Abstract:
Due to its lack of appreciable thickness, the reticle used in charged-particle-beam (CPB) microlithography is prone to bending and flexing, causing instability in reticle axial height position relative to the projection-lens system, with consequent errors in image focus, rotation and magnification. Apparatus and methods are disclosed for monitoring changes in axial height position of the reticle, to facilitate making compensatory changes. Representative apparatus include a device for detecting the axial height position of the reticle. The device produces one or more beams of light (IR to visible) to strike the reticle at an oblique angle of incidence, detects light reflected from the reticle surface, and detects lateral shifts of the reflected light as received by a height detector. Hence, reticle focus is detected easily and in real time. Multiple detection beams can be used, thereby allowing detection of both axial height position and inclination of the reticle with high accuracy. Reticle-position data can be used to regulate one or more parameters of exposure and/or axial position of the reticle or wafer.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/694,089, filed on Oct. 19, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, to a suitable substrate using an energy beam). Microlithography is a key technology used in the manufacture of microelectronic devices (e.g., semiconductor integrated circuits), displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to detecting and adjusting the axial height position of the reticle (“reticle focus”) relative to a projection-lens system used to project an image of an illuminated region of the reticle onto the substrate.  
         BACKGROUND OF THE INVENTION  
         [0003]    Several techniques currently are used to perform charged-particle-beam (CPB) microlithography. One conventional technique is the so-called cell projection or character projection, in which a portion of a pattern that is repeated many times in the pattern is defined on a reticle. The reticle includes an arrangement of beam-transmissive regions and beam-blocking regions that, as an illumination beam passes through the reticle, forms a “patterned beam” or “imaging beam.” An example is a reticle defining a highly repeated portion of an overall pattern for a memory chip. To expose a single die on a wafer or other substrate, the reticle is exposed many times, each time at a different location in the die so as to re-form the entire pattern contiguously on the die. Unique portions of the die pattern (i.e., portions that are not composed of repetitive pattern-portion units and that typically are located mainly at the periphery of the die) can be exposed using a variable-shaped beam, wherein a charged particle beam of a desired size and shape is obtained by selectively blocking portions of the beam from propagating to the substrate. These techniques are described, for example, in Rai-Choudhury (ed.),  Handbook of Microlithography, Micromachining, and Microfabrication,  Vol. 1, SPIE Press, 1997, p. 184, § 2.5.6).  
           [0004]    In the cell projection technique summarized above, each of the highly repeated portions exposed per single “shot” of the beam typically has an area of approximately (5 μm) square. Hence, hundreds to thousands of shots are required to expose a single die, which adversely affects throughput greatly. As the size and density of microelectronic devices has continued to increase, throughput tends to decrease progressively.  
           [0005]    Accordingly, considerable interest lies in developing CPB microlithography methods and apparatus that can achieve higher throughput. One possible technique is to expose the entire die pattern from a reticle in a single shot. Unfortunately, this technique requires enormous CPB optical systems that are extremely difficult and expensive to manufacture, exhibit excessive aberrations (especially off-axis), and are extremely difficult to provide with a reticle (CPB reticles of the required size are extremely difficult or impossible to fabricate using conventional methods). Consequently, development has progressed toward development of systems that do not expose the entire reticle pattern in one shot, but rather expose sequential regions of the pattern in a stepping or scanning manner.  
           [0006]    Typically, in these methods, a highly accelerated charged particle beam is used to improve resolution and reduce space-charge effects. Unfortunately, highly accelerated charged particle beams exhibit problems such as excessive reticle heating by absorbed particles of the beam. Such heating causes reticle deformation, which causes deformations of the pattern being transferred to the substrate.  
           [0007]    To alleviate this problem, a scattering-contrast technique is used in which no actual charged-particle absorption occurs in the reticle. Rather, a scattering aperture is used, wherein the degree of charged-particle blocking by the scattering aperture varies with differences in the scattering angle of the particles, thereby generating contrast. Suitable reticles include scattering-stencil reticles (in which a pattern is defined by a corresponding pattern of apertures in a particle-scattering membrane), and scattering-membrane reticles (in which a pattern is defined by a corresponding pattern of particle-scattering bodies arranged on a particle-transmissive membrane). In any event, substantially all reticles used for CPB microlithography are reinforced structurally by “struts” extending between subfields or other exposure units of the reticle.  
           [0008]    Unfortunately, whenever CPB microlithographic pattern transfer is performed using methods as described above, problems of pattern-image defocus (blur), magnification deviations, and image rotation tend to occur at levels exceeding specifications. The respective magnitudes of these problems vary in repeated exposure experiments using the same reticle. As a result, yields of microelectronic devices drop to unacceptable levels and manufacturing costs are increased.  
           [0009]    One proposed method for achieving accurate correction of positional relationships between the reticle and the projection-optical system is disclosed in U.S. Pat. No. 5,796,467. According to that patent, multiple exposures are performed using a scanning type CPB microlithography apparatus. During the scanning exposures, the reticle and wafer are moved in mutually opposite directions. The optimal image plane variation obtained from the exposures is stored in a memory as a variation of the positional relationship between the reticle and the projection-optical system. An actual exposure is performed while making a correction according to the coordinates in the scanning direction. Unfortunately, results obtained using that method were not entirely satisfactory.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide charged-particle-beam (CPB) microlithography apparatus and methods that achieve detection of the axial height position of the reticle in a manner resulting in reduced defocus (blur) of the pattern image.  
           [0011]    To such end, and according to a first aspect of the invention, CPB microlithography apparatus are provided, of which a representative embodiment comprises an illumination-optical system, a projection-lens system, and a reticle-focus-detection device (i.e., a device for detecting the axial height position of the reticle). The illumination-optical system is situated and configured to illuminate a region of a pattern-defining reticle with a charged-particle illumination beam passing through the illumination-optical system. The projection-optical system is situated and configured to projection-transfer an image of the illuminated region of the reticle onto a corresponding region of a sensitive substrate using an imaging beam passing through the projection-optical system. The reticle-focus-detection device is situated and configured to detect an axial height position of the reticle relative to the projection-lens system. The reticle-focus-detection device can be used to detect an axial height position of a stencil reticle or a scattering-membrane reticle relative to the projection-lens system.  
           [0012]    Compared to a conventional apparatus with which exposure is performed after determining a correction of reticle position relative to the projection-lens system, an apparatus according to the invention as summarized above can provide real-time data on reticle axial height position relative to the projection-lens system. Hence, higher-accuracy projection exposure of the reticle pattern onto the substrate can be performed with high precision.  
           [0013]    The reticle-focus-detection device comprises a focus-detection-beam source situated and configured to produce a focus-detection light beam (desirably IR to visible) and to direct the focus-detection beam onto a surface of the reticle such that the focus-detection beam is incident on the reticle at an oblique angle of incidence (i.e., an incidence angle other than 0°). The device also includes a height detector situated and configured to detect light, of the focus-detection beam, reflected from the reticle surface and to produce a corresponding focus-detection (height-detection) signal. In this context, the reticle can be of a type including a reticle membrane and support struts extending from non-pattern-defining regions of the reticle membrane. With such a reticle, the focus-detection-beam source can be configured to produce multiple focus-detection light beamlets directed at the reticle surface in a manner in which the focus-detection light beamlets are incident on the non-pattern-defining regions of the reticle membrane.  
           [0014]    The reticle-focus-beam source can be configured to direct the focus-detection beamlets to the reticle, and the height detector can be configured to produce the focus-detection signal, only whenever the non-pattern-defining regions of the reticle membrane are being illuminated by the focus-detection beamlets. In this manner, by obtaining a focus-detection signal in synchrony with irradiation of non-pattern-defining regions of the reticle (e.g., membrane regions at which the support struts are located), an accurate height-detection (focus-detection) signal is obtained without interference generated by light reflected from apertures in the membrane.  
           [0015]    The height detector desirably comprises a light-receiving surface including a light sensor. In such an instance, the light sensor can be configured to measure a lateral displacement of the focus-detection light beam on the light-receiving surface. For example, the light sensor can be a one-dimensional light-sensor array, a two-dimensional light-sensor array, or a point-sensitive detector (PSD), wherein a plurality of these sensors is arranged on the light-receiving surface.  
           [0016]    For exposure, the reticle desirably is mounted to a reticle stage and the substrate desirably is mounted to a substrate stage. The reticle stage and substrate stage usually are movable in opposite directions during exposure of the reticle pattern onto the substrate. With such a configuration, the focus-detection-beam source can be configured to produce multiple focus-detection light beamlets directed at the reticle surface. Use of multiple beamlets allows measurements to be made simultaneously at multiple locations on the reticle. This allows detection not only of axial height position of the reticle but also of inclination of the reticle relative to an optical axis of the projection-lens system. The beamlets can be incident on the reticle from an incidence direction that is perpendicular to a scanning direction of the reticle stage.  
           [0017]    According to another aspect of the invention, methods are provided for performing projection-transfer of a pattern, defined on a reticle, to a sensitive substrate using a charged particle beam. A region of the reticle is illuminated with a charged-particle illumination beam to produce an imaging beam, and the imaging beam is directed to the substrate. The illumination beam and imaging beam pass through a CPB optical system. To detect a focus condition (axial height condition) of the reticle, a focus-detection beam of light is provided, directed at an oblique angle of incidence to a surface of the reticle to produce a reflected beam. The reflected beam is detected using a height detector configured to produce a corresponding height-detection signal from the detected light. The height-detection signal is processed to produce data concerning an axial height position of the reticle relative to the CPB optical system. The reticle typically comprises a reticle membrane and support struts extending from non-pattern-defining regions of the reticle membrane. In such an instance, multiple focus-detection beamlets can be directed at the reticle surface in a manner in which the beamlets are incident on the non-pattern-defining regions of the reticle membrane.  
           [0018]    Another embodiment of a CPB microlithography apparatus according to the invention comprises an illumination system, a projection system, and a reticle-focus-detection device. The illumination system is situated and configured to illuminate a region of a pattern-defining reticle with a charged-particle illumination beam passing through the illumination system. The projection system is situated and configured to projection-transfer an image of the illuminated region of the reticle onto a corresponding region of a sensitive substrate using an imaging beam passing through the projection system. The reticle-focus-detection device is situated and configured to detect an axial height position of the reticle relative to the projection system. The reticle-focus-detection device comprises a focus-detection-beam source and a height detector. The focus-detection-beam source is situated and configured to produce a focus-detection light beam and to direct the focus-detection beam onto a surface of the reticle such that the focus-detection beam is incident on the reticle at an oblique angle of incidence. The height detector is situated and configured to detect light, of the focus-detection beam, reflected from the reticle surface and to produce a corresponding focus-detection signal. The height detector comprises a light-receiving surface and is configured to measure a lateral displacement of the focus-detection light beam on the light-receiving surface.  
           [0019]    The height detector can comprise a light sensor selected from the group consisting of one-dimensional light-sensor arrays, two-dimensional light-sensor arrays, and point-sensitive detectors.  
           [0020]    The focus-detection-beam source can be configured to produce, from the focus-detection light beam, multiple focus-detection beamlets, and to direct the focus-detection beamlets onto respective height-detection loci on the surface of the reticle. In this instance, the reticle can comprise support struts having respective edge surfaces, wherein the height-detection loci are located on the edge surfaces of the support struts. The loci can be spaced from each other at an equal locus-spacing interval in a direction perpendicular to a reticle-scanning direction. Alternatively or in addition, the support struts can be spaced from each other at a strut-spacing interval in the reticle-scanning direction, in which instance the locus-spacing interval can be an integral multiple of the strut-spacing interval.  
           [0021]    According to another aspect of the invention, reticle-focus-detection devices are provided in the context of CPB microlithography apparatus. The CPB microlithography apparatus typically includes an illumination system and a projection system as summarized above. The reticle-focus-detection device is operable to detect an axial height position of the reticle relative to the projection system. An embodiment of the reticle-focus-detection device comprises a focus-detection-beam source and a height detector. The source is situated and configured to produce multiple separate beamlets of focus-detection light and to direct the beamlets at an oblique angle of incidence onto a surface of the reticle, such that the beamlets are incident at respective height-detection loci on the surface of the reticle. The height detector is situated and configured to detect light of the beamlets reflected from the reticle surface and to produce a corresponding focus-detection signal. The height detector comprises a light-receiving surface including a respective light detector for each beamlet, and each light detector is configured to measure a lateral displacement of the respective beamlet on the light-receiving surface and produce a respective height-encoding signal.  
           [0022]    The focus-detection-beam source can be configured to produce at least three beamlets that are incident at respective height-detection loci arranged on the reticle surface relative to an exposure region of the reticle surface that can be illuminated by a corresponding deflection of the illumination beam. In such an instance, the height detector can be configured to produce an aggregate signal from the respective height-encoding signals produced by the respective light detectors for the at least three beamlets. The aggregate signal corresponds to a height measured at a center of the exposure region. The exposure region can include opposing ends each including multiple height-detection loci. In such an instance, the focus-detection beam source can be further configured to produce a respective beamlet for each height-detection locus at each end.  
           [0023]    The reticle can comprise support struts having respective edge surfaces. In such an instance, the height detector can be further configured to detect respective beamlets reflected from height-detection loci located on the edge surfaces of the support struts. The loci are spaced from each other at an equal locus-spacing interval in a direction perpendicular to a reticle-scanning direction. The support struts can be spaced from each other at a strut-spacing interval in the reticle-scanning direction. In such an instance, the locus-spacing interval can be an integral multiple of the strut-spacing interval.  
           [0024]    Alternatively, the height detector can be configured to detect respective beamlets reflected from height-detection loci located on the edge surfaces of the support struts, wherein the loci are spaced from each other at an equal locus-spacing interval in the reticle-scanning direction. In such an instance, the support struts can be spaced from each other at a strut-spacing interval in a direction perpendicular to the reticle-scanning direction. The locus-spacing interval can be, for example, an integral multiple of the strut-spacing interval or a integral multiple of one-half the strut-spacing interval.  
           [0025]    The reticle-focus-detection device can further comprise a processor to which the light detectors of the height detector are connected. The processor is configured to calculate respective heights of the height-detection loci, based on the respective height-encoding signals. The processor can further comprise an interpolating circuit configured to calculate respective interpolated heights of locations situated between flanking height-detection loci. In such an instance, the interpolated heights can be calculated based on the respective height-encoding signals from the flanking height-detection loci. The interpolating circuit can be further configured to calculate respective interpolated heights of locations, situated between flanking height-detection loci, lined up in a direction perpendicular to a reticle-scanning direction. If the reticle is segmented into multiple subfields, then at least one of the locations at which interpolated heights are calculated can be situated adjacent a respective subfield of the reticle.  
           [0026]    If the reticle comprises multiple subfields, wherein at least some of the subfields are flanked by respective multiple height-detection loci, then the processor can further comprise a height-determining circuit configured to calculate respective heights of the subfields based on determined heights of the respective flanking height-detection loci. The processor in this instance can further comprise a predicting circuit configured to predict respective heights of subfields lined up in a direction perpendicular to a reticle-scanning direction. The predictions typically are based on the heights of subfields calculated by the height-determining circuit.  
           [0027]    The light-receiving surface can constitute a main light-receiving portion of the height detector. In such an instance the main light-receiving portion can be situated so as to receive beamlets reflected from locations, on the reticle surface, at which respective height detections are determined. The height detector can further comprise multiple auxiliary light-receiving portions each situated so as to receive respective beamlets reflected from locations, on the reticle surface, at which respective height detections are to be determined. The auxiliary light-receiving portions can be situated and configured to receive respective beamlets reflected from locations, on the reticle surface, that are displaced in a reticle-scanning direction from locations detected by the main light-receiving portion. In this configuration, a processor desirably is used to calculate respective heights of the height-detection loci, based on the respective height-encoding signals.  
           [0028]    The processor can further comprise a direction-determining circuit configured to detect a direction of scanning movement of the reticle. The processor can further include a sensor selector configured to select a respective auxiliary light-receiving portion based on the respective direction of scanning movement of the reticle as detected by the direction-determining circuit.  
           [0029]    The reticle-focus-detection device can further comprise a stage-detection device situated and configured to detect a position of the reticle stage. The stage-detection device can be further configured to detect a detection-enable position of the reticle stage and to output an AF-enable signal to the height detector whenever the reticle stage is in the detection-enable position. The height detector can be further configured to produce the focus-detection signals upon receiving the AF-enable signal. The detection-enable position can correspond to a reticle-stage position at which the beamlets are incident on the respective light-receiving loci. If the reticle comprises support struts, then the detection-enable position can correspond to the reticle-stage position at which the beamlets are incident on respective light-receiving loci situated on edge surfaces of the support struts.  
           [0030]    According to another aspect of the invention, methods are provided (in the context of performing projection-transfer of a pattern using a charged particle beam) for detecting a focus condition of the reticle. In an embodiment of such a method, a reticle is provided that is segmented into multiple subfields arrayed in a two-dimensional array and separated from one another by support struts. The reticle is mounted on a reticle stage movable at least in a stage-scanning direction. A position of the reticle stage is detected. While the reticle stage is at the detected position, a focus-detection beam of light is directed at an oblique angle of incidence to a surface of the reticle to produce a reflected beam. Light of the reflected focus-detection beam is detected using a height detector configured to produce a corresponding detection signal from the detected light. The detection signal is processed to produce data concerning an axial height position of the reticle relative to the CPB optical system. If the axial height position of the reticle is outside pre-set tolerance limits, then a correction is applied to at least one of the axial height position and the CPB optical system until the axial height position is within the tolerance limits.  
           [0031]    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  
       [0032]    [0032]FIG. 1 is an oblique elevational diagram of certain aspects of a charged-particle-beam (CPB) microlithography apparatus, as described in connection with the first representative embodiment.  
         [0033]    [0033]FIG. 2 is an elevational schematic depiction of certain aspects of the projection-lens system and beam trajectories of the apparatus of FIG. 1.  
         [0034]    [0034]FIG. 3 is an oblique elevational diagram of certain aspects of a reticle as used with the embodiment of FIG. 1.  
         [0035]    [0035]FIG. 4 is a plan view of a reticle, discussed in connection with the first representative embodiment, having two pattern-defining regions.  
         [0036]    [0036]FIG. 5 is an elevational schematic view of a CPB microlithography apparatus, as described in connection with the first representative embodiment, including a reticle-focus-detection device.  
         [0037]    [0037]FIG. 6 is a plan view showing an exemplary arrangement of focus-detection beams as usable for detecting wafer “focus” (i.e., axial height position of the wafer).  
         [0038]    FIGS.  7 (A)- 7 (B) are respective planar views showing, as described in the first representative embodiment, impingement of reticle-focus-detection beamlets on the “membrane side” (non-strut side) of the reticle, within regions from which, on the opposite side of the reticle, the struts extend.  
         [0039]    [0039]FIG. 7(A) is a reticle in which individual square subfields (intended to be exposed one at a time) are separated from one another by minor struts, and  
         [0040]    [0040]FIG. 7(B) is a reticle in which the subfields (intended to be exposed in a scanning manner) are extended longitudinally in a beam-deflection direction.  
         [0041]    [0041]FIG. 8 is a plan view of a reticle, discussed in connection with the second representative embodiment, comprising multiple rows each including multiple subfields.  
         [0042]    [0042]FIG. 9 is a plan view of a reticle, discussed in connection with the second representative embodiment, comprising a column of multiple one-dimensional subfields.  
         [0043]    [0043]FIG. 10 is an elevational section of a portion of a stencil reticle, configured generally as shown in FIG. 8, showing two subfields.  
         [0044]    [0044]FIG. 11 is an elevational section of a single subfield of a scattering-membrane reticle, configured generally as shown in FIG. 8.  
         [0045]    FIGS.  12 (A)- 12 (B) are respective planar views showing, as described in the fourth representative embodiment, impingement of reticle-focus-detection beamlets on the upstream-facing surfaces of minor struts.  
         [0046]    [0046]FIG. 12(A) is a reticle in which individual square subfields (intended to be exposed one at a time) are separated from one another by minor struts, and  
         [0047]    [0047]FIG. 12(B) is a reticle in which the subfields (intended to be exposed in a scanning manner) are extended longitudinally in a beam-deflection direction (row direction).  
         [0048]    [0048]FIG. 13 is an elevational schematic diagram of a device, as described in the third representative embodiment, for controlling movement and position of the reticle stage, movement and position of the wafer stage, and auto-focus operation in a CPB microlithography apparatus.  
         [0049]    [0049]FIG. 14 is a block diagram of certain details of the reticle-stage-detection device shown in FIG. 13.  
         [0050]    [0050]FIG. 15 is a schematic oblique view showing certain details of the illumination system, reticle stage, and reticle-focus-detection mechanism shown in FIG. 13.  
         [0051]    [0051]FIG. 16 is a transverse section of three rows of beamlets of AF-detection light as viewed toward the surfaces of the minor struts on which the beamlets impinge, as discussed in connection with the third representative embodiment.  
         [0052]    [0052]FIG. 17 is a plan view of three rows of subfields on the reticle discussed in the third representative embodiment, showing height-detection loci (located on upstream-facing surfaces of the minor struts) on which the beamlets of AF-detection light, arranged as shown in FIG. 16, can impinge.  
         [0053]    [0053]FIG. 18 is a transverse section of five rows of beamlets of AF-detection light as viewed toward the surfaces of the minor struts on which the beamlets impinge, as discussed in connection with the third representative embodiment.  
         [0054]    [0054]FIG. 19 is similar to FIG. 17, but shows the height-detection loci on which the beamlets of AF-detection light, arranged as shown in FIG. 18, can impinge.  
         [0055]    [0055]FIG. 20 is a plan view of three rows of subfields on the reticle discussed in the third representative embodiment, showing possible height-detection loci, in the row direction, at which beamlets of AF-detection light can impinge, as discussed in connection with the third representative embodiment.  
         [0056]    [0056]FIG. 21 is similar to FIG. 17 except that, in FIG. 21, the height-detection loci are rotated 90 degrees from the orientations of the respective height-detection loci in FIG. 17.  
         [0057]    FIGS.  22 (A)- 22 (B) are plan views that are similar to FIGS.  12 (A)- 12 (B), respectively, except that the incident focus-detection beams in FIGS.  22 (A)- 22 (B) extend in directions rotated 90 degrees from the directions shown in FIGS.  12 (A)- 12 (B), respectively.  
         [0058]    [0058]FIG. 23 is a plan view of a reticle, such as shown in FIG. 9, showing the locations of certain height-detection loci on upstream-facing surfaces of minor row struts, as discussed in connection with the fourth representative embodiment.  
         [0059]    [0059]FIG. 24 is a plan view of the light-receiving surface with which the height-detection system shown in FIG. 15 receives AF-reflected light from a reticle, as discussed in connection with the fourth representative embodiment.  
         [0060]    [0060]FIG. 25 is a block diagram showing certain functions of the processor shown in FIG. 15, as discussed in connection with the fourth representative embodiment.  
         [0061]    [0061]FIG. 26 is similar to FIG. 25, except that, in the processor configuration of FIG. 26, the sensor selector is replaced with a data selector, as discussed in connection with the fourth representative embodiment.  
         [0062]    [0062]FIG. 27 is a plan view depicting certain aspects of operation of the interpolating circuit, as discussed in connection with the fourth representative embodiment.  
         [0063]    [0063]FIG. 28 is a plot of the relationship between two measured subfield “heights” and a third predicted subfield height, based on an extrapolation using a linear (first-order) function, as performed by the predicting circuit discussed in the fourth representative embodiment.  
         [0064]    [0064]FIG. 29 is a plan view of the three subfields shown in FIG. 28, including the respective centers of the subfields.  
         [0065]    [0065]FIG. 30 is a block diagram of certain steps in a method for manufacturing a microelectronic device, involving a microlithography step, as described in the fifth representative embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0066]    General Considerations  
         [0067]    The invention is based on an analysis of the following problems experienced with the types of membrane reticles summarized above:  
         [0068]    (1) Bending of the reticle membrane caused by gravity is greater than bending experienced by a conventional reticle.  
         [0069]    (2) Vibration of the reticle in an axial direction is caused by the movements of the reticle stage performed to move the desired region of the reticle pattern within the visual field of the CPB illumination-optical and projection-optical systems.  
         [0070]    (3) Especially in the case of pattern-transfer type CPB microlithography apparatus, the demagnification ratio at which projection occurs is kept to values of 1/(several ones) to avoid making the reticle extremely large. As a result, the variation in position of the reticle in the axial direction has an effect on the focusing performance of the image on the wafer that cannot be ignored.  
         [0071]    The present invention is based on the discovery that these problems cannot be corrected entirely in a satisfactory manner using correction values measured prior to exposure. Rather, it is necessary to correct positional variations of the reticle in real time during exposure.  
         [0072]    In conventional CPB microlithography, stencil-type reticles frequently are used. Typically, the reticle comprises a membrane made of a metal that absorbs the charged particle beam. The membrane, having a thickness of several tens of micrometers, defines openings that, in combination with remaining portions of the membrane, define the pattern. The mechanical rigidity of such a reticle is relatively high. Also, the demagnification of the image as projected normally is 1/(several tens). As a result, the effects of reticle deformation in the axial direction on image focusing on the wafer usually can be ignored, and there is no need to detect reticle focus. The present invention originates in a phenomenon that could not be predicted from this conventional apparatus.  
         [0073]    The invention is described below in the context of representative embodiments. However, it will be understood that the invention is not limited to those embodiments.  
         [0074]    First Representative Embodiment  
         [0075]    A CPB microlithography apparatus according to this embodiment is depicted in FIG. 1. This embodiment is described in the context of using an electron beam as an exemplary charged particle beam. However, it will be understood that the general principles readily can be applied to other types of charged particle beams, such as an ion beam.  
         [0076]    In FIG. 1, a reticle  21  is mounted on a reticle stage  22  that is movable in two dimensions (X- and Y-dimensions). Downstream of the reticle  21 , a wafer  24  (constituting a “sensitive,” or resist-coated substrate) is mounted on a wafer stage  25  that also is movable in two dimensions (X- and Y-dimensions). Between the reticle  21  and the wafer  24  is a projection-lens system  23 , and upstream of the reticle  21  is an illumination-optical system  27 . The illumination-optical system  27  trims an electron beam  26  to a square transverse profile and causes the beam (as an “illumination beam”) to strike the reticle  21  perpendicularly to the plane of the reticle  21 . On the reticle  21 , the pattern region that can be transferred in a single shot is termed a “subfield.” The subfield typically has a size of (1 mm) square on the reticle  21 . From the reticle  21 , the beam passes (as an “imaging beam”) through the projection-lens system  23  to the wafer  24 . The illumination-optical system  27  and the projection-lens system  23  each include at least one deflector  19 ,  20 , respectively.  
         [0077]    The reticle  21  used in the configuration of FIG. 1 is a “scattering-stencil” reticle in which beam-transmissive apertures are defined in a CPB-scattering membrane material. FIG. 2 depicts the reticle  21  being illuminated from upstream by the electron beam (“illumination beam”  26 ). The illumination beam passes readily through the apertures in the reticle  21  but is scattered by passage through the membrane portion of the reticle. The scattered electrons  32  (indicated by broken lines) in the resulting “imaging beam” are blocked by a scattering aperture  31  in the projection-lens system  23  from propagating further downstream to the surface of the wafer  24 . Electrons (indicated by solid lines in the figure) passing freely through the apertures in the reticle  21  propagate as the imaging beam to the surface of the wafer  24  and expose a resist applied to the surface of the wafer  24 . By way of example, if the demagnification ratio of the projection-lens system  23  is  1 / 4  and the size of a subfield on the reticle  21  is (1 mm) square, then size of a corresponding subfield on the wafer  24  is (250 μm) square.  
         [0078]    Returning to FIG. 1, the illumination beam  26  illuminating the reticle  21  is moved laterally (to the left and right in the figure) in an appropriate manner by the deflector  19  of the illumination-optical system  27 . As shown in FIG. 1 (see also FIG. 3), the reticle  21  includes a grid structure formed of “minor struts”  41 . As the illumination beam  26  is illuminating a subfield on the reticle  21 , the illumination beam  26  is trimmed and blanked appropriately to illuminate the membranous portion  43  of a subfield  44  but not the surrounding minor struts  41 . The membranous portion  43  of the subfield includes the respective patterned region  45  surrounded by a respective non-patterned skirt  42 . During exposure of a subfield  44 , the respective skirt  42  prevents beam-shape errors and blanking-timing errors of the illumination beam  26  from adversely affecting the transfer-exposure of the subfield to the wafer  24 .  
         [0079]    The illumination beam  26  successively illuminates the respective patterned regions  45  of a row of subfields on the reticle  21  by appropriate lateral beam deflections within a controllable range. The corresponding imaging (“patterned”) beam  28  is transferred onto the wafer surface by the projection-lens system  23 . For each subfield in the row being exposed, the actual respective transfer position on the wafer is adjusted finely using the deflector  20  inside the projection-lens system  23 , so that the portions corresponding to the minor struts  41  and skirts  42  on the reticle do not appear on the wafer  24 . By carefully controlling operation of the deflector  20  in the projection-lens system  23  with respective movements of the reticle stage  22  and wafer stage  25  (in the figure, in the forward and rearward directions perpendicular to the beam-deflection directions), the individual subfield images as formed on the wafer  24  are “stitched” together to form a contiguous two-dimensional pattern on the wafer. After all subfields in a row are exposed (taking into account the limitations imposed by the size of the reticle  21  and respective ranges of stage movement), the reticle stage  22  and wafer stage  25  move in mutually opposite directions along a dimension perpendicular to the directions of stage movement during exposure to position the next row of subfields for exposure.  
         [0080]    The reticle  21  can have a single pattern-defining region comprising subfields  44  defining respective portions of the pattern. Alternatively, the reticle  21  can have multiple pattern-defining regions each defining a respective major portion of the pattern and each comprising subfields  44  defining respective portions of the pattern. A representative reticle  21  having multiple pattern-defining regions  53   a ,  53   b  is shown in FIG. 4. The reticle  21  is formed, by way of example, from a silicon wafer having a diameter of 200 mm. To provide ease of wafer handling, improved accuracy and precision of wafer-conveying, and reduced wafer bending whenever the reticle is mounted on different reticle stages, the reticle  21  includes a supporting frame  51 . Also, to increase the rigidity of the overall reticle structure, the reticle also may include one or more relatively wide major struts  52  extending between and separating the pattern-defining regions  53   a ,  53   b.    
         [0081]    Reference now is made to FIG. 5. For exposure, a wafer  24  is placed on the wafer stage  25  and a reticle  21  is placed on the reticle stage  22 . A focus-detection beam  9  produced by a light-emitting system  7  is directed to be incident on the wafer  24  at an inclined angle. Light of this beam reflected from the wafer surface is detected by a light-receiving system  8 . In response to focus and positional data provided by the detected beam  9 , controlled vertical (axial) and tilt movements (relative to the optical axis AX) of the wafer stage  25  are made by actuators (not shown) located in at least three places beneath the stage  25 . Hence, the wafer  24  is provided with an “auto-focus” detection scheme.  
         [0082]    Wafer auto-focus detection normally is performed photoelectrically. According to this principle, the focus-detection beam  9  is caused to vibrate on a slit situated upstream of the light-receiving system  8 . Vibration of the beam is achieved by reflecting the beam  9  from a vibrating mirror (not shown, but vibrating at a frequency of, e.g., several kHz). Hence, a light beam vibrating at twice the vibrational period of the mirror is detected to determine a position of best focus. I.e., as the focus-detection beam is caused, by reflection from a mirror vibrating at a frequency f, to sweep over a slit, the beam sweeps over the slit twice per each back and forth movement of the mirror. One forward movement the mirror (constituting a half swing) produces one output from the light-receiving system, and one backward movement of the mirror produces one output from the light-receiving system. Therefore, the frequency of the output signal is twice the frequency of mirror vibration. This same technique can be exploited with the reticle auto-focus detection scheme.  
         [0083]    To accommodate the continuously scanning movement of the wafer stage  25  during exposure, the focus-detection beam  9  is divided into multiple one-dimensional beamlets  55  arranged in a plurality of rows (three rows A, B, C of beamlets shown in FIG. 6). To produce the multiple beamlets, the light-emitting system  7  is provided with a corresponding slit array and a lens system that produce, from a single focus-detection beam  9 , multiple parallel beamlets  55  of light of a defined size and having a defined spacing. One row of beamlets  55  (row B in FIG. 6) is used for the feed-forward of data concerning wafer indentation and projection during stage scanning in a first direction. Another row (the middle row, row A in FIG. 6) is used for servo control of reticle axial height correction (as well as of other imaging corrections such as focus, rotation, and magnification). The remaining row (row C in FIG. 6) is used for feed-forward of data concerning wafer indentation and inclination during stage scanning in a second direction opposite the first direction. In “feed-forward,” pre-exposure information is obtained on wafer indentation and inclination by comparing, for example, the respective axial heights of two rows before exposing a subfield in the first of the rows. “Indentation” of a wafer pertains to surface topography features or characteristics that result in the wafer surface not being planar. Detection of indentation normally is required before exposing a corresponding region on the wafer, so as to obtain the best resolution. The Y-axis in the figure is the axis to which scanning movement of the wafer stage  25  is parallel. Whenever the wafer stage  25  moves in the Y-direction indicated by the arrow B, the row B of beamlets  55  is used for feed-forward; and whenever the wafer stage  25  moves in the Y-direction indicated by the arrow C, the row C of beamlets  55  is used for feed-forward.  
         [0084]    In FIG. 6, the 45-degree tilt of the beamlets is effective for detecting focus of a region of the wafer on which the pattern already has been formed. Usually, a pattern on the wafer  24  includes horizontal lines as well as vertical lines, along with some indentation. If the beamlets  55  were oriented exactly vertically, then focus information obtained from a patterned region of the wafer  24  would not be entirely correct due to effects of indentation. A similar error would arise if all the beamlets  55  were oriented exactly horizontally. Hence, the 45-degree tilt.  
         [0085]    The “beam spacing” BS between adjacent rows of beamlets  55  is determined by factors such as the flatness (planarity) of the wafer  24 , surface roughness of the wafer, and the inclination of the wafer stage  25  relative to the optical axis AX (FIG. 5). (I.e., respective degrees of indentation and inclination can be obtained from axial-height data obtained using the beamlets  55  of row B and row C. In the present example, the beam spacing BS is set at 1 mm. Residual controllable differences (residual errors left after making corrections using, for example, piezo-electric actuators to adjust wafer position) are controlled by the dynamic focus control of the projection-lens system  23 , in addition to the reticle-focus-tracking error described later.  
         [0086]    Referring further to FIG. 5, “UC” denotes an upper column containing the illumination-optical system  27  and an electron gun (serving as the source of the illumination beam). “LC” denotes a lower column containing the projection-lens system  23 . The reticle  21  is mounted on the reticle stage  22  that includes a stage plate  1  supported relative to a base  3  by vertical actuators  2 . The actuators  2  are located actually in three different places between the plate  1  and the base  3  to permit vertical and tilting control of the reticle stage  22 . Exemplary actuators  2  are piezoelectric elements. Item  6  is a reticle-focus-detection beam having an inclined angle of incidence to the reticle  21 . The reticle-focus-detection beam  6  is produced by a light-emitting system  4 , and is detected (after reflecting from the reticle  21 ) by a height detector  5  that receives the reticle-focus-detection beam.  
         [0087]    During operation, the projection-lens system  23  generates heat from passage of electrical energy through various coils of the constituent lenses. Ordinarily, the lower column LC is cooled to avoid performance drift of the projection-lens system  23  from thermal expansion. Accordingly, it is desirable that the light-emitting system  4  and the height detector  5  be attached to the lower column by coupling members  11  made of a low-thermal-expansion material (e.g., “Zerodur”).  
         [0088]    As will be understood readily by persons skilled in the relevant art, at least the reticle  21 , reticle stage  22 , lower part of the upper column UC, and upper part of the lower column LC are contained inside a vacuum chamber (not shown). The reticle-focus-detection beam  6  emitted from the light-emitting system  4  is transmitted into the vacuum chamber through a window (not shown) attached to a flange on the vacuum chamber.  
         [0089]    The height detector  5  includes a light-receiving surface including a light sensor. The light sensor can be any suitable device that can detect and measure the position of the reticle-focus-detection beam  6  incident thereon. For example, the light sensor can be a CCD comprising a one- or two-dimensional array of light-sensitive pixels, a sensor of which the light-sensitive portion simply is split into multiple portions (each requiring a respective driver), or a PSD (position-sensitive-detector) in which the light-sensitive portion is not split, but position information is obtained nevertheless. Any of these configurations fills the following need: the reticle-focus-detection beam  6  emitted from the light-emitting system  4  is reflected by the reticle  21 . If the “height” (axial position) of the reticle  21  relative to the projection-lens system  23  varies, then the beam  6  reflected from the reticle  21  exhibits a lateral deviation from the specified trajectory of the beam  6 . This lateral deviation is detected as a corresponding change in the position at which the light  6  is incident on the light-receiving surface of the height detector  5 .  
         [0090]    The reticle-focus-detection beam  6  desirably is a portion of the light output from an LED or halogen lamp (wavelength within the range of visible to infrared light, i.e., approximately 600 to 900 nm). These exemplary sources are sufficient for providing a quantity of reflected light even for silicon-type reticle materials. Individual focus-detection beamlets are produced by passing the reticle-focus-detection beam  6  from the source simultaneously through multiple slits or other suitable openings. The number of beamlets can vary, depending upon the required accuracy with which detection of the axial height position of the reticle  21  is to be performed, and on the number of subfields in a row. For some applications, the number of beamlets corresponds at least to the number of subfields in a row so that the axial height position of each subfield can be detected without changing the beam-irradiation position. The beamlets are shaped by a lens system (not shown) to have individual beamlet diameters of approximately 0.05 mm immediately before the beamlets strike the reticle. Hence, when the beamlets strike the reticle  21 , the resulting illumination spots on the reticle are shaped as ellipses each having a minor axis length of 0.05 mm.  
         [0091]    The reticle auto-focus mechanism desirably is usable with any of various types of reticles, including stencil-type reticles and scattering-membrane reticles. With a stencil-type reticle, a narrow beamlet impinging on the reticle at a feature-defining aperture will pass at least partly through the aperture. Consequently, according to this embodiment, the respective loci on the reticle  21  impinged by the beamlets can be located on the membrane side (non-strut side) of the minor struts  41  because these regions include no apertures. Normally, the membrane side is the downstream-facing surface of the reticle  21 , as shown in FIG. 5. This is shown in FIGS.  7 (A)- 7 (B). In FIG. 7(A), multiple beamlets  61  are incident on the surface of the reticle  21  on the membrane side  62 . The struts  41  extend away from the viewer on the opposite side of the reticle  21 . The beamlets  61  are incident within regions  63  from which, on the opposite side of the reticle, the struts  41  extend. The regions  63  do not define any pattern elements. Hence, even with a stencil-type reticle, the beamlets  61  are not incident on any pattern-defining apertures. FIG. 7(A) depicts a reticle  21  comprised of square subfields  64 , and FIG. 7(B) depicts a reticle  21  comprised of horizontally elongated subfields  65 .  
         [0092]    Alternatively, as discussed below, the loci impinged by the beamlets can be on upstream-facing surfaces of the minor struts  41 . Such surfaces are also examples of non-pattern-defining surfaces.  
         [0093]    To ensure that loci impinged by the beamlets are at minor strut positions regardless of any vertical movement of the reticle  21 , it is desirable that the orientation of beamlet incidence be in a direction along which the minor struts extend across the reticle. This is evident in FIGS.  7 (A)- 7 (B).  
         [0094]    Second Representative Embodiment  
         [0095]    This embodiment is directed to exemplary details of the reticle referred to in the first representative embodiment.  
         [0096]    A reticle R 1  according to this embodiment is shown in FIG. 8, providing a plan view from an upstream direction (i.e., from a direction in which the reticle R 1  is illuminated by the illumination beam). The reticle R 1  includes a pattern region RP 1 , which is the region defining the pattern to be projected onto the wafer. In this embodiment, the pattern region RP 1  has a rectangular profile.  
         [0097]    The pattern region RP 1  is not limited to a single region on the reticle R 1 . Alternatively to a single region, the pattern region RP 1  can comprise two or more regions (see, e.g., FIG. 4, discussed above).  
         [0098]    The pattern region RP 1  comprises multiple subfields SF 0101 -SF 2020  (generally referred to collectively as the subfields SF). The subfields SF are disposed positionally in the manner of a rectilinear lattice in the pattern region RP 1 . Specifically, the subfields SF are lined up in straight lines in the row direction and in the column direction. In FIG. 8, each row extends horizontally and each column extends vertically. The “row direction” is the direction (horizontal in FIG. 8) in which each row extends, and the “column direction” is the direction (vertical in FIG. 8) in which each column extends. In the configuration shown in FIG. 8, 20 subfields SF are disposed in the row direction, and 20 subfields SF are disposed in the column direction. It will be understood that the array of subfields SF is not limited to 20 subfields in the row direction and 20 subfields in the column direction; any number of subfields may be disposed in the row and column directions.  
         [0099]    Directions on the reticle R 1  are as follows. The direction in which the illumination beam is successively deflected for exposure is a direction perpendicular to the direction in which the reticle stage is scanned. In FIG. 8, the direction in which the illumination beam is successively deflected for exposure is parallel to the row direction (i.e., the horizontal direction in FIG. 8). The direction in which the reticle stage is scanned is parallel to the column direction (i.e., the vertical direction in FIG. 8).  
         [0100]    The subfield SF 0101  is located at the intersection of the first row and the first column; the subfield SF 0201  is located at the intersection of the first row and the second column; and the subfield SF 2001  is located at the intersection of the first row and the twentieth column. Similarly, the subfield SF 0102  is located at the intersection of the second row and the first column; the subfield SF 0120  is located at the intersection of the twentieth row and the first column; and the subfield SF  2020  is located at the intersection of the twentieth row and the twentieth column. Generally, the length of each row is equal to the length of the region that can be illuminated by lateral deflection of the illumination beam for purposes of exposure. (In the reticle configuration of FIG. 4, the length of the region that can be illuminated by lateral deflection of the illumination beam for purposes of exposure is the shorter dimension of each region  44 .)  
         [0101]    It will be understood that the profile of each subfield SF is not limited to a square profile. A subfield SF can have any of various profiles that allow the subfields to be lined up in the manner of a regular array. For example, each subfield can have a regular hexagonal profile.  
         [0102]    A row minor strut RG 1  is disposed between the row containing the subfields SF 0101 -SF 2001  and the row containing the subfields SF 0102 -SF 2002 . Similarly, respective row minor struts RG 2  through RG 19  are disposed between the other rows of subfields SF. The row minor struts RG 1 -RG 19  (collectively referred to as the row minor struts RG) are spaced equally apart from each other in the column direction. The row minor struts RG constitute a portion of the overall support structure for the reticle R 1 , and serve in part to separate and define the subfields SF. The shape of each row minor strut RG, as viewed from upstream, is rectangular, extending in the row direction the full length of a row of subfields.  
         [0103]    A column minor strut CG 1  is disposed between the column containing the subfields SF 0101 -SF 0120  and the column containing the subfields SF 0201 -SF 0220  that are lined up in the column direction. Similarly, respective column minor struts CG 2  through CG 19  are disposed between the respective subfields SF that are lined up in the column direction. The column minor struts CG 1 -CG 19  (collectively referred to as the column minor struts CG) are spaced equally apart from each other in the row direction. The column minor struts CG constitute a portion of the overall support structure for the reticle R 1 , and serve in part to separate and define the subfields SF. The shape of each column minor strut CG, as viewed from an upstream direction, is rectangular, extending in the column direction the full length of a column of subfields.  
         [0104]    Alternatively, a reticle can be configured as the reticle R 2  shown in FIG. 9, providing a plan view as seen from upstream. The reticle R 2  comprises a pattern region RP 2  defining the pattern. Multiple one-dimensional subfields RSF 1 -RSF 20  (collectively referred to herein as “one-dimensional subfields” RSF) are disposed in the pattern region RP 2 . The one-dimensional subfields RSF define respective portions of the pattern. Each one-dimensional subfield RSF, as viewed from upstream, has a rectangular profile extending in the row direction. The one-dimensional subfields RSF are disposed side-by-side, with their longitudinal dimensions (long sides) parallel to each other. One-dimensional minor struts RRG 1 -RRG 19  (collectively referred to as “one-dimensional minor struts” RRG) are disposed between the respective one-dimensional subfields RSF. The one-dimensional minor struts RRG provide mechanical support for the one-dimensional subfields RSF. Each one-dimensional minor strut RRG, as viewed from upstream, has a rectangular profile extending in the row direction.  
         [0105]    Continuing with the description of the reticle R 1  of FIG. 8, an elevational section (along the line A-B in FIG. 8) is shown in FIG. 10. In the reticle R 1  of FIG. 10, each subfield SF appears as a deep recess in the framework represented by the minor struts RG, CG. At the bottom of each recess is a respective membrane region Ml that defines the respective portion of the pattern defined by the reticle R 1 . As an illumination beam IB (propagating from upstream) impinges on a selected subfield SF, the illumination beam IB illuminates the respective membrane region M 1 . In the reticle of FIG. 10, the respective pattern portion defined by a given subfield SF is configured as a corresponding arrangement of through-holes (openings)  56  in the respective membrane region M 1 . The through-holes  56  are transmissive to charged particles of the illumination beam IB, which experience little to no scattering as they pass through the through-holes  56 . The membrane portions  57  of the membrane region M 1 , on the other hand, transmits particles of the illumination beam IB with substantial scattering at a wide scattering angle. This type of reticle is termed a “scattering-stencil” or simply a “stencil” reticle.  
         [0106]    Alternatively, the reticle R 1  can be configured with membrane regions M 2  as shown in FIG. 11, which depicts an elevational section of the membrane region M 2  of a single subfield SF. The illumination beam IB is incident from upstream on the membrane region M 2 . The membrane region M 2  comprises a film M 21  including thin regions  57  that transmit charged particles of the illumination beam IB with little to no scattering, and thick regions  58  that transmit charged particles of the illumination beam IB with substantial scattering at a wide angle. This type of reticle is termed a “scattering-membrane” or simply a “membrane” reticle.  
         [0107]    Referring further to FIG. 10, the upstream-facing surfaces of the row minor struts RG (these surfaces are referred to as the “row-minor-strut surfaces” RGP) and the surfaces of the column minor struts CG (these surfaces are referred to as the “column-minor-strut surfaces” CGP) are reflective to incident laser light. The row-minor-strut surfaces RGP and column-minor-strut surfaces CGP are referred to collectively as the “minor strut surfaces” GP.  
         [0108]    Third Representative Embodiment  
         [0109]    Reviewing certain details set forth in the first representative embodiment, in FIG. 5, the wafer  24  is carried on the wafer stage  25 . The wafer-focus-detection beam  9  is incident, at an inclined angle, on the wafer  24  from the light-emitting system  7 ; light reflected from the wafer  24  is detected by the light-receiving system  8 . Vertical and tilt control of the wafer stage  25  is achieved by means of actuators (not shown) situated in three locations in a tripod manner below the wafer stage  25 . This auto-focus device for the wafer  24  operates according to a photoelectric-detection principle set forth in, for example, Japan Kôkai Patent Document No. 56-42205 by the present applicant. According to this principle, the wafer-focus-detection beam  9  is caused to vibrate on a slit just upstream of the light-receiving system  8 . The vibration is achieved by means of a vibrating mirror (vibrating at a frequency of, e.g., several kHz). The best focus position of the wafer  9  is determined by detecting a wave having a frequency that is double the vibrational frequency of the mirror. To accommodate the scanning movement of the wafer stage  25  during exposure, multiple rows (three shown in FIG. 6) of one-dimensional multi-point beamlets  55  are used, e.g., as disclosed in Japan Kôkai Patent Document Nos. 6-283403 and 8-064506 by the present applicant. One row (row B) of beamlets  55  is used for feed-forward of data during scanning of the wafer stage  25  in one direction parallel to the Y-axis, the middle row (row A) is used for servo control, and the remaining row (row C) is used for feed-forward of data during scanning of the wafer stage  25  in the reverse direction parallel to the Y-axis. In FIG. 6, the Y-direction (vertical direction) represents the axis along which the wafer stage  25  moves. Whenever the wafer stage  25  moves in the Y-axis direction denoted by the arrow B, the row B is used for feed-forward; whenever the wafer stage  25  moves in the Y-axis direction denoted by the arrow C, the row C is used for feed-forward. The spacing BS of these beamlets  55  from each other is a function of the planarity of the wafer  24  and the inclination of the wafer stage  25 .  
         [0110]    Turning now to FIG. 13, certain details of a device, according to this embodiment, for controlling the reticle stage  22 , the wafer stage  25 , and the auto-focus operation of a CPB microlithography apparatus  100  are illustrated. The apparatus  100  is similar to the apparatus shown in FIG. 5, but is depicted in block form in FIG. 13. A source  111  (e.g., electron gun) produces the illumination beam IB that passes through the illumination system  12  to the reticle  21  situated on the reticle stage  22 . The patterned beam PB resulting from transmission of charged particles of the illumination beam IB passing through the reticle  21  pass through a projection-lens system  23  to the wafer  24  situated on the wafer stage  25 . The apparatus  100  includes a reticle-stage-detection device  102  and a reticle-focus-detection mechanism  103 .  
         [0111]    The reticle-stage-detection device  102  is configured to detect the position of the reticle stage  22 , and is described with reference to FIG. 14. The reticle-stage-detection device  102  comprises at least one interferometer (IF)  121  and a strut detector  122 . The interferometer  121  desirably is a laser interferometer that detects the position of the reticle stage  22  (e.g., in the Y-direction) and outputs interferometer data DY corresponding to the position of the reticle stage  22 . Desirably, another interferometer (not shown, but similar to the interferometer  121 ) is included that detects the position of the reticle stage  22  in the X-direction. The strut detector  122  detects the reticle stage  22  whenever the stage is in a detection-enable position PEN. The reticle stage  22  is located at the detection-enable position PEN whenever an AF-detection light  132  (described later below) is incident on surfaces RGP of row struts. The reticle-stage-detection device  102  also desirably includes a memory  123  that stores previously obtained interferometer data DY corresponding to the detection-enable position PEN. The strut detector  122  outputs an AF-enable signal SEN whenever the interferometer data DY stored in the memory  123  match the interferometer data DY currently being output by the interferometer  121 . The AF-enable signal SEN indicates that the reticle stage  22  is in the detection-enable position PEN.  
         [0112]    As an alternative to the configuration described above, the strut detector  122  can be located in the reticle-focus-detection mechanism  103 . The reticle-focus-detection mechanism  103  is configured to detect “heights” HSF of the respective subfields SF of the reticle  21 . (As is shown in FIG. 10, the “heights” HSF are located substantially at the same elevation as the upstream-facing row strut surfaces RGP, and are respective positions corresponding to the centers of gravity of the respective subfields SF viewed from an upstream direction.) The reticle-focus-detection mechanism  103  outputs height data DSF corresponding to actual heights HSF, and predicts the heights HSF of subfields SF located adjacent to the subfield SF of which the height HSF is being detected. The height data DSF usually represents an average of multiple readings. The reticle-focus-detection mechanism  103  also outputs the predicted height-prediction data DSFP.  
         [0113]    Details of the illumination system  12 , reticle stage  22 , and reticle-focus-detection mechanism  103  now are described with reference to FIG. 15, wherein the Y-direction is the direction of stage movement. The X-direction is perpendicular to the Y-direction. The Z-direction is perpendicular to the X-Y plane, and extends from the reticle R 1  toward the illumination system  12 . In the embodiment of FIG. 15, the AF-detection light  132  is incident on the strut-surface side (upstream-facing side) of the reticle R 1 ; in the embodiment of FIG. 5, in contrast, the AF-detection light  6  is incident on the membrane side (downstream-facing side) of the reticle  21  (i.e., the side on which the pattern is defined). Otherwise, there are no essential differences in the manner of focus detection in the embodiment of FIG. 15 versus the embodiment of FIG. 5.  
         [0114]    The reticle stage  22  holds the reticle R 1  such that the row direction of the subfields SF on the reticle R 1  is parallel to the X-direction and the column direction is parallel to the Y-direction. The “height” of the reticle R 1  is in the Z-direction.  
         [0115]    The illumination system  12  illuminates rows of subfields SF by scanning the illumination beam in the row direction. Meanwhile, the illumination system  12  receives height data DSF from a processor  135 . According to this height data DSF, the illumination system  12  imparts corrections to the illumination beam IB as required to reduce various types of distortion and aberration, especially at the positions of the respective subfields SF indicated by the height data DSF.  
         [0116]    The reticle-focus-detection mechanism  103  first detects the respective heights of the minor strut surfaces GP of the reticle R 1 . To such end, the reticle-focus-detection mechanism  103  includes an AF-illumination device  131  (similar to the light-emitting system  4  of the focus-detection beam  6  in FIG. 5), a height detector  134 , and the processor  135 . The AF-illumination device  131  emits an AF-detection light  132  (similar to the reticle-focus-detection beam  6  in FIG. 5) toward the row minor strut surfaces RGP. Emission of the AF-detection light  132  is constant as the reticle stage  22  is moving in the positive and negative Y-directions. The relative orientation of the AF-detection light  132  is fixed relative to the illumination system  12 . Accordingly, whenever the reticle stage  22  moves in the positive Y-direction, the location on the row minor strut surfaces RGP on which the AF-detection light  132  is incident “moves” in the negative Y-direction.  
         [0117]    The AF-detection light  132  and the location on the minor strut surfaces GP to which the AF-detection light  132  is incident are described further with reference to FIGS. 16 and 17. As discussed above, the AF-detection light  132  is divided into multiple beamlets. FIG. 16 is a transverse section (perpendicular to the propagation direction) showing the beamlets of AF-detection light  132  as viewed from the AF-illumination device  131  toward the minor strut surfaces GP. In FIG. 16, the AF-detection light  132  is an aggregate of multiple beamlets LA 1 -LA 5 , LB 1 -LB 5 , LC 1 -LC 5 . Each beamlet consists of light having a wavelength of 600 to 900 nm, desirably as emitted by a laser, LED, or halogen light source. The beamlets LA 1 -LA 5 , LB 1 -LB 5 , LC 1 -LC 5  are lined up in three rows extending in the row direction and five columns extending in the column direction of the reticle R 1 .  
         [0118]    Of the beamlets, three rows A, B, C are shown, wherein each row contains respective five beamlets. Row C is the first depicted row and includes the beamlets LC 1 -LC 5 , group A is the second depicted row and includes the beamlets LA 1 -LA 5 , and row B is the third depicted row and includes the beamlets LB 1 -LB 5 . The beamlets also are grouped into five columns. The beamlets in the first depicted column all include the “1” denotation, the beamlets in the second depicted column all include the “2” denotation, and so on.  
         [0119]    The respective locations on the minor strut surfaces GP on which the beamlets of AF-detection light  132  are incident (these respective locations are called the “height-detection loci” PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5 ) are described with reference to FIG. 17, depicting a region of the reticle R 1  as viewed from upstream. The height-detection locus PA 1  is where the beamlet LA 1  is incident, the location PB 1  is where the beamlet LB 1  is incident, the location PC 1  is where the beamlet LC 1  is incident, and so on. The respective height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5  are all located in the vicinity of a corner of a respective subfield SF.  
         [0120]    In FIG. 17, the height-detection loci PA 1 , PB 1 , PC 1  are located near the left end of the row minor strut RG 1  (to the left of the subfields SF 0101 , SF 0102 ) and are lined up in the column direction. The spacing between the height-detection loci PA 1  and PB 1  desirably is the same dimension as the pitch of the subfields SF in the column direction, which desirably is equal to the spacing between the height-detection loci PA 1  and PC 1 . Hence, the spacing between each of the loci PA 1 , PB 1 , PC 1  desirably is equal to the pitch of the row minor struts. Similarly, the height-detection loci PA 5 , PB 5 , PC 5  are located near the right end of the row minor strut RG 1  (to the right of the subfields SF 2001 , SF 2002 ) and are lined up in the column direction. The spacing between the height-detection loci PA 5  and PB 5  desirably is the same dimension as the pitch of the subfields SF in the column direction, which desirably is equal to the spacing between the height-detection loci PA 5  and PC 5 . Hence, the spacing between each of the loci PA 5 , PB 5 , PC 5  desirably is equal to the pitch of the row minor struts. The height-detection loci PA 3 , PB 3 , PC 3  are situated near the center of the row minor strut RG 1  (in the row direction), and are lined up in the column direction on the column minor strut CG 10 . The spacing between the height-detection loci PA 3  and PB 3  desirably is the same dimension as the pitch of the subfields SF in the column direction, which desirably is equal to the spacing between the height-detection loci PA 3  and PC 3 . Hence, the spacing between each of the loci PA 3 , PB 3 , PC 3  desirably is equal to the pitch of the row minor struts. The height-detection loci PA 2 , PB 2 , PC 2  are situated on the column minor strut CG 5 , midway between the respective loci PA 1 , PB 1 , PC 1  (as arrayed in the column direction) and the respective loci PA 3 , PB 3 , PC 3  (as arrayed in the column direction). The respective spacings between the loci PA 2  and PB 2 , and between the loci PA 2  and PC 2 , desirably are as described above. Finally, the height-detection loci PA 4 , PB 4 , PC 4  desirably are situated on the column minor strut CG 15 , midway between the loci PA 3 , PB 3 , PC 3  (as arrayed in the column direction) and the loci PA 5 , PB 5 , PC 5  (as arrayed in the column direction). The respective spacings between the loci PA 4  and PB 4 , and between PA 4  and PC 4 , are as described above.  
         [0121]    Accordingly, the height-detection loci PA 1 -PC 1 , PA 2 -PC 2 , PA 3 -PC 3 , PA 4 -PC 4 , and PA 5 -PC 5  are arrayed essentially symmetrically and at essentially equal intervals in the row direction. Furthermore, in the configuration shown in FIG. 17, the respective spacings between the height-detection loci PA 1 -PC 1  and PA 2 -PC 2 , for example, are 5 times the pitch of the subfields in the row direction. In other words, this spacing is 5 times the pitch of the minor struts extending in the column direction.  
         [0122]    Furthermore, the height-detection loci PA 1 -PC 1  are lined up at equal intervals from each other in the column direction, as are the loci PA 2 -PC 2 , PA 3 -PC 3 , PA 4 -PC 4 , and PA 5 -PC 5 . The height-detection loci PA 1 -PA 5  are situated near respective subfields SF that are illuminated by the illumination system  12 . The height-detection loci PB 1 -PB 5  are situated near respective subfields SF that are adjacent, in one direction in the column direction, to the subfields SF illuminated by the illumination system  12 . Similarly, the height-detection loci PC 1 -PC 5  are situated near respective subfields SF that are adjacent, in the opposite direction in the column direction, to the subfields SF illuminated by the illumination system  12 .  
         [0123]    Since the orientation of the AF-detection light  132  is fixed with respect to the illumination system  12  as described above, the height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5  move over the reticle R 1  in the column direction as the reticle R 1  moves in the positive Y-direction. Whenever the reticle R 1  moves in the negative Y-direction, the height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5  move over the reticle R 1  in the opposite direction in the column direction.  
         [0124]    An alternative configuration is shown in FIG. 18, in which the AF-detection light  132  further includes beamlets LD 1 -LD 5  and LE 1 -LE 5  arrayed in the row direction. (FIG. 18 is a transverse section of the AF-detection light  132 , perpendicular to the propagation direction, as viewed from the AF-illumination device  131  toward the minor strut surfaces GP.)  
         [0125]    The respective loci on the minor strut surfaces GP at which the beamlets of AF-detection light  132  (shown in FIG. 18) are incident are illustrated in FIG. 19, which is a plan view of the reticle R 1  as viewed from upstream. The locus PD 1  is where the beamlet LD 1  is incident, and the locus PE 1  is where the beamlet LE 1  is incident. Similarly, the loci PD 2 -PD 5  are where the beamlets LD 2 -LD 5 , respectively, are incident, and the loci PE 2 -PE 5  are where the beamlets LE 2 -LE 5 , respectively, are incident. The locus PD 1  is situated midway between the loci PA 1  and PB 1 . Similarly, the loci PD 2 -PD 5  are situated midway between respective pairs of loci PA 2  and PB 2 , PA 3  and PB 3 , PA 4  and PB 4 , and PA 5  and PB 5 . The locus PE 1  is situated midway between the loci PA 1  and PC 1 . Similarly, the loci PE 2 -PE 5  are situated midway between respective pairs of loci PA 2  and PC 2 , PA 3  and PC 3 , PA 4  and PC 4 , and PA 5  and PC 5 . Accordingly, the pitch at which the beamlets LA 1 -LE 1  are lined up in the column direction is approximately one-half the pitch of the subfields SF in the column direction and one-half the pitch of the minor struts extending in the row direction. Also, the beamlets LA 1 -LE 1  are arranged with left-to-right symmetry in the column direction, and the number of beamlets LA 1 -LE 1  is an odd number.  
         [0126]    As shown in FIG. 20, the beamlets of AF-detection light  132  may reach any of the height-detection loci PA 1 -PA 20 , PB 1 -PB 20 , and PC 1 -PC 20 . (FIG. 20 is a plan view of the reticle R 1  as viewed from upstream.) The number of loci in any row is equal to the number of subfields in the row direction. Specifically, in FIG. 20, the locus PA 1  is in the vicinity of the subfield SF 0101 , and the loci PA 2 -PA 20  are in the vicinities of the respective subfields SF 0201 -SF 2001 . Each of the loci PB 1 -PB 20  is separated from the respective locus PA 1 -PA 20  in the negative column direction by a distance equal to the pitch of the subfields SF in the column direction. Similarly, each of the loci PC 1 -PC 20  is separated from the respective locus PA 1 -PA 20  in the positive column direction by a distance equal to the pitch of the subfields in the column direction.  
         [0127]    [0127]FIG. 21 is similar to FIG. 17, except that, in FIG. 21, the height-detection loci are oriented at right angles to the orientation of height-detection loci in FIG. 17. As shown in FIG. 21, at a given instant in time the height-detection loci PA 2 -PC 2  may be situated on the column minor strut CG 5 , the loci PA 3 -PC 3  may be situated on the column minor strut CG 10 , and the loci PA 4 -PC 4  may be situated on the column minor strut CG 15 . (FIG. 21 is a plan view of the reticle R 1  as viewed from upstream.) In FIG. 21, the position of the locus PA 2  is in the vicinity of the subfield SF 0501 . Similarly, the respective positions of the loci PA 3  and PA 4  are in the respective vicinities of the subfields SF 1001  and SF 1501 . Each of the loci PB 2 , PB 3 , PB 4  is separated from the respective locus PA 2 , PA 3 , PA 4  in the negative column direction by a distance equal to the pitch of the subfields SF in the column direction. Each of the loci PC 2 , PC 3 , PC 4  is separated from the respective locus PA 2 , PA 3 , PA 4  in the positive column direction by a distance equal to the pitch of the subfields SF in the column direction.  
         [0128]    Fourth Representative Embodiment  
         [0129]    The foregoing discussion mainly referred to an exposure scheme in which a reticle pattern is lithographically transferred subfield-by-subfield. A representative reticle used for such exposure is shown in FIG. 12(A), in which the illumination beam is deflected during exposure left-to-right (in the figure) within a controlled range, as described above. (This left-to-right direction is the row direction.) In such a scheme, the illumination beam is blanked between each subfield of the row. The reticle-focus-detection beamlets  71 , indicated by shaded ellipses, are arranged in three rows as incident on the reticle. In the direction of stage scanning (column direction), each row of beamlets extends along a respective minor strut  41 . The illuminated minor struts  41  are separated by a respective non-illuminated minor strut. Note that the direction of beamlet incidence also is in the stage-scan direction (downward or upward in the figure). This allows the beamlets (reflected from the reticle) to be detected regardless of whether the reticle has experienced any vertical movement.  
         [0130]    Alternatively, the reticle pattern may be transferred while scanning the beam in a direction perpendicular to the stage-scanning direction (i.e., scanning the beam in the row direction) without blanking the beam during exposure of a row. FIG. 12(B) illustrates a representative reticle used in such an instance, and also shows a representative manner in which rows of focus-detection beamlets are incident on such a reticle. In this scheme, however, since the number of minor struts  41  extending in the vertical direction in the figure (stage-scanning or column direction) is small, a smaller number of beamlets  71  can be directed onto the reticle than on the reticle shown in FIG. 12(A). This could result in less accuracy of height measurements.  
         [0131]    A way of solving this problem is shown in FIGS.  22 (A) and  22 (B), in which the orientation of the incident beamlets  71  is in a direction perpendicular to the stage-scanning direction (i.e., the beamlets are oriented left-to-right, or in the row direction, in the figure). A sufficient number of beamlets  71  are incident on the minor struts  41  to ensure accurate reticle-height detection. However, as the reticle stage and wafer stage move in the indicated stage-scanning directions, the beamlets  71  (or portions thereof) may be reflected by stenciled portions of the reticle membrane, resulting in erroneous height-detection data being produced. To avoid this problem, detections can be timed to occur only whenever the beams  71  are incident on the minor struts  41  as shown.  
         [0132]    For detecting the reflected one-dimensional beamlets  71 , one-dimensional light-sensor arrays (e.g., one-dimensional CCDs) can be used, each corresponding to an individual beamlet. Alternatively, a two-dimensional light-sensor array (e.g., a two-dimensional CCD) can be used that simultaneously detects the multiple beamlets. Whichever type of light sensor is employed for situations in which reticle-height measurements are performed using one-dimensional beamlets arranged in multiple rows, it is housed in the height detector  134 .  
         [0133]    This aspect of the invention is described further below in the context of the AF-detection light  132  and the height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5 , with reference to FIG. 23, depicting a plan view of a reticle R 2  as viewed from upstream. The height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5  are located on respective one-dimensional minor row struts RCG 2 , RCG 3 , RCG 1  of the reticle R 2 . Respective beamlets of AF-detection light  132  are incident on the height-detection loci PA 1 -PA 5 , PB 1 -PB 5 , PC 1 -PC 5 . Thus, the reticle-focus-detection mechanism  103  detects the “heights” HG of the one-dimensional minor struts RCG of the reticle R 2 .  
         [0134]    The composition of the detection beamlets is as described above. Referring back to FIG. 15, the reticle-focus-detection mechanism  103  is described. Beamlets of the AF-detection light  132  are regularly reflected by the row minor strut surfaces RGP. This AF-reflected light is referred to as item  133  in the figure. This AF reflected light  133  is an aggregate of the light of the multiple beamlets LA 1 -LA 5 , LB 1 -LB 5 , LC 1 -LC 5  reflected from the row minor strut surfaces RGP. The AF reflected light  133  enters the height-detection system  134  that is configured to detect the “heights” HG of the minor strut surfaces GP on the basis of the AF reflected light  133 . The height-detection system  134  is described below with reference to FIGS.  24 - 29 .  
         [0135]    [0135]FIG. 24 depicts the light-receiving surface  341  with which the height-detection system  134  receives the AF-reflected light  133 . Disposed on the light-receiving surface  341  are a main light-receiving portion  342  and first and second auxiliary light-receiving portions  344 ,  346 , respectively. The position where the main light-receiving portion  342  is disposed is designated as a “main light-receiving position  343 ”, and the respective positions where the first and second auxiliary light-receiving portions  344 ,  346  are disposed are designated as “auxiliary light-receiving positions”  345 ,  347 , respectively. The main light-receiving portion  342  is configured to receive AF-reflected light  133  from the height-detection loci PA 1 -PA 5 . Specifically, the main light-receiving part  342  receives the AF-reflected light  133  from the vicinities of subfields SF illuminated by the illumination system  12 . For detecting AF-reflected light  133  from the height-position loci PA 1 -PA 5 , the main light-receiving portion  342  comprises respective photosensors SSA 1 -SSA 5 . The first auxiliary light-receiving portion  344  is configured to receive AF-reflected light  133  from the height-detection loci PB 1 -PB 5 . Specifically, the first auxiliary light-receiving portion  344  receives the AF-reflected light  133  from the vicinities of subfields SF that are adjacent (in a first direction in the column direction) to the subfields SF illuminated by the illumination system  12 . For detecting AF-reflected light from the height-position loci PB 1 -PB 5 , the first auxiliary light-receiving portion  344  comprises respective photosensors SSB 1 -SSB 5 . The second auxiliary light-receiving portion  346  is configured to receive the AF-reflected light  133  from the height-position loci PC 1 -PC 5 . Specifically, the second auxiliary light-receiving portion  346  receives the AF-reflected light  133  from the vicinities of subfields SF that are adjacent (in a second direction, opposite the first direction, in the column direction) to the subfields SF illuminated by the illumination system  12 . For detecting AF-reflected light from the height-position loci PC 1  -PC 5 , the second auxiliary light-receiving portion  346  comprises respective photosensors SSC 1 -SSC 5 .  
         [0136]    The height-detection system  134  is configured to determine the respective “heights” HG of the minor strut surfaces GP located at the respective height-detection loci PA 1 -PA 5 , according to calculations performed using data output by the respective photosensors SSA 1 -SSA 5 . Similarly, the height-detection system  134  is configured to determine the “heights” HG of the minor strut surfaces GP located at the respective height-detection loci PB 1 -PB 5  and PC 1 -PC 5 , according to calculations performed using data output by the respective photosensors SSB 1 -SSB 5  and SSC 1 -SSC 5 .  
         [0137]    Turning to FIG. 25, A “select” signal SL is used by the height-detection system  134  to select one of the auxiliary light-receiving portions  344 ,  346  to provide “height” data along with the main light-receiving portion  342 . Thus, two sets of height-detection data can be output by the height-detection system  134 , based on the select signal SL as the reticle stage moves in the positive Y-direction. For example, as the reticle stage moves in the positive Y-direction, the respective select signal SL results in selection of the first auxiliary light-receiving portion  344 . The resulting sets of data output by the height-detection system  134  comprise a set of height data DG regarding the “heights” HG of the loci PA 1 -PA 5  and a set of height data DG regarding the “heights” HG of the loci PB 1 -PB 5 . Alternatively, as the reticle stage moves in the negative Y-direction, the respective select signal SL results in selection of the second auxiliary light-receiving portion  346 . The resulting sets of data output by the height-detection system  134  comprise a set of height data DG regarding the “heights” HG of the loci PA 1 -PA 5  and a set of height data DG regarding the “heights” HG of the loci PC 1 -PC 5 . The height data DG corresponding to the “heights” HG of the loci PA 1 -PA 5  are designated as “height data DGA 1 -DGA 5 ”. Similarly, the respective height data DG corresponding to the “heights” HG of the loci PB 1 -PB 5  and PC 1 -PC 5  are designated as “height data DGB 1 -DGB 5 ” and “height data DGC 1 -DGC 5 ,” respectively. The term “height data DG” is a collective designation of the height data DGA 1 -DGA 5 , DGB 1 -DGB 5 , and DGC 1 -DGC 5 .  
         [0138]    Based on the height data DG, the processor  135  calculates the heights HSF of respective subfields SF. The processor  135  is described with reference to FIG. 25, which depicts in block form certain functions of the processor. The processor  135  comprises a direction-determining circuit  351 , a sensor selector  352 , an interpolating circuit  353 , a height-determining circuit  354 , and a predicting circuit  355 .  
         [0139]    The direction-determining circuit  351  determines the direction of movement of the reticle stage RS (in either the positive or negative Y-direction), based on interferometer data DY. For example, if the data DY increases in value with movement of the reticle stage RS, then the direction-determining circuit  351  determines that the reticle stage is moving in the positive Y-direction.  
         [0140]    As the reticle stage RS moves in the positive Y-direction, it is as if illumination of the reticle is progressing in the negative Y-direction (i.e., the column direction). Accordingly, whenever the direction-determining circuit  351  determines that the reticle stage RS is moving in the positive Y-direction, the reticle R 1  actually is moving in the column direction. Conversely, whenever the direction-determining circuit  351  determines that the reticle stage is moving in the negative Y-direction, the reticle R 1  actually is moving a direction opposite the column direction. The direction-determining circuit  351  outputs direction data DD, corresponding to whether the reticle stage RS is moving in the positive Y-direction or the negative Y-direction, to the sensor selector  352 . Alternatively to a direction-determining circuit, the direction of motion can be determined from data produced by an exposure sequencer, for example.  
         [0141]    The sensor selector  352  outputs a select signal SL to the height-detection system  134 . Whenever the direction data DD indicates movement of the reticle stage RS in the positive Y-direction, the select signal SL results in selection of the first auxiliary light-receiving portion  344 . Whenever the direction data DD indicates movement of the reticle stage RS in the negative Y-direction, the select signal SL results in selection of the second auxiliary light-receiving portion  346 .  
         [0142]    As an alternative to using a sensor selector  352  that outputs a select signal SL, it is possible to employ a data selector  356  (FIG. 26) that selects height data DG on the basis of the direction data DD. In such a configuration, the height-detection system  134  may always output height data DG from the main light-receiving portion  342  and auxiliary light-receiving portions  344  and  346  regardless of the direction of movement of the reticle stage.  
         [0143]    The interpolating circuit  353  performs calculations that interpolate the height data DG as required. More specifically, the interpolating circuit  353  interpolates between the two sets of data output by the height-detection system  134 . In an instance in which the height-detection system  134  outputs height data DGA 1 -DGA 5  and DGB 1 -DGB 5  (reticle stage moving in the positive Y-direction), one of the interpolated sets of data is the height data DGA 1 -DGA 5  and the other set is the height data DGB 1 -DGB 5 . In an instance in which the height-detection system  134  outputs height data DGA 1 -DGA 5  and DGC 1 -DGC 5  (reticle stage moving in the negative Y-direction), one of the interpolated sets of data is the height data DGA 1 -DGA 5  and the other set is the height data DGC 1 -DGC 5 .  
         [0144]    The interpolating circuit  353  is described with reference to FIG. 27, using a case in which the interpolation is between the height data DGA 1  and DGA 2  as an example. FIG. 27 also shows subfields SF and the height-detection loci PA 1 , PA 2 , PB 1 , and PB 2  (shaded). In the figure, the height-detection loci PB 1  and PB 2  are located on the row minor strut RG 1 . As described above, the height-detection locus PA 1  is located at a respective end of the row minor strut RG adjacent the subfield SF 0101 . The height-detection locus PA 2  is located on the column minor strut CG 5 . The subfields SF 0101 , SF 0201 , SF 0301 , SF 0401 , SF 0501  and the column minor struts CG 1 , CG 2 , CG 3 , CG 4 , CG 5  are located between the loci PA 1  and PA 2 . In FIG. 27, the locations interpolated by the interpolating circuit  353  are the height-detection loci PA 1 - 1 , PA 1 - 2 , PA 1 - 3 , and PA 1 - 4 . These loci divide a line segment connecting the locus PA 1  and the locus PA 2  into five essentially equal parts. The height-detection loci PA 1 - 1 , PA 1 - 2 , PA 1 - 3 , and PA 1 - 4  are respectively located on the column minor struts CG 1 , CG 2 , CG 3  and CG 4 . Whenever an AF-enable signal SEN is input into the interpolating circuit  353 , the interpolating circuit  353  determines the height data DGA 1 - 1 , DGA 1 - 2 , DGA 1 - 3 , and DGA 1 - 4  for the loci PA 1 - 1 , PA 1 - 2 , PA 1 - 3 , and PA 1 - 4 , respectively. This calculation is performed by a linear interpolation from the height data DGA 1  for the locus PA 1  and the height data DGA 2  for the locus PA 2 . The interpolating circuit  353  may determine the height data DGA 1 - 1 , DGA 1 - 2 , DGA 1 - 3 , and DGA 1 - 4  not only from the height data DGA 1  and DGA 2 , but also from the height data DGA 1 , DGA 2 , DGA 3 , DGA 4 , and DGA 5 . The interpolating circuit  353  outputs the height data DGA 1 , DGA 1 - 1 , DGA 1 - 2 , DGA 1 - 3 , DGA 1 - 4 , and DGA 2 . If the interpolating circuit  353  does not receive an AF-enable signal SEN, no linear interpolation is performed, and no height data DGA 1 , DGA 1 - 1 , DGA 1 - 2 , DGA 1 - 3 , DGA 1 - 4 , DGA 2  are output from the interpolating circuit.  
         [0145]    The strut detector  122  outputs an AF-enable signal SEN whenever the AF-detection light  132  reaches the row minor strut surfaces RGP. In such instances, the interpolating circuit  353  outputs the direct height data DGA 1  and DGA 2 , as well as the interpolated height data DGA 1 - 1 , DGA 1 - 2 , DGA 1 - 3 , and DGA 1 - 4  to the height-determining circuit  354 . The interpolating circuit  353  also interpolates between the height data DGA 2 -DGA 3 , DGA 3 -DGA 4 , and DGA 4 -DGA 5 . Also, the interpolating circuit  353  interpolates between the height data DGB 1 -DGB 2 , DGB 2 -DGB 3 , DGB 3 -DGB 4 , and DGB 4 -DGB 5 , or alternatively between the height data DGC 1 -DGC 2 , DGC 2 -DGC 3 , DGC 3 -DGC 4 , and DGC 4 -DGC 5 . The direct and interpolated height data from DGA 1  to DGA 5  are denoted as respective “height data IDGA 1 , IDGA 1 - 1 , . . . , and IDGA 5 ,” respectively. Similarly, the direct and interpolated height data from DGB 1  to DGB 5  are denoted as respective “height data IDGB 1 , IDGB 1 - 1 , . . . , and IDGB 5 ,” respectively, and the direct and interpolated height data from DGC 1  to DGC 5  are denoted as “height data IDGC 1 , IDGC- 1 , . . . , IDGC 5 ,” respectively. The interpolating circuit  353  outputs the appropriate sets of direct and interpolated data to the height-determining circuit  354 .  
         [0146]    The height-determining circuit  354  determines the heights HSF of certain subfields SF by direct calculation (using direct data), and calculates the respective heights HSF of intervening subfields SF (using the interpolating data). Thus, the height-determining circuit  354  determines either the respective heights HSF of subfields SF that provided the direct and interpolated height data IDGA 1 -IDGA 5  and IDGB 1 -IDGB 5 , or the respective heights HSF of subfields SF that provided the direct and interpolated height data IDGA 1 -IDGA 5  and IDGC 1 -IDGC 5 . In other words, as the reticle stage moves in the positive Y-direction, the height-determining circuit  354  determines the respective heights HSF of subfields SF that produce the height data IDGA 1 -IDGA 5  and IDGB 1 -IDGB 5 . As the reticle stage moves in the negative Y-direction, the height-determining circuit  354  determines the respective heights HSF of subfields SF that produce the height data IDGA 1 -IDGA 5  and IDGC 1 -IDGC 5 .  
         [0147]    In this regard, the height-determining circuit  354  is described with reference to FIG. 27, using a case in which the height HSF of the subfield SF 0101  (located between the height-detection loci PA 1  and PA 1 - 1 , and between the height-detection loci PB 1  and PB 1 - 1  for example. The position of the locus PA 1  is indicated by the height data IDGA 1 , and the position of the locus PA 1 - 1  is indicated by the height data IDGA 1 - 1 . The position of the height-detection locus PB 1  is indicated by the height data IDGB 1 , and the position of the height-detection locus PB 1 - 1  is indicated by the height data IDGB 1 - 1 .  
         [0148]    The height-determining circuit  354  determines the position of the center P 0101  of a rectangle (corresponding to the subfield SF 0101 ) having respective corners at the loci PA 1 , PA 1 - 1 , PB 1 , and PB 1 - 1 . Accordingly, the center P 0101  essentially coincides with the center of gravity of the subfield SF 0101 . The height-determining circuit  354  determines the mean of the height data IDGA 1 , IDGA 1 - 1 , IDGB 1 , and IDGB 1 - 1 . This mean is the height HSF of the subfield SF 0101 , and the position of this height HSF is the center P 0101 . Similarly, the height-determining circuit  354  determines the respective heights HSF of other subfields SF that produce data output by the interpolating circuit  353 .  
         [0149]    Returning the FIG. 26, the description of the processor  135  is continued. As described above, the interpolating circuit  353  outputs one of two sets of data, depending upon the direction of movement of the reticle stage RS. Thus, the height-determining circuit  354  also determines heights HSF based on the direction of reticle stage movement. The height-determining circuit  354  outputs height data DSF indicating respective heights HSF.  
         [0150]    The predicting circuit  355  predicts, from multiple items of height data DSF, the heights HSFP of subfields SF that are situated adjacent one another in the column direction (subfields that produced the data DSF). Since the particular height data DSF that are output are based on the movement direction of the reticle stage, the predicting circuit  355  predicts the heights HSFP on the basis of movement direction of the reticle stage. Specifically, in cases in which the reticle stage moves in the positive y-direction, the predicting circuit  355  predicts the heights HSFP of subfields SF that are adjacent, in the negative Y-direction, to the subfields SF that produced the height data DSF. In cases in which the reticle stage moves in the negative Y-direction, the predicting circuit  355  predicts the heights HSFP of subfields SF that are adjacent, in the positive Y-direction to the subfields SF that produced the height data DSF.  
         [0151]    The predicting circuit  355  is described with reference to FIGS. 28 and 29. In the depicted example, the predicted height HSFP of the subfield SF 0103  (lined up in the negative Y-direction with respect to the subfield SF 0101 ) is determined as the reticle stage moves in the positive Y-direction. FIG. 28 is a plot of the relationship between height HSF and predicted height HSFP. In the figure, HSF 0101  is the height HSF of the subfield SF 0101 , HSF 0102  is the height HSF of the subfield SF 0102 , and HSFP 0103  is the predicted height HSFP of the subfield SF 0103 . FIG. 29 shows the subfields SF 0101 -SF 0103 . In FIG. 29, the center P 0102  indicates the position of the height HSF 0102  of the subfield SF 0102 , and the center P 0103  indicates the position of the predicted height HSF 0103  of the subfield SF 0103 . As indicated in FIG. 29, the subfield SF 0103  is positioned farther in the negative Y-direction than the subfield SF 0102  from the subfield SF 0101 . The predicting circuit  355  determines a first-order function that connects the height HSF 0101  and the height HSF 0102 . The point at which this first-order function intersects the center P 0103  is the predicted height HSFP 0103  of the subfield SF 0103 .  
         [0152]    The predicting circuit  355  is not limited to determining predicted heights HSFP from two heights HSF. The circuit  355  also may determine predicted heights HSFP from three or more HSF values using higher-order curves. Using higher-order curves is advantageous because the accuracy of the predicted height HSFP is increased over using linear plots. The predicting circuit  355  similarly determines the predicted heights HSFP of the subfields SF 0202 -SF 2002 , lined up in the negative Y-direction, for the subfields SF 0201 -SF 2001 .  
         [0153]    Whenever the predicting circuit  355  thus determines the predicted heights HSFP based on the direction of movement of the reticle stage, the resulting height-prediction data DSFP (indicating the predicted heights HSFP) are output to the illumination system  12 .  
         [0154]    Returning to FIG. 15, the description of the illumination system  12  and the reticle-focus-detection mechanism  103  is continued. The illumination system  12  stores the height data DSF for the subfields SF of three rows of which the exposure is completed. However, the illumination system  12  is not limited to storing height data DSF for the subfields SF of three rows. Alternatively, the illumination system  12  can be configured to store height data DSF for the subfields SF of any number of rows, so long as the number of rows is two or more. The illumination system  12  determines (by calculation) a function that expresses a curve passing through the stored height data DSF and predicted-height data DSFP. A correction to reticle height, and/or to the illumination beam produced by the illumination system is performed in accordance with the determined curve.  
         [0155]    The CPB microlithography apparatus  100  and its operation may now be summarized with reference to FIG. 13. The illumination beam IB illuminating the reticle  21  is patterned as the beam passes through the illuminated portion of the reticle. The resulting patterned beam PB propagates to the projection-lens system  23 . The projection-lens system  23  projects the patterned beam PB onto the wafer  24 . During such projection, the images of the subfields SF on the reticle  21 , as projected onto the wafer  24 , are “reduced” or “demagnified.” Also, during such projection of successive subfields SF, the illumination beam IF and patterned beam PB are scanned in opposite directions in the X-direction. Meanwhile, the wafer  24  is held on the wafer stage  25 , which moves in a direction opposite the direction of motion of the reticle stage  22 .  
         [0156]    After a reticle  21  is mounted onto the reticle stage  22  and a wafer  24  is mounted onto the wafer stage  25 , exposure operation of the CPB microlithography apparatus  100  is initiated. The reticle stage  22  holds the reticle  21  such that a direction opposite the row direction of the reticle  21  is parallel to the positive Y-direction. At initiation of exposure, the reticle stage  22  moves the reticle  21  in the Y-direction as the source  111  emits the illumination beam IB. As the illumination beam IB passes through the illumination system  12 , the subfields SF on the reticle  21  are illuminated successively as the illumination beam IB is scanned in the row direction.  
         [0157]    The reticle-focus-detection mechanism  103  emits beamlets of AF-detection light  132  toward the reticle  21 . The reticle-stage-detection device  102  detects the position of the reticle stage  21 , and outputs corresponding interferometer data DY. Whenever the reticle stage  21  is in the detection-enable position PEN, the reticle-stage-detection device  102  outputs an AF-enable signal SEN. The reticle-focus-detection mechanism  103  determines the direction of movement of the reticle stage  21 , either by inputting the interferometer data DY or based on data from an exposure sequencer. Whenever an AF-enable signal SEN is input to the reticle-focus-detection mechanism  103 , and the direction of movement of the reticle stage  21  is the positive Y-direction, the reticle-focus-detection mechanism  103  determines the heights HSF of the subfields SF illuminated by the illumination system  12 . The reticle-focus-detection mechanism  103  also determines the predicted heights HSFP of the subfields SF adjacent in the negative Y-direction to these subfields SF. These determinations are made based on AF-reflection light  133  from the height-detection loci PA 1 -PA 5  and PB 1 -PB 5 . Whenever an AF-enable signal SEN is input to the reticle-focus-detection mechanism  103 , and the direction of movement of the reticle stage is the negative Y-direction, the reticle-focus-detection mechanism  103  determines the heights HSF of the subfields SF illuminated by the illumination system  12 . The reticle-focus-detection mechanism  103  also determines the predicted heights HSFP of the subfields SF adjacent in the positive Y-direction to these subfields SF. These determinations are made based on AF-reflection light  133  from the height-detection loci PA 1 -PA 5  and PC 1 -PC 5 . The reticle-focus-detection mechanism  103  outputs height data DSF indicating the heights HSF, as well as the height-prediction data DSFP indicating the predicted heights HSFP.  
         [0158]    The illumination system  12  corrects the illumination beam IB so that various types of distortion and various types of aberration of the beam are minimized at the height indicated by the height data DSF. The illumination system  12  also corrects the illumination beam IB along a curve passing through the height data DSF and the height-prediction data DSFP stored in a memory.  
         [0159]    The illumination beam IB illuminating the reticle  21  is patterned by passage through the illuminated region of the reticle  21 . The resulting patterned beam PB propagates to the projection-lens system  23 . The projection-lens system  23  reduces (demagnifies) the images of the reticle subfields SF as projected onto the wafer  24 , while scanning the patterned beam PB in the X-direction opposite to the scanning direction of the illumination beam IB. Meanwhile, the wafer stage  25  moves the wafer  24  in a direction opposite to the direction in which the reticle stage  22  moves. Hence, the reticle pattern is projected onto the wafer  24 .  
         [0160]    As an example of focus control, and referring to FIG. 5, focus detection of the reticle R can be performed using the output of the height detector  5  receiving the reticle-focus-detection beam  6 . In such a scheme, the output from the sensor is fed back to control one or more vertical actuators  2 . To correct any residual difference, the focus of the projection lens LC can be caused to track the correct value by means of dynamic focus control in which the respective currents flowing to respective coils in the columns LC, UC are controlled. To achieve this control, an output signal from the reticle-focus height detector  5  and an output signal from the wafer-focus height detector  8  are received by a processor. Data regarding the detected positions of the stages  1 ,  25  are received from the respective distance-measuring interferometer systems. Height-position-correction amounts for the subfields on the reticle  21  and wafer-focus correction amounts are determined. During exposure, actuator control of both the reticle  21  and the wafer  24 , and dynamic focus control of the CPB-optical components in the columns UC, LC are performed on the basis of the output of the correction calculations.  
         [0161]    Fifth Representative Embodiment  
         [0162]    This embodiment is directed to a microelectronic-device manufacturing method, including a microlithography step using a CPB microlithography apparatus as described herein, as shown in FIG. 30.  
         [0163]    In Step  101 , a metal film is deposited on the wafer W.  
         [0164]    In Step  102 , a photoresist is applied to the metal film deposited on the wafer W.  
         [0165]    In Step  103 , the elements of a pattern defined by a reticle R are exposed sequentially onto the wafer W using the CPB microlithography apparatus.  
         [0166]    In Step  104 , the photoresist (with imprinted pattern) is developed.  
         [0167]    In Step  105 , the wafer W is etched using the developed resist as a mask. During etching, elements corresponding to the features of the pattern defined by the reticle R are formed on the wafer W.  
         [0168]    After completing Step  105 , other circuits (layers) can be formed on the wafer W as required atop the layer formed in Steps  101 - 105 , until manufacture of the respective microelectronic devices on the wafer W is completed.  
         [0169]    Hence, according to the invention, reticle-focus-detection devices and methods are provided for use in CPB microlithography. The reticle-focus-detection device provides an array of multiple focus-detection beamlets that are incident at an oblique angle on the reticle. Light of the beamlets reflected from the reticle is detected by a sensor operable to determine respective positions of the reflected beamlets. From the resulting beam-position data, data is obtained regarding the axial height position of the reticle relative to the projection-lens system. Obtaining of the reticle-position data can be synchronized so that the data is obtained only whenever minor struts of the reticle are coincident with the respective positions of the focus-detection beamlets. Such synchronization prevents erroneous position signals that otherwise might be produced whenever, for example, the reticle is a stencil reticle. As the beams are reflected from the reticle, variations in axial height position of the reticle relative to the projection-lens system are detected as variations in the positions of the beamlets as incident on the sensor. To such end, the sensor can be split into multiple portions (configured as a two-dimensional sensor) or can be a one-dimensional (linear) sensor. The reticle-focus detection can be performed at high speed and with high stability regardless of the direction of movement of the reticle stage.  
         [0170]    Whereas the invention has been described in connection with a preferred embodiment, it will be understood that the invention is not limited to that embodiment. 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.