Abstract:
The specification describes a method and apparatus for electron beam lithography wherein a Wehnelt electron gun is modified to improve the uniformity of the electron beam. The bias on the Wehnelt aperture is reversed from the conventional bias so that it is biased positively with respect to the cathode. The Wehnelt opening is tapered with a disk emitter inserted into the taper. The result of these modifications is an electron beam output with low brightness which is highly uniform over the beam cross section.

Description:
FIELD OF THE INVENTION 
     This invention relates to electron beam lithography apparatus used for the manufacture of semiconductor integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Electron beam exposure tools have been used for lithography in semiconductor processing for more than two decades. The first e-beam exposure tools were based on the flying spot concept of a highly focused beam, raster scanned over the object plane. The electron beam is modulated as it scans so that the beam itself generates the lithographic pattern. These tools have been widely used for high precision tasks, such as lithographic mask making, but the raster scan mode is found to be too slow to enable the high throughput required in semiconductor wafer processing. The electron source in this equipment is similar to that used in electron microscopes, i.e. a high brightness source focused to a small spot beam. 
     More recently, a new electron beam exposure tool was developed based on the SCALPEL (SCattering with Angular Limitation Projection Electron-beam Lithography) technique. In this tool, a wide area electron beam is projected through a lithographic mask onto the object plane. Since relatively large areas of a semiconductor wafer (e.g. 1 mm 2 ) can be exposed at a time, throughput is acceptable. The high resolution of this tool makes it attractive for ultra fine line lithography, i.e sub-micron. 
     The requirements for the electron beam source in SCALPEL exposure tools differ significantly from those of a conventional focused beam exposure tool, or a conventional TEM or SEM. While high resolution imaging is still a primary goal, this must be achieved at relatively high (100-1000 μA) gun currents in order to realize economic wafer throughput. The axial brightness required is relatively low, e.g. 10 2  to 10 4  Acm −2 sr −1 , as compared with a value of 10 6  to 10 9  Acm −2 sr −1  for a typical focused beam source. However, the beam flux over the larger area must be highly uniform to obtain the required lithographic dose latitude and CD control. 
     A formidable hurdle in the development of SCALPEL tools was the development of an electron source that provides uniform electron flux over a relatively large area, has relatively low brightness, and has an electron emitter with a sufficient lifetime to avoid excessive downtime. Lanthanum hexaboride (LaB 6 ) emitters in a modified Wehnelt electron gun arrangement were found to be promising for this application, and the first SCALPEL tools were built with these electron sources. Efforts to improve the uniformity of the electron emission profile over the surface of the LaB 6  have continued, but with limited success. Replacement of the LaB 6  emitter with a simple tantalum disk was found to improve the surface emission uniformity and stability. While SCALPEL systems are regarded as highly successful fine line lithographic exposure tools, there is a continuing search for electron gun designs that improve the efficiency and uniformity of the electron beam source. 
     STATEMENT OF THE INVENTION 
     We have developed a new electron beam source for SCALPEL systems which uses a modified Wehnelt gun design wherein the Wehnelt electrode is biased in reverse of the Wehnelt gun used in conventional SCALPEL tools. The modified Wehnelt gun also has a tapered opening, with the electron emitter recessed into the tapered opening. The result of these modifications is a laminar electron beam with low brightness and essentially no crossover. These beam properties are ideal for a SCALPEL tool, which does not require focusing optics in the near field. 
     The invention will be described more specifically in the following detailed description which, taken with the drawing, will provide a greater understanding of the features that distinguish this invention from conventional electron beam sources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram illustrating the principles of the SCALPEL exposure system. 
     FIG. 2 is a schematic diagram of a conventional Wehnelt electron gun with a tantalum disk emitter; 
     FIG. 3 is a schematic diagram of a Wehnelt electron gun modified in accordance with the invention; 
     FIG. 4 is a plot of the brightness variation of the electron emission profile from the cathode of a conventional Wehnelt electron gun; 
     FIG. 5 is a plot of the brightness variation of the electron emission profile from the reverse biased Wehnelt according to the invention; 
     FIG. 6 is an electron emission profile from the cathode of a conventional Wehnelt electron gun; 
     FIG. 7 is an electron emission profile from the modified Wehnelt electron gun of the invention; 
    
    
     DETAILED DESCRIPTION 
     As indicated above the electron gun of the invention is most advantageously utilized as the electron source in a SCALPEL electron beam lithography machine. Fabrication of semiconductor devices on semiconductor wafers in current industry practice contemplates the exposure of polymer resist materials with fine line patterns of actinic radiation, in this case, electron beam radiation. This is achieved in conventional practice by directing the actinic radiation through a lithographic mask and onto a resist coated substrate. The mask may be positioned close to the substrate for proximity printing or may be placed away from the substrate and the image of the mask projected onto the substrate for projection printing. 
     SCALPEL lithography tools are characterized by high contrast image patterns at very small linewidths, i.e 0.1 μm or less. They produce high resolution images with wide process latitude, coupled with the high throughput of optical projection systems. The high throughput is made possible by using a flood beam of electrons to expose a relatively large area of the wafer. Electron beam optics, comprising standard magnetic field beam steering and focusing, are used to image the flood beam onto the lithographic mask, and thereafter, onto the substrate, i.e. the resist coated wafer. The lithographic mask is composed of regions of high electron scattering and regions of low electron scattering, which regions define the features desired in the mask pattern. Details of suitable mask structures can be found in U.S. Pat. No. 5,079,112 issued Jan. 7, 1992, and U.S. Pat. No. 5,258,246 issued Nov. 2, 1993, both to Berger et al. 
     An important feature of the SCALPEL tool is the back focal plane filter that is placed between the lithographic mask and the substrate. The back focal plane filter functions by blocking the highly scattered electrons while passing the weakly scattered electrons, thus forming the image pattern on the substrate. The blocking filter thus absorbs the unwanted radiation in the image. This is in contrast to conventional lithography tools in which the unwanted radiation in the image is absorbed by the mask itself, contributing to heating and distortion of the mask, and to reduced mask lifetime. 
     The principles on which SCALPEL lithography systems operate are illustrated by FIG.  1 . Lithographic mask  12  is illuminated with a uniform flood beam  11  of 100 keV electrons. The membrane mask  12  comprises regions  13  of high scattering material and regions  14  of low scattering material. The weakly scattered portions of the beam, i.e. rays  11   a,  are focused by magnetic lens  15  through the aperture  17  of the back focal plane blocking filter  16 . The back focal plane filter  16  may be a silicon wafer or other material suitable for blocking electrons. The highly scattered portions of the electron beam, represented here by rays  11   b  and  11   c,  are blocked by the back focal plane filter  16 . The electron beam image that passes the back focal plane blocking filter  16  is focused onto a resist coated substrate located at the optical plane represented by  19 . Regions  20  replicate the features  14  of the lithographic mask  12 , i.e. the regions to be exposed, and regions  21  replicate the features  13  of the lithographic mask, i.e. the regions that are not to be exposed. These regions are interchangeable, as is well known in the art, to produce either negative or positive resist patterns. 
     The vitalizing feature of the SCALPEL tool is the positioning of a blocking filter at or near the back focal plane of the electron beam image. Further details of SCALPEL systems can be found in U.S. Pat. No. 5,079,112 issued Jan. 7, 1992, and U.S. Pat. No. 5,258,246 issued Nov. 2, 1993, both to Berger et al. These patents are incorporated herein by reference for such details that may be found useful for the practice of the invention. 
     It should be understood that the figures included with this description are schematic and not necessarily to scale. Device configurations, etc., are given by way of example and are not intended to convey any limitation on the device structures covered. 
     The electron flood beam  11  in a conventional SCALPEL tool is typically produced by a Wehnelt electron gun. Referring to FIG. 2, a conventional Wehnelt electron gun assembly is shown with base  23 , cathode support arms  24 , cathode filament  25 , Wehnelt electrode comprised of Wehnelt support arms  27  and conventional Wehnelt aperture  28 . The base  23  may be ceramic, the support members  24  may be tantalum, steel or molybdenum. The filament  25  may be tungsten wire, the material forming the Wehnelt aperture may be steel or tantalum, and the electron emitter  26  is, e.g., tantalum. The effective area of the electron emitter is typically in the range 0.5-3.0 mm 2  . The electron emitter is preferably a disk with a diameter in the range 0.5-2.0 mm. The emitter is spaced from the Wehnelt opening as shown, typically by a distance in the range 0.1-1.0 mm. For simplicity, the anode and the beam control apparatus are not shown. These elements are conventional and well known in the art. 
     The Wehnelt electron gun in a conventional SCALPEL tool is biased with the cathode, i.e. the Wehnelt opening, negative with respect to the anode by several hundred volts. 
     An important feature of the electron source for SCALPEL exposure tools is relatively low electron beam brightness, as mentioned earlier. For most effective exposures, it is preferred that beam brightness be limited to a value less than 10 4  Acm −2 sr −1 . This is in contrast with conventional scanning electron beam exposure tools which are typically optimized for maximum brightness. See e.g. U. S. Pat. No. 4,588,928 issued May 13, 1986 to Liu et al. 
     The modified Wehnelt electron gun according to the invention is shown in FIG.  3 . The principle departure from the standard SCALPEL Wehnelt is that the Wehnelt  37  is biased positively, with a bias in the range 2000-8000 V. This is a reverse bias compared to the standard SCALPEL Wehnelt, and results in a laminar output beam  41  with essentially parallel electron flux lines, i.e. there is little or no crossover in the electron beam pattern. 
     To enhance the beam characteristics from this reverse bias Wehnelt computer simulations show that the Wehnelt opening is preferably tapered, as shown in FIG.  3 . FIG. 3 shows Wehnelt electrode  37  which comprises a disk with a circular opening  38 . The disk  37  has a first surface and a second surface, and the opening comprises a cylindrical portion and a conical portion, with the cylindrical portion extending from the first surface of the disk to a distance X in the opening, and the conical portion extending from the cylindrical portion to the other surface of the disk. The angle of the conical portion is designated α. The electron beam emitter  36  with supports  34  and  35  is positioned within the conical portion of the opening with electron beam  41  and anode  39  as shown. 
     The preferred design for the Wehnelt opening is: 
     t=1.5-3.5 mm and preferably 2.0-3.0 mm 
     α=15-40 0    
     X=0.2-0.6 mm, and 
     ratio of d 1  to d 2 =1.2-4.0 
     where d 1  is the diameter of the Wehnelt opening and d 2  is the diameter of the electron emitter. 
     Results of computer simulations comparing the output beam characteristics of the conventional Wehnelt gun with the reverse biased Wehnelt gun of the invention are shown in FIGS. 4 and 5. Curve  45  in FIG. 4, simulating a conventional Wehnelt, shows beam brightness falling off considerably at less than 8.2 mrad. By contrast, the beam brightness curve  46 , for the modified Wehnelt of the invention, shows uniform, and lower, beam brightness to 14.609 mrad. 
     A comparison of the beam profiles of the conventional and modified Wehnelts made by actual computer simulation is given by FIGS. 6 and 7. FIG. 6 shows the beam profile for a conventional Wehnelt. The crossover in the beam is evident. FIG. 7 shows the beam profile for the modified Wehnelt of the invention, with the absence of crossover evident. 
     For the purpose of definition here, and in the appended claims, the term electron emitter is intended to define a solid metal body with an approximately flat emitting surface, said flat emitting surface being symmetrical, i.e. having the shape of a circle or regular polygon. Also for the purpose of definition, the term substrate is used herein to define the object plane of the electron beam exposure system whether or not there is a semiconductor workpiece present on the substrate. The term electron optics plane may be used to describe an x-y plane in space in the electron beam exposure system between the electron emitting surface of the electron gun and the surface onto which the electron beam image is focused, i.e. the object plane where the semiconductor wafer is situated. 
     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.