Abstract:
Reduced-pressure (“vacuum”) chambers, and microlithographic exposure systems including one or more of such chambers, are disclosed. The vacuum chamber exhibits reduced deformation of a bulkhead of the chamber during evacuation of the chamber during a change in pressure differential across the bulkhead. A secondary wall is situated relative to the bulkhead outside the chamber and at a gap distance from the bulkhead, so as to form a secondary reduced-pressure chamber in the gap. The secondary reduced-pressure chamber is isolated from the pressure outside the chamber and from the subatmospheric pressure inside the chamber. The differential between the pressure outside the chamber and the pressure inside the secondary reduced-pressure chamber is exerted on the secondary wall, but has substantially no effect on the bulkhead, thereby reducing deformation of the bulkhead. Reducing bulkhead deformation prevents degradations of performance accuracy, otherwise caused by pressure-change-induced deformation of the bulkhead, of any instruments mounted to the bulkhead.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of, and claims the benefit of, co-pending U.S. patent application Ser. No. 10/209,738, filed on Jul. 31, 2002. The entire &#39;738 application is incorporated by reference into the instant application. 
     
    
     
       FIELD  
         [0002]    This disclosure pertains to systems in which a workpiece is placed inside a chamber evacuated to a subatmospheric pressure. Such systems are used, for example, in any of various irradiation and transfer-exposure apparatus that irradiate an object with an energy beam inside such a chamber or that contain an object for observation or tests performed on the object. The disclosure also pertains to microlithography systems, comprising at least one such chamber, that include one or more measuring instruments (e.g., position-measuring and/or alignment-measuring instruments) mounted to a bulkhead of such a chamber. The chamber is configured to prevent reductions in the operational accuracy and precision of the instrument(s) by controlling deformation of the bulkhead caused by evacuation of the chamber or by changes in the pressure differential across the chamber bulkhead.  
         BACKGROUND  
         [0003]    Many types of apparatus are known that utilize a charged particle beam (e.g., electron beam) or other energy beam for imaging, displaying, workpiece processing, or other practical application. An exemplary apparatus of this general type is a projection-exposure apparatus, also termed a “microlithography” system, used for transferring a pattern to a suitable substrate. Whereas most conventional microlithography systems utilize a beam of vacuum ultraviolet light for making the exposure, an emerging class of microlithography systems utilizes a charged particle beam (e.g., electron beam or ion beam) or an X-ray beam for making the exposure.  
           [0004]    The summary below is set forth in the context of an electron-beam (EB) microlithography system, by way of example, which is used mainly for transferring intricate circuit patterns for integrated circuits and the like onto semiconductor wafers. In a typical EB microlithography system an electron beam is directed onto a layer of “resist” coated on a surface of a semiconductor wafer. Since an electron beam is blocked, and thus attenuated, by collisions with gas molecules, the inside of the microlithography system (especially in the beam trajectory) is maintained at high vacuum.  
           [0005]    To create and contain the high-vacuum environment, a vacuum chamber is used. In the context of EB microlithography systems, exemplary vacuum chambers include vacuum chambers configured for holding a substrate (“wafer”) undergoing lithography and vacuum chambers configured for holding a reticle defining, for example, a pattern to be transferred lithographically to the substrate. The vacuum chamber typically is defined by at least one bulkhead and additional walls as required. The “bulkhead” is a stationary wall characterized by increased strength and rigidity (compared to other walls) for use as a mounting support for any of various instruments, windows, optical components, and other things in the vacuum chamber. Whenever this vacuum chamber is evacuated to a high vacuum, the walls (including the bulkhead) of the vacuum chamber exhibit some deformation due to the resulting pressure differential of the inside of the chamber (high vacuum) versus the outside of the chamber (normally at ambient atmospheric pressure). Changes in atmospheric pressure also can cause an accompanying change in deformation of the chamber walls and bulkhead. Whenever a bulkhead of such a chamber deforms, the attitude and position of, for example, a measuring instrument attached to the bulkhead change accordingly. For example, in an EB microlithography system, certain auto-focus (AF) and alignment (AL) instruments and/or optical microscopes or the like typically are installed on a bulkhead of the vacuum chamber. A change in attitude or position of an AF or AL instrument mounted on a bulkhead experiencing deformation can produce a corresponding decrease in the accuracy of pattern transfer performed in the chamber using the microlithography system.  
           [0006]    According to conventional thinking, the way to prevent deformation of a bulkhead of a vacuum chamber (and the consequential adverse effect on accuracy of AF and AL instruments mounted on the bulkhead) is to increase the rigidity and stoutness of the bulkhead by forming, for example, strong ribs on the bulkhead and/or by constructing the bulkhead of a material having a relatively high Young&#39;s modulus. However, with such approaches, increasingly stringent demands for measurement accuracy and precision of AF and AL systems are accompanied by corresponding substantial increases in the size and mass of the overall vacuum-chamber structure, which unavoidably increases the overall size and cost of the apparatus. Therefore, other countermeasures are needed to avoid this trend.  
         SUMMARY  
         [0007]    In view of the problems experienced with conventional apparatus and methods as summarized above, the invention provides, inter alia, systems respectively comprising a vacuum chamber that is more resistant to decreases in the accuracy and precision of instruments mounted on a bulkhead of the vacuum chamber. These ends are met by reducing the effects of deformation of the chamber bulkhead during evacuation of the chamber or during changes in the pressure differential of the pressure inside the chamber relative to the pressure outside the chamber.  
           [0008]    According to a first aspect of the invention, chambers are provided for holding an object (e.g., a workpiece) at a pressure that is lower (i.e., higher vacuum) inside the chamber than outside the chamber. An embodiment of such a chamber comprises walls and at least one bulkhead that collectively define the chamber. A secondary wall is situated outside the chamber relative to the bulkhead, and defines a gap between the secondary wall and the bulkhead. The gap defines a secondary reduced-pressure chamber that is maintained at a pressure that is lower than the pressure outside the chamber. For example, if the inside of the chamber is maintained at a particular vacuum level, the pressure maintained in the secondary reduced-pressure chamber can be less than (i.e., at a higher vacuum level than) the pressure inside the chamber and the pressure outside the chamber (the latter usually being atmospheric pressure). Alternatively, the pressure maintained in the secondary reduced-pressure chamber can be intermediate the pressure outside the chamber and the pressure inside the chamber. In either case, the pressure inside the secondary reduced-pressure chamber is lower than the pressure outside the chamber. The secondary wall is deformable relative to the bulkhead in response to this differential of pressure inside the secondary reduced-pressure chamber relative to outside the chamber. The secondary reduced-pressure chamber desirably is isolated from the pressure outside the chamber and from the pressure inside the chamber.  
           [0009]    As noted above, the chamber can be configured to be evacuated to a particular vacuum level relative to atmospheric pressure outside the chamber. In this and other configurations, the secondary reduced-pressure chamber can be connected to a vacuum pump configured to evacuate the secondary reduced-pressure chamber to a lower pressure than outside the chamber.  
           [0010]    The chamber further can comprise a measurement instrument and a seal means. In this configuration the measurement instrument is mounted to the bulkhead and has a portion extending through the secondary wall to outside the chamber. The seal means is situated and configured to establish a seal between the secondary wall and the measurement instrument such that the secondary wall can move relative to the bulkhead (and hence relative to the measurement instrument), without breaching the seal, in response to the differential of pressure. The measurement instrument can be configured to measure a characteristic of the object inside the chamber. The seal means can comprise an elastomeric sealing member extending from the secondary wall to the measurement instrument.  
           [0011]    By way of example, the chamber can be a wafer chamber of a microlithography system, wherein the object is a semiconductor wafer being exposed lithographically in the wafer-vacuum chamber. In this configuration the measurement instrument can be used for measuring at least one of position and alignment of the wafer inside the wafer-vacuum chamber. Alternatively, the chamber can be a reticle-vacuum chamber of a microlithography system, wherein the object is a reticle, and the measurement instrument can be used for measuring at least one of position and alignment of the reticle inside the reticle-vacuum chamber.  
           [0012]    The relative pressures inside the chamber, inside the secondary reduced-pressure chamber, and the pressure outside the chamber can be as summarized earlier above.  
           [0013]    According to another aspect of the invention, apparatus are provided for housing an object in a subatmospheric-pressure environment. An embodiment of such an apparatus comprises a chamber collectively defined by vessel walls and at least one bulkhead. The chamber is sized to contain the object and to contain an atmosphere evacuated to the subatmospheric pressure. The apparatus includes at least one instrument mounted to the bulkhead outside the chamber, wherein the instrument is configured to measure a characteristic of the object in the chamber. The apparatus also includes a deformation-reducing device for reducing deformation of the bulkhead in response to the pressure differential of the subatmospheric pressure inside the chamber relative to the pressure outside the chamber. The deformation-reducing device desirably comprises a secondary wall situated outside the chamber relative to the bulkhead and defining a gap between the bulkhead and the secondary wall, wherein the gap defines a secondary reduced-pressure chamber that is evacuated to a pressure that is lower than the pressure outside the chamber. The secondary wall desirably deforms relative to the bulkhead in response to the pressure differential of the pressure inside the secondary reduced-pressure chamber relative to the pressure outside the chamber, thereby greatly reducing deformation of the bulkhead. The apparatus further can comprise a seal means and/or vacuum pump as summarized above.  
           [0014]    The apparatus further can comprise a stage situated inside the chamber and configured to hold the object inside the chamber. If the object is a reticle or substrate, then the stage can be, for example, a reticle stage or wafer stage, respectively, of a microlithographic projection-exposure system. In this instance, the instrument can be, for example, a reticle-autofocus device, a reticle-alignment device, a wafer-autofocus device, or a wafer-alignment device.  
           [0015]    According to another aspect of the invention, systems are provided for irradiating an object with an energy beam. An embodiment of such a system comprises a chamber collectively defined by vessel walls and at least one bulkhead. The chamber is sized to contain the object for irradiation with the energy beam and to contain an atmosphere evacuated, at least during the irradiation, to a desired subatmospheric pressure. The system also includes an optical system situated so as to irradiate the object in the chamber with the energy beam. The system also includes an instrument mounted to the bulkhead outside the chamber, wherein the instrument is configured to measure a characteristic of the object in the chamber. The system also includes a deformation-reducing device for reducing deformation of the bulkhead in response to the differential of pressure inside the chamber relative to pressure outside the chamber. The deformation-reducing device desirably comprises a secondary wall situated outside the chamber relative to the bulkhead and defining a gap between the bulkhead and the secondary wall, wherein the gap defines a secondary reduced-pressure chamber that is evacuated to a pressure that is lower than the pressure outside the chamber.  
           [0016]    As summarized above, the secondary wall desirably is configured to deform relative to the bulkhead in response to the differential of pressure inside the secondary reduced-pressure chamber relative to the pressure outside the chamber. The system further can include a seal means and/or vacuum pump as summarized above.  
           [0017]    If the object is a lithographic wafer substrate, then the optical system can be a projection-optical system situated relative to the chamber and configured to illuminate the substrate inside the chamber with an energy beam so as to expose the substrate lithographically. In this configuration the energy beam can be, for example, a beam of electromagnetic radiation (e.g., vacuum-UV light, extreme UV light, or X-ray light) or a charged particle beam.  
           [0018]    According to yet another aspect of the invention, lithographic exposure systems are provided for exposing a substrate with a pattern. An embodiment of such a system comprises a first chamber collectively defined by respective chamber walls and at least one respective bulkhead. The first chamber is configured: (a) to contain the substrate for exposure, (b) to allow irradiation of the substrate with an energy beam capable of imprinting the pattern on the substrate, and (c) to contain an atmosphere evacuated, at least during the exposure, to a respective subatmospheric pressure. The system also includes an energy-beam source situated relative to the first chamber to direct the energy beam into the first chamber to expose the substrate. The source can comprise a projection-optical system mounted to the bulkhead of the first chamber. A respective instrument is mounted to the respective bulkhead, wherein the instrument is configured to measure a characteristic (such as position and/or alignment) of the substrate in the first chamber. The system includes a respective deformation-reducing device for reducing deformation of the bulkhead of the first chamber in response to the differential of pressure inside the first chamber relative to the pressure outside the first chamber.  
           [0019]    The system further can comprise a second chamber collectively defined by respective chamber walls and at least one respective bulkhead. The second chamber is configured: (a) to contain a reticle, (b) to allow irradiation of the reticle with an illumination beam, (c) to allow the illumination beam, propagating downstream of the reticle, to pass from the second chamber to the first chamber, and (d) to contain an atmosphere evacuated, at least during irradiation, to a respective subatmospheric pressure. An illumination-optical system is situated relative to the second chamber and configured to direct the illumination beam into the second chamber to illuminate the reticle. A respective instrument is mounted to the respective bulkhead, wherein the instrument is configured to measure a characteristic (e.g., position and/or alignment) of the reticle in the second chamber. The system includes a respective deformation-reducing device for reducing deformation of the bulkhead of the second chamber in response to the differential of pressure inside the second chamber relative to pressure outside the second chamber.  
           [0020]    With respect to either chamber, the respective deformation-reducing device desirably comprises a respective secondary wall situated outside the respective chamber relative to the respective bulkhead. The secondary wall defines a gap, between the bulkhead and the secondary wall, that defines a respective secondary reduced-pressure chamber that is evacuated to a subatmospheric pressure that is lower than the pressure outside the respective chamber. The secondary wall desirably is configured to deform relative to the respective bulkhead in response to the differential of pressure inside the respective secondary reduced-pressure chamber relative to pressure outside the respective chamber, thereby preventing deformation of the respective bulkhead. Each secondary reduced-pressure chamber can include a respective seal means and/or vacuum pump as summarized above. The vacuum pump can be configured to change the subatmospheric pressure in the respective secondary reduced-pressure chamber in response to a change in pressure outside the respective chamber and/or a change in pressure inside the respective chamber.  
           [0021]    According to yet another aspect of the invention, methods are provided (in the context of any of various methods involving holding a workpiece or other object under a subatmospheric-pressure condition established within a chamber collectively defined by vessel walls and at least one bulkhead) for reducing deformations of the bulkhead resulting from changes in the differential of pressure inside the chamber relative to pressure outside the chamber. An embodiment of such a method comprises placing a secondary wall outside the chamber relative to the bulkhead so as to define a gap between the secondary wall and the bulkhead, wherein the gap defines a secondary reduced-pressure chamber. The secondary reduced-pressure chamber is evacuated to a subatmospheric pressure that is lower than the pressure outside the chamber, wherein the secondary wall deforms relative to the bulkhead in response to a pressure differential, as summarized above.  
           [0022]    According to yet another aspect of the invention, microlithography systems are provided that illuminate a selected region on a reticle with an energy beam and that project and focus the energy beam, propagating from the reticle, onto a selected region on a sensitive substrate so as to transfer features from the reticle to the sensitive substrate. An embodiment of such a system comprises a reticle-vacuum chamber containing a reticle stage or other reticle holder on which the reticle is mounted. The reticle-vacuum chamber is defined by respective walls and at least one respective bulkhead. The system also includes a wafer-vacuum chamber that contains a wafer stage or other substrate holder, on which the sensitive substrate is mounted, wherein the wafer-vacuum chamber is defined by respective walls and at least one respective bulkhead. A respective instrument is mounted on the bulkhead of the reticle-vacuum chamber for measuring a characteristic of the reticle, and a respective instrument is mounted on the bulkhead of the wafer-vacuum chamber for measuring a characteristic of the substrate. For at least one of the chambers, a respective deformation-reducing device is provided for reducing deformation of the respective bulkhead in response to a pressure differential, as summarized above.  
           [0023]    The deformation-reducing device desirably comprises a respective secondary wall situated outside the respective chamber relative to the respective bulkhead and defining a gap between the respective bulkhead and respective secondary wall. The gap defines a respective secondary reduced-pressure chamber that is evacuated to a respective pressure that is lower than the pressure outside the respective chamber. The secondary wall desirably deforms relative to the respective bulkhead in response to a pressure differential, as summarized above. The system can include a seal means and/or vacuum pump as summarized above.  
           [0024]    In a more specific embodiment of the system, a first deformation-reducing device is provided for reducing deformation of the bulkhead of the reticle-vacuum chamber, and a second deformation-reducing device is provided for reducing deformation of the bulkhead of the wafer-vacuum chamber, in response to respective pressure differentials. In this system, each deformation-reducing device desirably comprises a respective secondary wall situated outside the respective chamber relative to the respective bulkhead and defining a gap between the respective bulkhead and respective secondary wall. Each gap defines a respective secondary reduced-pressure chamber that is evacuated to a respective pressure that is lower than the pressure outside the respective chamber. As noted above, each secondary wall desirably is configured to deform relative to the respective bulkhead in response to pressure differentials. Seal means and vacuum pumps, as summarized above, can be included.  
           [0025]    An instrument can be mounted on the bulkhead of the reticle-vacuum chamber. This instrument can be, for example, a reticle-position-measurement system (e.g., a reticle-autofocus system) or a reticle-alignment-measurement system. Similarly, an instrument can be mounted on the bulkhead of the wafer-vacuum chamber. This instrument can be, for example, a substrate-position-measurement system (e.g., a substrate-autofocus system) or a substrate-alignment-measurement system.  
           [0026]    The bulkhead of the reticle-vacuum chamber and the bulkhead of the wafer-vacuum chamber can be mounted to opposite respective ends of a projection-optical system extending between and outside the chambers. In such a system the bulkhead of the reticle-vacuum chamber can be configured as a reticle-optical plate, and the bulkhead of the wafer-vacuum chamber can be configured as a wafer-optical plate. As used herein, an “optical plate” is a bulkhead especially configured, such as in terms of enhanced planarity, strength, and/or rigidity, for serving as a mounting foundation or base for optical components or an optical system mounted thereto.  
           [0027]    The reticle-vacuum chamber can comprise a second bulkhead situated opposite the respective bulkhead, relative to the respective walls. In such a configuration the second bulkhead can be connected to an illumination-optical system.  
           [0028]    The reticle-vacuum chamber can be coupled to a reticle-loader chamber and a reticle load-lock chamber, and the wafer-vacuum chamber can be coupled to a wafer-loader chamber and a wafer load-lock chamber, as desired or required.  
           [0029]    Since the various systems summarized above include respective devices that reduce deformation of a bulkhead occurring during evacuation or other pressure change in the respective chamber, misalignments and/or positional shifts of instruments mounted on the bulkhead are reduced. This allows higher-accuracy work to be performed on an object or workpiece located in the respective chamber, such as workpiece measuring, workpiece processing, workpiece irradiation, or pattern transfer from a reticle to the workpiece.  
           [0030]    Exemplary energy-beam irradiation systems include, but are not limited to, lithographic-exposure systems, coordinate-measurement systems, scanning electron microscopes, etc. Exemplary instruments include, but are not limited to, autofocus (AF) devices (see, e.g., Japan Kôkai Patent Document Nos. Hei 6-283403 and Hei 8-64506, referred to herein as “AF” devices), alignment devices (see, e.g., Japan Kôkai Patent Document No. Hei 5-21314, referred to herein as “AL” devices), and interferometers.  
           [0031]    With respect to any of the secondary reduced-pressure chambers referred to above, by making the pressure inside the chamber and the pressure inside the secondary reduced-pressure chamber nearly equal to each other, deformation of the bulkhead is further reduced. This is because, under such conditions, the differential of internal versus external pressure across the bulkhead has virtually no effect on the bulkhead, especially near instruments mounted to the bulkhead. If there is a change in the pressure differential, then the respective secondary wall (rather than the bulkhead) is deformed. Also, by moving the secondary wall instead of the bulkhead in response to the pressure differential, any instruments mounted on the bulkhead experience correspondingly less disturbance in response to the change in pressure differential. The seal means established between the secondary wall and the instruments or their mountings can be configured as a sliding or otherwise deformable gasket between the instruments (or instrument mounts) and the secondary wall. The seal means can be, for example, O-rings or diaphragms extending between the secondary wall and the instrument mounts or instruments.  
           [0032]    Reducing deformation of the bulkhead generally results in substantially reduced tilting, misalignment, distortion, or other undesired movement of the instrument(s) mounted to it. For example, a “distortion” to an instrument can arise in a situation in which there is no actual tilting of the instrument itself but rather a shift of the position of the instrument relative to an object inside the chamber that is being monitored by the instrument. If this distortion is very slight, the measurement accuracy of the instrument is not affected significantly in many instances. But, a more pronounced distortion (as experienced in conventional apparatus) substantially can reduce the performance accuracy of the instrument.  
           [0033]    The pressure inside a secondary reduced-pressure chamber can be regulated according to changes in the pressure outside the respective chamber. Thus, the positioning of instrument(s) and/or their respective mount(s) mounted to a bulkhead of the respective chamber can be optimized by intentional control of the pressure in the secondary reduced-pressure chamber.  
           [0034]    The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    [0035]FIG. 1 is a schematic elevational diagram showing the overall configuration of a representative embodiment of a microlithographic projection-exposure system.  
         [0036]    [0036]FIG. 2 is a plan view of an exemplary wafer-optical plate of the microlithographic exposure system of FIG. 1, showing certain components associated with this particular wafer-optical plate.  
         [0037]    [0037]FIG. 3 is an elevational section, along the line X-X, of the wafer-optical plate of FIG. 2, showing the location of the wafer auto-focus (AF) device.  
         [0038]    [0038]FIG. 4 is an enlarged elevational section showing details of the wafer AF device shown in FIG. 3.  
         [0039]    [0039]FIG. 5 is an elevational section viewed in the direction of the arrow Y in FIG. 4.  
         [0040]    [0040]FIG. 6( a ) is a schematic elevational depiction of deformation of the wafer-optical plate that occurs whenever no secondary wall is provided in association with the wafer-optical plate.  
         [0041]    [0041]FIG. 6( b ) schematically shows the absence of deformation of the wafer-optical plate achieved by including a secondary wall in association with the wafer-optical plate.  
         [0042]    [0042]FIG. 7 is a schematic elevational diagram showing certain optical relationships in a charged-particle-beam (notably electron-beam) microlithography system. 
     
    
     DETAILED DESCRIPTION  
       [0043]    Several representative embodiments are described below that are not intended to be limiting in any way. Also, the description is made largely in the context of an electron-beam microlithography system as a representative charged-particle-beam (CPB) microlithography system and as a representative system employing a vacuum chamber. It will be understood that the details described below can be applied with equal facility to any of various other types of microlithography systems and to other systems employing a vacuum chamber, such as an ion-beam, X-ray, or extreme ultraviolet (EUV) microlithography system or other system that utilizes one or more charged particle beams or beams of electromagnetic radiation.  
         [0044]    An overview of the overall construction of an exemplary electron-beam (EB) projection-microlithography system and of the imaging relationships in such a system is provided in FIG. 7. In the depicted system, an electron gun  1  is situated at the extreme upstream end of an EB optical system and emits an electron beam (“illumination beam” IB) in the downstream direction. A condenser lens  2  and an illumination lens  3  are situated downstream of the electron gun  1 , and the illumination beam IB passes through the lenses  2 ,  3  to illuminate a pattern-defining reticle  10 .  
         [0045]    In FIG. 7, the EB optical system upstream of the reticle  10  (termed the “illumination-optical system”) also includes other components such as a shaping aperture, a blanking deflector, a blanking aperture, and an illumination-beam deflector that are not shown but are well understood in the art. The primary components in the illumination-optical system are the lenses  2 ,  3 . The illumination beam IB, shaped and appropriately deflected in the illumination-optical system, sequentially scans the reticle  10  to illuminate “subfields” on the reticle. Each subfield defines a respective portion of the overall pattern defined by the reticle  10 . The lateral distance on the reticle over which the illumination beam IB is scanned is within the optical field of the illumination-optical system.  
         [0046]    As noted above, the reticle  10  has a multiple subfields that typically are arranged on the reticle in a rectilinear array. The reticle is mounted on a movable reticle stage  11 . Subfields on the reticle located outside the optical field of the illumination-optical system are brought to within the optical field (for illumination) by movement of the reticle stage  11  within a plane perpendicular to the optical axis A.  
         [0047]    Downstream of the reticle  10  is the “projection-optical system” comprising a primary projection lens  15  and a secondary projection lens  19  for projecting and forming respective images of the illuminated subfields on appropriate locations on a “sensitive” substrate (resist-coated wafer)  23 . The projection-optical system also includes deflectors  16  (denoted  16 - 1 ,  16 - 2 ,  16 - 3 ,  16 - 4 ,  16 - 5 ,  16 - 6  in the figure) used for aberration correction and for achieving a desired image registration on the wafer. Portions of the illumination beam passing through an illuminated subfield on the reticle  10  thus become a “patterned beam” that carries an aerial image of the illuminated subfield. The aerial image is formed at a specified position on the wafer  23  by means of the projection lenses  15 ,  19  and the deflectors  16 . As noted, the upstream-facing surface of the wafer  23  is coated with a suitable resist that, upon receiving an appropriate “dose” of the patterned beam, becomes imprinted with the respective image. Thus, the pattern on the reticle  10  is transferred onto the wafer. The transferred image normally is demagnified according to a demagnification ratio (reduction ratio) of, e.g., 1/4.  
         [0048]    A crossover C.O. is formed at a point on the optical axis at which the axial distance between the reticle  10  and wafer  23  is divided according to the demagnification ratio. A contrast aperture  18  is disposed at the position of the crossover. The contrast aperture  18  blocks electrons of the patterned beam that have experienced substantial forward-scattering during passage through non-patterned portions of the reticle  10 . Thus, these scattered electrons do not reach the wafer  23 .  
         [0049]    The wafer  23  is mounted by an electrostatic chuck on a wafer stage  24  that is movable in the X and Y directions perpendicular to the optical axis A. By synchronously scanning the reticle stage  11  and wafer stage  24  in opposite directions, the various portions of the pattern situated beyond the optical field of the projection-optical system are exposed sequentially.  
         [0050]    Turning now to FIGS. 1-5, an electron-beam projection-microlithography (“projection-exposure”) system  100  according to a representative embodiment is shown, wherein the system  100  is representative of any of various systems that include a vacuum chamber. In the depicted apparatus, an illumination-optical-system (IOS) column  101  is situated at the upstream end of the apparatus  100  (top of the figure, labeled the “illumination-system electron optics” (EO)). The electron gun  1 , condenser lens  2 , illumination lens  3 , and other components of the illumination-optical system discussed above are disposed inside the IOS column  101 . A reticle-vacuum chamber  103 , situated just downstream of the IOS column  101 , contains the reticle stage  11 .  
         [0051]    A reticle-loader chamber  105  and reticle load-lock chamber  107 , shown at the right in FIG. 1, are connected to the reticle-vacuum chamber  103 . A robotic manipulator (not shown), used for reticle handling, is situated inside the reticle-loader chamber  105 . The manipulator operates, for example, to replace an existing reticle on the reticle stage  11  with a new reticle waiting inside the reticle-loader chamber  105 . Whenever reticles are moved into the reticle-vacuum chamber  103  from outside the projection-exposure system or out of the reticle-vacuum chamber  103  to outside the projection-exposure system, such movements are made by the manipulator via the reticle-loader chamber  105  though the reticle load-lock chamber  107 . Evacuation means, such as respective vacuum pumps (not shown, but well understood in the art), are connected to the reticle-vacuum chamber  103  and the reticle load-lock chamber  107 . The interior of the IOS column  101 , as well as the interior of the projection-optical-system (POS) column  111  discussed below, normally are evacuated to high vacuum.  
         [0052]    A reticle interferometer (IF)  109 , shown at the left in FIG. 1, extending into the reticle-vacuum chamber  103 . The reticle interferometer  109  is connected to a controller  25 . Accurate data regarding the position of the reticle stage  11  are produced by the reticle interferometer  109  and routed to the controller  25 . The controller  25 , in turn, produces reticle-movement commands routed to the reticle stage  11  as required in response to the reticle-position data. Thus, the position of the reticle stage  11  is controlled accurately in real time.  
         [0053]    The reticle stage  11  is mounted to an upstream-facing surface of a “reticle-optical plate”  131  (serving as a chamber bulkhead and instrument-mounting plate for the reticle-vacuum chamber  103 ). Downstream of the reticle-vacuum chamber  103  is a wafer-vacuum chamber  113  described later below. The wafer-vacuum chamber  113  is defined in part by a “wafer-optical plate”  132  that is a bulkhead of the wafer-optical system. The POS column  111  is disposed between and mounted to the optical plates  131 ,  132 . In the depicted embodiment, each optical plate  131 ,  132  is configured in this embodiment as a respective octagonal plate fabricated from mild steel plate or the like (see FIG. 2). The primary projection lens  15  and secondary projection lens  19  are disposed inside the POS column  111 , which is evacuated to high vacuum.  
         [0054]    A reticle-autofocusing (AF) system  141  and reticle-alignment (AL) system  142  (as exemplary “instruments”) are mounted on the downstream-facing (“bottom”) surface of the reticle-optical plate  131 , and a wafer AF system  151  and wafer AL system  152  (as exemplary “instruments”) are mounted on the upstream-facing (“top”) surface of the wafer-optical plate  132 , around the perimeter of the POS column  111 , as discussed in detail below. A “main body”  130  extends between the two optical plates  131 ,  132 .  
         [0055]    The wafer-vacuum chamber  113  contains the wafer stage  24  and related components. A wafer-loader chamber  115  and wafer load-lock chamber  117 , shown on the right in FIG. 1, are connected to the wafer-vacuum chamber  113 . Evacuation means, such as respective vacuum pumps (not shown), are connected to the wafer-vacuum chamber  113  and the wafer load-lock chamber  117 .  
         [0056]    A wafer interferometer (IF)  119 , shown at the left in FIG. 1, extends into the wafer-vacuum chamber  113 . The wafer interferometer  119  is connected to the controller  25 . Accurate data concerning the position of the wafer stage  24  are produced by the wafer interferometer  119  and routed to the controller  25 . The controller  25 , in turn, produces wafer-movement commands routed to the wafer stage  24  as required in response to the wafer-position data. Thus, the position of the wafer stage  24  is controlled accurately in real time.  
         [0057]    The wafer-vacuum chamber  113  is supported by a stand  122  mounted to a base plate  126 . The main body  130 , discussed above, is supported on the base plate  126  by a stand  128  providing active attenuation of vibrations between the base plate  126  and the main body  130 .  
         [0058]    Exemplary structures associated with the wafer AF system  151  are shown in FIGS. 2-5. The respective structures of the wafer AF system  151  and reticle AF system  141  are similar to each other, and the respective structures of the wafer AL system  152  and reticle AL system  142  are similar to each other.  
         [0059]    The wafer AF system  151 , as shown in FIGS. 2-3, comprises a light-transmission device  153  and a light-reception device  155  mounted to the “outer” surface of the wafer-optical plate  132  (i.e., outside the wafer-vacuum chamber  113 ). The devices  153 ,  155  extend through the wafer-optical plate  132  into the interior of the wafer-vacuum chamber  113 , and are situated on opposite sides of the POS column  111 , with the POS column situated between them. Signal light emitted from the light-transmission device  153  impinges on the “top” (upstream-facing) surface of the wafer W on the wafer stage  24  inside the wafer-vacuum chamber  113 , and signal light reflected from the wafer surface is received by the light-reception device  155 . Meanwhile, the wafer AL system  152  (not shown in FIG. 3) is mounted to the outer surface of the wafer-optical plate  132  at a location just outside the perimeter of the POS column  111  and separately from the light-transmission device  153  and light-reception device  155  of the wafer AF system  151 . Measurement data produced by the wafer AF system  151  pertain to the measured position of an existing pattern on the wafer or of a mark plate on the wafer stage  24 . These data are used for registering the relative positions of the existing alignment-mark pattern provided on the wafer  23  or on a pattern to be formed next on the wafer.  
         [0060]    The wafer AF system  151  can have a conventional configuration such as disclosed in Japan Kôkai Patent Publication No. Hei 6-283403 and Japan Kôkai Patent Publication No. Hei 8-64506, and the wafer AL system  152  can have a conventional configuration such as disclosed in Japan Kôkai Patent Publication No. Hei 5-21314.  
         [0061]    Structures in the vicinity of the light-transmission device  153  of the wafer AF system  151  are shown in FIGS. 4 and 5. Turning first to FIG. 5, the light-transmission device  153  comprises a vertical lens column  156 , a horizontal lens column  157 , and a light source  158 . The vertical lens column  156  includes an objective lens  156   b  and vacuum-bulkhead window  156   e  situated at the “bottom” and “top,” respectively, of an AF lens column  156   a . A mirror  156   c  and window  156   d  are situated at the “upper” end of the AF lens column  156   a.    
         [0062]    As shown in FIGS. 4 and 5, a box-shaped mirror chamber  161  is attached to the “bottom” of the AF lens column  156   a . A flange  161   a  extends outward around the circumference of an opening at the “top” of the mirror chamber  161 . The mirror chamber  161  extends from the outer surface of the wafer-optical plate through an opening in the wafer-optical plate  132  and through a corresponding opening in an upper lip  113   a  of the wafer-vacuum chamber  113 , such that the “lower” portion of the mirror chamber  161  extends into the interior of the wafer-vacuum chamber  113 . The flange  161   a  of the mirror chamber  161  is attached to the “top” surface (outer surface) of the wafer-optical plate  132 , with an O-ring seal  162  therebetween. A mirror  161   c  and window  161   d  are situated inside the mirror chamber  161  (FIG. 4).  
         [0063]    As shown in FIG. 5, the horizontal lens column  157  and light source  158  are attached to a platform  165 . The platform  165  is supported firmly by legs  166  mounted to the “top” surface of the wafer-optical plate  132 .  
         [0064]    As shown in FIG. 2, a “pan”  170  is disposed over nearly the entire “top” surface of the wafer-optical plate  132 . Thus, in this embodiment, the pan  170  is situated outside the wafer-vacuum chamber  113  relative to the wafer-optical plate  132 . The pan  170  serves as a secondary wall to the wafer-optical plate  132  (the latter being an exemplary bulkhead), and defines a gap H (FIGS. 4 and 5) between the pan  170  and the wafer-optical plate  132 . Thus, a secondary reduced-pressure chamber S 1  is defined in the space between the pan  170  and the wafer-optical plate  132 . The pan  170  desirably is made from a sheet of relatively low-mass metal, such as aluminum, to allow the pan to flex, as described further below. As shown in FIGS. 4 and 5, the pan  170  is situated “above” the flange  161   a  of the mirror chamber  161 . The secondary reduced-pressure chamber S 1  is connected to and evacuated by a vacuum pump (not shown in FIGS. 4 and 5, but see item  171  in FIG. 2). The secondary reduced-pressure chamber S 1  is connected to a space S 2 , in which the mirror  161   c  is located, inside the mirror chamber  161 .  
         [0065]    The pan  170  defines a hole  170   a  through which the vertical lens column  156  extends and defines respective holes  170   b  through which the legs  166  of the stand  165  extend. An annular closure member  186  extends radially on the “top” surface of the pan  170  to close most of the space between the hole  170   a  and the outside diameter of the AF lens column  156   a . The mounting of the closure member  186  to the pan  170  is sealed with an O-ring  187  (or analogous elastomeric seal, such as a diaphragm), and the space between the inside diameter of the closure member  186  and the outside diameter of the AF lens column  156   a  is sealed with an O-ring  188  (or analogous elastomeric seal). The O-ring  182  allows movement of the pan  170  relative to the AF lens column  156   a . Meanwhile, respective annular closure members  192  extend radially on the “top” surface of the pan  170  to close respective spaces between the holes  170   b  and the outer surfaces of the legs  166 . The mounting of each closure member  192  with the pan  170  is sealed with a respective O-ring  193 , and the space between the inside diameter of each closure member  192  and the outside diameter of each leg  166  is sealed with a respective O-ring  194 .  
         [0066]    The secondary reduced-pressure chamber S 1  between the pan  170  and the wafer-optical plate  132  is isolated from the environment outside the system (which is usually, but not necessarily, at atmospheric pressure) and from the vacuum environment inside the wafer-vacuum chamber  113 . The vacuum pump  171  (FIG. 2) connected to the secondary reduced-pressure chamber S 1  operates to reduce and regulate the pressure inside the secondary reduced-pressure chamber S 1 . A distortion sensor (not shown) can be mounted on the inner surface of the mirror chamber  161  or other suitable location for measuring deformation of the mirror chamber  161  and pan  170 , allowing the pressure inside the secondary reduced-pressure chamber S 1  to be regulated appropriately in real time.  
         [0067]    Item  175  in FIG. 4 is an annular member situated between the “bottom” surface of the POS lens column  111  and the “top” surface of the wafer-optical plate  132 . The annular member  175  desirably is made from a non-magnetic material, such as stainless steel, and serves to interrupt an electromagnetic circuit that otherwise could form between the POS column  111  and the wafer-optical plate  132 , both of which are made of magnetic materials.  
         [0068]    Turning now to FIG. 6( a ), a wafer AF system  151  (or wafer AL system  152 ) and wafer-optical plate  132  lacking a pan  170  are depicted schematically. Atmospheric pressure is exerted on the “top” surface (outside the wafer-vacuum chamber  113 ) of the wafer-optical plate  132 . The “lower” surface of the wafer-optical plate  132  (situated inside the wafer-vacuum chamber  113 ) normally is subjected to a high vacuum (e.g., 10 −6  Torr). In the absence of the pan  170 , during evacuation of the wafer-vacuum chamber  113 , or whenever there is a change in atmospheric pressure outside the wafer-vacuum chamber, a corresponding pressure differential (or change in pressure differential) is exerted directly on the wafer-optical plate  132 . The pressure differential tends to distort the wafer-optical plate  132  relative to the wafer-vacuum chamber  113  (downward in the figure), as shown by the dotted line in the figure. Whenever such deformation occurs, an instrument such as the wafer AF system  151 , mounted on and supported by the wafer-optical plate  132 , is affected adversely by experiencing an alignment and/or positional shift.  
         [0069]    In contrast, referring now to FIG. 6( b ), the secondary reduced-pressure chamber S 1  and the pan  170  are located on the “top” surface (outside the wafer-vacuum chamber  113 ) of the wafer-optical plate  132 . The prevailing external pressure (usually atmospheric) is exerted on the “top” surface of the pan  170 , but not directly on the “top” surface of the wafer-optical plate  132 . This is because the secondary reduced-pressure chamber S 1  located between the pan  170  and the wafer-optical plate  132  serves to isolate the “top” surface of the wafer-optical plate from the pressure outside the wafer-vacuum chamber  113 . To such end, the secondary reduced-pressure chamber S 1  is evacuated by the vacuum pump  171  (FIG. 2) to a vacuum of approximately 10 −4  Torr, for example, which is a lower pressure than the pressure outside the wafer-vacuum chamber  113 . Further by way of example, if the inside of the wafer-vacuum chamber  113  is at a high vacuum (e.g., 10 −6  Torr), the secondary reduced-pressure chamber S 1  is at approximately 10 −4  Torr, and the external pressure is atmospheric pressure, most of the pressure differential with respect to outside the wafer-vacuum chamber  113  is imparted to the pan  170 , not to the wafer-optical plate  132 . This pressure differential of the external pressure relative to the subatmospheric pressure inside the secondary reduced-pressure chamber S 1  causes the pan  170  to deform, as indicated by the dotted line in FIG. 6( b ), rather than causing deformation of the wafer-optical plate  132 . Since the pressure differential thus has virtually no effect on the wafer-optical plate  132 , deformation of the wafer-optical plate  132  is substantially reduced compared to conventional systems lacking a secondary reduced-pressure chamber. Since the respective spaces between the pan  170  and the wafer AF system  151  are sealed by the respective closure members  186 ,  192  and O-rings  188 ,  194  (in a manner allowing a small amount of movement of the pan  170  relative to the wafer-optical plate  132 ), deformation of the pan  170  has substantially no effect on the wafer AF system  151 .  
         [0070]    Meanwhile, since deformation of the wafer-optical plate  132  is reduced substantially, as described above, movements of the AF lens column  156   a , the mirror chamber  161  supporting the wafer AF system  151 , and the legs  166  supporting the stand  165  are reduced substantially. This reduction of deformation of the wafer-optical plate  132  allows focusing and registration to be performed with higher accuracy than previously, which, in turn, allows higher-accuracy lithographic exposures to be made.  
         [0071]    If any residual deformation or a change in deformation of the wafer-optical plate  132  becomes problematic, these deformations can be detected using a pressure sensor or deformation sensor (e.g., strain gauge). Data from the sensor can be used in feedback control of the pressure in the secondary reduced-pressure chamber S 1 , making it possible to cancel the residual or change in deformation.  
         [0072]    Whereas the invention has been described in the context of representative embodiments, the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.