An electrostatic clamp

Abstract

An electrostatic clamp for clamping a substrate (e.g. a silicon wafer) comprises: a body; at least one central electrode; and at least one peripheral electrode. The body defines a surface and a plurality of protrusions (also known as burls) extend from the surface. The plurality of protrusions comprises a first, central set of protrusions and a second, peripheral set of protrusions. The at least one central electrode is adjacent the first, central set of protrusions and the at least one peripheral electrode is adjacent the second, peripheral set of protrusions. The second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions. That is the outermost burls have a greater height that the central burls. The second, peripheral set of protrusions may have a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to the protrusions.

Description

AN ELECTROSTATIC CLAMPCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167847.3 which was filed on 29 March 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an electrostatic clamp. In particular, the electrostatic clamp arrangement may be configured to clamp an object such as a lithographic substrate. The electrostatic clamp may form part of a lithographic apparatus or lithographic tool. The present invention relates to a corresponding method of clamping a substrate using an electrostatic clamp.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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] 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 extreme ultraviolet (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] A substrate which is being patterned by a lithographic apparatus is held on a substrate table (which may be movable). The substrate table includes a clamp which is configured to securely hold the substrate on the substrate table whilst the substrate is being patterned, and then to allow the substrate to be removed from the substrate table once patterning has been completed. Within EUV lithographic systems such clamps are typically electrical (or electrostatic) clamps. The electrostatic clamp includes an electrode which applies an electrostatic clamping force to the substrate. Current designs of clamp include raised support portions or protrusions, known as burls, which contact the substrate clamped by the clamp arrangement in order to support the substrate. Any deviation in the flatness of the substrate may result in errors in the patterning of the substrate by the lithographic apparatus.

[0006] It may be desirable to provide new electrostatic clamps that at least partially address one or more problems associated with prior art arrangements, whether such problems are disclosed herein or otherwise.SUMMARY

[0007] According to a first aspect of the present disclosure there is provided an electrostatic clamp for clamping a substrate, the electrostatic clamp comprising: a body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions comprises a first, central set of protrusions and a second, peripheral set of protrusions; at least one central electrode adjacent the first, central set of protrusions; and at least one peripheral electrode adjacent the second, peripheral set of protrusions; wherein the second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions.

[0008] The electrostatic clamp according to the first aspect of the present disclosure is advantageous as it can allow for better clamping of a relatively thin substrate (for example a silicon wafer), particularly at the edges (or peripheral portions) of such a substrate, as now discussed.

[0009] Such an electrostatic clamp may, for example, be used to clamp a silicon wafer to a wafer stage within a lithographic apparatus. For such embodiments, the main function of the electrostatic clamp is to support the wafer (during pre-exposure measurements and during exposure) thereby retaining flatness and thermal conditioning. It is known for such clamps to comprise a surface comprising a plurality of protrusions (known as burls), which provide physical contact between the clamp and the wafer and which limit the apparent contact surface between wafer and clamp to approximately 1.5% of the surface area of the wafer. This can limit van der Waals sticking and the probability of wafer backside contamination between the clamp surface and the wafer.

[0010] Any variations in the flatness of the upper surface of a wafer within a lithographic apparatus can result in overlay (an offset between the features formed in two or more sequential lithographic exposures), which is undesirable. Typically larger overlay is experienced near the edge of the wafer as compared to the centre of the wafer. The flatness of this portion will be largely dictated by an intrinsic shape of the wafer and therefore this portion is typically less flat than a central portion of the wafer.

[0011] The inventors of the present application have realized that the intrinsic shape of the wafer may be such that at the edge of the wafer the wafer may be warped down (which may be referred to as an umbrella-shaped wafer) or may be warped up (which may be referred to as a bowl-shaped wafer). Furthermore, the inventors have realized that simply providing increased clamping pressure at the edge (or closer to the edge) of the wafer may not improve wafer flatness for wafers that are intrinsically umbrella- shaped.

[0012] Advantageously, the combination of features of the electrostatic clamp according to the first aspect of the present disclosure allow for the flatness of wafers to be improved regardless of the intrinsic shape of the wafers being clamped, as now discussed.

[0013] The second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions (i.e. the outer burls are taller than the central burls). Therefore, when a wafer is clamped to the electrostatic clamp using the at least one central electrode, the second, peripheral set of protrusions will tend to deform the edge of the wafer up (unless it is very bowl-shaped). Therefore, advantageously, with such a clamping force from the at least one central electrode all wafers (evenumbrella- shaped wafers) can be distorted so as to become either flat or bowl-shaped wafer. Furthermore, since the electrostatic clamp comprises both: (a) at least one central electrode; and (b) at least one peripheral electrode different clamping pressures may be applied to: (a) the (central) portion of the wafer supported by the first, central set of protrusions and (b) the (peripheral) portion of the wafer supported by the second, peripheral set of protrusions. In particular, this allows for the amount of clamping pressure applied to the peripheral or edge portions of the wafer (applied by the at least one peripheral electrode) to be controlled (independently of the main clamping pressure applied by the at least one central electrode). In turn, in combination with the second, peripheral set of protrusions extending farther from the surface than the first set of protrusions (i.e. the outer burls being taller than the central burls), this allows for all types of wafer to be flattened.

[0014] When an intrinsically flat or bowl-shaped wafer is clamped using the at least one central electrode, the larger peripheral protrusions (burls) force the wafer bowl-shaped. A relatively large clamping pressure may be provided by the at least one peripheral electrode to compress the second, peripheral set of protrusions (i.e. the outer-most burls) slightly more than the first, peripheral set of protrusions (i.e. the central burls) so as to flatten the wafer. When an intrinsically umbrella- shaped wafer is clamped using the at least one central electrode, the larger peripheral protrusions (burls) force the wafer to become flat or slightly bowl-shaped. A relatively small clamping pressure may be provided by the at least one peripheral electrode to flatten the edge portion of the wafer.

[0015] The at least one central electrode may, for example, comprise two electrodes. These may be referred to as a first central electrode and a second central electrode. The first and second central electrodes may be independently controlled. In use, during clamping of a wafer, a positive voltage may be applied to the first central electrode and negative voltage may be applied to the second central electrode. The first and second central electrodes may each be generally semi-circular.

[0016] The at least one central electrode may be embedded in the body, below the first, central set of protrusions (burls). Alternatively, the at least one central electrode may be disposed (for example bonded) to the surface of the body from which the first, central set of protrusions (burls) protrude. That is, the first, central set of protrusions (burls) may protrude through the central electrode(s). Alternatively, the at least one central electrode may be disposed (for example bonded) to a surface of the body which is parallel to the surface of the body from which the first, central set of protrusions (burls) protrude.

[0017] The at least one peripheral electrode may be embedded in the body, below the second, peripheral set of protrusions (burls). Alternatively, the at least one peripheral electrode may be disposed (for example bonded) to the surface of the body from which the second, peripheral set of protrusions (burls) protrude. That is, the second, peripheral set of protrusions (burls) may protrude through the peripheral electrode(s). Alternatively, the at least one peripheral electrode may be disposed (for example bonded) to a surface of the body which is parallel to the surface of the body from which the second, peripheral set of protrusions (burls) protrude.

[0018] The second, peripheral set of protrusions may have a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body.

[0019] Advantageously, with such an arrangement, with similar clamping pressure applied to both sets of protrusions, the second, peripheral set of protrusions will be compressed more than the first, central set of protrusions.

[0020] It will be appreciated that the stiffness of an object in a first direction is given by a ratio of an applied force in the first direction to a displacement (or compression) in the first direction caused by the applied force.

[0021] It will be appreciated that a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body is, in use, generally perpendicular to a clamped wafer.

[0022] A stiffness of the second, peripheral set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body may be smaller than a stiffness of the first, central set of protrusions in that direction by at least a factor of 10.

[0023] In some embodiments, a stiffness of the second, peripheral set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body is smaller than a stiffness of the first, central set of protrusions in that direction by at least a factor of 100.

[0024] The second, peripheral set of protrusions may be formed from different material(s) to the first, central set of protrusions.

[0025] By using different material for the first and second sets of protrusions, they can have different properties. For example, the second, peripheral set of protrusions may be formed from, or at least comprise a portion formed from, a material having a reduced stiffness relative to the material(s) from which the first, central set of protrusions are formed.

[0026] The second, peripheral set of protrusions may have a different thickness to the first, central set of protrusions in a direction generally perpendicular to the direction in which the protrusions extend from the surface of the body.

[0027] For example, the second, peripheral set of protrusions may be thinner than the first, central set of protrusions.

[0028] The electrostatic clamp may further comprise a resilient biasing member for the protrusions of the second, peripheral set of protrusions.

[0029] The resilient biasing member may form part of the protrusions. Additionally or alternatively, the resilient biasing member may be provided between the body and (a main portion of) the second, peripheral set of protrusions.

[0030] The resilient biasing member may comprise any form of spring or elastic member. For example, the resilient biasing member may comprise a layer of material that is more flexible than a bulk material of the body.

[0031] A single resilient biasing member may be provided for each of the protrusions of the second set of protrusions. Alternatively, a resilient biasing member may be provided for a plurality of (for example all of) the protrusions of the second set of protrusions.

[0032] The second, peripheral set of protrusions may extend farther from the surface than the first, central set of protrusions by at least 50 nm, preferably by at least 100 nm, further preferably by at least 200 nm.

[0033] The extent of one of the protrusions from the surface may be referred to as the height of that protrusion (burl). That means, the respective distal ends of the second, peripheral set of protrusions are at a height level of at least 50 or 100 or 200 nm above the height level of the respective distal ends of the first, central set of protrusions.

[0034] The first, central set of protrusions may extend from the surface by a distance of the order of 10pm.

[0035] The first, central set of protrusions may define a discontinuous support surface for, in use, supporting a workpiece. Such a workpiece may comprise a substrate or a lithographic wafer.

[0036] The surface defined by the body may be generally planar. Similarly, the (discontinuous) support surface defined by the first, central set of protrusions may be generally planar (and may be generally parallel to the surface defined by the body).

[0037] The electrostatic clamp, or at least some parts thereof, may be generally rotationally symmetric about an axis. The axis may be generally perpendicular to the surface defined by the body.

[0038] The first, central set of protrusions may extend from a generally circular portion of the surface and second, peripheral set of protrusions may extend from a generally annular portion of the surface.

[0039] The generally circular portion of the surface and the generally annular portion of the surface may be concentric and may be centered on a central axis of the electrostatic clamp.

[0040] The at least one central electrode and / or the at least one peripheral electrode may be generally planar.

[0041] The at least one central electrode may be generally circular and at least one peripheral electrode may be generally annular.

[0042] The at least one central electrode and the at least one peripheral electrode may be concentric and may be centered on a central axis of the electrostatic clamp.

[0043] In some embodiments, the at least one central electrode comprises two generally semicircular electrodes. Together these two generally semi-circular electrodes may be considered to be generally circular.

[0044] The body may define a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and a radial extent of the at least one central electrode relative to the central axis may generally match a radial extent of the first, central set of protrusions relative to the central axis.

[0045] In some embodiments, the radial extent of the at least one central electrode relative to the central axis is slightly greater than the radial extent of the first, central set of protrusions relative to the central axis. For example, a distance between a radially outermost portion of the at least one central electrode and the central axis may be greater than a distance between a radially outermost portion of the first, central set of protrusions and the central axis.

[0046] The body may define a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and a radial extent of the at least one peripheral electrode relative to the central axis may generally match a radial extent of the second, peripheral set of protrusions relative to the central axis.

[0047] In some embodiments, the radial extent of the at least one peripheral electrode relative to the central axis is slightly greater than the radial extent of the second, peripheral set of protrusions relative to the central axis. For example, a distance between a radially innermost portion of the at least one peripheral electrode and the central axis may be less than a distance between a radially innermost protrusion of the second set and the central axis. Similarly, a distance between a radially outermost portion of the at least one peripheral electrode and the central axis may be greater than a distance between a radially outermost protrusion of the second set and the central axis.

[0048] The body may comprise a dielectric material. The at least one central electrode and / or the at least one peripheral electrode may be encased in said dielectric material.

[0049] The electrostatic clamp may further comprise a voltage supply operable to supply a voltage to the at least one central electrode and / or the at least one peripheral electrode.

[0050] The electrostatic clamp may further comprise a controller operable to control a voltage applied to the at least one central electrode and / or the at least one peripheral electrode.

[0051] The controller may be operable to independently control a voltage applied to the at least one central electrode and a voltage applied to the at least one peripheral electrode. Advantageously, such a controller can independently control the clamping pressure applied by the at least one central electrode and the at least one peripheral electrode.

[0052] The controller may be operable to vary a voltage applied to the at least one peripheral electrode.

[0053] In some embodiments, the second, peripheral set of protrusions comprises a small number of rows or rings of protrusions (e.g. one or two rows).

[0054] The second, peripheral set of protrusions may comprise a single row or ring of protrusions.

[0055] The body may define a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend. A ratio of: (a) a radial extent of the at least one peripheral electrode relative to the central axis to (b) a radial extent of the at least one central electrode relative to the central axis may be 1: 100 or less.

[0056] For example, the ratio of: (a) the radial extent of the at least one peripheral electrode relative to the central axis to (b) the radial extent of the at least one central electrode relative to the central axismay be of the order of 1:148. For example, in one embodiment the radial extent of the at least one peripheral electrode may be of the order of 1 mm whereas the radial extent of the at least one central electrode may be of the order of 148 mm.

[0057] In some embodiments, the body and the first, central set of protrusions may be monolithic, the body and the first, central set of protrusions comprising a dielectric material.

[0058] The plurality of protrusions may comprise an outer coating.

[0059] The outer coating may be configured, in use, to contact a supported workpiece. The outer coating may be formed of an electrically conductive material. For example, the electrically conductive material may comprise chromium nitride or titanium nitride.

[0060] The electrostatic clamp may be configured to, in use, support a workpiece in the form of a semiconductor wafer or lithographic substrate.

[0061] According to a second aspect of the present disclosure there is provided a lithographic apparatus comprising an electrostatic clamp according to the first aspect of the present disclosure.

[0062] According to a third aspect of the present disclosure there is provided a method of clamping a substrate to an electrostatic clamp according to the first aspect of the present disclosure, the method comprising: placing the substrate onto the plurality of protrusions; providing a first voltage to the at least one central electrode so as to provide a first clamping pressure on a central portion of the substrate; and optionally providing a second voltage to the at least one peripheral electrode so as to provide a second clamping pressure on a peripheral portion of the substrate.

[0063] The inventors of the present application have realized that, due to variations in (a) the intrinsic shape of wafers (whether they are warped or not and, if so, by how much) and (b) a coating provided on the back surface of the wafers (i.e. the surface supported by the burls) it is advantageous to provide an independently controllable electrode (or set of electrodes) for a relatively small portion at the edge of the wafer. For some topologies of wafer the at least one peripheral electrode may be used more and for some topologies the at least one peripheral electrode may be used less. The provision of this additional, independently controllable at least one peripheral electrode may allow for burls to be provided closer to the edge of the wafer, further improving wafer flatness.

[0064] The electrostatic clamp may define a central axis which is perpendicular to a plane of the surface (and parallel to a direction in which the plurality of protrusions extend). Placing the substrate onto the plurality of protrusions may comprise placing the substrate such that a centroid of the substrate lies on the central axis.

[0065] The method may further comprise selecting the second voltage that is applied to the at least one peripheral electrode in dependence on a topology of an upper surface of the substrate once clamped to the electrostatic clamp.

[0066] That is, the second voltage is selected in dependence on the resultant shape of the upper surface of the substrate. For example, the second voltage may be selected so as to optimize a shape of the upper surface of the substrate. For example, the second voltage may be selected so as to minimizevariations of the upper surface of the substrate from a flat surface (or to maximize flatness of the substrate).

[0067] It will be appreciated that the upper surface of the substrate is intended to mean a surface of the substrate opposite a surface of the substrate that is supported by the plurality of protrusions.

[0068] For example, this may be achieved by a feed-forward method. That is, a measurement of the intrinsic shape of a wafer and / or a thickness of a coating provided on the back surface of the wafer (i.e. the surface supported by the burls) may be made. These measurements may then be used to select the second voltage. Alternatively, this may be achieved by a feed-back method. For example, a topology of the upper surface of the wafer may be measured while the wafer is clamped to the electrostatic clamp and the second voltage may be varied until a flatness of the wafer has been optimized.

[0069] The second voltage may be selected in dependence on a difference in the distance that the first and second sets of protrusions extend from the surface of the body.

[0070] The second, peripheral set of protrusions may have a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body. The second voltage may be selected in dependence on a difference in stiffness of the first and second sets of protrusions.

[0071] Features of different aspects of the invention may be combined together, where the person skilled in the art deems it to be appropriate.BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a substrate table according to an embodiment of the present disclosure;Figure 2 shows a schematic cross-sectional view of a portion of a first electrostatic clamp;Figure 3 is a schematic illustration of a new electrostatic clamp for clamping a substrate (for example a silicon wafer) according to an embodiment of the present disclosure;Figure 4A schematically shows an arrangement wherein a wafer is clamped to the electrostatic clamp shown in Figure 3 using only the at least one central electrode and showing that the second, peripheral set of protrusions will tend to deform the edge of the wafer up (unless it is very bowl-shaped);Figure 4B schematically shows the arrangement shown in Figure 4A wherein an amount of clamping pressure is also applied to the peripheral or edge portions of the wafer by the at least one peripheral electrode, which can be controlled independently of the main clamping pressure applied by the at least one central electrode, so as to flatten the wafer;Figure 5A schematically shows an embodiment of the electrostatic clamp of the type shown in Figure 3 wherein the second, peripheral set of protrusions are formed from different material(s) to thefirst, central set of protrusions so that the two different sets of protrusions can have different properties and, in particular, the second, peripheral set of protrusions may have a reduced stiffness relative to the first, central set of protrusions;Figure 5B schematically shows an embodiment of the electrostatic clamp of the type shown in Figure 3 wherein the second, peripheral set of protrusions have a reduced thickness relative to the first, central set of protrusions in a direction generally perpendicular to the direction in which the protrusions extend from the surface of the body so that the second, peripheral set of protrusions may have a reduced stiffness relative to the first, central set of protrusions;Figure 5C schematically shows an embodiment of the electrostatic clamp of the type shown in Figure 3 further comprising a resilient biasing member for the protrusions of the second, peripheral set of protrusions so that the second, peripheral set of protrusions may have a reduced stiffness relative to the first, central set of protrusions;Figure 6 is a schematic representation of the surface of the body of the electrostatic clamp shown in Figure 3 from which the protrusions extend, also showing the central axis, a generally circular region (bounded by a dotted line) corresponding to a footprint of the at least one central electrode and a generally annular region (bounded by two dotted lines) corresponding to a footprint of the at least one peripheral electrode;Figure 7 schematically shows a parameter space spanned by: (i) stiffness of the second, peripheral set of protrusions; and (ii) a height difference, Ah, between the second, peripheral set of protrusions and the first, central set of protrusions; also shown is a region of this parameter space that best allows an umbrella shaped wafer having a given difference Az in height of the peripheral portion of the wafer relative to the center of the wafer to be flattened; andFigure 8 schematically shows a method of clamping a substrate to an electrostatic clamp of the type shown in Figure 3 according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0073] 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, a measurement system MS, and a substrate loading system SL. The lithographic system further comprises two substrate tables WT1, WT2 which are configured to support a substrate W. Each substrate table WT1, WT2 may be according to an embodiment of the present disclosure. The support structure MT may be according to an embodiment of the present disclosure. As explained further below, the substrate clamp may comprise an electrostatic clamp and a Johnsen-Rahbek clamp.

[0074] 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.

[0075] 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 table WT2. 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).

[0076] 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.

[0077] 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 environment of the projection system PS (which may also contain the measurement system MS).

[0078] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0079] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelengthsuch as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0080] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0081] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6 to form an image at the intermediate focus 6 of the plasma 7 present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0082] The measurement system MS of the lithographic apparatus is configured to perform measurements of properties of a substrate W held on a substrate table WT1. The measurement system MS comprises an alignment system, which is configured to measure the positions of alignment marks on the substrate with reference to alignment marks on the substrate table WT1. The measurement system further comprises a level sensor which is configured to measure the topology of the substrate W. These measured properties are used to ensure accuracy when a pattern is subsequently projected onto the substrate W.

[0083] The lithographic apparatus LA is a dual-stage lithographic apparatus. That is, the lithographic apparatus includes two substrate tables WT1, WT2 and is configured such that a substrate W held on one of the substrate tables WT1 is measured by the measurement system MS simultaneously with a pattern being applied to a substrate W held on the other substrate table WT2. For ease of terminology, one of the substrate tables WT1 may be referred to as a first substrate table, and the other substrate table WT2 may be referred to as a second substrate table. Dual-stage lithographic apparatus advantageously provide higher throughput (i.e. exposure of substrates per hour) than single stage lithographic apparatus. Once the substrate W held on the first substrate table WT1 has been measured, and the substrate W held on the second substrate table WT2 has been exposed, the substrate table WT1 supporting the measured substrate W is moved to be beneath the projection system PS. At the same time, the exposed substrate W supported by the second substrate table WT2 is moved to the substrate loading system SL.

[0084] The substrate loading system SL includes a substrate handler (not depicted) which is configured to remove the patterned substrate W from the second substrate table WT2, and then load a new substrate to be patterned onto the second substrate table. Once the substrate has been loaded onto the second substrate table WT2, the measurement system MS is used to measure alignment markpositions and the topology of the substrate. Simultaneously, the substrate W held on the first substrate table WT1 is exposed by the lithographic apparatus LA.

[0085] The above method is repeated many times in order to expose many substrates using the lithographic apparatus LA.

[0086] Each substrate table WT1, WT2 comprises a substrate clamp according to an embodiment of the present disclosure. The substrate clamp is switched off before an exposed substrate W is to be removed from the substrate table, thereby allowing a substrate handler of the substrate loading system to easily remove the substrate W from the substrate table. An unexposed substrate W is then placed on the substrate table WT1 (or WT2), following which the substrate clamp is switched on.

[0087] Figure 2 is a schematic cross-sectional view of a portion of a known electrostatic clamp 20. The electrostatic clamp 20 comprises a body 22, which defines a surface 24 from which a plurality protrusions 26, or raised support burls, extend. Note that Figure 2 is schematic and the sizes and positions of the burls 26 are not to scale. Furthermore, for the sake of clarity a total of 9 burls are shown and it will be appreciated that there may be significantly more burls. However, the total number of burls 26 is not important for understanding the electrostatic clamp 20.

[0088] Distal ends of the support burls 26 define a discontinuous support surface 28. In use, the discontinuous support surface 28 supports a workpiece (in this case substrate 30). In the present case, each of the support burls 28 has a generally cuboid shape in cross-section. The discontinuous support surface 28 is formed by the upper surface (i.e. the surface which receives the workpiece 30) of each of the support burls 26. Depending on the shape of the support burls 26, the shape of the surface of each support burl 26 upon which the substrate is supported in use, and hence the shape of the discontinuous support surface 28, will be different.

[0089] The body 22 comprises a dielectric layer 32. The electrostatic clamp 20 further comprises a generally planar electrode 34 embedded or encased in the dielectric layer 32.

[0090] The electrostatic clamp 20 defines a central axis A which is generally perpendicular to a plane of the electrode 34. The electrostatic clamp 20 is configured to, in use, support the workpiece 30 such that a centroid of the workpiece 30 lies on the central axis A. The centroid of the workpiece 30 may be considered to be the centre of mass of the workpiece 30. It is common that the workpiece in the form of a substrate 30 is generally disc-shaped (e.g. it may comprise a silicon wafer). In this case, the centroid will generally be located at the geometric centre of the disc.

[0091] In an example of a substrate 30 which may be supported by the electrostatic clamp 20, the substrate 30 may include a semiconductor wafer 30a, the underside of which has a dielectric coating 30b.

[0092] The electrostatic clamp 20 may further comprises a raised seal 36 which protrudes from the surface 24 of the body 22 and which is located radially outboard of the radially outermost support burls 26. The seal 36 acts to reduce the potential for contaminants located radially outboard of the supported substrate 30 to ingress between the body 22 and the supported substrate 30. A maximum height of theraised seal 36 in a direction parallel to the central axis A is less than a maximum height of the support burls 26. In one known example the maximum height of the raised seal 36 is 2 pm less than the maximum height of the support burls 26. For example, the maximum height of the support burls may be 10 pm and the maximum height of the raised seal may be 8 pm. The maximum height of the raised seal 36 may be defined as the maximum distance parallel to the central axis A between a portion of the seal 36 and the surface 24 defined by the body 22.

[0093] The height of the support burls 26 may be the maximum distance, parallel to the central axis A, between a portion of the support burls 26 and the surface 24 defined by the body 22.

[0094] The body 22 and the plurality of raised support burls 26 are monolithic. That is to say, the body 22 and the plurality of raised support burls 26 are all formed from the same piece of material. The body 22 and plurality of raised support burls 26 can be formed from a dielectric material.

[0095] The plurality of raised support burls 26 include an outer coating 38 configured, in use, to contact the supported workpiece. In some examples the outer coating 38 is formed of an electrically conductive material. In some examples, the electrically conductive material may be chromium nitride. In other examples, the electrically conductive material may be titanium nitride.

[0096] The seal 36 may also be coated in the same outer coating 38 as the support burls 26. However, given that the seal 36 is not designed to contact the supported substrate 30, the outer coating 38 on the seal is not configured, in use to contact the supported workpiece 30.

[0097] Some embodiments of the present disclosure relate to a new electrostatic clamp for clamping a substrate (for example a silicon wafer). An example of such a new electrostatic clamp 100 is now described with reference to schematic Figures 3 to 6. Figure 3 is a schematic illustration of the new electrostatic clamp 100 in cross section.

[0098] The electrostatic clamp 100 comprises: a body 110 defining a surface 112 and a plurality of protrusions extending from the surface 112. The plurality of protrusions comprises a first, central set of protrusions 120 and a second, peripheral set of protrusions 122. The second, peripheral set of protrusions 122 extend farther from the surface 112 than the first, central set of protrusions 120. That is, an extent 124 of the first, central set of protrusions 120 from the surface 112 is less than an extent 126 of the second, peripheral set of protrusions 122 from the surface 112.

[0099] In some embodiments, the body 110 and the first, central set of protrusions 130 may be monolithic, the body 130 and the first, central set of protrusions 130 comprising a dielectric material.[000100] The electrostatic clamp 100 further comprises: at least one central electrode 130 adjacent the first, central set of protrusions 122; and at least one peripheral electrode 132 adjacent the second, peripheral set of protrusions 124.[000101] In some embodiments, the second, peripheral set of protrusions 122 may extend farther from the surface 112 of the body 110 than the first, central set of protrusions 120 by at least 50 nm or at least 100 nm or at least 200 nm, for example about 100 nm or about 150 nm or about 200 nm or about 250 nm. The extent of one of the protrusions 120, 122 from the surface 112 may be referred to as theheight of that protrusion 120, 122 (burl). The first, central set of protrusions 120 may extend from the surface 112 by a distance of the order of 10 pm.[000102] The first, central set of protrusions 120 may define a discontinuous support surface 128 for, in use, supporting a workpiece. Such a workpiece may comprise a substrate or a lithographic wafer W.[000103] The surface 112 defined by the body 110 may be generally planar. Similarly, the (discontinuous) support surface 128 defined by the first, central set of protrusions 120 may be generally planar (and may be generally parallel to the surface 112 defined by the body 110).[000104] The electrostatic clamp 100, or at least some parts thereof, may be generally rotationally symmetric about an axis B. The axis B may be generally perpendicular to the surface 112 defined by the body 110.[000105] The new electrostatic clamp 100 shown in Figure 3 is advantageous as it can allow for better clamping of a relatively thin substrate (for example a silicon wafer), particularly at the edges (or peripheral portions) of such a substrate, as now discussed.[000106] The electrostatic clamp 100 may be configured to, in use, support a workpiece in the form of a semiconductor wafer W or lithographic substrate. The electrostatic clamp 100 may, for example, be used to clamp a silicon wafer W to a wafer stage WT within a lithographic apparatus LA. For such embodiments, the main function of the electrostatic clamp 100 is to support the wafer W (during preexposure measurements, for example using the measurement system MS, and during exposure, for example using the projection system PS) thereby retaining flatness and thermal conditioning. It is known for such clamps to comprise a surface comprising a plurality of protrusions (known as burls), which provide physical contact between the clamp and the wafer W and which limit the apparent contact surface between wafer and clamp to approximately 1.5% of the surface area of the wafer W. This can limit van der Waals sticking and the probability of wafer backside contamination between the clamp surface and the wafer W.[000107] Any variations in the flatness of the upper surface of a wafer W within a lithographic apparatus LA can result in overlay (an offset between the features formed in two or more sequential lithographic exposures), which is undesirable. Typically larger overlay is experienced near the edge of the wafer W as compared to the centre of the wafer W. One reason for this is that there is typically a radially outer portion of the wafer that is unsupported by burls. Therefore, the flatness of this portion will be largely dictated by an intrinsic shape of the wafer W and therefore this portion is typically less flat than a central portion of the wafer W.[000108] The inventors of the present application have realized that the intrinsic shape of the wafer W may be such that at the edge of the wafer W, the wafer W may be warped down (which may be referred to as an umbrella-shaped wafer W) or may be warped up (which may be referred to as a bowlshaped wafer W). Furthermore, the inventors have realized that simply providing increased clampingpressure at the edge (or closer to the edge) of the wafer may not improve wafer flatness for wafers that are intrinsically umbrella-shaped.[000109] Advantageously, the combination of features of the electrostatic clamp 100 shown schematically in Figure 3 allow for the flatness of wafers W to be improved regardless of the intrinsic shape of the wafers W being clamped, as now discussed.[000110] The second, peripheral set of protrusions 122 extend farther from the surface 112 than the first, central set of protrusions 120 (i.e. the outer burls 122 are taller than the central burls 120). Therefore, when a wafer W is clamped to the electrostatic clamp 100 using the at least one central electrode 130, the second, peripheral set of protrusions 122 will tend to deform the edge of the wafer up (unless it is very bowl-shaped). Such an arrangement, wherein a clamping force is provided only by the at least one central electrode 130, is shown schematically in Figure 4 A. Therefore, advantageously, with such a clamping force from the at least one central electrode 130 all wafers W (even umbrellashaped wafers) can be distorted so as to become either flat or bowl-shaped wafer. Furthermore, since the electrostatic clamp 100 comprises both: (a) at least one central electrode 130; and (b) at least one peripheral electrode 132 different clamping pressures may be applied to: (a) the (central) portion of the wafer supported by the first, central set of protrusions 120 and (b) the (peripheral) portion of the wafer supported by the second, peripheral set of protrusions 122. In particular, this allows for the amount of clamping pressure applied to the peripheral or edge portions of the wafer (applied by the at least one peripheral electrode 132) to be controlled (independently of the main clamping pressure applied by the at least one central electrode 130). In turn, in combination with the second, peripheral set of protrusions 122 extending farther from the surface 112 than the first set of protrusions 120 (i.e. the outer burls 122 being taller than the central burls 120), this allows for all types of wafer W to be flattened.[000111] When an intrinsically flat or bowl-shaped wafer W is clamped using the at least one central electrode 130, the larger peripheral protrusions 122 (burls) force the wafer to be bowl-shaped. A relatively large clamping pressure may be provided by the at least one peripheral electrode 132 to compress the second, peripheral set of protrusions 122 (i.e. the outer-most burls) slightly more than the first, peripheral set of protrusions 120 (i.e. the central burls) so as to flatten the wafer. When an intrinsically umbrella-shaped wafer W is clamped using the at least one central electrode, the larger peripheral protrusions (burls) force the wafer W to become flat or slightly bowl-shaped. A relatively small clamping pressure may be provided by the at least one peripheral electrode 132 to flatten the edge portion of the wafer W.[000112] An arrangement wherein clamping forces are provided by the at least one central electrode 130 and the at least one peripheral electrode 132 so as to flatten the wafer W is shown schematically in Figure 4B.[000113] In some embodiments, the second, peripheral set of protrusions 122 has a smaller stiffness than the first, central set of protrusions 120 in a direction that is generally parallel to a direction in which the protrusions 120, 122 extend from the surface 112 of the body 110. It will be appreciated that thestiffness of an object in a first direction is given by a ratio of an applied force in the first direction to a displacement (or compression) in the first direction caused by the applied force.[000114] Advantageously, with such an arrangement, with similar clamping pressure applied to both sets of protrusions 120, 122, the second, peripheral set of protrusions 122 will be compressed more than the first, central set of protrusions 120.[000115] It will be appreciated that a direction that is generally parallel to a direction in which the protrusions 120, 122 extend from the surface 112 of the body 110 is, in use, generally perpendicular to a clamped wafer W.[000116] In some embodiments, a stiffness of the second, peripheral set of protrusions 122 in a direction that is generally parallel to a direction in which the protrusions 120, 122 extend from the surface 112 of the body 110 is smaller than a stiffness of the first, central set of protrusions 120 in that direction by at least a factor of 10.[000117] In some embodiments, a stiffness of the second, peripheral set of protrusions 122 in a direction that is generally parallel to a direction in which the protrusions 120, 122 extend from the surface 112 of the body 110 is smaller than a stiffness of the first, central set of protrusions 120 in that direction by at least a factor of 100.[000118] This difference in stiffness of the first set of protrusions 120 and the second set of protrusions 122 may be achieved in a number of different ways, as now discussed with reference to Figures 5A to 5C.[000119] In some embodiments, the second, peripheral set of protrusions 122 may be formed from different material(s) to the first, central set of protrusions 120. By using different material for the first and second sets of protrusions 120, 122, the two different sets of protrusions 120, 122 can have different properties. For example, the second, peripheral set of protrusions 122 may be formed from, or at least comprise a portion formed from, a material having a reduced stiffness relative to the material(s) from which the first, central set of protrusions 120 are formed. Such an arrangement is illustrated schematically in Figure 5A, wherein the second, peripheral set of protrusions 122 and the body 110 are not monolithic (as illustrated by a joining line 140).[000120] In some embodiments, the second, peripheral set of protrusions 122 have a different thickness to the first, central set of protrusions 120 in a direction generally perpendicular to the direction in which the protrusions 120, 122 extend from the surface 112 of the body 110. For example, the second, peripheral set of protrusions 122 may be thinner than the first, central set of protrusions 120. Such an arrangement is illustrated schematically in Figure 5B.[000121] In some embodiments, the electrostatic clamp 100 may further comprise a resilient biasing member for the protrusions of the second, peripheral set of protrusions 122.[000122] The resilient biasing member may form part of the protrusions 122. Additionally or alternatively, the resilient biasing member may be provided between the body 110 and (a main portion of) the second, peripheral set of protrusions 122. Such an arrangement is shown schematically in Figure5C, which shows a resilient biasing member 150 provided between the body 110 and (at least a main portion of) the second, peripheral set of protrusions 122.[000123] The resilient biasing member 150 may comprise any form of spring or elastic member. For example, the resilient biasing member 150 may comprise a layer of material that is more flexible than a bulk material of the body 110.[000124] A single resilient biasing member 150 may be provided for each of the protrusions of the second set of protrusions 122. Alternatively, a resilient biasing member 150 may be provided for a plurality of (for example all of) the protrusions of the second set of protrusions 122.[000125] In some embodiments, a low stiffness of the second, peripheral set of protrusions 122 may be achieved by creating cavities below the second, peripheral set of protrusions 122. In an embodiment, such cavities may be provided below the peripheral electrode.[000126] The at least one central electrode 130 may, for example, comprise two electrodes. These may be referred to as a first central electrode and a second central electrode. The first and second central electrodes may be independently controlled. In use, during clamping of a wafer W, a positive voltage may be applied to the first central electrode and negative voltage may be applied to the second central electrode. The first and second central electrodes may each be generally semi-circular.[000127] The body 110 may comprise a dielectric material. The at least one central electrode 130 and / or the at least one peripheral electrode 132 may be encased in said dielectric material.[000128] The at least one central electrode 130 may be embedded in the body 110, below the first, central set of protrusions 120 (burls). Alternatively, the at least one central electrode 130 may be disposed (for example bonded) to the surface 112 of the body 110 from which the first, central set of protrusions 120 (burls) protrude. That is, the first, central set of protrusions 120 (burls) may protrude through the central electrode(s) 130. Alternatively, the at least one central electrode 130 may be disposed (for example bonded to) a surface 114 of the body 110 which is parallel to the surface 112 of the body 110 from which the first, central set of protrusions 120 (burls) protrude.[000129] The at least one peripheral electrode 132 may be embedded in the body 110, below the second, peripheral set of protrusions 122 (burls). Alternatively, the at least one peripheral electrode 132 may be disposed (for example bonded) to the surface 112 of the body 110 from which the second, peripheral set of protrusions 122 (burls) protrude. That is, the second, peripheral set of protrusions 122 (burls) may protrude through the peripheral electrode(s) 132. Alternatively, the at least one peripheral electrode 132 may be disposed (for example bonded) to a surface 114 of the body 110 which is parallel to the surface 112 of the body 110 from which the second, peripheral set of protrusions 122 (burls) protrude.[000130] In some embodiments, the first, central set of protrusions 120 extend from a generally circular portion of the surface 112 and second, peripheral set of protrusions 122 extend from a generally annular portion of the surface 112. The generally circular portion of the surface 112 and the generallyannular portion of the surface 112 may be concentric and may be centered on a central axis B of the electrostatic clamp 100.[000131] The at least one central electrode 130 and / or the at least one peripheral electrode 132 may be generally planar.[000132] In some embodiments, the at least one central electrode 130 is generally circular and at least one peripheral electrode is generally annular 132. The at least one central electrode 130 and the at least one peripheral electrode 132 may be concentric and may be centered on a central axis B of the electrostatic clamp 100. In some embodiments, the at least one central electrode 130 comprises two generally semi-circular electrodes. Together these two generally semi-circular electrodes may be considered to be generally circular.[000133] The body 110 may define a central axis B which is generally perpendicular to the surface 112 from which the plurality of protrusions 120, 122 extend. In some embodiments, a radial extent 134 of the at least one central electrode 130 relative to the central axis B generally matches a radial extent of the first, central set of protrusions 120 relative to the central axis B.[000134] As shown schematically in Figure 3, in some embodiments, the radial extent 134 of the at least one central electrode 130 relative to the central axis B is slightly greater than the radial extent of the first, central set of protrusions 120 relative to the central axis B. For example, a distance 134 between a radially outermost portion of the at least one central electrode 130 and the central axis B may be greater than a distance between a radially outermost portion of the first, central set of protrusions 120 and the central axis B.[000135] In some embodiments, a radial extent 136 of the at least one peripheral electrode 132 relative to the central axis B generally matches a radial extent of the second, peripheral set of protrusions 122 relative to the central axis B.[000136] A radial extent 136 of the at least one peripheral electrode 132 may be a distance 136 between a radially outermost portion of the at least one peripheral electrode 132 and a radially innermost portion of the at least one peripheral electrode 132. As shown schematically in Figure 3, in some embodiments the radial extent 136 of the at least one peripheral electrode 132 relative to the central axis B is slightly greater than the radial extent of the second, peripheral set of protrusions 122 relative to the central axis B. For example, a distance between a radially innermost portion of the at least one peripheral electrode 132 and the central axis B may be less than a distance between a radially innermost protrusion of the second set 122 and the central axis B. Similarly, a distance between a radially outermost portion of the at least one peripheral electrode 132 and the central axis B may be greater than a distance between a radially outermost protrusion of the second set 122 and the central axis B.[000137] Optionally, the electrostatic clamp 100 may further comprise a voltage supply 160 operable to supply a voltage to the at least one central electrode 130 and / or the at least one peripheral electrode 132.[000138] Optionally, the electrostatic clamp 100 may further comprise a controller 170 operable to control a voltage applied to the at least one central electrode 130 and / or the at least one peripheral electrode 132.[000139] The controller 170 may be operable to independently control a voltage applied to the at least one central electrode 130 and a voltage applied to the at least one peripheral electrode 132. Advantageously, such a controller 170 can independently control the clamping pressure applied by the at least one central electrode 130 and the at least one peripheral electrode 132.[000140] In some embodiments, the controller 170 may be operable to vary a voltage applied to the at least one peripheral electrode 132.[000141] In some embodiments, the second, peripheral set of protrusions 122 comprises a small number of rows or rings of protrusions (e.g. one or two rows). In some embodiments, the second, peripheral set of protrusions 122 comprises a single row or ring of protrusions.[000142] It will be appreciated that although eleven protrusions 120, 122 are shown in Figure 3, in practice, the electrostatic clamp 100 may comprise any number of protrusions 120, 122. In some embodiments, the electrostatic clamp 100 may comprise of the order of 10,000 protrusions 120, 122.[000143] Figure 6 is a schematic representation of the surface 112 of the body 110 from which the protrusions 120, 122 extend. Also shown in Figure 112 is the central axis B, a generally circular region 112a (bounded by a dotted line) and a generally annular region 112b (bounded by two dotted lines). The generally circular region 112a corresponds to a footprint of the at least one central electrode 130. The generally annular region 112b corresponds to a footprint of the at least one peripheral electrode 132. The radial extent 134 of the at least one central electrode 130 and the radial extent 136 of the at least one peripheral electrode 132 are also shown in Figure 6.[000144] It will be appreciated that the first, central set of protrusions 120 may be provided within the circular region 112a of the surface 112 and that the second, peripheral set of protrusions 122 may be provided within the annular region 122b of the surface 112.[000145] In some embodiments, a ratio of: (a) a radial extent 136 of the at least one peripheral electrode 132 relative to the central axis B to (b) a radial extent 134 of the at least one central electrode 130 relative to the central axis may be 1 : 100 or less. For example, the ratio of: (a) the radial extent 136 of the at least one peripheral electrode 132 relative to the central axis B to (b) the radial extent 134 of the at least one central electrode 130 relative to the central axis B may be of the order of 1:148. For example, in one embodiment the radial extent 136 of the at least one peripheral electrode 132 may be of the order of 1 mm whereas the radial extent 134 of the at least one central electrode 130 may be of the order of 148 mm.[000146] Optionally, in some embodiments the plurality of protrusions 120, 122 may comprise an outer coating 180. The outer coating 180 may be configured, in use, to contact a supported workpiece. The outer coating 180 may be formed at least on a distal end of the plurality of protrusions 120, 122.The outer coating 180 may be formed of an electrically conductive material. For example, the electrically conductive material may comprise chromium nitride or titanium nitride.[000147] In general, how well the electrostatic clamp 100 can flatten wafers W may depend on: (a) a difference Ah in the heights 126, 124 of the second, peripheral set of protrusions 122 and the first, central set of protrusions 120; (b) the stiffness of the first and second sets of protrusions 120, 122; and (c) an expected range of differences Az in the height of the peripheral portion of the each wafer W relative to the center of that wafer W. Put differently, given an expected range of differences Az in the height of the peripheral portion of each wafer W relative to the center of that wafer W, how well the electrostatic clamp 100 can flatten wafers W may depend on: (a) the difference Ah in the heights 126, 124 of the second, peripheral set of protrusions 122 and the first, central set of protrusions 120; and (b) the stiffness of the first and second sets of protrusions 120, 122.[000148] Various combinations of these parameters have been studied to assess the feasibility of such a new electrostatic clamp 100, as now discussed.[000149] An initial investigation was made into which region of a parameter space, spanned by: (i) stiffness of the second, peripheral set of protrusions 122; and (ii) the height difference Ah, the electrostatic clamp 100 of the type shown in Figure 3 could flatten an umbrella shaped wafer having a difference Az in height of the peripheral portion of the each wafer W relative to the center of the wafer W of 100 pm. The results of this study are now discussed with reference to Figure 7, which shows this parameter space. The following steps were performed.[000150] First, simulations were performed to check, for each combination of stiffness and height difference Ah, to which extent the wafer W can be pushed into a flat or bowl shape only using the at least one central electrode 130. If the second, peripheral set of protrusions 122 have a relatively low stiffness or the height difference is relatively small then this may be more difficult. This region of the parameter space (with such relatively low stiffness or such relatively small height difference for the wafer W to be pushed into a flat or bowl shape only using the at least one central electrode 130) is labelled 180.[000151] Second, simulations were performed to check, for each combination of stiffness and height difference, Ah, to which extent when switching on the at least one peripheral electrode 132 with the same clamping pressure as the at least one central electrode 130, the bowl shape can be removed (i.e. the wafer can be flattened). If the second, peripheral set of protrusions 122 are relatively stiff or the height difference is relatively large then this will be more difficult. This region of the parameter space (with a relatively high stiffness of the second, peripheral set of protrusions 122 or a relatively large height difference) is labelled 182.[000152] The remaining region 186 of the parameter space corresponds to combinations of parameters that would best allow the electrostatic clamp 100 to flatten the umbrella shaped wafer with a difference Az in height between the peripheral and central portions of the wafer of 100 pm when theat least one peripheral electrode 132 applies some clamping pressure. For some parts of this region 186 of parameter space, using the at least one peripheral electrode 132 to apply the same clamping pressure as the at least one central electrode 130 may result in a relatively high pressure being applied. To optimize performance in such situations, a voltage of the at least one peripheral electrode 132 can be reduced to reduce the clamping pressure applied by the at least one peripheral electrode 132.[000153] Typically, a side of a semiconductor wafer that contacts the electrostatic clamp may be provided with a coating (for example a dielectric coating). This coating may be referred as a wafer backside coating. In order to ensure flatness of the wafer once clamped, it is generally desirable for a thickness of the coating to be uniform. However, in practice, it is difficult to ensure a uniform thickness of this coating at the edge of the wafer and typically the thickness of the wafer backside coating will be reduced at the edge of the wafer.[000154] Note that for the simulations performed to produce the results shown in Figure 7, it was assumed that wafer backside coating thickness is constant across the entire wafer radius. However, in practice it may be that the backside coating thickness may decrease at the peripheral portion of the wafer (that will be supported by the second, peripheral set of protrusions 122) by a non-zero amount. Therefore, in practice, it would be preferable to choose a region of parameter space with a height difference that is greater than a maximum expected reduction in backside coating thickness. This maximum expected reduction in backside coating thickness is represented in Figure 7 by line 188. It may therefore be preferable to choose a portion of the allowable region 186 of the parameter space with a height difference that is greater than this maximum expected reduction in backside coating thickness (i.e. a portion of region 186 that it on the right-hand side of line 188).[000155] The type of study described above with reference to Figure 7 has been repeated for a range of different wafer warpages. By selecting parameters that fall within an overlap between all of the allowable regions 186 of parameter space from such studies, the electrostatic clamp 100 shown in Figure 3 can to the largest extent flatten wafers with all of the studied warpages.[000156] It may be that within this region of parameter space (that is suitable for the range of different wafer warpages) a suitable stiffness can be selected and that a range of different height differences can be used with the selected stiffness. If so, it is proposed that the largest allowable height difference be selected since this allows the electrostatic clamp to flatten the largest reduction in wafer backside coating thickness at the edge of the wafer.[000157] The reduction in wafer backside coating thickness at the edge of the wafer has typically meant that there is a limit to the radial position of the protrusions (burls) in existing electrostatic clamps. However, advantageously, since the second, peripheral set of protrusions 122 extend farther from the surface 112 than the first, central set of protrusions 120, the new electrostatic clamp 100 shown in Figure 3 allows for the protrusions to be provided closer to the edge of the wafer W.[000158] Note that in some embodiments of the new electrostatic clamp 100, the first, central set of protrusions 120 may be provided on a region of the surface 112 corresponding to the position ofprotrusions in existing electrostatic clamps. Furthermore, in some embodiments, the second, peripheral set of protrusions 122 may be provided on a region of the surface 112 that is radially outboard of the positions of protrusions in existing electrostatic clamps.[000159] Some embodiments of the present disclosure relate to lithographic apparatus comprising a new electrostatic clamp 100 for clamping a substrate (for example a silicon wafer) of the type shown in Figure 3 and described above. Such a lithographic apparatus may, for example, be generally of the form of the lithographic apparatus LA shown in Figure 1 and may have any of the features described above with reference to Figure 1. The electrostatic clamp 100 may, for example, form part of one or more substrate tables WT1, WT2 of the lithographic apparatus.[000160] Some embodiments of the present disclosure relate to a method of clamping a substrate W to an electrostatic clamp 100 of the type shown in Figure 3 and described above. An example of such a method 200 is shown schematically in Figure 8.[000161] The method 200 comprises a step 210 of placing the substrate W onto the plurality of protrusions 120, 122.[000162] The method 200 further comprises a step 220 of providing a first voltage to the at least one central electrode 130 so as to provide a first clamping pressure on a central portion of the substrate W. [000163] The method 200 optionally further comprises a step 230 of providing a second voltage to the at least one peripheral electrode 132 so as to provide a second clamping pressure on a peripheral portion of the substrate W.[000164] The inventors of the present application have realized that, due to variations in (a) the intrinsic shape of wafers W (whether they are warped or not and, if so, by how much) and (b) a coating provided on the back surface of the wafers (i.e. the surface supported by the burls 120, 122) it is advantageous to provide an independently controllable electrode 132 (or set of electrodes) for a relatively small portion at the edge of the wafer W. For some topologies of wafer W the at least one peripheral electrode 132 may be used more and for some topologies the at least one peripheral electrode 132 may be used less. The provision of this additional, independently controllable at least one peripheral electrode 132 may allow for burls 122 to be provided closer to the edge of the wafer W, further improving wafer flatness.[000165] The electrostatic clamp 100 may define a central axis B which is perpendicular to a plane of the surface 112 (and parallel to a direction in which the plurality of protrusions 120, 122 extend). Placing the substrate W onto the plurality of protrusions 120, 122 (i.e. step 210) may comprise placing the substrate W such that a centroid of the substrate W lies on the central axis B.[000166] The method 200 may further comprise a step 240 of selecting the second voltage that is applied to the at least one peripheral electrode 132 in dependence on a topology of an upper surface of the substrate W once clamped to the electrostatic clamp 100.[000167] That is, the second voltage may be selected in dependence on the resultant shape of the upper surface of the substrate W. For example, the second voltage may be selected so as to optimize ashape of the upper surface of the substrate W. For example, the second voltage may be selected so as to minimize variations of the upper surface of the substrate W from a flat surface (or to maximize flatness of the substrate).[000168] It will be appreciated that the upper surface of the substrate W is intended to mean a surface of the substrate W opposite a surface of the substrate W that is supported by the plurality of protrusions 120, 122.[000169] For example, this may be achieved by a feed-forward method. That is, a measurement of the intrinsic shape of a wafer W and / or a thickness of a coating provided on the back surface of the wafer W (i.e. the surface supported by the burls 120, 122) may be made. These measurements may then be used to select the second voltage. Alternatively, this may be achieved by a feed-back method. For example, a topology of the upper surface of the wafer W may be measured while the wafer W is clamped to the electrostatic clamp 100 and the second voltage may be varied until a flatness of the wafer has been optimized.[000170] In some embodiments, the second voltage may be selected in dependence on a difference Ah in the distance 124, 126 that the first and second sets of protrusions 120, 122 extend from the surface 112 of the body 110.[000171] In some embodiments, the second, peripheral set of protrusions 122 may have a smaller stiffness than the first, central set of protrusions 120 in a direction that is generally parallel to a direction in which the protrusions 120, 122 extend from the surface 112 of the body 110. The second voltage may be selected (at step 230) in dependence on a difference in stiffness of the first and second sets of protrusions 120, 122.[000172] 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.[000173] Although specific reference has been 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 apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions. A substrate clamp according to an embodiment of the invention may form part of a lithographic tool.[000174] 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 thatmodifications may be made to the invention as described without departing from the scope of the claims set out below.[000175] A number of aspects of the present disclosure are defined in the following numbered paragraphs:1. An electrostatic clamp for clamping a substrate, the electrostatic clamp comprising: a body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions comprises a first, central set of protrusions and a second, peripheral set of protrusions; at least one central electrode adjacent the first, central set of protrusions; and at least one peripheral electrode adjacent the second, peripheral set of protrusions; wherein the second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions.2. The electrostatic clamp of paragraph 1 wherein the second, peripheral set of protrusions has a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body.3. The electrostatic clamp of paragraph 2 wherein a stiffness of the second, peripheral set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body is smaller than a stiffness of the first, central set of protrusions in that direction by at least a factor of 10.4. The electrostatic clamp of any preceding paragraph wherein the second, peripheral set of protrusions are formed from different material(s) to the first, central set of protrusions.5. The electrostatic clamp of any preceding paragraph wherein the second, peripheral set of protrusions have a different thickness to the first, central set of protrusions in a direction generally perpendicular to the direction in which the protrusions extend from the surface of the body.6. The electrostatic clamp of any preceding paragraph further comprising a resilient biasing member for the protrusions of the second, peripheral set of protrusions.7. The electrostatic clamp of any preceding paragraph wherein the second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions by at least 50 nm, preferably by at least 100 nm, further preferably by at least 200 nm; or by a value in the range of 50 to 250 nm, for example; or for example by about 100 nm or about 150 nm or about 200 nm or about 250 nm.8. The electrostatic clamp of any preceding paragraph wherein the first, central set of protrusions extend from a generally circular portion of the surface and second, peripheral set of protrusions extend from a generally annular portion of the surface.9. The electrostatic clamp of any preceding paragraph wherein the at least one central electrode is generally circular and at least one peripheral electrode is generally annular.10. The electrostatic clamp of any preceding paragraph wherein the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend andwherein a radial extent of the at least one central electrode relative to the central axis generally matches a radial extent of the first, central set of protrusions relative to the central axis.11. The electrostatic clamp of any preceding paragraph wherein the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and wherein a radial extent of the at least one peripheral electrode relative to the central axis generally matches a radial extent of the second, peripheral set of protrusions relative to the central axis.12. The electrostatic clamp of any preceding paragraph further comprising a voltage supply operable to supply a voltage to the at least one central electrode and / or the at least one peripheral electrode.13. The electrostatic clamp of any preceding paragraph further comprising a controller operable to control a voltage applied to the at least one central electrode and / or the at least one peripheral electrode.14. The electrostatic clamp of paragraph 13 wherein the controller is operable to vary a voltage applied to the at least one peripheral electrode.15. The electrostatic clamp of any preceding paragraph wherein the second, peripheral set of protrusions comprises a single row or ring of protrusions.16. The electrostatic clamp of any preceding paragraph wherein the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and wherein a ratio of: (a) a radial extent of the at least one peripheral electrode relative to the central axis to (b) a radial extent of the at least one central electrode relative to the central axis is 1 : 100 or less.17. The electrostatic clamp of any preceding paragraph wherein the plurality of protrusions comprise an outer coating.18. A lithographic apparatus comprising an electrostatic clamp according any preceding paragraph.19. A method of clamping a substrate to an electrostatic clamp according to any one of paragraphs 1 to 17, the method comprising: placing the substrate onto the plurality of protrusions; providing a first voltage to the at least one central electrode so as to provide a first clamping pressure on a central portion of the substrate; and optionally providing a second voltage to the at least one peripheral electrode so as to provide a second clamping pressure on a peripheral portion of the substrate.20. The method of paragraph 19 further comprising selecting the second voltage that is applied to the at least one peripheral electrode in dependence on a topology of an upper surface of the substrate once clamped to the electrostatic clamp.21. The method of paragraph 20 wherein the second voltage is selected in dependence on a difference in the distance that the first and second sets of protrusions extend from the surface of the body.22. The method of paragraph 21 wherein the second, peripheral set of protrusions has a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to a direction inwhich the protrusions extend from the surface of the body; and wherein the second voltage is selected in dependence on a difference in stiffness of the first and second sets of protrusions.

Claims

CLAIMS1. An electrostatic clamp for clamping a substrate, the electrostatic clamp comprising: a body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions comprises a first, central set of protrusions and a second, peripheral set of protrusions; at least one central electrode adjacent the first, central set of protrusions; and at least one peripheral electrode adjacent the second, peripheral set of protrusions; wherein the second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions.

2. The electrostatic clamp of claim 1 wherein the second, peripheral set of protrusions has a smaller stiffness than the first, central set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body.

3. The electrostatic clamp of claim 2 wherein a stiffness of the second, peripheral set of protrusions in a direction that is generally parallel to a direction in which the protrusions extend from the surface of the body is smaller than a stiffness of the first, central set of protrusions in that direction by at least a factor of 10.

4. The electrostatic clamp of any preceding claim wherein the second, peripheral set of protrusions are formed from different material(s) to the first, central set of protrusions.

5. The electrostatic clamp of any preceding claim wherein: the second, peripheral set of protrusions have a different thickness to the first, central set of protrusions in a direction generally perpendicular to the direction in which the protrusions extend from the surface of the body; and / or the electrostatic clamp further comprises a resilient biasing member for the protrusions of the second, peripheral set of protrusions.

6. The electrostatic clamp of any preceding claim wherein the second, peripheral set of protrusions extend farther from the surface than the first, central set of protrusions by at least 50 nm, preferably by at least 100 nm, further preferably by at least 200 nm.

7. The electrostatic clamp of any preceding claim wherein the first, central set of protrusions extend from a generally circular portion of the surface and second, peripheral set of protrusions extend from a generally annular portion of the surface.

8. The electrostatic clamp of any preceding claim wherein the at least one central electrode is generally circular and at least one peripheral electrode is generally annular.

9. The electrostatic clamp of any preceding claim wherein: the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and wherein a radial extent of the at least one central electrode relative to the central axis generally matches a radial extent of the first, central set of protrusions relative to the central axis; and / or the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and wherein a radial extent of the at least one peripheral electrode relative to the central axis generally matches a radial extent of the second, peripheral set of protrusions relative to the central axis.

10. The electrostatic clamp of any preceding claim further comprising a controller operable to control a voltage applied to the at least one central electrode and / or the at least one peripheral electrode, wherein optionally the controller is operable to vary a voltage applied to the at least one peripheral electrode.

11. The electrostatic clamp of any preceding claim wherein the second, peripheral set of protrusions comprises a single row or ring of protrusions.

12. The electrostatic clamp of any preceding claim wherein the body defines a central axis which is generally perpendicular to the surface from which the plurality of protrusions extend and wherein a ratio of: (a) a radial extent of the at least one peripheral electrode relative to the central axis to (b) a radial extent of the at least one central electrode relative to the central axis is 1: 100 or less.

13. A lithographic apparatus comprising an electrostatic clamp according any preceding claim.

14. A method of clamping a substrate to an electrostatic clamp according to any one of claims 1 to 12, the method comprising: placing the substrate onto the plurality of protrusions; providing a first voltage to the at least one central electrode so as to provide a first clamping pressure on a central portion of the substrate; and optionally providing a second voltage to the at least one peripheral electrode so as to provide a second clamping pressure on a peripheral portion of the substrate.

15. The method of claim 14 further comprising selecting the second voltage that is applied to the at least one peripheral electrode in dependence on a topology of an upper surface of the substrate once clamped to the electrostatic clamp.

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