Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from solid or gel state
Reexamination Certificate
1999-01-21
2001-04-17
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from solid or gel state
C117S008000, C117S009000, C117S930000
Reexamination Certificate
active
06217647
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a production of a monocrystalline layer (hereinafter referred to as a “monocrystalline material layer”) of a conducting or semiconducting material (hereinafter referred to as “material”) on a monocrystalline porous layer (hereinafter referred to as “porous material layer”) of the same material. If the material is silicon, for example, SOI (silicon on insulator) wafers can be produced starting with such a layer structure. These wafers can be advantageously used in semiconductor electronics and in silicon micromechanics.
BACKGROUND INFORMATION
In a conventional method of producing SOI wafers, two wafers of monocrystalline silicon having at least one oxidized surface are thermally bonded to one another with the silicon dioxide surfaces and then the one wafer is thinned back.
In the SIMOX (“separation by implanted oxygen”) method, oxygen ions are implanted in a monocrystalline silicon wafer, forming, after activation with silicon, a silicon dioxide layer which separates the wafer from a thin layer of silicon.
In another conventional method, silicon is deposited epitaxially on a porous monocrystalline silicon layer and then the porous silicon is oxidized.
In another conventional method, silicon is applied epitaxially to a porous, monocrystalline silicon layer, partially oxidized on its surface, after etching back.
All these conventional methods have in common the high manufacturing costs, caused by high consumption of materials or very expensive process steps, e.g., in the production of thick epitaxial layers.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a simple and inexpensive method of producing monocrystalline material layers on porous monocrystalline layers of the same material in a reproducible and time-saving manner.
This object is achieved with a method according to the present invention by applying a layer of amorphous material (hereinafter referred to as “amorphous material layer”) to the porous material layer and converting it to the monocrystalline material layer by a tempering procedure. Producing the monocrystalline material layer in two steps using the amorphous material layer is much less complicated and consumes much less energy than when the monocrystalline material layer is produced in one step, e.g., by epitaxy. The seeds supplied by the porous material layer, whose size is optionally (see below) reduced by oxide-masked surface areas, are sufficient to ensure complete conversion of the amorphous material to monocrystalline material. The porosity of the porous material layer is necessary in a subsequent step to permit their selective removal or selective conversion to the oxide form in the presence of the monocrystalline material layer plus optionally a wafer of the material.
It is advantageous to select the material from the group of aluminum, silicon, silicon carbide and gallium arsenide.
If the material is silicon, it is advantageously tempered at temperatures between about 600° C. and about 800°
0
C. Within this temperature range, a complete conversion of the amorphous silicon to monocrystalline silicon within approximately 15 to 24 hours is ensured.
It is advantageous to surface oxidize the porous material layer superficially before applying the amorphous material layer and to thin back the surface oxidized porous material layer to the extent that the porous material is exposed again in some areas. If the porous material layer is silicon, it is especially advantageous if it is surface oxidized dry or wet at about 400° C. to 800° C. Thinning back is advantageously performed by partially dissolving with a solvent for the oxide just formed, resputtering, regrinding, plasma etching or with HCl gas in a tempering tube. Growth of the oxide layer can be controlled with these methods, and in thinning back, essentially the oxidized porous material layer is removed in the direction of the layer normal so that the oxide is removed first from elevations projecting out of the plane of the layer, and then these elevations serve as seeds in conversion to monocrystalline material in the subsequent tempering. Therefore, the monocrystalline material layer formed in tempering is in contact with the porous material only at said elevations, but not in the pores. This facilitates the subsequent removal or conversion of the porous oxide, and the levelness of the surface of the monocrystalline material layer facing the porous material layer is improved without interfering with formation of the monocrystalline material layer.
It is advantageous if the amorphous material layer is applied by sputtering onto a target plate made of the material or by LPCVD or PECVD in an atmosphere containing at least one volatile compound of the material, for example, in SiH
4
or di- or trichlorosilane in applying silicon. These methods are not very complicated, can be controlled well and use the conventional equipment of semiconductor manufacture.
A structure according to the present invention can be advantageously used by bonding the monocrystalline material to a carrier substrate, and then the porous material layer is dissolved away, and the bond between the monocrystalline material layer and the substrate, normally a wafer of the same material, is dissolved. Because of its structure, the solubility of the porous material layer differs so greatly from that of the wafer and the monocrystalline material layer that the latter is practically not attacked when dissolving the porous material. Instead, the monocrystalline material layer retains its original thickness and the wafer can be reused. The monocrystalline material layer has excellent electrical and mechanical properties and can therefore be used for numerous applications, such as (if the material is semiconducting) the production of high-grade thin-film electronics.
The structure including the porous material layer, the monocrystalline material layer covering it and a wafer of the same material covered by it can be advantageously used to produce a structure which, like an SOI wafer, for example, includes the wafer, an insulation layer of mainly oxidized porous material applied to it and the monocrystalline material layer; this is accomplished by exposing the structure to oxidizing conditions for a specified period of time. In order for oxidation of the porous material to be complete, a great deal of time would be required, because the oxygen would have to diffuse from the side into the porous material layer, which has a very great diameter in relation to its thickness. To obtain a good insulator, it is not generally critical if needles of porous material completely surrounded by oxide are still present. This is true in particular when the structure is subjected to a reflow step after oxidation to compress the insulator. If oxidation of the porous material is to be complete, however, it is advantageous to provide the monocrystalline material layer with a pattern of through openings before oxidation, and it is especially advantageous to distribute the openings as uniformly as possible over the monocrystalline layer, there being enough tolerance so that it is not necessary to accept any problems in use of the SOI wafer, for example, because of the pattern. In oxidation of the porous material layer, the wafer and the monocrystalline material layer are not subject to any mentionable oxidative attack. If the insulator thus produced is made to collapse by reflow after oxidation, then the resulting structure not only has good insulating properties, but also has excellent mechanical and optical properties—assuming film stress is minimized. Film stress is minimal when the porous material has a porosity of approximately 50% to 60%.
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patent: 585412
Frey Wilhelm
Laermer Franz
Anderson Matt
Kenyon & Kenyon
Robert & Bosch GmbH
Utech Benjamin L.
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