Abstract:
Reticles and reticle blanks are disclosed for performing charged-particle-beam (CPB) microlithography. The reticles typically include a rigid peripheral frame attached to a reticle portion. Such attachment can cause warping, and thus deformation, of the reticle portion. To reduce such warp, the reticle portion comprises an inner supporting part (surrounding a pattern-defining region) surrounded by an outer supporting part. Situated between the inner and outer supporting parts are multiple connecting structures. The connecting structures can have spring characteristics that collectively absorb warp. Alternatively, the connecting structures can include respective driving mechanisms. The driving mechanisms are especially adapted to cause, when electrically activated, local electrostatic attraction between a respective first conductive region (located on the outer supporting part) and a respective second conductive region (located on the inner supporting part). Selective energization of the connecting structures causes micro movement of inner supporting part (and thus the pattern-defining region) relative to the outer supporting part, thereby canceling reticle warp.

Description:
FIELD OF THE INVENTION 
     This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate such as a semiconductor wafer). Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography using a charged particle beam (electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to methods for making reticles as used in charged-particle-beam (CPB) microlithography, to reticles made using such methods, and to CPB microlithography methods performed using such reticles. 
     BACKGROUND OF THE INVENTION 
     In recent years, as semiconductor integrated circuits have become increasingly miniaturized, the resolution limits of optical microlithography (i.e., projection-transfer of a pattern performed using ultraviolet light as an energy beam) have become increasingly apparent. As a result, considerable development effort currently is being expended to develop microlithography methods and apparatus that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. For example, considerable effort has been directed to use of X-rays. However, a practical X-ray system has not yet been developed because of many technical problems with that technology. Another candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam. 
     A current type of electron-beam pattern-transfer system is an electron-beam system that literally “draws” a pattern on a substrate using an electron beam. In such a system, no reticle is used. Rather, the pattern is drawn line-by-line. These systems can form intricate patterns having features sized at 0.1 μm or less because, inter alia, the electron beam itself can be focused down to a spot diameter of several nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to draw the pattern satisfactorily. Also, drawing a pattern line-by-line requires large amounts of time; consequently, this technology has very little utility in the mass production of semiconductor wafers where “throughput” (number of wafers processed per unit time) is an important consideration. 
     In view of the shortcomings in electron-beam drawing systems and methods, charged-particle-beam (CPB) projection-microlithography systems have been proposed in which a reticle defining the desired pattern is irradiated with a charged particle beam. The portion of the beam passing through the irradiated region of the reticle is “reduced” (demagnified) as the image carried by the beam is projected onto a corresponding region of a wafer or other suitable substrate using a projection lens. 
     The reticle is generally of two types. One type is a scattering-membrane reticle  21  as shown in FIG.  15 ( a ), in which pattern features are defined by scattering bodies  24  formed on a membrane  22  that is relatively transmissive to the beam. A second type is a scattering-stencil reticle  31  as shown in FIG.  15 ( b ), in which pattern features are defined by beam-transmissive through-holes  34  in a particle-scattering membrane  32 . The membrane  32  normally is silicon with a thickness of approximately 2 μm. 
     Because, from a practical standpoint, an entire reticle pattern cannot be projected simultaneously onto a substrate using a charged particle beam, conventional CPB microlithography reticles are divided or segmented into multiple “subfields”  22   a ,  32   a  each defining a respective portion of the overall pattern. The subfields  22   a ,  32   a  are separated from one another on the membrane  22 ,  32  by boundary regions  25 ,  35 , in which no pattern elements are defined. In order to provide the membrane  22 ,  32  with sufficient mechanical strength and rigidity, support struts  23 ,  33  extend from the boundary regions  25 ,  35 . 
     Each subfield  22   a ,  32   a  typically measures approximately 1-mm square. The subfields  22   a ,  32   a  are arrayed in columns and rows across the reticle  21 ,  31 . For projection-exposure, the subfields  22   a ,  32   a  are illuminated in a step-wise or scanning manner by the charged particle beam (serving as an “illumination beam”). As the illumination beam passes through each subfield, the beam becomes “patterned” according to the configuration of pattern elements in the subfield. As depicted in FIG.  15 ( c ), the patterned beam propagates through a projection-optical system (not shown) to the sensitive substrate  27 . (By “sensitive” is meant that the substrate is coated on its upstream-facing surface with a material, termed a “resist,” that is imprintable with an image of the pattern as projected from the reticle.) The images of the subfields have respective locations on the substrate  27  in which the images are “stitched” together (i.e., situated contiguously) in the proper order to form the entire pattern on the substrate. 
     Conventionally, reticles of the types summarized above are manufactured using semiconductor-fabrication technology. Fabrication begins with a silicon reticle substrate (typically having a thickness of 1 mm or less). The reticle membrane, subfields, and support struts are fabricated from the reticle substrate. The reticle conventionally is attached circumferentially to a peripheral frame typically having a thickness of about 10 mm. The peripheral frame, normally also made of silicon, strengthens the reticle for routine handling and during use of the reticle in the CPB projection-microlithography apparatus. 
     A conventional scattering-stencil reticle mounted to a peripheral frame is shown in FIGS.  16 ( a )- 16 ( b ). FIG.  16 ( a ) depicts a reticle assembly  39  comprising a stencil-reticle portion  41  that includes a pattern-defining region  45  and a peripheral region  44 . The pattern-defining region  45  includes multiple subfields  42  (each with a respective membrane portion) and support struts  43 . The membrane portions have a thickness of about 2 μm and define respective portions of the reticle pattern, as described above. If the stencil-reticle portion  41  has an outer diameter of about 8 inches, then the thickness of the peripheral region  44  is about 700 μm. The edge region  46  of the stencil-reticle portion  41  is attached to a peripheral frame  40  having a thickness of about 10 mm. 
     Unfortunately, with reticles made by conventional technology, attachment of the stencil-reticle portion  41  to a peripheral frame  40  generates a stress throughout the stencil-reticle portion  41  that tends to cause warping (deformation) of the pattern-defining region  45 . The warping extends to the subfields  42  and thus to the respective pattern portions defined by the subfields  42 . This warping is especially a problem if the stencil-reticle portion  41  is attached to the peripheral frame  40  after the pattern has been formed on the pattern-defining region  45 . The warping prevents attainment of sufficiently accurate pattern transfer. 
     Hence, there is a need for a reticle (for CPB microlithography) that is attached to a peripheral frame  40  but that exhibits substantially reduced warp in the pattern-defining region  45 , compared to conventional reticles. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide reticles in which pattern warp is substantially reduced or reducible. 
     To such end and according to a first aspect of the invention, reticles are provided, for charged-particle-beam (CPB) microlithography, that comprise a reticle portion. In an embodiment, the reticle portion comprises a pattern-defining region, an inner supporting part, and an outer supporting part. The pattern-defining region comprises multiple subfields separated from one another by support struts. Each subfield defines a respective portion of a pattern defined by the reticle. The inner supporting part is attached peripherally to the pattern-defining region, and is configured to support the pattern-defining region integrally. The outer supporting part surrounds the inner supporting part and is connected to the inner supporting part by multiple connecting structures each having a spring characteristic. The outer supporting part is configured so as to support the inner supporting part and pattern-defining region in a peripheral manner. The reticle can further comprise a peripheral frame peripherally attached to the reticle portion. With such a reticle, stress triggered in the periphery of the reticle as a result, especially, of attaching a peripheral frame to the reticle is absorbed by deformation of the connecting structures rather than warping of the pattern-defining region. 
     The pattern-defining region can be configured as a stencil reticle in which pattern elements are defined as respective voids in a CPB-scattering reticle membrane. With such a reticle, the temperature of pattern-defining region does not increase excessively during use because the amount of charged-particle absorption by the pattern-defining region is relatively small, even with high illumination-beam currents. Thus, thermally induced warp is reduced. In any event, thermal warp and mechanically engendered warp are dissipated in the connecting structures. 
     Alternatively, the pattern-defining region can be configured as a scattering-membrane reticle in which pattern elements are defined as respective spaces between CPB-scattering bodies situated on a CPB-transmissive reticle membrane. Even with this type of reticle, temperature increase of the reticle during use is not excessive because the amount of absorption of charged particles by the reticle is small, even at high illumination-beam currents. In any event, thermal warp and mechanically engendered warp are dissipated in the connecting structures. 
     Each connecting structure can have an H-shaped configuration having two pairs of H-ends. In such a configuration, a first pair of H-ends is connected to the inner supporting part and a second pair of H-ends is connected to the outer supporting part. Alternatively, each connecting structure can have an X-shaped configuration having two pairs of X-ends. In this alternative configuration, a first pair of X-ends is connected to the inner supporting part and a second pair of X-ends is connected to the outer supporting part. With such structures, it is possible to define spring constants by matching the spring constant of connecting structure to a characteristic of mechanical strength (especially an elastic characteristic) of the reticle portion. 
     The reticle can comprise a number (n) of connecting structures each satisfying a relationship nK f =K s /β, wherein K s  is an in-plane elastomeric constant of the reticle portion, β is a connection-relaxation coefficient of the connecting structure, and K f  is a spring constant of the connecting structure. With such a configuration, if the number of connecting structures is excessive, then additional mechanical stress is imparted to the reticle portion, which is rendered easily warped. On the other hand, if the number of connecting structures is too low, then proper support of the reticle portion becomes too difficult to achieve. By satisfying this relationship, the reticle portion is supported adequately while inhibiting propagation of warp from the outer supporting part to the inner supporting part (and pattern-defining region). 
     According to another aspect of the invention, methods are provided for making a reticle for CPB microlithography. Inc an embodiment of such methods, a silicon-on-insulator (SOI) reticle substrate is provided. The reticle substrate comprises a base layer, a silicon oxide layer on an obverse surface of the base layer, and a silicon layer on the silicon oxide layer. An etching mask is applied to a reverse surface of the base layer. The etching mask defines respective openings at anticipated locations of reticle subfields in a patter-defining region. The etching mask also defines respective locations of an inner supporting part surrounding the pattern-defining region, an outer supporting part surrounding the inner supporting part, and multiple connecting structures connecting the inner supporting part to the outer supporting part. The base layer is etched anisotropically at openings in the etching mask. The etching is allowed to proceed depthwise through the base layer to the silicon oxide layer, so as to define the subfields, the inner supporting part, the outer supporting part, and the connecting structures. Afterward, the exposed regions of silicon oxide are removed. Desirably, each connecting structure is composed of silicon and is formed in the anisotropic etching step by selectively etching away complementary regions of the base layer by anisotropic etching. The connecting structures can be formed, in the anisotropic etching step, at the same time as supporting struts separating the subfields from each other in the pattern-defining region. By fabricating the connecting structures at the same time as the support struts, the time (and cost), required to fabricate the reticle is reduced. 
     The method summarized above can include the step of defining a chip pattern in the pattern-defining region, and/or the step of attaching a peripheral frame to the outer supporting part. 
     According to another aspect of the invention, CPB microlithography reticles are provided that are formed by any of the methods according to the invention. 
     According to another embodiment, CPB-microlithography reticles according to the invention comprise a reticle portion that comprises (1) a:pattern-defining region comprising multiple subfields separated from one another by support struts, wherein each subfield defines a respective portion of a pattern defined by the reticle; (2) an inner supporting part peripherally attached to the pattern-defining region and configured so as to integrally support the pattern-defining region; (3) an outer supporting part peripherally surrounding the inner supporting part; and (4) multiple connecting structures connecting the inner supporting part to the outer supporting part. Each connecting structure comprises a first conductive region situated on the outer supporting part and a second conductive region situated on the inner supporting part. At least the first conductive regions are selectively energizable electrically so as to cause, in a selective manner, the respective first and second conductive regions to move relative to each other, thereby displacing the pattern-defining region so as to cancel, at least partially, a warp of the patter-defining region. 
     In each connecting structure, the first and second conductive regions can exhibit an electrostatic attraction with respect to each other under appropriate conditions of electrical energization of at least the respective first conductive region. 
     The reticles can further comprise a peripheral frame peripherally attached to the outer supporting part. In such a configuration, the peripheral frame can comprise a conductive pad from which a wiring connection is made to a respective first conductive region. 
     Each of the first conductive regions can comprise a first flexible membrane member connected to the outer supporting part. Similarly, each of the second conductive regions can comprise a second flexible membrane member connected to the inner supporting part. In such a configuration, each connecting structure desirably further comprises an insulating member situated between the respective first and second flexible membrane members. 
     According to another aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system, a projection-optical system, a reticle stage, and a substrate stage. The illumination-optical system is situated and configured to irradiate a charged-particle illumination beam onto a selected region of any of the various embodiments of a reticle, according to the invention, as summarized above. The reticle stage is situated and configured to: (i) hold the reticle as the reticle is being illuminated by the illumination beam, and (ii) selectively energize the conductive regions so as to reduce reticle warp. The projection-optical system is situated and configured to receive a patterned beam, formed by passage of the illumination beam through the reticle and carrying an image of the irradiated region of the reticle, and to focus the image onto a predetermined position on a sensitive substrate. The substrate stage is situated and configured to hold the substrate as the substrate is being exposed by the patterned beam. 
     According to yet another aspect of the invention, methods are provided for microlithographically exposing a pattern onto a sensitive substrate using a charged particle beam. In an embodiment of such a method, a reticle is provided that comprises: (i) a pattern-defining region comprising multiple subfields each defining a respective portion of a pattern defined by the reticle, (ii) an inner supporting part peripherally attached to the pattern-defining region and configured so as to support the pattern-defining region integrally, (iii) an outer supporting part peripherally surrounding the inner supporting part, and (iv) multiple connecting structures connecting the inner supporting part to the outer supporting part. Each connecting structure comprises a first conductive region situated on the outer supporting part and a second conductive region situated on the inner supporting part. At least one of the conductive regions is energized electrically in a selective manner so as to cause, in a selective manner, the respective first and second conductive regions to move relative to each other, thereby displacing the pattern-defining region so as to cancel, at least:partially, a warp of the pattern-defining region. The charged particle beam is irradiated selectively onto the subfields in an ordered manner to transfer the reticle pattern to the substrate. 
     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 
     FIGS.  1 ( a )- 1 ( b ) are an obverse plan view and elevational section (along the line A—A), respectively, of a reticle according to a first representative embodiment of the invention. 
     FIGS.  2 ( a )- 2 ( j ) are elevational views of the results of certain respective steps of a method, according to the invention, for manufacturing a reticle of the first representative embodiment. 
     FIG.  3 ( a ) is a reverse plan view of a portion of the reticle of the first representative embodiment, and FIG.  3 ( b ) is a plan view of certain details of a connecting structure in the reticle of FIG.  3 ( a ). 
     FIG. 4 is a plan view of a reticle according to a second representative embodiment. 
     FIGS.  5 ( a )- 5 ( b ) are a plan view and elevational section (along the line A-A′), respectively, showing certain features of a reticle according to a third representative embodiment. 
     FIGS.  6 ( a )- 6 ( b ) depict certain details of a drivable connecting structure in a reticle as shown in FIG.  5 ( a ). FIG.  6 ( a ) includes two sections, one along the line A-A′ and the other along the line B-B′, providing further detail of the connecting structure. 
     FIG. 7 is a plan view showing the arrangement of the drivable connecting structures of the reticle according to the third representative embodiment. 
     FIGS.  8 ( a )- 8 ( j ) schematically depict respective modes of motion of the pattern-defining region of a reticle according to the third representative embodiment whenever certain indicated drivable connecting structures are actuated. 
     FIGS.  9 ( a )- 9 ( b ) depict the results of certain respective steps in the manufacture of a reticle according to the third representative embodiment. FIG.  9 ( a ) shows a plan view and elevational section along the line X-X′, and FIG.  9 ( b ) shows a plan view and elevational section along the line Y-Y′. 
     FIGS.  10 ( a )- 10 ( b ) depict the results of certain respective steps, continued from FIGS.  9 ( a )- 9 ( b ), in the manufacture of a reticle according to the third representative embodiment. FIG.  10 ( a ) shows an elevational section only, and FIG.  10 ( b ) shows both a plan view and an elevational section along the line Z-Z′. 
     FIGS.  11 ( a )- 11 ( d ) depict the results of certain steps, continued from FIGS.  10 ( a )- 10 ( b ), in the manufacture of a reticle according to the third representative embodiment. Each of FIGS.  11 ( a )- 11 ( d ) provides a respective elevational section. 
     FIG. 12 is a schematic elevational diagram of a charged-particle-beam microlithography apparatus according to a fourth representative embodiment of the invention. 
     FIG. 13 is a process flowchart for manufacturing a semiconductor device, wherein the process includes a microlithography method utilizing a reticle according to the invention. 
     FIG. 14 is a process flowchart for performing a microlithography method utilizing a reticle according to the invention. 
     FIG.  15 ( a ) is a schematic elevational view of certain aspects of a conventional scattering-membrane reticle. 
     FIG.  15 ( b ) is a schematic elevational view of certain aspects of a conventional scattering-stencil reticle. 
     FIG.  15 ( c ) is a schematic oblique view of certain aspects of conventional microlithographic transfer of a reduced image from a reticle to a substrate using a charged particle beam. 
     FIGS.  16 ( a )- 16 ( b ) depict certain aspects of a conventional scattering-stencil reticle, for charged-particle-beam microlithography, incorporating a peripheral frame. 
    
    
     DETAILED DESCRIPTION 
     The following description is directed to scattering-stencil reticles, as exemplary reticles, according to the invention, for charged-particle-beam (CPB) microlithography. It will be understood, however, that embodiments of the invention are not limited to scattering-stencil reticles. The principles described below can be applied with equal facility to other types of reticles for CPB microlithography, such as scattering-membrane reticles. 
     The invention is described below in the context of representative embodiments. However, it will be understood that the invention is not limited to those embodiments. 
     FIRST REPRESENTATIVE EMBODIMENT 
     A reticle  49  according to this embodiment is shown in FIGS.  1 ( a )- 1 ( b ), and comprises a stencil-reticle portion  51  and a peripheral frame  50 . The stencil-reticle portion  51  comprises an outer supporting part  54 , an inner supporting part  57 , a pattern-defining region  55 , and multiple connecting structures  58  for connecting together the outer supporting part  54  and the inner supporting part  57 . The pattern-defining region  55  is divided into multiple subfields  52   a  separated from one another by support struts  53 . Each subfield  52   a  includes a respective portion of the reticle membrane  52   b . The peripheral frame  50  is attached to the edge region  56  of the stencil-reticle portion  51 . 
     The reticle  49  can be fabricated using semiconductor-fabrication technology. FIGS.  2 ( a )- 2 ( j ) schematically depict the results of certain respective steps in a fabrication process for making the reticle  49 . In a first step (FIG.  2 ( a )), a silicon.-on-oxide (SOI) reticle substrate  60  is prepared. By way of example, the reticle substrate  60  has an outer diameter of 8 inches and a thickness of 725 μm. The reticle substrate  60  includes a base layer  60   c , a silicon oxide layer  60   b , and a silicon layer  60   a . The silicon layer  60   a  has a thickness of approximately 2 μm and normally comprises doped silicon. The silicon oxide layer  60   b  has a thickness of approximately 1 μm and serves as an intermediate layer. The base layer,  60   c  is made of silicon. A layer  61  of an organic resist is formed on the reverse side of the base, layer  60   c  (FIG.  2 ( b )), followed by patterning of the resist  61  (FIG.  2 ( c )). Material of the base layer  60   c  is removed selectively by dry etching, in the depthwise direction, from the regions unprotected by the resist  61  (FIG.  2 ( d )). In other words, the patterned resist  61  serves as an etching mask. As shown in FIG.  2 ( d ), depthwise etching stops automatically at the silicon oxide layer  60   b . The dry etching defines subfields  62 , support struts  65 , and the inner supporting part  63 . Next, the remaining resist  61  is removed (FIG.  2 ( e )), and the exposed regions of the silicon oxide  60   b  are removed (FIG.  2 ( f )) using hydrogen fluoride or other suitable reagent. Thus, the silicon layer  60   a  becomes a reticle membrane. Next, a layer of an organic resist  66  is coated on the upper surface of the SOI substrate  60  (specifically on the upper surface of the silicon layer  60   a , FIG.  2 ( g )), and a desired stencil pattern is imprinted in the resist  66  (FIG.  2 ( h )). Using the remaining resist  66  as an etching mask, a reticle stencil pattern  67  is formed in the silicon layer  60   a , and the remaining resist  66  is removed (FIG.  2 ( i )). Finally, a peripheral frame  68 , made of a material such as silicon, ceramic, or glass, is attached peripherally to the stencil-reticle portion (FIG.  2 ( j )), desirably using an adhesive, or by anodic welding or eutectic welding. 
     In the method of FIGS.  2 ( a )- 2 ( j ), connecting structures (see item  58  in FIG.  1 ( a )) can be formed at the same time as the support struts  65 . Alternatively, the connecting structures  58  can be formed independently of the struts  65 . Also, the connecting structures  58  can be formed so as to be surrounded by thin membrane regions as shown in FIG.  1 ( b ), or to be surrounded by through-holes (represented by regions  75   a  and  75   b  in FIG.  3 ( a )). 
     In a CPB microlithographic reticle fabricated as described above, attachment of the peripheral frame  50  (FIG.  1 ( a )) to the outer supporting part  54  can generate a warp that is transmitted to the inner supporting part  57  and the pattern-defining region  55 . To achieve a substantial reduction (e.g., ten-fold) in warp transmitted to the pattern-defining region  55  each connecting structure  58  desirably is configured to have a spring constant that is approximately one tenth the spring constant of the combined inner supporting part  57  and pattern-defining region  55 . 
     For example, consider a warp of 100 nm arising by connecting the peripheral frame  50  to the stencil-reticle portion  51 . This warp at the pattern-defining region  55  can be reduced to 10 nm by using a reticle  49  configured according to this embodiment. More specifically, the spring constant of a connecting structure  58  can be defined from the size of the pattern-defining region  55 , the number of support struts  53 , the width of each support strut  53 , and the spacing between the support struts  53 . In general, the stated 10-fold reduction in warp transmission to the pattern-defining region  55  is achieved by employing at least ten to less than  20  connecting structures  58 , each having a spring constant of about 1 N/μm between the inner supporting part  57  and the outer supporting part  54 . More accurately, if the in-plane elastic constant of the stencil-reticle portion  51  is denoted as K s , the connection-relaxation coefficient is denoted as β, and the spring constant of the connecting structure  58  is denoted as K f , then the number “n” of connecting structures  58  and their spring constants can be configured to satisfy the relation: nK f =K s /β. 
     FIGS.  3 ( a )- 3 ( b ) show a stencil-reticle portion  70  that can be produced using the method described above and shown in FIGS.  2 ( a )- 2 ( j ). The stencil-reticle portion  70  includes an outer supporting part  71 , an inner supporting part  72 , and a pattern-defining region  73  (comprising multiple subfields separated from one another by support struts). The inner supporting part  72  is connected to the outer supporting part via multiple connecting structures  74  (see detail in FIG.  3 ( b )). The pattern-defining region  73  is supported by the struts and by the inner supporting part  72 . 
     Although the connecting structures  74  of this embodiment have the simple configuration shown in FIG.  3 ( b ), the configuration of the connecting structures  74  is not so limited. In general, to facilitate adjustment of the spring constant, it is desirable that, at the location of each connecting structure, the inner supporting part  72  and the outer supporting part  71  each have two connections. With such a configuration, the connecting structure  74  has an “H” configuration (FIG.  3 ( b )). An alternative configuration providing generally the same effect is a connecting structure having an X-shaped configuration. In any event, the shape and spring constant of the connecting structure  74  can be determined by finite-element analysis using a material constant of the connecting structure  74  such as Young&#39;s modulus of elasticity. The edges of the connecting structure  74  are defined by through-holes  75   a ,  75   b  and edges of adjacent membrane structures. 
     SECOND REPRESENTATIVE EMBODIMENT 
     A reticle assembly  80  according to this embodiment is shown in FIG. 4, and is especially suitable in instances in which multiple (two in this embodiment) pattern-defining regions are provided on the same reticle substrate. More specifically, the reticle assembly  80  is a stencil reticle  81  comprising two separate pattern-defining regions  82   a ,  82   b . The pattern-defining regions are surrounded by respective inner supporting parts  83   a ,  83   b . The inner supporting parts  83   a ,  83   b  are connected to an outer supporting part  84  by multiple connecting structures  85 . Finally, the stencil reticle  81  is connected to a peripheral frame  86 . 
     THIRD REPRESENTATIVE EMBODIMENT 
     A reticle  89  according to this embodiment is shown in FIGS.  5 ( a )- 5 ( b ), and comprises a stencil-reticle portion  91  and a peripheral frame  90 . The stencil-reticle portion  91  comprises an outer supporting part  94 , an inner supporting part  97  (collectively constituting a support part  96 ), a pattern-defining region  95 , and multiple drivable (electrically actuatable) connecting structures  98 ( 1 )- 98 ( 12 ) for connecting together the outer supporting part  94  and the inner supporting part  97 . The pattern-defining region  95  is divided into multiple subfields  92   a  separated from one another by support struts  93 . Each subfield  92   a  includes a respective portion of the reticle membrane  92   b . An obverse surface of the peripheral frame  90  is attached circumferentially to the stencil-reticle portion  91 . 
     Each of the drivable connecting structures  98  is electrically actuatable. To such end, pads  99  are provided on the reverse surface of the peripheral frame, wherein wiring  100  connects each pad  99  to a respective connecting structure  90 . In general, and by way of example, the wiring  100  has a diameter of 30 μm, and each pad  99  measures 30 μm square. The wiring  100  is bonded to the pads  99  and connecting structures  98  using conventional semiconductor fabrication techniques. Further details of a drivable connecting structure  98  are shown in FIGS.  6 ( a )- 6 ( b ). Each connecting structure  98  comprises a first flexible membrane member  101  connected to the outer supporting part  94 , a second flexible membrane member  102  connected to the inner supporting part  97 , and an electrically insulating member  103  situated between the first flexible membrane member  101  and the second flexible membrane member  102 . The first and second flexible membrane members  101 ,  102  are electrically conductive and can be formed by doping impurities into intrinsic silicon. As noted above, in this embodiment, twelve (by way of example) drivable connecting structures  98 ( 1 )- 98 ( 12 ) are provided. Since each connecting structure  98  has respective first and second flexible membrane members  101 ,  102  and a respective insulating member  103 , the respective reference numbers for the first flexible membrane members are  101 ( 1 )- 101 ( 12 ), the respective reference numbers for the second flexible membrane members are  102 ( 1 )- 102 ( 12 ), and the respective reference numbers for the insulating members are  103 ( 1 )- 103 ( 12 ). 
     The first flexible membrane member  101 ( 1 ) of the first connecting structure  98 ( 1 ), connected to the outer supporting part  94 , is part of a respective conductive region  104  provided in the outer supporting part  94 . The second flexible membrane member  102 ( 1 ) of the first connecting structure  98 ( 1 ), connected to the inner supporting part  97 , is part of a conductive region  105  provided in the inner supporting part  97 . The conductive regions  104 ,  105  can be formed by doping impurities into intrinsic silicon. The conductive regions  101 ,  104  desirably are made of the same material, and the conductive regions  102 ,  105  desirably are made of the same material. 
     The arrangements of connecting structures  98 ( 1 )- 98 ( 12 ) in this embodiment, and associated conductive regions  104 ( 1 )- 104 ( 12 ), are shown in FIG.  7 . Each of the conductive regions  104 ( 1 )- 104 ( 12 ) is separate from one another. Connected to each of the conductive regions  104 ( 1 )- 104 ( 12 ) is a respective wire  100 ( 1 )- 100 ( 12 ) (not shown, but see FIGS.  6 ( a )- 6 ( b )). The wires  100 ( 1 )- 100 ( 12 ) deliver respective electrical driving signals (from a power source, not shown) to the conductive regions  104 ( a )- 104 ( 12 ). FIG. 7 also shows the conductive region  105 . The conductive region  105  can be the inner support part  97  or a portion of the inner support part  97 . 
     As shown in FIG.  6 ( b ), upon application of different respective electrical voltages to each conductive region  104 , an electrostatic-charge attraction is generated between the conductive regions  104 ,  105 , respectively. Specifically, the conductive region  105  is floated electrically, and the conductive regions  104  receive respective applied voltages. As a result, the conductive regions  104 ,  105  move toward each other (arrows  106 ), causing the flexible membrane members  101 ,  102  to flex. The attractive force is a function of the applied voltage, the area of the conductive region (relative to the opposing conductive region), and the distance between opposing conductive regions. 
     For the following discussion, the conductive regions  104 ( 1 )- 104 ( 12 ) are denoted A-L, respectively, as indicated in FIG.  7 . FIGS.  8 ( a )- 8 ( j ) depict respective modes of motion of the inner supporting part (and pattern-defining region  95 ) whenever certain respective groups of conductive regions A-L are energized (arranged as shown: in FIG.  7 ). Selective energization of the conductive regions  104 ( 1 )- 104 ( 12 ) is performed by selectively applying voltages to them. Before applying the voltages, deformation of the reticle is determined. For example, if the adjacent region of the connecting structure A is deformed (i.e., smaller than required), then voltage is applied to the conductive region  104 ( 1 ). From measurements of such deformation and calculations of the relationship, between applied voltage and deformation by attractive force, the required voltage to correct the deformation by attractive force is determined. In general, applied voltages range from 0-140 KV at an accuracy of 140 mV. For example, at 140 KV, 1 μm linear deformation or 3 mdeg rotational deformation can be achieved. 
     For example, whenever a voltage is applied to each of the conductive regions A,D,G, and J, the inner supporting part  97  (and thus the pattern-defining region  95 ) is rotated to a limited extent in a clockwise direction in the figure (FIG.  8 ( a )). Similarly, whenever a voltage is applied to each of the conductive regions C, F, I, and L, the inner supporting part  97  (and thus the pattern-defining region  95 ) is rotated to a limited extent in a counterclockwise in the figure (FIG.  8 ( b )). If the rotation is symmetrical, then no deformation of the pattern-defining region  95  occurs. However, if this rotation is not symmetrical, then some deformation of the pattern-defining region  95  can occur, which can be corrected by selective energization of other conductive regions as described below. Referring to FIG. 4, if the regions  83   a  and  83   b  are rotated in opposite directions, then electrically actuated corrective rotations as described above can be used to correct the rotations. 
     The achievable angle of rotation in each of FIGS.  8 ( a )- 8 ( b ) is approximately 3 μdeg to 3 mdeg. (The rotation of 3 mdeg is achieved at about 140 KV of applied voltage.) The degree of rotation is controllable to with an accuracy of 1 μdeg by appropriately controlling the applied voltage. 
     To continue, whenever a voltage is applied to each of the conductive regions A, B, and C, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves upward in the figure (FIG.  8 ( c )). Similarly, whenever a voltage is applied to each of the conductive regions G, H, and  1 , the inner supporting part  97  (and thus the pattern-defining region  95 ) moves downward in the figure (FIG.  8 ( d )). Similarly, whenever a voltage is applied to each of the conductive regions I, K, and L, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves to the left in the figure (FIG.  8 ( e )). Similarly, whenever a voltage is applied to each of the conductive regions D, E, and F, the inner supporting part  97  (and thus the pattern-defining region  95 ) to the right in the figure of the drawing (FIG.  8 ( f )). In each of these instances, the displacement distance of the inner supporting part  97  is 1 nm to 1 μm. The accuracy of this movement can be controlled to an accuracy of 1 nm or less. Again, by way of example, the range of applied voltage is 0-140 KV, with an accuracy of 140 mV. At 140 KV, a deformation of about 1.4 μm is obtainable. 
     To continue, whenever a voltage is applied to each of the conductive regions G, H, I, J, K, and L, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves diagonally downward to the left in the figure (FIG.  8 ( g )). Similarly, whenever a voltage is applied to each of the conductive regions D, E, F, G, H, and I, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves diagonally downward to the right in the figure (FIG.  8 ( h )). Similarly, whenever a voltage is applied to each of the conductive regions A, B, C, D, E, and F, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves diagonally upward to the right in the figure (FIG.  8 ( i )). Similarly, whenever a voltage is applied to each of the conductive regions A, B, C, J, K, and L, the inner supporting part  97  (and thus the pattern-defining region  95 ) moves diagonally upward to the left in the figure (FIG.  8 ( j )). The displacement distance in each instance of the inner supporting part  97  is 1.4 nm to 1.4 μm. The accuracy of motion can be controlled to within 1.4 nm. 
     Methods for fabricating a reticle according to this embodiment are now described with reference to FIGS.  9 ( a )- 9 ( b ),  10 ( a )- 10 ( b ), and  11 ( a )- 11 ( d ). 
     An SOI (silicon on insulator) reticle substrate  110  is prepared that comprises a silicon layer  113 , a silicon oxide layer  112 , and a base layer  111  of silicon. The SOI reticle substrate  110  is fabricated by conventional techniques as summarized above (regarding FIG.  2 ( a )). Conductive regions  114 ( 1 )- 114 ( 12 ) and  115  are then formed on (and extending into the thickness dimension of) the silicon base layer  111  (FIG.  9 ( a )). 
     The conductive regions  114 ( 1 )- 114 ( 12 ) and  115  are formed by doping impurities (e.g., P and/or B) into predetermined regions of the base layer  111  using ion injection or thermal diffusion. The predetermined regions are defined by using a suitable mask (not shown) having openings corresponding to the desired locations. 
     Next, an etching mask  116  defining a predetermined pattern is applied to the under-surface (in the figure) (FIG.  9 ( b )) using conventional techniques. Using the etching mask  116  as an etching guide, anisotropic etching is performed of the conductive regions  114 ( 1 )- 114 ( 12 ) and  115  to the silicon oxide layer  112  (FIG.  10 ( a )). After etching, the remaining mask  116  is removed. During the anisotropic etching, the base layer  111  is etched to the silicon oxide layer  112  due to substantially different etch rates of silicon versus silicon oxide. The silicon oxide  112  exposed in the trenches formed by etching is removed using hydrofluoric acid, thereby forming the outer supporting part  117 , the drivable connecting structures  118 , the inner supporting part  119 , and support struts  120  (FIG.  10 ( b )). The silicon layer  113  becomes a silicon reticle membrane  113   a  in the resulting reticle blank (FIG.  10 ( b )). 
     A layer of resist  160  is coated on the reticle membrane  113   a  (FIG.  11 ( a )). The resist is patterned microlithographically with the desired reticle pattern. The resist is cured and baked to form an etching mask  161  (FIG.  11 ( b )). The reticle blank is etched according to the etching mask  161  to produce a stencil reticle  121  (FIG.  11 ( c )). 
     The process described above is a so-called “back-etch preceding process” in which the stencil-reticle pattern is formed in the reticle membrane after completing formation of the reticle blank, an alternative process that can be used is the so-called “back-etch successive process” in which the stencil-reticle pattern is formed in the membrane before completing formation of the reticle blank. I.e., in the back-etch successive process, the silicon base layer is etched after the reticle pattern is formed on the silicon layer  113 . 
     The stencil reticle  121  of FIG.  11 ( c ) is attached to a peripheral frame  162  by eutectic or anodic welding, use of an adhesive, or use of mechanical fasteners. The peripheral frame  162  is attached to the outer supporting part  117  (FIG.  11 ( d )). The peripheral frame  162  desirably is made separately, before attachment to the outer supporting part  117 , from a unit of silicon, glass, or ceramic. The peripheral frame  162  desirably has an inside diameter that is smaller than the outside diameter of the outer supporting part  117  and larger than the inside diameter of the outer supporting part  117 , and an outside diameter that is larger than the outside diameter of the outer supporting part  117 . The peripheral frame  162  has a thickness desirably in the range of 5 to 10 mm (the thickness is a function of the radius of the peripheral frame  162 ). The profile of the inside diameter of the peripheral frame  162  is not limited to circular; for example, it alternatively can be polygonal. 
     Eutectic bonding of the peripheral frame  162  to the outer supporting part  117  is performed as follows: A gold layer (having a thickness of 200 to 500 nm) is layered in a predetermined region(s) on the peripheral frame  162  that will be bonded to the outer supporting part  117 . The gold layer can be formed by a conventional vacuum evaporation technique. Desirably, the surface of the peripheral frame  162  on which the gold layer is formed is mirror-polished (before applying the gold layer) to achieve maximal adhesion. Gold-silicon eutectic bonds are formed by heating in an electric furnace at a temperature of 400° C. for 5 hours. 
     The gold-silicon eutectic bond need not be formed entirely around the periphery of the outer supporting part  117 . Alternatively, eutectic “spot welds” can be formed, each having an area of several square millimeters. Spot welds can be formed by forming a ring-shaped gold layer, as described above, and then partially removing portions of the gold layer by etching or the like. Alternatively, the gold can be applied selectively to desired locations using a mask or the like. The number of spot welds and the area of each spot weld are determined by the warp tolerance of the reticle and the desired strength of the welds. After forming the gold spots, the peripheral frame  162  and outer supporting part  17  are brought into contact with each other and heated, such as in an electric furnace at 400° C. for 5 hours. 
     The reticle pattern can be formed in the reticle membrane after attaching the peripheral frame  162 . 
     Wire-connecting pads  163  made of an electrically conductive material (e.g., gold) can be applied to the peripheral frame  162  before or after the peripheral frame  162  is bonded to the outer supporting part  117 . The wire-connecting pads  163  are used for connecting respective wires  164  connected to respective conductive regions  114 ( 1 )- 114 ( 12 ) via respective wire-connecting pads  165 . For example, a wire  164 ( 1 ) connects the wire-connecting pad  163 ( 1 ) to a corresponding wire-connecting pad  165 ( 1 ) associated with the conductive region  114 ( 1 ), as shown in FIG.  11 ( d ). Because the conductive regions  114 ( 1 )- 114 ( 12 ) are doped silicon, wire connections as described above facilitate electrical energization of the respective conductive regions. 
     FOURTH REPRESENTATIVE EMBODIMENT 
     An electron-beam microlithography apparatus  140  according to this embodiment is shown in FIG.  12 . The apparatus  140  comprises an illumination-optical system  141  that directs an electron beam (“illumination beam”  151 ) from an electron gun (not shown) to a reticle  142   a . The apparatus also comprises a reticle stage  142  for holding the reticle as described above. Downstream of the reticle stage  142  are a projection-optical system  143  and a substrate stage  144  for holding a suitable substrate  144   a  (e.g., semiconductor wafer) for exposure with the pattern defined on the reticle  142   a . The projection-optical system  143  receives portions of the illumination beam (i.e., a “patterned beam”  152 ) passing through the illuminated region of the reticle and focuses the beam (beam  153 ) on a corresponding region of the substrate  144   a.    
     Any warp of the reticle  142   a  (i.e., warp of pattern elements defined by the reticle) can be measured at time of using the reticle in the microlithography apparatus  140 . Warp can be measured using, for example, a Nikon optical-wave-coherence-type coordinate-measuring tool. 
     After measuring reticle warp, the reticle is mounted on the reticle stage  142 . If any warp was detected, selective energization of the conductive regions  114  is made (FIGS.  8 ( a )- 8 ( j )) as required to achieve countervailing motion of the reticle, thereby canceling the warp. The necessary electrical connections to the reticle are made via connectors provided in the reticle stage. The connectors in the reticle stage are connected to a power source (FIG. 12) that is connected to a processor (e.g., the central processor of the microlithography apparatus). The processor supplies appropriate commands to the power source, based on warp-data input to the processor. The processor calculates voltages necessary to cancel the deformation of the reticle. Warp correction can be made in this manner within a tolerance of 5 nm to 20 nm. 
     After making the warp correction, the illumination beam  151  passing through the illumination-optical system  141  is directed at the reticle mounted on the reticle stage  142 . The resulting patterned beam  152  is directed to the substrate by the projection-optical system  143 . 
     The substrate can be a silicon wafer, for example, coated with a suitable resist that can be exposed in an image-forming way by the patterned beam  153 . The substrate typically is imaged multiple times with different patterns (with intervening process steps) to form a many-layered semiconductor device on the wafer. 
     FIFTH REPRESENTATIVE EMBODIMENT 
     FIG. 13 is a flowchart of an exemplary semiconductor fabrication method to which reticles according to the invention readily can be applied. The fabrication method generally comprises the main steps of wafer production (wafer preparation), reticle production (reticle preparation), wafer processing, device assembly, and inspection. Each step usually comprises several sub-steps. 
     Among the main steps, wafer processing is key to achieving the smallest feature sizes (critical dimensions) and best inter-layer registration. In the wafer-processing step, multiple circuit patterns are successively layered atop one another on the wafer, wherein the formation of each layer typically involves multiple sub-steps. Usually, many operative semiconductor devices are produced on each wafer. 
     Typical wafer-processing steps include: (1) thin-film formation involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires; (2) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (3) etching or analogous step to etch the thin film or substrate according to the resist pattern, or doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (4) resist stripping to remove the resist from the wafer; and (5) chip inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired semiconductor chips on the wafer. 
     FIG. 14 provides a flow chart of typical steps performed in microlithography, which is a principal step in wafer processing. The microlithography, step typically includes: (1) resist-coating step, wherein a suitable resist is coated on the wafer substrate (which can include a circuit element formed in a previous wafer-processing step); (2) exposure step, to expose the resist with the desired pattern; (3) resist-developing step, to develop the exposed resist; and (4) optional resist-annealing step, to enhance the durability of the resist pattern. 
     During microlithography, a charged-particle illumination beam is irradiated onto a reticle made according to the invention. The portion of the illumination beam passing through the irradiated region on the reticle (now termed the “patterned beam”) is projected on the substrate (wafer) by a projection-optical system, thereby exposing a corresponding region on the substrate. As discussed above, the reticle is divided into multiple subfields, and images of the subfields are formed on the substrate in such a way that the images are stitched together. The reticle is divided due to, inter alia, the difficulty of providing a projection-optical system having an optical field sufficiently large to expose an entire reticle pattern in one shot without excessive aberrations. Also as discussed above, the subfields on the reticle are separated from one another by support struts that add rigidity and strength to the reticle. To obtain an image of the entire pattern on the substrate, the reticle and substrate are synchronously moved relative to each other during exposure. Further details of this exposure scheme are set forth in Japanese Kôkai Patent Document No. Hei 9-283405. 
     Reticles and microlithographic methods according to the invention reduce the effects of reticle warp, thereby reducing semiconductor fabrication costs. Providing a reticle with support structures as described above can be performed at the same time as forming the support struts; hence, reticles according to the invention can be produced with no increase in reticle production time over conventional reticles. 
     In any event, reducing reticle warp also results in less pattern warp as projected onto the substrate. Certain embodiments within the scope of the invention permit reduction of reticle warp immediately before using the reticle for making a microlithographic exposure, allowing greater accuracy of pattern transfer. 
     Whereas the invention has been described in connection with multiple representative embodiments, it will be apparent that 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.