Patent Number: 042007941
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a new and improved micro lens array and micro deflector sub-assembly fabricated from silicon semiconductor plates processed in accordance with semiconductor microcircuit fabrication technology, metalized in part and held together in assembled relationship by glass rodding to which the silicon plates are used or otherwise secured either directly or through the medium of suitable metal mounting rings. 2. Prior Art Problem The desirability of using a matrix of micro-electron optical elements arranged in the manner of a fly's eye lens is a now well-established fact in that such an arrangement provides large field coverage without sacrifice of resolution, large beam current, deflection sensitivity or accuracy and other desirable attributes as described in a paper entitled "Electron Beam Memories" by D. E. Speliotis, D. O. Smith, K. J. Harte and F. O. Arntz, presented at the ELECTRO/76 held at Bostom, Mass. on May 11-14, 1976 and in an article entitled "Advances in Fly's Eye Electron Optics" appearing in the Proceedings of the National Electronics Conference, vol. 23, pgs. 746-751 (1967) by S. P. Newberry et al. While the desirable characteristics of the fly's eye electron optical system are well established, as the requirements for the number of channels in the matrix increases and the linear dimensions of the matrix correspondingly decrease in efforts to increase its storage capacity and minimize the size, complexity and weight of the equipment, the problems of fabrication of fly's eye electron beam systems using known materials and fabrication techniques become increasingly difficult if not insurmountable. In the known prior art fly's eye electron optics system heretofore available to the art as described in the above-noted National Electronics Conf. article, the micro lens array sub-assembly has been fabricated in the form of a "top hat" structure as shown in FIG. 2 of the article. In this form of micro lens array, the focusing element of the micro lens consists of an array of holes formed in thin metal plates. The thin metal plates in turn are tightly stretched and bonded to a strong metal ring and the holes are produced by a variety of methods such as drilling, punching and photo-chemical etching to mention a few. The problems encountered with these known micro lens array structures are: (1) Photo-chemical etching of metal is expensive and does not result in lens aperture openings having required roundness, smoothness and uniformity between holes in the array. PA1 (2) While punching of holes does reduce cost substantially, and if followed by a finishing operation such as shaving, does produce uniform diameters and smooth surfaces, these procedures cannot be accomplished on a matrix of holes (lens aperture openings) in which the hole diameter equals or even approaches the optimum ratio to the spacing between holes. PA1 (3) The use of heavy metal rings to support the thin plates does not permit close spacing of the plates as the spacing between lens aperture openings (channels) is decreased to optimize density of channels and minimize size. If the "top hat" structure shown in FIG. 2 of the National Electronics Conference article is employed, while permitting close spacing between lens plates, it is expensive and uses space inefficiently, but most seriously, it prevents close approach to one side of the lens elements of neighboring elements of the overall fly's eye electron optical system. PA1 (4) If an attempt is made to avoid the above-discussed difficulties encountered with the use of thick mounting rings or the "top hat" configuration by using metal plates which are thick enough to be self-supporting, eventually the impossible condition would be reached in large arrays (e.g., arrays having lens elements numbering 128.times.128) where the plate thickness required for mechanical rigidity exceeds the spacing between the plates required for optimum electron optical performance. Additionally, thick plates are more costly to process in the fabrication of the lens aperture openings (holes), are more severely limited in hole size permitted, and are inclined to warp during bake-out temperature cycling due to built-up strains. Finally, as with thin metals, the desired optimum hole diameter to spacing between holes cannot be achieved. PA1 (1) Micro deflector systems which depend upon production of individual deflector plates which are subsequently stacked together with spacers require unreasonable tolerance control because the position error is cumulative. Single blade metal deflectors are better than metal deflectors sawed from solid stock, but they are expensive and too thin to remain straight unless placed in tension by the assembly. PA1 (2) Thin metal plates are microphonic at some resonant frequency and this resonance can be excited by the application of periodic changes in the deflection voltage such as a raster scan. PA1 (3) In micro deflector systems which use deflector bars sawed from blocks, the ceramic blocks must be sawed in the fired state (i.e., very hard) in which state they are so abrasive that even diamond tools wear rapidly and the dimensions are very difficult to hold. Thus, they are costly to produce. PA1 (1) In electron beam accessed memories, thermal match is obtained between the recording media and the micro lens array and micro deflector elements since such elements are formed of silicon and glass rodding which has a temperature coefficient of expansion very near to that of silicon. PA1 (2) The high purity and regularity of the material (single crystal silicon) permits construction of the micro lens elements by known microcircuit photoetch techniques and better quality holes and straighter edges are obtained in comparison to holes formed in metals or amorphous materials. PA1 (3) Fewer problems are encountered with the flatness of the materials. PA1 (4) It is not necessary to mount the micro lens plates on a supporting ring of substantial thickness thereby permitting closer spacing between the micro lens plates. PA1 (5) As will be explained more fully hereafter, it is possible by appropriate fabrication techniques to make bi-layer lens elements without bimetallic thermal effects thus permitting the construction of highly conductive, buttressed outer lens plates having ultra thin lens aperture openings formed on a silicon lens plate of substantial thickness and conductive layers on each of the opposite sides thereof. PA1 (6) Metalization (if needed) and bonding techniques for silicon plates are well established and proven. PA1 (7) Extreme cleanliness and stability at bake-out can be obtained for the resulting structure. PA1 (8) Polycrystalline silicon is easier to saw and metalize than ceramic thus making the problem of micro deflector bar fabrication much less costly and better controlled. PA1 (9) In addition to producing smoother more uniform lens aperture openings (holes) in silicon plates, the photochemical etching techniques used in producing the holes permit hole size to center spacing to be controlled to optimum values. Turning attention now to the micro deflector structure for achieving fine deflection, the above-mentioned National Electronics Conference article describes a micro deflector construction which has been successfully applied to the fly's eye lens and comprises two sets of parallel conductive bars in tandem. The use of metal plates to produce the deflection bars has not been satisfactory, however, for reasons to be discussed hereafter. Sawing of bars from ceramic blocks and metalization of the ceramic bars, has produced electron optically acceptable fine deflectors but the cost has been unacceptable and the yield very low. In summary, experience with the known fine deflector sub-assembly design has taught the following lessons: In addition to the component fabrication problems discussed above, the overall structure, i.e., the micro lens array plus micro deflector and target electrode member, has further constraints. Since a single piece of dirt can spoil an assembly for many applications, the assembled structure must either be capable of disassembly for cleaning or fabricated by techniques which leave it electron optically clean. Additionally, the assembly must not permit relative motion of the parts by environmental factors such as vibration or thermal excursions. Two of the most important applications for fly's eye type electron beam tubes are in electron beam accessed semiconductor target memories for use with computers and in microcircuit pattern fabrication. In these applications, if the target area covered is large, then temperature excursions pose a severe problem with the mixing of construction materials such as metals, ceramics and semiconductor targets each with a different temperature coefficient of expansion and pattern displacement of several microns can occur due to normal room temperature variations. Thus, it will be appreciated that the above-listed requirements can make the overall assembly of a fly's eye electron beam tube micro lens array and micro deflector a very difficult problem. From the foregoing discussion, it would be appreciated that new materials and methods of construction of micro lens arrays and micro deflector sub-assemblies are required if the benefit of higher density, larger arrays are to be achieved for the industry. SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a new and improved micro lens array and micro deflector sub-assembly for use in electron beam tubes of the fly's eye type and which is fabricated from silicon, either in single crystal or polycrystalline form, to the greatest extent possible and wherein certain parts are processed in accordance with silicon semiconductor microcircuit fabrication techniques and other parts of which are metallized and the various parts held together in an assembled structure by glass rodding. The advantages obtained by fabricating the fly's eye electron optical assembly from silicon in this manner are: In practicing the invention a combined fine focusing micro lens array and micro deflector sub-assembly is provided for use in electron beam tubes of the fly's eye type. The assembly comprises a fine focusing micro lens array sub-assembly formed by a plurality of spaced-apart, stacked, parallel, thin, planar, apertured lens plates each fabricated from silicon semiconductor material and having an array of micro lens aperture openings formed therein by photolithographic semiconductor microcircuit fabrication techniques. The apertured silicon lens plates each have highly conductive surfaces and are secured to glass rods for holding the plates in stacked, parallel, spaced-apart relationship with the longitudinal axes of the glass support rods extending at right angles to the planes of the silicon plates. The apertures in all of the silicon lens plates are axially aligned in parallel with a longitudinal axis passing through the center of the array to form an array of fine focusing lens elements. The assembly further includes a micro deflector sub-assembly mounted immediately adjacent to the fine focusing micro lens array and defining a honeycomb matrix of sets of orthogonally disposed micro deflector elements, there being a set of orthogonally disposed micro deflector elements axially aligned with each respective fine focusing lens element formed by the axially aligned aperture openings of the stacked parallel spaced-apart silicon lens plates for deflecting an electron beam passing through a respective axially aligned fine focusing micro lens array element along orthonal x-y directional axes of movement in the plane normal to the electron beam path. The honey-comb matrix of sets of micro deflector elements are comprised by two orthogonally arrayed sets of parallel, spaced-apart deflector bars which define the respective orthogonally arrayed sets of micro deflector elements with alternate bars of each set of deflector bars being interconnected electrically for common connection to a respective source of fine x-y deflection potential. In one preferred embodiment, the thin planar apertured lens plates comprise a thin planar wafer of single crystalline silicon about 2 microns thick and having a matrix of lens aperture openings formed therein by etching from one side only all the way through the thickness of the wafer. In a second preferred embodiment, the thin planar apertured lens elements each comprise a thin planar single silicon wafer of about 1/2 millimeter in thickness etched from each of the opposite planar sides thereof through openings defined by a masking area formed on both planar surfaces of the wafer where no openings are to be formed and application of a suitable etchant to both sides of the wafer. In preferred embodiments, the deflector bars of the micro deflector sub-assembly are fabricated from polycrystalline silicon having metalized surfaces. The two orthogonally arrayed sets of parallel, spaced-apart deflection bars are held in assembled spaced-apart, parallel relationship by respective sets of supporting glass rods whose longitudinal axes extend in a plane parallel to the plane of the deflector bars but at right angles thereto and to which the ends of the deflector bars are fused. In one preferred embodiment, the micro lens array sub-assembly and the micro deflector sub-assembly have the glass support rods thereof thermally bonded to respective, annularly shaped outer support rings comprised of molybdenum, tungsten or other suitable metal and having alignment notches formed around the periphery thereof for facilitating alignment of the respective sub-assemblies. The outer support rings in turn then are thermally bonded to an additional set of glass support rods whose longitudinal axis extend at right angles to the planes of the apertured silicon plates and to the plane of the deflector bars for holding the two sub-assemblies in assembled relationship. In another preferred embodiment, the thin apertured silicon lens plates are thermally bonded directly to a set of glass support rods whose longitudinal axis extends at right angles to the planes of the lens plates and to which the support rods for the micro deflector bars also are thermally bonded. The micro deflector sub-assembly further comprises end deflector bars which have extensions of malleable metal material extending beyond the points of connection of the ends of the end deflector bars for use as mounting tabs either to the outer support ring, or directly to the glass support rods which extend at right angles to the plane of the micro deflector bars. In structures which do not employ the outer support rings having alignment notches, alignment of the micro deflector elements is obtained by light optical or electron optical alignment techniques and fusion of the various silicon elements to the glass support rods can be obtained by electron beam heating or laser beam heating and fusion jointure. In assemblies where bonding rings are not employed, the set of glass support rods to which both the micro lens array and the micro deflector sub-assemblies are secured, have the ends thereof shaped to seat with and be fused to a precision insulating sapphire ball that is in turn seated in and fused to a socket formed in an annularly shaped supporting ring for mounting the assembly within the housing of a fly's eye type electron beam tube. The structure thus comprised also has a target electrode member fabricated from silicon semi-conductor material mounted in parallel to the micro lens array and micro deflector bar but spaced therefrom and secured by fusion to the common glass support rods. Electrical connection to at least one of the thin apertured silicon lens plates of the micro lens array sub-assembly is obtained by trapping an exposed portion of a conductive wire between the hot glass of at least one of the glass support rods and the conductive surface of the respective plate during thermal bonding or fusion of the plates to the glass support rods. The conductive wire thereafter may be connected by conventional lead-in insulated conductor to a source of electric energy. In another preferred embodiment, the glass support rods at the point of thermal bonding to the silicon lens plates have projections formed thereon extending inwardly to contact the peripheral edge portions of the silicon plates at the point of fusion to thereby provide greater effective insulator distance between adjacent silicon plates while maintaining minimum physical spacing or plate separation distance between the plates. If desired, the lens aperture openings formed in at least one of the thin apertured silicon lens plates need not be round but may be semi-elliptical or of another configuration for reducing third order aberrations. In embodiments wherein because of the intended application it may be necessary to disassemble the assembly from time to time, ring-shaped metal pads of compatible conductive material are brazed or otherwise secured to points around the peripheral edge of the thin silicon apertured lens plates for increasing the thickness thereof and a plurality of insulating ball spaces are seated in the ring-shaped metal pads for assembling the thin silicon plates in a stacked, spaced-apart, parallel array upon being clamped together in a self-supporting structure. Alternatively, a plurality of support holes may be formed around the peripheral edge portion of at least one of the thin silicon apertured lens plates and a plurality of small insulating ball spacers seated in and fused to the holes for providing an insulating mounting means for the respective thin silicon lens plates and insulating balls. The combined micro lens array and micro deflector assembly thus comprised may be used with a planar target electrode member of silicon semiconductor material for an electron beam accessed memory, mounted in common with the assembly in a vacuum-tight housing or alternatively may be used with a target member of electron sensitive material (such as a photosensitive target or an electron etchable photo resist target member used in the fabrication of microcircuits) removably mounted by a vacuum-tight enclosure in common with an in a plane parallel to the thin apertured micro lens silicon plates and the plane of the micro deflector bars. With either type of application, the assembly may be used with a coarse deflector electrode system or alternatively with a graded field electrode system located intermediate the electron gun of the electron beam tube and the fly's eye electron optical system thereby allowing the new and improved fly's eye electron optical system to be used either with a coarse deflected beam of electrons, or a uniform flood of electrons.