Abstract:
Apparatuses for and methods of maximizing particle protection while enabling temporary concurrent illumination of a reticle with exposure radiation through an aperture and auto focus beams or while mounting a reticle to or removing a reticle from a reticle stage are disclosed.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Embodiments disclosed herein relate to an apparatus for and method of protecting a reticle on a reticle stage in a lithography system, such as an extreme ultraviolet lithography (“EUVL”) system. 
         [0003]    2. Related Art 
         [0004]    Protection from particulate matter (i.e., dust, dirt, etc.) contaminating objects of interest is required in many fields of application, including applications in semiconductor manufacturing such as microlithography. As microprocessors become faster and more powerful, an ever increasing number of transistors are required to be positioned on a semiconductor chip. The increased transistor density necessitates closer placement of the transistors, smaller device sizes, and interconnects that take less space. To achieve such great circuit density, the exposure radiation wavelengths used in microlithography are decreasing from visible to VUV, EUV, and smaller in next generation lithography (“NGL”) tools. 
         [0005]    In a microlithography exposure process, a reticle with a desired pattern on one side is illuminated by the radiation, and the radiation transfers an image of the pattern to the substrate to create a part of the desired circuit. Conventional reticles are typically for use with longer wavelength exposure radiation. As a result, a clear faceplate, called a pellicle, can be utilized to cover and protect the pattern side of a reticle from particulate matter that would obscure the pattern. 
         [0006]    As the features grow smaller, resulting in the need for shorter wavelengths, e.g. EUV radiation, the pellicle can not be utilized as present materials absorb too much of the radiation for process efficiency and deteriorate quickly. Therefore, using other methods of protecting the pattern side of the reticle in a lithography system from contamination may be used. The structures and methods for particle protection must not interfere with the exposure of the reticle or any other required calibration procedures. 
         [0007]    Referring to wafer processing equipment,  FIG. 1  illustrates a portion of one type of lithographic exposure system  50 . It should be noted that  FIG. 1  is not to scale, nor are the components&#39; sizes necessarily proportional. The depicted system is a projection-exposure system that performs step-and-scan lithographic exposures using light. 
         [0008]    Reticle  52  can be mounted via a reticle chuck  56  on a reticle stage  58 . Reticle stage  58  can be operable to hold and position reticle  52  in at least the X- and Y-axis directions as required for proper alignment of reticle  52  relative to the substrate  54  for accurate exposure. Reticle stage  58  may also be operable to rotate reticle  52  as required about the Z-axis. Reticle stage  58  can be moveably coupled to a supporting reticle stage frame or base  60 , which can be coupled to a main supporting frame  62  of lithography system  50 . 
         [0009]    A projection-optical system  64  and substrate  54  can be disposed in the path of reflected patterned beam from reticle  52 . Projection-optical system  64  can include several optical elements (not shown). Patterned beam reflecting from reticle  52 , carrying an aerial image of the illuminated portion of reticle  52 , can be “reduced” (demagnified) by a desired factor (e.g., ¼ or some other appropriate factor) by projection-optical system  64  and focused on a surface of substrate  54 , thereby forming a latent image of the illuminated portion of the pattern on substrate  54 . The top surface of substrate  54  can be coated with a suitable resist to form the image carried by the patterned beam. Projection-optical system  64  can be coupled to a supporting projection-optical system frame  66 , which can be coupled in fixed relation via vibration isolators  69  to main supporting frame  62 . 
         [0010]    Substrate  54  may be mounted by an electrostatic or other appropriate mounting force via a substrate “chuck” (not shown but well understood in the art) to a substrate table  70  mounted to a substrate stage  72 . Substrate stage  72  can be configured to move substrate table  70  (with attached substrate) in the X-direction, Y-direction, and theta Z (rotation about the Z axis) direction relative to the projection-optical system  64 , in addition to the three vertical degrees of freedom. Desirably, substrate stage  72  can be mounted on and supported by vibration-attenuation devices  73  which are well understood in the art. Substrate stage  72  can be moveably coupled to a supporting substrate frame  61 , which can be coupled to main supporting frame  62  of lithography system  50 . The position of the substrate stage  72  is detected interferometrically, in a manner known in the art. 
         [0011]    During a lithographic exposure performed using system  50  shown in  FIG. 1 , light is directed onto a selected region of a reflective surface  74  of reticle  52 . As exposure progresses, reticle  52  and substrate  54  are scanned synchronously (by their respective stages  58 ,  72 ) relative to projection-optical system  64  at a specified velocity ratio determined by the demagnification ratio of projection-optical system  64 . Normally, because not all of the pattern defined by reticle  52  can be transferred in one “shot,” successive portions of the pattern, as defined on reticle  52 , are transferred to corresponding shot fields on substrate  54  in a step-and-scan manner. By way of example, a 25 mm×25 mm square chip can be exposed on substrate  54  with an IC pattern having a 0.07 μm line spacing at the resist on substrate  54 . 
         [0012]    When a particular reticle  52  is first mounted on a reticle stage  58  and, occasionally, at other times, its position on reticle stage  58  may need to be determined. Then the best position of substrate stage  72  relative to reticle  52  may need to be determined for the best focus and calculation of the actual magnification of the system at the best focus position of substrate stage  72 . Both the best focus and magnification measurement procedures involve exposing a portion of reticle  52  to exposure light through an aperture (not shown) as previously discussed. Other sensors (not shown) used in conjunction with the interferometers (not shown) to detect the relative positions of reticle  52  and substrate  54  during the step and scan exposure project must be calibrated (in effect “zeroed”) in the best focus position to use their information accurately. Calibration may be accomplished through the use of auto-focus (“AF”) beams (not shown) that illuminate a portion of reticle  52 . That portion is typically larger than the portion exposed during the step and scan process as previously described. 
         [0013]    In some instances, use of a reticle in an EUV lithography system may involve simultaneous exposure to the EUV beam that requires the presence of an appropriate aperture frame over the reticle and to the auto-focus beams for calibrating associated positioning sensors that require no interference from structures between the auto-focus beam source and the portion of the reticle to be illuminated. Therefore, there is a need for a particle contamination protection system that maximizes protection but still allows full functionality of the reticle, including illuminating a portion with auto-focus beams. 
       SUMMARY 
       [0014]    As embodied and broadly described herein, embodiments consistent with the invention can include an EUV lithography tool having a particle contamination reduction element, a particle contamination reduction apparatus for an object, a method of maximizing particle contamination protection with a gas flow system, a lithography method, and a method of performing auto-focus on a reticle protected by a gas flow system. 
         [0015]    An EUV lithography tool to project an image onto a substrate using EUV radiation according to some embodiments of the invention can include a reticle defining an image and a particle contamination reduction element position adjacent the reticle and configured to substantially reduce particles from contaminating the reticle. The particle contamination reduction element can include a planar shield provided a predetermined distance away from the reticle and one or more movable protrusions extending from the planar shield toward the reticle. The one or more movable protrusions form a variable-sized opening adjacent the reticle and vary the size of the opening when one or more of the movable protrusions moves. 
         [0016]    A particle contamination reduction apparatus for an object according to some embodiments of the invention can include at least one object shield having a variable-sized opening therein, two or more gas ports coupled to the at least one object shield, and an aperture frame disposed in the variable-sized opening between at least two of the two or more gas ports. 
         [0017]    The object shield can include a planar portion at a distance, d, away from the surface of an object to be protected from particle contamination, and one or more portions projecting from the planar portion toward the surface of the object to be protected from particle contamination and forming at least a part of the perimeter of a variable-sized opening in the object shield. The object shield covers the surface of the object to be protected from particle contamination except for a portion of the surface exposed by the variable-sized opening. 
         [0018]    At least two or more gas ports are positioned adjacent the variable-sized opening and positioned between the planar portion and the surface of the object to be protected so as to emit gas flow parallel to the surface of the object to be protected from particle contamination and away from the perimeter of the variable-sized opening. At least one of the two or more gas ports may move with respect to the planar portion, thereby varying the size of the variable-sized opening. 
         [0019]    A method of maximizing particle contamination protection with a gas flow system according to some embodiments consistent with the invention can include providing two or more gas ports, at least one of which is movable, wherein at least two of the two or more gas ports emit gas parallel to a face of an object to be protected from particle contamination, providing an aperture frame positioned between at least two of the two or more gas ports, positioning at least one of the at least one movable gas ports close to the aperture frame to maximize protection of the object from particle contamination and moving at least one of the gas ports of the two or more gas ports apart from the aperture frame when necessary to enlarge the space between them to permit a predetermined process to be performed on the object. 
         [0020]    A lithography method according to some embodiments consistent with the invention can include illuminating a reticle with auto focus beams to calibrate interferometer sensors and moving at least two gas ports closer together after illuminating a reticle with auto focus beams. 
         [0021]    A method of performing auto-focus on a reticle protected by a gas flow system according to some embodiments consistent with the invention can include moving gas ports apart to enlarge a space formed between them, directing auto focus beams through the space between the gas ports, wherein the auto focus beams illuminate a reticle without interference from an aperture frame disposed in the space between the gas ports. 
         [0022]    A method of mounting a reticle on or removing a reticle from a reticle stage while maintaining thermophoretic gas pressure around the reticle according to some embodiments of the invention can include horizontally moving a reticle transport device toward the reticle stage in a space between a stationary reticle shield and a plane containing a patterned surface of the reticle when mounted on the reticle stage to a first reticle transport position, horizontally moving the reticle stage to a first stage position wherein the reticle can be mounted on or released from the reticle stage, vertically moving the reticle transport device to a second reticle transport position, wherein the reticle can be mounted on or released from the reticle stage, vertically moving the reticle transport device from the second reticle transport position to the first reticle transport position, and horizontally moving the reticle transport device away from the reticle stage. 
         [0023]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments consistent with some embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings, 
           [0025]      FIG. 1  shows a side view of a lithography system with a reticle stage, metrology frame, a projection-optical system, and a substrate stage; 
           [0026]      FIG. 2  shows a side view of a “chucked” reticle and reticle shields according to some embodiments of the invention, showing an end effector with thickness “t” having space to move between the right reticle shield and the reticle; 
           [0027]      FIG. 3  shows a side view of the chucked reticle and reticle shields shown in  FIG. 2 , but the reticle chuck has translated to the right and the end effector has translated up such that it is directly below the reticle; 
           [0028]      FIG. 4  shows a side view of the chucked reticle and reticle shields shown in  FIGS. 1 and 2 , with a “skirt” according to some embodiments of the invention close to the reticle shields when the reticle chuck is translated to the right; 
           [0029]      FIG. 5  shows a cross-sectional view of a reticle with a gas flow protection system according to some embodiments of the invention in an EUV beam scanning position; 
           [0030]      FIG. 6  shows a top view of a gas flow protection system according to some embodiments of the invention and an aperture frame in an EUV beam scanning position; 
           [0031]      FIG. 7  shows a top view of the gas flow protection system shown in  FIG. 6  in an AF calibration position; 
           [0032]      FIG. 8  shows an X-Z cross-sectional view of a microlithographic system according to some embodiments of the invention; 
           [0033]      FIG. 9  shows a Y-Z cross-sectional view of the microlithographic system shown in  FIG. 8 ; 
           [0034]      FIG. 10  shows a lithography system according to some embodiments of the invention; 
           [0035]      FIG. 11  a diagram of a process of fabricating semiconductor devices; 
           [0036]      FIG. 12  is a detailed flow diagram of step  1004  of the process shown in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Reference will now be made in detail to exemplary embodiments consistent with the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0038]      FIG. 2  illustrates reticle shields according to some embodiments of the invention. A reticle  52  is illustrated mounted to reticle stage  58 . An end effector  102  having a thickness “t” is illustrated in the space between reticle shield  106  and reticle  52 . In some embodiments, reticle shield  106 , on the right, has a portion  108  projecting from the horizontal, planar portion of reticle shield  106  toward patterned side  74  of reticle  52  and forming a vertical gap between portion  108  and patterned side  74  of about 1 mm. In some embodiments, reticle shield  110 , on the left, also has a portion  108  projecting from a horizontal, planar portion of reticle shield  110  toward patterned side  74  of reticle  52  and forming a vertical gap between portion  108  and patterned side  74  of about 1 mm. The perimeter of the horizontal opening defined in part by edge portions  108  may function as an aperture for irradiating radiation. The vertical gap defined in part by edge portions  108  and reticle  52  may function as a low conductance seal for gases present near reticle  52  and reticle chuck  56 , maintaining a pressure differential and preventing any significant volumetric flow of gas and/or particles from migrating. If there is no vertical gap, portion  108  contacts patterned side  74  of reticle  52 , which may be undesirable due to possible damage to patterned side  74 . A pressure differential of 45 mtorr (50 mtorr in the space around the reticle, also referred to as the reticle stage area, and 5 mtorr in photo-optics chamber  64 ) may be maintained with vertical gaps up to about 2 mm. Other pressure differentials can be maintained depending on sizing of the vertical gap. 
         [0039]    In some embodiments, reticle shields  106  and  110  may be configured as shown in  FIG. 2 , with a horizontal, planar portion of the reticle shield a distance “d” from reticle  52 , where “d” is greater than 1 mm. In some embodiments, “d” may be from about 10 mm to about 15 mm. In some embodiments, like the one illustrated in  FIG. 2 , “d” may accommodate an end effector  102  of thickness “t”. 
         [0040]    Thermophoretic particle protection, in some embodiments, relies on the reticle chamber pressure and a temperature gradient calculated as ΔT/d, where ΔT is the difference between the average temperature of patterned surface of reticle  52  and the average temperature of reticle shields  106  and  110 . For a given temperature difference, as reticle chamber pressure decreases, “d” must increase to maintain effective thermophoretic particle protection. For a given reticle chamber pressure, if d is increased, then ΔT should be increased to maintain the same gradient. In some embodiments, where thermophoretic particle protection is intended for reticle  52 , “d” may be at least 110 mm, given a reticle chamber pressure of 50 mtorr. If different reticle chamber pressures are used, the minimum “d” may vary accordingly. 
         [0041]    The angle theta, θ, formed between the planar portion of reticle shield that is a distance “d” away from reticle  52  and the projecting edge portion  108  in  FIG. 2  is approximately 120 degrees. When edge portions  108  are intended to provide an aperture, an angle between 90 and 180 degrees may be desirable, depending on the intended angle of incidence on patterned side  74  of reticle  52 . In some embodiments, a mirror  112  for use in measuring the location of reticle stage  58  along the Y axis may be attached to reticle stage  58 . Reticle shields must not mechanically interfere with mirror  112 , and thus “d” may be sized, in those embodiments, to be greater than the distance mirror  112  extends downward from reticle stage  58 . Moreover, the angled portion of reticle shield  110 , in those embodiments, must also not mechanically interfere with mirror  112 . Other design configurations may be envisioned by one skilled in the art. 
         [0042]      FIG. 3  illustrates how end effector  102  gains access to reticle  52  in order to support it when it is released from reticle chuck  56 . A reticle transport device, such as end effector  102  moves horizontally in the space between a plane containing the bottom surface of reticle  102  and a plane containing the top surface of the planar portion of reticle shielding plate  106 . Reticle stage  58  translates, for example, via linear bearings and actuator (not shown) to the right to position reticle  52  directly in line with end effector  102 . End effector  102  moves vertically to position itself under reticle  52 . Once in the position illustrated in  FIG. 3 , reticle  52  may be released from reticle stage  58  and then be supported by end effector  102 . End effector  102  with supported reticle  52  may then lower and remove reticle  52  from the reticle stage area. A similar process, but in reverse, may be used to transport reticle  52  on an end effector  102  for mounting on reticle stage  104 . 
         [0043]    In some embodiments, like the one illustrated in  FIG. 3 , the low conductance seal formed between edge portions  108  and reticle  52  disappears once reticle  52  is translated past an edge portion  108 . Thus, in  FIG. 3 , gases containing contaminants may flow from the reticle stage area, which may be higher pressure, in some embodiments, than the surrounding environment within the microlithography system. 
         [0044]      FIG. 4  illustrates a solution according to some embodiments of the invention to preserve the low conductance seal when reticle  52  is translated past an edge portion  108 . In some embodiments, a member  114  may be disposed in any location next to reticle  52  that may be above an edge portion  108  of reticle shield  106  or  110 . When member  114  surrounds reticle  52 , it may be called a “skirt.” “Skirt” may be used herein to refer to member  114 , but does not necessarily mean that it must completely encompass the perimeter of reticle  52 . It should be noted that the space, or gap, between reticle  52  and member  114  should be small to preserve the low conductance seal. The gap in  FIG. 3  is not drawn to scale, but is enlarged to demonstrate that it may exist to improve the ease with which reticle  52  may be positioned with end effector  102  and still maintain the low conductance seal. 
         [0045]    Another aspect of the invention that may be used in combination with the aspects of the invention described in conjunction with  FIGS. 1-4 , is to provide an aperture frame separate from reticle shields and to blow gas parallel to the patterned side  74  of reticle  52  from gas ports positioned on the reticle shields. 
         [0046]      FIG. 5  illustrates a reticle  52  mounted via reticle chuck  56  (not shown) to reticle stage  58  (not shown) and surrounded by a skirt  114 . As illustrated in  FIG. 5 , in some embodiments, an aperture frame  116  may be positioned below reticle  52  for use as an aperture with EUV or other radiation. In some embodiments, aperture frame  116  is attached to supporting metrology frame  68  with a mounting bracket (not shown). As a result, in some embodiments, aperture frame  116  can be fixed relative to supporting metrology frame  68 . In some embodiments, aperture frame  116  can be fixed relative to reticle stage base  60 . In some embodiments, aperture frame  116  can be fixed relative to both metrology frame  68  and reticle stage base  60  (not shown). On the right side of aperture frame  116 , in  FIG. 5 , is a right-facing gas port  118 , which emits a flow of gas  120  to the right past part of reticle  52  and skirt  114 . In some embodiments, right-facing gas port  118  is attached to a reticle shield  122 . In some embodiments, an upper face of gas port  118 , if positioned close enough to reticle  52 , e.g., about 1 mm away, may provide at least a part of a low conductance seal as described above. 
         [0047]    On the left side of aperture frame  116 , in  FIG. 5 , is a left-facing gas port  118 , which emits a flow of gas  120  to the left past part of reticle  52  and skirt  114 . In some embodiments, an upper face of left-facing gas port  118  can be positioned close enough to reticle  52  to form a low conductance seal as described above. Left-facing gas port  118  may be mounted to a reticle shield  124 . 
         [0048]    Gas may exit left-facing and right-facing gas ports  118  through small orifices or a section of porous material  126 . In  FIG. 5 , left-facing and right-facing gas ports  118  are shown providing a gas supplying pathway or manifold connected to porous material  126 . Examples of porous materials for use in gas ports  118  include a polycarbonate membrane filter, available from Structure Probe, Inc., an electrostatically charged polypropylene fibrous filter, available from 3M, or a porous nickel metal filter, available from Mott Corporation. Any filter material will work which functions to remove harmful particles from the gas flow and prevents them from migrating to the reticle. 
         [0049]    In  FIG. 6 , a top view of system  150  (looking down from reticle  52 ), aperture frame  116  may be disposed in a variable-sized opening (“window”), the perimeter of which may be formed by left-facing and right-facing gas ports  118  and low conductance seal-providing-structures  128 . In some embodiments, aperture frame  116  can be of a constant width “w.” In some embodiments, w may be of varying dimension. In some embodiments, w can be about 2 mm. A narrower, as well as a wider, aperture frame can be used. An aperture frame of a different configuration may also be used. However, the narrower aperture frame  116  is, the better for gas flow particle contamination protection for patterned surface  74  of reticle  52 , as it will permit closer positioning of gas ports  118 , thereby increasing the amount of patterned surface  74  protected by gas flow  120  (see  FIG. 5 ) emitted from gas ports  118 . 
         [0050]    Gas ports  118 , in some embodiments, extend the entire width of reticle  52  (not shown). Positioning of gas ports  118  may be fixed, as when a gas port is attached to fixed reticle shield  122 , or moveable, as when a gas port is attached to a retractable reticle shield  124 . 
         [0051]    Structures  128  function like the projecting edge portion  108  of reticle shields  106  and  110 , as described in  FIGS. 2-4 , to form low conductance seals to prevent significant volumetric flows of gas in either direction and maintain different pressures on either side of the seal. When a gas port  118  is attached to a retractable reticle shield  124 , structure  128  may extend the expected distance of travel of retractable reticle shield, if keeping a low conductance seal around the “window” is important. In some embodiments, structures  128  may contain a gas port that emits gas to flush or purge any contaminants from outgassing parts present near reticle  52  and or reticle stage  58  from passing to the space external to the reticle stage where it could possibly enter the projection-optics chamber  64  (see  FIG. 1 ) and contaminate the optical elements within. 
         [0052]      FIG. 6  also illustrates ring seals  130  that, in some embodiments, may surround a portion of microscope  132  that may protrude through a hole  134  in fixed reticle shield  122 . Microscope  132  may be mounted on metrology frame  68 , but needs to be close to, and maintain a line of vision to, the plane containing patterned side  74  of reticle  52  for its inspection. In some embodiments, ring seals  130  may be shaped like a flat washer. Ring seal  130  may have an outer diameter sized to be larger than through-hole  134  it covers, in excess of the designed relative motion between reticle shield  122  and microscope  132 . Ring seal  130  may have an inner diameter sized just slightly larger than the diameter of microscope  132 . Thus, the gap  136  between ring seal  130  and microscope  132  may be designed to be very small and function as a low conductance seal around microscope  132 . In the same way, any gap between ring seal  130  and reticle shield  122  also may function as a low conductance seal. In some embodiments, a ring seal  130 ′ may surround an interferometer reference mirror  138  that may protrude through through-hole  140 . Like ring seal  130 , ring seal  130 ′ is dimensioned to provide a low conductance seal around interferometer reference mirror  138 , due to the small size of any gap  142 . Lastly,  FIG. 6  illustrates, by dotted line rectangles, auto focus optics  144  that may be mounted to metrology frame  68  (see  FIGS. 1-2 ) below fixed reticle shield  122 . 
         [0053]    During normal step and scan lithography process, gas ports  118  can be positioned close to aperture frame  116  to reduce the space between aperture frame  116  and gas ports  118 . Positioning gas ports  118  close to aperture frame  116 , such as in  FIG. 6 , permits maximum protection of patterned side  74  of reticle  52  from contamination by particles that may enter the flow of gas. Gas flow  120  (see  FIG. 5 ) also forms a gas purge to flush/dilute molecular contaminants that may be present in the reticle stage area. Gas flow  120  may purge the volume near low conductance seals to prevent or at least minimize migration of molecular contaminants to projection-optics chamber  64  (see  FIG. 1 ). 
         [0054]      FIG. 7 , like  FIG. 6 , illustrates a top view of system  150  except that retractable reticle shield  124  and attached left-facing gas port  118  is in its fully retracted position to enlarge the “window” formed between at least left-facing and right-facing gas ports  118 . Such an enlarged window provides room for auto-focus beams to form an array of points on reflective surface  74  of reticle  52 . The square areas  148  represent an example of areas through which the auto-focus beams may pass on their way to and from patterned side  74 . In some embodiments, the auto focus area on patterned side  74  of reticle  52  is 110 mm×20 mm. In some embodiments, the angle of incidence is approximately 5 degrees from the patterned side  74  of reticle  52 . In some embodiments, the AF beam comprises approximately 50 beam points. In some embodiments, an AF beam point is shaped like a 1.4 mm line angled at 45 degrees from the scanning axis, as illustrated by the diagonal lines in square areas  148 . The beams are typically arranged in a grid pattern with uniform spacing along the x and y axes. One possible arrangement is a 5×10 beam point layout. Other arrangements will be apparent to one skilled in the art. 
         [0055]    As described above, structure  128  extends at least as far as the expected travel of retractable reticle shield  124 . Some embodiments of a reticle particle protection system will provide improved particle contamination protection, while enabling auto focus beams to illuminate patterned surface  74  of reticle  52  between gas ports  118  and aperture frame  116 . 
         [0056]      FIG. 8  illustrates a cross-sectional view in the X-Z plane of a microlithography system according to some embodiments of the invention. In this figure, reticle  52  is mounted via reticle chuck  56  to reticle stage  58 . Reticle stage  58  may be movably coupled to main supporting frame  62  either directly or indirectly through reticle stage base  60  (not shown) as discussed in conjunction with  FIG. 1 . As illustrated in  FIG. 8 , reticle  52  may be surrounded by a skirt  114 , and an aperture frame  116  may be disposed closely below patterned side  74  of reticle  52 . For example, aperture frame  116  may be about 1 mm below patterned side  74  of reticle  52 . The closer aperture frame  116  is to patterned side  74 , the sharper, or, stated another way, the less blurred, the edges of the image reflected will be. In some embodiments, structure  128  may be positioned with an upper face close to skirt  114  to form a low conductance seal therebetween. 
         [0057]    Metrology frame  68  may contain several components used in interferometric measurements or other inspections of reticle  52 . For example, auto-focus beam emitter (light source)  148  projects beams to auto-focus optics  144 , which in turn projects beams  150  onto patterned side  74  of reticle  52  without interference from aperture frame  116 . Horizontal lines  148  are a side view of the areas through which incident beams  150  and reflected beams  152  pass. Reflected beams  152  may be collected by auto-focus optics  144  and passed to auto-focus beam receiver or detector  154 . 
         [0058]    Other components mounted on metrology frame  68  include “Z” distance interferometer  156 , a reference mirror  138 ′, and X distance interferometer  158 . In  FIG. 8 , double-headed arrows show light path between an interferometer and a respective reference mirror. In  FIG. 8 , these components protrude through through-holes in fixed reticle shield  122  and have low conductance ring seals  130  and  130 ′″ around the through-holes in reticle shield  122 . 
         [0059]    Reticle stage  58  may also have components for interferometric measurement mounted thereon. In some embodiments, such components include mirror  160  for use with Z interferometer  156  and mirror  112 ′ for use with X interferometer  158 . In some embodiments, a fiducial glass  164  with a reticle fiducial mark (R-FM) thereon may be mounted on reticle stage  58 . In some embodiments, skirt  114  and fiducial glass  164  may be one part. 
         [0060]    Reticle alignment marks are typically used as reference marks in positioning reticle  52  in the X, Y, and theta-Z degrees of freedom. Reticle alignment marks present on each reticle  52  are illuminated with visible light and then measured by alignment microscopes  132 . The measured positions of reticle alignment marks may then be used to align reticle  52  as desired with respect to photo-optics chamber  64 . 
         [0061]    With reticle  52  aligned, in some embodiments, an aerial imaging sensor (AIS)  162  disposed on substrate stage table  70  may then be used for determining a best focus position and the actual magnification of the projected pattern in that position. AIS measurement marks (not shown) on reticle  52  are used in a “best focus” measurement and a magnification measurement. A “best focus” measurement includes irradiating AIS measurement marks with exposure light that passes through aperture frame  116  and sensing the resulting projected image with AIS  162  on substrate stage table  70 . A controller (not shown) steps substrate stage table  70  up and down. At each step, the contrast in the projected image is measured and compared to the previous step&#39;s contrast(s). The step with the greatest contrast is the best focus of the projection lens (not shown) located in projection-optics chamber  64 . 
         [0062]    In some embodiments, a magnification calibration may subsequently be performed. Exposure light (EUV) irradiates AIS measurement marks with wafer table  70  in the best focus position. The coordinates of the projected images of the at least two AIS measurement marks are then measured. A controller (not shown) compares the measured distance in the X direction between the projected image of the AIS measurement marks with the designed value. Any disparity is used by the controller to calculate a magnification error from the designed magnification. 
         [0063]    After reticle  52  and substrate stage table  70  are in the relative positions that create the best focus of the projected pattern, the values of relative position sensors (interferometers) and auto-focus receiver or detector  154  are measured to create a baseline, thereby in effect zeroing the sensors. After the baseline is established, the auto focus system is then used to scan and map the topography of reticle patterned surface  74 . 
         [0064]    It may be desirable to add an additional method of particle protection to such a microlithographic system according to some embodiments of the invention, as depicted in  FIG. 8 . In such embodiments, an electro-magneto phoresis apparatus  160  may be disposed between reticle shield  122  and PO chamber  64 . In some embodiments, electro-magneto phoresis apparatus  160  does not interfere with auto-focus incident beams  150  or reflected beams  152 . Details regarding electro-magneto phoresis units and methods of particle protection may be found in U.S. Pat. Appl&#39;n Publication No. U.S. 2002/0096647 A1, which is herein explicitly incorporated by reference. 
         [0065]      FIG. 9  illustrates the embodiment of system  200  shown in  FIG. 8  in a cross section in the Y-Z plane. In  FIG. 9 , alignment microscope  132  and reference mirror  138 , as described in  FIG. 6 , are depicted, as is a ring seal  130   IV  providing a low conductance seal around microscope  132  and reference mirror  138 . Y distance interferometer  166  may also be mounted on metrology frame  68 . A double headed arrow illustrating the light path between interferometer  166  and reference mirrors  138  and  112  is shown. In some embodiments, the function of  164  and  114  may be achieved by a single part. 
         [0066]    Referring to wafer processing equipment,  FIG. 10  illustrates one example of an EUV (or soft-X-ray “SXR”) lithographic exposure system  150 . The depicted system is a projection-exposure system that performs step-and-scan lithographic exposures using light in the extreme ultraviolet (“soft X-ray”) band, typically having a wavelength in the range of λ≈11-14 nm (nominally 13 nm). Lithographic exposure involves directing an EUV illumination beam to a pattern-defining reticle  52 . The illumination beam  288  reflects from reticle  52  while acquiring an aerial image of the pattern portion defined in the illuminated portion of reticle  52 . The resulting “patterned beam” is directed to an exposure-sensitive substrate  54 , which upon exposure becomes imprinted with the pattern. 
         [0067]    The EUV beam can be produced by a laser-plasma source  252  excited by a laser  254  situated at the most upper end of the depicted system  50 . Laser  254  generates laser light at a wavelength within the range of near-infrared to visible. For example, laser  254  can be a YAG or an excimer laser, but other lasers can be used. Laser light emitted from laser  254  is condensed by a condensing optical system  256  and directed to downstream laser-plasma source  252 . 
         [0068]    A nozzle (not shown), disposed in laser-plasma light source  252 , discharges xenon gas. As the xenon gas is discharged from the nozzle in laser-plasma light source  252 , the gas is irradiated by the high-intensity laser light from the condensing optical system  256 . The resulting intense irradiation of the xenon gas causes sufficient heating of the gas to generate a plasma. Subsequent return of Xe molecules to a low-energy state results in the emission of SXR (EUV) radiation with good efficiency having a wavelength of approximately 13 nm. 
         [0069]    Since EUV light has low transmissivity in air, its propagation path preferably is enclosed in a vacuum environment produced in a vacuum chamber  258 . Also, since debris tends to be produced in the environment of the nozzle from which the xenon gas is discharged, vacuum chamber  258  desirably is separate from other chambers of system  300 . 
         [0070]    A paraboloid mirror  260 , provided with, for example, a surficial multi-layer Mo/Si coating, is disposed relative to laser-plasma source  252  so as to receive EUV light radiating from laser plasma source  252  and to reflect the EUV light in a downstream direction as a collimated beam  262 . The multi-layer film on parabolic mirror  260  is configured to have high reflectivity for EUV light of which λ=approximately 13 nm. 
         [0071]    Collimated beam  262  passes through a visible-light-blocking filter  264  situated downstream of the parabolic mirror  260 . By way of example, filter  264  can be made of beryllium (Be), with a thickness of about 0.15 nm. Of the EUV radiation  262  reflected by parabolic mirror  260 , only the desired 13 nm wavelength of radiation passes through filter  264 . Filter  264  is contained in a vacuum chamber  266  evacuated to high vacuum. 
         [0072]    An exposure chamber  267  can be situated downstream of pass filter  264 . Exposure chamber  267  contains an illumination-optical system  268  that comprises at least a condenser-type mirror and a fly-eye-type mirror (not shown, but well understood in the art). Illumination-optical system  268  also is configured to shape EUV beam  270  (propagating from filter  264 ) to have an arc-shaped transverse profile. Shaped “illumination beam”  272  is irradiated toward the left in  FIG. 10  and is received by mirror  274 . 
         [0073]    Mirror  274  has a circular, concave reflective surface  274 A, and is held in a vertical orientation (in the figure) by holding members (not shown). Mirror  274  can be formed from a substrate made, e.g., of quartz or low-thermal-expansion material such as Zerodur (Schott). Reflective surface  274 A is shaped with extremely high accuracy and coated with a Mo/Si multi-layer film that is highly reflective to EUV light. Whenever EUV light having a wavelength in the range of 10 to 15 nm is used, the multi-layer film on surface  274 A can include a material such as ruthenium (Ru) or rhodium (Rh). Other candidate materials are silicon, beryllium (Be), and carbon tetraboride (B 4 C). 
         [0074]    A bending mirror  276  may be disposed at an angle relative to mirror  274 , and is shown to the right of mirror  274  in  FIG. 10 . Reflective reticle  52 , that defines a pattern to be transferred lithographically to the substrate  54 , may be situated “above” bending mirror  276 . Note that reticle  52  may be oriented horizontally with a reflective surface directed downward to avoid deposition of any debris on the patterned surface of reticle  52 . Additional particle protection systems in accordance with the present invention may reduce the deposition of any debris on patterned surface  74  of reticle  52 . Illumination beam  272  of EUV light emitted from illumination-optical system  268  may be reflected and focused by mirror  274 , and reaches the reflective surface of reticle  52  via bending mirror  276 . 
         [0075]    As described previously, reticle  52  typically has an EUV-reflective surface configured as a multi-layer film. Pattern elements, corresponding to pattern elements to be transferred to the substrate (or “wafer”)  67 , can be defined on or in the EUV-reflective surface. Reticle  52  can be mounted via a reticle chuck  56  on a reticle stage  58  that may be operable to hold and position reticle  52  in at least the X- and Y- axis directions as required for proper alignment of reticle  52  relative to the substrate  54  for accurate exposure. Reticle stage  58  can, in some embodiments, be operable to rotate reticle  52  as required about the Z-axis. The position of reticle stage  58  may be detected interferometrically in a manner known in the art. Hence, illumination beam  272  reflected by bending mirror  276  may be incident at a desired location on the reflective surface of reticle  52 . 
         [0076]    Again, as previously described, a projection-optical system  64  and substrate  54  can be disposed downstream of reticle  52 . Projection-optical system  64  can include several EUV-reflective mirrors and apertures. Patterned beam  288  from reticle  52 , carrying an aerial image of the illuminated portion of reticle  52 , can be “reduced” (demagnified) by a desired factor (e.g., ¼) by projection-optical system  64  and may be focused on the surface of substrate  54 , thereby forming an image of the illuminated portion of the pattern on substrate  54 . So as to be imprintable with the image carried by patterned beam  288 , the upstream-facing surface of the substrate  54  can be coated with a suitable resist. 
         [0077]    Reticle  52  as mounted on reticle stage  58  may be separated by the various structures and gas flow as described with respect to  FIGS. 2-8  from projection-optical system  64 . 
         [0078]    As previously described, substrate  54  may be mounted by an electrostatic or other appropriate mounting force via a substrate “chuck” (not shown but well understood in the art) to a substrate table  70  mounted to a substrate stage  72 . Substrate stage  72  may be configured to move substrate table  70  (with attached substrate) in the X-direction, Y-direction, and theta Z (rotation about the Z axis) direction relative to projection-optical system  64 , in addition to the three vertical degrees of freedom. Desirably, substrate stage  72  may be mounted on and supported by vibration-attenuation devices. The position of substrate stage  72  may be detected interferometrically, in a manner known in the art. 
         [0079]    A pre-exhaust chamber  292  (load-lock chamber) may be connected to exposure chamber  267  by a gate valve  294 . A vacuum pump  296  may be connected to pre-exhaust chamber  292  and serves to form a vacuum environment inside pre-exhaust chamber  92 . 
         [0080]    During a lithographic exposure performed using the system shown in  FIG. 10 , EUV light  272  may be directed by illumination-optical system  268  onto a selected region of the reflective surface of reticle  52 . As exposure progresses, reticle  52  and substrate  54  are scanned synchronously (by their respective stages  58 ,  72 ) relative to projection-optical system  64  at a specified velocity ratio determined by the demagnification ratio of projection-optical system  64 . Normally, because not all of the pattern defined by reticle  52  can be transferred in one “shot,” successive portions of the pattern, as defined on reticle  52 , are transferred to corresponding shot fields on substrate  54  in a step-and-scan manner. By way of example, a 25 mm×25 mm square chip can be exposed on substrate  54  with an IC pattern having a 0.07 μm line spacing at the resist on substrate  54 . 
         [0081]    Coordinated and controlled operation of system  50  may be achieved using a controller (not shown) coupled to various components of system  50  such as illumination-optical system  268 , reticle stage  58 , projection-optical system  64 , and substrate stage  72 . For example, the controller operates to optimize the exposure dose on substrate  54  based on control data produced and routed to the controller from the various components to which the controller may be connected, including various sensors and detectors (not shown). 
         [0082]    Many of the components and their interrelationships in this system are known in the art, and hence are not described in detail herein. 
         [0083]    As described above, a photolithography system according to the above described embodiments can be built by assembling various subsystems, including each element listed in the appended claims, in such a manner that prescribed mechanical accuracy, electrical accuracy and optical accuracy are maintained. In order to maintain the various accuracies, prior to and following assembly, every optical system may be adjusted to achieve its optical accuracy. Similarly, every mechanical system and every electrical system are adjusted to achieve their respective mechanical and electrical accuracies. The process of assembling each subsystem into a photolithography system includes mechanical interfaces, electrical circuit wiring connections and air pressure plumbing connections between each subsystem. Needless to say, there is also a process where each subsystem is assembled prior to assembling a photolithography system from the various subsystems. Once a photolithography system is assembled using the various subsystems, total adjustment may be performed to make sure that every accuracy is maintained in the complete photolithography system. Additionally, it may be desirable to manufacture an exposure system in a clean room where the temperature and humidity are controlled. 
         [0084]    Further, semiconductor devices can be fabricated using the above described systems, by process  1000  shown generally in  FIG. 11 . In step  1001 , the device&#39;s function and performance characteristics are designed. Next, in step  1002 , a mask (reticle) having a pattern designed according to the previous designing step is made. In a parallel step  1003 , a wafer is made from a silicon material. The mask pattern designed in step  1002  may be exposed onto the wafer from step  1003  in step  1004  by a photolithography system described hereinabove according to the principles of the present invention. In step  1005  the semiconductor device may be assembled (including the dicing process, bonding process and packaging process), then finally the device may be inspected in step  1006 . 
         [0085]      FIG. 12  illustrates a detailed flowchart example of the above-mentioned step  1004  in the case of fabricating semiconductor devices. In step  1011  (oxidation step), the wafer surface may be oxidized. In step  1012  (CVD step), an insulation film may be formed on the wafer surface. In step  1013  (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step  1014  (ion implantation step), ions are implanted in the wafer. The above mentioned steps  1011 - 1014  form the preprocessing steps for wafers during wafer processing, and selection of specific steps and sequence of steps is done according to processing requirements. 
         [0086]    At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, initially, in step  1015  (photoresist formation step), photoresist is applied to a wafer. Next, in step  1016 , (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) to a wafer. Then, in step  1017  (developing step), the exposed wafer is developed, and in step  1018  (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step  1019  (photoresist removal step), unnecessary photoresist remaining after etching is removed. 
         [0087]    Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps. 
         [0088]    Other embodiments consistent with some embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.