Abstract:
Control of particle contamination on the reticle and carbon contamination of optical surfaces in photolithography systems can be achieved by the establishment of multiple pressure zones in the photolithography systems. The different zones will enclose the reticle, projection optics, wafer, and other components of system. The system includes a vacuum apparatus that includes: a housing defining a vacuum chamber; one or more metrology trays situated within the vacuum chamber each of which is supported by at least one support member, wherein the tray separates the vacuum chamber into a various compartments that are maintained at different pressures; and conductance seal devices for adjoining the perimeter of each tray to an inner surface of the housing wherein the tray is decoupled from vibrations emanating from the inner surface of the housing.

Description:
[0001] The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to licence others on reasonable terms as provided for by the terms of Contract No. DE-AC04-94AL85000 awarded by the Department of Energy. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to projection lithography employing soft x-rays and in particular to a lithographic system including a multi-chamber housing the reticle, optics, e.g., camera, and wafer zones. The zones are vibrationally isolated and maintained at different pressures with the aid of conductance limiting seals.  
         BACKGROUND OF THE INVENTION  
         [0003]    In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a “transparency” of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the “projecting” radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.  
           [0004]    Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. “Long” or “soft” x-rays (a.k.a. Extreme UV) (wavelength range of λ=100 to 200 Å (“Angstrom”)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.  
           [0005]    One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning, e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015, available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.  
           [0006]    The present state-of-the-art for Very Large Scale Integration (“VLSI”) involves chips with circuitry built to design rules of 0.25 μm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (“UV”) delineating radiation. “Deep UV” (wavelength range of λ=0.3 μm to 0.1 μm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 μm or slightly smaller.  
           [0007]    To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.  
           [0008]    Two x-ray radiation sources are under consideration. One source, a plasma x-ray source, depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (“YAG”) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 μm to 250 μm spot, thereby heating a source material to, for example, 250,000° C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Another source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays.  
           [0009]    A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object: image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term “proximity”), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength λ≦16 Å is required for 0.25 μm or smaller patterning to limit diffraction at feature edges on the mask.  
           [0010]    Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV (“EUVL”) lithography (a.k.a., soft x-ray projection lithography (“SXPL”)).  
           [0011]    A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.  
           [0012]    It is expected that effort toward adaptation of electron storage ring synchrotron sources for EUVL will continue. Economical high-throughput fabrication of 0.25 μm or smaller design-rule devices is made possible by use of synchrotron-derived x-ray delineating radiation. Large angle collection over at least 100 mrad will be important for device fabrication. Design of collection and processing optics for the condenser is complicated by the severe mismatch between the synchrotron light emission pattern and that of the ringfield scan line.  
           [0013]    Aside from the quality of the optics that are employed in EUVL systems, factors that influence the quality of the printed wafers fabricated include the ability of the systems to prevent contaminants from depositing onto the surfaces of lens and mirrors and other optical devices. A possible source of contaminants are the hydrocarbons generated by the wafer upon exposure to radiation. Reducing the amount of such deposits will enhance overall quality and performance. Another factor that will affect the quality of the printed wafer is the ability of projection photolithography systems to be vibrationally isolated.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention is based in part on the recognition that control of particle contamination on the reticle and carbon contamination of optical surfaces in photolithography systems can be achieved by the establishment of multiple pressure zones in the photolithography systems. The different zones will enclose the reticle, projection optics, wafer, and other components of system.  
           [0015]    Accordingly, in one aspect, the invention is directed to a seal assembly for connecting first and second members that are spaced apart and for providing a conductance limiting path, the seal assembly including:  
           [0016]    a support that is attached to the first member;  
           [0017]    means for adjusting the height of the support, wherein the support and the second member define an aperture; and  
           [0018]    means for sealing the aperture.  
           [0019]    In another aspect of the invention is directed to a vacuum apparatus that includes:  
           [0020]    housing defining a vacuum chamber;  
           [0021]    a tray situated within the vacuum chamber which is supported by at least one support member, wherein the tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures; and  
           [0022]    means for adjoining the perimeter of the tray to an inner surface of the housing wherein the tray is decoupled from vibrations emanating from the housing and wherein the means for adjoining the perimeter of the tray comprises a conductance limiting seal.  
           [0023]    In a further aspect, the invention is directed to a vacuum apparatus that includes:  
           [0024]    a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;  
           [0025]    a tray situated within the vacuum chamber which is supported by at least one support member, wherein the tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;  
           [0026]    means for adjoining the perimeter of the tray to an inner surface of the inner enclosure wherein the tray is decoupled from vibrations emanating from the inner enclosure; and  
           [0027]    means for releasably attaching the inner enclosure to outer inner enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.  
           [0028]    In yet another aspect, the invention is directed to a photolithography system that includes:  
           [0029]    a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;  
           [0030]    a first tray situated within the vacuum chamber which is supported by at least one first support member, wherein the first tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;  
           [0031]    a second tray that is spaced apart from the first tray and which is situated within the vacuum chamber and which is supported by at least one second support member, wherein the second tray defines a third chamber that is at a different pressure from that of the first chamber and second chamber;  
           [0032]    means for adjoining the perimeter of the first tray to a first inner surface of the inner enclosure wherein the first tray is decoupled from vibrations emanating from the inner enclosure;  
           [0033]    means for adjoining the perimeter of the second tray to a second inner surface of the inner enclosure wherein the second tray is decoupled from vibrations emanating from the inner enclosure; and  
           [0034]    means for releasably attaching the inner enclosure to the outer enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.  
           [0035]    In a yet another aspect, the invention is directed to a photolithography system that includes:  
           [0036]    a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;  
           [0037]    a first tray situated within the vacuum chamber which is supported by at least one first support member, wherein the first tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;  
           [0038]    a second tray that is spaced apart from the first tray and which is situated within the vacuum chamber and which is supported by at least one second support member, wherein the second tray defines a third chamber that is at a different pressure from that of the first chamber and second chamber;  
           [0039]    a reticle stage positioned within the first chamber that supports a reflective reticle;  
           [0040]    a wafer stage positioned within the third chamber that supports a wafer;  
           [0041]    a projection optics device positioned in the second chamber that projects extreme ultraviolet radiation toward the reflective reticle;  
           [0042]    a camera that collects extreme radiation reflected from the reflective reticle and directing the radiation toward the wafer;  
           [0043]    means for adjoining the perimeter of the first tray to a first inner surface of the inner enclosure wherein the first tray is decoupled from vibrations emanating from the inner enclosure;  
           [0044]    means for adjoining the perimeter of the second tray to a second inner surface of the inner enclosure wherein the second tray is decoupled from vibrations emanating from the inner enclosure; and  
           [0045]    means for releasably attaching the inner enclosure to the outer enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.  
           [0046]    In a preferred embodiment, the means for adjoining the perimeter of the first tray comprises a conductance limiting seal and the means for adjoining the perimeter of the second tray comprises a conductance limiting seal. In addition, the means for releasably attaching the outer enclosure comprises a conductance limiting seal. The trays typically are metrology trays supporting various instruments integral to controlling and operating the photolithography process. The metrology trays in effect function as dividing planes in vacuum chamber. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]    [0047]FIG. 1 is a schematic of the main elements of an exemplary photolithography apparatus;  
         [0048]    [0048]FIG. 2 is a schematic of a photolithography apparatus including the reticle, optics, and wafer zones;  
         [0049]    [0049]FIGS. 3A, 3B, and  3 C illustrate seal assemblies;  
         [0050]    [0050]FIG. 4 illustrates the bridge-tray seal; and  
         [0051]    [0051]FIG. 5 illustrates the chamber-bridge seal. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0052]    [0052]FIG. 1 schematically depicts an apparatus for EUV lithography that comprises a radiation source  11 , such as a synchrontron or a laser plasma source, that emits x-rays  12  into condenser  13  which in turn emits beam  14  that illuminates a portion of reticle or mask  15 . The emerging patterned beam is introduced into the imaging optics  16  which projects an image of mask  15 , shown mounted on mask stage  17 , onto wafer  18  which is mounted on stage  19 . Element  20 , an x-y scanner, scans mask  15  and wafer  18  in such direction and at such relative speed as to accommodate the desired mask-to-image reduction.  
         [0053]    As described in further detail herein, the wafer is housed in a wafer chamber that is separated from the other elements of the photolithography system located upstream as illustrated in FIG. 1. These other elements can be housed in one or more chambers which are preferably maintained in vacuum to minimize attenuation of the x-rays. EUV radiation projected from the mask and translated by the camera travels through an aperture in the wafer chamber.  
         [0054]    The EUV lithography device of the present invention is particularly suited for fabricating integrated devices that comprise at least one element having a dimension of ≦0.25 μm. The process comprises construction of a plurality of successive levels by lithographic delineation using a mask pattern that is illuminated to produce a corresponding pattern image on the device being fabricated, ultimately to result in removal of or addition of material in the pattern image regions.  
         [0055]    Typically, where lithographic delineation is by projection, the collected radiation is processed to accommodate imaging optics of a projection camera and image quality that is substantially equal in the scan and cross-scan directions, and smoothly varying as the space between adjacent lines varies. In a preferred embodiment, projection comprises ringfield scanning comprising illumination of a straight or arcuate region of a projection mask. In another preferred embodiment, projection comprises reduction ringfield scanning in which an imaged arcuate region on the image plane is of reduced size relative to that of the subject arcuate region so that the imaged pattern is reduced in size relative to the mask region.  
         [0056]    The individual elements that form the EUV lithography device as shown in FIG. 1 can comprise conventional optical devices, e.g., condensers, cameras, and lens, for projection EUV lithography. Preferably the EUVL device employs a condenser that collects soft x-rays for illuminating a ringfield camera. A particularly preferred EUVL device that employs a condenser having a diffraction grating on the surface of a mirror upstream from the reflective mask that enhances critical dimension control is described in Sweatt et al., U.S. patent application Ser. No. 09/130,224 entitled “Diffractive Element in Extreme-UV Lithography Condenser” filed on Aug. 6, 1998, which is incorporated by reference. The condenser illustrated therein has the ability to separate the light from a line or quasi point source at the entrance pupil into several separated lines or transform point foci that are still superimposed on each other at the ringfield radius, thus maximizing the collection efficiency of the condenser and smoothing out any inhomogeneities in the source optics.  
         [0057]    [0057]FIG. 2 depicts a cross-sectional view of a photolithography vacuum apparatus  30  that preferably has a cylindrical cross section. The apparatus includes a housing having an outer-enclosure  32  and an inner support structure or bridge  34  that is isolated from environmental noise and vibrations. The outer enclosure  32  and bridge define gap  41  there-in-between and they both rest on base or pedestal  54 .  
         [0058]    The photolithography vacuum apparatus  30  preferably has three zones or regions into which the various EUV lithography components are positioned. The zones are created by reticle metrology tray  40  and wafer metrology tray  60 . Positioned near the center of the vacuum apparatus  30  is platform  36  which comprises a disk-shaped member having an inner aperture. Platform  36  is supported by a plurality of vibration isolators  66  and  68  which serve to dampen vibrations that may come from the surrounding environment. A suitable vibration isolator is commercially available from Integrated Dynamics Engineering, Inc. In a preferred embodiment, three vibration isolators positioned 120 degrees apart are employed. The vibration isolators in turn are supported by isolator support frame  38  which comprises a hollow cylinder that rests on base  54 . The outer perimeter of platform  36  and the inner surface of bridge  34  are separated by an open gap for vibration isolation.  
         [0059]    Projecting from the upper surface of platform  36  is upper truss  52  which comprises a hollow cone-shaped cylinder. The lower end of upper truss  52  is attached to the upper surface of platform  36  via ring  48 . Upper truss  52  supports reticle metrology tray  40  which has a circular perimeter that generally matches the contour of the inner surface of bridge  34 . Specifically, the upper end of upper truss  52  is attached to the lower surface of reticle metrology tray  40  via ring  46 . A seal assembly  75 , which is described in herein, provides a conductance, i.e., gas, limiting seal between the outer perimeter of reticle metrology tray  40  and bridge  34 . Similarly, a seal assembly  78  is positioned between outer enclosure  32  and bridge  34 . In this fashion, reticle metrology tray  40  separates reticle zone  31  from optics zone  33 . It should be noted that the optics zone encompasses the interior region between the reticle metrology tray  40  and wafer metrology tray  60 .  
         [0060]    In a similar manner, wafer metrology tray  60 , which is positioned at a lower part of the interior region defined by isolator support frame  38 , separates wafer zone  35  from optics zone  33 . The wafer metrology tray  60  has a circular perimeter which generally matches the contour of the inner surface of isolator support frame  38 . The wafer metrology tray is supported by lower truss  72  which comprises a hollow cone-shape cylinder. The upper end of the lower truss  72  is attached to the lower surface of platform  36  via ring  70  and the lower end of the lower truss  72  is attached to the upper surface of wafer metrology tray  60  via ring  71 . A seal assembly  77  at the interface of the perimeter of the optics metrology tray  60  and isolator support frame  38  provides a conductance, i.e., gas, limiting seal, as further described herein.  
         [0061]    The reticle zone  31  encases reticle stage  80  is connected to bridge  34  and has reflective reticle or mask  81  attached thereto. The optics zone  33  encases projection optics devices that are collectively situated in projection optics box  84 . In a preferred embodiment, EUV radiation is collected from a source (not shown) and a plurality of beams from the source are transformed into a plurality of substantially parallel arc-shaped light beams which are directed through opening  88  to processing and imaging device  84 A which transmit a plurality of arc-shaped light beams  90  to reflective mask  81 . The reflected light  92  is then translated as beam  94  by camera  84 B to wafer  83  that is positioned on wafer stage  82  within wafer zone  35 .  
         [0062]    To accommodate the transmission of the EUV radiation, reticle metrology tray  40  has aperture  42  through which the light beams  90  and  92  enter and exit. Alternatively, instead of having a single aperture, the reticle metrology tray can comprise two smaller apertures, one for each beam. The size of the aperture(s) should be kept to a minimal since the reticle zone and optics zone are preferably maintained at different pressure levels. Wafer metrology tray  60  includes a window  62  that is made of material having a high EUV radiation transmission factor such as beryllium. As is apparent, wafer metrology tray  60  also serves a barrier that prevents contaminants in the wafer zone from entering the optics zone. Contaminants of primary concern are gaseous hydrocarbons that formed when a wafer typically having a layer of photoresist thereon is exposed to EUV radiation.  
         [0063]    The projection optics box  84  has a circular outer surface which is attached to the surface of the aperture of platform  36 . In a preferred embodiment, the outer enclosure  32 , bridge  34 , and isolator support frame  38  form three concentric hollow cylinders at the lower portion of vacuum apparatus  30 . As further described herein, conductance limiting seal assembly  73  is situated between the space defined by bridge  34  and outer enclosure  32  and conductance limiting seal assembly  74  is situated between the space defined by bridge  34  and isolator support frame  38 .  
         [0064]    It should be noted that the upper truss  52 , lower truss  72 , projection optics box  84 , and isolator support frame  38  are typically not solid structures. Specifically, in order to minimize the weight of the photolithography vacuum apparatus these structures have perforations that permit lateral gas movement. One feature of this is that the pressure within optic zone  33  will be substantially uniform throughout.  
         [0065]    To maintain the proper vacuum level in each zone, two or more pumps are employed. Reticle zone  31 , optics zone  33 , and wafer zone  35  are connected to pump  97  through port  97 A, pump  98  through port  98 A, and pump  99  through port  99 A, respectively. In a preferred embodiment, no pump is directly connected to reticle zone  31 , rather one or more pumps connected to optics zone  33  is sufficient to maintain the vacuum levels in both zones. The reticle zone is maintained at a vacuum pressure of typically of less than about 100 mTorr and preferably at about 30 mTorr. The optics zone is maintained at a vacuum pressure of less than about 5 mTorr. The wafer zone is maintained at a vacuum pressure of less than about 500 mTorr and preferably at about 200 mTorr. The atmosphere in the zones comprise any suitable inert gas such as, for example, argon and helium.  
         [0066]    As illustrated in FIG. 2, the seal assemblies in the photolithography vacuum chamber help maintain the pressure differentials in the three zones. FIG. 3A shows the general features of a seal assembly that connects an outer circular vertical wall  150  to an inner cantilever structure  160 . As is apparent, the vertical wall and cantilever structure represent any adjacent pair of structures where a conductance limiting seal is desired. As shown, the vertical wall includes a cantilever portion  152  that defines a hole through which adjustable screw  170  is inserted. The adjustable screw passes through a flanged seal ring  154  before engaging threaded hole  158  in flanged cylindrical ring  156 . A sliding vertical seal  166  covers the aperture  182  that is between the lower end of flanged seal ring  154  and flanged cylindrical ring  156 . The vertical seal is held in place with band  176 .  
         [0067]    The upper portion of flanged cylindrical ring  156  is separated from the adjacent inner cantilever structure  160  by aperture or gap  180 . Horizontal seal  164  is attached to the upper portions of the flanged cylindrical ring  156  with screw  174 . As is apparent, the vertical and horizontal seals prevent gases from flowing between the upper region  184  and lower region  186 .  
         [0068]    Typically in constructing the seal assemblies for the photolithography apparatus of FIG. 2, the cantilever structure  160  (e.g., reticle or wafer tray) is positioned in place first and is held in place by appropriate supports means as shown in FIG. 2. Thereafter, the various elements of the seal assembly are lowered into position adjacent the cantilever  160 . The flanged cylindrical ring  156  is raised or lowered by moving the head  172  of adjustable screw  170  until the upper surface of the flanged cylindrical ring is substantially flushed with the upper surface of the cantilever structure  160 .  
         [0069]    [0069]FIG. 3B illustrates another embodiment of a seal assembly that connects an outer circular vertical wall  150  to an inner cantilever structure  160 . The components of this seal assembly are essentially the same as those of FIG. 3A but this embodiment does not employ the horizontal seal  164 , rather, a sheet of polymer material  300  is suspended over aperture  180 . Any suitable conductance limiting polymer such as KAYTON or TEFLON (tetrafluoroethylene fluorocarbons) can be employed. The sheet is held in place on one side by outer ring  310  which is secured to the upper surface of flanged cylindrical ring  156  by screw  350 . The other side of the sheet is held in place with an inner clamp ring which includes upper ring  320  and lower ring  330 . An edge of the sheet is secured between the upper and lower rings which are connected by screw  340 . Preferably, sheet  300  is suspended into aperture  180  to a distance ( 1 ) of about 1 inch. Typically, aperture  180  has a gap distance (d) of about 0.75 inch. In this fashion, sheet  300 , which is preferably about 0.001 inch thick or less, affords sufficient flexibility for vibration isolation. Preferably, the lower ring  330  is not secured to cantilever structure  160 . In this manner, when flanged cylindrical ring  156  is raised, attached sheet  300  and the inner clamp ring are also removed.  
         [0070]    [0070]FIG. 3C illustrates a further embodiment of a seal assembly which is particularly suited for being employed as seal assembly  75  or  78  between the reticle zone  31  and the optics zone  33  (FIG. 2). The components of this embodiment are essentially the same as those of FIGS. 3A but it also does not employ the horizontal seal  164 , rather plate  360 , which preferably is made of metal (e.g., stainless steel), is attached to the upper surface of flanged cylindrical ring  156  so as to substantially cover aperture  180 . Preferably, plate  360  is secured by screw  370 . The plate does not touch cantilever structure  160 , rather a small gap  380  of about 0.008 inch separates the surfaces of the two structures. As is apparent, a small amount of gas will permeate through the gap. It is estimated that the the pressure differential between the reticle zone and the optics zone will cause gas to flow through gap  380  at a rate of about 2 liters per second. However, this amount is negligible in comparison to the estimated 200 l/s that flows through aperture  42  of reticle metrology tray  40 . Since plate  360  is not in contact with cantilever structure  160 , vibrational isolation is achieved.  
         [0071]    As is apparent, the seal assemblies as shown in FIGS. 3A, 3B, and  3 C each defines an aperture and means for sealing the aperture. It is evident, that the term “sealing” includes essentially completely covering the aperture as in the first two embodiments and partially covering the aperture as in the third embodiment. In any case, the seal assemblies provide vibration isolation.  
         [0072]    Referring to FIG. 4, the seal assembly  75  includes an adjustment screw  110  that passes through a hole  114  in a cantilever portion  134  of the bridge  34  (FIG. 2) and engages a threaded hole  116  in flanged cylindrical ring  106 . The threaded hole  116  extends parallel to the axial centerline of the flanged cylindrical ring  106 , and turning the adjustment screw  110  raises or lowers the flanged cylindrical ring  106 . The adjustment means may alternatively comprise any other suitable means, including but not limited to bolts, nuts or ratches.  
         [0073]    A through hole  120 , also extends parallel to the axial centerline of the flanged cylindrical ring  106 , and permits a threaded shoulder screw  122  to pass through and threadingly engage a flanged seal ring  117 . The head of the shoulder screw  122  may abut against the bottom surface  124  of the flanged cylindrical ring  106 , thereby limiting the distance an operator may lower the flanged cylinder. A band clamp  112  secures a circumferentially fitted sleeve seal  108  around flanged cylindrical ring  106 . The band clamp  112  may alternatively secure the circumferentially fitted sleeve seal  108  around the flanged seal ring  117 .  
         [0074]    A ring seal washer  102  is attached to the flanged cylindrical ring  106  and is held in place by a retainer ring  104  with screw  105 . By lowering the flanged cylindrical ring  106 , the ring seal washer  102  contacts the top surface of the reticle metrology tray  40  or alternatively contacts a thermal shield  100  located above the reticle metrology tray. The ring seal washer  102  effectively prevents gas from moving through the gap  125  between reticle metrology tray  40  and flanged cylindrical ring  104 .  
         [0075]    The fitted sleeve seal  108  and the ring seal washer  102  are made of a durable material that is impermeable to gas and that is sufficiently flexible to absorb vibrations. Suitable materials include polymers for example. A preferred material is a polyimide that is commercially available as KAPTON. Typically, the fitted sleeve seal  108  is about 0.001 in. to 0.010 in. thick and the ring seal washer  102  is about 0.001 in. to 0.010 in. thick. In one embodiment, the fitted sleeve seal and the ring seal washer  102  both comprise 0.003 in. thick KAPTON. The ring and washer  102  prevent vibrations from transmitters from cantilever portion  134  to reticle metrology tray  40 . The fitted sleeve seal  108  allows vertical adjustment of the ring seal washer  102 . Both provide a conductance limited gas seal between the reticle zone and the optics zone.  
         [0076]    As illustrated in FIG. 2, seal assembly  75  and seal assembly  77  both are positioned between the perimeter of a metrology tray and an essentially vertical structure. For this reason, both seal assemblies can comprise essentially identical configurations. Therefore, the structure of seal assembly  75  as shown in FIG. 4 can also be employed between the wafer metrology tray  60  and isolator support frame  38 .  
         [0077]    Referring to FIG. 5, the resealable seal device  78  includes a ring seal washer  202  attached to an inverted L-shaped flanged cylindrical ring  130  and is held in place by a retainer ring  204 . The ring seal washer  202  contacts a flanged portion  234  of the bridge  34  (FIG. 2), thereby creating a seal against the bridge. An annular groove  140  in the top surface of the outer enclosure  32  may receive an O-ring or other sealing device to seal vacuum chamber sections not related to zone seals. A band clamp  212  secures a circumferentially fitted sleeve seal  208  around a lower ring  136  located below the inverted L-shaped flanged cylindrical ring  130 . The band clamp  212  may alternatively secure the circumferentially fitted sleeve seal  208  around the inverted L-shaped flanged cylindrical ring  130 .  
         [0078]    The inverted L-shaped flanged cylindrical ring  130  has a threaded axial hole  138  which contains an adjustment screw  210 , and turning the adjustment screw raises or lowers the L-shaped flanged cylinder. The retainer ring  204  may abut against the inside surface wall  132  of the outer chamber  32  or against the top surface of lower rings  136 , thereby limiting the distance an operator may raise or lower, respectively, the inverted L-shaped flanged cylindrical ring  130 .  
         [0079]    The ring seal washer  202  is made of the same material and has the same thickness as ring seal washer  102  (FIG. 4). Similarly, fitted sleeve seal  208  is made of the same material and has the same thickness as fitted sleeve seal  108  (FIG. 4).  
         [0080]    The structure of seal assembly  78  as shown in FIG. 5 can also represent the structure for seal assembly  77  between the isolator support frame  38  and bridge  34  as well as the structure of seal assembly  73  between bridge  34  and outer enclosure  32 , as shown in FIG. 2.  
         [0081]    Although only preferred embodiments of the invention are specifically disclosed and described above, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.