bearing

Abstract

A bearing comprises: a support and a plurality of bearing surfaces. The bearing surfaces are supported by the support and configured to at least partially define an aperture for receipt of a shaft, the aperture having a finite order of rotational symmetry about a centroid of the aperture. At least some of the bearing surfaces are configured so as to be independently displaceable relative to the support so as to allow a shaft received in the aperture to rotate about one or more axes of rotation orthogonal to a nominal axis of the shaft whilst substantially preventing rotation of the shaft about its nominal axis.

Description

BEARINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23181410.4 which was filed on lune 26, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a bearing, which may be suitable for use within a lithographic apparatus. The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus.BACKGROUND

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

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on that substrate. A lithographic apparatus that uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] A patterning device (e.g., a mask) that is used to impart a pattern to a radiation beam in a lithographic apparatus may form part of a mask assembly.

[0006] Reticle masking blades may be used to define a generally rectangular region of the patterning device which can receive radiation. In use, the reticle masking blades may undergo linear motion. This linear motion may be transmitted to the reticle masking blades via one or more shafts. The one or more shafts may be supported by bearings. In use, each shaft may move relative to the bearings in a direction that is generally parallel to its axis whilst being supported by the bearing. Each shaft and the reticle masking blade supported thereby may be subject in use to high accelerations. It is desirable for the reticle masking blades to be accurately positioned such so as to ensure that they operate properly.

[0007] It may be desirable to provide an apparatus that obviates or mitigates one or more problems associated with the prior art.SUMMARY

[0008] According to a first aspect of the disclosure there is provided a bearing comprising: a support; and a plurality of bearing surfaces supported by the support and configured to at least partially define an aperture for receipt of a shaft, the aperture having a finite order of rotational symmetry about a centroid of the aperture; wherein at least some of the bearing surfaces are configured so as to beindependently displaceable relative to the support so as to allow a shaft received in the aperture to rotate about one or more axes of rotation orthogonal to a nominal axis of the shaft whilst substantially preventing rotation of the shaft about its nominal axis.

[0009] The term displaceable should be taken to indicate capability to move translationally and / or rotationally.

[0010] In some embodiments, the bearing surfaces may be configured so as to be independently rotatable relative to the support. In further alternative embodiments the bearing surfaces may be configured so as to be exclusively rotatable relative to the support.

[0011] The shaft may be a reciprocating shaft which, in use, may move relative to the bearing in a direction that is generally parallel to its axis whilst being supported by the bearing. Under high accelerations, such a reciprocating shaft may be subject to deflections in directions normal to the nominal axis of the shaft. Deflection of the reciprocating shaft may cause local rotation of the shaft, or local tilt, at the bearing. The shaft may, for example, support a reticle masking blade of a lithography apparatus. Rotation of the reticle masking blades about the nominal axis of the shaft is undesirable.

[0012] Advantageously, the degrees of rotational freedom afforded to the bearing surfaces of the bearing according to the first aspect allow the bearing surfaces to conform to the local tilt of the reciprocating shaft by rotating. The rotation of the bearing surfaces reduces non-uniformity (i.e. local concentration) of loads across the bearing surfaces. Where a fluid film is present between the bearing surfaces and the shaft, the rotation of the bearing surfaces assists the maintenance of an adequate film between the bearing surfaces and reciprocating shaft. This allows for a film capable of supporting the reciprocating shaft without excessive contact between the reciprocating shaft and the bearing surfaces.

[0013] The nominal axis of a shaft may be an axis of the aperture defined by the plurality of bearing surfaces when the plurality of bearing surfaces are disposed in nominal positions or orientations.

[0014] Using a right-handed set of Cartesian coordinates, if the nominal axis of the shaft is parallel to the z-axis, at least some of the bearing surfaces may be rotatable so as to allow the shaft to rotate about the x-axis and / or the y-axis. It will be appreciated that after such rotation, in general, the axis of the shaft will be at a non- zero angle relative to the nominal axes of the shaft (i.e. the shaft will no longer be perfectly parallel to the z-axis)

[0015] The bearing according to the first aspect tolerates deflection of the reciprocating shaft, which allows the reciprocating shaft to be subjected to higher accelerations and velocities.

[0016] The bearing according to the first aspect requires less stringent manufacturing tolerances, making production of the bearing more economical.

[0017] The plurality of bearing surfaces may be arranged in one or more pairs of generally opposed bearing surfaces and the bearing surfaces of each pair of bearing surfaces may be separated in a first direction and may each be rotatable about a rotation axis. The rotation axes of the two bearing surfaces of a given pair of bearing surfaces may be parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft.

[0018] Opposed bearing surfaces rotatable about axes in a second direction (i.e. parallel axes of rotation) are capable of remaining mutually -parallel when rotated in common - for example, in response to local tilt of a reciprocating shaft. The prismatic geometry of the aperture is preserved under rotations in common of the pairs of bearing surfaces, allowing maintenance of a uniform film thickness, aiding operation of the bearing. In particular, if each such pair of generally opposed bearing surfaces rotate by the same angle about their respective rotation axes, since these two rotations axes are both parallel to the second direction, the shape of the aperture that is at least partially defined by the bearing surfaces will be the same although scaled in the first direction by a factor of the cosine of the angle.

[0019] The plurality of bearing surfaces may comprise at least one bearing surface that is constrained so as to substantially prevent rotation of the at least one bearing surface relative to the support about the nominal axis of the shaft.

[0020] The plurality of bearing surfaces may comprise only one bearing surface that is constrained so as to substantially prevent rotation of the at least one bearing surface relative to the support about the nominal axis of the shaft.

[0021] Constraint of only one bearing prevents undesirable rotation of the shaft about its nominal axis, and avoids overconstraint of the bearing.

[0022] The plurality of bearing surfaces may be arranged in one or more pairs of generally opposed bearing surfaces and the bearing surfaces of each pair of bearing surfaces are separated in a first direction.

[0023] The bearing surfaces of each pair of bearing surfaces are each rotatable about a rotation axis, the rotation axes of the two bearing surfaces of a given pair of bearing surfaces being parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft.

[0024] The rotation axes of the bearing surfaces, are such that they are substantially unable to rotate about axes parallel to the nominal axis of the shaft.

[0025] Each pair of bearing surfaces may comprise a rotatable bearing surface and a biased bearing surface.

[0026] The combination of a rotatable and biased bearing surface allows the bearing to effectively constrain the shaft without requiring close tolerances between the bearing surfaces and shafts.

[0027] The rotatable bearing surface of at least one of the pairs may only be rotatable about an axis parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft.

[0028] The rotatable bearing surface of at least one of at least one of the pairs is rotatable about either of two axes perpendicular to the first direction of the pair of bearing surfaces.

[0029] The rotatable bearing surface of at least one of the pairs of bearing surfaces may be connected to the support via a cardan hinge. The cardan hinge may comprise one or more flexures.

[0030] Advantageously, the bearing surface supports the shaft in position without adding additional rotational constraint. Additional rotational constraint may require finer tolerances of the bearing and shaft.

[0031] Each biased bearing surface may be urged toward the centroid of the aperture by one or more biasing means.

[0032] In use, the biased bearing surfaces are urged toward the shaft, allowing the bearing surfaces to adapt to variations in the shaft and bearing dimensions.

[0033] Each of the one or more biased bearing surfaces may comprise a means for preventing the rotation of biased bearing surfaces along a rotation axis in the first direction. The means for preventing the rotation of biased bearing surfaces may comprise complementary anti-rotation features.

[0034] The one or more biasing means of the one or more biased bearing surfaces may comprise a pneumatic and / or mechanical biasing means.

[0035] The one or more biasing means may comprise a pneumatic piston.

[0036] The one or more biased bearing surfaces may be urged toward the centroid of the aperture by mechanical means in a first configuration, and by pneumatic means in a second configuration.

[0037] In the first configuration, the bearing may be disconnected from external sources of pneumatic pressure, such as during transportation. In the second configuration, the bearing may be connected from external sources of pneumatic pressure, such as during operation of the bearing.

[0038] Advantageously during transportation of the bearing, the tension provided by the mechanical means allows the shaft to be securely held by the bearing passively, without external power.

[0039] Mechanical biasing means may comprise helical springs, wave washers or elastomeric elements.

[0040] The bearing may comprise two pairs of bearing surfaces, whose rotation axes are mutually orthogonal.

[0041] Two pairs of bearing surfaces allow the bearing to maintain an adequate film in response to deflection of the reciprocating shaft in any direction normal to the nominal shaft axis. It will be appreciated that other arrangements having more pairs of bearing surfaces are possible (e.g. 3, 4, 5, 6, 7 or more) although such arrangements may be mechanically more complex.

[0042] The bearing may be an air bearing.

[0043] In particular, the bearing may be an aerostatic bearing. Air bearings operate with low friction, vibration and high precision. By conforming to local tilt of a reciprocating shaft, air bearings of the present disclosure can maintain an adequate gas film across the bearing surfaces. An adequate gas film can support the reciprocating shaft without any contact between the reciprocating shaft and the bearing surfaces, minimising wear and friction.

[0044] The bearing may further comprise a gas inlet for connection to a gas supply and one or more channels configured to fluidly connect the gas inlet to each of the plurality of bearing surfaces.

[0045] In use, the gas inlet may be connected to a gas supply system. In use, the gas inlet may be supplied with pressurised gas (for example via one or more flexible tubes). It will be appreciated that the gas inlet may comprise a plurality of gas inlets. For example, in some embodiments, one gas inlet may be provided for each of the plurality of bearing surfaces. For such embodiments, one or more channels may be configured to fluidly connect each gas inlet to a corresponding one of the plurality of bearing surfaces.

[0046] The bearing may further comprise a plurality of bearing members, each bearing member comprising a body and defining one of the plurality of bearing surfaces. The body of each bearing member may define a gas manifold comprising an inlet and configured to evenly distribute gas received at the inlet over the bearing surface defined by the body.

[0047] Each bearing member may be considered to be a support underlying one of the bearing surfaces. The supports may further comprise a manifold, supplied with gas through the flexible tubes. In use, the inlet of each manifold of each bearing member may be connected to a gas supply system. In use, the inlet of each manifold of each bearing member may be supplied with pressurised gas (for example via flexible tubes). The gas may be air.

[0048] The bearing members may be composite members comprising a first portion that defines bearing surface and a second portion that supports the first portion. The first portion that defines the bearing surface may be substantially composed of porous graphite. In general, the first and second portions may be formed from different materials.

[0049] Each of the bearing surfaces may be connected to the support via a biasing mechanism that is arranged to bias the bearing surface towards a nominal position or orientation.

[0050] It will be appreciated that the biasing mechanism may comprise any combination of resilient biasing members such as springs, leaf springs, flexures or the like.

[0051] At least one of the bearing surfaces may be connected to the support via one or more flexures.

[0052] Advantageously, flexures are single-component and so do not exhibit significant friction or backlash. The flexures may be of the form of leaf springs or any other type of flexure. The flexures may be ‘notch’ flexures. The flexures may be mounted to the support.

[0053] In some embodiments, each of the bearing surfaces may be connected to the support via one or more flexures.

[0054] At least one of the bearing surfaces may have one degree of freedom.

[0055] In some embodiments, each of the bearing surfaces may have one degree of freedom.

[0056] Bearing surfaces having one degree of freedom inherently restrict rotation to axes of rotation orthogonal to the direction of shaft reciprocation, without requiring further mechanisms or structures. In an example, bearing surfaces connected to the support via notch flexures, have one degree of freedom.

[0057] The bearing surfaces may have a rotation range of up to 3 milliradians. In an example, at least one of the bearing surfaces may have a rotation range of ±1.5 milliradians. The air bearing may furthercomprise structures (e.g. bumpers or physical stops) configured to limit the rotation of the bearing surfaces within a desired range. In addition, or alternatively, the air bearing may comprise structures to further support the bearing surfaces, whilst still allowing the desired rotation.

[0058] The biasing mechanism of at least one of the bearing surfaces may be substantially composed of titanium or ferrous alloys.

[0059] In some embodiments, each of the bearing surfaces may be substantially composed of titanium or ferrous alloys.

[0060] Ferrous alloys may include stainless steel or carbon steel. Advantageously titanium alloy and ferrous alloy biasing mechanisms (e.g. flexures) have a long lifespan, due to the high fatigue limit of titanium alloys and ferrous alloys. In general, the biasing mechanisms may be fabricated from any material having a high fatigue strength and / or fatigue limit.

[0061] The bearing surfaces may be substantially planar.

[0062] Planar bearing surfaces allow the reciprocating shaft to freely tilt in axes normal to their planes.

[0063] The bearing surfaces may alternatively be curved (i.e. convex, concave or combinations thereof) in directions transverse to the direction of shaft reciprocation (i.e. the nominal axis of the shaft).

[0064] According to a second aspect there is provided a reticle-masking system of a lithographic apparatus, the reticle-masking system comprising: a shaft; a reticle masking blade, configured to partially define an optical aperture; and a bearing of the first aspect; wherein the shaft is received in the aperture defined by the bearing surfaces of the bearing and supported thereby. The bearing may comprise any of the optional features described above.

[0065] The shaft is configured to reciprocate within the bearing aperture. A reticle-masking system according to the second aspect can operate at high speeds and accelerations of the reticle masking blade. The reticle masking system according to the second aspect also obviates the need for additional mechanisms configured to restrict rotation of the reticle masking blades about the direction of shaft reciprocation, reducing the footprint of the reticle masking system.

[0066] The shaft may be at least partially non-circular in cross-section.

[0067] Non-circular apertures and / or shafts prevent rotation in the direction of shaft reciprocation.

[0068] The biasing means may comprise: a spring; an expansible pneumatic chamber; further wherein the biasing means is configurable between: the first configuration, wherein the spring is configured to urge the biased bearing surface toward the aperture centroid through the pneumatic chamber; and the second configuration, wherein the pneumatic chamber is pressurised, retracting the spring, and the pneumatic pressure of the pneumatic chamber urges the biased bearing surface toward the aperture centroid.

[0069] For a given air pressure, the relative position of the shaft and biased bearing surface (i.e. their separation) is approximately constant. Advantageously, the separation is substantially independent of the absolute positions of the components, within limits. Under any disturbance of the separation, thebearing surface will reposition itself at the correct separation in order to reach force equilibrium. The biased bearing surfaces automatically compensate for inaccuracies in the bearing and / or shaft dimensions.

[0070] The biasing means may further comprise: a sub piston, urged by the spring; a main piston, connected to the biased bearing surface; wherein the spring, sub-piston and main piston are captive within a bore provided in the support, and the main piston and sub-piston define the expansible pneumatic chamber in conjunction with the bore; and further wherein in the first configuration the sub piston and the main piston are in contact; and in the second configuration the sub piston and the main piston are separated by pneumatic pressure.

[0071] The main piston and / or the sub-piston may further comprise a piston seal ring, configured to seal against the bore.

[0072] The piston seal ring tolerates reciprocating movement of the main piston and / or the sub-piston and permits the main piston to tilt. The piston seal ring ensures an effective pneumatic seal between the main piston / sub-piston and bore.

[0073] The pneumatic chamber may be fluidly connected to the gas inlet and the biased bearing surface.

[0074] The shaft may be further supported by a second bearing.

[0075] Supporting the reciprocating shaft at two points translationally constrains the reciprocating shaft in directions normal to the nominal axis for the shaft.

[0076] The shaft may comprise a first portion having a non-circular cross-section, received by the bearing and a second portion having a circular cross-section, received by an aperture of the second bearing.

[0077] In some embodiments, the second portion of the shaft may also be non-circular, being received by a corresponding aperture of the second bearing. In such embodiments, any (slight) rotation of the front bearing about the nominal axis would cause a conflict in alignment with the second bearing (e.g. causing a reduction of air gap in air bearings). As a result, such embodiments with non-circular first and second shaft portions may need the reticle masking system and its components to have increased accuracy, precision, stability and rigidity to function optimally. Thus the circular cross section of the second portion, simplifies the fabrication of the reticle masking system.

[0078] The second bearing may be supported by a thin circumferential web of material.

[0079] The thin circumferential web of material allows the bearing surface of the second bearing to conform to local tilt of the reciprocating shaft by rotating. The rotation of the bearing surface assists the maintenance of an adequate film between the bearing surfaces and reciprocating shaft.

[0080] The second bearing may be suspended from a support frame by a plurality of suspension arms having greater flexibility than the support frame, projecting from the second bearing.

[0081] The above-described second bearing suspension provides a required degree of compliance, whilst being relatively simple to manufacture.

[0082] The second bearing may be supported by at least three flexures.

[0083] The plurality of suspension arms may project from the second bearing in a generally tangential fashion.

[0084] The second bearing may be supported by three flexures.

[0085] The reticle-masking system may further comprise a gas supply system, configured to supply gas to each of the plurality of bearing surfaces of the bearing. The gas supply system may also be configured to supply gas to one or more bearing surfaces of the second bearing.

[0086] Provision of a gas supply system allows maintenance of an adequate gas film between the bearing surfaces and reciprocating shaft.

[0087] According to a third aspect there is provided a lithographic apparatus comprising the reticlemasking system of the second aspect. The reticle-masking system may comprise any of the optional features described above.

[0088] It will be appreciated that one or more aspects or features described above or referred to in the following description may be combined with one or more other aspects or features.BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 is a schematic illustration of a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2A is a schematic plan view of a support structure and patterning device shown in Figure 1, disposed in a first end position;Figure 2B is a schematic plan view of the support structure and patterning device shown in Figure 1, disposed in a second end position;Figure 3A is a schematic illustration of a first cross section through a patterning device on the support structure and reticle masking blades of the lithographic apparatus of Figure 1 ;Figure 3B is a schematic illustration of a second cross section through a patterning device on the support structure and reticle masking blades of the lithographic apparatus of Figure 1;Figure 4 is a plan view showing the y masking blades and the x masking blades (dotted lines) of the lithographic apparatus of Figure 1 in a first configuration;Figure 5A is a schematic illustration of a transverse cross section through a bearing according to an embodiment of the present disclosure;Figure 5B is a schematic illustration of an axial cross section through a bearing according to the embodiment shown in Figure 5A;Figure 6 is a perspective illustration of a bearing according to another embodiment of the present disclosure;Figure 7A is a schematic illustration of an axial cross section through an air bearing according to an embodiment of the present disclosure;Figure 7B is a schematic illustration of a transverse cross section through a bearing according to the embodiment shown in Figure 7 A;Figure 8A is a schematic illustration of an axial cross section through a reticle masking system according to an embodiment of the present disclosure;Figure 8B is a schematic illustration of an axial cross section through a reticle masking system according to an embodiment shown in Figure 8A under deflection;Figure 9 is a perspective illustration of a reticle masking system according to an embodiment of the present disclosure.Figure 10 schematically illustrates a bearing in transverse cross-section according to an embodiment of the present disclosure.Figure 11 schematically illustrates a bearing surface and its connecting means according to an embodiment of the present disclosure.Figures 12a and 12b schematically illustrate a biasing means according to an embodiment of the present disclosure.Figure 13 schematically illustrates a main piston according to an embodiment of the present disclosure.Figures 14a and 14b schematically illustrate a bearing according to an alternative embodiment of the present disclosure.DETAILED DESCRIPTION

[0090] Figure 1 shows a lithographic system. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a reticle assembly 15 including a patterning device MA (e.g., a reticle or 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 radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the patterning device MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[0091] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g., hydrogen) may be provided in the radiation source SO. A vacuum may be provided in the illumination system IL and / or the projection system PS. A smallamount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and / or the projection system PS.

[0092] The radiation source SO shown in Figure 1 is of a type that may be referred to as a laser produced plasma (LPP) source. A laser 1, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) that is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g., in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.

[0093] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure that is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[0094] In other embodiments of a laser produced plasma (LPP) source the collector 5 may be a so- called grazing incidence collector that is configured to receive EUV radiation at grazing incidence angles and focus the EUV radiation at an intermediate focus. A grazing incidence collector may, for example, be a nested collector, comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be disposed axially symmetrically around an optical axis.

[0095] The radiation source SO may include one or more contamination traps (not shown). For example, a contamination trap may be located between the plasma formation region 4 and the radiation collector 5. The contamination trap may for example be a rotating foil trap, or may be any other suitable form of contamination trap.

[0096] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[0097] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0098] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facettedfield mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the reticle assembly 15 held by the support structure MT. The reticle assembly 15 includes a patterning device MA and a pellicle 19. The pellicle is mounted to the patterning device MA via a pellicle frame 17. The reticle assembly 15 may be referred to as a reticle and pellicle assembly 15. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.

[0099] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 that are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system PS may include any number of mirrors (e.g., six mirrors).[000100] The lithographic apparatus may, for example, be used in a scan mode, wherein the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a substrate W (i.e., a dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the demagnification and image reversal characteristics of the projection system PS. The patterned radiation beam that is incident upon the substrate W may comprise a band of radiation. The band of radiation may be referred to as an exposure slit. During a scanning exposure, the movement of the substrate table WT and the support structure MT may be such that the exposure slit travels over an exposure field of the substrate W.[000101] The radiation source SO and / or the lithographic apparatus that is shown in Figure 1 may include components that are not illustrated. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.[000102] In other embodiments of a lithographic system the radiation source SO may take other forms. For example, in alternative embodiments the radiation source SO may comprise one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation that may be provided to one or more lithographic apparatus.[000103] As was described briefly above, the reticle assembly 15 includes a pellicle 19 that is provided adjacent to the patterning device MA. The pellicle 19 is provided in the path of the radiation beam B such that radiation beam B passes through the pellicle 19 both as it approaches the patterning device MA from the illumination system IL and as it is reflected by the patterning device MA towardsthe projection system PS. The pellicle 19 comprises a thin film or membrane that is substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation). By EUV transparent pellicle or a film substantially transparent for EUV radiation herein is meant that the pellicle 19 is transmissive for at least 65% of the EUV radiation, preferably at least 80% and more preferably at least 90% of the EUV radiation. The pellicle 19 acts to protect the patterning device MA from particle contamination.[000104] Whilst efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of a pellicle 19, particles may be deposited onto the patterning device MA. Particles on the patterning device MA may disadvantageous^ affect the pattern that is imparted to the radiation beam B and therefore the pattern that is transferred to the substrate W. The pellicle 19 advantageously provides a barrier between the patterning device MA and the environment in the lithographic apparatus LA in order to prevent particles from being deposited on the patterning device MA.[000105] The pellicle 19 is positioned at a distance from the patterning device MA that is sufficient that any particles that are incident upon the surface of the pellicle 19 are not in a field plane of the lithographic apparatus LA. This separation between the pellicle 19 and the patterning device MA acts to reduce the extent to which any particles on the surface of the pellicle 19 impart a pattern to the radiation beam B that is imaged onto the substrate W. It will be appreciated that where a particle is present in the beam of radiation B, but at a position that is not in a field plane of the beam of radiation B (for example not at the surface of the patterning device MA), then any image of the particle will not be in focus at the surface of the substrate W. In the absence of other considerations, it may be desirable to position the pellicle 19 a considerable distance away from the patterning device MA. However, in practice the space which is available in the lithographic apparatus LA to accommodate the pellicle is limited due to the presence of other components. In some embodiments, the separation between the pellicle 19 and the patterning device MA may, for example, be approximately between 1 mm and 10 mm, for example between 1 mm and 5 mm, for example between 2 mm and 2.5 mm.[000106] A description of some additional features of an example type of lithographic apparatus LA, in particular some features and components that are close to the support structure MT, is now provided with reference to Figures 2 A to 4.[000107] The support structure MT may be movable in a scanning direction so as to expose a greater region of the patterning device MA of the reticle and pellicle assembly 15 in a single dynamic scanning exposure, as now discussed with reference to Figures 2 A and 2B. Figures 2 A and 2B show a schematic plan view of the support structure MT and the reticle and pellicle assembly 15 in two different positions.[000108] The support structure MT is movably mounted within a region 24. In particular, the support structure MT is movable in a scanning direction, as indicated by arrow 26, between a first end position (as shown in Figure 2 A) and a second end position (as shown in Figure 2B).[000109] Unless stated otherwise, throughout this specification, the following set of Cartesian co-ordinates will be used. The scanning direction is labelled as the y-direction. A direction which is also in the plane of the support structure MT and is perpendicular to the scanning direction is referred to as the non-scanning direction and is labelled as the x-direction. A direction which is perpendicular to the plane of the support structure MT is labelled as the z-direction.[000110] The lithographic apparatus LA may be considered to comprise a scanning module operable to move the support structure MT in the scanning direction between at least the first end position and the second end position. For example, the scanning module may be operable to move the support structure MT in the scanning direction relative to a supporting frame (indicated schematically by region 24), which the support structure MT may be considered to be movably mounted to.[000111] The reticle and pellicle assembly 15 may be considered to comprise a central portion 15a and a peripheral portion 15b surrounding the central portion 15a. The central portion 15a may be referred to as an image formation portion and may coincide with a portion the reticle MA that patterns the radiation beam B and a membrane of the pellicle 19. The peripheral portion 15b may coincide with a border portion of the pellicle 19 and a frame of the pellicle 19.[000112] The movement of the support structure MT between the first and second positions defines an extended first portion region 28 of the support structure MT defined by all of the regions that the central portion 15a of the reticle and pellicle assembly 15 can be disposed in. That is, the extended first portion region 28 of the support structure MT is a region defined by moving the central portion 15a of the reticle and pellicle assembly 15 from the first end position (as shown in Figure 2A) to the second end position (as shown in Figure 2B).[000113] The lithographic apparatus LA is provided with four reticle masking blades, which define the extent of the field on the substrate W which is illuminated, as now described with reference to Figures 3A, 3B and 4. The illumination system IL is operable to illuminate a region of the patterning device MA when disposed on the support structure MT. This region may be referred to as the slit of the illumination system IL and is at least partially defined by four reticle masking blades, which define a generally rectangular region of the patterning device which can receive radiation. The extent of the generally rectangular region in a first direction, which may be referred to as the x direction, is defined by a pair of x masking blades 32, 34. The extent of the generally rectangular region in a second direction, which may be referred to as the y direction, is defined by a pair of y masking blades 36, 38.[000114] Each of the masking blades 32, 34, 36, 38 is disposed close to, but slightly out of the plane of the patterning device on the support structure MT. The x masking blades 32, 34 are disposed in a first plane 40 and the y masking blades 36, 38 are disposed in a second plane 42.[000115] Each of the masking blades 32, 34, 36, 38 defines one edge of a rectangular field region 44 in the plane of the patterning device MA which can receive radiation. In practice the illumination system IL may only illuminate part of the rectangular field region 44. As shown in Figure 4, theillumination system IL may be arranged to illuminate a curved slit region 46, which may coincide with a part of the rectangular field region 44 (depending on the positions of the y masking blades 36, 38).[000116] The curved slit region 46 may be partially defined by optics within the illumination system IL and / or the projection system PS. In addition, the curved slit region 46 may be partially defined by a plurality of independently movable objects provided along one or both of the curved edges of the curved slit region 46. The plurality of independently movable objects may be referred to as unicorn fingers. The plurality of independently movable objects may be provided at different x positions and may be movable in the y direction to control an overlap between each of the movable objects and the radiation beam B produced by the illumination system IL. Controlling the y positions of the movable members may control a shape (or at least an intensity distribution) of one or both of the curved edges of the curved slit region 46. The movable members may be used to minimize variations in a dose of radiation provided by the radiation beam B at different positions in the non-scanning direction (i.e. the x direction). In addition, the curved slit region 46 may be partially defined by physical aperture, for example an entrance aperture of the projection system PS.[000117] Each of the masking blades 32, 34, 36, 38 may be independently movable between a retracted position wherein it is not disposed in the path of the radiation beam and an inserted position wherein it at least partially blocks the radiation beam projected onto the patterning device MA by the illumination system IL. By moving the masking blades 32, 34, 36, 38 into the path of the radiation beam, the radiation beam B can be truncated (in the x and / or y direction) thus limiting the extent of the field region 44 which receives radiation beam B.[000118] The x-direction corresponds to the non-scanning direction of the lithographic apparatus LA and the y-direction corresponds to the scanning direction of the lithographic apparatus LA. The patterning device MA is movable in the y-direction through the field region 44 (as indicated by again by arrow 26) so as to expose a greater region of the patterning device MA in a single dynamic scanning exposure.[000119] During a dynamic exposure of a target region of a substrate W the target region is moved through an exposure region in the plane of the substrate W, the exposure region being a portion of the substrate W that the exposure region 44 of the patterning device MA is imaged onto by projection system PS. As the target region of the substrate W moves into the exposure region, the first masking blade 36, 38 moves such that only the target region receives radiation (i.e. no parts of the substrate outside of the target region are exposed). At the start of the scanning exposure one of the y masking blades 36, 38 is disposed in the path of the radiation beam B, acting as a shutter, such that no part of the substrate W receives radiation. At the end of the scanning exposure the other y masking blade 36, 38 is disposed in the path of the radiation beam B, acting as a shutter, such that no part of the substrate W receives radiation. During a middle part of the scanning exposure, when there is no overlap between the exposure region 44 (which receives the radiation B) and either of the adjacent target regions of the substrate W, both of the y masking blades 36, 38 are disposed in retracted positions.[000120] Rays of radiation beam B are shown adjacent to each of the masking blades 32, 34, 36, 38. It will be appreciated that each point in the slit region 46 is illuminated with radiation from a range of angles. For example, each point in the slit region 46 may receive a cone of radiation. The rays of radiation beam B are shown adjacent to each of the masking blades 32, 34, 36, 38 indicate an average direction of the radiation received by the patterning device MA. The rays of radiation beam B are shown adjacent to each of the masking blades 32, 34, 36, 38 may be referred to as chief rays. As can be seen from Figures 3A and 3B, in this embodiment, as projected onto the x-z plane, a chief ray of the radiation is generally normally incident on the patterning device MA whereas as projected onto the y-z plane, a chief ray of the radiation is generally incident on the patterning device MA at an angle 48.[000121] The lithographic apparatus LA may further comprise a gas nozzle 50, which may be arranged to direct a flow of gas 52 adjacent to the support structure MT. In particular, the flow of gas 52 provided adjacent to the support structure MT by the gas nozzle 50 may be generally parallel to a surface of the patterning device MA and may be referred to as a cross-flow. The gas nozzle 50 may be disposed generally in the same plane (the first plane 40) as the x masking blades 32, 34. The gas nozzle may point in the scanning direction such that the flow of gas 52 is generally parallel to the scanning direction and flows between the x masking blades 32, 34. The gas nozzle 50 may be considered to form part of a gas supply module which is operable to provide a flow of gas adjacent to the support structure MT.[000122] The gas nozzle 50 may form part of a hydrogen supply operable to supply hydrogen in the vicinity of the pellicle 19 of the pellicle and reticle assembly 15 when supported by the support structure MT.[000123] Figure 4 shows a plan view of the y masking blades 36, 38 in the second plane 42 as viewed in the positive z-direction (i.e. upwards in Figure 3B). The position of the x masking blades 32, 34 and the gas nozzle (which are disposed in the first plane 40) are shown in dotted lines. In Figure 4, the four masking blades 32, 34, 36, 38 are disposed so as to define a generally rectangular field region 44, the slit region 46 being disposed within this generally rectangular field region 44. This may be a typical configuration of the four masking blades 32, 34, 36, 38 during the exposure of a central portion of a target region (for example a die on a substrate W). As explained above, each of the x masking blades 32, 34 is operable to move in the x direction and each of the y masking blades 36, 38 is operable to move in the y direction to control the size of the field region 44. The y masking blades 36, 38 are configured such that they can be actuated from the same side of the field region 44. To achieve this, the y masking blades 36, 38 are shaped such that (although they lie in substantially the same plane 42) each of the y masking blades 36, 38 is provided with one or more support portions which extend in the same direction (the negative y direction in Figure 4).[000124] The masking blades 32, 34, 36, 38 and the gas nozzle 50 may be mounted on a common masking blade assembly support (not shown). It will be appreciated that the masking blades 32, 34, 36,38 may be movably mounted on such a support such that they can move relative thereto. The gas nozzle may be statically mounted on such a support.[000125] In order to move with low friction and a precisely-defined movement envelope, one or more of the masking blades 32, 34, 36, 38 are provided with a shaft and bearing system. A bearing is now described with reference to Figures 5A and 5B, which schematically illustrate a bearing 100 in a transverse and axial cross section, respectively.[000126] In the following description, where multiple instances of substantially-identical components are present in a figure, only a single instance may be labelled in the interests of brevity and clarity.[000127] In addition, it should be noted that the axes referenced in the following description are distinct from those referenced in Figures 2A to 4. It will be appreciated from Figures 2A to 4 that the direction of movement (which the supporting shaft is parallel to) of some of the masking blades 32, 34 is the non-scanning direction of the lithographic apparatus LA whereas the direction of movement (which the supporting shaft is parallel to) of some of the masking blades 36, 38 is the scanning direction of the lithographic apparatus LA. In the following, this direction of movement (which the supporting shaft is parallel to) is referred to as the z-direction.[000128] The bearing 100 comprises a support 101 and four bearing surfaces 102 supported by the support 101. The bearing surfaces 102 are configured to at least partially define an aperture 104 for receipt of a shaft 108. The aperture 104 has a finite order of rotational symmetry (i.e. order 4) about a centroid 106 of the aperture 104.[000129] The bearing surfaces 102 are configured so as to be independently displaceable relative to the support 101 so as to allow the shaft 108 received in the aperture to rotate about one or more axes of rotation 112 orthogonal to a nominal axis 110 of the shaft whilst substantially preventing rotation of the shaft 108 about its nominal axis 110.[000130] The position and extent of a shaft 108 receivable by the bearing 100 and its nominal axis 110 are best indicated in Figure 5B.[000131] Displacement of the bearing surfaces 102 is facilitated by connecting members 114 which couple the bearing surfaces 102 to the support 101.[000132] The nominal axis 110 of a shaft may be an axis of the aperture defined by the plurality of bearing surfaces 102 when the plurality of bearing surfaces are disposed in nominal positions or orientations.[000133] The term displaceable should be taken to indicate a capability to move translationally and / or rotationally - displacement may comprise both rotation and translation.[000134] Although the embodiment described with respect to Figures 5A and 5B comprises four bearing surfaces 102, in other embodiments the bearing may comprise 3, 5, 6, 7, 8, 9 or more bearing surfaces. In general, the bearing may comprise a plurality of bearing surfaces.[000135] In some embodiments, not all of the bearing surfaces 102 may be configured to be independently displaceable relative to the support 101. Each bearing surface 102 may be in a fixed relationship relative to another bearing surface 102, for example, pairs of bearing surfaces may move in tandem.[000136] In some embodiments, the bearing surfaces 102 may be configured so as to be independently rotatable relative to the support 101. In further alternative embodiments, the bearing surfaces 102 may be configured so as to be exclusively (independently) rotatable relative to the support 101.[000137] The shaft 108 may, for example, support a reticle masking blade of a lithography apparatus (for example, the shaft 108 may support a masking blade 32, 34, 36, 38 of the type described above with reference to Figures 2A to 4). In view of the above-described linear movement of the reticle masking blades, the shaft 108 may be a reciprocating shaft which, in use, may move relative to the bearing 100 in a direction that is generally parallel to its axis whilst being supported by the bearing. Under high accelerations, such a reciprocating shaft may be subject to deflections in directions normal to the nominal axis of the shaft. Deflection of the reciprocating shaft may cause local rotation of the shaft, or local tilt, at the bearing. Rotation of the reticle masking blades about the nominal axis of the shaft is undesirable.[000138] The degrees of rotational freedom afforded to the bearing surfaces 102 of the bearing allow the bearing surfaces to conform to the local tilt of the reciprocating shaft 108 by rotating. The rotation of the bearing surfaces 102 reduces non-uniformity (i.e. local concentration) of loads across the bearing surfaces. Where a fluid film is present between the bearing surfaces 102 and the shaft 108, the rotation of the bearing surfaces assists the maintenance of an adequate film between the bearing surfaces and reciprocating shaft. This allows for a film capable of supporting the reciprocating shaft 108 without excessive contact between the reciprocating shaft and the bearing surfaces 102.[000139] The bearing 100 tolerates deflection of a reciprocating shaft 108, which allows the reciprocating shaft to be subjected to higher accelerations and velocities. The bearing 100 also requires less stringent manufacturing tolerances, making production of the bearing more economical.[000140] Using a right-handed set of Cartesian coordinates, if the nominal axis of the shaft is parallel to the z-axis, at least some of the bearing surfaces may be rotated so as to allow the shaft to rotate about the x-axis and / or the y-axis. It will be appreciated that after such rotation, in general, the axis of the shaft will be at a non-zero angle relative to the nominal axes of the shaft (i.e. the shaft will no longer be perfectly parallel to the z-axis). Note that these axes do not coincide with the axes defined in relation to Figures 2 A to 4.[000141] Figure 6 shows another embodiment of a bearing 200 of the present disclosure. The support is omitted for clarity and ease of understanding. Bearing 200 is generally analogous to bearing 100, described with reference to Figures 5A and 5B and like parts will be referred to by like numerals.[000142] The bearing 200 comprises a support (not shown in Figure 6), eight flexures 206 and four bearing surfaces 102. Each of the bearing surfaces 102 is connected to the support via a pair of flexures 206.[000143] The bearing surfaces 102 of bearing 200 are arranged in pairs of generally opposed bearing surfaces 202. Bearing 200 comprises two pairs of bearing surfaces 202. Each bearing surface of a pair 202 has a rotation axis 204. The rotation axes 204 of a pair are parallel. The rotation axes 204 of the different pairs of bearing surfaces 202 are mutually orthogonal. The bearing surfaces 102 of each pair of bearing surfaces 202 are separated in a first direction 205 and are each rotatable about a rotation axis 204. The rotation axes 204 of the two bearing surfaces 102 of a given pair of bearing surfaces 202 are parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft 110.[000144] The two mutually orthogonal pairs of bearing surfaces 202 partially define an aperture 104, having a square (or more generally rectangular) cross-section.[000145] The two pairs of bearing surfaces 202 allow the bearing to maintain an adequate film in response to deflection of the reciprocating shaft in any direction normal to the nominal axis of the shaft 110 (i.e. deflections in the x-y plane of Figure 6).[000146] It will be appreciated that the pairs of bearing surfaces 202 need not be mutually- orthogonal. The two pairs of bearing surfaces may be disposed at angles greater than, or less than the 90° to each other. In an example, the two pairs of bearing surfaces 202 may at least partially define an aperture having a parallelogram cross-section.[000147] Whilst bearing 200 has two pairs of bearing surfaces 202, other arrangements are possible. In some embodiments, the bearing may comprise more pairs of bearing surfaces (e.g. 3, 4, 5, 6, 7 or more) although such arrangements may be mechanically more complex.[000148] The opposed bearing surfaces 102 of bearing 200 are rotatable about axes 204 in a second direction (i.e. parallel axes of rotation). The opposed bearing surfaces 102 are capable of remaining mutually-parallel when rotated in common - for example, in response to local tilt of a reciprocating shaft. The prismatic geometry of the aperture 104 is preserved under rotations in common of the pairs of bearing surfaces 202, allowing maintenance of a uniform film thickness, aiding operation of the bearing 200. In particular, if each such pair of generally opposed bearing surfaces rotate by the same angle about their respective rotation axes 204, since these two rotation axes are both parallel to the second direction, the shape of the aperture that is at least partially defined by the bearing surfaces 102 will be the same, although scaled in the first direction by a factor of the cosine of the angle.[000149] Flexures 206 are notch flexures. The flexures 206 allow the above-described rotation of the bearing surfaces.[000150] Each of the bearing surfaces 102 has one degree of freedom. Notch flexures limit the bearing surfaces 102 to a single degree of freedom. One or more bearing surfaces having one degree offreedom inherently restrict rotation to axes of rotation orthogonal to the direction of shaft reciprocation, without requiring further mechanisms or structures.[000151] Flexures are single-component and so do not exhibit significant friction or backlash. Although the flexures 206 are notch flexures, the flexures may also be of the form of leaf springs or any other type of flexure.[000152] In some embodiments, the number of flexures per bearing surface may vary. For example, the bearing surfaces be connected to the support via 1, 3, 4, 5, 6 or more flexures.[000153] Flexures combine a rotation mechanism and a biasing mechanism in one structure. In some other embodiments, the rotation mechanism may comprise a ball joint, hinge or axle.[000154] It will be appreciated that in other embodiments, one or more of the bearing surfaces 102 may be connected to the support via a biasing mechanism that is arranged to bias the bearing surface towards a nominal position or orientation. The biasing mechanisms may comprise flexures, coil springs, or leaf springs or combinations thereof. In particular, bearing embodiments where the rotation mechanism comprises a ball joint, hinge or axle may be provided with separate biasing mechanisms. In other embodiments, biasing mechanisms may not be present.[000155] In general, biasing mechanisms of at least one of the bearing surfaces may be substantially composed of titanium or ferrous alloys.[000156] Ferrous alloys may include stainless steel or carbon steel. Advantageously titanium alloy and ferrous alloy biasing mechanisms (e.g. flexures) have a long lifespan, due to the high fatigue limit of titanium alloys and ferrous alloys. In general, the biasing mechanisms may be fabricated from any material having a high fatigue strength and / or fatigue limit.[000157] The bearing surfaces 102 may have a rotation range of up to ±3 milliradians. In an example, at least one of the bearing surfaces 102 may have a rotation range of ±1.5 milliradians. In general, the selection of rotation range may depend on a compromise between the tolerances, desired compliancy and stiffness of the bearing itself, the shaft or any other connected parts. The air bearing may further comprise structures (e.g. bumpers or physical stops) configured to limit the rotation of the bearing surfaces within a desired range. In addition, or alternatively, the air bearing may comprise structures to further support the bearing surfaces, whilst still allowing the desired rotation.[000158] The bearing surfaces 102 are substantially planar. Planar bearing surfaces allow a reciprocating shaft to freely tilt in axes normal to their planes.[000159] In other embodiments, the bearing surfaces may alternatively be curved (i.e. convex, concave or combinations thereof) in directions transverse to the direction of shaft reciprocation (i.e. the nominal axis of the shaft).[000160] Each of the bearing surfaces 102 of bearing 200 is connected to the support via a pair of flexures 206. Alternatively, one bearing surface of each pair of bearing surfaces may be connected to the support via another rotation mechanism (e.g. a ball joint). It will be appreciated that only one or more of the bearing surfaces needs to be restricted to one degree of freedom so as to prevent rotation.In an example, each pair of bearing surfaces may comprise a first bearing surface with one degree of freedom and a second, opposed bearing surface having a two or more degrees of freedom. The first bearing surface may be connected to the support via a notch flexure and the second, opposed bearing surface may be connected to the support via a ball joint.[000161] In some embodiments, the bearing 200 may be an air bearing. In particular, the bearing may be an aerostatic bearing. Air bearings operate with low friction, vibration and high precision. By conforming to local tilt of a reciprocating shaft, air bearings of the present disclosure can maintain an adequate gas film across the bearing surfaces. An adequate gas film can support the reciprocating shaft without any contact between the reciprocating shaft and the bearing surfaces, minimising wear and friction.[000162] Figures 7A and 7B schematically illustrate an axial and transverse cross section of an air bearing 300. It will be appreciated that the air bearing is mechanically analogous to bearing 200; the air bearing further comprises components and structures configured to generate and maintain a gas film. A description of its mechanical operation will therefore be omitted for brevity.[000163] The air bearing 300 comprises a gas inlet 302 for connection to a gas supply and a plurality of channels 304 configured to fluidly connect the gas inlet 302 to each of the plurality of bearing surfaces 312.[000164] The gas inlet 302 and channels 304 may be provided within a support 301.[000165] In use, the gas inlet 302 may be connected to a gas supply system. In use, the gas inlet 302 is supplied with pressurised gas (for example via one or more flexible tubes).[000166] The air bearing 300 further comprises a plurality of bearing members 310, each bearing member 310 comprising a body 311 and defining one of the plurality of bearing surfaces 312, and wherein the body 311 of each bearing member 310 defines a gas manifold 308 comprising an inlet 307 and configured to evenly distribute gas received at the inlet 307 over the bearing surface 312 defined by the body.[000167] Each bearing member 310 may be considered to be a support underlying one of the bearing surfaces 312. In use, the inlet 307 of each manifold 308 of each bearing member may be connected to a gas supply system. In use, the inlet 307 of each manifold 308 of each bearing member 310 is supplied with pressurised gas through flexible tubes 306. The gas may be air.[000168] Each bearing member is connected to the support via flexures 206. Flexures 206 are notch flexures. The flexures 206 allow bearing surfaces 312 to rotate in an analogous manner to that described with reference to Figure 6.[000169] Although the bearing members 310 of Figure 6 create a gas film by means of a manifold 308, in an alternative, the bearing members may be composite members comprising a first portion that defines a bearing surface and a second portion that supports the first portion. The first portion defining the bearing surface may be substantially composed of porous graphite or another porous material. In general, the first and second portions may be formed from different materials.[000170] It will be appreciated that the gas inlet may comprise a plurality of gas inlets. For example, in some embodiments, one gas inlet may be provided for each of the plurality of bearing surfaces. For such embodiments, one or more channels may be configured to fluidly connect each gas inlet to a corresponding one of the plurality of bearing surfaces.[000171] In order to facilitate proper operation of an air bearing, the bearing surfaces must be accurately-positioned in relation to the shaft. In an example, the bearings surfaces may be separated from the shaft by between 10 and 12 pm.[000172] In use, it may be desirable to adjust the bearing surfaces relative to their support. In some embodiments adjustments to within ±1 pm may be desirable. There are a number of alternative adjustment methods. In some embodiments bearing surface adjustment may be effected by disassembling the bearing, progressively machining components of the bearing toward their final configurations and inspecting the assembled bearing. On inspection, further disassembly, machining and inspection may be necessary. Thus the desired bearing dimensional configuration may be arrived at iteratively.[000173] Alternatively, the bearing surfaces may be adjusted by means of shims and / or bearing adjustment mechanisms. In an example, two or more of the bearing surfaces may be supported via an axially-moveable shaft allowing adjustment of the bearing surface position. Note that, such shafts extend, and are axially-movable, in a direction that is perpendicular to the axis of the shaft supporting the reticle masking blade. The proper bearing surface position may be fixed by means of friction from a bolt.[000174] In other embodiments, the air gap dimensions may be adjusted by means of separate assemblies comprising the bearing surfaces. The assemblies may be glued into the support. In an example, the glued assemblies may be positioned in a desired position and then supplied with pressurized liquid glue. Simultaneously, the bearing surfaces may be pressurized to a nominal, in use, gas pressure. The resulting bearing configuration will mimic nominal bearing pretension and have a desired bearing surface position. This bearing surface position is then fixed by curing the glue.[000175] Figures 8A and 8B schematically depict an axial cross-section of an embodiment of a reticle masking system 400.[000176] A lithographic apparatus LA shown in Figure 1 and described above may comprise reticle masking system 400.[000177] Figure 8A illustrates the reticle masking system 400 at rest and / or in an undeflected state. The reticle masking system 400 comprises a shaft 402, a reticle masking blade 404 (which may be a masking blade 32, 34, 36, 38 of the type described above with reference to Figures 2A to 4) and a bearing 406 according to an above-described embodiment (e.g. bearing 100, bearing 200 or air bearing 300). The shaft 402 is received in the apertures defined by the bearing surfaces 410 of the bearing 406 and bearing 408 and supported thereby. The reticle masking blade 404 is coupled to an end of shaft 402.[000178] The reticle masking system 400 further comprises a second bearing 408. The shaft 402 is further supported by the second bearing 408. Supporting the reciprocating shaft 402 at two points translationally constrains the reciprocating shaft in directions normal to the nominal axis of the shaft 402, as is desired.[000179] The shaft 402 is configured to reciprocate within the apertures defined by bearing surfaces 410 of bearing 406 and bearing 408. By extension, the reticle masking blade 404 is configured to reciprocate in tandem with the shaft. The reticle-masking system 400 can operate at high speeds and accelerations of the reticle masking blade 404.[000180] The effect of high speed and / or high acceleration of reticle masking system 400 is illustrated in Figure 8B. Under high speed and / or high acceleration, the shaft 402 may deflect normal to the shaft nominal axis 110 (cf. Figure 8A). The bearing surfaces 410 rotate by an angle 0 to conform to the deflection-induced local rotation of the shaft 402. It should be noted that the angle 0 shown in Figure 8B is exaggerated for the purposes of illustration. The rotation of the bearing surfaces assists the maintenance of an adequate gas film between the bearing surfaces and reciprocating shaft. Without the rotation of bearing surfaces 410, the shaft 402 may impinge the bearing surfaces 410.[000181] The reticle masking system 400 also obviates the need for additional mechanisms configured to restrict rotation of the reticle masking blades about the nominal axis of the shaft 402, reducing the footprint of the reticle masking system.[000182] In some embodiments of the reticle masking system, the bearing may be of a different type. Generally, at least one of the bearings should be according to an above-described embodiment - i.e. bearings restricting shaft rotation about the nominal axis.[000183] Figure 9 depicts a cross-sectional view of part of a reticle masking system 500 which comprises a shaft 502, a first bearing 506, a second bearing 508 and a linear motor 520. The shaft 502 itself comprises a first portion 503 having a square cross-section and a second portion 504 having a round cross-section. The first portion 503 is received by a square aperture of the first bearing 506. The second portion 504 is received by a cylindrical aperture of second bearing 508.[000184] Although omitted from Figure 9, the shaft 502 is configured to be coupled with a reticle masking blade.[000185] The first bearing 506 comprises four rotatable bearing surfaces 510, at least partially defining the square aperture. The first bearing 506 may be similar to bearing 100, bearing 200 or air bearing 300.[000186] The second bearing 508 comprises a single bearing surface 512, defining the cylindrical aperture. The second bearing 508 is supported by a support 513 via a thin circumferential web of material 514.[000187] The thin circumferential web of material 514 is easily-deformable. The deformation of the thin circumferential web of material 514 allows the bearing surface 512 of the second bearing 508 to conform to local tilt of the second portion 504 of the reciprocating shaft 502 by rotating relative tothe support 513. The rotation of the bearing surface assists the maintenance of an adequate gas film between the bearing surfaces and reciprocating shaft.[000188] In an example, the thin circumferential web of material 514 may be at least around 0.2mm thick. In another example, the thin circumferential web of material 514 may be at least around 0.3mm thick. In some embodiments the thin circumferential web of material may have an inner diameter around 38mm and an outer diameter around 48mm. The thin circumferential web of material 514 may be substantially composed of a steel alloy (e.g. a stainless steel alloy) or titanium alloy. In general, the thin circumferential web of material 514 may be configured (by material or dimensional selection) to be between a factor of around 20 and 100 less rotationally stiff, relative to the bearing surface 512.[000189] The linear motor is rigidly coupled to the reciprocating shaft 502. In use, the linear motor is configured to induce linear, reciprocating motion of the shaft 502 and by extension a reticle masking blade.[000190] The second bearing 508 may be supported in an alternative arrangement 550 schematically illustrated in a front and a side view in Figures 14a and 14b, respectively. As shown in Figure 14a the second bearing 508 is suspended from a support frame 552 by three suspension arms 554a, 554b, 554c. The three suspension arms 554a, 554b, 554c have greater flexibility than the support frame 552 and project from the second bearing 508. The suspension arms 554a, 554b, 554c project from the second bearing in a generally tangential fashion.[000191] In use, the flexible suspension arms permit the bearing surface 558 of the second bearing 508 to conform to local tilt of the second portion 504 of the reciprocating shaft 502 by rotating relative to the support 552.[000192] In some embodiments, the suspension arms may be flexures. In an example the flexures may be leaf spring-like metal elements.[000193] A method of manufacturing the second bearing 508 and its support arrangement 550 may comprise milling at least a portion second 508 and the support arrangement 550 from a single piece of material, simplifying manufacture.[000194] The bearing surface 558 may be a porous graphite component in fluid communication with a gas supply (not shown).[000195] In some embodiments, the second portion of the shaft may also be non-circular, being received by a corresponding aperture of the second bearing. In such embodiments, any (slight) rotation of the front bearing about the nominal axis would cause a conflict in alignment with the second bearing (e.g. causing a reduction of air gap in air bearings). As a result, such embodiments with non-circular first and second shaft portions may need the reticle masking system and its components to have increased accuracy, precision, stability and rigidity to function optimally. Thus the circular cross section of the second portion, simplifies the fabrication of the reticle masking system.[000196] Although reticle masking system 500 comprises bearings which are air bearings, it will be appreciated that in some embodiments, the one or more bearings (e.g. second bearing 508) need notbe air bearings and may be oil-lubricated bearings (although these may pose a greater contamination risk in a lithography application) or roller bearings, for example.[000197] Whilst the air bearings described with reference to Figures 5A-8B comprise four independently displaceable bearing surfaces, wherein each bearing can rotate only in axes perpendicular to the nominal axis of the shaft (e.g. bearing surfaces 102 of bearing 200). It will be appreciated that effective prevention rotation of a shaft about its nominal axis may be achieved by alternative arrangements.[000198] Figure 10 schematically illustrates a bearing 1000 in transverse cross-section according to an embodiment of the present disclosure. In a preferred embodiment, bearing 1000 is an air bearing. [000199] The bearing 1000 comprises a support 1101 and four bearing surfaces 1002, 1202a, 1202b, 1402 supported by the support 1101. The bearing surfaces are configured to at least partially define a square aperture 1004 for receipt of a shaft 1008. The aperture 1004 has a finite order of rotational symmetry (i.e. order 4) about a centroid 1006 of the aperture 1004.[000200] The bearing surfaces are arranged in (two) pairs of generally opposed bearing surfaces. Bearing 1000 comprises a first pair of bearing surfaces and second pair of bearing surfaces. The first pair of bearing surfaces comprises bearing surface 1002 and bearing surface 1202a. The second pair of bearing surfaces comprises bearing surface 1402 and bearing surface 1202b. The bearing surfaces of each pair of bearing surfaces are separated in a first direction. Bearing surface 1002 and bearing surface 1202a of the first pair of bearing surfaces are separated in a first direction 1005. Bearing surface 1402 and bearing surface 1202b of the second pair of bearing surfaces are separated in a first direction 1405. [000201] The central axis of aperture 1004, as defined by the plurality of bearing surfaces when the plurality of bearing surfaces are disposed in nominal positions or orientations, defines a nominal axis of the shaft.[000202] One of the plurality of bearing surfaces, bearing surface 1002 is only rotatable about a rotation axis 1012 parallel to a second direction that is generally perpendicular to the first direction 1005 and the nominal axis of the shaft. Put alternatively, bearing surface 1002 has one (rotational) degree of freedom. Bearing surface 1002 is connected to the support 1101 by connecting means 1014. It will be appreciated that bearing surface 1002 is analogous to the bearing surfaces (e.g. bearing surfaces 102 of Figure 6) described above with reference to Figures 5A-8B. As such, it may incorporate any of the details of construction described in connection with those bearing surfaces - for example, connecting means 114 may comprise one or more (notch) flexures,[000203] In alternative embodiments, two or more bearing surfaces of the plurality of bearing surfaces may have one degree of rotational freedom only.[000204] Bearing surface 1402 has two degrees of rotational freedom - bearing surface 1402 is capable of tilting about the y- and z- axes. Put alternatively, bearing surface 1402 is rotatable about either of two axes perpendicular to the first direction 1405 of the first pair of bearing surfaces. In some embodiments, the bearing surface 1402 is connected to the support 1101 via a connecting means 1414.The connecting means may be a cardan hinge. The cardan hinge may comprise one or more flexures. In other embodiments the bearing surface 1402 may be connected to the support by other means allowing two degrees of (rotational) freedom, such as a ball joint.[000205] Bearing surfaces 1202a and 1202b are biased bearing surfaces. Each pair of bearing surfaces pair comprises a rotatable bearing surface and a biased bearing surface.[000206] Bearing 1000 comprises two biased bearing surfaces 1202a, 1202b, which are each urged toward the centroid of the aperture 1006 by biasing means 1214. In use, biased bearing surfaces 1202a, 1202b push against the shaft 1008 toward the opposed bearing surfaces 1002 and 1402, respectively. It should be noted that the plurality of bearing surfaces, 1002, 1202a, 1202b, 1402 are not configured to contact the shaft 1008 directly - any forces are transmitted via an air film between the bearing surfaces and the shaft 1008.[000207] The biasing means 1214 of the two biased bearing surfaces 1202a, 1202b may comprise a pneumatic and / or mechanical biasing means. Pneumatic biasing means may comprise pneumatic pistons. Mechanical biasing means may comprise helical springs, wave washers or elastomeric elements. Alternatively, or additionally, the biasing means 1214 may comprise electromagnetic biasing means.[000208] It will be further appreciated that the biased bearing surfaces 1202a, 1202b are mounted to the support 1101 at least in part by the biasing means 1214. The biased bearing surfaces 1202a, 1202b may advance towards or retract from the centroid 1006.[000209] In use, the bearing 1000 can conform to deflections of the reciprocating shaft 1008, maintaining a uniform fluid (air) film. The bearing surface 1002 can tilt in response to deflections or local rotation of the shaft about the x-direction. The pressure provided by biased bearing surface 1202a constrains the shaft 1008 against the bearing surface 1002. Bearing surface 1002 only has a single axis of rotation 1012, and so the shaft 1008 is substantially prevented from rotating about the y-axis. The bearing surface 1402 can also tilt in response to deflections or local rotation of the shaft about the z- direction. In addition, bearing surface 1402 can tilt in the y-direction. The two degrees of freedom of bearing surface 1402 prevents over-constraint of the shaft 1008, allowing the bearing surface 1402 to conform with variations in the dimensions of the shaft. Because the biased bearing surfaces 1202a, 1202b can advance and retract, the aperture 1004 can change its extent in the first directions 1405 and 1005, allowing use of the bearing 1000 with shafts of looser dimensional tolerances.[000210] The position of the biased bearing surface 1202a, 1202b is controlled by the force exerted by the air film between the shaft 1008 and the bearing surface. For a given air pressure, the relative position of the shaft and biased bearing surface (i.e. their separation) is approximately constant. The separation is substantially invariant / independent of the absolute positions of the components, within a movement envelope of the main piston. Under any disturbance of the separation, the bearing surface will reposition itself at the correct separation. The biased bearing surfaces 1202a, 1202b automatically compensate for inaccuracies in the bearing and / or shaft dimensions.[000211] An embodiment of the bearing surface 1402 and its connecting means 1414 is schematically illustrated in Figure 11. The connecting means 1414 comprises a first flexure 1416, configured to allow rotation about the z-axis, and a second flexure 1418, configured to allow rotation about the y-axis. In combination, the first and second flexures 1416, 1418 form a cardan hinge connecting to bearing surface 1402. Bearing surface 1402 comprises a plurality of holes 1420 to provide, in use, an air film between the bearing surface 1402 and the shaft 1008. In addition, the bearing surface 1402 and its connecting means 1414 are very stiff in the x-direction. In conjunction with the biased bearing surface 1202b, the shaft 1008 is accurately constrained, or located, in its x position.[000212] An embodiment of the biasing means is described with reference to Figures 12a and 12b. Biasing means 1214 comprises a spring 1222, a sub piston 1224, and a main piston 1226, The sub piston 1224 is urged by the spring 1222 (toward the centroid of the aperture). The main piston is connected to the biased bearing surface 1202a. The spring 122, sub-piston 1224 and main piston 1226 are captive within a bore 1228 provided in the support 1101. The main piston 1226 and sub-piston 1224 define an expansible pneumatic chamber 1230 in conjunction with the bore 1228. The biasing means 1214 is configured to be operable between a first (or transport) and a second (or operational) configuration.[000213] Spring 1222 may be substituted for other biasing means - e.g. helical springs, wave washers, cone washers, elastomeric elements and / or other mechanical biasing means. In other embodiments the spring 1222 may be substituted for any passive biasing means.[000214] Figure 12a shows the biasing means 1214 in the first configuration, wherein the spring 1222 urges the main piston 1226 toward the aperture centroid via the sub-piston 1224. In the first configuration the main piston 1226 and the connected biased bearing surface are urged without the need for any external source of energy. Advantageously during transportation of a reticle-masking system, the tension provided by the spring allows a shaft to be securely held by the bearing passively.[000215] Figure 12b shows the biasing means 1214 in the second configuration, wherein the pneumatic chamber 1230 is pressurised, retracting the sub-piston 1224, and the pressure urges the main piston 1226 toward the aperture centroid by pneumatic pressure. The pneumatic chamber 1230 may be pressurised and expanded by pressurised air, supplied by means of an air conduit or inlet 1232. The expansion of the chamber 1230 separates the sub-piston 1224 and the main piston 1226. The force exerted by the piston is controlled by the cross sectional area of the bore 1228 and the pressure in chamber 1230. In some embodiments the pressure in the pneumatic chamber 1230 in the second configuration may be around 6 bar(a).[000216] Figure 13 shows a more detailed schematic cross-section view of the main piston 1226. The piston 1226 comprises a piston seal ring 1234 held captive in a circumferential groove 1238 of the piston 1226. The piston seal ring 1234 seals against the bore 1228, providing a reliable pneumatic seal in use. In addition, the piston seal ring allows the use of a slightly undersized main piston 1226, such that there is a clearance (indicated by A in Figure 13) between the main piston 1226 and the bore 1228.The clearance A can provide a degree of compliance to the biased bearing surface 1202. Put alternatively the biased bearing surface 1202a can tilt. Advantageously, the piston seal ring 1234 seals the main piston 1226 under any orientation (e.g. tilt about the x and / or y-axes) - the biased bearing surface 1202a can also conform to deflections of the shaft 1008 whilst maintaining the compressive force on the shaft. [000217] The pneumatic chamber 1230 is fluidly connected to the inlet 1232 and the biased bearing surface 1202a. The pneumatic chamber acts as a reservoir of pressurised air to supply the air film (via gas manifold 1203) between the biased bearing surface 1202a and the shaft 1008.[000218] Main piston 1226 further comprises a flange 1236 which is configured to prevent overinsertion of the main piston.[000219] Although the main piston 1226, and by extension the biased bearing surface 1202a, can tilt about the x and y-axes, it is undesirable for the biased bearing surface 1226 to rotate axially - that is, about the z-axis. The main piston 1226 further comprises one or more (e.g. two) of anti-rotation lugs 1238 which interface with complementary recesses 1239 defined in the support 1101. The anti-rotation lugs 1238 locate into the complementary recesses 1239 and resist rotation of the main piston 1226 about the z-axis. Anti-rotation lugs and complementary recesses are examples of complementary anti-rotation features.[000220] In some embodiments, the sub piston 1224 may also comprise a piston seal ring in order to provide a pneumatic seal against the bore 1228.[000221] The embodiment of the biasing means described with reference to Figures 12a, 12b, and 13 is described with respect to biased bearing surface 1202a of the first pair of bearing surfaces. It will be appreciated that a substantially similar biasing means may also connect biased bearing surface 1202b of the second pair of bearing surfaces to the support 1101.[000222] A lithographic apparatus LA may comprise reticle masking system 500. References to a mask or reticle in this document may be interpreted as references to a patterning device (a mask or reticle is an example of a patterning device) and the terms may be used interchangeably. In particular, the term mask assembly is synonymous with reticle assembly and patterning device assembly.[000223] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000224] The term “EUV radiation” may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.[000225] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.[000226] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A bearing comprising : a support; and a plurality of bearing surfaces supported by the support and configured to at least partially define an aperture for receipt of a shaft, the aperture having a finite order of rotational symmetry about a centroid of the aperture; wherein at least some of the bearing surfaces are configured so as to be independently displaceable relative to the support so as to allow a shaft received in the aperture to rotate about one or more axes of rotation orthogonal to a nominal axis of the shaft whilst substantially preventing rotation of the shaft about its nominal axis.

2. The bearing of claim 1 wherein the plurality of bearing surfaces comprises at least one bearing surface that is constrained so as to substantially prevent rotation of the at least one bearing surface relative to the support about the nominal axis of the shaft.

3. The bearing of claim 1 or 2 wherein the plurality of bearing surfaces comprises only one bearing surface that is constrained so as to substantially prevent rotation of the at least one bearing surface relative to the support about the nominal axis of the shaft.

4. The bearing of claim 2 or 3 wherein the plurality of bearing surfaces are arranged in one or more pairs of generally opposed bearing surfaces and the bearing surfaces of each pair of bearing surfaces are separated in a first direction.

5. The bearing of claim 4 when dependent on claim 2 wherein the bearing surfaces of each pair of bearing surfaces are each rotatable about a rotation axis, the rotation axes of the two bearing surfaces of a given pair of bearing surfaces being parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft.

6. The bearing of claim 4 wherein each pair of bearing surfaces comprises a rotatable bearing surface and a biased bearing surface.

7. The bearing of claim 6 wherein the rotatable bearing surface of at least one of the pairs is only rotatable about an axis parallel to a second direction that is generally perpendicular to the first direction and the nominal axis of the shaft.8 The bearing of claim 6 or 7 wherein the rotatable bearing surface of at least one of the pairs is rotatable about either of two axes perpendicular to the first direction of the pair of bearing surfaces.

9. The bearing of any of claims 6-8 wherein each biased bearing surface is urged toward the centroid of the aperture by one or more biasing means.

10. The bearing of claim 9 wherein the one or more biasing means of the one or more biased bearing surfaces comprise a pneumatic and / or mechanical biasing means.

11. The bearing of claim 10 wherein the one or more biasing means comprises a pneumatic piston.

12. The bearing of claim 10 or 11 wherein the one or more biased bearing surfaces are urged toward the centroid of the aperture by mechanical means in a first configuration, and by pneumatic means in a second configuration.

13. The bearing of any preceding claim wherein the bearing comprises two pairs of bearing surfaces, whose rotation axes are mutually orthogonal.

14. The bearing of any preceding claim wherein the bearing is an air bearing.

15. The bearing of any preceding claim further comprising a gas inlet for connection to a gas supply and one or more channels configured to fluidly connect the gas inlet to each of the plurality of bearing surfaces.

16. The bearing of any preceding claim further comprising a plurality of bearing members, each bearing member comprising a body and defining one or the plurality of bearing surfaces, and wherein the body of each bearing member defines a gas manifold comprising an inlet and configured to evenly distribute gas received at the inlet over the bearing surface defined by the body.

17. The bearing of any preceding claim wherein each of the bearing surfaces is connected to the support via a biasing mechanism that is arranged to bias the bearing surface towards a nominal position or orientation.

18. The bearing of any preceding claim wherein at least one of the bearing surfaces is connected to the support via one or more flexures.

19. The bearing of any preceding claim, wherein at least one of the bearing surfaces have one degree of freedom.

20. The bearing of claim 17 or any claim depending therefrom, wherein the biasing mechanism of at least one of the bearing surfaces are substantially composed of titanium or ferrous alloys.

21. The bearing of any preceding claim wherein the bearing surfaces are substantially planar.

22. A reticle-masking system of a lithographic apparatus, the reticle-masking system comprising: a shaft; a reticle masking blade, configured to partially define an optical aperture; and a bearing according to any preceding claim; wherein the shaft is received in the aperture defined by the bearing surfaces of the bearing and supported thereby.

23. The reticle-masking system of claim 22 wherein the shaft is at least partially non-circular in cross-section.

24. The reticle-masking system of claim 22 or 23, when dependent on claim 12 wherein the biasing means comprises: a spring; an expansible pneumatic chamber; further wherein the biasing means is configurable between: the first configuration, wherein the spring is configured to urge the biased bearing surface toward the aperture centroid through the pneumatic chamber; and the second configuration, wherein the pneumatic chamber is pressurised, retracting the spring, and the pneumatic pressure of the pneumatic chamber urges the biased bearing surface toward the aperture centroid.

25. The reticle-masking system of claim 24, wherein the biasing means further comprises: a sub piston, urged by the spring; a main piston, connected to the biased bearing surface; wherein the spring, sub-piston and main piston are captive within a bore provided in the support, and the main piston and sub-piston define the expansible pneumatic chamber in conjunction with the bore; and further wherein in the first configuration the sub piston and the main piston are in contact; and in the second configuration the sub piston and the main piston are separated by pneumatic pressure.

26. The reticle-masking system of claim 25 wherein the main piston and / or the sub-piston further comprise a piston seal ring, configured to seal against the bore.

27. The reticle-masking system of claim 25 or 26 wherein the pneumatic chamber is fluidly connected to the gas inlet and the biased bearing surface.

28. The reticle-masking system of any of claims 22 to 27 wherein the shaft is further supported by a second bearing.

29. The reticle-masking system of claim 28 wherein the shaft comprises a first portion having a non-circular cross-section, received by the bearing and a second portion having a circular cross-section, received by an aperture of the second bearing.

30. The reticle masking system of claim 29 wherein the second bearing is supported by a thin circumferential web of material.

31. The reticle masking system of claim 29 wherein the second bearing is suspended from a support frame by a plurality of suspension arms having greater flexibility than the support frame, projecting from the second bearing.

32. The reticle masking system of claim 31 wherein the plurality of suspension arms project from the second bearing in a generally tangential fashion.

33. The reticle masking system of claim 31 or 32 wherein the second bearing is supported by three flexures.

34. The reticle-masking system of any of claims 22 to 33 wherein the reticle-masking system further comprises a gas supply system, configured to supply gas to each of the plurality of bearing surfaces of the bearing.

35. A lithographic apparatus comprising the reticle-masking system of any one of claims 22 to 34.

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