US3892454A

US3892454A - Method of forming silicon storage target

Info

Publication number: US3892454A
Application number: US396028A
Authority: US
Inventors: Amos Picker
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1972-06-28
Filing date: 1973-09-10
Publication date: 1975-07-01
Anticipated expiration: 1992-07-01

Abstract

A cathode ray storage tube having a target structure of semiconductor material which supports areas of dielectric material on which the information is stored.

Description

United States Patent 1 1 I 1111 3,892,454

Picker 1 July 1, I975 METHOD OF FORMING SILICON STORAGE TARGET [56] References Cited [75] Inventor: Amos Picker, Sharon, Mass. UNITED STATES PATENTS 2,918,600 12/1959 Pensak 313/68 R X [73] Asslgnee Raythem' Company Lexmgwn 3,523,208 3/1970 Bodmer et a1. 1. 315/10 Mass- 3,541,384 11 1970 Shortes 315/11 L 10 1973 3,631,294 12/1971 Hofstein 1 315/12 [22] filed Sep 3,646,391 2/1972 Hofsiein 315/12 [21] Appl. No.1 396,028

Related (1.8. Application Data Primary erR0y Lake Assistant Examiner-James W. Davie 60 Division of Ser. No 266,863, June 28, 1972, I abandoned, which is a continuation of Ser. No, Attorney Agent or Firm-Joseph Pannone;

7 957 Sept 30 970 abandone D. Bartlett; Herbert W. Arnold [52] US. Cl. 316/19; 29/2514; 29/25.!5; [57] ABSTRACT 319/392 A cathode ray storage tube having a target structure [5 l] It. Cl. H01] f Semiconductor material which pp areas of [58] Field of Search 313/65 65 68 electric material on which the information is stored.

3l3/39l, 392, 393, 394, 395; 315/10, ll, 12', 316/17, 18, 19; 29/2514, 25.17, 25.18, 25.15 11 Claims, 4 a g Flglll'es METHOD OF FORMING SILICON STORAGE TARGET This is a division of application Ser. No. 266,863 filed June 28, 1972 (now abandoned), which is a continuation of application Ser. No. 76,957 filed Sept. 30, 1970 (now abandoned).

BACKGROUND OF THE INVENTION Cathode ray tubes are well-known in which a foraminous electrode structure has dielectric portions on which charges may be deposited with respect to a conductive substrate, such as, for example, the tubes shown in US. Pat. No. 2,547,638 issued Apr. 3, 1951 to Bernard C. Gardner and assigned to the same assignee as this application. Such target structures require a relatively expensive mesh structure for high definition, such as, for example, a mesh of 2,000 holes per linear inch or 4,000,000 holes per square inch and because of the fine wire required to produce such meshes they have a tendency to be structurally weak, particularly when subject to vibration. In addition, because of the small size of the apertures, care must be taken in applying the dielectric to avoid closing the apertures completely as well as to insure a uniform thickness of the dielectric.

SUMMARY OF THE INVENTION This invention provides for a storage target in which a semiconductor material, such as silicon, is used as the conductive substrate and the dielectric layer is superimposed thereon by oxidizing the semiconductor, for example, by thermal oxidation, vacuum deposition, sputtering, or by any other desired process. The thickness of the dielectric may be made any desired thickness and for silicon dioxide is preferably greater than 100 angstroms thick. The bulk resistance of the silicon substrate is generally less than 500 ohm centimeters and may be as low as 0.0I ohm centimeters, while the silicon dioxide dielectric will typically have a bulk resistance value of IO' ohm centimeters.

While a wide range of materials may be used, both for the conductive substrate and the dielectric material, preferably the dielectric material is formed as a compound of the element which constitutes the major constituent of the substrate. For example, in the case of a silicon substrate the dielectric may be silicon monoxide, silicon dioxide, silicon nitride or other compounds of silicon.

For high definition storage, the dielectric should have a bulk resistivity several orders of magnitude higher than the bulk resistivity of the substrate so that the substrate may be considered to be substantially a conductor and the dielectric an insulator. For high definition, the ratio of the resistivity of the dielectric to the resistivity of the substrate is preferably orders of magnitude or higher.

The device may be constructed either with a continuous layer of dielectric material over the semiconductor or with portions of the dielectric material removed to expose the semiconductor, the latter construction being particularly useful for nondestructive read-out of the stored material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a longitudinal cross-sectional view of a cathode ray storage tube structure illustrating an embodiment of the invention using a target structure having a foraminous coating of dielectric material;

FIG. 2 illustrates a sectional view of a portion of the target structure illustrated in FIG. 1;

FIG. 3 illustrates a graph of the secondary emission ratio versus voltage for the dielectric structure illustrated in FIGS. 1 and 2; and

FIG. 4 illustrates a longitudinal cross-sectional view of a cathode ray storage tube having a storage target with a continuous coating of dielectric material.

Referring now to FIGS. I and 2, there is shown a cathode ray storage tube 10 comprising an evacuated envelope 1] formed, for example, of glass. Extending inwardly from the base of the glass envelope 11 are a plurality of leads 12 which provide external electric connections to an electron gun structure in envelope 11. The gun structure is by way of example only and as illustrated herein comprises a heater I3, a cathode 14, a control grid 15, a focusing electrode 16 and an accelerating electrode structure 17. The electron gun formed by these electrodes produces an electron beam which is focused on a target structure 18 by the electron optics of the gun when suitable potentials are ap plied thereto and a longitudinal magnetic field supplied by focusing coil 19. The size and dimensions of the electron gun and the magnetic focusing field are selected in accordance with well-known practice to produce a high definition electron beam having a spot size preferably less than 0.001 inches in diameter. The elec tron is deflected across the face of the target structure 18 by deflection coils 20 in accordance with well known practice.

The target structure 18 is supported by a conductive support plate 21 connected to a conductive support rod 22 which extends out through a metallic tubular element 23 sealed through opposite end of envelope 11 from the leads 12. Rod 22 is sealed to tube 23 outside envelope 11 to produce a vacuum tight electrical connection to the target structure I8 from which an output signal may be derived. Plate 21 is made thick enough to provide for substantially rigid support of the semiconductor disc substrate 24.

Attached to plate 21 is a disc of semiconductor material 24 which may be, for example, 0.01 inches thick and I to 2 inches in diameter. Disc 24 may be made from commercially available discs of silicon which have formed by cutting slices from a single crystal silicon ingot grown from a melt in accordance with wellknown practice. It is, however, contemplated that a wide range of materials may be used for the substrate 24.

A foraminous dielectric layer 25 is positioned on the opposite side of the conductive substrate 24 from the support plate 21. Layer 25 is preferably formed by exposing the substrate 24 in an oxidizing atmosphere at an elevated temperature to form silicon dioxide in accordance with well-known practice. The layer 25 may be grown by oxidation to thicknesses of, for example. I00 angstroms to 15,000 angstroms thickness. If de sired, a thickness of 40,000 angstroms of the layer 25 may be formed by sputtering.

The apertures in the dielectric layer 25 may be produced, for example, by forming a pattern on the surface of the dielectric material by a photoresist process which produces a mask having apertures therein in a well-known manner. Exposed regions of the silicon dioxide layer 25 are subjected to any desired etch which will etch silicon dioxide. such as a hydrofluoric acid etch, to remove the silicon dioxide in these regions thereby exposing areas of the silicon layer 24. The resuit is a rugged relatively inexpensive dielectric storage target electrode structure which can be formed with apertures on the order of 0.00l inches in diameter with good uniformity and the high resistance which is required to store information for long periods without substantial degradation by lateral leakage.

Referring now to FIG. 3, there is shown a curve of the ratio of electrons emitted from the surface of the dielectric 25 to the electron impinging thereon as a function of the impinging velocity. The impingement velocity is plotted as the cathode to dielectric surface voltage along the X axis, and the ratio of the number of electrons emitted from the dielectric surface to the number of electrons impinging on the dielectric surface, called the secondary emission ratio, is plotted along the Y axis. At zero velocity of impingement, all the electrons are reflected so that as many electrons leave the surface as impinge thereon as shown at 30. When the voltage is l5 volts or so, the secondary emission ratio is decreased to approximately 0.5 as shown at 31. When the voltage is at the critical voltage of about 20 to 30 volts for most dielectrics, the number of electrons striking the layer 25 causes secondary emission of an equal number of electrons so that the secondary emission ratio is unity as shown at 32. At higher voltages, the secondary emission ratio is greater than unity. At 600 to 800 volts, the secondary emission ratio is on the order of 4 to 6 as indicated by the region 33 of the graph. Other dielectric materials have the same general secondary emission ratio versus target voltage characteristics but differ in magnitude and beam velocity at which minimum and peak emission occurs,

The storage tube may be operated in a manner similar to a conventional storage tube, for example, as a destructive readout device or as a nondestructive read out device. For operation in a nondestructive read-out storage system, a conditioning voltage of, for example, +20 volts is connected between the cathode and the target conductive substrate 20 and electrons caused to impinge over the face of the target. The surface areas of the dielectric storage material 25 charge negatively, since the device is operating in the region below unity secondary emission ratio, toward cathode potential, erasing any previously stored information.

Regions of the conditioned target may then have information written thereon by raising the potential of conductive substrate 24 to +35 volts with respect to the cathode. Areas of the dielectric surface are charged volts or so more negative with respect to substrate 24 by the electron beam to store information on the di electric surface.

To read the pattern in a nondestructive manner, the conductive substrate 24 is maintained at approximately volts with respect to cathode potential and the electron beam is scanned across the target surface. The portions of the foraminous dielectric 25 are charged 35 volts negatively with respect to the substrate 24 to repel the electron beam which is collected, for example, by a screen 26 on the end of the electron gun structure, or by a coating (not shown) on the wall of the envelope 11, in accordance with well-known practice. In those regions of the dielectric 25 having a negative charge of 20 volts, with respect to the substrate 24, or zero potential with respect to the cathode, portions of the electron beam will pass through the apertures in screen 25 and be collected by the substrate 24, thereby producing an output signal which is connected to any desired external circuit by rod 22, but no electrons will impinge on the dielectric layer 25, and hence, the read-out is nondestructive.

To increase writing speed, information may be written positive as follows. The storage surface, after conditioning, is first reduced to -15 volts with respect to the cathode by switching the substrate from +20 to +5 volts. The cathode 24 is then lowered to 200 volts with respect to the target 18 and the beam is modulated to dispose a positive charge pattern on the dielectric surface, whose magnitude is determined by the beam scan rate and intensity, raising the dielectric surface potentials by 5 to 20 volts. To read, the cathode is raised to cathode potential and the charged areas of the dielectric surface are between 0 and -15 volts. The reading beam scans across the target and produces an output signal in accordance with the charge pattern.

if desired, an additional collector mesh element may be positioned between the anode and the target whose potential is maintained at, for example, +25 volts with respect to the substrate in order to limit the magnitude of the positive charge stored on the dielectric surface.

In the event that a destructive read-out is desired, the target and anode voltages are operated at, for example, 800 volts positive with respect to the cathode as shown at 34, and the target scanned to equilibrium. To write, the anode is lowered by, for example, 15 volts and the target scanned with a modulated beam. To read, the anode is raised back to 800 volts and an output signal is derived either from the electrode 22 or from the screen 26 by scanning the target with an unmodulated beam which again produces equilibrium for the next write cycle.

Referring now to FIG. 4, there is shown an embodiment of the invention in a cathode ray storage tube which is similar to that illustrated in FIG. 1 except that the target electrode 18 has a dielectric layer 25 which has no apertures therein and, accordingly, it is used primarily for storage of information which will be destroyed upon read-out.

This completes the embodiment of the invention illustrated herein. However, many modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of this invention. For example, additional elements and/or shapes could be used in the target electrode structure. Various methods of scanning the target, such as electrostatic deflection, could be used, and the size and shape of the target electrode can be varied to produce any desired configuration. Accordingly, it is contemplated that this invention is not limited to the embodiments illustrated herein except as defined by the appended claims.

What is claimed is:

l. A method of forming an information storage de vice comprising:

forming a target structure by the steps comprising attaching a metal support structure to one side of a semiconductor wafer and oxidizing the other side of said semiconductor wafer;

supporting said target structure with an electrically conductive structure extending through an insulatin g portion of an envelope and providing an electrical connection from the exterior of said envelope to said metal support structure spaced from an electron source facing said oxide layer; and

evacuating said envelope.

2. The method in accordance with claim 1 wherein said step of forming said target structure comprises:

forming apertures in said oxide layer.

3. The method in accordance with claim 2 wherein:

said evacuated envelope contains a structure for forming electrons from said source into a beam and directing said beam toward said target structure.

4. The method in accordance with claim 3 wherein:

said semiconductor is silicon.

5. The method in accordance with claim 1 wherein:

said electrically conductive structure supporting said metal support structure for said target is a conductor extending through the face of said envelope on the other side of said target from said electron source.

6. The method in accordance with claim 5 wherein:

the major portion of said envelope comprises insulating material.

7. The method in accordance with claim 1 wherein:

said step of forming said target structure comprises oxidizing a portion of said semiconductor material to form said oxide layer.

8. The method in accordance with claim 7 wherein:

said step of forming said target structure further comprises etching said oxide surface to form apertures therein exposing regions of the surface of said semiconductor material.

9. The method of forming a storage tube comprising:

supporting at least an electron source. a beam forming electrode structure and a storage target in rigid spaced relation within an envelope;

said target structure being formed by the steps comprising attaching a metal support structure to one side of a semiconductor wafer and oxidizing the other side of said semiconductor wafer;

said metal support structure being positioned on the opposite side of said target structure from said electron source; and

evacuating said envelope.

10. The method in accordance with claim 9 wherein:

said target structure is positioned with the insulating surface facing said electron source.

11. The method in accordance with claim 10 wherein:

said semiconductor wafer is formed of single crystal silicon.

Claims

1. A METHOD OF FORMING AN INFORMATION STORAGE DEVICE COMPRISING: FORMING A TARGET STRUCTURE BY THE STEPS COMPRISING ATTACHING A METAL SUPPORT STRUCTURE TO ONE SIDE OF A SMICONDUCTOR WAFER AND OXIDIZING THE OTHER SIDE OF SAID SEMICONDUCTOR WAFER, SUPPORTING SAID TARGET STRUCTURE WITH AN ELACTRICALLY CONDUCTIVE STRUCTURE EXTENDING THROUGH AN INSULATING PRTION OF AN ENVELOPE AND PROVIDING AN ELECTRICAL CONNECTION FROM

2. The method in accordance with claim 1 wherein said step of forming said target structure comprises: forming apertures in said oxide layer.

3. The method in accordance with claim 2 wherein: said evacuated envelope contains a structure for forming electrons from said source into a beam and directing said beam toward said target structure.

4. The method in accordance with claim 3 wherein: said semiconductor is silicon.

5. The method in accordance with claim 1 wherein: said electrically conductive structure supporting said metal support structure for said target is a conductor extending through the face of said envelope on the other side of said target from said electron source.

6. The method in accordance with claim 5 wherein: the major portion of said envelope comprises insulating material.

7. The method in accordance with claim 1 wherein: said step of forming said target structure comprises oxidizing a portion of said semiconductor material to form said oxide layer.

8. The method in accordance with claim 7 wherein: said step of forming said target structure further comprises etching said oxide surface to form apertures therein exposing regions of the surface oF said semiconductor material.

9. The method of forming a storage tube comprising: supporting at least an electron source, a beam forming electrode structure and a storage target in rigid spaced relation within an envelope; said target structure being formed by the steps comprising attaching a metal support structure to one side of a semiconductor wafer and oxidizing the other side of said semiconductor wafer; said metal support structure being positioned on the opposite side of said target structure from said electron source; and evacuating said envelope.

10. The method in accordance with claim 9 wherein: said target structure is positioned with the insulating surface facing said electron source.

11. The method in accordance with claim 10 wherein: said semiconductor wafer is formed of single crystal silicon.