Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
  • H01J37/06—Electron sources; Electron guns
  • H01J37/065—Construction of guns or parts thereof
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B9/00—Recording or reproducing using a method not covered by one of the main groups G11B3/00 - G11B7/00; Record carriers therefor
  • G11B9/10—Recording or reproducing using a method not covered by one of the main groups G11B3/00 - G11B7/00; Record carriers therefor using electron beam; Record carriers therefor
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/30—Electron-beam or ion-beam tubes for localised treatment of objects
  • H01J37/305—Electron-beam or ion-beam tubes for localised treatment of objects for casting, melting, evaporating or etching
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/063—Electron sources
  • H01J2237/06308—Thermionic sources
  • H01J2237/06316—Schottky emission

Definitions

• 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.
• thermoplastic medium R4, R5, R6
• 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).
• 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) .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• FIG. 5 is a fragmentary sectional view of the FIG.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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;
• 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.
• 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
• a is the radius of the heated zone in the recording medium in centimeters.
• ____ 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.
• 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.
• thermal 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.
• 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.
• 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.
• 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.
• the emission pattern becomes smaller so that current densities increase.
• 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.
• 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
• 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.
• the FIG. 2 gun 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.
• the focus 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.
• 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.
• lens 54 wil 1 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.
• N is the number of turns of wire.
• NI is independent of the particular geometry that we might choose. It only depends upon the energy of the electrons.
• NI approximately 1500 ampere turns
• the coil is encased in a vacuum-tight can.
• Lenses of other configuration may be employed.
• 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.
• 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
• 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.
• 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.
• 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 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;
• L is the length of the electron optic column
• ⁇ is the semi-angle of convergence at the probe
• I is the beam current
• V is accelerating voltage
• k is a constant whose value can be obtained from theoretical considerations or experiment.
• a second contribution to total, probe radius diameter is due to diffraction and is given by:
• the spherical aberration probe radius limit is
• C g (cm) is the coefficient of spherical aberration.
• the fourth contribution to probe size is the chromatic aberration of the lens:
• ⁇ V is the variation in the energy of the electrons
• C c is the coefficient of chromtic aberration
• K a reasonable valuf would be 10 4 volt 3/ 2 amp .
• 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.
• K a reasonable value of K is 4 x 10 "6 amp volt -1 .
• 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
• FIG. 3 shows the box-like region on the log ⁇ 5 -v- log K plot.
• ⁇ 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 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
• the electron gun according to this invention is ultra-compact and of extremely low mass.
• 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.
• 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.
• 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
• FIGS. 4-5 embodiment is

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• 1987
  • 1987-01-29 EP EP19870901792 patent/EP0255542A1/de not_active Withdrawn
  • 1987-01-29 WO PCT/US1987/000187 patent/WO1987004846A1/en unknown

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