Patent Number: 
Section: description

In the drawings, like reference numerals indicate like parts. FIG. 1 schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises: a radiation system LA, Ex, IN, CO for supplying a projection beam PB of radiation (e.g. UV or EUV radiation); a first object table (mask table) MT for holding a mask MA (e.g. a reticle), and connected to first positioning means for accurately positioning the mask with respect to item PL; a second object table (substrate table) WT for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means for accurately positioning the substrate with respect to item PL; a projection system (xe2x80x9clensxe2x80x9d) PL (e.g. a refractive or catadioptric system, a mirror group or an array of field deflectors) for imaging an irradiated portion of the mask MA onto a target portion C (comprising one or more dies) of the substrate W. As here depicted, the apparatus is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type, for example. The radiation system may comprise a source LA (e.g. a Hg lamp, excimer laser, an undulator provided around the path of an electron beam in a storage ring or synchrotron, or an electron or ion beam source) which produces a beam of radiation. This beam is caused to traverse various optical components comprised in the illumination system, e.g. beam shaping optics Ex, an integrator IN and a condenser COxe2x80x94so that the resultant beam PB has a desired uniformity and intensity distribution in its cross-section. The beam PB subsequently intercepts the mask MA which is held on a mask table MT. Having passed through the mask MA, the beam PB traverses the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the interferometric displacement measuring means IF and the second positioning means, 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 interferometric displacement measuring means IF and the first positioning means 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. In general, movement of the object tables MT, WT will be realized with the aid of a long stroke module (course positioning) and a short stroke module (fine positioning), which are not explicitly depicted in FIG. 1. The depicted apparatus can be used in two different modes: In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at once (ie. a single xe2x80x9cflashxe2x80x9d) 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; In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single xe2x80x9cflashxe2x80x9d. 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=1/4 or 1/5). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution. FIG. 2 shows in cross-section the layout of a voice coil, or Lorentz-force, motor 100. The voice coil motor is comprised of a coil 3, two sets of permanent magnets 2a, 2b mounted on a yoke 1, and cooling jacket 4. The yoke 1 is mounted on the substrate table or the mask table and the combination of the table, the yoke and the magnets is moveable with respect to the coil. The permanent magnets are mounted on the yoke 1 with the north-south directions orientated as indicated by the arrows, the direction of the magnetic field created by the first set of permanent magnets 2a being orientated substantially opposite to the direction of the magnetic field created by the second set of permanent magnets 2b. The coil 3 contains a plurality of windings and the plane of the windings is perpendicular to the plane of the drawing and perpendicular to the direction of the magnetic fields. A first portion of the coil 3a passes through the magnetic field created by the first set of permanent magnets 2a, and a second portion of the coil 3b passes through the magnetic field created by the second set of permanent magnets 2b. The direction of an electric current in the windings at the first portion 3a and second portion 3b of the coil 3 will be opposite and perpendicular to the plane of the drawing. Thus when an electric current is flowing in the coil, a Lorentz force will be induced between each of the first set of permanent magnets 2a and the first portion of the coil 3a and the second set of permanent magnets 2b and the second portion of the coil 3b. In both cases the Lorentz force will be parallel to the plane of the drawing (but perpendicular to the direction of the magnetic fields created by the magnets) and both Lorentz forces will be in the same direction. However, when the electric current is flowing through the coil 3, it creates heat which needs to be dissipated. In the present invention the cooling jacket 4 is attached directly onto the upper and lower surfaces of the coil. The cooling jacket may be attached to and cool the coil as shown in FIG. 2. Alternatively the cooling jacket may cover the entire coil including the center portion where there are no windings. In addition a single cooling jacket may be attached to and provide cooling for a plurality of coils. The cooling jacket has connecting channels 5 which connect the cooling jacket to a water cooling system 6. The connecting channels may be comprised of flexible plastic tubes and/or rigid stainless steel tubing, however there may be a variety of other means for connecting the cooling jacket to the water cooling system and the invention is not intended to be limited to a particular means. As will be further described below, the cooling jacket 4 has channels 10, in which water from the cooling system can flow via the connecting channels 5. The heat created by the electric current flowing through the coil 3 passes into the cooling water in the channels 10 which flows back to the cooling system 6 where the heat is removed and the temperature of the cooling water is regulated to a pre-determined value. It can be appreciated that a variety of fluids could be used as the cooling medium instead of the cooling water described above. Furthermore a variety of cooling systems which could be connected to the cooling jacket 4 are already well known and will not be further discussed in this document as the choice of cooling system 6 is not critical to the present invention. As described above, in a preferred embodiment the cooling jacket is attached to the top and bottom sides of the coil. However, the invention is not limited to this arrangement. A variation of this embodiment is shown in FIG. 7. In this arrangement the coil-cooling jacket assembly is comprised of alternating layers of coil and cooling jacket. In the example shown there are two coil layers 41, 43. The lower coil layer 41 has lower cooling jacket layer 40 attached to its bottom side and middle cooling jacket layer 42 attached to its upper side. Coil layer 43 has middle cooling jacket layer 42 attached to its bottom side and upper cooling jacket layer 44 attached to its upper side. Each of the cooling jacket layers 40, 42, 44 has channels 10 in which cooling fluid can flow. The channels 10 in each cooling jacket layer are connected to one another by water connection manifolds 45 and are connected to the water cooling system 6 by connecting channels 5 as before. It will be appreciated that the layout and number of layers used can be varied within the scope of the invention. In voice coil motor 100 the clearance between the permanent magnets and the coil is limited by practical motor design considerations. This in turn will limit the maximum thickness that the cooling jacket can encompass. In practice in a lithographic apparatus this clearance will be about 2 to 4 mm on each side of the coil, limiting the maximum thickness of each cooling jacket to about 1.5 mm. FIG. 3 shows a cross-section of a portion of the cooling jacket 4 of the first embodiment of the invention. The cooling jacket is comprised of a substantially planar body 9 with channels 10 through it. The channels are substantially parallel to the plane of the cooling jacket (perpendicular to the plane of FIG. 3) and the cooling water flows through them. The lower surface 9a is attached to the surface of the coil with a thermally conductive but electrically insulating compound, and the upper surface is exposed to the environment. (It will be appreciated that when a portion of the cooling jacket is attached to the lower surface of the coil surface, 9a will become the upper surface of the cooling jacket and surface 9b will become the lower surface of the cooling jacket.) Thus the heat from the voice coil will be conducted through the lower surface 9a and into the cooling water in the channels 10. Ideally the heat flow passing through the cooling jacket to the upper surface 9b will be negligible. To facilitate this, an upper portion of the planar body 9 of the cooling jacket 4, which is adjacent to the upper surface 9b, may have a relatively low thermal conductivity and a lower portion of the planar body 9 of the cooling jacket 4, which is adjacent to the lower surface 9a, may have a relatively high thermal conductivity. Overall the thermal conductivity of the entire jacket may be lower than that of the prior art as the length of the desired heat-flow path to the cooling water is greatly reduced. As is apparent from the foregoing description at least a part of the cooling jacket 4 will be located in the magnetic field of the voice coil motor 100. It will therefore be advantageous if the cooling jacket is made from an electrically non-conducting material so as to prevent undesirable eddy-current damping. Such a material could be a plastic, a composite material, or preferably a ceramic. As previously stated, ceramic materials are advantageous in the present situation because of their relatively good thermal conductivity, and their fabrication methods and mechanical properties are well known. In the preferred embodiment of the invention the cooling jacket 4 will therefore be made from a ceramic material such as Al2O3 or AlN, but the invention should not be considered as limited to any particular type of material. The embodiment in FIG. 3 shows part of a cooling jacket 4 which has been fabricated as a single ceramic component 11. This is achieved by casting or cold-pressing the ceramic powder particles 13 onto a former 12 which resembles the shape of the channels 10. The former can subsequently be removed either by melting at a temperature which is lower than the sintering temperature of the ceramic which is being used or by chemically dissolving the former. The cooling jacket can then be completed by sintering. The resulting cooling jacket has a monolithic construction. As described above it may be desirable to use different ceramics for the top and bottom portions of the cooling jacket. It is possible to produce the cooling jacket of this embodiment using two different ceramics. However, it will be more difficult to realize because of differing sintering temperatures and amounts of shrinkage. A second embodiment of the cooling jacket 4 of the invention is shown in FIG. 4. In this case the cooling jacket is created by forming an upper portion, or component, 14 and a lower portion, or component, 15 of the cooling jacket. The upper portion 14 is comprised of a planar body with grooves 10b and the lower portion 15 is comprised of a planar body with grooves 10a, grooves 10a , 10b being formed in the contacting surfaces of portions 14, 15. Each of these portions are made by conventional ceramic processing methods and sintered. The two portions are subsequently bonded together along the bond lines 16 by a technique such as glass bonding. When the upper and lower portions are bonded together the grooves in each, 10a and 10b, are lined up such that the combination of the grooves 10b on the upper portion 14 and the grooves 10a on the lower portion 15 combine to form the channels 10. As described above it may be desirable to use different ceramic compositions which have different thermal conductivities for the upper and lower portions. Also, four grooves may be provided in any one of the upper portion 14 and lower portion 15, with the other having a flat surface. FIG. 5 shows the cooling jacket 4 of a third embodiment of the invention. This is a variation of the second embodiment. In this case the upper portion 17 and the lower portion 18 are again formed by conventional ceramic processing methods but are not immediately sintered. Instead the two unsintered portions (in the green state) are pressed together such that grooves 10a and 10b line up to form channels 10 and the two portions are joined at join line 19. The cooling jacket assembly is then co-fired to form a single monolithic component. Again different ceramic compositions can be used for the upper and lower portions. A fourth embodiment of the cooling jacket 3 is shown in FIG. 6. In this case the cooling jacket is composed of three layers: a substantially planar upper layer, or component, 20, a substantially planar lower layer, or component, 22 and a middle layer, or component, 21. The upper layer and the lower layer do not have grooves and can be formed and sintered according to conventional ceramic processing techniques. The middle layer is formed as a series of strips 21 of material with gaps 25 in between. The middle layer may be formed either as a planar layer from which strips of material are removed, for example by laser machining, to leave just the required strips of material or can be formed directly by conventional ceramic processes. The three layers are then bonded together at bond lines 23 by a method such as glass bonding. As in other embodiments each of the layers may be formed from different ceramic compositions. It will be appreciated that the third and fourth embodiments of the invention may be combined. That is to say, a cooling jacket may be formed by pressing together the three layers of the fourth embodiment whilst they are still in the green state and then co-firing the assembly. In the cooling jackets of the present invention the cooling jacket, and hence the channels 10 within, must generally be thin. This in turn results in high fluid flow resistance within the channels and therefore the cooling fluid circuit must be operated at high pressure P to maintain a sufficient flow rate. The fluid flow resistance and hence pressure P can be reduced by increasing the width W of the channels. However, as shown in FIG. 8a, increasing the effective length of the span 50 results in large bending stresses in the corner regions 51 of the cooling jacket. An alternative way to reduce the fluid flow resistance is to provide parallel channels as shown in FIG. 8b. The channel arrangements shown in FIGS. 8a and 8b have similar fluid flow resistances (and hence P and Pxe2x80x2 are similar) but the maximum bending stress in the corner 53 of the arrangement shown in FIG. 8b is much lower than the maximum bending stress in the corner 51 of the arrangement shown in FIG. 8a.  FIGS. 9a and 9b schematically show the arrangement of parallel channels 56, 58 in sections 55, 57 of cooling jackets 60, 61 of the present invention. The channels have a common cooling fluid inlet 54 and a common fluid outlet 59. In all of the embodiments of the invention the cooling jackets may have straight channels 56 or serpentine channels 58, as shown in FIGS. 9a and 9b respectively, but the invention should not be considered as limited to only these configurations. While we have described above specific embodiments of the invention it will be appreciated that the invention may be practiced otherwise than described. For example, the cooling jacket may be formed of any convenient number of sub-layers (more than three) which may be of similar or different materials and may be grooved as desired. The description is not intended to limit the invention.