| Method for making thin film devices intended for solar cells or silicon-on-insulator (soi) applications -> Monitor Keywords |
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Method for making thin film devices intended for solar cells or silicon-on-insulator (soi) applicationsRelated Patent Categories: Data Processing: Generic Control Systems Or Specific Applications, Specific Application, Apparatus Or Process, Product Assembly Or Manufacturing, Particular Manufactured Product Or Operation, Integrated Circuit Production Or Semiconductor FabricationMethod for making thin film devices intended for solar cells or silicon-on-insulator (soi) applications description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060184266, Method for making thin film devices intended for solar cells or silicon-on-insulator (soi) applications. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application is a divisional of U.S. application Ser. No. 10/627,576, filed Jul. 24, 2003 and claims the benefit of European Application No. EP02447146, filed Jul. 24, 2002, which are hereby incorporated by reference in their entirety herein. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention is related to micro-electronics and more particular to the field of thin film device applications such as silicon-on-insulator (SOI) structures or solar cells in particular. Furthermore, the present invention is relating to a manufacturing method of such devices. [0004] 2. Description of Related Technology [0005] The silicon in existing semiconductor devices usually has a thickness of several hundred microns. However, the electrically active domain of a wafer is limited to its surface; in fact, less than a few microns of thickness is needed. The remaining portion of the wafer is used as substrate. Unfortunately, this excess of material causes both a rise in power consumption and a fall in the operating speed of the device. The SOI wafers incorporate an insulating layer between its very thin (less than a few microns) active domain and its much thicker substrate. The substrate is isolated and can thus no longer deteriorate the speed or efficiency of the active layer. [0006] Silicon on insulator technology (SOI) involves the formation of a monocrystalline silicon semi-conductor layer on an insulating material such as silicon oxide. [0007] One important application of a thin film device is the manufacturing of solar cells. Solar cells usually comprise an active surface on the top of a silicon wafer in the form of a thin film device deposited on said silicon wafer. [0008] During the conversion of light into electrical energy, as mentioned above, only the top few microns of said top layer are really active. The major part of this silicon wafer only provides mechanical strength to the device. This function can be achieved by any other low-cost substrate compatible to the production process. The requirements for such a substrate, excepted low cost, are high temperature stability (1100.degree. C.), matching of the thermal expansion coefficients and low impurity contents. [0009] More generally, in the prior art, the preparation of a porous semiconductor layer on a substrate as a sacrificial layer for solar cell usually comprises several steps such as at least a porous semiconductor layer formation on an original substrate, epitaxial silicon layer deposition, device fabrication on said substrate and separation of the device from the original substrate and transfer to a foreign substrate in order to possibly re-use the original substrate. This sequence is largely illustrated in the documents U.S. Pat. No. 6,258,698 (Iwasaki et al, Canon), U.S. Pat. No. 6,211,038 (Nakagawa et al, Canon) and U.S. Pat. No. 6,326,280 (Tayanaka, Sony Corporation). [0010] In the prior art, several methods are known to separate thin (porous) semiconductor films from a substrate. All those methods use a lift-off or peeling-off process at the end of the production chain. The drawback of these methods is that during all the process steps, parameters such as temperature, pressure and chemicals are conditioned by the resistance of the original substrate. The film separation and its transfer is the last technological step that requires preserving the high-porous characteristics of the Si layer. The fact of maintaining said porous characteristics throughout many high-temperature steps allows only a narrow processing window in terms of process temperature and porosity. Moreover in said case, the transfer is difficult to achieve properly. [0011] In particular, a lift-off process is described in EP-A-1132952 where it is shown that a thin porous silicon film of 5 to 50 .mu.m can be separated from the silicon substrate whereon it is deposited. In such case, the substrate can be re-used many times for getting new porous silicon films. Other possible techniques for thin film separation are ion implantation or wafer bonding techniques. [0012] In EP-A-0993029, a method is disclosed for the production of a crystalline semiconductor film. This is done by forming a porous layer on a semiconductor substrate, lifting-off the porous layer, and either before or after the lifting-off applying a thermal annealing step such that the porous layer is at least partially recrystallized. For the lift-off step a method is disclosed in which the porous layer is attached to a `Hilfstrager`, which can be translated as a `sub-carrier`, or a `foreign substrate`. The porous layer is physically bonded or glued to said foreign substrate. SUMMARY OF CERTAIN INVENTIVE ASPECTS [0013] Various inventive aspects aim to provide an easy method for the preparation of thin-film devices for structures being highly efficient and low-cost. Examples of such structures are silicon-on-insulator (SOI) structures or solar cells. The use of thin film in SOI structures in general and in solar cells in particular allows the reduction of the amount of material consumed per structure, which significantly reduces the high costs of the active substrate, while the quality of the film provides the good characteristics of the whole device. [0014] Further inventive aspects may reduce the impact of device processing steps on the original substrate, porous silicon layer, porous silicon layer formation, necessary glues and target substrate. [0015] Yet further inventive aspects may improve control on the production-and-transfer process of thin films. [0016] Several documents listed below are incorporated by reference herewith. In particular, the document EP-A-1132952 is hereby incorporated by reference as a whole, especially with respect to the production of the thin porous film. [0017] In the art, the term `original substrate` has also been named `mother substrate`, `start-substrate` or `originating substrate`. The term `dummy support` in the art can also be named `intermediate support or substrate`, `support substrate`, `dummy substrate`. The term `foreign substrate` in the art can also be named `target substrate`, `final substrate` or `end-substrate`. The foreign substrate can be any substrate, e.g., glass. More examples of possible substrates are given in EP-A-0767486 and in U.S. Pat. No. 6,391,219, where the final substrate is called support substrate. To avoid any confusion, in the present context, the distinction between the dummy support and the final support is that the dummy support is a support on which the fabrication of a device can easily take place with full freedom of process parameters and/or without a negative impact of any device processing steps on the original substrate, porous silicon layer, porous silicon layer formation, necessary glues and/or target substrate. The dummy support is a support to which the porous layer is transferred intermediately without physical attachment thereto. The dummy substrate provides the necessary support and/or mechanical strength to the porous layer(s) during fabrication thereon of a device. In the absence of such supportive material, the porous layer(s) might break during device fabrication because too fragile. [0018] One aspect of the present invention relates to a method for manufacturing a semiconductor device. The method may comprise the following: [0019] (a) formation of a porous semiconductor layer in the form of a thin film on an original substrate, said formation being immediately followed by the step of [0020] (b) separation of said thin film by a lift-off process from said original substrate; [0021] (c) the transfer of said thin film to a dummy support, said thin film not being attached to said dummy support; [0022] (d) fabrication of a device on top of said thin film; [0023] (e) transfer and attachment of said device on said thin film on a foreign substrate. [0024] In one aspect, the method differs from methods known in the art in the sequence of its steps. It further differs from methods in the prior art by the fact that a device is fabricated on the thin film while placed on a support to which the thin film is not physically bonded and/or glued. In other words, the device is fabricated on a free-standing thin film. The advantageous effects that accompany these changes with respect to the prior art are discussed throughout the specification. [0025] How to perform the different steps per se is known in the art. For instance, several possible methods for steps (a) and/or (b) are disclosed in EP-A-0767486, EP-A-0993029, EP-A-1132952, U.S. Pat. No. 6,391,219 and references cited herein. In an embodiment of the present invention, cleaved surfaces may be smoothened before further processing, as described for instance in U.S. Pat. No. 6,391,219. In a preferred embodiment of the invention, the original substrate can be re-used directly and/or be prepared for re-use. [0026] In accordance with the present invention, the fabrication of a device, or at least part thereof, takes place on a free-standing thin film, before transfer of film and device to a final substrate which becomes part of the final thin film device, preferably of low-cost but highly efficient. 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