Substrate holder used by lithography apparatus, and production method of device

Abstract

To provide a substrate holder that enables effective pattern forming to a substrate having a high degree of distortion.SOLUTION: A substrate holder incudes: a main body having a main body surface; a plurality of bars constituted so as to support a substrate protruding from the main body surface; and an edge part seal protruding from the main body surface, where the edge part seal is separated from the plurality of bars so as to define gaps having a width of 75% or wider than a pitch of the plurality of bars between the edge part seal and the plurality of bars, the plurality of bars includes a first group of bars, and a second group of bars surrounding the first group of bars, and the rigidity per unit area in a direction vertical to a supporting surface of the second group of bars is approximately 150% or more of the rigidity per unit area in the direction vertical to the supporting surface of the first group of bars.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 19153181.3, filed January 23, 2019, the entire contents of which are incorporated by reference herein.

[0002] The present invention relates to a substrate holder for use in a lithographic apparatus and to a device manufacturing method. [Background technology]

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

[0004]

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing for decades, following a trend colloquially known as "Moore's Law". To keep up with Moore's Law, the semiconductor industry is pursuing technologies that allow it to create smaller and smaller features. To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm.

[0005]

[0005] In a lithographic apparatus, a substrate to be exposed (sometimes called a product substrate) is held on a substrate holder (sometimes called a wafer table). The substrate holder may be movable relative to the projection system. The substrate holder typically comprises a solid body made of a rigid material and having similar dimensions in plan as the product substrate to be supported. The surface of the solid body facing the substrate is provided with a number of protrusions (called burls). The tip faces of the burls conform to a plane and support the substrate. The burls offer several advantages: contaminant particles on the substrate holder or on the substrate are more likely to fall between the burls and thus not cause deformation of the substrate; it is easier to machine the burls so that their ends conform to a plane than to make the surface of the solid body flat; and the properties of the burls can be adjusted, for example to control the clamping of the substrate.

[0006]

[0006] Product substrates may become distorted during the process of manufacturing devices, especially when structures with significant heights, such as so-called 3D-NAND, are formed. Substrates are often "bowl-shaped", i.e., concave as viewed from above, or "umbrella-shaped", i.e., convex as viewed from above. In this disclosure, the surface on which the device structures are formed is referred to as the top surface. In this context, "height" is measured in a direction perpendicular to the reference surface of the substrate, sometimes referred to as the Z direction. Bowl-shaped and umbrella-shaped substrates are flattened to some extent when clamped to a substrate holder, for example by applying a vacuum to the space between the substrate and the substrate holder. However, if the amount of distortion, typically measured by the height difference between the lowest point on the substrate's surface and the highest point on the substrate's surface, is too large, various problems may occur. In particular, it may be difficult to clamp the substrate properly, the burls may wear excessively during loading and unloading of the substrate, and the residual height change of the substrate's surface may be too large to accurately pattern all parts of the substrate, especially near the edges. Summary of the Invention

[0007]

[0007] It is an object of the present invention to provide a substrate holder that allows effective patterning on substrates with high distortion.

[0008]

[0008] In an embodiment of the invention there is provided a substrate holder for use in a lithographic apparatus, configured to support a substrate, the substrate holder comprising: a body having a body surface; a plurality of burls projecting from the body surface, each having a tip surface configured to substantially conform to the support surface and to support a substrate; an edge seal protruding from the body surface and extending around the plurality of burls to restrict gas flow when a substrate is held by the substrate holder; an edge seal spaced from the plurality of burls to define a gap between the edge seal and the plurality of burls having a gap width that is greater than or equal to about 75% of a pitch of the plurality of burls; the plurality of burls includes a first group of burls and a second group of burls surrounding the first group of burls; A substrate holder is provided in which the stiffness per unit area of ​​the second group of burls in a direction perpendicular to the support surface is greater than or equal to about 150% of the stiffness per unit area of ​​the first group of burls in a direction perpendicular to the support surface.

[0009]

[0009] An embodiment of the present invention also includes holding the substrate on the substrate holder by reducing the pressure between the body surface and the substrate; and A device manufacturing method is provided, comprising: applying a pattern to a substrate. [Brief description of the drawings]

[0010]

[0010] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, in which:

[0011] [Figure 1]1 depicts a schematic representation of a lithographic apparatus; [Diagram 2] 1 shows a portion of a conventional substrate holder for clamping a bowl-shaped substrate. [Diagram 3] 1 shows a portion of a conventional substrate holder for clamping an umbrella-shaped substrate. [Figure 4] 1 shows a portion of a substrate holder according to an embodiment of the invention for clamping a bowl-shaped substrate; [Diagram 5] 1 shows a portion of a substrate holder according to an embodiment of the invention for clamping an umbrella-shaped substrate; [Figure 6] 6 shows a plan view of the substrate holder of FIG. 4 and FIG. 5. [Figure 7] 1 is a graph illustrating the results of a finite element analysis of the effect of clamping substrates with various amounts of warpage to a conventional substrate holder and to a substrate holder according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]

[0011] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 436 nm, 405 nm, 365 nm, 248 nm, 193 nm, 157 nm, 126 nm or 13.5 nm).

[0013]

[0012] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted to refer to a general purpose patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, corresponding to the pattern to be created in a target portion of a substrate. The term "light valve" can also be used in this context. Besides the classical mask (transmissive or reflective mask, binary mask, phase-shifting mask, hybrid mask, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0014]

[0013] Figure 1 illustrates diagrammatically a lithographic apparatus LA. The lithographic apparatus comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation or DUV radiation), a mask support (e.g. mask table) MT constructed to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. substrate holder) WT constructed to hold a substrate (e.g. resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support WT according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0015]

[0014] In operation, the illumination system IL receives a radiation beam B from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution across its cross-section in the plane of the patterning device MA.

[0016]

[0015] The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems including refractive optical systems, catadioptric optical systems, anamorphic optical systems, magnetic optical systems, electromagnetic optical systems, and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. When the term "projection lens" is used herein, this can be considered as synonymous with the more general term "projection system" PS.

[0017]

[0016] The lithographic apparatus may be of a type in which at least a part of the substrate W may be covered with a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system PS and the substrate W. This is also called immersion lithography. More information about immersion techniques is given in US patent 6,952,253, which is incorporated herein by reference.

[0018] The lithographic apparatus may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or a substrate W on one substrate support WT may be used for exposing a pattern thereon while a preparation step for a subsequent exposure of the substrate W is performed on a substrate W arranged on another substrate support WT.

[0019]

[0018] In addition to the substrate support WT, the lithographic apparatus may include a measurement stage (not shown in Figure 1). The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, such as for example part of the projection system PS or part of the system for providing immersion liquid. When the substrate support WT is separate from the projection system PS, the measurement stage may be moved below the projection system PS.

[0020]

[0019] In operation, the radiation beam B is incident on the patterning device MA, e.g. a mask, which is held on the mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. Using the second positioner PW and the position measurement system PMS, the substrate support WT can be accurately moved, e.g. to position the various target portions C at focused and aligned positions in the path of the radiation beam B. Similarly, the patterning device MA can be accurately positioned with respect to the path of the radiation beam B using the first positioner PM and possibly further position sensors (not explicitly shown in Figure 1). The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, it is also possible that they may be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-line alignment marks.

[0021]

[0020] A Cartesian coordinate system is used in this specification. The Cartesian coordinate system has three axes, namely, the x-axis, the y-axis, and the z-axis. Each of the three axes is orthogonal to the other two axes. Rotation about the x-axis is called the Rx rotation. Rotation about the y-axis is called the Ry rotation. Rotation about the z-axis is called the Rz rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is vertical. The Cartesian coordinate system is not a limitation of the invention and is used only for clarity. Alternatively, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0022]

[0021] In a lithographic apparatus, it is necessary to position with high precision the top surface of the substrate to be exposed in the plane of best focus of the aerial image of the pattern projected by the projection system. To achieve this, the substrate is held in a substrate holder. The surface of the substrate holder that supports the substrate is provided with a number of burls whose tips are in the same plane as the reference support surface. The burls are numerous but have a small cross-sectional area parallel to the support surface such that the total cross-sectional area of ​​their tips is a few percent of the surface area of ​​the substrate, for example less than 5%. The burls are usually, but not necessarily, conical. They reduce the gas pressure in the space between the substrate holder and the substrate compared to the pressure on the substrate, generating a force that clamps the substrate to the substrate holder. Alternatively, the substrate holder is provided with a number of electrodes that can clamp the conductive substrate using an electrostatic voltage.

[0023]

[0022] The burls serve several purposes. For example, if there is a contaminant particle on the substrate holder or substrate, it will probably not be located at the burr location and will not distort the substrate. Also, it is easier to manufacture burls whose tips precisely match the plane than to manufacture large areas with very poor flatness. Also, the properties of the burls can be changed, for example, by applying a coating or by changing the dimensions of the burls.

[0024]

[0023] Figures 2 and 3 show in cross-section the situation at the edge of a conventional substrate holder WH when clamping a bowl-shaped and umbrella-shaped substrate W, respectively. It should be noted that in Figures 2 and 3 the distortion of the substrate W is greatly exaggerated for illustrative purposes. When manufacturing devices, it is desirable to manufacture as many devices per substrate as possible. That is, it is desirable to image as close as possible to the edge of the substrate W. In many instances, it is desirable to form a pattern within a few mm, for example 3 mm or less, of the edge of the substrate W. Thus, the performance of the substrate holder WH can be judged by considering the shape of the surface of the clamped substrate W in the area of ​​interest AoI, including the outermost positions where the pattern is formed. In particular, the performance of the substrate holder WH can be judged based on the maximum height difference relative to the average height of the substrate W.

[0025] 2 and 3, it can be seen that when a bowl-shaped substrate is clamped, the edge of the substrate W may not be able to contact the outermost burl 20. When the space is evacuated to provide a pressure differential to clamp the substrate W, the edge seal 85, sometimes called a vacuum edge seal, reduces the gas flow into the space below the substrate W. For the same amount of distortion in the unclamped (unstressed) substrate, the distortion of the area of ​​interest AoI when clamped is higher for a bowl-shaped substrate than for an umbrella-shaped substrate. It is therefore desirable to provide a substrate holder that can improve performance when clamping a bowl-shaped substrate, preferably with little or no loss in performance when clamping an umbrella-shaped substrate.

[0026] To address at least some of the shortcomings of the prior art, an embodiment of the present invention provides a substrate holder WH with a gap between the burls and the edge seal 85. The gap is free of burls and other protrusions reaching the support surface and has a width of at least 75% of the pitch of the burls. The burls are divided into two groups, a first group and a second group surrounding the first group. The second group is thus the burls 20b closest to the edge seal 85. The burls 20b of the second group are configured such that their overall vertical stiffness in a given area (i.e. stiffness in a direction perpendicular to the support surface) is at least 150% of the overall stiffness of the burls 20a of the first group in a comparable area of ​​the same size.

[0027]

[0026] Experiments and simulations show that the substrate holder WH constructed as above can provide better performance in clamping bowl-shaped substrates without unacceptably compromising performance in clamping umbrella-shaped substrates. For example, by positioning the vacuum edge seal 85 closer to the edge of the substrate holder WH, by increasing the gap between the outermost burls 20b and the vacuum edge seal 85, a larger force is exerted on the outer portion of the substrate W, resulting in a flatter bowl-shaped substrate. As a result, the force opposed by each of the outermost burls 20b is increased, which is offset by the increased stiffness of the outermost burls 20b. Thus, the outermost burls 20b (second group) are not overly compressed.

[0028]

[0027] When a bowl-shaped substrate is clamped, its very edge is not supported such that the outer portion of the substrate W forms a cantilever away from the outermost burls 20b. When an umbrella-shaped substrate is clamped, the force exerted on the outermost burls 20b is also increased, which causes the outermost burls 20b to compress more, but the increased stiffness of the outermost burls 20b counters this effect. Thus, the performance of the substrate holder WH in the case of umbrella-shaped burls is not adversely affected, or at least the adverse effects are tolerable.

[0029]

[0028] In an embodiment, the substrate holder WH is circular in plan view and the burls are arranged in concentric rings. Some burls may not be arranged in concentric rings, for example near the through holes 89 of the substrate holder WH. The edge seal 85 is also annular and concentric with the ring of burls. The second group of burls 20b may be the outermost ring. The width D of the gap is determined with respect to the radial distance P between the innermost ring of the second group and the next ring (i.e. the outermost ring of the first group).

[0030] In embodiments where the burls are not arranged in concentric rings, the gap width is desirably determined relative to the average (e.g., mean, mode or median) pitch of the first group of burls. The pitch is defined as the center-to-center distance of adjacent burls. In most cases, the burl pitch will be uniform across the substrate holder WH. However, sometimes there may be variations near other features of the substrate holder WH, for example holes for e-pins or vacuum ports. If there is variation in the burl pitch, the mean, mode and median are likely to be fairly close.

[0031] In embodiments where the burls are not arranged in concentric rings, the second group of burls consists of burls that are within a distance less than the average pitch from the edge seal.

[0032]

[0031] Those skilled in the art will appreciate that the vertical stiffness of a burl is the ratio of the magnitude of the vertical compressive force applied to the tip of the burl to the amount of compression (change in height relative to the original height) that results. The area stiffness (stiffness per unit area) of a group of burls is determined by the stiffness of the individual burls and the density of the burls (number of burls per unit area). In some embodiments of the invention, one or both of the stiffness of the individual burls and the density of the burls may vary in the second group compared to the first group. In embodiments where the second group of burls consists of a single outermost ring of burls, the density of the burls 20b in the second group is determined by the circumferential spacing of the burls.

[0033]

[0032] The stiffness of an individual burl can be varied in a variety of ways, for example by changing the cross-sectional area of ​​the burl, by changing the shape of the burl (e.g. making the taper tighter or less steep), or by changing the composition of the burl.

[0034]

[0033] The gap width D is at least 75% of the pitch, preferably at least 80% of the pitch P. The gap width D is preferably at least 90% of the pitch P. In one embodiment, the gap width D is about 100% of the pitch P. It is believed that wider gaps can hold substrates with greater amounts of warpage. Preferably, the gap width D is no more than 125% of the pitch P.

[0035] In an embodiment, the vertical stiffness per unit area of ​​the second group of burls 20b is at least 150% of that of the first group, desirably at least 160% of that of the first group. In an embodiment, the vertical stiffness per unit area of ​​the second group of burls 20b is at least 180% of that of the first group. In an embodiment, the vertical stiffness per unit area of ​​the second group of burls 20b is no more than 200% of that of the first group.

[0036]

[0035] In Fig. 4 and Fig. 5, a portion of a substrate holder WH according to an embodiment is shown in cross-section. In Fig. 4, a substrate holder WH for clamping a bowl-shaped substrate is shown, and in Fig. 5, one for clamping an umbrella-shaped substrate is shown. As in Fig. 2 and Fig. 3, the distortion of the substrate W is greatly exaggerated in Fig. 4 and Fig. 5. In Fig. 4 and Fig. 5, the outer burls 20b are shown with different hatching and larger than the other burls only to facilitate visual differentiation from the other burls. However, this should not be taken as indicating that the outer burls 20b must be of a different material or size than the other burls. In Fig. 6, the entire substrate holder WH is shown in plan view. In Fig. 6, the size of the burls is exaggerated and the number of burls is significantly reduced for clarity.

[0037]

[0036] The substrate holder WH is attached to the substrate support WT. The substrate holder WH comprises a body 21 having a body top surface 22 and a number of burls 20 protruding from the body top surface 22. The substrate W may be supported by the tip surfaces of the burls 20, which match a substantially flat support surface to support the substrate W in a flat state. The body 21 and the burls 20 may be made of SiSiC, a ceramic material having silicon carbide (SiC) particles in a silicon matrix. The burls 20 may be integral with the body 21 and may be formed by additive or subtractive methods. Alternatively, the burls 20 may be deposited on the body 21 and may be made of a different material than the body 21. The second group of burls 20b may be formed using a different technique and / or material than the first group of burls 20a. Suitable materials and techniques for manufacturing the substrate holder WH are well known in the art.

[0038]

[0037] A number of through holes 89 are formed in the body 21. The through holes 89 allow the e-pins to penetrate the substrate holder WH to receive the substrate W and to evacuate the space between the substrate W and the substrate holder WH. Another through hole can be provided to evacuate the space between the substrate W and the substrate holder WH. The vacuum state of the space between the substrate W and the substrate holder WH can provide a clamping force to hold the substrate W in place by creating a pressure difference with the space above the substrate W. In a lithographic apparatus using EUV radiation, the space above the substrate W is at such a low pressure that an adequate pressure difference cannot be created. Therefore, an electrode can be provided on the substrate holder WH to form an electrostatic clamp. Other structures can also be provided, for example to control the gas flow and / or the thermal conductivity between the substrate holder WH and the substrate W. The substrate holder WH can also be provided with electronic components, for example heaters and sensors, to control the temperature of the substrate holder WH and the substrate W.

[0039]

[0038] An edge seal 85 is provided near the periphery of the substrate holder WH. The edge seal 85 is a protruding ridge on the outer periphery of the substrate holder WH. The edge seal 85 has a height slightly less than the burls 20 so as not to contact the undistorted substrate W while reducing gas flow into the space between the substrate W and the substrate holder WH to improve vacuum clamping. When vacuum clamping is applied, the gas pressure outside the edge seal 85 is greater than the gas pressure inside it. If the diameter of the edge seal 85 is smaller than the diameter of the substrate W, the radially outer portion of the edge seal 85 of the substrate W will not experience a clamping force or will experience a smaller clamping force compared to the inner portion of the edge seal 85.

[0040] In an embodiment, the burls 20 have a height in the range of 100 μm to 500 μm, for example, about 150 μm. The diameter of the tip surface of the burls 20 may be in the range of 100 μm to 500 μm, for example, about 200 μm, 210 μm, 270 μm or 350 μm. The pitch of the burls 20, i.e., the distance between the centers of two adjacent burls 20, may be in the range of about 0.5 mm to 3 mm, for example, about 1.5 mm, 2 mm or 2.5 mm. In an embodiment, the total area of ​​the tip surfaces of all the burls 20 is in the range of 1% to 3% of the total area of ​​the substrate W or substrate holder WH. The burls 20 may be frustum-shaped with slightly sloping sidewalls. In an embodiment, the sidewalls may be vertical or flared, if this is more convenient for manufacturing. In an embodiment, the burls 20 are circular in plan view. The burls 20 may also be formed in other shapes as required.

[0041] In an embodiment, one or more coatings may be applied to all or a portion of the burl 20 to control properties such as friction between the burl 20 and the substrate W. For example, a diamond-like carbon (DLC) layer may be provided on the tip surface of the burl 20 to form a release structure.

[0042]

[0041] Figure 7 shows the results of FEM simulations of clamping substrates W with different bowing amounts by different designs of substrate holders WH. The horizontal axis shows the bowing amount (peak to valley) (in μm) of the unloaded substrate, whereas the vertical axis shows the height change (in nm) from the average height of the substrate W in the area of ​​interest AoI, in this case 3 mm from the edge of a 300 mm substrate. The filled triangles (I) show the results of a substrate holder with a gap between the outermost ring of burls and the edge seal less than 75% of the pitch and with all burls having the same stiffness and density. The open diamonds (II) show the results of a substrate holder similar to that of example I, but with burls and edge seals arranged in accordance with an embodiment of the present invention.

[0043] It will be seen that the substrate holder of Example I works well with umbrella-shaped wafers, but not so well with bowl-shaped wafers. On the other hand, the holder II of the embodiment of the present invention provides a very significant improvement in performance with bowl-shaped wafers, while not providing a significant degradation in performance with umbrella-shaped substrates. It will be seen that the embodiment of the present invention can hold a wider range of substrates than a comparable substrate holder not in accordance with the present invention.

[0044]

[0043] Although particular reference is made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein has other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like. In light of these alternative applications, those skilled in the art will recognize that the use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion", respectively. The substrates described herein may be processed, before or after exposure, for example in a track (a tool that typically applies a layer of resist to the substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where appropriate, the disclosure herein may be applied to these and other substrate processing tools. Furthermore, the substrate may be processed multiple times, for example to produce a multi-layer IC, and thus the term substrate as used herein may also refer to a substrate that already includes multiple processed layers.

[0045]

[0044] Although specific reference has been made above to the use of embodiments of the present invention in relation to optical lithography, it will be appreciated that the present invention may be used in other applications.

[0046]

[0045] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0047]

[0046] The above description is for purposes of illustration and not limitation. Thus, it will be apparent to one skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. 1. A substrate holder for use in a lithographic apparatus, configured to support a substrate, the substrate holder comprising: a body having a body surface; a plurality of burls projecting from the body surface, each having a tip surface configured to substantially conform to a support surface and to support the substrate; an edge seal protruding from the body surface and extending around the plurality of burls to restrict gas flow when a substrate is held by the substrate holder; the edge seal is spaced from the plurality of burls to define a gap between the edge seal and the plurality of burls having a gap width greater than or equal to about 75% of a pitch of the plurality of burls; the plurality of burls includes a first group of burls and a second group of burls surrounding the first group of burls; a stiffness per unit area of ​​the second group of burls in a direction perpendicular to the support surface that is greater than or equal to about 150% of a stiffness per unit area of ​​the first group of burls in a direction perpendicular to the support surface.

2. 2. The substrate holder of claim 1, wherein the gap width is greater than or equal to about 80% of the pitch of the plurality of burls, greater than or equal to about 90% of the pitch of the plurality of burls, desirably greater than or equal to about 100% of the pitch of the plurality of burls.

3. The substrate holder of claim 1 or 2, wherein the gap width is less than or equal to about 125% of the pitch of the plurality of burls.

4. 4. The substrate holder of claim 1, 2 or 3, wherein the stiffness per unit area of ​​the second group of burls in a direction perpendicular to the support surface is greater than or equal to about 160% of the stiffness per unit area of ​​the first group of burls in a direction perpendicular to the support surface, desirably greater than about 180% of the stiffness per unit area of ​​the first group of burls in a direction perpendicular to the support surface.

5. 5. The substrate holder of claim 1 , wherein the stiffness per unit area of ​​the second group of burls in a direction perpendicular to the support surface is less than or equal to about 200% of the stiffness per unit area of ​​the first group of burls in a direction perpendicular to the support surface.

6. 6. A substrate holder as claimed in claim 1 , wherein the stiffness of each of the burls of the second group in a direction perpendicular to the support surface is greater than the average stiffness of the burls of the first group in a direction perpendicular to the support surface.

7. A substrate holder according to claim 1 , wherein the density of the second group of burls is greater than the density of the first group of burls.

8. A substrate holder according to claim 1 , wherein each of the burls of the second group has a cross-sectional area in a plane parallel to the support surface that is larger than a cross-sectional area of ​​the burls of the first group.

9. 9. A substrate holder according to claim 1, wherein the second group of burls and at least some of the first group of burls are arranged in concentric rings, of which the second group forms the outermost portion.

10. The substrate holder of claim 9 , wherein the pitch is the spacing between the second group of burls and an outermost ring of burls in the first group of burls.

11. A lithographic apparatus for applying a pattern to a substrate, said apparatus comprising a substrate holder according to any one of claims 1 to 10.

12. Holding a substrate in a substrate holder according to any one of claims 1 to 10 by reducing a pressure between the body surface and the substrate; and applying a pattern to the substrate.

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