Abstract:
Charged-particle-beam microlithography methods and apparatus are disclosed that employ a segmented reticle and provide high-accuracy pattern transfer even under conditions of drift of the charged particle beam. Beam-drift test patterns are defined on a reticle at the termini of deflection fields at one or both lengthwise ends of certain reticle stripes. Corresponding beam-test marks are situated on or at the wafer. A charged particle beam passing through a beam-test pattern on the reticle is a “detection beam” that is directed to and scanned over the corresponding beam-test mark on the wafer. Before performing actual pattern transfer, the beam-test marks on the wafer are scanned by the detection beam passing through the corresponding beam-drift test patterns on the reticle, and electrons emitted by impingement of the detection beam on the wafer beam-test marks are detected. The positions of the beam-test marks are detected multiple times and the corresponding data is used to measure beam drift. The beam-drift measurements provide data usable to perform a correction of the beam position so as to reduce or eliminate effects of beam drift.

Description:
FIELD OF THE INVENTION 
     This invention pertains to microlithography (pattern projection and transfer from a mask or reticle to a substrate) as used in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to microlithography using a charged particle beam (e.g., electron beam or ion beam) to transfer a circuit pattern or the like to a substrate (e.g., semiconductor wafer) at a resolution (minimum linewidth) of 0.1 μm or less on the substrate. 
     BACKGROUND OF THE INVENTION 
     As feature sizes in circuit patterns for integrated circuits, displays, and the like progressively have been miniaturized, the resolution limits of optical microlithography have become increasingly apparent. This has resulted in intensive efforts to develop practical microlithography apparatus and methods exploiting an exposure technology offering prospects of substantially greater resolution than obtainable using optical microlithography. Optical microlithography utilizes a beam of light (typically ultraviolet light) as a pattern-transfer energy beam. One candidate alternative technology to optical microlithography involves the use of a charged particle beam (e.g., electron beam or ion beam) rather than a light beam as an energy beam. 
     Whereas charged-particle-beam (CPB) microlithography (e.g., electron-beam microlithography) offers prospects of high resolution, many technical problems must be solved in order to develop practical CPB microlithography apparatus and methods. One technical problem pertains to beam drift, i.e., changes in actual beam position relative to desired beam position. As can be surmised, in order to achieve a pattern-feature resolution on the order of 0.1 μm or less, the position of a charged particle beam as used for pattern transfer must be controlled extremely accurately and precisely. If beam drift is excessive, then the “CPB optical system” (i.e., assembly of “lenses”, deflectors, and the like for shaping and guiding the beam from a source to the substrate) conventionally must be disassembled, cleaned, and reassembled. 
     Most instances of beam drift arise from the accumulation of contaminants in the CPB optical system. Deposits of contaminants in a CPB optical system tend to accumulate static charges that can have a significant electrostatic effect on the beam. I.e., propagation of the beam past contaminant deposits presenting an unwanted electrostatic charge to the beam can cause the beam to be deflected or distorted in undesirable ways. Some causes of beam drift can be attributed to parameters that can be controlled in the optical system such as variations in lens-induction current, deflection current, voltage, temperature, and the like. Nevertheless, beam drift (especially beam drift caused by factors that cannot be controlled directly) remains an important problem requiring effective solution. 
     Whereas the beam current in certain types of CPB microlithography apparatus (specifically, conventional electron-beam “variable-shaped beam apparatus”) is usually small (approximately 1 μA or less), a beam current of, e.g., 20 times greater (i.e., 20 to 25 μA) is used in other types of apparatus such as “divided-pattern” CPB microlithography. Exposure of a resist on the surface of a wafer with these higher beam currents typically generates large amounts of volatile by-products of the resist. The volatile by-products tend to deposit in various locations inside the CPB optical system, and the rate of deposition tends to increase with increases in beam current. To achieve and maintain maximal resolution of pattern transfer, beam stability (freedom from significant drift) must be maintained at a high level. However, to maintain such stability at high beam currents, affected components of the CPB optical system must be disassembled and cleaned progressively more frequently. In addition, especially at higher beam currents, resulting variations in temperature of the components in the CPB optical system can cause significant beam drift, even in instances in which the CPB optical system is not “dirty.” 
     SUMMARY OF THE INVENTION 
     This invention addresses these problems and its purpose is to provide, inter alia, methods for realizing high-precision pattern transfer, even when there is a certain amount of beam drift. 
     A charged-particle-beam CPB) microlithography (“projection-transfer” or “projection-exposure”) system according to the invention employs a reticle in which the pattern field is divided or “segmented” into multiple portions defining respective portions of the pattern. More specifically, the pattern field is divided into multiple “stripes” that are typically rectangular in shape. Each stripe has a width (shorter dimension) that is within the deflectable field of the CPB optical system. Each stripe is divided further into multiple parallel “deflection fields” each having a length extending the width of the stripe. The overall pattern field is transferred stripe-by-stripe and each stripe is transferred deflection-strip-by-deflection-strip. To transfer a stripe, the charged particle beam is deflected, in a scanning manner, across the width of the stripe to scanningly expose each deflection field. As each deflection field is exposed, the reticle and substrate are mechanically displaced as required (in the length dimension of the stripe) to bring the next deflection field into position for exposure. 
     The reticle includes beam-drift test patterns in certain deflection fields (a beam-drift test pattern desirably is located in a terminus of the respective deflection field) that is scanned by the charged particle beam (functioning as a “detection beam”). Corresponding beam-test marks are disposed on or at a substrate in locations where the respective test-pattern-containing deflection fields will be exposed by the charged particle beam. The beam-test marks on the substrate are irradiated scanningly by the detection beam (passing through corresponding beam-test patterns on the reticle) prior to pattern transfer. The positions of the test patterns are detected (desirably iteratively) by scanning the corresponding beam-test marks on the substrate multiple times with the detection beam passing through the corresponding beam-drift test patterns on the reticle, thereby providing positional data for detecting beam drift. From such positional data, the magnitude and directions of corrective deflections to the beam are calculated. The beam position during subsequent pattern transfer is corrected according to the results of these calculations to correct the beam drift and achieve a more accurate pattern transfer. 
     Whenever the charged particle beam is being used constantly under identical conditions, the magnitude of beam drift over time tends to be minimal. On the other hand, comparatively large beam drifts tend to occur whenever beam parameters are changed. For example, substantial beam drift can occur immediately after resuming use of a beam that has been “blanked” for a long period of time. Substantial beam drift also can occur whenever the beam current is suddenly and substantially changed from a high beam current to a low beam current, for example, or immediately after the beam is subjected to a large deflection. Even though such drifts are regarded as irregular, a degree of repeatability can be discerned in them under similar beam parameters. 
     In CPB microlithography apparatus that perform pattern transfer using reticles having identical specifications, essentially the same exposure operations normally are repeated at locations on the reticle at which the beam is either blanked or deflected, but not at locations on the reticle at which the beam current normally changes with a change in the pattern. Similar magnitudes and directions of the beam drift tend to be evident at the respective repeated locations. By correcting these repeatable components of beam drift, it is possible to perform high-accuracy pattern transfer even when there is a small residual amount of beam drift. 
     According to the invention, changing ratios of beam drift or simple differences in beam drift observed in various deflection fields can be measured and tabulated in advance using a test reticle defining a beam-drift test pattern in multiple deflection fields. Also, a test substrate can be used that possesses corresponding beam-test marks in respective regions on the substrate corresponding to the deflection fields on the reticle. Actual pattern transfer can be performed after storing these data (after replacing the test reticle and test substrate with an actual transfer reticle and substrate). During such exposure, beam drift is corrected based on the pre-tabulated ratios or differences. 
     Alternatively to using a test reticle and test substrate, it is possible to dispose beam-drift test patterns on a region of a normal patterned reticle and corresponding beam-test marks on a corresponding region of an actual wafer. In such a situation, beam drift can be measured in real time during actual pattern transfer. In any event, with a CPB microlithography apparatus used full time for semiconductor device fabrication, it is desirable to measure beam drift on a regular basis, such as at least once a week, using a dedicated test reticle and test wafer. The results of such periodic tests desirably are tabulated and used to perform corrections, as required, of beam drift during the remaining time the apparatus is used. By using a dedicated test reticle, more of an actual production reticle can be used for defining the pattern to be transferred to wafers for actual device manufacture. 
     It is also desirable for the feature density in the deflection fields containing the beam-drift test pattern or in the corresponding beam-test marks on the substrate to be substantially equal to the average feature density of an actual device pattern to be transferred. 
     Whenever beam current is high, the magnitude of beam drift tends to be correspondingly larger, and the opposite is experienced whenever the beam current is low. Nevertheless, beam drift can be measured under test conditions that are nearly identical to conditions during actual device-pattern transfer, and beam-drift correction under such conditions can be performed very accurately. Beam drift also can be measured while changing the beam current to various levels of magnitude, and the beam-drift correction can be calculated as a function of the beam current. Since beam current and drift magnitude may not be proportional, drift magnitude alternatively can be determined by interpolating from a mid-range value of beam current. 
     A CPB microlithography (pattern-transfer exposure) apparatus according to the invention comprises an illumination-optical system situated and configured to illuminate a reticle defining features of a pattern to be transferred to a sensitive substrate. The reticle is mounted on a movable reticle stage and illuminated with an “illumination beam.” The apparatus also comprises a projection-optical system situated and configured to project and form an image of the portion of the illumination beam (that has passed through the illuminated portion of the reticle and become a “patterned beam”) onto a desired location on the sensitive substrate. The substrate is mounted on a movable substrate stage. 
     A reticle according to the invention comprises a pattern-transfer field that is divided into multiple stripes, and each stripe is divided into multiple deflection fields. Each deflection field can comprise multiple subfields. Each stripe has a length dimension and a width dimension. The width dimension corresponds to the maximal lateral deflection that can be imparted to the illumination beam by the illumination-optical system of the CPB microlithography system with which the reticle is used. Each stripe is further divided into multiple strip fields (deflection fields) that extend in the direction of the width dimension of the respective stripe. 
     The overall pattern-transfer field of the reticle is transferred to the substrate by deflecting and scanning the illumination beam in the direction of the width dimension of the stripes (to sequentially expose the deflection fields), while mechanically scanning (using the respective stages) the reticle and the sensitive substrate in the direction of the length dimension of the stripes (to sequentially move deflection fields into position for exposure). 
     A beam-drift test pattern is defined on the reticle in a deflection field at the lengthwise end of a stripe. Similarly, a corresponding beam-test mark (to be irradiated with the detection beam passing through the corresponding beam-drift test pattern on the reticle) is defined on the substrate. The apparatus includes a beam-drift-correction unit that, before pattern transfer begins, detects the positions of the beam-drift test patterns multiple times by scanning the beam-test marks on the substrate with the detection beam passing through corresponding beam-drift test patterns on the reticle. The beam-drift-correction unit iteratively determines the magnitude of beam drift, calculates the amount of required correction of the beam position, and imparts the required correction to the beam position as required during actual pattern transfer, based on the measured magnitude and direction of beam drift. 
     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 
     FIG. 1 is a graph showing an example of beam-drift measurement results as used for determining beam-drift correction according to the invention. The vertical axis is beam drift in the X-direction (unit=mm), and the horizontal axis is amplitude by which the beam is deflected in the X-direction (unit=mm). 
     FIG. 2 is a schematic plan view of a representative disposition of beam-drift test patterns on a reticle (e.g., test reticle) according to the invention. The beam-drift test patterns are situated in certain deflection fields at the termini of certain stripes of the reticle. 
     FIG. 3 is a schematic plan view of the disposition of corresponding beam-test marks on a substrate (e.g., test wafer) used with the reticle of FIG.  2 . 
     FIG. 4 is a schematic elevational view of various components, imaging relationships, and control systems of an electron-beam microlithography apparatus according to a representative embodiment of the invention. 
     FIGS.  5 (A)- 5 (C) schematically depict certain features of a representative embodiment of a reticle according to the invention, as used for electron-beam microlithography, wherein FIG.  5 (A) is a plan view of the overall reticle, FIG.  5 (B) is a partial oblique view of the reticle, and FIG.  5 (C) is a plan view of a single 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 
     Reference is first made to FIG. 4 that depicts an overall configuration of a representative embodiment of a CPB microlithography apparatus according to the invention. FIG. 4 also depicts certain imaging and control relationships of the CPB optical system of the apparatus. FIG. 4 is depicted and discussed in the context of an electron-beam system. However, it readily will be understood that the general principles discussed below can be applied with equal facility to an ion-beam system or the like. 
     An electron gun  1  is situated at the extreme upstream end of the optical system of FIG.  4 . The electron gun  1  emits an electron beam (termed the “illumination beam”) in a downstream direction (downward in the figure) along an optical axis AX. A two-stage condenser-lens assembly (comprising first and second condenser lenses  2 ,  3 , respectively) is situated downstream of the electron gun  1 . The illumination beam passes through the condenser lenses  2 ,  3  and forms a crossover image C.O. at a blanking aperture  7 . 
     A beam-shaping aperture  4  is situated downstream of the second condenser lens  3 . The beam-shaping aperture  4  has a transverse profile (usually square or rectangular) that trims and shapes the illumination beam to have a transverse profile and dimensions sufficient to illuminate only one “subfield” on a downstream reticle  10 . A “subfield” is a unit of the reticle pattern that is exposed at any instant of time during exposure of the pattern, and thus represents an “exposure unit” of the reticle  10 . By way of example, the beam-shaping aperture  4  trims the illumination beam to have a square transverse profile measuring just over 1 mm on each side as illuminated on the reticle. An image of the beam-shaping aperture  4  is formed on the reticle  10  by a third condenser lens  9 . 
     The blanking aperture  7  is defined by a plate that, except for the illumination beam allowed to pass through the actual blanking aperture, blocks the illumination beam. A blanking deflector  5  is situated between the beam-shaping aperture  4  and the blanking aperture  7 . The blanking deflector  5  deflects the illumination beam to strike the plate of the blanking aperture  7  (thereby blocking the illumination beam from propagating further downstream) whenever the illumination beam is to be prevented from propagating to the reticle  10 . 
     A subfield-selection deflector  8  is situated downstream of the blanking aperture  7 . The subfield-selection deflector  8  illuminates each subfield on the reticle  10  within the field of the illumination-optical system by sequentially or continuously scanning the illumination beam primarily in the left-right direction in FIG. 4 (i.e., in the X-direction). The third condenser lens  9 , situated downstream of the subfield-selection deflector  8 , collimates the illumination beam for impingement on the reticle. Thus, the illumination beam forms an image of the beam-shaping aperture  4  on the reticle  10  whenever the illumination beam strikes the reticle  10 . 
     Even though only one subfield of the reticle  10  is shown (the depicted subfield is centered on the optical axis AX in FIG.  4 ), it will be understood that the reticle  10  actually comprises multiple subfields arrayed within the X-Y plane extending perpendicularly to the optical axis AX. The reticle  10  normally defines the entire pattern of, for example, a layer of a semiconductor-device chip to be transferred to a substrate (“wafer”)  15 . Because the reticle  10  is divided into subfields, it is a socalled “divided” or “segmented” reticle. Further detail regarding the configuration of the reticle is presented later. 
     The illumination beam is deflected as required by the subfield-selection deflector  8 , as discussed above, to illuminate the subfields sequentially or continuously within the field of the illumination-optical system. 
     The optical components (lenses and deflectors) discussed above that are situated between the electron gun  1  and the reticle  10  are regarded as components of the “illumination-optical system.” 
     The reticle  10  is mounted on a reticle stage  11  to facilitate mechanical movement of the reticle as required in the X- and Y-directions during exposure of the pattern. Thus, subfields located outside the optical field of the illumination-optical system can be moved to within the optical field. 
     The FIG. 4 apparatus also comprises first and second projection lenses  12 ,  14 , respectively, and a deflector  13  situated downstream of the reticle  10 . As the illumination beam strikes a particular subfield on the reticle  10 , the portion of the illumination beam passing through the illuminated subfield and propagating downstream of the reticle  10  is denoted the “patterned beam.” This is because the beam downstream of the reticle is “patterned” by passing through regular pattern features or test-pattern features defined in the illuminated subfield and thus acquires the ability to form an image, downstream of the reticle  10 , of the illuminated features. The projection lenses  12 ,  14  act in concert on the patterned beam to prepare the beam for forming the image on the upstream-facing surface of the wafer  15 . As the projection lenses  12 ,  14  converge the patterned beam onto the wafer  15 , the image carried by the patterned beam is “reduced” (demagnified) for projection onto the wafer  15 . By “reduced” or “demagnified” is meant that the image as formed on the wafer  15  is smaller (by an integer reciprocal factor termed the “demagnification ratio”) than the corresponding illuminated area on the reticle  10 . For each subfield on the reticle  10 , the corresponding image is formed at a specified respective location on the wafer  15 . 
     For imprinting of the images on the wafer surface, the upstream-facing surface of the wafer  15  is coated with a suitable “resist.” Portions of the resist that receive a dose of charged particles in the patterned beam undergo a latent chemical change that is “developed” to reveal the image. A wafer or substrate coated with a non-developed resist is termed “sensitive.” 
     A second crossover C.O. is formed at an axial location at which the axial distance from the reticle  10  and the wafer  15  is divided according to the demagnification ratio. A contrast aperture  18  is located at this second crossover. The contrast aperture  18  blocks electrons of the patterned beam that have been scattered by passing through or by non-patterned areas of the reticle  10 . Thus, such scattered electrons do not propagate to the wafer  15 . 
     The optical components (lenses and deflectors) discussed above that are situated between the reticle  10  and the wafer  15  are regarded as components of the “projection-optical system.” 
     A backscattered-electron (BSE) detector  19  is situated between the second projection lens  14  and the wafer  15 . The BSE detector  19  detects electrons emitted when the patterned beam strikes the wafer  15  (which causes some of the electrons of the patterned beam to be emitted in an upstream direction from the wafer  15 . The BSE detector  19  produces an electrical signal, corresponding to the electrons, emitted from the wafer and actually received by the detector  19 . The signal is routed to a controller  21  via a converter circuit  19   a . The converter circuit  19   a  includes an analog-to-digital (A/D) converter that converts the signals from the BSE detector  19  to corresponding digital signals that can be processed by the controller  21 . The relative positions of beam-test marks on the wafer  15  can be determined by processing the signal from the BSE detector, thereby allowing determinations of alignment between the wafer  15  and the electron-optical system or between the wafer  15  and the reticle  10 . These signals are also used for determining beam drift, as discussed further below. 
     The wafer  15  is mounted, desirably using an electrostatic chuck  16 , to a wafer stage  17  that is movable in X- and Y-directions. By appropriately moving the wafer stage  17  synchronously with movements of the reticle stage  11 , wide areas of the pattern as defined on the reticle  10  can be exposed sequentially onto the wafer  15 . Due to the image-inversion imparted by the projection lenses  12 ,  14 , the stage movements normally are in opposite directions relative to each other. The respective positions of the stages  11 ,  17  in the X-and Y-directions are determined very accurately using laser interferometers (not shown, but as known in the art). 
     The various lenses  2 ,  3 ,  9 ,  12  and deflectors  5 ,  8 ,  13  are controlled by the controller  21  via respective coil power supplies  2   a ,  3   a ,  9   a ,  12   a , and  5   a ,  8   a , and  13   a  connected to the controller  21 . In addition, the reticle stage  11  and wafer stage  17  are controlled by the controller  21  via stage drivers  11   a  and  17   a , respectively, connected to the controller  21 . Finally, the electrostatic chuck  16  is controlled by the main controller  21  via a chuck driver  16   a  connected to the controller  21 . Thus, the respective positions of the stages and respective energizations of the lenses and deflectors are accurately controlled to allow demagnified images of the subfields on the reticle  10  to be stitched together accurately on the wafer  15 , thereby forming one or more entire chip patterns on respective regions of the wafer. 
     The controller  21  includes a beam-drift correction unit  21   a  that includes a memory in which a table of beam-drift data obtained as described below can be stored. Based on data recalled from the table, the beam-drift correction unit  21   a  generates an appropriate beam-position correction signal and routes the signal to the deflector power supply  13   a.    
     Details of an exemplary reticle  10  as used in the FIG. 4 apparatus are shown in FIGS.  5 (A)- 5 (C). In FIG.  5 (A), the reticle  10  comprises multiple “stripes”  49  each containing multiple rows of subfields  41 . The rows each extend in the X-direction (representing the width dimension of the corresponding stripes), and the array of rows in each stripe  49  extends in the Y-direction (representing the length dimension of the corresponding stripe). Each row of subfields is termed a “deflection field”  44  because the length of the row (in the X-direction) corresponds to the maximum deflection range (in the X-direction) of the illumination beam as achieved by the subfield-deflection deflector  8  in the illumination-optical system. 
     As shown in FIG.  5 (B), each subfield  41  comprises a respective membrane region  41   m . The thickness (Z-dimension) of each membrane region  41   m  is 0.1 μm to several μm. As shown in FIG.  5 (C), each subfield  41  comprises a respective patterned region  42  surrounded by a skirt  43  that lacks any pattern features. The patterned region  42  defines the features of the respective portion of the overall pattern defined by the reticle  10 . During illumination of a subfield  41 , the respective patterned region  42  is illuminated by the illumination beam, wherein the edges of the illumination beam fall within the respective skirt  43 . 
     The patterned region  42  of each subfield  41  on the reticle  10  typically has an area (extending in the X- and Y-directions) of approximately 0.5 mm to 5 mm square. At a demagnification ratio of ⅕, the size of the corresponding image of the subfield as projected onto the wafer  15  is 0.1 mm to 1 mm square. 
     The reticle includes a grid-like “grillage”  45  comprising intersecting struts that surround each subfield  41 . The reticle  10  includes grillage  45  because the membrane regions  41   m  are too thin to provide the reticle  10  with any substantial rigidity. Each strut is approximately 0.5 mm to 1 mm thick (in the Z-direction) and approximately 0.1 mm wide in the respective X- or Y-direction. 
     Referring further to FIG.  5 (A), multiple stripes  49  are arrayed in the X-direction on the reticle  10 . Between adjacent stripes  49  and around the perimeter of the reticle  10  are wide struts  47  that provide additional rigidity to the reticle  10 . A wide strut  47  situated between adjacent stripes  49  is typically several mm wide (in the X-direction) and has the same thickness (in the Z-direction) as a regular strut located between adjacent subfields  41 . 
     Reticles also can be used in which no non-patterned regions (regular struts and skirts  43 ) exist between adjacent subfields  41  in each deflection field  44 . I.e., in such a reticle (and referring to FIGS.  5 (A) and  5 (C)), the patterned regions  42  of adjacent subfields  41  are contiguous within each deflection field  44  of each stripe  49 . 
     Using an apparatus as shown in FIG. 4 and a reticle as shown in FIG.  5 (A) for projection-exposure of the pattern defined on the reticle  10 , the subfields  41  in each deflection field  44  are exposed sequentially by appropriately deflecting the electron beam in the X-direction. As each deflection field  44  is exposed, the next deflection field  44  is moved into position for exposure by appropriately moving (in a scanning manner) the reticle stage  11  and the wafer stage  17 . (The stages  11 ,  17  are moved in opposite directions in the Y-direction.) After completing exposure of a stripe  49 , the next stripe  49  is moved into position for exposure by appropriately moving (in a start-and-stop manner) the reticle stage  11  and the wafer stage  17 . (The stages  11 ,  17  are moved in opposite directions in the X-direction.) 
     As each subfield  41  is projection-exposed onto the wafer  15 , the non-patterned portions (skirts  43  and grillage  45 ) are “canceled” on the wafer so as to place the images of the patterned regions  42  contiguously with each other on the wafer  17 . Such placement of the images of the patterned regions  42  on the wafer is termed “stitching” of the images. Upon completing exposure of the entire reticle  11 , the corresponding image of a layer of a chip on the wafer comprises all the individual images of the patterned regions  42  stitched together. At a demagnification ratio of ¼ or ⅕, a chip size of 27 mm×44 mm on the wafer (the size of a 4-Gigabit DRAM) would require a reticle measuring (including subfields and non-patterned areas) 120×150 mm to 230×350 mm. 
     FIG. 2 schematically depicts the disposition of beam-drift test patterns on a reticle (e.g., test reticle) according to the invention (and usable with the FIG. 4 apparatus). FIG. 3 schematically depicts the disposition of corresponding beam-test marks on a wafer (e.g., test wafer) used with the reticle of FIG.  2 . 
     In FIG. 2, three stripes  71 ,  72 ,  73  are shown arrayed side-by-side in the X-direction. A wide strut  74  is situated between each pair of adjacent stripes  71 ,  72 ,  73 . The stripe  71  is divided into deflection fields  81 ; the stripe  72  is divided into deflection fields  82 , and the stripe  73  is divided into deflection fields  83 . In each stripe  71 ,  72 ,  73 , the respective deflection fields  81 ,  82 ,  83  are arrayed serially in the Y direction. In each deflection field, the constituent subfields are array serially in the X-direction. In FIG. 2, non-patterned regions (grillage  45  and skirts) are not shown so as to eliminate detail not needed for the following discussion. 
     In this example, the stripe  71  comprises deflection fields  81 - 1  through  81 - 5 , the stripe  72  comprises deflection fields  82 - 1  through  82 - 5 , and the stripe  73  comprises deflection fields  83 - 1  through  83 - 5 . Each of these deflection fields includes beam-drift test patterns  87 ,  88 . So-called “X-direction beam-drift test patterns”  87  include linear features extending in the Y-direction and arrayed serially in the X-direction, and so-called “Y-direction beam-drift test patterns”  88  include linear features extending in the Z-direction and arrayed serially in the Y-direction. Deflection fields containing the beam-drift test patterns  87 ,  88  are situated at the ends (in the Y-direction) of each stripe  71 ,  72 ,  73 . In this example, although only five are shown, up to approximately ten sets of deflection fields that include these beam-drift test patterns can be disposed at the end of each stripe. 
     Each linear feature of a beam-drift test pattern  87 ,  88  is a respective area on the reticle that is readily transmissive to the illumination beam. Examples of such areas are voids or regions of the reticle membrane that exhibit low scattering. The corresponding beam-test marks  97 ,  98  on the wafer (FIG. 3; the beam-test marks  97 ,  98  have linear features corresponding to the linear features in the beam-drift test patterns  87 ,  88 ) desirably are defined by a metallic layer or the like that is highly reflective to the charged particles in the beam. 
     In FIG. 2, by way of example, the deflection fields  82  in the middle stripe  72  only have X-direction beam-drift test patterns  87 , whereas the deflection fields  83  in the right-hand stripe  73  only have Y-direction beam-drift test patterns  88 . Providing both X-direction test patterns and Y-direction test patterns allows beam-drift data to be obtained for both the X-direction and the Y-direction. The deflection fields  81  in the left-hand stripe  71  include subfields alternately containing X-direction test patterns  87  and Y-direction test patterns  88 . Other combinations of the beam-drift test patterns  87 ,  88  can be selected as appropriate. Different combinations allow more flexibility in obtaining beam-drift data at various magnitudes of beam deflection in both the X-and Y-directions. For example, if beam drift in the Y-direction is relatively small, then the beam-drift test patterns in the stripe  82  are usually sufficient (for testing the beam drift in the X-direction). The beam-drift test patterns in the stripe  81  are sufficient if a plot of beam drift versus magnitude of beam deflection (such as shown in FIG. 1) is a smooth curve. 
     Corresponding to the beam-drift test patterns  87 ,  88  in FIG. 2, deflection fields with multiple beam-test marks  97 ,  98  are formed on the wafer shown in FIG.  3 . However, on the wafer, there are no components situated between the stripes  91 ,  92 ,  93  that correspond to the wide struts situated between the stripes  71 ,  72 ,  73  on the reticle. Rather, on the wafer, the stripes  91 ,  92 ,  93  are joined together contiguously. Also, non-patterned regions located on the reticle between adjacent subfields and deflection fields are not formed on the wafer. Thus, on the wafer, the entire reticle pattern is formed with all the constituent pattern-portion-defining subfields, deflection fields, and stripes being stitched together. 
     In this example, the width and length of the linear test-pattern features are pre-determined so that the beam current at a subfield defining only the linear test-pattern features is 1 μA. In cases where the beam current is larger, relatively large voids  96 ,  99  can be defined in areas of the subfield aside from where the test patterns  94  exist, as shown in the enlarged view of a subfield  95  shown in FIG.  2 . For example, beam drift can be measured at beam currents of 5 μA, 10 μA, 15 μA, 20 μA, and 25 μA, etc., to ascertain the relationship between beam current and beam drift. Now, in this case, the illumination-beam current incident to a single subfield on the reticle is 100 μA. 
     FIG. 1 is a graph showing an example of beam-drift measurement results and of determining and applying a corresponding amount of beam-drift correction. In this example, the beam current is 10 μA. The ordinate (vertical axis) is magnitude of beam drift in the X-direction, and the abscissa (horizontal axis) is beam-drift in the X-direction (unit=nm). The curve defined by the open circles is of measured beam drift obtained by scanning the beam over the first deflection field  82 - 1  (FIG. 2, corresponding to the deflection field  102 - 1  in FIG. 3) in the Y-direction and detecting the test-pattern positions. The curve defined by the black (closed) circles is of measured beam drift obtained by scanning the beam over the second deflection field  82 - 2  in the Y-direction and detecting the test-pattern positions. The curve defined by the triangles is of measured beam drift obtained by scanning the beam over the third deflection field  82 - 3  in the Y-direction and detecting the test-pattern positions. The curve defined by the x&#39;s is of measured beam drift obtained by scanning the beam over the fourth deflection field  82 - 4  in the Y-direction and detecting the test-pattern positions. The curve defined by the open squares is of measured beam drift obtained by scanning the beam over the fifth deflection field  82 - 5  in the Y-direction and detecting the test-pattern positions. Finally, the curve defined by the open double circles is of measured beam drift obtained by scanning the beam over a sixth deflection field in the Y-direction and detecting the test-pattern positions. When obtaining these data, the scanning rate and timing for beam blanking, etc., were the same as during actual device-pattern scanning. 
     In this example, since the measurements proceeded from the minus direction toward the plus direction over the respective deflection fields, the magnitude of drift in each curve was greater for X-direction beam deflection in the minus direction than for X-direction beam deflection in the plus direction. In addition, the magnitude of drift was relatively high in the first deflection field  82 - 1  and was progressively less in the second deflection field  82 - 2 , third deflection field  82 - 3 , and so on. Even though not shown in FIG. 1, respective magnitudes of beam drift as measured in deflection fields after the sixth deflection field were essentially the same as in the sixth deflection field. The relatively high magnitude of drift in the first deflection field of a stripe is due principally to de-energization of a blanking deflector after a relatively long period of beam blanking (as encountered while waiting for a change of stripes or the like). 
     In the example shown in FIG. 1, reproducibility is relatively good and the data points smoothly define the respective curves. In instances in which beam drift exhibits poor reproducibility, scattered points appear on the respective curves, wherein the points do not clearly define the respective curve. The corresponding data will exhibit poor convergence, regardless of the deflection field. Since drift correction is very difficult under such conditions, equipment maintenance is indicated, such as cleaning the interior of lenses in the illumination-optical system and/or projection-optical system of the pattern-transfer apparatus. 
     A representative method for correcting beam drift (based on the exemplary measurement results in FIG. 1) is now explained. During normal projection-exposure of a device pattern, the beam-drift test patterns can be disposed in the first deflection field in the Y-direction end of a stripe. That stripe is exposed while performing beam-drift correction based on the detection data obtained with the test patterns. If beam-drift correction were not to be performed, then correction for all the deflection fields in the particular stripe would be determined based on the measurement data obtained from the first deflection field. 
     In this example, the respective magnitudes of beam drift of other deflection fields relative to the first deflection field are known in advance from prior testing. The magnitudes of beam drift for each strip are tabulated, corresponding to the amplitude of beam deflection in the X 30  direction, in as memory in the beam-drift correction unit  21   a . For example, if the amplitude of beam deflection for the second deflection field were −2.25 mm, then the value (4 nm) denoted by “C1” in FIG. 1 would correspond to the drift magnitude. If the amplitude of beam deflection for the third deflection field were −0.5 mm, then the value (3 nm) denoted by “C2” in FIG. 1 would correspond to the drift magnitude. These values are tabulated and stored in the beam-drift-correction unit  21   a  (FIG.  4 ). 
     Afterward, when an actual device pattern is projection-exposed onto a wafer, the magnitude of beam drift corresponding to the respective deflection amplitude for each stripe is corrected according to test-pattern-position data obtained at the first deflection field of the stripe. Thus, the subfield-transfer position in each deflection field is corrected. 
     The table in the foregoing example was created using difference values. However, a table alternatively can be created using ratios of beam-drift magnitude in a second deflection field to beam-drift magnitude in a first deflection field. For example, if the deflection amplitude for the second defection field were −2.25 mm, then the value of the beam-position drift ratio would be (10 nm)/(14 nm)=0.71, wherein 10 nm is the magnitude of beam drift in the second deflection field and 14 nm is the magnitude of beam drift in the first deflection field (see line “C1” in FIG.  1 ). According to this ratio, the beam position is corrected and actual exposure is made. 
     A table can be created for each magnitude of beam current; for example, six tables can be created. Then, the magnitude of drift correction is calculated according to the average beam current at the level of the actual device pattern. The required correction to the median beam current can be determined by interpolation. In such a manner, the magnitude of beam-position correction is calculated taking into account the beam current and other variables, based on magnitudes of beam drift previously determined. Beam position is corrected by the determined magnitude. 
     Correction in the Y-direction is measured in a manner similar to that described above regarding the X-direction. Also, the data are similarly tabulated and used to perform correction. Correction in the Y-direction desirably is performed together with correction in the X-direction. (Beam drift in the Y-direction is normally less than beam drift in the X-direction.) 
     Measurements as shown in FIG. 1 desirably are performed on a periodic basis. Under such conditions, if the magnitudes of beam drift are not observed to converge around a constant value in five or six attempts, as described above (or if variances of over 2 nm occur in the data even as they approach a constant magnitude of beam drift at the end of the deflection field), then it can be concluded that a non-reproducible variable is causing the beam drift. Cleaning of the apparatus is required. 
     As is clear from the foregoing, this invention provides methods and apparatus for achieving high-accuracy and high-precision pattern transfer, even when a certain amount of beam drift is evident. 
     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 ionimplantation 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 superposedly formed on the wafer. 
     Whereas the invention has been described in connection with example embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.