Patent Number: 
Section: description

The invention is described below in the context of multiple representative embodiments. However, it will be understood that the invention is not limited to those embodiments. First Representative Embodiment A plan view of a hollow-beam aperture according to this embodiment is shown in FIG. 1. The FIG. 1 embodiment is substantially planar in configuration and includes a first member 1 and multiple second members 2a, 2b. The first member 1 is attached to the second planar members 2a, 2b by respective flexible members 3a, 3b. The first planar member 1 includes opposing triangular portions 12 of which the respective apices are connected together by a narrow bridge 13. In the middle of the bridge 13 is a circular portion 11. The triangular portions 12 each have opposing edges 14a, 14b, and the second members 2a, 2b each have opposing edges 22a, 22b, respectively. The respective distal ends 15a, 15b of the second members 2a, 2b define respective semicircular cutouts 21a, 21b. Each of the semicircular cutouts 21a, 21b has a radius slightly larger (by approximately 20 xcexcm) than the radius of the circular portion 11. The edges 22a, 22b are configured to engage the edges 14a, 14b, respectively, in a conformable manner. In such a configuration, the bridge 13 and triangular portions 12 support the circular portion 11. The first member 1, second members 2a, 2b, and the flexible members 3a, 3b desirably are formed as a single unit as shown. Alternatively, these members can be separate from each other. Also, the flexible members 3a, 3b are not required. However, they are desirable because they simplify engagement of the second members 2a, 2b with the first member 1 (which occurs whenever the edges 14a, 14b engage the respective edges 22a, 22b). The flexible members 3a, 3b desirably are sufficiently thin to allow flexible expansion and contraction of the members. A unified structure (including the members 1, 2a-2b, and 3a-3b) can be manufactured (e.g., by EDM) from a 0.5-mm thick sheet of heat-resistant material (e.g., W, Mo, or sintered graphite). For use as a hollow-beam aperture, the FIG. 1 structure is configured further, as shown in FIGS. 2(a)-2(b), to include a support base 4, respective movable members 5a, 5b, nuts 6a, 6b, and screws 7a, 7b. FIG. 2(a) is a plan view, and FIG. 2(b) is an elevational section through the midline of FIG. 2(a). The first member 1 is mounted to the support base 4. The support base 4 defines a centrally located through-hole 8. The movable members 5a, 5b are affixed to the respective second members 2a, 2b, and the nuts 6a, 6b are attached to opposite ends of the first member 1. A respective screw 7a, 7b is attached rotatably to each movable member 5a, 5b is threaded through the respective nut 6a, 6b. By rotating the screws 7a, 7b, the respective second members 2a, 2b can be moved closer together or farther apart. Such movements of the second members 2a, 2b are opposed by a respective force produced by the respective flexible member 3a, 3b.  As the second members 2a, 2b move toward each other, the edges 14a, 14b eventually contact the edges 22a, 22b, respectively. After such contact, further turning of the screws 7a, 7b causes the respective second members to move further together relative to the triangular portions 12 until the respective distal edges 15a, 15b contact the lateral edges of the bridge 13. The first member 1 and second members 2a, 2b are shaped such that, whenever the second members 2a, 2b are moved maximally together in this manner, no gaps exist between the distal edges 15a, 15b and the bridge 13 or between the edges 14a, 22a and 14b, 22b. Also, whenever the second members 2a, 2b are moved maximally together in this manner, the circular portion 11 is concentric with the semicircular cutouts 21a, 21b. Thus, during use, the circular portion 11 serves as a beam-absorbing body, and the space between the circular portion 11 and the semicircular cutouts 21a, 21b defines a substantially annular beam aperture (i.e., the aperture is contiguously ring-shaped except where interrupted by the bridge 13) surrounding the circular portion 11. The annular aperture is transmissive to the charged particle beam (which also passes through the through-hole 8), while the circular portion 11 and structure surrounding the annular aperture absorb charged particles of the beam incident on such structure. In the foregoing description, the circular portion 11 and cutouts 21a, 21b are emphasized by making them appear disproportionately large in FIGS. 1 and 2(a)-2(b). However, it will be understood that the cutouts 21a, 21b actually are no more than 100 xcexcm in diameter, whereas the second members 2a, 2b and first member 1 are relatively quite large (a few millimeters and a few tens of millimeters wide, respectively). Second Representative Embodiment A hollow-beam aperture according to this embodiment is depicted in FIGS. 3(a)-3(b), and comprises a first member 31 and a second member 35 intended to be mounted together as shown in FIG. 3(b). The first member 31 comprises a central body 32a (desirably cylindrical in shape), a planar portion 32b surrounding the central body 32a, and radial supports 33 connecting the central body 32a to the planar portion 32b. The planar portion 32b defines fastening holes 34. The second member 35 defines fastening holes 36 and a beam-transmitting opening 37. Appropriate fasteners (e.g., screws, not shown) are inserted through respective fastening holes 34, 36 to mount the second member 35 to the first member 31. The first member 31 defines a substantially circular through hole 38 in which the central body 32a is disposed centrally and coaxially, supported by the supports 33 (e.g., three supports 33 as shown). The first member 31 desirably is made of a heat-resistant material such as W, Mo, or sintered graphite, formed desirably by EDM as a single unit shaped as shown. The central body 32a is structured so that it extends above the plane of the first member 31 by at least the thickness of the second member 35. The second member 35 desirably is made from an approximately 0.5-mm-thick sheet of W, Mo, or sintered graphite. In an assembled hollow-beam aperture according to this embodiment, the central body 32a functions as a beam-absorbing body. The central body 32a also can be used during fabrication of the hollow-beam aperture as an EDM electrode for cutting the opening 37 in the second member 35. The opening 37 desirably has a radius that is approximately 20 xcexcm larger than the radius of the central body 32a. To assemble the hollow-beam aperture, the respective fastening holes 34, 36 in each comer are aligned with each other (FIG. 3(b)) and secured with screws or the like. Certain aspects of a top plan view of the assembled hollow-beam aperture according to this embodiment are shown in FIG. 4. In this figure, the annular gap between the central body 32a and the edge of the opening 37 readily can be seen. During EDM as described above, the outer edge of the opening 37 is formed due to the existence of a spark gap between the central body 32a and the second member 35 as the central body 32a is passed through the graphite sheet of the second member 35. In FIGS. 3(a)-3(b) and 4 the central body 32a and opening 37 are emphasized by making them appear disproportionately large. Actually, the opening 37 typically is less than 100 xcexcm in diameter, as in the first representative embodiment. The sizes of the first and second members, on the other hand, is arbitrary. Third Representative Embodiment A hollow-beam aperture according to this embodiment is shown in FIGS. 5(a)-5(b), and the results of certain steps in the manufacture of this embodiment are depicted in FIGS. 6(a)-6(d), respectively. Each of FIGS. 5(a)-5(b) is an isometric view, wherein FIG. 5(a) is an upper oblique view and FIG. 5(b) is a lower oblique view. The hollow-beam aperture comprises a main body 52 machined to have structural elements including a beam-absorbing body 51 and a support bar 53. The main body 52 also defines openings 54, 56 and spaces 55 that are best understood from a description of how the hollow-beam aperture of this embodiment is manufactured. Finally, when viewed axially (FIG. 6(d)), the hollow-beam aperture defines a substantially annular aperture 57 surrounding the beam-absorbing body 51. Fabrication of the hollow-beam aperture is described with reference to FIGS. 6(a)-6(d). To make these figures easier to understand, the dimensions of the beam-absorbing body 51 and the annular aperture 57 are depicted larger than actual, compared to other structural features. In reality, the annular aperture 57 has an outer diameter of 100 xcexcm or less, whereas the main body 52 has a width on each side of several millimeters. Fabrication begins with a block 61 of a suitable rigid, beam-absorbing material such as W, Mo, or sintered carbon. The block 61 desirably has a rectangular parallelopiped shape. Turning the block 61 about an axis A allows a cylindrical portion 62 (rotationally symmetric about the axis A, destined to be the beam-transmission axis, and relatively thick in the axial direction) to be cut (FIG. 6(a)). The cylindrical portion 62 is destined to become the beam-absorbing body 51, and has a diameter D1 and a xe2x80x9cheightxe2x80x9d (along the axis A) of H1. Thus, the remaining block 61 has a thickness H2. Similarly, turning the block 61 about the axis A allows a cylindrical void 63 to be cut (rotationally symmetrical about the axis A; FIG. 6(b)). The void 63 essentially forms the circular opening 56, and has a diameter D3 (wherein D1 less than D3) and a xe2x80x9cheightxe2x80x9d H3. Thus, of the original block 61, a rotationally symmetric shoulder 64 (having a xe2x80x9cheightxe2x80x9d of H2-H3, and being relatively thin in the axial direction, i.e., (H2-H3) less than H1)) and a portion 65 (that is relatively thick in the axial direction) are left. Two cylindrical voids 66a, 66b are formed (e.g., by drilling) along respective axes that are perpendicular to the axis A (see FIG. 6(c), showing the full circumference of each void). The radius of each void 66a, 66b is selected such that, when the finished hollow-beam aperture is viewed from a direction along the axis A (FIG. 6(d)), the substantially annular aperture 57 is formed that is bisected by the support bar 53 and flanked by lateral bars 67. The three manufacturing steps described above and shown in FIGS. 6(a)-6(c), respectively, can be performed in any order. In this embodiment, the cylindrical portion 62 has a diameter equal to the intended inner diameter of the annular aperture 57. The cylindrical void 63 desirably has a diameter equal to the intended outer diameter of the annular aperture 57. Thus, a structure is formed that, when viewed along the axis A, is rotationally symmetric (FIG. 6(d)). Along the axial direction, the cylindrical portion 62 is relatively thick, the shoulder 64 is relatively thin, and the lateral bars 67 are relatively thick. As noted above, the voids 66a, 66b can be formed by drilling from a lateral direction perpendicular to the axis A, but such that the voids flank the axis A (FIG. 6(c)). Thus, the voids 66a, 66b remove part of the shoulder 64 that is relatively thin in the axial direction. (Compare FIG. 6(c) with FIG. 6(b); also, as shown in FIG. 5(a), the only remaining portion of the shoulder 64 defines the support bar 53.) The items shown in FIGS. 5(a)-5(b) and 6(a)-6(d) are related as follows when manufacture is complete: the cylindrical portion 62 is the beam-absorbing body 51, which is relatively thick in the axial direction; the wall 65 corresponds to the main body 52, which is relatively thick in the axial direction; portions remaining after machining of the rotationally symmetrical shoulder 64 correspond to the support bar 53, which is relatively thin in the axial direction; and the cylindrical void 63 corresponds to the opening 56. As shown in FIG. 5(a), the individual openings 54 are somewhat crescent-shaped especially when viewed from a non-orthogonal direction. However, when viewed orthogonally from above, as shown in FIG. 6(d), with the beam-absorbing body 51 centrally situated over the opening 56, the openings 54 collectively appear as a complete ring (except for the support bars 53). Fourth Representative Embodiment A hollow-beam aperture according to this embodiment is shown in FIGS. 7(a)-7(b). FIG. 7(a) depicts a step in the manufacture of the subject hollow-beam aperture. Two steps in the manufacture of this embodiment are the same as shown in FIGS. 6(a)-6(B), respectively, and described above in connection with the third representative embodiment. I.e., fabrication begins with a block of a suitable rigid, beam-absorbing material such as tungsten, molybdenum, or sintered carbon. Turning the block about the axis A allows the cylindrical portion 62 (rotationally symmetric about the axis A and relatively thick in the axial direction) to be cut. The cylindrical portion 62 is destined to become the beam-absorbing body 51. Similarly, turning the block about the axis A allows the cylindrical void 63 to be cut (rotationally symmetrical about the axis A). The diameter of the void 63 is destined to become the outer diameter of the annular aperture. Thus, of the original block, the cylindrical portion 62 (that is relatively thick in the axial direction), a rotationally symmetric shoulder 64 (that is relatively thin in the axial direction), and a portion 65 (that is relatively thick in the axial direction) are left (see FIG. 6(b)). In FIG. 7(a), in contrast with FIG. 6(c), machining in a direction perpendicular to the axis A is performed using a suitable rectangular EDM electrode 68 tilted at an angle a relative to the axis A and urged (arrows) obliquely toward the axis A. Each resulting cut 70 has a rectangular transverse profile as shown in FIG. 7(a) and produces a respective sloping surface 69 flanked by a lateral bar 67 (FIG. 7(b)). Other features of this embodiment are similar to corresponding features of the third representative embodiment and have the same respective reference numerals. After completion of the machining described above (in which the individual machining steps can be performed in any order), the resulting hollow-beam aperture has a substantially annular aperture 57 that is ring-shaped except for support bars 53 supporting the beam-absorbing body 51. In this embodiment the annular aperture 57 has an inner diameter that is established by the diameter of the cylindrical portion 62, an outer diameter that is established by the diameter of the cylindrical void 63, and a radial width equal to the difference in these two diameters. Hence, the radial width of the annular aperture 57 can be made extremely small. Also, the annular aperture 57 is flanked by the sloped surfaces 69. Portions of an incident charged particle beam striking a sloped surface 69 are reflected diagonally, which prevents disturbance of the incident beam from stray particles generated by multi-path reflections. An advantage of this embodiment over the third representative embodiment is that this embodiment requires less complex machining than the third representative embodiment, and thus can be made in less time. I.e., in the third representative embodiment the openings 16 are formed by drilling through the entire width (several millimeters) of the body 51, whereas the cuts 70 of the fourth representative embodiment are formed readily (typically about 100 xcexcm wide) by EDM at respective locations flanking the ring-shaped opening 17. Also, the hollow-beam aperture of the fourth representative embodiment has greater mechanical strength than the third representative embodiment. Fifth Representative Embodiment A hollow-beam aperture according to this embodiment is depicted in FIGS. 9(a)-9(b), and FIGS. 8(a)-8(d) depict the results of certain respective steps in the manufacture of a hollow-beam aperture according to this embodiment. The hollow-beam aperture is formed from a main body 71 having a center axis 72 (which will be the beam axis). The main body 71 defines a ring-shaped groove 73 having a triangular sectional profile as shown in FIG. 8(a), truncated pyramidal openings 74, a peripheral frame portion 75, a beam-absorbing body 76, a support bar 77, sloping surfaces 78, a planar surface 79, and a substantially ring-shaped opening 80. To form the hollow-beam aperture of this embodiment, reference is made to FIGS. 8(a)-8(d). As shown in FIG. 8(a), the ring-shaped groove 73 (with triangular sectional profile) is machined into a bottom (in the figure) surface of the main body 71, about the axis 72. Cutting the groove 73 can be performed either by holding the main block 71 stationary and rotating a cutting tool about the axis 72, or holding the cutting tool stationary and rotating the main block 71 about the axis 72. This machining step can be performed using a lathe or by EDM. As shown in FIG. 8(b), the truncated pyramidal openings 74 are cut into the top (in the figure) surface of the main block 71 using complementary-shaped EDM electrodes (profiled by dash-dot lines) engaged perpendicularly (arrows) to the top surface. Each electrode has sloping sides. EDM machining is performed at bilaterally symmetrical positions relative to the center axis 72. EDM machining depths are set such that the width of the resulting ring-shaped opening 80 extending through the main block 71 after machining from above and below will be equal to the prescribed diameter of the hollow beam to be formed by the hollow-beam aperture of this embodiment. Sectional and plan views of the resulting hollow-beam aperture produced by the foregoing machining steps are shown in FIGS. 8(c) and 8(d), respectively. Isometric views of the hollow-beam aperture as seen from above and below are shown in FIGS. 9(a) and 9(b), respectively. These figures show the peripheral frame portion 75 and centrally located beam-absorbing body 76 supported relative to the frame portion 75 by the support bars 77. The beam-absorbing body 76 is surrounded by the substantially ring-shaped opening 80 and is coaxial with the center axis 72. The planar surface 79 is oriented perpendicularly to the center axis 72. The radial gap between the diameter of the base of the cone-shaped beam-absorbing body 76 and the outer diameter of the substantially ring-shaped opening 80 is configured to the prescribed value by using appropriate specifications for the downward-machining step (FIG. 8(b)), in view of the specifications used for the upward-machining step (FIG. 8(a)). Also, multi-path reflection problems are avoided by making the planar surfaces 79 as small as possible. Because the beam-absorbing body 76 is made cone-shaped in this embodiment, it has a sharp edge residing in a plane perpendicular to the center axis 72 (i.e., the same plane as that of the planar surface 79). This configuration provides an ideal trimming of the beam regardless of the angle of incidence of the beam to the hollow-beam aperture. Also, the inner and outer circumferences of the substantially ring-shaped opening 80 can be made perfectly concentric because they are both defined by the same groove 73. The sides of the support bars 77 desirably are sloped as shown. This allows the support bars 77 to be made as thin as possible, thereby minimizing possible adverse effects from the support bars 77 interfering with an inclined beam. In each of representative embodiments three, four, and five, the beam-transmitting portion of the respective hollow-beam aperture was formed by machining voids in two locations. Alternatively, it will be understood that this machining could have been performed in three or more locations. Whenever the machining in performed in three locations, the beam-absorbing body is supported by three support bars. Also, in any of the third, fourth, and fifth representative embodiments, machining for forming the beam-transmitting portion desirably is performed at rotationally symmetric locations about the center of the aperture. In so doing, the formation of a ring-shaped beam having anisotropic characteristics can be avoided. Sixth Representative Embodiment A hollow-beam aperture according to this embodiment is shown in FIGS. 10(a)-10(c), in which item 81 is a main block, items 82 and 83 are respective truncated conical openings, item 84 is a beam-absorbing body, item 85 is a support bar, and item 86 is a substantially ring-shaped opening. The truncated conical opening 82 is formed in the main block 81 by machining from above (FIG. 10(a)). The opening 82 is formed with its axis A being perpendicular to the top (in the figure) surface of the main block 81. The axis A of the opening 82 will become the center axis of the beam-absorbing body 84. Four truncated conical openings 83 are machined with their respective axes B being perpendicular to the bottom (in the figure) surface of the main block 81. The respective axes B of the conical openings 83 are equidistant from, and rotationally-symmetric about, the axis A of the opening 82. Machining is performed such that portions of the opening 82 are situated over the openings 83 below it. The respective sizes, positions, and depths of the openings 82, 83 are such that a xe2x80x9cpassagexe2x80x9d (comprising openings 86) is formed through the structure. Plan and isometric views of a structure machined in this manner are provided in FIGS. 10(b) and 10(c), respectively. In the center of the structure is the beam-absorbing body 84, which is supported in the main block 81 (as the equivalent of a first member) by four supports 85. Formed around the beam-absorbing body 84 is a substantially ring-shaped opening comprising four segments 86. The outer diameter of the beam-absorbing body 84 (i.e., the diameter of a circle inscribed in the beam-absorbing body 84) and the outer diameter of the substantially ring-shaped opening 86 (i.e., the diameter of a circle circumscribing the segments 86) are established by using the proper respective values for the size, location, and depth of the openings 82 and 83 above and below it, respectively. Hence, a substantially ring-shaped aperture, having a prescribed width and thickness, is provided in this representative embodiment. In this embodiment, the side wall of the truncated conical opening 82 desirably is sloped. Such a configuration reflects an incident beam at an angle that prevents multi-path reflection problems. Also, because the substantially ring-shaped opening 86 is formed in a plane that is perpendicular to the center axis A of the beam-absorbing body 84, the ring-shaped opening can achieve an ideal trimming of the beam, regardless of the angle of incidence of the beam to the hollow-beam aperture. In addition, because the substantially ring-shaped (annular) opening 86 is rotationally symmetric about the axis A, anisotropism of a beam passing through the annular aperture can be eliminated. Seventh Representative Embodiment A hollow-beam aperture according to this embodiment is shown in FIGS. 11(a)-11(b), and 12. In a conventional hollow-beam aperture as shown in FIG. 17, the thickness in the axial direction is relatively thin. Consequently, the hollow-beam aperture has a correspondingly low rigidity, which can result in distortion or fracture of the hollow-beam aperture. On the other hand, if the axial thickness of the hollow-beam aperture of FIG. 17 were to be increased to reduce distortion, then manufacture of the hollow-beam aperture would be made correspondingly more difficult; also, propagation of the beam through the substantially annular aperture may be impaired due to collision of particles of the beam with side walls of the openings. Since a beam incident in a normal manner to such a hollow-beam aperture typically can have an inclination (relative to the axis) of, for example, 8 mrad, a hollow-beam aperture configured as shown in FIG. 17 exhibits a significant absorption and scattering of incident charged particles in the beam. Reference now is made to FIGS. 11(a)-11(b), wherein FIG. 11(a) is an elevational section along the line A-Axe2x80x2 of FIG. 11(b). The hollow-beam aperture according to this embodiment comprises three portions 101, 103, 104 that can be made as three separate respective items laminated together, or as a single integrated unit. The portion 101 is a xe2x80x9ccharged-particlexe2x80x9d (xe2x80x9cCPxe2x80x9d) stop member, the portion 103 is a support member, and the portion 104 is an optional reinforcing member. The CP-stop member 101 defines an annular-shaped cutout 107 that serves to define a beam-absorbing member 102. The support member 103 defines multiple (e.g., four) openings 105 each being, for example, desirably circular with a diameter of 170 xcexcm. The reinforcing member 104 is optional; if a combination of the CP-stop member 101 and support member 103 provides sufficient rigidity to prevent distortion of the CP-stop member, then the reinforcing member 104 can be deleted. If present, the reinforcing member 104 defines an opening 108 that desirably is circular with a diameter of, e.g., 300 xcexcm and that is concentric with the substantially annular opening 107. A representative thickness of the reinforcing member 104 is 300 xcexcm. The support member 103 supports the CP-stop member 101. Hence, the CP-stop member 101 can be thin with low rigidity. (However, the CP-stop member 101 desirably is sufficiently thick to absorb incident charged particles; a representative thickness is 50 xcexcm.). A representative thickness of the support member is 150 xcexcm. In the configuration of FIGS. 11(a)-11(b), the support member 103 defines four circular openings 105 equiangularly and equi-radially spaced about the axis A. The portions of the support member 103 extending between the openings 105 serve as support bars 109 for a region 106 of the support member 103 that support the beam-absorbing member 102. Each opening 105 can have a relatively small diameter (e.g., 170 xcexcm). But, if the support member 103 is sufficiently thick to maintain requisite rigidity of the hollow-beam aperture, then portions of an incident beam divergently propagating through the substantially annular aperture 107 may be blocked by collision with a wall of an opening 105. In such a case, the diameter of the openings 105 can be increased as appropriate and the reinforcing member 104 be included to maintain adequate rigidity of the hollow-beam aperture. If the reinforcing member 104 is included, then the opening 108 desirably has a relatively large diameter (e.g., 300 xcexcm) to prevent the beam colliding with a wall of the opening 108 during passage of the beam through the hollow-beam aperture. FIG. 12 shows representative positional relationships of the various openings 105, 107, 108 of this embodiment. In the configuration of FIG. 5, the center of each opening 105 is coincident at a respective apex A, B, C, D of a square 110. The square 110 has sides each having a length xe2x80x9csxe2x80x9d. The square 110 is tangent to a circle 11 having a radius xe2x80x9crxe2x80x9d that is the midline of the substantially annular aperture 107. Each support bar 109 has a width xe2x80x9cwxe2x80x9d, wherein d=sxe2x88x92w. An alternative configuration of a hollow-beam aperture according to this embodiment is shown in FIGS. 13(a)-13(b), wherein FIG. 13(a) is an elevational section along the line C-Cxe2x80x2 of FIG. 13(b), and components that are similar to components shown in FIGS. 11(a)-11(b) have the same respective reference numerals. The configuration of FIGS. 13(a)-13(b) includes only two openings 105xe2x80x2 (analogous to the openings 105 in the configuration of FIG. 11(b)). Each opening 105xe2x80x2 has a profile that is extended from circular (i.e., each opening 105xe2x80x2 is an elongated slot with full-radius ends). Such a configuration exhibits reduced beam blocking, compared to the configuration of FIGS. 11(a)-11(b). However, the circular openings 105 in the configuration of FIGS. 11(a)-11(b) generally are easier to make than the openings 105xe2x80x2. The material used to manufacture a hollow-beam aperture according to this embodiment desirably is tantalum, molybdenum, or graphite. Graphite has the lowest cost. The openings desirably are formed by EDM or mechanical machining. In any of the embodiments described herein, the member situated at the center of the hollow-beam aperture has been referred to as a xe2x80x9cbeam-absorbing memberxe2x80x9d that blocks propagation of the incident beam through it. The center member alternatively can be a xe2x80x9cbeam-scattering memberxe2x80x9d that transmits the beam (while scattering the beam). In the alternative situation, a scattering aperture desirably is situated axially downstream to block propagation of the particles scattered by the center member (see item 47 in FIG. 17, discussed below). Eighth Representative Embodiment This embodiment of a charged-particle-beam (CPB) microlithography apparatus is shown in FIG. 14. The apparatus comprises a CPB source 41, illumination lenses 42, a hollow-beam aperture 43 such as any of the embodiments described above, a first aperture stop 44, a reticle 45, projection lenses 46, a second aperture stop (xe2x80x9cscattering aperturexe2x80x9d) 47, and a substrate (wafer) 48. As in a conventional CPB microlithography apparatus, the illumination lenses 42 cause the reticle 45 to be illuminated uniformly by a charged particle beam emitted from the CPB source 41. A pattern defined by the reticle 45 is imaged on the substrate 48 by the projection lenses, which form a latent image in a layer of resist on the wafer 48. The aperture stops 44, 47 are provided to shield the wafer 48 from scattered charged particles. As noted above, the FIG. 14 embodiment includes a hollow-beam aperture 43 desirably situated at a beam crossover (an axial location where the charged particle beam converges). Placing the hollow-beam aperture 43 at a crossover provides maximal effectiveness of the hollow-beam aperture 43 in preventing Coulomb effects. Although a hollow-beam aperture 43 placed at a crossover normally experiences heating to an extremely high temperature by absorption of particles of the charged particle beam, a hollow-beam aperture according to the present invention withstands such high temperatures extremely well. This is because, inter alia, the hollow-beam aperture is made of a material that can withstand high temperatures, such as sintered graphite, tantalum, or molybdenum, and constructed as described above. Another problem with placing the hollow-beam aperture at a crossover is that the beam is very small in transverse profile at such a location. Consequently, an annular aperture must have an extremely narrow radial width to make the hollow center of the beam large enough while achieving the desired reduction in Coulomb effects. Hollow-beam apertures according to the invention provide, for the first time, annular apertures that are sufficiently narrow in radial width (e.g., 20 xcexcm) to be effective at the crossover. Ninth Representative Embodiment FIG. 15 is a flowchart of an exemplary microelectronic-device fabrication method to which apparatus and methods according to the invention readily can be applied. The fabrication method generally comprises the main steps of wafer production (wafer fabrication and preparation), wafer processing, device (chip) assembly (including dicing, lead connections, and chip packaging), and device inspection. Each step usually comprises several sub-steps. Among the main steps, wafer (substrate) 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 layered successively atop one another on the wafer, wherein the formation of each layer typically involves multiple sub-steps. Usually, many operative microelectronic devices (e.g., memory chips or CPUs) are produced on each wafer. Typical wafer-processing steps include: (1) thin-film layer formation involving formation of a dielectric layer for electrical insulation or a metal layer for forming interconnects and electrodes; (2) oxidation of the thin-film layer or wafer substrate; (3) microlithography to form a resist pattern, for selective processing of the thin film or the substrate itself, according to a reticle; (4) etching or analogous step to etch the thin film or substrate according to the resist pattern; (5) impurity doping or implantation (e.g., by ion bombardment or diffusion) as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (6) resist stripping to remove the resist from the wafer; and (7) wafer inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired microelectronic device(s) on the wafer. FIG. 16 provides a flowchart of typical steps performed in microlithography, which is a principal step in wafer (substrate) processing. The microlithography step typically includes: (1) resist-coating step, wherein a suitable resist is coated on the wafer or 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) optional resist-annealing step, to enhance the durability of the resist pattern. Details of the microelectronic-device manufacturing process outlined above are well known by persons of ordinary skill in the art, and hence do not require elaboration here. Whereas the invention has been described in connection with multiple representative embodiments, it will be apparent 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.