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 or occurrence of a change in pressure differential across the bulkhead. A “pan” (serving as a secondary wall) is situated at a gap distance from the bulkhead. A secondary reduced-pressure chamber is formed in the gap between the pan and the bulkhead. The secondary reduced-pressure chamber is isolated from atmospheric pressure outside the chamber and from the subatmospheric pressure inside the chamber. The differential between atmospheric pressure and the pressure inside the secondary reduced-pressure chamber is exerted on the pan, but the pressure differential has substantially no effect on the bulkhead, thereby reducing deformation of the bulkhead. Reducing deformation of the bulkhead prevents degradations of accuracy, otherwise caused by pressure-change-induced deformation of the bulkhead, of any instruments mounted to the bulkhead.

Description:
FIELD  
         [0001]    This disclosure pertains to systems configured to place and process a workpiece 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 a workpiece with an energy beam inside such a chamber. The disclosure also pertains to transfer-exposure apparatus, comprising such a chamber, that include one or more measuring instruments (e.g., alignment-measuring instruments) mounted to a bulkhead or wall of such a chamber. The exposure apparatus are 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 changes in the pressure differential across the chamber bulkhead (the latter being caused by, e.g., a change in atmospheric pressure).  
         BACKGROUND  
         [0002]    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 transfer-exposure apparatus, also termed a “microlithography” apparatus, 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 utilize a charged particle beam (e.g., electron beam or ion beam) or an X-ray beam for making the exposure.  
           [0003]    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.  
           [0004]    To create the high-vacuum environment, a vacuum chamber is used that typically comprises two portions, a wafer-vacuum chamber and a reticle-vacuum chamber. Whenever this vacuum chamber is evacuated to a high vacuum, the walls (bulkheads) of the 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 bulkheads. Whenever bulkheads of such chambers deform, the attitudes and positions of measuring instruments attached to the affected 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 in the vacuum chamber on a bulkhead of the 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 using the microlithography system.  
           [0005]    According to conventional thinking, the way to prevent deformation of the bulkheads of vacuum chambers (and the consequential adverse effect on accuracy of AF and AL instruments mounted on the bulkheads) is to increase the rigidity of the chamber by providing the bulkheads with stout ribs and/or constructing the bulkheads of materials 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 must be met by corresponding substantial increases in the size and mass of the overall vacuum-chamber structure, which unavoidably increases the overall size of the apparatus. Therefore, other countermeasures are needed to avoid this trend.  
         SUMMARY  
         [0006]    In view of the problems experienced with conventional apparatus and methods as summarized above, the invention provides, inter alia, systems comprising vacuum chambers that are more resistant to decreases in the accuracy and precision of instruments mounted on a bulkhead of the vacuum chambers. These ends are met by reducing the effects of deformation of chamber bulkheads during evacuation of the chamber or during changes in the ambient pressure outside the chamber.  
           [0007]    According to a first aspect of the invention, chambers are provided for performing a process on a workpiece at a pressure that is lower 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. The secondary wall defines a gap between the secondary wall and the bulkhead. The gap defines a secondary reduced-pressure chamber that is pressurizable at a pressure intermediate the respective pressures inside and outside the chamber. The secondary wall also is deformable relative to the bulkhead in response to a differential of pressure inside the secondary reduced-pressure chamber relative to outside the chamber. The secondary reduced-pressure chamber desirably is isolated from pressure outside the chamber and from pressure inside the chamber.  
           [0008]    The chamber can be configured to be evacuated to a high vacuum relative to atmospheric pressure outside the chamber. In this configuration, the secondary reduced-pressure chamber desirably is connected to a vacuum pump configured to evacuate the secondary reduced-pressure chamber to a less-high vacuum level than inside the chamber.  
           [0009]    The chamber can further comprise a measurement instrument and a seal means. In this configuration the measurement instrument is mounted to the bulkhead and extends through the secondary wall. 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 measurement instrument, without breaking the seal, in response to the differential of pressure. The measurement instrument can be configured to measure a characteristic of an object inside the chamber. The seal means can comprise a closure member extending radially from a surface of the secondary wall to the measurement instrument, and an elastomeric sealing member extending from the closure member to the measurement instrument.  
           [0010]    By way of example, the chamber can be a wafer chamber of a microlithography system, wherein the object is a semiconductor wafer being processed lithographically in the chamber. In this configuration the measurement instrument can be used for measuring at least one of focus and alignment of the object inside the chamber. Alternatively, the chamber can be a reticle chamber of a microlithography system.  
           [0011]    Further by way of example, the pressure inside the chamber can be a high vacuum, in which instance the pressure inside the secondary reduced-pressure chamber is an intermediate vacuum, and the pressure outside the chamber is ambient atmospheric pressure.  
           [0012]    According to another aspect of the invention, apparatus are provided for housing an object in a subatmospheric-pressure. 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 an instrument-mounting member mounted to the bulkhead outside the chamber, and an instrument mounted to the instrument-mounting member and 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 a differential of pressure 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 subatmospheric pressure intermediate the subatmospheric pressure in the chamber and the pressure outside the chamber. The secondary wall desirably deforms relative to the bulkhead in response to a differential of pressure inside the secondary reduced-pressure chamber relative to the pressure outside the secondary reduced-pressure chamber and outside the chamber. The apparatus can further comprise a seal means and/or vacuum pump as summarized above.  
           [0013]    The apparatus can further 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 exposure system. In this instance, the instrument can be a reticle autofocus system, a reticle alignment system, a wafer autofocus system, or a wafer alignment system.  
           [0014]    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 being sized to contain the object for irradiation with the energy beam and to contain an atmosphere evacuated, at least during the irradiation, to a 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-mounting member mounted to the bulkhead outside the chamber, and an instrument mounted to the instrument-mounting member and 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 a differential of pressure inside the chamber relative to pressure outside the chamber. The deformation-reducing device can comprise 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 subatmospheric pressure intermediate the subatmospheric pressure in the chamber and the pressure outside the chamber.  
           [0015]    As summarized above, the secondary wall desirably is configured to deform relative to the bulkhead in response to a differential of pressure inside the secondary reduced-pressure chamber relative to the pressure outside the secondary reduced-pressure chamber and outside the chamber. The system further can include a seal means and/or vacuum pump as summarized above.  
           [0016]    If the object is a lithographic wafer substrate, then the optical system can be a projection-optical system situated and configured to illuminate the substrate inside the chamber with an energy beam so as to expose the substrate lithographically with a pattern image. In this configuration the energy beam can be, for example, a beam of vacuum UV light, extreme UV light, or X-ray light, or a charged particle beam.  
           [0017]    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 chamber walls and at least one bulkhead. The first chamber is configured: (a) to contain the substrate for exposure, (b) to irradiate the substrate with an energy beam capable of imprinting the pattern on the substrate, and (c) to contain a respective atmosphere evacuated, at least during the exposure, to a respective subatmospheric pressure. The system also includes a source of the energy beam situated to direct the energy beam into the first chamber to expose the substrate. The source can comprise a projection-optical system coupled to the bulkhead of the first chamber. An instrument-mounting member is mounted to the bulkhead outside the first chamber, and an instrument is mounted to the instrument-mounting member and configured to measure a characteristic of the substrate in the first chamber. The system includes a respective deformation-reducing device for reducing deformation of the bulkhead in response to a differential of pressure inside the first chamber relative to the pressure outside the first chamber.  
           [0018]    The system can further comprise a second chamber collectively defined by chamber walls and at least one bulkhead. Similar to the first chamber, the second chamber is configured: (a) to contain a reticle defining a pattern to be exposed onto the substrate, (b) to irradiate the reticle with an illumination beam, and (c) to contain a respective atmosphere evacuated, at least during exposure, to a respective subatmospheric pressure. An illumination-optical system is situated and configured to direct the illumination beam into the second chamber to illuminate the reticle. An instrument-mounting member is mounted to the respective bulkhead outside the second chamber, and an instrument is mounted to the instrument-mounting member and configured to measure a characteristic 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 a differential of pressure inside the second chamber relative to pressure outside the second chamber.  
           [0019]    The instrument mounted to the instrument-mounting member of the second chamber can be, for example, a reticle autofocus system or a reticle alignment system.  
           [0020]    The deformation-reducing device can comprise a secondary wall situated outside the first chamber relative to the bulkhead. The secondary wall defines a gap between the bulkhead and the secondary wall. The gap defines a secondary reduced-pressure chamber that is evacuated to a subatmospheric pressure intermediate the subatmospheric pressure in the first chamber and the pressure outside the first chamber. The secondary wall desirably is configured to deform relative to the bulkhead in response to a differential of pressure inside the secondary reduced-pressure chamber relative to pressure outside the secondary reduced-pressure chamber and outside the first chamber. The system can include a seal means and/or vacuum pump as summarized above. The vacuum pump can be configured to change the subatmospheric pressure in the secondary reduced-pressure chamber in response to a change in pressure outside the first chamber.  
           [0021]    According to yet another aspect of the invention, methods are provided (in the context of methods for processing a workpiece 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 a differential of pressure inside of the chamber relative to pressure outside of 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, the gap defining a secondary reduced-pressure chamber. The secondary reduced-pressure chamber is evacuated to a subatmospheric pressure intermediate the subatmospheric pressure in the chamber and the pressure outside the chamber, wherein the secondary wall deforms relative to the bulkhead in response to a differential of pressure inside the secondary reduced-pressure chamber relative to the pressure outside the secondary reduced-pressure chamber and outside the chamber.  
           [0022]    According to yet another aspect of the invention, microlithography systems are provided that illuminate a selected region on a pattern-defining reticle with an energy beam, and project and focus the energy beam, that has passed through the reticle, onto a selected region on a sensitive substrate so as to transfer the pattern from the reticle to the sensitive substrate. An embodiment of such a system comprises a reticle-vacuum chamber that accommodates a reticle stage on which the reticle is mounted. The reticle-vacuum chamber is defined by walls and at least one bulkhead. The system also includes a wafer-vacuum chamber that accommodates a wafer stage, on which the sensitive substrate is mounted, wherein the wafer-vacuum chamber is defined by walls and at least one bulkhead. A respective instrument is mounted on the bulkhead of the reticle-vacuum chamber for measuring a characteristic of the reticle. A respective instrument is mounted on the bulkhead of the wafer-vacuum chamber for measuring a characteristic of the substrate. The system also includes a deformation-reducing device for reducing deformation of the respective bulkhead of at least one of the chambers in response to a pressure differential being established in the respective chamber relative to outside the respective chamber.  
           [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 subatmospheric pressure intermediate the subatmospheric pressure in the respective chamber and the pressure outside the respective chamber. The secondary wall desirably deforms relative to the respective bulkhead in response to a differential of pressure inside the respective secondary reduced-pressure chamber relative to pressure outside the respective secondary reduced-pressure chamber and outside the respective chamber. 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 wafer-vacuum chamber, in response to respective pressure differentials being established in the respective chambers relative to outside the respective chambers. 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 subatmospheric pressure intermediate the subatmospheric pressure in the respective chamber and 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 a differential of pressure inside the respective secondary reduced-pressure chamber relative to pressure outside the respective secondary reduced-pressure chamber and outside the respective chamber. Seal means and vacuum pumps, as summarized above, can be included.  
           [0025]    The respective instruments mounted on the bulkhead of the reticle-vacuum chamber can be, for example, a reticle autofocus system and/or a reticle alignment system. Similarly, the respective instruments mounted on the bulkhead of the wafer-vacuum chamber can be, for example, a wafer autofocus system and/or a wafer alignment system.  
           [0026]    The bulkhead of the reticle-vacuum chamber and the bulkhead of the wafer-vacuum chamber can be mounted to opposite ends of a projection-optical system extending between 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.  
           [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.  
           [0029]    Since various systems summarized above include a mechanism that controls deformation of the bulkhead occurring during evacuation of the respective chamber and/or in response to a change in atmospheric pressure, 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 chamber, such as workpiece processing, workpiece irradiation, or pattern transfer 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 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 instrument mounts attached to the bulkhead. If there is a change in the pressure differential, then the respective secondary wall is deformed rather than the bulkhead. Also, by moving the secondary wall instead of the bulkhead, any instruments mounted on the bulkhead experience correspondingly less movement in response to the change in pressure differential. The seal means established between the secondary wall and the instruments or their mountings provides 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]    Controlling deformation of the bulkhead generally results in substantially reduced tilting, misalignment, distortion, or other undesired movement of the instrument mounts or instruments themselves. For example, a “distortion” to an instrument can arise in a situation in which there is no actual tilting of the instrument but only a slight shift of the position of the instrument mounts (or instruments). If this distortion is very slight, the measurement accuracy of the instruments 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 instruments.  
           [0033]    The pressure inside any of the chambers referred to above can be regulated according to changes in the pressure external to the chambers. Thus, the positioning of the instrument mounts can be optimized by intentional control of the pressure of the respective secondary reduced-pressure chambers.  
           [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 exposure system according to the invention.  
         [0036]    [0036]FIG. 2 is a plan view of the wafer optical plate of the microlithographic exposure system of FIG. 1, showing certain components associated with the 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 pan 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 pan 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]    The invention is described below in the context of several representative embodiments 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 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 other systems employing a vacuum chamber, such as ion-beam, X-ray, or extreme ultraviolet (EUV) microlithography systems and to other systems that utilize one or more charged particle beams, EUV beams, or X-ray beams.  
         [0044]    An overview of the overall construction of an electron-beam (EB) 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 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 . 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 over which the illumination beam IB is scanned is within the optical field of the illumination-optical system.  
         [0045]    As noted above, the reticle  10  has a multiple subfields that 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.  
         [0046]    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 an image of the illuminated subfield on the appropriate location 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. Transfer normally is at demagnification, by a factor of, e.g., ¼.  
         [0047]    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 by the demagnification (reduction) 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 .  
         [0048]    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.  
         [0049]    Turning now to FIGS.  1 - 5 , a microlithography (“exposure”) system  100  according to a representative embodiment is shown, wherein the system  100  is representative of any of various systems including 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 “below” the IOS column  101 , contains the reticle stage  11 .  
         [0050]    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 exposure system or out of the reticle-vacuum chamber  103  to outside the exposure system, such movements are made by the manipulator via the reticle-loader chamber  105  though the reticle load-lock chamber  107 . Respective vacuum pumps (not shown, but well understood in the art) are connected to each of 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.  
         [0051]    A reticle interferometer (IF)  109 , shown at the left in FIG. 1, is mounted in the reticle-vacuum chamber  103 . The reticle interferometer  109  is connected to a controller (not shown). Accurate data regarding the position of the reticle stage  11  are produced by the reticle interferometer  109  and routed to the controller. The controller, 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.  
         [0052]    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). A “wafer optical plate”  132  (chamber bulkhead) is situated downstream of the reticle optical plate  131 . The POS column  111  is disposed between the optical plates  131 ,  132 , wherein each of the optical plates serves as a respective bulkhead of the respective chamber. In the depicted embodiment, each optical plate  131 ,  132  is configured 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.  
         [0053]    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  is situated laterally between the two optical plates  131 ,  132 .  
         [0054]    A wafer-vacuum chamber  113  is disposed downstream of the wafer optical plate  132 . The wafer stage  24  and related components are situated inside the wafer-vacuum chamber  113 . 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 . Respective vacuum pumps (not shown) are connected to each of the wafer-vacuum chamber  113  and the wafer load-lock chamber  117 .  
         [0055]    A wafer interferometer (IF)  119 , shown at the left in FIG. 1, is situated inside the wafer-vacuum chamber  113 . The wafer interferometer  119  is connected to the controller (not shown). Accurate data concerning the position of the wafer stage  24  are produced by the wafer interferometer  119  and routed to the controller. The controller, 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.  
         [0056]    The wafer-vacuum chamber  113  is situated on 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 .  
         [0057]    Structures associated with the wafer AF system  151 , by way of example, 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.  
         [0058]    The wafer AF system  151 , as shown in FIGS.  2 - 3 , comprises a light-transmission device  153  and a light-reception device  155 . The light-transmission device  153  and light-reception device  155  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 , 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 situated at a specified position just outside the perimeter of the POS column  111 , away from the light-transmission device  153  and light-reception device  155  of the wafer AL system  152 . 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.  
         [0059]    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.  
         [0060]    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.    
         [0061]    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 through an opening in the wafer optical plate  132  and 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 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).  
         [0062]    As shown in FIG. 5, the horizontal lens column  157  and light source  158  are attached to a stand  165 . The stand  165  is supported firmly by legs  166  mounted to the “top” surface of the wafer optical plate  132 .  
         [0063]    As shown in FIG. 2, a “pan”  170  is disposed over nearly the entire “top” surface of the wafer optical plate  132 . The pan  170  serves as a secondary wall to the wafer optical plate  132 , and defines a gap H (FIGS. 4 and 5) between the pan  170  and the wafer-optical plate  132 . A secondary reduced-pressure chamber S 1  is formed in the space between the “bottom” surface of the pan  170  and the “top” surface of the wafer optical plate  132 . The pan  170  desirably is made from a relatively low-mass metal plate, 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 .  
         [0064]    The pan  170  defines a hole  170   a  through which the vertical lens column  156  extends and 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 cover space between the hole  170   a  and the AF lens column  156   a . The mounting of the closure member  186  with 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 a small amount of 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 cover 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 .  
         [0065]    The secondary reduced-pressure chamber S 1  between the “bottom” surface of the pan  170  and the “top” surface of the wafer optical plate  132  is isolated from the atmospheric-pressure space outside the system and from the vacuum environment inside the wafer-vacuum chamber  113 . The vacuum pump  171  (FIG. 2) is connected to the secondary reduced-pressure chamber SI and operates to reduce and regulate the pressure inside the secondary reduced-pressure chamber SI. A distortion sensor (not shown) can be mounted on the inner surface of the mirror chamber  161  for measuring deformation of the mirror chamber  161  and pan  170 , allowing the pressure inside the secondary reduced-pressure chamber S I to be regulated appropriately.  
         [0066]    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 would form between the POS column  111  and the wafer optical plate  132 , both of which are made of magnetic materials.  
         [0067]    Turning now to FIG. 6(A), a wafer AF system  151  (or wafer AL system  152 ) and wafer optical plate  132  (pan  170  not shown) are depicted schematically. Atmospheric pressure is exerted on the “top” surface 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). 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) is exerted directly on the wafer optical plate  132 . The pressure differential tends to pull the wafer optical plate  132  toward the wafer-vacuum chamber  113  (downward in the figure), causing the wafer optical plate  132  to exhibit deformation as shown by the dotted line in the figure. Whenever such deformation occurs, 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.  
         [0068]    In contrast, referring now to FIG. 6(B), the secondary reduced-pressure chamber SI and the pan  170  are located on the “top” surface of the wafer optical plate  132 . Atmospheric pressure 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 SI between the pan  170  and the wafer optical plate  132 , evacuated by the vacuum pump  171  (FIG. 2) to a vacuum of approximately 10 −4  Torr, serves to isolate the “top” surface of the wafer optical plate from atmospheric pressure. Whenever the inside of the wafer-vacuum chamber  113  is at a high vacuum (e.g., 10 −6  Torr) and the secondary reduced-pressure chamber SI is at approximately 10 −4  Torr, most of the pressure differential with respect to atmospheric pressure is imparted to the pan  170 , not by the wafer optical plate  132 . The pressure differential between external atmospheric pressure and the subatmospheric pressure inside the secondary reduced-pressure chamber S 1  causes the pan  170  to deform, as indicated by the dotted line in the figure, rather than causing deformation of the wafer optical plate  132 . As a result, the pressure differential has virtually no effect on the wafer optical plate  132 , which substantially reduces any deformation of the wafer optical plate  132 . 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 slidability of the pan  170  relative to the wafer optical plate), deformation of the pan  170  has substantially no effect on the wafer AF system  151 .  
         [0069]    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 high-accuracy focusing and registration, which, in turn, allow high-accuracy lithographic exposures to be made.  
         [0070]    If any residual deformation or a change in deformation of the wafer optical plate  132  become 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 of the secondary reduced-pressure chamber S 1 , making it possible to cancel the residual or change in deformation.  
         [0071]    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.