Positioning apparatus for a stage apparatus

Abstract

Disclosed is a coarse positioning apparatus for a stage apparatus, the coarse positioning apparatus comprising: at least one gas spring, each said at least one gas spring comprising a piston; and at least one electromagnetic actuator; wherein said at least one gas spring and said at least one electromagnetic actuator are operable to each apply respective complementary forces to said stage apparatus so as to accelerate said stage apparatus for coarse positioning of said stage apparatus, and wherein at least said gas spring is coupled to said stage apparatus throughout each full stroke of said stage apparatus.

Description

POSITIONING APPARATUS FOR A STAGE APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24177846.3 which was filed on 24 May 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a stage apparatus for use in a lithographic apparatus, and in particular a positioning apparatus for a reticle stage apparatus.BACKGROUND

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

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Such a lithographic apparatus works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. Moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan does this. The wafer stage is then asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.

[0006] Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, onto an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer. The reticle may be transmissive or reflective depending on the wavelength of light used; EUV systems for example use a reflective reticle within a vacuum chamber to prevent absorption of the EUV radiation.

[0007] The reticle can be placed on a platform or stage (hereinafter, both are referred to as “stage”). The stage can be positioned according to parameters of the lithographic system. Similarly, the semiconductor wafer can be placed on a stage. The stage supporting either the reticle or the semiconductor wafer can be moved one or more directions and / or one or more degrees of freedom depending on how the image is to be formed on the semiconductor wafer.

[0008] A typical reticle stage comprises a coarse positioning apparatus or long stroke module, for coarse positioning of the reticle, and a fine positioning apparatus or short stroke module for fine positioning of the reticle. The long stroke module supports the short stroke module, while the short stroke module is configured to carry the reticle.

[0009] It is desirable to improve on the reticle stage, and in particular to actuation of the reticle stage. More specifically it is desirable to improve on actuation of the long stroke module of the reticle stage.SUMMARY

[0010] In the first aspect of the invention there is provided a coarse positioning apparatus for a stage apparatus, the coarse positioning apparatus comprising: at least one gas spring, each said at least one gas spring comprising a piston; and at least one electromagnetic actuator; wherein said at least one gas spring and said at least one electromagnetic actuator are operable to each apply respective complementary forces to said stage apparatus so as to accelerate said stage apparatus for coarse positioning of said stage apparatus; and wherein at least said gas spring is coupled to said stage apparatus throughout each full stroke of said stage apparatus.

[0011] In a second aspect of the invention there is provided a method for positioning a stage apparatus comprising: applying a first force on said stage apparatus using at least one gas spring; and applying a first force on said stage apparatus using at least one electromagnetic actuator; wherein said at least one gas spring and said at least one electromagnetic actuator wherein said first force and said second force comprise respective complementary forces so as to controllably accelerate said stage apparatus for coarse positioning of said stage apparatus; and wherein at least said gas spring is coupled to said stage apparatus throughout each full stroke of said stage apparatus.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts a reticle stage apparatus according to an embodiment;Figure 3 depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment;Figure 4(a) is a plot of force against position for an ideal stage actuator and Figure 4(b) is an equivalent plot for a real gas spring;Figure 5(a) is a plot of position against time for a reticle stage according to an embodiment;Figure 5(b) is a plot of the force applied to the stage apparatus by gas springs according to an embodiment; and Figure 5(c) is a plot of the force applied to the stage apparatus by gas springs and electromagnetic actuators according to an embodiment;Figure 6 depicts a further reticle stage apparatus according to an embodiment;Figure 7 depicts a further reticle stage apparatus according to an embodiment;Figure 8(a) and 8(b) each depict a gas spring usable in reticle stage apparatuses described herein according to an embodiment;Figure 9(a), 9(b) and 9(c) each depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment in different stages of operation;Figure 10 is a plot of force against position for various gas spring embodiments described herein, such as depicted in Figures 9, 11-14 and 16;Figure 11 depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment;Figure 12 depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment;Figure 13(a) and 13(b) each depict a gas spring usable in reticle stage apparatuses described herein according to an embodiment;Figure 14(a) and 14(b) each depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment in different stages of operation;Figure 15 depicts a further reticle stage apparatus according to an embodiment;Figure 16 depicts a gas spring usable in reticle stage apparatuses described herein according to an embodiment; andFigure 17 depicts a further reticle stage apparatus according to an embodiment.DETAILED DESCRIPTION

[0013] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask or reticle), a projection system PS and a substrate table WT configured to support a substrate W.

[0014] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL mayinclude other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0015] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B ’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0016] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0017] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0018] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0019] Though only drawn as a very basic schematic, it can be appreciated that the support structure MT for the patterning device (hereafter reticle) MA comprises a significantly more complex stage apparatus (hereafter reticle stage) than is depicted. The reticle stage may comprise a long stroke module (or coarse positioning apparatus) which supports a short stroke module (or fine positioning apparatus); the short stroke module in turn being configured to carry the reticle. The long stroke module is used for coarse positioning of the reticle stage, while the short stroke module is used for fine positioning of the reticle with respect to the projection system and / or wafer.

[0020] The stage apparatuses and arrangements disclosed herein are described in the context of an EUV lithographic apparatus, for which the reticle is a reflective reticle. However, it can be appreciated that the concepts herein are not limited to only reticle stages for EUV lithographic apparatuses and reflective reticles; the concepts are equally applicable to reticle stages for non-EUV lithographic apparatuses and transmissive reticles, and more to stages generally (e.g., also to the wafer / substrate stage for EUV lithographic apparatuses and for non-EUV lithographic apparatuses, metrology apparatuses etc.).

[0021] During exposures, the short stroke module may accurately position the reticle, moving the reticle along a meandering or zig-zag path so as to expose different portions of the substrate. However,the movement of the long stroke stage is always linear, repeatedly translating back and forth between two end positions, i.e., back and forth linear reticle scans. Present reticle stages reverse the linear reticle scan motion at the end of each scan without mechanical energy recovery. That means that the force required to reverse the linear motion at the end of each scan has to be generated by the electromagnetic actuators (hereafter electromagnetic forcers or simply forcers).

[0022] The force provided by electromagnetic forcers cannot be scaled endlessly. Based on the current state of technology, the maximum achievable acceleration of the reticle stage is in the region of 300- 350 m / s2, which is approximately a third of the desired acceleration. Additionally, the energy consumption of these forcers is very substantial.

[0023] One proposal for implementing energy recovery is to provide for a magnetic bouncer. A magnetic bouncer may use repelling magnets to create a contactless spring. Such a magnetic bouncer may be contactless, but suffers from the same problem as the long stroke forcers: it has an unfavorable force / moving mass ratio and is therefore not suitable for the desired acceleration magnitudes for a reticle stage within a lithographic apparatus. Such desired accelerations may be greater than 350m / s2, greater than 500m / s2, greater than 600m / s2, greater than 700m / s2, greater than 800m / s2, or greater than 900m / s2for example; e.g., more specifically an acceleration of 960m / s2.

[0024] For magnetic bouncing, where magnets of same polarity repel each other, a sufficient force density cannot be achieved for the magnetic bounce concept to provide accelerations of 960 tn / s2. As such, neither increasing efficiency of motors / forcers nor providing a magnetic bouncing arrangement (alone or in combination) can be used to achieve the desired acceleration magnitude as described above.

[0025] Proposed herein is a stage apparatus which provides a “spring action” at the end of each exposure move. A physical coil spring arrangement has been considered; however a suitable industrial coil spring would be very big and heavy. The two springs would need to be stationary mounted, so that they are not part of the moving mass during the reticle scan. The stage would have to physically contact the spring at some point where the speed reversal begins. This sudden contact with a large spring mass at standstill will generate particles and a huge jerk (e.g., 3rd-6th order motion derivatives).

[0026] As such, an alternative spring technology is preferred which fulfills the following requirements: minimal particle generation which may result from moving parts that contact each other, such as linkages, rollers, contact bearings and minimal vibration due to interaction or contact with the spring functionality. In addition, the force / mass ratio of the spring functionality should very high as poor force / mass ratio is a main drawback of electromagnetic forcers and the magnetic bouncers. An additional desirable feature would be to provide for adjustment during operation, e.g., to allow for reticle strokes of varying length and / or higher and lower accelerations.

[0027] As such, described herein is a stage apparatus, e.g., a reticle stage apparatus and more specifically a coarse positioning apparatus for a reticle stage apparatus, comprising a gas spring arrangement and a complementary electromagnetic actuator arrangement (e.g., forcers) which operatetogether for coarse positioning of the reticle. The gas spring arrangement may comprise at least one gas spring at each end of the (coarse) reticle stroke.

[0028] Gas springs have a very high force to moving mass ratio. For example, the gas spring may be able to provide and exceed the desired accelerating forces on their own. As such, it is proposed in some examples that the electromagnetic actuator arrangement is used (for at least a portion of the reticle stroke) predominately to reduce and / or minimize jerk and / or smooth movement of the reticle rather than actively accelerate the reticle, the required acceleration forces being predominately provided by the gas spring arrangement. Accordingly, in some examples, it may be that, for at least some portions of the reticle stroke (e.g., when reversing the direction of the reticle stage at the end of each stroke), the electromagnetic actuator arrangement is used against the force provided by the gas spring arrangement. For example, the gas spring arrangement may provide a force in excess of the force required for a desired acceleration when turning the stage around, and the electromagnetic actuator arrangement may provide a compensatory force in the opposite direction to the gas spring arrangement, thereby together generating the desired force and therefore desired reticle acceleration.

[0029] The gas spring arrangement and the electromagnetic actuator arrangement may be operable to each apply respective complementary forces to said stage apparatus so as to controllably accelerate said stage apparatus for coarse positioning of said stage apparatus.

[0030] The electromagnetic actuator arrangement may apply a force on said stage apparatus to compensate for non-linearity of the force applied on said stage apparatus by the gas spring(s), the gas springs otherwise predominately being used to accelerate the stage apparatus at the turnaround or reversal of the stage apparatus.

[0031] The gas springs may be coupled to the stage apparatus throughout each full stroke of the stage apparatus.

[0032] In an embodiment, the gas spring arrangement may provide a force which generates an acceleration on the reticle stage greater than 350m / s2,greater than 500m / s2, greater than 600m / s2, greater than 700tn / s2, greater than 800m / s2, greater than 900m / s2greater than 1000m / s2greater than 1200m / s2greater than 1400m / s2greater than 1500m / s2greater than 1700m / s2or greater than 1900m / s2for example.

[0033] In an embodiment, each gas spring of the gas spring arrangement may comprise an enlarged active area on which a gas pressure is exerted to accelerate the stage. For example, each piston active area may be larger than 50cm2, larger than 100cm2, larger than 130cm2, larger than 140cm2or larger than 150cm2.

[0034] In an embodiment, each gas spring may comprise a controllable gas supply for providing a controllable pressure within the piston chamber and therefore a controllable force on the piston. In some examples, one, some or all of the gas springs may comprise a controllable gas supply on both sides of the piston within the piston cylinder, thereby providing a controllable pressure on both sides of the piston within the piston chamber and therefore a controllable force on both sides of the piston. Notethat the term cylinder is used herein according to the definition of a chamber within which a piston moves, rather than its geometric meaning. As such, the cylinder and piston may each have any complementary cross-sectional shape and no limitation to a cylindrical cylinder (circular cross section) should be assumed or inferred.

[0035] Normal gas pistons have a seal to keep the gas inside, and use a lubricant (e.g., oil) to make the seal. This is not desired for the reticle stage, as such a seal would be a source of pollution and because friction will make the motion erratic. As such, at least some examples may use gas pistons which comprise gas bearings (or “air bearings”, although they may use any gas) to seal the gas springs.

[0036] Some of the examples herein may locate the gas springs in a low-pressure or vacuum environment. For example, the reticle stage environment may be divided into a “dirty vacuum” compartment and a “clean vacuum” compartment, the latter comprising the reticle. It is proposed that the gas springs (e.g., and other actuators) may be comprised within the dirty vacuum compartment. For example, the gas springs may comprise one or more air-bearings which seal towards the dirty vacuum. Gas bearings may operate at several bars of pressure, and with a narrow gap of the order of approximately 5 micrometers.

[0037] The gas bearing requires a higher pressure than the surrounding pressure adjacent to the bearing. This can be realized by application of a very high pressure gas bearing (e.g., greater than 50 bar). Alternatively, a lower pressure gas bearing (e.g., less than 20 bar, less than 10 bar, less than 8 bar or less than 6 bar) may be used, with such an arrangement further comprising a suction gap between the high pressure gas spring and gas bearing to maintain the pressure near the gas bearing below the gas bearing supply pressure. In such a suction gap, the pressure is only required to be maintained below the gas bearing pressure, a deep vacuum is not required. In all embodiments described herein which comprise gas bearing, the gas bearings may comprise a suction gap around the gas bearing.

[0038] Any gas bearing described herein may comprise for example an amorphous carbon gas bearing or porous gas bearings. By way of a specific example, carbon or graphite porous bearings have high stiffness and load capacity with good crash resistance.

[0039] Optionally, it may be that no gas bearings are required.

[0040] In an embodiment, each gas spring may comprise an exhaust system for removing gasses from the piston cylinder(s) and / or gas bearing(s).

[0041] Figure 2 is a schematic drawing of a basic arrangement implementing the concepts disclosed herein. The Figure shows a long stroke module 200 (coarse positioning apparatus) supporting a reticle 205 (more specifically supporting a short stroke module, not shown, which in turn supports the reticle 205. The long stroke module 200 is attached to shafts 210 (there may be two such shafts in parallel i.e., a second parallel shaft behind the shaft 210 as depicted in this view) via respective connecting structures or structural members 215. The shafts 210 may comprise support shafts or gas bearing shafts, for example.

[0042] Electromagnetic actuators or electromagnetic forcers 220 may act on the shafts 210 thereby moving the long stroke module 200 and reticle 205. Any suitable electromagnetic actuator arrangement may be used, which may be located in any suitable location of the stage apparatus so as to impose a force on the long stroke module 200.

[0043] One or more gas springs 225 may be provided respectively at each end of each shaft 210; each are similar and therefore only one gas spring 225 is labeled comprehensively in the drawing. In an example, there may be four gas springs, one at each end of two parallel shafts 210. The end of each shaft may form the piston 230 of the gas spring 225, the piston 230 being displaceable within a piston cylinder 235 within cylinder housing 240, e.g., such that the shafts 210, structural members 215, long stroke module 200 and reticle 205 comprise the moving mass and move together (left / right as illustrated), with the shaft ends / pistons 230 traveling within the fixed cylinder housings 240.

[0044] A gas supply 245 may introduce gas into piston cylinder 235. For example, supply 245 may introduce gas controllably into piston cylinder 235, e.g., thereby enabling the cylinder pressure to be regulated at a controllable desired pressure. For example, the gas supply 245 may comprise pressure regulator 250 and valve 255 (although other arrangements are possible) between a gas source and the piston cylinder 235. The supplied gas (for all embodiments) may comprise hydrogen (Hi) gas, for example. Other gases may be supplied in addition to or instead of hydrogen. Hydrogen may be preferred as it is often already present in the dirty vacuum environment and therefore any leak will not substantially change the gas composition within that environment. In some circumstances, for example, gasses which are easier handled and / or less volatile may be preferred such as nitrogen, a noble gas or air (e.g., a clean dry air). This may be case during a maintenance action such as servicing of the gas springs.

[0045] Gas bearings 260 may be provided for the gas springs 225. For example, as illustrated here, the gas bearings 260 may be incorporated within the cylinder housings 240. A benefit of this is that there is no additional gas leak source with respect to the gas bearings presently used on the shafts, and there is no vibration or alignment penalty. Gas bearing gas may comprise nitrogen (N2) gas for example.

[0046] In known arrangements, the reticle stage may be slidably mounted on (fixed) shafts via gas bearings. In this example arrangement, the gas bearings may be incorporated within the cylinder housing 240, with the shafts slidably mounted within the fixed cylinder housings 240, the reticle stage being fixed to the shafts 210. However, as will be described, other arrangements are possible.

[0047] A moving seal 265 (e.g., which moves with the long stroke module 200) may provide a barrier between a dirty vacuum area 270 and clean vacuum area 275.

[0048] Though not shown in this Figure, the gas spring cylinders in this and many of the examples described herein may be mounted on a balance mass framework to control / manage the reaction forces.

[0049] It can be appreciated that there is limited moving mass required to create the spring functionality. Also, it is proposed that there is always some spring force present on the stage to prevent any sudden onset of spring force (such as a hammer action).

[0050] Note that the stroke of the cylinder is longer in reality than depicted here (in all the Figures); the stroke would likely be of the order of 50 cm.

[0051] Since the force applied by the gas spring is proportional to the product of the compressed gas pressure and the area to which the pressure is applied, it may be preferred to increase this area.

[0052] Figure 3 illustrates a gas spring 325 design which may be used in place of gas springs 225 of Figure 2. This gas spring 325 comprises a piston 330 with increased piston area. This piston area may be larger than 50cm2, larger than 100cm2, larger than 130cm2, larger than 140cm2or larger than 150cm2for example.

[0053] Because the piston 330 has a larger diameter than the shaft 210, it divides the cylinder volume into a first cylinder volume 335 and second cylinder volume 380 respectively on either side of piston 330. As before, the first cylinder volume 330 may comprise a gas supply 245.

[0054] The piston may comprise additional gas bearings 360 to enable frictionless movement along the cylinder wall (although this may not be necessary). The second cylinder volume 380 may comprise a venting arrangement 385 to vent any inflowing gas from the gas bearings 360. This can either go into the dirty vacuum, or with a dedicated removal line.

[0055] In an embodiment, in place of venting arrangement 385, a second gas supply (not depicted) may be provided for the second cylinder volume, i.e., such that the first cylinder volume 335 and second cylinder volume 380 each comprise a respective dedicated gas supply. By selectively varying the amount of gas individually in each of the first cylinder volume 335 and second cylinder volume 380, it is possible to position the working point of the gas spring along the direction of travel. By regulating the pressure in each volume prior to operation, while taking into account the positioning information of the long stroke, it is possible to select the desired force vs position curve of the gas spring. These setpoint possibilities (the pressure and volume of gas in each compartment at rest) can help in adapting the spring action to the required reticle expose length and / or required acceleration. It is envisioned that the gas spring may be sufficiently long to support the complete stroke of the reticle stage (e.g., about 300 mm maximum), so that during operation, the piston can move back and forth without leaving the cylinder. By providing double action gas springs with gas supplies either side of the piston, there are 8 remotely settable parameters (for two gas springs) which may be used to optimally match the required stroke and acceleration (4 gas pressures and 4 gas volumes).

[0056] The system is also backwards compatible: if the gas volumes are evacuated, the piston runs without friction, and the system has no longer any spring action, though it incurs the extra mass of the pistons. An alternative way to achieve this would be to have a communicating opening / connection between the two gas volumes in a double acting gas spring; the opening being actuatable such that it can be selectively opened and closed.

[0057] In all examples described herein, the gas supply (or each gas supply) may provide a peak pressure within the gas spring of the order of 5-50 bar. For example, for a piston area of 100 cm2, and a delta pressure in compressed state of 50 bar, the max force is 100*500 = 50kN. To create a substantialportion of the required force to accelerate to e.g., 960 m / s2with such gas springs requires very large springs, multiple springs and / or very high peak pressures. On the other hand, the pressure with the reticle stage in rest (centered in the stroke) may be substantially lower, for example 5-10 bar. Therefore tuning of gas cylinder pressures is likely most conveniently done when the reticle stage is at rest.

[0058] It is desirable to begin the reticle stage with limited preparation time. If linear forcers are provided which can accelerate the reticle stage from standstill to 8 m / s within the length of a reticle (e.g., in this specific example: 132 mm), then the system can be ready for scanning at 8 m / s after a single bounce.

[0059] It is proposed that the reticle stage may be capable of acceleration in excess of 900 m / s2, with the long stroke forcers generating substantially less than half of the required turnaround acceleration, the remaining turnaround acceleration being provided by the gas springs. In this context, turnaround acceleration comprises the acceleration applied to the reticle stage to reverse its direction at the end of each reticle stroke. It is envisioned that the total long stroke force is a summation of the force provided by the gas spring(s) and the force provided by the long stroke electromagnetic actuators. Optionally, the gas springs may provide a larger maximum spring force than is required, with this excess force being compensated by driving the long stroke motors counter to the gas spring force at the maximum spring compression.

[0060] Figure 4(a) is a plot 400 of a desired (total) force F profile on the reticle stage with respect to reticle displacement D from center (0) for a reticle stage speed reversal. The profile corresponds to only half a stroke, and will be mirror symmetric around the force axis for the other half of the stroke. The profile comprises three distinct portions. In a first portion, between center (zero displacement) and position a (e.g., up to 82 mm assuming a scan length of 132mm), the desired applied force is zero. This portion corresponds to the stage traveling at a constant 8m / s (during the main exposure scan) and therefore no acceleration is desirable. A second profile portion relates to the region between stage position a and stage position b. During this portion, it is desirable that an increasing force is applied with displacement away from the center (e.g., where the force increases substantially linearly with displacement). During this portion, the force applied is limited by jerk. Jerk linearly increases with acceleration (i.e., force). A third profile portion relates to the region between stage position b and stage position c, corresponding to the end of the stroke (turnaround portion). This flat portion describes the maximum force (the turnaround force) being applied so as to change the direction of the stage. It is desirable to turn the stage around as fast as possible. Therefore, during this portion, a stage acceleration greater than 900m / s2, e.g., at 960m / s2may be desirable, to accelerate the stage from positive to negative velocity (or vice versa).

[0061] The acceleration and the corresponding force provided by the gas springs may, for example, be greater than the desired stage acceleration during this portion, e.g., greater than 1200m / s2, greater than 1300m / s2greater than 1400m / s2or greater than 1500m / s2. In that manner, the force delivered by the gas springs is better matched, and the long stroke actuators have to deliver less counterforce.

[0062] It can be seen that the desired force remains constant once the max acceleration is reached. The graph is traversed in each direction, i.e., once position c is reached the desired profile follows the same line in the opposite direction as the stage bounces back towards the center (zero displacement).

[0063] Figure 4(b) is an equivalent plot 410 to that of Figure 4(a), for a typical force profile obtain by a (e.g., dual acting) gas spring. It can be seen that there is a long, relatively flat area between the center position and position a, corresponding to a small applied force. This is followed by a steeply increasing force when nearing the end of the travel range. This is quite a good match for the desired profile of Figure 4(a): minimal force during the scan portion of the reticle, and a linearly increasing force for the speed reversal at the end of the stroke. However, it is not a perfect match and therefore the long stroke forcers are still required, which act together with the gas spring force to create a better match to the desired force profile 400.

[0064] Figure 5 comprise plots which illustrate how the gas springs on each side may work together to provide a reversal force on the stage, and how this gas spring supplied force may be used in combination with forcer supplied force to obtain a desired stage acceleration.

[0065] Figure 5(a) is a plot 500 of stage displacement D against time t during a full stroke comprising a first reversal period during which the direction of travel of the reticle stage is reversed, a reticle scan period S during which the reticle is scanned at a substantially constant scan velocity, and second reversal period R2 during which the direction of travel of the reticle stage is again reversed.

[0066] Figure 5(b) is a plot of force F applied by the gas springs against time t during the same full scan period in an example stage control strategy. Three plots are shown, a first plot 510L of the force applied by the left hand side gas spring(s), a second plot 51 OR of the force applied by the right hand side gas spring(s) (both gray lines) and the resultant combined applied gas spring force 520 (black line), comprising a sum of the first plot 510L and second plot 51 OR.

[0067] Figure 5(c) is a plot of total applied force (gas springs and forcers) to the reticle stage against time t during the same full scan period. This plot shows the total force applied to the stage 530 (dotted gray line, corresponding to desired / ideal profile 400 of Figure 4(a)), the gas spring force 540 applied (black solid line) and the forcer force applied 550 in an example stage control strategy.

[0068] In this strategy, during a large proportion of the reversal periods Rl, R2, e.g., in particular when the maximum applied force Fmaxhas been reached, the force 540 applied by the gas springs may be deliberately set to be in excess of the desired force 530, with the forcers applying a counter force 550 (force in the opposite direction) to compensate such that the combined applied force 530 (sum of the gas spring force 540 and forcer force 550) substantially comprises the desired force. However, it can also be seen that there are times during the reversal period where the force applied by the gas springs and forcers act in the same direction.

[0069] During the scan period S, the forcer force 550 is used to counteract the non-zero gas spring force 540 applied, such that substantially zero combined force is applied to obtain a substantially constant scan speed. It is preferable to always maintain a gas spring force 540 on the stage even whenno acceleration is wanted, and counteracting the gas spring force with the forcers, rather than (for example) decoupling the gas spring from the stage during the central scan period S. This is because the required recoupling of the gas spring at the end of each scan would cause a large jerk.

[0070] Note that this is an optional control strategy, and other control strategies may use smaller maximum forces closer to and possibly below the desired maximum force (in which case the gas springs and forcers may act together in the same direction to achieve the maximum force).

[0071] Figure 6 schematically illustrates a more complete example of one side of a gas spring arrangement for a reticle stage. The gas spring in this example comprises of a dual working cylinder. The other side of the arrangement would be essentially identical or similar around the axis of symmetry AS.

[0072] Shown is reticle stage structural member 615, which connects the shafts 610 to the long stroke module and reticle (not shown). The shafts are actuated via electromagnetic forcers 620 and at least one gas spring 625 (per side). As with all embodiments described herein, there may be a pair of shafts 610 and two gas springs 625 per side. The gas spring(s) comprise a piston 630 within a cylinder housing 640. Being a dual working cylinder, there is a first gas supply 645a (comprising for example pressure regulator 650a and valve 655a) for controlling the pressure in a first (primary) cylinder volume 635 acting on the front side of the piston 630 and a second gas supply 645b (comprising for example pressure regulator 650b and valve 655b) for controlling the pressure in a second (primary) cylinder volume 680 acting on the back side of the piston 630.

[0073] The illustrated example comprises first gas bearing 660a, e.g., a relatively low pressure gas bearing (such as less than 10 bar, less than 8 bar or less than 6 bar for example), fed via a (e.g., low pressure) gas bearing supply 617. The example shown may also comprise a second gas bearing 660b on the piston 630. This gas bearing may be a high pressure gas bearing (e.g., having a pressure greater than the maximum pressure of the gas supplies 645a, 645b) fed by a (e.g., high pressure) gas bearing gas supply 637. The gas bearing gas supply 637 may transport the gas to the bearing 660b via a supply line located within the corresponding shaft 610 and piston 630.

[0074] The example arrangement also comprises a pressure lowering stage (e.g., e.g., to lower the pressure to less than 5 bar, less than 3 bar or less than 2 bar between the cylinder and first gas bearing 660a). This may be implemented by a first exhaust 607 before the first gas bearing 660a towards the dirty vacuum. The example arrangement may also comprise a final vacuum area implemented by a vacuum pump 627 to reduce the outflow of gas bearing gas into the dirty vacuum. The vacuum pump may lower the pressure on an order of magnitude of e.g., 10 mbar.

[0075] It can be appreciated that many of these details (pressure lowering stage, vacuum pump, gas bearings and supplies, cylinder gas supplies) are equally applicable and may be implemented on the other examples described herein, but will not be explicitly described in each case for brevity.

[0076] Figure 7 is another example reticle stage arrangement comprising gas springs 725. In the examples described above, the gas springs were comprised integrally with the reticle stage gas bearingsystem. If this is not desired, the gas springs 725 can also be realized separately from the (main) gas bearings 718. In this arrangement, the main gas bearing shafts 710 are fixed, such that the forcers 720 and gas springs 725 move the structural member 715 along the shafts 710. The gas springs 725 are not described in detail, but may comprise any of the gas springs described herein; only an example is illustrated.

[0077] Each actuation rod 728 connecting the structural member 715 (or more generally the reticle stage / moving body) to a respective gas spring 725 may comprise a spring or flex arrangement 738, e.g., a leaf spring arrangement, so as to minimize lateral stiffness between gas spring system and the moving mass.

[0078] Figures 8(a) and 8(b) illustrate two alternative gas spring arrangements which may be used in place of any of the gas springs in the example stage apparatuses and arrangements disclosed herein. In this embodiment, the gas spring comprises a central piston 830 which is supported at both sides by a shaft 833 and gas bearing 860. The gas spring may comprise a dual acting gas spring, having respective gas supplies 845a, 845b either side of the piston 830. Detail of the gas supplies 845a, 845b, and other design elements such as gas lines to the gas bearings and additional exhausts around the gas bearings are omitted for clarity.

[0079] The piston 830 is attached to the shaft 833 such that they move as one body. The shaft 833 can move with respect to the cylinder housing 840, 840a, 840b comprising gas bearings 860. A benefit of this example is that the shaft 833 and piston 830 position are determined by the two gas bearings 860. This means that the piston 830 does not require an gas bearing, simplifying the design considerably (a sufficiently large gas bearing for the piston diameter may be cumbersome).

[0080] The piston 830 may be made longer than the pistons of other examples, so as to provide a larger hydraulic resistance in the gap between piston and cylinder wall.

[0081] The difference between the gas spring of Figure 8(a) and of Figure 8(b) is in the cylinder housing. In Figure 8(a) the cylinder housing 840 is a one-piece housing, while in Figure 8(b) the cylinder housing 840a, 840b comprises a cylindrical component 840a, and end components 840b. End components 840b comprise the gas bearings 860. A seal 843 may be provided (e.g., an O-ring seal) to seal the cylinder chamber from the environment. The gas spring of Figure 8(b) is simpler to manufacture. It can be appreciated that many of the other gas spring examples described herein may also be manufactured with separate cylinder and end components, and it should be assumed that this basic concept may be equally applicable to all the gas spring examples described herein.

[0082] The gas springs of Figure 8 may be used in two ways: with the shaft connected to the reticle stage or with the cylinder housing connected to the reticle stage. The gas springs could fulfil the reticle stage gas bearing and spring function at the same time, as is the case for the examples of Figure 2 and 6. If the gas bearing functionality is not desired for load carrying the reticle stage, the shaft or the cylinder housing could be mounted on flexible mountings (stiff in the travel direction).

[0083] The gap between the piston 830 and cylinder wall may be, for example between 12pm and 3 pm, between 11pm and 4pm or between 10pm and 5pm. When the stage is in rest (centered), the pressure on both sides of the piston 830 is the same, and there is no flow through the air gap. The provision of gas supplies either side of the piston 830 means that an unbalanced pressure either side of the piston 830 may be actively maintained (e.g., by introducing gas on one side and / or removing gas on the other side).

[0084] Returning to Figure 4, it is apparent that the main difference between the Force-Displacement relationship of a real gas spring 410 and the ideal Force-Displacement relationship 400 is that for the real relationship 410, the force keeps increasing with displacement, while it would be preferred if this force was to flatten to a constant maximum force.

[0085] Figure 9 is an example gas spring arrangement which is designed to provide a substantially flatter force-position response once a certain piston displacement from the center is reached. The basic concept is to increase the effective cylinder volume at a certain threshold displacement from center. This may be achieved, for example, by an arrangement where a primary cylinder volume is coupled to (e.g., brought into communication with) a secondary cylinder volume at the threshold displacement.

[0086] Figure 9 shows the example gas spring at three different piston 930 displacements. The basic gas spring design is similar to that of Figure 8, comprising gas bearings 960 at each end of the cylinder housing 940. However, on each side of the piston 930 are respective primary cylinder volumes 935a, 935b and respective secondary cylinder volumes 943a, 943b. Each primary cylinder volume 935a, 935b is selectively connected to its respective secondary cylinder volume 943a, 943b via a respective coupling conduit 944a, 944b. The shaft on either side of the piston 930 comprises a relatively thinner shaft portion 933b (second shaft portion) adjacent the piston 830 and thicker shaft portion 933a (first shaft portion), e.g., with a step change therebetween, starting at a point along the shaft length away from the piston 930.

[0087] Figures 9(a) to 9(c) show such a gas spring at three different displacements, while Figure 10 is a plot of force F against displacement D for such a gas spring. Positions a, b and c correspond to positions a, b and c of Figure 4. Figure 10 shows the ideal relationship 410 and actual relationship 1000 obtained using a gas spring of Figure 9 (and Figures 11-14).

[0088] In Figure 9(a) the gas spring is shown centered. The thicker shaft portions 933a prevent any communication between the primary cylinder volumes 935a, 935b and their respective secondary cylinder volumes 943a, 943b, as thicker shaft portions 933a block each of the coupling conduits 944a, 944b, substantially sealing the primary cylinder volumes 935a, 935b from the secondary cylinder volumes 943a, 943b.

[0089] Figure 9(b) shows the gas spring after a gas 935 has been admitted into primary cylinder volume 935b and the piston 930 is displaced from the center (e.g., towards the right as shown), thereby compressing the gas 935. The thicker shaft portion 933a is still preventing communication between primary cylinder volume 935b and secondary cylinder volume 943b via coupling conduit 944b.

[0090] The position range between Figure 9(a) and Figure 9(b) largely corresponds to the position range bounded by positions a and b in Figure 10. This portion of the force position relationship is unchanged from that illustrated in Figure 4(b) between positions a and b.

[0091] At a displacement shortly after the situation in Figure 9(b) (moving away from the center), the step between the thicker shaft portion 933a and thinner shaft portion 933b moves beyond the position of coupling conduit 944b. This step defines the threshold displacement at which the smaller diameter of the thinner shaft portion 933b brings the primary cylinder volume 935b into communication with the secondary cylinder volume 943b via coupling conduit 944b. When this happens, the gas 953 that was contained within the primary cylinder volume 935b is now contained within the larger volume provided by the sum of primary cylinder volume 935b and secondary cylinder volume 943b (and coupling conduit 944b). This is illustrated in Figure 9(c). It can be appreciated that the pressure in the primary and secondary volumes will be equalized, and the two volumes have the same pressure during the time that the volumes are connected.

[0092] The effect of this is illustrated in Figure 10. Position b represents the threshold displacement from center corresponding to the step between the thicker shaft portion and thinner shaft portion being moved beyond the coupling conduit. The resultant sudden increase in volume prevents the pressure, and therefore the force 1000 provided by the gas spring, from continuing to rapidly rise between position b and position c (the full extent of the stroke). Instead of following the dotted line (e.g., the line 410 observed in Figure 4(b)), the force relationship significantly flattens (i.e., rises more slowly) with displacement between positions b and c, such that the ideal relationship 400 is more closely followed.

[0093] The piston 930 keeps moving until the velocity is zero. The direction then reverses and the pressure slowly reduces until the secondary cylinder volume 943b is decoupled from the primary cylinder volume 943a (i.e., when the thicker shaft portion again blocks the coupling conduit). At that point, the secondary cylinder volume 943b stays at a fixed pressure, while the primary cylinder volume 943a continues to drop as the stage moves towards and past the center position and on to the other side. After having bounced in the same way at the other side (coupling and decoupling the other secondary cylinder volume 943a to the other primary cylinder volume 933a), the stage returns to again couple secondary volume 943b to primary cylinder volume 933b, at which point the pressure in the primary and secondary volume will be substantially the same, so that no shock or vibration will result. As such, if for any reason the two volumes were not equalized at the start of operations, after a few bounces they will be at equal pressure at the moment that the secondary volume is connected.

[0094] It can be appreciated, of course, that the thinner shaft portion may instead be embodied as grooves in the shaft (i.e., a reduced thickness for only a portion of the outer surface), a hollow shaft or any other suitable arrangement for selectively blocking communication between the primary and secondary cylinder volumes. As such, the thinner or second shaft portion may comprise a second thickness which is smaller than the first thickness of the first / thicker shaft portion for at least a portion of an outer circumference of said second shaft portion.

[0095] Figures 11 to 14 illustrate a number of variations on the basic idea illustrated in Figure 9 usable in stage embodiments described herein. Figure 11 shows an arrangement where the secondary cylinder volumes 1143a, 1143b are comprised within the cylinder housing 1140. The cylinder housing 1140 may also still house the gas bearings 960. This example operates in the same manner as that illustrated in Figure 9 and as such, the manner of operation will not be described again.

[0096] Figure 12 shows another gas spring arrangement usable in stage embodiments described herein, where the threshold displacement and therefore the start-point of the flattening off of the force / displacement relationship is adjustable. In this embodiment, instead of a shaft comprising fixed thinner and thicker portions, the shaft 1290 is relatively thin and comprises a sleeve or sheath 1291 over a portion of its length. By any suitable mechanism (e.g., a motorized mechanism and / or worm screw mechanism), the sleeve 1292 on either side of the piston 930 may be movable with respect to the shaft 1290, therefore moving the position of the step thickness change 1292 at which the secondary cylinder volumes 1143a, 1143b come into communication with the primary cylinder volume 935a, 935b.

[0097] By making the flattening off start-point adjustable, the gas spring may be tailored for different reticle scan lengths. An alternative strategy may be to ensure that the turn around speed is so fast that it is not necessary to reverse immediately after the scan is completed; e.g., the reticle stage always does full field scans regardless of the requested length. If the reticle stage is too fast, the wafer stage cannot keep up in any case.

[0098] It can be appreciated that the sleeve may comprise grooves (i.e., a reduced thickness for only a portion of the outer surface) along at least a portion of its length which bring the two volumes into communication at a configurable displacement, rather than effectively increasing the shaft thickness, or may comprise any other suitable arrangement

[0099] This example otherwise operates in the same manner as that illustrated in Figures 9 and 11 and as such, how it operates will not be described again. However, it can be appreciated that the shaft / sleeve arrangement may equally be incorporated with the Figure 9 example.

[0100] Figure 13 illustrates another gas spring arrangement usable in stage embodiments described herein. Figure 13(a) shows a gas spring where the primary cylinder volumes 1335a, 1335b are adjustable, e.g., by displacement of displaceable rear cylinder wall members 1391 with respect to shafts 1390. This may be achieved via any suitable mechanism (e.g., a motorized mechanism and / or worm screw mechanism 1394).

[0101] Figure 13(b) describes a combination of the gas spring of Figure 13(a) and the gas spring of Figure 11. As such, it comprises displaceable rear cylinder wall members 1395 mounted on a shaft comprising a thinner shaft portion 1133b and thicker shaft portion 1133a (or grooves along part of its length etc.). The displaceable rear cylinder wall members 1395 comprise an aperture 1396 sized substantially the same as the thicker shaft portion 1133a. Aperture 1396 provides a coupling conduit which couples the primary cylinder volume 1335a, 1335b to its respective secondary cylinder volume 1143a, 1143b when the piston 930 is displaced such that the thinner shaft portion 1133b is within theaperture 1396. When the thicker shaft portion 1133a is located within aperture 1396, aperture 1396 is blocked and the primary and secondary volumes decoupled. This embodiment provides adjustable primary and secondary volume sizes and flattening of the force-position relationship.

[0102] Figure 14 illustrates another gas spring arrangement usable in stage embodiments described herein. In this embodiment, the coupling conduit 1444a, 1444b between the primary cylinder volume 1435a, 1435b and secondary cylinder volume 1443a, 1443b is comprised within the piston 1430. In Figure 14(a), the piston 1430 is centered and neither coupling conduit 1444a, 1444b is aligned with its respective secondary cylinder volume 1443a, 1443b and the gas 1453 is confined to the primary cylinder volume 1435b. At a point along the scan, as illustrated in Figure 14(b) the coupling conduit 1444b is aligned with an inlet for the secondary cylinder volume 1443b, at which point the gas 1453 pressure is equalized between the primary cylinder volume 1435b and the secondary cylinder volume 1443b. The same applies when the displacement is in the other direction to connect the primary cylinder volume 1435a and the secondary cylinder volume 1443a via coupling conduit 1444a. It can be appreciated that the general operation of this gas spring is as has already been described.

[0103] In another embodiment, not shown, instead of the coupling conduit being comprised within a piston, it may be comprised within at least one of the piston shafts (e.g., connecting the piston to the stage apparatus). For example, the shafts may be hollow, with apertures or aperture arrangement, such that at least one of the apertures only becomes unblocked when the stage is at a certain displacement from center, thereby coupling the secondary gas volume(s) to the primary gas volume(s). On the return travel toward the center, the same aperture(s) will again become blocked decoupling the volumes again. An advantage of using a hollow shaft is that the components will be lighter while serving the same purpose. As another alternative, the shaft may be solid and comprise bores into the shaft body, connected by a channel connecting. The arrangement works the same as the hollow shaft embodiment just described, with the bores acting as the apertures.

[0104] Figure 15 is an additional embodiment of a stage apparatus comprising a single gas spring 1525, or single pair of parallel gas springs, in which the cylinder housing 1540 is affixed to the reticle stage; e.g., to structural members 1515 connecting the long stroke module 1500 and reticle 1505 to the cylinder housing 1540. In this embodiment, the piston 1530 is fixed in position, such that the cylinder housing 1540, structural members 1515 and long stroke module 1500 etc. comprise the moving mass, moving along shaft(s) 1533 on gas bearings 1560. The shaft(s) 1533 may be mounted to a balance mass arrangement 1599. As with the other embodiments, such an arrangement will comprise electromagnetic actuators or forcers (not shown).

[0105] Figure 16 illustrates another gas spring arrangement usable in stage embodiments described herein. This example gas spring comprises two pistons 1630a, 1630b. A primary piston 1630a and a secondary piston 1630b. An end stop 1655 prevents the secondary piston from traveling past a certain position towards the primary piston 1630a. In such an arrangement, the cylinder volume is divided into a primary cylinder volume 1635a, secondary cylinder volume 1635b and a tertiary cylinder volume1635c. Each of these cylinder volumes may be connected to a respective gas supply 1645a, 1645b, 1645c (alternatively the tertiary cylinder volume 1635c may simply be vented).

[0106] This gas spring arrangement enables a threshold pressure P2 for the secondary cylinder volume 1635b to be set. This threshold pressure should be above the pressure Pl of the primary cylinder volume 1635a for a first portion of the primary piston 1630a travel. At a point during this travel where the pressure Pl of the primary cylinder volume 1635a exceeds the threshold pressure P2 in the secondary cylinder volume 1635b, the secondary piston 1630b will move away from the primary piston 1630a, increasing the primary cylinder volume 1635a. This flattens the force-position relationship at larger displacements as has been described in relation to Figure 10. The position at which this flattening begins can be adjusted by appropriate setting of the threshold pressure within the secondary cylinder volume 1635b.

[0107] A coil spring between the secondary piston 1630a and the end wall of the cylinder housing 1640 may be used instead of a pressurized secondary cylinder volume.

[0108] Each of the pistons may comprise a respective gas bearing 1660b, 1660c, in addition to the shaft 1610 gas bearing 1660a.

[0109] Figure 17 illustrates a similar stage arrangement to that illustrated in Figure 6. Where elements are the same, they will not be described again. This gas springs 1725 of this arrangement each comprise a single gas supply 645a supplying gas to the first cylinder volume 635. Also, there is no bearing on the piston 630. The second cylinder volume 680 is pumped, e.g., using an exhaust pump 1707, so as to remove leakage past the piston 630 from the first cylinder volume 635 and from the gas bearing 660a.

[0110] In all the embodiments described, one or more of the gas supply lines may be comprised within a cable slab of the reticle stage.

[0111] In each of the embodiments described, the gas spring volumes and pressures may be set such that the spring contributes the force required given a certain reticle scan length. As has been mentioned, some examples and embodiments may impose a gas spring force greater than required for the acceleration requirement (e.g., equivalent to an acceleration greater than 1300 tn / s2greater than 1500 m / s2greater than 1700 m / s2or greater than greater than 1900 m / s2at the maximum extension. This ensures that the force requirement on the long stroke forcers are lower, while turning around the stage more quickly. In some embodiments, it is possible that the force requirement for forcers may be sufficient for generating only less than 500 m / s2, less than 400 m / s2or less than 350 m / s2of the stage acceleration.

[0112] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0113] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic apparatuses or lithographic tools. Such a lithographic apparatus may use vacuum conditions or ambient (non-vacuum) conditions.

[0114] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A coarse positioning apparatus for a stage apparatus, the coarse positioning apparatus comprising: at least one gas spring, each said at least one gas spring comprising a piston; and at least one electromagnetic actuator; wherein said at least one gas spring and said at least one electromagnetic actuator are each operable to apply respective complementary forces to said stage apparatus so as to accelerate said stage apparatus for coarse positioning of said stage apparatus; and wherein at least said gas spring is coupled to said stage apparatus throughout each full stroke of said stage apparatus.

2. A coarse positioning apparatus as claimed in claim 1, wherein said gas spring is configured to apply a force to said stage apparatus for substantially each full stroke of said stage apparatus.

3. A coarse positioning apparatus as claimed in claim 1 or 2, wherein each said gas spring comprises at least one gas supply inlet for controllably regulating a pressure in at least one primary cylinder volume of the gas spring.

4. A coarse positioning apparatus as claimed in any preceding claim, wherein each said gas spring comprises at least a first gas supply inlet for controllably regulating a pressure in a first primary cylinder volume of the gas spring and a second gas supply inlet for regulating a pressure in a second primary cylinder volume of the gas spring, said first primary cylinder volume and said second primary cylinder volume being comprised on respective opposite sides of said piston.

5. A coarse positioning apparatus as claimed in claim 3 or 4, comprising respective gas supplies for each said gas supply inlet.

6. A coarse positioning apparatus as claimed in claim 3, 4, or 5, comprising at least one displaceable rear cylinder wall member for controllably varying said at least one primary cylinder volume.

7. A coarse positioning apparatus as claimed in claim 3, 4, or 5, wherein each said gas spring comprises a primary piston and a secondary piston, said secondary piston being operable to displace and increase a respective said primary cylinder volume when a threshold pressure within the respective said primary cylinder volume is reached.

8. A coarse positioning apparatus as claimed in claim 7, comprising a secondary piston gas supply for setting a gas pressure behind said secondary piston and therefore setting said threshold pressure.

9. A coarse positioning apparatus as claimed in any preceding claim, comprising at least one gas spring on each side of said stage apparatus in a scan direction.

10. A coarse positioning apparatus as claimed in any preceding claim, comprising two gas springs on each side of said stage apparatus in a scan direction.

11. A coarse positioning apparatus as claimed in any of claims 1 to 8, wherein said at least one gas spring is arranged such that each said piston has a fixed position and a respective cylinder of each said at least one gas spring moves with said stage apparatus.

12. A coarse positioning apparatus as claimed in claim 11, wherein each said piston is positioned substantially in alignment with a center of each stroke.

13. A coarse positioning apparatus as claimed in any preceding claim, wherein said stage apparatus is a reticle stage apparatus.

14. A coarse positioning apparatus as claimed in any preceding claim, wherein said stage apparatus is an EUV reticle stage apparatus, for an EUV lithographic apparatus.

15. A coarse positioning apparatus as claimed in any preceding claim, comprising at least one gas bearing for substantially sealing each said gas spring with respect to its environment.

16. A coarse positioning apparatus as claimed in claim 15, wherein said at least one gas bearing comprises at least one gas bearing located around a shaft connecting the gas spring to said stage apparatus.

17. A coarse positioning apparatus as claimed in claim 15 or 16, wherein said at least one gas bearing comprises at least one gas bearing located around each said piston.

18. A coarse positioning apparatus as claimed in any preceding claim, wherein each said piston comprises a central piston with a shaft on each side of said piston being operable to support said piston.

19. A coarse positioning apparatus as claimed in claim 18, wherein each said shaft is mounted on an gas bearing.

20. A coarse positioning apparatus as claimed in any preceding claim, each said gas spring comprises at least one primary cylinder volume, at least one secondary cylinder volume and at least one coupling conduit, said at least one coupling conduit for selectively coupling a respective primary cylinder volume of said at least one primary cylinder volume to a respective secondary cylinder volume of said at least one secondary cylinder volume at a threshold displacement of said stage apparatus.

21. A coarse positioning apparatus as claimed in claim 20, wherein each of said at least one secondary cylinder volume is comprised within a cylinder housing of its respective gas spring.

22. A coarse positioning apparatus as claimed in claim 20 or 21, wherein each said at least one coupling conduit is comprised within said piston of its respective gas spring and / or comprised within a shaft connected to said piston of its respective gas spring.

23. A coarse positioning apparatus as claimed in claim 20, 21 or 22, wherein said selective coupling is effected by each at least one shaft coupled to said piston comprising a first shaft portion and a second shaft portion, said first shaft portion being operable to block a respective coupling conduit when said displacement is below said threshold displacement and said second shaft portion being operable to not block the respective coupling conduit when said displacement is above said threshold displacement.

24. A coarse positioning apparatus as claimed in claim 23, wherein said first shaft portion comprises a first thickness and said second shaft portion comprises a second thickness, wherein said second thickness is smaller than said first thickness for at least a portion of an outer circumference of said second shaft portion.

25. A coarse positioning apparatus as claimed in claim 23 or 24, wherein said first shaft portion is formed by said shaft comprising a sleeve, said sleeve being displaceable along said shaft to vary said threshold displacement.

26. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one gas spring is operable to impose a force on said stage apparatus sufficient to accelerate said stage apparatus at over 350 m / s2.

27. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one gas spring is operable to impose a force on said stage apparatus sufficient to accelerate said stage apparatus at over 900 m / s2.

28. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one gas spring is operable to impose a force on said stage apparatus sufficient to accelerate said stage apparatus at over 1400 m / s2.

29. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one gas spring and said at least one electromagnetic actuator are operable, for at least a portion of each said full stroke, such that said at least one gas spring imposes a force on said stage apparatus greater than a desired maximum force and at least one electromagnetic actuator imposes an opposing force on said stage apparatus.

30. A coarse positioning apparatus as claimed in any preceding claim, wherein each piston comprises an active area larger than 50cm2.

31. A coarse positioning apparatus as claimed in any preceding claim, wherein each piston comprises an active area larger than 100cm2.

32. A coarse positioning apparatus as claimed in any preceding claim, wherein each piston comprises an active area larger than 150cm2.

33. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one electromagnetic actuator applies a force on said stage apparatus to compensate for non-linearity of the force applied on said stage apparatus by said at least one gas spring.

34. A coarse positioning apparatus as claimed in any preceding claim, wherein said at least one gas spring and said at least one electromagnetic actuator are each operable to apply respective complementary forces to said stage apparatus so as to controllably accelerate said stage apparatus for coarse positioning of said stage apparatus35. A stage apparatus comprising: a coarse positioning apparatus as claimed in any preceding claim for coarse positioning of said stage apparatus; and a fine positioning apparatus for fine positioning of said stage apparatus.

36. A lithographic apparatus comprising: a stage apparatus as claimed in claim 35, constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam.

37. A lithographic apparatus as claimed in claim 36, comprising an illumination system configured to condition the radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate.

38. A method for positioning a stage apparatus comprising: applying a first force on said stage apparatus using at least one gas spring; and applying a second force on said stage apparatus using at least one electromagnetic actuator; wherein said first force and said second force comprise respective complementary forces so as to accelerate said stage apparatus for coarse positioning of said stage apparatus; and wherein at least said gas spring is coupled to said stage apparatus throughout each full stroke of said stage apparatus.

39. A method as claimed in claim 38, wherein said gas spring applies a force to said stage apparatus for substantially each full stroke of said stage apparatus.

40. A method as claimed in claim 38 or 39, wherein said first force is greater than a desired maximum force and said second force comprises a force opposing said first force on said stage apparatus, for at least a portion of each full stroke of said stage apparatus.

41. A method as claimed in claim 38, 39 or 40, wherein said second force is applied to compensate for non-linearity in said first force.

42. A method as claimed in any of claims 38 to 41, wherein said first force and said second force comprise respective complementary forces so as to controllably accelerate said stage apparatus for coarse positioning of said stage apparatus.

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