PLUGGED PINHOLE THIN FILM AND METHOD OF MAKING SAME

Abstract: A thin film and method of making the same involving a film (11) in which pinholes (17, 18, 19) in the film are plugged with insoluble photoresist material (21). The film (11) is substantially opaque to the light wavelength range causing said photoresist material to become insoluble. The method for making such thin film involves coating said thin film (11) on one side with a layer of insulating photoresist material so that a substantial portion of substantially all of the pinholes (17, 18, 19) are filled and then exposing the other side of the film to the light wavelength range causing said photoresist material to become insoluble.

Description: PLUGGED PINHOLE THIN FtLM AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION This invention relates to a plugged pinhole thin film and a method of making the same which Is particularly useful in the production of thin film insulating or semiconductor layers and thin film devices such as photoconductive and photovoltaic devices. As described in the Handbook Of Thin Film Technology" edited by Leon 1. Malssel and Reinhard Glang, McGraw-HIII Book Company (1970), on pages 7 - 46 to 7 - 48 and 23 - II, pinholes occur in thin films due to a variety of causes. The most dreaded result of pinholes occurs when the film Is an insulating or semiconductor film or an active element which becomes electrl catty shorted because of the pinholes. To date there has been no known method of plugging such pinholes after they are formed in the thin film. -Consequently the prior art approach to the pinhole problem has involved trying to avoid pinholes as much as possible. One method of avoidance has been to pay scrupulous attention to the process cleanliness and to material puritles In the hope that the resulting film would be pinhole-free or substantially pinhole-free. Needless to say such approach necessarily Involves a balancing between the expenses and efforts incurred In the thin film process to avoid pinholes against the number and size of The pinholes resulting. Another approach to avoiding pinholes is to increase the thickness of the film so that most pinholes are covered over. However, such approach necessarily Involves excessive material costs and Incurs undesireable effects In device applications where a very thin film Is necessary. In response to such pinhole problem and the relatively unsatisfactory methods of avoiding the pinhole problem which have been utilized in the past, an object of the present Inven tion is a thin film wherein the pinholes are plugged with an Insulating material. Another object of the present invention is a simple method of plugging pinholes in a thin film which utilizes well known techniques and materials. SUMMARY OF THE INVENTION In general the present invention Involves a thin film in which its pinholes are plugged with insoluble insulating photoresist material and which Is substantially opaque to the light wavelength range which causes said photoresist materlal to become insoluble. Such thin film may be either one or more Integral Insulating or semiconductor layers or one or more active devices such as a photovoltaic or a photoconductive device and Is supported by a substrate transparent to the light wavelength range which causes said photoresist material to become Insoluble. The method of the present invention Involves plugging the pinhole In the thin film by coating one side of the thin film with a layer of Insulating photoresist material, exposing the other side of the thin film to the light wavelength range which causes said photoresist material to become insoluble and then removing the unexposed photoresist material from the one side of thin cell. In order to facilitate understanding of the present invention, reference will now be made to the drawings and to a preferred specific embodiment of the present invention and specific examples thereof. Such examples, like the preceding summary, should not be construed as limiting the present invention which is properly set forth in the appended claims. DESCRIPTION OF DRAWINGS Fig. I is a schematic cross sectional view of a por tlon of a thin film photovoltaic device made In accordance with current methods prior to the step of depositing thereon a back electrode. Fig. 2 is a schematic cross sectional view of a por tlon of a thin film photovoltaic device made In accordance with current methods including the final step of depositing a back electrode. Fig. 3 shows the device of Fig. I after the step of coating the one side with a layer of insulating photoresist materlal and the step of exposing the other side of said device to light in the light wavelength range which causes said photoresist material to become insoluble have both been carried out. Fig. 4 shows the device of Fig. 3 after the unexposed photoresist material has been removed from the one side of the device. Fig. 5 shows the devi ce of Fig. 4 after a back electrode has been deposited and corresponds to the device of Fig. 2 after the method of the present invention has been Incorporated In the method of making the device. Fig. 6 is a graph of typical current-voltage curves for thin film photovoltaic devices made both as shown In Fig. 2 and Fig. 5. DESCRIPTION OF THE PREFERRED Et#43ODlMENT OF THE PRESENT INVENTION The plugged pinhole thin film of the present Tnven- tton and the method of making the same can best be described by reference to a specific semiconductor device particularly a thin film photovoltaic device wherein the presence of pinholes can effectively prevent the device from operating. Except for the step of the electrodeposition of cadmium sulfide (described below), the method of making the photovoltaic device illustrated in Fig. I is described In detail In Canadian Patent No. 280,011 of Monosolar, Inc., entitled "PHOTO-VOLTAIC POWER GENERATING MEANS AND METhODS'. For purposes of the present patent application, it is sufficient to note that in Fig. I the device 10 includes a thin film II which is deposited on a substrate 12. The substrate 12 Is composed of a sheet 13 of glass on which is deposited a layer 14 of indium tin oxide by we I I I known techniques. The thin film II Includes a first layer 15 of cadmium sulfide which Is electrodeposited using the same equipment and generally the same process as described in the aforementioned patent, except a single carbon anode was used. The solution used was 0.2M CdCI2 and O.OlM Na2S2O3 with a pH of 4.5. The solution was stirred and heated to 900C. The deposition voltage was -0.690V with reference to a calomel electrode. A second layer 16 of cadmium telluride Is then electrodeposited as described above. For illustrative purposes there is shown in Fig. I a number of pinholes 17, 18, and 19 which extend to various depths through the film II. Thus, pinhole 17 extends from the surface of the cadmium telluride layer 16 through to the glass sheet 13. Pinhole 18 extends from the surface of the cadmium telluride layer 16 to the cadmium sulfide layer 15. Pinhole 19 extends from the surface of the cadmium telluride layer 16 to the indium tin oxide layer 14. As shown In Fig. 2, the deposition of the gold layer 20 for a back electrode by the well known vacuum evaporation process also causes deposition of the gold In the pinholes 17, 18 and 19. Consequently, there are conductive paths from the gold surface to the indium tin oxide surface in the case of pinholes 18 and 19. Such conductive paths cause a shunting effect that at least degrades the effectiveness of the device and may, depending on the size and location, substantially prevent effective operation by short-circuiting the device. As set forth in Fig. 3, the film 11 and the substrate 12 are composed of the same set of layers and contain the illustrative pinholes 17, 18 and 19. However, prior to depositing the gold layer 20, the one side of thin film It is coated with a layer 21 of insulating photoresist material so that the photoresist material fills at least a substantial portion of substantially all of the pinholes In the thin film 11. Such thin film formed by the layers of cadmium sulfide 15 and cadmium telluride 16 is substantially opaque to the light wavelength range which causes such photoresist material to become insoluble. The opposite side of the substrate 12 Is then exposed to light 22 In a wavelength range which causes said photoresist material to become insoluble. Such light travels through the glass sheet 13 and the Indium tin oxide layer 14 and thus enters the back side of the film 11 so as to cause the photoresist material in the pinholes 17 and 19 to become insoluble. In the specific example shown in FIg. 3, the cadmium sulfide layer is sufficiently thin and thus transparent so as to cause the photoresist material in pinhole 18 also to become insoluble. As shown in Fig. 4, the unexposed photoresist material 21 on the surface of the film II is then removed leaving the exposed insoluble photoresist material plugs in the pinholes 17, 18 and 19. Finally, as illustrated in Fig. 5, the layer of gold is deposited In the same fashion as illustrated in Fig.2. However, as also illustrated In Fig. 5, because of the plugs In pinholes 17, 18 and 19, the gold layer 20 no longer contacts the indium tin oxide conductive layer 14 on the back side of film 11. As Illustrated in Fig. 5, It Is not necessary for the pinhole plugging to be complete as shown In pinhole 19 wherein the photoresist material only partially fills such hole, Such partial plug should be sufficient to prevent the electrical contact between the gold layer 20 and the indium tin oxide layer 14. A specific example of the comparison of the l-V characteristIcs of dev Ices resulting from the above prior art procedure shown in Fig. 2 and the procedure of the present invention shown in Figs. 3 - 5 Is illustrated in Fig. 6. In Fig. 6, curve I Is the l-V plot of devices made by the prior art procedure and curve li Is the same for devices made by the procedure of the present Invention except as noted below. Such devices were produced by applying pinhole plugging to only one hald of the area of a photovoltaic device as Illustrated In ilg. I measurlng approximately 12 X 1-1/3V; in size. The plugging process was generally in accordance with the procedures set forth in the application bulletin of the Eastman Kodak Company No. T-306 entitled iKODAK MICRORESIST 747'6 published In 1975. Consequently the treated and untreated areas were made the same way and only the treated area was subjected to the method of the present Invention so as to compare the results of the present invention wIth the conventional processing technique. Such treated area was coated with a thin film of the Kodak Microresist 747 liquid and Then was spun at 5,000 rpm for approximately 20 seconds to provide a uniform thin coat of about 0.5 micrometer thickness. After spinning, the resistcoated photovoltaic device was baked In an oven for 15 minutes at a temperature of 800C. After baking, the film area was exposed to a 200 watt high pressure mercury vapor lamp from the glass side. The photoresist layer was then developed In Kodak Microresist Developer by immersion for 60 sec. following which it was immersed In Kodak Microresist Rinse for 20 sec. to strip away the uncured resist leaving only the cured resist plugs In the pinholes. It was then washed in double distilled water and dried in a jet of pure nitrogen. Next, the film was baked at 850 for 15 minutes. As would be expected In any photographic process, the steps of coating with photoresist and a subsequent development were done under dark room conditions with room illumination being provided by a gold fluorescent safe light lamp. The resist Is kept In a dark bottle and handled like any photographic emulsion. Next on both the treated and untreated areas, a series of five circular gold back electrode spots 0.2 cm2 in area each were deposited by vacuum evaporation. The resulting solar cells were then tested for I-V characteristics. For light l-V measurements the cells were Illuminated through the glass side by a lamp, I.e., General Electric EFR, 150W, 15V Quartz Halogen simulating AMI solar radiation, and the resulting curves were plotted as shown In Fig. 6 whereIn curves I and ill are the typical light i-V curves of all of the five spots In the untreated area and curve 11 Is typical for four of the five spots in the treated area. The remaining spots in the treated area gave a curve similar to curves 1 and 111 and It Is believed that it was shorted out due to causes unrelated to the process of the present Invention. Curve IV is the typical dark l-V curve for the spots in the treated areas. As is obvious from a comparison of curve I with curve li, the process of the present Invention Is a very effective process for plugging pinholes In thin film photovol- talc devices which otherwise would normally short out such devices. There are many features In the present Invention which clearly show the signiffeant advance which the present invention achieves over the prior art. Consequently, only a few of the more outstanding features will be pointed out to illustrate the unexpected and unusual results obtained by the present invention. One feature is that pinholes In any Insulating or semi conductive thin film which Is opaque to The light wavelength range curing the selected photoresist material can be readily plugged by a well-known process. Another feature of the present invention Is that efficiency of large are thin film photovoltaic devices can be substantially Improved by plugging the pinholes therein by means of the present Invention. It will be understood that the foregoing description Is only illustrative of the present invention and Is not Intended that the Invention be limited thereto. All substitutions, alterations and modifications of the present invention which come within the scope of the following claims to which the present invention is readily susceptible without departing from the spirit and scope of this disclosure are considered part of the present Invention.

Claims: CLAItslS I. A thin film in which its pinholes are plugged with insoluble insulating photoresist material and which is substantially opaque to the light wavelength range causing sale photoresist material to become insoluble. 2. A thin fllm as claimed In Claim I wherein said film is an insulating film. 3. A thin film as claimed in Claim I wherein saId film is a semiconductor film. 4. A thin film as claimed in Claim I wherein said film Is a photoconductive device. 5. A thin film as claimed in Claim I wherein said film is comprised of one or more photovoltaic devices. 6. A thin film as claimed in Claim I wherein the thin film is supported by a substrate transparent to the light wavelength range causing said photoresist material to become insoluble. 7. A method of plugging pinholes in thin films comprising: (a) coating one side of said thin film with a layer of insulating photoresist material so that said photoresist material fills at least a substantial portion of substantially all of the pinholes in said thin film, said Thin film being substantially opaque to the light wavelength range causing said photo resist material to become insoluble; and (b) exposing the other side of said thin film to the light wavelength range causing said photoresist material to become insoluble so as to cause the photoresist material in said pinholes to become insoluble.

Inventors: BASOL, BULENT M.
Applicants: BP PHOTOVOLTAICS LTD., SOHIO COMMERCIAL DEVELOPMENT CO.
Application year: 1982
Main classification: H01L31
IPC codes: G03F7, H01L31