US2423125A - Photoelectromotive force cell of the silicon-silicon oxide type and method of making the same - Google Patents

Description

G. K. TEAL PHOTOELEbTROMOTIVE FORCE CELL v0F TH 5 1 2 1 a e 3 h w w n a SE M2 N O c I L I s E July 1, 1947.

OXIDE TYPE AND METHOD. OF MAKING THE SA Original Filed Jan. 30. 1943 20 2/ 2 I3 23 D I I6 FIG 2 //v VN TOR BY G. K TEAL ATTORNEY July 1, 1947.

' PHOTOELECTROMOTIVE G LAYER TMLUCENT- LAYER CWT! MATERIAL 3:0: H15 INSULATM Arron/ Patented July 1,

"UNITED STATES- PHOTOELECTROMOTIVE FORCE CELL, THE SILICON-SILICON, oxrps ,TYPE,

METHOD or MAKING THE SAME Gordon K, Teal, Summit, N. 'J., assigno'r Ito Bell l I Telephone Laboratories, Incorporated, w,rNcwflr York, N. Y., a' corporation of New York a, 1 j Y Original application January 30, 1943, S erial No." T p 474,205. Divided and this application May]16,' 1

' I 1944, Serial N0. 535,798

u j J 8 Claims;

This invention relates to electro-optical devices and more specifically to photo-EL M. F. devices and to methods of making them..

This application is a division of application S'erial No.;474,205, filed January 30, 1943.

A photo-E. device may be defined as a material, combination of materials or cell which when electromagnetic radiations of certain short wavelength, like those, for example, to which the eye issensitive, areapplied thereto will producean electromotive force across itslterminals, or across two portions thereof which can serve as terminals. The voltageset up between the terminals of a photo-E. M, F. device or cell when such waves are applied thereto may be used, by

way of example, to controlan electron beam. In describing the invention, such Waves will be called light, but this term is used in a broad sense to be inclusive also of radiations towhich'the eye is not sensitive. v

Objects of this invention are to provide sensitive and cheap photo-E. M. F. cells and methods of making them.

In accordance with a specific embodiment of the invention, chosen by wayof example for purposes oitillustration, there is provided a photo- E., 'cell comprising a metal base a coating comprising a mixture of silicon and ametal of higher conductivity than silicon on the base, a layer of silicon on the coating, andla layer, of an oxide ofsilicon, such as quartz, mixed with a metallic material on said silicon layer, and a metallic member, such as a thin metallic coating, on the last-mentioned layer.v In accordance with the inventin,-this cell, by way of example, can be, formed by simultaneously evaporating silicon and a metal of higher conductivity than silicon, such as aluminumgarsenic, titanium,"copper or silver, on a metal base, evaporating silicon alone on this,

combination or mixed layerycovering theisilicon I layer with quartz (silicon dioxide) and tungsten oranother metal, and then applying a th'in metallic coating on the quartz and metallic mate rial. The: efiect of the metal, of higher conduc-., V tivity is to give the cella lower-internal resist:

ance than would otherwise be obtained; Instead of simultaneous evaporations, the silicon-(and the metal of greater conductivity canbel a'pplied in alternate ,very, thin layers which partially merge witheacn'other. I i i t 1 Photo-E. M. F. cells or devices of the type described above can be utilized in a television eleca tron camera tube which comprises an evacuated envelope enclosing means for generating a beam of electrons, and an apertured target for said i plate carrying'on the side remoteifromthe beam generating means a multiplic'ity. of elemental photo-E. MsFifcells i a egor n object is projected upon the photo E.,M. F.1ce1ls and the other side of the oielectrons. g The principleof operatiohbffthis'tube follows: The beam 'generating {means produces a relatively high velocity beam andthis is'caused to scan the sidelot thetarget remote from the photo-E. M.; F." fcfellsq These cells have applied thereto'light radiations fromthe object-to be; televised and ,there is"produced acrossgeach cell avoltage proportional to the light striking the cell from acorresponding "elementalareaofthe object. The'scanning-beam strikes the target plate and causes the emission of secondary electrons which pass through the} apertures in the target member to a collecting electrode placedwithin the tubebetween the jtarget and the object. The number of electronsfwhich pass-through any aperture is controlle'd by the voltageacross the neighboring elemental photo-EpM. F; cells) 7 The collectingfelectrode is connected to a" signal re Theinventl'ontwillibe lore readily understood,

by referring to the following description taken-in connection with the accompanyingdrawing formfm a part'thereof, in whichi liig. 1- is a schematic representation of a oath ode ray tubeandj-oe tain of'it the tupe-e pmy" g: h'otocordance with'the invention i Figigisfafschem ticfview-showing in'greatly rtion oftheftarget and of the d ofthe tub'e' shown l a ifp 'ii fi f thel ar tian'd,I Figs; 4 to 19', inclusiv A g sentatipns to aid "in? ainingfthe invention. l. [Referringmoreflparticularly t the mosaic target ii. an electron gun l3 for gentarget scanned with the beam sistor through which-passes the video current,

associatedjcircuitsj The tube crating, focusing and accelerating a beam of high velocity electrons towards this target, a secondary electron collecting electrode It on the side of the target ll remote from the electron gun It, and two sets of electrostatic deflecting plates l5 and I6 for causing the beam of electrons to scan every elemental area in turn of a field of view on the mosaic target ll. Radiations from an object or field of view are applied to the side of the mosaic target ll remote from the electron gun by means of any suitabl optical system represented schematically by the lens l1.

' The electron gun It preferably comprises a cathode 20, a control electrode or member 2|, a first anode member 22, and a second and final anode member comprising cylindrical member 23 and a coating 24 of conducting material on the inside walls of the envelope 1 2 extending from the region of the cylinder 23 to the region of the mosaic target. The collecting electrode M for the secondary electrons emitted from the target II when it is struck by the beam of high velocity electrons preferably consists of mesh material or, if desired. it may consist of a ring of metallic material.

Th control electrode 21 is placed at any suitable negative potential with respect to the potential of the cathode II by means of an adjustable source and the first anode 22 and the final anode '23, 24 are placed at appropriate positive potentials with respect to the cathode 20 by means of the source 3! and the source 32. As an example, the final anode. 24 can be about 1.000 volts positive with respect to the cathode and the first anode 22 can be 300 volts positive with respect to the cathode. Any suitable source 35 can be utilised to heat the cathode 20. The negative terminal of the source ll is connected to the cathode 20 and the positive terminal thereof is connected to thefirst anode 22, while the negative terminal of thesource 32 is connected to the posifive terminal of the source 3| and the positive terminal of "source 321s connected to the second anode 23, I. Preferably the positive terminal of th'e source 12 is connected to ground through a source Tl which is used to make the apertured plate 50 (see Fig. '2) of the target Ii positive with respect to the final anode of the electron gun for a purpose which will be pointed out hereinafter. The voltage of this source 31 can be, for example, 20 to 50 volts. The apertured plate 50 is connected to ground and is also connected through a source 33 and a signal resistor 34 to the collecting electrode I4. Any suitable amplifier '38 is connected 'to the signal resistor 34 and is in turn connected to the other elements of the television transmitter circuit which prepare the video current for transmission to receiving station. The -potentials applied to the various electrode members and their configuration and shape are such that a beam :of f ocussedh'igh velocity electrons strikes the target ii :and this beam is deflected over asuitable i'fleld thereon by means of appropriate potentials applied to the deflecting plates l and It by electrostatic sweep circuits (not shown) to produce secondary electrons, the action of which will be considered more fully below. As examples of satisfactory sweep circuits, reference may be made to Patent No. 2,178,464, dated October 31, 1939, to M. W. Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable for this purpose. Connections can be made from the balanced sweep circuits to the pairs of plates I I and It by means of coupling condensers 40, 4| and 42,

43, respectively, of about one microfarad capacity each. Coupling resistances 44 and 45 of the order of many megoh'ms each are respectively connected across the pairs of plates Ii and It. Th mid-points of the resistances 44 and 45 are connected to the positive terminal of the source 32 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potential of the anode 23, 24. This relationship is maintained to avoid changes in the sensitivity of the deflecting system. and the consequent distortion of the image which would otherwise result. For more complete descriptions of the advantages of balanced sweep circuits for use with cathode ray television tubes, reference is made to the above=mentioned Baldwin patent and also to Patent 2,209,199, issued July 23, 1940, to Frank Gray.

Reference will now be made to Figs. 2 and 3 which are enlarged schematic views of a preferred form of mosaic target -l|. Fig. 2 is a schematic side view. in cross section, of a portion of the target I I, while Fig. 3 is an enlarged showing of a portion of the target ll, viewed from the right in Fig. 2, to show the general relationship of the apertures 52 in the target and the photo- E. M. F. cells 5| thereof. Certain dimensions in Figs. 2 and 3 have been exaggerated at the expense of others in order to more clearly show the screen structure. The mosaic target ii preferably comprises an apertured plate I of any suitable material. such as nickel, carrying thereon on the side remote from the beam generating means a multiplicity of photo-E. M. 1'. cell ele- 'ments SI, each aperture 52 inthe plate being effectively surrounded by cell elements. M0! the elements Ii can comprise. forexample, a small copper-oxide photo-E. M. F. cell which cell. as is well known in the art. generally comprises a layer of copper, a, semiconducting layer of cuprous oxide thereon treated by means wellknown in th art to produce on the semiconducting layer a blocking layer, and a semitrahsparent conducting element such as silver or gold covering the blocking layer. In the preferred embodiment, however, th element; :are of the silicon-silicon oxide type to be described more fully below.

Reference will now be made songs. 4 to 8, in-

clusive, which illustrate .a method of making a suitable target structure :ll. Figs. 4 to 8 are.

schematic views showing anumber onsmall photo-E. M. F; 'cells on a target (but not showing all that would be normally shown by a cross-section'al view in order to simplify the drawings). The relative dimensions of Figs. 4 to 8, inclusive. are, of course, not exactly correct, the only purpose of these figures being to illustrate the process. A satisfactory method of making the target structure ll of the tube and utilizing coppercopper oxide type 'cells is as follows: A perforated screen or plate "of nickel or any other suitable metal, made by :afiywe'll-known commercial process, has a greased or 'waxed surface It thereof insuillated with asuita'ble protective covering material, such as asphaltum particles small in comparison with the diameter of holes 52 in the screen. The screen III has, for example, to 200 or more apertures per inch. It is slightly warmed to cause the asphaltum deposit to flow and produce a multiplicity of dot-like elements 82 fairly evenly distributed over the surface 01' the plate as shown in Fig. 4. The back of the plate is then covered with an asphaltum covering 03' (see Fig. 5) which can be done without the asphaltum flated side by pouring asphaltum varnish of the correct consistency on the back of the screen and allowing it to dry. A silver or other suitable metallic mask 64 is. then plated on the unprotected areas of the plate 50, the greasy or waxy coating 60 making it possible for the silver to be later stripped oif. This is shown in Fig. 6. Due to the fact that the grease film is very thin, there are various minute openings therein and thus the silver reaches through these openings to the metal layer underneath and attaches itself to the metal layer. It is not as firm a coating, however, as

going completely through to the insufwould be the case if the grease film were not there and hence the silver can be easily stripped off at the proper time in the method. The grease can be applied either before or after the asphalt, but preferably it is applied first. All the asphaltum is then dissolved in benzene. A copper film 65 is then deposited on the plated side of the member 50 by evaporation. The appearance of the target after these last two steps is schematically represented by Fig. 7. The copper is deposited directly on the metal since all the grease and asphaltum has been dissolved by the henzene. Hence the copper is firmly adherent to the metal electrode whereas the silver, being not so firmly adherent, can be stripped off without the copper being stripped off with it. The silver mask or screen 64 is then carefully stripped away leaving small dots 65 of copper where the asphaltum dots had formerly been. The silver can be stripped away, for example, by grasping one end of the mask by a suitable holding tool and very carefully lifting the mask while holding the target firmly until the whole mask is stripped from the target. The copper dots are then oxidized by any suitable means (such as by heating the entire target in oxygen) and the oxide surfaces 56 treated by electron or ion bombardment to produce a blocking 'layer 61 for each small cell. A very thin semitransparent film 68 of gold or other suitable conducting material is evaporated onto the small cell surfaces. The gold film is not confined to the cell surfaces nor need it be since it is so' thin that its resistance is very high laterally of the target. By this process there is prowhen the cells are of the silicon-silicon oxide type, they can be made as follows: First, silicon (instead of copper) is deposited on the silverplated side of the mosaic after the asphaltum has been dissolved. A more to be preferred method is to deposit the silicon in. two layers, one by simultaneous evaporation of silicon and a metal of higher conductivity than silicon, such as aluminum, arsenic, titanium, copper or silver, and

the second by evaporation of silicon alone. The effect or the metal of higher conductivity is to give the cell a lower internal resistance than would otherwise be obtained. The silicon (or silicon and the conducting metal) layer is then covered by evaporation of quartz (silicon dioxide) or another suitable insulator with tungsten or another suitable elementsuch as silicon, molybdenum, or tantalum. The evaporations can be efiected either simultaneously or in alternate thin layers as the layers are very thin and not very smooth, thus permitting one to partially merge with the other. Finally, a thin semi-transparent film of gold, silver or other suitable conducting material is evaporated onto the small cell surfaces and the silver mask and the films on it are stripped away, leaving small sandwich-type sillduced a number of small sandwich-type photow E. M. F. cells distributed at random over the screen. If desired, the copper dots 65 can be oxidized, treated, and the semitransparent film 68 evaporated thereon before the silver mask 64 is stripp d off. These cells are of the type known as a. front wall cell. In these cells, a negative potential with respect to the plate is acquired by the semitransparent metal layer 68 when light radiations are applied to the cells from the object 0. If desired, the cells can beproduced by any suitable process which produces a back wall type of cell but cells of this latter type are generally not so sensitive as those of the front wall type.. If the cells 5| are of the back wall" type, the semitransparent metal layer becomes positive with respect to the plate 50 when light radiations are applied to the cells. The arrangement in accordance with this invention is operable with either type of cell and, moreover, is not limited to the use of the copper oxide type or of the sillcon-silicon oxide type now to be described,

If desired, the surface of the target nearer the beam can be treated in a manner to enhance the production of low velocity secondary electrons by evaporating a thin layer of a suitable metal, such as magnesium, on this surface and then wholly or partially oxidizing this metal layer.

con oxide photo-E. M. F. cells distributed at random over the screen. Such photo-E. M. F. cells, greatly enlarged, are shown in Fig 9. The silicon-silicon oxide cell described above is of particular advantage in the camera tube in accordance with this invention because of its low impedance. Because of this, electrons which fall back on the surface of the target remote from the beam do not charge this surface.

The operation of the arrangement shown in Fig. 1 is as follows, reference also being made to Figs. 2 and 8. Radiations from an object or field of view are projected upon the right-hand side of the mosaic target H by means of the lens system IT. A high velocity beam of electrons generated by the electron gun I3 is deflected over a field of the mosaic target H corresponding to the area covered by the radiations from the object by means of deflecting potentials applied to the deflecting plates l5 and I6. As shown in Fig. 2, the cathode ray beam is generally of such size that it covers several apertures 52 and the spaces between them. It will be appreciated that the lines indicating the beams are not accurately representative of the electron paths since obviously the electric fields adjacent the apertures of the member 50 are not uniform and produce a certain amount of convergence or divergence. When the primary beam strikes the metal member 50, secondary electrons are given off and these secondary electr'ons (mixed with some primary electrons which are not shown in the drawing in order to simplify it) pass through the aperture or apertures 52 adjacent the scanning spot to the collecting electrode ll. The source 31 places the plate 50 at a positive potential of from 20 to 50 volts with respect to the electrode 24 to allow only electrons originating next to an aperture to go through it, or, in other words, a field is establi'shed which discourages secondary electrons from traveling from one point of the target to another. The number of secondary electrons which pass through any particular aperture 52 is dependent upon the potential generated across the adjacent individual photo-E. M. F. cells 5| by the radiations applied thereto from the corresponding elemental area of the object. The action of the photo-E. M. F. cells 5| is somewhat analogous to that of a grid in a triode. This voltage between the semitransparent metal member 68 and the metal screen member 50 varies the number of secondary electrons which pass through the corresponding aperture 52. This voltage has practically no effect on the primary electrons but as the number of them passing through the target H in a given time interval remains substantially constant, these electrons contribute only to the average or background current flowing through the resistor 34 and do not affect the signal current. Moreover, secondary electrons emitted from the plate 50 which.

do not go through the apertures 52 and are picked up by the plate 50 do not introduce distortion in the signal current as the plate 50 is maintained at a fixed potential. If any secondary electrons fall back on the side of the cells remote from the electron beam, they are immediately conducted to the plate 50 as the resistance of the cells is relatively low. Unlike the storage type of camera tube using photoemissive elements wherein each photoemissive element must be discharged once per cycle, the photo-E. M. F. 08115 5| can have the potential thereacross varied at will (by the change in light) and need not have this potential brought to zero once every scanning cycle. I

The variable current passing through the resistor 34 constitutes the video signal current and this current is amplified by the amplifier 36 in a manner well known in the art. The video current is then caused to modulate a carrier for transmission to a distant station by means well known.

various modifications can be made in the embodiment described above without departing from the spirit of the inv'ention, the scope of which is indicated by the appended claims.

What is claimed is:

1. A photo-E. M. F. cell comprising a metal base, a coating comprising a mixture of silicon and a metal of higher conductivity than silicon on said base, a layer of silicon on said coating, a layer of quartz mixed with a metallic material on said silicon layer, and a metallic member in contact with said last mentioned layer.

2. A photo-E. M. F. cell comprising a metal base, a coating comprising a mixture of silicon and a metal of higher conductivity than silicon on said base, a layer of silicon on said coating, a

layer of an oxide of silicon {nixed with a metallic material on said silicon layer, and a metallic member in contact with said last mentioned layer.

3. A method of making a photo-E. M. F. cell comprising the steps of simultaneously evaporating silicon and a metal of higher conductivity on a metal base to form a composite coating of the two materials thereon, forming a layer of silicon on said coating, forming a layer of an oxide of silicon on said silicon layer, and applying a thin,

semi-transparent metallic coating on said last mentioned layer.

4. A photo-E. M. F. cell comprising a metal base, a coating comprising a mixture of silicon and one of the group consisting of aluminum, arsenic, titanium, copper and silver on said base, a layer of silicon on said coating, a layer of an oxide or silicon mixed with an element from the group consisting of tungsten, silicon, molybdenum and tantalum on said silicon layer, and a metallic member in contact with said last mentioned layer.

5. A method of making a photo-E. M. F. cell comprising the steps of simultaneously evaporating silicon and a metal of higher conductivity on a metal base to form a composite coating of the two materials thereon, forming a layer of silicon on said coating, forming a layer of an oxide of silicon on said silicon layer, and applying a thin, semi-transparent metallic coating on said oxide layer.

6. A method of making a photo-E. M. F. cell comprising the steps of simultaneously evaporating a coating of silicon and a metal of higher conductivity on a metal base, forming a layer of silicon on said coating, applying quartz and a metallic material simultaneously to said silicon layer, and applying a thin, semi-transparent metallic coating on said quartz and metallic material.

7. A method of making a photo-E. M. F. cell comprising the steps of simultaneously evaporating a coatin of silicon and a metal of higher conductivity on a metal base, forming a layer of silicon on said coating, applying alternate thin layers of quartz and metallic material to said silicon layer, and applying a thin, semi-transparent metallic coating on said oxide layer.

8. A method of making a photo-E. M. F. cell comprising the steps of evaporating a coating of silicon and tungsten on a metal base, forming a layer of silicon on said coating, forming a layer of an oxide of silicon on said silicon layer, and applying a thin, semi-transparent metallic coating on said oxide layer.

GORDON K. TEAL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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