Patent Number: 
Section: description

In the Figures, corresponding reference symbols indicate corresponding parts. FIG. 1 schematically depicts a lithographic projection apparatus 1 according to an embodiment of the invention. The apparatus 1 includes a base plate BP; a radiation system Ex, IL constructed and arranged to supply a projection beam PB of radiation (e.g. EUV radiation), which in this particular case also comprises a radiation source LA; a first object table (mask table) MT provided with a mask holder that holds a mask MA (e.g. a reticle), and connected to a first positioning device PM that accurately positions the mask with respect to a projection system or lens PL; a second object table (substrate table) WT provided with a substrate holder that holds a substrate W (e.g. a resist-coated silicon wafer), and connected to a second positioning device PW that accurately positions the substrate with respect to the projection system PL. The projection system or lens PL (e.g. a mirror group) is constructed and arranged to image an irradiated portion of the mask MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. As here depicted, the apparatus is of a reflective type (i.e. has a reflective mask). However, in general, it may also be of a transmissive type, for example (with a transmissive mask). Alternatively, the apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above. The source LA (e.g. a discharge or laser-produced plasma source) produces a beam of radiation. This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed a conditioning device, such as a beam expander Ex, for example. The illuminator IL may comprise an adjusting device AM that sets the outer and/or inner radial extent (commonly referred to as "sgr"-outer and "sgr"-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section. It should be noted with regard to FIG. 1 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors). This latter scenario is often the case when the source LA is an excimer laser. The present invention encompasses both of these scenarios. The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning device PW and interferometer IF, the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 1. However, in the case of a wafer stepper (as opposed to a step and scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. The mask MA and the substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The depicted apparatus can be used in two different modes: 1. In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at once, i.e. a single xe2x80x9cflash,xe2x80x9d onto a target portion C. The substrate table WT is then shifted in the X and/or Y directions so that a different target portion C can be irradiated by the beam PB; 2. In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single xe2x80x9cflash.xe2x80x9d Instead, the mask table MT is movable in a given direction (the so-called xe2x80x9cscan directionxe2x80x9d, e.g., the Y direction) with a speed v, so that the projection beam PB is caused to scan over a mask image. Concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=xc2xc or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution. FIG. 2 illustrates the substrate table WT and the second positioning device PW in more detail. The second positioning device PW is a planar magnetic positioning device having a stator 10 comprising a plurality of magnets (not illustrated) arranged in rows and columns in a single plane. The magnets in the stator 10 may be arranged according to a Halbach array, i.e. the magnetic orientation of successive magnets in each row and each column rotates 90xc2x0 counterclockwise. The substrate table WT is provided with an electric coil system in its base. The electric coil system comprises two types of coils, one type having an angular offset of +45xc2x0, and the other having an offset of xe2x88x9245xc2x0 with respect to the checker board configuration of the magnets. The substrate table WT may be moved relative to the stator 10 by driving current through the electric coil system in the base of the substrate table WT. A mechanical limiter 100 limits rotation of the substrate table WT about a direction orthogonal to the stator 10. As shown in FIG. 2, reference number 12 denotes the X-axis and reference number 14 denotes the Y-axis and the mechanical limiter 100 limits rotation around the Z-axis (i.e. Rz). The mechanical limiter 100 includes an actuator 20 which moves in a direction 16 in the plane of the stator 10. The actuator 20 moves in direction 16 which is parallel to the Y-axis 14 along a guide or track 25. An elongate member 110 is fixedly attached to the actuator 20. A sleeve 120 which at least partly surrounds the elongate member 110 is fixedly attached to the substrate table WT. The sleeve 120 is slidable along the elongate member 110. In use, movement of the substrate table WT in the X-direction 12 results in the sleeve 120 sliding along the elongate member 110. If the substrate table WT moves in the Y-direction, the actuator 20 is also moved a corresponding amount in the Y-direction 14 so that the substrate table WT may move freely unhindered by the mechanical limiter 100. This is achieved by having a sensor for measuring the position of the actuator 20 with respect to the substrate table WT and which initiates the actuator 20 to follow the substrate table WT in the Y-direction. The substrate table WT can be moved by the second positioning device PW without interference from the mechanical limiter 100. Frictionless bearings such as air bearings may be used between the sleeve 120 and the elongate member 110. The mechanical limiter 100 may be provided with incremental or linear encoders 125, 127 to aid in alignment and control of the substrate table WT. Only one encoder is illustrated in FIG. 2. The encoder is of an interferometer type consisting of a sensor 125 attached to the sleeve 120 and a diffraction grating 127 attached to the elongate member 110. Other types of encoders, such as rotational potentiometers, can be used and the position in the Y-direction may also be measured. The position at which the encoders are fixed may also be different. Should the substrate table WT experience a rotational force about the Z-axis, engagement of the sleeve 120 and the elongate member 110 will substantially prevent rotation of the substrate table WT. Upon rotation of the substrate table WT around the Z-axis, each end of the sleeve 120 will come in contact on opposite sides of the elongate member 110. The amount of rotation of the substrate table WT about the Z-axis allowed before rotation is stopped by the mechanical limiter 100 may be adjusted by a choice of the length of the sleeve 120 in the elongate direction of the elongate member 110 and by a choice of difference in external dimension of the elongate member 110 and the internal dimension of the sleeve 120. Alternatively, the elongate member 110 and sleeve 120 may be attached such that some limited rotation relative to the actuator 20 or substrate table WT is possible. The mechanical limiter 100 may be designed to substantially prevent any rotation or may be designed to allow for rotation of up to 3xc2x0, 5xc2x0 or 10xc2x0. Software may be provided to limit the rotation around the Z-axis. However, mechanical solutions may also be provided as software may fail. Conduits 30 provide the substrate table WT with utilities. The conduits 30 may be attached between the substrate table WT and the actuator 20. In an alternative embodiment, the conduits 30 may be guided by the mechanical limiter 100 from the actuator 20 to the substrate table WT. In this way large mechanical loads on the conduits 30 can be substantially prevented as only one degree of freedom of the conduits is then required. The actuator 20 may be mounted on the track 25 and driven by a suitable device, such as a liner electric motor or a worm drive. Referring to FIG. 3, a mechanical limiter 200 includes at least one pair of crossed elongate arms 210, 220, 230. Each pair of crossed elongate arms are pivoted relative to each other at a median portion 211, 221, 231. A first pair 210 of the at least one pair of crossed elongate arms is an outer pair of elongate arms. An outer end 212 of one arm of the outer pair of arms 210 is rotatably attached to the substrate table WT. An outer end 216 of the other elongate arm of the pair 210 is in sliding engagement with a slot 214 in the substrate table WT. The slot 214 is parallel to the Y-axis 14. A second pair 220 of the at least one pair of crossed elongate arms is an outer pair of elongate arms. An outer end 222 of one arm of the outer pair of arms 220 is rotatably attached to the actuator 20. An outer end 226 of the other elongate arm of the pair 220 is in sliding engagement with a slot 224 in the actuator 20. The slot 224 is parallel to the slot 214 on the substrate table WT. Between the first pair 210 and the second pair 220 of crossed elongate arms, a third pair 230 of elongate arms is situated. The ends of the elongate arms of the first pair 210 and the second pair 220 not attached to the substrate table or actuator 20, are attached to ends of the elongate arms of the third pair 230. The invention also works with only a single pair of elongate arms in which case one end of each elongate arm will be attached to the actuator 20 and the other end to the substrate table WT. Alternatively only two pairs may be used, or any other number. In use, the mechanical limiter 200 is similar to the mechanical limiter 100 for movement in the Y-direction. For movement of the substrate table WT in the X-direction, the pairs of crossed elongate arms 210, 220, 230 pivot relative to each other in a scissor action to extend and retract. The ends 216, 226 engaged in slots 214 and 224 (which are parallel) allow this freedom to expand and contract and ensure that the limiter 200 can limit rotation. Dimensioning of the thickness of the elongate slots 214, 224 and the engagement portion of ends 216, 226 results in the desired degree of rotatability of the substrate table WT before limitation of rotation. The limiter 200, and the limiter 300 described below, may also have incremental or linear encoders attached to substrate table WT or the mechanical limiter 200, 300 (or the actuator in the case of the limiter 200) to measure the position of the substrate table WT and may have conduits 30 guided by the mechanical limiter 200, 300 to the substrate table WT. Referring to FIGS. 4a and 4b, a mechanical limiter 300 may be used in a lithographic apparatus having only one substrate table WT. The range of movement of the single substrate table WT in such an apparatus is small compared to the range of movement required of the substrate tables WT in an apparatus with dual substrate tables WT. Thus, the mechanical limiter 300 is more suited to an apparatus with only a single substrate table WT. The difference in the mechanical limiter 300 and the mechanical limiters 100 and 200 is that one end of the mechanical limiter 300 is fixed in a position relative to the stator 10 of the second positioning device PW rather than being movable in a direction of the plane of the stator 10. The required two degrees of freedom of the end of the mechanical limiter 300 attached to the substrate table WT is achieved by use of two four-bar mechanisms 301 and 302. These replace the elongate member 110 and slide 120 of the limiter 100. The first four-bar mechanism 301 includes of a first pair of elongate arms 310, 315 of equal length which are attached between the substrate table WT and a joining member 340. The first pair of elongate arms 310, 315 are pivotably attached at each end and the separation distance between their attachment portions on the substrate table WT and on the joining member 340 are equidistant such that the elongate arms 310, 315 always remain parallel. The second four-bar mechanism 302 includes a second pair of elongate arms 320, 325 of equal length. The second pair of elongate arms 320, 325 are attached at one end to the joining member 340 and at another end fixedly in relation to the stator 10 on a platform 330. Again, the ends of the second pair of arms 320, 325 are rotatably fixed equidistant from each other at each end. The first arm 310 of the first pair of arms and the first arm 320 of the second pair of arms are pivoted about the same point on the joining member 340. However, the arms 310, 320 may pivot at different positions on the joining member 340. The positioning of the ends of the elongate arms and the fact that both of the first pair of arms are the same length and that both of the second pair of arms are the same length means that when the substrate table WT moves the substrate table WT will be constrained in its rotation about the Z-axis by the mechanical limiter 300. If a wider range of rotation is required, the platform 330 may be attached to an actuator (i.e. actuator 20) such that a larger range of motion of the substrate table WT is possible. While specific embodiments of the invention have been described above, it would be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.