MEMBRANE MEMORY SYSTEM

Abstract: A membrane memory system comprises a substrate of a selected metal, anodized to provide a surface layer of a microporous ceramic matrix membrane having substantially regular vertical pores. The microporous ceramic matrix membrane may be separated from the metal substrate by a barrier layer at the bottom of each pore. Data is recorded on the membrane memory system by ablating the barrier layer at the bottom of selected pores so a pattern of ablated and nonablated pores denote positive and negative binary information bits. A method using a modified electron microscope having a rapid, precision swinging electron beam or a laser beam to insert information into or extract information from the membrane memory system is also disclosed.

Description: MEMBRANE MEMORY SYSTEM DESCRIPTION OF METHOD AND EXPERIMENTAL INVESTIGATION This invention relates to a method of recording electronic data with high bit density on a substrate of a selected metal and to achieve a high bit density quick access electronic recording or storage medium prepared by the said metho . At present information storage technology uses a variety of memory recording systems for data storage. Known memory 7 systems are limited to a bit density of about 5 - 10 per square centimetre, when quick access is required. A tape memory recording system offers a larger storage capability (up to 10 13 bits) but access times for such tape systems are of the order of minutes. Typical existing quick access memory recording systems comprise magnetic or optical disk units including appropriate means of reading and writing the disk under the direction of laser systems controlled by semiconductor computer devices. Such known memory recording systems are vulnerable to high radiation fluxes such as those caused by a nuclear bomb exploded in the atmosphere and no satisfactory solution has been found yet for the protection of existing memory systems against such damage. The production of a quick access memory recording system resistant to damage caused by such high radiation fluxes is becoming increasingly important. For military electronics and computers, active steps are being taken to return to the hard valve technology of the 1940's and 1950's which is not subject to the problem outlined above. One object of the present invention is to provide a high bit density, quick access, electronic memory recording or storage device for general use in the storage of information and with the special advantage that the storage device is not vulnerable to damage by high radiation fluxes. In a book by L. Young entitled "Anodic Oxide Films" published by Academic Press (London) in 1961, there is a description of an anodic process conducted in electrolytes in which aluminium oxide is partially soluble, as a result of which. Nature produces on an aluminium substrate a microporous ceramic matrix membrane structure with beautiful vertical micropores in the diameter range 20 to 2000 Angstroms. The micropores persist nearly to the interface between the ceramic structure and the aluminium substrate but there is a thin barrier layer at the bottom of the ceramic pore structure which is of the order of a few hundred Angstroms in thickness, the actual thickness of the barrier layer depending upon the precise anodizing operating conditions at the time. The pores are not all of precisely the same size and shape as in the case of the pores of a honeycomb but the pores produced by the anodizing process described by L. Young have a generally regular pattern and have thin walls. The thickness of the ceramic membrane may be anything between 200 Angstroms and 100 microns or more and the pore diameter is determined by the operating conditions of the process. The process described by L. Young is interesting and in a paper by Rigby, Cowleson, Davles and Furneaux a porous membrane structure similar to that described by L. Young is proposed for use in a wide variety of laboratory filtration applications by separating the porous membrane structure from the substrate. USA Patent No. 4678547 describes the production of a memory disk substrate using an aluminium alloy substrate which was anodized in a mixed acid solution of chromic acid and sulphuric acid and after which the anodized substrate was polished so as to leave behind the anodic film, but USA Patent No. 4678547 is silent on the precise way in which the memory discs so formed can be used to achieve the desired object. The present invention takes advantage of the fact that the micro porous structure of an anodized substrate has an ideal form for use as a memory recording medium due to the fact that the pores provide a substantially regular honeycomb matrix membrane above the barrier layer, between the membrane and the substrate, and that the barrier layer can be penetrated or not penetrated as desired to denote positive and negative binary information. According to the present invention there is provided a method of recording electronic data with high bit density on a substrate of a selected metal which has been anodized to provide a surface layer of a microporous ceramic matrix membrane of substantially regular vertical pores separated from the selected metal of the substrate by a barrier layer at the bottom of each pore, wherein the method comprises ablating the barrier layer at the bottom of selected pores in accordance with a predetermined plan to provide a memory, storage device comprising a pattern of ablated and non ablated pores to denote positive and negative binary information bits. With pore diameters around 20 Angstroms the pore density is about 10 per centimetre squared. The ceramic membrane is produced on the substrate using the anodization procedure, the basis of which is described in the book by L. Young referred to above, to produce the required microporous structure. The metals capable of forming porous oxide films when anodized are the group of so called 'valve* metals, namely. Aluminium, Tantalum, Niobium, Zirconium, Hafnium, and other selected metals such as Tungsten, Bismuth, Antimony, Beryllium, Magnesium, Silicon, Germanium, Tin, Titanium, Uranium and Zinc, all of the above metals being referred to herein as 'selected metals' and the preferred order of the selected metals is as written. One method of inserting and extracting information in to and from the microporous structure is to use a slightly modified form of the electron optics of a scanning electron microscope (SEM) . This provides a rapid, precision swinging electron beam deflected by electrostatic and electromagnetic fields. For information insertion the modification required to a standard unit is to make it possible for the voltage driving the beam to be rapidly increased when a positive bit is required to be inserted. This increase in voltage must be sufficient to increase the energy of the beam to a level which causes the beam to ablate away the thin barrier layer to provide direct contact to the metal substrate beneath. During information extraction the increased beam current occurring when the beam alights on an ablated pore, as compared with the beam from an unablated pore is used to distinguish between positive and negative bits. Full details of a raster scan electron beam system and its use to draw lines about 100 Angstroms wide and as little as 200 Angstroms apart can be found in the following two papers by workers at the IBM Research Centre at Yorktown Heights, NY, USA. 1. A.N. Broers J. Vac Sc; Technol 16 (1979) 1692 2. A.N. Broers and M. Pomerance, Thin Solid Films 99, 1983, 323-329 It is believed that the necessary modification to provide the increased voltage capability will be well understood by those skilled in the art. Another method of information insertion and extraction is to employ a narrow laser beam with a diameter of the order of 0.25 microns, which is close to the limiting diameter of the laser beams determined by diffraction. Either an infrared (IR) or an ultra-violet (UV) laser may be used for data insertion, the IR laser piercing the barrier layer thermally and the UV laser piercing the barrier layer by chemical ionization. The laser head used for insertion and extraction may be mounted above the microporous structure and may be moved using electro-acoustic field effects or mechanically on a spiral or rectilinear raster. In operation the distinction between positive and negative bits is made by measuring the strength of the reflected beam. The difference in magnitude between the reflective characteristics of the ceramic barrier layer and that of the substrate is sufficient for effective measurement and is usually at least an order of magnitude. The laser system may be operated at atmospheric pressure and is thus less expensive to manufacture and to operate than the SEM system, that must be operated at reduced pressure. To access each pore of the smallest microporous structure individually requires the two dimensional coordinates of the pores to be recorded to an appropriate precision of around 20 Angstroms. Clearly this requires a large amount of information to be stored. The system may therefore be arranged so that a single positive or negative binary bit is represented by a group comprising a plurality, for example seven adjacent pores. The pore groups may be arranged so that the groups may be scanned by a beam on a square, rectilinear, crossed line or spiral raster thus reducing the information overhead required to locate an information bit or bit group. The use of bit groups as described above enables the required beam width to be increased, each bit group preferably being selected so that a central pore is surrounded by six other pores symmetrically disposed substantially in hexagonal formation. The groups of seven pores may be scanned by a rapidly moving beam of three times the diameter of that appropriate to a single pore location system. It will be understood that the microporous structure forming the memory storage device of the present invention is resistant to damage caused by high radiation fluxes and is complementary to recently emerging hard valve and attack computers now being developed for military purposes. In order that the invention may be more clearly understood reference is now directed to the accompanying drawings given by way of example which: Figure 1 shows a plan view of a typical high density microporous structure for use as a memory storage device in accordance with the invention. Figure 2 shows an idealized side elevation of the vertical pore structure and Figure 3 shows diagrammatically an impression of a laser beam of 6000 Angstroms in diameter covering seven pore groups of 2000 Angstrom pores. In the drawings Figure 1 indicates an array of single pores of 2000 Angstroms diameter and Figure 3 indicates a group of seven such pores that may be used to represent a single positive or negative binary bit. Beam searching for such seven pore groups may be done on a square rectilinear crossed line raster or spiral raster and much less total information bits are required to find a particular individual hexagonal group of pores than by using individual pores. Although the foregoing description deals mainly with groups of seven pores, it will be understood such groups are merely the preferred arrangement and groups of any desired number of pores may be used as required. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims: CLAIMS 1. A method of recording electronic data with high bit density on a substrate of a selected metal which has been anodized to provide a surface layer of a microporous ceramic matrix membrane of substantially regular vertical pores separated from the selected metal of the substrate by a barrier layer at the bottom of each pore, said method comprising ablating the barrier layer at the bottom of selected pores in accordance with a predetermined plan to provide a memory storage device comprising a pattern of ablated and nonablated pores to denote positive and negative binary information bits. 2. A method of preparing an electronic high bit density quick access memory storage device and of recording data thereon comprising the steps of anodizing a substrate of a selected metal to provide a microporous ceramic matrix membrane of substantially regular vertical pores on the surface of the substrate, the matrix membrane being separated from the metal substrate by a barrier layer at the bottom of each pore and recording data on the matrix membrane by ablating the barrier layer at the bottom of selected pores in accordance with a predetermined plan to provide a pattern of ablated and nonablated pores to denote positive and negative binary information bits. 3. A method of inserting and/or extracting information into and from a memory storage device prepared by the method of either of claims 1 or 2 characterised in that a slightly modified form of electron microscope is used to provide a rapid, precision swinging electron beam deflected by electrostatic or electromagnetic fields, the standard electron microscope being modified to make it possible for the voltage driving the beam to be rapidly increased when a positive bit is required to be inserted. 4. A method of inserting and/or extracting information in to and from a memory storage device prepared by the method of either of claims 1 or 2 characterised in that a narrow laser beam is employed. 5. The method of claim 3 characterised in that said electron beam is arranged to scan a group of pores instead of a single pore. 6. The method of claim 4 characterized in that said laser beam is arranged to scan a group of pores instead of a single pore. 7. The method of claim 5 characterised in that the beam is arranged to scan groups of seven pores using a rectilinear or spiral raster. 8. The method of claim 6 characterized in that the beam is arranged to scan groups of seven pores using a rectilinear or spiral raster. 9. A memory storage device prepared by the method of any one of claims 1-8.

Inventors: DESTY DENIS HENRY, DESTY, DENIS, HENRY
Applicants: TECHNOLOGY DEVELOPMENTS COMPAN, TECHNOLOGY DEVELOPMENTS COMPANY
Application year: 1991
Main classification: G11C13
IPC codes: G11B7, G11B9, G11B11