Patent Publication Number: US-2003230729-A1

Title: Positioning stage with stationary and movable magnet tracks

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a stage for supporting, moving, and positioning articles, and more specifically, a stage for positioning an article in an electron beam or EUV-light lithography system used for manufacturing semiconductor devices.  
       BACKGROUND OF THE INVENTION  
       [0002] Many devices such as reticles, semiconductor circuits and liquid crystal displays are fabricated using lithographic equipment, such as an electron beam lithography system. In the fabrication of circuits a article must be repeatedly and precisely positioned under the optics of the lithography system. Such precise positioning is necessary to ensure accurate alignment of the microscopic features being formed in a new layer with other microscopic features in the layers previously formed on the article during the fabrication process for semiconductor circuits.  
       [0003] Complex systems have been developed to precisely position an article, such as a wafer or reticle beneath the lithographic optics. A step and repeat system often uses an x-y positioning system to position the article on a positioning stage beneath the lithographic equipment, expose a portion of the article to a pattern of light or charged particles generated by the lithographic equipment, and reposition the article at another location to again expose the article to the pattern of light or charged particles. Many different types of positioning stages and linear motors which move the positioning stage into the desired position, such as beneath the lithographic equipment, have been developed in an attempt to provide improved accuracy of article placement.  
       [0004] The articles being worked upon are typically supported and positioned using x-y guides with moving motors. Typically, such guides include separate x and y guide assemblies, with one guide assembly mounted on and movable with respect to the other guide assembly. Often a separate wafer stage is mounted on top of the guide assemblies. As the guides move during the positioning of the wafer, the magnet assemblies of the motors as well as other magnetic permeable materials also move. As a result, the shifting magnetic fields created by the magnet assemblies and other materials may interfere with an electron beam of an electron beam lithography system.  
       [0005] Electron beam lithography is used in the production of high quality patterns. The electron beam passes through magnetic or electrostatic lenses and deflectors capable of focusing the beam into the wafer plane and directing the beam in an x-y direction on the wafer. An electron beam projection system typically includes an electron beam source, a deflecting system for deflecting the electron beam in a predetermined pattern, and magnetic projection lenses for focusing the electron beam. The deflected and focused beam is directed to an article which may be, for example, a semiconductor substrate or mask (reticle).  
       [0006] Conventional positioning stages do not typically shield the magnetic fields created by the moving motors or other moving magnetic permeable components from the electron beam lithography system. The magnetic fields may shift the electron beam and cause misalignment of the pattern on the article. Thus, it is desirable to provide a positioning stage which limits the movement of the magnetic fields during positioning of the stage while exposing the article to the electron beam. It is also desirable to shield the magnetic fields from the electron beam to accurately, reliably and timely move and position articles in an electron beam lithography system.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention overcomes the deficiencies of the prior art by providing a stage positioning system which minimizes interference with an electron beam of an electron beam lithography system by magnetic fields created by the motors of the stage positioning system as well as other magnetic permeable components. The invention also provides a stage positioning system that can be used in an EUV light lithography system.  
       [0008] A stage positioning system of the present invention comprises a stationary frame, a slide movable relative to the frame in a first direction and a support platform connected to the slide and movable therewith in the first direction. The support platform is movably attached to the slide for movement in a second direction generally orthogonal to the first direction. The stage positioning system further includes first and second linear motors.  
       [0009] In the preferred embodiment, the first linear motor includes a first magnet assembly attached to the frame and a first coil device attached to the slide. As current is applied to the first coil device the slide will move in the first direction. The second linear motor includes a second magnet assembly attached to the slide and a second coil device attached to the support platform. As current is applied to the second coil device the support platform will move in the second direction.  
       [0010] The first magnet assembly may include a parallel pair of magnet tracks spaced apart a distance sufficient for receiving the slide therebetween. The first coil device includes coil members extending from opposite ends of the slide to interact with the magnet tracks. The frame may include a rail and the slide may include a slider block movably engagable with the rail. The slider block may contain bearings selected from rotating roller bearings, needle bearings, ball bearings, or gas bearings. The magnet tracks are preferably substantially shielded to prevent interference of the magnetic fields created by the magnetic assemblies with an electron beam.  
       [0011] The slide includes a pair of shafts extending generally parallel to a central longitudinal axis of the slide, and the support platform includes a pair of sleeves movably mounted on the shafts. The pair of sleeves may contain gas bearings to support the sleeves about the shafts.  
       [0012] The invention is also of a method of exposing an article in a lithography system, the method including providing a slide movably attached to a stationary frame such that the article can be positioned in a first direction, providing a support platform movably attached to the slide such that the article disposed on the support platform can be positioned in a second direction, providing a first linear motor to move the slide in the first direction and providing a second linear motor to move the support platform in a second direction, positioning the support platform by moving the slide to a selected position in the first direction, and exposing the article to light or an electron beam as the support platform is moved in the second direction. The procedure of moving the slide in the first direction followed by moving the support platform while exposing the article to light or an electron beam, is repeated until the selected exposure operation for the article is completed.  
       [0013] The positioning stage can be used in an electron beam lithography system, or an extreme ultraviolet (“EUV”) light lithography system. The lithography systems will comprise an electron beam source or an EUV light source, an optical projection system to project a pattern defined by a mask onto a surface of the article; a deflector system operable to position the electron beam on the article, and the stage positioning system of the invention. When an EUV light source is used, the deflector may not be necessary.  
       [0014] The stage positioning system of both the electron beam and the EUV light lithography systems preferably move the support platform in the first direction while the article is not being exposed to the electron beam or EUV light. The support platform is then moved in the second direction while the article is being exposed to the electron beam or EUV light. This allows the magnet assemblies that move the support platform in the second direction to remain stationary during the exposure of the article. It should be noted that electron beams are sensitive to magnetic fields whereas EUV light sources are not. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached drawings, wherein:  
     [0016]FIG. 1 is a perspective of the stage positioning system of the present invention;  
     [0017]FIG. 2 is a perspective of the positioning system of FIG. 1 with parts removed and broken away to show detail;  
     [0018]FIG. 3 is a cross-sectional view taken through and including line  3 — 3  of FIG. 2;  
     [0019]FIG. 4 is a perspective of a slide of the positioning system of FIG. 1;  
     [0020]FIG. 5 is a perspective of the slide and a portion of an x linear motor of the positioning system of FIG. 1 with a support platform removed to show further detail;  
     [0021]FIG. 6 is a perspective of a support platform assembly of the positioning system of FIG. 1;  
     [0022]FIG. 7 is a perspective of a portion of an electron beam projection system with parts broken away to show detail; and  
     [0023]FIG. 8 is a representation of an EUV light lithography system using the positioning system of the invention. 
    
    
     [0024] Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.  
     DESCRIPTION OF THE INVENTION  
     [0025] Electron beam lithography is one type of lithography system which uses an electron beam to expose an article. The electron beam is very sensitive to magnetic fields, which may effect the exposure performance of the beam. For this reason the movement of magnetic permeable materials during exposure needs to be minimized. Most if not all of the present positional stages contain magnetic permeable components that move during exposure. Of primary concern is the movement of the magnetic assemblies that position the article to the desired coordinate positions. Other magnetic materials used in the stage such as bearings, support members, or magnetic shielding may also effect exposure performance and should be taken into account when calibrating the system. As these magnetic materials move, the corresponding magnetic fields associated with these materials also move. It is the movement of the magnetic fields which can effect the performance of the electron beam and hence the exposure performance of the system.  
     [0026] The positional stage of the invention is designed to minimize the movement of these magnetic components during the exposure of the article. In the preferred embodiment, the support platform is moved in a first direction while the electron beam is deflected away or shielded from the article. One method of shielding the article from the electron beam is to use a blanking device that turns off the beam. Alternatively, a shutter device could be placed between the beam and the article to block the beam. As the support platform moves with the slide in the first direction the magnetic assembly associated with moving the support platform in a second direction also moves. Other magnetic components associated with the slide also move. However, because the article is not being exposed during this time the movement of the magnetic assembly and the other magnetic components have no effect on exposure performance. The support platform is then moved in the second direction, usually orthogonal to the first, while the article is exposed to the electron beam. Because the support platform contains mostly non-magnetic components, there is little if any movement of magnetic permeable materials during the exposure of the article.  
     [0027] Minimizing the movement of relatively heavy and bulky components during exposure of the article also minimizes the amount of potential vibrations that may occur as the support platform is moved. These vibrations can also have a negative effect on the exposure performance of the system. The positional stage is designed to minimize movement of such components during exposure.  
     [0028] Referring now to the drawings, and first to FIG. 1, a stage positioning system of the present invention is generally indicated at  20 . The stage positioning system  20  may be used for positioning a semiconductor wafer W or reticle (not shown) during semiconductor processing, for example. The positioning system  20  is particularly advantageous for applications such as electron beam lithography since motors of the stage positioning system are configured to reduce the effect of magnetic fields of the motors on the electron beam, as further described below. It is to be understood that the stage positioning system  20  of the present invention may be easily adapted for use in other types of systems for article processing such as an EUV lithography system. The general reference to an electron beam lithography system is purely for illustrating an embodiment of an environment in which the concept of the stage positioning system  20  of the present invention may be advantageously adopted. Further, the stage positioning system  20  is described below with reference to a wafer stage, but may also be used as a reticle stage.  
     [0029] The stage positioning system  20  comprises a stationary frame  28 , a slide  30  movable relative to the frame in an x (first) direction, and a support platform assembly  32  having a support platform  33  configured for supporting a semiconductor wafer W. The support platform assembly  32  is movably attached to the slide  30  for movement along the slide in an y (second) direction. The positioning stage  20  further includes two x linear motors (first motor)  34  for moving the slide  30  in the x direction and a y linear (second) motor  36  for moving the support platform  33  in the y direction. Each of the x motors  34  include a magnet track  40  and a coil member  42  operable to interact with magnetic fields of the magnetic track to generate a force to move the slide  30  in the x direction (FIGS. 1 and 3). Similarly, the y motor  36  includes a magnet track  44  and coil member  46  operable to interact with magnetic fields of the magnetic track to generate a force to move the support platform  33  in the y direction (FIGS. 2 and 6). The magnet tracks  40  of the x linear motors  34  form a magnet assembly. One or more coil members  42  form a coil device.  
     [0030] It is to be understood that the arrangement and configuration of the magnet assembly and coil device may be different than shown and described herein without departing from the scope of the invention. For example, as shown in FIG. 1, the first magnetic assembly is attached to the frame, and as shown in FIG. 2, the first coil device is attached to each end of the slide. Alternatively, it is possible that the first magnetic assembly can be attached to the slide and the first coil device attached to the frame.  
     [0031] In addition, FIGS. 2 and 4 show the preferred embodiment, with the second magnetic assembly attached to the slide, and the second coil device attached o the support platform. Alternatively, it is possible that the second magnetic coil could be attached to the slide and the second magnetic assembly attached to the support platform. However, this embodiment would result in the second magnetic assembly being moved as the article is exposed.  
     [0032] The positioning stage  20  is preferably arranged so that the support platform  33  moves in the y direction during a scan operation of the lithography system (FIGS. 1 and 2). The x and y magnet tracks  40 ,  44  remain stationary during movement of the support platform  33  in the y direction. Thus, any shifting magnetic fields associated with the magnet tracks  40 ,  44  do not interfere with the electron beam during the scan process. When the support platform assembly  32  moves in the x direction the y magnet track  44  also moves. However, the movement of the y magnet track  44  is of little consequence because the lithography system is not scanning at this time. The electron beam is either turned off, shielded from, or directed away from the support platform. Since there is no scanning while the platform moves in the x-direction, the effect of moving the y linear motor has no effect on the patterning of the article.  
     [0033] As is well known by those skilled in the art, a force sufficient to move the coil members  42 ,  46  is generated between the coil members and the magnet tracks  40 ,  44  by application of appropriate current to the coil members. By synchronously actuating the x linear motors  34 , a force is exerted on the x coil members  42  which are connected to the slide  30  to force the slide to move in the x direction. Similarly, by actuating the y linear motor  36 , a force is exerted on the y coil member  46  which is connected to the support platform assembly  32  to thereby position the support platform  33  along the y axis. An example of a type of linear motor for use in the positioning stage system  20  is described in copending U.S. patent application Ser. No. 09/054,766, by A. Hazelton et al., filed Apr. 3, 1998, the entirety of which is incorporated herein by reference.  
     [0034] The provision of two generally parallel x linear motors  34  facilitates in reducing or preventing vibration of the support platform  33  as well as reducing or preventing the creation of a moment about the z axis. In particular, the provision of two generally parallel x linear motors  34  facilitates in driving the slide  30  through the center of gravity or through a location near the center of gravity in the y direction.  
     [0035] The frame  28  is formed from two elongated frame members  28   a ,  28   b  spaced from one another along the y axis a distance sufficient to permit movement of the slide  30  therebetween (FIGS. 1 and 2). Each frame member  28   a ,  28   b  includes a pair of parallel rails  50  extending longitudinally along the frame member (FIG. 2). The rails  50  are disposed on inner walls  52  of the frame members  28   a ,  28   b  and positioned along upper and lower edges of the inner walls. The rails  50  are provided as guides for U-shaped slider blocks  54  extending from opposite ends of the slide  30  (FIG. 2). Two pairs of slider blocks  54 , one pair for each rail  50 , are attached to each end of the slide  30  for sliding engagement with the rails. The slider blocks  54  and rails  30  may be any suitable slider block and rail system, such as those utilizing roller balls. An example of a suitable slider block and rail system is a guide system available from THK America Inc., of Schaumburg, Ill., under product designation SSR LM. It is to be understood that the rail  50  and slider block  54  arrangement may be different than shown herein without departing from the scope of the invention. For example, air bearings or ball bearings may be used rather than roller bearings.  
     [0036] Another advantage of scanning only while the platform is moving in the y-direction is that vibrations caused by the x-bearings  54  along the tracks  50  occurs only when the slide  30  moves in the x-direction. Since the slide  30  does not move in the x-direction during exposure of the article the amount of vibrations caused by the bearings  54  have no effect on exposure performance. As a result, one embodiment may include incorporating less expensive, conventional roller-type bearings to move the slide  30  in the x-direction, and more sophisticated gas bearings to propel the support platform  33  along the slide  30  in the y-direction.  
     [0037] The x magnet tracks  40  of the x linear motors  34  are each disposed within one of the elongated frame members  28   a ,  28   b  (FIGS. 1 and 3). Each magnet track  40  comprises a generally U-shaped support member  56  and a magnet array comprising a plurality of magnets  58 . The magnet track  40  forms a longitudinal slot  60  for movably receiving the coil member  42  therein. The magnet track  40  has a length preferably greater than a length Lc of the coil member  42  plus the stroke of the x linear motor  34  in the x direction (FIGS. 2 and 4). The magnet track  40  is surrounded by a shield  62  and fixedly connected thereto (FIG. 3). As shown in FIG. 1, the shields  62  form the elongated members  28   a ,  28   b  and each comprise a U-shaped member  63  and the inner wall  52 . The magnet track  40  is completely shielded by the shield  62  except for a longitudinal slot  64  formed in the inner wall  52  and extending along the length of the magnet track to permit movement of the coil member  42  along the length of the track. The shield  62  is preferably formed from steel or other suitable material to shield the magnetic fields generated by the magnet track  40  and prevent interference by the magnetic fields from the magnet track with the electron beam.  
     [0038] The coil member  42  is a generally planar member having a thickness t slightly less than a distance d between the magnets  58  (FIG. 3). The coil members  42  are attached to opposite ends of the slide  30  by a connecting member  68  extending outwardly from the ends of the slide (FIGS. 3 and 4). The connecting member  68  has a thickness t c  slightly less than a width L w  of the slot  64  of the shield  62 .  
     [0039] The slide  30  comprises an elongated member  69  extending along a central longitudinal axis A of the slide and two cylindrical shafts  70  extending generally parallel to the elongated member on opposite sides thereof (FIGS. 1 and 5). The elongated member  69  and shafts  70  are yoked together at each end to form the slide  30 . The elongated member  69  is formed from the y magnet track  44  and shield  72  which are similar to the x magnet track  40  and shield  62  described above and shown in FIG. 3. The shield  72  has a longitudinal slot  74  formed in a sidewall  76  thereof for receiving a connecting member  78  which couples the coil member  46  to the support platform assembly  32  (FIGS. 5 and 6). The sidewall  76  is removably attached to the other portion of the shield  72  so that the coil  46  can be positioned within the magnet track  44  during assembly.  
     [0040] The support platform assembly  32  includes two cylindrical sleeves  82  configured for movably receiving the shafts  70 . An air bearing (not shown) is positioned within central openings  84  of the sleeves. The air bearing facilitates sliding of the support platform assembly  32  along the shafts  70  in the y direction. Any suitable bearing including mechanical bearings such as roller, needle, or ball bearings, or gas bearings may be used. Preferably, gas bearings support the mass of the support platform  33  by pressurized air, nitrogen, or other suitable gas which provides an air cushion between the inner surface of the sleeves  82  and the outer surface of the shafts  70 . The use of the gas bearings minimizes vibrations which can extend to the support platform  33  during the scanning operation. The support platform assembly  32  has a central opening  86  extending longitudinally therethrough generally parallel to the sleeves  82  and sized to fit over the shield  72 . The coil member  46  extends longitudinally through a portion of the central opening  86  and is positioned generally along a center of gravity line Cg of the support platform assembly  32 . This facilitates in driving the support platform  33  through its center of gravity and reduces or prevents vibration or yaw of the support platform.  
     [0041] As shown in FIG. 5, three mounting or flexure pads  90  are positioned on an upper surface of the sleeves  82  for mounting the support platform  33  thereon. The support platform assembly  32  further includes a central support  94  comprising two cross members  94   a ,  94   b  for preventing contact of the support platform  33  with the shield  72  during movement of the support platform.  
     [0042] It is to be understood that the sleeves  82  and shafts  70  may have configurations other than shown herein without departing from the scope of the invention. For example, although each of the shafts  70  is shown to have a circular cross-section, any other suitable cross-sectional shape such as ellipsoid or rectangular, may be used. The corresponding sleeves would also have similar corresponding cross-sectional shapes.  
     [0043] The support platform  33  is configured to support one or more articles such as a semiconductor wafer W or reticle for movement and positioning relative to the exposure system. The articles may be secured on the platform  33  by clamps, vacuum chuck, or any other suitable device. The dimensions of the stage may vary, depending on the specific application. Various devices such as an interferometer (not shown) may be utilized to measure and determine the orientation and position of the support platform  33 . The interferometer utilizes signals reflected from mirrors positioned on faces  96   a ,  96   b  of the support platform  33  to measure and determine the orientation and position of the support platform (FIG. 6). The support platform  33  preferably includes an extension  98  which provides increased length of the mirrored face  96   a  for maintaining the mirrored face within sight of the interferometer to provide the reflected signals to the interferometer.  
     [0044] A feedback controller (not shown) may be provided to send different levels of current to the coil members  42 ,  46  in response to the orientation and position of the support platform  33 . An interferometer or other suitable position sensor may send output signals indicative of the orientation and position of the support platform  33  to the feedback controller. The x direction linear motors  34  may be differentially driven to prevent and overcome any tendency of the support platform  33  to yaw, i.e., rotate about the z axis (FIG. 1). Such differential driving of the x direction linear motors  34  compensates for the tendency of the slide  30  to pivot, i.e., move faster on one side versus the other. This tendency of the slide  30  to pivot may be caused by the non-ideal response of the linear motors  34  to the applied currents.  
     [0045] The stage positioning system  20  may comprise any suitable material such as steel, aluminum, ceramics, and plastics, for example. For electron beam lithography applications, all movable components are preferably formed from non-conducting, non-magnetic materials, such as ceramics or plastics.  
     [0046]FIG. 7 shows a portion of an electron beam lithography system  100  in which the stage positioning system  20  of the present invention may be utilized. The electron beam projection system includes an electron beam source  102 , an electron beam column  104 , and the stage movable in a number of degrees of freedom (e.g., three or six degrees of freedom) for positioning a workpiece such as a semiconductor wafer W relative to the electron beam column  104  to provide accurate alignment of the wafer with the optical systems for processing. The electron beam column  104  generally consists of a vertical arrangement of separate stages including a condenser lens, alignment stages, demagnification lens stages, a projection lens, a deflector system, and magnification lens stages, for example. The use of any one, or any one or more in combination,+ of these separate stages is defined as an optical projection system. The optical projection system is used to project a pattern defined by a mask onto a surface of the article. The electron beam system  100  operates under vacuum conditions to prevent gas molecules from perturbing the electron beam.  
     [0047] The electron beam source (gun)  102  emits a diverging beam E of electrons downwardly in the z direction along axis A through an illuminating aperture  106 . After passing through the aperture  106 , the beam E is collimated (rendered parallel) by a conventional magnetic lens acting as a condenser. The electron beam E may be gaussian in profile, or it may have a simple geometric shape such as a rectangle or triangle, or as an element of a repetitive pattern to be printed on the wafer W, for example. The beam E may also pass through a patterned area that imparts the final wafer pattern on it. The electron beam column  104  includes magnetic or electrostatic lenses  108  operable to focus the beam E onto a surface of the wafer W and deflectors  110  for directing the beam to specific positions on the wafer where photoresist placed on an upper surface of the wafer is to be exposed.  
     [0048] As shown schematically in the electron beam projection system of FIG. 7, the lens assemblies are aligned along the central longitudinal axis A of the electron beam column  104 . For clarity, parts of the system are removed to show detail. A reticle (mask) R having a circuit pattern formed therein is placed between the lens assembles  108 . The reticle R represents a pattern on a layer of an integrated circuit. The electron beam E will step in sequence through portions of the reticle R, the totality of which represents the pattern of the integrated circuit. As the beam E passes through the reticle R, the beam is patterned with the information contained in the reticle.  
     [0049] A representative embodiment of an EUV light lithography system  120  according to the invention is depicted schematically in FIG. 8. The depicted embodiment is a projection-exposure apparatus employing light in the UV range as the exposure-illumination light. The EUV light will have a wavelength between 0.1 and 400 nm preferably between 1 and 50 nm. Projection-imaging is performed using an imaging-optical system  122 , which forms a “reduced” (demagnified) image of the pattern defined by the mask  124  on the wafer  126 . In FIG. 8, the optical axis of the imaging-optical system  120  extends in the Z-direction, and the Y-direction is perpendicular to the plane of the page.  
     [0050] As noted above, the pattern to be transferred onto the wafer  126  is defined by the reflection-type mask  124 , which is mounted on a mask stage  128 . The wafer  126  is mounted on a wafer stage  130 . Typically, exposure is performed in a step-and-scan manner, wherein the mask pattern is projected in successive portions (“shot regions”) while synchronously moving the mask stage  128  and wafer stage  130  relative to each other as exposure progresses. Scanning of the mask  124  and wafer  126  typically is performed in a single dimension relative to the imaging-optical system  122 . Upon exposing all the shot regions on the mask  124  onto respective regions of the wafer surface, exposure of the pattern onto a die of the wafer  126  is complete. Exposure can then progress stepwise to the next die on the wafer  126 .  
     [0051] The EUV light used as the illumination light for exposure has low transmittance through the atmosphere. Hence, the optical path through which the EUV light passes desirably is enclosed in a vacuum chamber  132 . The vacuum chamber  132  is evacuated using a suitable vacuum pump  134 . The EUV light desirably is produced by a laser-plasma X-ray source comprising a xenon target gas. The laser-plasma X-ray source comprises a laser source  136  (serving as an excitation-light source) and a xenon gas supply  138 . The laser-plasma X-ray source is enclosed by a vacuum chamber  140 . The EUV light produced by the laser-plasma X-ray source passes through a window  141  in the vacuum chamber  140 . Window  141  may also be formed as an aperture that permits the laser plasma X-ray source to pass unhindered. It is preferred that the vacuum chamber  140  is separate from the vacuum chamber  132  because debris tends to be generated by a nozzle  142  that discharges the xenon gas.  
     [0052] The laser source  136  is configured to generate laser light having a wavelength that can be within the range from infrared to ultraviolet. For example, a YAG laser or excimer laser can be used. The laser light from the laser source  136  is condensed and irradiated onto the stream of xenon gas (supplied from a gas supply  138 ) discharged from the nozzle  142 . Irradiation of the stream of xenon gas causes heating of the xenon gas sufficiently to form a plasma. Photons of EUV light are emitted as the laser-excited molecules of xenon gas drop to a lower energy state.  
     [0053] A parabolic mirror  144  is situated in the vicinity of xenon-gas discharge. The parabolic mirror  144  collects and condenses the EUV light produced by the plasma. The parabolic mirror  144  constitutes herein the condenser optical system, and the parabolic mirror  144  is situated such that its focal point is nearly at the locus of discharge of the xenon gas from the nozzle  142 . The parabolic mirror  144  comprises a multilayer film suitable for reflecting the EUV light. The multilayer film typically is provided on the concave surface of the parabolic mirror  144 . The EUV light reflected from the multilayer film passes through the window  141  of the vacuum chamber  140  to a condenser mirror  146 . The condenser mirror  146  condenses and reflects the EUV light to the reflection-type mask  124 . To such end, the condenser mirror  146  also comprises a surficial multilayer film that is reflective to EUV light. EUV light reflected from the condenser mirror  146  illuminates the prescribed shot region on the reflection-type mask  124 . As referred to herein, the parabolic mirror  144  and condenser mirror  146  collectively comprise the “illumination system” of the FIG. 8 apparatus.  
     [0054] The reflection-type mask  124  is configured with a multilayer EUV-reflective surface as described above, as further description of the mask  124  is omitted here. As the EUV light reflects from the mask  124 , the EUV light becomes “patterned” with pattern data from the mask  124 . The patterned EUV light passes through the projection system  122  to the wafer  126 .  
     [0055] In one embodiment, the imaging-optical system  122  comprises four reflection mirrors: a concave first mirror  150   a , a convex second mirror  150   b , a convex third mirror  150   c , and a concave fourth mirror  150   d . Each of the mirrors  150   a - 150   d  comprises a multilayer film (reflective to EUV light) applied to a backing material (article). The mirrors  150   a - 150   b  in this embodiment are arranged so that their respective optical axes are coaxial with each other.  
     [0056] To prevent obstructing the optical path defined by the respective mirrors  150   a - 150   d , appropriate cutouts are provided in the first mirror  150   a , the second mirror  150   b , and the fourth mirror  150   d . (In FIG. 8, the dashed-line portions of the mirrors indicate the respective cutouts.) An aperture stop (not shown) is provided at the position of the third mirror  150   c.    
     [0057] The EUV light reflected by the reflection-type mask  18  is reflected sequentially by the first mirror  150   a  through the fourth mirror  150   d  to form a reduced image of the mask pattern, based on a prescribed demagnification ratio β (for example α—¼, ⅕, or ⅙) within the respective shot region on the wafer  126 . The projection system  122  is configured so as to be telecentric on its image side (wafer side).  
     [0058] The reflection-type mask  124  is supported, at least in the X-Y plane, by the movable reticle stage  128 . The wafer  126  is supported, desirably in each of the X-, Y-, and Z-directions by the movable wafer stage  130 . During exposure of a die on the wafer  126 , while EUV light is irradiated to each shot region on the mask  124  by the illumination system, the mask  124  and wafer  126  are moved in a coordinated manner relative to the imaging-optical system  122  at a prescribed velocity according to the demagnification ratio of the imaging-optical system  122 . Thus, the mask pattern is scanned progressively and exposed within a prescribed shot range (for a die) on the wafer  126 .  
     [0059] During exposure, to prevent gases generated from the resist on the wafer  126  from depositing on and adversely affecting the mirrors  150   a - 150   d  of the imaging-optical system  122 , the wafer  126  desirably is situated behind a partition  152 . The partition  152  defines an aperture  152   a  through which the EUV light can pass from the mirror  150   d  to the wafer  126 . The space defined by the partition  152  is evacuated by a separate vacuum pump  154 . Thus, gaseous contaminants produced by irradiation of the resist are prevented from depositing on the mirrors  150   a - 150   d  or on the mask  126 , thereby preventing deterioration of optical performance of these components.  
     [0060] It will be observed from the foregoing that the stage positioning system  20  of the present invention has numerous advantages. Importantly, the system  20  provides movement of the support platform  33  in a scanning direction without movement of the magnet tracks  40 ,  44 . Because the magnet tracks  40 ,  44  are stationary during exposure of the article, magnetic field shifts are minimized. Also, vibrations that may result from the movement of the slide in the first direction are eliminated. Moreover, the magnet tracks  40 ,  44  are substantially shielded to prevent interference of the magnetic fields with the electron beam. As a result, the stage positioning system  20  provides an accurate and reliable method for aligning articles such as semiconductor wafers or reticles in electron beam or EUV light lithography systems.  
     [0061] As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.