Abstract:
In the context of charged-particle-beam microlithography as used to transfer a pattern, defined by a segmented reticle, to a sensitive substrate, methods are disclosed for reducing the occurrence of defects caused by subfield-stitching errors and/or overlayer errors. The methods are especially useful in semiconductor-device fabrication situations in which multiple pattern layers are projection-transferred to a wafer. In defining the segmented reticle for a pattern layer, subfield boundaries are established so as to cross pattern elements at locations other than where stitching or overlayer accuracy is critical. E.g., subfield boundaries are not passed through gate, source, or drain regions of transistors.

Description:
FIELD OF THE INVENTION 
     This invention pertains to microlithography (projection-transfer) of a pattern, defined by a reticle, to a sensitive substrate using a charged particle beam as an energy beam. Microlithography is used generally in the fabrication of semiconductor integrated circuits and displays. More specifically, the invention pertains to microlithography methods in which adverse effects of errors of pattern overlayer and subfield stitching are reduced. 
     BACKGROUND OF THE INVENTION 
     Charged-particle-beam (CPB) microlithography is a promising method for used in fabrication of semiconductor integrated circuits, displays, and other devices demanding the accurate transfer of extremely fine patterns (having linewidths of 0.1 μm or less). 
     Much current development effort is aimed at producing a practical CPB microlithography apparatus that can achieve desired transfer accuracy at an acceptable throughput. One current impediment to achieving acceptable throughput is that the pattern for an entire semiconductor device (“chip”) cannot be projection-transferred at one time using CPB microlithography. This is because, inter alia, producing a CPB optical system having an optical field sufficiently large to image an entire chip-layer pattern has not been accomplished. Hence, the pattern, as defined on a reticle, is divided or segmented into pattern portions usually termed “subfields” that are individually exposed according to a pre-established order. 
     The projected images of the subfields on the wafer (or other suitable sensitive substrate) desirably are situated contiguously with respect to each other so that the images are “stitched” together in a manner that forms the entire pattern on the wafer. A reticle for this type of pattern transfer is termed a “divided” or “segmented” reticle, and microlithography performed using such a reticle is termed “divided-reticle” microlithography. 
     In a segmented reticle, each subfield is dimensioned to fit within the optical field of the CPB optical system. Progression of exposure from one subfield to the next can occur either by scanning the charged particle beam in a continuous manner or by stepwise motion of the respective stages on which the reticle and wafer are mounted. 
     According to a conventional approach, the reticle is divided into subfields all identically sized and each having a square or rectangular profile. When dividing a pattern in this manner, it is inevitable that subfield boundaries will extend across conductor lines and other pattern features. When projected onto the wafer, such conductor lines and the like must be imaged such that intact connections of conductors are established properly between adjacent subfields (i.e., the subfields are properly “stitched” together on the wafer). As a first example, if the projected images of adjacent subfields containing an interconnecting conductor are not located properly on the wafer, then a break or short can be formed in the conductor as imaged on the wafer. As a second example, if a pattern element extending between a source and a gate of a transistor or between a gate and a drain of a transistor of a semiconductor device crosses a subfield boundary, and if a subfield-stitching defect occurs at that intersection, then the yield of fully functional semiconductor devices is compromised. The incidence frequency of “subfield-stitching” defects (i.e., defects in the manner in which pattern elements extending across subfield boundaries are connected together between adjacent subfields as projected) increases with an increase in the number of locations on the wafer where subfield boundaries and pattern features intersect. As semiconductor devices (e.g., microprocessors and memories) become increasingly complex and miniaturized, subfield-stitching and layer-registration defects experienced during fabrication steps involving CPB microlithography tend to increase. This results in substantial loss of production efficiency and loss of salable product through rejects. 
     Interconnection defects as summarized above can occur in a single layer of a semiconductor device; single-layer errors generally are termed “subfield-stitching” errors as noted above. Similar errors can also occur between layers as formed on the wafer, and multi-layer errors generally are termed “overlayer” errors or “pattern-registration” errors. 
     Therefore, there is a need for CPB microlithography methods exhibiting a significantly reduced incidence of subfield-stitching and overlayer defects in the patterns as projected onto a wafer. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide CPB microlithography methods in which defects arising from subfield-stitching errors and/or overlayer errors are reduced. 
     According to a first aspect of the invention, methods are provided in which a pattern, to be transferred to a sensitive substrate (“wafer”) by CPB microlithography, is divided on the reticle into multiple subfields. As the subfields are projection-transferred onto the wafer, the projected subfields are positioned so as to stitch together the subfield images in a way that forms the entire pattern. On the reticle, subfield boundaries are established apart from regions and areas of the pattern where stitching accuracy of the projected pattern must be relatively high compared to other regions and areas of the pattern. For example, a source or drain of a transistor is normally defined in a diffusion layer. A gate electrode, defined in a subsequently applied layer, must be applied with very high positional accuracy to the diffusion layer. A subfield boundary is not provided in such a region. 
     According to another embodiment of methods according to the invention, if a subfield boundary must cross a pattern element, such crossing of the element by the boundary is made where the element has an internal angle of 225 degrees or greater. By making the intersection in such a region, the probability of a break occurring in the pattern element (as projected onto the wafer) is low in the event of a subfield-stitching error or dislocation in the respective subfields as projected. I.e., at a region of a pattern element characterized by such a large internal angle, proximity effects tend to compensate for situations that otherwise would have a higher probability of causing a break in the element crossing the boundary. The internal angle desirably is at least 225 degrees so as to adequately distinguish from the very common internal angle of 180 degrees. 
     According to yet another embodiment, the subfield boundaries of each layer are at different relative locations, established independently in each layer. Hence, it is possible to make the subfield (and pattern feature) connections in each pattern layer inconspicuous. 
    
    
     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 
     According to another embodiment of methods according to the invention, if a subfield boundary must cross a pattern element, such crossing of the element by the boundary is made where the element has an internal angle of 225 degrees or greater. By making the intersection in such a region, the probability of a break occurring in the pattern element (as projected onto the wafer) is low in the event of a subfield-stitching error or dislocation in the respective subfields as projected. i.e., at a region of a pattern element characterized by such a large internal angle, proximity effects tend to compensate for situations that otherwise would have a higher probability of causing a break in the element crossing the boundary. The internal angle desirably is at least 225 degrees so as to adequately distinguish from the very common internal angle of 180 degrees. 
     FIG. 1 is a plan view of exemplary pattern elements of an SRAM cell, including a subfield having boundaries established (relative to pattern features) according to a representative embodiment of the invention. 
     FIG. 2 is a plan view of a first (relative to the FIG. 1 layer) polycrystalline Si layer of the FIG. 1 device. 
     FIG. 3 is a plan view of a second (relative to the FIG. 1 layer) polycrystalline Si layer of the FIG. 1 device pattern. 
     FIG. 4 is a schematic elevational depiction of principal components of the CPB-optical system and associated control systems of a divided-reticle CPB microlithography apparatus. 
     FIGS.  5 (A)- 5 (C) schematically depict structural aspects of a representative segmented reticle as used in an electron-beam microlithography apparatus as shown in FIG.  4 . FIG.  5 (A) is a plan view of the reticle, FIG.  5 (B) is an oblique view of a portion of the reticle, and FIG.  5 (C) is a plan view of a subfield of the reticle. 
     FIG. 6 is a flow chart of steps in a process for manufacturing a semiconductor device such as a semiconductor chip. 
    
    
     DETAILED DESCRIPTION 
     The following description is given in the context of electron-beam microlithography. However, it will be understood that the general principles described herein can be applied with ready facility to use of an alternative charged particle beam such as an ion beam. 
     Certain aspects of a divided-reticle electron-beam microlithography system, to which methods according to the invention are applied, are depicted in FIG. 4, in which the most upstream component shown is an electron gun  51 . The electron gun  51  emits an electron beam EB that propagates in a downstream direction along an optical axis AX. Downstream of the electron gun  51  are first and second condenser lenses  52 ,  53 , respectively. The electron beam EB passes through the condenser lenses  52 ,  53  to form a first crossover C.O. 1 . The first crossover C.O. 1  is located on the optical axis AX at a blanking aperture  57 . 
     A beam-shaping aperture  54  is situated between the second condenser lens  53  and the blanking aperture  57 . The beam-shaping aperture  54  defines an axial opening that is sized and shaped to pass therethrough only a portion of the electron beam EB sufficient to illuminate a single exposure unit (or “subfield”) of a downstream reticle  60 . For example, if the subfields on the reticle  60  are rectangular in shape (each subfield on the reticle usually is sized and shaped identically), then the beam-shaping aperture  54  defines a corresponding rectangular axial opening. By way of another example, if the subfields on the reticle  60  are square in shape and have an area of (1 mm) 2 , then the beam-shaping aperture  54  defines an axial opening sufficient to provide the electron beam, as incident on the reticle, with a square transverse profile with each side of the square being slightly greater than 1 mm. An image of the axial opening defined by the beam-shaping aperture is formed on the reticle  60  by a collimating lens  59  situated between the blanking aperture  57  and the reticle  60 . 
     The electron beam EB propagating between the electron gun  51  and the reticle  60  is termed herein the “illumination beam” IB. The corresponding portion of the electron-optical system (including the lenses  52 ,  53 ,  59  and the apertures  54 ,  57 ) is termed herein the “illumination-optical system.” 
     The illumination-optical system also includes a blanking deflector  55  disposed downstream of the beam-shaping aperture  54 . The blanking deflector  55 , when energized, deflects the illumination beam IB laterally so as to cause the entire illumination beam IB to be blocked as required by the blanking aperture  57  during moments when no exposure is occurring or desired. Thus, the illumination beam IB can be prevented from impinging on the reticle  60 . 
     The illumination-optical system also includes a selection deflector  58  situated downstream of the blanking aperture  57 . The selection deflector  58  is operable to deflect the illumination beam IB in the X-, or left-right, direction (note axes shown in the figure) in a scanning manner. By scanning the illumination beam IB in this manner, successive subfields on the reticle  60  located within the field of the illumination-optical system can be illuminated. Thus, the embodiment shown in FIG. 4 can expose the subfields of the reticle  60  in a scanning manner. The collimating lens  59 , situated downstream of the selection deflector  58 , collimates the illumination beam IB for impingement on a desired subfield of the reticle  60 . Thus, an image of the axial opening defined by the beam-shaping aperture  54  is focused on the reticle  60 . 
     In FIG. 4, only a single subfield (centered on the optical axis AX) is shown. It will be understood that an actual reticle  60  extends outward in the X-Y plane and defines many subfields, as discussed below with reference to FIGS.  5 (A)- 5 (B). In any event, the reticle  60  defines a pattern (chip pattern) for a single semiconductor device (“die”) to be formed on a downstream substrate  65 , and each subfield defines a respective portion of the pattern. 
     As noted above, regarding the FIG. 4 embodiment, the illumination beam IB is deflected laterally to illuminate successive subfields situated within the field of the illumination-optical system. (Hence, multiple subfields desirably fall within the field of the illumination-optical system.) To illuminate a subfield situated outside the field of the illumination-optical system, the reticle  60  is moved relative to the illumination-optical system. To such end, the reticle  60  is mounted on a reticle stage  61  that is movable in the X- and Y-directions. 
     As the illumination beam IB passes through the illuminated subfield on the reticle  60 , the electron beam becomes capable of forming an image of the illuminated subfield on the substrate  65 . Hence, the electron beam propagating downstream of the reticle  60  is termed herein the “patterned beam” PB. The electron-optical system located between the reticle  60  and the substrate  65  is concerned primarily with projecting the image onto the desired location on the reticle  65 . Hence, the electron-optical system located between the reticle  60  and substrate  65  is termed herein the “projection-optical system.” 
     The projection-optical system includes first and second projection lenses  62 ,  64 , respectively, and a deflector  63 . The first and second projection lenses operate in concert to form a “reduced” image of the illuminated reticle subfield on the substrate  65 . By “reduced” is meant that the image as formed on the substrate is smaller than, or “demagnified” relative to, the corresponding illuminated portion of the reticle  60  by a factor termed the “demagnification ratio.” The demagnification ratio is usually an integer factor such as ¼ or ⅕. 
     The deflector  63  laterally deflects the patterned beam as required to cause the patterned beam to form the image of the illuminated subfield at a desired location on the substrate  65 . The surface of the substrate (or “wafer”)  65  is coated with an appropriate resist so as to be imprinted with the demagnified image when dosed by the patterned beam PB. The demagnified images of successively illuminated subfields are formed on the wafer  65  such that all the images are contiguous with each other (i.e., “stitched” together) in the proper order and arrangement to form the complete die pattern on the wafer. Proper stitching together of the images on the wafer  65  is facilitated by mounting the wafer  65  on a wafer stage  67  that is moved controllably as required in the X- and Y-directions and by deflecting each image using the deflector  63 . 
     The first projection lens  62  causes the patterned beam PB to form a second crossover C.O. 2  on the optical axis upstream of the second projection lens  64 . At the second crossover C.O. 2 , the axial distance between the reticle  60  and wafer  65  is divided such that the axial distance from the reticle  60  to the second crossover C.O. 2 , divided by the axial distance from the second crossover C.O. 2  to the wafer  65 , is equal to the demagnification ratio. The second crossover C.O. 2  is also the location, along the optical axis AX, of a “contrast aperture”  68 . The contrast aperture  68  blocks charged particles in the patterned beam PB that were scattered upstream by particles of the illumination beam encountering non-patterned regions of the reticle  60 . Thus, the scattered particles are prevented from propagating to the wafer  65 . 
     A backscattered-electron (BSE) detector  69  is situated between the second projection lens  64  and the wafer  65 . The BSE detector  69  detects backscattered electrons produced when the patterned beam PB strikes certain regions (e.g., alignment marks or analogous features) on the wafer  65 . The positions of the alignment marks on the wafer  65  can be ascertained from characteristics of the BSE signal produced by the BSE detector  69 . Thus, basic data concerning alignments between the wafer  65  and the electron-optical system or between the wafer  65  and reticle  60  can be obtained. 
     The wafer  65  desirably is mounted on an electrostatic chuck  66  that, in turn, is mounted on the wafer stage  67 . The wafer stage  67  moves the chuck  66  (and thus the wafer  65 ) in the X- and Y-directions. The various subfields of the chip pattern on the reticle  60  can be exposed successively by synchronously scanning the reticle stage  61  and the wafer stage  67  in opposite directions. The axis along which these scannings of the stages are performed is perpendicular to the axis along which lateral beam-scanning is performed. The respective position of each stage is determined accurately using a position-measurement system employing one or more laser interferometers as known in the art. Each stage  61 ,  67  is equipped with its own position-measurement system. Extremely accurate positional measurements are required to obtain accurate stitching together of the demagnified images, on the wafer  65 , of the illuminated reticle subfields. 
     The lenses  52 ,  53 ,  59 ,  62 ,  64  and the deflectors  55 ,  58 ,  63  are controlled by a main controller (e.g., microprocessor)  71  via respective coil power supplies,  52   a,    53   a,    59   a,    62   a,    64   a,    55   a,    58   a,    63   a.  Also, the reticle stage  61  and wafer stage  67  are controlled by the main controller  71  via respective stage drivers  61   a,    67   a.  The electrostatic chuck  66  is controlled by the main controller  71  via a chuck driver  66   a.    
     A divided reticle  60  as used with the apparatus of FIG. 4 is described with reference to FIGS.  5 (A)- 5 (C). The reticle  60  comprises a reticle membrane  92  partitioned into multiple subfields  91 . The subfields  91  in this example are square-shaped and similarly sized. Depending upon the type of reticle (e.g., scattering-membrane type or scattering-stencil type), the membrane  92  has a thickness ranging from about 0.1 μm to several μm. Each subfield  91  defines a respective portion of the pattern defined by the reticle. The subfields  91  on a given reticle typically have identical size and shape (typically square or rectangular), ranging from 0,5 mm square to 5 mm square on the reticle. When projected at a demagnification ratio of ⅕, these subfields produce corresponding images on the wafer  65  measuring 0.1 mm square to 1 mm square, respectively. 
     Referring to FIG.  5 (C), each individual subfield  91  is surrounded by a skirt  93  that is defined on the membrane  92  but does not define any portion of the pattern. Referring to FIG.  5 (B), projecting (in the Z-direction) from the membrane  92  between individual subfields  91  are struts  95  that extend lengthwise in the X- and Y-directions. Thus, in each space between intersecting struts  95  is a respective subfield  91  surrounded by a skirt  93 . The struts  95  collectively form a supporting lattice (termed “grillage”) for the subfield membranes  92 . The thickness of each strut  95  in the Z-direction is typically 0.5 to 1 mm, and the width of each strut  95  in the X- or Y-direction is typically about 0.1 mm. 
     These dimensions provide the reticle  60  with substantial rigidity. 
     Referring to FIG.  5 (A), the subfields  91  are arrayed in groups  99  termed “stripes.” The stripes  99  are separated from one another by large struts  97 . The large struts  97  are integral with the struts  95  separating individual subfields  91  within each stripe  99 . The thickness of each large strut  97  in the Z-direction is typically 0.5 to 1 mm, and the width of each large strut  97  in the X- or Y-direction is typically several mm. The large struts  97 , in combination with the grillage formed by the struts  95 , further increase the rigidity and mechanical strength of the reticle  60 . Each stripe  99  comprises multiple subfields  91  linearly arrayed in multiple rows  94  (each row  94  is longitudinally extended in the X-direction). Each such row  94  is termed a “deflection field” because the length of a row (in the X-direction) falls within the field of the illumination-optical system, and the length (in the X-direction) represents the maximum lateral deflection that can be imparted to the illumination beam IB by the selection deflector  58  (FIG.  4 ). The width of each deflection field  94  (in the Y-direction) corresponds to the width (in the Y-direction) of each subfield  91  of the deflection field  94 . Thus, each deflection field  94  has a “band-shaped” profile that is longitudinally extended in a first direction (here, the X-direction) perpendicular to the optical axis AX, relative to a second direction (here, the Y-direction) perpendicular to the optical axis AX. Each stripe  99  consists of an array (extending in the Y-direction) of multiple deflection fields  94 . In view of the length (in the X-direction) of each deflection field, the width of each stripe  99  is also no greater than the maximum lateral deflection, at the reticle, that can be imparted to the illumination beam IB by the selection deflector  58 . The array of stripes extends across the reticle  60  in the X-direction. 
     The stripes  99  (and deflection fields  94  of each stripe  99 ) are arranged such that the subfields  91  in each deflection field  94  of each stripe can be illuminated sequentially by the illumination beam IB. That is, in a given stripe  99 , the subfields  91  in each deflection field  94  are illuminated sequentially, and the deflection fields  94  are illuminated sequentially. The subfields  91  in each deflection field  94  are exposed sequentially by continuously scanning the illumination beam in the X-direction. As the beam scans the subfields  91  in a deflection field, the reticle stage  61  and wafer stage  67  are moving in the Y-direction (but in opposite directions) to position the next deflection field  94  for scanning after completing scanning of the current deflection field. When exposure of a stripe  99  is complete, scanning motions of the illumination beam and stages stop momentarily for the reticle stage  61  and wafer stage  67  to move stepwise to respective positions appropriate for beginning exposure of the next stripe  99 . 
     During projection exposure of the subfields  91 , areas of the reticle in the skirts  93  (including the grillage) are not exposed. Hence, the positions of the corresponding subfield images on the wafer  65  must be such that all the images of the pattern are contiguous with each other on the wafer (i.e., the images are properly “stitched together”). By way of example, if the demagnification ratio is ⅕, a die as formed on the wafer measuring 27×44 mm (e.g., for a 4-gigabit DRAM) would have a size on the reticle (including grillage) of approximately 150×230 mm to 200×350 mm. 
     A representative scheme, according to the invention, for dividing a pattern is now explained with reference to FIGS. 1,  2 , and  3 . FIGS. 1,  2 , and  3  are schematic plan views of exemplary pattern elements and respective subfield division boundaries situated according to the invention. The pattern elements are of a static random-access memory cell (SRAM cell) comprising four transistors and polycrystalline load resistors. FIG. 1 shows mainly the elements defined in a diffusion layer  2 , FIG. 2 shows elements defined in a first polycrystalline Si layer  3 , and FIG. 3 shows features defined in a second polycrystalline Si layer  7 . FIG. 1 also depicts the respective locations of the four transistors TR 1 -TR 4  and their respective sources (S), gates (G), and drains (D). 
     These figures are based on drawings published in  Nikkei Microelectronics,  1987, No. 2, page 72. They are not design drawings for an actual SRAM device but rather are schematic drawings to show, by way of example, the inventive concept of pattern division. 
     Turning first to FIG. 1, elements (delineated by solid lines) defined in a diffusion layer  2  are formed on a Si wafer surface. Elements defined on an overlying first polycrystalline Si layer  3  are delineated by broken lines. Also shown are a bit-line pair  1  and contact holes  4 . The diffusion layer  2  forms the respective source S of each transistor TR 1 -TR 4 . The first polycrystalline Si layer  3  forms the respective gate G of each transistor TR 1 -TR 4 . 
     The inventive manner of establishing a subfield-division boundary in the diffusion layer  2  is now described. A subfield boundary  6  extends in the lateral direction (i.e., a direction parallel to the X-axis) in FIG.  1 . Particular aspects of the subfield boundary  6  are as follows. 
     (1) The subfield boundary  6  does not cross transistor regions (in the vicinity of the gates  2 ), which are regions where the tolerance for pattern defects is especially strict. As a result, each transistor region is transferred within a respective subfield so as to avoid subfield-stitching errors within transistors. Each subfield can have more than one transistor. 
     (2) The bit-line pair  1  and its respective surrounding diffusion layer  2 ′ fall within the same subfield. Hence, in these regions, subfield-stitching errors are not added to overlayer errors, and excellent stitching accuracy is realized. 
     (3) Intersections of the subfield boundary  6  with the diffusion layer  2 , indicated by points  11  in the drawing, desirably are “corners” where the internal angle of the pattern element (diffusion layer  2 ) has an angle of greater than 225 degrees (here, the angles are 270 degrees). 
     By configuring the subfield boundary  6  according to the foregoing, even if a subfield-stitching error (e.g., underlap) occurs in the X-axis direction between the adjacent subfields, for example, breaking of the diffusion layer is avoided. This is because the point  11  is a region where enlargement of the projected element occurs readily due to proximity effects. Such enlargement tends to compensate for the adverse effects of subfield-stitching errors (e.g., underlap) that would otherwise cause a break in the pattern element traversed by a subfield boundary. 
     The subfield boundary  6  extending in the X-axis direction desirably is in the same location on other layers also (i.e., the first polycrystalline Si layer and a second polycrystalline Si layer). Such a configuration reduces overlayer errors between the layers (i.e., between the diffusion layer and the first polycrystalline layer or second polycrystalline layer). Because subfield-stitching errors thus are not added to overlayer errors, positioning accuracy of the diffusion layer with respect to the other layers is further improved. 
     Next, the pattern-subfield boundary in the vertical direction (i.e., the direction parallel to the Y-axis) is explained. The boundary  9  in the Y-axis direction can be established so as to pass through a different location on each pattern layer as required so as to have the boundary pass through pattern elements at locations where extreme stitching accuracy is not required. Hence, the locations can be established so that subfield-stitching errors do not become conspicuous in each respective layer, and interception of subfield boundaries with pattern-element edges is minimized in each respective layer. 
     In the diffusion layer  2  shown in FIG. 1, the subfield boundary  9  running parallel to the Y-axis is located so as to avoid transistors, bit-line pairs  1 , and contact holes  4 . Rather, in regions through which the subfield boundary  9  extends, the diffusion-layer pattern element desirably is relatively large, so little adverse effect occurs even if the stitching together of elements of the diffusion layer extending across the boundary  9  is not ideal. Each point  12  at the intersection of the boundary line  9  and the diffusion layer  2  desirably has an internal angle of at least 180 degrees. Also, at each point  12 , the diffusion-pattern element is relatively large, which greatly reduces the effects of subfield-stitching errors. 
     A representative desirable boundary of the pattern subfield for the first polycrystalline Si layer is now explained with reference to FIG.  2 . 
     FIG. 2 shows the bit-line pair  1  and contact holes  4 . FIG. 2 also shows a word line  3 ′, a gate  3 , and other elements defined by the first polycrystalline Si layer. 
     The subfield-division boundary  6 , extending parallel to the X-axis, is at the same position as on other layers (e.g., at the same position as shown in FIG. 1 ). The boundary  6  is situated between the bit-line pair  1  and the word line  3 ′, and does not intersect any pattern element in this layer. 
     A subfield-division boundary  19 , extending parallel to the Y-axis, desirably does not directly cross the word line  3 ′, which extends a long distance in the X-axis direction. To avoid a direct crossing, in this example, the boundary  19  bends twice (bend points  26  and  27 ) in the word line  3 ′. The points  25  and  28  where the boundary  19  intersects the word line  3 ′ intersect are displaced in the X-axis direction. By placing the boundary and element-edge intersection points at different positions in the X-axis direction, even if there is a relatively small subfield-stitching error between the right and left (in the figure) subfields, the word line  3 ′ is not broken. 
     Next, a representative desirable boundary of the pattern subfield for the second polycrystalline Si layer is explained with reference to FIG.  3 . In FIG. 3, the bit-line pair  1  and contact holes  4  are shown in the second polycrystalline Si layer  7 . A portion of the second polycrystalline Si layer  7  defines load-resistor segments  5 . 
     The subfield-division boundary  6 , extending parallel to the X-axis, is at the same position as on other layers. Specifically, the boundary  6  is situated between the bit-line pair  1  and the second polycrystalline Si layer  7 , and does not intersect a pattern element in this layer. 
     The subfield-division boundary  39  extends parallel to the Y-axis. Where the subfield-division boundary  39  crosses the second polycrystalline Si layer  7  (extending in the X-axis direction), the boundary  39  is bent at points  31 ,  15 ,  16 , and  32 . The reason for such a bend is to separate the boundary  39  from the transistor region (around the load-resistor pattern  5 ) as much as possible, and to pass the boundary  39  through locations on the pattern feature (defined by the second polycrystalline Si layer  7 ) where the internal angle is large (greater than 225 degrees). Here, the angle at points  15  and  16  is 270 degrees. 
     By establishing subfield-division boundaries as described above, defects caused by subfield-stitching errors and overlayer errors are reduced compared to conventional methods. 
     FIG. 6 is a flow chart of steps in a process for manufacturing a semiconductor device such as a semiconductor chip (e.g., an integrated circuit or LSI device), a display panel (e.g., liquid-crystal panel), or CCD, for example. In step  1 , the circuit for the device is designed. In step  2 , a reticle (“mask”) for the circuit is manufactured. In step  3 , a wafer is manufactured from a material such as silicon. 
     Steps  4 - 12  are directed to wafer-processing steps, specifically “pre-process” steps. In the pre-process steps, the circuit pattern defined on the reticle is transferred onto the wafer by microlithography. Step  13  is an assembly step (also termed a “post-process” step) in which the wafer that has been passed through steps  4 - 12  is formed into semiconductor chips. This step can include, e.g., assembling the devices (dicing and bonding) and packaging (encapsulation of individual chips). Step  14  is an inspection step in which any of various operability and qualification tests of the device produced in step  13  are conducted. 
     Afterward, devices that successfully pass step  14  are finished, packaged, and shipped (step  16 ). 
     Steps  4 - 12  also provide representative details of wafer processing. 
     Step  4  is an oxidation step for oxidizing the surface of a wafer. Step  5  involves chemical vapor deposition (CVD) for forming an insulating film on the wafer surface. Step  6  is an electrode-forming step for forming electrodes on the wafer (typically by vapor deposition). Step  7  is an ion-implantation step for implanting ions (e.g., dopant ions) into the wafer. Step  8  involves application of a resist (exposure-sensitive material) to the wafer. Step  9  involves microlithographically exposing the resist so as to imprint the resist with the reticle pattern, as described elsewhere herein. Step  10  involves developing the exposed resist on the wafer. Step  11  involves etching the wafer to remove material from areas where developed resist is absent. Step  12  involves resist separation, in which remaining resist on the wafer is removed after the etching step. By repeating steps  4 - 12  as required, circuit patterns as defined by successive reticles are formed superposedly on the wafer. 
     Whereas the invention is described in connection with 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.