Substrate holder for use in a lithographic apparatus

A substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder having a main body having a main body surface, a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate, a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member, and a plurality of minor burls projecting from the upper surface of the first seal member, wherein each minor burl has a distal end surface configured to support the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT Patent Application No. PCT/EP2018/075293, filed Sep. 19, 2018, which claims the benefit of priority of European Patent Application No. 17196086.7, which was filed on Oct. 12, 2017 and of European Patent Application No. 18163985.7, which was filed on Mar. 26, 2018, each of which are incorporated herein in their entirety by reference.

FIELD

The present invention relates to a substrate holder for use in a lithographic apparatus.

BACKGROUND

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. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

In an immersion lithographic apparatus an immersion liquid is interposed in a space between a projection system of the apparatus and a substrate. This immersion liquid can find its way past the edge of the substrate to an under surface of the substrate. This can be deleterious due to contamination of the under surface of the substrate resulting from this immersion liquid and/or due to thermal loads applied to the under surface of the substrate at positions close to the edge of the substrate, due to evaporation of the immersion liquid. A substrate holder which is configured to support the substrate can have features which reduce the quantity and/or distance the immersion liquid moves radially inwardly along the under surface of the substrate. Such features can deleteriously effect the achieved flatness and cleanliness of the substrate as well as the ease of removal.

SUMMARY

It is an object of the present invention to provide a substrate holder with an acceptable compromise between performance of the substrate holder in terms of flatness and cleanliness of the substrate and reduction of the passage of the immersion liquid along the under surface of the substrate.

In an embodiment of the present invention there is provided a substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising: a main body having a main body surface; a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate: a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member; and a plurality of minor burls projecting from the upper surface of the first seal member, wherein each minor burl has a distal end surface configured to support the substrate.

In an embodiment of the present invention, there is provided a substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising: a main body having a main body surface; a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate: a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member; a second seal member projecting from the main body surface, the second seal member surrounding the first seal member and configured for restricting the passage of liquid between the substrate and the main body surface radially inward past the second seal member; a plurality of extraction openings formed in the main body between the first seal member and the second seal member for the extraction of fluid into the main body from between the main body and the substrate; and a plurality of outer burls projecting from the main body surface between the first seal member and the second seal member, each outer burl having a distal end surface configured to support the substrate; wherein the plurality of outer burls and the plurality of extraction openings are arranged alternately in a line surrounding the first seal member and the plurality of main burls.

In an embodiment of the present invention, there is provided a substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising: a main body having a main body surface; a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate: a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member; a plurality of minor burls projecting from the upper surface of the first seal member, wherein each minor burl has a distal end surface configured to support the substrate;a second seal member projecting from the main body surface, the second seal member surrounding the first seal member and configured for restricting the passage of liquid between the substrate and the main body surface radially inward past the second seal member; a third seal member projecting from the main body surface, the third seal member surrounding the first seal member and the second seal member and configured for restricting the passage of liquid between the substrate and the main body surface radially inward past the third seal member; a plurality of inlet openings formed in the main body between the first seal member and the second seal member; and a plurality of extraction openings formed in the main body between the second seal member and the third seal member for the extraction of fluid into the main body from between the main body and the substrate.

In an embodiment of the present invention, there is provided a substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising: a main body having a main body surface; a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate; a seal member projecting from the main body surface and having an upper surface, the seal member surrounding the plurality of main burls; a plurality of extraction openings formed in one or more first recesses in the upper surface of the seal member; a plurality of inlet openings formed in one or more second recesses in the upper surface of the seal member; a barrier between the one or more first recesses and the one or more second recesses and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the barrier.

DETAILED DESCRIPTION

The lithographic apparatus may be of a type wherein at least a portion of the substrate may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fill a space11between the projection system PS and the substrate W—which is also referred to as immersion lithography. More information on immersion techniques is given in U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

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.

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 a space11between a projection system 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 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. The effect of the immersion liquid is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid than 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.)

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 space11as immersion liquid. Other liquids with a high refractive index can be used besides water can be used 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.

In this specification, reference will be made in the description to localized immersion in which the immersion liquid is confined, in use, to the space11between 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 structure12present between the projection system PS and the substrate support WT is used to confine the immersion liquid to the immersion space11. The space11filled by the immersion liquid is smaller in plan than the top surface of the substrate W and the space11remains substantially stationary relative to the projection system PS while the substrate W and substrate support WT move underneath.

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 space11is 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.

The fluid handling structure12is a structure which supplies the immersion liquid to the immersion space11, removes the immersion liquid from the space11and thereby confines the immersion liquid to the immersion space11. 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 space11and 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 space11between the final element of the projection system PS and the substrate support WT or substrate W, so as to in part define the space11.

The fluid handing structure12may have a selection of different functions. Each function may be derived from a corresponding feature that enables the fluid handling structure12to achieve that function. The fluid handling structure12may 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.

As a barrier member, the fluid handling structure12is a barrier to the flow of the immersion liquid from the space11. As a liquid confinement structure, the structure confines the immersion liquid to the space11. As a seal member, sealing features of the fluid handling structure form a seal to confine the immersion liquid to the space11. The sealing features may include an additional gas flow from an opening in the surface of the seal member, such as a gas knife.

In an embodiment the fluid handling structure12may supply immersion fluid and therefore be a fluid supply system.

In an embodiment the fluid handling structure12may at least partly confine immersion fluid and thereby be a fluid confinement system.

In an embodiment the fluid handling structure12may provide a barrier to immersion fluid and thereby be a barrier member, such as a fluid confinement structure.

In an embodiment the fluid handling structure12may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid.

The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure12may be referred to as a seal member; such a seal member may be a fluid confinement structure.

In an embodiment, immersion liquid is used as the immersion fluid. In that case the fluid handling structure12may 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.

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 structure12between 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 structure12. The fluid handling structure12may confine the immersion liquid under the final element and above the facing surface.

FIGS. 2aand 2bshow different features which may be present in variations of fluid handling structure12. The designs may share some of the same features asFIGS. 2aand 2bunless described differently. The features described herein may be selected individually or in combination as shown or as required.

FIG. 2ashows a fluid handling structure12around the bottom surface of a final element100. The final element100has an inverted frusto-conical shape. The frusto-conical shape having a planar bottom surface and a conical surface. The frusto-conical shape protrudes from a planar surface and having a bottom planar surface. The bottom planar surface is the optically active portion of the bottom surface of the final element100, through which the radiation beam B may pass. The final element100may have a coating30. The fluid handling structure12surrounds at least part of the frusto-conical shape. The fluid handling structure12has an inner-surface which faces towards the conical surface of the frusto-conical shape. The inner-surface and the conical surface have complementary shape. A top surface of the fluid handling structure12is substantially planar. The fluid handling structure12may fit around the frusto-conical shape of the final element100. A bottom surface of the fluid handling structure12is substantially planar and in use the bottom surface may be parallel with the facing surface of the substrate support WT and/or substrate W. The distance between the bottom surface and the facing surface may be in the range of 30 to 500 micrometers, desirably in the range of 80 to 200 micrometers.

The fluid handling structure12extends closer to the facing surface of the substrate W and substrate support WT than the final element100. A space11is therefore defined between the inner surface of the fluid handling structure12, the planar surface of the frusto-conical portion and the facing surface. During use, the space11is filled with immersion liquid. The immersion liquid fills at least part of a buffer space between the complementary surfaces between the final element100and the fluid handling structure12, in an embodiment at least part of the space between the complementary inner-surface and the conical surface.

The immersion liquid is supplied to the space11through an opening formed in a surface of the fluid handling structure12. The immersion liquid may be supplied through a supply opening20in the inner-surface of the fluid handling structure12. Alternatively or additionally, the immersion liquid is supplied from an under supply opening23formed in the undersurface of the fluid handling structure12. The under supply opening23may surround the path of the radiation beam B and it may be formed of a series of openings in an array. The immersion liquid is supplied to fill the space11so that flow through the space11under the projection system PS is laminar. The supply of the immersion liquid from the opening23under the fluid handling structure12additionally prevents the ingress of bubbles into the space11. This supply of the immersion liquid functions as a liquid seal.

The immersion liquid may be recovered from a recovery opening21formed in the inner-surface. The recovery of the immersion liquid through the recovery opening21may be by application of an under pressure; the recovery through the recovery opening21as a consequence of the velocity of the immersion liquid flow through the space11; or the recovery may be as a consequence of both. The recovery opening21may be located on the opposite side of the supply opening20, when viewed in plan. Additionally or alternatively, the immersion liquid may be recovered through an overflow opening24located on the top surface of the fluid handling structure12. In an embodiment, the supply and recovery openings20,21can have their function swapped (i.e. the flow direction of liquid is reversed). This allows the direction of flow to be changed depending upon the relative motion of the fluid handling structure12and substrate W.

Additionally or alternatively, immersion liquid may be recovered from under the fluid handling structure12through a recovery opening25formed in its bottom surface. The recovery opening25may serve to hold (or ‘pin’) a meniscus33of the immersion liquid to the fluid handling structure12. The meniscus33forms between the fluid handling structure12and the facing surface and it serves as border between the liquid space and the gaseous external environment. The recovery opening25may be a porous plate which may recover the immersion liquid in a single phase flow. The recovery opening in the bottom surface may be a series of pining openings32through which the immersion liquid is recovered. The pining openings32may recover the immersion liquid in a two phase flow.

Optionally radially outward, with respect to the inner-surface of the fluid handling structure12, is an gas knife opening26. Gas may be supplied through the gas knife opening26at elevated speed to assist liquid confinement of the immersion liquid in the space11. The supplied gas may be humidified and it may contain substantially carbon dioxide. Radially outward of the gas knife opening26is a gas recovery opening28for recovering the gas supplied through the gas knife opening26. Further openings, for example open to atmosphere or to a gas source, may be present in the bottom surface of the fluid handling structure12. For example, further openings may be present between gas knife opening26and gas recovery opening28and/or between pining openings32and gas knife opening26.

Features shown inFIG. 2bwhich are common toFIG. 2ashare the same reference numbers. The fluid handling structure12has an inner surface which complements the conical surface of the frusto-conical shape. The undersurface of the fluid handling structure12is closer to the facing surface than the bottom planar surface of the frusto-conical shape.

Immersion liquid is supplied to the space11through supply openings34formed in the inner surface of the fluid handling structure12. The supply openings34are located towards the bottom of the inner surface, perhaps below the bottom surface of the frusto-conical shape. The supply openings34are located around the inner surface, spaced apart around the path of the radiation beam B.

Immersion liquid is recovered from the space11through recovery openings25in the undersurface of the fluid handling structure12. As the facing surface moves under the fluid handling structure12, the meniscus33may migrate over the surface of the recovery opening25in the same direction as the movement of the facing surface. The recovery openings25may be formed of a porous member. The immersion liquid may be recovered in single phase. In an embodiment the immersion liquid is recovered in a two phase flow. The two phase flow is received in a chamber35within the fluid handling structure12where it is separated into liquid and gas. The liquid and gas are recovered through separate channels36,38from the chamber35.

An inner periphery39of the undersurface of fluid handling structure12extends into the space11away from the inner surface to form a plate40. The inner periphery39forms a small aperture which may be sized to match the shape and size of the radiation beam B. The plate40may serve to isolate the immersion liquid at either side of it. The supplied immersion liquid flows inwards towards the aperture, through the inner aperture and then under the plate40radially outwardly towards the surrounding the recovery openings25.

In an embodiment the fluid handling structure12may be in two parts as shown on the right hand side ofFIG. 2b: an inner part12aand an outer part12b. The inner part12aand the outer part12bmay move relatively to each other, in a plane parallel to facing surface. The inner part12amay have the supply openings34and it may have the overflow recovery24. The outer part12bmay have the plate40and the recovery opening25. The inner part12amay have an intermediate recovery42for recovering the immersion liquid which flows between the inner part12aand the outer part12b.

The substrate support WT comprises a substrate holder200which is configured to support the substrate W.FIG. 3illustrates, in cross-section in an upper portion and in plan in a lower portion, an edge region of the substrate holder200and the associated substrate W (in the upper part) according to an embodiment. The substrate holder200comprises a main body210having a main body surface212. In use the main body surface212faces an under surface of the substrate W.

In a central region of the main body surface212(to the left hand side inFIG. 3), a plurality of main burls220project from the main body surface212. Each main burl220has a distal end surface configured to support the substrate W. The main burls220are arranged relative to another in a pattern, in plan. The pattern is such as to support the substrate W and to reduce any bowing of the substrate W towards the main body surface212to an acceptable amount.

The area in plan of each main burl220is relatively small compared to the area, in plan, of the substrate W. Therefore the main burls220contact only a small area of the under surface of the substrate W. This reduces the opportunity for contamination to be transferred from the substrate holder200to the substrate W.

A pressure differential across the substrate W is established. For example, the space between the main body210of the substrate holder200and the substrate W is connected to an under pressure that is lower than a higher pressure above the substrate W. The pressure difference gives rise to a force holding the substrate W to the substrate holder200.

In an immersion lithographic apparatus, liquid will, at least at certain times during exposure of the substrate W, be present adjacent an edge of the substrate W. Due to the under pressure between the main body210of the substrate holder200and the under surface of the substrate W, this liquid will be drawn in around the edge of the substrate W and under the substrate W. In order to reduce occurrence of liquid being in contact with the under surface of the substrate W and especially at the area where main burls220contact with the substrate W, a first seal member230projecting from the main body surface212of the main body210is provided. The first seal member230surrounds the plurality of main burls212. The first seal member230is configured to restrict the passage of liquid between the substrate W and the main body surface212radially inward past the first seal member230. The first seal member230is a continuous (though not necessarily uniform in cross-section) barrier surrounding the main burls220.

One purpose of the first seal member230is to limit the flow of gas (which may undesirably be humid) radially inward towards the main burls220. This enables the under pressure to be generated around the main burls220which is necessary for clamping the substrate W to the substrate holder200. It is advantageous to allow some flow of gas over the first seal member230so that when the under pressure source which generates the under pressure around the main burls220is switched off, the substrate W can quickly be removed from the substrate holder200. If the gas flow is too low past the first seal member230, then the time taken for the pressure around the main burls220to equalise with the pressure above the substrate W, thereby releasing the substrate W, is too high.

The first seal member230has an upper surface232which is configured, in use, to form a gap between it and the under surface of the substrate W. That is, the upper surface232is configured to be somewhat closer to the main body surface212than the distal end surface of the main burls220. This is advantageous because this arrangement allows for gas to be drawn in over the first seal member230(under the substrate W) just before substrate W removal whilst allowing restriction of the passage of liquid in the same direction. This is achieved without contacting a large area of the under surface of the substrate W which would lead deleteriously to transfer of contamination from the first seal member230to the substrate W. This would also make removal of the substrate W from the substrate holder200more problematic.

The cross-sectional area of the first seal member230, in plan, is very much greater than that of the main burls220. The relatively large area, in plan, of the first seal member230results in a greater resistance to the passage of liquid between the substrate W and the main body surface212radially inward past the first seal member230.

As can be seen inFIG. 3, the main burl220which is the most radially outward of the plurality of main burls220, is quite a distance from the edge of the substrate W. In the absence of any other features supporting the substrate W radially outward of the most radially outward main burl220, bending of an edge of the substrate W downwards can occur. This is due to the under pressure underneath the substrate W compared to above the substrate W. In the present invention, in order to support the substrate W radially outward of the radially outward most main burl220, a plurality minor burls240are provided. The minor burls240project from the upper surface232of the first seal member230. Each minor burl240has a distal end surface configured to support the substrate W.

The plurality of minor burls240are provided circumferentially along the first seal member230. The plurality of minor burls240may be spaced apart. The plurality of minor burls240may all be at the same or at different radial distances from the centre of the substrate holder200. The cross-sectional area, in plan, of each minor burl240is very much less than that of the first seal member220. For instance, the sum of the cross-sectional areas of the plurality of minor burls240on the first seal member230is very much less than the total cross-sectional area of the first seal member230, for example at least 10 or 15 times less.

Radially outwardly of the first seal member230are a plurality of extraction openings250. The extraction openings250are formed in the main body210. The extraction openings250are connected to an under pressure source. Thereby any liquid which reaches the extraction openings250is extracted through the main body210. This means that the liquid is restricted from entering further into the space between the main body surface212and the substrate W. The extraction openings250can also extract gas, for example when there is no liquid present to be extracted. A mixture of liquid and gas can be extracted through the extraction openings250.

The extraction openings250are spaced apart from one another all the way around the first seal member230. Although the extraction openings250are illustrated inFIG. 3as being discrete openings in the main body surface212, this may not be the case. For example, a groove can be formed in the main body surface212and the extraction openings250can emerge from the main body210at the bottom of the groove. The groove may be segmented with one or more openings in each segment. The segments may be seen as a plurality of recesses.

By connecting the extraction openings250to an under pressure, liquid which does find its way to the edge of the substrate W can be removed through the extraction openings250. Once the edge of the substrate W is no longer covered in liquid, the under surface of the substrate W is dried as the liquid is removed.

Radially outward of the extraction openings250is a second seal member260. The second seal member260surrounds the extraction openings250. The second seal member260also surrounds the first seal member230.

The second seal member260may be similar to the first seal member230with an upper surface262from which a plurality of minor burls270project. An advantage of providing the plurality of minor burls270on the upper surface262of the second seal member260is that the substrate W is supported even closer to its edge. This further reduces deformation of the substrate W due to it edge being unsupported.

Although in the embodiment ofFIG. 3minor burls240,270are shown on both of the first seal member230and second seal member260, this may not be the case. For example, minor burls240may only project from the upper surface232of the first seal member230or minor burls270may only project from the upper surface262of the second seal member260. In both cases deformation of the substrate W is reduced compared to the case where the minor burl240,270is not present.

Although both first and second seal members230,260are illustrated inFIG. 3, it may be that only the first seal member230or only the second seal member260is present. If only the second seal member260is present, this arrangement can be seen as having a first seal member with a plurality of extraction openings250radially inward of that first seal member. If the first seal member230radially inward of the extraction openings250is present in such an arrangement, this can be seen as being a second seal member.

In the embodiment ofFIG. 3, in use, an under pressure in a central region of the substrate holder200between the main body surface212and the substrate W is provided. This under pressure is the reason the substrate W is clamped to the substrate holder200. This clamping under pressure may have a lower magnitude (i.e. is a less severe vacuum) than the under pressure at a region adjacent the extraction openings250. This arrangement results in a gas flow radially outward from a position around the main burls220towards the extraction openings250and a radially inward flow of fluid from the edge of the substrate W towards the extraction openings250. In this way, because of the gas flow radially outwardly to a position adjacent the extraction openings250, liquid and humidified gas is constrained from moving further radially inward than a position of the extraction openings250. Thereby the extent of liquid penetration under the substrate W is reduced. Because the first seal member230does not have liquid on it or is not present, removing the substrate W from the substrate holder200is achieved more easily, resulting in less wear. Wear is deleterious because this can result in contamination of the substrate W as well as a change in the clamping characteristics of the substrate holder200and thereby deformation of the substrate W. The presence of liquid between main burls220and substrate W underside can also lead to wear (if the substrate holder200is a ceramic), and possibly friction variation. Deformation of the substrate W can lead to imaging errors (e.g. overlay errors and/or focus errors), as can contamination. The presence of liquid on the underside of the substrate W is generally deleterious because this can result in thermal stability issues of the substrate W or difficulties when liquid droplets are lost during unloading of the substrate W. Therefore the substrate holder200ofFIG. 3alleviates some of these difficulties by having in place extra support for the substrate edge, measures for maintaining the first seal member230dry and for preventing the ingress of liquid radially inward past the extraction openings250. Additionally a humid atmosphere between the main body surface212and the substrate W is avoided by preventing the ingress of liquid past the extraction openings250. The disadvantage of a humid atmosphere is the possibility of oxidation of the main burls220. Oxidation of the main burls220is deleterious as this reduces the achievable flatness of the substrate W supported by the main burls220.

FIG. 4shows another embodiment of a substrate holder200according to the present invention. The Figure shows, in cross-section in an upper portion and in plan in a lower portion, an edge region of the substrate holder200. The embodiment ofFIG. 4is the same as that ofFIG. 3except as described below. Like reference numerals are used to illustrate corresponding features.

In the embodiment ofFIG. 4, a plurality of inlet openings280are formed in the main body surface212. The inlet openings280are radially inward of the extraction openings250. The inlet openings280are open to the atmosphere or connected to a gas source. As a result, a flow of gas radially outward from the plurality of inlet openings280towards the plurality of extraction openings250is created. This flow of gas forms a barrier to the ingress of liquid and humid gas radially inwards under the substrate W. A flow of gas radially inwards from the plurality of inlet openings280is also created, due to the under pressure formed in the center of the substrate holder200for clamping the substrate W to the substrate holder200. The under pressure generated in a center of the substrate holder200in a region of the main burls220may be of the same magnitude as the under pressure generated above the extraction openings250. The under pressure at the extraction openings250may be deeper (i.e. have a greater magnitude) than that at main buds220, causing a radially outward flow over the upper surface232as well.

Main burls220(and/or minor burls270) may be arranged alternately with the inlet opening280or extraction opening250or between the inlet opening280and extraction opening250, depending on the depth of the groove between the first seal member230and the second seal member260.

The plurality of inlet openings280may be formed in the bottom of a groove in the main body surface212. The groove may be in the form of a continuous or discontinuous circumferential channel. Alternatively the plurality of inlet openings280may be formed as a plurality of discrete openings in the main body surface212, as illustrated. The number, size and spacing of the inlet openings280may be selected as appropriate. The number, size and spacing of the inlet openings280may be similar to that of the extraction openings250, as illustrated.

Because the gas which exits the inlet openings280can be dehumidified gas, this further reduces the humidity of the gas present around the main burls220. Alternatively in the case that gas exiting the inlet openings280is drawn towards the extraction openings250, humidified gas can be provided out of the inlet openings280. This reduces evaporation of liquid in the resulting gas flow, thereby reducing evaporative thermal load on the underside of the substrate W and/or in extraction openings250and further downstream. The provision of the inlet openings280also allows the under pressure connected to the extraction openings250to be the same as that of the under pressure surrounding the main burls220. This is advantageous as the under pressure does not need to be lower than the under pressure surrounding the main burls220thereby drawing more liquid underneath the substrate W from the edge of the substrate W, as in the embodiment ofFIG. 3.

In an embodiment, the gas flow radially inward towards the extraction openings250results in a superficial liquid flow and a superficial gas flow which find a balance. This means that liquid will flow down the outer side of the through hole forming the extraction opening250and gas will flow on the inner side of the through hole. If any liquid is present this type of two phase flow will occur and a smooth flow is achievable. Desirably in the case of no liquid being present, the under pressure which is determined by the flow restrictions of the inlet openings280and the second seal member260, should be higher than the maximum capillary under pressure of the second seal260. That is, the gas flow is great enough to overcome capillary pressure of liquid in the gap between the upper surface262and the substrate W. If this condition is met, then the gas flow over the second seal member260should always be enough to remove any liquid present between the upper surface262of the second seal member260and the substrate W. This is desirable as it allows removal of the substrate W without needing to overcome attraction to the substrate holder200due to liquid being present between the second seal member260and the substrate W. No liquid is present between the first seal member230and the substrate W, as described above. The absence of liquid between the first and second seal member230,260and the substrate W will also increase flatness of the substrate W.

FIG. 5illustrates a further embodiment which is the same as the embodiment ofFIG. 4except as described below.

In the embodiment ofFIG. 5the second seal member260has a larger width in the radial direction than that of the first seal member230. The extraction openings250are formed in the upper surface262of the second seal member260. As illustrated, a groove252is formed in the upper surface262of the second seal member260and the extraction openings250are formed in the bottom of the groove252.

As illustrated inFIG. 5two minor burls270are formed on the upper surface262of the second seal member260, on either side of the extraction openings250. Such minor burls270may or may not be present and they may be on only the radial inner side or only on the radial outer side of the extraction openings250. The radial inner and outer minor burls270on the second seal member260may be lined up in the radial direction, as illustrated, or may be staggered relative to one another in the radial direction.

In one sense the embodiment ofFIG. 5can be seen as having three seal members, the first seal member230, a seal member on the radially inward side of the extraction openings250and a third seal member radially outward of the extraction openings250. This would be more apparent if the groove252were so deep that its bottom surface was substantially coplanar with the main body surface212of the main body210. In this view it can be seen that the middle seal member is positioned between the inlet openings280and the extraction openings250as well as between the first seal member230and the second seal member260.

Main burls220(and/or minor burls270) may be arranged alternately with the inlet opening280or extraction opening250.

An advantage of theFIG. 5embodiment is that a flow of gas radially outwardly from the inlet openings280to the extraction openings250passes over the constriction between the upper surface262of the radially inward part of the second seal member260and the substrate W. This results in an acceleration of the gas flow, thereby enhancing the sealing capability because any liquid which does find its way radially inward of extraction openings250is pushed by the accelerated gas flow back radially outwards towards the extraction opening250if the force generated by the gas flow is greater than the capillary force of the liquid between the upper surface232and the substrate W.=

In the embodiment ofFIG. 5, instead of or in addition to the extraction openings250being positioned in the second seal member260, the plurality of inlet openings280can be positioned in the first seal member230. This arrangement has similar advantages to the arrangement illustrated inFIG. 5as the gas flow radially outward from the inlet openings280towards the extraction openings250must pass through the narrow gap between the first seal member230and the substrate W so that the gas flow is accelerated thereby enhancing the sealing capability of the arrangement.

In an alternative embodiment, the groove252may be replaced with a plurality of recesses similar to the recesses284described below with reference toFIG. 6and the inlet openings280.

The embodiment ofFIG. 6is the same as the embodiment ofFIG. 5except as described below.

In the embodiment ofFIG. 6the arrangement of the extraction openings250in the bottom of a groove252formed in the upper surface262of the second seal member260is the same as in the embodiment ofFIG. 5. However instead of the inlet openings280being provided in the bottom of a deep recess between the first seal member230and second seal member260, the inlet openings280in the embodiment ofFIG. 6are provided at the bottom of individual recesses284formed in the upper surface232of the first seal member230. This embodiment can be viewed as having a first seal member230in which the inlet openings280are formed, a second seal member260with no inlets or openings formed in it, and a shallow groove252with extraction openings250formed in the bottom of the groove252between the first seal member230and second seal member260.

The recesses284are a pressure divider and make the gas flow more defined in position. Additionally the recesses284create a tangential flow between the inlet openings280. This tangential flow can remove liquid from between the inlet openings280. This effect is also achieved with the groove252.

Although the arrangement illustrated shows that each recess284has a corresponding inlet opening280, the arrangement may be different whereby one recess284has two or more associated inlet openings280.

In this and all other embodiments the minor burls240may be arranged alternately with the inlet openings280, rather like the outer burls300being arranged alternately with the extraction openings250in the embodiment ofFIG. 9described below. Additionally or alternatively the minor burls240may be positioned radially inwardly and/or radially outwardly of the inlet openings280, as well as in a line at the same radial distance from the center of the substrate holder200as the inlet openings280.

The embodiment ofFIG. 7is the same as the embodiment ofFIG. 5 or 6except as described below.

InFIG. 7a groove290is formed in the upper surface232of the first seal member230. The groove290does not have any openings formed in its bottom surface.

As can be seen in the lower half ofFIG. 7, the groove290may have a shape to form a labyrinth seal for the first passage of gas from radially outward of the first seal member230to radially inward of the first seal member230. That is, the groove290extends from a radially inward side of the first seal230to a radially outward side of the first seal member230following a tortuous path. As with all the other embodiments, the main burls220and/or minor burls240,270may be arranged alternately with (and/or radially inwards/outwards of) the inlet opening280and/or extraction openings250.

FIG. 8illustrates an embodiment which is the same as the embodiment ofFIG. 5except as described below.

In the embodiment ofFIG. 8instead of having the extraction openings250formed in the upper surface262of the second seal member260and the inlet openings280formed between the first seal member230and second seal member260, the extraction openings250are formed between the first seal member230and second seal member260and the inlet openings280are formed in the upper surface232of the first seal member230. Like with the embodiment ofFIG. 5this can be seen as a three seal member embodiment. A continuous groove or individual recesses may or may not be provided in the upper surface232of the first seal member230with the inlet openings280formed in the bottom surface of the groove or individual recesses like shown and described with reference toFIG. 5around the extraction openings250. Minor burls240may be provided on the upper surface232of the first seal member230on one or both radially inward and outward sides of the extraction openings280(or alternatingly). Like the minor burls270on the second seal member260of theFIG. 5embodiment, the minor burls240may or may not be radially aligned.

FIG. 9illustrates an embodiment which is the same as the embodiment ofFIG. 8except as described below.

In the embodiment ofFIG. 9, a meniscus pinning feature290is provided in the upper surface232of the first seal member230. The meniscus pinning feature290extends around the region of the main burls220. The meniscus pinning feature290is radially outward of the inlet openings280. The meniscus pinning feature290is radially inward of the extraction openings250.

The meniscus pinning feature290has a feature, for example a sharp edge292, which is effective to pin a meniscus of liquid in place. The meniscus pinning feature290applies a force to the meniscus meaning that extra energy is required for the meniscus to move past the meniscus pinning feature290. In this way, a further barrier to the radially inward movement of liquid is present.

As with the embodiment ofFIG. 8, the minor burls240may be in any position on the upper surface232of the first seal member230. Additionally or alternatively minor burls270may be present on the upper surface262of the second seal member260.

In the embodiment ofFIG. 9the extraction openings250are held at an underpressure which is greater than the underpressure applied in the region of the main burls220. In an embodiment an underpressure is also applied to the inlet openings280. The underpressure applied to inlet openings280has a magnitude which is between that of the underpressure applied in the region of the main buds220and that applied to the extraction openings250. In this way, a flow of gas is generated radially outwardly past the inlet openings280. This radially outward flow of gas presents another force on the meniscus of liquid between the substrate W and the substrate holder200.

Although the meniscus pinning feature290is illustrated as a groove with a sharp edge292, any feature which functions as an meniscus pinning feature290may be used. An alternative feature may be a change in contact angle of the upper surface262with the immersion liquid at the position of the meniscus pinning feature290.

An meniscus pinning feature290such as that illustrated inFIG. 9could be used in any of the other embodiments. The position of the meniscus pinning feature290is best located radially inwardly of the extraction openings250.

The embodiment ofFIG. 10is the same as the embodiment ofFIG. 3except as described below.

In the embodiment ofFIG. 10minor burls240,270are optional features. Instead or additionally, in order to support the edge of the substrate W radially further outward than the outer most main burl220, a plurality of outer burls300are provided radially outward of the first seal member230. The plurality of outer burls300project from the main body surface212. Each of the plurality of outer burls300has a distal end surface configured to support the substrate W. The plurality of outer burls230may be provided radially inward of the second seal member260, as illustrated. In one embodiment the plurality of outer burls300are arranged alternately with the extraction openings250in a line surrounding the first seal member230and the main burls220.

In this way the edge of the substrate W has a support. This can reduce deformation of the substrate W at its outer edge. In an embodiment the outer burls300may be positioned radially outward of the second seal member260. An outer burl300such as that illustrated inFIG. 10may optionally be provided in the embodiments as described with reference toFIGS. 3-9 and 11-13.

FIG. 11illustrates an embodiment which is the same as the embodiment ofFIG. 10except as described below.

The variation in the geometry of the first seal member230and/or second seal member260illustrated inFIG. 11and described below can be applied to the first seal member230and/or second seal member260of any of the embodiments.

In the embodiment ofFIG. 11, as can be seen in the lower portion of that Figure, the second seal member260follows a meandering path around the circumference of the substrate holder200. As also illustrated inFIG. 11, minor burls240are formed on the top surface232of the first seal member230. The minor burls240are positioned at apexes, for example where adjacent concave curved portions320meet. Thus, the plurality of minor burls240project from parts of the first seal member230which extend further from the center of the substrate holder200than other parts of the first seal member230. The first seal member230and the second seal member260may have any shape. What is desirable is that the minor burls240positioned on the first seal member230are positioned on a circumferential line which has an equal area of the groove between the first seal member230and second seal member260radially inwards of it as radially outward of it. Therefore the underpressure generated between the first seal member230and the second seal member260is equal on either side of the line of minor burls240, so that no great bending moment is introduced due to the large underpressure in the area between the first seal member230and second seal member260.

In one embodiment, the first seal member230is formed by a plurality of concave curved portions320joined together. In one embodiment, the second seal member260has an overall shape, in plan, which is defined by a plurality of curved portions310having a smaller radius than the overall shape, which are joined together thereby to form the overall shape. The second seal member260is formed of a plurality of convex curved portions310(relative to the radial direction from inward to outward of the substrate holder200).

Inlet openings280may also be provided, for example radially inward of the extraction openings250between the first seal member230and the second seal member260.

The embodiment ofFIG. 12incorporates features of the embodiment ofFIG. 6as well as the embodiment ofFIG. 11.

The embodiment ofFIG. 12has a single seal member230. The single seal member230is shown in plan in the middle ofFIG. 12and in cross-section on the left and right hand sides ofFIG. 12. The left hand cross-section is through line A-A illustrated in the plan view ofFIG. 12. The right had side ofFIG. 12illustrates the cross-section through line B-B in the plan view ofFIG. 12.

The seal member230includes a plurality of inlet openings280in the upper surface232. Extraction openings250are also formed in the upper surface232of the seal member230. The inlet openings280may be connected to an ambient pressure source or an underpressure which the has a lower magnitude than an underpressure to which extraction openings250are connected.

In the embodiment ofFIG. 12one or more first recesses510are formed in the upper surface232of the seal member230. Each of the first recesses510has at least one associated extraction opening250. However there may be more than one extraction opening250in the or each first recess510.

One or more second recesses520are formed in the upper surface232of the seal number230. Each of the second recesses520has one or more corresponding inlet opening280formed in it. However there may be more than one inlet opening280in the or each second recess520.

The first and second recesses510,520are shaped and positioned to result in a barrier550being formed between the first recesses510and the second recesses520. The barrier550is configured to restrict the passage of liquid between the substrate W and the main body surface212radially inwardly past the barrier550.

Due to the relatively higher pressure of gas in the inlet openings280, gas is sucked out of the inlet openings280over the barrier550towards the extraction openings250. This flow of gas is accelerated as it passes over the barrier550, thereby forming an effective gas seal between the barrier550and the underside of the substrate W. The gas flow advantageously has a tangential component from the inlet openings280towards the extraction openings250so as to direct radially incoming fluid towards the extraction openings250.

The relative positioning of the plurality of extraction openings250and inlet openings280and the size and shape of the barrier550result in the fluid flows illustrated by arrows in the plan view ofFIG. 12. That is, due to the underpressure applied to the extraction openings250, fluid is sucked from radially outside of the seal member230into the extraction openings250and focused by tangential movement towards the extraction openings250.

In an embodiment, first extensions560extend from the barrier550radially outwardly. Second extensions570extend from the barrier550radially inwardly. The first extension560and the second extension570are effective to define sidewalls of the first recess510and the second recess520, respectively. The first extension560bridges from the barrier550to a radially outer portion of the seal member230. The second extension570bridges from the barrier550to a radially inner portion of the seal member230.

The inlet openings280and extraction openings250alternate in the circumferential direction such that gas drawn out of an inlet opening280flows towards extraction openings250on either side of that inlet opening280. In this way a tangential flow of gas is achieved substantially around the entire circumference of the seal member230resulting in superior sealing properties.

Due to each extraction opening250being formed in a first recess510, and the extraction openings250being spaced closely next to each other around the circumference of the seal member230, the force experienced by the substrate W due to the underpressure applied to the extraction openings250is evened out in the circumferential direction resulting in lower deformation of the substrate W.

Due to the inlet openings280being positioned in the bottom of a corresponding second recess520, the gas flow out of the inlet openings280is spread out over a larger area of the barrier550than if the inlet openings280were not formed in a second recess520.

Optionally the inlet openings280are spaced radially outward of the extraction openings250. This helps in splitting of the radially incoming fluid flow into two streams towards adjacent extraction openings250. This splitting of the fluid flow towards one of two extraction openings250is also aided by the shape of the barrier550and also the (optional) presence of the first extensions560, explained further below.

So that the extraction openings250may be radially inward of the inlet openings280, the one or more first recesses510extend radially inward so that the innermost part of the first recesses510is closer to the center of the substrate holder200than the outermost part of the one or more second recesses520. The shape of the first recess510and the second recess520includes a tangential narrowing towards the center of the substrate W in the case of the first recesses510and away from the center of the substrate W in the case of second recesses520.

The barrier550can be seen as forming a side wall of the first recesses510on one side (radially outward) and a side wall of the second recess520on the other side (radially inward). The side wall is formed by sets of first portion552and second portion554of the barrier550.

The first portion552and the second portion554of a set converge on one another towards a respective first apex556as the first portion552and the second portion554extend in the radially inward direction. By positioning one of the plurality of extraction openings250adjacent the first apex556, the flow of fluid radially inwardly is directed by the first portion552and the second portion554as well as by the flow of gas from the inlet openings280towards the extraction openings250. This directing of the flow of fluid towards the extraction openings250results in better extraction efficiency of the extraction openings250. The flow of gas is also aided by the first extension510as this forms a tangential barrier to the flow of fluid.

First portion552and second portion554of adjacent sets also converge on one another towards a respective second apex558of the barrier550as the first portion552and the second554portion extend in the radially outward direction. Advantageously each of the second apexes558has an associated inlet opening280.

The first extensions560extend from the second apexes558. The second extensions570extend from the first apexes556.

Also helping with guiding of the fluid flow towards the extraction openings250is the overall shape, in plan, of the seal member230. That is, a radially outer most part of the seal member230varies in distance from the center of the substrate holder200. This results in the wavy pattern illustrated in the plan view ofFIG. 12. The extraction openings250are positioned circumferentially aligned with parts of the seal member230which extend further from the center of the substrate holder200than other parts of the seal member230. That is, an imaginary line (such as line A-A) passing through the center of the substrate holder200passes through an extraction opening250and through a radially outer most part of the seal member230. A similar arrangement is optionally made, as illustrated, on the inner surface of the seal member230with the inlet openings280being radially aligned with parts of the seal member230which extend closer to the substrate holder200than other parts of the seal member230.

Although not illustrated, the seal member230of the embodiment ofFIG. 12may comprise a plurality of minor burls240, such as discussed in connection with the other embodiments of the present invention. The minor burls240may be positioned radially inwardly and/or radially outwardly of the extraction openings250and/or inlet openings280or may be positioned substantially in line with the inlet openings280and/or extraction openings250.

The embodiment ofFIG. 13is the same as the embodiment ofFIG. 12except that only a single first recess510is formed. That is, the first extension560is missing. Alternatively or additionally the second extension570may be missing.

Embodiments are provided according to the following clauses:

1. A substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising:a main body having a main body surface;a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate;a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member; anda plurality of minor burls projecting from the upper surface of the first seal member, wherein each minor burl has a distal end surface configured to support the substrate.

2. The substrate holder of clause 1, further comprising a plurality of extraction openings formed in the main body for the extraction of fluid into the main body from between the main body and the substrate.

3. The substrate holder of clause 2, wherein the plurality of extraction openings are arranged radially outward of the first seal member, and/or wherein the substrate holder further comprising a second seal member projecting from the main body surface, the second seal member having an upper surface and surrounding the plurality of main burls and configured for restricting the passage of liquid between the substrate and the main body surface radially inward past the second seal member.

4. The substrate holder of clause 3, wherein the second seal member surrounds the first seal member, and preferably wherein the substrate holder further comprises a plurality of second minor burls projecting from an or the upper surface of the second seal member, and each second minor burl has a distal end surface configured to support the substrate, and preferably wherein the plurality of extraction openings are in the upper surface of the second seal member.

5. The substrate holder of clause 4, wherein each extraction opening is formed at a bottom surface of an extraction groove formed in the second seal member or wherein each extraction opening is formed at a bottom surface of a corresponding recess formed in the upper surface of the second seal member.

6. The substrate holder of clause 2, wherein the plurality of extraction openings are arranged radially inward of the first seal member.

7. The substrate holder of clause 2 or clause 6, further comprising a second seal member projecting from the main body surface, the second seal member surrounding the plurality of main burls and configured for restricting the passage of liquid between the substrate and the main body surface radially inward past the second seal member.

8. The substrate holder of clause 7, wherein the first seal member surrounds the second seal member, and/or wherein the second seal member has an upper surface and the substrate holder further comprises a plurality of second minor burls projecting from the upper surface of the second seal member, wherein each second minor burl has a distal end surface configured to support the substrate.

9. The substrate holder of any of clause 4, clause 6, clause 7 or clause 8, wherein the plurality of extraction openings are in the main body surface between the first seal member and the second seal member.

10. The substrate holder of any of clauses 2 to 9, further comprising a plurality of outer burls projecting from the main body surface, wherein each outer burl has a distal end surface configured to support the substrate, the outer burls being radially outward of the first seal member, and preferably wherein the outer burls and the extraction openings are arranged alternately in a line surrounding the first seal member and the plurality of main burls.

11. The substrate holder of any of clauses 2 to 10, further comprising a plurality of inlet openings formed in the main body for the drawing therethrough of gas to a space between the substrate and the main body, and preferably wherein the substrate holder further comprising a plurality of inlet openings formed in the main body for the drawing therethrough of gas to a space between the substrate and the main body.

12. The substrate holder of clause 11, wherein the plurality of inlet openings are formed in the main body surface radially outward of the first seal member, or wherein the plurality of inlet openings are in the upper surface of the first seal member, and preferably wherein the plurality of inlet openings are provided at the bottom of individual recesses formed in the upper surface of the first seal member.

13. The substrate holder of any of clauses 1 to 12, further comprising a groove formed in the upper surface of the first seal member, wherein the groove provides a tortuous path for gas from a radially outward side of the first seal member to a radially inward side of the first seal member, and/or wherein the first seal member has an overall shape, in plan, such that a radially outer most part of the first seal member varies in distance from the center of the substrate holder around its circumference and wherein the plurality of minor burls project from parts of the first seal member which extend further from the center of the substrate holder than other parts of the first seal member.

14. The substrate holder of any of clauses 1 to 13, further comprising a meniscus pinning feature on the upper surface of the first seal member.

15. A lithographic apparatus including a substrate holder of any of clauses 1-14.