Substrates that are intended for supporting electrode layers of semiconductor devices such as solar batteries, semiconductor layers, light-emitting layers, etc., have to possess such basic characteristics as resistance to deformations, strains, or property deterioration under the effect of high temperatures inherent in processes for the formation of light-emitting layers.
They also should have precision surface flatness, stability against deformations that may be caused by humidity of air, exhibit prevention of such defects as pinholes in the semiconductor films, demonstrate resistance to buckling, peeling, breaking, etc. Substrates known heretofore that could satisfy such requirements are those made from glass and ceramic.
However, from the viewpoint of cost efficiency, versatility, and weight reduction of current electronic and semiconductor devices, attention is drawn nowadays to thin-film semiconductor elements and devices used in thin-film type solar batteries, thin-film transistors (TFT) for reflection-type liquid crystal displays, thin-film electroluminescent displays, or the like. A demand exists for highly efficient thin-film type solar batteries, and one especially important problem in this connection is the need to decrease in weight of solar batteries intended for use in space satellites.
For example, the following is stated in Japanese Examined Patent Application Publication [Kokoku] H4-63551: “Japanese Unexamined Patent Application Publication [hereinafter referred to as “JP Kokai”] S54-149489 discloses a solar battery that utilizes a substrate made from a thin polyimide resin film.
A disadvantage of such thin polyimide resin film is that it is deformed and cannot be uniformly heated at the high temperatures of deposition processes. Therefore, thin-film solar batteries have been developed with photovoltaic element layers of an amorphous silicon on a thin resin film, e.g., of polyimide that is highly resistant to heat and deformation and is formed on a metal foil.”
The following is stated in JP Kokai H10-286907: “When a thin layer of a polyimide resin is formed on the surface of a thin metal plate, and pinholes are developed under the effect of such defects as irregularities on the surface of the thin metal plate, in order to prevent development of the pinholes the plate has to be polished. Introduction of this additional process increases the production cost. In order to solve this problem, a heat-resistant flexible substrate that was composed of a thin metal plate with a polyimide film applied onto it via an epoxy resin type adhesive was invented. Such a substrate finds application in solar batteries, reflection-type liquid crystal displays, electroluminescent displays, or similar thin-film type semiconductor devices.”
The following statement is contained in JP Kokai 2000-323732: “A thin semiconductor layer (photoelectric conversion layer) can be formed at temperatures exceeding 400° C. on a substrate composed of a conductive sheet of stainless steel, aluminum, or the like, the surface of which and the inner peripheral surface of connecting holes formed in the plate are coated with an insulating layer of a polyimide resin, aramid resin, or a similar heat-resistant polymer.”
As it follows from JP Kokai 2003-179238: “A thin-film CIS-type solar battery is produced by forming an electrode film on a flexible substrate (made., e.g., from stainless steel or polyimide) or on a substrate having a SiO2 film, forming on said electrode film another thin film that contains an Ib group element (e.g., Cu, IIIb group element (e.g., In), and VI group element (e.g., Se), and then forming a semiconductor film (an light-absorbing layer) by subjecting the unit to heat treatment at a temperature within the range of 500 to 600° C.”
Claims of JP Kokai 2005-79405 relate to a stainless steel foil coated with a silica-based inorganic polymer film consisting mainly of siloxane bonds, as well as to a stainless steel foil coated with a silica-based inorganic polymer film having a recessed and projected structure on the surface of the silica-based inorganic polymer film, wherein at least a part of the Si that constitutes the silica-based inorganic polymer film is chemically bonded with one or both of an organic radical and hydrogen. The claims also relate a thin-film silicon type solar battery utilizing the aforementioned coated foil as a substrate.
Practical examples of the aforementioned patent application describe only stainless steel foil coated with an organosilicon polymer having a recessed and projected structure of a submicron level and obtained by coating a stainless steel foil with a tetraethoxytitanium and dimethylsiloxane carbinol-modified at both molecular terminals or with a sol-like product of a reaction between a polymethylphenylsiloxane and water with subsequent thermal curing of the coating. A comparative example discloses a stainless steel foil coated with an insulating film obtained by preparing an aqueous solution of a tetraethoxytitanium and tetramethoxysilane, applying the solution onto a stainless steel foil, and thermally curing the coating.
JP Kokai 2000-260570 states that “a thin-film EL element can be obtained by heat-treating, at a temperature above 600° C., a structure composed of a substrate capable of withstanding a temperature exceeding 600° C. (e.g., of alumina or mullite), a conductive electrode layer (e.g., of an impurity-doped silicon), and a light-generating layer (e.g., of ZnS, SrS:Ce).
JP Kokai 2003-318119 discloses “a thin-film transistor for use in a liquid-crystal display unit, where the aforementioned thin-film transistor is produced by applying an optically-curable silane onto a substrate (made, e.g., of quartz), exposing the coating to light through an image pattern, forming a higher silane, and forming the thin-film transistor by heat-treating the higher silane at a temperature of about 550° C.”
JP Kokai 2001-011611 describes “a process of obtaining a positive electrode of a thin lithium cell (LiMn2O4) by successively sputtering manganese trioxide onto a film substrate (e.g., stainless steel SUS304) for forming a thin film of β-manganese dioxide at a temperature below 400° C., vapor depositing lithium at a 400° C., and vapor-depositing β-manganese dioxide for the second time for obtaining the aforementioned LiMn2O4.”
However, all the above described substrates (i.e., a substrate made from polyimide; a substrate composed of a metal foil coated with a thin heat-resistant resin film such as a polyimide film; a heat-resistant flexible substrate composed of a thin metal plate coated with a layer of a polyimide film attached through an epoxy type adhesive agent; a substrate composed of a thin metal plate, e.g., of stainless steel or aluminum the surface of which is coated with an electroinsulating layer of heat-resistant polymer such as polyimide or aramid resin; and a stainless steel foil coated with an organosilicon polymer having a recessed and projected structure of a submicron level and obtained by coating a stainless steel foil with a tetraethoxytitanium and dimethylsiloxane carbinol-modified at both molecular terminals or with a sol-like product of a reaction between a polymethylphenylsiloxane and water with subsequent thermal curing of the coating) utilize an organic resin, such as a polyimide resin, aramid resin, or organosilicon polymer as their structural material, and therefore at the high temperatures required for the formation of electrode layers, semiconductor layers, light-emitting layers, etc., such substrates are subject to damage and deformation.
At first glance, the surfaces of inorganic and metal substrate made, e.g., from stainless steel, aluminum, alumina, and mullite look smooth. On a microscopic level, however, these surfaces have irregularities and holes, and therefore, when they are coated with thin electrode layers, semiconductor layers, light-emitting layers, etc., the coating acquires a non-uniform thickness and a problem occurs in connection with the fact that some areas remain uncovered. The same is applicable to substrates made from heat-resistant polymers such as polyimide or aramid resin.