Abstract:
Electron-beam sources are disclosed that exhibit substantially reduced spherical aberration compared to conventional sources. In a beam produced by the cathode of such a source, axially propagating electrons are subjected to a lens action by voltage applied to a Wehnelt electrode and an extraction electrode. The cathode includes a peripheral portion that is “drawn back” (displaced along the axis of the source away from the beam-propagation direction) relative to a center portion of the cathode. With such a cathode, the percentage of dimensions of the crossover involved in spherical aberration of the crossover is reduced. This improves the uniformity of beam current at a lithographic substrate and minimizes location-dependency of the aperture angle. Since the Wehnelt voltage can be reduced, positional changes in the electrical field at the cathode surface are reduced, and the distribution of electrons in the beam propagating from the cathode surface is made more uniform than conventionally.

Description:
FIELD OF THE INVENTION 
     This invention pertains to high-emittance electron-beam sources for use in any of various electron-beam devices, particularly electron-beam microlithography apparatus and related exposure apparatus. Electron-beam microlithography is a type of charged-particle-beam microlithography, which represents one of several “next-generation” microlithography technologies currently experiencing intensive development effort due to its potential for achieving substantially greater resolution than obtainable using conventional optical microlithography technology. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, micromachines, and the like. 
     BACKGROUND OF THE INVENTION 
     A conventional thermionic electron-beam source (also termed an “electron gun”) is shown in FIG.  7 . The depicted source includes a cathode  1  (serving as the electron-emitting surface), a Wehnelt electrode  3 , and an “extraction electrode” (anode)  4  arranged along an axis Ax. The cathode  1  normally is heated by a heating means (not shown but well understood in the art) to cause the cathode to emit hot electrons. The emitted electrons are formed into a beam  2  by the Wehnelt electrode  3  and anode  4 . Specifically, the anode  4  extracts electrons emitted from the cathode  1  and urges them to propagate in a downstream direction (to the right in the figure) from the cathode to the anode  4  and beyond. Electrons of a beam  2  emitted from the cathode  1  and propagating initially parallel to the axis Ax (i.e., axially propagating electrons) are subjected to respective lens actions by the respective voltages applied to the Wehnelt electrode  3  and anode  4 . The electrons of the beam  2  converge at a gun crossover  5 . The axial position of the gun crossover  5  is a function of the respective voltages applied to the Wehnelt electrode  3  and anode  4  (e.g., a higher negative voltage applied to the Wehnelt electrode  3  will tend to move the gun crossover  5  to the left in the figure). In a microlithography context, the beam  2  propagating downstream of the anode  4  is acted upon by a downstream electron-optical system (not shown) that shapes and conditions the beam for use as an illumination beam for illuminating a desired region of a reticle or mask or other object (not shown). 
     In certain types of electron-beam apparatus the emittance of the electron gun is critical, especially if the apparatus is used for making one-shot lithographic exposures of respective portions of a pattern, or for making reduced (demagnified) transfer-exposures of a pattern. “Emittance” is a quantitative expression of the ability of the beam to achieve uniform irradiation of a defined surface, and is expressed as the range of uniform beam current in an area irradiated by the electron beam  2  multiplied by the aperture half-angle of the beam at the irradiated region. 
     In electron microscopes and microlithography apparatus utilizing an electron beam having a transversely Gaussian distribution but configured as a spot beam for pattern drawing, emittance normally is not a significant variable. This is because the area illuminated by a spot beam at any instant in time is only 1 to 10 nm in diameter, which is effectively at the apex of the distribution. In contrast, in microlithography apparatus utilizing a reticle divided into subfields that are exposed individually with demagnification, merely forming the beam to irradiate a spot is insufficient for achieving proper pattern transfer because the area illuminated by the electron beam is substantially larger than 1-10 nm across. Rather, it is necessary to achieve uniform irradiation in an area measuring 10 μm square (typical of one-shot partial pattern block exposures) to 1 mm square (typical of one-shot reduced transfer exposures from respective subfields of a divided reticle). These latter areas encompass not only the apex of the Gaussian distribution but also the tails (distal or outlying portions) of the distribution. In addition, the aperture half-angle in these latter two cases is several mrad. As a result, to achieve the required uniform illumination over the desired one-shot area, high emittance from the electron gun is necessary. 
     To improve the transverse uniformity of the energy of the electron beam emitted from the electron gun used in apparatus for performing partial pattern block exposures and reduced transfer exposures from a divided reticle, the cathode normally is made transversely wide and planar as shown in FIG. 7. A wide planar cathode also improves the uniformity of beam current as incident on a substrate such as a semiconductor wafer when forming a microlithographic image of the cathode on the substrate. However, whenever electrons are emitted from a wide cathode surface, beam current tends to be excessive. Hence, various means conventionally are employed to prevent emission of extraneous electrons from the cathode. Exemplary conventional means include fabricating the cathode of a material having a high work function or applying a substance having a high work function to portions of the cathode surface located off axis. 
     If the cathode is a thermionic-emission type, the electron gun generally exhibits a relationship between emission-current density J c  and anode voltage V a  as shown in FIG.  10 . In the figure T c  is cathode temperature. For example, the relationship of J c  versus V a  for T c3  is indicated by the solid-line curve. The region where the relationship of J c  versus V a  is nearly according to J c ∝V a   2/3  is termed a “space-charge-limited” region. The more distal region is a “temperature-limited” region. 
     As the temperature of a thermionic cathode rises, beam current can become excessively high, and operation of the electron gun becomes space-charge limited. Whereas operation of the gun in a space-charge-limited manner can be performed in a stable manner, the presence of a high-charge field at or adjacent the cathode surface can cause the emitted electrons to lose characteristics reflective of the cathode surface from which they were emitted. If the electric field is substantially non-uniform, then electron emission from the cathode surface is not uniform. Under such conditions, the uniformity of current at the cathode surface conventionally cannot be utilized. Hence, there is a need for a way in which to utilize the electron gun in a temperature-limited region having a relatively low temperature and low beam current at the cathode. 
     Meanwhile, the distribution of the aperture angle is determined by the transverse energy distribution of the beam, which is determined by the cathode temperature of the cathode as electrons are being emitted from the cathode. The trajectories of electrons, emitted from the cathode  1  at a point of intersection of the optical axis Ax with the cathode surface, and emitted at an angle relative to the optical axis Ax are shown in FIG.  8 . Near-axis trajectories  6  determine the configuration and dimensions of a crossover  7  formed at the crossover point  5 . The spatial intensity of the beam at the crossover  7  is a function of the distribution of electron emission at the cathode surface, which (as discussed above) usually is a Gaussian distribution. 
     The emittance at a surface irradiated by the beam is determined not only by the uniformity of beam current and aperture angle at the cathode of the electron gun, but also by aberrations generated by lens actions generated by respective voltages applied to the electrodes of the electron gun. Whereas emittance can be preserved if the downstream optical system is free of aberrations, emittance is degraded by an optical system exhibiting significant aberrations. If emittance has deteriorated, it generally cannot be restored by the downstream optical system. 
     In conventional electron guns, substantial aberrations are imparted to the image at the crossover point  5  by the respective lens actions of the Wehnelt electrode  3  and the anode  4 , as shown in FIG. 9, depicting a spherical aberration. Whenever spherical aberration is evident at the crossover point  5 , the beam current is irregular and aperture angle becomes location-dependent at the wafer or other substrate. This, in turn, causes a substantial degradative deviation of actual emittance from the desired emittance, and results in a deterioration of the uniformity of imaging performed by the beam. 
     Therefore, there is a demand for electron-beam sources that exhibit substantially reduced spherical aberration at a crossover compared to conventional electron-beam sources. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide electron-beam sources that exhibit substantially reduced spherical aberration compared to conventional sources. 
     To such end, and according to a first aspect of the invention, electron-beam sources are provided. An embodiment of an electron-beam source according to the invention comprises a cathode situated on an axis and configured to emit an electron beam propagating along the axis in a downstream direction from the cathode. The cathode comprises a transversely planar center portion and a transversely planar peripheral portion. The peripheral portion is “drawn back” in an upstream direction along the axis relative to the center portion. 
     Drawing back the peripheral portion of the cathode relative to the center portion provides several advantages. One advantage is that peripheral portions of a beam exhibiting spherical aberrations can be effectively blocked using a downstream aperture situated at a crossover image plane. This results in a more uniform transverse beam-intensity distribution immediately downstream of this image plane. Another advantage is that a more uniform angular distribution of the beam at the reticle and wafer is obtained. Hence, if the beam source is used in an electron-beam microlithography apparatus, drawing back the peripheral portion of the cathode yields a corresponding improvement of the uniformity of beam current at the wafer and reduces the location-dependency of the aperture angle of the beam. This drawing back also allows the voltage applied to a Wehnelt electrode of the source to be reduced, yielding improved uniformity of the electrical field at the electron-emitting surface of the cathode, with a correspondingly improved uniformity of the distribution of electrons extracted from the electron-emitting surface. These benefits yield a substantially improved emittance compared to conventional electron-beam sources. 
     As suggested in the summary above, the subject electron-beam source can further include a Wehnelt electrode situated coaxially downstream of the cathode, and an extraction electrode (also termed an anode) situated coaxially downstream of the Wehnelt electrode. 
     According to another aspect of the invention, methods are provided (in the context of producing an electron beam using an electron gun including a cathode and a Wehnelt electrode arranged along an optical axis) for increasing uniformity of a field at an electron-emitting surface of the cathode. In an embodiment of such a method, the cathode is configured with a transversely planar center portion and a transversely planar peripheral portion. The peripheral portion is drawn back, relative to the center portion, in an upstream direction along the optical axis. This drawing back weakens a field imposed at the electron-emitting surface. 
     According to another aspect of the invention, electron-beam microlithography apparatus are provided that include an electron-beam source such as any of those, according to the invention, summarized above. Since the electron-beam source is characterized by having a high emittance, the subject apparatus is especially suitable for use in achieving uniformity of illumination of a selected region on a reticle. The subject apparatus also is especially suitable for use in lithographic situations requiring high emittance, such as partial pattern block exposure and/or reduced transfer-exposure of a pattern from a divided (“segmented”) reticle. 
     An electron-beam microlithography apparatus according to the invention can include an illumination-optical system and a projection-optical system. The illumination-optical system is situated along the axis downstream of the source. The illumination-optical system is configured to receive the electron beam, propagating as an illumination beam from the source, and to direct the electron beam to a reticle so as to illuminate a region on the reticle. The projection-optical system is situated along the axis downstream of the reticle. The projection-optical system is configured to receive the electron beam, propagating as a patterned beam from the reticle, and to direct the patterned beam to a substrate (e.g., semiconductor “wafer” coated with a suitable resist). 
     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 is a sagittal section of an electron-beam source according to a representative embodiment of the invention 
     FIG. 2 shows plots of the dimensional percentage of the crossover significantly affected by spherical aberration, versus the voltage impressed on the extraction electrode (anode) of the electron-beam source. In the plots, the coordinates indicated by diamonds are of an example of the embodiment shown in FIG. 1 (wherein the peripheral portion of the cathode is “drawn back” 4 mm relative to the center portion of the cathode), the coordinates indicated by squares are of a conventional electron-beam source such as that shown in FIG. 7 (wherein the cathode has a peripheral portion that is flush (i.e., coplanar with) the center portion thereof), and the respective coordinates indicated by triangles and circles are of respective examples in which the peripheral portion of the cathode is “drawn forward” 1 and 2 mm, respectively, relative to the center portion thereof. 
     FIG. 3 shows plots of the applied voltage to the Wehnelt electrode versus the voltage impressed on the extraction electrode of the electron-beam source. In the plots, the respective coordinates indicated by diamonds, squares, triangles, and circles are as described above in the summary of FIG.  2 . 
     FIG. 4 is an elevational schematic diagram of a representative embodiment of an electron-beam microlithography apparatus according to the invention. 
     FIG. 5 is a flow chart of certain steps in a process for manufacturing a microelectronic device. 
     FIG. 6 is a flow chart of certain steps in the microlithography step of the process depicted in FIG.  5 . 
     FIG. 7 is a sagittal section of a conventional electron-beam source. 
     FIG. 8 depicts exemplary trajectories of electrons from the cathode of the electron-beam source of FIG.  7 . 
     FIG. 9 depicts a spherical aberration at the crossover, as obtained with the electron-beam source of FIGS. 7 and 8. 
     FIG. 10 is a representative plot of emission-current density J c  and anode voltage V a  for an electron gun. 
    
    
     DETAILED DESCRIPTION 
     The invention is described below in the context of representative embodiments, which are not to be regarded as limiting in any way. 
     A first representative embodiment of an electron-beam source  10  according to the invention is depicted in FIG.  1 . The source  10  comprises a cathode  11  including a center portion  11   a  and a peripheral portion  11   b . The center portion  11   a  produces an electron beam  2  that propagates axially downstream (to the right in the figure) of the cathode  11  along an axis Ax. The source  10  also comprises a Wehnelt electrode  13  and an “extraction electrode” (anode)  14 . The beam  12  forms a gun crossover  15  on the axis Ax in the vicinity of the anode  14 . 
     Comparing FIGS. 1 and 7, it can be seen that, in FIG. 1, the peripheral portion  11   b  of the cathode  11  is displaced (“drawn back”), along the axis Ax, from the center portion  11   a  in a direction opposite the direction of electron propagation from the cathode  11 . 
     The performance of the electron-beam source  10  of FIG. 1 is depicted in FIGS. 2 and 3. Specifically, FIG. 2 shows plots of the dimensional percentage of the crossover significantly affected by spherical aberration, versus the voltage (kV) impressed on the extraction electrode  15 , by electron-beam sources having various respective configurations of the peripheral portion  11   b  relative to the center portion  11   a . The plot connecting coordinates indicated by diamonds corresponds to an example in which the peripheral portion  11   b  is drawn back 4 mm from the center portion  11   a . The respective plots connecting coordinates indicated by triangles and circles correspond to respective comparison examples in which the peripheral portion  11   b  is “drawn forward” 1 mm and 2 mm, respectively, relative to the center portion  11   a . “Drawn forward” means that the peripheral portion  11   b  is situated downstream (to the right in FIG. 1) relative to the center portion  11   a . The plot connecting coordinates indicated by squares corresponds to a comparison example in which the center portion  11   a  and peripheral portion  11   b  are transversely flush with each other, similar to the conventional configuration shown in FIG.  7 . FIG. 3 shows similar plots, but in which the ordinate is the applied voltage to the Wehnelt electrode. 
     To obtain the plots shown in FIGS. 2 and 3, the voltage applied to the Wehnelt electrode  13  was changed while maintaining a constant axial position of the gun crossover  15 . As can be discerned in FIG. 2, the dimensional percentage of the crossover significantly affected by spherical aberration is smallest whenever the peripheral portion  11   b  is drawn back from the center portion  11   a  (curve connecting the diamonds) according to the invention. As can be discerned from FIG. 3, the voltage applied to the Wehnelt electrode is lowest whenever the peripheral portion  11   b  is drawn back from the center portion  11   a , according to the invention. 
     Further with respect to the results shown in FIG. 3, with an increase in voltage applied to the Wehnelt electrode  13 , the electrical field imposed on the surface of the cathode  11  is increased correspondingly. This causes the field distribution at the cathode surface to become irregular. Such irregularity causes a deterioration of the desired uniformity with which electrons are drawn from the cathode surface. By drawing the peripheral portion  11   b  of the cathode  11  back from the center portion  11   a , the Wehnelt voltage can be reduced. This weakens the field at the cathode surface, with a corresponding improvement of the uniformity of the field at the cathode surface. 
     FIG. 4 depicts a representative embodiment of an electron-beam microlithography apparatus that comprises an electron-beam source  20  according to the invention (e.g., a source  10  as shown in FIG.  1 ). Arranged along an optical axis Ax are the electron-beam source  20 , a first illumination lens  21 , a second illumination lens  22 , a third illumination lens  23 , a beam-shaping aperture  24 , an illumination aperture  25 , a reticle  26  that defines a pattern, a first projection lens  27 , a second projection lens  27 , a projection aperture  29 , and a substrate (“wafer”)  30 . The portion of the system between the source  20  and the reticle  26  is the “illumination-optical system” IOS, and the portion of the system between the reticle  26  and the wafer  30  is the “projection-optical system” POS. An electron beam emitted from the electron-beam source  20  propagates as an “illumination beam” through the illumination-optical system IOS and as a “patterned beam” through the projection-optical system POS. 
     The illumination beam emitted from the electron-beam source  20  is formed by the illumination lenses  21 - 23  and shaped by the apertures  24 ,  25  to illuminate a desired region on the reticle  26 . The beam-shaping aperture  24  trims the beam as required to illuminate the region on the reticle  26  having a desired dimension. The plane of the aperture  25  desirably is conjugate to the gun crossover  15  of the electron-beam source  10  shown in FIG. 1 (i.e., an image of the crossover  15  is formed on the plane of the aperture  25 ). Beam uniformity at this plane defines the uniformity of the angular distribution of the beam at the reticle  26  and/or wafer  30 . The beam profile at the gun crossover  15  is Gaussian, and the plate defining the aperture  25  normally trims (blocks) the tails of the Gaussian distribution of the beam. However, if the beam exhibits significant spherical aberration at the aperture  25 , then tail portions of the beam distribution pass through the aperture  25  to the reticle rather than being blocked by the aperture plate. This results in deterioration of beam uniformity at the reticle. 
     An image of the illuminated region on the reticle  26  is formed on the wafer  30  by the first and second projection lenses  27 ,  28  working in concert with each other. So as to be imprinted with an image of the illuminated pattern projected from the reticle  26 , the upstream-facing surface of the wafer  30  includes a layer of a suitable exposure-sensitive agent (termed a “resist”). The apertures  25  and  29  trim away scattered electrons and limit the aperture angle on the reticle and wafer, respectively. These various components (except for configuration of the electron-beam source  20 ) can have the same respective structures and functions as corresponding elements in a conventional microlithography apparatus. 
     In the embodiment shown in FIG. 4, the electron-beam source  20  can be similar to that shown in FIG. 1, discussed above. With such a configuration of a microlithography apparatus, a comparatively large area of the reticle  26  can be illuminated uniformly. Hence, it now is possible to transfer-expose patterns having finer linewidths than conventionally possible, with excellent accuracy and precision, even when the per-shot area of the reticle has a relatively large surface area. 
     FIG. 6 is a flowchart of an exemplary microelectronic-fabrication method in which apparatus and methods according to the invention can be applied readily. The fabrication method generally comprises the main steps of wafer production (wafer manufacturing or preparation), reticle (mask) production or preparation; wafer processing, device (chip) assembly (including dicing of chips and rendering the chips operational), and device (chip) 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 layered successively atop one another on the wafer, forming multiple chips destined to be memory chips or main processing units (MPUs), for example. The formation of each layer typically involves multiple sub-steps. Usually, many operative microelectronic devices are produced on each wafer. 
     Typical wafer-processing steps include: (1) thin-film formation (by, e.g., sputtering or CVD) involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires or electrodes; (2) oxidation step to oxidize the substrate or the thin-film layer previously formed; (3) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (4) etching or analogous step (e.g., dry-etching) to etch the thin film or substrate according to the resist pattern; (5) doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (6) resist stripping to remove the remaining resist from the wafer; and (7) wafer inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired microelectronic devices on the wafer. 
     FIG. 7 provides a flowchart of typical steps performed in microlithography, which is a principal step in the wafer-processing step shown in FIG.  6 . The microlithography step typically includes: (1) resist-application step, wherein a suitable resist is coated on the wafer substrate (which an include a circuit element formed in a previous wafer-processing step); (2) exposure step, to expose the resist with the desired pattern by microlithography; (3) development step, to develop the exposed resist to produce the imprinted image; and ( 4 ) optional resist-annealing step, to enhance the durability of and stabilize the resist pattern. 
     The process steps summarized above are all well known and are not described further herein. 
     Whereas the invention has been described in connection with a representative embodiment, it will be understood that the invention is not limited to that embodiment. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.