Electrostatic clamp

Abstract

An electrostatic clamp is disclosed. The electrostatic clamp is for holding an object by electrostatic force. The clamp comprises a first conductive element and a second conductive element disposed between the first conductive element and a plane in which the object is held. The second conductive element is separated from the first conductive element by less than 100 micrometres. Also disclosed is a method of manufacturing an electrostatic clamp for holding an object by electrostatic force in a lithographic apparatus.

Description

ELECTROSTATIC CLAMPCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 571,544 which was filed on March 29, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to electrostatic clamps, and in particular electrostatic clamps for holding a substrate or reticle in a lithographic apparatus.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) 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 that are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i- line), 248 nm (KrF), 193 nm (ArF) and 13.5 nm (EUV). 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] At such short wavelengths, precise positioning of the patterning device and / or substrate within the lithographic apparatus is essential.

[0006] Such lithographic apparatuses may be provided with one or more clamps to clamp the patterning device and / or substrate to an object support, such as a mask table or a wafer table respectively. The clamp may be, for example, a mechanical clamp, a vacuum clamp, or an electrostatic clamp. Electrostatic clamps may be particularly suited to operating at EUV wavelengths, since regions of an EUV lithographic apparatus necessarily operate under near vacuum conditions.

[0007] Electrostatic clamps are often maintained in a low-pressure hydrogen-rich environment, which is a generally non-conductive environment. As such, residual electric charges may accumulateon dielectric or ungrounded surfaces of the clamp, which may for example be non-uniformly distributed across the surfaces.

[0008] In some prior art example electrostatic clamps, electric fields due to residual charge may be excessively high at edge regions of electrodes, which may in some instances result in electrical charge being injected from the electrode edge into a dielectric substrate or member in which the electrode is formed. Such charge injection may be known in the art as “buried charge” or “Buried Barrier Leakage”. Known techniques for discharging residual charge may be ineffective against such buried leakage.

[0009] Furthermore, in use during a clamping operation, any defects such as in the dielectric substrate of layers in immediate proximity to the edge of the electrode may be at increased risk of electrical breakdown, due to the effectively amplified field at the edge of the electrode. Likely future increases in a required clamping pressure and hence applied voltage may increase a likelihood of such an electrical breakdown.

[0010] It is therefore desirable to provide an electrostatic clamping method and / or apparatus that is not subject to excessive Buried Barrier Leakage.

[0011] It is an object of at least one embodiment of at least one aspect of the present invention to obviate or at least mitigate at least one of the above identified shortcomings of the prior art.SUMMARY

[0012] According to a first aspect of the disclosure, there is provided an electrostatic clamp for holding an object by electrostatic force, the clamp comprising: a first conductive element; and a second conductive element disposed between the first conductive element and a plane in which the object is held and separated from the first conductive element by less than 100 micrometres.

[0013] In prior art electrostatic clamps implementing only a single conductive element, a relatively strong strength electric field may be emitted from a single radiating point at an edge of the conductive element, resulting in excessive Buried Barrier Leakage and / or formation of a breakdown risk point. However, by advantageously implementing two conductive elements as described above, the two conductive elements may collectively function as less of a point-source for emitting an electric field. That is, since an electric field is emitted perpendicular to a surface of the conductive elements, by implementing two conductive elements in relatively close proximity the conductive elements may collectively emit a weaker electric field around the edge or end-point region of the conductive elements.

[0014] Furthermore, since the use of two conductive elements may reduce an electric field strength and hence amount of leakage into the buried barrier, in use a clamping voltage may be increased beyond that of a single-conductive element clamp before a baseline -equivalent E-field strength in the buried barrier is reached. This may advantageously result in an increase in a clamping pressure.

[0015] The first and / or second conductive element may each be formed as layer of conductive material, e.g. a metal. In use, the first and / or second conductive element may each operate as an electrode.

[0016] The second conductive element may be separated from the first conductive element by less than 50 micrometres.

[0017] The second conductive element may be separated from the first conductive element by less than 20 micrometres.

[0018] Advantageously, by disposing the first and second conductive elements so close together, the first and second conductive elements may collectively operate as a single electrode in use, yet produce a relatively weak electric field at an edge region compared to a single conductive element implementation of the electrostatic clamp.

[0019] The electrostatic clamp may comprise a dielectric layer. The dielectric layer may extend between the first conductive element and the second conductive element.

[0020] The electrostatic clamp may comprise a first dielectric substrate and a second dielectric substrate. The first and second conductive elements may be disposed between the first and second dielectric substrates.

[0021] The electrostatic clamp may comprise a plurality of burls extending from a surface to define the plane in which the object is held. The first and second conductive elements may extend between the plurality of burls.

[0022] Each burl of the plurality of burls may extend through a respective hole in each of the first and second dielectric substrates.

[0023] A spacing may exist between an outer surface of each burl and a respective opposing surface of each of the first and second dielectric substrates.

[0024] The outer surface of each burl and the respective opposing surfaces of each of the first and second dielectric substrates may comprise a conductive layer.

[0025] The first and second dielectric substrates may extend closer to each burl than the first and second conductive elements.

[0026] One of the first and second conductive elements may extend further towards each burl than the other of the first and second conductive elements.

[0027] The dielectric layer may comprises a polymer. Optionally, the polymer may comprise benzocyclobutene.

[0028] The first and / or second conductive elements may comprise a metal. Optionally, the metal may comprise chromium.

[0029] The first and / or second dielectric substrate may comprises a glass. Optionally, the glass may comprise at least one of: borosilicate; fused silica; Corning® EAGLE XG® glass; ULE (UltraLow Expansion) glass; and / or Schott BOROFLOAT®.

[0030] One of the first and second dielectric substrates may be affixed to the surface from which the plurality of burls extend by an adhesive.

[0031] The object may be a substrate, e.g. a wafer, used in lithographic projection techniques.

[0032] The object may be a lithographic projection reticle or reticle blank in at least one of a lithographic projection apparatus

[0033] The object may be a reticle handling apparatus, and / or a reticle manufacturing apparatus.

[0034] According to a second aspect of the disclosure, there is provided a method of use of the electrostatic clamp of the first aspect, the method comprising configuring the electrostatic clamp to hold the object by an electrostatic force by biasing the first and second conductive elements to a same potential.

[0035] In examples, the same potential of the first and second conductive elements may be a first potential that is different form a second potential at a surface of each of the plurality of burls.

[0036] According to a third aspect of the disclosure, there is provided a lithographic apparatus comprising an electrostatic clamp according to the first aspect.

[0037] According to a fourth aspect of the disclosure, there is provided a method of manufacturing an electrostatic clamp for holding an object by electrostatic force in a lithographic apparatus, the method comprising the steps of: forming a first conductive element on a dielectric substrate; forming a second conductive element separated from the first conductive element by less than 100 micrometres; and disposing the dielectric substrate such that the second conductive element is disposed between the first conductive element and a plane in which the object is held.

[0038] The method may comprise a step of forming a dielectric layer. The dielectric layer may be formed such that it extends between the first conductive element and the second conductive element. The dielectric layer may be formed on one or both of the first conductive element and the second conductive element.

[0039] According to a fifth aspect of the disclosure, there is provided an electrostatic clamp for holding an object by electrostatic force, the clamp comprising: a conductive element formed on a dielectric substrate and extending between a plurality of burls defining a plane in which the object is held; wherein an edge portion of the conductive element comprises a curved and / or overhanging section.

[0040] The above summary is intended to be merely exemplary and non-limiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilized, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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 lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts examples of cross-sections of portion of prior art electrostatic clamps;Figure 3 depicts an example of an electrode layer as implemented in the prior art electrostatic clamps of Figure 2;Figure 4 depicts a simulation of an electric field at the electrode layer of Figure 2;Figure 5 depicts an example of an arrangement of conductive elements in an electrostatic clamp, according to an embodiment of the disclosure;Figure 6 depicts an example of the electric field strength in the region of an edge of the conductive elements of the electrostatic clamp depicted in Figure 5;Figure 7 depicts an example of an arrangement of conductive elements in an electrostatic clamp, according to a further embodiment of the disclosure;Figure 8 depicts an example of the electric field strength in the region of an edge of the conductive elements of the electrostatic clamp depicted in Figure 7;Figure 9 depicts an example of an arrangement of a conductive element in an electrostatic clamp, according to a further embodiment of the disclosure; andFigure 10 depicts a method of manufacturing an electrostatic clamp for holding an object by electrostatic force in a lithographic apparatus.DETAILED DESCRIPTION

[0042] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate support WT, also known as a substrate table, configured to support a substrate W.

[0043] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0044] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate support WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0045] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’ , with a pattern previously formed on the substrate W.

[0046] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0047] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0048] The substrate support comprises a clamp 100, also known as a chuck, configured to clamp the substrate W to the substrate support WT. The clamp 100 may be held within a recess in the substrate support WT. The clamp 100 is an electrostatic clamp 100, and is described in more detail with reference to Figures 5 to 9.

[0049] The main body of the electrostatic clamp 100 generally corresponds in shape and size to the substrate W. At least on an upper surface of the clamp, e.g. a surface adjacent the substrate W in use, the clamp has projections, known in the art as burls. The burls extend from the upper surface of the clamp to define a plane in which the substrate W is held.

[0050] It will be appreciated that the term ‘upper’ is used in the context of the example lithographic apparatus LA of Figure 1, wherein the electrostatic clamp 100 is depicted in a particular orientation. It will be understood that the disclosed clamp may be disposed in various orientations, and therefore the term ‘upper’ should be taken in the context of a particular described use case.

[0051] Furthermore, it will also be appreciated that in examples, such an electrostatic clamp 100 may additionally or alternatively be implemented on the support structure MT, for clamping the patterning device MA in a desired position.

[0052] Figure 2 depicts examples of cross-sections of portion of a first prior art electrostatic clamp 200 and a second prior art electrostatic clamp 250, and Figure 3 depicts an example of an electrode layer as implemented in the prior art electrostatic clamps 200, 250 of Figure 2.

[0053] It will be appreciated that Figure 2, and the ensuing figures, are not true-to-scale illustrations of electrostatic clamps, but are intended to clarify a provision of conductive element(s), e.g. electrodes, in a spacing between burls, as described in more detail below.

[0054] The first example prior art electrostatic clamp 200 comprises a base 210. The base may be formed from silicon-infiltrated silicon carbide (SiSiC). The base 210 comprises a plurality of burls 215, e.g. projections, extending from the base 210. An upper side of the plurality of burls 215 defines a plane 295 for supporting an object, such as a substrate, e.g. a wafer, used in lithographic projection techniques, or a lithographic projection reticle or reticle blank in at least one of a lithographic projection apparatus, a reticle handling apparatus, and / or a reticle manufacturing apparatus.

[0055] In the example, each burl 215 of the plurality of burls 215 in the first example prior art electrostatic clamp 200 comprises a substantially frustoconical shape. A diameter of an upper portion of each burl may for example be in the range of 100 to 500 micrometres; a height of each burl, e.g. a distance each burl extends from the base, may for example be in the range of 300 to 2000 micrometres. It will be appreciated that such geometries and dimensions are provided for purposes of example only, and in other examples the burls 215 may have, the shape of cuboids, cylinders, pyramids, cones, truncated pyramids and / or truncated cones, or the like, and may have different dimensions.

[0056] The plurality of burls 215 may, for example, be arranged with a spacing between adjacent burls in the region of one to several millimetres, such that a total area of all of the burls 215 extending from the upper surface of the base 210 is less than about 10% of the total area of the total surface area of the upper surface.

[0057] As a further example, the second example prior art electrostatic clamp 250 comprises a plurality or burls 265, wherein each burl is disposed in a recess 290.

[0058] In a practical example, there can be many hundreds, thousands, or tens of thousands of burls 215, 265 distributed across an electrostatic clamp of diameter, 200 mm, 300 mm or 450 mm or larger.

[0059] An electrode device 220, denoted “E-sheef ’ in Figure 2, is provided, wherein the electrode device 220 extends between the plurality of burls 215, 265. The electrode device 220 comprises a layered structure with a first (lower) dielectric substrate 225, at least one conductive element 240, and a second (upper) dielectric substrate 235.

[0060] In use, the at least one conductive element 240 may be connected via a conductive path to a voltage source (not shown). The at least one conductive element 240 may, for example, be formed from polycrystalline silicon, gold, chromium or aluminium.

[0061] The at least one conductive element 240 may be formed next to, e.g. adjacent, a dielectric layer 255. The dielectric layer 255, may, for example, comprise a polymer such as benzocyclobutene (BCB). In an example, the dielectric layer may be in the region of 1 to 100 micrometres, for example approximately 9 micrometres thick. A primary purpose of the dielectric layer may be for bonding the first and second dielectric substrates 225, 235.

[0062] Although not depicted in the figures, a portion of the dielectric layer 255 may extend between the at least one conductive element 240 and the first and / or second dielectric substrates 225, 235.

[0063] In examples, the first and / or second dielectric substrates 225, 235 may formed from a glass, such as an alkali ion-free glass, e.g. a borosilicate glass. The glass may, for example, comprise at least one of: borosilicate; fused silica; Corning® EAGLE XG® glass; ULE (Ultra-Low Expansion) glass; and / or Schott BOROFLOAT®. The first dielectric substrate 225 may be referred to as “Bottom Glass”. The second dielectric substrate 235 may be referred to as “Top Glass”.

[0064] The electrode device 220 has openings 270, the extent of which along the electrode device 220 is greater than the diameter of the burls 215, 265 and through which the burls 215, 265 extend. Between the inner edges of the electrode device 220 at the openings 270 and the burls 215, 265, a spacing is formed. For this purpose, the openings 270 have an extent which is, for example, less than 200 micrometres, for example 30 micrometres to 100 micrometres greater than the diameter of the burls 215, 265. The openings 270 may be, for example, circular with a diameter of 600 micrometres to 800 micrometres, or may be any other shape that generally conforms to a shape of the burls 215, 265.

[0065] The electrode device 220 may be connected to the base 210 by means of an adhesive 275. The adhesive 275 may be electrically insulating so that preferably no, or only a negligibly small, electrical leakage current from the electrode device 220 to the base 210 occurs.

[0066] An overall thickness of the electrode device 220 may be selected such that between an upper side of the second dielectric layer 235 and the support plane 295 defined by the burls, a gap (denoted “vacuum gap” in Figure 3) in the range of, for example, of 1 micrometres to 50 micrometres is formed.

[0067] As more clearly depicted in Figure 3, the above-described spacing exists between an outer surface of each burl 215, 265 and a respective opposing surface of each of the first and second dielectric substrates 225, 235.

[0068] The outer surface of each burl 215, 265 and the respective opposing surfaces of each of the first and second dielectric substrates 225, 235 comprises a conductive layer. In use, when the electrostatic clamp is configured to hold the object by an electrostatic force, said conductive layer may be coupled to a voltage source (or ground) at a different potential than the at least one conductive element 240.

[0069] Figure 4 depicts a simulation of an electric field at the electrode device 220 of Figure 2. It can be seen that the highest electric field strength is at a region at the edge of the at least one conductive element 240.

[0070] It is this region where, due to the amplified field strength close to the edge of the at least one conductive element 240, negative charge may be injected from the edge of the at least one conductive element 240 into a dielectric layer 255. Such injected negative charge may be known in the art as “Buried Barrier Leakage”, wherein the dielectric layer 255 may be considered a “buried barrier”.

[0071] Conversely, at an opposite polarity, charge may be extracted from an interface between the dielectric layer 255 and the first and / or second dielectric substrate 225, 235, leaving a positive charge after removal of a clamping voltage. Above a certain threshold, such a residual charge may generate a disturbance force in the electrostatic clamp, potentially deforming an object held by the electrostatic clamp, and potentially even causing cause the object to stick to the electrostatic clamp. Because this charge is effectively buried, known charge cleaning technique may be relatively ineffective.

[0072] Furthermore, due to such charge a risk of electrical breakdown may be increased near the edge E of the at least one conductive element 240 due to the amplified field, especially in the case of one or more defects during manufacturing in the dielectric layer 255 or the first and / or second dielectric substrates 225, 235 near to the edge E of the at least one conductive element 240. Likely future increases in a required clamping pressure and voltage may further increase a likelihood of such electrical breakdown.

[0073] Figure 5 depicts an example of an arrangement of conductive elements in an electrostatic clamp 500, according to an embodiment of the disclosure. The electrostatic clamp 500 may be an embodiment of the electrostatic clamp 100 of Figure 1.

[0074] The electrostatic clamp 500 of Figure 5, comprises an electrode device comprising a first (lower) dielectric substrate 525 and an second (upper) dielectric substrate 535, similar to the first dielectric substrate 225 and second dielectric substrate 235 of the example electrostatic clamp 200, 250 of Figures 2 to 4.

[0075] Similarly, the electrostatic clamp 500 of Figure 5 also comprises burls 515, and a spacing between the burls 515 and the electrode device. That is, each burl 515 of the plurality of burls extends through a respective hole in each of the first and second dielectric substrates 525, 535, in a similar manner to the examples of Figure 2. As such, the burls 515 and first and second dielectric substrates 525, 535 are not described in further detail at this juncture, for purposes of brevity. Similarly, the outer surface of each burl 515 and the respective opposing surfaces of each of the first and second dielectric substrates 525, 535 each comprises a conductive layer.

[0076] Although only a portion of a single burl 515 is depicted in Figure 5, it will be appreciated that the electrostatic clamp 500 comprises a plurality of burls 515 extending from a surface, e.g. from a base, to define a plane 595 in which the object is held.

[0077] However, a salient different between the prior art electrostatic clamps 200, 250 and the electrostatic clamp 500 of Figure 5 is an implementation of an electrode layer comprising a first conductive element 540a (e.g. a first electrode) and a second conductive element 540b (e.g. a second electrode).

[0078] In embodiments, the first conductive element 540a and the second conductive element 540b are separated by less than 100 micrometres.

[0079] Indeed, in some embodiments, the first conductive element 540a and the second conductive element 540b may be separated by less than 50 micrometres, or even by less than 20 micrometres. Inthe depicted example, the first conductive element 540a and the second conductive element 540b are separated by approximately 9 micrometres.

[0080] The second conductive element 540b is disposed between the first conductive element 540a and the plane 595 in which the object is held, e.g. a plane 595 defined by upper surfaces of the burls 515. The first and second conductive elements 540a, 540b extend between the plurality of burls 515.

[0081] By advantageously implementing two conductive elements 540a, 540b as described above, the two conductive elements 540a, 540b may collectively function as less of a point-source for emitting an electric field than the single conductive element 240 of the examples of Figures 2 to 4. That is, since an electric field is emitted perpendicular to a surface of the conductive elements 540a, 540b, implementing two conductive elements 540a, 540b in relatively close proximity the conductive elements 540a, 540b may collectively emit a weaker electric field around the edge E or end-point region of the conductive elements 540a, 540b.

[0082] Furthermore, since the use of two conductive elements 540a, 540b may reduce an electric field strength and hence amount of leakage into the buried barrier, in use a clamping voltage may be increased beyond that of a single-conductive element clamp 200, 250 before a baseline-equivalent E- field strength in the buried barrier is reached. This may advantageously result in an increase in a clamping pressure.

[0083] In use, the electrostatic clamp 500 may be configured to hold the object by an electrostatic force by biasing the first and second conductive elements 540a, 540b to a same potential. The same potential of the first and second conductive elements 540a, 540b may be a first potential that is different from a second potential (e.g. ground potential) at a surface of each of the plurality of burls 515.

[0084] Each of the first conductive element 540a and the second conductive element 540b may be formed as layer of conductive material, e.g. polycrystalline silicon, gold, chromium or aluminium.

[0085] A dielectric layer 555, e.g. the “buried barrier11, extends between the first conductive element 540a and the second conductive element 540b. That is, the first and second conductive elements 540a, 540b are disposed between the first and second dielectric substrates 525, 535.

[0086] Similar to the examples of Figures 2 to 4, the dielectric layer 555, may, for example, comprise a polymer such as benzocyclobutene (BCB). In an example, the dielectric layer 555 may be in the region of approximately 9 micrometres thick. A primary purpose of the dielectric layer 555 may be for bonding the first and second dielectric substrates 525, 535.

[0087] Although not depicted in the figures, a portion of the dielectric layer 555 may extend between the first conductive element 235 and the first and / or second dielectric substrates 225, 235.

[0088] Figure 6 depicts an example of the electric field strength in the region of an edge of the conductive elements of the electrostatic clamp depicted in Figure 5. The graph in Figure 6 provides a direct comparison between a single conductive element 240 implementation of an electrostatic clamp 200, 250 (e.g. Figures 2 to 4) and a double conductive element 540a, 540b implementation of an electrostatic clamp 500 (e.g. the embodiment of Figure 5).

[0089] It can be seen that an electric field at the edge of the second conductive element 540b is about 1.36x lower compared to the nominal design for the same clamping pressure at wafer level for a distance from the second conductive element 540b up to about 1 micrometres. Furthermore, this is still improved up to a distance of approximately ~10 micrometres.

[0090] Advantageously, this means that relative to the electrostatic clamps 200, 250 of Figure 2, for the new electrostatic clamp 500 embodiment of Figure 5 a voltage may be increased by a factor of approximately 1.36x before a baseline-equivalent electric- field strength in the buried barrier is reached. This would be equivalent to increasing the pressure by about 1.85x, assuming all other factors stay the same.

[0091] It can be seen in the embodiment of Figure 5 that the first and second dielectric substrates 525, 535 extend closer to each burl 515 than the first and second conductive elements 540a, 540b. In this example embodiment the first and second conductive elements 540a, 540b are aligned, such that the first and second conductive elements 540a, 540b extend a same distance as each other in a direction towards the burl 515.

[0092] In contrast, Figure 7 depicts a further example of an arrangement of conductive elements 740a, 740b in an electrostatic clamp 700, according to an embodiment of the disclosure. The electrostatic clamp 700 may be an embodiment of the electrostatic clamp 100 of Figure 1.

[0093] In this example, a first conductive element 740a extends further in a direction towards the burl 715 than a second conductive element 740b.

[0094] Advantageously, a slightly improved edge electric field reduction may be achieved by increasing the area of the first (bottom) conductive element 740a slightly, e.g. approximately ~1.39x, equivalent to a pressure increase of about 1 ,94x, for the same clamping pressure (compared to the above-mentioned identical first and second conductive elements 540a, 540b embodiment). This is shown in Figure 8, which depicts an example of the electric field strength in the region of an edge E of the conductive elements 740a, 740b of the electrostatic clamp 700 described by Figure 7. The inset in the graph depicts the top and bottom electric field strengths (denoted “Top 900um” and “Bottom 900um”) separately in a case where a distance from the second (top) conductive element 740b to the burl 715 is smaller than the distance from first (bottom) conductive element 740a to the burl 715. The line between depicted top and bottom electric field strengths (denoted “898 ,6um”) depicts the case where the distance from bottom conductive element 740a to the burl 715 is 1.4 micrometres smaller than the top conductive element 740b to the burl 715, exactly providing (optimizing) an equal electric field strength at the edges of the top and bottom conductive elements 740a, 740b..

[0095] Figure 9 depicts an example of an arrangement of a conductive element in an electrostatic clamp 900, according to a further embodiment of the disclosure. The electrostatic clamp 900 may be an embodiment of the electrostatic clamp 100 of Figure 1.

[0096] The electrostatic clamp 900 of Figure 9, comprises an electrode device comprising a first (lower) dielectric substrate 925 and an second (upper) dielectric substrate 935, similar to the firstdielectric substrate 225 and second dielectric substrate 235 of the example electrostatic clamp 200, 250 of Figures 2 to 4.

[0097] Similarly, the electrostatic clamp 900 of Figure 9 also comprises burls (not shown), and a spacing between the burls and the electrode device. As such, the burls and first and second dielectric substrates 925, 935 are not described in further detail at this juncture, for purposes of brevity. Similarly, the outer surface of each burl and the respective opposing surfaces of each of the first and second dielectric substrates 925, 935 each comprises a conductive layer.

[0098] However, a salient different between the prior art electrostatic clamps 200, 250 and the electrostatic clamp 900 of Figure 9 is an implementation of an electrode layer comprising conductive element 940 wherein an edge portion of the conductive element 940 comprises a curved section 995 and / or overhanging section. Such a curved section 995 and / or overhanging section may, for example, be formed by a lithographic process, by process of etching, such as etching at an angle or selective etching, and / or selective deposition, or the like.

[0099] Furthermore, in such an embodiment, a thickness of the conductive element 940 may be substantially greater than a thickness of each conductive element 540a, 540b, 740a, 740b of the abovedescribed embodiments. In one example, the conductive element 940 may be in the region of 9 to 10 micrometres thick, e.g. extending substantially all of, or at least 90% of a distance between the first and second dielectric substrates 925, 935.[000100] Due to the curved edge portion, the conductive element 940 may act less like a pointed edge, than any of the above described conductive elements. Advantageously, this may also have the effect of reducing an electric field strength in a region of the curved edge.[000101] Figure 10 depicts a method of manufacturing an electrostatic clamp 500, 700 for holding an object by electrostatic force in a lithographic apparatus, according to an embodiment of the disclosure.[000102] The method comprises a step 1010 of forming a first conductive 540a element on a first substrate 525. The first substrate 525 may comprise a dielectric substrate. The first substrate 525 may, for example, comprise at least one of: borosilicate; fused silica; Corning® EAGLE XG® glass; ULE (Ultra-Low Expansion) glass; and / or Schott BOROFLOAT®. The first conductive 540a element may be formed by known deposition or lithography methods. The first conductive 540a may for example be the first conductive 540a of the embodiment of Figure 5.[000103] The method comprises a step 1020 of forming a second conductive element 540b separated from the first conductive element 540a by less than 100 micrometres. The second conductive element 540b may for example be the second conductive element 540b of the embodiment of Figure 5.[000104] In some embodiments, the step 1020 may comprise forming a dielectric layer 555 over the first conductive 540a element and subsequently forming the second conductive element 540b over the dielectric layer 555, and then disposing a second substrate 535 over the second conductive element 540b.[000105] In other embodiments, the step 1020 may comprise forming the second conductive element 540b on the second substrate 535, and forming the dielectric layer 555 over one or both of the first and second conductive elements, before disposing the second substrate 535 over the first substrate 525.[000106] The method comprises a step 1030 of disposing the first substrate 525 such that the second conductive element 540b is disposed between the first conductive element 540a and a plane in which the object is held. That is, the first substrate 525 may be adhered to a surface of a base from which burls extend, wherein the burls define the plane in which the object is held, as described above with reference to the examples of Figures 5 to 9.[000107] 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000108] 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.[000109] 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.[000110] 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.REFERENCE NUMERALSLA lithographic apparatus 270 openingsMA patterning device 275 adhesiveMT support structure 290 recessSO radiation source295 planeW substrate 500 electrostatic clampWT substrate support 515 burlsPS projection system 525 first dielectric substrateIL illumination system 535 second dielectric substrateB radiation beam 35 540a first conductive elementB’ radiation beam 540b second conductive element10 faceted field mirror device 555 dielectric layer11 facetted pupil mirror device 595 plane13 mirror 700 electrostatic clamp14 mirror 40 715 burls100 clamp 740a first conductive element200 electrostatic clamp 740b second conductive element210 base 900 electrostatic clamp215 burls 925 first dielectric substrate220 electrode device 45 935 second dielectric substrate225 first dielectric substrate 940 conductive element235 second dielectric substrate 995 curved section240 conductive element 1010 first step250 electrostatic clamp 1020 second step255 dielectric layer1030 third step265 burls

Claims

CLAIMS1. An electrostatic clamp (500, 700) for holding an object by electrostatic force, the clamp comprising: a first conductive element (540a, 740a); and a second conductive element (540b, 740b) disposed between the first conductive element and a plane (595) in which the object is held and separated from the first conductive element by less than 100 micrometres.

2. The electrostatic clamp (500, 700) of claim 1, wherein the second conductive element (540b, 740b) is separated from the first conductive element (540a, 740a) by less than 50 micrometres.

3. The electrostatic clamp (500, 700) of claim 1 or 2, wherein the second conductive element (540b, 740b) is separated from the first conductive element (540a, 740a) by less than 20 micrometres.

4. The electrostatic clamp (500, 700) of any preceding claim, comprising a dielectric layer (555) extending between the first conductive element (540a, 740a) and the second conductive element (540b, 740b).

5. The electrostatic clamp (500, 700) of any preceding claim, comprising a first dielectric substrate (525) and a second dielectric substrate (535), wherein the first and second conductive elements (540a, 540b, 740a, 740b) are disposed between the first and second dielectric substrates.

6. The electrostatic clamp (500, 700) of any preceding claim, comprising a plurality of burls (515, 715) extending from a surface to define the plane in which the object is held, wherein the first and second conductive elements (540a, 740a, 540b, 740b) extend between the plurality of burls.

7. The electrostatic clamp (500, 700) of claim 6, when dependent on claim 5, wherein: each burl of the plurality of burls (515, 715) extends through a respective hole in each of the first and second dielectric substrates; a spacing exists between an outer surface of each burl and a respective opposing surface of each of the first and second dielectric substrates (525, 535); and the outer surface of each burl and the respective opposing surfaces of each of the first and second dielectric substrates comprises a conductive layer.

8. The electrostatic clamp (500, 700) of claims 6 or 7, wherein the first and second dielectric substrates (525, 535) extend closer to each burl than the first and second conductive elements (540a, 740a, 540b, 740b).

9. The electrostatic clamp (500, 700) of any of claims 6 to 8, wherein one of the first and second conductive elements (740a) extends further towards each burl than the other of the first and second conductive elements (740b).

10. The electrostatic clamp (500, 700) of any of claims 4 to 9, wherein at least one of: the dielectric layer (555) comprises a polymer, and optionally the polymer comprises benzocyclobutene; the first and / or second conductive elements (540a, 540b, 740a, 740b) comprise a metal, and optionally the metal comprises chromium; the first and / or second dielectric substrate (525, 535) comprises a glass, and optionally the glass comprises a borosilicate.

11. The electrostatic clamp (500, 700) of any of claims 6 to 10, wherein one of the first and second dielectric substrates (525, 535) is affixed to the surface from which the plurality of burls (515) extend by an adhesive.

12. The electrostatic clamp (500, 700) of any preceding claim, wherein the object is at least one of: a substrate (W) used in lithographic projection techniques; and a lithographic projection reticle or reticle blank in at least one of a lithographic projection apparatus, a reticle handling apparatus, and / or a reticle manufacturing apparatus.

13. A method of use of the electrostatic clamp (500, 700) of claim 12, the method comprising configuring the electrostatic clamp to hold the object by an electrostatic force by biasing the first and second conductive elements (540a, 540b, 740a, 740b) to a same potential.

14. A lithographic apparatus comprising an electrostatic clamp (500, 700) according to any of claims 1 to 13.

15. A method of manufacturing an electrostatic clamp (500, 700) for holding an object by electrostatic force in a lithographic apparatus, the method comprising the steps of: forming a first conductive element (540a, 540a) on a dielectric substrate; forming a second conductive element (540b, 740b) separated from the first conductive element by less than 100 micrometres; and disposing the dielectric substrate such that the second conductive element is disposed between the first conductive element and a plane (595) in which the object is held.

16. An electrostatic clamp (900) for holding an object by electrostatic force, the clamp comprising: a conductive element (940) formed on a dielectric substrate (925) and extending between a plurality of burls defining a plane in which the object is held; wherein an edge portion of the conductive element comprises a curved and / or overhanging section (995).

Images