus of any preceding clause, wherein the voltage source is provided within the lock-in detection system.11. A lithographic apparatus comprising an illumination system and a projection system, wherein the lithographic apparatus further comprises the substrate height measuring apparatus of any preceding clause.12. A method of measuring a substrate height using a measuring apparatus comprising a cantilever extending from a body and having a generally spherical tip provided at a distal end, the generally spherical tip and the cantilever having a conductive coating; the method comprising applying a periodic position modulation to a substrate table supporting a substrate; applying a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; measuring movement of the cantilever whilst the position modulation and the voltage modulation are being applied; using a lock-in detection system to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and using the output signals to determine a substrate height measurement.13. The method of clause 12, further comprising adjusting the DC voltage to minimise the output signal indicative of electrostatic force acting upon the cantilever.14. The method of clause 12 or clause 13, further comprising performing step changes of the distance between the generally spherical tip and the substrate W, and obtaining the output signals for each distance.15. The method of any of clause 12 to 14, wherein a plurality of cantilevers with generally spherical tips are provided, and height measurements are obtained from the plurality of cantilevers with generally spherical tips.16. The method of any of clause 12 to 15, wherein multiple height measurements are performed to obtain a height map.17. The method of any of clause 12 to 16, wherein the periodic position modulation and the periodic voltage modulation both have a frequency of less than 300Hz.

Claims

CLAIMS1. A substrate height measuring apparatus comprising: a cantilever extending from a body and having a generally spherical tip provided at a distal end, wherein the generally spherical tip and the cantilever have a conductive coating; a modulation system configured to apply a periodic position modulation to a substrate table; a voltage source configured to apply a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; an interferometric detection system configured to measure movement of the cantilever whilst the position modulation and the voltage modulation are being applied; a lock-in detection system configured to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and a processor configured to provide a substrate height measurement based upon the output signals.

2. The substrate height measuring apparatus of claim 1, wherein the interferometric detection system comprises a laser, optical fiber, an optical interferometer and an optical detector.

3. The substrate height measuring apparatus of claim 2, wherein the optical fiber has an end which is above the distal end of the cantilever.

4. The substrate height measuring apparatus of claim 2 or claim 3, wherein the optical fibrer is received in a groove provided in the body from which the cantilever extends.

5. The substrate height measuring apparatus of any preceding claim, wherein the body from which the cantilever extends has a conductive coating.

6. The substrate height measuring apparatus of any preceding claim, wherein the cantilever is one of a plurality of cantilevers which extend from the body.

7. The substrate height measuring apparatus of claim 6, wherein an interferometric detection system and a lock-in detection system are provided for each cantilever.

8. The substrate height measuring apparatus of claim 6, wherein a multiplexing arrangement using the same interferometric detection system and lock-in detection system is provided for multiple cantilevers.

9. The substrate height measuring apparatus of any preceding claim, wherein the voltage source is provided within the lock-in detection system.

10. A lithographic apparatus comprising an illumination system and a projection system, wherein the lithographic apparatus further comprises the substrate height measuring apparatus of any preceding claim.

11. A method of measuring a substrate height using a measuring apparatus comprising a cantilever extending from a body and having a generally spherical tip provided at a distal end, the generally spherical tip and the cantilever having a conductive coating; the method comprising applying a periodic position modulation to a substrate table supporting a substrate; applying a DC voltage and a periodic voltage modulation to the cantilever and generally spherical tip; measuring movement of the cantilever whilst the position modulation and the voltage modulation are being applied; using a lock-in detection system to receive a signal from the interferometric detection system and provide an output signal indicative of hydrodynamic force acting upon the cantilever and an output signal indicative of electrostatic force acting upon the cantilever; and using the output signals to determine a substrate height measurement.

12. The method of claim 11, further comprising adjusting the DC voltage to minimise the output signal indicative of electrostatic force acting upon the cantilever.

13. The method of claim 11 or claim 12, further comprising performing step changes of the distance between the generally spherical tip and the substrate W, and obtaining the output signals for each distance.

14. The method of any of claims 11 to 13, wherein a plurality of cantilevers with generally spherical tips are provided, and height measurements are obtained from the plurality of cantilevers with generally spherical tips.

15. The method of any of claims 11 to 14, wherein the periodic position modulation and the periodic voltage modulation both have a frequency of less than 300Hz.

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