Apparatus incorporating a gas lock

An apparatus, which may form part of a lithographic apparatus, comprises a substrate table, a projection system, a gas lock and a gas flow guide. The substrate table is suitable for supporting a substrate. The projection system has a body which defines an interior and an opening. The projection system is configured and arranged to project a radiation beam through the opening onto a substrate supported by the substrate table. The gas lock is suitable for providing a gas flow from the opening away from the interior. The gas flow guide is configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 18157048.2 which was filed on Feb. 16, 2018 and which is incorporated herein in its entirety by reference.

FIELD

The present invention relates to an apparatus incorporating a gas lock. In particular, the apparatus may form part of a lithographic apparatus comprising substrate table for supporting a substrate (e.g. a silicon wafer) and a projection system arranged to project a patterned radiation beam onto the substrate. The gas lock may be arranged to at least partially protect the interior of the projection system from ingress of contaminants.

BACKGROUND

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 can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 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.

Since EUV radiation is strongly absorbed by matter, within an lithographic apparatus using EUV radiation the optical path of the EUV radiation is under vacuum conditions (i.e. at pressures significantly below atmospheric pressure). In particular, the projection system, which comprises a system of optical elements for projecting the EUV radiation onto a substrate, may be held under vacuum conditions. The EUV radiation is projected through an opening defined by the projection system onto the substrate. It is desirable to limit ingress of contaminants into the projection system. It is known to provide a dynamic gas lock for this purpose.

It may be desirable to provide alternative arrangements of gas locks, and apparatus comprising such gas locks, that at least partially address one or more problems associated with known arrangements, whether identified herein or otherwise.

SUMMARY

According to a first aspect of the invention there is provided an apparatus comprising: a substrate table for supporting a substrate; a projection system having a body, the body defining an interior and an opening, the projection system being configured and arranged to project a radiation beam through the opening onto a substrate supported by the substrate table; a gas lock for providing a gas flow from the opening away from the interior; and a gas flow guide configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table.

The apparatus according to the first aspect of the invention may form part of a lithographic apparatus. For example, in use the substrate table may support a silicon wafer and the projection system may be arranged to project a patterned radiation beam onto the wafer (for example to form a diffraction limited image of a reticle). In use a plurality of distinct target regions (which may correspond to one or more dies) may be sequentially exposed by moving the substrate table relative to the projection system between exposures. In addition, each such exposure may be a scanning exposure during which the substrate table moves in a scanning direction relative to the opening.

The apparatus according to the first aspect of the invention is advantageous since it provides a gas lock that can protect the interior of the projection system from the ingress of contaminants. For example, it can prevent such contaminants impinging on optical elements (for example mirrors or lenses). In turn, this can improve the optical performance of the projection system.

Furthermore, the gas flow guide is configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table. This is in contrast to conventional gas locks provided between projections systems and substrate tables wherein a primary gas flow from the opening of the projection system passes is directed towards the substrate table. This distinction over conventional gas locks has a number of advantages, as now discussed.

With an existing arrangement where the gas flow from the opening passes over the substrate and substrate table, the gas flow will deliver a heat load to the substrate. In turn, this can cause thermal deformation of the substrate, which can be detrimental to the quality of the formed image.

It will be appreciated that, in general, in addition to the portion of gas provided by the gas lock which forms the gas flow from the opening away from the interior, another portion of the gas which is provided by the gas lock will flow into the interior of the projection system. The inventors have realized that in conventional arrangements (wherein a primary gas flow from the opening of the projection system is directed towards the substrate table) the proportion of the total gas provided by the gas lock which forms the gas flow from the opening away from the interior is dependent on the position of the substrate table relative to the opening. This is because in different positions the flow path away from the opening of the housing will, in general, have a different fluid conductance as a different restriction of the gas is provided at the opening of the projection system. The inventors have further realized that with such an arrangement during use, as the substrate table moves relative to the opening, and when the rate of gas production is constant, this will result in variations in the pressure both within the interior of the projection system and outside the projection system, for example in a volume or space within which the substrate table (and any substrate supported thereby) is disposed. Such a volume or space may be referred to as a substrate table compartment or, alternatively, a wafer stage compartment. It will be appreciated that, unless stated to the contrary, as used herein “proximate” the substrate table is intended to mean within in a volume or space within which the substrate table (and any substrate supported thereby) is disposed. The use of the term “proximate” should not be interpreted as implying that any limitation on the dimensions of the volume or space within which the substrate table (and any substrate supported thereby) is disposed. These pressure variations are undesirable, as now explained.

First, these variations in the pressure can result in time varying thermal distortions of the substrate caused by the heat load provided by the gas flow.

Second, it will be appreciated that it may be important to accurately determine and control the position of the substrate table relative to the opening of the projection system. Within a typical lithographic apparatus, such determinations of the position of the substrate table relative to the opening of the projection system are typically made using interferometric devices. Such interferometric devices typically comprise components mounted on the substrate table and mounted on a reference object relative to which it is desirable to determine the position of the substrate table (for example an isolated frame to which the projection system is connected). For example, such interferometric devices may comprise a light source mounted on an isolated frame to which the projection system is connected and a mirror mounted on the substrate table arranged to reflect light from the light source. However, if the pressure surrounding the substrate table varies with time then this will result in an error in the determined position. In particular, the light may travel from the light source to the mirror over any desired distance within the volume or space within which the substrate table (and any substrate supported thereby) is disposed and, therefore, any pressure variations within this volume or space may result in an error in the determined position. In turn, this can contribute, for example, to overlay errors in the formed image.

In contrast to such known arrangements, the gas flow guide of the apparatus of the first aspect of the invention is configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table. Advantageously, this avoids the above-described problems.

It will be appreciated that providing a gas flow “from the opening away from the interior” is intended to mean providing a gas flow generally from the vicinity of the opening away from the interior. For example, the gas flow may be provided within the interior of the body of the projection system (for example proximate the opening), in which case the gas flow from the opening away from the interior flows through the opening. Alternatively, the gas flow may be provided outside of the interior of the body of the projection system but proximate to the opening. Additionally or alternatively, the opening may be considered to have a non-zero dimension in a direction generally perpendicular to the opening (i.e. in a direction which is generally aligned with a propagation direction of the radiation beam projected through the opening). For example, the opening may be defined by a wall with a non-zero thickness. For embodiments wherein the opening is considered to have a non-zero dimension in a direction generally perpendicular to the opening the gas flow may be provided at the opening.

The gas lock may be arranged to provide the gas flow through the opening and away from the interior.

The gas flow guide may be configured to guide the at least a portion of the gas flow along a fluid pathway of substantially fixed fluid conductance.

It will be appreciated that the gas flow guide may be configured to guide the at least a portion of the gas flow along a fluid pathway of substantially fixed fluid conductance when the apparatus is disposed in a first, operating configuration.

It will be appreciated that as used in this context the fluid conductance of a fluid pathway is a measure of how easily gas flows along the fluid pathway. For example, the fluid conductance may be proportional to a ratio of a total throughput of gas in the fluid pathway to a pressure differential across the fluid pathway.

Advantageously, by guiding the at least a portion of the gas flow along a fluid pathway of substantially fixed fluid conductance, when the rate of gas production by the fluid lock is constant there will be substantially no variation in the pressure within the interior of the projection system or proximate the substrate table.

The gas flow guide may be configured such that more than half of the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table.

It will be appreciated that the gas flow guide may be configured such that at more than half of the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table when the apparatus is disposed in a first, operating configuration.

That is to say, the gas flow guide may be considered to direct gas along a primary pathway, which receives more than half of the gas flow from the opening away from the interior.

Ensuring that more than half of the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table may be achieved by ensuring that the fluid conductance of a fluid pathway along which the at least a portion of the gas flow is directed by the gas guide is sufficiently high.

In some embodiments, the gas flow guide may be configured such that at least 80% the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table. In some embodiments, the gas flow guide may be configured such that at least 90% the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table.

The gas flow guide may be configured to guide the at least a portion of the gas flow along a fluid pathway having a greater fluid conductance than other available fluid pathways.

It will be appreciated that the gas flow guide may be configured to guide the at least a portion of the gas flow along a fluid pathway having a greater fluid conductance than other available fluid pathways when the apparatus is disposed in a first, operating configuration.

With such an arrangement gas will preferentially flow along said fluid pathway.

Additionally or alternatively, ensuring that more than half of the gas flow from the opening away from the interior is directed away from the substrate supported by the substrate table may be achieved by ensuring that a pressure difference across a fluid pathway along which the at least a portion of the gas flow is directed by the gas guide is sufficiently high.

The gas flow guide may comprise a pump arranged to draw the at least a portion of the gas flow along a fluid pathway.

The pump may, for example, comprise a vacuum pump.

The apparatus may further comprise a movement mechanism for moving the substrate table relative to the opening of the projection system.

The movement mechanism may be used to move a substrate relative to the opening between exposures of two different target regions (for example to place the next target region to be exposed in a position for receiving the radiation beam. In addition, the movement mechanism may be used during a scanning exposure of one or more target regions.

The apparatus may further comprise an intermediate member arranged between the opening and the substrate table and the gas flow guide may comprise a pathway defined between the intermediate member and the projection system.

It will be appreciated that the intermediate member may comprise an opening that is generally aligned with the opening of the projection system for transmission of the radiation beam. This allows the radiation that is projected through the opening of the projection system to irradiate a substrate supported by the substrate table.

In use, the intermediate member may be disposed in a first, operating position wherein it is in close proximity to a region of the substrate surrounding the region of the substrate which is being irradiated by the radiation beam. With such an arrangement, the intermediate member can act as a seal, or at least a restriction, so as to at least partially block the gas flow from flowing towards the region of the substrate surrounding the region of the substrate which is being irradiated by the radiation beam. It will be appreciated that, as used here the intermediate member being in close proximity to a region of the substrate surrounding the region of the substrate which is being irradiated by the radiation beam is intended to mean sufficiently close that the fluid conductance through the clearance between the intermediate member and the substrate is significantly smaller than the fluid conductance through the pathway defined between the intermediate member and the projection system.

In addition, the intermediate member may be configured such that gas flowing along the pathway defined between the intermediate member and the projection system is generally directed away from the substrate table.

The intermediate member may be configured to direct gas flowing along the pathway defined between the intermediate member and the projection system into a cavity formed within a wall of the body of the projection system.

It will be appreciated that such a cavity within the wall of the body of the projection system may be isolated from the interior of the body of the projection system.

The wall of the body may be considered to comprise and inner wall, which defines the interior, and an outer wall. The cavity may be defined by the inner and outer walls.

The pathway defined between the intermediate member and the projection system may be defined between the intermediate member and the inner wall of the body of the projection system. In order to direct gas flowing along the pathway defined between the intermediate member and the projection system into a cavity formed within a wall of the body of the projection system, the intermediate member may comprise a sealing portion which extends towards, and is in sealing relationship with, the outer wall of the body of the projection system.

The intermediate member may comprise a sealing element which extends towards, and is in sealing relationship with, the wall of the body of the projection system around substantially the entire perimeter of the opening of the projection system.

It will be appreciated that, as used here the sealing element being in sealing relationship with the wall of the body of the projection system around substantially the entire perimeter of the opening of the projection system is intended to mean that the fluid conductance through any clearance between the sealing element and the wall of the body of the projection system is significantly smaller than the fluid conductance through an alternative exhaust pathway (for example into a cavity formed within a wall of the body of the projection system).

The sealing element may comprise a rigid flange portion and a clearance may be provided between the rigid flange portion and a portion of the wall of the body of the projection system which it is in sealing relationship with.

The clearance may allow the sealing element to be in sealing relationship with the wall of the body of the projection system whilst allowing the intermediate member to move relative to the wall.

The apparatus may further comprise a flexible membrane connected between the intermediate member and the wall of the body of the projection system.

The flexible membrane may allow the intermediate member to be in sealing relationship with the wall of the body of the projection system whilst allowing the intermediate member to move relative to the wall.

The intermediate member may be movable relative to the projection system between at least a first, operating position and a second, retracted position.

The position of the intermediate member may define a configuration of the apparatus. When the intermediate member is disposed in the first, operating position the apparatus may be considered to be in a first, operating configuration. When the intermediate member is disposed in the second, retracted position the apparatus may be considered to be in a second, retracted configuration.

In use, when a substrate is being irradiated by the radiation beam the intermediate member may be disposed in the first, operating position. As discussed above, when disposed in the first, operating position the intermediate member may be in close proximity to a region of the substrate surrounding the region of the substrate which is being irradiated by the radiation beam. However, it may be desirable to allow the intermediate member to be movable relative to the projection system to a second, retracted position, for example to prevent the intermediate member from contacting a substrate, for example, if the substrate table moves rapidly and/or unpredictably towards the intermediate member. Movement of the intermediate member, for example by of the order of ±4 mm may be desirable in case an out-of-control situation occurs. For instance, in case a substrate table positioning module makes an unplanned fast upward movement, there will be significant damage to the substrate table and intermediate member unless the intermediate member can move sufficiently to avoid contact.

It may also be desirable to be able to adjust the clearance between the intermediate member and a substrate supported by the substrate table.

The intermediate member may be operable to cool a region of the substrate surrounding the region of the substrate which is irradiated by the radiation beam.

The intermediate member may comprise a cooling member which may be maintained at a lower temperature than the substrate table (and any substrate supported thereby). For example, the cooling member may be maintained at a temperature of around −70° C. The substrate table may be maintained at a suitable temperature to ensure stability of substrates supported thereby such as, for example, around 22° C. (which is, for example, suitable for silicon wafers). It will be appreciated that the cooling member may be provided with a suitable refrigeration system (for example a closed loop around which coolant is circulated) so as to maintain it at a desired temperature.

The intermediate member may be operable to direct a cooling gas flow to a region of the substrate surrounding the region of the substrate which is irradiated by the radiation beam.

The intermediate member may comprise a cooling member which is maintained at a lower temperature than the substrate table any may further comprise a heat shield which is arranged to thermally insulate the substrate table and/or the projection system from the cooling member.

According to a second aspect of the invention there is provided a lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; and the apparatus according to the first aspect of the invention, wherein the projection system is configured receive the patterned radiation beam and to project it onto a substrate supported by the substrate table.

According to a third aspect of the invention there is provided a method of operating an apparatus comprising a projection system having a body, the body defining an interior and an opening, the method comprising the steps of: arranging a substrate on a substrate table; projecting a radiation beam through the opening of the projection system onto the substrate supported by the substrate table; providing a gas flow from the opening away from the interior; and guiding at least a portion of the gas flow away from the substrate supported by the substrate table.

The method according to the third aspect of the invention is advantageous since a gas flow is provided from the opening away from the interior, which can protect the interior of the projection system from the ingress of contaminants.

Furthermore, the method involves guiding at least a portion of the gas flow away from the substrate supported by the substrate table. As with the apparatus of the first aspect of the invention, the method according to the third aspect of the invention is advantageous for a number of reasons. First, it prevents the gas flow from delivering a heat load to the substrate, which could cause thermal deformation of the substrate, to the detriment of the quality of the formed image. Second, for the reasons set out above in relation to the first aspect of the invention, it can prevent variations in the pressure both within the interior of the projection system and proximate the substrate table (and any substrate supported thereby). Such pressure variations are undesirable, since they can result in time varying thermal distortions of the substrate (caused by the heat load provided by the gas flow) and can result in errors in position measurements of the substrate table, which can contribute, for example, to overlay errors in the formed image.

The apparatus may, for example, comprise an apparatus according to the first aspect of the invention. It will be appreciated that the method according to the third aspect of the invention may comprise any of the features, of equivalent features, to those of the apparatus of the first aspect of the invention as appropriate. In particular, the apparatus may comprise a gas flow guide configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table.

In the method according to the third aspect of the invention the apparatus may comprise the apparatus according to the first aspect of the invention.

The method may further comprise the step of drawing the gas flow from the opening away from the interior and away from the substrate supported by the substrate table.

The gas flow may be drawn from the opening away from the interior and away from the substrate supported by the substrate table using a pump. The pump may, for example, comprise a vacuum pump.

DETAILED DESCRIPTION

FIG. 1shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device10and a facetted pupil mirror device11. The faceted field mirror device10and faceted pupil mirror device11together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device10and faceted pupil mirror device11.

After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B′ is generated. The projection system PS is configured to project the patterned EUV radiation beam B′ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors13,14which are configured to project the patterned EUV radiation beam B′ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B′, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors13,14inFIG. 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B′, with a pattern previously formed on the substrate W.

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

The projection system PS is connected to an isolated frame. For example the isolated frame (which may be referred to as a metro frame) may be supported by a base frame, which may be supported on the ground, such that it is substantially isolated from external influences (such as vibrations in the base frame). This isolation may be achieved by the use of acoustically damping mounts, which support the isolated frame on the base frame BF. The acoustically damping mounts may be actively controlled to isolate vibrations which are introduced by the base frame and/or by the isolated frame itself.

The support structure MT is movably mounted to the isolated frame via a first positioning device. The first positioning device may be used to move the patterning device MA, and to accurately position it, relative to the isolated frame (and the projection system PS which is connected to the frame). The substrate table WT is movably mounted to the isolated frame via a second positioning device. The second positioning device may be used to move the substrate W, and to accurately position it, relative to the isolated frame (and the projection system PS which is connected thereto).

The projection system PS has a body or housing, which defines an interior and an opening. The two mirrors13,14are disposed within the interior and the projection system PS is configured and arranged to project the patterned EUV radiation beam B′ through the opening onto a substrate W supported by the substrate table WT.

The projection system PS is configured to project the patterned EUV radiation beam B′ onto the substrate W. In particular, the projection system PS is configured to project the patterned EUV radiation beam B′ onto a generally rectangular exposure region in an image plane of projection system PS such that, in this image plane (which generally coincides with a surface of the substrate W) the radiation beam B′ is a generally rectangular band of radiation. Therefore, the opening may be generally rectangular. The generally rectangular exposure region in the image plane of projection system PS or the generally rectangular band of radiation may be referred to as a slit or an exposure slit.

In use a plurality of distinct target regions (which may correspond to one or more dies) of a substrate W may be sequentially exposed by moving the substrate table WT relative to the projection system PS between exposures (so as to change which of the target regions is disposed in the exposure region of the image plane). In addition, each such exposure may be a scanning exposure during which the substrate table WT moves in a scanning direction relative to the projection system PS such that the target region being exposed moves, or scans through the exposure region in a scanning direction. To achieve this functionality, the lithographic apparatus LA is provided with actuators for moving the substrate table WT relative to the projection system PS. These actuators may be considered to form a movement mechanism.

Each point in the exposure region of the image plane will, in general, receive a cone of radiation (i.e. will receive radiation beams with a range of angles of incidence). Therefore, proximate the opening, the interior of the body may taper inwards towards the opening (in both directions perpendicular to an optical axis of the patterned EUV radiation beam B′). This tapered portion of the housing proximate the opening may be described as generally funnel shaped and may be referred to as a funnel portion.

An embodiment of the present invention is now described with reference to schematicFIG. 2.

FIG. 2shows an apparatus20comprising a substrate table WT for supporting a substrate W and a projection system (only part of the projection system is shown). The apparatus20may form part of the lithographic apparatus LA shown schematically inFIG. 1.FIG. 2shows a portion of a body or housing of the projection system, which defines an interior21and an opening22(only a tapered portion of the interior21proximate the opening22is shown).

The apparatus20further comprises a gas lock arranged to provide a gas flow through the opening away from the interior of the projection system PS. This gas flow is indicated schematically by arrows23, indicating that the gas is directed through apertures in an interior wall of the body or housing of the projection system and into interior21. The gas flow23may comprise any suitable gas such as, for example, hydrogen or nitrogen. The apertures are arranged to direct the gas flow23generally towards opening22. By doing so, the gas lock protects the interior of the projection system PS from the ingress of contaminants. For example, it can prevent such contaminants impinging on the mirrors13,14which, in turn, improves the optical performance of the projection system PS.

Embodiments of the present invention further comprise a gas flow guide that is configured to guide at least a portion of the gas flow (which is directed through the opening by the gas lock) away from the substrate W supported by the substrate table WT, as described further with reference toFIGS. 2 to 6.

In some embodiments, the apparatus further comprises an intermediate member24arranged between the opening22and the substrate table WT. The intermediate member24forms part of the gas flow guide, which is arranged to guide at least a portion of the gas flow23which passes though the opening22along a pathway defined between the intermediate member24and the projection system PS.

The intermediate member24comprises an opening25that is generally aligned with the opening22of the projection system for transmission of the radiation beam. This allows the radiation that is projected through the opening22of the projection system PS to irradiate a substrate W supported by the substrate table WT.

Furthermore, the apparatus20further comprises a barrier26that defines a surface27that is generally parallel to the substrate table WT (and any substrate W supported thereby). The barrier26surrounds and extends away from a portion of the projection system proximate the opening22. The barrier26may be considered to form part of, or an extension of, the body or housing of the projection system PS.

It will be appreciated that the barrier26may be formed from a plurality of different components.

For example, the barrier26may comprise a substrate table heat shield (not shown), which may be arranged to at least partially thermally isolate the substrate table WT (and any substrate W supported thereby), which it may be desired to maintain at a particular temperature, from other parts of the apparatus (for example other parts of lithographic apparatus LA). Such a substrate table heat shield may comprise a water-cooled plate.

Additionally or alternatively, the barrier26may comprise or incorporate one or more mirrors (not shown) that may be used as part of a measurement system that is arranged to measure the position of the substrate table WT relative to the barrier26and/or the projection system PS (or, equivalently, relative to the isolated frame to which the barrier26and projection system PS are attached). For example, the barrier26may comprise one or more mirrors that are used for reflecting interferometer beams that are used as part of an interferometric positioning measurement system. Such a measurement system may be arranged to determine a position of the substrate table WT relative to the barrier26and/or the projection system PS (or, equivalently, relative to the isolated frame to which the barrier26and projection system PS are attached) in a direction generally perpendicular to a plane of the substrate table WT (which may be referred to as a z-direction).

Additionally or alternatively, the barrier26may comprise an access hatch or door, which can be moved, or removed, to provide access to the interior21of the projection system PS.

Additionally or alternatively, the barrier26may comprise components arranged to support the intermediate member24. For example such components which support the intermediate member24may be connected to the projection system PS or to the isolated frame to which the projection system PS is attached. As will be described further below, with reference toFIG. 6, the intermediate member24may be mounted to the projection system PS, or to the isolated frame to which it is attached, such that the intermediate member24is movable relative to the projection system PS (and substrate table WT). Therefore, it will be appreciated that components which support the intermediate member24and which may form part of the barrier26may include actuators that facilitate this movement.

As shown schematically inFIG. 2, the apparatus20further comprises sealing element28which extends between the intermediate member24and the barrier26. The sealing element28is in sealing relationship with both the intermediate member24and the barrier26around substantially the entire perimeter of the opening22of the projection system PS.

The intermediate member24and the sealing element28may be considered to form a gas flow guide. The intermediate member24and the sealing element28define a pathway along which at least a portion of the gas flow23(which is directed through the opening22by the gas lock) is directed. This portion of the gas flow23is indicated by arrow30and is directed away from the substrate W supported by the substrate table WT. As shown rather schematically inFIG. 2, a passageway32is provided in the barrier26and/or the body or housing of the projection system PS. The portion30of the gas flow23is directed by the intermediate member24and the sealing element28through passageway32and away from the substrate table WT (and any substrate W supported thereby).

Since the barrier26may be considered to form part of, or an extension of, the body or housing of the projection system PS, the apparatus shown inFIG. 2may be considered to be configured to direct gas flowing along a pathway defined between the intermediate member24and the projection system PS into a cavity formed within a wall of the body of the projection system PS. It will be appreciated that such a cavity within the wall of the body of the projection system may be isolated from the interior21of the body of the projection system PS.

The wall of the body may be considered to comprise and inner wall, which defines the interior21, and an outer wall. The cavity may be defined by the inner and outer walls.

The pathway defined between the intermediate member24and the projection system PS may be defined between the intermediate member24and the inner wall of the body of the projection system. The sealing member28is provided so as to direct gas flowing along this pathway defined between the intermediate member24and the projection system PS into a cavity formed within a wall of the body of the projection system PS.

The apparatus20shown inFIG. 2, is arranged to direct a portion30of the gas flow23away from the substrate W supported by the substrate table WT. This is in contrast to conventional gas locks provided between projections systems and substrate tables wherein a primary gas flow from the opening of the projection system passes is directed towards the substrate table. This distinction over conventional gas locks has a number of advantages, as now discussed, with reference toFIGS. 3 and 4A-4C.

FIG. 3shows a prior art apparatus40comprising a substrate table WT for supporting a substrate W and a projection system (only part of the projection system is shown). The prior art apparatus40is generally of the form of the apparatus20shown inFIG. 2although it does not comprise a gas flow guide that is configured to guide a portion of the gas flow away from the substrate W supported by the substrate table WT. It will be appreciated that components of prior art apparatus40that are generally equivalent to corresponding components of the apparatus20inFIG. 2share common reference numerals therewith.

With the prior art arrangement, as shown inFIG. 3, the gas flow34from the opening22passes over the substrate W and substrate table WT. The gas flows generally between barrier26and substrate W. With such an arrangement, the gas flow will deliver a heat load to the substrate W. In turn, this can cause thermal deformation of the substrate W, which can be detrimental to the quality of the image formed by the lithographic apparatus LA.

It will be appreciated that, in general, in addition to the portion of the gas23which forms the gas flow through the opening22away from the interior21, a portion of the gas23which is provided by the gas lock will flow into the interior21of the projection system. The inventors have realized that in conventional arrangements, as shown inFIG. 3, (wherein a primary gas flow from the opening22of the projection system PS is directed towards the substrate table WT) the proportion of the total gas23provided by the gas lock which forms the gas flow through the opening22away from the interior21is dependent on the position of the substrate table WT relative to the opening22. This is because, when the substrate table WT is disposed in different positions the flow path away from the opening22of the housing21will, in general, have a different fluid conductance as a different restriction of the gas is provided at the opening22of the projection system PS.

It will be appreciated that as used herein the fluid conductance of a fluid pathway is a measure of how easily gas flows along the fluid pathway. For example, the fluid conductance may be proportional to a ratio of a total throughput of gas in the fluid pathway to a pressure differential across the fluid pathway.

This can be seen inFIGS. 4A-4C, which schematically shows barrier26and the opening22of the projection system PS disposed above substrate table WT and substrate W, the substrate table WT and substrate W being disposed in three different positions relative to the opening22of the projection system PS.

InFIG. 4A, the substrate table WT is disposed such that the opening22of the housing generally overlies a central portion of the substrate W. In this position, the substrate W and barrier26form a restriction for gas flowing out of the opening22generally in all directions away from the exposure region. InFIG. 4B, the substrate table WT is disposed such that the opening22of the housing generally overlies a peripheral portion of the substrate table WT. In this position, the substrate WT and barrier26form a restriction for gas flowing out of the opening22. On one side of the opening22(the right hand side inFIG. 4B) the restriction is similar to the restriction provided (in all directions) when the substrate table WT is disposed in the position shown inFIG. 4A. However, on the other side of the opening22(the left hand side in FIG. FB) the substrate table WT provides substantially no restriction to gas flow such that this forms a pathway of increased fluid conductance. InFIG. 4C, the substrate table WT is disposed such that the opening22is generally clear of (i.e. not disposed adjacent to) the substrate table WT. In this position, the substrate WT provides substantially no restriction to gas flow such that a pathway of increased fluid conductance is formed.

The inventors have further realized that with such a prior art arrangement (as described above with reference toFIGS. 3 to 4C), during use, as the substrate table WT moves relative to the opening22, and when the rate of gas production23is constant, this will result in variations in the pressure both within the interior21of the projection system and outside the projection system, for example in a volume or space within which the substrate table WT (and any substrate W supported thereby) is disposed. It will be appreciated that, unless stated to the contrary, as used herein “proximate” the substrate table is intended to mean within a volume or space within which the substrate table (and any substrate supported thereby) is disposed. These pressure variations are undesirable, as now explained.

First, these variations in the pressure can result in time varying thermal distortions of the substrate W caused by the heat load provided by the gas flow.

Second, it will be appreciated that it may be important to accurately determine and control the position of the substrate table WT relative to the opening22of the projection system PS. Within a typical lithographic apparatus LA, such determinations of the position of the substrate table WT relative to the opening of the projection system PS are typically made using interferometric devices. Such interferometric devices typically comprise components mounted on the substrate table WT and mounted on a reference object relative to which it is desirable to determine the position of the substrate table WT (for example an isolated frame to which the projection system PS is connected). For example, such interferometric devices may comprise a light source mounted on an isolated frame to which the projection system is connected and a mirror mounted on the substrate table arranged to reflect light from the light source. Such interferometric devices may be referred to as position sensors. Two such interferometric devices35are shown schematically inFIGS. 2, 3 and 4A-4Cmounted on the substrate table WT. It will be appreciated that these interferometric devices35will also comprise other components (not shown) mounted on a reference object, for example an isolated frame to which the projection system PS is connected. These interferometric devices35can be used so as to accurately control the position of the substrate table WT relative to the isolated frame to which the projection system PS is attached, for example so as to position different target portions of the substrate W in the path of the patterned radiation beam B′.

However, if the pressure surrounding the substrate table WT varies with time then this will result in an error in the position determined by such interferometric devices35. In particular, light may travel from a light source (for example on an isolated frame to which the projection system PS is connected) to part of the interferometric device35on substrate table WT over any desired distance within the volume or space within which the substrate table WT (and any substrate W supported thereby) is disposed and, therefore, any pressure variations within this volume or space may result in an error in the determined position. In turn, this can contribute, for example, to overlay errors in the formed image.

In contrast to such known arrangements, the gas flow guide (formed by the intermediate member24and the sealing element28) of the apparatus22is configured to guide at least a portion of the gas flow away from the substrate supported by the substrate table. Advantageously, this avoids the above-described problems.

Further details of the intermediate member24are now discussed with reference toFIG. 5, which shows some parts of the apparatus20shown inFIG. 2in greater detail. For clarity, the sealing member28is not shown inFIG. 5(but will be discussed below with reference toFIG. 6).

The gas flow23discussed above with reference toFIGS. 2 and 3is provided from channels33in a wall of the projection system PS, via apertures in an interior wall of the body or housing of the projection system, and into interior21.

In this embodiment, the intermediate member24is operable to cool a region of the substrate W surrounding the region37of the substrate W which is irradiated by the radiation beam (which may be referred to as an exposure region37). To achieve this, the intermediate member24may comprise a cooling apparatus. A suitable cooling apparatus for cooling the substrate W is disclosed in WO2017/005463 which is hereby incorporated by reference.

As shown inFIG. 5, the intermediate member24comprises a cooling member36which may be maintained at a lower temperature than the substrate table WT (and any substrate supported thereby). For example, the cooling member36may be maintained at a temperature of around −70° C. The substrate table WT may be maintained at a suitable temperature to ensure stability of substrates W supported thereby such as, for example, around 22° C. (which is, for example, suitable for silicon wafers). The substrate table WT may comprise a clamp38(for example an electrostatic clamp) which is suitable for clamping a substrate W to the substrate table WT. The clamp38and other parts of the substrate table WT may be provided with a suitable conditioning system arranged to maintain the temperature thereof (and the temperature of any substrate W supported thereby) at a desired temperature (for example 22° C.).

It will be appreciated that the cooling member36may be provided with a suitable refrigeration system (for example a closed loop around which coolant is circulated) so as to maintain it at a desired temperature. The cooling member may be formed from a metal and is provided with channels40through which a suitable coolant is circulated. Suitable coolants may comprise, for example, nitrogen.

In some embodiments, the intermediate member24may be operable to direct a cooling gas flow to a region of the substrate W surrounding the exposure region37of the substrate W which is irradiated by the radiation beam B′.

The cooling member36of the intermediate member24is maintained at a lower temperature than the substrate table WT. The intermediate member24further comprises a heat shield43which is arranged to thermally insulate the substrate table WT (and any substrate W supported thereby) and the projection system PS and barrier26from the cooling member36.

The intermediate member24is movable relative to the projection system PS and the substrate table WT, as shown schematically by arrow42. In particular, the intermediate member24is movable in a direction that is generally perpendicular to a plane of the substrate table WT. The intermediate member24is moveable between at least a first, operating position and a second, retracted position, as now described.

The position of the intermediate member24may define a configuration of the apparatus20. When the intermediate member24is disposed in the first, operating position the apparatus20may be considered to be in a first, operating configuration. When the intermediate member24is disposed in the second, retracted position the apparatus20may be considered to be in a second, retracted configuration.

In use, when a substrate W is being irradiated by the radiation beam B′ the intermediate member24may be disposed in the first, operating position. When disposed in the first, operating position the intermediate member24may be in close proximity to a region of the substrate W surrounding the exposure region37of the substrate W which is being irradiated by the radiation beam B′. With such an arrangement, the intermediate member24can act as a seal, or at least a restriction, so as to at least partially block the gas flow from flowing towards the region of the substrate surrounding the exposure region37of the substrate W which is being irradiated by the radiation beam B′. It will be appreciated that, as used here the intermediate member24being in close proximity to a region of the substrate surrounding the exposure region37of the substrate W which is being irradiated by the radiation beam B′ is intended to mean sufficiently close that the fluid conductance through the clearance44between the intermediate member24and the substrate W is significantly smaller than the fluid conductance through the pathway defined between the intermediate member24and the projection system PS. That is, the fluid conductance of the pathway46that flows through the cooling member36and then between the intermediate member24and the substrate W is significantly smaller than the fluid conductance through the pathway48defined between the intermediate member24and the projection system PS.

As will be discussed further with reference toFIG. 6, the intermediate member24is configured such that gas flowing along the pathway48defined between the intermediate member24and the projection system PS is generally directed away from the substrate table WT.

Although in use, when a substrate W is being irradiated by the radiation beam B′, the intermediate member24is disposed in the first, operating position (in close proximity to the substrate W), it may be desirable to allow the intermediate member to be movable relative to the projection system PS and substrate table WT (as indicated by arrow42) to a second, retracted position, for example to prevent the intermediate member24from contacting a substrate W. For example, if the substrate table WT moves rapidly and/or unpredictably towards the intermediate member24it may be desirable to move the intermediate member24so as to avoid contact between the substrate W and the intermediate member24. Movement of the intermediate member24, for example by of the order of ±4 mm may be desirable in case an out-of-control situation occurs. For instance, in case a substrate table WT positioning module makes an unplanned fast upward movement, there will be significant damage to the substrate table WT and intermediate member24unless the intermediate member can move sufficiently to avoid contact.

Additionally or alternatively, It may be desirable to be able to adjust the clearance44between the intermediate member24and a substrate W supported by the substrate table WT.

In some embodiments, the gas flow guide (which may be formed by the intermediate member24and the sealing member28is configured to guide at least a portion of the gas flow through the opening22along a fluid pathway of substantially fixed fluid conductance. For example, the gas flow guide formed by the intermediate member24and the sealing element28guides a portion30of the gas23along a pathway (including passageway32) of substantially fixed fluid conductance. It will be appreciated that the gas flow guide may be configured to guide the at least a portion of the gas flow along a fluid pathway of substantially fixed fluid conductance when the apparatus20is disposed in a first, operating configuration (for example, when intermediate member24is disposed in a first, operating position).

Advantageously, by guiding at least a portion of the gas flow through opening22along a fluid pathway of substantially fixed fluid conductance, when the rate of gas23production by the fluid lock is constant there will be substantially no variation in the pressure within the interior21of the projection system PS or proximate the substrate table WT.

In some embodiments, the gas flow guide may be configured such that more than half of the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT. It will be appreciated that the gas flow guide may be configured such that more than half of the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT when the apparatus20is disposed in a first, operating configuration (for example, when intermediate member24is disposed in a first, operating position).

That is to say, the gas flow guide may be considered to direct gas along a primary pathway, which receives more than half of the gas flow through the opening22away from the interior21. Ensuring that more than half of the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT may be achieved by ensuring that the fluid conductance of a fluid pathway along which the at least a portion of the gas flow is directed by the gas guide is sufficiently high.

In some embodiments, the gas flow guide may be configured such that at least 80% the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT. In some embodiments, the gas flow guide may be configured such that at least 90% the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT.

In some embodiments, the gas flow guide is configured to guide a portion of the gas flow through the opening22away from the interior21along a fluid pathway having a greater fluid conductance than other available fluid pathways. It will be appreciated that the gas flow guide may be configured to guide the portion of the gas flow through the opening22away from the interior21along a fluid pathway having a greater fluid conductance than other available fluid pathways when the apparatus20is disposed in a first, operating configuration. With such an arrangement gas will preferentially flow along said fluid pathway.

Additionally or alternatively, ensuring that more than half of the gas flow through the opening22away from the interior21is directed away from the substrate W supported by the substrate table WT may be achieved by ensuring that a pressure difference across a fluid pathway along which the portion of the gas flow is directed by the gas guide is sufficiently high. In some embodiments the gas flow guide may comprises a pump arranged to draw a portion of the gas flow along a fluid pathway. The pump may, for example, comprise a vacuum pump.

It will be appreciated that, as used herein the sealing element28being in sealing relationship with the wall of the body of the projection system around substantially the entire perimeter of the opening22of the projection system PS is intended to mean that the fluid conductance through any clearance between the sealing element28and the wall of the body of the projection system PS or barrier26is significantly smaller than the fluid conductance through an alternative exhaust pathway (for example into a cavity formed within a wall of the body of the projection system such as is provided, for example, by passageway32).

A specific embodiment of the sealing member28is now discussed with reference toFIG. 6, which shows an apparatus50according to an embodiment of the present invention. It will be appreciated that components of the apparatus50inFIG. 6that are generally equivalent to corresponding components of the apparatus20inFIGS. 2 and 5share common reference numerals therewith. For clarity, the substrate table and substrate are not shown inFIG. 6.

In this embodiment, a wall of the body of the projection system PS may be considered to comprise and inner wall52, which defines the interior21(and opening22), and an outer wall54. A cavity56is defined by the inner and outer walls52,54.

A pathway58is defined between the intermediate member24and the inner wall52of the projection system PS. A sealing member is provided so as to direct gas flowing along this pathway58defined between the intermediate member24and the projection system PS into the cavity56formed within the wall of the body of the projection system PS.

In this embodiment, the sealing member comprises a flange portion60provided on the heat shield43of the intermediate member24. The flange portion60extends from a main portion of the heat shield43of the intermediate member24generally perpendicular thereto. The flange portion60extends into the cavity56formed between the inner and outer walls52,54of the projection system PS. The flange portion60is in sealing relationship with a lower flange portion62of outer wall54. A clearance64is provided between the flange portion60and lower flange portion62of the outer wall54of the body of the projection system PS. The clearance64allows the flange portion60(which provides the sealing element28) to be in sealing relationship with the outer wall54of the body of the projection system PS whilst allowing the intermediate member24to move relative thereto.

It will be appreciated that the flange portion60of the apparatus50shown inFIG. 5is equivalent to, and may be considered to be an embodiment of, the sealing element28of the apparatus20shown inFIG. 2. It will be further appreciated that the cavity56formed between the inner and outer walls52,54of the projection system PS of the apparatus50shown inFIG. 5is equivalent to, and may be considered to be an embodiment of, the passageway32of the apparatus20shown inFIG. 2.

In some embodiments, the sealing member28may comprise a flexible membrane connected between the intermediate member24and a wall of the body of the projection system PS. For example, in some embodiments, such a flexible membrane may be provided in place of the flange portion60of the heat shield43of the intermediate member24and may be connected between the heat shield43of the intermediate member24and the lower flange portion62of the outer wall54of the body of the projection system PS. Alternatively, in some embodiments, such a flexible membrane may be provided in addition to the flange portion60of the heat shield43of the intermediate member24and may be connected between said flange portion60of the heat shield43and the lower flange portion62of the outer wall54of the body of the projection system PS. Such a flexible membrane may allow the intermediate member24to be in sealing relationship with the wall of the body of the projection system PS whilst allowing the intermediate member24to move relative to the wall.