US3750117A

US3750117A - Electron beam addressable archival memory

Info

Publication number: US3750117A
Application number: US00185125A
Authority: US
Inventors: A Chen; J Wang
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-09-30
Filing date: 1971-09-30
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

An electron beam addressable memory is disclosed wherein information is stored as deformation and phase changes in a noncrystalline thin film. The thin film comprises a material having metastable structural characteristics. Information is stored by locally heating the film to change it from one state to another, thereby deforming it. Readout is accomplished by scanning the film with an electron beam and determining the stored information by variations in secondary electron emission yield due to the deformations and phase changes in the film.

Description

Unite States Patet Chen et al.

1111 3,750,117 1451 July 31,1973

[54] ELECTRON BEAM ADDRESSABLE ARCHIVAL MEMORY inventors: Arthur C. M. Chen; Jish-Min Wang,

both of Schenectady, NY.

General Electric Company, Schenectady, NY.

Filed: Sept. 30, 1971 Appl. No.: 185,125

Assignee:

11.8. Cl. 340/173 LS, 346/74 EB Int. Cl Gllc 11/42 Field of Search 340/173 R, 173 LS,

340/173 CR; 346/74 EB References Cited UNITED STATES PATENTS 3,l8i,l25 4/1965 Vadopalas ..340/i73CR 3,636,526 1 1972 Feinleib 340/173 LS OTHER PUBLICATIONS Electronics-Sept. 28, 1970 pg. 61-72.

Primary Examiner-James W. Moffitt Attorney-Frank L. Neuhauser et al.

[57] ABSTRACT 10 Claims, 5 Drawing Figures V v t Q??? Pmimaw I 3.750.111

sutz uorz .F/Gfl. I F/6.2

' RELATIVE voLumEm) 8 8 8 8.

I aussr A? GA i so I ATENI JUL 3 1 ms SHfEI 2 [If 2 ELECTRON BEAM ADDRESSABLE ARCHIVAL MEMORY This invention relates to information storage devices and, in particular, to an archival memory utilizing electron beam readout.

In the prior art, a wide variety of proposals for storing large amounts of information have met with varying degrees of success. However, as storage capacities increase, the cost of the memory increases as well. Simi-- larly, the desire to reduce the size of the memoryhas increased the complexity of fabricating the memory further increasing its cost.

There is also a need in the art for archival" memories, i.e., long term permanent storage memories in which information is written once and never changed, but which can be read out frequently. In addition, the usual desiderata apply: high bit density, rapid access and low cost per bit.

Beam accessable memory systems appear to be an attractive approach for building a large, high density memory with fast access time and low 'cost. However, one form of this approach laser holographic storage in photographic medium suffers from the disadvantage that the writing means differs from the reading means and that complicated photographic developing processes are. involved. This complicates the memory system and prevents the addition of new information once the hologram has been formed.

An electronbeam offers an alternative access mechanismfor the'memory. With an electron beam, to obtain high signal to noise ratio, a large reading beam current should be used. However, a large reading beam current tends to produce a destructive rather than the desired non-destructive readout.

Other storage targets for electron beam memory have been proposed and implemented in the past. Among'them is thermoplastic recording. Thermoplastic recording, in which the information is stored as erasable deformation of the material, in-general requires optical readout; i.e., the writing and reading mechanisms are different.

In view of the foregoing, it is therefore an object of the present invention to provide an archival memory utilizing an electron beam for both writing and reading.

It is a further object of the present invention to provide an archival memory utilizing a non-crystalline or an amorphous material exhibiting metastable structural characteristics.

It is another object of the present invention to provide an archival memory utilizing a homogeneous, structureless target of non-crystalline or amorphous material in conjunction with electron beam deflection apparatus. I

It is a further object of the present invention to provide an archival memory utilizing secondary electron emission effects for readout.

The foregoing objects are achieved in one embodiment of the present invention by providing a thin, e.g. less than 3,000A, film of non-crystalline material, e.g. vapor deposited amorphous germanium, as a target for an electron beam. Also located adjacent the target, and off-axis from the electron beam, is a secondary electron detector.

Writing is accomplished by irradiating selected bit sites of the target to change the state of the material from amorphous to crystalline. By utilizing a metastapoint of the material used for layer 12. Point A on the ble material, the change from the amorphous to the crystalline state is energetically very favorable while the change from crystalline to amorphous is impossible except under extreme conditions. Thus, information once stored is extremely difficult to erase; hence, the memory is archival. If desired, blank areas can be left on the target so that additional information may be added to the memory as required.

In a second embodiment of the present invention, the target is covered with a metal layer, e.g. molybdenum.

A more complete understanding of the present invention may be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of the present invention.

FIG. 2 illustrates the relationship between volume and temperature for the various states of the target material.

FIG. 3 illustrates an example of a ternary diagram giving the composition of suitable materials.

FIG. 4 illustrates an alternative form of the target in accordancewith the present invention.

comprises a non-crystalline material target, an electron beam writing and reading apparatus and a secondary electron detector.

Specifically, target 10 comprises a high'thermal conductivity substrate havingdeposited thereon a layer 12 of non-crystalline material. Electron beam generating and control apparatus comprises heater element 13, cathode l4 and deflection elements 15. These three elements serve to generate and direct a beam of electrons 16 at target 10.

During an initial writing mode, the intensity of electron beam 16 is sufficient to cause local heating at selected bit sites within non-crystalline layer 12 of target 10. The local heating serves to crystalline portions of layer 12 forming crystalline areas such as crystalline material 17.

During the reading operation, a beam of electrons of lower intensity is directed at target 10 and the emitted secondary'electrons 18 are monitored by the electron detector 19. Alternatively, the current variations due to secondary electron emission yield can be read out as changes in voltage drop across output resistor 21 at output-terminal 22.

The overall operation of the embodiment illustrated in FIG. 1 may be understood by also considering FIG. 2 which illustrates an, important parameter of noncrystalline and crystalline materials, the variation in volume as a function of temperature for bulk glassy materials. Point B on this curve represents the melting amorphous-liquid curve representsan operating point of the material, that is, above this temperature the material comprising layer 12 is switched from the noncrystalline or amorphous state to the crystalline state. As can be seen by inspection of FIG. 2, when the amorphous material is switched to the crystalline state there is a decrease in the volume occupied by the material. For amorphous germanium this decrease in volume amounts to approximately 20-30 percent of the original volume.

Thus, as originally constructed, target comprises a substrate 11 having deposited thereon a uniform planar layer of non-crystalline material 12. The entire target is structureless, that is the storage site on the target are not defined by any physical features of the target. In an initial writing operation a directed beam of electrons 16 is deflected over the target to selected bit sites upon which it is desired to write. The impingement of electrons upon the selected bit sites locally raises the temperature of layer 12. At this point the material at the bit site changes from the amorphous to the crystalline state and undergoes a change in volume.

To read out the information stored in target 10, a lower beam exposure, as more fully explained below, is used. During the reading operation, the emission of secondary electrons 18 from the target is monitored by the electron detector 19. Assuming the directed beam of electron 16 impinges upon a storage site of decreased volume (crystalline), the yield of secondary electrons is reduced as compared to when the beam of directed electron l6 impinges upon an area of layer 12 in the amorphous state. This variation in secondary electron yield as the directed electron beam is impinged upon various storage sites in the target provides an output signal from detector 19 indicative of the information stored in target 10. Alternatively, similar information can be obtained by monitoring the current flowing through target 10 and readout resistor 21, i.e., variations in secondary electron yield causes variations in the current flowing through resistor 21.

As is well known, secondary electron emission" refers to the emission of electrons from a material bombarded by some form of primary" radiation. More specifically, it refers to the emission of electrons from a solid bombarded by higher energy electrons. At least two factors appear to contribute to the change in the secondary electron yield from a switched memory site in accordance with the present invention. A first is the change of the material into the crystalline state. This.

can produce a lower yield than the material in the amorphous state, as discussed by Chen, Norton and Wang, Applied Physics Letter 18, 443 (1971). Other materials, such as antimony trisulphide, Sb S produce a higher secondary emission yield in the crystalline state than in the amorphous state. Whether or not a given target produces a higher or lower yield is empirically determined. Insofar as the present invention is concerned, it is immaterial whether the yield from the target area in the crystalline state is higher or lower than that in the amorphous state, so long as a variation is obtained.

A second factor in secondary emission is the depression itself, or, strictly speaking, the slope of the sides of the depression causing the electron beam to strike the target at an angle other than normal to the target. This factor increases the secondary electron yield, as noted by A. J. Dekker in Solid State Physics, Vol. 6, Ed. by

Seitz and Turnbull, 1958, Academic Press, New York. In the second embodiment of the present invention, with a metal overlayer, it is only required that the material deform, since the second embodiment relies on this second factor alone.

FIG. 3 illustrates a ternary diagram as is frequently used in the art to designate suitable material for useas layer 12 in accordance with the present invention. Memories in the prior art utilizing amorphous materials generally rely on theamorphous material as having bistable characteristics. That is, the amorphous material can be readily switched between the amorphous and crystalline states. These suitable materials are empirically found and generally designated as preferred areas such as

areas

31 and 32 in a ternary diagram.

The present invention, on the other hand, is concerned with materials whose composition is defined by substantially the remainder of the ternary diagram, in which the bulk materials are usually crystalline rather than glassy. A metastable, amorphous state is produced in the material by vapor depositing the material on a substrate cooled below the crystalline temperature. The resulting film is amorphous as vapor deposited but can be locally changed to the crystalline state by an electron beam. These amorphous, vapor deposited films exhibit poor bistable characteristics since they transform irreversibly to the crystalline state upon irradiation by an electron beam.

By utilizing a material for layer 12 that exhibits poor bistable characteristics, that is, a material which will switch irreversibly from the amorphous to the crystalline state, one can initially write on the target and subsequently read out the target a great many times without destroying the information, as well as permanently store the information therein.

For example, the bulk glassy region within the ternary system of (Si, Ge), (Te, Se, 8,), (As, P, Sb) has been described by A. R. Hilton, C. E. Jones and M. Brau, Physics and Chemistry of Glasses, Vol. 7, Page I05 (Aug. 1966). The compositions suitable for the archival memory of the present invention are generally those compositions which are crystalline in the bulk form and lie outside of thebulk glassy regions described by A. R. Hilton et al. The suitability of any composition depends on whether vapor deposited thin film will be non-crystalline and on whether the films crystallization temperature can be attained by electron beam heating. For example, various Ge-Te thin films vapor deposited on cooled substrates are noncrystalline. The crystallization temperatures of these films occurs from C to 500C depending on the richness of Ge content. In principle, these films are all suitable for use in the present invention.

For the ternary system Ge-Te-Aa, films comprising Ge Te As and Ge 'le As, have been found suitable for archival memory applications. These are but some of the examples of compositions which are crytalline in bulk form but form vapor deposited noncrystalline thin films as described above. In general, for

greater than about 15 percent provides suitable films.

Other non-crystalline thin films can be formed by the vapor deposition of complex oxides with low substrate temperature. For example, E. K. Miller, B. .I. Nicholson and M. H. Francombe, Electrochemical Tech. Vol. 1, page 158 (1963) shows that a grain by grain evaporation method produces non-crystalline glassy BaTiO thin films. Upon heating to above 400C, the amorphous BaTiO thin film readily transforms irreversibly to the denser crystalline form.

Similarly, the non-crystalline thin film may be a vapor-deposited, elemental semiconductor such as Ge on a cooled, maintained below 450C, substrate, The elemental amorphous semiconductor films also transform irreversibly to denser crystalline form upon heating to above its crystallization temperature.

The film thickness of the non-crystalline material is preferably from about 500A to about I t. Again the thickness used depends upon the other memory parameters. The desired characteristics of the substrate are its chemical inertness with the non-crystalline material and its high thermal conductivity characteristics. A 1,000A to l 11. thick molybdenum layer on a silicon wafer is a suitable substrate. Other possibilities are tungsten on quartz, or a solid refractory metal such as molybdenum or tungsten.

Since the writing process is a thermal crystallization process, it involves complex interactions among the electron beam parameters, the material parameters, and the total thermal characteristics of the target. The goal is to achieve high density (fine beam diameter) electron beam induced crystallization in the noncrystalline thin film. For non-destructive read-out, the reading'electron beam energy should not induce any change in the non-crystalline target. This can be achieved by lowering the beam voltage or the beam current. Alternatively, the beam dwell time at any spot on the target can be reduced to a sufficiently short time to prevent any heating to occur.

For example, writing has been accomplished in a target consisting of a 1,650A film of Ge Te- As, amorphous semiconductor deposited upon 2,000A molybdenum coated Si wafer. The molybdenum film is used to prevent any possible chemical reaction between the Ge,,,Te As and the Si wafer during the vapor deposition process. The writing electron beam parameters were 4.5Kv, lOOnA, 1,000A in diameter and the beam was line swept at a rate which cover 1,000A distance in SOnS (i.e. dwell time of 50nS). The resultant electron beam recording in the target was l,500-2,000A wide. Non-destructive electron beam reading was accomplished by the same electron beam parameters except the sweep rate covered 1,000A in 5nS (i.e. dwell time of 5nS). Thus, reading is possible with the same power electron beam but with decreased dwell time to prevent any alternation of the target. Dwell time is defined as the time required for the beam to travel its diameter.

In general, the specific values of the electron beam parameters, the physical dimensions of the various layer of the thin film target and the dwell time depend on the material and thermal characteristics of the noncrystalline thin film, the substrate and on each other.

In particular, the crystallization temperature is different for different non-crystalline thin films. Thus, if all other parameters of the writing process are identical, the beam voltage, current and the dwell time vary with the crystallization temperature. Taking into account the electronics, electron optics and material characteristics requirements, in general the writing process requires a beam voltage of about l.5l(v to 30Kv, and a beam current of approximately l0nA to 10 uA. The possible dwell time depends in part on the thermal characteristics and in part on the crystallization kinetics but is approximately lnS to l0 #8.

While deflection system 15 is illustrated in FIG. 1 as comprising an electrostatic deflection system, any suitable deflector may be utilized. FIG. 4 illustrates a pre ferred embodiment of the present invention wherein deflector 15 is utilized as a coarse deflector and a matrix deflection system 40 is used as a fine deflector, such as disclosed and claimed byS. P. Newberry in US. Pat. No. 3,534,219. This deflection system comprises a course deflector illustrated as two pairs of orthogonal electrostatic plates and a fine deflector composed of a matrix of lenslets forprecisely directing the electron beam over adjacent areas of a target. Selection of the lenslct and storage site to be either written on or read out is made by an address command module controlling the amplifiers coupled to the deflectors.

Specifically, a beam of electrons is deflected by coarse deflector 15 to the matrix deflection system 40 which comprises sets of orthogonal conductors electrically connected to one another. That is, conductors 41 which are parallel and extending in one direction are connected together and orthogonal conductor 42 are each connected one to the other. Coarse deflector l5 deflects stream of electrons 16 to a particular aperture as defined by orthogonal conductors 41 and 42. The potential applied to orthogonal conductors 41 and 42 then further directs electron beam 16 to a particular site on target 50 downstream therefrom. Target 50 comprises a substrate 11 of a good thermal conductivity material having applied thereover a layer of noncrystalline material 12 as previously described. In addition, target 50 has applied over non-crystalline layer 12 an additional thin metal layer 43, for example, approximately A thick, such as molybdenum. Any metal layer from 10 to 2,000A thick may be used as the overlayer.

During the initial writing operation, electron beam 16 is directed at selected ,bitsites in storage target 50. As with the embodiment of FIG. 1, electron beam 16 causes localized heating thereby changing the noncrystalline material of layer 12 into the crystalline state as illustrated in H6. 4 by reference numeral 17. Metal layer 43 serves to seal target 50 so that should the material on layer 12 be volatized during writing it cannot escape from the surface of target 50, but rather remains trapped within target 50 as a minute crystalline mass at the selected bit site.

Readout of target 50 is similar to that of target 10. However, the effect relied upon to vary the yield of secondary electrons is different from that of the embodiment illustrated in FIG. 1.

As previously noted, secondary electron emission yield is affected by the geometric effect, that is, changes in the angle of incidence of the primary electrons will change the yield of secondary electrons. Thus as electron beam 16 traverses the previously irradiated bit site, the yield of secondary electrons will show an appreciable increase as the beam traverses the sloped portions of the sides of the bit site. These variations in the secondary emission yield are detected by electron detector 19. Alternatively, the effects of secondary emission can be detected by monitoring the current flowing through target 50 and output resistor 21. Variations in current due to variations in secondary electron emission yield indicate whether a non-crystalline or crystalline storage site is being irradiated.

FIG. 5 illustrates a pair of

waveforms

51 and 53 illustrating the outputs obtained from

targets

10 and 50 respectively. In reading out target 10, as the primary electron beam traverses a previously radiated bit site in which thedensification and a phase change that has been induced reduces the yield of secondary electrons. Thus, the output signal from the electron detector 19 will contain a pronounced clip 52 at the point representing the previously irradiated bit site. In contrast to this, output waveform 53 contains a pair of

peaks

54 and 55 produced as primary electron beam 16 traverses the sloped side portions of the densified bit site.

Having thus described the invention it will be apparent to those skilled in the art that various modifications may be made within the spirit and scope of the present invention. For example, as noted above, in the choice of material for amorphous layer 12, compounds other than germanium, tellurium and arsenic may be utilized. What we claim as new and desire to secure by Letters Patent of the United States is:

1. An archival memory system comprising target means comprising a thin film of metastable, non-crystalline material overlying a high thermal conductivity substrate; electron beam deflection means for initially writing on said target means by electron beam heating selected bit sites on said target to change the material at said sites from the non-crystalline state at one volume to the crystalline state at another volume; said electron beam deflection means also reading said target by irradiating bit sites therein; and secondary electron emission detection means for sensing variations in secondary electron emission yield during subsequent reading operations as said deflection means irradiates bit sites in said target means. 2. An archival memory as set forth in claim 1 wherein said electron beam deflection means comprises:

electron beam generating means; a first deflection means; and a second deflection means comprising a planar array of a plurality of deflection elements arranged in a matrix, wherein the electron beam from said generating means is deflected by said first deflection means to one of the deflection elements of said second deflection means which, in turn, directs said electron beam to said target means.

3. An archival memory system as set forth in claim 1 wherein said material comprises a complex oxide vaper-deposited upon a cooled substrate.

4. An archival memorysystem as set forth in claim 1 wherein said material comprises amorphous germanium.

5. An archival memory system as set forth in claim 4 wherein said substrate comprises a molybdenum layer overlying a silicon wafer.

6. An archival memory as set forth in claim 1 wherein said target comprises a molybdenum layer overlying said amorphous material.

7. An archival memory as set forth in claim 6 wherein said material comprises a 3 ,000A thick layer composed of germanium, arsenic and tellurium overlying a substrate and having a A layer of molybdenum thereover.

8. An archival memory as set forth in claim 7 wherein said material comprises at least 15 percent germanium.

9. An archival memory as set forth in claim 8 wherein said material comprises a vapor deposited layer having the composition Ge Te As 10. An archival memory as set forth in claim 8 wherein said material comprises a vapor deposited layer having the composition Ge Te As I i i

Claims

1. An archival memory system comprising target means comprising a thin film of metastable, noncrystalline material overlying a high thermal conductivity substrate; electron beam deflection means for initially writing on said target means by electron beam heating selected bit sites on said target to change the material at said sites from the noncrystalline state at one volume to the crystalline state at another volume; said electron beam deflection means also reading said target by irradiating bit sites therein; and secondary electron emission detection means for sensing variations in secondary electron emission yield during subsequent reading operations as said deflection means irradiates bit sites in said target means.

2. An archival memory as set forth in claim 1 wherein said electron beam deflection means comprises: electron beam generating means; a first deflection means; and a second deflection means comprising a planar array of a plurality of deflection elements arranged in a matrix, wherein the electron beam from said generating means is deflected by said first deflection means to one of the deflection elements of said second deflection means which, in turn, directs said electron beam to said target means.

3. An archival memory system as set forth in claim 1 wherein said material comprises a complex oxide vapor-deposited upon a cooled substrate.

4. An archival memory system as set forth in claim 1 wherein said material comprises amorphous germanium.

7. An archival memory as set forth in claim 6 wherein said material comprises a 3,000A thick layer composed of germanium, arsenic and tellurium overlying a substrate and having a 100A layer of molybdenum thereover.

9. An archival memory as set forth in claim 8 wherein said material comprises a vapor deposited layer having the composition Ge39.4Te60.3As0.3.

10. An archival memory as set forth in claim 8 wherein said material comprises a vapor deposited layer having the composition Ge26Te70As4.