IL27769A

IL27769A - Electrode system,particularly semiconductor electrode system and method of producing the same

Info

Publication number: IL27769A
Application number: IL27769A
Authority: IL
Original assignee: Philips Nv
Priority date: 1966-04-14
Filing date: 1967-04-11
Publication date: 1971-01-28
Also published as: DE1614235A1; AT269238B; CH499184A; JPS4527028B1; ES339178A1; US3620832A; NL6604959A; BE697072A; NO121223B; FR1519071A; SE333025B; OA02586A; GB1186074A

Description

niun ]Ti3 -mnrn ·π PATENT ATTORNEYS · D'Dl-9 'Dill] DR. REINHOLD COHN |Π 3 Tjini"n m DR. MICHAEL COHN | Π -J ■) H 3 ' D Ί Τ ISRAEL SHACHTER B.Sc. .Ώ.Ώ "I D 30] "J N T HI * File CI 26318 PATENTS AND DESIGNS ORDINANCE SPECIFICATION Electrode system, particularly semiconductor electrode system and method of producing the same I/We ff.V. PHILIPS' GLOEILMPEIT A RIB BH, incorporated under the laws of the Bfetherlands, of 29 Emmasingel, Eindhoven, The Netherlands. do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement :- PHN.1461 Electrode system, particularly semiconduc tor electrode system and method of producing the same. II The invention relates to a method manufacturing an electrode system comprising a granular layer, for example., a semiconductor granular layer substantially of the thickness of one grain, the grains being embedded in an insulating filling substance, said granular layer being coated by at least one electrode layer, which is in contact with the grains and which consists of a pattern of coherent, electrically good conducting domains between the grains on the filling substance and of electrically usually less good conductive domains on the grains in which method, in order to obtain said electrode layer, a coherent, uninterrupted, electrically good conducting electrode layer is applied to the granular layer, after which portions of the electrode layer on the grains are selectively removed. The invention furthermore relates to an electrode system manufactured by the method according to the invention.

Such an electrode system, as well method of manufacturing the same, is the subject a method of the kind set forth, in which a good con- PH .H61 ducting electrode layer is applied to the whole granular layer, after which either by a selective etching process or by using photo-resist methods this good conducting electrode layer is selectively removed from the heads of the grains. If desired, a second electrode layer may be applied to the grains.

Electrode systems of the kind set forth may comprise radiation-sensitive grains and may be used as radiation detectors, in which case radiation energy is incident to the photo-sensitive granular layer, in which it produces electric voltage or impedance differences, which can be measured by means of electrodes provided on the granular layer, at least one of which electrodes has to be transparent to the incident ra-diation. Examples of these uses are inter alia photo-resistors and photo-cells for exposure meters; such electrode systems may furthermore be employed for converting radiation energy into electric energy, inter alia in so-called solar batteries. For converting elec-trie energy into radiation energy, in which case in the grains radiation can be produced for example by recombination of charge carriers at a p-n junction or by other forms of electro-luminescence, electrode systems comprising a granular layer are interesting.

In all these cases not only grains having approximately the same dimensions in all directions but also grains in the form of scales or needles may be used. The electrode systems of the kind set forth involve often the problem of having to provide on the granular layer at least one electrode layer for current input or current output, which electrode layer has to satisfy conflicting requirements. On the one hand the electrode layer should have a low sheet resistance, whereas on the other hand at the contact area with the grains the electrode should have properties which may counteract said low sheet resistance.

If the electrode system comprising a granular layer is, for example, an electric-optical device, at least one. of the electrodes applied to the granular layer has to be transparent, at least on the grains, to the radiation incident to the grains or emanating from the grains. In connection with the usually poor conductivity of such transparent electrode layers an electrode layer of unhomogeneous structure is de-sired, in which coherent, good conducting domains between the grains and less good conducting domains of the desired radiation-transparent properties on top of the grains are provided. In view of the required transparency it may in some cases be desirable to omit partly the electrode layer on the grains.

The invention has for its object to provide a particularly rapid method for applying such an unhomogeneous electrode layer.

The invention is based on the recog-nition of the fact thati in the case of a granular layer, on the electrode side of which the filling substance exhibits subsidences between the grains, parts of the electrode layer located on the grains can be selectively removed by abrading this side of the electrode- o e - ΡΜ„1461 trode layer located in the subsidences of the filling substance between the grains are substantially not affected by the abrasive.

According to the invention a method of manufacturing an electrode system of the kind referred to above is characterized in that the manufacture starts from a granular layer-, the filling sub- . stance between the grains having a thickness considerably smaller than the average grain thickness, so that at least on one side the granular layer exhibits subsidences between the grains and in that on said side the granular layer is coated by an electrically good conducting electrode layer, after which by abrading the granular layer on said side only portions of the electrode layer on the grains are removed owing to the subsidences between the grains 0 Abrasion may be carried out by various grinding or abrading agents. It is advantageous to use an abrasive whose grains have such a size that they cannot attain the bottom of the subsidence.

Therefore a preferred embodiment of the method according to the invention is characterized in that the granular layer is abraded by means of an abrasive whose grains have a diameter exceeding the average distance between the grains on the granular layer, so that they cannot reach down to the bottom of the depressions. It is preferred to use an abrasive, the grain diameter of which is more than twice and less than five times the average distance between the grains of the granular layer. It is furthermore advantageous to use an abrasive PHN.1461 whose grains have a considerably smaller diameter than the average grain diameter of the granular layer and a further, . important embodiment of the method according to the invention is characterized in that the granular layer is abraded with the aid of an abrasive whose grains have a diameter which is considerably smaller than the average grain diameter of the granular layer, said abrasive being applied to a carrier. In this case the carrier is desired for preventing the abrading grains from attacking the electrode layer in the subsidences between the grains of the granular layer. The carrier should preferably have such a flexibility that it can follow the shape of the granular layer, so that in practice each grain of the granular layer comes into contact with the abrasive.

It is advantageous to use an abrasive in this case, the grain diameter of which is less than one fifth, preferably less than one tenth of the average grain diameter of the granular layer.

In the above embodiments of the method according to the invention it is possible to start with a granular layer, the filling substance of which extends over the grains. The parts of the filling substance located on the heads of the grains is then removed simul-taneously with the electrode layer by abrasion. However, owing to the disadvantages involved herein, for example, the plastic properties of most filling substances, which are therefore less suitable for an abrading or grinding process, it is preferred to start with a granular layer whose grains protrude from the filling substance on the PHU.14 1 side of the electrode layer, so that the abrasive need • not come into contact with the filling substance.

It is advantageous to start in this case with a granular layer obtained by embedding the grains in a hardening filling substance which contracts between the grains so that parts of the grains are made free of the filling substance.

In an important, preferred embodiment of the method according to the invention a granular layer is used, the grains of which consist of cadmium sulphide and the filling substance of which consists of polyurethane .

In a further important, preferred embodiment of the method according to the invention the-abrasion produces openings in the electrode layers on the heads of the grains, whilst the contact between the electrode layer and the grains is maintained.

This method has the advantage that in a single operation an electrode is formed, which is good conducting between the grains whilst on the heads of the grains the incident and the emerging radiations are not hindered, whilst the resultant contact, though only present on part of the grain surface, is in many cases sufficient. For improving the contact it is advantageous to subject the granular layer, subsequent to abrasion, to an ion- or electron-bombardment.

Although a satisfactory contact can be obtained in this manner without subjecting the grain parts free of the electrode layer to a further process, it will be desirable in other cases to cover the free . i grain parts with further contact material in order to obtain given, desired contact properties. The initial electrode layer may, if desired, be completely removed from the grains. A further preferred embodiment of the method according to the invention is characterized in that subsequent to the removal of only the parts of the electrode layer from the heads of the grains a second electrode layer, conductively connected to the first electrode layer is applied to at least the free grain portions. In those cases in which the electrode layer on the grains has to be transparent to incident or emerging radiation a second electrode layer is used in an important, preferred embodiment, which layer has a greater transparency than the first electrode layer for electro-magnetic radiation to be emitted by the grains or for a radiation to which the grains are sensitive.

The method described above may be used with different combinations of grains and electrode materials. The invention is, however, particularly im-portant for the manufacture of electrode systems having a granular layer whose grains consist mainly of photo-conductive sulphides and/or selenides of cadmium and zinc, whilst the grains are covered by an electrode layer containing indium or an indium alloy. Between the electrode layer and the grains an ohmic contact is then established.

The electrode layer may be applied without preliminary treatment of the granular layer. In order to obtain a satisfactory ohmic contact it is, how-ever, often desired to subject the granular layer to an PHN.1461 ion or electron bombardment prior to the application of the electrode layers The invention furthermore relates to an electrode system manufactured by the method according to the invention.

The invention will now be described more fully with reference to a few embodiments shown in the drawing, in which Pigs. 1 to 3 illustrate diagrammati-cally in cross sectional views consecutive steps of the manufacture of part of a photo-resistor according to a method according to the invention.

Fig„ 4 is a diagrammatic plan view of a photo-resistor the manufacture of which is illustrated in Figs . 1 to 3 in $ cross sectional views taken on the line I-I in Pig^. 4 Pi So 5 to 8 illustrate dia rammatically in cross sectional views consecutive stages of the manufacture of part of a solar cell obtained by a method according to the invention,, A first embodiment of a method of manufacturing an electrode system will now be described with reference to Figs. 1 to 4; said system comprises a granular layer (1, 2), having grains 1 (Fig. 3) of a semiconductor material having substantially the thickness of one grain, the grains being embedded in an insulating filling substance 2; this granular layer (1, 2) is covered by at least one electrode layer (3> 4), which is in contact with the grains 1 and which consists of a pattern of coherent, electrically good conducting domains 3 between ΡΗΝβ14β1 the grains on the filling substance 2 and electrically-less good conducting domains 4 on the heads of the grains; in order to obtain said electrode layer on t¾e granular layer (1, 2) a coherent, uninterrupted, electrically good conducting electrode layer 3 (see Pig. 1) is applied^ after which the parts 5 of the electrode layer 3 located on the grains are selectively removed.

The starting material is again a .granular layer (1, 2), stuck by an adhesive layer 7 (Pig. 1) to a support 8 and cohering by means of a filling substance 2, operating as a binder, and having granular portions 9 free of the filling substance; this granular layer is covered by an electrode layer 3. The grains 1 consist of photo-conductive cadmium sulphide, activated by about 10"^ to 0~^$ by weight of copper and gallium.

Such a granular layer may be obtained by various means. In connection with the necessity of forming subsidences between the grains, the following, very appropriate method is carried out, A carrier, for example, a glass plate 1 is coated by immersion in a solution of gelatine in water, by a gelatine layer 7 of a few microns thick.

Photo-conductive cadmium sulphide grains of a diameter of about 35 u are sunk into the still swollen gelatine layer, after which the gelatine is hardened by drying and the grains not adhering to the support are removed. Then polyurethane is applied as a binder between the grains on the gelatine. This may be carried out by dipping the carrier with the granular layer in ..a solution of materials known commercially under the trademarks of "Desmophen" PHN-14 1 and '!Desmodur" in ethylacetate ; when pulling the plate up, a thin layer is left on the granular layer, which layer is converted by a hardening process of, for example, 8 hours, at 75° > into polyurethane . Then the initially less viscous mixture withdraws from the heads of the grains, so that subsequent to hardening a granular layer is obtained, the grains of which project from the filling substance on the side remote from the carrier, whilst the filling substance between the grains has a thickness which is considerably smaller than the average grain thickness, so that on the side remote from the carrier the granular layer exhibits subsidences between the grains.

The resultant granular layer may be directly coated by vapour deposition by an electrode layer 3 . In order to improve the contact between the grains and the electrode layer, the granular layer is preferably subjected to an ion- or electron-bombardment prior to the application of the electrode layer 3 . In the present embodiment this is achieved by exposing the granular layer to a gas discharge for about 4 minutes at a voltage of 1 kV (discharge current about 50 mA with an electrode surface of 100 cms ). The side of the granular layer remote from the carrier is then covered by the electrode layer 3 » which establishes a substantially ohmic contact with the cadmium sulphide grains; for example, an indium layer of a thickness of 5000 $ is applied from the vapour phase.

On the side remote from the carrier (see Pig. 1 ) the granular layer is abraded by means of . grains 10 of alumina having a diameter lying between about 150 and 250 ^x. This diameter is greater than twice and smaller than five times the average distance between the grains of the layer, in this case 70 to 100 u. The abrasive grains 10 are used in the dry state and rubbed by means of a piece of cloth or another soft object 16 (see Fig. 1) with a light pressure across the granular layer. Owing to their size the abrasive grains cannot reach the coherent portions 3 of the electrode layer located in the subsidences between the grains ¾n the filling substance, so that only the parts of the electrode layer on the heads of the grains (5) are removed. Thus holes 4 are made on the grains (see Figs. 2 and 4) in the electrode layer, whilst portions 6 of the electrode layer remain in contact with the grains at the edges of the holes. After the formation of the holes, which is checked by a microscope, abrading is ceased and, if desired, any residues of polyurethane on the abraded grain parts may be removed by a dissolving or etching process. In order to improve the contact on the grains the abraded side of the granular layer is then again exposed to a gas dis charge „ Then (see Fig. 3) an approximately 50 ^u thick, hardening radiation-pervious, flexible layer 11 of a synthetic resin is then applied to the granular layer, for example, of polyurethane, and after hardening of said layer the granular layer is removed . from the -carrier 8 by dissolving the gelatine layer 7 in water,,, so that on the side of the carrier the grain portions 12 are free. After the gelatine residues are removed in PHK.1461 ~f water, the free side of the granular layer is again exposed to a gas discharge and a second electrode layer' 1 3 (see Pig. 3) is applied thereto by vapour deposition of indium to a thickness of about 5000 _? .

In this manner a photo resistor formed by a flexible sheet is formed as is shown in Pig. 3 in a cross sectional view and in Pig. 4 in a plan view.

On the electrode layer 1 3 and on a portion 14 of the electrode layer (3, 4, 6) free of the layer 1 1 contacts may be provided, between which the impedance of the granular layer can be measured. Through the synthetic resin layer 1 1 and the holes 4 a radiation 15 can strike the grain portions not covered by the electrode layer ( 3 » 4, 6 ) , whilst all grains are connected in parallel between the electrode layer 1 3 and the portion 6 of the electrode layer ( 3 , 4, 6 ) , which establish a substantially ohmic contact with the grains 1 .

A second example of the method according to the invention will be described with reference to Pigs. 5 to 8 for the manufacture of a solar cell. The same reference numerals of the two embodiments designate corresponding parts. The starting material is again (see Pig. 5) a granular layer (1, 2 ) , stuck by means of a gelatine layer 7 to a carrier 8 and consisting of grains 1 of photo-conductive cadmium sulphide of an average diameter of about 35 / , embedded in a binder 2 of poly-urethane and manufactured in the manner described above; on the side remote from the carrier the granular layer exhibits subsidences between the grains and is coated completely, for example by vapour " deposition with an . electrode layer (3, 5). The electrode layer (3, 5) consists in this example of an about 0.1 ^u thick copper layer.

The granular layer is then abraded on the side remqte from the carrier (see Pig. 1) by means of abrasive grains 21 of alumina of a diameter of about 5 u, applied to a flexible support 22 of a sheet of silicon rubber of about 0.2 mm thickness. The diameter of the abrasive grains is smaller than one fifth of the mean grain diameter of the granular layer. Under given conditions it may be advantageous to use still smaller abrasive grains of a diameter smaller than one tenth of the average diameter of the grains of the granular layer. The abrasive grains 21 may be applied to the support 22, for example, by pouring out the silicon rubber in the liquid state on a glass plate and strewin alumina grains after which the silicon rubber is caused to harden to form a flexible sheet, in which the abrasive grains are partially embedded. Abrasion is performed by arranging a cushion 23 of foam plastics or another very elastic material on the carrier 22 with the abrasive grains (Pig. 5) and thereon a non-elastic f^at object 24 and by rubbing the carrier of the abrasive grains by hand or by a machine with slight pressure across the granular layer. The carrier 22 follows the contours of the granular layer and only the portions of the electrode layer (3, 5) on the heads of the grains are removed so that (see Pig. 6) a structure similar to- that of Pig. 2 of the first embodiment is obtained. Portions 6 of the electrode layer re-main in contact with the grains 1 , although this is not . required in this example with a view to the electrode layer 25 to be applied afterwards „ On the side remote from the carrier the resultant layer is coated with a second, radiation-pervious, less good conductive electrode layer 25 of copper by vapour deposition to a thickness of about 100 , which layer establishes a contact with the portions 3 of the initial copper layer (3, 5) and a rectifying contact with the grains 1. In this example, in which a rectifying contact instead of an ohmic contact is formed with the grains, a gas discharge prior to the application of the electrode layer 25 is, of course, omitted. Subsequently, the layer 25 is coated with a radiation-pervious, hardening, preferably flexible plastics layer 11, for example, of polyurethane of a thickness of about 50 u,, After hardening of the layer 11 the granular layer is removed from the carrier by dissolving the gelatine layer 7 in water, so that on the side of the carrier grain portions are exposed. These exposed sides of the grains are subjected to a gas discharge and (see Pig. 8) an electrode layer 13 is applied by vapour deposition of an about 5000 j? thick indium layer. This electrode layer 13 establishes an ohmic contact with the grains.

In this manner (see fig. 8) a solar cell is obtained, which also has the shape of a flexible sheet; radiation 26 can strike through the pervious plastics layer 11 and the pervious copper layer 25, the rectifying copper cadmium sulphide contact„ The electrode layer 13 and a portion 14 of the electrode layer (3, 25) PHH. 1 461 clear of the layer 11 may be provided with contacts, fr •om which the photo-voltage produced by the indicent radiation can be derived.

It will be obvious that the invention is not restricted to the above examples and many variants are possible for those skilled in the art. Under given conditions it will be possible to start from a granular layer whose grains do not project from the filling substance, in which case by abrasion not only the portions of the electrode layer located on the grains but also the subjacent filling substance are removed. Since in this case the remaining portions of the electrode layer are no longer in contact with the grains, it is necessary to apply a second electrode layer to the relevant side of "the granular as described with reference to the second embodiment. Instead of photo-resistors and · photo-cells electrode systems may be manufactured which comprise granular layers capable emitting injection recombination radiation under the action of an applied voltage. More— over, grains of other material than cadmium sulphide may . be employed and in dependence upon the use of the resultant electrode system they need not be photo-conductive, for example, for the manufacture of diafides, capacitors,- bolometers, non-linear resistors and the like, As stated above, filling substances of quite different materials may be used, for example epoxyresins or photo-hardening lacquers in accordance with the use and the technique used for the formation of the starting granular layer, whilst also the covering layer 11 (see Figs, 3, 7 and 8) may consist not only of polyurethane but . also of, for example, methylmetacrylate or another hardening synthetic resin pervious or not pervious to radiation.

Under given conditions it may be advantageous to omit the electrode layer 13 (see Figs, 3 and 8) and be replaced by a flow of charged particles, for example, ions or electrons, which are incident to the granular layer and convey the charge. When, for example, the granular layer is used as a photo-conductive layer in xerography, an electrode layer may be replaced by a gas discharge for local discharge of the photo-conductive layer.

Finally it may be desirable for given uses not to remove the granular layer from the carrier 8 j in this case an electrode layer may be provided previously between the carrier and the granular layer.

Claims

PHN.H61 HAVING NOW particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. A method of manufacturing an electrode system comprising a granular layer, for example, a semiconductor granular layer having substantially the thickness of one grain, the grains being embedded in an insulating filling substance, the granular layer being coated by at least one electrode layer in contact with the grains, said electrode layer consisting of a pattern of coherent, electrically good conducting domains between the grains on the filling substance and of electrically usually less good conductive domains on the grains in which, in order to obtain said electrode layer, the granular layer is provided with a coherent, uninterrupted, electrically good conducting electrode layer, after which portions of the electrode layer on the grains are selectively removed, characterized in that the manufacture starts from a granular layer, in which the filling substance between the grains has a thickness which is considerably smaller than the average grain thickness so that at least on one side the granular layer exhibits subsidences between the grains and in that on said side the granular layer is coated by an electrically good conducting electrode layer, after which by abrading the granular layer on said side only portions of the •electrode layer on the grains are removed owing to the subsidences between the grains. PHN0 461 I

2. A method as claimed in claim 1, characterized in that the granular layer is abraded by means of an abrasive, the grains of which have a diameter exceeding the mean distance between the grains of the granular layer, so that they cannot reach the bottoms of the subsidences,

3. A method as claimed in claim 2, characterized in that there is used an abrasive, the grain diameter of which is more than twice and less than five times the mean distance between the grains of the granular layer.

4. A method as claimed in claim 1, characterized in that the granular layer is abraded by means of an abrasive, the grains of which have a diameter which is considerably smaller than the average grain diameter of the granular layer, said abrasive being provided on a carrier,,

5. A method as claimed in claim 4, characterized in that there is used an abrasive, the grain diameter of which is less than one fifth, preferably less than one tenth of the mean diameter of the grains of the granular layer.

6. A method as claimed in any of the preceding claims, characterized in that the manufacture starts from a granular layer, the grains of which project from the filling substance on the side where the electrode layer is provided.

7. A method as claimed in claim 6, characterized in that the manufacture starts from a granular layer obtained by embedding the grains in a hardening , filling substance, which contracts between the grains, so PHN0 461 * that parts of the grains are clear of the filling sub• stance,

8. A method as claimed in claim 7, characterized in that the grains used consist of cadmium sulphide, whilst the filling substance is formed by polyurethane.

9. A method as claimed in any of claims 6 to 8, characterized in that by the abrasion apertures are formed in the electrode layer on the heads of the grains, whilst the contact between the electrode layer and the grains is maintained.

10. A method as claimed in claim 9, characterized in that subsequent to the abrasion the granular layer is subjected to an ion- or electron-bombardment.

11. A method as claimed in any of the preceding claims, characterized in that subsequent to the removal of only those portions of the electrode layer which are found on the grains, at least the clear grain portions are coated by a second electrode layer, which is conductively connected with the first electrode layer.

12. A method as claimed in claim 11, cha racterized in that the second electrode layer has a greater transparency than the first electrode layer for electro-magnetic radiation which the grains can emit or to which the grains are sensitive.

13. A method as claimed in any of the preceding claims, characterized in that the manufacture PHN01461 ' starts, from a granular layer, the grains of which consist mainly of photo-conductive sulphides and/or selenides of cadmium and zinc, whilst the grains are coated by an electrode layer containing indium or an indium alloy.

14. A method as claimed in any of the preceding claims, characterized in that prior to the application Of an electrode layer the granular layer is exposed 'to an ion- or electron-bombardment.

15. An electrode system manufactured by carrying out a method as claimed in any of the preceding claims. Dated this 10th day of April, 1967 For thjfe Applicants, Dr. iohold Pohn & By: