Abstract:
Methods are disclosed for reducing distortions, differences in focal point-positions, and astigmatic blurring of a pattern defined on a reticle and projected onto a sensitive substrate using a charged particle beam. The methods reduce variations in the distribution of beam current as projected onto the substrate. To such end, a charged particle beam passing through pattern features as defined on the reticle is projected onto a region on the substrate. The reticle is provided with multiple “micro features” each having a size less than the resolution limit of the projection-optical system. The micro features can be provided on a portion of the reticle having a low feature density.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to microlithography reticles and methods for their use in transferring a pattern defined on the reticle to a suitable sensitized substrate using a charged particle beam (e.g., electron beam or ion beam). Such reticles and methods have especial utility in the manufacture of semiconductor devices and displays. More specifically, the invention is directed to reducing errors in focal-point positions, distortions, and astigmatic blurring of transferred patterns caused by the Coulomb effect and related effects. 
     BACKGROUND OF THE INVENTION 
     Conventional electron-beam microlithography apparatus offer prospects of high-accuracy and high-resolution exposures but suffer from low throughput. Various technologies have been investigated in efforts to correct this fault. For example, certain pattern-portion batch exposure methods such as “cell projection,” “character projection,” and “block exposure” methods have received considerable attention. In a pattern-portion batch exposure method, small portions (e.g., 5 μm square units of a pattern defining a memory portion) of an overall pattern (e.g., of an entire integrated circuit) that are repeated many times in the overall pattern are defined by respective regions on the reticle. The region on the reticle corresponding to the small portion is typically used repeatedly many times during the transfer of a die pattern to the substrate (e.g., semiconductor wafer) to form the overall pattern on the substrate. Portions of the overall pattern that are not repeated are typically transferred using a different method such drawing using a variable shaped beam. Unfortunately, such methods have very low throughput. 
     An electron-beam “reduction” (i.e., demagnifying) projection-transfer apparatus has been proposed that purportedly achieves higher throughput than pattern-portion batch exposure methods. In this type of projection-transfer apparatus, the reticle defines the entire die pattern (i.e., the entire pattern destined to be exposed onto a separate “chip” on the substrate). The pattern on the reticle is typically divided (“segmented”) into multiple exposure units that are exposed sequentially by the electron beam onto the substrate. As the electron beam passes through an exposure unit, an image of the respective exposure unit is formed on a corresponding region of the substrate using a projection lens. The image is demagnified as projected onto the substrate, by which is meant that the image is smaller than the corresponding exposure unit as defined on the reticle. 
     In an attempt to improve the throughput of divided projection-transfer methods and apparatus, simultaneous irradiation of the entire reticle (i.e., “batch” exposure of the entire reticle defining an entire die pattern or even multiple die patterns) has been proposed. Unfortunately, such a technique exhibits poor transfer accuracy and poor edge resolution. It is also very difficult to produce a reticle that defines an entire die pattern (or multiple die patterns) to be transferred in one “shot” to the substrate. 
     Hence, divided projection exposure remains the favored technique for achieving projection exposure using a charged particle beam. According to one approach in divided projection exposure, the optical field of the projection-optical system is increased to allow projection of larger portions of the pattern during each shot. In any event, in divided projection-exposure methods, aberrations can arise during exposure of each exposure unit. Certain conventional divided projection-exposure apparatus achieve real-time correction of aberrations such as distortion or variations in the focal points of the images of the exposure units as formed on the substrate. Such corrections tend to improve the resolution and accuracy of pattern transfer over the entire die region compared to batch-transfer methods. 
     In exposure apparatus that employ a charged particle beam, exposed patterns can exhibit blurring (e.g., astigmatic blurring) and distortion. In a conventional variable-spot method or cell-projection method, each exposure unit is typically less than about 5 μm square. In the conventional divided transfer methods and apparatus summarized above, the exposure units are typically larger, approximately 100 μm square or larger (to increase throughput). With such large exposure units (each defining a respective portion of the overall pattern), if the features of the respective pattern portion are not evenly distributed, then the Coulomb effect can have a variable effect on image quality depending upon the distribution of pattern features in the exposure unit. 
     An example of an exposure unit having a non-uniform distribution of pattern features is shown in FIG.  5 . In FIG. 5, the exposure unit  81  comprises multiple pattern features  87 ,  89 . The features  87  are smaller than and spaced farther apart than the features  89 . Also, the features  87  are congregated in a region  83  and the features  89  are congregated in a region  85 . Hence, the feature density in the region  83  is lower than the feature density in the region  85 . Each of the features  87  and  89  is defined on the reticle as an aperture (if the reticle is a stencil reticle) or a local region highly transmissive to charged particles (if the reticle is a membrane reticle). Hence, charged particles passing through any of the features  87 ,  89  apply a corresponding local dosage of charged particles on the substrate. (Such features are termed “positive” features.) The complementary portions of the exposure unit  81  tend to block transmission of charged particles and are termed “negative” features. 
     In FIG. 5, the higher-density region  85  within the exposure unit  81  has a feature density of 50% and the lower-density region  83  has a feature density of 10%. The local beam current of the beam passing through the higher-density region  85  will be higher than the local beam current of the beam passing through the lower-density region  85 . As a result, the Coulomb effect will be more pronounced in the higher-density region  85 . The differential impact of the Coulomb effect causes the point of best focus of the beam passing through the higher-density region  85  to be axially displaced relative to the point of best focus of the beam passing through the lower-density region  83 . 
     Conventionally, transfer of the exposure unit  81  is performed at a “compromise” focal point for the regions  83  and  85 . The compromise focal point, however, is not optimal for either of the regions  83 ,  85 . This results in a corresponding decrease in overall resolution of the transferred image of the exposure unit  81  than if each region  83 ,  85  were exposed separately. The distortion in an image of an exposure unit  81  as projected is also different than any distortion in an image of an exposure unit with a more uniform feature density. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of conventional methods and apparatus as summarized above, an object of the invention is to provide, inter alia, charged-particle-beam (CPB) projection-exposure methods that reduce variations in focal-point positions, distortions, or astigmatic blurring of the transferred pattern caused by variations in the Coulomb effect originating from variations in the beam-current distribution over the pattern as transferred to the substrate. 
     To such end, and according to a first aspect of the invention, methods are provided for projection-exposing a pattern onto a sensitive substrate using a charged particle beam and a projection-optical system. According to a representative embodiment of such a method, the pattern is defined on a reticle. The reticle defines multiple features of the pattern. The reticle also defines multiple “micro features” (as defined herein) each sized below a resolution limit of the projection-optical system. A region of the reticle is illuminated using the charged particle beam such that a portion of the charged particle beam passing through the illuminated region of the reticle becomes an imaging beam. The imaging beam is projected through the projection-optical system onto the sensitive substrate to form an image of the region of the reticle on a corresponding region of the sensitive substrate. The micro features transmit charged particles in the beam from the reticle to the substrate and thus change an exposure condition of the corresponding region of the substrate relative to the exposure condition that would otherwise be achieved if the reticle lacked the micro features. A representative exposure condition is a beam-current distribution over the region that serves to reduce distortion over the region, astigmatic blurring over the region, and/or local focus variations over the region caused by the Coulomb effect. For example, the methods can make a beam-current distribution on the corresponding region of the substrate more uniform over the region than would otherwise be achieved if the reticle lacked the micro features. 
     Various types of reticles can be used with such methods. For example, the reticle can be a stencil reticle, in which instance the pattern as defined on the reticle can be divided into multiple exposure units, wherein at least one exposure unit can include a highfeature-density portion and a low-feature-density portion. Stencil reticles comprise a reticle “plate.” The reticle plate can be an “absorption” type by which is meant that the reticle plate is made of a material that tends to absorb charged particles of an incident charged particle beam. Alternatively, the reticle plate can be a “scattering” type by which is meant that the reticle plate is made of a material that tends to scatter charged particles of an incident beam. In either instance, the features of the pattern are defined by respective apertures extending through the thickness dimension of the reticle plate, and the micro features are located at least in the low-feature-density portion. 
     Alternatively, the reticle can be a so-called “membrane” reticle. A membrane reticle comprises a reticle membrane having a relatively high transmissivity to charged particles in the beam. The reticle membrane has formed thereon multiple negative features formed of a material tending to scatter or absorb more charged particles than absorbed or scattered by the reticle membrane. The reticle also defines multiple micro features each sized below a resolution limit of the projection-optical system. A region of the reticle is illuminated using a charged particle beam such that a portion of the charged particle beam passing through the illuminated region of the reticle becomes an imaging beam. The imaging beam is projected through the projection-optical system onto the sensitive substrate to form an image of the region of the reticle on a corresponding region of the sensitive substrate. The micro features transmit charged particles in the beam from the reticle to the substrate and thus make a beam-current distribution on the corresponding region of the substrate more uniform over the region than would otherwise be achieved if the reticle lacked the micro features. The pattern as defined on the reticle can have a highfeature-density portion and a low-feature-density portion, wherein the micro features are defined at least in the low-feature-density portion. The micro features can be defined also in the high-feature-density portion. Further alternatively, the micro features can be defined over the entire reticle regardless of feature density of various portions of the reticle. 
     Any of the foregoing methods can further include the step of correcting, based on the more uniform beam-current distribution that is achieved, at least one of a focal-point position, distortion, and astigmatic blurring of the image as formed on the substrate. 
     According to another aspect of the invention, reticles are provided that define a pattern to be transferred to a region on a sensitive substrate using a charged particle beam passing through the reticle and a projection-optical system to the substrate. One embodiment of such a reticle comprises features of the pattern and defines micro features extending at least partly through a thickness dimension of the reticle. The micro features are smaller than a resolution limit of the projection-optical system. The micro features transmit charged particles in the beam from the reticle to the substrate so as to achieve a beam-current distribution on the region of the substrate that is more uniform over the region than would otherwise be achieved if the reticle lacked the micro features. 
     Regardless of whether the reticle is a stencil reticle or a membrane reticle, the pattern as defined on the reticle can be divided into multiple exposure units. At least one exposure unit can include a high-feature-density portion and a low-feature-density portion. The micro features are defined in the low-feature-density portion. In a stencil reticle, the pattern features are defined by apertures (through holes) in the reticle plate. The reticle plate can be made of a CPB-scattering material or a CPB-absorbing material, and the micro features are defined by apertures (through holes) in the reticle plate. 
     According to another embodiment, the reticle defines a pattern to be transferred to a region on a sensitive substrate using a charged particle beam passing through the reticle and a projection-optical system to the substrate. The reticle comprises a reticle membrane having a relatively high transmissivity to charged particles in the beam. The reticle membrane has formed thereon multiple negative features formed of a material tending to scatter or absorb more charged particles than absorbed or scattered by the reticle membrane. The reticle also defines multiple micro features each sized below a resolution limit of the projection-optical system. The micro features can be defined by the reticle membrane having a relatively high transmissivity or by through-holes in the reticle. The micro features desirably transmit charged particles in the beam from the reticle to the substrate. As a result, the beam-current distribution on the corresponding region of the substrate is more uniform over the region than would otherwise be achieved if the reticle lacked the micro features. 
     By providing the micro features on the reticle, especially in a low-feature-density region of the reticle, the feature density in the corresponding region of the reticle is made more uniform overall. Because the micro features are not resolved by the projection-exposure system, images of the micro features are projected onto the substrate in a blurred manner. Thus, the specific pattern of micro features defined on the reticle is not transferred to the substrate. However, because charged particles of the beam pass from the micro features to the substrate, such charged particles contribute to the Coulomb effect. Such a contribution desirably causes any distortion and/or blurring at the substrate to be made more uniform. Such greater uniformity allows better correction of projection accuracy and resolution overall. 
     In addition to methods (e.g., “divided” projection-exposure methods involving a divided or “segmented” reticle), the present invention is also applicable to methods in which the pattern is transferred using, e.g., a cell-projection method. In a cell-projection method, portions of the whole pattern are transferred using separate reticles, and any remaining portions of the pattern are drawn on the substrate using, e.g., a variable-shaped beam or a Gaussian beam. 
     In yet other methods according to the invention, a reticle is provided that defines, in addition to actual (“real”) pattern features, “dummy” features. Dummy features desirably are located in low-feature-density regions of the reticle and are situated such that, when projected onto the substrate, they do not interfere with the function of the “real” features. Hence, a dummy feature can be of a size that is resolvable by the projection-optical system. But, even if the dummy feature is resolved onto the substrate, it is inconsequential because it has no effect on the overall pattern. Dummy features do, however, provide any of the various advantages of the micro features discussed herein. 
     The foregoing and other features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a stencil reticle according to a first representative embodiment of the present invention. 
     FIG. 2 includes plots showing differences in local beam intensity produced by an ordinary pattern feature on the FIG. 1 reticle versus by an array of micro features on the reticle. The horizontal axis is the position on the wafer and the vertical axis is the beam intensity on the wafer. 
     FIGS.  3 (A)- 3 (B) show vertical sections of two possible membrane reticles according to first and second representative embodiments of the invention. 
     FIG. 4 is a schematic optical diagram of image-forming relationships in a charged-particle-beam projection-exposure apparatus that can utilize a reticle and/or a method according to the invention. 
     FIG. 5 is a plan view of a stencil reticle illustrating differences in feature density in two different regions of an exposure unit of the reticle. 
     FIG. 6 is a plan view of a portion of a reticle, according to the invention, including “dummy” features as well as “real” features. 
     FIG. 7 is a process flowchart for manufacturing a semiconductor device, wherein the process includes a microlithography method according to the invention. 
     FIG. 8 is a process flowchart for performing a microlithography method that includes a projection-exposure method according to the invention. 
    
    
     DETAILED DESCRIPTION 
     A representative embodiment of the invention is described below that utilizes an electron beam as a representative charged particle beam. However, it will be understood that the principles discussed below can be applied with equal facility to an embodiment utilizing an alternative charged particle beam such as an ion beam. 
     Certain optical relationships are schematically depicted in FIG. 4, which shows an entire optical system of an electron-beam projection-exposure apparatus according to the invention. The electron beam EB is produced by an electron gun  101  and propagates downstream away from the electron gun  101  along an optical axis A. The beam then passes through first and second condenser lenses  103 ,  105  situated downstream of the electron gun  101 . The condenser lenses  103 ,  105  converge the beam EB at a crossover CO 1  located on the axis A at a blanking aperture  107 . 
     A beam-shaping aperture  106  (e.g., defining a rectangular opening) is situated just downstream of the condenser lens  105 . The beam-shaping aperture  106  only transmits a portion of the electron beam having a transverse profile that matches the dimensions of an exposure unit on the reticle  110 . An image of the beam-shaping aperture  106  is formed on the reticle  110  by a collimating lens  109 . 
     Upstream of the reticle  110 , the beam is termed an “illumination beam,” and downstream of the reticle  110 , the beam is termed an “imaging beam.” Between the blanking aperture  107  and the collimating lens is a deflector  108 . The deflector  108  successively scans the illumination beam in the X-direction of FIG. 4 so as to sequentially illuminate each exposure unit of the reticle  110 . The collimating lens  109 , situated downstream of the deflector  108 , forms the illumination beam into a parallel beam that is incident on the reticle  110 . As the illumination beam strikes the reticle  110 , the beam forms an image of the beam-shaping aperture  106  on the illuminated exposure region of the reticle  110 . 
     In FIG. 4, the reticle  110  is represented by one exposure unit situated on the optical axis A. Actually, the reticle  110  comprises many separate exposure units extending in the X and Y directions. To illuminate each exposure unit, the illumination beam is deflected as required by the deflector  108 . 
     The reticle  110  is mounted on a reticle stage  111  that is movable in the X and Y directions. In addition, the substrate (“wafer”)  114  is mounted on a wafer stage  115  movable in the X and Y directions. During exposure, the reticle stage  111  and the wafer stage  115  are scanned in opposite directions along the Y-axis to sequentially select successive exposure units for exposure. Each exposure unit of the die pattern defined on the reticle  110  is illuminated and exposed. Each stage  111 ,  115  includes a respective stage-position measurement system (not shown) each employing at least one laser interferometer (not shown). As exposed onto the wafer  114 , images of the exposure units are accurately joined (“stitched”) together by the action of an alignment means and by adjustment of each deflector in the optical system. 
     First and second projection lenses  112 ,  113  and a deflector system  131  (wherein the lenses  112 ,  113  and deflector system  131  are collectively termed a “projection-optical system”) are situated downstream of the reticle  110 . The illumination beam illuminates one exposure unit at a time on the reticle  110 . The imaging beam, patterned by passage through the illuminated exposure unit on the reticle  110 , is demagnified by passage through the projection lenses  112 ,  113 . The imaging beam is deflected as required by the deflector system  131  to form an image of the exposure unit at the desired fixed location on the wafer  114 . 
     The wafer  114  is coated with an appropriate resist so as to be imprinted with the projected images. As each exposure unit is illuminated, each corresponding region on the wafer surface is exposed with a respective dose of electrons to form the demagnified image of the illuminated exposure unit on the wafer  114 . The wafer stage  114  is movable in the X and Y directions to ensure that the image of the illuminated exposure unit is placed at a desired location on the wafer  114 . 
     A first representative embodiment of a reticle and method for its use is shown in FIG.  1 . The reticle of this embodiment is a stencil reticle of which an exemplary exposure unit  1  is shown. The exposure unit  1  comprises features  11 ,  15 . Each feature is defined as a corresponding aperture in a thin silicon membrane or “plate” (having a thickness of, e.g., 1 μm). The silicon plate scatters or absorbs charged particles of the illumination beam, and the feature apertures transmit charged particles of the illumination beam. Consequently, the feature apertures define “positive” features and the reticle plate defines “negative” features. The exposure unit  1  is shaped, e.g., as a square with 1-mm sides on the reticle. The entire exposure unit  1  is exposed with the illumination beam; an image of the exposure unit is demagnified and transferred to a corresponding region on the wafer  114 . If the demagnification ratio is 1/4, the exposed region (corresponding to the exposure unit  1 ) on the wafer is a square with 250-μm sides. To form the entire die on the wafer (corresponding to the entire die for a layer of a semiconductor device formed on the wafer), the reticle will typically have a few thousand to several tens of thousands of exposure units. 
     The exposure unit  1  shown in FIG. 1 comprises a low-density portion  3  in which multiple small rectangular features (positive features)  11  are dispersed comparatively sparsely. Because the FIG. 1 reticle is a stencil reticle, each positive feature  11  is defined as a corresponding aperture in the reticle plate. The exposure unit  1  also comprises a high-density portion  5  in which multiple large rectangular positive features  15  are dispersed comparatively densely. Again, because this reticle is a stencil reticle, each feature  15  is defined as a corresponding aperture in the reticle membrane. By way of example, the feature density of the low-density portion  3  is 10% and the feature density of the high-density portion  5  is 50%. 
     In this embodiment, the low-density portion  3  also comprises multiple “micro features”  13  situated between the positive features  11  (i.e., the micro features are situated in negative features). As used herein, a “micro feature” is an aperture defined by the reticle having a size that cannot be resolved as a distinct corresponding positive feature on the substrate by the projection-optical system. I.e., each micro feature is sized below the resolution limit of the projection-optical system. Each micro feature  13  can be, e.g., a square aperture (not detailed in drawing) measuring 0.08 μm on each side, and the micro features  13  can be spaced apart from one another by intervals of 0.12 μm. Other exemplary shapes of the micro features are circles and rectangles. The presence of the micro features  13  in addition to the positive features  11  in the low-density portion  3  yields a total feature density in the low-density portion  3  of approximately 50%. The micro features  13  also provide a concomitant improvement in the uniformity of the feature density of the low-density portion  3  compared to the high-density portion  5 . 
     Assuming a demagnification ratio of 1/4 and assuming the micro features  13  can be resolved on the wafer, each micro feature  13  (if square shaped as described above) would form a corresponding 0.02-μm square feature on the wafer. However, the resolution of the projection-optical system is typically approximately 0.08 μm as projected onto the wafer. Consequently, the image of each micro feature  13  as projected on the wafer is blurred without any resolution of the image of the micro feature. 
     This is illustrated in FIG. 2, which includes a graph showing typical differences in beam intensity (on the wafer) between an ordinary feature (such as the positive feature  11 ) and a micro feature  13 . In FIG. 2, the abscissa is the position on the wafer and the ordinate is beam intensity. Normally, the beam intensity passing through the positive feature  11  rises sharply along the edges of the feature as projected, as indicated by the plot  21 . In contrast, the beam passing through adjacent micro features  13  produces low and extended intensity profiles as indicated by the plots  23 . Thus, the exposures represented by the plots  23  produce background noise only with very little contrast. Therefore, the micro features  13  are not transferred as recognizable corresponding features on the wafer. 
     Particles of the beam passing through the micro features  13  are influenced by the Coulomb effect. However, due to the improved feature-density uniformity in the low-density portion  3  relative to the high-density portion  5  as a result of micro features  13  being present in the low-density portion  3 , distortion and blurring within an image of the entire exposure unit  1  are more uniform. As a result, distortion correction over the entire image of the exposure unit  1  is simplified and provides greater accuracy and resolution of the positive features  11 ,  15  as transferred to the wafer. 
     A second representative embodiment of a reticle and method for its use is shown in FIGS.  3 (A) and  3 (B). The reticle of FIG.  3 (A) (only a single exposure unit  31  is shown) is a “membrane” reticle comprising scattering bodies  35 ,  37  arranged on a membrane  33 . (The “scattering” bodies  35 ,  37  can either scatter or absorb charged particles of the illumination beam, depending upon the specific reticle. However, for ease of discussion herein, they are referred to simply as “scattering” bodies.) Each scattering body  35 ,  37  has a thickness of, for example, 0.5 μm, and comprises a heavy metal such as tungsten or tantalum. The scattering bodies  35 ,  37  constitute negative features arranged on the membrane  33 . The membrane  33  is typically made of silicon (and has a thickness of, for example, 0.1 μm). Regions of the membrane  33  lacking a scattering body  35 ,  37  do not cause significant scattering or absorption of electrons in the illumination beam passing through such regions. In contrast, electrons in the beam that encounter a scattering body  35 ,  37  are substantially scattered or absorbed. 
     Portions of the beam passing only through the membrane  33  have a relatively high beam current compared to beam portions that pass through both a scattering body and the membrane  33 . In any event, highly scattered electrons passing through the reticle can be blocked from reaching the wafer by placing a scattering aperture between the projection lenses  112 ,  113  of FIG.  4 . Thus, an image with acceptable contrast can be formed on the wafer. 
     The exposure unit  31  of FIG.  3 (A) comprises a high-feature-density portion  41  in which the feature density (as projected onto the wafer) is comparatively high and a low-feature-density portion  43  in which the feature density (as projected onto the wafer) is comparatively low. I.e., the distribution of negative features  35  in the high-feature-density portion  41  is sparse, yielding a correspondingly dense distribution of positive features in that portion. Similarly, the distribution of negative features  37  in the low-feature-density portion  43  is dense, yielding a correspondingly sparse distribution of positive features in that portion. As a result, the average current of the beam passing through the high-feature-density portion  41  is substantially higher than the average current passing through the low-feature-density portion  43 . This discrepancy in beam current yields a different magnitude of the Coulomb effect in the high-feature-density portion  41  compared to the low-feature-density portion  43 . 
     To alleviate the difference in Coulomb effect within a single exposure unit  31 , multiple small “micro features”  39  are defined by and extend through the scattering bodies  37  parallel to the optical axis in the low-density portion  43 . The micro features  39  serve to increase the average beam current passing through the low-density portion  43  and hence reduce the difference in beam current between the portions  41 ,  43 . The micro features  39  can be formed in the scattering bodies  37  by any of various suitable techniques such as etching. 
     Turning now to the reticle of FIG.  3 (B), an exposure unit  31 ′ is shown that comprises a high-feature-density portion  41 ′ and a low-feature-density portion  43 ′. The high-feature-density portion  41 ′ comprises scattering bodies  35 ′, and the low-feature-density portion  43 ′ comprises scattering bodies  37 ′. The scattering bodies  35 ′,  37 ′ each define a respective negative feature as projected onto the wafer. Micro features  45  extend (desirably parallel to the optical axis) not only through the scattering bodies  35 ′ and the scattering bodies  37 ′ (i.e., through the negative features) but also through the underlying membrane  33 ′ without distinction between the high-feature-density portion  41 ′ and the low-feature-density portion  43 ′. In addition, micro features  45  can be provided that extend through the membrane  33 ′ in positive features in the low-feature-density portion  41 ′ and optionally also in the high-feature-density portion  43 ′. Because the micro features  45  can be formed by any of various techniques such as ion-beam etching, the FIG.  3 (B) reticle can be manufactured simply. 
     As an alternative to methods and reticles configured as described above, the present invention also encompasses reticles (and methods for their use) that include “dummy” features. Dummy features are features that are resolvable by the CPB optical system but do not contribute to the overall functional pattern as projected onto the substrate. Nevertheless, dummy features can provide a more uniform beam current from one area to another as projected onto the substrate. An example of a reticle defining dummy features is shown in FIG. 6, which specifically depicts a portion  200  of a reticle. The portion  200  defines multiple actual pattern features  201  represented by solid-line squares dispersed in a high-feature-density region  203 . The portion  200  also defines dummy features  202  represented by dashed-line squares dispersed in a low-feature-density region  204 . By way of example, the features  201  are sized on the reticle to produce (0.1 μm) 2  corresponding features on the reticle. Similarly, the dummy features  202  are sized on the reticle to produce (0.1 μm) 2  corresponding features on the substrate. (However, it will be understood that the dummy features need not be the same size or configuration as the “real” features.) The dummy features  202  differ from the “real” features  201  in that the dummy features, as formed on the substrate, contribute nothing to the overall functional pattern transferred to the substrate, even though the dummy features  202 , like the “real” features  201 , can be resolved by the projection-optical system. Even though the dummy features  202  are resolved, it is of no consequence because the dummy features are situated deliberately in regions not occupied by “real” features and that do not interfere with the function of the “real” features. 
     FIG. 7 is a flowchart of an exemplary semiconductor fabrication method to which reticles and methods according to the invention can be readily applied. The fabrication method generally comprises the main steps of wafer production (wafer preparation), reticle production (reticle preparation), wafer processing, device assembly, and inspection. Each step usually comprises several sub-steps. 
     Among the main steps, wafer processing is key to achieving the smallest feature sizes (critical dimensions) and best inter-layer registration. In the wafer-processing step, multiple circuit patterns are successively layered atop one another on the wafer, wherein the formation of each layer typically involves multiple sub-steps. Usually, many operative semiconductor devices are produced on each wafer. 
     Typical wafer-processing steps include: (1) thin-film formation involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires; (2) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (3) etching or analogous step to etch the thin film or substrate according to the resist pattern, or doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (4) resist stripping to remove the resist from the wafer; and (5) chip inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired semiconductor chips on the wafer. 
     FIG. 8 provides a flow chart of typical steps performed in microlithography, which is a principal step in wafer processing. The microlithography step typically includes: (1) resist-coating step, wherein a suitable resist is coated on the wafer substrate (which can include a circuit element formed in a previous wafer-processing step; (2) exposure step, to expose the resist with the desired pattern; (3) development step, to develop the exposed resist; and (4) annealing step, to enhance the durability of the resist pattern. Microlithography also requires a reticle, which can be a reticle as described herein. 
     Reticles and methods according to the invention can be applied to a semiconductor fabrication process, as summarized above, to provide substantially improved pattern-transfer resolution and accuracy without sacrificing throughput. 
     Therefore, the present invention provides reticles, and methods for their use, that render more uniform image distortion and blurring within individual exposure units and from one exposure unit to another. As a result, projection microlithography using a charged particle beam can be performed with greater accuracy and resolution of pattern features on the wafer. 
     Whereas the invention has been described in connection with multiple representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.