EP4728326A1 - Substrate support - Google Patents

Abstract

Disclosed herein is a substrate support (200) configured to support a substrate (W), the substrate support comprising: an upper surface (225); a plurality of support members (241) extending above the upper surface; an extraction gutter (230) defined by a first circumferential wall and a second circumferential wall, wherein the first circumferential wall and the second circumferential wall extend above the upper surface; an extraction opening (217) formed in the upper surface within the extraction gutter; and one or more fluid-retaining features (250) formed within the extraction gutter.

Description

SUBSTRATE SUPPORT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23180090.5 which was filed on 19 June 2023 and EP application 23204104.6 which was filed on 17 October 2023 and EP application 24159835.8 which was filed on 27 February 2024 which are incorporated herein in their entirety by reference.

FIELD

[0002] The present invention relates to a substrate support, a lithographic apparatus comprising a substrate support, a method of manufacturing a substrate support and a method of manufacturing a device comprising supporting a substrate on 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, use a projection system to 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 liquid having a relatively high refractive index, such as water, on the substrate during exposure. The effect of the immersion liquid 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 liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus. [0006] The immersion liquid 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] When a gap between the substrate and a surrounding cover ring passes under the fluid handling structure, immersion liquid may enter an extraction gutter between the substrate and the substrate support. When immersion liquid is present on the surfaces of the extraction gutter, a thermal, i.e., cold load may be exerted on the surfaces of the extraction gutter. After a period of time, the immersion liquid may fully evaporate. If the immersion liquid has fully evaporated, the thermal load is no longer exerted on the surfaces of the extraction gutter. This may lead to transient structural deformation, which can increase overlay error, and, specifically, wafer load grid (WLG).

[0008] The present invention is directed to providing a configuration in which immersion liquid is retained within the extraction gutter. Consequently, a consistent thermal load may be exerted on the surfaces of the extraction gutter, and the magnitude of transient thermal deformation may be reduced. The transient thermal load may be delayed to a time when its affect is less. This may lead to an improvement in overlay error, and, specifically, in wafer load grid (WLG).

[0009] According to a first aspect of the invention, there is provided a substrate support configured to support a substrate, the substrate support comprising: an upper surface; a plurality of support members extending above the upper surface; an extraction gutter defined by a first circumferential wall and a second circumferential wall, wherein the first circumferential wall and the second circumferential wall extend above the upper surface; an extraction opening formed in the upper surface within the extraction gutter; and one or more fluid-retaining features formed within the extraction gutter.

[0010] According to a second aspect of the invention, there is provided a lithographic apparatus comprising the substrate support described above.

[0011] According to a third aspect of the present invention, there is provided a method of manufacturing a substrate support as described above, the method comprising forming the one or more recessed portions by removing material from the upper surface using laser structuring.

[0012] According to a fourth aspect of the present invention, there is provided a method of manufacturing a device comprising supporting a substrate on a substrate support as described above.

[0013] 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

[0014] 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 a lithographic apparatus.

Figure 2 depicts, in cross-section, a substrate support.

Figure 3 depicts, in plan-view, a substrate support.

Figure 4 depicts a schematic representation of the path of a fluid handling structure over a substrate during exposure of the substrate.

Figure 5 depicts a circumferential sectional view of a substrate support not in accordance with the present invention, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 6 depicts a circumferential sectional view of a substrate support in accordance with a first embodiment of the present invention, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 7 depicts an extraction gutter of a substrate support in accordance with the first embodiment of the present invention.

Figure 8 depicts a circumferential sectional view of a substrate support in accordance with a second embodiment of the present invention, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 9 depicts an extraction gutter of a substrate support in accordance with the second embodiment of the present invention.

Figure 10 depicts a circumferential sectional view of a substrate support in accordance with a third embodiment of the present invention, the cutting plane of the sectional view passing through the radially outmost ring of burls within the extraction gutter.

Figure 11 depicts an extraction gutter of a substrate support in accordance with the third embodiment of the present invention.

Figure 12 depicts a circumferential sectional view of a substrate support in accordance with a fourth embodiment of the present invention, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 13 depicts an extraction gutter of a substrate support in accordance with the fourth embodiment of the present invention.

Figure 14 depicts a substrate and a substrate support.

Figures 15A and 15B depict a burl.

Figure 16 depicts a burl.

Figure 17 depicts a plan view of a substrate support in which capillary bridges of immersion liquid have formed between adjacent burls.

Figure 18A depicts circumferential sectional view of a substrate support with a bridging feature, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 18B depicts an extraction gutter of the substrate support depicted in Figure 18 A. Figure 18C depicts a plan view of the substrate support depicted in Figure 18 A.

Figure 19A depicts circumferential sectional view of a substrate support with a bridging feature, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 19B depicts an extraction gutter of the substrate support depicted in Figure 19 A.

Figure 19C depicts a plan view of the substrate support depicted in Figure 19 A.

Figure 20A depicts circumferential sectional view of a substrate support with a bridging feature, the cutting plane of the sectional view passing through the radially outermost ring of burls within the extraction gutter.

Figure 20B depicts an extraction gutter of the substrate support depicted in Figure 20A.

Figure 20C depicts a plan view of the substrate support depicted in Figure 20A.

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

[0015] 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).

[0016] 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.

[0017] 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. [0018] 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.

[0019] 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.

[0020] 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 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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 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.

[0026] 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 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.

[0027] 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 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. The immersion space filled by the immersion liquid is smaller in plan than the top surface of the substrate W and the immersion space remains substantially stationary relative to the projection system PS while the substrate W and substrate support WT move underneath.

[0028] 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 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.

[0029] The fluid handling structure IH is a structure which supplies the immersion liquid to the immersion space, removes the immersion liquid from the immersion space and thereby confines the immersion liquid to the immersion space. 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 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 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.

[0030] 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..

[0031] Immersion liquid may be used as the immersion liquid. 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.

[0032] 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. [0033] As depicted in Figure 1 , the lithographic apparatus comprises a controller 500. The controller 500 is configured to control the substrate support WT.

[0034] Within the lithographic apparatus, the substrate W is held by a support body (e.g. a pimple or burl table). The support body may be referred to as a substrate support.

[0035] Figures 2 and 3 illustrate a substrate support 20. Figure 2 depicts the substrate support 20 in cross-section, with a substrate W clamped thereon. Figure 3 depicts the substrate support 20 in planview. The substrate support 20 depicted in Figures 2 and 3 and described below may be used within the lithographic apparatus described in relation to Figure 1.

[0036] The substrate support 20 may comprise one or more burls 41 (i.e., projections or protrusions from the surface). The burls 41 may also be referred to as support members. The burls 41 may protrude from an upper surface 25 of the substrate support 20. Distal ends of the burls 41 form a 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.

[0037] The substrate support 20 is an example of an object holder. Another example of an object holder is a mask holder. An under-pressure applied between the substrate W and the substrate support 20 helps ensure that the substrate W is held firmly in place.

[0038] In an embodiment, the substrate support 20 comprises one or more conditioning channels 61 of a thermal conditioner. A gap 5 exists between an edge of the substrate W and an edge of the substrate support 20. 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 filled with immersion 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 immersion liquid from the immersion space entering the gap 5. If immersion liquid gets between the substrate W and the support body 21 this can lead to difficulties, particularly when unloading the substrate W.

[0039] In order to deal with the immersion liquid entering the gap 5 at least one drain 10, 12 is provided at the edge of the substrate W to remove immersion liquid which enters the gap 5. In the example of Figure 2 two drains 10, 12 are illustrated though there may only be one drain or there could be more than two drains. In an embodiment, each of the drains 10, 12 is annular so that the whole periphery of the substrate W is surrounded.

[0040] A primary function of the first 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 where the immersion liquid from the fluid handling structure IH is present. Such bubbles may deleteriously affect the imaging of the substrate W. The first drain 10 is present to help avoid gas in the gap 5 escaping from the fluid handling structure IH into the immersion space filled by the immersion liquid. If gas does escape into the immersion space, this can lead to a bubble which floats within the immersion space. Such a bubble, if in the path of the projection beam, may lead to an imaging error. The first 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 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. In the x/y plane, the cover ring 101 may be shaped, as a ring that surrounds the outer edge of the substrate W. The first drain 10 extracts mostly gas and only a small amount of immersion liquid.

[0041] The second 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 WT after imaging. The provision of the second drain 12 reduces or eliminates any problems which may occur due to immersion liquid finding its way underneath the substrate W.

[0042] In the present disclosure, terms such as “radially inward”, “radially inner”, “radially outward” and “radially outer” are used to define the radial positions of various features relative to the centre of the substrate support 20 in the horizontal plane, i.e. relative to the centre of the substrate support 20 in a plane that is parallel to the plane formed by the distal ends of the plurality of burls 41.

[0043] As depicted in Figure 2, the lithographic apparatus comprises a first extraction channel 102 and a second extraction channel 113 for the passage therethrough of a two phase flow. The first and second extraction channels 102, 113 may be formed within the support body 21 or as a separate component. The first and second drains 10, 12 are each provided with a respective opening (a first extraction opening 107 and a second extraction opening 117) and a respective extraction channel (a first extraction channel 102 and a second extraction channel 113). The extraction channels 102, 113 are in fluid communication with their respective openings 107, 117 through respective passageways (a first extraction passageway 103 and a second extraction passageway 114).

[0044] The second extraction opening 117 may be provided within an extraction gutter 130. The extraction gutter 130 may correspond to a radial portion of a region between the substrate support 20 and the substrate W. All of the extraction gutter 130 may be located above the upper surface 25 of the substrate support 20. The extraction gutter 130 may be defined by circumferential seals 131, 132. The circumferential seals 131, 132 may be walls extending circumferentially around the substrate support 20 and protruding above upper surface 25 of the support body 21 of the substrate support 20. The extraction gutter 130 may be a portion extending radially between an inner circumferential seal 131 and an outer circumferential seal 132. The inner circumferential seal 131 may be referred to as a first circumferential wall and the outer circumferential seal 132 may be referred to as a second circumferential wall. The extraction gutter 130 may be substantially or wholly beneath the substrate W when the substrate W is supported by the substrate support 20. That is, the extraction gutter 130 may be radially inward of the edge of the substrate W when the substrate W is supported by the substrate support 20. [0045] In context of the term “extraction gutter” 130, “gutter” may be considered to mean an open channel or a trough. The extraction gutter 130 may be defined by a radially outward vertical wall of the inner circumferential seal 131, a radially inward vertical wall of the outer circumferential seal 132, and the upper surface 25 of the substrate support 20. When considering the substrate support 20 in isolation, the extraction gutter 130 may not have a top surface. Thus, the gutter defined by the surfaces described above may be open. In use, when a substrate W is supported by the substrate support 20, a lower surface of the substrate W may form an upper surface of the extraction gutter 130.

[0046] When a substrate W is supported by the substrate support 20, a distance between upper surfaces of the circumferential seals 131, 132 and the lower surface of the substrate W may be such that a partial seal is formed. Thus, fluid within the extraction gutter 130 may be substantially prevented from moving radially inwards or radially outwards of the extraction gutter 130.

[0047] Whilst the extraction gutter 130 described above is part of a substrate support 20 that may not be in accordance with the present invention, the description of the extraction gutter 130 itself may apply to substrate supports 20 in accordance with the present invention. For example, this description of the extraction gutter 130 may apply for the first, second third and fourth embodiments of the present invention described below.

[0048] The plurality of burls 41 may be distributed across the upper surface 25 of the substrate support 20. A radially outermost circumferential ring of burls 41 may be provided within the extraction gutter 130. A plurality of second extraction openings 117 may be provided within the extraction gutter 130. The plurality of second extraction openings 117 may be distributed circumferentially around the extraction gutter 130. The substrate support 20 may further comprise other openings. For example, the substrate support 20 may comprise clamp openings 127. The clamp openings 127 may be provided in a radially inward region of the substrate support 20, relative to the extraction gutter 130. The clamp openings 127 may be configured to extract fluid, such that when a substrate W is supported by the substrate support 20, the clamp openings 127 can create an under pressure in the region between the substrate support 20 and the substrate W such that the substrate W can become clamped to the substrate support 20.

[0049] The gap 5 between the cover ring 101 and the substrate W is caused by a central opening in the cover ring 101 having a larger diameter that the substrate W. In places, the gap 5 may be about 200pm. When the fluid handling structure IH passes across the gap 5, at least some of the immersion liquid flows through the gap 5. After flowing through the gap 5, some immersion liquid may enter the extraction gutter 130 and a region corresponding to the first extraction opening 107.

[0050] Immersion liquid entering the extraction gutter 130 may be extracted by the second extraction opening 117. For example, the second extraction channel 113 may be connected to a vacuum pressure source, i.e., a device or component which is able to provide an extraction pressure that is less than the environment surrounding the substrate support 20. This may be such that immersion liquid entering the extraction gutter 130 is pulled towards the second extraction opening 117. In this manner, immersion liquid can be made to flow through the second extraction passageway 114 and out through the second extraction channel 113.

[0051] Despite the extraction of immersion liquid from the extraction gutter 130, some immersion liquid may remain present on the surfaces within the extraction gutter 130. For example, immersion liquid may remain present on the upper surface 25 of the substrate support 20 within the extraction gutter 130. Immersion liquid may also remain present on the vertical surfaces of the circumferential seals 131, 132 which define the boundaries of the extraction gutter 130 in the radial direction. When the majority of the immersion liquid has been removed from the extraction gutter 130, dry air may enter the extraction gutter 130 via the gap 5 between the cover ring 101 and the substrate W. Particularly, dry air may be drawn into the extraction gutter 130 and out through the second extraction channel 113 by the extraction pressure which is provided to the second extraction channel 113.

[0052] The flow of dry air within the extraction gutter 130 may lead to the evaporation of immersion liquid present on the surfaces of the extraction gutter 130. The evaporation of immersion liquid from the surfaces of the extraction gutter 130 may cause a thermal load to be exerted on the surfaces of the extraction channel 130. Specifically, a cold load may be exerted on the surfaces of the extraction gutter 130. This cold load may cause the temperature of the support body 21 of the substrate support 20 around the extraction gutter 130 to be reduced.

[0053] Once all of the immersion liquid has evaporated from the surfaces of the extraction gutter 130, the cold load may cease to be exerted on the substrate support 20. This means that the temperature of the support body 21 of the substrate support 20 around the extraction gutter 130 may increase. Thus, in general, the temperature of the support body 21 of the substrate support 20 around the extraction gutter 130 may depend on whether immersion liquid is present on the surfaces of the extraction gutter 130. During use, there may be times at which immersion liquid is present on the surfaces of the extraction gutter 130, and times at which immersion liquid is not present on the surfaces of the extraction gutter 130. Consequently, the temperature of the support body 21 of the substrate support 20 around the extraction gutter 130 may vary over time.

[0054] Variation in the temperature of the support body 21 over time may cause structural deformation. Specifically, when the temperature of the support body 21 increases, the support body 21 may expand. When the temperature of the support body 21 decreases, the temperature of the support body 21 may contract. Consequently, the exact shape and size of the substrate support 20 (and, therefore, the flatness of the support plane formed by the plurality of burls 41) may vary over time.

[0055] If structural deformation of the substrate support 20 occurs during one or more critical periods in the operation of a lithographic apparatus, overlay error may be increased. Overlay error is a measure of misalignment between corresponding features in different layers of a device manufactured by lithography. Thus, if the thermal load on the substrate support 20 changes during a critical period, overlay error may be increased. This may occur if, for example, at the start of a critical period, immersion liquid is present within the extraction gutter 130, but over the course of the critical period, all of the immersion liquid evaporates (i.e., the extraction gutter 130 dries out). Critical periods during the operation of a lithographic apparatus may include: (i) when a substrate W is loaded onto a substrate support 20; (ii) during processes where vertical displacement and in-plane deformation of the substrate W are measured and mitigated; and (iii) during exposure of the substrate W.

[0056] Overlay error may also be increased if the conditions in the extraction gutter 130 (i.e., the presence or absence of immersion liquid) are different for each of a series of operations formed on a substrate W. For example, overlay error may be increased in the case that, when exposing a substrate W for a first layer, the extraction gutter 130 is dry, and, when exposing a substrate W with subsequent, the extraction gutter 130 is, at least partially, wet.

[0057] Figure 5 depicts a circumferential sectional view of a substrate support 20 not in accordance with the present invention (i.e., a comparative example). The sectional view is taken such that it passes through a radially outermost ring of burls 41 within the extraction gutter 130 of the substrate support 20. The substrate support 20 may be similar in structure to the substrate support 20 depicted in Figures 2 and 3 and described above.

[0058] As depicted in Figure 5, an upper surface 25 of the substrate support 20 within the extraction gutter 130 is substantially flat. That is, apart from the extraction openings 117 and the plurality of burls 41, there are no features. In some cases, there may be additional features. However, the extraction gutter 130 does not comprise any features which are configured to retain fluid.

[0059] In such a substrate support 20, the length of time required for the extraction gutter 130 to dry out (i.e., the time required for the extraction gutter 130 to transition from being filled with immersion liquid to being substantially devoid of immersion liquid) may be between 10 and 30 seconds. For example, the time taken for the extraction gutter 130 to dry out may be approximately 20 seconds. Such a drying time may mean that it is possible for immersion liquid to be present within the extraction gutter

130 at the start of a critical period, but for the immersion liquid to fully evaporate over the course of the critical period. Further, such a drying time may mean that the immersion liquid fully dries out between subsequent operations. Consequently, a substrate support 20 as depicted in Figure 5 may exhibit a relatively high overlay error.

[0060] The substrate support 20 may be configured to operate in a dry edge mode in which, when a substrate W is clamped to the substrate support 20, an upper surface of the inner circumferential seal

131 remains dry (i.e. immersion liquid is not present on the upper surface of the inner circumferential seal 131). The substrate support 20 may also be configured to operate in a bypass mode in which the upper surface of the inner circumferential seal 131 becomes wetted (i.e. immersion liquid is present on the upper surface of the inner circumferential seal 131). In the dry edge mode, a pressure difference between the extraction gutter 130 and a region between the substrate W and the substrate support 20 radially inward of the inner circumferential seal 131 may be larger than in the bypass mode. The larger pressure difference in the dry edge mode may mean that the drying out of the extraction gutter 130 occurs particularly quickly. [0061] The presence (or absence) of immersion liquid within the extraction gutter 130 also affects the characteristics of the contact between the outer circumferential ring of burls 41 and the underside of the substrate W. When immersion liquid is present in the extraction gutter 130 and a substrate W is clamped to the substrate support 20, capillary forces mean that upper surfaces of the burls 41 (that is, surfaces which are in contact with the underside of the substrate of W) are kept wetted by the immersion liquid. After the substrate W is removed from the substrate support 20, the upper surfaces of the burls may remain wetted for a period, which may be approximately 0.1s to 10s.

[0062] Immersion liquid on the upper surfaces of the burls 41 lubricates the upper surface of the burls 41. Consequently, when immersion liquid is present on the upper surfaces of the burls 41, the coefficient of friction between the upper surfaces of the burls 41 and the underside of the substrate W is reduced relative to when immersion liquid is not present on the upper surfaces of the burls 41. The coefficient of friction between the upper surfaces of the burls 41 and the underside of the substrate W affects inplane deformation within the substrate W, which occurs as a substrate W is loaded onto a substrate support W. If, when clamping a substrate W to a substrate support 20 to project a pattern for a first layer onto the substrate W, the burls 41 are dry, but when clamping the substrate W to the substrate support 20 to project a pattern for a second layer on the substrate W, the burls 41 are wet, the in-plane deformations in the substrate W will be different for the first and second layers. This can lead to an increase in overlay error. A characteristic pattern of overlay error that arises from such in-plane deformation is referred to as “wafer load grid” (WLG).

[0063] Figure 4 depicts a schematic representation of an example of a path taken by a fluid handling structure IH relative to a moving substrate W during exposure of the substrate W. For ease of description, the substrate W will be referred to as having a “North” portion N and a “South” portion S, where the North portion N and South portion S are radially outer portions of the substrate W separated from one another by 180°. Whilst not shown in Figure 4, the substrate W is supported on a substrate support WT, such as the substrate support 20 described above. The fluid handling structure IH is described as moving over the substrate W, but, as described above, relative movement of the substrate W and the fluid handling structure IH may be provided by movement of the substrate support 20, with the fluid handling structure IH remaining stationary.

[0064] In the example of the path depicted in Figure 4, the fluid handling structure IH begins (t = 11) at the South portion S of the substrate W. Specifically, the fluid handling structure IH begins (t = ti) in a position overlapping the edge of the substrate W at the South portion S of the substrate W. Consequently, immersion liquid is introduced into the South portion S of the extraction gutter 130 of the substrate support 20 (i.e. a portion of the extraction gutter 130 corresponding to the South portion S of the substrate W).

[0065] From t = ti to t = ts, the fluid handling structure IH relatively moves over substantially all of the substrate W so that substantially all of a surface of the substrate W can be exposed. The path of the fluid handling structure IH may be a zig-zag path. In other words, the fluid handling structure IH may snake across the substrate W.

[0066] At t = t2, the immersion liquid on the burls in placed in the South portion S of the extraction gutter are dye (i.e. substantially free of immersion liquid). At t = t?, the fluid handling structure IH finishes (i.e. reaches the end of its path) at the North portion N of the substrate W. Specifically, the fluid handling structure IH finishes in a position overlapping the edge of the substrate W at the North portion N of the substrate W. Consequently, immersion liquid is introduced into a North portion N of the extraction gutter 130 of the substrate support 20 (i.e. a portion of the extraction gutter 130 corresponding to the North portion N of the substrate W).

[0067] At t = t4, the substrate W is unloaded from (i.e. removed from) the substrate support 20. When the substrate W is unloaded from the substrate support 20, the time that has elapsed since the fluid handling structure IH introduced immersion liquid into the South portion S of the extraction gutter 130 (i.e. t4-ti) is greater than the time that has elapsed since the fluid handling structure IH introduced immersion liquid into the North portion N of the extraction gutter 130 (i.e. t4-ts). This may mean that, when the substrate W is unloaded from the substrate support 20, upper surfaces of the burls 41 in the South portion S of the extraction gutter 130 are dry (i.e. substantially free of immersion liquid) and upper surfaces of the burls 41 in the North portion N of the extraction gutter 130 are wetted (i.e. coated in immersion liquid).

[0068] Mechanical wear (e.g. abrasive wear) of the upper surfaces of the burls 41 during unloading of the substrate W from the substrate support 20 may occur to a greater extent when the upper surfaces of the burls 41 are dry compared to when the upper surfaces of the burls 41 are wet. Thus, if the path of the fluid handling structure IH is as described above, wear of the burls 41 in the extraction gutter 130 may be asymmetric. Specifically, burls 41 in the South portion S of the extraction gutter 130 may be worn at a greater rate than burls 41 in the North portion N of the extraction gutter 130. Over time, this can lead to overlay error. This may mean that the substrate support 20 may need to be replaced. It may be difficult or expensive to recondition substrate supports 20 for re-use in such a way that asymmetric wear is corrected.

[0069] The present invention is directed to a substrate support 20 which is configured to mitigate the problems described above. Specifically, the present invention is directed to substrate support 20 which is configured to retain immersion liquid within the extraction gutter 130 for a longer period of time. In doing so, it can be ensured that the extraction gutter 130 does not dry out during critical periods (or that the risk of the extraction gutter 130 drying out during critical periods is reduced), and the conditions within the extraction gutter 130 (i.e., the presence of immersion liquid) can be made to be consistent for series of operations. Further, by retaining immersion liquid within the extraction gutter 130 for a longer period of time, the risk that, during unloading of a substrate W from the substrate support 20, the chance that burls 41 in one portion of the extraction gutter 130 are wetted and burls 41 in another portion of the extraction gutter 130 are dry can be reduced. Consequently, retaining immersion liquid in the substrate support 20 means that the occurrence of the following can be reduced: (i) transient thermal deformation of the substrate support 20 during and between critical periods; (ii) changes to the coefficient of friction between the upper surfaces of the burls 41 and the underside of the substrate W during and between critical periods; and (iii) asymmetric wear of the burls 41 in the extraction gutter 130 over time. This means that overlay error, and, for example, wafer load grid (WLG) can be improved. [0070] In accordance with the present invention, the substrate support 20 comprises a support body 21 having an upper surface and a plurality of burls 41 (which may also be referred to as support members). The plurality of burls 41 extend above the upper surface. The substrate support 20 further comprises an extraction gutter 130 defined by a first circumferential wall and a second circumferential wall. Like the plurality of burls 41, the first circumferential wall and the second circumferential wall extend above the upper surface. The substrate support 20 further comprises an extraction opening formed in the upper surface within the extraction gutter 130. The extraction opening referred to in relation to the present invention may be similar to the second extraction opening 117 referred to in relation to the substrate support 20 depicted in Figures 2 and 3. The substrate support 20 further comprises one or more fluid-retaining features formed within the extraction gutter 130. The one or more fluid-retaining features are defined by the support body 21. In this context, “defined by” may mean that the profile (i.e. shape) of the fluid-retaining features are formed by the support body 21. That is, surfaces which define boundaries of the fluid-retaining features may be surfaces of the support body 21. For example, the upper surface 25 of the support body 21 may not be flat, and may have contours, so that immersion liquid can be retained. By providing the one or more fluid-retaining features within the extraction gutter 130, immersion liquid that enters the extraction gutter 130 can be maintained within the extraction gutter 130 for a longer period of time. Consequently, overlay error may be improved, as explained above.

[0071] A substrate support in accordance with the present invention may be similar to the substrate support 20 depicted in Figures 2, 3 and 5 and described above. The main difference may be the provision of fluid-retaining features within the extraction gutter 230, 330, 430, 530. Other features, for example, the structure of the extraction gutter 230, 330, 430, 530 itself, may be the same in a substrate support 200, 300, 400, 500 in accordance with the present invention as the substrate support 20 depicted in Figures 2, 3 and 5 and described above.

[0072] Figures 6 and 7 depict a substrate support 200 in accordance with a first embodiment of the present invention. Figure 6 depicts a circumferential sectional view of the substrate support 200. The sectional view passes through a radially outermost ring of burls 241 in the extraction gutter 230. Figure 7 depicts the structure of the extraction gutter 230, which may be the same as the extraction gutter 130 described above. The extraction gutter 230 is defined by an inner circumferential seal 231 (first circumferential wall), an outer circumferential seal 232 (second circumferential wall) and the upper surface 225 of the substrate support 200. In particular, a lower horizontal surface of the extraction gutter 230 is defined by the upper surface 225 of the substrate support 200 and vertical surfaces of the extraction gutter 230, which define the radial boundaries of the extraction gutter 230, are a radially outer vertical surface 231a of the radially inner circumferential seal 231 and a radially inner surface 232a of the radially outer circumferential seal 232. The radially inner circumferential seal 231 may comprise an upper surface 231b and the radially outer circumferential seal may comprise an upper surface 231a. [0073] Within the extraction gutter 230, there may be a plurality of extraction openings 217 and a plurality of burls 241 distributed circumferentially. The plurality of extraction openings 217 and the plurality of burls 241 may be distributed uniformly around the circumference of the extraction gutter 230.

[0074] In some cases, during the drying out of the extraction gutter 230, immersion liquid may remain present on the surfaces of the burls 241 for longer than on other surfaces of the extraction gutter 230 (e.g. the upper surface 225 of the substrate support 200 within the extraction gutter 230). The drying out of the surfaces of the burls 241 may not be uniform around the circumference of the substrate support 200. Specifically, burls 241 that are positioned further away from an extraction opening 217 may dry out faster than burls 241 that are positioned closer to an extraction opening 217. Thus, during the drying out of the extraction gutter 230, burls 241 that are positioned around the midpoint of adjacent extraction openings 217 may dry out first, and burls 241 that are positioned adjacent to extraction openings 217 may dry out last.

[0075] Further, in some cases, burls 241 that are positioned further away from extraction openings 217 (e.g. burls 241 around the midpoint of adjacent extraction openings 217) may not become wetted at all when immersion liquid is present in the extraction gutter 230. Whether or not burls 241 that are positioned further away from extraction openings 217 (e.g. burls 241 around the midpoint of adjacent extraction openings 217) become wetted when immersion liquid is present in the extraction gutter 230 is unpredictable.

[0076] In general, burls 241 in close proximity to the extraction openings 217 may have a longer, and more consistent, dry-out time than burls 241 that are not in close proximity to the extraction openings 217. Consequently, it is more likely that burls 241 in close proximity to extraction openings 217 remain wetted throughout critical operations of the substrate support 200 and between subsequent operations of the substrate support 200.

[0077] Considering the above, to improve spatial homogeneity of the drying out of the burls 241, it may be preferable for each of the burls 241 to be in close proximity to an extraction opening 217 (i.e. close enough to an extraction opening 217 for the dry out time to be relatively consistent and relatively long). To achieve this, it may be preferable to increase the number of extraction openings 217. For example, the number of extraction openings 217 may be greater than 10, preferably greater than 50, preferably greater than 100, and further preferably greater than 200, e.g. 220. In some cases, the preferred number of extraction openings 217 may be dependent on the radius of the extraction gutter 230. The numbers of extraction openings 217 provided above may be particularly applicable a substrate support 200 with a radius of approximately 150 mm. [0078] A distance between adjacent extraction openings 217 along the circumference on which the plurality of burls 241 in the extraction gutter 230 are disposed may be referred to as a separation distance. The separation distance may be measured between the centre points of adjacent extraction openings 217. The separation distance between adjacent extraction openings 217 may be less than 90 mm, preferably less than 20 mm, and further preferably less than 10 mm. The number of burls 241 in the extraction gutter 230 between adjacent extraction openings 217 (i.e. the number of burls 241 disposed along a single separation distance) may be less than 50, preferably less than 10, and further preferably less than or equal to 6.

[0079] An additional benefit of increasing the number of extraction openings 217 is that the velocity of the air flow within the extraction gutter 230 is reduced. This results in a decrease in the rate of evaporation within the extraction gutter 230 and a reduction of the drag force exerted on the immersion liquid by the air flow within the extraction gutter 230. Consequently, the extraction gutter 230, (and any fluid-retaining features therein) may be able to retain a greater volume of immersion liquid for a longer length of time. That is, the dry-out time in the extraction gutter 230 is lengthened.

[0080] Providing an excessive number of extraction openings 217 may unnecessarily complicate the design and manufacturing of the substrate support 200. Consequently, the number of extraction openings 217 may be less than 1000, preferably less than 500, and further preferably less than 250. Additionally or alternatively, the separation distance between adjacent extraction openings 217 may be greater than 2 mm, and preferably greater than 6 mm. Additionally or alternatively, the number of burls 241 in the extraction gutter 230 between adjacent openings 217 (i.e. the number of burls 241 disposed along a single separation distance) may be greater than 1, preferably greater than 2, and further preferably greater than or equal to 4, and optionally greater than 5.

[0081] It will be appreciated that the above-described configuration of the extraction openings 217 may be applied to any substrate support 200, irrespective of whether the substrate support 200 includes other features described in the present disclosure. For example, the above-described configuration of extraction openings 217 may be applied to a substrate support 20 which does not comprise any fluidretaining features, such as the one depicted in Figures 2 and 3.

[0082] The height of each of the burls 241 may be approximately 100 pm to approximately 200 pm. The height of the inner circumferential seal 231 and the outer circumferential seal 232 may be several pm less than the height of the burls 241, such that, when a substrate W is supported by the substrate support 200, a partial seal is formed between the circumferential seals 231 , 232 and the underside of the substrate W, but the circumferential seals 231, 232 do not contact the underside of the substrate W. [0083] As shown in Figures 6 and 7, the upper surface 225 of the substrate support 200 within the extraction gutter 230 may comprise one or more fluid-retaining features. These fluid-retaining features may be recessed portions 250, e.g., indentations, within the upper surface 225 of the extraction gutter 230. The recessed portions 250 may comprise lower surfaces 252 and side surfaces 251. The lower surfaces 252 and side surfaces 251 of the recessed portions 250 may define reservoirs. That is, the recessed portions 250 may be configured to collect and store immersion liquid.

[0084] The lower surfaces 252 of the recessed portions 250 may be lower than the surface in which the extraction openings 217 are defined. For instance, if the extraction openings 217 are formed in the upper surface 225 of the substrate support 200, the lower surfaces 252 of the recessed portions 250 may be lower than the upper surface 225 of the substrate support 200. By providing recessed portions 250 with lower surfaces 252 that are lower than the other surface 225 of the substrate support 200, the recessed portions 250 may be able to inhibit the extraction of immersion liquid through the extraction opening 217. That is, the extraction opening 217 may not be able to extract immersion liquid that has been collected within the recessed portions 250. Thus, immersion liquid that collects within the recessed portions 250 may remain within the extraction gutter 230 for a longer period of time.

[0085] The recessed portions 250 may include indentations, recesses, pits or pockets in the upper surface 225 of the substrate support 200. In general, the recessed portions 250 may take any form that allows them to act as reservoirs. That is, the recessed portions 250 may take any form that allows them to collect and store (even temporarily) immersion liquid in the extraction gutter 230.

[0086] The recessed portions 250 may be manufactured using any suitable method. For example, the process used to manufacture the recessed portions 250 may comprise drilling. Additionally or alternatively, the process used to manufacture the recessed portions 250 may comprise laser structuring. [0087] Even if immersion liquid that has been collected within the recessed portions 250 is substantially prevented from being extracted from the extraction gutter 230 via the extraction opening 217, the immersion liquid may evaporate over time. Consequently, it may be preferable to ensure that the depth of the recessed portions 250 is such that the recessed portions 250 do not become dry (i.e., that all of the immersion liquid does not evaporate) before the expiry of the certain period of time. The depth of the recessed portions 250 may be a distance between the upper surface 225 of the substrate support 200 and a lowest point of the recessed portions 250. The certain period of time may be greater than 10 s, preferably greater than 30 s, and further preferably greater than 50 s. For example, the certain period of time may be 60 s. Ensuring that immersion liquid remains in the extraction gutter 230 for the certain period of time may mean that it can be ensured that the extraction gutter 230 does not dry out during critical periods, and the conditions within the extraction gutter 230 (i.e., the presence of immersion liquid) can be made to be consistent during and between a series of critical periods. Desirably, the certain period of time is greater than the length of time taken to complete all exposures on a substrate W or longer that the total duration spent by a substrate W on the substrate support 200 during a production process.

[0088] To ensure that immersion liquid remains present within the recessed portions 250 for the certain period of time, a depth of the recessed potions 250 may be greater than 10 pm and preferably greater than 100 pm. [0089] Providing recessed portions 250 with an excessive depth may mean that flow within the extraction gutter 230 is affected. Further, providing recessed portions 250 with an excessive depth, may complicate the manufacturing process used to form the recessed portions 250. For example, laser structuring may not be suitable for forming the recessed portions 250 if the depth of the recessed portions 250 is excessively large. Considering this it may be preferable for the depth of the recessed portions 250 to be less 500 pm, preferably less than 200 pm, and further preferably less than 120 pm. [0090] The size, shape, and arrangement of the recessed portions 250 may not be particularly limited. However, it may be preferable for the plurality of recessed portions 250 to be arranged as an array and distributed across the upper surface 225 of the extraction gutter 230 substantially uniformly in the circumferential and radial directions. This may ensure that the amount of immersion liquid retained within the extraction gutter 230 is substantially uniform around the extraction gutter 230. This may ensure that the thermal load applied to the substrate support 200 is substantially uniform around the extraction gutter 230, and variable deformation around the extraction gutter 230 can be reduced. Further, distributing the recessed portions 250 uniformly may ensure that the advantageous fluidretaining properties and technical effects thereof are present throughout the extraction gutter 230.

[0091] The cross-sectional shape (in a plane parallel to the support plane) of the recessed portions 250 may be circular, rectangular or any other suitable shape. Circular recessed portions 250 may be less complex to manufacture if the manufacturing process involves drilling. It may be possible to arrange recessed portions 250 in a spatially efficient manner if they have a rectangular cross-section.

[0092] The profile of the recessed portions 250 below the level of the upper surface 225 of the substrate support 200 may also not be particularly limited. For example, when the cross-section of the recessed portions 250 is circular, the overall profile of the recessed portions 250 may be hemispherical, cylindrical or conical. When the cross-section of the recessed portions 250 is rectangular, the profile of the recessed portions 250 may be cuboidal or pyramidal.

[0093] The recessed portions 250 may have a maximum width. In a case where the recessed portions 250 are circular, the maximum width may be the diameter of the circular cross-section. In a case where the recessed portions 250 are rectangular, the maximum width may be a maximum dimension of the rectangle, i.e., a distance between opposite corners. The maximum width of the plurality of recessed portions 250 may be greater than 10 pm, preferably greater than 100 pm and further preferably greater than 150 pm. This may be to ensure that immersion liquid can be effectively collected in the plurality of recessed portions 250. The maximum width of the plurality of recessed portions 250 may be less than 2,000 pm, preferably less than 1,000 pm, and further preferably less than 500 pm. If the maximum width of the plurality of recessed portions 250 is too large, it is possible that immersion liquid that is flowing within the extraction gutter 230 towards the extraction opening 217 will cause immersion liquid that has been collected within the recessed portions 250 to be drawn out from the recessed portions 250 and into the flow of extracted immersion liquid in the extraction gutter 230. This may lead to all of the immersion liquid being extracted through the extraction opening 217. Consequently, by providing recessed portions 250 with a maximum width that is less than the values described above, immersion liquid can be effectively retained in the extraction gutter 230.

[0094] The plurality of recessed portions 250 may be arranged on the upper surface 225 such that the proportion of the upper surface 225 at which a recessed portion 250 is present is greater than 50%, preferably greater than 75% and further preferably greater than 85%. By providing recessed portions 250 in such a way that they cover this amount of the upper surface 225 of the substrate support 200, it can be ensured that the thermal load (cold load) applied to the substrate support 200 in the extraction gutter 230 is substantially uniform. Further, it can be ensured that the cold load applied to the substrate support 200 at times when immersion liquid is only present in the recessed portions 250 is sufficiently similar to the cold load applied to the substrate support 200 when immersion liquid is present throughout the extraction gutter 230. Consequently, transient structural deformation of the substrate support 200 can be substantially reduced.

[0095] Whilst, generally, the size, shape and density of recessed portions 250 may be uniform around the extraction gutter 230, in some embodiments, this may not be the case. For example, at least one of the size, the shape and the density of the recessed portions 250 may be adjusted depending on the circumferential position within the extraction gutter 230. This may be done if measurement and analysis determine that it is preferable to retain more immersion liquid in one or more portions of the extraction gutter 230 than other portions of the extraction gutter 230. For example, in cases where the fluid handling structure IH relatively moves across the substrate W as depicted in Figure 4, it may be preferable to provide a greater amount of fluid-retaining capacity in the South portion S of the extraction gutter 230 (i.e. in a portion of the extraction gutter 230 for which an elapsed time between immersion liquid being provided to that portion of the extraction gutter 230 and the substrate W being unloaded from the substrate support 200 is greater).

[0096] In some embodiments, the recessed portions 250 may comprise a plurality of grooves (not shown). In this context, a groove means a relatively long and/or relatively narrow cut or depression in the upper surface 225 of the substrate support 200 within the extraction gutter 230. The function of the grooves may be the same as described in relation to the other types of recessed portion 250 above.

[0097] For example, the recessed portions 250 may comprise a plurality of grooves which extend circumferentially around the extraction channel 230. The plurality of grooves may be arranged in the radial direction and/or as an array. That is, subsequent circumferential grooves may be arranged at different radial positions.

[0098] Figure 8 depicts a circumferential sectional view of a substrate support 500 in accordance with a second embodiment of the present invention, the cutting plane of the sectional view passing through the radially outermost ring of burls 541 within the extraction gutter 530. Figure 9 depicts an extraction gutter 530 of a substrate support 500 in accordance with the second embodiment of the present invention. In the second embodiment, the substrate support 500 compromises a plurality of grooves 550 which extend in a radial direction. Each of the grooves 550 may extend from a position close to the radially outer surface 531a of the radially inner circumferential seal 531 to a position close to the radially inner surface 532a of the radially outer circumferential seal 532. The radially inner circumferential seal 531 may comprise an upper surface 531b and the radially outer circumferential seal 532 may comprise an upper surface 532b. The grooves 550 may each comprise a lowermost surface 552 and vertical surfaces 551. The lowermost surface 552 of a groove 550 may be below the upper surface 525 of the substrate support 500.

[0099] The grooves 550 may be arranged in the circumferential direction. That is, different grooves 550 may have different circumferential positions. A distance between adjacent grooves 550 in the circumferential direction (i.e. a groove pitch, p) may be greater than 25 pm and preferably greater than 50 pm. The distance between adjacent grooves 550 in the circumferential direction may be less than 150 pm and preferably less than 100 pm. The distance between adjacent grooves 550 may be the distance between the two closest edges of the grooves 550. For example, as shown in Figure 8, the distance between adjacent grooves 550 may be the distance between a right-hand edge of a first groove 550 and a left-hand edge of a second groove 550.

[0100] The grooves 550 may be provided in such a way that they do not interfere with other features within the extraction gutter 530. For example, the grooves 550 may be arranged such that they do not interfere with (i.e. coincide with or intersect or contact) the burls 541 and/or the extraction openings 517. In circumferential regions of the substrate support 500 where burls 541 and/or extraction openings 517 are present, the grooves 550 may not be provided (i.e. grooves 550 may be omitted where burls 541 and/or extraction openings 517 are provided). Thus, in these circumferential regions, the distance between adjacent grooves 550 may be greater than the values for the spacing provided above. In general, in the majority of the extraction gutter 530 (e.g. greater than 50%, preferably greater than 75% and further preferably greater than 90% of the extraction gutter 530), the distance between adjacent grooves 550 may be as described above (i.e. 25 pm to 150 pm, preferably 50 pm to 100 pm). In the remainder of the extraction gutter 530, the distance between adjacent grooves 550 may be greater to allow for the presence of other features (e.g. burls 541 and/or extraction openings 517). Alternatively, the grooves 550 may be diverted such that they go around the burls 541 and/or extraction openings 517. [0101] The values provided for depths of the recessed portions 250 above may also apply for the grooves 550. For the grooves 550, a maximum width may be a dimension perpendicular to the direction in which the groove 550 extends. The width of the grooves 550 may be greater than 50 pm and less than 100 pm. Preferably, the width of the grooves 550 may be greater than 85 pm and less than 100 pm. Grooves 550 with a width in this range may exert strong capillary forces on immersion liquid within the extraction gutter 530 to draw the immersion liquid into the grooves 550. Consequently, when immersion liquid is present in the extraction gutter 530 (e.g. during a time in which immersion liquid is being extracted from between the substrate W and the substrate support 500 via the extraction opening 517), the grooves 550 may be filled with immersion liquid. Strong adhesive forces will be present between the surfaces 551 , 552 of the grooves 550. This may mean that less immersion liquid is available at the surfaces of the extraction gutter 530. Consequently, the rate of evaporation of the immersion liquid within the extraction gutter 530 may be reduced. As such, immersion liquid is retained within the extraction gutter 530 for a longer period of time.

[0102] Figure 10 and 10 depict a substrate support 300 in accordance with a third embodiment of the present invention. Figure 10 depicts a circumferential sectional view of the substrate support 300. The sectional view passes through the radially outermost ring of burls 341 within the extraction gutter 330. Figure 11 depicts an extraction gutter 330 of the substrate support 300. As in the first embodiment, the extraction gutter 330 may be as described in relation the substrate support 20 depicted in Figures 2 and 3.

[0103] In the third embodiment, an extraction opening wall 351 may extend circumferentially around the extraction opening 317. The extraction opening wall 351 may protrude above the upper surface 325 of the substrate support 300.

[0104] The extraction opening wall 351 may be configured to inhibit the extraction of immersion liquid from the extraction gutter 330. Specifically, the extraction opening wall 351 may be configured to inhibit the extraction of immersion liquid which is below the level of an upper surface 352 of the extraction opening wall 351. When immersion liquid enters the extraction gutter 330, it therefore may be collected within the extraction gutter 330 up to the level of the upper surface 352 of the extraction opening wall 351. That is, a reservoir 350 (i.e. a region in which immersion liquid can collect) may be defined by: a radially outward surface 331b of a radially inward circumferential seal 331; a radially inward surface 332a of a radially outward circumferential seal 332; the upper surface 325 of the substrate support 300 within the extraction gutter 330; and the extraction opening circumferential wall 351.

[0105] To ensure that immersion liquid remains present within the extraction gutter 330 for the certain period of time referred to above, a height of the extraction opening wall 351 (i.e., a distance between the upper surface 325 of the substrate support 300 and the upper surface 352 of the extraction opening wall 351) may be greater than 10 pm, preferably greater than 100 pm and further preferably greater than 150 pm. The height of the extraction opening circumferential wall 351 may be less than 2000 pm, preferably less than 1000 pm and further preferably less than 500 pm. This may be to ensure that the extraction opening circumferential wall 351 does not interfere with the ability of the extraction opening 317 to extract the majority of the immersion liquid from the extraction gutter 330.

[0106] As with the recessed portions 250, the proportion of the upper surface 325 within the extraction gutter 330 which is covered by reservoir 350 (defined at least partially by an extraction opening wall 351) is greater than 50%, preferably greater than 75% and further preferably greater than 85%. Reservoir 350 may cover the whole of the extraction gutter 330 except the areas occupied by the burls 341 and the extraction opening circumferential wall 351.

[0107] Figures 12 and 13 depict a substrate support 400 in accordance with a fourth embodiment of the present invention. Figure 12 depicts a circumferential sectional view of the substrate support 400. The sectional view passes through the radially outermost circumferential ring of burls 441 in the extraction gutter 430. Figure 13 depicts an extraction gutter 430 of the substrate support 400. In accordance with the fourth embodiment of the present invention, trenches 450 may be disposed around the bases of the plurality of burls 441 within the extraction gutter 430. In this context, trench 450 may mean a narrow channel cut into or otherwise formed in the upper surface 425. The trenches 450 may be referred to as moats.

[0108] The trenches 450 may have lower surfaces 452. The lower surfaces 452 may be lower than the surface in which the extraction opening 417 is formed. For instance, if the extraction opening 417 is formed in the upper surface 425 of the substrate support 400, the lower surface 452 of the trenches 450 may be lower than the upper surface 425 of the substrate support 400. The trenches 450 may be configured to collect and store immersion liquid within the extraction gutter 430. Immersion liquid that has been collected within the trenches 450 may not be extracted through the extraction opening 417.

[0109] By providing circumferential trenches 450 around the plurality of burls 441 in the extraction gutter 430, the period of time in which immersion liquid is available to lubricate the upper surfaces of the burls 441 may be extended. Consequently, it can be ensured that immersion liquid is available to lubricate the burls 441 during and between a series of critical periods. As explained above, this means that the coefficient of friction will be consistent during and between the series of critical periods. Consequently, overlay, and specifically wafer load grid (WLG) may be improved.

[0110] A maximum width of the circumferential trenches 450 around the bases of the plurality of burls 441 may be the same as the maximum width described above in relation to the recessed portions 250. The minimum and maximum depths of the circumferential trenches 450 around the bases of the plurality of burls 441 may be the same as discussed in relation to the plurality of recessed portions 250 discussed above.

[0111] The features of the first to fourth embodiments are not mutually exclusive. That is, some embodiments may include features of the first embodiment, the second embodiment, the third embodiment and the fourth embodiment. For example, a substrate support 200 in accordance with the present invention may include a plurality of recessed portions 250 distributed across the upper surface 225 of the substrate support 200 within the extraction gutter 230, and also include an extraction opening circumferential wall 351. Such a substrate support 200 may additionally or alternatively include trenches 450 disposed circumferentially around the bases of the plurality of burls 241 in the extraction gutter 230.

[0112] Fluid-retaining features may be disposed within portions of the substrate support 20 other than the extraction gutter 130. For example, fluid-retaining features may be provided radially outward of the outer circumferential seal 132. For example, fluid-retaining features may be provided in the upper surface 25 of a substrate support 20 within a portion of the substrate support 20 in which the first extraction opening 107 is formed, such as depicted in Figure 14. A part or all of this portion of the substrate support 20 may be radially outward of the edge of the substrate W. The fluid-retaining features may be between the outer circumferential seal 132 and the first extraction opening 107. The fluidretaining features may be any of those described herein. For example, the fluid-retaining features may be grooves, such as the grooves 550 described in relation to Figures 8 and 9.

[0113] Figure 14 depicts a substrate W and a substrate support 20. The substrate support 20 may be substantially the same as the substrate support 20 depicted in Figure 2, except as described below. The substrate support 20 depicted in Figure 14 may comprise any of the fluid-retaining features described above, and depicted in Figures 6-13.

[0114] As has been described above, the substrate support 20 comprises a plurality of burls 41a, 41b. A radially outermost ring of burls 41b may be disposed within the extraction gutter 130. That is, the radially outermost ring of burls 41b may be disposed radially outward of the inner circumferential seal 131 and radially inward of the outer circumferential seal 132. The remainder of the burls 41a may be disposed outside of the extraction gutter 130, i.e. radially inward of the radially inner seal 131.

[0115] The burls in the radially outermost ring of burls 41b may have a different shape to the other burls 41a of the substrate support 20. Specifically, the shape of the burls in the radially outermost ring of burls 41b may be configured such that immersion liquid can be retained on said burls 41b. This may mean that distal ends of the burls in the radially outermost ring of burls 41b are wetted throughout a period of time in which a series of operations (e.g. clamping the substrate W to the substrate support 20) are performed. Consequently, the coefficient of friction between upper surfaces of the burls 41b and the underside of the substrate W may remain substantially constant throughout the series of operations. Consequently, overlay error (and, specifically, overlay error arising from in-plane deformation) can be reduced, as explained above.

[0116] Figures 15 A and 15B depict burls 41b which are configured to retain immersion liquid in their vicinity. The burls 41b depicted in Figures 15A and 15B are stepped burls. The burls 41b comprise a base portion and a tip portion. A proximal end 43 of the base portion is connected to the upper surface 25 of the substrate support 20. The base portion of the burls 41b extends distally (i.e. away from the upper surface 25) to a distal end of the base portion. The distal end of the base portion may be connected to a proximal end of the tip portion at an interface I. The tip portion extends above the distal end of the base portion to a distal end 42 of the tip portion. The distal end 42 of the tip portion may be configured to contact the underside of the substrate W. That is, the distal end 42 of the tip portion may form the upper surface of the burl 41b on which the substrate W is supported.

[0117] As described above, the profile of the burls in the radially outermost circumferential ring of burls 41b may be frustoconical. That is, the diameter of the burls 41b may gradually decrease as the burls 41b extend distally. However, for one or more of the burls in the radially outermost circumferential ring of burls 41b, at an interface I between the base portion and the tip portion of a burl 41b, there may be a sudden change in the diameter of the burl 41b. That is, there may be a step change in the diameter of the burl 41b. The diameter of the proximal end of the tip portion may be substantially less than the diameter of the distal end of the base portion. [0118] The tip portion may extend above a top surface 47 of the base portion. The top surface 47 may be a surface of the base portion that is substantially parallel to the upper surface 25 of the substrate support 20 at the distal end of the base portion. Some of the top surface 47 of the base portion may be exposed. This exposed top surface 47 may extend circumferentially around the base of the tip portion. [0119] In general, one or more of the burls in the radially outermost circumferential ring of burls 41b may have a step profile, with the step being made up of a substantially vertical face of the base portion, the top surface 47 of the base portion, and a substantially vertical surface 44 of the tip portion. This configuration may allow immersion liquid to be retained around the distal end 42 of the tip portion. Thus, the distal end 42 of the burl 41b may remain wetted throughout a series of operations performed by the lithographic apparatus.

[0120] The burls 41b may have a total height H. The total height H may be made up of the height of the base portion hi and the height of the tip portion h2. A diameter of the distal end 42 of the tip portion d2 may be substantially the same as the diameter of the distal ends of the other burls 41a of the substrate support 20. A diameter of the proximal end 43 of the base portion di may be greater than the diameter of the proximal ends of the other burls 41a of the substrate support 20, to account for the step-decrease in the diameter of the burls 41b at the interface I between the base portion and the tip portion.

[0121] The burl 41b depicted in Figure 15B may be substantially the same as the burl 41b depicted in Figure 15A, except from the fact that the burl 41b depicted in Figure 15Bfurther comprises a trench 45. The trench 45 may be defined in the base portion of the burl 41b. The trench 45 may be recessed into the top surface 47 of the base portion of the burls 41b. The trench 45 may extend circumferentially around the proximal end of the tip portion. The trench 45 may allow immersion liquid to be collected therein. Thus, the trench 45 may act as a reservoir of immersion liquid, which is able to provide immersion liquid to the distal end 42 of the burl 41b, thus keeping the distal end 42 of the burl wetted during the series of operations of the lithographic apparatus. The trench 45 may have a depth h ;.

[0122] Figure 16 depicts an example of a burl 41b configured to collect immersion liquid at a distal end portion thereof. The burl 41b may be a burl 41b of the outermost ring of burls 41b. The burl 41b extends from a proximal end 43 (at which the burl 41b is connected to the main body 21 of the substrate support 20) to a distal end 42. The distal end 42 of the burl 41b may provide the upper surface 48 of the burl 41b on which the substrate W is supported.

[0123] The burl 41b depicted in Figure 16 further comprises a reservoir 49 defined therein. The reservoir 49 is formed at the distal end 42 of the burl 41b. The reservoir 49 may be a recess (which may otherwise be referred to as a hole or an indentation) in the distal end 42 of the burl 41b. The reservoir 49 may be recessed into the upper surface 48 (i.e. a distal surface) of the burl 41b, and specifically recessed into a radially inward portion (i.e. middle portion) of the upper surface 48 of the burls 41b. Thus, the shape of the portion of the upper surface 48 which comes into contact with the substrate W when the substrate W is supported on the substrate support 20 may be annular, with the annulus surrounding the reservoir 49. The reservoir 49 may be configured to hold immersion liquid, and so is able to provide immersion liquid to the upper surface 48 at the distal end 42 of the burl 41b, thus keeping the upper surface 48 of the burl wetted during the series of operations of the lithographic apparatus.

[0124] A depth (L4) of the reservoir 49 (i.e. a distance between the upper surface 48 of the burl 41b and a lowermost surface of the reservoir 49) may be greater than 10 pm, preferably greater than 50 pm and further preferably greater than 80 pm. A diameter of the reservoir 49 (i.e. a dimension of the reservoir 49 in a horizontal direction) may be greater than 10 pm, preferably greater than 50 pm and further preferably greater than 80 pm. Increasing the size of the reservoir 49 may increase the volume of immersion liquid that the reservoir 49 is able to store. With an increased storage capacity, the burl 41b may be able to keep the upper surface 48 of the burl 41b wetted for a longer period of time. The depth (L4) of the reservoir 49 may be less than 200 pm, preferably less than 150 pm and further preferably less than 100 pm. The diameter of the reservoir 49 may be less than 200 pm, preferably less than 150 pm and further preferably less than 100 pm. For example, the diameter of the reservoir may be 90 pm. In general, the diameter of the reservoir may be less than half, and preferably less than a third, of the diameter of the distal end 42 of the burl 41 in which the reservoir 49 is formed. In some cases, the provision of the reservoir 49 at the distal end 42 of the burl 41b may impact the structural integrity of the burl 41b. Ensuring that the size of the reservoir 49 is not excessive means that the impact that the provision of the reservoir 49 has on the structural integrity of the burl 41b is not larger than necessary.

[0125] In some embodiments, the burls 41b in the outermost circumferential ring of burls 41b may have a standard shape (e.g. be frustoconical), without any of the modifications depicted in Figures 15A, 15B and 16). In such embodiments, an asymmetric drift rate of the burls 41b in the outermost ring of burls 41b (i.e. a rate at which the asymmetry of the burls 41b in the outermost ring of burls 41b increases) may be decreased by increasing the surface area of the upper surfaces of the burls 41b (i.e. the flat surfaces of the distal ends of the burls 41b on which the underside of the substrate W is supported. It may be preferable for the surface area of the upper surface of a burls 41b that is in direct contact with the underside of the substrate W to be greater than 20% of the nominal area of the upper surface of the burl 41b, preferably greater than 30% of the nominal area of the upper surface of the burl 41b, and further preferably greater than 40% of the nominal area of the upper surface of the burl 41b. It may be preferable for the surface area of the upper surface of the burl 41b that is in direct contact with the underside of the substrate W to be less than 60% of the nominal area of the upper surfaces of the burls. The nominal surface area of the upper surface of a burl may be the surface area of the upper surface of the burl having a standard shape (i.e. without adaptations to the geometry around the distal end 42 to improve fluid retention).

[0126] In some embodiments, a substrate support 20 comprising one or more of the burls 41b described above with reference to Figures 15 A, 15B and 16 may also comprise any of the other fluidretaining features described above. In other embodiments, a substrate support 20 may comprise one of the burls 41b described above with reference to Figures 15 A, 15B and 16 but not any of the other fluidretaining features described above. That is, the burls 41b described above with reference to Figures 15 A, 15B and 16 may be implemented within a substrate support 20 in the absence of the other features described in the present disclosure.

[0127] It has been observed that, during the drying out of the extraction gutter 130, a region of immersion liquid may form between adjacent burls 41. The region of immersion liquid may be a capillary bridge of immersion liquid. The capillary bridge may extend between the adjacent burls 41 so as to connect: (i) immersion liquid coating one burl 41 and (ii) immersion liquid coating the adjacent burl 41.

[0128] Figure 17 depicts a plan view of a substrate support 600 in which a first capillary bridge of immersion liquid IL has been formed between a first burl 641 A and a second burl 641B, and a second capillary bridge of immersion liquid IL has been formed between the second burl 64 IB and the third burl 641C. The first, second and third burls 641 A, 64B, 641C are disposed within the extraction gutter 630 between the radially inner wall 631 and the radially outer wall 632. As described above, the radially inner wall 631 comprises a radially outer surface 631a and an upper surface 631b, and the radially outer wall 632 comprises a radially inner surface 632a and an upper surface 632b (referring to Figures 18B, 19B and 20B). The extraction gutter 630 further comprises a plurality of extraction openings 617, each in fluid communication with a second extraction passageway 614 (referring to Figures 18 A, 19A and 20A). The burls 641A, 641B, 641C protrude from a support body 621 of the substrate support 600 (referring to Figures 18B, 19B and 20B). The shape of the capillary bridges may correspond to an area of the extraction gutter 630 in which turbulence is relatively low. The relatively low turbulence may mean that the drying time for immersion liquid in the capillary bridge is longer than the drying time for immersion liquid in other parts of the extraction gutter 630.

[0129] The first and second capillary bridges may be continuous (i.e. connected to one another). Similar capillary bridges may be formed between other one of the plurality of burls 641 distributed around the extraction gutter 630 of the substrate support 600. Capillary bridges may not be formed where an extraction opening 617 is present. For example, if a fourth burl (not shown) is adjacent to the first burl 641A, but an extraction opening 617 is interposed between the fourth burl and the first burl 641 A, a capillary bridge may not be formed between the first burl 641 A and the fourth burl.

[0130] A capillary bridge may follow an arcuate path between adjacent burls. For example, the capillary bridge formed between the first burl 641 A and the second burl 64 IB may follow an arcuate path between the first burl 641 A and the second burl 641B. The arcuate path along which the capillary bridge extends between the first burl 641 A and the second burl 64 IB may be referred to as a first arcuate path.

[0131] The first arcuate path of the first capillary bridge between the first burl 641 A and the second burl 64 IB may curve radially inward between the first burl 641 A and the second burl 641B. That is, when looking at the capillary bridge between the first burl 641 A and the second burl 64 IB from a position that is radially outward of the first burl 641 A and the second burl 641B, the shape of the capillary bridge may appear concave. In other words, a midpoint of the first arcuate path may be radially inward of the first burl 641 A and the second burl 64 IB. The midpoint of the first arcuate path may be a point which bisects the first arcuate path.

[0132] A radially outer edge of the capillary bridge may extend along a second arcuate path, and a radially inner edge of the capillary bridge may extend along a third arcuate path. A degree of curvature of the second arcuate path may be greater than a degree of curvature of the third arcuate path. Degree of curvature is inversely proportional to radius of curvature. In some embodiments, the second arcuate path and the third arcuate path may be, or may be approximated to, arcuate paths of a circle or an ellipse. In such embodiments, a radius of the third arcuate path (rs) may be greater than a radius of the second arcuate path ( ). In the context of the arcuate paths, the terms “radially inner” and “radially outer” are used in the same way as in the rest of the disclosure, i.e. the terms “radially inner” and “radially outer” are used to refer to the position of the arcuate paths relative to the centre of the substrate support 600. In general, the first arcuate path may represent the overall shape of the capillary bridge between adjacent burls 641 A, 641B, and the second and third arcuate paths represent inner and outer edges of the capillary bridge. Thus, the first arcuate path may be disposed between the second arcuate path and the third arcuate path.

[0133] The formation of the first capillary bridge between the first burl 641 A and the second burl 64 IB may increase the length of time taken for an upper surfaces of the first burl 641 A and the second burl 641B to dry out, for example to approximately 20 seconds. Thus, formation of a capillary bridge may be desirable because the capillary bridge reduces the risk of upper surfaces of the burls drying out during and between critical periods. Consequently, the coefficient of friction between upper surfaces of the burls and the underside of the substrate W may remain substantially constant during and between the critical periods. Consequently, overlay error (and, specifically, overlay error arising from in-plane deformation of the substrate W) can be reduced, as has been explained above.

[0134] In some cases, the capillary bridge may be formed naturally (i.e. without external influence). However, whether or not a capillary bridge is formed between adjacent burls is inconsistent and unpredictable (spatially and temporally). For example, in some cases, a capillary bridge may be formed naturally between some of the burls (e.g. between the first burl 641 A and the second burl 641B) but not between other burls (e.g. between the second burl 641B and the third burl 641C). In some cases, a capillary bridge may be formed between the burls 641 when the immersion liquid is provided to the extraction gutter 630 for a first time, but not when the immersion liquid is provided to the extraction gutter 630 for a second time (where the first and second time refer to any times that immersion liquid is provided to the extraction gutter 630). Whether or not a capillary bridge is naturally formed may depend of the surface characteristics of the substrate support 600 between the burls.

[0135] To improve the consistency (spatially and temporally) with which capillary bridges are formed between burls 641 in the extraction gutter 630, the substrate support 600 may be provided with one or more bridging features. A bridging feature may be any feature provided to, or defined by, the substrate support 600 which promotes the formation of a capillary bridge between adjacent burls. A bridging feature may extend between adjacent burls in the extraction gutter 630. For instance, there may be a first bridging feature between the first burl 641 A and the second burl 641B, and a second bridging feature between the second burl 64 IB and the third burl 641C. Particular examples of such bridging features are depicted in Figures 18A to 20C.

[0136] The bridging features (e.g. the position and shape of the bridging features) may follow the path along which the capillary bridges naturally form. That is, a bridging feature may follow an arcuate path between adjacent burls. For example, the first bridging feature formed between the first burl 641 A and the second burl 64 IB may follow an arcuate path between the first burl 641 A and the second burl 641B. The arcuate path along which the first bridging feature extends between the first burl 641 A and the second burl 641B may be referred to as a first arcuate path of the first bridging feature.

[0137] The first arcuate path of the first bridging feature between the first burl 641 A and the second burl 64 IB may curve radially inward between the first burl 641 A and the second burl 641B. That is, when looking at the bridging feature between the first burl 641 A and the second burl 64 IB from a position that is radially outward of the first burl 641 A and the second burl 64 IB, the first bridging feature may appear concave. In other words, a midpoint of the first arcuate path of the first bridging feature may be radially inward of the first burl 641 A and the second burl 641B.

[0138] A radially outer edge of the bridging feature may extend along a second arcuate path, and a radially inner edge of the bridging feature may extend along a third arcuate path. Like the first arcuate path, midpoints of the second and third arcuate paths of the first bridging feature may be radially inward of the first burl 641 A and the second burl 641B. A degree of curvature of the second arcuate path may be greater than a degree of curvature of the third arcuate path.

[0139] Figures 18A to 18C depict an embodiment of a substrate support 600 comprising bridging features 6501AB, 6501BC. Figure 18A depicts circumferential sectional view of a substrate support 600 with bridging features 6501AB, 6501BC. The cutting plane of the sectional view passes through the radially outermost ring of burls 641 within the extraction gutter 630. Figure 18B depicts the extraction gutter 630 of the substrate support 600 depicted in Figure 18 A. Figure 18C depicts a plan view of the substrate support 600 depicted in Figure 18 A.

[0140] In the embodiment depicted in Figures 18A to 18C, the bridging features 6501AB, 6501BC are recessed portions formed in the upper surface 625 of the substrate support 600, and the recessed portions each extend between two or more of the support members or burls. For instance, a first bridging feature 6501 AB (which, in the embodiment depicted in Figures 18A to 18C, is a recessed portion) may extend between the first burl 641 A and the second burl 641B, and a second bridging feature 6501BC (which, in the embodiment depicted in Figures 18A to 18C, is a recessed portion) may extend between the second burl 64 IB and the second burl 641C. During operation of the lithographic apparatus, immersion liquid may collect in the recessed portions, thus promoting the formation of a capillary bridge.

[0141] The recessed portions may extend between adjacent burls and around the adjacent burls. For instance, the recessed portion corresponding to the first bridging feature 6501 AB may extend between the first burl 641 A and the second burl 641 B, and also around the first burl 641 A and around the second burl 641B. In other words, the burls may be disposed within the recessed portions corresponding to the bridging features. Immersion liquid collected within the first bridging feature 6501AB may be able to move to the upper surfaces of the first and second burls 641 A, 64 IB by capillary action.

[0142] Recessed portions corresponding to different bridging features 6501AB, 6501BC may be connected to each other. For instance, the recessed portion corresponding to the first bridging feature 6501 AB may be connected to the recessed portion corresponding to the second bridging feature 6501AB, BC. For example, in the embodiment depicted in Figures 18A to 18C, a single recessed portion forms the first bridging feature 6501AB and the second bridging feature 6501BC, and the first burl 641 A, the second burl 641B and the third burl 641C are disposed within the single recessed portion. [0143] In the embodiment depicted in Figures 18A to 18C, the second arcuate path may follow a radially outward edge of the recessed portion corresponding to the first bridging feature 6501 AB, and the third arcuate path may follow a radially inward edge of the recessed portion corresponding to the first bridging feature 6501AB.

[0144] Figures 19A to 19C depict another embodiment of a substrate support 600 comprising bridging features 6502AB, 6502BC. Figure 19A depicts circumferential sectional view of a substrate support 600 with bridging features 6502AB, 6502BC. The cutting plane of the sectional view passes through the radially outermost ring of burls within the extraction gutter 630. Figure 19B depicts the extraction gutter 630 of the substrate support 600 depicted in Figure 19 A. Figure 19C depicts a plan view of the substrate support 600 depicted in Figure 19 A.

[0145] In the embodiment depicted in Figures 19A to 19C, the bridging features are elongated protrusions. The elongated protrusions may extend between two or more support members or burls. For instance, a first bridging feature 6501AB (which, in the embodiment depicted in Figures 19A to 19C, is an elongated protrusion) may extend between the first burl 641 A and the second burl 641B, and a second bridging feature 6501BC (which, in the embodiment depicted in Figures 19A to 19C, is an elongated protrusion) may extend between the second burl 64 IB and the second burl 641C.

[0146] The elongated protrusions may be referred to as ridges, e.g. ridges that extend between adjacent burls. The elongated protrusions may protrude from the upper surface 625 of the substrate support 600. For example, the elongated protrusions may protrude upward from the upper surface 625 of the substrate support 600. During operation of the lithographic apparatus, immersion liquid may adhere to surfaces of the elongated protrusions, thus promoting the formation of a capillary bridge.

[0147] In the embodiment depicted in Figures 19A to 19C, the second arcuate path may follow a radially outer surface of the elongated protrusion corresponding to the first bridging feature 6501 AB, and the third arcuate path may follow a radially inner surface of the elongated protrusion corresponding to the first bridging feature 6501 AB.

[0148] Figures 20A to 20C depict another embodiment of a substrate support 600 comprising bridging features 6503 AB, 6503BC. Figure 20A depicts circumferential sectional view of a substrate support 600 with bridging features 6503 AB, 6503BC. The cutting plane of the sectional view passes through the radially outermost ring of burls 641 within the extraction gutter 630. Figure 20B depicts the extraction gutter 630 of the substrate support 600 depicted in Figure 20A. Figure 20C depicts a plan view of the substrate support 600 depicted in Figure 20A.

[0149] In the embodiment depicted in Figures 20A to 20C, the bridging features are reservoirs. The reservoirs extend between two or more adjacent burls. The reservoirs are formed by walls protruding from the upper surface 625 of the substrate support 600. The reservoirs may extend between two or more support members or burls. For instance, a first bridging feature 6501AB (which, in the embodiment depicted in Figures 20A to 20C, is a reservoir) may extend between the first burl 641A and the second burl 641B, and a second bridging feature 6501BC (which, in the embodiment depicted in Figures 19A to 19C, is a reservoir) may extend between the second burl 64 IB and the second burl 641C. During operation of the lithographic apparatus, immersion liquid may be collected in the reservoirs, thus promoting the formation of a capillary bridge.

[0150] In the embodiment depicted in Figures 20A to 20C, the second arcuate path may follow a radially outer edge of the reservoir (i.e. an inner surface of a wall which defines a radially outward boundary of the reservoir) corresponding to the first bridging feature 6501AB, and the third arcuate path may follow a radially inward edge of the reservoir (i.e. an inner surface of a wall which defines a radially inward boundary of the reservoir) corresponding to the first bridging feature 6501 AB. In this context, inner surface of a wall means a surface of a wall facing the internal volume of the reservoir.

[0151] In some embodiments, a substrate support 600 comprising one or more bridging features may also comprise any of the other fluid-retaining features described above. In other embodiments, a substrate support 600 may comprise one or more bridging features but not any of the other fluidretaining features described above. The bridging features may possess similar or corresponding features to the fluid-retaining features described above. For instance, in the case that a bridging feature is a recessed portion extending between adjacent burls, a depth of the recessed portions may be the same as, for example, the recessed portions 250 depicted in Figure 7).

[0152] The material from which the substrate support 200, 300, 400 is formed may comprise at least one of SiSiC, lithium-aluminosilicate glass-ceramic (e.g., Zerodur®), cordierite, SiC or diamond SiSiC. However, the material from which the substrate support 200, 300, 400 is formed is not limited to these examples.

[0153] In some embodiments, the support body of the substrate support 200, 300, 400 may be formed of SiSiC. The SiSiC layer may have a diamond-like carbon (DLC) coating thereon. The DLC coating may have a thickness of approximately 100 nm to 1,000 nm, e.g., 500 nm. The DLC coating may be less hydrophilic than the SiSiC which forms the support body. Therefore, for example, when a plurality of recessed portions 250 are formed in the upper surface 225 within the extraction gutter 230, the surfaces 251 , 252 which form the interior of the recessed portions 250 may be more hydrophilic than the surrounding surfaces. Consequently, the recessed portions 250 may be able to effectively collect and retain immersion liquid.

[0154] The roughness of surfaces within the extraction gutter 230, 330, 430 may also be adapted in order to further retain immersion liquid within the extraction gutter 230, 330, 430. For example, the roughness of the upper surface 225, 325, 425 of the extraction gutter 230, 330, 430 may be increased. Increasing the roughness of surfaces may lead to those surfaces trapping more immersion liquid in the rough surface. Other surface treatments to make surfaces in the extraction gutter 230, 330, 430 more hydrophilic may also cause the treated surfaces to retain a greater amount of immersion liquid.

[0155] The surface area roughness (S_a) of one or more of the surfaces within the extraction gutter 230, 330, 430 (e.g., the upper surface 225, 325, 425 within the extraction gutter 230, 330, 430) may be greater than 5 pm, preferably greater than 10 pm, preferably greater than 30 pm, and preferably greater than 50 pm. In general, the roughness may be sufficiently large so that the surfaces within the extraction gutter 230, 330, 430 can retain fluid for the certain period of time discussed above. The surface area roughness (S_a) may be a measure of the average distance between the heights of the points on the surface and the mean height of the surface. Values for roughness may be determined using white light interferometry.

[0156] The roughness of surfaces within the extraction gutter 230, 330, 430 may be increased by any suitable method. The method may involve selecting alternate manufacturing processes when forming the surfaces of the extraction gutter 230, 330, 430 which naturally result in a higher surface roughness. Additionally or alternatively, texturing processes may be performed on the surfaces within the extraction gutter 230, 330, 430 after they have been formed. Texturing processes may comprise grinding or sand-blasting.

[0157] Instead of adapting the roughness of the surfaces within the extraction gutter 230, 330, 430, a hydrophilic coating (not shown) may be applied to the surfaces within the extraction gutter 230, 330, 430 to increase their ability to retain immersion liquid. Hydrophilic coatings may be, for example, glass-based coatings. Additionally or alternatively, the hydrophilic coating may comprise SiCh.

[0158] To further increase the retention of immersion liquid within the extraction gutter 230, 330, 430, the outermost circumferential ring of burls 241, 341, 441, which are located within the extraction gutter 230, 330, 430, may be positioned sufficiently close to the outer circumferential seal 232, 332432 for the effect of the capillary action between the outermost circumferential ring of burls 241, 341, 441 and the outer circumferential seal 232, 332, 432 to be more significant than the pull on the immersion liquid by the extraction opening 217, 317, 417. As a result, immersion liquid may collect in the space between the outermost circumferential ring of burls 241, 341, 441 and the outer circumferential seal 232, 332, 432. This may further enable the substrate support 200, 300, 400 to be able to retain fluid within the extraction gutter 230, 330, 430 for a longer period of time.

[0159] In another aspect of the present disclosure, there is provided a method of reducing asymmetric wear of the burls 41 in the outermost circumferential ring of burls 41b by increasing the uniformity of the presence of immersion liquid in the extraction gutter 130 prior to the unloading of the substrate W from the substrate support 20. The method comprises using the fluid handling structure IH to provide immersion liquid to the extraction gutter 130 after the fluid handling structure IH has relatively moved across substantially all of the substrate W (i.e. after exposure of the substrate W has finished). Specifically, the fluid handling structure IH may provide immersion liquid to portions of the extraction gutter 130 from which the immersion liquid has dried by the time that the substrate W is unloaded from the substrate support 20. In this way, immersion liquid is made to be present throughout the extraction gutter 130 when the substrate W is unloaded form the substrate support 20.

[0160] A specific example will now be described for the case that the lithographic apparatus is configured such that the fluid handling structure IH relatively moves over the substrate W as shown in Figure 4. As described above, if the fluid handling structure IH relatively moves over the substrate W as shown in Figure 4, at the time at which the substrate W is unloaded form the substrate support 20 (i.e. at t = t4), the North portion N of the extraction gutter 130 is wetted (i.e. immersion liquid is present in the North portion N of the extraction gutter 130), but the South portion S of the extraction gutter 130 is not wetted (i.e. immersion liquid is not present in the South portion S of the extraction gutter 130). Thus, the method may comprise, after the fluid handling structure IH has relatively moved over substantially all of the substrate W (i.e. t = ts) and before the substrate W is unloaded from the substrate support 20 (i.e. t = t4), a re-wetting step in which the South portion S of the extraction gutter 130 is rewetted with immersion liquid.

[0161] The re-wetting step may comprise moving the fluid handling structure IH relative to the substrate W such that the fluid handling structure IH moves over the South portion S of the substrate W, and thus provides immersion liquid to the South portion S of the extraction gutter 130. By performing this step, the uniformity of the presence of immersion liquid in the extraction gutter 130 is increased between the North portion N of the extraction gutter 130 and the South portion S of the extraction gutter 130. Consequently, burls 41b in the North portion N of the extraction gutter 130 are worn to the same extent as the burls 41b in the South portion S of the extraction gutter 130 when the substrate W is unloaded from the substrate support 20. Thus, the increase of asymmetric wear on the burls 41b is suppressed. It will be appreciated that the re-wetting step may comprise re-wetting any other portions of the extraction gutter 130 in which immersion liquid is no longer present by the time that the substrate W is unloaded from the substrate support 20.

[0162] Implementing a step of re-wetting a portion of the extraction gutter 130, as described above, may increase the length of time required to process a substrate W. Thus, implementing a step of rewetting a portion of the extraction gutter 130 may reduce the throughput of the lithographic apparatus. However, in some cases, the suppression of asymmetric wear of the burls 41b may be to the extent that the effect on throughput is justified.

[0163] It will be appreciated that a method comprising the step of re-wetting a portion of the extraction gutter 130 may be performed on a substrate support 20, 200, 400, 300, 500, 600 having the features described herein (e.g. fluid-retaining features, bridging features, etc.). However, this is not essential, and the method comprising the step of re-wetting a portion of the extraction gutter 130 may be successfully implemented on any substrate support 20 having an extraction gutter 130.

[0164] 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 support WT, etc..

[0165] 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 300, 400, 500 as described in any of the above embodiments and variations.

[0166] 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.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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, the substrate support comprising: a support body having an upper surface; a plurality of support members extending above the upper surface; an extraction gutter defined by a first circumferential wall and a second circumferential wall, wherein the first circumferential wall and the second circumferential wall extend above the upper surface; an extraction opening formed in the upper surface within the extraction gutter; and one or more fluid-retaining features formed within the extraction gutter, wherein the one or more fluidretaining features are defined by the support body.

2. The substrate support according to claim 1 , wherein the one or more fluid-retaining features are configured to inhibit the extraction, through the extraction opening, of fluid collected by the fluidretaining features, and/or wherein the fluid-retaining features are reservoirs, and/or wherein the fluidretaining features are recessed portions formed in the upper surface, and/or wherein the one or more fluid-retaining features are an array of indentations distributed across the upper surface within the extraction gutter.

3. The substrate support according to claim 2, wherein a depth of the fluid-retaining features is greater than 10 pm and preferably greater than 100 pm, less than 500 pm, preferably less than 200 pm, and further preferably less than 120 pm, and/or wherein the indentations are cylindrical, hemispherical, conical or cuboidal, and/or wherein a maximum width of the indentations is greater is greater than 10 pm, preferably greater than 100 pm, further preferably greater than 150 pm, less than 2000 pm, preferably less than 1000 pm, and further preferably less than 500 pm.

4. The substrate support according to claim 1 or 2, wherein the fluid-retaining features comprise a plurality of grooves formed in the upper surface, desirably wherein the size of the plurality of grooves is such that they are capillary grooves, and/or desirably wherein the plurality of grooves have a width of greater than 50 pm and less than 100 pm, and preferably greater than 85 pm and less than 100 pm, and/or desirably wherein the one or more grooves extend in a radial direction, desirably wherein the one or more grooves are distributed circumferentially around the substrate support, and a distance between adjacent grooves in the circumferential direction is greater than 25 pm and preferably greater than 50 pm, and less than 150 pm and preferably less than 100 pm, and/or desirably wherein the one or more grooves extend circumferentially around the substrate support.

5. The substrate support according to claim 1 or 2, wherein the one or more fluid-retaining features are trenches extending circumferentially around the base of one or more of the plurality of support members or further comprising an extraction opening wall disposed around the circumference of the extraction opening, desirably wherein there is a single fluid-retaining feature defined at least partially by the extraction opening wall, the first circumferential wall and the second circumferential wall, and/or desirably wherein there are a plurality of extraction openings distributed circumferentially around the extraction gutter, and the substrate support comprises an extraction opening wall for each of the extraction openings, such that the fluid-retaining feature is defined by a plurality of extraction opening walls, the first circumferential wall and the second circumferential wall, and/or desirably wherein the height of the extraction opening wall is greater than 10 pm, preferably greater than 100 pm, further preferably greater than 150 pm, less than 2000 pm, preferably less than 1000 pm, and further preferably less than 500 pm.

6. The substrate support according to any of the preceding claims, wherein a fluid-retaining feature is present at greater than 50%, preferably greater than 75%, and further preferably greater than 85% of the surface area of the upper surface within the extraction gutter.

7. The substrate support according to claim 1 or 2, wherein the fluid-retaining features comprise one or more bridging features, and wherein the bridging features each extend between two or more support members, optionally between two or more support members in an outermost circumferential ring of support members, desirably wherein the one or more of the bridging features comprise recessed portions formed in the upper surface of the substrate support, and the recessed portions extend between the two or more support members, or desirably wherein the one or more of the bridging features comprise elongated protrusions protruding from the upper surface of the substrate support, and the elongated protrusions extend between the two or more support members, or desirably wherein the one or more of the bridging features comprise reservoirs extending between the two or more support members, wherein the reservoirs are formed by walls protruding from the upper surface of the substrate support.

8. The substrate support according to claim 7, wherein a bridging feature extends between a first support member and a second support member along a first arcuate path, desirably wherein a midpoint of the first arcuate path is radially inward of the first support member and the second support member, and/or wherein a radially outer edge of the bridging feature extends along a second arcuate path, and a radially inner edge of the bridging feature extends along a third arcuate path, and a degree of curvature of the second arcuate path is greater than a degree of curvature of the third arcuate path.

9. The substrate support according to any of the preceding claims, wherein the second circumferential wall is radially outward of the first circumferential wall, and a radial distance between one or more radially outermost support members and the second circumferential wall is less than 500 pm, preferably less than 250 pm, further preferably less than 200 pm, greater than 10 pm, preferably greater than 50 pm and preferably greater than 100 pm, and/or comprising a plurality of extraction openings distributed circumferentially around the extraction gutter, optionally wherein the plurality of extraction openings are distributed uniformly around the extraction gutter, desirably wherein the number of extraction openings is greater than 10, preferably greater than 50, further preferably greater than 75, further preferably greater than 100, less than 1000, preferably less than 500, and further preferably less than 150, and/or desirably wherein a distance between adjacent extraction openings is less than 90 mm, preferably less than 20 mm, further preferably less than 10 mm, greater than 2 mm, and preferably greater than 6 mm, and/or desirably wherein a number of support members between adjacent extraction openings in the extraction gutter is less than 50, preferably less than 10, further preferably less than or equal to 6, greater than 1 , preferably greater than 2, and further preferably greater than or equal to 4.

10. The substrate support according to any of the preceding claims, wherein a surface area of an upper surface of a support member in the extraction gutter that is in direct contact with an underside of the substrate is greater than 20% of a nominal area of the upper surface of the support member, preferably greater than 30% of the nominal area of the upper surface of the support member, further preferably greater than 40% of the nominal area of the upper surface of the support member, and less than 60% of the nominal area of the upper surface of the support member, and/or wherein a lowermost horizontal surface of the extraction gutter is the upper surface of the substrate support, such that substantially all of the extraction gutter is above the upper surface, and/or wherein distal ends of the plurality of support members are configured to support a bottom surface of the substrate, and/or wherein the first circumferential wall and the second circumferential wall have a height less than that of the plurality of support members so as to form at least a partial seal with a bottom surface of the substrate when the substrate is supported by the substrate support, and/or wherein the substrate support is formed of at least one of SiSiC, lithium-aluminosilicate glass-ceramic, Zerodur, cordierite, SiC or diamond SiSiC.

11. A lithographic apparatus including the substrate support according to any of the preceding claims.

12. A method of manufacturing the substrate support according to any of claims 1-10, the method comprising forming the one or more recessed portions by removing material from the upper surface using laser structuring.

13. A method of manufacturing a device comprising supporting a substrate on the substrate support according to any of claims 1-10.