Patent Number: 
Section: description

FIG. 1 schematically depicts a lithographic projection apparatus 1 according to the invention. The apparatus comprises: a radiation system LA, IL for supplying a projection beam PB of EUV radiation; a first object table (mask table) MT provided with a first object (mask) holder for holding a mask MA (e.g. a reticle), and connected to first positioning means PM for accurately positioning the mask with respect to item PL; a second object table (substrate table) WT provided with a second object (substrate) holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means PW for accurately positioning the substrate with respect to item PL; and a projection system (xe2x80x9clensxe2x80x9d) PL (e.g. a refractive, catadioptric or reflective system) for imaging an irradiated portion of the mask MA onto a target portion C (die) 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. The radiation system comprises a discharge plasma source LA that produces a beam of radiation. This beam is passed along various optical components included in illumination system (xe2x80x9clensxe2x80x9d) IL so that the resultant beam PB is collected in such a way as to give illumination of the desired shape and intensity distribution at the entrance pupil of the projection system and the mask. The beam PB subsequently impinges upon the mask MA which is held in the mask holder on the mask table MT. Having been selectively reflected by the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target area C of the substrate W. With the aid of the interferometric displacement measuring means IF and positioning means PW, the substrate table WT can be moved accurately, e.g. so as to position different target areas C in the path of the beam PB. Similarly, the positioning means PM and interferometric displacement measuring means IF can be used to accurately position the mask MA with respect to the path of the beam PB. 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: 1. In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected in one go (i.e. a single xe2x80x9cflashxe2x80x9d) onto a target area C. The substrate table WT is then shifted in the X and/or Y directions so that a different target area C can be irradiated by the beam PB; and 2. In scan mode, essentially the same scenario applies, except that a given target area 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=xc2xc or ⅕). In this manner, a relatively large target area C can be exposed, without having to compromise on resolution. FIG. 2 depicts the various vacuum chambers of the lithographic apparatus of FIG. 1, the various parts shown in FIG. 1 being located in the various chambers shown in FIG. 2. The vacuum chambers are separated by walls in which openings are present for passing the projection beam of radiation PB from one vacuum chamber to the next one. In FIG. 2 one can distinguish a source chamber 10 containing the source LA, an illumination optics box 20 containing a collector mirror and the illumination optics, a chamber 30 containing the mask table and mask MA, a projection optics box 40 containing the projection system, and a chamber containing the substrate table and substrate W. In the various vacuum chambers a different vacuum level is maintained, the optics boxes requiring the highest vacuum level to keep the reflective optics clean. The apparatus is provided with a discharge plasma EUV radiation source, which employs a gas or vapor, such as Xe gas or Li vapor, in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing a partially ionized plasma of an electrical discharge to collapse onto an optical axis of the system, and the region in which the plasma collapses will, in general, have a finite length along the optical axis. The elongated region into which the plasma collapses is designated LA in FIGS. 2, 3 and 4. Partial pressures of 0.1 mbar Xe, Li vapor or any other suitable gas or vapor may be required for efficient generation of EUV radiation. The radiation source LA is contained in its own source chamber 10, shown in more detail in FIG. 3, to confine those rather high partial pressures to the region of the source. Radiation emitted by the source is subsequently passed from the source chamber 10 to the illumination optics box 20. A filter having a channel structure 11 is incorporated on the optical axis OA in the vacuum chamber wall 15 that separates the source chamber from the illumination optics box. The channel structure 11 comprises adjacent narrow channels 12 separated by walls that are substantially parallel to a propagation direction of radiation emitted by the radiation source LA, the propagation direction being substantially directed along the optical axis OA. The channel structure passes the EUV radiation emitted by the source and at the same time functions as a flow resistance, or barrier, in between the source chamber and the illumination optics box so as to be able to maintain the illumination optics box, or illuminator box, at a much higher vacuum level (lower pressure) than the source chamber. The form and length of the channels in the channel structure should be chosen so as to provide a high EUV transparency and a high enough flow resistance. FIG. 3 also shows that the radiation is sort of radially emitted from the elongated region LA. To increase the transparency of the channel structure, the width of the channels in the structure increases along the optical axis OA in the propagation direction of the radiation, both in the plane of the drawing and in a plane perpendicular to the plane of the drawing of FIG. 3. In such a configuration the walls of the channels are directed more parallel to the propagation direction of the radiation emitted from the source. The channel structure is rotationally symmetric around the optical axis OA, as is further shown in FIG. 4. FIG. 4 only depicts about a quarter of the channel structure 11. FIGS. 3 and 4 further show that the region LA from which EUV radiation is emitted, is elongated along the optical axis, i.e. it has a finite length along the optical axis. To further increase the transmittance of radiation through the channel structure for the whole elongated region that emits radiation, the width wr of the of the channels in a direction perpendicular to the optical axis, i.e. the radial direction RD with respect to the optical axis OA, is chosen considerably larger than the width wt of the channels in a tangential direction TD around the optical axis OA. FIG. 4 better shows those width dimensions. In case the channels would have a narrow width in the radial direction RD, only radiation from a very small part from the elongated emitting region LA would be transmitted through the channels, while radiation from other parts would hit the channel walls. Only a small fraction of the radiation energy of the source would then pass the channel structure 11. FIG. 3 shows two rays 17 in dotted lines, which are emitted from opposite ends of the elongated region LA, the rays not being parallel and entering one channel. A channel that is to narrow in the radial direction RD would not pass both rays. The gas flow conductance of the channel structure, or its resistance to gas flow, can be derived as follows. It shows that the result of the division of an opening in a vacuum chamber wall in a number of adjacent channels drastically reduces the conductance of the opening. In a simplified calculation, which is accurate within approximately 10%, the conductance CT of the channel structure can be written as CT=CM+CV, where CM represents the molecular conductance and CV the laminar conductance. A further background for such calculations can be found in xe2x80x9cFoundations of Vacuum Science and Technologyxe2x80x9d, edited by J. M. Lafferty, Wiley and Sons Inc., 1998, ISBN 0-471-17593-5. For high Knudsen numbers ( greater than 0.5) CM dominates, whereas CV dominates for low Knudsen numbers. In the so-called transition regime, which is the case for the situation considered, both contributions have to be taken into account. Taking the structure of FIG. 4 and assuming that it covers 2.4 steradians at 7 cm distance from the source, it has a surface area of approximately 114 cm2. Having channels that are approximately 20 mm long, 0.5 mm wide in the tangential direction and 20 mm wide in the radial direction with respect to the optical axis and having Xe at a source pressure of 0.1 mbar, it results in CM=50 1/s, CV=4 l/s and CT=54 l/s. In an equilibrium situation the gas flow through the channel structure equals the flow into a vacuum pump VPI connected to the illuminator box 20: (Psourcexe2x88x92Pilluminator)xc3x97CT=Pilluminatorxc3x97Seff, wherein Psource represents the pressure in the source chamber, Pilluminator the pressure in the illuminator box and Seff the effective pumping speed of the vacuum pump VPI connected to the illuminator box. Another vacuum pump VPS will generally be connected to the source chamber 10. Aiming at Pilluminator less than  less than Psource one obtains: Pilluminator/Psource=CT/Seff=54/6000=1/111=0.009, when having a vacuum pump VPI, such as a turbomolecular pump, of Seff=6000 l/s effective pumping speed. The above calculation shows that due to the channel structure a sufficient flow resistance can be reached to have more than a factor 100 reduction in Xe pressure. For the case above the source pressure is at such a low level that the molecular regime dominates. In general, CM scales with the inverse square root of the molecular mass of the gas considered, and CV scales with source pressure. For higher source pressures and lower molecular masses the channel structure would be less efficient. By increasing the aspect ratio (ratio of channel length over smallest channel width) of the channels, the resistance can be increased, as required, to regain efficiency. When keeping the aspect ratio constant, the length of the channels can be decreased while keeping CT constant. In case the channel structure is positioned closer to the source, its surface area is smaller and its conductance decreases accordingly. A drawback would be vignetting due to the finite dimension of the source and a higher heat load of the source on the channel structure. The channel structure should be made of a suitable material to withstand the heat load of the radiation source. The walls of the channels are preferably very thin (foils) to present a very low obscuration to the radiation from source LA. Channels in the middle of channel structure 11 around optical axis OA are not shown in FIG. 4. The channels in this part will more or less xe2x80x9cseexe2x80x9d a point source and it is therefore less important to have a large width in the radial direction RD. The embodiment shown comprises channels having a honeycomb structure as is depicted in FIG. 5 in the middle part of the channel structure 11. The diameter of the channels in the honeycomb structure is approximately 0.3 mm and their length approximately 20 mm. For certain source configurations the honeycomb structure might also be employed for the whole channel structure 11. The radiation source used may emit particles that are detrimental to any optics arranged along the optical axis. Those particles can be charged and/or move with a high velocity. The channel structure as described above will also act as a filter to prevent those particles from reaching the optics, and it need not be incorporated in a vacuum wall to perform such filtering. Particles emitted by the source will become trapped in the channel structure. Such a filter comprising a channel structure as described may be mounted in any manner so as to be able to prevent those particle from reaching components that may be damaged by the emitted particles. Whilst a specific embodiment of the invention is disclosed above it will be appreciated that the invention may be practiced other than described. The description is not intended to limit the invention. For instance, the channel structure may be employed with other gases to present a flow resistance, and a decreased flow conductance accordingly. Further, the channel structure may be incorporated in a vacuum wall separating other vacuum chambers, such as in the vacuum wall separating the projection optics box and the substrate chamber to present a barrier to contaminants that may escape from the radiation-sensitive resist layer on the substrate upon exposure. Also the considerations with regard to the channel width along the optical axis for a diverging beam of radiation may hold at other locations. For a converging beam of radiation the width of the channels should decrease along the optical axis in the propagation direction of the radiation. Since elongated images of the elongated source may be formed at locations on the optical axis, the considerations with regard the widths of the channels in the radial and tangential directions with respect to the optical axis will also hold at those locations.