Abstract:
Electron guns, and electron-beam optical systems including same, are disclosed that allow adjustment and attainment of a uniform transverse beam-intensity distribution in an electron beam. Such electron guns and systems are especially useful in electron-beam microlithography apparatus and methods. A representative electron gun includes a cathode having an electron-emitting surface, an anode for drawing electrons away from the cathode, and a filament array for applying electrical energy to a rear (upstream-facing) surface of the cathode. The filament array includes multiple independently controllable filaments.

Description:
FIELD OF THE INVENTION 
     The invention pertains to electron guns as used in apparatus and methods utilizing an electron beam, especially apparatus and methods in which an electron beam is used to perform projection of an image of a pattern (such as an integrated circuit pattern), as defined on a reticle, to a sensitive substrate (such as a semiconductor wafer). The invention also pertains to apparatus including such guns and to methods for manufacturing devices (e.g., semiconductor integrated circuits), wherein the methods utilize such projection apparatus. 
     BACKGROUND OF THE INVENTION 
     A key technology in manufacturing integrated circuits and displays is microlithography (image-transfer and imprinting technology). Feature sizes and line widths of integrated circuits progressively are becoming more miniaturized and have now reached the resolution limit of light (visible and ultraviolet light as used in “optical” microlithography). Electron-beam microlithography currently is under intensive investigation as a possible successor to optical microlithography, especially in view of the potentially greater resolving power of electron-beam microlithography compared to optical microlithography. 
     In electron-beam microlithography, an electron beam is produced by an electron gun. The beam is directed to a reticle (sometimes termed a “mask”) that defines the pattern to be transferred. The beam illuminates the pattern, or a selected portion thereof on the reticle, and the portion of the beam passing through the illuminated portion of the reticle is directed to a selected region of the substrate. More specifically, the electron beam propagating from the electron gun to the reticle is termed the “illumination beam,” which passes through an “illumination-optical system” to the reticle. The illumination-optical system typically includes multiple electromagnetic lenses that converge the illumination beam appropriately for illuminating the desired region of the reticle. Upon passing through the reticle, the illumination beam acquires an ability to form an image of the illuminated portion of the reticle; thus, the beam propagating downstream of the reticle is termed the “patterned beam.” The patterned beam passes through a “projection-optical system” to the substrate. The projection-optical system typically includes a pair of electromagnetic projection lenses that form a focused image, of the illuminated portion of the reticle, of a desired size on a corresponding region of the substrate. Hence, the image defined by the reticle is projected onto the substrate, usually portion-by-portion. This general process is also termed “pattern transfer” because the pattern defined by the reticle effectively is “transferred” to the substrate. 
     Conventional microlithography apparatus as summarized above normally produce a “demagnified” (or “reduced”) image on the substrate. This means that the image as formed on the substrate is smaller, usually by an integer factor, than the corresponding illuminated region on the reticle. The reciprocal of the integer factor is termed the “demagnification ratio,” of which a representative value is 1/4 or 1/5. 
     Electron guns used in conventional electron-beam microlithography apparatus of the type summarized above generally include three electrodes. The first electrode is a cathode used within a temperature-limitation region of its intensity-temperature (I-T) profile (FIG.  3 ). The second electrode is an anode that is charged appropriately to pull electrons away from the cathode to propagate through an axial aperture defined by the anode. The third electrode is a Wehnelt electrode (also termed a “Wehnelt cylinder”) that serves, inter alia, to guide electrons from the cathode through the anode aperture and thus, by preventing impingement of the electrons on the anode, reduce heating of the anode. In this conventional electron-gun configuration, the cathode and Wehnelt electrode are insulated electrically from each other, and have different electrical potentials (voltages) applied to them. 
     Many types of conventional electron-beam microlithography systems (e.g., variable-shaped pattern systems, character-projection systems, and divided-pattern projection systems) utilize a “solid” electron beam having a transverse profile (e.g., gaussian or rectangular) in which the beam intensity at the contrast aperture is greatest at the center of the beam. However, it has been found that, in such systems, a solid beam is subject to “space-charge effects” that are manifest as, e.g., focal-point shift, increase in beam blur, and distortion of the pattern as projected onto a wafer or other suitable substrate. In effort to solve problems associated with space-charge effects, electron guns have been investigated that produce a “hollow beam” in which, at the contrast aperture, the most intense portion of the beam is not located at the center of the beam, but rather at peripheral regions of the beam. 
     Unfortunately, no effective methods or apparatus exist to date for evaluating or adjusting a hollow beam. 
     As shown in FIG. 3, within the temperature-limitation region of the cathodic I-T profile, even a slight change in cathode temperature causes a substantial change in the intensity of beam current produced by the cathode. Consequently, in an electron gun in which the cathode is operated under temperature-limitation conditions, any irregularity in cathode-surface temperature or irregularity in the work function of the cathode surface causes the intensity distribution of the electron beam to be not uniform. An electron beam produced under such conditions does not provide a desired uniform illumination of the reticle. As a result, the dimensional accuracy of the pattern as transferred onto the substrate is degraded. This problem is difficult especially whenever a hollow beam is used. 
     For example, in order for an electron-beam microlithography apparatus to be viable commercially for high-volume production, it must have a per-shot exposure area of at least 1 mm×1 mm at the reticle, which requires a cathodic surface having an area of 3 to 10 mm 2 . Variations arising from the cathode being operated under temperature-limitation conditions can be substantial, especially with a gun having a cathode configured to produce a hollow beam. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide electron guns, for use in electron-beam microlithography apparatus that produce a demagnified image of the reticle pattern on the substrate, in which the transverse distribution of electron-beam intensity can be made uniform or otherwise adjusted as required. 
     To such end, and according to a first aspect of the invention, electron guns are provided that comprise a cathode, an anode, and a filament array. The cathode comprises an electron-emitting surface that emits a beam of electrons whenever the cathode is energized electrically. The anode is situated downstream of the cathode and can be energized at a voltage appropriate for drawing electrons from the cathode. The filament array is situated adjacent the cathode (e.g., adjacent an upstream-facing surface of the cathode) and is configured to energize respective regions of the cathode in a selective manner. The filament array comprises multiple filaments that are controllable independently to allow independent adjustment of electrical energy from the filaments to respective regions of the cathode. 
     Typically, the cathode and anode are arranged on an axis (“optical axis”), and the multiple filaments are arranged equidistantly from one another radially around the axis. For example, the filament array can comprise eight independently controllable filaments. In a particularly advantageous configuration, the electron-emitting surface is ring-shaped about the axis so as to emit a hollow beam of electrons, where each filament is adjacent a respective region of the ring-shaped electron-emitting surface. 
     The electron gun can include a control anode situated between the cathode and the anode. The electron gun also can include a Wehnelt electrode. 
     Each filament in the filament array is connected typically to a respective power supply and a respective bombardment-voltage power supply. The power supplies and bombardment-voltage power supplies are connected desirably to a CPU interface (or analogous controller) configured to energize the filaments and bombardment-voltage power supplies independently. For example, the filament array can be configured to bombard, when energized by the bombardment-voltage supplies, electrons onto an upstream-facing surface of the cathode. In such a configuration, each bombardment-voltage supply is controllable independently to allow independent adjustment of respective currents of electrons from the filaments bombarding the upstream-facing surface of the cathode. 
     The CPU interface can be connected to a computer or the like that is configured to receive and process data concerning a transverse beam-intensity profile of the electron beam and to route control signals to the bombardment-voltage power supplies as required to change the transverse beam-intensity profile of the electron beam. Alternatively or in addition, the CPU interface can be connected to a display. The display is configured to display data concerning a transverse beam-intensity profile of the electron beam. Such data can be used by an operator who inputs control commands to the CPU interface appropriate for causing the CPU interface to route control signals to the bombardment-voltage power supplies as required to change the transverse beam-intensity profile of the electron beam. 
     According to another aspect of the invention, electron-beam optical systems are provided, especially for use in an electron-beam microlithography apparatus. A representative embodiment of such a system comprises, on an optical axis, an electron gun, an illumination-optical system, and a projection-optical system. 
     The electron gun of the system comprises a cathode comprising an electron-emitting surface that emits an illumination beam of electrons whenever the cathode is energized electrically. The electron gun also comprises an anode situated downstream of the cathode. The anode can be energized at a voltage appropriate for drawing electrons from the cathode. 
     The electron gun can include a filament array situated adjacent the cathode and configured to energize respective regions of the cathode in a selective manner. The filament array comprises multiple filaments that are independently controllable to allow independent adjustment of electron current from the filaments to the respective regions of the cathode. 
     The illumination-optical system is situated downstream of the electron gun and is configured to direct the illumination beam to a region on a reticle situated downstream of the illumination-optical system. The region is illuminated by the illumination beam so as to produce a patterned beam propagating downstream of the reticle. The projection-optical system is situated downstream of the reticle and is configured to direct the patterned beam to a region on a substrate so as to imprint the substrate with a pattern defined on the reticle. 
     The system also comprises a first aperture situated off-axis, a first deflector, and a first detector. The first deflector is situated and configured to deflect, whenever the first deflector is energized, either the illumination beam or the patterned beam to the first aperture and to scan the beam relative to the first off-axis aperture. The first detector is situated relative to the first aperture and configured to obtain data concerning a transverse beam-intensity profile as the beam is scanned relative to the first off-axis aperture. 
     The first off-axis aperture and first deflector can be situated in the illumination-optical system in which the first deflector deflects and scans the illumination beam relative to the first aperture. A system having such a configuration also can comprise a second off-axis aperture, a second deflector, and a second detector. The second off-axis aperture is situated off-axis in the projection-optical system. The second deflector is situated in the projection-optical system and configured to deflect, whenever the second deflector is energized, the patterned beam to the second off-axis aperture and to scan the patterned beam relative to the second off-axis aperture. The second detector is situated relative to the second off-axis aperture and is configured to obtain data concerning a transverse beam-intensity profile as the patterned beam is scanned relative to the second off-axis aperture. 
     Alternatively, the first off-axis aperture, first deflector, and first detector can be situated in the projection-optical system. In such a configuration, the first deflector deflects and scans the patterned beam relative to the first off-axis aperture. 
     According to another aspect of the invention, methods are provided for detecting and adjusting a transverse beam-intensity profile of an electron beam produced in an electron-beam microlithography apparatus. (The apparatus includes, along an optical axis, an electron gun that produces an electron beam, an illumination-optical system that directs the electron beam to a reticle, and a projection-optical system that receives the electron beam from the reticle and directs the beam to a substrate.) In a representative embodiment of the method, the electron gun is provided with multiple filaments adjacent a cathode of the electron gun. Each filament is connected to a respective power supply and a respective bombardment-voltage supply, and each filament can be energized selectively to adjust an output of electrons from a respective region of the cathode. An off-axis aperture is provided on a plane at a position at which either the illumination-optical system or the projection-optical system forms an image of a beam crossover (e.g., gun crossover). A detector is situated downstream of the off-axis aperture and a deflector is situated upstream of the off-axis aperture. The deflector is energized to cause the deflector to deflect the electron beam laterally to the off-axis aperture. The electron beam is scanned across the off-axis aperture. Using the detector, electrons are detected that have passed through the off-axis aperture so as to produce a data signal corresponding to a transverse beam-intensity profile of the electron beam. Based on data in the data signal, electrical energy provided to at least some of the power supplies and bombardment-voltage supplies can be adjusted selectively as required to cause a change to the transverse beam-intensity profile. 
     The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a simplified elevational view of an electron gun according to a representative embodiment of the invention. 
     FIG.  1 ( b ) is a simplified plan view of the FIG.  1 ( a ) embodiment, showing detail of the upstream-facing surface of the cathode. 
     FIG. 2 is a schematic elevational depiction of a first representative embodiment of an electron-beam microlithography apparatus according to the invention that includes an electron gun of the type shown in FIGS.  1 ( a ) and  1 ( b ). 
     FIG. 3 is a graph of representative “I-T profiles,” i.e., plots of the variation in beam-current density (quantity of emitted electrons in a particular region of the transverse profile of the beam) as a function of cathode temperature, in an electron gun having a prescribed potential difference between the cathode and anode of the gun. 
     FIG. 4 is a schematic elevational depiction of a second representative embodiment of an electron-beam microlithography apparatus according to the invention. 
     FIG. 5 is a plan view of the aperture plate of the contrast aperture as used in the FIG. 4 embodiment. 
     FIG. 6 is a graph of beam-intensity distributions as measured by scanning the beam in the X- and Y-directions over a small off-axis aperture defined by the contrast-aperture plate of the FIG. 4 embodiment. The abscissa is scanned beam position in the X- or Y-direction, and the ordinate is beam intensity. 
     FIG. 7 is a graph of the beam-intensity distribution as measured by scanning the beam circularly over the small off-axis aperture defined by the contrast-aperture plate of the FIG. 4 embodiment. The abscissa is rotational angle θ and the ordinate is maximum beam intensity as a function of θ (i.e., the ordinate is I max (θ)). I max  is as shown in FIG. 6, and θ max  and θ min  represent angles of circular beam scanning in which I max  has a maximum value and a minimum value, respectively. 
     FIG. 8 is a flow chart of steps in a process for manufacturing a semiconductor device such as a semiconductor chip. 
    
    
     DETAILED DESCRIPTION 
     A representative embodiment of an electron gun according to the invention is described below with reference to FIGS.  1 ( a ) and  1 ( b ), showing an elevational view and a plan view, respectively. In FIG.  1 ( a ), the downstream direction extends upward in the figure. The electron gun comprises an anode  1 , a control anode  2 , a cathode  3 , and an array of sub-cathodic filaments  5  arranged along an “optical axis” AX. Although FIG.  1 ( a ) does not show a Wehnelt electrode, it will be understood that a Wehnelt electrode can be, and desirably is, included with this embodiment. 
     The anode  1  desirably is configured as a plate defining an axial aperture (opening). The anode  1  normally is at ground (zero voltage) potential. The anode  1  serves to draw electrons away from the cathode  3 , to which a negative voltage (acceleration voltage) is applied. 
     The control anode  2  desirably is configured as a plate defining an axial aperture (opening). The control anode  2  is situated, on the axis AX and parallel to the anode  1 , between the cathode  3  and the anode  1 . 
     The respective diameter and axial position of the aperture in the anode  1  and the aperture in the control anode  2  are optimized, by simulation, for high-emittance conditions with minimum generation of lens-effect aberrations. 
     The cathode  3  desirably is made of tantalum with a film  4  of iridium or rhenium selectively formed on the downstream-facing tantalum surface. By “selectively” is meant that the downstream-facing tantalum surface is covered with the film  4  except for an area of the tantalum surface defining a ring-shaped electron-emitting surface  6 . By way of example, with a cathode  3  having a diameter of 10 mm, the iridium or rhenium film  4  is deposited (e.g., by vacuum deposition) within a cathode radius of 0 to 4 mm and also within the cathode radius of 4.5 to 5 mm. Thus, a “ring”  6  of bare tantalum is left exposed within the remaining cathode region between radii of 4 and 4.5 mm. Because electrons are emitted from this exposed tantalum ring  6 , the ring  6  is also termed herein the “electron-emitting surface”  6 . 
     The work functions of iridium and rhenium are 5.3 eV and 4.7 eV, respectively, while the work function of tantalum is 4.19 eV. Hence, iridium and rhenium have larger respective work functions than tantalum. For this reason, the temperature of the cathode  3  can be established such that an electron beam is emitted only from the electron-emitting surface  6 . 
     Because the material of the film  4  is electrically conductive, the film is essentially at the same potential as the electron-emitting surface  6  on the cathode  3 . Consequently, the film  4  does not perturb the electric field near the electron-emitting surface  6 . 
     The cathode  3  is mounted centrally to a support member  10  made of an electrically conductive but thermally insulating material. Desirably, the support member  10  is made of ceramic with an electrically conductive metal coating. The metal coating is connected to ground. 
     In the FIG.  1 ( a ) embodiment, eight filaments (also termed “sub-cathodes”)  5  are placed adjacent the cathode  3 , more specifically adjacent a rear (upstream-facing) surface  3   a  (underside in the drawing) of the cathode  3 . As shown in FIG.  1 ( b ), the filaments  5  are spaced evenly from each other in a radial pattern around the center of the cathode  3 , “under” (i.e., upstream of) the ring-shaped electron-emitting surface  6 . Each filament  5  is connected to a respective d.c. power supply  7  and a respective bombardment-voltage supply  8 . The d.c., power supplies  7  simply heat the respective filaments  5 . The bombardment-voltage supplies  8  provide a continuous respective d.c. voltage to the respective filaments  5  sufficient to keep the respective filaments at desired respective negative voltages. Electrons from the filaments  5  locally bombard the rear surface  3   a  of the cathode  3 . Such bombardment causes localized heating of the electron-emitting surface  6 . Each bombardment-voltage supply  8  is controllable independently by a central-processing-unit (CPU) interface  9  to which the bombardment-voltage supplies  8  are connected. The bombarding electrons are returned to the bombardment-voltage supplies  8  through the support member  10 . Note that, by connecting the ground terminal of each bombardment-voltage supply  8  to the support member  10 , the ground connections are common. 
     Thus, the cathode  3  is not heated directly by the filaments  5  but rather by bombardment of electrons produced by the filaments  5  and directed to respective regions of the cathode. At each respective region of the cathode, the current of bombarding electrons can be changed rapidly using the bombardment-voltage supplies, which allows rapid changes of localized cathode temperature in contrast to the slow changes of localized cathode temperature otherwise achievable using direct heating of the cathode. 
     FIG.  1 ( b ) depicts a configuration in which eight filaments  5  are situated adjacent the rear surface  3   a  of the cathode  3  in an evenly spaced radial arrangement around the center of the cathode. However, the number and arrangement of the filaments  5  can be varied from the depicted configuration, depending upon the size of the cathode  3  and size and shape of the electrode-emitting surface  6 . 
     A first representative embodiment of a method (and corresponding system), according to the invention, for controlling an electron gun  21  as used in a reduced-image-type of electron-beam microlithography apparatus, is now described with reference to FIG.  2 . In FIG. 2, the dotted ray lines permit identification of axial locations that are conjugate to the surface of the cathode  3 . The solid ray lines permit identification of axial locations that are conjugate to a flat portion of a transverse intensity distribution of the electron beam formed near a beam crossover but located downstream of the cathode. 
     The electron gun  21  emits an electron beam (termed an “illumination beam”) in a downstream direction (downward in the drawing) along an optical axis AX. The illumination beam desirably is a hollow beam. The illumination beam is converged by a first condenser lens  22  and additionally converged by a second condenser lens  23 . The illumination beam is shaped, by passage through a beam-shaping aperture  24 , to have a transverse outer-edge profile (e.g., square) sufficient to illuminate a desired region on a reticle  30 . The beam-shaping aperture  24  desirably is situated within the second condenser lens  23 . The shaped illumination beam then passes through an illumination lens  29  to illuminate the desired region on the reticle  30 . The “illumination-optical system” of the FIG. 2 embodiment comprises the condenser lenses  22 ,  23 , the beam-shaping aperture  24 , and the illumination lens  29 . 
     The reticle  30  defines a pattern containing elements that differentially transmit and scatter electrons of the illumination beam. Thus, as the illumination beam passes through the illuminated portion of the reticle  30 , the beam becomes a “patterned beam” having an ability to form a downstream image of the illuminated portion of the reticle. 
     The patterned beam propagates through a “projection-optical system” which, in the FIG. 2 embodiment, comprises a first projection lens  31 , a second projection lens  32 , and a contrast aperture  37 . The projection-optical system forms the image, carried by the patterned beam, on the surface of a suitable substrate  33  (e.g., semiconductor wafer). Because the image formed on the substrate  33  is smaller than the corresponding illuminated region on the reticle  30 , the projection-optical system “demagnifies” the image by a “demagnification ratio” (e.g., 1/4 or 1/5). Thus, a “reduced” or “demagnified” image of the reticle pattern is formed on the surface of the substrate  33 . The upstream-facing surface of the substrate is coated with a resist that, when exposed by the patterned beam, becomes imprinted with the pattern. 
     The beam-shaping aperture  24  is configured essentially as an aperture plate that defines an axial opening through which the illumination beam passes. In the FIG. 2 embodiment, the aperture plate of the beam-shaping aperture  24  also defines a relatively small aperture (in this example, approximately 2 μm in diameter) situated off-axis. Thus, the off-axis aperture  25  in the figure is in the same plane as the beam-shaping aperture  24 . (The off-axis aperture  25  alternatively can be placed at any plane that is optically conjugate with the beam-shaping aperture.) Situated downstream of the off-axis aperture  25  is a first detector  26  (desirably configured as a “Faraday cage” as known in the art) used for detecting electrons of the illumination beam passing through the off-axis aperture  25 . 
     The contrast aperture  37  is located axially where an image of the cathode is formed between the first and second projection lenses  31 ,  32 . Such a location in this embodiment is also where the entrance pupil of the projection lenses  31 ,  32 , regarded collectively, is located. The contrast aperture  37  is configured essentially as an aperture plate that defines an axial opening through which the patterned beam passes. In the FIG. 2 embodiment, the aperture plate of the contrast aperture  37  also defines a relatively small aperture  35  (in this example, approximately 2 μm in diameter) situated off-axis. Thus, the off-axis aperture  35  is in the same plane as the contrast aperture  37 . Situated downstream of the off-axis aperture  35  is a second detector  36  (desirably configured as a “Faraday cage”) used for detecting electrons that pass through the off-axis aperture  35 . 
     The off-axis aperture  35  (with downstream detector  36 ) alternatively can be located on a plane where an image of the cathode is formed between the first condenser lens  22  and the illumination lens  29 . This alternative location is optically conjugate to the location shown in FIG.  2 . 
     FIG. 2 also depicts the CPU interface  9 , discussed above, to which the bombardment-voltage supplies are connected (see FIG.  1 ( b )). The CPU interface  9  is also connected to each detector  26 ,  36 , to a display  27  (e.g., a cathode-ray tube, or “CRT”), and to a computer  11 . A first deflector  28  located upstream of the beam-shaping aperture  24  and a second deflector  34  located upstream of the contrast aperture  34  are also connected to the display  27 . The switch in the line connecting the deflectors  28 ,  34  together is used to change the source of the displayed intensity distribution in a selective manner, i.e., to select the source as being the beam-shaping aperture or the contrast aperture. The switch in the line connecting the second deflector  36  to the display  27  is used to select the intensity distribution at the contrast aperture that can be seen by the operator. 
     During operation of the FIG. 2 embodiment, a “coarse” beam-uniformity adjustment can be performed. To such end, as the electron beam is propagating downstream from the electron gun  21 , the deflector  34  is energized to deflect the beam laterally to the off-axis aperture  35 . As the beam is being deflected in such a manner, the beam is scanned over the off-axis aperture  35  in the X- and Y-directions (in the figure, horizontally and along a line perpendicular to the plane of the page of the drawing). As the off-axis aperture  35  is being scanned, signals representative of the amount of deflection imparted to the beam by the second deflector  34 , and the electron intensity as detected by the second detector  36 , are routed to the display  27 . From the amount of deflection can be determined the portion of the beam diameter (beam transverse section) that is passing through the off-axis aperture  35 . The display  27  displays a plotted representation  27   p  of electron intensity (on the vertical axis) versus position along the diameter of the beam (on the horizontal axis). 
     The switch in the line connecting the second detector  36  to the CPU interface  9  is used to select whether the display  27  is being used in a “manual” mode or in an automatic-control mode as controlled by the CPU interface  9 . 
     The electron-intensity distribution as displayed on the display  27  can be observed (such as by a human operator) who can enter appropriate commands via the computer  11  connected to the CPU interface  9 . Exemplary commands would include a specification of which filaments  5  require voltage adjustment and control, and a determination of appropriate values of bombardment voltage to be provided by the respective supplies  8 . Such commands achieve appropriate adjustment of the filaments  5  to achieve an optimal distribution of beam intensity. 
     Since the portion of the beam scanned for making the “coarse” adjustment is at a location, along the optical axis AX, where an image of the cathode is formed, the displayed intensity distribution at such a point would be expected to have a two-peak curve profile representative of a ring-shaped beam. 
     The degree of rotation of the beam achieved by each of the various lenses in the FIG. 2 embodiment desirably is computed in advance. The data from such computations allow the degree of rotation of the electron beam emitted from the electron-emitting surface  6  to be taken into account when determining, based on the electron-intensity distribution on the display  27 , the particular filaments  5  requiring voltage control. From the CPU interface  9 , commands from the computer  11  are routed as voltage-control signals to the appropriate bombardment-voltage supply(ies)  8 . These calculations can be repeated as required to obtain a desired or prescribed transverse intensity profile of the beam. 
     After a particular “coarse” intensity profile of the beam is obtained, fine adjustments of the beam can be performed as required. To perform a “fine” adjustment, the first deflector  28  is energized to scan the beam in the X- and Y-directions over the first off-axis aperture  25 . Data obtained from such scanning are used to determine the particular filaments  5  requiring further adjustment of energizing voltage, in a manner similar to the “coarse” adjustment described above. The “fine” adjustment can be repeated as required to obtain a desired or prescribed electron-intensity distribution. 
     By way of example, if the electron intensity is low on the left and high on the right, as indicated by the profile  27   p  on the display  27  shown in FIG. 2, then the location on the cathode corresponding to the left side as detected (taking into account the degree of beam rotation) has a lower temperature than a location on the cathode corresponding to the right side as detected. (A region of the cathode at a lower temperature produces relatively fewer emitted electrons.) 
     Generally, it is sufficient to perform an adjustment of the electron gun  21 , as described above, only after replacing the electron gun in a particular microlithography apparatus. However, it is possible for an electron gun to exhibit unstable performance, under which condition the electron gun desirably is adjusted more frequently, such as every time a new reticle is used. 
     Instead of having a technician observe the transverse intensity profile  27   p  as shown on the display  27  and manually enter corresponding data to the computer  11 , it is possible for the computer  11  directly to process data routed to the display. In the latter instance, the results of such data processing can be used directly, without human intervention, to control the output from the cathode  3 . 
     As described above, an electron gun  21  according to the present invention desirably comprises multiple independently voltage-controllable filaments  5  situated and arranged adjacent the cathode  3 . The respective voltages supplied to the filaments  5  can be controlled individually so as to make adjustments to the transverse intensity of the electron beam. Such adjustments can yield a more uniform intensity profile, which yields a more accurate pattern transfer by the microlithography apparatus. 
     Because the filaments  5  desirably are situated adjacent the upstream-facing surface of the cathode  3 , and desirably are arranged in an equally spaced radial pattern around the center (axis) of the cathode, the uniformity of the electron-beam intensity can be controlled with high accuracy. 
     In addition, because a ring-shaped beam thus can be emitted from the electron-emitting surface  6 , the influence of space-charge effects on the reticle image can be reduced. 
     FIG. 4 depicts a second representative embodiment of a method and system according to the invention. An electron gun  41  is situated at the upstream end of the system shown in FIG.  4  and emits a beam of electrons in a downstream direction (downward in the figure). The electron gun  41  comprises three electrodes: a cathode  41   a,  a control anode  41   c,  and an anode  41   d.  The cathode  41   a  has a ring-shaped electron-emitting surface  41   b.  The ring-shaped electron-emitting surface  41   b  can be formed using a material having a relatively low work function, with surrounding and central regions of the cathode  41   a  being formed of a material having a relatively high work function. 
     The anode  41   d  defines a respective aperture through which electrons from the cathode  41   a  can pass. The control anode  41   c,  located between the cathode and the anode, defines a respective aperture through which electrons from the cathode  41   a  can pass. The cathode, control anode, and anode are connected to a power supply  42  supplying respective potentials to these electrodes. By way of example, the cathode  41   a  is energized with −100 KV, the anode  41   d  has a ground potential, and the control anode  41   c  is energized at −82 KV. 
     The electron beam (illumination beam) emitted from the cathode  41   a  of the FIG. 4 embodiment is hollow at the cathode and its conjugate plane(s), with a ring-shaped transverse profile. The illumination beam forms a “gun crossover” downstream of the cathode  41   a.  I.e., electrons emitted from the electron-emitting surface  41   b  converge at an axial region, denoted by the numeral  43 , at which the beam has its most uniform intensity distribution. 
     From the electron gun  41  to the reticle  50 , the illumination beam passes through an illumination-optical system including first and second condenser lenses  45 ,  46 , respectively, and first and second illumination lenses  48 ,  49 , respectively. A beam-shaping aperture  47  is situated between the second condenser lens  46  and the first illumination lens  48 . The beam-shaping aperture  47  has a function as described with respect to the FIG. 2 embodiment, serving generally to trim the outside edge of the transverse profile of the illumination beam. At the beam-shaping aperture  47 , the illumination beam is not hollow but rather has an even distribution (due to the imaging at the beam-shaping aperture  47  of the location  43 ). The first and second illumination lenses  48 ,  49  collectively form an image of the beam-shaping aperture  47  on the reticle  50 . The image of the beam-shaping aperture  47  is formed on a region of the reticle  50  termed an “exposure unit” or “subfield.” 
     Downstream of the beam-shaping aperture  47  is one or more deflectors (not shown) used for selectively deflecting the illumination beam to a desired subfield on the reticle  50  and for sequentially scanning the illumination beam, in a horizontal direction, to illuminate sequentially all the subfields within an optical field of the deflectors. 
     The reticle  50  and substrate (“wafer”)  56  are mounted on respective stages (not shown) that facilitate the sequential exposure of subfields on the reticle. 
     Downstream of the reticle  50  are first and second projection lenses  51 ,  55 , respectively (collectively defining a two-stage projection-lens system), a contrast aperture  54  (defined by a contrast-aperture plate), and deflectors (not shown). In the figure, one subfield of the reticle  50  is being illuminated by the illumination beam. Electrons of the illumination beam passing through the reticle  50  form a “patterned beam” that is reduced (demagnified) and converged as appropriate by the projection lenses  51 ,  55  for producing an image of the illuminated subfield on the wafer  56 . The location of the image on the wafer  56  can be adjusted as required by the deflectors (not shown). The wafer  56  is coated with an appropriate resist so as to be imprinted with the reticle images upon receiving a proper dose of electrons from the patterned beam. 
     The contrast aperture  54  is situated at a position at which the axial distance from the reticle  50  to the wafer  56  is divided by the “demagnification ratio” of the projection lenses  51 ,  55 . The contrast aperture  54  is situated on a plane that is optically conjugate with the electron-emitting plane of the cathode  41   a.  (In FIG. 4, the position of the contrast aperture  54  is the entrance pupil of the two-stage projection lens formed by the first and second projection lenses  51 ,  55 .) The contrast-aperture plate is used to block electrons, scattered by the reticle  50  but lacking any image-forming information, from propagating to the wafer  56 . 
     The contrast-aperture plate also defines a small off-axis aperture  53  used for beam adjustment, as described later. 
     Downstream of the first projection lens  51  is a deflector  52  used to scan the patterned beam over the off-axis aperture  53  for adjusting the beam. 
     Downstream of the second projection lens  55  but upstream of the wafer  56  is a secondary-electron detector  57 . The detector  57  captures secondary electrons emitted from the wafer  56  and detects them. During beam adjustment, the detector  57  is also used to detect electrons passing through the off-axis aperture  53  defined by the contrast-aperture plate. The detector  57  is connected to a display  58  that, upon receiving a corresponding electrical signal from the detector  57 , processes the signal to produce a displayed profile of the beam-intensity distribution. This signal can be processed synchronously with energizations of the deflector  52 . 
     In FIG. 4, the ray trace indicated by the dotted lines denotes regions in which imaging has uniform transverse beam intensity and allows identification of regions that are optically conjugate to the axial location  43 . The solid-line trace allows identification of regions that are optically conjugate to the plane of the electron-emitting surface of the cathode  41   a.  As shown in the figure, the cathode  41  a and contrast aperture  54  are conjugate. Also conjugate are the axial location  43  of uniform beam intensity, the beam-shaping aperture  47 , the reticle  50 , and the wafer  56 . 
     FIG. 5 is a plan view of a representative embodiment of a contrast-aperture plate  54   a  that defines the contrast aperture  54  used in the FIG. 4 embodiment. The actual contrast aperture  54  is circular and located in the center of the plate  54   a.  The contrast aperture  54  has a diameter “D” that is desirably 1.2 to 1.3 times the diameter (e.g., 1 mm) of the electron beam at the axial position of the contrast aperture. The diameter of the aperture plate  54   a  is about 20D. The aperture plate  54   a  also defines multiple off-axis apertures  53   a - 53   d.  By way of example, each off-axis aperture  53   a - 53   d  is about 0.1 mm in diameter, and situated apart from the contrast aperture  54  by a distance “S”, wherein S is 5D to 10D. Since the contrast aperture  54  is conjugate with the electron-emitting surface  41   b  of the cathode, the beam at the contrast aperture  54  is a hollow beam. The beam, even though hollow, can be evaluated using the off-axis apertures  53   a - 53   d.  To such end, the beam is scanned over an off-axis aperture  53   a - 53   d.  Electrons of the beam passing through an off-axis aperture  53  are detected using the secondary-electron detector  57 . The apertures  53   a  and  53   b  are used for calibrating deflection sensitivity of X-direction deflection, and the apertures  53   b  and  53   d  are used for calibrating deflection sensitivity of Y-direction deflection. 
     Even though the beam produced by the FIG. 4 embodiment is a hollow beam, any of various situations can cause the beam not to have an ideal hollow profile. Such situations include, but are not limited to, lens aberrations, focus shifts, and a non-uniform emission of electrons from the electron-emitting surface  41   b  of the cathode (see discussion above regarding the embodiment of FIGS.  1 ( a )-( b ) and  2 ). A procedure as described below can be followed to ascertain whether the hollow beam has a desired transverse profile. 
     FIG. 6 is a representative graph of transverse beam intensity as measured when scanning the beam in the X- or Y-direction across an off-axis aperture  53  in the contrast aperture plate  54   a.  The abscissa is the position in the X- or Y-direction, and the ordinate is beam intensity. Two peaks  31 ,  35  (solid-line curves) and the two peaks  33 ,  37  (dashed-line curves) are shown. Each set of curves  31 ,  35  and  33 ,  37  is of beam intensity measured as the beam is scanned over an off-axis aperture  53 . 
     The curves  31 ,  35  exhibit steep peaks, indicating that the hollow beam exhibits a desirable minimal blur. In contrast, the curves  33 ,  37  exhibit shallow peaks, indicating an excessively blurred beam. With respect to the solid-line curves  31 ,  35 , I max /h is an expression of peak sharpness, wherein I max  is the peak height, and h is the full width at half maximum (0.5)(I max ) serving as an approximate measure of the “hollow width” (outside radius minus inside radius) of the hollow beam. With respect to the dashed-line curves  33 ,  37 , I max ′ and h′ have similar respective meanings. As can be seen readily, I max  is high and h is small, whereas I max ′ is low and h′ is large. When obtaining a transverse intensity profile of the beam, if curves such as  33 ,  37  are obtained, the profile can be adjusted by, for example, changing the voltage applied to the control anode  41   c  of the electron gun  41  to change the axial position of an image of the cathode without changing the real position of the cathode. Alternatively, the excitation current applied to one or both of the condenser lenses  45 ,  46  can be changed as required to form the cathode image exactly at the contrast aperture  54 . The transverse profile of the hollow beam is optimized when I max /h is maximized. 
     FIG. 7 depicts a graph of the distribution of beam intensity whenever the beam is scanned over an off-axis aperture (e.g., aperture  53 ) while circularly scanning the beam. The abscissa is the rotational angle (θ) of circular scanning, and the ordinate is the peak beam intensity as a function of θ, i.e., I max (θ). In FIG. 7, I max  varies markedly, and the difference between I max (θ max ) and I max (θ min ) is fairly large. It is desired that the ratio of I max (θ max ) to I max (θ min ) be minimal (i.e., unity or nearly unity). A large value of this ratio can arise from any of various causes such as the condition of the electron gun (e.g., whether the electron-emission surface has a uniform temperature, see above). If temperature of the cathode is not uniform, it can be adjusted as discussed above with respect to FIGS.  1 ( a )- 1 ( b ). 
     FIG. 8 is a flow chart of steps in a process for manufacturing a semiconductor device such as a semiconductor chip (e.g., an integrated circuit or LSI device), a display panel (e.g., liquid-crystal panel), or CCD, for example. In step  1 , the circuit for the device is designed. In step  2 , a reticle (“mask”) for the circuit is manufactured. In step  3 , a wafer is manufactured from a material such as silicon. 
     Steps  4 - 12  are directed to wafer-processing steps, specifically “pre-process” steps. In the pre-process steps, the circuit pattern defined on the reticle is transferred onto the wafer by microlithography. Step  13  is an assembly step (also termed a “post-process” step) in which the wafer that has been passed through steps  4 - 12  is formed into semiconductor chips. This step can include, e.g., assembling the devices (dicing and bonding) and packaging (encapsulation of individual chips). Step  14  is an inspection step in which any of various operability and qualification tests of the device produced in step  13  are conducted. Afterward, devices that successfully pass step  14  are finished, packaged, and shipped (step  16 ). 
     Steps  4 - 12  also provide representative details of wafer processing. Step  4  is an oxidation step for oxidizing the surface of a wafer. Step  5  involves chemical vapor deposition (CVD) for forming an insulating film on the wafer surface. Step  6  is an electrode-forming step for forming electrodes on the wafer (typically by vapor deposition). Step  7  is an ion-implantation step for implanting ions (e.g., dopant ions) into the wafer. Step  8  involves application of a resist (exposure-sensitive material) to the wafer. Step  9  involves microlithographically exposing the resist so as to imprint the resist with the reticle pattern, as described elsewhere herein. Step  10  involves developing the exposed resist on the wafer. Step  11  involves etching the wafer to remove material from areas where developed resist is absent. Step  12  involves resist separation, in which remaining resist on the wafer is removed after the etching step. By repeating steps  4 - 12  as required, circuit patterns as defined by successive reticles are formed superposedly on the wafer. 
     Whereas the invention has been described in connection with representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.