Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Total dielectric isolation
Reexamination Certificate
2000-07-14
2001-12-04
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Total dielectric isolation
Reexamination Certificate
active
06326280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film semiconductor and method for making a thin film semiconductor, a solar cell, and a light emitting diode. More particularly, it relates to a thin film semiconductor and a method for forming a semiconductor film layer on a substrate having a plurality of porous layers defined therein having controlled and differing relative porosities.
As the material of the solar cell, various materials have been studied. Silicon, for which there are abundant reserves and which is free from apprehension of pollution, is the center of these efforts. Ninety percent or more of the amount of production of solar cells in the world are silicon solar cells as well. The tasks in solar cells are how to achieve a low cost, a high efficiency of conversion of light to electricity, a high reliability, and a small number of years for energy recovery. For the requests for high conversion efficiency and high reliability, single crystal silicon is most suitable, but it is difficult to fabricate single crystal silicon at a low cost. Therefore, at present, in the field of solar cells, particularly solar cells having a large surface area, active research and development is proceeding on solar cells using thin film polycrystalline silicon or thin film amorphous silicon.
In a thin film polycrystalline Si solar cell, the purity of the silicon is raised by refining techniques from metal class silicon using plasma or the like. An ingot is prepared by a casting process, and a wafer is prepared by a multi-wire or other high speed slicing technology. However, process for removing the boron and phosphorus from the metal class silicon, the preparation of an ingot of a good quality crystal by a casting process, enlargement of the surface area of the wafer, and a multi-wire or other high speed slicing technology require a very high grade of technology, so a substrate which is sufficiently cheap and has a good quality has not yet been fabricated at present. Further, the film thickness of the wafer is approximately 200 &mgr;m, therefore a flexible substrate cannot be formed.
Amorphous silicon can be formed on the surface of a plastic substrate by a CVD (chemical vapor deposition) process. Therefore, it is possible to form flexible thin film amorphous silicon. As a result, solar cells having a wide range of applications can be formed. However, there are drawbacks in that the conversion efficiency is lower than that of the polycrystalline silicon and single crystal silicon, and the conversion efficiency deteriorates during use.
Single crystal silicon offers the promise of a high conversion efficiency and a high reliability. Thin film single crystal silicon can be fabricated by the SOI (Silicon On Insulator) technique, which is a manufacturing technique of integrated circuits etc., but the productivity is low. Using the SOI technique, the manufacturing cost becomes considerably high, this is a problem in application to a low cost solar cell. Further, the processing temperature in the preparation of single crystal silicon is relatively high, so it is difficult to form this on a plastic substrate or glass substrate having a low heat resistance. Since it is difficult to form single crystal silicon on a plastic substrate, the manufacture of flexible thin film single crystal silicon is difficult.
When constructing window glass equipped with solar cells, in other words, solar cells are arranged on a surface of a window glass, solar cars with solar cells arranged on the roof, etc., the use of a flexible solar cell is desirable from the viewpoint of the simplification of the manufacture and the ease of rational arrangement for enlarging the light receiving surface. Nevertheless, the only semiconductor silicon which can be used to make the flexible solar cells at the present time is amorphous silicon.
SUMMARY OF THE INVENTION
The present invention provides a thin film semiconductor and a method for making a thin film semiconductor with which a thin film semiconductor, for example, thin film single crystal silicon, can be reliably produced on a mass production basis and a device constructed in accordance with the method. Therefore, a reduction of cost can be achieved and processes for producing a solar cell and a light emitting diode with which a solar cell having a high opto-electric conversion efficiency can be reliably and easily produced at a low cost.
In an embodiment, the present invention provides a process for producing a solar cell with which the terminal of the solar cell can be easily and reliably led outside with a low resistance.
In an embodiment, a new and improved method for making a thin film semiconductor is provided comprising the steps of providing a semiconductor substrate having a surface. The surface portion of the substrate is treated in at least one anodization process to define a plurality of porous layers having varying degrees of porosity adjacent the surface. In a preferred embodiment, the substrate is anodized in a first anodization step at a first current density to provide a first porous layer adjacent the surface having a first porosity. A second anodization step is performed at a second, higher current density to provide a second porous layer adjacent the first porous layer opposite the surface having a porosity greater than the first porosity. The difference in porosity between the first porous layer and the second porous layer provides an inherent line or zone of relative weakness located in the second porous layer or at or adjacent to the interface between the first porous layer and the second porous layer. The line of weakness or fragility introduced by the strain caused by the difference in the lattice constants of the adjacent porous layers permits separation of the surface layer and any film grown thereon from the remainder of the second porous layer and the substrate. In an especially preferred embodiment, a third anodization step at a third higher current density is performed to define third porous layer having a third porosity higher then the second porosity. The third porous layer is disposed within the second porous layer or adjacent to the second porous layer. In accordance with this embodiment, a relative line of weakness is defined by the third porous layer or at or adjacent an interface formed between the third porous layer and the second porous layer. After a plurality of porous layers are defined adjacent the surface of the substrate, at least one semiconductor film layer is formed on the first porous layer and surface. Thereafter, the semiconductor film is separated from the semiconductor substrate along the line of relative weakness to provide a thin film semiconductor product.
According to the present invention, a thin film semiconductor is prepared by a changing a surface of a semiconductor substrate to form a porous structure comprising two or more porous layers having different porosities; growing a semiconductor film on the surface of the porous structure; and separating, removing and/or peeling the semiconductor film from the semiconductor substrate in a controlled or directed manner along the line of weakness created in the porous structure layers.
Further, in the process for producing a solar cell according to the present invention, a solar cell is produced by method comprising the steps of changing the surface of a semiconductor substrate to form a porous structure comprising two or more porous layers having different porosities; epitaxially growing a semiconductor film comprising multiple layers constituting the solar cell on the surface of this porous layer; and peeling or otherwise separating the multi-layer epitaxial semiconductor film from the semiconductor substrate along a line of weakness defined in the porous structure layers.
Furthermore, in a process for producing a light emitting diode according to the present invention, the light emitting diode is prepared by a method comprising the steps of changing the surface of a semiconductor substrate to form a porous structure layer comprising two or mor
Bowers Charles
Christianson K
Sonnenschein Nath & Rosenthal
Sony Corporation
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