TW202501171A - Bearing - Google Patents

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

Bearings

The present invention relates to a bearing which can be suitable for use in a lithography apparatus. The lithography apparatus can be an extreme ultraviolet (EUV) lithography apparatus.

A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithography apparatus may be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithography apparatus may project a pattern from a patterning device (such as a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

The wavelength of radiation used by a lithography apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on that substrate. Lithography apparatus using EUV radiation, which is electromagnetic radiation having a wavelength in the range of 4 to 20 nm, can be used to form smaller features on a substrate than conventional lithography apparatus (which may, for example, use electromagnetic radiation having a wavelength of 193 nm).

A patterning device (eg, a mask) for imparting a pattern to a radiation beam in a lithography apparatus may form part of a mask assembly.

The zoom mask shield blade can be used to define a generally rectangular area of the patterned device that can receive radiation. In use, the zoom mask shield blade can undergo linear motion. This linear motion can be transmitted to the zoom mask shield blade via one or more shafts. One or more shafts can be supported by bearings. In use, each shaft can move relative to the bearing in a direction generally parallel to its axis while being supported by the bearing. Each shaft and the zoom mask shield blade supported thereby can withstand high accelerations in use. Accurate positioning of the zoom mask shield blade is required to ensure its proper operation.

It may be desirable to provide a device that prevents or mitigates one or more of the problems associated with the prior art.

According to a first aspect of the present disclosure, a bearing is provided, comprising: a support member; and a plurality of bearing surfaces supported by the support member and configured to at least partially define a void for receiving a shaft, the void having a finite rotational symmetry order about a center of mass of the void; wherein at least some of the bearing surfaces are configured so as to be independently displaceable relative to the support member so as to allow a shaft received in the void to rotate about one or more rotational axes orthogonal to a nominal axis of the shaft while substantially preventing the shaft from rotating about its nominal axis.

The term displaceable should be used to indicate the ability to move in translation and/or rotation.

In some embodiments, the bearing surface may be configured so as to be independently rotatable relative to the support. In other alternative embodiments, the bearing surface may be configured so as to be exclusively rotatable relative to the support.

The shaft may be a reciprocating shaft which, in use, may move relative to the bearing in a direction generally parallel to its axis while being supported by the bearing. Under high accelerations, such a reciprocating shaft may experience deflections in a direction orthogonal to the nominal axis of the shaft. Deflections of the reciprocating shaft may cause local rotation or local tilting of the shaft at the bearing. For example, the shaft may support a multiplying reticle shielding blade of a lithography apparatus. Rotation of the multiplying reticle shielding blade about the nominal axis of the shaft is undesirable.

Advantageously, the rotational freedom provided to the bearing surface of the bearing according to the first aspect allows the bearing surface to conform to the local tilt of the reciprocating shaft by rotating. The rotation of the bearing surface reduces the non-uniformity (i.e., local concentration) of the load across the bearing surface. In the event that a fluid film exists between the bearing surface and the shaft, the rotation of the bearing surface assists in maintaining sufficient film between the bearing surface and the reciprocating shaft. This allows the film to support the reciprocating shaft without excessive contact between the reciprocating shaft and the bearing surface.

The nominal axis of the shaft may be the axis of the aperture defined by the plurality of bearing surfaces when the plurality of bearing surfaces are disposed in a nominal position or orientation.

Using a right-hand Cartesian set of coordinates, if the nominal axis of the shaft is parallel to the z-axis, at least some of the bearing surfaces may be rotatable 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 axis of the shaft (i.e., the shaft will no longer be perfectly parallel to the z-axis).

The bearing according to the first aspect tolerates deflections of the reciprocating shaft, which allows the reciprocating shaft to withstand higher accelerations and speeds.

Bearings according to the first aspect require less stringent manufacturing tolerances, thereby making the bearings less expensive to manufacture.

The plurality of bearing surfaces may be arranged in one or more pairs of substantially opposed bearing surfaces, and the bearing surfaces of each pair of bearing surfaces may be spaced apart in a first direction and may each rotate about an axis of rotation. The axes of rotation of both bearing surfaces of a given pair of bearing surfaces may be parallel to a second direction substantially perpendicular to the first direction and the nominal axis of the shaft.

Opposing bearing surfaces rotatable about the shaft in a second direction (i.e., parallel to the axis of rotation) are able to remain parallel to each other when rotated together (e.g., in response to local tilt of the reciprocating shaft). The prismatic geometry of the pores is preserved when the pair of bearing surfaces rotate together, thereby allowing uniform film thickness to be maintained, facilitating operation of the bearings. In particular, if each pair of substantially opposing bearing surfaces is rotated about their respective axes of rotation by the same angle, then because both axes of rotation are parallel to the second direction, the shape of the pores at least partially defined by the bearing surfaces will be the same, although scaled by a factor of the cosine of the angle in the first direction.

The plurality of bearing surfaces may include at least one bearing surface that is constrained so as to substantially prevent the at least one bearing surface from rotating relative to the support member about a nominal axis of the shaft.

The plurality of bearing surfaces may include only one bearing surface that is constrained so as to substantially prevent at least one bearing surface from rotating relative to the support member about a nominal axis of the shaft.

Constraint of only one bearing prevents unwanted rotation of the shaft about its nominal axis and avoids overconstraint of the bearing.

The plurality of bearing surfaces may be arranged in one or more pairs of substantially opposing bearing surfaces, and the bearing surfaces of each pair of bearing surfaces are spaced apart in a first direction.

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

The axis of rotation of the bearing surface is such that it is substantially unable to rotate about an axis parallel to the nominal axis of the shaft.

Each pair of bearing surfaces may include a rotatable bearing surface and an offset bearing surface.

The combination of a rotatable bearing surface and an offset bearing surface allows the bearing to effectively constrain the rod without requiring close tolerances between the bearing surface and the rod.

The rotatable bearing surface of at least one of the pairs can rotate only about an axis parallel to a second direction that is substantially perpendicular to the first direction and the nominal axis of the shaft.

The rotatable bearing surface of at least one of the pairs can rotate about either of two axes perpendicular to the first direction of the pair of bearing surfaces.

The rotatable bearing surface of at least one of the bearing surface pairs may be connected to the support member via a universal hinge. The universal hinge may include one or more flexures.

Advantageously, the bearing surfaces hold the shaft in place without adding additional rotational restraint, which might require finer tolerances on the bearing and shaft.

Each offset bearing surface may be urged toward the centroid of the aperture by one or more offset members.

In use, the offset bearing surface is pushed toward the shaft, thereby allowing the bearing surface to adapt to changes in shaft and bearing size.

Each of the one or more offset bearing surfaces may include means for preventing rotation of the offset bearing surface along the axis of rotation in a first direction. The means for preventing rotation of the offset bearing surface may include complementary anti-rotation features.

One or more biasing members of one or more biased bearing surfaces may include pneumatic biasing members and/or mechanical biasing members.

The one or more biasing members may include a pneumatic piston.

One or more offset bearing surfaces may be urged toward the center of mass of the aperture by a mechanical component in a first configuration and by a pneumatic component in a second configuration.

In a first configuration, the bearing may be disconnected from the external pneumatic pressure source, such as during transportation. In a second configuration, the bearing may be connected from the external pneumatic pressure source, such as during operation of the bearing.

Advantageously, during transportation of the bearing, the tension provided by the mechanical member allows the shaft to be passively and securely held by the bearing without the need for external power.

The mechanical biasing member may include a coil spring, a wave washer or a resilient element.

The bearing may comprise two pairs of bearing surfaces whose axes of rotation are orthogonal to each other.

Two pairs of bearing surfaces allow the bearings to maintain adequate film in response to deflection of the reciprocating shaft in any direction perpendicular to the nominal shaft axis. It should be understood that other configurations with more pairs of bearing surfaces are possible (e.g., 3, 4, 5, 6, 7 or more), but such configurations may be more mechanically complex.

The bearing may be an air bearing.

In particular, the bearing may be a hydrostatic bearing. Air bearings operate with low friction, vibration and high precision. By conforming to the local tilt of the reciprocating shaft, the air bearing of the present disclosure can maintain a sufficient air film across the bearing surface. The sufficient air film can support the reciprocating shaft without any contact between the reciprocating shaft and the bearing surface, thereby minimizing wear and friction.

The bearing may further include a gas inlet for connecting to a gas supply and one or more channels configured to fluidly connect the gas inlet to each of the plurality of bearing surfaces.

In use, the gas inlet may be connected to a gas supply system. In use, the gas inlet may be supplied with pressurized gas (e.g., via one or more flexible tubes). It will be appreciated that the gas inlet may include a plurality of gas inlets. For example, in some embodiments, a gas inlet may be provided for each of a plurality of bearing surfaces. For such embodiments, one or more channels may be configured to fluidically connect each gas inlet to a corresponding bearing surface of the plurality of bearing surfaces.

The bearing may further include a plurality of bearing components, each bearing component including a body and defining one of a plurality of bearing surfaces. The body of each bearing component may define a gas manifold including an inlet and configured so that gas received at the inlet is evenly distributed over the bearing surface defined by the body.

Each bearing component may be considered as a support member below one of the bearing surfaces. The support member may further comprise a manifold, which is supplied with gas via a flexible tube. In use, the inlet of each manifold of each bearing component may be connected to a gas supply system. In use, the inlet of each manifold of each bearing component may be supplied with pressurized gas (e.g. via a flexible tube). The gas may be air.

The bearing component may be a composite component including a first portion defining a bearing surface and a second portion supporting the first portion. The first portion defining the bearing surface may be substantially composed of porous graphite. In general, the first portion and the second portion may be formed of different materials.

Each of the bearing surfaces may be connected to the support via a biasing mechanism configured to bias the bearing surface toward a nominal position or orientation.

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

At least one of the bearing surfaces may be connected to the support member via one or more flexure members.

Advantageously, the flex member is a single component and therefore does not exhibit significant friction or backlash. The flex member may be in the form of a leaf spring or any other type of flex member. The flex member may be a "notch" flex member. The flex member may be mounted to a support member.

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

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

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

A bearing surface with one degree of freedom inherently constrains rotation to an axis of rotation orthogonal to the reciprocating direction of the shaft without requiring additional mechanisms or structures. In one example, a bearing surface connected to a support member via a notched flexure has one degree of freedom.

The bearing surfaces may have a range of rotation of up to 3 milliradians. In one example, at least one of the bearing surfaces may have a range of rotation of ±1.5 milliradians. The air bearing may further include structure (e.g., a safety lever or a physical stop) configured to limit rotation of the bearing surface to a desired range. Additionally or alternatively, the air bearing may include structure to further support the bearing surface while still allowing the desired rotation.

The biasing mechanism of at least one of the bearing surfaces may consist essentially of titanium or an iron alloy.

In some embodiments, each of the bearing surfaces may consist essentially of titanium or an iron alloy.

The iron alloy may include stainless steel or carbon steel. Advantageously, titanium alloy and iron alloy biasing mechanisms (e.g., bends) have a long life due to their high fatigue limits. Generally speaking, biasing mechanisms may be made of any material with high fatigue strength and/or fatigue limits.

The bearing surface may be substantially planar.

A flat bearing surface allows the reciprocating rod to tilt freely about an axis normal to its plane.

The bearing surface may alternatively be curved (ie, convex, concave, or a combination thereof) in a direction transverse to the reciprocating direction of the shaft (ie, the nominal axis of the shaft).

According to a second aspect, a multiplying mask shielding system for a lithography apparatus is provided, the multiplying mask shielding system comprising: a shaft; a multiplying mask shielding blade configured to partially define an optical aperture; and the bearing of the first aspect; wherein the shaft is received in the aperture defined by and supported by a bearing surface of the bearing. The bearing may include any of the optional features described above.

The shaft is configured to reciprocate within the bearing aperture. The doubling mask shielding system according to the second aspect can operate at high speeds and accelerations of the doubling mask shielding blades. The doubling mask shielding system according to the second aspect also avoids the need for an additional mechanism configured to limit the rotation of the doubling mask shielding blades around the reciprocating direction of the shaft, thereby reducing the footprint of the doubling mask shielding system.

The shaft may be at least partially non-annular in cross-section.

Non-annular apertures and/or shafts will prevent rotation in the shaft reciprocating direction.

The biasing member may include: a spring; an expandable pneumatic chamber; and wherein the biasing member may be configured between: a first configuration, wherein the spring is configured to push the biased bearing surface toward the center of mass of the pore through the pneumatic chamber; and a second configuration, wherein the pneumatic chamber is pressurized, thereby retracting the spring, and the pneumatic pressure of the pneumatic chamber pushes the biased bearing surface toward the center of mass of the pore.

For a given gas pressure, the relative position of the shaft and the offset bearing surfaces (i.e., their separation) is approximately constant. Advantageously, the separation is substantially independent of the absolute position of the components within limits. With any disturbance in the separation, the bearing surfaces will reposition themselves at the correct separation to achieve force balance. The offset bearing surfaces automatically compensate for inaccuracies in the bearing and/or shaft dimensions.

The biasing member may further include: a sub-piston, which is pushed by a spring; a main piston, which is connected to the biased bearing surface; wherein the spring, the sub-piston and the main piston are confined in a hole set in the support member, and the main piston and the sub-piston together with the hole define an expandable pneumatic chamber; and further wherein in a first configuration, the sub-piston and the main piston are in contact; and in a second configuration, the sub-piston and the main piston are separated by pneumatic pressure.

The main piston and/or the sub-piston may further include a piston sealing ring configured to seal the hole.

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

The pneumatic chamber can be fluidly connected to the gas inlet and the offset bearing surface.

The shaft may be further supported by a secondary bearing.

Supporting the reciprocating shaft at two points constrains the translation of the reciprocating shaft in a direction perpendicular to the nominal axis of the shaft.

The shaft may include a first portion having a non-annular cross-section received by the bearing and a second portion having an annular cross-section received by the aperture of the second bearing.

In some embodiments, the second portion of the shaft may also be non-annular, being received by a corresponding aperture of the second bearing. In such embodiments, any (slight) rotation of the front bearing about the nominal axis will cause a conflict with the alignment of the second bearing (e.g. causing a reduction in the air gap in the air bearing). Therefore, such embodiments with non-annular first and second shaft portions may require the zoom mask shielding system and its components to have increased accuracy, precision, stability and rigidity to function optimally. Therefore, the annular cross-section of the second portion simplifies the manufacture of the zoom mask shielding system.

The secondary bearing may be supported by a thin annular web of material.

The thin annular web of material allows the bearing surface of the secondary bearing to conform to the local tilt of the reciprocating shaft by rotating. The rotation of the bearing surface assists in maintaining an adequate film between the bearing surface and the reciprocating shaft.

The secondary bearing may be suspended from the support frame by a plurality of suspension arms protruding from the secondary bearing and having greater flexibility than the support frame.

The second bearing suspension described above provides the required degree of compliance while being relatively simple to manufacture.

The secondary bearing may be supported by at least three flexures.

A plurality of suspension arms may project from the second bearing in a generally tangential manner.

The secondary bearing may be supported by three flexures.

The zoom mask shielding system may further include 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.

Providing a gas supply system allows an adequate air film to be maintained between the bearing surface and the reciprocating shaft.

According to a third aspect, a lithography apparatus is provided, which includes the multiplication mask shielding system of the second aspect. The multiplication mask shielding system may include any one of the optional features described above.

It should be understood that one or more aspects or features described above or mentioned in the following description may be combined with one or more other aspects or features.

FIG1 shows a lithography system. The lithography system comprises a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a multiplying mask assembly 15 (e.g., a multiplying mask or a shadow mask) including a patterning device MA, 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 the radiation beam B is incident on the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the patterning device MA) onto a substrate W. The substrate W may include a previously formed pattern. In this case, the lithography equipment aligns the patterned radiation beam B with the pattern previously formed on the substrate W.

The radiation source SO, the illumination system IL and the projection system PS may all be constructed and arranged so that they can be isolated from the external environment. A gas (e.g. hydrogen) at a pressure lower than atmospheric pressure 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 small amount of gas (e.g. hydrogen) at a pressure sufficiently lower than atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

The radiation source SO shown in FIG. 1 is of a type that may be referred to as a laser produced plasma (LPP) source. The laser 1, which may be, for example, a CO2 laser, is configured to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided from a fuel emitter 3. Although tin is mentioned 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 an alloy. The fuel emitter 3 may include a nozzle configured to guide tin, for example in the form of droplets, along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy is deposited into the tin to generate a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of ions in the plasma.

EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). Collector 5 may have a multi-layer structure configured to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). Collector 5 may have an elliptical configuration with two elliptical foci. The first focus may be at the plasma formation region 4, and the second focus may be at the middle focus 6, as discussed below.

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

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

The laser 1 may be separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable steering mirrors and/or beam expanders and/or other optical components. The laser 1 and the radiation source SO may be considered together as a radiation system.

The radiation reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at a point 6 to form an image of the plasma forming zone 4, which image serves 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 an intermediate focus. The radiation source SO is configured such that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

A radiation beam B is transmitted from a radiation source SO to an illumination system IL which is configured to condition the radiation beam. The illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the 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 is transmitted from the illumination system IL and is incident on a zoom mask assembly 15 held by a support structure MT. The zoom mask 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 zoom mask assembly 15 may be referred to as a zoom mask and pellicle assembly 15. The patterning device MA reflects and patterns the radiation beam B. In addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may comprise other mirrors or devices.

After 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 which are configured to project the radiation beam B onto a substrate W held by a substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, thereby forming an image having components that are smaller than corresponding components on the patterning device MA. For example, a reduction factor of 4 may be applied. Although in FIG1 the projection system PS has two mirrors 13, 14, the projection system PS may comprise any number of mirrors (e.g. six mirrors).

The lithography apparatus may, for example, be used in a scanning 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 the substrate W (i.e. dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the magnification and image inversion characteristics of the projection system PS. The patterned radiation beam incident on the substrate W may comprise a swath of radiation. The swath of radiation may be referred to as an exposure slit. During a scanning exposure, movement of the substrate table WT and the support structure MT may cause the exposure slit to travel across the exposure field of the substrate W.

The radiation source SO and/or the lithography apparatus shown in Fig. 1 may include components not shown. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may substantially transmit EUV radiation but substantially block radiation of other wavelengths, such as infrared radiation.

In other embodiments of the lithography system, the radiation source SO may take other forms. For example, in an alternative embodiment, the radiation source SO may include one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation that can be provided to one or more lithography devices.

As briefly described above, the reticle assembly 15 includes a pellicle 19 provided adjacent to the patterning device MA. The pellicle 19 is provided in the path of the radiation beam B so that the radiation beam B passes through the pellicle 19 both when it approaches the patterning device MA from the illumination system IL and when it is reflected by the patterning device MA towards the projection system PS. The pellicle 19 comprises a film or membrane that is substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation). In this context, EUV transparent pellicle or substantially transparent film for EUV radiation means that the pellicle 19 transmits at least 65% of the EUV radiation, preferably at least 80% and more preferably at least 90% of the EUV radiation. The pellicle 19 serves to protect the patterning device MA from particle contamination.

Despite efforts to maintain a clean environment inside the lithography apparatus LA, particles may still be present inside the lithography apparatus LA. In the absence of the pellicle 19, particles may be deposited on the patterning device MA. Particles on the patterning device MA may adversely affect the pattern imparted to the radiation beam B and therefore the pattern transferred to the substrate W. The pellicle 19 advantageously provides a barrier between the patterning device MA and the environment in the lithography apparatus LA so as to prevent particles from being deposited on the patterning device MA.

The pellicle 19 is positioned a distance from the patterning device MA that is sufficient so that any particles incident on the surface of the pellicle 19 are not in the field plane of the lithography apparatus LA. This spacing between the pellicle 19 and the patterning device MA serves to reduce the extent to which any particles on the surface of the pellicle 19 impart a pattern to the radiation beam B imaged onto the substrate W. It will be appreciated that in the event that a particle is present at a location in the radiation beam B but not in the field plane of the radiation beam B (e.g., not at the surface of the patterning device MA), then any image of the particle will not be focused 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 from the patterning device MA. However, in practice, the space available in the lithography apparatus LA to accommodate the pellicle is limited due to the presence of other components. In some embodiments, the spacing between the pellicle 19 and the patterning device MA may be, for example, approximately between 1 mm and 10 mm, such as between 1 mm and 5 mm, such as between 2 mm and 2.5 mm.

A description of some additional features of an example type of lithography apparatus LA, particularly some features and components proximate to the support structure MT, is now provided with reference to FIGS. 2A to 4 .

The support structure MT can be moved in the scanning direction to expose a larger area of the patterning device MA of the multiplying mask and pellicle assembly 15 in a single dynamic scanning exposure, as now discussed with reference to Figures 2A and 2B. Figures 2A and 2B show schematic plan views of the support structure MT and the multiplying mask and pellicle assembly 15 in two different positions.

The support structure MT is movably mounted in the region 24. In particular, the support structure MT is movable in the scanning direction indicated by the arrow 26 between a first end position (shown in FIG. 2A ) and a second end position (shown in FIG. 2B ).

Unless otherwise stated, throughout this specification, the following set of Cartesian coordinates will be used. The scanning direction is denoted as the y-direction. The direction that is also in the plane of the support structure MT and perpendicular to the scanning direction is called the non-scanning direction and is denoted as the x-direction. The direction perpendicular to the plane of the support structure MT is denoted as the z-direction.

The lithography apparatus LA may be considered to include a scanning module operable to move the support structure MT in a scanning direction between at least a first end position and a second end position. For example, the scanning module may be operable to move the support structure MT in the scanning direction relative to a support frame (schematically indicated by area 24), to which the support structure MT may be considered to be movably mounted.

The zoom mask and pellicle assembly 15 can be considered to include a central portion 15a and a peripheral portion 15b surrounding the central portion 15a. The central portion 15a can be referred to as an image forming portion and can overlap with a portion of the patterned radiation beam B of the zoom mask MA and the diaphragm 19. The peripheral portion 15b can overlap with a border portion of the pellicle 19 and the frame of the pellicle 19.

The movement of the support structure MT between the first position and the second position defines an extended first partial area 28 of the support structure MT, which is defined by all areas where the central portion 15a of the zoom mask and pellicle assembly 15 can be placed. That is, the extended first partial area 28 of the support structure MT is the area defined by moving the central portion 15a of the zoom mask and pellicle assembly 15 from the first end position (shown in FIG. 2A ) to the second end position (shown in FIG. 2B ).

The lithography apparatus LA is provided with four reticle shielding blades that define the extent of the field to be illuminated on the substrate W, as now described with reference to FIGS. 3A , 3B and 4 . The illumination system IL is operable to illuminate a region of the patterned device MA when the patterned device MA is disposed on the support structure MT. This region may be referred to as the aperture of the illumination system IL and is at least partially defined by the four reticle shielding blades that define a generally rectangular region of the patterned device that may 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 shielding 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 shielding blades 36, 38.

Each of the shielding leaves 32, 34, 36, 38 is arranged close to the patterned device on the support structure MT, but slightly out of the plane of the patterned device. The x shielding leaves 32, 34 are arranged in a first plane 40 and the y shielding leaves 36, 38 are arranged in a second plane 42.

Each of the shielding leaves 32, 34, 36, 38 defines an edge of a rectangular field 44 in the plane of the patterned device MA that can receive radiation. In practice, the illumination system IL can illuminate only part of the rectangular field 44. In FIG. 4 , it is shown that the illumination system IL can be configured to illuminate a curved gap area 46, which can coincide with a part of the rectangular field 44 (depending on the position of the y shielding leaves 36, 38).

The bend gap region 46 may be defined in part by optics within the illumination system IL and/or the projection system PS. Additionally, the bend gap region 46 may be defined in part by a plurality of independently movable objects provided along one or both of the curved edges of the bend gap region 46. The plurality of independently movable objects may be referred to as unicom fingers. The plurality of independently movable objects may be provided at different x positions and may be moved in the y direction to control the overlap between each of the movable objects and the radiation beam B generated by the illumination system IL. Controlling the y position of the movable component may control the shape (or at least the intensity distribution) of one or both of the curved edges of the bend gap region 46. The movable components may be used to minimize variations in the radiation dose provided by the radiation beam B at different locations in the non-scanning direction (ie, the x-direction). Additionally, the bend gap region 46 may be defined in part by a solid aperture (eg, an entrance aperture of the projection system PS).

Each of the shielding leaves 32, 34, 36, 38 can be independently moved between a retracted position in which the shielding leaf is not disposed in the path of the radiation beam and an inserted position in which the shielding leaf at least partially blocks the radiation beam projected by the illumination system IL onto the patterning device MA. By moving the shielding leaf 32, 34, 36, 38 into the path of the radiation beam, the radiation beam B can be intercepted (in the x and/or y direction), thereby limiting the range of the field 44 that receives the radiation beam B.

The x-direction corresponds to the non-scanning direction of lithography apparatus LA, and the y-direction corresponds to the scanning direction of lithography apparatus LA. Patterning device MA may be moved in the y-direction through field 44 (as again indicated by arrow 26) in order to expose a larger area of patterning device MA in a single dynamic scanning exposure.

During dynamic exposure of a target area of substrate W, the target area moves through an exposure zone in the plane of substrate W, which is a portion of substrate W onto which the exposure area 44 of patterning device MA is imaged by projection system PS. When the target area of substrate W moves into the exposure zone, the first shielding blades 36, 38 move so that only the target area receives radiation (i.e., portions of the substrate outside the target area are not exposed). At the beginning of the scanning exposure, one of the y shielding blades 36, 38 is placed in the path of radiation beam B, acting as a light shield, so that no part of substrate W receives radiation. At the end of the scanning exposure, the other of the y shielding blades 36, 38 is placed in the path of radiation beam B, acting as a light shield, so that no part of substrate W receives radiation. During the middle portion of the scanning exposure, when there is no overlap between the exposure area 44 (which receives radiation B) and any of the adjacent target areas of the substrate W, both of the y-shielding blades 36, 38 are disposed in the retracted position.

The rays of the radiation beam B are shown adjacent to each of the shielding leaves 32, 34, 36, 38. It should be understood that each point in the gap area 46 is illuminated by radiation from a range of angles. For example, each point in the gap area 46 can receive a radiation cone. The rays of the radiation beam B shown adjacent to each of the shielding leaves 32, 34, 36, 38 indicate the average direction of the radiation received by the patterning device MA. The rays of the radiation beam B shown adjacent to each of the shielding leaves 32, 34, 36, 38 can be referred to as primary rays. As can be seen from Figures 3A and 3B, in this embodiment, when projected onto the x-z plane, the main ray of radiation is substantially incident on the patterning device MA, and when projected onto the y-z plane, the main ray of radiation is substantially incident on the patterning device MA at an angle of 48.

The lithography apparatus LA may further comprise a gas nozzle 50 which may be configured to direct a gas flow 52 adjacent to the support structure MT. In particular, the gas flow 52 provided by the gas nozzle 50 adjacent to the support structure MT may be substantially parallel to the surface of the patterning device MA and may be referred to as a cross flow. The gas nozzle 50 may be disposed substantially in the same plane (the first plane 40) as the x-shielding blades 32, 34. The gas nozzle may be pointed in the scanning direction such that the gas flow 52 is substantially parallel to the scanning direction and flows between the x-shielding blades 32, 34. The gas nozzle 50 may be considered to form part of a gas supply module operable to provide a gas flow adjacent to the support structure MT.

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 multiplication reticle assembly 15 when the pellicle and multiplication reticle assembly 15 is supported by the support structure MT.

FIG. 4 shows a plan view of the y shielding leaves 36, 38 in a second plane 42 viewed in the positive z-direction (i.e., upward in FIG. 3B ). The positions of the x shielding leaves 32, 34 and the gas nozzles (which are disposed in the first plane 40) are shown with dotted lines. In FIG. 4 , the four shielding leaves 32, 34, 36, 38 are disposed so as to define a generally rectangular field 44 within which a gap region 46 is disposed. This may be a typical configuration of the four shielding leaves 32, 34, 36, 38 during exposure of a central portion of a target area (e.g., a die on a substrate W). As explained above, each of the x shielding leaves 32, 34 is operable to move in the x-direction and each of the y shielding leaves 36, 38 is operable to move in the y-direction to control the size of the field 44. The y shielding leaves 36, 38 are configured so that they can be actuated from the same side of the field 44. To achieve this, the y shielding leaves 36, 38 are shaped so that (although they are located in substantially the same plane 42) each of the shielding leaves 36, 38 has one or more support portions extending in the same direction (negative y direction in FIG. 4).

The shielding blades 32, 34, 36, 38 and the gas nozzle 50 may be mounted on a common shielding blade assembly support (not shown). It should be understood that the shielding blades 32, 34, 36, 38 may be movably mounted on such a support so that they can move relative to such a support. The gas nozzle may be statically mounted on such a support.

In order to enclose the movement with low friction and precisely defined movement, one or more of the shielding blades 32, 34, 36, 38 is provided with a shaft and bearing system. The bearing is now described with reference to Figures 5A and 5B, which schematically show the bearing 100 in a transverse cross section and in an axial cross section, respectively.

In the following description, where there are multiple instances of substantially the same component in the figures, only a single instance may be labeled for the sake of brevity and clarity.

In addition, it should be noted that the axes referenced in the following description are different from the axes referenced in FIGS. 2A to 4. It should be understood from FIGS. 2A to 4 that the movement direction (parallel to the support shaft) of some of the shielding leaves 32 and 34 is the non-scanning direction of the lithography apparatus LA, while the movement direction (parallel to the support shaft) of some of the shielding leaves 36 and 38 is the scanning direction of the lithography apparatus LA. Hereinafter, this movement direction (parallel to the support shaft) is referred to as the z direction.

The bearing 100 includes a support 101 and four bearing surfaces 102 supported by the support 101. The bearing surfaces 102 are configured to at least partially define a cavity 104 for receiving a shaft 108. The cavity 104 has a finite rotational symmetry order (ie, 4th order) about a center of mass 106 of the cavity 104.

The bearing surface 102 is configured to be independently displaceable relative to the support 101 so as to allow a shaft 108 received in the aperture to rotate about one or more rotation axes 112 orthogonal to the shaft's nominal axis 110 while substantially preventing the shaft 108 from rotating about its nominal axis 110 .

The location and range of shaft 108 that can be received by bearing 100 and its nominal axis 110 is best indicated in Figure 5B.

The displacement of the bearing surface 102 is facilitated by a connecting member 114 coupling the bearing surface 102 to the support 101 .

The nominal axis 110 of the shaft may be the axis of the aperture defined by the plurality of bearing surfaces 102 when the plurality of bearing surfaces are disposed in a nominal position or orientation.

The term displaceable should be used to indicate the ability to move in translation and/or in rotation - displacement may include both rotation and translation.

Although the embodiment described with respect to Figures 5A and 5B includes four bearing surfaces 102, in other embodiments, the bearing may include 3, 5, 6, 7, 8, 9 or more bearing surfaces. In general, the bearing may include a plurality of bearing surfaces.

In some embodiments, not all bearing surfaces 102 may be configured to be independently displaceable relative to the support member 101. Each bearing surface 102 may be in a fixed relationship relative to another bearing surface 102, for example, a pair of bearing surfaces may be movable in series.

In some embodiments, the bearing surface 102 may be configured so as to be independently rotatable relative to the support member 101. In other alternative embodiments, the bearing surface 102 may be configured so as to be exclusively (independently) rotatable relative to the support member 101.

Shaft 108 may, for example, support a reticle shield blade of a lithography apparatus (e.g., shaft 108 may support shield blades 32, 34, 36, 38 of the type described above with reference to FIGS. 2A-4). In view of the above-described linear movement of the reticle shield blade, shaft 108 may be a reciprocating shaft that, when in use, may move relative to bearing 100 in a direction substantially parallel to its axis while being supported by the bearing. Under high accelerations, such a reciprocating shaft may experience deflection in a direction orthogonal to the nominal axis of the shaft. Deflection of the reciprocating shaft may cause local rotation or local tilt of the shaft at the bearing. It is undesirable for the zoom mask to obscure the rotation of the blades about the nominal axis of the shaft.

The rotational freedom provided to the bearing surface 102 of the bearing allows the bearing surface to conform to the local tilt of the reciprocating shaft 108 by rotating. The rotation of the bearing surface 102 reduces the non-uniformity (i.e., local concentration) of the load across the bearing surface. In the event that a fluid film exists between the bearing surface 102 and the shaft 108, the rotation of the bearing surface assists in maintaining sufficient film between the bearing surface and the reciprocating shaft. This allows the film to support the reciprocating shaft 108 without excessive contact between the reciprocating shaft and the bearing surface 102.

The bearing 100 tolerates deflection of the reciprocating shaft 108, which allows the reciprocating shaft to withstand higher accelerations and speeds. The bearing 100 also requires less stringent manufacturing tolerances, making the bearing less expensive to manufacture.

Using a right-hand Cartesian set of coordinates, if the nominal axis of the shaft is parallel to the z-axis, at least some of the bearing surfaces may be rotated to allow the shaft to rotate about the x-axis and/or the y-axis. It will be appreciated that, following such rotation, the axis of the shaft will, in general, be at a non-zero angle relative to the nominal axis of the shaft (i.e., the shaft will no longer be perfectly parallel to the z-axis). It should be noted that these axes do not coincide with the axes defined with respect to FIGS. 2A-4.

FIG. 6 shows another embodiment of a bearing 200 of the present disclosure. For clarity and ease of understanding, the support members are omitted. The bearing 200 is generally similar to the bearing 100, described with reference to FIGS. 5A and 5B, and the same parts will be referred to by the same symbols.

The bearing 200 includes a support member (not shown in FIG. 6 ), eight flexure members 206 and four bearing surfaces 102 . Each of the bearing surfaces 102 is connected to the support member via a pair of flexure members 206 .

The bearing surfaces 102 of the bearing 200 are arranged in pairs as substantially opposed bearing surfaces 202. The bearing 200 includes two pairs of bearing surfaces 202. Each bearing surface of a pair 202 has an axis of rotation 204. The axes of rotation 204 of a pair are parallel. The axes of rotation 204 of different pairs of bearing surfaces 202 are orthogonal to each other. The bearing surfaces 102 of each pair of bearing surfaces 202 are separated in a first direction 205 and each can rotate around the axis of rotation 204. The axes of rotation 204 of the two bearing surfaces 102 of a given pair of bearing surfaces 202 are parallel to a second direction substantially perpendicular to the first direction and the nominal axis 110 of the shaft.

Two mutually orthogonal bearing surface pairs 202 partially define a void 104 having a square (or more generally rectangular) cross-section.

The two pairs of bearing surfaces 202 allow the bearings to maintain adequate film in response to deflection of the reciprocating shaft in any direction orthogonal to the nominal axis 110 of the shaft (i.e., deflection in the x-y plane of FIG. 6 ).

It should be understood that the bearing surface pairs 202 need not be orthogonal to each other. The two pairs of bearing surfaces can be disposed at an angle greater than or less than 90° to each other. In one example, the two pairs of bearing surfaces 202 can at least partially define a void having a parallelogram cross-section.

Although the bearing 200 has two pairs of bearing surfaces 202, other configurations are possible. In some embodiments, the bearing may include more pairs of bearing surfaces (e.g., 3, 4, 5, 6, 7 or more), but such configurations may be more complex mechanically.

The opposing bearing surfaces 102 of the bearing 200 can rotate about the axis 204 in a second direction (i.e., parallel to the axis of rotation). The opposing bearing surfaces 102 can remain parallel to each other when they rotate together (e.g., in response to local tilt of the reciprocating shaft). The prismatic geometry of the pores 104 is preserved when the bearing surface pair 202 rotates together, thereby allowing a uniform film thickness to be maintained, facilitating operation of the bearing 200. In particular, if each pair of substantially opposing bearing surfaces is rotated about their respective axes of rotation 204 by the same angle, then because both axes of rotation are parallel to the second direction, the shape of the aperture at least partially defined by the bearing surfaces 102 will be the same, albeit scaled by a cosine factor of the angle in the first direction.

The flexure 206 is a notched flexure. The flexure 206 allows the above-mentioned rotation of the bearing surface.

Each of the bearing surfaces 102 has one degree of freedom. The notched flexure constrains the bearing surface 102 to a single degree of freedom. One or more bearing surfaces having one degree of freedom inherently constrains rotation to an axis of rotation orthogonal to the reciprocating direction of the shaft without the need for additional mechanisms or structures.

The flexure is a single component and therefore does not exhibit significant friction or backlash. Although the flexure 206 is a notched flexure, the flexure may also be in the form of a leaf spring or any other type of flexure.

In some embodiments, the number of flexures per bearing surface can vary. For example, the bearing surface is connected to the support member via 1, 3, 4, 5, 6 or more flexures.

The flexure combines the rotation mechanism and the biasing mechanism into one structure. In some other embodiments, the rotation mechanism may include a ball joint, a hinge or a shaft.

It should be appreciated that in other embodiments, one or more of the bearing surfaces 102 may be connected to the support via a biasing mechanism configured to bias the bearing surface toward a nominal position or orientation. The biasing mechanism may include a flexure, a coil spring, or a leaf spring, or a combination thereof. In particular, bearing embodiments where the rotational mechanism includes a ball joint, hinge, or shaft may have a separate biasing mechanism. In other embodiments, there may be no biasing mechanism.

Generally speaking, the biasing mechanism of at least one of the bearing surfaces may consist essentially of titanium or an iron alloy.

The iron alloy may include stainless steel or carbon steel. Advantageously, titanium alloy and iron alloy biasing mechanisms (e.g., bends) have a long life due to their high fatigue limits. Generally speaking, biasing mechanisms may be made of any material with high fatigue strength and/or fatigue limits.

The bearing surfaces 102 may have a range of rotation of up to ±3 milliradians. In one example, at least one of the bearing surfaces 102 may have a range of rotation of ±1.5 milliradians. In general, the selection of the range of rotation may depend on a balance between tolerances, desired compliance, and stiffness of the bearing itself, the shaft, or any other connected parts. The air bearing may further include structure (e.g., a safety rod or a physical stop) configured to limit rotation of the bearing surface to a desired range. Additionally or alternatively, the air bearing may include structure to further support the bearing surface while still allowing the desired rotation.

The bearing surface 102 is substantially planar. A planar bearing surface allows the reciprocating rod to freely tilt about an axis normal to its plane.

In other embodiments, the bearing surface may alternatively be curved (ie, convex, concave, or a combination thereof) in a direction transverse to the reciprocating direction of the shaft (ie, the nominal axis of the shaft).

Each of the bearing surfaces 102 of the bearing 200 is connected to the support member via a pair of deflections 206. Alternatively, one of the bearing surfaces in each pair of bearing surfaces may be connected to the support member via another rotational mechanism, such as a ball joint. It should be understood that it is only necessary to restrict one or more of the bearing surfaces to one degree of freedom in order to prevent rotation. In one example, each pair of bearing surfaces may include a first bearing surface having one degree of freedom and a second opposing bearing surface having two or more degrees of freedom. The first bearing surface may be connected to the support member via a notched deflection member, and the second opposing bearing surface may be connected to the support member via a ball joint.

In some embodiments, the bearing 200 may be an air bearing. Specifically, the bearing may be a hydrostatic bearing. Air bearings operate with low friction, vibration, and high precision. By conforming to the local tilt of the reciprocating shaft, the air bearing of the present disclosure can maintain a sufficient air film across the bearing surface. The sufficient air film can support the reciprocating shaft without any contact between the reciprocating shaft and the bearing surface, thereby minimizing wear and friction.

7A and 7B schematically illustrate axial and transverse cross-sections of air bearing 300. It should be understood that the air bearing is mechanically similar to bearing 200; the air bearing further includes components and structures configured to generate and maintain an air film. For the sake of brevity, a description of its mechanical operation will therefore be omitted.

The air bearing 300 includes a gas inlet 302 for connecting to a gas supply and a plurality of channels 304 configured to fluidly connect the gas inlet 302 to each of a plurality of bearing surfaces 312 .

The gas inlet 302 and the channel 304 may be disposed in the support member 301 .

In use, the gas inlet 302 can be connected to a gas supply system. In use, the gas inlet 302 is supplied with pressurized gas (e.g., via one or more flexible tubes).

The air bearing 300 further includes a plurality of bearing components 310, each bearing component 310 including a body 311 and defining one of a plurality of bearing surfaces 312, and wherein the body 311 of each bearing component 310 defines a gas manifold 308, the gas manifold including an inlet 307 and configured so that gas received at the inlet 307 is evenly distributed throughout the bearing surface 312 defined by the body.

Each bearing component 310 can be considered as a support under one of the bearing surfaces 312. In use, the inlet 307 of each manifold 308 of each bearing component can be connected to a gas supply system. In use, the inlet 307 of each manifold 308 of each bearing component 310 is supplied with pressurized gas through the flexible tube 306. The gas can be air.

Each bearing member is connected to the support member via a flex member 206. The flex member 206 is a notched flex member. The flex member 206 allows the bearing surface 312 to rotate in a manner similar to that described with reference to FIG. 6 .

Although the bearing component 310 of FIG. 6 generates an air film by means of the manifold 308, in an alternative embodiment, the bearing component may be a composite component including a first portion defining a bearing surface and a second portion supporting 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 portion and the second portion may be formed of different materials.

It should be understood that the gas inlet may include a plurality of gas inlets. For example, in some embodiments, a gas inlet may be provided for each of a plurality of bearing surfaces. For these embodiments, one or more channels may be configured to fluidically connect each gas inlet to a corresponding bearing surface in the plurality of bearing surfaces.

To facilitate proper operation of the air bearing, the bearing surface must be accurately positioned relative to the shaft. In one example, the bearing surface may be separated from the shaft by between 10 μm and 12 μm.

In use, it may be necessary to adjust the bearing surfaces relative to their supports. In some embodiments, adjustment may be required to within ±1 μm. There are a number of alternative adjustment methods. In some embodiments, bearing surface adjustment may be achieved by disassembling the bearings, progressively machining the components of the bearings toward their final configuration, and testing the assembled bearings. During testing, further disassembly, machining, and testing may be necessary. Thus, the desired bearing size configuration may be achieved iteratively.

Alternatively, the bearing surfaces may be adjusted by means of gaskets and/or bearing adjustment mechanisms. In one example, two or more of the bearing surfaces may be supported by axially movable rods, thereby allowing the bearing surface positions to be adjusted. It should be noted that these rods extend in a direction perpendicular to the axis of the rods supporting the zoom mask shielding blades and are axially movable. The proper bearing surface position may be secured by means of friction from screws.

In other embodiments, the air gap size may be adjusted by means of a separate assembly including the bearing surface. The assembly may be glued into the support. In one example, the glued assembly may be positioned in the desired position and then supplied with pressurized liquid glue. At the same time, the bearing surface may be pressurized to the nominal gas pressure in use. The resulting bearing configuration will mimic the nominal bearing pretension and have the desired bearing surface position. This bearing surface position is then fixed by curing the glue.

8A and 8B schematically depict axial cross-sections of an embodiment of a zoom mask system 400.

The lithography apparatus LA shown in FIG. 1 and described above may include a reticle shielding system 400.

FIG8A shows a zoom mask shielding system 400 in a stationary and/or undeflected state. Zoom mask shielding system 400 includes a shaft 402, a zoom mask shielding blade 404 (which may be a shielding blade 32, 34, 36, 38 of the type described above with reference to FIGS. 2A to 4), and a bearing 406 (e.g., bearing 100, bearing 200, or air bearing 300) according to the above embodiments. Shaft 402 is received in an aperture defined by and supported by bearing surfaces 410 of bearings 406 and 408. Zoom mask shielding blade 404 is coupled to the end of shaft 402.

The zoom reticle shielding system 400 further includes a second bearing 408. The shaft 402 is further supported by the second bearing 408. Optionally, supporting the reciprocating shaft 402 at two points constrains the translation of the reciprocating shaft in a direction orthogonal to the nominal axis of the shaft 402.

The shaft 402 is configured to reciprocate within an aperture defined by the bearing 406 and the bearing surface 410 of the bearing 408. By extension, the reticle shielding blade 404 is configured to reciprocate in series with the shaft. The reticle shielding system 400 can operate at high speeds and accelerations of the reticle shielding blade 404.

The effects of high speed and/or high acceleration of the zoom mask system 400 are illustrated in FIG8B . Under high speed and/or high acceleration, the shaft 402 may deflect orthogonally to the shaft nominal axis 110 (see FIG8A ). The bearing surface 410 rotates an angle θ to account for the local rotation induced by the deflection of the shaft 402. It should be noted that the angle θ shown in FIG8B is exaggerated for illustration purposes. The rotation of the bearing surface assists in maintaining an adequate air film between the bearing surface and the reciprocating shaft. Without the rotation of the bearing surface 410, the shaft 402 may impact the bearing surface 410.

The reticle shielding system 400 also avoids the need for an additional mechanism configured to limit the rotation of the reticle shielding blades about the nominal axis of the shaft 402, thereby reducing the footprint of the reticle shielding system.

In some embodiments of the zoom mask shielding system, the bearings may be of different types. Typically, at least one of the bearings should be according to the above-described embodiments, that is, a bearing that limits the rotation of the shaft around a nominal axis.

FIG9 depicts a cross-sectional view of a portion of a zoom mask system 500 including a shaft 502, a first bearing 506, a second bearing 508, and a linear motor 520. The shaft 502 itself includes a first portion 503 having a square cross-section and a second portion 504 having a circular cross-section. The first portion 503 is received by the square aperture of the first bearing 506. The second portion 504 is received by the cylindrical aperture of the second bearing 508.

Although omitted from FIG. 9 , shaft 502 is configured to couple with a zoom reticle shielding blade.

The first bearing 506 includes four rotatable bearing surfaces 510 that at least partially define a square aperture. The first bearing 506 can be similar to the bearing 100, the bearing 200, or the air bearing 300.

The second bearing 508 includes a single bearing surface 512 defining a cylindrical aperture. The second bearing 508 is supported by a support member 513 via a thin annular web of material 514.

The thin annular web of material 514 can be easily deformed. The deformation of the thin annular web of material 514 allows the bearing surface 512 of the second bearing 508 to conform to the local tilt of the second portion 504 of the reciprocating shaft 502 by rotating relative to the support member 513. The rotation of the bearing surface assists in maintaining an adequate air film between the bearing surface and the reciprocating shaft.

In one example, the thin annular web of material 514 may be at least about 0.2 mm thick. In another example, the thin annular web of material 514 may be at least about 0.3 mm thick. In some embodiments, the thin annular web of material may have an inner diameter of about 38 mm and an outer diameter of about 48 mm. The thin annular web of material 514 may be substantially composed of a steel alloy (e.g., a stainless steel alloy) or a titanium alloy. In general, the thin annular web of material 514 may be configured (by material or size selection) to be between about 20 and 100 times less rotationally stiff than the bearing surface 512.

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, the shroud shield blades.

The second bearing 508 may be supported in an alternative configuration 550 schematically depicted in the front and side views in FIGS. 14A and 14B , respectively. As shown in FIG. 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 are more flexible than the support frame 552 and protrude from the second bearing 508. The suspension arms 554a, 554b, 554c protrude from the second bearing in a generally tangential manner.

In use, the flexible suspension arm allows the bearing surface 558 of the second bearing 508 to conform to the local tilt of the second portion 504 of the reciprocating shaft 502 by rotating relative to the support member 552.

In some embodiments, the suspension arm can be a flexure. In one example, the flexure can be a leaf spring type metal element.

A method of manufacturing the second bearing 508 and its support arrangement 550 may include grinding at least a portion of the second bearing 508 and the support arrangement 550 from a single piece of material, thereby simplifying manufacturing.

The bearing surface 558 may be a porous graphite component that is in fluid communication with a gas supply (not shown).

In some embodiments, the second portion of the shaft may also be non-annular, being received by a corresponding aperture of the second bearing. In such embodiments, any (slight) rotation of the front bearing about the nominal axis will cause a conflict with the alignment of the second bearing (e.g. causing a reduction in the air gap in the air bearing). Therefore, such embodiments with non-annular first and second shaft portions may require the zoom mask shielding system and its components to have increased accuracy, precision, stability and rigidity to function optimally. Therefore, the annular cross-section of the second portion simplifies the manufacture of the zoom mask shielding system.

Although the zoom reticle shielding system 500 includes bearings that are air bearings, it should be understood that in some embodiments, one or more bearings (e.g., the second bearing 508) need not be air bearings and may be, for example, oil-lubricated bearings (although such bearings may pose a greater risk of contamination in lithography applications) or drum bearings.

Although the air bearings described with reference to Figures 5A-8B include four independently displaceable bearing surfaces, each of which can rotate only about an axis perpendicular to the nominal axis of the shaft (e.g., bearing surface 102 of bearing 200), it will be appreciated that effective prevention of rotation of the shaft about its nominal axis may be achieved by alternative arrangements.

Figure 10 schematically illustrates a bearing 1000 in a transverse cross section according to an embodiment of the present disclosure. In a preferred embodiment, the bearing 1000 is an air bearing.

The bearing 1000 includes 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 receiving a shaft 1008. The aperture 1004 has a finite rotational symmetry order (i.e., order 4) about a center of mass 1006 of the aperture 1004.

The bearing surfaces are arranged in (two) pairs of substantially opposite bearing surfaces. Bearing 1000 includes a first pair of bearing surfaces and a second pair of bearing surfaces. The first pair of bearing surfaces includes bearing surface 1002 and bearing surface 1202a. The second pair of bearing surfaces includes 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 in the first pair of bearing surfaces are separated in a first direction 1005. Bearing surface 1402 and bearing surface 1202b in the second pair of bearing surfaces are separated in a first direction 1405.

When the plurality of bearing surfaces are disposed in a nominal position or orientation, the center axis of the aperture 1004 defined by the plurality of bearing surfaces defines the nominal axis of the shaft.

One of the plurality of bearing surfaces (bearing surface 1002) can only rotate about a rotation axis 1012 that is parallel to a second direction that is substantially perpendicular to the first direction 1005 and the nominal axis of the shaft. Alternatively, the bearing surface 1002 has one (rotational) degree of freedom. The bearing surface 1002 is connected to the support member 1101 by a connecting member 1014. It should be understood that the bearing surface 1002 is similar to the bearing surfaces described above with reference to Figures 5A to 8B (e.g., the bearing surface 102 of Figure 6). As such, it may incorporate any of the details of the construction described in conjunction with those bearing surfaces—for example, the connecting member 114 may include one or more (notch) deflection members.

In alternative embodiments, two or more of the plurality of bearing surfaces may have only one rotational degree of freedom.

The bearing surface 1402 has two rotational degrees of freedom - the bearing surface 1402 can tilt about the y-axis and the z-axis. Alternatively, the bearing surface 1402 can rotate about either of two axes that are 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 member 1414. The connecting member can be a universal hinge. The universal hinge can include one or more flexures. In other embodiments, the bearing surface 1402 can be connected to the support by other members that allow two (rotational) degrees of freedom, such as a ball joint.

Bearing surfaces 1202a and 1202b are offset bearing surfaces. Each pair of bearing surfaces includes a rotatable bearing surface and an offset bearing surface.

The bearing 1000 includes two offset bearing surfaces 1202a, 1202b, each of which is urged toward the center of mass of the aperture 1006 by an offset member 1214. In use, the offset bearing surfaces 1202a, 1202b urge the shaft 1008 toward the opposing bearing surfaces 1002 and 1402, respectively. It should be noted that the plurality of bearing surfaces 1002, 1202a, 1202b, 1402 are not configured to directly contact the shaft 1008 - any forces are transmitted through the air film between the bearing surfaces and the shaft 1008.

The biasing member 1214 of the two biased bearing surfaces 1202a, 1202b may include pneumatic and/or mechanical biasing members. The pneumatic biasing member may include a pneumatic piston. The mechanical biasing member may include a coil spring, a wave washer, or an elastic element. Alternatively or in addition, the biasing member 1214 may include an electromagnetic biasing member.

It should be further understood that the offset bearing surfaces 1202a, 1202b are mounted to the support 1101 at least in part by the offset member 1214. The offset bearing surfaces 1202a, 1202b can advance toward or retract from the center of mass 1006.

In use, bearing 1000 can conform to deflections of reciprocating shaft 1008, thereby maintaining a uniform fluid (air) film. Bearing surface 1002 can tilt in response to deflection or partial rotation of the shaft about the x-direction. The pressure provided by offset bearing surface 1202a constrains shaft 1008 to bearing surface 1002. Bearing surface 1002 has only a single axis of rotation 1012, and thus substantially prevents shaft 1008 from rotating about the y-axis. Bearing surface 1402 can also tilt in response to deflection or partial 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 the bearing surface 1402 prevents the shaft 1008 from being over-constrained, thereby allowing the bearing surface 1402 to conform to changes in the size of the shaft. Because the offset bearing surfaces 1202a, 1202b can advance and retract, the aperture 1004 can change its range in the first direction 1405 and 1005, thereby allowing the use of the bearing 1000 with a shaft having a looser dimensional tolerance.

The position of the offset bearing surfaces 1202a, 1202b is controlled by the force exerted by the gas film between the shaft 1008 and the bearing surfaces. For a given gas pressure, the relative position of the shaft and the offset bearing surfaces (i.e., their separation) is approximately constant. The separation is substantially constant within the movement envelope of the main piston/independent of the absolute position of the assembly. With any perturbation of the separation, the bearing surfaces will reposition themselves at the correct separation. The offset bearing surfaces 1202a, 1202b automatically compensate for inaccuracies in the bearing and/or shaft dimensions.

An embodiment of a bearing surface 1402 and its connecting member 1414 is schematically illustrated in FIG11 . The connecting member 1414 includes 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 universal hinge connected to the bearing surface 1402. The bearing surface 1402 includes a plurality of holes 1420 to provide an air film between the bearing surface 1402 and the shaft 1008 in use. In addition, the bearing surface 1402 and its connecting member 1414 are extremely strong in the x-direction. In conjunction with the offset bearing surface 1202b, the shaft 1008 is accurately constrained or located in its x position.

An embodiment of a biasing member is described with reference to FIGS. 12A and 12B. The biasing member 1214 includes a spring 1222, a sub-piston 1224, and a main piston 1226. The sub-piston 1224 is pushed (toward the center of mass of the aperture) by the spring 1222. The main piston is connected to the offset bearing surface 1202a. The spring 122, the sub-piston 1224, and the main piston 1226 are confined within a hole 1228 disposed in the support member 1101. The main piston 1226 and the sub-piston 1224 together with the hole 1228 define an expandable pneumatic chamber 1230. The biasing member 1214 is configured to be operable between a first (or transport) and a second (or operating) configuration.

Spring 1222 can replace other biasing members—such as coil springs, wave washers, conical washers, elastic elements and/or other mechanical biasing members. In other embodiments, spring 1222 can replace any passive biasing member.

FIG. 12A shows the biasing member 1214 in a first configuration, wherein the spring 1222 pushes 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 pushed without any external energy source. Advantageously, during transportation of the telescopic shielding system, the tension provided by the spring allows the shaft to be passively held firmly by the bearing.

FIG. 12B shows the biasing member 1214 in a second configuration, wherein the pneumatic chamber 1230 is pressurized, thereby retracting the sub-piston 1224, and the pressure pushes the main piston 1226 toward the center of mass of the aperture by the pneumatic pressure. The pneumatic chamber 1230 can be pressurized and expanded by pressurized air, which is supplied by means of an air line or inlet 1232. The expansion of the chamber 1230 separates the sub-piston 1224 from the main piston 1226. The force applied by the piston is controlled by the cross-sectional area of the bore 1228 and the pressure in the chamber 1230. In some embodiments, the pressure in the pneumatic chamber 1230 in the second configuration can be about 6 bar(a).

FIG. 13 shows a more detailed schematic cross-sectional view of the main piston 1226. The piston 1226 includes a piston sealing ring 1234 that is retained and confined in an annular groove 1238 of the piston 1226. The piston sealing ring 1234 seals the bore 1228, thereby providing a reliable pneumatic seal in use. In addition, the piston sealing ring allows the use of a slightly undersized main piston 1226, so that there is a gap (indicated by Δ in FIG. 13) between the main piston 1226 and the bore 1228. The gap Δ can provide a certain degree of compliance for the offset bearing surface 1202. Alternatively, the offset bearing surface 1202a can be tilted. Advantageously, the piston seal ring 1234 seals the main piston 1226 in any orientation (e.g., tilted about the x-axis and/or y-axis) - the offset bearing surface 1202a can also accommodate deflection of the shaft 1008 while maintaining a compressive force on the shaft.

The pneumatic chamber 1230 is fluidly connected to the inlet 1232 and the offset bearing surface 1202a. The pneumatic chamber acts as a reservoir for pressurized air to supply an air film between the offset bearing surface 1202a and the shaft 1008 (via the gas manifold 1203).

The main piston 1226 further includes a flange 1236 configured to prevent over-insertion of the main piston.

Although the master piston 1226 and, by extension, the offset bearing surface 1202a can tilt about the x-axis and the y-axis, it is not desirable for the offset bearing surface 1226 to rotate axially—that is, about the z-axis. The master piston 1226 further includes one or more (e.g., two) anti-rotation lugs 1238 that interface with complementary grooves 1239 defined in the support 1101. The anti-rotation lugs 1238 are positioned into the complementary grooves 1239 and prevent the master piston 1226 from rotating about the z-axis. The anti-rotation lugs and complementary grooves are examples of complementary anti-rotation features.

In some embodiments, the sub-piston 1224 may also include a piston sealing ring to provide a pneumatic seal to the hole 1228.

The embodiment of the offset member described with reference to Figures 12A, 12B and 13 is described with respect to the offset bearing surface 1202a of the first pair of bearing surfaces. It should be understood that substantially similar offset members can also connect the offset bearing surface 1202b of the second pair of bearing surfaces to the support member 1101.

The lithography apparatus LA may include a 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 a reticle assembly and a patterning device assembly.

Although specific reference may be made herein to embodiments of the invention in the context of lithography 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 a mask (or other patterned device). Such apparatus may generally be referred to as a lithography tool. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

The term "EUV radiation" may be considered to cover electromagnetic radiation having a wavelength in the range of 4 nm to 20 nm, for example in the range of 13 nm to 14 nm. EUV radiation may have a wavelength less than 10 nm, for example in the range of 4 nm to 10 nm, such as 6.7 nm or 6.8 nm.

Although specific reference may be made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guiding and detecting patterns for magnetic resonance memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

1: Laser 2: Laser beam 3: Fuel emitter 4: Plasma formation zone 5: Collector 6: Intermediate focus 7: Plasma 8: Opening 9: Enclosure 10: Faceted field mirror device 11: Faceted pupil mirror device 13: Mirror 14: Mirror 15: Zoom mask assembly/film assembly 15a: Central portion 15b: Peripheral portion 17: Film frame 19: Film 24: Zone 26: Arrow 28: Extended first section zone 32: x shielding blade 34: x shielding blade 36: y shielding blade 38: y shielding blade 40: First plane 42: Second plane 44: generally rectangular field area 46: bend gap area 48: angle 50: gas nozzle 52: gas flow 100: bearing 101: support member 102: bearing surface 104: aperture 106: center of mass 108: shaft 110: nominal axis 114: connecting member 200: bearing 202: bearing surface 204: rotation axis 205: first direction 206: deflection member 300: air bearing 301: support member 302: gas inlet 304: passageway 306: flexible tube 307: inlet 308: Manifold 310: Bearing Component 311: Body 312: Bearing Surface 400: Zoom Mask Shielding System 402: Reciprocating Shaft 404: Zoom Mask Shielding Blade 406: Bearing 408: Secondary Bearing 410: Bearing Surface 500: Zoom Mask Shielding System 502: Reciprocating Shaft 503: First Section 504: Second Section 506: First Bearing 508: Secondary Bearing 510: Rotatable Bearing Surface 512: Bearing Surface 513: Support 514: Material 520: Linear Motor 550: Alternative Configurations 552: support frame 554a: suspension arm 554b: suspension arm 554c: suspension arm 558: bearing surface 1000: bearing 1002: bearing surface 1004: aperture 1005: first direction 1006: center of mass 1008: shaft 1012: axis of rotation 1014: connecting member 1101: support member 1202a: offset bearing surface 1202b: offset bearing surface 1203: gas manifold 1214: offset member 1222: spring 1224: sub-piston 1226: main piston 1228: hole 1230: expandable pneumatic chamber 1232: air duct or inlet 1234: piston seal ring 1236: flange 1238: annular groove 1239: complementary groove 1402: bearing surface 1405: first direction 1414: connecting member 1416: first deflection member 1418: second deflection member 1420: hole B: radiation beam IL: illumination system LA: lithography equipment MA: patterning device MT: support structure PS: projection system SO: radiation source W: substrate WT: substrate stage

An embodiment of the present invention will now be described as an example only with reference to the accompanying schematic drawings, in which: -     FIG. 1 is a schematic illustration of a lithography system including a lithography apparatus and a radiation source; -     FIG. 2A is a schematic plan view of the support structure and patterning device shown in FIG. 1 disposed in a first end position; -     FIG. 2B is a schematic plan view of the support structure and patterning device shown in FIG. 1 disposed in a second end position; -     FIG. 3A is a schematic illustration of a first cross-section of the patterning device and the doubling mask shielding blade on the support structure of the lithography apparatus of FIG. 1; -    FIG. 3B is a schematic illustration of a second cross-section through the patterning device and the doubling mask shielding blade on the support structure of the lithography apparatus of FIG. 1; -     FIG. 4 is a plan view showing the y shielding blade and the x shielding blade (dotted line) of the lithography apparatus of FIG. 1 in a first configuration; -     FIG. 5A is a schematic illustration of a transverse cross-section through a bearing according to an embodiment of the present disclosure; -     FIG. 5B is a schematic illustration of an axial cross-section through a bearing according to the embodiment shown in FIG. 5A; -     FIG. 6 is a perspective illustration of a bearing according to another embodiment of the present disclosure; -     FIG. 7A is a schematic illustration of an axial cross-section through an air bearing according to an embodiment of the present disclosure; -    FIG. 7B is a schematic illustration of a transverse cross section through a bearing according to the embodiment shown in FIG. 7A; -     FIG. 8A is a schematic illustration of an axial cross section through a doubling mask shielding system according to an embodiment of the present disclosure; -     FIG. 8B is a schematic illustration of an axial cross section through a doubling mask shielding system according to an embodiment of the present disclosure under deflection; -     FIG. 9 is a perspective illustration of a doubling mask shielding system according to an embodiment of the present disclosure. -     FIG. 10 schematically illustrates a bearing in a transverse cross section according to an embodiment of the present disclosure. -     FIG. 11 schematically illustrates a bearing surface and its connecting components according to an embodiment of the present disclosure. -     Figures 12A and 12B schematically illustrate a biasing member 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.

100: Bearings

101: Support parts

102: Bearing surface

104: Porosity

106: Center of mass

108: Shaft

114: Connecting parts

Claims (35)

A bearing comprising: a support member; and a plurality of bearing surfaces supported by the support member and configured to at least partially define a bore for receiving a shaft, the bore having a finite rotational symmetry order about a center of mass of the bore; wherein at least some of the bearing surfaces are configured to be independently displaceable relative to the support member so as to permit a shaft received in the bore to rotate about one or more rotational axes orthogonal to a nominal axis of the shaft while substantially preventing the shaft from rotating about its nominal axis. A bearing as claimed in claim 1, wherein the plurality of bearing surfaces includes at least one bearing surface that is constrained so as to substantially prevent the at least one bearing surface from rotating relative to the support member about the nominal axis of the shaft. A bearing as claimed in claim 1 or 2, wherein the plurality of bearing surfaces includes only one bearing surface which is constrained so as to substantially prevent the at least one bearing surface from rotating relative to the support member about the nominal axis of the shaft. A bearing as claimed in claim 2, wherein the plurality of bearing surfaces are arranged as one or more pairs of substantially opposing bearing surfaces, and the bearing surfaces of each pair of bearing surfaces are separated in a first direction. A bearing as claimed in claim 4, wherein the bearing surfaces of each pair of bearing surfaces can each rotate around a rotation axis, and the rotation axes of the two bearing surfaces of a given pair of bearing surfaces are parallel to a second direction that is substantially perpendicular to the first direction and the nominal axis of the shaft. A bearing as claimed in claim 4, wherein each pair of bearing surfaces includes a rotatable bearing surface and an offset bearing surface. A bearing as in claim 6, wherein the rotatable bearing surface of at least one of the pairs is rotatable only about an axis parallel to a second direction substantially perpendicular to the first direction and the nominal axis of the shaft. A bearing as claimed in claim 6 or 7, wherein the rotatable bearing surface of at least one of the pairs can rotate around any of two axes perpendicular to the first direction of the pair of bearing surfaces. A bearing as claimed in claim 6 or 7, wherein each offset bearing surface is urged toward the centre of mass of the aperture by one or more offset members. A bearing as claimed in claim 9, wherein the one or more offset members of the one or more offset bearing surfaces include a pneumatic and/or mechanical offset member. A bearing as claimed in claim 10, wherein the one or more biasing members include a pneumatic piston. A bearing as in claim 10, wherein the one or more offset bearing surfaces are urged toward the center of mass of the aperture by the mechanical component in a first configuration and by the pneumatic component in a second configuration. A bearing as claimed in claim 1 or 2, wherein the bearing comprises two pairs of bearing surfaces, and the rotation axes of the two pairs of bearing surfaces are orthogonal to each other. A bearing as claimed in claim 1 or 2, wherein the bearing is an air bearing. A bearing as claimed in claim 1 or 2, further comprising a gas inlet for connecting to a gas supply and one or more channels configured to fluidly connect the gas inlet to each of the plurality of bearing surfaces. A bearing as claimed in claim 1 or 2, further comprising a plurality of bearing components, each bearing component comprising a body and defining one or more bearing surfaces, and wherein the body of each bearing component defines a gas manifold, the gas manifold comprising an inlet and configured so that the gas received at the inlet is evenly distributed throughout the bearing surface defined by the body. A bearing as claimed in claim 1 or 2, wherein each of the bearing surfaces is connected to the support member via a biasing mechanism, the biasing mechanism being configured to bias the bearing surface toward a nominal position or orientation. A bearing as claimed in claim 1 or 2, wherein at least one of the bearing surfaces is connected to the support member via one or more flexure members. A bearing as claimed in claim 1 or 2, wherein at least one of the bearing surfaces has a degree of freedom. A bearing as claimed in claim 17, wherein the biasing mechanism of at least one of the bearing surfaces is substantially composed of titanium or a ferrous alloy. A bearing as claimed in claim 1 or 2, wherein the bearing surfaces are substantially planar. A multiplying reticle shielding system for a lithography apparatus, the multiplying reticle shielding system comprising: a shaft; a multiplying reticle shielding blade configured to partially define an optical aperture; and a bearing as in any one of claims 1 to 21; wherein the shaft is received in the aperture defined by and supported by the bearing surfaces of the bearing. A zoom mask shielding system as in claim 22, wherein the shaft is at least partially non-annular in cross-section. A zoom mask shielding system as claimed in claim 22 or 23, wherein the shaft is further supported by a second bearing. A zoom mask shielding system as claimed in claim 24, wherein the shaft includes a first portion having a non-annular cross-section received by the bearing and a second portion having an annular cross-section received by a hole in the second bearing. A zoom mask shielding system as in claim 25, wherein the second bearing is supported by a thin annular web of material. A zoom mask shielding system as in claim 25, wherein the second bearing is suspended from the support frame by a plurality of suspension arms protruding from the second bearing and having greater flexibility than the support frame. A zoom mask shielding system as in claim 27, wherein the plurality of suspension arms protrude from the second bearing in a substantially tangential manner. A doubling photomask shielding system as claimed in claim 27, wherein the second bearing is supported by three flexure members. A zoom mask shielding system as claimed in claim 22 or 23, wherein the zoom mask shielding system further comprises a gas supply system configured to supply gas to each of the plurality of bearing surfaces of the bearing. A zoom mask shielding system for a lithography apparatus, the zoom mask shielding system comprising: a shaft; a zoom mask shielding blade configured to partially define an optical aperture; and a bearing as claimed in claim 12; wherein the shaft is received in the aperture defined by and supported by the bearing surfaces of the bearing, wherein the biasing member comprises: a spring; an expandable pneumatic chamber; further wherein the biasing member is configurable between: a first configuration in which the spring is configured to push the biased bearing surface toward the center of mass of the aperture through the pneumatic chamber; and The second configuration, wherein the pneumatic chamber is pressurized, thereby retracting the spring, and the pneumatic pressure of the pneumatic chamber pushes the biased bearing surface toward the center of mass of the aperture. A zoom mask shielding system as claimed in claim 31, wherein the biasing member further comprises: a sub-piston pushed by the spring; a main piston connected to the biased bearing surface; wherein the spring, sub-piston and main piston are confined in a hole disposed in the support member, and the main piston and sub-piston together with the hole define the expandable pneumatic chamber; and further wherein in the first configuration, the sub-piston is in contact with the main piston; and in the second configuration, the sub-piston is separated from the main piston by pneumatic pressure. A zoom mask shielding system as claimed in claim 32, wherein the main piston and/or the sub-piston further includes a piston sealing ring configured to seal the hole. A zoom mask shielding system as in claim 32 or 33, wherein the pneumatic chamber is fluidly connected to the gas inlet and the offset bearing surface. A lithography apparatus comprising a zoom mask shielding system as claimed in any one of claims 22 to 34.