US3763476A

US3763476A - Method and apparatus for storing and reading out charge in an insulating layer

Info

Publication number: US3763476A
Application number: US00234786A
Authority: US
Inventors: R Wilson; G Possin; G Ellis
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1972-03-15
Filing date: 1972-03-15
Publication date: 1973-10-02
Anticipated expiration: 1990-10-02
Also published as: FR2176029A1; FR2176029B3; JPS494919A

Abstract

A diode storage array, including a diode array on one face of a semiconductor wafer, an insulating layer overlying the opposite face of said wafer and a conductive layer overlying the insulating layer, is written upon by irradiating the conductive layer side of said wafer to induce charge storage in the insulating layer. The radiation may be high energy photons, a scanned electron beam or electrons from a photo-emitter. Readout is accomplished by irradiating the target with lower energy radiation to form electron-hole pairs in the wafer. The holes are selectively driven to the diode side of the wafer under the control of the stored charge where selected, reverse biased, diodes are discharged. Subsequent scanning of the diode array by an electron beam produces a variable output signal, indicative of the information stored. Since the charge on the insulating layer is not dissipated, the information can be read as often as desired.

Description

United States Patent Wllson et al.

Oct. 2, 1973 METHOD AND APPARATUS FOR STORING AND READING OUT CHARGE IN AN lNSULATlNG LAYER [75] Inventors: Ronald H. Wilson; George E. Possin,

both of Schenectady; George W.

Ellis, Burnt Hills, all of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Mar. 15, 1972 [21] Appl. No.1 234,786

[52] [1.8. CI. 340/173 LS, 313/65 AB, 317/235 NA,

[51] Int. Cl. Gllc 5/02, G1 1c 11/34, Gllc 13/04 [58] Field of Search 340/173 LS, 173 CR;

317/235 NA; 313/65 A, 65 T, 89, 65 AB [56] References Cited UNITED STATES PATENTS 3,701,979 10/1972 Smith 340/173 R 3,668,473 6/1972 Miyashiro 317/235 NA 3,676,715 7/1972 Brojdo 340/173 LS 3,576,392 4/1971 l-lofstein..... 317/235 NA Crowell......

Primary ExaminerBernard Konick Assistant Examiner-Stuart Hecker AttorneyJohn F. Ahern et al.

[57] ABSTRACT A diode storage array, including a diode array on one face of a semiconductor wafer, an insulating layer overlying the opposite face of said wafer and a conductive layer overlying the insulating layer, is written upon by irradiating the conductive layer side of said wafer to induce charge storage in the insulating layer. The radiation may be high energy photons, a scanned electron beam or electrons from a photo-emltterfR'ad o'ut is'ac complished by irradiating the target with lower energy radiation to form electron-hole pairs in the wafer. The holes are selectively driven to the diode side of the wafer under the control of the stored charge where selected, reverse biased, diodes are discharged. Subsequent scanning of the diode array by an electron beam produces a variable output signal, indicative of the information stored. Since the charge on the insulating layer is not dissipated, the information can be read as often as desired. I

10 Claims, 1 Drawing Figure WRITE Patented Oct. 2, 1973 ERASE WRITE I I6 1 WM- 24 1 2 METHOD AND APPARATUS FOR STORING AND Another object of the present invention is to provide READING OUT CHARGE IN AN INSULATING a diode array storage target in which the reading and LAYER writing operations can be performed simultaneously.

The foregoing objects are achieved in the present in- ThiS invention elates to diode array storage targets 5 vention wherein information is stored as an electric and, in-particular, to a method for writing and reading charge in an insulating layer overlying one Surface 0f 3 with h targets, semiconductor wafer. Reading is accomplished by in- In the prior art, diode array targets are used f a ducing electron-hole pairs in the semiconductor and riety f f ti such as imaging devices and scan allowing the holes to diffuse through the wafer to the converters. In such applications attempts at providing opposite suffaceawhich Contains the p-n junction array. storage capability in excess of the typical 1/30 second, At the llmfmon the holes recombme the approximate time needed for scanning the entire electronsPrevlousl'y deposltefi by an electron beam to target once, have resulted in storage times on the reverse bias the diodes. During a subsequent scan of order of a few seconds. Longer storage times have been the array.by the electron vananons m curobtained by mechanisms that, in effect, interfere with rent F i {estqre the dlqdes to a.reverse blased the operation of the target, thereby Slowing the condition are indicative of the information stored.

. Since the charge is stored in the insulator it is not s ons of t p 6 he arget For example a msulatmg dissipated. By inducing the electron-hole pairs from an layer has been interposed between the diode array and th 1 t b t d h ff f h external source, the information can be read as often e e cc 0 re uce rec argmg e o t e as desired or continuously. The information stored can electron beam during readout, thereby retaining some be erased or chan ed as desired b suitabl irradiatin charge indicative of the information stored. However, g y y g the insulating layer with the more energetic radiation. Storage tfirgats of thls type that have an appreciable A more complete understanding of the present invenstorage time generally require a longer time for readtion can be obtained by considering the following detailed description in conjunction with the accompany- Alternatively, it has been proposed to store charge on ing drawings, in which;

the oxide on the diode side of the target. This, however, T FIGURE ill a di d array target system can compromise the operating characteristics of the i bl f use i h present invention g -g-. Charge P the oxide layer during The FIGURE illustrates a preferred embodiment of non-storing operation. Some diode array targets utilize the present invention wherein a diode array camera one or more protective layers to prevent charge buildtube target is modified by the application of charge up on the oxide. However, to provide storage capabilstorage layers to the substrate on the opposite side of ity, these protective layers cannot be used. the diode array. The net result is a combination that In addition, the information stored is destructively provides a unique cooperation between the charge read out in many of the systems of the prior art, i.e., the stored and diode array elements so as to provide a storcharge representing the information stored is utilized g Camera tube target that is x r m ly fle b e n 0pdirectly in the read operation and is partially or totally eratiofldissipated. Further, many systems of the prior art are l? y target m comprises subtfate 11 not capable of carrying out the read and write functions Pnsmg an yp Semlconductor material Onto which simultaneously p-type conductivity regions 12 are grown through a Thus, there is a need in the art f a method of writ plurality of apertures in apertured insulating layer 13 ing and reading in diode array targets that provides collpled to Substrate is long term, i.e., virtually indefinite, erasable storage across Much the vldfao outpu} Slgna] while also providing non-destructive readout, simultatamed1connected Feslstor 20 a l of neous read and write capability, and read times compa potential 19 for suitably biasing the target relative to thode l7. rable to non-storing targets. ca

In View of the foregoing it is therefore an object of The target as thusdescribed is similar to a target described by William E. Engeler in application Ser. No. invention to pmvlde a method. for f g 60,767, filed Aug. 3, 1970 and assigned to the same asinformation in a diode array memory so that the inforras bl b t a b tored for [on s de signee as the present invention. While this particular IS 6 a e u c n e s as g a form of diode array is shown and described it should be h f h t understood that any suitable diode array target may be not er 0 f? t e presen l to prov e utilized in carrying out the present invention. However, a method for writing and reading diode array targets d d this particular diode array does not require the protecproducing long term storage and nonestructive rea five layers noted above out.

The face of substrate 11 opposite the face in which A further ob ect of the present invention lS.tO provide Hype conductivity regions 12 are formed is covered long Storage and rapld readout of dlode array by insulating layer 14, which may conveniently commemol'les- 6o prise an oxide of substrate 1].. Layer M in turn is cov- Another object of the P invention is to Provide ered by a transparent conductive layer 15. Layer 15 is a writing and reading method for diode array Storage connected to sources of

operating potential

24 and 25 targets in which the information stored is not directly b way of potentiometer 16, fo l sources of utilized in reading. operating

potential

24 and 25 are further connected to A further object of the present invention is to provide a mm n oint comprising the junction of potential a writing and reading method for diode array storage source 19 and output resistor 20. Cathode 21 is biased targets in which the electrical effect of charge reprerealtive to the target by way of bias means 23 which is senting stored information is utilized for reading. also connected to the common point. Also illustrated in the FIGURE, is a source of light 26 producing photon energy illustrated by rays 27.

The overall operation of the present invention shall be described in three parts. The first relating to the diode array, left-hand portion of the target illustrated in the FIGURE, the second relating to the operation of V the right-hand portion of the target as illustrated in the FIGURE, and finally the cooperation of these two sections together.

Ignoring for the moment the presence of

layers

14 and 15, the operation of a diode array target is relatively well known and may be summarized as follows: As an initial step, the diode array is scanned by electrons 18 eminating from cathode 17. This scan negatively charges p-type conductivity regions 12 relative to substrate 11. Input information, generally an optical image, applied to the opposite face of the substrate 11 forms a pattern of electron hole pairs, the holes of which diffuse to the diodes and discharge the diodes in proportion to the intensity of the light absorbed in that area of the substrate. As the electron beam rescans the diode side of the substrate, the current necessary to recharge the p-type conductivity regions is proportional to the amount by which the p-type conductivity regions were discharged. This current flows in a circuit comprising cathode 17, electron beam 18, target and output resistor 20. Thus, the current flowing through output resistor 20 provides a video signal corresponding to the pattern of light incident upon the substrate. It may be noted that the operation of this type of target requires the continuous application of input information on the opposite face of the substrate. The storage time of the diode portion of the target is dependent upon the time it takes the dark current to completely discharge the diodes.

The right-hand side of target 10 as illustrated in the FIGURE operates to store charge in proportion to input information in insulating layer 14 for relatively long periods of time, e.g., several tens of hours. The storage of charge in insulator 14 is not permanent, i.e., the information can be readily erased.

The charge in insulating layer 14 can be created by a variety of means, such as a scanned electron beam, electrons from a photoemitter, or high energy photons. For the preferred embodiment of the present invention, it will be assumed that the pattern of charge is to be obtained from an electron beam.

Electron beam 22 emanating from cathode 21 is directed so as to write the information in a predetermined pattern as charge in insulating layer 14. This is accomplished by utilizing a high energy electron beam, for example, 10 kilovolts. During the writing operation, transparent conductive layer is biased positively with respect to n-type conductivity substrate 11. In so doing, mobile electrons induced in insulating layer 14 by electron beam 22 are drawn off through transparent conductive layer 15. Electron beam 22 is then terminated and a pattern of positive charges 30 is stored in insulating layer 14.

The charge stored in insulating layer 14 can be removed in the same way it was created, except that transparent conductive layer 15 is biased negatively or not at all with respect to substrate 11. Thus, the changeover from one mode of operation to another is very simply and easily accomplished. After all or part of the information is erased, a subsequent writing operation is performed to store new information in the erased areas.

The pattern of positive charges 30 is retained by insulating layer 14 for a relatively long time. As will be apparent from the following description of the operation of the two halves of the target together, the charge stored in insulating layer 14 is not used directly in the read out of the pattern of charged storage. Further, it should be noted that the writing operation and the reading operation are independent so that no restriction is placed on the operation of either half of target 10.

During readout, the storage face of target 10 is illuminated by light from source 26. Since the energy contained in a photon is proportional to the frequency (v) of the photon it is preferable to illuminate the target with light of frequency in the range of visible light so that the positive charge pattern 30 is not changed during the reading operation, regardless of the setting of potentiometer 16.

The incident photons penetrate into n-type conductivity substrate 11 thereby forming electron-hole pairs. Due to the storage of positive charge, the holes of the electron-hole pairs are repelled away from the storage side of target 10 to the diode side of target 10 where they discharge those diodes approximately opposite the location of the stored positive charge. In the absence of stored charge the electron-hole pairs diffuse t0 the interface between substrate 11 and insulator 14 where they recombine and are lost. For a silicon wafer, blue light is preferred so that the holes are created near the storage layer thereby increasing the modulation efficiency of the stored charge pattern. Thus, the pattern of charge storage is transferred from the right-hand side of target 10 as illustrated in the FIGURE to the left-hand side of target 10. Readout of the diode array is then accomplished as described above, wherein the diodes are scanned by an electron beam and the amount of charge necessary to restore the charged condition is monitored across output resistor 20.

As noted above, the readout and charge storage operations are completely independent since the stored charge is only used indirectly in discharging the diodes. This enables what may be generally described as special effects to be performed with the stored information. For example, the storage of information and the scanning by electron beam 18 can occur at different rates. Also, the pattern of scanning need not be the same for both the storage of information and the reading of information. For example, apparent motion can be obtained in an image by modifying the location of the starting point of the scan by electron beam 18.

It should be emphasized that radiation in the visible region incident upon the storage side of target 10 has no effect on the stored charge regardless of the setting of potentiometer 16. Information is written only with higher energy radiation when transparent conductive layer 15 is biased positively relative to substrate 11. Erasure of the entire stored charge is carried out by negatively biasing transparent conductive layer 15 during higher energy irradiation, as by electron beam 22. In this case, mobile electrons are induced and recombine with the positive charge stored in layer 14 to bring the net charge on insulating layer 14 to zero. Obviously, erasure can be either partial (one area or only part of the charge) or total (all of layer 14 or all of the charge). It should be further noted that no structure is required on the storage side of target 10, thereby enabling a maximum of resolution to be obtained. The storage side of target operates by controlling the recombination of induced electron-hole pairs by repelling the holes from the storage side of the target so that they diffuse to the diode side of the target where they discharge the diodes in the same pattern as the stored charge.

Target 10 can be fabricated in any suitable fashion, as, for example, set forth in the above-noted application of William E. Engeler, with the addition of layers 14 and 115. Substrate 11 can be a silicon wafer on the order of p. thick and the apertures through which ptype conductivity regions 12 are grown can be on the order of 8 p. in diameter. Insulating layer 14, which may comprise several layers of insulating material, can be on the order of 5,000A. thick, necessitating a write/erase beam in excess of about SkV. The intensity of the illumination from blue light source 26 can be on the order of (10) watts per square cm. The bias voltage on transparent conductive layer 15 can be fiO volts. It is to be understood that the foregoing values are exemplary only and not limiting.

Having thus described the invention it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present invention. For example, as previously noted, p-type conductivity regions 12 need not be epitaxially grown but may be formed simply as diffused regions. The source of bias potential for transparent conductive layer need not be as shown but may employ other, more elaborate sources, such as pulses for selecting write, neutral and erase in synchronism with the deflection of beam 22. Also, any suitable deflecting means may be employed.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An information storage target comprising:

a substrate of a first type conductivity semiconductor having first and second opposite sides;

a diode array formed on said first side;

an insulating layer, overlying said second side, for

storing information in the form of electric charge; and

a conductive layer, overlying said insulating layer, for

controlling the writing and erasing of information in said insulating layer.

2. The method of writing, storing, reading and erasing information in a diode array target, containing a diode array on one face of a semiconductor wafer, an insulating layer overlying the opposite face of said wa fer, and a transparent, conductive layer overlying the insulating layer, comprising the steps of:

selectively irradiating the insulating layer to induce mobile electrons therein;

drawing off said mobile electrons to selectively charge said insulating layer by biasing said conductive layer to one polarity;

inducing electron-hole pairs in said wafer;

preventing recombination of said pairs and allowing at least one of said electrons and holes to diffuse to the diode side of said wafer; and

5 scanning the diode side of said wafer with an electron beam to read out said stored information.

3. The method as set forth in claim 2 and further comprising the steps of:

biasing said conductive layer to a second, opposite 10 polarity; and

irradiating the insulating layer to induce mobile electrons in said insulator for neutralizing the charge on said insulating layer. 4. The method as set forth in claim 2 wherein said inl 5 ducing step comprises:

irradiating said target with photons of light.

5. The method as set forth in claim 2 wherein the step of selectively irradiating comprises:

selectively irradiating the insulating layer with an electron beam penetrating said conductive layer to induce mobile electrons in said insulating layer.

6. An information storage system comprising a semiconductor diode array target comprising a planar semiconductor substrate having a diode array on one side and an insulating layer covering the opposite side;

means for storing information in the form of electric charge in said insulating layer; means for irradiating said opposite side of said target for inducing electron-hole pairs in said substrate, said stored charge preventing recombination of said pairs to allow diffusion of at least one of said electrons and holes toward the diode array side of said target, thereby transferring the information from one side of the target to the other; and

means for reading said information from said diode array.

7. The information storage system as recited in claim 6 wherein said target further comprises a transparent, conductive layer overlying said insulating layer and said means for storing information comprises:

a source of electron beam for irradiating selected portions of said insulating layer; and

bias means coupled to said transparent, conductive layer and said substrate for biasing said conductive layer relative to said substrate.

8. The information storage system as set forth in claim 6 wherein said means for reading said information comprises:

a source of an electron beam for irradiating selected portions of said diode array.

9. The information storage system as set forth in claim 6 wherein said means for irradiating said target comprises a source of light.

10. The information storage system as set forth in claim 9 wherein said light is blue.

Claims

1. An information storage target comprising: a substrate of a first type conductivity semiconductor having first and second opposite sides; a diode array formed on said first side; an insulating layer, overlying said second side, for storing information in the form of electric charge; and a conductive layer, overlying said insulating layer, for controlling the writing and erasing of information in said insulating layer.

2. The method of writing, storing, reading and erasing information in a diode array target, containing a diode array on one face of a semiconductor wafer, an insulating layer overlying the opposite face of said wafer, and a transparent, conductive layer overlying the insulating layer, comprising the steps of: selectively irradiating the insulating layer to induce mobile electrons therein; drawing off said mobile electrons to selectively charge said insulating layer by biasing said conductive layer to one polarity; inducing electron-hole pairs in said wafer; preventing recombination of said pairs and allowing at least one of said electrons and holes to diffuse to the diode side of said wafer; and scanning the diode side of said wafer with an electron beam to read out said stored information.

3. The method as set forth in claim 2 and further comprising the steps of: biasing said conductive layer to a second, opposite polarity; and irradiating the insulating layer to induce mobile electrons in said insulator for neutralizing the charge on said insulating layer.

4. The method as set forth in claim 2 wherein said inducing step comprises: irradiating said target with photons of light.

5. The method as set forth in claim 2 wherein the step of selectively irradiating comprises: selectively irradiating the insulating layer with an electron beam penetrating said conductive layer to induce mobile electrons in said insulating layer.

6. An information storage system comprising a semiconductor diode array target comprising a planar semiconductor substrate having a diode array on one side and an insulating layer covering the opposite side; means for storing information in the form of electric charge in said insulating layer; means for irradiating said opposite side of said target for inducing electron-hole pairs in said substrate, said stored charge preventing recombination of said pairs to allow diffusion of at least one of said electrons and holes toward the diode array side of said target, thereby transferring the information from one side of the target to the other; and means for reading said information from said diode array.

7. The information storage system as recited in claim 6 wherein said target further comprises a transparent, conductive lAyer overlying said insulating layer and said means for storing information comprises: a source of electron beam for irradiating selected portions of said insulating layer; and bias means coupled to said transparent, conductive layer and said substrate for biasing said conductive layer relative to said substrate.

8. The information storage system as set forth in claim 6 wherein said means for reading said information comprises: a source of an electron beam for irradiating selected portions of said diode array.