ELECTRON BEAM MEMORY SYSTEM WITH ULTRA-COMPACT, HIGH CURRENT DENSITY ELECTRON GUN

Abstract: A rapid random accessed electron beam memory system (10) comprises a disc (13) mounted for rotation and supporting an information storage medium. An electron gun (18, 19, 20) is mounted for movement across the disc. The gun has ultra-compactness and extremely low mass, yet capable of developing a finely focused electron beam probe at high beam current densities. The gun comprises a low-mass field emission cathode and means for heating the cathode, the cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at the tip. A low-mass electrode receives a predetermined accelerating potential for receiving electrons from the tip to form an electron beam. A single focus lens of low mass is positioned a relatively short object distance from the tip for receiving the beam and forms a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom. The probe is of such high current density due to the brightness of the source and the power of the lens that the minimum diameter of the probe is significantly affected by space charge effects in said beam. The sum of the object and image distances are so small as to suppress the space charge contribution to probe diameter and thereby make practicable a finely focused, yet extremely intense electron beam probe. The total mass of the gun is so low as to be readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of data stored on the information storage medium.

Description: ELECTRON BEAM MEMORY SYSTEM WITH ULTRA-COMPACT, HIGH CURRENT DENSITY ELECTRON GUN SPECIFICATION BACKGRODND OF THE INVENTION Prior Electron Beam Memory Systems The technological society in which we live appears to have an insatiable appetite for the storage of data, but there is a significant mismatch between the amount of data (particularly in digital form) and the devices currently available to store such data. Examples of the need for digital storage are not difficult to find. Computer programs of 10-100 megabytes are not uncommon. To simply record social data in our population of 250 million people might require 10 megabytes of storage. Images of various forms need considerable storage capacity -- a single television frame may require two megabytes while a full color page of a magazine needs 10 times that amount. A year's supply of images from a single LANDSTAT satellite system has been estimated at 10 bits. It is also very apparent that the new Fifth Generation Computer will require a vast data base for its effective implementation. One might consider a base as large as the Library of Congress with perhaps 10 16 bits. Presently available data storage devices are totally inadequate to store this volume of digital information. For example, a magnetic hard disc system g can typically store about 4 x 10 bits of information. Optical disc systems can store in the order of 5 x 10 10 bits. There is an obvious need, then, for a system which could store 10 bits or more of information in digital form. There are also substantial benefits which are not quite so apparent. For example, the existence of a vast memory on line would obviate destroying interim data and would allow its retrieval at any time. In such areas as image processing for a program development, this could be a very substantial benefit. It is perhaps obvious that in order to develop memories of substantially greater storage capacity, resort must be had to the use of electrons or other charged particles with wavelengths less than the wavelength of light, or a system which does not depend upon radiation. Visible light is entirely inadequate; optical storage systems are already close to the wavelength limit of visible light. Electron beam recording and retrieval of information has been explored for many years in laboratories and in some cases, practiced commercially. In the television field, there has been developed the now-commercial diffraction-type optical projection system in which electrostatic charges are deposited by an electron beam on a thermally softenable tape. Upon heating of the tape, the surface of the tape is deformed in accordance t?ith the pattern of electrostatic charges The television picture information stored on the surface-deformed tape is displayed by projecting a coherent beam of light through the transparent tape and spatially filtering the information diffracted by the surface deformations (Rl, R2). An exhaustive treatment of television recording is given by Abramson (R2A). Another approach is disclosed in the literature which follows this same general electron recording beam technique, but electron beams are used to read out the stored information by detection of emitted secondary electrons (R3). The literature describes a similar disc-based system which can be played mechanically, as well as by the use of electron beams to stimulate secondary emission from the thermoplastic medium (R4, R5, R6). There is also disclosed the application of the same thermoplastic recording technique in a real¬ time disc, tape or drum mass memory electron beam reading/erasing system with simultaneous optical read¬ out in vacuum (R7). Others have suggested using electron beam recording to make mechanically readable video disc grooves by using a laterally vibrated electron beam to form the grooves in the disc (R6, R8). Electron beams have been used to read out the surface deformations formed on an electrostatically deformed thermoplastic medium which has been negatively charged to create an electron mirror at the medium surface (R9) . Substantial engineering efforts have been expanded developing video disc recording and retrieval technology — mainly optical and mechanical, but also using electron beams. An electron beam recording system has been developed for making the masters for capacitance-based (mechanical) video disc systems (RIO). The prior art also includes a technique for an archival electron beam accessed memory in which a high intensity electron beam selectively melts columnar bits in a two-dimensional lattice supported by a thin membrane (RID. It is clear that a disc-based system is best for quick access to high density information. In spite of the obvious need for an ultra-high density memory system and the obvious choice of electron beam radiation as the means by which the information is stored, no practical electron beam memory system has yet been developed because of the inability of the art to develop an electron gun having the requisite properties and capabilities. Requirements of a Writing Gun For A Practical Rapid Random Accessed Electron Beam Memory System The writing gun must be ultra-compact and have a mass of no more than a few hundred grams to make possible random accessing in a few seconds or less of any selected file or area on the disc. Rapid random access to any disc file is necessary for making additions or corrections to files at any location on the disc. Further, since the gun or guns used to read out the information will inevitably have to have the capability of rapid random accessing of any file or area of the target disc, the writing electron gun, in order to be functionally and structurally compatible with the reading gun system, must also be movable relative to the disc. The writing gun must be ultra-compact also for the reason that it not physically interfere with the reading gun or guns which may be operating simultaneously with the writing gun. Because of the vast amount of information storable in an electron beam memory system, it will not be desirable in many applications to permit the awaiting of storage of all information, which might take weeks or months, before any of the stored information can be accessed. rience the need for a system with multiple reading and writing guns. Further, the writing electron gun must be capable of developing a writing probe current of sufficiently high current density to make possible surface ablation or other power-intensive no-develop recording. The advantage of no-develop recording is that- the information recorded can be read out immediately without the need to devacuate the system, - develop the record medium, and re-evacuate the system. Yet, the gun's accelerating voltage can be no more than a few kilovolts if undue beam penetration and spreading is to be avoided. Low voltages are desirable also in the interest of minimizing the bulk and mass of the voltage insulating structures required and thus minimizing the size and mass of the gun. The writing electron gun for a practical electron beam memory system must also be capable of produci g a writing probe having a diameter sm ll enough to achieve the desired ultra-high recording densities. Limitations of Prior Electron Beam Memory Guns Render Them Useless For The Contemplated Application It is important to an application of the significance of the invention to understand why previous electron beam memory guns have failed to meet the minimum needs of a practical electron beam memory system. There have been a number of disc-based electron beam memory systems disclosed in the literature in which the" election beam writing gun or head is movable. RCA has disclosed in the literature a disc-type electron beam recording system for use in making video disc masters (P12- RIO). In this system the electron probe diameter is said to be 4 microns by .1 micron -- far too large for use in the ultra-high density electron beam memory system with which this invention is concerned. The disclosed electron optical column is massive and adapted to be moved unidirectional ly across the master disc upon which information is being recorded. The column is of far too great a mass and size to be usable requiring a rapid random accessing writing head. Teldec discloses in the patent literature a system for recording video information on a disc which employs a movable electron beam head (R6). The head is shown schematically, however the beam probe diameter is said to be 1 micron -- far too large for use in a practical ultra-high density electron beam memory system. The prior art also discloses a number of disc¬ based electron beam memory systems employing stationary electron beam writing columns. A number optically read out the stored information (R13, R14, R15, R5). In every case the electron optical column is far too cumbersome to give the requisite rapid random access capability of the commercially practicable electron beam memory system which I contemplate. Also, in every case the requisite combination of high beam current density and small probe size is not taught. The prior art literature also discloses other systems which, while full details are not available, clearly appear to fall far short of the requisite capabilities in current density and probe size as well as compactness and low mass (R7, R16). As will be discussed in more detail below, there is a basic incompatibility at relatively high electron beam current densities between the amount of electron current which can be developed in an electron beam probe and its minimum diameter. This is due to space charge effects which become significant as the electron density of an electron beam probe increases. In 1978 I showed in a very general context that the effects of space charge in an electron beam can be represented by a term proportional to the current, the length of the system, and the focal length and inversely proportional to the angle of convergence of the probe (R17). At the time I failed to realize the implications of this work in electron beam memory systems. As I will describe, I now see in the design of a practical electron gun for electron beam memory systems of the rapid random accessed type, the powerful implications of controlling the length of the gun's source-to-image distance. Nor is an Electron Gun Having The Requi- site Properties Available in Other Arts Typical scanning electron microscope electron optical columns are monolithic structures totally unsuited for rapid movement across an electron beam memory medium. Further, they typically develop probe currents which would be, at best, marginal for a practical high rate, ultra-high density electron beam memory system of the type I contemplate. In recent years, as a result of the development of field emission type electron guns, of which development I was instrumental (R18, R19), a scanning electron microscope with a less cumbersome, single focusing stage was developed (R20, R21, R22, R23, R24). The scanning electron microscope- developments utilizing field emission guns represented a step in the direction toward an electron gun useful in an electron beam memory system of the type I contemplate, however, even these guns fall far short of what is needed. Whereas they are of reduced size and mass, these guns are nevertheless too massive to be utilized in an electron beam system having a rapid random accessing writing electron gun. These guns also fall short of the minimum electron beam probe current needed in a no- develop high recording rate electron beam memory system. Electron beam lithography systems are capable of developing adequately high current densities in the electron beam probe, however, they are massive monolithic devices having no useful applicability in an electron beam memory system of the type I envision. In conclusion, 1 am not aware of any devices or disclosures either in the field of electron beam memory systems, or any other field, of an electron gun having the aforedescribed minimum properties and capabilities necessary for use xn a rapid random accessed electron beam memory system. Prior Art References A. Referenced above: R 1 - USP 3,113,179 R 2 - USP 3,116,962 R2A - A Short History of Television Recording, A. Abramson, JSMPTE Vol. 82, March, 1973, pages 188-198 R 3 - USP 3,168,726 R 4 - USP 3,750,117 R 5 - USP 3,952,146 R 6 - USP 3,737,598 R 7 - USP 3,239,602 R 8 - USP 3,842,217 R 9 - USP 3,278,679 RIO - USP 4,010,318 Rll - Target Design of an Archival Electron Beam Memory, J. Wolfe, J. App.Phys. 53 (12), Dec, 1982, pages 8429-8435 R12 - USP 4,074,313 R13 - USP 3,381,097 R14 - USP 3,361,873 R15 - USP RE: 30,974 R16 - USP 4,001,493 R17 - "Some Space Charge Effects in Electron Probe Devices", Optik, A.V. Crewe, 52 (1978/1979) N. 4, 337-34 R18 - "Electron Gun Using A Field Emission Source", A.V. Crewe, Rev. Sci. Instru., Vol. 39, No. 4, 576-583, Apr., 1985 R19 - "A High Resolution Scanning Elecjtorn Microscope, A.V. Crewe, Jour. App. Phys., Vol. 39, No. 13, 5861-5868, Dec, 1968 R20 - "Dramatic Desk-Type S.E.M. Performance Upgrade; Realtime Energies with a 100A Resolution", Jour, of Elec. Eng. (Japan), Nov/Dec, 1975 (page unknown) R21 - "Field Emission Scanning Electron Microscope - S310A" (Sales brochure, 7 pages, Hitachi, Ltd., Tokyo, Japan) R22 - USP 4,274,035 R23 - USP 4,020,353 R31 - USP 3,978,338 R24 - USP 4,099,055 R32 - USP 4,534,016 B. Not Referenced Above: R25 - USP 4,427,886 R26 - USP 3 , 731 , 095 R27 - USP 3 , 786 , 268 R29 - Recent Advances in Electron Beam Memories, J. Kelly, in Advances in Electronics and Electron Physics, ed. by L. Morton, Academic Press, 1977 R30 - USP 4,245,159 Objects of the Invention It is an object of this invention to provide an electron beam memory system having an electron gun capable, for the first time, of high enough electron probe current densities to permit no-develop recording and small enough probe sizes to permit ultra-high density recording, yet compact enough to make feasible rapid random accessing of any area on the system's recording medium. It is an object of this invention to provide a rapid random accessed electron beam memory system having an electron gun which is capable of developing electron beam probe current densities high enough to make feasible ablative or other no-develop recording at rates of 100 megahertz and above. It is another object of this invention to provide a rapid random accessed electron beam memory system having an electron gun which achieves such current levels using accelerating voltages of no more than a few ki iovoIts. It is still another object to provide a rapid random accessed electron beam memory system having an electron gun which is also capable of producing at such minimum probe currents and maximum accelerating voltages electron beam probe diameters of 500 angstroms or less in order to make feasible recording densities of at least 10 bits of information on a 12 inch disc. It is yet another object to provide a rapid random accessed electron beam memory system having an electron gun which is capable of these achievements, and yet is ultra-compact and has a mass of only a few hundred grams in order to make feasible accessing of any selected file or area on the recording medium within a few seconds or less. Brief Description of the Drawings FIG.1 is a highly schematic illustration of an electron beam memory system constructed according to the teachings of the present invention; FIG. 2 is a sectional view of a writing head constituting part of the FIG. 1 system and containing a writing electron gun implementing an aspect of the present invention; FIG. 3 is a plot of the factors limiting performance of the writing gun of the present invention; FIG. 4 is an exploded view of an alternative embodiment of a writing electron gun implementing the principles of the present invention; and FIG. 5 is a fragmentary sectional view of the FIG. 4 electron gun assembled. Description of the Preferred Embodiment Wheras the electron gun according to the present invention has numerous applications in which an electron gun of ultra-compactness, extremely low mass and a relatively high current density has utility, the most promising application envisioned is in an electron beam memory system. FIG. 1 is a schematic view of an electron beam memory system 10 embodying the present invention. The FIG. 1 system 10 is shown as including a vacuum enclosure, depicted schematically in dotted line form at 12. Within the enclosure is a storage medium 13 supported on a rotatable disc 14. The disc 14 is rotated by a disc drive shown schematically at 16. In nearly all disc-type electron beam memory systems disclosed in the prior art, the electron optical head or column is monolithic and immovable, requiring that the turntable be translated within the vacuum enclosure. Vacuum compatibility for such drive systems introduces lubrication and other problems. In the present system, the drive 16 is stationary and is therefore preferably located outside the vacuum enclosure 12. Further, compared with a system in which the turntable is moved, the vacuum enclosure volume is greatly reduced. The electron beam memory system 10 includes a plurality of electron beam heads adapted for simultaneous operation. In the illustrated embodiment, I have shown three heads -- a writing head 18 containing an electron gun for recording information, a verification head 19 containing an electron gun for verifying the fact and integrity of the stored information, and a reading head 20 containing an electron gun for retrieving the stored information. The electron beam memory system 10 is illustrated schematically as including head drives 21, 22, 23 interconnected with the heads 18, 19, 20 by support arms 24, 25, 26, for moving the heads 18, 19, 20 across the disc 14. Auxiliary electronic and electrical apparatus, shown schematically at 27, provides the necessary drive signals through conductors 28, 29, 30 for energizing head drives 21, 22, 23. Apparatus 27 also supplies through conductor 31 suitable drive signals for disc drive 16, as well as the necessary drive currents for the focus lens, heater current for the field emission source heater and energization potentials for the gun electrodes through bundles of conductors 33, 34, 35. FIG. 2 illustrates a writing electron gun 36 contained within writing head 18. As will be described, the FIG. 2 gun is capable of developing a finely focused electron beam probe at high beam current densities, yet is ultra-compact and of extremely low mass. For the first time, the electron gun of this invention makes possible a truly random accessed electron beam memory system for high rate, ultra-high density electron beam data recording, and yet with recording power making possible no-develop recording, i.e., recording w : hout the need for developing the recording medium after exposure. With the writing electron gun according to this invention, a very high capacity electron beam storage medium which is supported, e.g., on a rotatable disc, can be employed using multiple accessory verification and reading heads to permit simultaneous recording and reading over long periods of time -- a critically important capability for a great many applications. As noted above, because of the vast storage capability of an electron beam memory svstem such as this, it may be total ly impractical in many applications to delay access to the memory until the memory is filled (which could take weeks or months) , during which time the stored information is inaccessible. An electron beam memory system becomes truly universally useful only when it has the capability as is now provided by this invention, to record without any development of the medium using a rapid random accessing head and with simultaneously operable pick-up heads for verifying and/or retrieving the stored information as soon as it is recorded. Before engaging in a detailed discussion of the FIG. 2 electron gun practicing this invention, I will first discuss in general terms the performance requirements imposed on a writing gun of an electron beam memory system of the character described herein. A few of the performance, packaging and other requirements imposed on the gun are, in general terms, as follows: (1) as stated, the gun must be of sufficient compactness and low mass as to be readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of the electron beam memory medium; (2) the gun must be capable of producing an extremely fine probe to permit ultra-high density recording on the medium; (3) the probe produced must not only be extremely fine, but must have high current densities, in order that no-develop recording can be achieved -- that is, recording characterized by an alteration of the physical state of the recording medium which can be detected immediately after recording, as by use of an electron beam probe; (4) the gun must be capable of working with relatively low accelerating voltages in order that the beam penetration and spreading is not excessive, and so that the insulation requirements do not drive up the size and mass of the gun; (5) the head must have a modest power consumption in order that massive cooling structures are not required; (6) the gun must have an electron source which not only is extremely bright, but is also stable and of long life in order that the system in practice is easy to use by operators of ordinary skill; and (7) the gun must have a commercially tolerable cost of manufacture. The simplest way to effect an irreversible change in the physical state of the recording medium is to induce melting or boiling of the recording material to create a depression or pit in the medium. The pit can be detected, for example, with a less-intense electron beam probe and accompanying means for detecting secondary, back-scattered or transmitted electrons. The melting temperature "Tm" of a material can be approximated by the relation TM = „ π , wh _.re __ li _ SL "I" is the electron beam current in amperes, "V" is the beam voltage in volts, "K" is the thermal conduc t ivity of the material in calories per centimeter squired per second, and "a" is the radius of the heated zone in the recording medium in centimeters. Using bismuth as the recording material, for example, having "K" = .02 and "Tm" = 270 degrees centigrade, and assuming a beam voltage "V" of 5 kilovolts and a heated zone "a" of .08 microns, then "I" = 160 nanoamperes. In practice, a temperature in excess of the melting point would be required in order to create a hole or pit. Recording rates are limited by the rate at which the medium and be heated: γ = per a ~ , where ___ is the specific heat and p is the density of the material. The recording rates for most materials and the probe diameters of interest is in the order of 10 —8 to 10 seconds, allowing recording rates of 100 megahertz and above. In terms of beam power requirements, for beams having current values in the range of 100-500 nanoamperes with accelerating voltages of 3-5 kilovolts, for example, the beam power is in the range of 300-500 -microwatts. This is more than adequate power to produce melting in materials of interests such as bismuth, tellurium, arsenides of such materials and mixtures thereof, for example, which require only a few microwatts of power to be melted, using probes with a diameter of .1 micron or less. A detailed discussion of the FIG. 2 electron gun will now be engaged. In order to achieve the high current densities required for high rate, no-develop recording according to this invention, a heated field emission cathode is employed. Field emission sources are the only ones which can meet the gun requirements stated above. All other sources require more than one lens to demagnify the source and this makes the system long and more massive. It is possible that a number of different types of thermal field emission sources may be employed. The value of thermal field emission sources is built on the premise that by heating a field emission source, condensation of gas molecules can be prevented and the damage to the source by returning ions can be annealed out. The basic cold thermal field emission source also operates well but is limited in its current density capabilities. An improved type has the source coated with zirconium or zirconium oxide in order to reduce the emission angle and thereby increase the available angular current density. Zirconium and zirconium- oxide-coated heated field emission cathodes are known to work satisfactorily and have long source lifetimes. One serious drawback, however, is that electron emission may disappear entirely at random intervals for reasons which are at present not completely understood. Yet another type of thermal field emission source which may be used is the oxygen-treated type. Oxygen treatment reduces the angle of the cone of emission, thereby increasing the current density. According to some sources, a thin crystal of tungsten oxide appears on the tip. This crystal can be maintained for very long periods of time. The only objection to this source is that the emission is noisy and the tiny crystal of oxide is susceptible of being lost. My choice of thermal field emission source is a source of a type known as the built-up oxide type. It is well known that a thermal field emission source will increase in radius if it is overheated because it becomes liquid and tends to form a large drop. This will usually produce a drastic decrease in emission current. On the other hand, this tendency of the tip to become blunt can be counteracted by increasing the electric field at the surface of the tip. In fact, it is possible to operate in a stable region where these two effects are balanced. If the electric field is increased to a point where it is greater than that required to balance surface tension, the tip will become sharper but it no longer has a hemispherical shape. The tip grows in preferred crystal orientation and shape. The most favorable is along the <100> direction. As the "build-up" process continues, the emission pattern becomes smaller so that current densities increase. In addition, a lower voltage is required to produce the emission. This type of source meets all the requirements of the gun of the present invention. The current density can exceed 1 milliampere per steradian, the emission noise can be quite low, the energy spread appears to be no more than about .5 volts and the lifetime is in the thousands of hours. In FIG. 2, the field emission source tip is shown at 38. A tip can is shown at 40 and heater leads at 42, 43. A silicon ball 44 supports the heater in can 40. I have found good success in operating the tip at _ Q -1 π vacuum levels of 10 to 10 torr. Source currents up to 1,000 microamperes can be produced. Emission noise levels for these operating parameters are in the order of a few percent. An insulator 46 supports the tip assembly comprising the can 40 and tip 38 and isolates it electrically from the other parts of the gun and the gun enclosure 48. The insulator 46 is, in turn, supported by a support element 49. In order to draw electrons from the tip, the FIG. 2 gun includes a truncated conical accelerating " anode electrode 50 which is spaced from a beam tube assembly 52 by an insulating ring 57. Electrically conductive hold-down pins 51, 53 hold anode electrode 50 against ring 57. Appropriate electrical potential is applied to anode electrode 50 through lead 59. Of critical importance to the gun of the present invention is the focus lens 54. In a preferred- form of the invention, the lens 54 is a single lens positioned a relatively short object distance from the ti 38 for receiving a beam of electrons from the anode electrode 50. The single focus lens 54 forms a finely focused electron beam probe 55 (the beam focus) on the medium 10 at a relatively short focal distance therefrom. As will be explained, in accordance with an aspect of this invention, the sum of the object and image distances must be so small as to suppress the space charge contribution to probe diameter to make feasible electron beam probes with diameters as small as a few hundred angstroms. A number o± design constraints imposed on lens 54 wil 1 be discussed. As the gun of the present invention is ultra-compac + and of extremely low mass, so must be the lens 54. A single lens coil 56 is preferred. The very small size of the required lens coil 56 makes power dissipation a challenge. The excitation of the coil 56 which is required to produce a field B is given by E - 4HN- I, where N is the number of turns of wire. Then 4UNI. B&= 10 We see that NI is independent of the particular geometry that we might choose. It only depends upon the energy of the electrons. If we consider a coil 56 whose inner radius is r, , outer radius __ • , length & and mean resistivity p , then the power "W" dissipated by the coil 56 is given temperature is 1.6 x 10 ohm cm. This can increase to 2 x 10 ohm cm when hot and in addition an al lowance should be made for the insulation between the turns of wire. An average value of 3 x 10 ohm cm wil 1 be assumed. Then, assuming NI=2500 ampere turns, we have We must now choose r- j _, tz an - & xn order to satisfy all our mechanical constraints. A value for r- j _ of 4 millimeters would be close tc the minimum value that we could accept since we must leave space inside the lens for the deflector and stigmator. Then we would have where R = ~ 2^__' Beyond R=3 we gain very little. Therefore, we will take X2 equal to 12 millimeters. In that case 30 W = _ (watts) = 7.2 (calories/sec) 5, This amount of heat can be removed without difficulty. Another problem associated with this type of lens is the question of the outgassing of the coil. For NI equal to approximately 1500 ampere turns, and since we would like to limit the current to a few amperes (in order to reduce the size of the lead-in wires) we must necessarily use many turns. This means there will be a great deal of trapped gas which will be difficult to remove because the pumping speed will below. Preferably, the coil is encased in a vacuum-tight can. Lenses of other configuration may be employed. For example, a lens could be devised which would have greater or lesser length and different inner and outer radii. Selection of the physical dimensions of the coil 56 are also affected by such factors as e .se of access to the tip and other mechanical considerations. Located within the compass of the coil 56 are interwound stigmator coils 58 for reducing to an acceptable level any astigmatism which may be present in the electron beam. Axial ly separated from the stigmator coils 58 are a pair of interwound deflection coils 62 for deflecting the electron beam in orthogonal directions across the medium 13. Gross positioning of the electron beam probe 55 is by movement of the head 18 across the storage medium 13. Fine positioning of the electron beam probe 55 on the storage medium 13 is accomplished by appropriate selection of driving currents for the deflection coils 62. A beam tube 66, which may be of conventional construction, extends from the anode electrode 50 to the point of beam exit from the gun 36. To optimize the compactness of the system, the stigmator coils 58 and deflection coils 62 are preferably located concentric to the lens coil 56 and surrounding the beam tube 66 and are configured and arranged to lie substantially completely within the axial compass of the lens coil 56. As stated, whereas the exact configuration of the lens coil 56 is not critical to the invention, the lens nevertheless must be designed, whatever its configuration, to meet the very demanding constraints that it should have such power as to develop a short focal distance from a source located extremely close to the lens and yet consume modest power while having low aberration coefficients so as not to effect undue enlargement of the probe diameter. The diameter of the beam probe (focus) is dependent upon a number of factors, each of which can contribute to probe size. These include: (1) space charge; (2) electron diffraction; (3) chromatic aberration; (4) spherical aberration; (5) source size; (6) astigmatism; (7) beam-related electrostatic charging of gun parts; (8) defects in the lens; and (9) high order aberrations. The first four of these are the most important contributions to probe diameter. Of the remaining, a field emission source gives an adequately small, bright source. Astigmatism can be satisfactorily controlled with stigmator coils 58. The other factors can either be controlled by the use of care in the fabrication of the gun, or are insignificant in this application. The first four factors and their effect in determining the optimum operating and design paramets of the system will now be discussed. Since we are attempting to produce a large current density, we can presume that the effects of space charge will be important. The simplest description of the contribution to probe radius due to space charge effects are contained in the equation: δ = kLI/ 0 V 3/2 sc where "L" is the length of the electron optic column, α "is the semi-angle of convergence at the probe, "I" is the beam current, "V" is accelerating voltage, and "k" is a constant whose value can be obtained from theoretical considerations or experiment. Conservation of brightness indicates the existence of an upper limit: I L = Kα 2 V which we can insert into the space charge equation above to give a limit: A second contribution to total, probe radius diameter is due to diffraction and is given by: 6 =7.56xl0 "8 /αV 1 2 Another limit to the performance of the system is the spherical aberration of the lens. The spherical aberration probe radius limit is where C g (cm) is the coefficient of spherical aberration. The fourth contribution to probe size is the chromatic aberration of the lens: where Δ V is the variation in the energy of the electrons, and C c is the coefficient of chromtic aberration. There are four basic contributions to probe diameter which define the performance limits of the system. In order to proceed any further, it is necessary to obtain numerical values for the various unknown quantities. For "k", a reasonable valuf would be 10 4 volt 3/ 2 amp . For Cs, a model lens system will be used which consists of a uniform field of length L from source to image, in which C equals L/2. C c will be assumed to be L/2. For thermal field emission, a reasonable value of K is 4 x 10 "6 amp volt -1 . Using these values, the various parameters reduce to the following: <5 d = 7.6 x 10 " 8 /a V L/2 δ s = Lα 3 /8 Selection of L and V will completely define the system. These are not independent parameters. One wishes to make L smal 1 but insulators must be provided to hold the voltage V. In addition, the magnetic field of the lens can be calculated from the equation BL = 21 V 1/2 so that large values of B could be required if small values of L were used. These equations can be combined in graphical form, as shown in FIG. 3. To do so we take an example from the figures given above. Specifically, assume V = 5,000 volts and L = 4 centimeters. FIG. 3 shows the box-like region on the log <5 -v- log K plot. Using this graph a value of α can be chosen to give the required values of probe current and " probe radius. For example, assuming the example of bismuth given above where it was noted that a beam current of 160 nanoamperes with a probe radius of 'J8 microns was required to achieve melting, it is seen that this can readily be achieved, and can in fact be exceeded. An electron beam memory system according to this invention must necessarily be very small with a total overall length from source tip 38 to probe of no greater than about 5 centimeters. The operating - voltage of the electron beam will be in the range of about 3 to 10 Kv, the probe size will be in the range o of 100-500 A with a probe current in the range of 100-500 nA. This will allow a writing speed of 100 MHz or more. In accordance with one aspect of this invention, as aforestated, the electron gun according to this invention is ultra-compact and of extremely low mass. Specifically, the electron gun of this invention preferably has a total mass of no more than about 200 grams. The electron gun 36 according to this invention is extraordinarily compact. By way of illustration, the total length of the gun is no more than about 4 centimeters. The anode electrode 50 occupies no more than about 1 centimeter; the illustrated lens coil 56 has an axial length of about 3 centimeters. A gun having such extreme compactness and low mass is susceptible of being quickly moved to any part of the recording medium in order to effectuate rapid random accessing of any selected area on the medium for the purpose of adding information to any selected file or area on the medium. FIGS. 4 and 5 depict a second embodiment of the invention. To avoid redundancy, in FIGS. 4 and 5, only the disc 68 and recording medium 70 of an electron beam memory system are shown. FIGS. 4-5 illustrate a gun 71 somewhat modified relative to the design of the FIG. 2 electron gun. Gun 71 is illustrated as comprising a field emitting tip assembly 72, a tip mounting insulator 74, anode electrode 76, anode insulator 78, anode mounting ring 80, beam unit housing cap 82 with beam tube 84, a coil mandrel assembly 86 supporting interwound stigmator coils 91 and interwound deflection coils 93, 0 both shown schematically, a magnetic lens coil 88 and a gun housing 89 having a cap 90. Conductive hold-down pins 96, 98 hold anode electrode 76 against anode insulator 78. Pin 98 receives anode potential on head 100. I5 The FIGS. 4-5 embodiment differ from the FIG. 2 embodiment in a number of respects -- the anode electrode 76 has a cylindrical rather than a conical configuration. The lens coil 88 is shown as having a disc configuration which is axial ly more compact than 20 the lens coil 56 in the FIG. 2 embodiment. The mechanical mounting of the various parts is slightly different. The same operating principles apply to the FIGS. 4-5 embodiment as described above with respect to the FIG. 2 embodiment. The FIGS. 4-5 embodiment is 25 depicted to show that alternative structural configurations of the essential parts of this invention can be employed within the spirit and scope of the teachings of this invention. The above embodiments are included merely as 0 illustrative and it is contemplated that ot^er structures may be devised to practice the teachings of the present invention. The following claims are intended to cover such other structures.

Claims: HAT IS CLAIMED IS: 1. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said gun having ultra-compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: a low-mass field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip; low-mass electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam, and a single magnetic focus lens of low mass positioned a relatively short object distance from said tip for receiving said beam, said lens having turns in such number and being adapted to receive a focusing current of such magnitude as to form a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small as to suppress the space charge contribution- to probe diameter and thereby make practicable a fine focused, yet extremely intense electron beam probe. 2. The apparatus defined by Claim 1 wherein said lens has a pancake configuration with an axial dimension of at most half its outer diameter, and wherein said gun includes a beam tube located concentrically within said lens for passing said beam, said gun further including stigmator coil means and deflection coil means located concentric to said lens and surrounding said beam tube and configured and arranged to lie substantially completely within the axial compass of said lens. 3. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and comprising: a field emission cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip: electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam; and a single magnetic focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such current density due to the brightness of said source and power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small as to suppress the space charge contribution to probe diameter and thereby make practicable a finely focused, yet extremely intense electron beam probe, said gun having a total mass of no more than about 200 grams and thereby being readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of said information storage medium. 4. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and comprising: a field emission cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip; electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam; and a single magnetic focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused electron beam probe at a relatively short focal distance therefrom, said gun having a total mass of no more than about 200 grams and thereby being readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of said information storage medium. 5. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: a field emission cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip; electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam; and a single magnetic focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such current density due to the brightness of said source and power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object distance and said focal distance, and thus the total distance from cathode tip to storage medium being no more than 5 centimeters, whereby the space charge contribution to probe diameter is suppressed, making practicable a finely focused, yet extremely intense electron beam probe. 6. A rapid random accessed electron beam memory system, comprising: - disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: a field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential for forming a high brightness electron source at said tip; electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam; and a focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such current density due to the brightness of said source and power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being no more than about 5 centimeters so as to suppress the space charge contribution to probe diameter and thereby make practicable a finely focused yet extremely intense electron beam probe, said gun having a total mass of no more than about 200 grams and thereby being readily capable of being rapidly accelerated and decelerated to effectutate a rapid random accessing of the electron beam memory target or the like. 7. A rapid random accessed electron beam memory system, comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means, for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a ~ finely focused electron beam probe at high beam current densities, said gun comprising: fiel- ' l emission cathode and means for heating said cathode said cathode having an emitting tip and being adapted to receive a predetermined electrical potential for forming a high brightness electron source at said tip; electrode means adapted to receive a predetermined accelerating potential for receiving electrons from said tip to form an electron beam; and a single magnetic focus lens positioned a relatively short object distance from said tip for receiving said beam, said lens having turns in such number and being adapted to receive a focusing current of such magnitude as to form a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said lens having a pancake configuration with an axial dimension of at most half its outer diameter; a beam tube located concentrically within said lens for passing said beam; stigmator coil means and deflection coil means located concentric to said lens and surrounding said beam tube and configured and arranged to lie substantially completely within the axial compass of said lens, said probe being of such high current density due to the brighness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being no more than about 5 centimeters, so as to suppress the space charge contribution to probe diameter and thereby make possible a finely focused, yet extremely intense electron beam probe, said gun having a total mass of no more than about 200 grams and thereby being readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of the electron beam memory target or the like. 8. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: a low mass field emission cathode means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip, low mass electrode means adapted j receive a predetermined accelerating potential in the range of about 3 to 10 kilovolts for receiving electrons from said tip to form an electron beam, the operating characteristics of said cathode and said electrode means being selected to cause an intense electron beam to be developed having a current value in the range of about 100-500 nanoamperes or greater, and a single focus lens of low mass positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small as to suppress the space charge contribution to probe diameter such that said electron beam may have a diameter as smal 1 as a few hundred angstroms. 9. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at h-.gh beam current densities, said gun comprising: a low mass field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip, low mass electrode means adapted to receive a predetermined accelerating potential in the range of about 3 to 10 kilovolts for receiving electrons from said tip to form an electron beam, the operating characteristics of said cathode and said electrode means being selected to cause an intense electron beam to be developed having a current value in the range of about 100-500 nanoamperes or greater, and a single magnetic focus lens of low mass positioned a relatively short object distance from said tip for receiving said beam, said lens having turns in such number and being adapted to receive a focusing current of such magnitude as to form a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small as to suppress the space charge contribution to proe diameter such that said electron beam may have a diameter of as smal 1 as a few hundred angstroms. 10. A rapid random accessed electron beam memory system, comprising: disc means mounted for roration and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet being capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: a low mass field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermiend electrical potential to form a high brightness electron source at said tip, electrode means adapted to receive a predetermined low accelerating potential in the range of about 3-10 kilovolts for receiving electrons from said tip to form an electron beam, the operating characteristics of said cathode and said electrode means being selected to cause an intense electron beam to be developed having a current value in the range of about 100-500 nanoamperes or greater; and a single focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small as to suppress the space charge contribution to probe diameter such that said electron beam probe may have a diameter as smal 1 as a few hundred angstroms, said gun having a total mass of no more than about 200 grams, said gun thereby being readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of said information storage medium. 11. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and comprising: a field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at __aid tip, low mass electrode means adapted to receive a predetermined accelerating potential in the range of about 3-10 kilovolts for receiving electrons from said tip to form an electron beam, the operating characteristics of said cathode and said electrode means being selected to cause an intense electron beam to be developed having a current value greater than about 100 nanoamperes, and a single focus lens of low mass positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and image distances being so small, i.e., no more than about 5 centimeters, as to suppress the space charge contribution to probe diameter such that said electron beam probe may have a diameter as small as a few hundred angstroms. 12. A rapid random accessed electron beam memory system comprising: disc means mounted for rotation and supporting an information storage medium; disc rotating means for rotating said disc; and an electron gun and means for moving said gun across said disc, said electron gun having ultra- compactness and extremely low mass, yet capable of developing a finely focused electron beam probe at high beam current densities, said gun comprising: field emission cathode and means for heating said cathode, said cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at said tip; electrode means adapted to receive a predetermined accelerating potential in the range of 3 to 10 kilovolts for receiving electrons from said tip to form an electron beam, the operating characteristics of said cathode and said electron means being selected to cause an intense electron beam to be developed having a current value in the rang eof about 100 to 500 nanoamperes or greater; and a single magnetic focus lens positioned a relatively short object distance from said tip for receiving said beam and for forming a finely focused, yet intense electron beam probe at a relatively short focal distance therefrom, said probe being of such high current density due to the brightness of said source and the power of said lens that the minimum diameter of said probe is significantly affected by space charge effects in said beam, the sum of said object and ..mage distances being so small, i.e., no more than about 5 centimeters, as to suppress the space charge contribution to probe diameter such that said electron beam probe may have a diameter as small as a few hundred angstroms, said gun having a total mass of no more than about 200 grams and thereby being readily capable of being rapidly accelerated and decelerated to effectuate a rapid random accessing of the information storage medium. 13. The apparatus as defined by 12, wherein said lens has a pancake configuration with an axial dimension at most half its outer diameter, and wherein said gun includes a beam tube located concentrical ly within said lens for passing said beam, said gun further including stigmator coil means and deflection coil means located concentric to. said lens and surrounding said beam tube and configured and arranged to lie substantially completely within the axial compass of said lens.

Inventors: CREWE ALBERT V, CREWE, ALBERT, V.
Applicants: CREWE ALBERT V, CREWE, ALBERT, V.
Application year: 1987
Main classification: G11B9
IPC codes: G11B9, H01J37