Abstract:
Microlithography reticles are disclosed that include a high-contrast reticle-identification code (bar code). The bar code is configured as a pattern (usually linearly arrayed) of high-scattering regions (bar-code elements) each exhibiting a relatively high degree of reflection-scattering of irradiated probe light. The high-scattering regions are separated from one another by respective low-scattering regions each exhibiting a relatively low degree of reflection-scattering of incident probe light. For example, the low-scattering regions have smooth surfaces from which very little probe light is reflection-scattered, wherein each high-scattering region includes multiple scattering features such as line, channels, projections, or the like that provide multiple edges and/or points that reflection-scatter probe light. The edges in a high-scattering region can be arranged with a line-space (L/S) pitch that is below the resolution limit of an optical system that delivers probe light to the bar code and detects probe light reflection-scattered from the bar code.

Description:
FIELD  
         [0001]    This disclosure pertains to microlithography, which is a key technique used in the manufacture of microelectronic devices (e.g., semiconductor integrated circuits and displays) and micromachines, for example. More specifically, the disclosure pertains to microlithography of a pattern, defined by a mask (including the pattern master plate and reticle) and exposure method used therein. It particularly relates to a mask that has an identification code that is easy to read for identifying a plurality of different masks.  
         BACKGROUND  
         [0002]    Recent years have witnessed a relentless drive to achieve progressively finer pattern resolution in microlithography. The impetus for this drive is the persistent demand, in the microelectronics-fabrication industry, for integrated circuits (e.g., microprocessor and memory chips) having increasingly denser arrays of active circuit elements (e.g., transistors or memory cells) and/or circuits having increasingly greater numbers of active circuit elements. Achieving these goals has exceeded the ability of optical microlithography (i.e., lithography performed using deep-UV light) to provide adequate linewidth resolution, which has led to efforts to develop “next-generation lithography” (NGL) systems capable of achieving substantially finer resolution than obtainable with optical microlithography.  
           [0003]    A key NGL technology utilizes a charged particle beam (notably an electron beam) as a lithographic energy beam. Electron-beam microlithography potentially achieves finer resolution than optical microlithography for reasons similar to reasons for which electron microscopy achieves finer imaging resolution than optical microscopy. Most optical microlithography systems project onto an exposure-sensitive substrate a pattern from a master plate called a “mask” or “reticle” (the term “reticle” is used herein). The earliest electron-beam microlithography systems were direct-writing apparatus similar to apparatus conventionally used for mask-writing. Rather than projecting a pattern from a reticle, these early apparatus “wrote” the pattern line-by-line using a fine electron beam. Consequently, they exhibited extremely low throughput (number of production units, such as exposed wafers, that can be processed per unit time). Later electron-beam microlithography systems achieved better throughput by exposing small groups (“cells”) of pattern elements (especially of highly repeated pattern units such as in memory chips) simultaneously in respective shots. Nevertheless, whereas in optical microlithography an entire LSI pattern can be exposed in a single exposure “shot,” it currently is impossible to projection-expose an entire LSI pattern in a single shot using electron-beam microlithography. Despite this disadvantage, more recent electron-beam microlithography systems have been developed that exhibit throughput sufficient for use in mass-production wafer-fabrication facilities. These more recent systems utilize a “divided” reticle that defines the pattern, to be exposed, as an array of subfields that are individually exposed in a sequential manner. Each subfield defines a respective portion of the overall pattern and is substantially larger than a “cell” exposed by earlier electron-beam systems. The subfield images are projected onto the substrate in a manner by which the images are formed contiguously with each other. Projection normally is performed with demagnification (“reduction”), which means that the images formed on the substrate are smaller than the corresponding pattern on the reticle.  
           [0004]    An exemplary divided-reticle, electron-beam microlithography system useful for mass-production of exposed wafers is shown in FIG. 5, which depicts an overview of imaging and control relationships of the system.  
           [0005]    Situated at the extreme upstream end of the system is an electron gun  1  that emits an electron beam propagating in a downstream direction generally along an optical axis Ax. Downstream of the electron gun  1  are a first condenser lens  2  and a second condenser lens  3  collectively constituting a two-stage condenser-lens assembly. The condenser lenses  2 ,  3  converge the electron beam at a crossover C.O. situated on the optical axis Ax at a blanking diaphragm  7 .  
           [0006]    Downstream of the second condenser lens  3  and situated at the crossover C.O. is a “beam-shaping diaphragm”  4  comprising a plate defining an axial aperture (typically square or rectangular in profile) that trims and shapes the electron beam passing through the aperture. Thus, the beam is appropriately sized for illumination of only one subfield on the reticle  10  at a time. An image of the beam-shaping diaphragm  4  is formed on the reticle  10  by an illumination lens  9 .  
           [0007]    The electron-optical components situated between the electron gun  1  and the reticle  10  collectively constitute an “illumination-optical system” of the depicted microlithography system. The electron beam propagating through the illumination-optical system is termed an “illumination beam” because it illuminates a desired region of the reticle  10 . As the illumination beam propagates through the illumination-optical system, the beam actually travels in a downstream direction through an axially aligned “optical column” (not shown but well understood in the art) that can be evacuated to a desired vacuum level.  
           [0008]    A blanking deflector  5  is situated downstream of the beam-shaping aperture  4 . The blanking deflector  5  laterally deflects the illumination beam as required to cause the illumination beam to strike the aperture plate of the blanking diaphragm  7 , thereby preventing the illumination beam from being incident on the reticle  10  during times in which exposure is not being performed.  
           [0009]    A subfield-selection deflector  8  is situated downstream of the blanking diaphragm  7 . The subfield-selection deflector  8  laterally deflects the illumination beam as required to illuminate a desired subfield situated on the reticle  10  within the optical field of the illumination-optical system. Thus, subfields of the reticle  10  are scanned sequentially by the illumination beam in a horizontal direction (X-direction in the figure). The illumination lens  9  is situated downstream of the subfield-selection deflector  8 .  
           [0010]    The reticle  10  typically defines a large number (e.g., thousands) of subfields. The subfields collectively define the pattern for a layer to be formed at a single die (“chip”) on a lithographic substrate. The reticle  10  is mounted on a movable reticle stage  11 . The reticle stage  11  moves the reticle  10  in a direction (X- and Y-directions and combinations thereof) that is perpendicular to the optical axis Ax, thereby allowing respective subfields on the reticle  10 , extending over a range that is wider than the optical field of the illumination-optical system, to be illuminated. The position of the reticle stage  11  in the XY plane is determined using a “position detector”  12  that typically is configured as a laser interferometer. The laser interferometer is capable of measuring the position of the reticle stage  11  with extremely high accuracy in real time.  
           [0011]    In general, fabricating a microlithographic device on a substrate involves forming many superposed layers each involving at least one lithographic transfer of a respective pattern from a reticle. To make sure that the proper reticle is being used for a particular exposure step, each reticle bears a unique identification code  14 , which is “read” using an optical system  13  (discussed later below). During a day&#39;s production use of the microlithography system, the respective identification codes  14  on multiple reticles used by the system are read by the optical system  13  to ensure that the proper reticle is being used.  
           [0012]    Situated downstream of the reticle  10  are first and second projection lenses  15 ,  19 , respectively, and an imaging-position deflector  16 . The illumination beam, by passing through an illuminated subfield of the reticle  10 , becomes a “patterned beam” because the beam downstream of the reticle carries an aerial image of the illuminated subfield. The patterned beam is imaged at a specified location on a substrate  23  (e.g., “wafer”) by the projection lenses  15 ,  19 . To ensure imaging at the proper location on the substrate surface, the imaging-position deflector  16  imparts the required lateral deflection of the patterned beam.  
           [0013]    So as to be imprintable with the image carried by the patterned beam, the upstream-facing surface of the substrate  23  is coated with a suitable “resist” that is imprintably sensitive to exposure by the patterned beam. When forming the image on the substrate, the projection lenses collectively “reduce” (demagnify) the aerial image. Thus, the image as formed on the substrate  23  is smaller (usually by a defined integer-ratio factor termed the “demagnification factor”) than the corresponding region illuminated on the reticle  10 . By thus causing imprintation of the surface of the substrate  23 , the system of FIG. 4 achieves “transfer” of the pattern image from the reticle  10  to the substrate  23 .  
           [0014]    The components of the depicted electron-optical system situated between the reticle  10  and the substrate  23  collectively are termed the “projection-optical system.” The substrate  23  is mounted to a substrate stage  24  situated downstream of the projection-optical system. As the patterned beam propagates through the projection-optical system, the beam actually travels in a downstream direction through an axially aligned “beam tube” (not shown but well understood in the art) that can be evacuated to a desired vacuum level.  
           [0015]    The projection-optical system forms a crossover C.O. of the patterned beam on the optical axis Ax at the back focal plane of the first projection lens  15 . The position of the crossover C.O. on the optical axis Ax is a point at which the axial distance between the reticle  10  and substrate  23  is divided according to the demagnification ratio. Situated between the crossover C.O. (i.e., the rear focal plane) and the reticle  10  is a contrast-aperture diaphragm  18 . The contrast-aperture diaphragm  18  comprises an aperture plate that defines an aperture centered on the axis Ax. With the contrast-aperture diaphragm  18 , electrons of the patterned beam that were scattered during transmission through the reticle  10  are blocked so as not to reach the substrate  23 .  
           [0016]    A backscattered-electron (BSE) detector  22  is situated immediately upstream of the substrate  23 . The BSE detector  22  is configured to detect and quantify electrons backscattered from certain marks situated on the upstream-facing surface of the substrate  23  or on an upstream-facing surface of the substrate stage  24 . For example, a mark on the substrate  23  can be scanned by a beam that has passed through a corresponding mark pattern on the reticle  10 . By detecting backscattered electrons from the mark at the substrate  23 , it is possible to determine the relative positional relationship of the reticle  10  and the substrate  23 .  
           [0017]    The substrate  23  is mounted to the substrate stage  24  via a wafer chuck (not shown but well understood in the art), which presents the upstream-facing surface of the substrate  23  in an XY plane. The substrate stage  24  (with chuck and substrate  23 ) is movable in the X- and Y-directions. Thus, by simultaneously scanning the reticle stage  11  and the substrate stage  24  in mutually opposite directions, it is possible to transfer each subfield within the optical field of the illumination-optical system as well as each subfield outside the optical field to corresponding regions on the substrate  23 . The substrate stage  24  also includes a “position detector”  25  configured similarly to the position detector  12  of the reticle stage  11 .  
           [0018]    Each of the lenses  2 ,  3 ,  9 ,  15 ,  19  and deflectors  5 ,  8 ,  16  is controlled by a controller  31  via a respective coil-power controller  2   a,    3   a,    9   a,    15   a,    19   a  and  5   a,    8   a,    16   a.  Similarly, the controller  31 , via respective stage drivers  11   a  and  24   a,  controls operation of the reticle stage  11  and substrate stage  24 . The position detectors  12 ,  25  produce and route respective stage-position signals to the controller  31  via respective interfaces  12   a,    25   a  each including amplifiers, analog-to-digital (A/D) converters, and other circuitry for achieving such ends. In addition, the BSE detector  22  produces and routes signals to the controller  31  via a respective interface  22   a.    
           [0019]    From the respective data routed to it, the controller  31  ascertains, inter alia, errors of the respective stage positions as a subfield is being transferred. To correct such errors, the imaging-position deflector  16  is energized appropriately to deflect the patterned beam. Thus, a reduced image of the illuminated subfield on the reticle  10  is transferred accurately to the desired target position on the substrate  23 . This real-time correction is made as each respective image of a subfield is transferred to the substrate  23 , and the images are positioned such that they are stitched together in a proper manner on the substrate  23 .  
           [0020]    A reticle as used in an electron-beam microlithography system, such as the system shown in FIG. 5, is depicted in FIGS.  6 (A)- 6 (C). FIG. 6(A) is an overall plan view, FIG. 6(B) is a perspective view of a portion of the reticle, showing multiple small membrane regions (each including a respective subfield), and FIG. 6(C) is a plan view of a single small membrane region. As shown in FIG. 6(A), the reticle is divided into a large number of small membrane regions  41  each typically including a respective subfield. The small membrane regions  41  are arranged in several groups, termed “mechanical stripes”  49 , each containing multiple rows (“electrical stripes”)  44  of small membrane regions  41 . Four mechanical stripes  49  are shown, and each row  44  contains multiple small membrane regions  41 . Each small membrane region  41  has a thickness that typically is in the range of 1 to 2 μm, for example, for a “stencil”-type reticle. As shown in FIG. 6(C), each small membrane region  41  includes a central patterned portion that is the actual respective subfield  42  and a peripheral region (“skirt”)  43  that surrounds the subfield  42  in the manner of a picture frame. The subfield  42  defines a respective portion of the pattern defined by the reticle  10 . The skirt  43  is not patterned, and is a zone in which fall the edges of the illumination beam illuminating the respective small membrane region  41  during exposure.  
           [0021]    The reticle  10  can be a “stencil”-type reticle, in which pattern elements are defined by respective beam-transmissive (non-scattering) openings in the relatively beam-scattering membrane. Alternatively, the reticle  10  can be a “scattering-membrane” type, in which pattern elements are defined by respective units of a highly scattering layer formed on a relatively low-scattering membrane. In FIG. 6(B), the actual units of the scattering layer are formed on the under-surfaces of the small membrane region  41 .  
           [0022]    Note that the small membrane regions  41  are separated from each other in each mechanical stripe  49  by a latticed network of minor struts  45  called “grillage.” The minor struts  45  intersect each other at right angles and, in the nature of structural beams, provide substantial rigidity to and mechanical support for the reticle  10 . The thickness (in the Z-direction) of each minor strut  45  is 0.7 mm, for example, and the small membrane regions  41  extend from the “lower” portions of the minor struts  45  to cover the respective openings between the minor struts.  
           [0023]    In FIG. 6(A), each row of small membrane regions  41  extends in the X-direction and forms a respective “electrical stripe”  44 . Each mechanical stripe  49  comprises multiple electrical stripes  44  arrayed in the Y-direction. The length of each electrical stripe  44  (corresponding to the width of the respective mechanical stripe  49 ) is established by the width of the optical field of the illumination-optical system. The width of the optical field is limited by the maximum range (in the X-direction) of beam deflection that can be achieved by the subfield-selection deflector  8 . The electrical stripes  44  are so named because beam deflection required to illuminate the small membrane regions  41  in each electrical stripe  44  is performed electrically by the subfield-selection deflector  8 . The mechanical stripes  49  are so named because the constituent electrical stripes  44  are brought into position for exposure by mechanical movements of the reticle stage  11  and of the substrate stage  24 .  
           [0024]    In the reticle  10  shown in FIG. 6(A) multiple mechanical stripes  49  are arrayed in the X-direction. Between the mechanical stripes  49  are respective major struts  47  to which the minor struts  45  are contiguous. The major struts  47  typically have the same thickness (in the Z-direction) as the minor struts  45  but are substantially wider in the X-direction. The major struts  47  effectively prevent sagging and bending of the reticle  10 . The outer periphery  50  of the reticle  10  typically is circular and has the same thickness (in the Z-direction) as the minor struts  45  and major struts  47 .  
           [0025]    The reticle  10  also includes a band-shaped identification code  14  that, in FIG. 6(A), extends in the Y-direction and is situated to the left of the array of mechanical stripes  49 . The identification code  14  serves to uniquely identify the reticle, thereby facilitating control of the automated traffic of multiple reticles  10  into and out of the microlithography system. Typically, the identification code  14  is in the form of a bar code, which is the name used generally herein.  
           [0026]    The bar code  14  is “read” by the optical system  13  (FIG. 5). To such end, the optical system  13  includes a light source (e.g., an infrared LED) that illuminates the bar code  14  with a beam of “probe light,” and a reflected-light sensor (e.g., a CCD sensor) that detects probe light reflecting from the bar code  14 . Between the bar code  14  and the reflected-light sensor is a lens or the like for focusing probe light reflected from the bar code  14  on the surface of the sensor. The sensor is connected to a processor (e.g., the controller  31 ) that performs image-processing and image-discrimination of data produced by the sensor.  
           [0027]    For electron-beam microlithography, the reticle  10  is fabricated from a “reticle substrate” that typically is silicon or diamond-like carbon (DLC). According to conventional practice, the elements of the bar code  14  are formed on the reticle substrate either by lithography and vapor deposition or lithography and etching. In the vapor-deposition method, the elements of the bar code are formed of respective zones of a material, deposited on the surface of the reticle substrate, exhibiting a reflectivity to incident probe light that is substantially greater than the reflectivity of the reticle substrate to probe light. In the etching method, the elements of the bar code are formed as corresponding high- and low-elevation features defined in the reticle substrate (see FIG. 7(B), for example). The vapor-deposition method is not favored, largely for reasons of high cost.  
           [0028]    Turning now to FIGS.  7 (A)- 7 (C), certain details are shown of the conventional etching method that results in bar-code elements being defined by local differences in reticle-substrate thickness. FIG. 7(A) is a plan view of an exemplary bar code  72 ; FIG. 7(B) is an elevational section along the line K-L of the bar code of FIG. 7(A); and FIG. 7(C) is an exemplary detection signal produced by probe light that is reflection-scattered from the bar code. Referring to FIGS.  7 (A) and  7 (B), the bar code  72  is configured as an arrangement of multiple bar-code elements  74  separated from each other by intervening regions  73 . The intervening regions  73  extend upward to the surface plane of the reticle substrate  71  and thus have planar tops. The bar-code elements  74  are configured as respective voids defined in the reticle substrate  71 . As probe light is irradiated on the bar code  72 , the probe light is reflection-scattered only at the edges  75   a,    75   b . . . ,    75   n  of the bar-code elements  74  (FIG. 7(B)). This reflection-scattered probe light is detected using the optical system  13  (FIG. 5), which produces the detection signal shown in FIG. 7(C). The peaks  75   a ′,  75   b ′, . . . ,  75   n ′ of the detection signal correspond to the edges  75   a,    75   b, . . . ,    75   n  of the bar-code elements  74  (FIG. 7(B)). In other words, only the edges  75   a,    75   b, . . . ,    75   n  produce signal. As a result, wide bar-code elements  74  produce no more signal than narrow bar-code elements  74 . For example, the edges  75   e  and  75   f  of a relatively wide bar-code element  74  produce no more signal than the edges  75   d  and  75   e  of a relatively narrow bar-code element  74 . Also, since the edges  75   a,    75   b, . . . ,    75   n  also effectively are edges of the intervening regions  73 , the bar-code elements  74  produce no more signal than the intervening regions  73 . The resulting lack of signal distinction can result in the signal contrast being insufficient for accurate reading of the bar code  72 . In such instances the bar code  72  easily can be misread, which hinders achievement of a desired level of control of reticle traffic to and from the reticle stage  11 .  
         SUMMARY  
         [0029]    In view of the shortcomings of conventional technology as summarized above, provided hereby are reticles that include respective identification codes having sufficient contrast to ensure error-free reticle identification and control of reticle traffic to and from the microlithography system.  
           [0030]    To such end and according to a first aspect of the invention, reticles are provided for use in microlithography of a device pattern to an exposure-sensitive substrate using an energy beam. An embodiment of such a reticle comprises a reticle substrate on which is defined a device pattern to be transfer-exposed onto the exposure-sensitive substrate. A reticle-identification code is defined on the surface of the reticle substrate. The reticle-identification code comprises multiple high-scattering regions each exhibiting a relatively high degree of reflection-scattering of irradiated probe light. The high-scattering regions are separated from one another by respective low-scattering regions exhibiting a low degree of reflection-scattering of the irradiated probe light, relative to the high-scattering regions.  
           [0031]    Reflection of probe light from the high-scattering regions, relative to the low-scattering regions, provides a higher-contrast code-detection signal than obtainable using a conventional reticle-identification code. The higher-contrast signal provides more error-free identification of each code, and more trouble-free movement of reticles to and from the microlithography apparatus and/or in any of various other reticle-movement or storage processes.  
           [0032]    The low-scattering regions desirably present respective surfaces that are sufficiently smooth to avoid significant reflection-scattering, from the surfaces, of probe light incident on the surfaces. In contrast, the high-scattering regions comprise multiple scattering features that reflection-scatter incident probe light. The surfaces of the low-scattering regions, on which probe light is incident, can be coplanar with the surface of the reticle substrate. The scattering features in each of the high-scattering regions can comprise multiple points such as defined by respective pyramidal or conical projections. Alternatively, the scattering features can comprise multiple edges. For example, the edges in each high-scattering region can be configured to flank at least one channel extending in the high-scattering region and subdividing the respective high-scattering region. By way of another example, each high-scattering region can have multiple respective channels that subdivide the high-scattering region, wherein the multiple channels extend parallel to each other and/or perpendicularly to each other.  
           [0033]    In general, in view of the presence of two edges per bar-code element in a conventional reticle-identification code, each high-scattering region in a reticle as claimed herein defines three or more edges that reflection-scatter incident probe light. These edges can extend parallel to each other and/or perpendicularly to each other. The multiple edges in each high-scattering region can be defined by multiple respective channels extending in each high-scattering region, wherein the channels can extend parallel to each other and/or perpendicularly to each other.  
           [0034]    Further by way of example, the features in each high-scattering region of the reticle-identification code can comprise a line-and-space pattern defining multiple edges. The line-and-space pattern can have a pitch that is below the resolution limit of an optical system used for reading probe light reflected from the identification code.  
           [0035]    Even further by way of example, the features in each high-scattering region comprise a checkerboard pattern of projections and recesses that collectively define multiple edges. The checkerboard pattern can have a pitch that is below the resolution limit of an optical system used for reading probe light reflected from the identification code.  
           [0036]    By having the pitch be below the resolution limit of the optical system, the optical system is not able to read the respective borderlines of the bar-code elements. Rather, only the reflection-scattering features inside the bar-code elements are readable, which provides better assurance against mis-reading of the bar code.  
           [0037]    According to another aspect of the invention, microlithographic-exposure apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system situated and configured to illuminate a reticle with a lithographic energy beam. The reticle comprises: (a) a reticle substrate having a surface on which is defined a device pattern to be transfer-exposed onto an exposure-sensitive substrate, and (b) a reticle-identification code defined on the surface of the reticle substrate. The identification code comprises multiple high-scattering regions each exhibiting a relatively high degree of reflection scattering of irradiated probe light. The high-scattering regions are separated from one another by respective low-scattering regions exhibiting a relatively low degree of reflection scattering of the irradiated probe light. The microlithographic-exposure apparatus also includes a probe-light optical system situated relative to the reticle and configured to direct a beam of probe light to the identification code on the reticle and to sense probe light reflected from the identification code so as to provide an identification of the reticle.  
           [0038]    Yet another aspect of the invention is a method set forth in the context of a microlithographic method in which a pattern, defined on a reticle, is transfer-exposed from the reticle to an exposure-sensitive lithographic substrate. The subject method is for identifying a reticle, and comprises the step of providing on the reticle an identification code. The identification code is defined on a surface of the reticle and comprises multiple high-scattering regions each exhibiting a relatively high degree of reflection scattering of irradiated probe light. The high-scattering regions are separated from one another by respective low-scattering regions exhibiting a relatively low degree of reflection scattering of the irradiated probe light. In another step of the method a beam of probe light is irradiated on the identification code. Probe light reflected from the identification code is sensed. From the sensed probe light, a determination is made of the identity of the reticle corresponding to the identification code.  
           [0039]    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  
       [0040]    [0040]FIG. 1 is a plan view of a reticle-identification code (“bar code”) according to a first representative embodiment. Also shown is a bar-code image, such as would be projected onto a downstream substrate, of the bar code.  
         [0041]    [0041]FIG. 2(A) is an elevational section (along the line M-N) of a portion of the bar code shown in FIG. 1.  
         [0042]    [0042]FIG. 2(B) is an elevational section (along the line M-N) of a portion of a variation of the FIG. 2(A) embodiment having V-shaped channels rather than the ␣-shaped channels in the FIG. 2(A) configuration.  
         [0043]    [0043]FIG. 2(C) is a plot of an exemplary detection signal produced by detecting probe light that has been reflection-scattered from the bar code shown in FIG. 2(A).  
         [0044]    [0044]FIG. 3 is a plan view of a bar code according to a second representative embodiment. Also shown is a bar-code image, such as would be projected onto a downstream substrate, of the bar code.  
         [0045]    [0045]FIG. 4(A) is an oblique view of a high-scattering bar-code element, of the bar code of FIG. 3, in which the bar-code element is made highly scattering by multiple channels that subdivide the bar-code element in the X- and Y-directions into multiple planar-topped projections providing a large number of edges that reflection-scatter incident probe light.  
         [0046]    [0046]FIG. 4(B) is an oblique view of a variation of the bar-code element of FIG. 4(A), wherein, in FIG. 4(B) the bar-code element comprises multiple conical projections each having a respective point that reflection-scatters incident probe light.  
         [0047]    [0047]FIG. 5 is an elevational schematic diagram showing overall imaging relationships and control systems in a conventional divided-reticle, electron-beam, projection-microlithography system.  
         [0048]    [0048]FIG. 6(A) is a plan view of a conventional divided reticle including an identification code (bar code) used for reticle identification.  
         [0049]    [0049]FIG. 6(B) is an oblique view of a portion of the reticle shown in FIG. 6(A), wherein FIG. 6(B) provides detail of the grillage.  
         [0050]    [0050]FIG. 6(C) is a plan view of a single small membrane region of the reticle of FIG. 6(A), showing the respective subfield and skirt.  
         [0051]    [0051]FIG. 7(A) is a plan view of a conventional reticle-identification code (“bar code”).  
         [0052]    [0052]FIG. 7(B) is an elevational section (along the line K-L) of a portion of the bar code shown in FIG. 7(A).  
         [0053]    [0053]FIG. 7(C) is a plot of the detection signal produced by detecting probe light that has been reflection-scattered from the conventional bar code shown in FIG. 7(A).  
     
    
     DETAILED DESCRIPTION  
       [0054]    The invention is described below in the context of representative embodiments that are not intended to be limiting in any way.  
         [0055]    A first representative embodiment of a bar code  82 , as formed on a reticle, is shown in FIG. 1 in plan view. An enlarged elevational section, along the line M-N in FIG. 1, of a portion of the bar code  82  is shown in FIG. 2(A). The bar code  82  is configured as an arrangement of multiple bar-code elements  84  separated from each other by intervening regions  83 . The intervening regions  83  present respective “top” surfaces that are coplanar with each other and with the surface plane of the reticle substrate  81 . Hence, the intervening regions  83  have planar “top” surfaces in a manner similar to the intervening regions  73  of the conventional bar code  72  shown in FIG. 7(A).  
         [0056]    In contrast with the conventional bar code  72  of FIG. 7(A), each bar-code element  84  of the bar code  82  in FIG. 2(A) is subdivided to include multiple recessed portions (“channels”)  86  extending depthwise into the reticle substrate  81 . As a result, each bar-code element  84  presents more than two light-scattering edges  85 , compared to the only two edges per bar-code element  74  of the conventional bar code  72 . The “top” surfaces of the intervening regions  83  typically are sufficiently smooth to prevent significant scattering of incident probe light, and have no edges that would cause reflection-scattering of the incident probe-light beam. Each bar-code element  84 , in contrast, is “crowded” with edges  85  (more than two edges per element) each exhibiting a high level of reflection-scattering of the incident probe-light beam. As a result, the bar-code elements  84  are “high-scattering” regions and the intervening regions  83  are “low-scattering” regions of the bar code  82 . The bar-code elements  84  and intervening regions  83  are arrayed in a linear manner to form the bar code  82 .  
         [0057]    Referring further to FIG. 2(A), the edges  85  in each bar-code element  84  are defined by multiple channels  86  formed in the reticle substrate  81 . These channels  86  can be formed by, e.g., lithography followed by an etching step. In FIG. 2(A) the channels  86  are rectilinear and extend perpendicularly into the thickness dimension of the reticle substrate  81 .  
         [0058]    An alternative embodiment of a bar code  82 ′ is shown in FIG. 2(B), in which the channels  86 ′ in each bar-code element  84 ′ are V-shaped. The embodiment of FIG. 2(B) does not have as many edges  85 ′ per bar-code element  84 ′ as the embodiment of FIG. 2(A), but both embodiments nevertheless present more than the conventional two edges per bar-code element. In other words, both embodiments have bar-code elements that are “crowded” with edges  85 ,  85 ′ that perform reflection-scattering of incident probe light. Thus, each bar-code element  84 ,  84 ′ is a respective high-scattering region, compared to the intervening regions  83 , of the respective bar code  82 ,  82 ′.  
         [0059]    An exemplary probe-light-detection signal produced by the bar code  82  of FIG. 2(A) is shown in FIG. 2(C). The signal comprises “null” portions  83   s  corresponding to the intervening regions  83  of the bar code  82 . The signal also comprises “signal” portions  84   s  each corresponding to a respective bar-code element  84 . It readily can be seen that each signal portion  84   s  includes a group of signal peaks (at least four peaks per element in this embodiment) crowded together. Each signal peak corresponds to and is produced by reflection-scattering of probe light from a respective edge  85 . The production of many signal peaks by each bar-code element  84  (i.e., by each high-scattering region of the bar code) produces correspondingly enhanced “brightness” of the signal portions compared to the signal produced by a conventional bar code. Consequently, the bar code  82  produces a substantially higher-contrast signal than a conventional bar code and is more easily detected without errors than a conventional bar code.  
         [0060]    The bar-code embodiment  82 ′ of FIG. 2(B) also produces a signal having more than two peaks per bar-code element. For example, the left-most element  84 ′ has three edges  85 ′ and hence produces three peaks, and the right-most element  84 ′ has six edges and hence produces six peaks. As a result, the respective signal portions produced by the bar-code elements  84 ′ have enhanced brightness, and the bar code  82 ′ produces a higher-contrast signal, compared to the signal produced by a conventional bar code.  
         [0061]    In the conventional bar code  72  shown in FIG. 7(A), the bar-code elements  74  are not highly scattering compared to the intervening regions  73 . As a result, the bar-code elements  74  do not produce as “bright” a signal as bar-code elements  84  of the embodiments of FIGS.  2 (A) and  2 (B). By configuring each bar-code element  84  with more than two edges, as in the embodiments of FIGS.  2 (A) and  2 (B), higher-contrast bar-code-detection signals are produced. These higher-contrast signals, in turn, allow the bar codes to be detected by the optical system  13  more accurately more often, yielding smoother and more trouble-free control of reticle traffic to and from the microlithography system.  
         [0062]    Providing the bar code  82  with high-scattering regions  84  and low-scattering regions  83  as described above also provides a high-contrast bar-code image  87  (FIG. 1) as projected from the reticle  10  to the substrate  23 . This bar-code image  87  is suitable for detection purposes.  
         [0063]    In the bar codes of FIGS. 1 and 2(A)- 2 (B), each bar-code element (i.e., each high-scattering region)  84  can be configured, for example, in the manner described above, such that the channels  86  are sized and spaced apart from one another at line-and-space (L/S) intervals above the resolution limit of the optical system  13  reading probe light reflected from the bar code  82 . Alternatively, the L/S interval can be below the resolution limit of the optical system  13 . For example, in this alternative configuration the array of edges  85  has a pitch that is below the resolution limit of the optical system  13 . With such a fine pitch, the respective outer edge of each high-scattering element  84  is not detected, resulting in greater resolution of the bar-code elements as read by the optical system  13 . Nevertheless, the signal from the high-scattering regions  84  has enhanced brightness compared to the signal from a conventional bar code.  
         [0064]    Yet another representative embodiment of a bar code  92  is depicted in FIG. 3, in which the high-scattering regions (bar-code elements)  94  present a large number of edges that are two-dimensionally arrayed (in the X- and Y-directions) in the manner of a checkered pattern. Desirably, the pitch of edges in each of the X- and Y-directions is below the resolution limit of the optical system  13  reading reflected probe light. The respective pitch in the X- and Y-directions need not be identical. Detail of a portion of a high-scattering region  94  is shown in FIG. 4(A), in which each bar-code element is configured as an X-Y matrix of small square pillars  96 . An alternative embodiment is shown in FIG. 4(B), in which each bar-code element  94 ′ is configured as an X-Y matrix of cones  97  or pyramids. Both configurations present a respective crowded array of a very large number of edges (of the pillars  96 ) or points (of the apices of the cones  97 ) that reflection-scatter incident probe light.  
         [0065]    The high-scattering bar-code elements  94  in the embodiment of FIG. 3 are very “bright” compared to the intervening regions  93 . In fact, the high-scattering bar-code elements  94  are brighter than the high-scattering bar-code elements  84 ,  84 ′ of the embodiments of FIGS.  2 (A) and  2 (B). This is because, in the embodiment of FIG. 3, by subdividing each bar-code element  94  in both the X- and Y-directions, correspondingly more edges and/or points are produced compared to subdividing each bar-code element in only one direction (e.g., the X-direction as in FIG. 1).  
         [0066]    Providing the bar code  92  with high-scattering regions  94  and low-scattering regions  93  as described above also provides a high-contrast bar-code image  91  (FIG. 3) as projected from the reticle  10  to the substrate  23 . This bar-code image  91  is suitable for detection purposes.  
         [0067]    It is desirable that the line-width and pitch of the edges and/or points in the high-scattering regions  84 ,  94  be as narrow as possible in each applicable direction, taking into account the resolution limit of the optical system  13  reading the bar code as well as the resolution of the CCD detector used in the optical system.  
         [0068]    By configuring and using a bar code such as described in any of the representative embodiments, the contrast between high-scattering portions and low-scattering portions of the bar code is substantially enhanced compared to conventional bar codes. As a result, the bar codes are detected more reliably more often, compared to conventional bar codes. This is of substantial benefit during use of microlithography systems utilizing multiple reticles that must be routed into and out of the microlithography system because fewer bar-code-reading errors arise and reticle traffic into and out of the microlithography system can be better controlled. If the edges, points, or other scattering features in each bar-code element are arrayed at a L/S pitch that is below the resolution limit of the optical system  13 , then the border-edges of the bar-code elements are not detected, thereby yielding a greater contrast of detected signal from the bar code.  
         [0069]    Whereas the invention has been described in connection with representative embodiments, the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.