EP4616252A1

EP4616252A1 - Substrate support and lithographic apparatus

Info

Publication number: EP4616252A1
Application number: EP23782973.4A
Authority: EP
Inventors: Thilo Nils Martin SEEGER; Zhuangxiong HUANG; Niek Jacobus Johannes ROSET
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2022-11-11
Filing date: 2023-10-03
Publication date: 2025-09-17
Also published as: JP2025535595A; KR20250109688A; TW202435001A; CN120188104A; WO2024099640A1

Abstract

A substrate support configured to support a substrate in a lithographic apparatus comprising: a first circumferential (31) wall having a first height; a first drain (12) radially outwards of the first circumferential wall and configured to extract fluid; a second circumferential (32) wall having a second height and radially outwards of the first opening; a second drain (119) radially outwards of the second circumferential wall and configured to extract fluid; a flow restriction (33) having a third height and radially outwards of the second opening; and a third drain (10) radially outwards of the flow restriction and the substrate and configured to extract fluid; wherein the first opening, the second opening and the third opening are distributed around the circumference of the substrate support; and the third height is less than the first height and the second height.

Description

SUBSTRATE SUPPORT AND LITHOGRAPHIC APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 22206863.7 which was filed on November 11, 2022 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a substrate support configured to support a substrate in a lithographic apparatus, a lithographic apparatus including a substrate support, and a method of manufacturing a device using a substrate support.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation- sensitive material (resist) provided on a substrate (e.g., a wafer). Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm.

[0005] Further improvements in the resolution of smaller features may be achieved by providing an immersion fluid having a relatively high refractive index, such as water, on the substrate during exposure. The effect of the immersion fluid is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the fluid than in gas. The effect of the immersion fluid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.

[0006] The immersion fluid may be confined to a localized area between the projection system of the lithographic apparatus and the substrate by a fluid handling structure. SUMMARY

[0007] In a semiconductor manufacturing process, a substrate is supported on a substrate support. Specifically, the substrate is supported on a plurality of burls protruding from the a surface of the substrate support. A backside coating is often provided on the surface of the substrate that is in contact with the burls. The backside coating may be used to control friction between the substrate and the burls and/or to assist in dicing and packaging processes after manufacture of the devices. The backside coating may have a variable thickness near the edge of the substrate causing variation in the size of a gap between a wall on the substrate support intended to control fluid flow and the backside of the substrate. Variation in the size of this gap leads to variation in the time taken to remove immersion liquid from under the edge of the substrate. This in turn leads to a variation in the heat load (due to evaporative cooling) on the substrate and hence to overlay errors. Therefore it is desirable to increase the speed and consistency of removal of immersion liquid from under the edge of the substrate.

[0008] According to the present invention, there is provided a substrate support configured to support a substrate in a lithographic apparatus comprising: a first circumferential wall having a first height; a first drain radially outwards of the first circumferential wall and configured to extract fluid; a second circumferential wall having a second height and radially outwards of the first opening; a second drain radially outwards of the second circumferential wall and configured to extract fluid; a flow restriction having a third height and radially outwards of the second opening; and a third drain radially outwards of the flow restriction and the substrate and configured to extract fluid; wherein the first opening, the second opening and the third opening are distributed around the circumference of the substrate support; and the third height is less than the first height and the second height.

[0009] According to the present invention, there is also provided a lithographic apparatus including a substrate support.

[0010] According to the present invention, there is also provided a method of manufacturing a device using a substrate support.

[0011] Further embodiments, features and advantages of the present invention, as well as the structure and operation of the various embodiments, features and advantages of the present invention are described in detail below with reference to the accompanying drawings. DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the 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, and in which:

Figure 1 depicts a schematic overview of the lithographic apparatus;

Figures 2 and 3 depict, in cross-section, two different versions of a fluid handling system for use in a lithographic projection apparatus;

Figure 4 depicts, in cross-section, part of a substrate support in accordance with the present invention;

Figure 5 depicts, in cross-section, the substrate support of Figure 5 during extraction of immersion liquid;

Figure 6 depicts in cross-section, the substrate of Figure 4 when extraction of immersion liquid has stopped in the outer drain;

Figure 7 depicts, in cross-section, a variation of the substrate support of Figure 4;

Figure 8 depicts, in plan, the substrate support of Figure 4; and

Figure 9 depicts, in cross-section, part of a further substrate support.

[0013] The features shown in the Figures are not necessarily to scale, and the size and/or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature.

DETAILED DESCRIPTION

[0014] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

[0015] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase- shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0016] Figure 1 schematically depicts a lithographic apparatus. The lithographic apparatus includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a substrate table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support WT in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0017] In operation, the illumination system IL receives the radiation beam B from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at a plane of the patterning device MA.

[0018] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0019] The lithographic apparatus is of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fill an immersion space 11 between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0020] The lithographic apparatus may be of a type having two or more substrate supports WT (also named “dual stage”). In such a “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W. [0021] In addition to the substrate support WT, the lithographic apparatus may comprise a measurement stage (not depicted in figures). The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS. [0022] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0023] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz- rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0024] Immersion techniques have been introduced into lithographic systems to enable improved resolution of smaller features. In an immersion lithographic apparatus, a liquid layer of immersion liquid having a relatively high refractive index is interposed in the immersion space 11 between a projection system PS of the apparatus (through which the patterned beam is projected towards the substrate W) and the substrate W. The immersion liquid covers at least the part of the substrate W under a final element of the projection system PS. Thus, at least the portion of the substrate W undergoing exposure is immersed in the immersion liquid.

[0025] In commercial immersion lithography, the immersion liquid is water. Typically the water is distilled water of high purity, such as Ultra-Pure Water (UPW) which is commonly used in semiconductor fabrication plants. In an immersion system, the UPW is often purified and it may undergo additional treatment steps before supply to the immersion space 11 as immersion liquid. Other liquids with a high refractive index can be used besides water as the immersion liquid, for example: a hydrocarbon, such as a fluorohydrocarbon; and/or an aqueous solution. Further, other fluids besides liquid have been envisaged for use in immersion lithography. [0026] In this specification, reference will be made in the description to localized immersion in which the immersion liquid is confined, in use, to the immersion space 11 between the final element and a surface facing the final element. The facing surface is a surface of substrate W or a surface of the supporting stage (or substrate support WT) that is co-planar with the surface of the substrate W. (Please note that reference in the following text to surface of the substrate W also refers in addition or in the alternative to the surface of the substrate support WT, unless expressly stated otherwise; and vice versa). A fluid handling structure IH present between the projection system PS and the substrate support WT is used to confine the immersion liquid to the immersion space 11. The immersion space 11 filled by the immersion liquid is smaller in plan than the top surface of the substrate W and the immersion space 11 remains substantially stationary relative to the projection system PS while the substrate W and substrate support WT move underneath.

[0027] Other immersion systems have been envisaged such as an unconfined immersion system (a so-called ’All Wet’ immersion system) and a bath immersion system. In an unconfined immersion system, the immersion liquid covers more than the surface under the final element. The liquid outside the immersion space 11 is present as a thin liquid film. The liquid may cover the whole surface of the substrate W or even the substrate W and the substrate support WT co-planar with the substrate W. In a bath type system, the substrate W is fully immersed in a bath of immersion liquid.

[0028] The fluid handling structure IH is a structure which supplies the immersion liquid to the immersion space 11, removes the immersion liquid from the immersion space 11 and thereby confines the immersion liquid to the immersion space 11. It includes features which are a part of a fluid supply system. The arrangement disclosed in PCT patent application publication no. WO 99/49504 is an early fluid handling structure comprising pipes which either supply or recover the immersion liquid from the immersion space 11 and which operate depending on the relative motion of the stage beneath the projection system PS. In more recent designs, the fluid handling structure extends along at least a part of a boundary of the immersion space 11 between the final element of the projection system PS and the substrate support WT or substrate W, so as to in part define the immersion space 11.

[0029] The fluid handing structure IH may have a selection of different functions. Each function may be derived from a corresponding feature that enables the fluid handling structure IH to achieve that function. The fluid handling structure IH may be referred to by a number of different terms, each referring to a function, such as barrier member, seal member, fluid supply system, fluid removal system, liquid confinement structure, etc..

[0030] As a barrier member, the fluid handling structure IH is a barrier to the flow of the immersion liquid from the immersion space 11. As a liquid confinement structure, the structure confines the immersion liquid to the immersion space 11. As a seal member, sealing features of the fluid handling structure IH form a seal to confine the immersion liquid to the immersion space 11. The sealing features may include an additional gas flow from an opening in the surface of the seal member, such as a gas knife. [0031] The fluid handling structure IH may supply immersion fluid and therefore be a fluid supply system.

[0032] The fluid handling structure IH may at least partly confine immersion fluid and thereby be a fluid confinement system.

[0033] The fluid handling structure IH may provide a barrier to immersion fluid and thereby be a barrier member, such as a fluid confinement structure.

[0034] The fluid handling structure IH may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid.

[0035] The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure IH may be referred to as a seal member; such a seal member may be a fluid confinement structure.

[0036] Immersion liquid may be used as the immersion fluid. In that case the fluid handling structure IH may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.

[0037] A lithographic apparatus has a projection system PS. During exposure of a substrate W, the projection system PS projects a beam of patterned radiation onto the substrate W. To reach the substrate W, the path of the radiation beam B passes from the projection system PS through the immersion liquid confined by the fluid handling structure IH between the projection system PS and the substrate W. The projection system PS has a lens element, the last in the path of the beam, which is in contact with the immersion liquid. This lens element which is in contact with the immersion liquid may be referred to as ‘the last lens element’ or “the final element”. The final element is at least partly surrounded by the fluid handling structure IH. The fluid handling structure IH may confine the immersion liquid under the final element and above the facing surface.

[0038] As depicted in Figure 1, the lithographic apparatus comprises a controller 500. The controller 500 is configured to control the substrate table WT.

[0039] Figure 2 schematically depicts a localized liquid supply system or fluid handling system. The liquid supply system is provided with a fluid handling structure IH (or liquid confinement structure), which extends along at least a part of a boundary of the space 11 between the final element of the projection system PS and the support table WT or substrate W. The fluid handling structure IH is substantially stationary relative to the projection system PS in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). In an example, a seal is formed between the fluid handling structure IH and the surface of the substrate W and may be a contactless seal such as a gas seal (such a system with a gas seal is disclosed in EP 1,420, 298) or liquid seal.

[0040] The fluid handling structure IH at least partly confines the immersion liquid in the space 11 between the final element of the projection system PS and the substrate W. The space 11 is at least partly formed by the fluid handling structure IH positioned below and surrounding the final element of the projection system PS. Immersion liquid is brought into the space 11 below the projection system PS and within the fluid handling structure IH by one of liquid openings 13. The immersion liquid may be removed by another of liquid openings 13. The immersion liquid may be brought into the space 11 through at least two liquid openings 13. Which of liquid openings 13 is used to supply the immersion liquid and optionally which is used to remove the immersion liquid may depend on the direction of motion of the support table WT.

[0041] The immersion liquid may be confined in the space 11 by a contactless seal such as a gas seal 16 formed by a gas which, during use, is formed between the bottom of the fluid handling structure IH and the surface of the substrate W. The gas in the gas seal 16 is provided under pressure via inlet 15 to the gap between the fluid handling structure IH and substrate W. The gas is extracted via outlet 14. The overpressure on the gas inlet 15, vacuum level on the outlet 14 and geometry of the gap are arranged so that there is a high-velocity gas flow inwardly that confines the immersion liquid. Such a system is disclosed in US 2004/0207824, which is hereby incorporated by reference in its entirety. In an example, the fluid handling structure IH does not have the gas seal 16.

[0042] Figure 3 is a side cross-sectional view that depicts a further liquid supply system or fluid handling system. The arrangement illustrated in Figure 3 and described below may be applied to the lithographic apparatus described above and illustrated in Figure 1. The liquid supply system is provided with a fluid handling structure IH (or a liquid confinement structure), which extends along at least a part of a boundary of the space 11 between the final element of the projection system PS and the support table WT or substrate W.

[0043] The fluid handling structure IH at least partly confines the immersion liquid in the space 11 between the final element of the projection system PS and the substrate W. The space 11 is at least partly formed by the fluid handling structure IH positioned below and surrounding the final element of the projection system PS. In an example, the fluid handling structure IH comprises a main body member 53 and a porous member 33. The porous member 33 is plate shaped and has a plurality of holes (i.e., openings or pores). The porous member 33 may be a mesh plate wherein numerous small holes 84 are formed in a mesh. Such a system is disclosed in US 2010/0045949 Al, which is hereby incorporated by reference in its entirety.

[0044] The main body member 53 comprises supply ports 72, which are capable of supplying the immersion liquid to the space 11, and a recovery port 73, which is capable of recovering the immersion liquid from the space 11. The supply ports 72 are connected to a liquid supply apparatus 75 via passageways 74. The liquid supply apparatus 75 is capable of supplying the immersion liquid to the supply ports 72 through the corresponding passageway 74. The recovery port 73 is capable of recovering the immersion liquid from the space 11. The recovery port 73 is connected to a liquid recovery apparatus 80 via a passageway 79. The liquid recovery apparatus 80 recovers the immersion liquid recovered via the recovery port 73 through the passageway 29. The porous member 33 is disposed in the recovery port 73. Performing the liquid supply operation using the supply ports 72 and the liquid recovery operation using the porous member 33 forms the space 11 between the projection system PS and the fluid handling structure IH on one side and the substrate W on the other side.

[0045] Figure 4 illustrates part of a substrate support for a lithographic apparatus in accordance with the present invention. The arrangement illustrated in Figure 4 and described below may be a part of the substrate table WT and applied to the lithographic apparatus described above and illustrated in Figure 1. Figure 4 is a cross-section through a substrate support 20 and a substrate W. In an embodiment, the substrate support 20 comprises one or more conditioning channels (not shown) of a thermal conditioner. The substrate W is held by a support body 21 (e.g. a pimple or burl table) comprising one or more burls 41 (i.e., projections from the surface). The support body 21 is an example of an object holder. Another example of an object holder is a mask holder. An underpressure applied between the substrate W and the substrate support 20 helps ensure that the substrate W is held firmly in place.

[0046] A gap 5 exists between an edge of the substrate W and an edge of a recess in the substrate support 20. The edge of the recess in the substrate support 20 may be defined by a cover ring 101 which is optionally separate from the support body 21 of the substrate support 20. The cover ring 101 may be shaped, in plan, as a ring and surrounds the outer edge of the substrate W. When the edge of the substrate W is being imaged or at other times such as when the substrate W first moves under the projection system PS (as described above), the immersion space 11 filled with liquid by the fluid handling structure IH (for example) will pass at least partly over the gap 5 between the edge of the substrate W and the edge of the substrate support 20. This can result in liquid from the immersion space 11 entering the gap 5.

[0047] If immersion liquid gets between the substrate W and the support body 21 this can lead to difficulties, particularly when unloading the substrate W. In order to deal with the immersion liquid entering that gap 5, two drains 10, 12 are provided at the edge of the substrate W to remove immersion liquid which enters the gap 5. In an embodiment, each of the drains 10, 12 is annular so that the whole periphery of the substrate W is surrounded.

[0048] A primary function of the outer drain 10 (which is radially outward of the edge of the substrate W/support body 21) is to help prevent bubbles of gas from entering the immersion space 11 where the liquid of the fluid handling structure IH is present. Such bubbles may deleteriously affect the imaging of the substrate W. The outer drain 10 is present to help avoid gas in the gap 5 escaping into the immersion space 11 in the fluid handling structure IH. If gas does escape into the immersion space 11 , this can lead to a bubble which floats within the immersion space 11. Such a bubble, if in the path of the radiation beam B, may lead to an imaging error. The outer drain 10 is configured to remove gas from the gap 5 between the edge of the substrate W and the edge of the recess in the substrate support 20 in which the substrate W is placed. The outer drain 10 extracts mostly gas and only a small amount of immersion liquid.

[0049] The inner drain 12 (which is radially inward of the edge of the substrate W/support body 21) is provided to help prevent liquid which finds its way from the gap 5 to underneath the substrate W from preventing efficient release of the substrate W from the substrate support 20 after imaging. The provision of the inner drain 12 reduces or eliminates any problems which may occur due to liquid finding its way underneath the substrate W.

[0050] As depicted in Figure 4, in an embodiment the lithographic apparatus comprises a first extraction channel 102 and a second extraction channel 113. The first and second extraction channels 102, 113 may be adapted for the passage therethrough of a two phase flow. The first extraction channel 102 may be formed within the support body 21 or a separate block. The outer and inner drains 10, 12 are each provided with a respective opening 107, 117. The extraction channels 102, 113 are in fluid communication with the respective opening 107, 117 through respective passageways 103, 114.

[0051] As depicted in Figure 4, the cover ring 101 has an upper surface. The upper surface extends circumferentially around the substrate W on the support body 21. In use of the lithographic apparatus, the substrate support 20 moves relative to the fluid handling structure IH. During this relative movement, the fluid handling structure IH moves across the gap 5 between the cover ring 101 and the substrate W. In an embodiment the relative movement is caused by the substrate support 20 moving under the fluid handling structure IH. In an alternative embodiment the relative movement is caused by the fluid handling structure IH moving over the substrate support 20. In a further alternative embodiment the relative movement is provided by movement of both the substrate support 20 under the fluid handling structure IH and movement of the fluid handling structure IH over the substrate support 20. In the following description, movements of the fluid handling structure IH will be used to mean the relative movement of the fluid handling structure IH relative to the substrate support 20.

[0052] The support body 21 comprises a plurality of burls 41. When the substrate W is supported by the support body 21, the substrate W comes into direct contact with the burls 41 of the support body 21. The support body 21 is the part of the substrate support 20 that physically supports the underside of the substrate W. The distal ends of the burls 41 define a support plane at which the underside of the substrate W is supported. The underside of the substrate W comes into contact with the distal ends of the burls 41. The burls 41 are at the upper side of the support body 21.

[0053] When a substrate W is loaded onto the substrate support 20, the substrate W is first received by a plurality of e-pins (not shown) in their extended position. The e-pins are then retracted such that the substrate W is lowered towards the substrate support 20. When the underside of the substrate W comes into contact with the plurality of burls 41, the e-pins continue to retract such that the substrate W is no longer in contact with the e-pins, and the substrate W is fully supported by the plurality of burls 41. [0054] During the loading process, the pressure of the gas between the substrate W and the substrate support 20 may be controlled to control the loading process. For example, a relatively high pressure may established during the loading process to generate an upward force to deform the edges of the substrate W upwards. This may be such that umbrella-shaped deformation of the substrate W is reduced or completely eliminated. After loading is completed, gas may be extracted by via one or more clamp openings (not shown) such that an under-pressure (pressure that is less than the ambient pressure) is established below the substrate W. A force is therefore applied to the substrate W in a direction that is towards the substrate support 20, such that the substrate W is clamped to the substrate support 20.

[0055] To unload the substrate W from the substrate support 20 after exposure of the substrate W has been completed, the pressure below the substrate W is gradually increased towards the ambient pressure, such that the clamping force exerted on the substrate W diminishes. The e-pins (not shown) are extended from their retracted position. As the e-pins are extended, their distal portions come into contact with the underside of the substrate W. As the e-pins continue to extend, the substrate W is lifted off the plurality of burls 41.

[0056] As shown in Figure 4, the substrate support 20 further comprises a plurality of seals 31, 32. The seals 31, 32 are circumferential walls protruding from the substrate support 20. In this example, there are at least two seals: an inner seal 31 and an outer seal 32 radially outwards of the inner seal 31. The inner drain 12 is between the inner seal 31 and outer seal 32. When the substrate W is supported by the substrate support 20, top surfaces of the plurality of seals 31, 32 (that is, surfaces of the plurality of seals 31, 32 that are substantially parallel with and closest to the substrate W) do not contact the underside of the substrate W. However, the distances between the top surfaces of the plurality of seals 31, 32 and the underside of the substrate W are such that at least a partial seal is formed between the top surfaces of the seals 31, 32 and the underside of the substrate W. That is, the plurality of seals 31, 32 inhibit, but do not fully prevent, the flow of fluid.

[0057] The distance between the top surfaces of the seals 31, 32 and the underside of the substrate W is preferably smaller than 10 pm and further preferably smaller than 5 pm, and preferably larger than 1 pm and further preferably larger than 3 pm. In an embodiment, the distance between the top surface of a seal 31, 32 and the underside of the substrate W may not be the same for each seal 31, 32. [0058] For a substrate support 20 configured to support a substrate W with a diameter of 300 mm, the following dimensions are preferable. For substrates W that do not have a diameter of 300 mm, the dimensions may be similar or scaled in accordance with the diameter of the substrate W. The distance between the outer seal 32 and the circumferential edge of the substrate W is preferably less than 5 mm, further preferably less than 3 mm, and further preferably less than 2.5 mm. The distance between the outer seal 32 and the circumferential edge of the substrate W is preferably greater than 1 mm. [0059] The width of each of the plurality of seals 31, 32 (that is, the distance in the radial direction between the seal’s inner circumferential edge and the seal’s outer circumferential edge) is preferably greater than 0.1 mm, and further preferably greater than 0.2 mm. The width of each of the plurality of seals 31, 32 is preferably less than 1 mm and further preferably less than 0.6 mm.

[0060] The width of each of the plurality of seals 31, 32 may not be the same. The width of one of the seals 31, 32 may be made larger in order to ensure that a ring of burls 42 can be situated on the top surface of the seal 32 (not shown). In an embodiment, the width of the outer seal 32 is greater than the width of the inner seal 31. Preferably, the width of the outer seal 32 is greater than 0.4 mm and less than 0.6 mm, such as 0.5 mm. The width of the inner seal 31 is preferably less than 0.3 mm and greater than 0.2 mm, such as 0.25 mm.

[0061] In an embodiment, the plurality of burls 41 are arranged in circumferential rings. A radially outermost circumferential ring of burls 42 may be situated between the inner seal 31 and the outer seal 32. When the substrate support 20 supports a substrate W, it is preferable for the outer circumferential ring of burls 42 to remain dry, i.e. not to come into contact with the immersion fluid. This is to prevent wear to the outer circumferential ring of burl 42.

[0062] In an embodiment, when a substrate W with a diameter of 300 mm is clamped onto the substrate support 20, the radially outermost circumferential ring of burls 42 are preferably less than 10 mm from the circumferential edge of the substrate W, further preferably less than 5 mm from the circumferential edge of the substrate W, further preferably less than 4 mm from the circumferential edge of the substrate W, and further preferably less than 3.5 mm from the circumferential edge of the substrate W. When the substrate W with a diameter of 300 mm is clamped onto the substrate support 20, the radially outermost circumferential ring of burls 42 are preferably more than 1 mm away from the circumferential edge of the substrate W.

[0063] The diameter of each of the burls 41, 42 may not be the same. For example, each circumferential ring of burls 41, 42 may have a different diameter. The diameter of burls 41, 42 in a circumferential ring may depend on the radial distance from the centre of the substrate support 20. This is because the contact stiffness of a burl is proportional to its diameter. Consequently, by changing the diameter of the burls 41, 42, the amount of deformation at the burl-substrate interface can be adjusted. This means that the diameter of the burls 41, 42 can be controlled to ensure that the substrate W remains within the required flatness tolerance, despite the complex pressure profile on the underside of the substrate W. The required diameter for each ring of burls 41, 42 may be determined by optimization through experimentation or simulation.

[0064] Preferably, the diameter of the burls 42 in the radially outermost circumferential ring (or rings) is between 200 pm and 350 pm, and the diameter of the burls 41 in the innermost rings is between 150 pm and 250 pm. Further preferably, the diameter of the burls 42 in the radially outermost circumferential ring(s) is between 250 pm and 330 pm, and the diameter of the burls 41 in other circumferential rings is between 190 pm and 240 pm, Further preferably, the diameter of the burls 42 in the radially outermost circumferential ring(s) is between 260 |im and 280 |im, such as 270 |im, and the diameter of burls 41 in other rings is between 200 |im and 220 |im, such as 210 |im.

[0065] Burls with a smaller diameter are likely to wear down faster, which would mean that the substrate support 20 (or the support body 21) would need to be replaced more regularly. To avoid the burls 41, 42 wearing down too quickly, the diameters of all of the burls 41, 42 should be greater than 150 pm.

[0066] Other techniques could be used to adjust the stiffness of the burls 41, 42, such as changing the material, or applying a coating, e.g., diamond and DLC. However, the substrate W typically has a lower stiffness than the burls 41, 42, so the deformation at the substrate-burl interface is not significantly affected by the burl material or coating. Consequently, these techniques are not particularly effective for controlling the flatness of the substrate W.

[0067] In the above embodiment, the burl pitch (the distance between burls 41, 42) is preferably greater than 0.5 mm, further preferably greater than 1 mm, and further preferably greater than 1.4 mm. The burl pitch is preferably less than 3 mm, further preferably less than 2 mm, and further preferably less than 1.6 mm, such as 1.5 mm.

[0068] The burls 41 may have a height (that is, a dimension from the surface of the support body 21 to the burl’s distal end) of approximately 150 pm. However, the burls 41 may be any suitable height.

[0069] The material of the substrate support 20 is not particularly limited, and could be any suitable material known in the art. Preferably, the substrate support 20 may be made out of silicon carbide (SiSiC).

[0070] Manufacture of the substrate support 20 may involve standard techniques known in the art. Some openings may be too small for methods such as electrical discharge machining (EDM). In this case, laser drilling may be utilized.

[0071] In an embodiment of the invention, an intermediate drain 119 is provided outside outer seal 32 and within outer drain 10. Desirably intermediate drain 119 is inside the outer periphery of substrate W. A flow restriction 33 is provided between intermediate drain 119 and outer drain 12. The intermediate drain 119 and flow restriction 33 work together to increase the rate of extraction of immersion liquid and also make the extraction of immersion liquid more consistent even when variations in the thickness of a backside coating on the substrate W cause variation in the size of the gap between the backside of the substrate W and the upper surface of the substrate support 20.

[0072] Intermediate drain 119 comprises an opening 121 in the upper surface of support body 21 that is connected via a passageway 118 to a source of under pressure, e.g. extraction channel 102. First extraction channel 102 is desirably connected to a low pressure source in order to achieve efficient extraction of immersion liquid via intermediate drain 119 as well as outer drain 10.

[0073] Flow restriction 33 may comprise a raised portion of the support body 21 or a porous member mounted on the support body 21. Desirably, the height of flow restriction 33 is less than the heights of the inner seal 31 and outer seal 32 so that the gap between substrate W and the top of flow restriction 33 is greater than the gap between the substrate W and the top surfaces of inner seal 31 and outer seal 32. Desirably, the gap between substrate W and flow restriction 33 is at least 10 pm, preferably at least 20 pm and more preferably at least 30 pm. Desirably, the distance between the support plane and top surface of the flow restriction 33 is less than 100 pm, preferably less than 70 pm, and more preferably less than 50 pm. Flow restriction 33 desirably has a width in the radial direction of at least 0.1 mm, preferably at least 0.25 mm. Desirably the flow restriction 33 has a width in the radial direction less than 2 mm, more desirably less than 0.25 mm. Flow restriction 33 can have several functions. For example, flow restriction 33 can ensure that the extraction flow through intermediate drain 119 is not too high in order to prevent excessive evaporation and the resulting cooling load that might occur with a high gas flow. Flow restriction 33 can assist in making the pressure in the vicinity of intermediate drain 119 more consistent and provide more uniform extraction of immersion liquid. Flow restriction 33 can prevent a situation where only air is extracted and liquid is left behind in some places.

[0074] Desirably, the flow restriction 33 is the outermost feature of the support body 21 that is within the outer periphery of the substrate W. In other words, the flow restriction 33 is the feature of the upper surface of the substrate body 21 that is within the footprint of the substrate W and closest to the edge of the substrate W. Flow restriction 33 can thereby be located opposite that part of the backside of substrate W where the thickness of the backside coating is most variable. On the other hand, the inner seal 31 and outer seal 32 can be located at positions where the thickness of the backside coating of substrate W is relatively uniform. Because flow restriction 33 is relatively wide and the gap between the top of flow restriction 33 and the backside of substrate W is relatively large compared to variations in thickness of the backside coating, the effect of flow restriction 33 is sufficiently uniform irrespective of variations in the thickness of the backside coating of substrate W. It should be noted that in the outermost parts of the substrate W, the thickness of backside coating may vary between substrates and around the circumference of substrates. The thickness of the backside coating of substrate W at the position of flow restriction 33 may vary because of variations in the extent of coverage of the backside of substrate W. The dimensions of flow restriction 33, in particular its height and width, can be optimised based on simulation or empirical evidence, in particular in view of expected variations in the thickness of the backside coating of substrate W in the outer part thereof.

[0075] Figures 4, 5 and 6 illustrate the operation of substrate support 20 in removing immersion liquid. Figure 4 depicts the situation when the immersion space 11 containing immersion liquid is located above gap 5 between substrate W and substrate support 20. Gap 5 and the spaces below the substrate W outside of inner drain 12 quickly become filled with immersion liquid. Since first and second extraction channels 102, 113 are connected to an under pressure, immersion liquid will be extracted therethrough. After the immersion space 11 has moved away from above gap 5, immersion liquid in gap 5 and in the vicinity of outer drain 10 is quickly removed, as depicted in Figure 5. However, immersion liquid may still remain in the vicinity of intermediate drain 119 and inner drain 12. Therefore, extraction of immersion liquid will continue until the situation depicted in Figure 6 is reached. As depicted in Figure 6, only thin films of immersion liquid remain in the most of the various conduits and passages of the substrate support 20. Immersion liquid may remain in first extraction channel 102 if the extraction of the outer drain 10 is switched off when exposure is not being performed. It is desirable that any residual thin films of immersion liquid are minimised in order to minimise the cooling load caused by evaporation of the immersion liquid. Compared to a similar substrate support lacking the intermediate drain 119 and flow restriction 33, the process of extraction of immersion liquid is both quicker and more consistent in an embodiment of the invention. [0076] The inner drain 12, intermediate drain 119 and outer drain 10 may comprise a groove or gutter formed in the upper surface of support body 21. The openings 107, 117, 121 through which the immersion liquid is extracted may in each case comprise a continuous slit around the whole circumference of the body 21 or a series of discrete openings. Such discrete openings may comprise circular holes, which are convenient to manufacture, or elongate slits, which may provide for a higher fluid flow.

[0077] Figure 8 depicts the substrate support 20 in plan from above. As depicted, desirably, the inner drain 12, intermediate drain 119 and outer drain 10 extend all the way around the circumference of the support body 21. Any or all of the inner drain 12, intermediate drain 119 and outer drain 10 may be segmented with separate segments independently selectively connectable to an under pressure so that fluid is only extracted from selected regions of the periphery of the substrate W. By not extracting fluid from locations where there is no immersion liquid, power consumption may be reduced and the capacity of pumps creating the under pressure can be reduced. In some cases, it may not be necessary to provide the intermediate drain 119 all the way around the periphery of the support body 21. For example, if edge crossings by the immersion liquid will only occur in certain positions, it may be sufficient to provide the intermediate drain 119 only in the vicinity of locations where edge crossings will occur.

[0078] Figure 7 depicts a variant of the substrate support 21 of Figure 4. In the arrangement of Figure 7, the intermediate drain 119 is connected to an intermediate extraction channel 120 rather than to the first extraction 102. In this variation, the pressure in intermediate extraction channel 120 can be controlled independently of the pressure in first extraction channel 102 and second extraction channel 113, thereby providing additional control over the rate of extraction of fluid from below substrate W. [0079] An alternative substrate support 200 is depicted in Figure 9. Substrate support 200 comprises three major parts, support body 221 which supports substrate W but has a slightly smaller diameter, cove ring 211 which provides an upper surface coplanar with upper surface of substrate W and extraction ring 231 which surrounds support body 221 and supports cover ring 211. The cover ring 211 is optionally separate from the extraction ring 231. In the substrate support 200, the functions of supporting the substrate W and removing immersion liquid are separated between the support body 221 and the extraction ring 231. The support body 221 and extraction ring 231 may be thermally and/or mechanically isolated from one another by a gap 240. The gap 240 may comprise a vacuum or a gas. A seal (not shown) may be optionally provided to seal the gap 240.

[0080] Support body 221 has burls 41, 42 on its upper surface as described above in relation to Figures 4 to 7. Additional burls (not shown) may optionally be provided to the extraction ring 231 to support the substrate W. The support body 221 also has inner seal 31 and outer seal 32 as described above. Gas supply/extraction channels 222, 223 are provided to control the pressure below substrate W for clamping during operation and control of the shape of substrate W during loading and unloading operations. For example, gas opening may be configured to supply gas during the unloading operation. Extraction ring 231 comprises an outer drain 232 below cover ring 211 and an inner drain 233 below substrate W. In other words, inner drain 233 is inside gap 5 between substrate W and cover ring 211 whilst outer drain 232 is outside gap 5. Inner drain 233 and outer drain 232 are connected to respective extraction channels 234, 235. Flow control structures 236 may be provided in the vicinity of the inner drain 233. Desirably, pressures in the extraction channels 234 and 235 are arranged to maximise speed of extraction of immersion fluid from the gap 5 and to prevent any immersion fluid reaching the support body 221.

[0081] The present invention may provide a lithographic apparatus. The lithographic apparatus may have any/all of the other features or components of the lithographic apparatus as described above. For example, the lithographic apparatus may optionally comprise at least one or more of a source SO, an illumination system IL, a projection system PS, a substrate table WT, etc..

[0082] Specifically, the lithographic apparatus may comprise the projection system PS configured to project the radiation beam B towards the region of the surface of a substrate W. The lithographic apparatus may further comprise the substrate support 20 as described in any of the above embodiments and variations.

[0083] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin film magnetic heads, etc..

[0084] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0085] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools.

[0086] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography.

[0087] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A substrate support configured to support a substrate in a lithographic apparatus comprising: a first circumferential (31) wall having a first height; a first drain (12) radially outwards of the first circumferential wall and configured to extract fluid; a second circumferential (32) wall having a second height and radially outwards of the first opening; a second drain (119) radially outwards of the second circumferential wall and configured to extract fluid; a flow restriction (33) having a third height and radially outwards of the second opening; and a third drain (10) radially outwards of the flow restriction and the substrate and configured to extract fluid; wherein the first opening, the second opening and the third opening are distributed around the circumference of the substrate support; and the third height is less than the first height and the second height.

2. The substrate support according to claim 1, wherein the second drain is distributed around at least 80%, desirably at least 90%, of the circumference of the substrate support.

3. The substrate support according to claim 1 or 2, wherein the second drain is distributed around all parts of the circumference of the substrate support that are crossed by immersion liquid confined by a liquid handling system of the lithographic apparatus.

4. The substrate support according to claim 1, 2 or 3, wherein the flow restriction extends around the same extent of the circumference of the substrate support as the second drain.

5. The substrate support according to any preceding claim, further comprising a plurality of burls that define a support plane for supporting the substrate and wherein the distance between the support plane and the top surface of at least one of the first and second circumferential walls is between 1 pm and 10 pm, preferably between 1 pm and 5 pm, and further preferably between 3 pm and 5 pm, and/or further comprising a plurality of burls that define a support plane for supporting the substrate and wherein the distance between the support plane and the top surface of the flow restriction is between 10 pm and 100 pm, preferably between 20 pm and 70 pm and further preferably between 30 pm and 50 pm.

6. The substrate support according to any preceding claim, wherein the flow restriction comprises a third circumferential wall, and/or wherein the flow restriction comprises a porous member, and/or wherein the flow restriction has a width in the range of from 0.1 mm to 2 mm, preferably between 0.25 mm and 0.5 mm.

7. The substrate support according to any preceding claim, wherein the distance between the second drain and the outer edge of the substrate is less than 3 mm.

8. The substrate support according to any preceding claim, wherein the second drain is within the outer circumference of the substrate.

9. The substrate support according to any preceding claim, wherein the first drain is in fluid communication with a first extractor channel via at least one first passageway, the second drain is in fluid communication with a second extractor channel via at least one second passageway, and the third drain is in fluid communication with the second extractor channel via at least one third passageway, or wherein the first drain is in fluid communication with a first extractor channel via at least one first passageway, the second drain is in fluid communication with a second extractor channel via at least one second passageway, and the third drain is in fluid communication with a third extractor channel via at least one third passageway.

10. The substrate support according to claim 9, wherein the second passageway is within the body of the substrate support.

11. The substrate support according to any preceding claim, wherein at least one of the first drain and the second drain comprises a gutter in the surface of the substrate support facing the substrate and a plurality of openings in the gutter.

12. The substrate support according to any preceding claim, wherein at least one of the first drain and the second drain comprises a first segment and a second segment.

13. The substrate support according to claim 12, wherein the first segment and the second segment are separately selectively connectable to an extractor channel.

14. A lithographic apparatus including the substrate support according to any of claims 1-13.

15. A method of manufacturing a device using the substrate support according to any of claims