Electrostatic clamp

Abstract

An electrostatic clamp for holding a substrate (e.g., a silicon wafer) includes: a body; at least one central electrode; and at least one peripheral electrode. The body defines a surface and a plurality of protrusions (also referred to as nodes) extending from the surface. The plurality of protrusions includes a first set of central protrusions and a second set of peripheral protrusions. The at least one central electrode is adjacent to the first set of central protrusions, and the at least one peripheral electrode is adjacent to the second set of peripheral protrusions. The second set of peripheral protrusions extends further away from the surface than the first set of central protrusions. That is, the outermost protrusions have a greater height than the central protrusions. In a direction generally parallel to the protrusions, the second set of peripheral protrusions may have less stiffness than the first set of central protrusions.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to European application 24167847.3, filed on 29 March 2024, and that European application is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to electrostatic clamps. Specifically, the arrangement of electrostatic clamps can be configured to hold objects such as photolithography substrates. The electrostatic clamps can be part of a photolithography apparatus or tooling. This invention also relates to a corresponding method of using electrostatic clamps to hold a substrate. Background Technology

[0004] A photolithography apparatus is a machine configured to apply a desired pattern onto a substrate. Photolithography apparatus can be used, for example, to manufacture integrated circuits (ICs). It can project a pattern from a patterning apparatus (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern onto a substrate, a photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to photolithography apparatus that uses radiation with a wavelength of, for example, 193 nm, photolithography apparatus that uses extreme ultraviolet (EUV) radiation in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] A substrate being patterned by a photolithography apparatus is held on a substrate stage (which may be movable). The substrate stage includes a clamp configured to securely hold the substrate on the stage while it is being patterned and then allow removal of the substrate from the stage once patterning is complete. In EUV lithography systems, such clamps are typically electrical clamps (or electrostatic clamps). Electrostatic clamps include electrodes that apply electrostatic clamping forces to the substrate. Current designs of clamps include raised support portions or protrusions (referred to as tabs) that contact the substrate held by the clamping device to support it. Any deviation in the flatness of the substrate can lead to errors in the patterning of the substrate by the photolithography apparatus.

[0007] It may be desirable to provide novel electrostatic clamps that at least partially address one or more problems associated with arrangements in the prior art, whether such problems are disclosed herein or otherwise. Summary of the Invention

[0008] According to a first aspect of this disclosure, an electrostatic clamp for holding a substrate is provided, the electrostatic clamp comprising: a body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions includes a first set of central protrusions and a second set of peripheral protrusions; at least one central electrode adjacent to the first set of central protrusions; and at least one peripheral electrode adjacent to the second set of peripheral protrusions; wherein the second set of peripheral protrusions extends further away from the surface than the first set of central protrusions.

[0009] The electrostatic clamp according to the first aspect of this disclosure is advantageous because it allows for better clamping of relatively thin substrates (e.g., silicon wafers), particularly at the edges (or peripheral portions) of such substrates, as now discussed.

[0010] Such electrostatic chucks can be used, for example, to hold silicon wafers onto a wafer stage within a photolithography apparatus. In such embodiments, the primary function of the electrostatic chuck is to support the wafer (during pre-exposure measurements and during exposure) to maintain flatness and thermal conditioning. Known for such chucks are surfaces comprising multiple protrusions (referred to as protrusions) that provide physical contact between the chuck and the wafer, limiting the apparent contact surface between the wafer and the chuck to approximately 1.5% of the wafer's surface area. This limits the possibility of van der Waals adhesion and back-side contamination between the chuck surface and the wafer.

[0011] Any variation in the flatness of the upper surface of a wafer within a photolithography apparatus can lead to overlap errors (misalignment between features formed in two or more sequential photolithography exposures), which are undesirable. Overlap errors are typically more pronounced near the edges of the wafer compared to the center. The flatness of this portion is largely determined by the inherent shape of the wafer, and therefore this portion is generally less flat than the central portion of the wafer.

[0012] The inventors of this application have recognized that the inherent shape of a wafer may cause it to warp downwards (which may be referred to as an umbrella-shaped wafer) or upwards (which may be referred to as a bowl-shaped wafer) at its edges. Furthermore, the inventors have recognized 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 inherently umbrella-shaped.

[0013] Advantageously, the combination of features of the electrostatic clamp according to the first aspect of this disclosure allows for improved wafer flatness, regardless of the inherent shape of the clamped wafer, as now discussed.

[0014] The second set of peripheral protrusions extends further away from the surface than the first set of central protrusions (i.e., the outer protrusions are higher than the central protrusions). Therefore, when the wafer is clamped to the electrostatic jig using at least one central electrode, the second set of peripheral protrusions will tend to deform the wafer's edges upwards (unless it is completely bowl-shaped). Thus, advantageously, using this clamping force from at least one central electrode, all wafers (even umbrella-shaped wafers) can be reshaped to become flat or bowl-shaped wafers. Furthermore, since the electrostatic jig includes both (a) at least one central electrode and (b) at least one peripheral electrode, different clamping pressures can be applied to: (a) the (central) portion of the wafer supported by the first set of central protrusions, and (b) the (peripheral) portion of the wafer supported by the second set of peripheral protrusions. In particular, this allows control (independent of the main clamping pressure applied by at least one central electrode) of the amount of clamping pressure applied to the peripheral or edge portion of the wafer (which is applied by at least one peripheral electrode). Furthermore, the combination of the second set of peripheral protrusions extending further away from the surface than the first set of central protrusions (i.e., the outer protrusions are higher than the central protrusions) allows all types of wafers to be planarized.

[0015] When a wafer with an inherently flat or bowl-shaped structure is held using at least one central electrode, the larger peripheral protrusions (protrusions) force the wafer into a bowl shape. A relatively large clamping pressure can be provided by at least one peripheral electrode to compress the second set of peripheral protrusions (i.e., the outermost protrusions) slightly more than the compression of the first set of peripheral protrusions (i.e., the central protrusions) in order to planarize the wafer. When a wafer with an inherently umbrella shape is held using at least one central electrode, the larger peripheral protrusions (protrusions) force the wafer to become flat or slightly bowl-shaped. A relatively small clamping pressure can be provided by at least one peripheral electrode to planarize the edge portions of the wafer.

[0016] At least one central electrode may comprise, for example, two electrodes. These electrodes may be referred to as a first central electrode and a second central electrode. The first central electrode and the second central electrode may be controlled independently. In use, during wafer clamping, a positive voltage may be applied to the first central electrode, and a negative voltage may be applied to the second central electrode. The first central electrode and the second central electrode may each be approximately semi-circular.

[0017] At least one central electrode may be embedded in the body below the first set of central protrusions (protrusions). Alternatively, at least one central electrode may be disposed (e.g., bonded) to a surface of the body from which the first set of central protrusions (protrusions) protrude. That is, the first set of central protrusions (protrusions) may protrude through (or be more than one) the central electrode(s). Alternatively, at least one central electrode may be disposed (e.g., bonded) to a surface of the body parallel to the surface of the body from which the first set of central protrusions (protrusions) protrude.

[0018] At least one peripheral electrode may be embedded in the body below the second set of peripheral protrusions (protrusions). Alternatively, at least one peripheral electrode may be disposed (e.g., bonded) to a surface of the body from which the second set of peripheral protrusions (protrusions) protrude. That is, the second set of peripheral protrusions (protrusions) may protrude through (or be more than one) of the peripheral electrodes. Alternatively, at least one peripheral electrode may be disposed (e.g., bonded) to a surface of the body parallel to the surface of the body from which the second set of peripheral protrusions (protrusions) protrude.

[0019] In a direction roughly parallel to the direction along which the protrusions extend from the surface of the body, the second group of peripheral protrusions has less stiffness than the first group of central protrusions.

[0020] Advantageously, with this arrangement, when similar clamping pressure is applied to the two sets of protrusions, the second set of peripheral protrusions will be compressed more than the first set of central protrusions.

[0021] It will be understood that the stiffness of an object in the first direction is given by the ratio of the force applied in the first direction to the displacement (or compression) in the first direction caused by the applied force.

[0022] It will be understood that the direction which is roughly parallel to the direction along which the protrusion extends from the surface of the body is roughly perpendicular to the wafer being held during use.

[0023] The stiffness of the second set of peripheral protrusions in a direction roughly parallel to the direction in which the protrusions extend from the surface of the body is less than the stiffness of the first set of central protrusions in that direction, and is at most 1 / 10 of the stiffness of the first set of central protrusions in that direction.

[0024] In some embodiments, the stiffness of the second set of peripheral protrusions in a direction substantially parallel to the direction in which the protrusions extend from the surface of the body is less than the stiffness of the first set of central protrusions in that direction and is at most 1 / 100 of the stiffness of the first set of central protrusions in that direction.

[0025] The second set of peripheral protrusions may be formed of one or more materials that are different from the central protrusions of the first set.

[0026] By using different materials for the first group of protrusions and the second group of protrusions, they can have different properties. For example, the second group of peripheral protrusions can be formed of a material with reduced stiffness relative to one or more materials used to form the first group of central protrusions, or at least include a portion formed of such a material with reduced stiffness.

[0027] The second set of peripheral protrusions may have a different thickness than the first set of central protrusions in a direction that is approximately perpendicular to the direction in which the protrusions extend from the surface of the body.

[0028] For example, the peripheral protrusions of the second group can be thinner than the central protrusions of the first group.

[0029] The electrostatic clamp may also include an elastic biasing member for the protrusions in the second set of peripheral protrusions.

[0030] The resilient biasing member may form part of the protrusion. Alternatively or additionally, the resilient biasing member may be disposed between the body and the second set of peripheral protrusions (the main body portion).

[0031] The resilient biasing member can include any form of spring or elastic member. For example, the resilient biasing member can include a layer of material that is more flexible than the body material of the body.

[0032] Each of the protrusions in the second group can be provided with a single resilient biasing member. Alternatively, multiple (e.g., all) of the protrusions in the second group can be provided with resilient biasing members.

[0033] The second set of peripheral protrusions may extend at least 50 nm, preferably at least 100 nm, and more preferably at least 200 nm further away from the surface than the first set of central protrusions.

[0034] The extent of a protrusion away from the surface can be referred to as the height of that protrusion (protrusion). This means that the respective distal ends of the second set of peripheral protrusions are at least 50 nm, 100 nm, or 200 nm above the height level of the respective distal ends of the first set of central protrusions.

[0035] The first set of central protrusions can extend approximately 10 μm from the surface.

[0036] The first set of central protrusions can define a discontinuous support surface for supporting a workpiece during use. Such a workpiece may include a substrate or a photolithographic wafer.

[0037] The surface defined by the body can be generally planar. Similarly, the (discontinuous) support surface defined by the first set of central protrusions can be generally planar (and can be generally parallel to the surface defined by the body).

[0038] The electrostatic clamp, or at least some portions thereof, may be approximately rotationally symmetric about an axis. This axis may be approximately perpendicular to the surface defined by the body.

[0039] The first set of central protrusions can extend from the generally circular portion of the surface, and the second set of peripheral protrusions can extend from the generally annular portion of the surface.

[0040] The generally circular portion and the generally annular portion of the surface may be concentric and may be centered on the central axis of the electrostatic clamp.

[0041] At least one central electrode and / or at least one peripheral electrode may be generally planar.

[0042] At least one central electrode may be generally circular, and at least one peripheral electrode may be generally annular.

[0043] At least one central electrode and at least one peripheral electrode may be concentric and may be centered on the central axis of the electrostatic clamp.

[0044] In some embodiments, at least one central electrode comprises two generally semi-circular electrodes. These two generally semi-circular electrodes together can be considered generally circular.

[0045] The body may define a central axis that is substantially perpendicular to the surface from which a plurality of protrusions extend, and the radial extent of at least one central electrode relative to the central axis substantially matches the radial extent of the first set of central protrusions relative to the central axis.

[0046] In some embodiments, the radial extent of at least one central electrode relative to the central axis is slightly larger than the radial extent of the first group of central protrusions relative to the central axis. For example, the distance between the outermost radial portion of at least one central electrode and the central axis may be greater than the distance between the outermost radial portion of the first group of central protrusions and the central axis.

[0047] The body may define a central axis that is substantially perpendicular to the surface from which a plurality of protrusions extend, and the radial extent of at least one peripheral electrode relative to the central axis may substantially match the radial extent of a second set of peripheral protrusions relative to the central axis.

[0048] In some embodiments, the radial extent of at least one peripheral electrode relative to the central axis is slightly larger than the radial extent of the second group of peripheral protrusions relative to the central axis. For example, the distance between the innermost radial portion of at least one peripheral electrode and the central axis may be smaller than the distance between the innermost radial protrusion of the second group of protrusions and the central axis. Similarly, the distance between the outermost radial portion of at least one peripheral electrode and the central axis may be greater than the distance between the outermost radial protrusion of the second group of protrusions and the central axis.

[0049] The body may contain a dielectric material. At least one central electrode and / or at least one peripheral electrode may be encapsulated in the dielectric material.

[0050] The electrostatic clamp may also include a voltage supply that is operable to supply voltage to at least one central electrode and / or at least one peripheral electrode.

[0051] The electrostatic clamp may also include a controller that is operable to control the voltage applied to at least one central electrode and / or at least one peripheral electrode.

[0052] The controller is operable to independently control the voltage applied to at least one central electrode and the voltage applied to at least one peripheral electrode. Advantageously, such a controller can independently control the clamping pressure applied by at least one central electrode and at least one peripheral electrode.

[0053] The controller is operable to change the voltage applied to at least one peripheral electrode.

[0054] In some embodiments, the second set of peripheral protrusions includes a small number of rows or rings of protrusions (e.g., one or two rows).

[0055] The second group of peripheral protrusions may include a single row of protrusions or a single ring of protrusions.

[0056] The body may define a central axis that is substantially perpendicular to the surface from which a plurality of protrusions extend. The ratio of the radial extent of (a) at least one peripheral electrode relative to the central axis to the radial extent of (b) at least one central electrode relative to the central axis may be 1:100 or less.

[0057] For example, the ratio of (a) the radial extent of at least one peripheral electrode relative to the central axis to (b) the radial extent of at least one central electrode relative to the central axis may be approximately 1:148. For example, in one embodiment, the radial extent of at least one peripheral electrode may be approximately 1 mm, while the radial extent of at least one central electrode may be approximately 148 mm.

[0058] In some embodiments, the body and the first set of central protrusions may be integral, and the body and the first set of central protrusions may contain dielectric material.

[0059] Multiple protrusions may include an outer coating.

[0060] The outer coating can be configured to contact the supported workpiece during use. The outer coating can be formed of a conductive material. For example, conductive materials may include chromium nitride or titanium nitride.

[0061] Electrostatic clamps can be configured to support workpieces in the form of semiconductor wafers or photolithographic substrates during use.

[0062] According to a second aspect of this disclosure, a photolithography apparatus including an electrostatic clamp according to a first aspect of this disclosure is provided.

[0063] According to a third aspect of this disclosure, a method for clamping a substrate to an electrostatic clamp according to a first aspect of this disclosure is provided, the method comprising: placing the substrate onto a plurality of protrusions; providing a first voltage to at least one central electrode to provide a first clamping pressure on a central portion of the substrate; and optionally, providing a second voltage to at least one peripheral electrode to provide a second clamping pressure on a peripheral portion of the substrate.

[0064] The inventors of this application have recognized that, due to variations in (a) the inherent shape of the wafer (whether or not the wafer is warped, and if so, how much) and (b) variations in the coating disposed on the back surface of the wafer (i.e., the surface supported by the protrusions), it is advantageous to provide independently controllable electrodes (or electrode groups) for a relatively small portion at the edge of the wafer. For some wafer topologies, more of at least one peripheral electrode can be used, and for others, fewer. Providing such additional independently controllable at least one peripheral electrode allows the protrusions to be positioned closer to the edge of the wafer, thereby further improving wafer flatness.

[0065] The electrostatic clamp may define a central axis that is perpendicular to the plane of the surface (and parallel to the direction along which the protrusions extend). Placing the substrate onto the protrusions may include placing the substrate such that its center of mass is located on the central axis.

[0066] The method may further include selecting a second voltage to be applied to the at least one peripheral electrode based on the topology of the upper surface of the substrate when it is clamped to an electrostatic fixture.

[0067] In other words, the second voltage can be selected based on the shape of the upper surface of the substrate being clamped. For example, the second voltage can be selected to optimize the shape of the upper surface of the substrate. For example, the second voltage can be selected to minimize the variation of the upper surface of the substrate relative to a flat surface (or to maximize the flatness of the substrate).

[0068] It should be understood that the upper surface of the substrate is intended to represent the surface of the substrate opposite to the surface of the substrate supported by multiple protrusions.

[0069] For example, this can be achieved using a feedforward method. That is, measurements can be taken of the inherent shape of the wafer and / or the thickness of the coating disposed on the back surface of the wafer (i.e., the surface supported by protrusions). These measurements can then be used to select a second voltage. Alternatively, this can be achieved using a feedback method. For example, the topology of the upper surface of the wafer can be measured while the wafer is clamped to an electrostatic chuck, and the second voltage can be varied until the flatness of the wafer has been optimized.

[0070] The second voltage can be selected based on the difference in the distances from which the first group of protrusions and the second group of protrusions extend from the surface of the body.

[0071] In a direction substantially parallel to the direction along which the protrusions extend from the surface of the body, the second set of peripheral protrusions may have less stiffness than the first set of central protrusions. The second voltage can be selected based on the stiffness difference between the first set of protrusions and the second set of protrusions.

[0072] Features of different aspects of the present invention may be combined together, provided that such combination is appropriate to those skilled in the art. Attached Figure Description

[0073] Embodiments of the invention will now be described by way of example only, with reference to the accompanying schematic drawings, in which:

[0074] - Figure 1 A lithography system including a lithography apparatus and a substrate stage is schematically depicted according to an embodiment of the present disclosure;

[0075] - Figure 2 A schematic cross-sectional view of a portion of the first electrostatic clamp is shown;

[0076] - Figure 3 This is a schematic illustration of a novel electrostatic clamp for holding a substrate (e.g., a silicon wafer) according to embodiments of the present disclosure;

[0077] - Figure 4A This schematically illustrates how a wafer is clamped to a plate using only at least one central electrode. Figure 3 The arrangement of the electrostatic clamps is shown, and the second set of peripheral protrusions is shown to tend to deform the edge of the wafer upward (unless it is completely bowl-shaped).

[0078] - Figure 4B It schematically shows that in Figure 4A The arrangement shown in the figure also includes applying a certain amount of clamping pressure to the periphery or edge portion of the wafer via at least one peripheral electrode, which can be controlled independently of the main clamping pressure applied by at least one central electrode in order to planarize the wafer.

[0079] -Figure 5A It schematically shows that in Figure 3 An embodiment of the electrostatic clamp of the type shown is provided, wherein the second set of peripheral protrusions is formed of one or more materials different from the first set of central protrusions, such that the two sets of different protrusions can have different properties, and in particular, the second set of peripheral protrusions can have reduced stiffness relative to the first set of central protrusions.

[0080] - Figure 5B It schematically shows that in Figure 3 An embodiment of the electrostatic clamp of the type shown is provided, wherein the second set of peripheral protrusions has a reduced thickness relative to the first set of central protrusions in a direction substantially perpendicular to the direction along which the protrusions extend from the surface of the body, such that the second set of peripheral protrusions can have reduced stiffness relative to the first set of central protrusions.

[0081] - Figure 5C It schematically shows that in Figure 3 An embodiment of the electrostatic clamp of the type shown in the figure further includes an elastic biasing member for the protrusions in the second set of peripheral protrusions, such that the second set of peripheral protrusions can have reduced stiffness relative to the first set of central protrusions.

[0082] - Figure 6 Is Figure 3 The schematic diagram of the body of the electrostatic clamp shown shows a protrusion extending from its surface, and also shows a central axis, a generally circular region (defined by dashed lines) corresponding to the coverage area of ​​at least one central electrode, and a generally annular region (defined by two dashed lines) corresponding to the coverage area of ​​at least one peripheral electrode.

[0083] - Figure 7 The parameter space is schematically illustrated by (i) the stiffness of the second set of peripheral protrusions and (ii) the height difference Δh between the second set of peripheral protrusions and the first set of central protrusions; the region in this parameter space that optimally allows the planarization of the umbrella-shaped wafer (with a given height difference Δz between the peripheral portion of the wafer and the center of the wafer) is also shown; and

[0084] - Figure 8 The illustration schematically shows the substrate being clamped onto an object according to an embodiment of the present disclosure. Figure 3 Method for using electrostatic clamps of the type shown. Detailed Implementation

[0085] Figure 1The lithography system shown includes a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a patterning apparatus MA (e.g., a mask), a projection system PS, a measurement system MS, and a substrate loading system SL. The lithography system also includes two substrate stages WT1, WT2 configured to support a substrate W. Each substrate stage WT1, WT2 may be consistent with an embodiment of this disclosure. The support structure MT may be consistent with an embodiment of this disclosure. As further explained below, the substrate fixture may include an electrostatic fixture and a Johnsenk-Rahbek fixture.

[0086] The irradiation system IL is configured to modulate the EUV radiation beam B before it is incident on the patterning apparatus MA. Furthermore, the irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. The faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 together provide the EUV radiation beam B with the desired cross-sectional shape and the desired intensity distribution. In addition to, or in place of, the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11, the irradiation system IL may include other mirrors or devices.

[0087] After such adjustment, the EUV radiation beam B interacts with the patterning apparatus MA. This interaction produces a patterned EUV radiation beam B'. A projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT2. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B' to form an image with features smaller than the corresponding features on the patterning apparatus MA. For example, a reduction factor of 4 or 8 can be applied. Although... Figure 1 The projection system PS shown has only two mirrors 13 and 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0088] The substrate W may include a previously formed pattern. In this case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0089] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure much lower than atmospheric pressure, can be provided in the radiation source SO, in the irradiation system IL, and / or in the environment of the projection system PS (which may also include the measurement system MS).

[0090] Figure 1 The radiation source SO shown is of the type that can be referred to as a laser-generated plasma (LPP) source, for example. A laser system 1 (which may include, for example, a CO2 laser) is arranged to deposit energy into a fuel, such as tin (Sn), provided, for example, by a fuel emitter 3, via a laser beam 2. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form, or it can be, for example, a metal or alloy. The fuel emitter 3 may include a nozzle configured to guide tin (e.g., in droplet form) along a trajectory toward the plasma-forming region 4. The laser beam 2 is incident on the tin at the plasma-forming region 4. The laser energy deposited into the tin generates tin plasma 7 at the plasma-forming region 4. During the de-excitation and recombination of electrons and ions in the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

[0091] EUV radiation from the plasma is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referred to as a normal incident radiation collector). Collector 5 may have a multi-layered mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength (e.g., 13.5 nm)). Collector 5 may have an elliptical configuration with two focal points. The first of the two focal points may be located at plasma formation region 4, and the second of the two focal points may be located at intermediate focal point 6, as described below.

[0092] Laser system 1 can be spatially separated from radiation source SO. In this case, laser beam 2 can be transmitted from laser system 1 to radiation source SO by means of a beam delivery system (not shown), which includes, for example, suitable guide mirrors and / or beam expanders and / or other optical devices. Laser system 1, radiation source SO and beam delivery system can be considered together as a radiation system.

[0093] The radiation reflected by collector 5 forms an EUV radiation beam B. EUV radiation beam B is focused at intermediate focal point 6 to form an image of plasma 7 present in plasma formation region 4 at intermediate focal point 6. This image at intermediate focal point 6 serves as a virtual radiation source for the irradiation system IL. Radiation source SO is arranged such that intermediate focal point 6 is located at or near opening 8 in the enclosure structure 9 of radiation source SO.

[0094] The measurement system MS of the photolithography apparatus is configured to perform measurements of the properties of a substrate W held on a substrate stage WT1. The measurement system MS includes an alignment system configured to measure the position of alignment marks on the substrate by referencing alignment marks on the substrate stage WT1. The measurement system also includes a level sensor 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.

[0095] The lithography apparatus LA is a dual-stage lithography apparatus. That is, the lithography apparatus includes two substrate stages WT1 and WT2, and is configured such that a substrate W held on one of the substrate stages WT1 is measured by a measurement system MS while a pattern is applied to a substrate W held on the other substrate stage WT2. For ease of terminology, one of the substrate stages WT1 can be referred to as the first substrate stage, and the other substrate stage WT2 can be referred to as the second substrate stage. Compared to a single-stage lithography apparatus, a dual-stage lithography apparatus advantageously provides higher throughput (i.e., the number of substrates exposed per hour). Once the substrate W held on the first substrate stage WT1 has been measured, and the substrate W held on the second substrate stage WT2 has been exposed, the substrate stage WT1 supporting the measured substrate W is moved under the projection system PS. Simultaneously, the exposed substrate W supported by the second substrate stage WT2 is moved to the substrate loading system SL.

[0096] The substrate loading system SL includes a substrate handler (not depicted) configured to remove the patterned substrate W from the second substrate stage WT2 and then load a new substrate to be patterned onto the second substrate stage. Once the substrate has been loaded onto the second substrate stage WT2, the alignment mark positions and topology of the substrate are measured using a measurement system MS. Simultaneously, the substrate W, held on the first substrate stage WT1, is exposed by a photolithography apparatus LA.

[0097] Repeat the above method multiple times in order to expose many substrates using a photolithography (LA) device.

[0098] Each substrate stage WT1, WT2 includes a substrate fixture according to an embodiment of the present disclosure. Before removing the exposed substrate W from the substrate stage, the substrate fixture is shut down, thereby allowing the substrate handler of the substrate loading system to easily remove the substrate W from the substrate stage. Then, the unexposed substrate W is placed on the substrate stage WT1 (or WT2), and the substrate fixture is turned on.

[0099] Figure 2This is a schematic cross-sectional view of a portion of a known electrostatic clamp 20. The electrostatic clamp 20 includes a body 22 defining a surface 24 from which a plurality of protrusions 26 or raised support protrusions extend. Note that Figure 2 This is illustrative, and the dimensions and positions of the protrusions 26 are not drawn to scale. Furthermore, for clarity, a total of nine protrusions are shown, and it will be understood that significantly more protrusions may exist. However, the total number of protrusions 26 is not important for understanding the electrostatic clamp 20.

[0100] The distal end of the support protrusion 26 defines a discontinuous support surface 28. In use, this discontinuous support surface 28 supports a workpiece (in this case, a substrate 30). In the present case, each of the support protrusions 28 has a cross-section that is approximately cubic or cuboid in shape. The discontinuous support surface 28 is formed by the upper surface of each of the support protrusions 26 (i.e., the surface that receives the workpiece 30). Depending on the shape of the support protrusions 26, the shape of the surface of each support protrusion 26 on which the substrate is supported in use will be different, and therefore the shape of the discontinuous support surface 28 will be different.

[0101] The body 22 includes a dielectric layer 32. The electrostatic clamp 20 also includes a generally planar electrode 34 embedded or encapsulated in the dielectric layer 32.

[0102] The electrostatic clamp 20 defines a central axis A, which is substantially perpendicular to the plane of the electrode 34. The electrostatic clamp 20 is configured to support the workpiece 30 in use such that the center of mass of the workpiece 30 is located on the central axis A. The center of mass of the workpiece 30 can be considered as the center of mass of the workpiece 30. Typically, the workpiece in the form of a substrate 30 is generally disk-shaped (e.g., the workpiece may include a silicon wafer). In this case, the center of mass is approximately located at the geometric center of the disk.

[0103] In an example where the substrate 30 can be supported by the electrostatic clamp 20, the substrate 30 may include a semiconductor wafer 30a having a dielectric coating 30b on its underside.

[0104] The electrostatic clamp 20 may also include a raised seal 36 that protrudes from the surface 24 of the body 22 and is located radially outside the outermost support protrusion 26. The seal 36 serves to reduce the likelihood of contaminants located radially outside the supported substrate 30 penetrating between the body 22 and the supported substrate 30. The maximum height of the raised seal 36 in a direction parallel to the central axis A is less than the maximum height of the support protrusion 26. In a known example, the maximum height of the raised seal 36 is 2 μm less than the maximum height of the support protrusion 26. For example, the maximum height of the support protrusion may be 10 μm, and the maximum height of the raised seal may be 8 μm. 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.

[0105] The height of the support protrusion 26 can be the maximum distance between a portion of the support protrusion 26 and the surface 24 defined by the body 22, parallel to the central axis A.

[0106] The body 22 and the multiple raised support protrusions 26 are integral. That is, the body 22 and the multiple raised support protrusions 26 are all formed from the same material. The body 22 and the multiple raised support protrusions 26 can be formed from a dielectric material.

[0107] The multiple protruding support protrusions 26 include an outer coating 38 configured to contact the supported workpiece during use. In some examples, the outer coating 38 is formed of a conductive material. In some examples, the conductive material may be chromium nitride. In other examples, the conductive material may be titanium nitride.

[0108] The seal 36 may also be coated with the same outer coating 38 as the support protrusion 26. However, since the seal 36 is not designed to contact the supported substrate 30, the outer coating 38 on the seal is configured not to contact the supported workpiece 30 during use.

[0109] Some embodiments of this disclosure relate to novel electrostatic clamps for holding substrates (e.g., silicon wafers). Reference is now made to illustrative examples. Figures 3 to 6 An example describing this novel electrostatic clamp 100. Figure 3 This is a schematic diagram of the cross-section of the novel electrostatic clamp 100.

[0110] The electrostatic clamp 100 includes: a body 110 defining a surface 112; and a plurality of protrusions extending from the surface 112. The plurality of protrusions includes a first set of central protrusions 120 and a second set of peripheral protrusions 122. The second set of peripheral protrusions 122 extends further away from the surface 112 than the first set of central protrusions 120. That is, the extent 124 of the first set of central protrusions 120 away from the surface 112 is smaller than the extent 126 of the second set of peripheral protrusions 122 away from the surface 112.

[0111] In some embodiments, the body 110 and the first set of central protrusions 130 may be integral, and the body 130 and the first set of central protrusions 130 may contain dielectric material.

[0112] The electrostatic clamp 100 further includes at least one central electrode 130 adjacent to the first group of central protrusions 122, and at least one peripheral electrode 132 adjacent to the second group of peripheral protrusions 124.

[0113] In some embodiments, the second set of peripheral protrusions 122 may extend at least 50 nm, at least 100 nm, or at least 200 nm further from the surface 112 of the body 110 than the first set of central protrusions 120, for example, about 100 nm, about 150 nm, about 200 nm, or about 250 nm. The extent to which one of the protrusions 120, 122 extends away from the surface 112 may be referred to as the height of that protrusion 120, 122 (protrusion). The first set of central protrusions 120 may extend from the surface 112 by a distance of approximately 10 μm.

[0114] The first set of central protrusions 120 can define a discontinuous support surface 128 for supporting a workpiece in use. Such a workpiece may include a substrate or a photolithographic wafer W.

[0115] The surface 112 defined by the body 110 may be generally planar. Similarly, the (discontinuous) support surface 128 defined by the first set of central protrusions 120 may be generally planar (and may be generally parallel to the surface 112 defined by the body 110).

[0116] The electrostatic clamp 100, or at least some portions thereof, may be substantially rotationally symmetric about axis B. Axis B may be substantially perpendicular to surface 112 defined by body 110.

[0117] exist Figure 3 The novel electrostatic clamp 100 shown is advantageous because it allows for better clamping of relatively thin substrates (e.g., silicon wafers), especially at the edges (or peripheral portions) of such substrates, as now discussed.

[0118] The electrostatic clamp 100 can be configured to support a workpiece in the form of a semiconductor wafer W or a photolithography substrate during use. The electrostatic clamp 100 can, for example, be used to clamp a silicon wafer W onto a wafer stage WT within a photolithography apparatus LA. In such embodiments, the primary function of the electrostatic clamp 100 is to support the wafer W (during pre-exposure measurements, for example, using a measurement system MS, and during exposure, for example, using a projection system PS), thereby maintaining flatness and thermal conditioning. Known for such clamps are surfaces comprising a plurality of protrusions (referred to as protrusions) that provide physical contact between the clamp and the wafer W, and limit the apparent contact surface between the wafer and the clamp to approximately 1.5% of the surface area of ​​the wafer W. This limits the possibility of van der Waals adhesion and wafer back-side contamination between the clamp surface and the wafer W.

[0119] Any variation in the flatness of the upper surface of the wafer W within the lithography apparatus LA can lead to overlap errors (offsets between features formed in two or more sequential lithography exposures), which are undesirable. Overlap errors are typically experienced near the edges of the wafer W compared to its center. One reason for this is the presence of radially outer portions of the wafer that are not supported by protrusions. Therefore, the flatness of these portions is largely determined by the inherent shape of the wafer W, and thus they are generally less flat than the central portion of the wafer W.

[0120] The inventors of this application have recognized that the inherent shape of the wafer W may cause it to warp downwards (which may be referred to as an umbrella-shaped wafer W) or upwards (which may be referred to as a bowl-shaped wafer W) at its edges. Furthermore, the inventors have recognized 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 inherently umbrella-shaped.

[0121] Advantageously, in Figure 3 The combination of features of the electrostatic clamp 100, schematically shown, allows for improved flatness of the wafer W, regardless of the inherent shape of the clamped wafer W, as now discussed.

[0122] The second set of peripheral protrusions 122 extends further away from the surface 112 than the first set of central protrusions 120 (i.e., the outer protrusions 122 are higher than the central protrusions 120). Therefore, when the wafer W is clamped to the electrostatic chuck 100 using at least one central electrode 130, the second set of peripheral protrusions 122 will tend to deform the edges of the wafer upwards (unless it is completely bowl-shaped). Figure 4AThe diagram schematically illustrates this arrangement, in which clamping force is provided solely by at least one central electrode 130. Therefore, advantageously, utilizing this clamping force from at least one central electrode 130, all wafers W (even umbrella-shaped wafers) can be reshaped to become flat or bowl-shaped wafers. Furthermore, since the electrostatic clamp 100 includes both: (a) at least one central electrode 130; and (b) at least one peripheral electrode 132, different clamping pressures can be applied to: (a) the (central) portion of the wafer supported by a first set of central protrusions 120, and (b) the (peripheral) portion of the wafer supported by a second set of peripheral protrusions 122. In particular, this allows control (independent of the main clamping pressure applied by at least one central electrode 130) of the amount of clamping pressure applied to the peripheral or edge portion of the wafer (applied by at least one peripheral electrode 132). Furthermore, the combination of the second set of peripheral protrusions 122 extending further away from the surface 112 than the first set of central protrusions 120 (i.e., the outer protrusions 122 are higher than the central protrusions 120) allows all types of wafers W to be planarized.

[0123] When a wafer W that is essentially flat or bowl-shaped is clamped using at least one central electrode 130, the larger peripheral protrusions 122 (protrusions) force the wafer into a bowl shape. A relatively large clamping pressure can be provided by at least one peripheral electrode 132 to compress the second set of peripheral protrusions 122 (i.e., the outermost protrusions) slightly more than the first set of peripheral protrusions 120 (i.e., the central protrusions) to planarize the wafer. When a wafer W that is essentially umbrella-shaped is clamped using at least one central electrode, the larger peripheral protrusions (protrusions) force the wafer W to become flat or slightly bowl-shaped. A relatively small clamping pressure can be provided by at least one peripheral electrode 132 to planarize the edge portions of the wafer W.

[0124] exist Figure 4B The diagram schematically illustrates an arrangement in which clamping forces are provided by at least one central electrode 130 and at least one peripheral electrode 132 to planarize the wafer W.

[0125] In some embodiments, the second set of peripheral protrusions 122 has less stiffness than the first set of central protrusions 120 in a direction substantially parallel to the direction along which the protrusions 120, 122 extend from the surface 112 of the body 110. It should be understood that the stiffness of an object in the first direction is given by the ratio of the force applied in the first direction to the displacement (or compression) in the first direction caused by the applied force.

[0126] Advantageously, with this arrangement, when similar clamping pressure is applied to the two sets of protrusions 120, 122, the second set of peripheral protrusions 122 will be compressed more than the first set of central protrusions 120.

[0127] It should be understood that the direction which is approximately parallel to the direction along which the protrusions 120 and 122 extend from the surface 112 of the body 110 is approximately perpendicular to the clamped wafer W during use.

[0128] In some embodiments, the stiffness of the second set of peripheral protrusions 122 in a direction substantially parallel to the direction along which the protrusions 120, 122 extend from the surface 112 of the body 110 is less than the stiffness of the first set of central protrusions 120 in that direction and is at most 1 / 10 of the stiffness of the first set of central protrusions 120 in that direction.

[0129] In some embodiments, the stiffness of the second set of peripheral protrusions 122 in a direction substantially parallel to the direction along which the protrusions 120, 122 extend from the surface 112 of the body 110 is less than the stiffness of the first set of central protrusions 120 in that direction and is at most 1 / 100 of the stiffness of the first set of central protrusions 120 in that direction.

[0130] This stiffness difference between the first set of protrusions 120 and the second set of protrusions 122 can be achieved in a variety of different ways, as now referred to Figures 5A to 5C As discussed.

[0131] In some embodiments, the second set of peripheral protrusions 122 may be formed of one or more materials different from the first set of central protrusions 120. By using different materials for the first set of protrusions 120 and the second set of protrusions 122, the two different sets of protrusions 120, 122 can have different properties. For example, the second set of peripheral protrusions 122 may be formed of a material with reduced stiffness relative to the one or more materials used to form the first set of central protrusions 120, or at least include a portion formed of such a material with reduced stiffness. Figure 5A The diagram illustrates this arrangement in which the second set of peripheral protrusions 122 and the body 110 are not integral (as shown by line 140).

[0132] In some embodiments, the second set of peripheral protrusions 122 has a different thickness than the first set of central protrusions 120 in a direction substantially perpendicular to the direction in which the protrusions 120, 122 extend from the surface 112 of the body 110. For example, the second set of peripheral protrusions 122 may be thinner than the first set of central protrusions 120. Figure 5B The diagram illustrates this arrangement.

[0133] In some embodiments, the electrostatic clamp 100 may further include an elastic biasing member for the protrusions in the second set of peripheral protrusions 122.

[0134] A resilient biasing member may form part of the protrusion 122. Additionally or alternatively, the resilient biasing member may be disposed between the body 110 and the second set of peripheral protrusions 122 (the main body portion). Figure 5C This arrangement is schematically shown in the figure, which shows an elastic biasing member 150 disposed between the body 110 and the second set of peripheral protrusions 122 (at least the main body portion).

[0135] The resilient bias member 150 may include any form of spring or resilient member. For example, the resilient bias member 150 may include a layer of material that is more flexible than the body material of the body 110.

[0136] Each of the protrusions in the second set of protrusions 122 may be provided with a single resilient biasing member 150. Alternatively, multiple (e.g., all) of the protrusions in the second set of protrusions 122 may be provided with resilient biasing members 150.

[0137] In some embodiments, the low stiffness of the second set of peripheral protrusions 122 can be achieved by creating a cavity below the second set of peripheral protrusions 122. In one embodiment, such a cavity may be disposed below the peripheral electrode.

[0138] At least one central electrode 130 may include, for example, two electrodes. These electrodes may be referred to as a first central electrode and a second central electrode. The first central electrode and the second central electrode can be controlled independently. In use, during the clamping of the wafer W, a positive voltage can be applied to the first central electrode, and a negative voltage can be applied to the second central electrode. The first central electrode and the second central electrode may each be approximately semi-circular.

[0139] The body 110 may include a dielectric material. At least one central electrode 130 and / or at least one peripheral electrode 132 may be encapsulated in the dielectric material.

[0140] At least one central electrode 130 may be embedded in the body 110 below the first set of central protrusions 120 (protrusions). Alternatively, at least one central electrode 130 may be disposed (e.g., bonded) to a surface 112 of the body 110 from which the first set of central protrusions 120 (protrusions) protrude. That is, the first set of central protrusions 120 (protrusions) may protrude through (or through) the central electrode(s) 130(s). Alternatively, at least one central electrode 130 may be disposed (e.g., bonded) to a surface 114 of the body 110, which is parallel to the surface 112 of the body 110 from which the first set of central protrusions 120 (protrusions) protrude.

[0141] At least one peripheral electrode 132 may be embedded in the body 110 below the second set of peripheral protrusions 122 (protrusions). Alternatively, at least one peripheral electrode 132 may be disposed (e.g., bonded) to a surface 112 of the body 110 from which the second set of peripheral protrusions 122 (protrusions) protrude. That is, the second set of peripheral protrusions 122 (protrusions) may protrude through (or more than one) the peripheral electrode 132(s). Alternatively, at least one peripheral electrode 132 may be disposed (e.g., bonded) to a surface 114 of the body 110, which is parallel to the surface 112 of the body 110 from which the second set of peripheral protrusions 122 (protrusions) protrude.

[0142] In some embodiments, a first set of central protrusions 120 extends from a generally circular portion of surface 112, and a second set of peripheral protrusions 122 extends from a generally annular portion of surface 112. The generally circular portion and the generally annular portion of surface 112 may be concentric and may be centered on the central axis B of the electrostatic clamp 100.

[0143] At least one central electrode 130 and / or at least one peripheral electrode 132 may be generally planar.

[0144] In some embodiments, at least one central electrode 130 is generally circular, and at least one peripheral electrode 132 is generally annular. The at least one central electrode 130 and at least one peripheral electrode 132 may be concentric and may be centered on the central axis B of the electrostatic clamp 100. In some embodiments, at least one central electrode 130 comprises two generally semicircular electrodes. These two generally semicircular electrodes together may be considered generally circular.

[0145] The body 110 may define a central axis B that is generally perpendicular to the surface 112 from which the plurality of protrusions 120, 122 extend. In some embodiments, the radial extent 134 of at least one central electrode 130 relative to the central axis B generally matches the radial extent of the first set of central protrusions 120 relative to the central axis B.

[0146] like Figure 3 As schematically shown, in some embodiments, the radial extent 134 of at least one central electrode 130 relative to the central axis B is slightly larger than the radial extent of the first group of central protrusions 120 relative to the central axis B. For example, the distance 134 between the outermost radial portion of at least one central electrode 130 and the central axis B may be greater than the distance between the outermost radial portion of the first group of central protrusions 120 and the central axis B.

[0147] In some embodiments, the radial extent 136 of at least one peripheral electrode 132 relative to the central axis B is substantially matched with the radial extent of the second set of peripheral protrusions 122 relative to the central axis B.

[0148] The radial range 136 of at least one peripheral electrode 132 can be the distance 136 between the outermost radial portion and the innermost radial portion of the at least one peripheral electrode 132. For example... Figure 3 As schematically shown, in some embodiments, the radial extent 136 of at least one peripheral electrode 132 relative to the central axis B is slightly larger than the radial extent of the second set of peripheral protrusions 122 relative to the central axis B. For example, the distance between the radially innermost portion of at least one peripheral electrode 132 and the central axis B may be smaller than the distance between the radially innermost protrusion of the second set of protrusions 122 and the central axis B. Similarly, the distance between the radially outermost portion of at least one peripheral electrode 132 and the central axis B may be greater than the distance between the radially outermost protrusion of the second set of protrusions 122 and the central axis B.

[0149] Optionally, the electrostatic clamp 100 may also include a voltage supply 160, which is operable to supply voltage to at least one central electrode 130 and / or at least one peripheral electrode 132.

[0150] Optionally, the electrostatic clamp 100 may also include a controller 170, which is operable to control the voltage applied to at least one central electrode 130 and / or at least one peripheral electrode 132.

[0151] The controller 170 is operable to independently control the voltage applied to at least one central electrode 130 and the voltage applied to at least one peripheral electrode 132. Advantageously, such a controller 170 can independently control the clamping pressure applied by at least one central electrode 130 and at least one peripheral electrode 132.

[0152] In some embodiments, the controller 170 is operable to change the voltage applied to at least one peripheral electrode 132.

[0153] In some embodiments, the second set of peripheral protrusions 122 includes a small number of rows or rings of protrusions (e.g., one or two rows). In some embodiments, the second set of peripheral protrusions 122 includes a single row of protrusions or a single ring of protrusions.

[0154] Will understand, despite Figure 3The diagram shows eleven protrusions 120, 122, but in practice, the electrostatic clamp 100 may include any number of protrusions 120, 122. In some embodiments, the electrostatic clamp 100 may include approximately 10,000 protrusions 120, 122.

[0155] Figure 6 This is a schematic representation of the surface 112 from which the protrusions 120, 122 extend of the body 110. Figure 112 also shows a central axis B, a generally circular region 112a (defined by dashed lines), and a generally annular region 112b (defined by two dashed lines). The generally circular region 112a corresponds to the footprint of at least one central electrode 130. The generally annular region 112b corresponds to the footprint of at least one peripheral electrode 132. Figure 6 The diagram also shows a radial range 134 of at least one central electrode 130 and a radial range 136 of at least one peripheral electrode 132.

[0156] It will be understood that the first set of central protrusions 120 can be disposed within the circular region 112a of the surface 112, and the second set of peripheral protrusions 122 can be disposed within the annular region 122b of the surface 112.

[0157] In some embodiments, the ratio of (a) the radial extent 136 of at least one peripheral electrode 132 relative to the central axis B to (b) the radial extent 134 of at least one central electrode 130 relative to the central axis B may be 1:100 or less. For example, the ratio of (a) the radial extent 136 of at least one peripheral electrode 132 relative to the central axis B to (b) the radial extent 134 of at least one central electrode 130 relative to the central axis B may be approximately 1:148. For example, in one embodiment, the radial extent 136 of at least one peripheral electrode 132 may be approximately 1 mm, while the radial extent 134 of at least one central electrode 130 may be approximately 148 mm.

[0158] Optionally, in some embodiments, the plurality of protrusions 120, 122 may include an outer coating 180. The outer coating 180 may be configured to contact a supported workpiece in use. The outer coating 180 may be formed at least on the distal ends of the plurality of protrusions 120, 122. The outer coating 180 may be formed of a conductive material. For example, the conductive material may include chromium nitride or titanium nitride.

[0159] Generally, the degree to which the electrostatic clamp 100 can planarize the wafer W can depend on: (a) the difference Δh between the height 126 of the second set of peripheral protrusions 122 and the height 124 of the first set of central protrusions 120; (b) the stiffness of the first set of protrusions 120 and the second set of protrusions 122; and (c) the expected range of the height difference Δz of the peripheral portion of each wafer W relative to the center of the wafer W. In other words, given the expected range of the height difference Δz of the peripheral portion of each wafer W relative to the center of the wafer W, the degree to which the electrostatic clamp 100 can planarize the wafer W can depend on: (a) the difference Δh between the height 126 of the second set of peripheral protrusions 122 and the height 124 of the first set of central protrusions 120; and (b) the stiffness of the first set of protrusions 120 and the second set of protrusions 122.

[0160] As discussed hereafter, various combinations of these parameters have been studied to evaluate the feasibility of this novel electrostatic clamp 100.

[0161] A preliminary study was conducted on the region in the parameter space encompassed by (i) the stiffness of the second set of peripheral protrusions 122 and (ii) the height difference Δh. Figure 3 An electrostatic clamp 100 of the type shown can planarize an umbrella-shaped wafer (each wafer W has a 100 μm height difference Δz between its peripheral portion and the center of the wafer W). Now refer to Figure 7 Discuss the results of this study. Figure 7 The parameter space is shown. Proceed with the following steps.

[0162] First, simulations are performed to examine the extent to which the wafer W can be flattened or cupped using only at least one central electrode 130 for each combination of stiffness and height difference Δh. This may be more difficult if the second set of peripheral protrusions 122 has relatively low stiffness or a relatively small height difference. This region in the parameter space (with such relatively low stiffness or such a small height difference for flattening or cupping the wafer W using only at least one central electrode 130) is labeled 180.

[0163] Next, simulations are performed for each combination of stiffness and height difference Δh to examine the extent to which the bowl-shaped shape (i.e., wafer planarization) can be eliminated when at least one peripheral electrode 132 is switched on, under the same clamping pressure as at least one central electrode 130. This will be more difficult if the second set of peripheral protrusions 122 is relatively stiff or has a relatively large height difference. This region in the parameter space (where the second set of peripheral protrusions 122 has relatively high stiffness or a relatively large height difference) is labeled 182.

[0164] The remaining region 186 of the parameter space corresponds to a combination of parameters that optimally allow the electrostatic clamp 100 to planarize an umbrella-shaped wafer in which the height difference Δz between the peripheral and central portions of the wafer is 100 μm when at least one peripheral electrode 132 applies some clamping pressure. For some portions of this region 186 of the parameter space, applying the same clamping pressure using at least one peripheral electrode 132 as using at least one central electrode 130 may result in relatively high pressure being applied. To optimize performance in this case, the voltage of at least one peripheral electrode 132 can be reduced to decrease the clamping pressure applied by that at least one peripheral electrode 132.

[0165] Typically, a coating (e.g., a dielectric coating) may be applied to one side of the semiconductor wafer that contacts the electrostatic clamp. This coating may be referred to as the wafer back-side coating. To ensure the flatness of the wafer after clamping, it is generally desirable that the coating thickness be uniform. However, in practice, it is difficult to ensure a uniform coating thickness at the wafer edges, and the thickness of the wafer back-side coating typically decreases at the wafer edges.

[0166] Note that for the purpose of generating Figure 7 The simulations performed based on the results shown assume that the back-side coating thickness is constant across the entire wafer radius. However, in practice, the back-side coating thickness may decrease by a non-zero amount at the peripheral portion of the wafer (which will be supported by the second set of peripheral protrusions 122). Therefore, in practice, it is preferable to select a region in the parameter space where the height difference is greater than the maximum expected decrease in the back-side coating thickness. This maximum expected decrease in the back-side coating thickness occurs in... Figure 7 The figure is represented by line 188. Therefore, it is preferable to select a portion of the maximum expected reduction in the height difference in the allowable region 186 of the parameter space that is greater than the thickness of the back coating (i.e., a portion of region 186 to the right of line 188).

[0167] The above reference has been repeated for different chip warpages within a certain range. Figure 7 The type of study described. By selecting parameters from this type of study that fall within the overlapping portions of all permissible regions 186 in the parameter space, in... Figure 3 The electrostatic clamp 100 shown can planarize wafers with all the warps studied to the greatest extent possible.

[0168] It is possible that within this region of the parameter space (which is suitable for a range of different wafer warpages), a suitable stiffness can be selected, and a range of different height differences can be used in conjunction with the selected stiffness. If so, it is recommended to select the maximum permissible height difference, as this allows the electrostatic clamp to planarize the wafer backside coating thickness at the wafer edge to the maximum extent.

[0169] The reduction in the thickness of the back-side coating at the wafer edge typically implies limitations on the radial position of protrusions (protrusions) in existing electrostatic jigs. However, advantageously, since the second set of peripheral protrusions 122 extends further away from the surface 112 than the first set of central protrusions 120, therefore... Figure 3 The novel electrostatic clamp 100 shown allows the protrusion to be positioned closer to the edge of the wafer W.

[0170] Note that in some embodiments of the novel electrostatic clamp 100, a first set of central protrusions 120 may be disposed on a region of surface 112 corresponding to the position of a protrusion in a conventional electrostatic clamp. Furthermore, in some embodiments, a second set of peripheral protrusions 122 may be disposed on a region of surface 112 radially outward of the position of a protrusion in a conventional electrostatic clamp.

[0171] Some embodiments of this disclosure relate to a lithography apparatus, which includes... Figure 3 A novel electrostatic clamp 100 of the type described above is shown in the figure for holding a substrate (e.g., a silicon wafer). For example, such a photolithography apparatus can typically have… Figure 1 The lithography equipment shown is in the form of LA, and can have the features described in the above reference. Figure 1 Any features described. The electrostatic fixture 100 may, for example, form part of one or more substrate stages WT1, WT2 of a photolithography apparatus.

[0172] Some embodiments of this disclosure relate to clamping a substrate W to... Figure 3 The method of using an electrostatic clamp 100 of the type described above is shown in the figure. Figure 8 An example of this method 200 is illustrated schematically.

[0173] Method 200 includes step 210 of placing a substrate W onto a plurality of protrusions 120, 122.

[0174] Method 200 further includes step 220 of providing a first voltage to at least one central electrode 130 to provide a first clamping pressure on the central portion of the substrate W.

[0175] Method 200 may optionally further include step 230 of providing a second voltage to at least one peripheral electrode 132 to provide a second clamping pressure on a peripheral portion of the substrate W.

[0176] The inventors of this application have recognized that, due to variations in (a) the inherent shape of the wafer W (regardless of whether the wafer is warped, and if so, how much) and (b) variations in the coating disposed on the back surface of the wafer (i.e., the surface supported by the protrusions 120, 122), it is advantageous to provide independently controllable electrodes 132 (or electrode groups) for a relatively small portion at the edge of the wafer W. For some topologies of the wafer W, at least one peripheral electrode 132 can be used more extensively, and for some topologies, at least one peripheral electrode 132 can be used less extensively. Providing such additional, independently controllable at least one peripheral electrode 132 allows the protrusions 122 to be disposed closer to the edge of the wafer W, thereby further improving wafer flatness.

[0177] The electrostatic clamp 100 may define a central axis B that is perpendicular to the plane of the surface 112 (and parallel to the direction along which the plurality of protrusions 120, 122 extend). Placing the substrate W onto the plurality of protrusions 120, 122 (i.e., step 210) may include placing the substrate W such that the center of mass of the substrate W is located on the central axis B.

[0178] Method 200 may further include step 240 of selecting a second voltage to be applied to at least one peripheral electrode 132 based on the topology of the upper surface of the substrate W when it is clamped onto the electrostatic jig 100.

[0179] In other words, the second voltage can be selected based on the shape of the upper surface of the substrate W after it has been clamped. For example, the second voltage can be selected to optimize the shape of the upper surface of the substrate W. For example, the second voltage can be selected to minimize the variation of the upper surface of the substrate W relative to a flat surface (or to maximize the flatness of the substrate).

[0180] It should be understood that the upper surface of substrate W is intended to represent the surface of substrate W opposite to the surface of substrate W supported by a plurality of protrusions 120, 122.

[0181] For example, this can be achieved using a feedforward method. That is, measurements can be taken of the inherent shape of the wafer W and / or the thickness of the coating disposed on the back surface of the wafer W (i.e., the surface supported by the protrusions 120, 122). These measurements can then be used to select the second voltage. Alternatively, this can be achieved using a feedback method. For example, the topology of the upper surface of the wafer W can be measured while the wafer W is clamped to the electrostatic chuck 100, and the second voltage can be varied until the flatness of the wafer has been optimized.

[0182] In some embodiments, the second voltage can be selected based on the difference Δh between the distances 124 and 126 that the first set of protrusions 120 and the second set of protrusions 122 extend from the surface 112 of the body 110.

[0183] In some embodiments, the second set of peripheral protrusions 122 may have less stiffness than the first set of central protrusions 120 in a direction substantially parallel to the direction along which the protrusions 120, 122 extend from the surface 112 of the body 110. The second voltage may be selected (at step 230) based on the stiffness difference between the first set of protrusions 120 and the second set of protrusions 122.

[0184] While specific references are made to the use of lithography equipment in IC manufacturing in this article, it should be understood that the lithography equipment described herein can have other applications. Other possible applications include manufacturing integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0185] Although embodiments of the invention may be specifically referred to herein in the context of lithography equipment, embodiments of the invention can be used in other equipment. Embodiments of the invention can form part of mask inspection equipment, metrology equipment, or any equipment for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning apparatus). These devices are generally referred to as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions. A substrate fixture according to embodiments of the invention can form part of a lithography tool.

[0186] Although specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in ways other than those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below.

[0187] Several aspects of this disclosure are defined in the following numbered sections:

[0188] 1. An electrostatic clamp for holding a substrate, the electrostatic clamp comprising:

[0189] A body, the body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions includes a first set of central protrusions and a second set of peripheral protrusions;

[0190] At least one central electrode adjacent to the first group of central protrusions; and

[0191] At least one peripheral electrode adjacent to the second set of peripheral protrusions;

[0192] The second set of peripheral protrusions extends further away from the surface than the first set of central protrusions.

[0193] 2. The electrostatic clamp according to section 1, wherein the second set of peripheral protrusions has less stiffness than the first set of central protrusions in a direction substantially parallel to the direction along which the protrusions extend from the surface of the body.

[0194] 3. The electrostatic clamp according to section 2, wherein the stiffness of the second group of peripheral protrusions in a direction substantially parallel to the direction along which the protrusions extend from the surface of the body is less than the stiffness of the first group of central protrusions in that direction and is at most 1 / 10 of the stiffness of the first group of central protrusions in that direction.

[0195] 4. The electrostatic clamp according to any of the preceding segments, wherein the second set of peripheral protrusions is formed of a different material than the first set of central protrusions.

[0196] 5. The electrostatic clamp according to any of the preceding segments, wherein the second set of peripheral protrusions has a thickness different from that of the first set of central protrusions in a direction substantially perpendicular to the direction along which the protrusions extend from the surface of the body.

[0197] 6. The electrostatic clamp according to any of the foregoing segments further includes an elastic biasing member for the protrusions in the second set of peripheral protrusions.

[0198] 7. The electrostatic clamp according to any of the preceding segments, wherein the second set of peripheral protrusions extends at least 50 nm further away from the surface than the first set of central protrusions, preferably at least 100 nm further away from the surface than the first set of central protrusions, more preferably at least 200 nm further away from the surface than the first set of central protrusions, or for example, the second set of peripheral protrusions extends further away from the surface than the first set of central protrusions by a value in the range of 50 nm to 250 nm, or for example, the second set of peripheral protrusions extends further away from the surface than the first set of central protrusions by about 100 nm, or about 150 nm, or about 200 nm, or about 250 nm.

[0199] 8. The electrostatic clamp according to any of the preceding segments, wherein the first set of central protrusions extends from a generally circular portion of the surface, and the second set of peripheral protrusions extends from a generally annular portion of the surface.

[0200] 9. The electrostatic clamp according to any of the preceding segments, wherein the at least one central electrode is generally circular and the at least one peripheral electrode is generally annular.

[0201] 10. The electrostatic clamp according to any of the preceding segments, wherein the body defines a central axis that is substantially perpendicular to the surfaces from which the plurality of protrusions extend, and wherein the radial extent of the at least one central electrode relative to the central axis substantially matches the radial extent of the first set of central protrusions relative to the central axis.

[0202] 11. The electrostatic clamp according to any of the preceding segments, wherein the body defines a central axis that is substantially perpendicular to the surfaces from which the plurality of protrusions extend, and wherein the radial extent of the at least one peripheral electrode relative to the central axis substantially matches the radial extent of the second set of peripheral protrusions relative to the central axis.

[0203] 12. The electrostatic clamp according to any of the preceding segments further includes a voltage supply operable to supply voltage to the at least one central electrode and / or the at least one peripheral electrode.

[0204] 13. The electrostatic clamp according to any of the preceding segments further includes a controller operable to control the voltage applied to the at least one central electrode and / or the at least one peripheral electrode.

[0205] 14. The electrostatic clamp according to section 13, wherein the controller is operable to change the voltage applied to the at least one peripheral electrode.

[0206] 15. The electrostatic clamp according to any of the preceding segments, wherein the second set of peripheral protrusions includes a single row of protrusions or a single ring of protrusions.

[0207] 16. An electrostatic clamp according to any of the preceding segments, wherein the body defines a central axis that is substantially perpendicular to the surfaces from which the plurality of protrusions extend, and wherein (a) the ratio of 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 axis is 1:100 or less.

[0208] 17. The electrostatic clamp according to any of the preceding segments, wherein the plurality of protrusions include an outer coating.

[0209] 18. A photolithography apparatus comprising an electrostatic fixture according to any of the foregoing segments.

[0210] 19. A method of clamping a substrate to an electrostatic clamp according to any one of segments 1 to 17, the method comprising:

[0211] Place the substrate onto the plurality of protrusions;

[0212] A first voltage is provided to the at least one central electrode to provide a first clamping pressure on the central portion of the substrate; and

[0213] Optionally, a second voltage is provided to the at least one peripheral electrode to provide a second clamping pressure on the peripheral portion of the substrate.

[0214] 20. The method according to section 19 further includes: selecting the second voltage applied to the at least one peripheral electrode based on the topology of the upper surface of the substrate when it is clamped to the electrostatic clamp.

[0215] 21. The method according to segment 20, wherein the second voltage is selected based on the difference between the distances by which the first set of protrusions and the second set of protrusions extend from the surface of the body.

[0216] 22. The method according to section 21, wherein the second set of peripheral protrusions has less stiffness than the first set of central protrusions in a direction substantially parallel to the direction along which the protrusions extend from the surface of the body; and wherein the second voltage is selected based on the stiffness difference between the first set of protrusions and the second set of protrusions.

Claims

1. An electrostatic clamp for holding a substrate, the electrostatic clamp comprising: A body, the body defining a surface and a plurality of protrusions extending from the surface, wherein the plurality of protrusions includes a first set of central protrusions and a second set of peripheral protrusions; At least one central electrode adjacent to the central protrusion of the first group; as well as At least one peripheral electrode adjacent to the second set of peripheral protrusions; The second set of peripheral protrusions extends further away from the surface than the first set of central protrusions.

2. The electrostatic clamp according to claim 1, wherein, In a direction substantially parallel to the direction along which the protrusions extend from the surface of the body, the second set of peripheral protrusions has less stiffness than the first set of central protrusions.

3. The electrostatic clamp according to claim 2, wherein, The stiffness of the second set of peripheral protrusions in a direction substantially parallel to the direction in which the protrusions extend from the surface of the body is less than the stiffness of the first set of central protrusions in that direction and is at most 1 / 10 of the stiffness of the first set of central protrusions in that direction.

4. The electrostatic clamp according to any of the preceding claims, wherein, The second set of peripheral protrusions is formed of a different material than the first set of central protrusions.

5. The electrostatic clamp according to any of the preceding claims, wherein: The second group of peripheral protrusions has a different thickness than the first group of central protrusions in a direction substantially perpendicular to the direction along which the protrusions extend from the surface of the body; and / or The electrostatic clamp also includes an elastic biasing member for the protrusions in the second set of peripheral protrusions.

6. The electrostatic clamp according to any of the preceding claims, wherein, The second set of peripheral protrusions extends at least 50 nm further from the surface than the first set of central protrusions, preferably at least 100 nm, and more preferably at least 200 nm.

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

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

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

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

11. The electrostatic clamp according to any of the preceding claims, wherein, The second group of peripheral protrusions includes single-row protrusions or single-ring protrusions.

12. The electrostatic clamp according to any of the preceding claims, wherein, The body defines a central axis that is substantially perpendicular to the surface from which the plurality of protrusions extend, and wherein (a) the ratio of 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 axis is 1:100 or less.

13. A photolithography apparatus comprising an electrostatic clamp according to any of the preceding claims.

14. A method of clamping a substrate to an electrostatic clamp according to any one of claims 1 to 12, the method comprising: Place the substrate onto the plurality of protrusions; A first voltage is provided to the at least one central electrode to provide a first clamping pressure on the central portion of the substrate; as well as Optionally, a second voltage is provided to the at least one peripheral electrode to provide a second clamping pressure on the peripheral portion of the substrate.

15. The method of claim 14, further comprising: The second voltage applied to the at least one peripheral electrode is selected based on the topology of the upper surface of the substrate when it is clamped to the electrostatic clamp.

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