Capacitive sensor system and method

Abstract

The present disclosure provides a capacitive sensor system, comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.

Description

CAPACITIVE SENSOR SYSTEM AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24218742.5 which was filed on 10 December 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a capacitive sensor system and to a method for capacitive sensing. The capacitive sensor system may be included in an exposure apparatus. The system and method can be applied, for instance, for sensing the position of an object, such as a movable optical element in the exposure apparatus.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] 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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. 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 can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 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] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be alternatingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm)actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] Eithographic apparatus in general typically use a number of sensors to measure or sense the position of objects. For instance, for accurate processing of a substrate it is adamant to know with a predetermined accuracy where the respective substrate is located with respect to a reference position. An example of the latter includes the substrate stage, which should be accurately positioned with respect to the patterned beam of the projection optics. However, many other objects can move and it is typically relatively important to know the position thereof up to a certain accuracy.

[0007] To sense the position of a (moveable) object, a capacitive sensor can be used. Capacitive sensors can be very accurate. However, one of the main contributors to the accuracy error of a capacitive sensor is the tilt between the sensor head and the sensor target, i.e. the object. This error is well understood and can be quantified and corrected for when the tilt between sensor head and sensor target is known. Nevertheless, the tilt cannot always be determined and / or is not always known, which limits the accuracy of the respective sensor.

[0008] JP2001241948A in the name of Tohoku Techno Arch provides a capacitance-type inclination angle sensor. The capacitance-type inclination angle sensor 100 is provided with a protective ring 110, insulator 120, and guard ring 130 on a horizontal face and comprises a sensor electrode 150 in the guard ring 130. The sensor electrode 150 is an electrode in the shape of a circle divided into four and is constituted of electrodes 151, 152, 153, and 154 arranged symmetrically with respect to the x-axis and y-axis. The protective ring 110, guard ring 130, and four electrodes (child sensors) 151-154 of the same shape of the sensor are insulated from one another by the insulators 120 and 140. By measuring the capacitance between the four electrodes and a target surface 160, the inclination of the sensor to the target surface 160 is measured.

[0009] Gian Bartolo Picotto et al., 2009 Meas. Sci. Technol. 20 084011, "A multi-electrode plane capacitive sensor for displacement measurements and attitude controls", discloses a multi-electrode capacitive sensor. The circular active electrode of a guarded plane-parallel capacitive sensor is subdivided into four sectors of equal area. The four output signals from each independent sensor are acquired, normalized and summed, to obtain the displacement. Similarly, by a combination of summing and differencing, the tip and tilt between the relative electrodes can be determined. The measuring electronics was set for a full-scale displacement range of several hundreds of micrometers.

[0010] The disclosures described above provide more accurate capacitive sensors having multiple electrodes. Nevertheless, these prior art capacitive sensors require relatively complex, and therefore relatively expensive and bulky, electronics to drive the respective electrodes and to process the resulting measurement signals.

[0011] It is an aim to provide an improved system and method for sensing the position of an object.SUMMARY

[0012] The present disclosure provides a capacitive sensor system, comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.

[0013] In an embodiment, in the second mode all other of the at least two segments is connected to the guard electrode.

[0014] In an embodiment, the ground electrode encloses the guard electrode.

[0015] In an embodiment, the measurement electrode comprises three segments.

[0016] In an embodiment, the measurement electrode is substantially round, and wherein each of the at least two segments has substantially the same shape.

[0017] In an embodiment, the at least two segments have different shapes.

[0018] In an embodiment, one of the at least two segments has a larger surface than the other segments, to accommodate for expected tilt of a target.

[0019] The disclosure provides a sensor wherein the active sensor area is divided into multiple, for instance two or three, segments. In normal measuring mode, active segments can be connected to a single demodulator channel. When measuring tilt, the segments can be contacted separately.

[0020] In an embodiment, the two segments can be measured sequentially, instead of in parallel.

[0021] According to an aspect, the disclosure provides an exposure apparatus comprising at least one capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.

[0022] In an embodiment, in the second mode all other of the at least two segments are connected to the guard electrode.

[0023] In an embodiment, the exposure apparatus comprises at least one moveable optical element provided with at least two capacitive sensor systems for measuring a distance with respect to the respective optical element.

[0024] In an embodiment, the exposure apparatus comprises a source of radiation for exposure of a pattern on a substrate, the radiation being in the deep ultraviolet (DUV) or the extreme ultraviolet (EUV) spectrum.

[0025] According to another aspect, the disclosure provides an exposure apparatus comprising at least one sensor system as described above.

[0026] In an embodiment, the exposure apparatus comprises a source of radiation for exposure of a pattern on a substrate, the radiation being in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) spectrum.

[0027] According to another aspect, the disclosure provide a method of measuring a position of an object, the method comprising the steps of:- providing a capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode;- operating the sensor system in a second mode wherein only one of the at least two segments is active for measuring a second position with respect to the object; and- operating the sensor system in a first mode, wherein all segments of the measurement electrode are active for measuring a first position with respect to the object.

[0028] In an embodiment, while the only one segment is active for measuring a position with respect to the object in the second mode, the other segments of the at least two segments are connected to the guard electrode.

[0029] In an embodiment, the step of operating the sensor system in the second mode comprises:- operating the sensor system wherein one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode;- operating the sensor system wherein another one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode; and- repeating the steps above until all segments have measured a position with respect to the object.

[0030] In an embodiment, the method comprises the steps of:- providing a moveable object;- arranging a number of sensor systems for measuring a distance with respect to the moveable object;- operating the number of sensor systems in the second mode, for measuring a first distance of each segment of each sensor system with respect to the object;- using the measured first distances for determining a first tilt of the object; and- operating the number of sensor systems in the first mode, for measuring a second distance with respect to the object.

[0031] In an embodiment, the step of operating the number of sensor systems in the first mode for measuring a second distance with respect to the object includes using the measured distance for determining a second tilt of the object which differs from the first tilt.

[0032] In an embodiment, the step of determining a second tilt of the object includes using the determined first tilt for correcting the measured second distance.

[0033] In an embodiment, the step of operating the sensor system in the second mode includes storing the measured second position(s) in a calibration table; and the method comprising the step of using the using the second position(s) in the calibration table to correct the measured first position when operating the sensor system in the first mode.

[0034] The system and method of the present disclosure enable to more accurately measure the position of an object, such as a movable optical element in an exposure apparatus. The system and method enable to correct for tilt, while also providing a relatively simple yet robust and accurate position measurement. The system and its method of operation is relatively simple, providing cost benefits both relating to capital expenditure as well as operating expenditure.BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 shows a schematical representation of a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2A shows a front view of a capacitive sensor;Figures 2B to 2D show a front view of a capacitive sensor according to embodiments of the present disclosure;Figures 3A to 3C show a side view of a capacitive sensor in respective steps of an embodiment of a method according to the disclosure; andFigure 4 shows a side view of a moveable object provided with a number of sensors according to the disclosure.DETAILED DESCRIPTION

[0036] 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, 157or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5- 100 nm).

[0037] 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.

[0038] 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) MT constructed 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may beused in parallel, and / or steps in preparation of 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.

[0043] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0044] 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 aid of 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.

[0045] 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 axes. 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.

[0046] Figure 2A shows a front view of a capacitive sensor 1. The sensor comprises a first or measurement electrode 2. A second or guard electrode 4 may enclose the measurement electrode 2. A third or ground electrode 6 may enclose the second electrode. Respective electrodes are separated by strips of insulator 7.

[0047] Figure 2B shows a front view of a capacitive sensor 10 in accordance with the present disclosure. Herein, the first or measurement electrode 2 may be subdivided in multiple segments. The measurement electrode may comprise, for instance, two (Fig. 2B), three (Fig. 2C), or four segments 12, 14, 16, 18 (Fig. 2D). The at least two segments of the measurement electrode 2 are electrically isolated with respect to each other.

[0048] The second or guard electrode 4 may enclose the measurement electrode 2. The third or ground electrode 6 may enclose the second electrode 4.

[0049] Although the insulators 7 are indicated as strips, the general principle of the sensor system 1 of the present disclosure includes having an insulator between respective electrodes. Various methods are conceivable to fabricate the insulators 7. For instance, lines between respective electrodes may be laser printed into a conductive layer which was previously deposited on top of a layer of insulator. Subsequently, electrical connections to each electrode may be provided, penetrating the insulating layer. Thus, all electrodes are substantially planar.

[0050] Generally referring to Figure 3A, in operation, the third electrode 6 is typically connected to ground 20. Ground herein refers to a reference point in the electrical circuit from which voltages on the other electrodes are measured. Ground herein provides a common return path for electric current, or a direct physical connection to the Earth.

[0051] The second or guard electrode 4 may typically be connected to a voltage source 22 to provide a second reference level or guard level. Herein, the second reference level is able to have the guard electrode create an electric field. Said electric field may be rereferred to as a guard electric field or electrical guard field.

[0052] The respective segments 12, 14, 16, 18 of the measurement electrode 2 can be connected to respective electronics 24.

[0053] The sensor 10 according to the present disclosure can operate in at least two modes of operation. In a first mode, schematically depicted in Figure 3C, all measurement segments 12, 14 of the measurement electrode 2 are connected to the measurement electronics 24. Herein, all of the segments 12, 14 are active for measuring a first position with respect to an object 30.

[0054] Respective first and second connections 32, 34 may electrically connect the respective segments 12, 14 to the measurement electronics 24.

[0055] First position herein may relate to a distance of the sensor 10 with respect to the object 30. Said distance, when the sensor is operating in the first mode (Fig. 3C), may be an average of a first distance dl and a second distance d2 between the object and a first measurement electrode 12 and a second measurement electrode 14 respectively.

[0056] Generally referring to Figures 3 A and 3B, the sensor system can also operate in a second mode. Herein, only one of the at least two segments 12, 14 is active for measuring a second position with respect to the object 30.

[0057] Herein, only one of the respective connections 32, 34 relating to one of the measurement segments 12, 14 is electrically connected to the measurement electronics 24. In a first position, see Fig. 3 A, only segment 14 is connected to the measurement electronics 24. In a second position, see Fig. 3B, only segment 12 is connected to the measurement electronics 24.

[0058] While only one segment is active for measuring a position or respective distance dl, d2 with respect to the object 30 while the system is in the second mode, the other segments of the at least two segments may be connected to the guard electrode 4. In the first position, see Fig. 3 A, segment 12 is connected to the guard electrode 4. In the second position, see Fig. 3B, segment 14 is connected to the guard electrode 4.

[0059] In a preferred embodiment, operating the sensor system 1 in the second mode comprises the steps of operating the sensor system with one of the segments, i.e. one of the first electrodes 12 to 18, active for measuring a position with respect to the object 30. Herein, said one segment is connected to the electronics 24. During the measurement, all other segments of the first electrodes 12 to 18 are connected to the guard electrode 4. Thus, the other, non-active segments provide the electric guard field, thereby enhancing the measurement of the active electrode, i.e. the distance dl, d2 as measured by the electrode which is connected to the electronics 24.

[0060] In a practical embodiment, the sensor system may be provided with two respective plugs, a first plug representing the first mode of operation (Fig. 3C), and a second plug representing the second mode of operation (Figs. 3A, 3B).

[0061] In use, an operator may connect the second plug, in a first step, as indicated in Figure 3A. In a second step, the operator may re-connect the same plug yet this time as indicated in Figure 3B. These steps may be repeated until all respective segments of the measurement electrode have provided their respective measurement signals, indicating the respective distances dl, d2 with respect to the object 30.

[0062] Subsequently, the first plug may be connected to the sensor system. Herein, the situation of Figure 3C occurs. Herein, the respective signal lines 32, 34 may basically be interconnected, causing the measurement electronics 24 to basically only measure an average of the respective distances dl, d2.

[0063] The above provides a simple, reliable yet very accurate method to obtain sensor readings. Herein, the option to connect non-active measurement segments to the guard electrode enables to obtain more accurate measurement values of respective distances dl, d2.

[0064] Having only a single channel for measurement saves on electronics, rendering the electronics 24 and the interconnection to the sensor system relatively simple and straightforward, saving costs and space. The latter may become relatively important in relatively complex machines, such as a lithographic apparatus or comparable exposure apparatus. The latter may typically require not one, but often many capacitive sensor systems 1 as described herein. Saving space and limiting complexity of electronics may generally have a significant and positive impact on the baseline of such apparatus, both in terms of capital expenditure and operating expenses.

[0065] Generally referring to Figure 4, an example of using the method and sensor system 1 of the present disclosure may include providing a movable object 30 with a multitude of sensor systems 10 according to the disclosure. For instance, depending on the degrees of freedom (DoF) in which the object 30 can move, for instance two or three sensors 10A, 10B, 10C may be provided. The object may be an optical element in an exposure apparatus, such as a lens or mirror. If the object can move in one degree of freedom, two sensors 10 may be sufficient. If the object can move in two degrees of freedom, three sensors 10 provided in a triangular setup with respect to the object may be suitable. If the object 30 can move in more than three degrees of freedom, more than three sensors 10 may be required to obtain position measurements at an accuracy exceeding a set threshold.

[0066] Figure 4 shows the object 30, provided with three sensors 10A, 10B, 10C according to the disclosure. The object is movable, as exemplified by tilted position 30A.

[0067] In a method of operation, in a first step, the sensor systems 10 may perform an initial position measurement of the object. For instance, assuming that the object has a reference position as indicated by the solid outline, yet due to circumstances, such as vibration, use, friction, etc., is positioned in the tilted position 30A indicated by the dashed line. Herein, before commencing use of the apparatus comprising the object 30, the sensors 10 will be operated in their respective second modes. The second mode of operation is explained herein above, with reference to Figures 3A and 3B.

[0068] Herein, separate segments 12-18 of the sensor systems are activated one at a time. Said measurements provide the respective distances dl, d2 (Fig. 3 A) between the respective segment and the object.

[0069] Given that the sensor system includes multiple, for instance two or three, segments, the respective distance measurements enable a second step, wherein the differences between the distance measurements of respective segments is used to calculate the tilt, in one or two degrees of freedom, with respect to the respective sensor system 10A, 10B, 10C.

[0070] The step of operating the respective sensor systems in the second mode may typically include storing the positions or distances as measured by each respective segment 12 to 18. The distances may be stored, for instance, in a calibration table. The calibration table may be stored in any suitable manner, for instance in a computer memory device or a digital storage device.

[0071] Subsequently, the respective sensor systems 10A to 10C are operated in their respective first modes of operation. Herein, all segments of each respective sensor system 10 are active, connected to the measurement electronics. The tilt of the object, as calculated during the step outlined above, is used to adjust the average distance as measured by the segments in conjunction.

[0072] Herein, the method may comprise the step of using the positions or distances in the calibration table, as stored in a step outlined above, to correct the measured position when operating the respective sensor system(s) 1 in the first mode.

[0073] As a bit of background to the above, in the case of tilting of the object with respect to the one or more capacitive sensors, a measurement error can be assumed as a geometric condition of the fieldfor the target change. The average distance of the sensor with respect to the object remains constant. Yet, the edge areas move closer of further away from the target. The latter results in electric field distortions, which affect the capacity C as measured by the capacitive sensor. Said capacity may be corrected for tilt of the target, for instance using the following model:andAx= 100

[0074] Herein, C is capacity; 0 is tilt angle of the object with respect to a reference position; R is the measurement area radius,

[0075] In practice, there will always be a certain amount of tilt between the sensor head and sensor target, for instance due to tolerances and alignment capabilities. The information outlined above, and knowledge of the tilt between sensor head and sensor target, i.e. the object 30, the induced error can be compensated. The system and method of the present disclosure provide a simple robust and cost effective means to first measure and determine the tilt between sensor head and target, and use the tilt to correct further measurements as described above.

[0076] 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. Possible other applications include 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.

[0077] 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 substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0078] 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.

[0079] Embodiments include the following numbered clauses:1. A capacitive sensor system, comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.2. The sensor system of clause 1 , wherein in the second mode all other of the at least two segments is connected to the guard electrode.3. The sensor system of clause 1 or 2, wherein the ground electrode encloses the guard electrode.4. The sensor system of one of the previous clauses, wherein the measurement electrode comprises three segments.5. The sensor system of one of the previous clauses, wherein the measurement electrode is substantially round, and wherein each of the at least two segments has substantially the same shape.6. The sensor system of one of clauses 1 to 5, wherein the at least two segments have different shapes.7. The system of clause 6, wherein one of the at least two segments has a larger surface than the other segments, to accommodate for expected tilt of a target.8. An exposure apparatus comprising at least one capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.9. The exposure apparatus of clause 8, wherein in the second mode all other of the at least two segments are connected to the guard electrode.10. The exposure apparatus of clause 8 or 9, comprising at least one moveable optical element provided with at least two capacitive sensor systems for measuring a distance with respect to the respective optical element.11. The exposure apparatus of clause 8 or 9, comprising a source of radiation for exposure of a pattern on a substrate, the radiation being in the deep ultraviolet (DUV) or the extreme ultraviolet (EUV) spectrum.12. Method of measuring a position of an object, the method comprising the steps of:- providing a capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode;- operating the sensor system in a second mode wherein only one of the at least two segments is active for measuring a second position with respect to the object; and- operating the sensor system in a first mode, wherein all segments of the measurement electrode are active for measuring a first position with respect to the object.13. The method of clause 12, wherein while the only one segment is active for measuring a position with respect to the object in the second mode, the other segments of the at least two segments are connected to the guard electrode.14. The method of clause 12 or 13, wherein the step of operating the sensor system in the second mode comprises:- operating the sensor system wherein one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode;- operating the sensor system wherein another one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode; and- repeating the steps above until all segments have measured a position with respect to the object.15. The method of one of clauses 12 to 14, comprising the steps of:- providing a moveable object;- arranging a number of sensor systems for measuring a distance with respect to the moveable object;- operating the number of sensor systems in the second mode, for measuring a first distance of each segment of each sensor system with respect to the object;- using the measured first distances for determining a first tilt of the object; and- operating the number of sensor systems in the first mode, for measuring a second distance with respect to the object.16. The method of clause 15, wherein the step of operating the number of sensor systems in the first mode for measuring a second distance with respect to the object includes using the measured distance for determining a second tilt of the object which differs from the first tilt.17. The method of clause 16, wherein the step of determining a second tilt of the object includes using the determined first tilt for correcting the measured second distance.18. The method of one of clauses 12 to 17, wherein the step of operating the sensor system in the second mode includes storing the measured second position(s) in a calibration table; and the method comprising the step of using the using the second position(s) in the calibration table to correct the measured first position when operating the sensor system in the first mode.

[0080] 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.

Claims

CLAIMS1. A capacitive sensor system, comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second mode wherein only one of the at least two segments is active for measuring a position with respect to the object.

2. The sensor system of claim 1 , wherein in the second mode all other of the at least two segments is connected to the guard electrode.

3. The sensor system of claim 1 or 2, wherein the ground electrode encloses the guard electrode.

4. The sensor system of one of the previous claims, wherein the measurement electrode comprises three segments.

5. The sensor system of one of the previous claims, wherein the measurement electrode is substantially round, and wherein each of the at least two segments has substantially the same shape.

6. The sensor system of one of claims 1 to 5, wherein the at least two segments have different shapes.

7. The system of claim 6, wherein one of the at least two segments has a larger surface than the other segments, to accommodate for expected tilt of a target.

8. An exposure apparatus comprising at least one capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode; wherein the sensor system is adapted to operate in a first mode, wherein all segments of the measurement electrode are active for measuring a position with respect to an object, or a second modewherein only one of the at least two segments is active for measuring a position with respect to the object.

9. The exposure apparatus of claim 8, wherein in the second mode all other of the at least two segments are connected to the guard electrode.

10. The exposure apparatus of claim 8 or 9, comprising at least one moveable optical element provided with at least two capacitive sensor systems for measuring a distance with respect to the respective optical element.

11. The exposure apparatus of claim 8 or 9, comprising a source of radiation for exposure of a pattern on a substrate, the radiation being in the deep ultraviolet (DUV) or the extreme ultraviolet (EUV) spectrum.

12. Method of measuring a position of an object, the method comprising the steps of:- providing a capacitive sensor system, the sensor system comprising: a measurement electrode comprising at least two segments; a guard electrode enclosing the measurement electrode; and a ground electrode;- operating the sensor system in a second mode wherein only one of the at least two segments is active for measuring a second position with respect to the object; and- operating the sensor system in a first mode, wherein all segments of the measurement electrode are active for measuring a first position with respect to the object.

13. The method of claim 12, wherein while the only one segment is active for measuring a position with respect to the object in the second mode, the other segments of the at least two segments are connected to the guard electrode.

14. The method of claim 12 or 13, wherein the step of operating the sensor system in the second mode comprises:- operating the sensor system wherein one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode;- operating the sensor system wherein another one of the at least one segments is active for measuring a position with respect to the object while all other segments are connected to the guard electrode; and- repeating the steps above until all segments have measured a position with respect to the object.

15. The method of one of claims 12 to 14, comprising the steps of:- providing a moveable object;- arranging a number of sensor systems for measuring a distance with respect to the moveable object;- operating the number of sensor systems in the second mode, for measuring a first distance of each segment of each sensor system with respect to the object;- using the measured first distances for determining a first tilt of the object; and- operating the number of sensor systems in the first mode, for measuring a second distance with respect to the object.

16. The method of claim 15, wherein the step of operating the number of sensor systems in the first mode for measuring a second distance with respect to the object includes using the measured distance for determining a second tilt of the object which differs from the first tilt.

17. The method of claim 16, wherein the step of determining a second tilt of the object includes using the determined first tilt for correcting the measured second distance.

18. The method of one of claims 12 to 17, wherein the step of operating the sensor system in the second mode includes storing the measured second position(s) in a calibration table; and the method comprising the step of using the using the second position(s) in the calibration table to correct the measured first position when operating the sensor system in the first mode.

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