Abstract:
Methods and apparatus are disclosed for performing charged-particle-beam (CPB) microlithography, in which methods and apparatus certain position-measurement marks are detected by appropriate deflections of a charged particle beam. The deflections are performed using a primary deflector and a mark-scanning deflector. For example, the beam is deflected by the primary deflector to illuminate a position-measurement mark on the reticle and a corresponding position-measurement mark on the substrate. The position-measurement mark on the substrate is scanned by minute deflections of the beam as performed by the mark-scanning deflector. Meanwhile, charged particles backscattered from the position-measurement mark on the substrate (as the mark is being scanned) are captured and detected by a detector. The marks are detected at timing moments during normal operation of the primary deflector. Thus, during detection of the marks, the resulting positional determinations are less affected by extraneous variables such as changes in temperature and/or hysteresis of the primary deflector.

Description:
FIELD  
         [0001]    This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam.  
           [0002]    Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography methods and apparatus in which certain alignment marks are detected so as to provide improved accuracy of lithographic exposure.  
         BACKGROUND  
         [0003]    Microlithographic pattern transfer using a charged particle beam is regarded as highly accurate and capable of achieving very fine pattern-transfer resolution. However, charged-particle-beam (CPB) microlithography disadvantageously has low “throughput” compared to optical microlithography (performed using deep UV light). (“Throughput” as used herein refers to the number of lithographic substrates, such as semiconductor wafers, that can be processed lithographically per unit time.) Various approaches have been investigated with the object of substantially improving throughput.  
           [0004]    For example, several types of partial-pattern single-shot exposure systems (termed “cell projection,” “character projection,” and “block exposure” systems) have been devised. In each of the partial-pattern single-shot systems, certain circuit sub-patterns that are highly repeated in the layer being formed are repetitively transferred and exposed using an aperture mask on which one or more of the basic sub-patterns have been defined. An example of such a highly repeated sub-pattern is a memory cell dimensions of approximately 5-μm square on the lithographic substrate (“sensitive substrate”). Unfortunately, with any of these techniques, variable-shaped-beam tracing is required to form on the substrate those portions of the pattern that are relatively non-repetitive. Consequently, overall throughput is too low for practical application for mass-production of wafers.  
           [0005]    An attractive solution to the problem of substantially improving the throughput of CPB microlithography is the so-called “one-shot” pattern-transfer approach, in which the entire pattern for a layer in a single die (“chip”) or even for multiple dies is exposed in one “shot.” Unfortunately, this approach has not been realized from a practical standpoint for two main reasons. The first reason is that reticles suitable for exposing an entire die pattern in one shot are currently impossible to fabricate. The second reason is that CPB optical systems having optical fields sufficiently large for exposing an entire die pattern without significant off-axis aberrations are currently impossible to fabricate. Consequently, whereas the excitement over the potential of this approach remains high, engineering development work has been directed to other, more feasible, approaches.  
           [0006]    One approach receiving much current attention involves dividing a reticle, defining a die pattern, into multiple portions usually termed “subfields.” Each subfield defines a respective portion of the overall pattern. The subfields are arrayed on the reticle in an ordered manner and are exposed in a sequential manner from the reticle to the substrate. This approach is termed the “divided reticle” method, and apparatus configured for performing this method are termed “divided reticle” projection-microlithography apparatus. By performing exposure subfield-by-subfield, the optical field can be sufficiently small to keep aberrations within specifications. Furthermore, any specific aberrations or other errors (e.g., distortion or errors in focus) that arise while exposing a particular subfield can be corrected, on the fly, in a manner that is most suitable for the particular subfield being exposed. The subfield images are placed contiguously on the substrate so as to form, after all the subfields are exposed, the complete die pattern on the substrate surface. Thus, overall exposure is performed with excellent resolution, accuracy, and precision across an optically much wider range than possible with the one-shot transfer method.  
           [0007]    So as to expose various subfields on the reticle and form the respective images at proper locations on the substrate, a deflector is provided in the CPB optical system of the microlithography apparatus. This deflector imparts appropriate lateral deflections of the beam to reach the selected subfields. Conventionally, the deflector is an electromagnetic deflector. Since the subfields (which normally are arranged in a highly ordered array) are exposed sequentially by laterally deflecting the beam as required, the deflector experiences a predetermined, repetitive, energization sequence during exposure of the subfields. But, this ordered scheme of energizing the deflector must be stopped temporarily to allow use of the deflector in detecting alignment marks on the substrate by beam irradiation.  
           [0008]    An electromagnetic deflector exhibits certain magnetic hysteresis characteristics that are best controlled when the deflector is being energized in a highly ordered manner. The reproducibility of its deflection characteristics is significantly lowered whenever the deflector is being energized in a non-ordered manner (e.g., for irradiating alignment marks). In addition, whenever the electromagnetic deflector is not being energized, the deflector temperature is reduced. Even a slight temperature reduction normally experienced after a shift from a highly ordered energization sequence (for exposing subfields) to a non-ordered energization (for exposing alignment marks) causes significant changes in the deflection characteristics exhibited by the deflector. These changes in deflection characteristics, in turn, cause corresponding errors in positional measurements performed using the beam, such as errors in detecting the positions of alignment marks relative to the axis of the optical system and/or the axes of the reticle stage and substrate stage. These errors generate subtle shifts in the subfield images as formed on the substrate, which decreases the accuracy of pattern transfer.  
         SUMMARY  
         [0009]    In view of the shortcomings of conventional methods and apparatus as summarized above, the invention provides, inter alia, charged-particle-beam (CPB) microlithography apparatus and methods that achieve more accurate detection of position-measurement marks than conventionally. I.e., detections of such marks is less influenced by temperature changes and hysteresis effects of deflection(s) used for performing such detection, which allows more accurate positional detections than currently achievable by current methods and apparatus.  
           [0010]    To such ends and according to a first aspect of the invention, methods are provided, in the context of a CPB microlithography method, for detecting a position of a position-measurement mark situated within a deflection field of a CPB optical system. The CPB microlithography method is performed using a CPB optical system that is configured to projection-transfer respective images of exposure units of a pattern, defined on a divided reticle, to a sensitive substrate. In an embodiment of the subject method a charged particle beam (e.g., an electron beam) is deflected within the deflection field, according to a predetermined exposure sequence. While the charged particle beam is being deflected within the deflection field in a sequential manner, the position-measurement mark is irradiated with the charged particle beam. The position of the mark is detected at a moment in which the charged particle beam deflected according to the exposure sequence encounters the position-measurement mark in the deflection field.  
           [0011]    The charged particle beam is deflected within the deflection field using a primary deflector in the CPB optical system. In this instance, the method can further comprise the step of scanning the charged particle beam over the position-measurement mark at the moment in which the charged particle beam encounters the position-measurement mark. Scanning of the charged particle beam over the position-measurement mark can be performed using a mark-scanning deflector separate from the primary deflector. After scanning the position-measurement mark, the mark-scanning deflector is de-activated, and sequential deflection is continued using the primary deflector.  
           [0012]    The method can further comprise the step of moving the reticle and substrate as required for placing the position-measurement mark in the deflection field. In this instance at least one of the reticle and substrate can be moved to place the position-measurement mark on an optical axis of the CPB optical system. The position of the position-measurement mark is measured while the position-measurement mark is on the optical axis.  
           [0013]    According to another aspect of the invention, methods are provided for detecting a relative position of the reticle and substrate, in the context of a CPB microlithography method. In an embodiment of such a methodrespective position-measurement marks of the reticle and substrate are situated, within a deflection field of the CPB optical system, on the reticle and substrate. Using a primary deflector of the CPB optical system, a charged particle beam is deflected within the deflection field, according to a predetermined exposure sequence. While the charged particle beam is being deflected by the primary deflector within the deflection field in a sequential manner, the position-measurement marks are irradiated with the charged particle beam. The relative positions of the marks are detected at a moment in which the charged particle beam deflected by the primary deflector according to the exposure sequence encounters the position-measurement marks in the deflection field.  
           [0014]    At the moment in which the charged particle beam encounters the position-measurement marks, a mark-scanning deflector in the CPB optical system can be actuated so as to scan the charged particle beam over the position-measurement mark on the substrate. After scanning the position-measurement marks, the mark-scanning deflector is de-actuated, and sequential deflection of the charged particle beam in the deflection field using the primary deflector is continued.  
           [0015]    Desirably, the reticle and substrate are mounted on respective stages, in which instance the reticle stage and substrate stage are moved as required to place the position-measurement marks in the deflection field. The reticle stage and substrate stage can be moved to place the position-measurement marks on an optical axis of the CPB optical system, in which instance the relative positions of the position-measurement marks of the reticle and substrate desirably are measured while the position-measurement marks are on the optical axis.  
           [0016]    In the methods summarized above, since mark detection is performed while the deflector is activated normally for exposure purposes, the temperature and hysteresis conditions of the deflector during mark detection can be substantially the same as during exposure. Thus, exposure accuracy and precision are improved. Desirably, the mark-scanning deflector has a relatively small deflection angle, and desirably only is operated at the moment, during normal deflection of the charged particle beam for exposure purposes, when the charged particle beam is to be used for mark detection. Also, the mark-scanning deflector exhibits low hysteresis.  
           [0017]    In any of the methods summarized above, for example, if a weak detection signal is obtained, the relative positions of the marks can be detected multiple times to obtain multiple mark-detection signals. The mark-detection signals can be combined to obtain a stronger cumulative signal.  
           [0018]    According to another aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises a reticle stage, a substrate stage, and a CPB optical system. The reticle stage is configured to hold and move the reticle to place the subfields for illumination according to a predetermined exposure sequence. The substrate stage is configured to hold and move the sensitive substrate for imprinting of images of the illuminated subfields at respective locations on the sensitive substrate. The CPB optical system is situated relative to the reticle stage and has a deflection field. The CPB optical system is configured to direct a charged-particle illumination beam so as to illuminate a subfield on the reticle and to project a resulting charged-particle patterned beam, propagating downstream of the reticle, to form an image of the illuminated subfield on the sensitive substrate. The CPB optical system comprises a primary deflector and a mark-scanning deflector. The primary deflector is configured: (a) to direct the illumination beam so as to illuminate the subfields on the reticle sequentially within a deflection field, (b) to direct, as the subfields on the reticle are being illuminated, the resulting patterned beam so as to form images of the illuminated subfields at respective locations on the sensitive substrate within a corresponding deflection field, and (c) to direct the illumination and patterned beams as required to illuminate the respective position-measurement marks on the reticle and substrate. The mark-scanning deflector is configured to scan the patterned beam, which has passed through the respective position-measurement mark on the reticle, over the position-measurement mark on the sensitive substrate.  
           [0019]    The CPB optical system desirably comprises an illumination-optical system situated upstream of the reticle and a projection-optical system situated between the reticle and the substrate. Each of the illumination-optical system and projection-optical system comprises a respective primary deflector. Desirably, each of the primary deflectors is an electromagnetic deflector, and the mark-scanning deflector is an electrostatic deflector or small hollow-core electromagnetic deflector.  
           [0020]    The illumination-optical system and projection-optical system have respective deflection fields. The primary deflector of the illumination-optical system is configured to deflect the illumination beam within the deflection field of the illumination-optical system, and the primary deflector of the projection-optical system is configured to deflect the patterned beam within the deflection field of the projection-optical system. The mark-scanning deflector desirably is located in the projection-optical system, wherein the mark-scanning deflector is configured to scan the patterned beam, as the illumination beam is illuminating the position-measurement mark on the reticle, over the respective position-measurement mark on the substrate.  
           [0021]    The length of the deflection field of the illumination-optical system corresponds to a length of an electrical stripe of subfields arranged on the reticle. Similarly, the length of the deflection field of the projection-optical system corresponds to a length of an electrical stripe of subfields as projected onto the sensitive substrate.  
           [0022]    Typically, the CPB optical system has an optical axis. In such a configuration, the reticle stage can be configured to place the position-measurement mark, on the reticle, on the optical axis as the illumination beam illuminates the position-measurement mark. Similarly, the substrate stage can be configured to place the position-measurement mark, on the substrate, on the optical axis as the patterned beam from the illuminated position-measurement mark on the reticle impinges on the position-measurement mark on the substrate. The mark-scanning deflector can be configured to scan the patterned beam, as the illumination beam is illuminating the position-measurement mark on the reticle, over the respective position-measurement mark on the substrate.  
           [0023]    The apparatus can further comprise a detector situated and configured to detect backscattered charged particles from the position-measurement mark on the sensitive substrate.  
           [0024]    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  
       [0025]    [0025]FIG. 1 is an elevational schematic diagram of imaging relationships and control systems of a representative embodiment of a CPB microlithography apparatus employing a segmented reticle, position-measurement marks, primary deflector(s), and mark-scanning deflector as described herein.  
         [0026]    [0026]FIG. 2 is a plan view showing general aspects of a segmented reticle as used, for example, in the apparatus of FIG. 1.  
         [0027]    [0027]FIG. 3 is a schematic oblique diagram showing certain aspects of pattern transfer from a reticle to a substrate using, for example, the apparatus of FIG. 1.  
         [0028]    [0028]FIG. 4 is a flowchart of steps in a process for manufacturing a microelectronic device such as a semiconductor chip (e.g., integrated circuit or LSI), liquid-crystal panel, CCD, thin-film magnetic head, or micromachine, the process including performing microlithography using a microlithography apparatus as described herein. 
     
    
     DETAILED DESCRIPTION  
       [0029]    It will be understood that representative embodiments of the invention as described below are not intended to be limiting in any way. The embodiments are described in the context of using an electron beam as an exemplary charged particle beam. However, it will be understood that the principles as described herein are applicable with equal facility to use of another type of charged particle beam as a lithographic energy beam, such as an ion beam.  
         [0030]    First, a general description of an electron-beam projection-exposure (microlithography) apparatus and method, employing a divided reticle, is provided below, referring to FIG. 1. FIG. 1 also depicts general imaging and control relationships of the subject system.  
         [0031]    Situated at the extreme upstream end of the system is an electron gun  1  that emits an electron beam propagating in a downstream direction generally along an optical axis Ax. Downstream of the electron gun  1  are a first condenser lens  2  and a second condenser lens  3  collectively constituting a two-stage condenser-lens assembly. The condenser lenses  2 ,  3  converge the electron beam at a crossover C.O. situated on the optical axis Ax at a blanking diaphragm  7 .  
         [0032]    Downstream of the second condenser lens  3  is a “beam-shaping diaphragm”  4  comprising a plate defining an axial aperture (typically rectangular or square in profile) that trims and shapes the electron beam passing through the aperture. The aperture is sized and configured to trim the electron beam sufficiently to illuminate one exposure unit (e.g., subfield) on the reticle  10 . An image of the beam-shaping diaphragm  4  is formed on the reticle  10  by an illumination lens  9 .  
         [0033]    The electron-optical components situated between the electron gun  1  and the reticle  10  collectively constitute an “illumination-optical system” of the depicted microlithography system. The electron beam propagating through the illumination-optical system is termed an “illumination beam” because it illuminates a desired region (exposure unit) of the reticle  10 . As the illumination beam propagates through the illumination-optical system, the beam actually travels in a downstream direction through an axially aligned “beam tube” (not shown but well understood in the art) that can be evacuated to a desired vacuum level.  
         [0034]    A blanking deflector  5  is situated downstream of the beam-shaping aperture  4 . The blanking deflector  5  laterally deflects the illumination beam as required to cause the illumination beam to strike the aperture plate of the blanking diaphragm  7 , thereby preventing the illumination beam from being incident on the reticle  10  during blanking.  
         [0035]    A subfield-selection deflector  8  is situated downstream of the blanking diaphragm  7 . The subfield-selection deflector  8  is a “primary deflector” of the illumination-optical system, and is used for laterally deflecting the illumination beam as required to illuminate a desired reticle subfield (or other exposure unit) situated within the optical field of the illumination-optical system. Thus, as described further later below, subfields of the reticle  10  are scanned sequentially by the illumination beam in a horizontal direction (X-direction in the figure). The illumination lens  9  is situated downstream of the subfield-selection deflector  8 .  
         [0036]    The reticle  10  extends in a plane (X-Y plane) that is perpendicular to the optical axis Ax. The reticle  10  typically defines many exposure units (e.g., thousands of subfields). The exposure units collectively define the pattern for a layer to be formed at a single die (“chip”) on a lithographic substrate, wherein each exposure unit defines a respective portion of the pattern. (The exposure units collectively defining an entire pattern can be located on a single reticle or divided between multiple reticles.) One or more position-measurement marks (discussed below in connection with FIG. 2) are defined on the surface of the reticle  10 . For example, one X-mark and one Y-mark are disposed at the top of each stripe, and each mark area is of the same size as a subfield.  
         [0037]    The reticle  10  is mounted on a movable reticle stage  11 . Using the reticle stage  11 , by moving the reticle  10  in a direction (Y-direction and/or X-direction) perpendicular to the optical axis Ax, it is possible to illuminate the respective exposure units on the reticle  10  extending over a range that is wider than the optical field of the illumination-optical system. The position of the reticle stage  11  in the X-Y plane is determined using a “position detector”  12  typically configured as a laser interferometer. A laser interferometer is capable of measuring the position of the reticle stage  11  with extremely high accuracy in real time.  
         [0038]    Situated downstream of the reticle  10  but upstream of a substrate  23  is the “projection-optical system” portion of the electron-optical system. The projection-optical system comprises first and second projection lenses  15 ,  19 , respectively, an imaging-position deflector  16 , a mark-scanning deflector  17 , and a contrast-aperture diaphragm  18 . The illumination beam, by passage through an illuminated exposure unit of the reticle  10 , becomes a “patterned beam” because the beam carries an aerial image of the illuminated exposure unit. The patterned beam is imaged at a specified location on the substrate  23  (e.g., “wafer”) by the projection lenses  15 ,  19  collectively functioning as a “projection-lens assembly,” as described further below with reference to FIG. 3.  
         [0039]    To ensure imaging at the proper location, the imaging-position deflector  16  imparts the required lateral deflection of the patterned beam. The imaging-position deflector  16  is a “primary deflector” of the projection-optical system, and can deflect the patterned beam laterally over a relatively wide angular range (relative to the axis Ax). The imaging-position deflector  16  typically is configured as an electromagnetic deflector.  
         [0040]    In the depicted embodiment, the mark-scanning deflector  17  typically is smaller than the imaging-position deflector  16  and desirably is disposed at the same axial position as the contrast-aperture diaphragm  18 . The mark-scanning deflector  17  desirably is configured as an electrostatic deflector (exhibiting a very rapid response) that deflects the patterned beam over a relatively small lateral deflection angle (relative to the axis Ax and compared to the deflector  16 ). For example, the mark-scanning deflector  17  can be configured as a hollow-core deflector having a small number of coil-winding turns. Desirably, the mark-scanning deflector  17  exhibits extremely low hysteresis. By way of example, the mark-scanning deflector  17  deflects the patterned beam within the range of about ±10 μm on the substrate  23 . The mark on the reticle is projected and scanned by the mark-scanning deflector  17 .  
         [0041]    So as to be imprintable with the image carried by the patterned beam, the upstream-facing surface of the substrate  23  is coated with a suitable “resist” that is imprintably sensitive to exposure by the patterned beam. When forming the image on the substrate  23 , the projection-lens assembly “reduces” (demagnifies) the aerial image. Thus, the image as formed on the substrate  23  is smaller (usually by a defined integer-ratio factor termed the “demagnification factor”) than the corresponding region illuminated on the reticle  10 . By thus causing imprinting on the surface of the substrate  23 , the apparatus of FIG. 2 achieves “transfer” of the pattern image from the reticle  10  to the substrate  23 .  
         [0042]    The substrate  23  is situated on a substrate stage  24  situated downstream of the projection-optical system. As the patterned beam propagates through the projection-optical system, the beam actually travels in a downstream direction through an axially aligned “beam tube” (not shown but well understood in the art) that can be evacuated to a desired vacuum level.  
         [0043]    The projection-optical system forms a crossover C.O. of the patterned beam on the optical axis Ax at the rear focal plane of the first projection lens  15 . The position of the crossover C.O. on the optical axis Ax is a point at which the axial distance between the reticle  10  and substrate  23  is divided according to the demagnification ratio. Situated at the crossover C.O. (i.e., the rear focal plane) is the contrast-aperture diaphragm  18 . The contrast-aperture diaphragm  18  comprises an aperture plate that defines an aperture centered on the axis Ax. With the contrast-aperture diaphragm  18 , electrons of the patterned beam that were scattered during transmission through the reticle  10  are blocked so as not to reach the substrate  23 .  
         [0044]    The substrate  23  is mounted to the substrate stage  24  via a wafer chuck (not shown but well understood in the art), which presents the upstream-facing surface of the substrate  23  in an X-Y plane. The wafer chuck typically is an electrostatic chuck. The substrate stage  24  (with chuck and substrate  23 ) is movable in the X and Y directions. Thus, by simultaneously scanning the reticle stage  11  and the substrate stage  24  in mutually opposite directions in a synchronous manner, it is possible to transfer each exposure unit within the optical field of the illumination-optical system as well as exposure units located outside the optical field to corresponding regions on the substrate  23 . The substrate stage  24  also includes a “position detector”  25  configured similarly to the position detector  12  of the reticle stage  11 .  
         [0045]    At least one position-measurement mark (discussed below with reference to FIG. 2) is situated at the plane of the upstream-facing surface of the substrate  23 , desirably on the upstream-facing surface of the substrate. For example, a position-measurement mark  62  can be located on a scribe line on the substrate  23 . The position-measurement mark can be formed by etching mark-element grooves in the surface of the substrate or by forming a patterned layer of a heavy metal (e.g., Ta, W, or the like) that is highly reflective to electrons incident on the substrate  23 .  
         [0046]    A backscattered-electron (BSE) detector  22  is situated immediately upstream of the substrate  23 . The BSE detector  22  is configured to detect and quantify electrons backscattered from the position-measurement mark at the substrate  23 . The position-measurement mark on the substrate  23  desirably is scanned by a beam that has passed through the corresponding position-measurement mark  61  on the reticle  10 . By detecting backscattered electrons from the position-measurement mark  62  at the substrate  23 , it is possible to determine the relative positional relationship of the reticle  10  and the substrate  23 .  
         [0047]    Each of the lenses  2 ,  3 ,  9 ,  15 ,  19  and deflectors  5 ,  8 ,  16 ,  17  is controlled by a controller  31  via a respective coil-power controller  2   a ,  3   a ,  9   a ,  15   a ,  19   a  and  5   a ,  8   a ,  16   a ,  17   a . Similarly, the reticle stage  11  and substrate stage  24  are controlled by the controller  31  via respective stage drivers  11  a,  24   a . The position detectors  12 ,  25  produce and route respective stage-position signals to the controller  31  via respective interfaces  12   a ,  25   a  and other circuitry for achieving such ends. In addition, the BSE detector  22  produces and routes signals to the controller  31  via a respective interface  22   a.    
         [0048]    From the respective data routed to the controller  31 , as an exposure unit is being transferred the controller  31  ascertains, inter alia, any control errors of the respective stage positions. To correct such control errors, the imaging-position deflector  16  is energized appropriately to deflect the patterned beam. Thus, a reduced image of the illuminated exposure unit on the reticle  10  is transferred accurately to the desired target position on the substrate  23 . This real-time correction is made as each respective exposure-unit image is being transferred to the substrate  23 , and the exposure-unit images are positioned such that they are stitched together properly on the substrate  23 .  
         [0049]    [0049]FIG. 2 shows in plan view the general configuration of an exemplary “segmented” or “divided” reticle  10  comprising many subfields as representative exposure units. This reticle can be fabricated by, e.g., performing electron-beam drawing of the pattern on a surface of a silicon wafer, followed by etching of the patterned surface. In the figure, multiple regions  49  are shown each including a large respective array of subfields  42 . The regions  49  generally are termed “mechanical stripes,” and each mechanical stripe  49  extends lengthwise in the Y-direction and widthwise in the X-direction in the figure. The mechanical stripe  49  is so named because exposure of it in the Y-direction is achieved by corresponding mechanical motion of the reticle stage  11 . Each mechanical stripe  49  includes multiple rows  44  of subfields  42 , wherein each row  44  extends lengthwise in the X-direction in the figure. Each row  44  is termed an “electrical stripe” because the row represents the area swept by the illumination beam as the beam is being deflected laterally by the deflector  8 . In other words, the length of each row  44  (equal to the width of each mechanical stripe  49 ) corresponds to the maximum achievable deflection range achievable by the deflector  8  within the optical field of the illumination-optical system. This deflection range is termed the “deflection field.” 
         [0050]    Each subfield  42  comprises a respective portion of the reticle membrane that defines a respective portion of the pattern. The membrane portion of a subfield is surrounded by a non-patterned region (not shown) termed a “skirt.” Depending somewhat upon the particular type of reticle (scattering-membrane reticle or scattering-stencil reticle), the membrane has a thickness of, e.g., 0.1 μm to several μm. In the depicted configuration, the subfields  42  in each electrical stripe  44  and the electrical stripes  44  in each mechanical stripe  49  are separated from each other by respective “minor struts”  45 . The mechanical stripes  49  are separated from each other by major struts  47 . The major struts  47  and minor struts  45  collectively constitute a support structure termed “grillage.” 
         [0051]    The upstream-facing surface of the reticle embodiment shown in FIG. 2 also defines position-measurement marks  61 . For example, in the depicted embodiment, position-measurement marks  61  are situated adjacent a lower-left corner of the first mechanical stripe  49 .  
         [0052]    In an electron-beam microlithography system such as the system discussed above with reference to FIG. 1, the patterned beam is deflected by the imaging-position deflector  16  to expose the subfields  42  in a row (i.e., in an electrical stripe  44 ) of a mechanical stripe  49  in a sequential manner. Meanwhile, the stages  11 ,  24  are moved continuously in a scanning manner so as to expose the rows  44  in the Y-direction within the mechanical stripe  49  in a sequential manner.  
         [0053]    General aspects of the exposure events summarized above are shown in FIG. 3. Shown at the upper portion of the figure is an end of one mechanical stripe  49  on the reticle  10 , wherein the end is shown containing three electrical stripes  44 . As described above, each electrical stripe  44  contains multiple constituent subfields  42  separated from each other by intervening minor struts  45 . Minor struts  45  also separate individual electrical stripes  44  from each other in the mechanical stripe  49 . Downstream of the reticle  10  along the optical axis OA is a corresponding region of the substrate  23  situated in opposition to the reticle. In the figure, the subfield  42 - 1  in the left corner of the electrical stripe  44  nearest the viewer is being illuminated by the illumination beam IB propagating from upstream. The necessary lateral deflection of the illumination beam IB is imparted by the subfield-selection deflector  8  in the illumination-optical system. The resulting patterned beam PB downstream of the illuminated subfield  42 - 1  is being exposed (by the projection-optical system, not shown) onto a corresponding region  52 - 1  on the surface of the substrate  23 . As can be seen, the image in the region  52 - 1  is reduced (demagnified) relative to the corresponding subfield  42 - 1 . The position of the region on the substrate surface at which the image is formed is achieved by appropriate actuation of the imaging-position deflector  16  (FIG. 1).  
         [0054]    Between the reticle  10  and the substrate  23 , the patterned beam PB is deflected twice by the collective action of the projection lenses  15 ,  19 . The first deflection (performed by the first projection lens  15 ) is from a direction parallel to the optical axis OA to a direction in which the patterned beam intersects the optical axis OA. The second deflection (performed by the second projection lens  19 ) is in a direction opposite the first deflection.  
         [0055]    The exact position  52  on the surface of the substrate  23  at which a transferred subfield image is formed is established such that the respective subfield images are contiguous with one another with no intervening spaces or the like. The respective positions  52  of the subfield images relative to each other typically correspond to the respective positions of the corresponding subfields  42  on the reticle  10 . As noted above, this alignment of individual subfield images on the substrate  23  is achieved by appropriate deflections of the patterned beam by the imaging-position deflector  16  (FIG. 1). The subfield images as formed on the substrate  23  lack intervening grillage and skirts. To eliminate the grillage and skirts, the respective transfer position of each subfield image  52  is shifted appropriately, due to actuation of the imaging-position deflector  16 , by an amount that corresponds to the combined widths of the skirt and minor strut  45  associated with the respective subfield  42  on the reticle. Since the subfields  42  on the reticle  10  are arranged in a regular array (which tends to be the same from one reticle to the next), actuation of the imaging-position deflector  16  tends to be of a pre-determined, repetitive, and cyclical nature during exposure.  
         [0056]    This positional adjustment of subfield images on the substrate  23  must be performed in both the X-direction and Y-direction, and is achieved by appropriate deflections of the patterned beam PB as mediated by the imaging-position deflector  16 . To such end, the deflector  16  actually includes separate respective deflectors for each of the X-direction and Y-direction.  
         [0057]    Before exposure of the mechanical stripe  49 , the reticle stage  11  is moved as required to place the position-measurement marks  61   X  (on the reticle  10 ) on the optical axis OA. Similarly, the substrate stage  24  also is moved to place a corresponding position-measurement mark  62   X  (on the surface of the substrate  23 ) on the optical axis OA. As the illumination beam illuminates the position-measurement mark  61   X , portions of the beam passing through the mark  61   X  impinge on the mark  62   X . Thus, the patterned beam from the position-measurement mark  61   X  is scanned over the position-measurement mark  62   X  on the substrate during the normal use of the deflectors  8 ,  16 . Also, whenever the beams IB, PB are on the optical axis in the course of being continuously deflected for performing exposure, the mark-scanning deflector  17  (FIG. 1) is actuated as required to deflect and scan the patterned beam PB (produced by passage of the illumination beam IB through the position-measurement mark  61   X  on the reticle) over the position-measurement mark  62   X  on the substrate. The BSE detector  22  (FIG. 1) detects charged particles backscattered from the mark  62   X . The same procedure is applied for the Y-marks  61   Y ,  62   Y . Thus, the relative positional relationship of the reticle  10  and substrate  23  is determined.  
         [0058]    During irradiation of the marks  61   X ,  62   X  the mark-scanning deflector  17  typically is actuated for only a short time (e.g., 200 μs). As a result of such short irradiation, situations may arise in which an adequate signal level cannot be obtained. In such an instance, a stronger signal can be obtained by repeating the irradiation of the position-measurement marks  61   X ,  62   X  while actuating the mark-scanning deflector  17 , and accumulating the signals.  
         [0059]    In the mark-scanning protocol described above, charged particles transmitted by the mark  61   X  and backscattered from the alignment mark  62   X  are detected during the course of actuating the primary deflectors  8 ,  16 . During the instances in which the position-measurement marks  61   X ,  62   X  are being irradiated, the mark-scanning deflector  17  is being actuated to scan the mark  62   X . Thus, mark detection is performed using the low-hysteresis mark-scanning deflector  17  without having to stop exposure-actuation of the imaging-position deflectors  8 ,  16 . This manner of mark detection allows position detection to be performed more accurately under deflection-actuation conditions that are closer to the actual exposure conditions.  
         [0060]    [0060]FIG. 4 is a flow chart of steps in a process for manufacturing a microelectronic device such as a semiconductor chip (e.g., an integrated circuit or LSI device), a display panel (e.g., liquid-crystal panel), charged-coupled device (CCD), thin-film magnetic head, micromachine, for example. In step S 1 , the circuit for the device is designed. In step S 2  a reticle (“mask”) for the circuit is manufactured. In step S 2 , local resizing of pattern elements can be performed to correct for proximity effects or space-charge effects during exposure. In step S 3 , a wafer (substrate) is manufactured from a material such as silicon.  
         [0061]    Steps S 4 -S 13  are directed to wafer-processing steps (described below), in which the circuit pattern defined on the reticle is transferred onto the wafer by microlithography. Steps S 14 -S 16  are “post-process” steps. Specifically, step S 14  is an assembly step in which the wafer that has been passed through steps S 4 -S 13  is formed into semiconductor chips. This step can include, e.g., assembling the devices (dicing and bonding) and packaging (encapsulation of individual chips). Step S 15  is an inspection step in which any of various operability, qualification, and durability tests of the device produced in step S 14  are conducted. Afterward, devices that successfully pass step S 115  are finished, packaged, and shipped (step S 16 ).  
         [0062]    Steps S 4 -S 13  provide representative details of wafer processing. Step S 4  is an oxidation step for oxidizing the surface of a wafer. Step S 5  involves chemical vapor deposition (CVD) for forming an insulating film on the wafer surface. Step S 6  is an electrode-forming step for forming electrodes on the wafer (typically by vapor deposition). Step S 7  is an ion-implantation step for implanting ions (e.g., dopant ions) into the wafer. Step S 8  involves application of a resist (exposure-sensitive material) to the wafer. Step S 9  involves microlithographically exposing the resist using a charged particle beam so as to imprint the resist with the reticle pattern of the reticle produced in step S 2 . In step S 9 , a CPB microlithography apparatus as described above can be used. Step S 10  involves microlithographically exposing the resist using optical microlithography. This step also can be performed using a reticle as produced in step S 2 . Also, during Step S 10 , an exposure can be performed (before, during, or after the pattern exposure) in a manner serving to correct proximity effects (e.g., normalizing backscattered electrons of the patterned beam). Step S 11  involves developing the exposed resist on the wafer. Step S 12  involves etching the wafer to remove material from areas where developed resist is absent. Step S 13  involves resist separation (resist “stripping”), in which remaining resist on the wafer is removed after the etching step. By repeating steps S 4 -S 13  as required, circuit patterns as defined by successive reticles are formed superposedly on the wafer.  
         [0063]    In the foregoing description, mark detections are performed in the course of continuous “primary” deflection of the beam for exposure. Mark scanning (and hence detection) desirably is performed at an instant in which the beam, as a result of primary deflection, is on the optical axis. Alternatively, mark detection during primary deflection of the beam need not be on-axis; rather, mark detection can be performed at any of various other specified locations relative to the axis.  
         [0064]    Whereas the invention has been described in connection with representative embodiments, it will be understood 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.