Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1998-09-15
2001-10-30
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S003000, C134S028000, C423S348000, C423S350000
Reexamination Certificate
active
06309467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to polycrystalline silicon, a method for producing it and a device for producing it.
2. The Prior Art
High-purity semiconductor material is needed for the production of solar cells or electronic components, for example, memory elements or microprocessors. The dopants introduced in a controlled manner are the sole impurities which should be present in such a material. An attempt is therefore made to minimize the concentrations of harmful impurities. It is frequently observed that even semiconductor material produced under high-purity conditions becomes contaminated again in the course of the further processing to produce the final products. Thus, expensive cleaning steps are repeatedly necessary to reacquire the original purity. Foreign metal atoms which are built into the crystal lattice of the semiconductor material disturb the charge distribution and may reduce the functioning of the subsequent component or result in its failure. Consequently, contamination of the semiconductor material by metallic impurities should be avoided. This applies to silicon which is the semiconductor material most frequently used in the electronics industry.
High-purity silicon is obtained, for example, by thermal decomposition of readily volatile silicon compounds. These compounds, such as trichlorosilane, can therefore be purified easily by distillation methods. Under these circumstances, polycrystalline silicon is produced in the form of ingots having typical diameters of 70 to 300 mm and lengths of 50 to 2500 mm. A large proportion of the ingots is used to produce crucible-drawn monocrystals, strips and foils or to produce polycrystalline solar-cell raw material. Since these products are produced from high-purity molten silicon, it is necessary to melt solid silicon in crucibles. In order to make this operation as efficient as possible, large-volume, solid pieces of silicon, such as, the polycrystalline ingots mentioned, have to be comminuted before melting. This is normally always associated with a superficial contamination of the semiconductor material. This is because the comminution is carried out with metallic crushing tools, such as jaw crushers or roll-type crushers, hammers or chisels.
During the comminution, care has to be taken to ensure that the surfaces of the fragments are not contaminated with impurities. Contamination by metal atoms is to be regarded as critical since these may alter the electrical properties of the semiconductor material in a harmful way. If the semiconductor material to be comminuted is comminuted with mechanical tools, such as steel crushers, the fragments have to be subjected to a surface cleaning before melting.
Mechanically machined polycrystalline silicon or polycrystalline silicon granules which have been produced from mechanically machined products can be used as silicon starting material. This starting material is for the production of monocrystalline silicon. However, it is necessary to reduce the concentration of iron atoms and/or chromium atoms which are present on the surface of the mechanically machined polycrystalline silicon.
Mechanically machined particles of polycrystalline silicon can be used as starting material for monocrystalline silicon. In this case, the surface of the mechanically machined polycrystalline silicon is etched with a mixture of nitric acid and hydrofluoric acid. This process is widely used, but it cannot adequately reduce the concentration of iron atoms and/or chromium atoms on the surface of the polysilicon.
DE-A1 195 29 518 discloses that polycrystalline silicon is first cleaned with a mixture of aqua regia (a mixture of hydrochloric acid and nitric acid) and is furthermore cleaned with hydrofluoric acid. This process, however, achieves only a mean iron value of 73.32×10
−11
g/cm
2
.
JP 051-54466 describes a cleaning process in which hydrofluoric acid and nitric acid are used. A mean iron value of 23.31×10
−11
g/cm
2
was achieved.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide semiconductor material, in particular polycrystalline silicon, which has a very low iron/chromium content.
It is a further object of the invention to provide a cleaning device with which the low iron/chromium content can be achieved.
These objects are achieved by the present invention, which relates to a composition comprising a semiconductor material which has a low metal concentration on the surface. This semiconductor material has an iron content and/or chromium content on the surface of less than 6.66×10
−11
g/cm
2
. This semiconductor material has a particle diameter of from 0.1 to 350 mm, preferably from 20 to 150 mm.
The semiconductor material includes silicon, indium phosphide, germanium or gallium arsenide. Silicon, in particular polycrystalline silicon, is preferred.
A polycrystalline silicon is comminuted by means of metallic crushing tools. These are made of steel which has been hardened with iron carbide and/or chromium carbide.
During the comminution of the polycrystalline silicon with these metals, the metal particles are pressed into the oxide layer and the uppermost layer of the silicon lattice. For this reason, cleaning mixtures which do not remove silicon, for example HCl/H
2
O
2
, cannot completely remove the abraded metal material. A complete cleaning is consequently necessary.
The invention furthermore relates to a method for producing semiconductor material which has a low metal concentration, which method comprises washing polycrystalline silicon in a preliminary cleaning in at least one stage with an oxidizing cleaning solution, washing it in a main cleaning in a further stage with a cleaning solution which contains nitric acid and hydrofluoric acid, and, during hydrophilization, washing it in yet a further stage with an oxidizing cleaning liquid.
In a preliminary cleaning step, the semiconductor material, in particular the polycrystalline silicon fragment, is preferably first rinsed with fully demineralized water having a resistivity of 15-18 Mohm and then washed with an oxidizing cleaning solution. This oxidizing cleaning solution is preferably hydrogen peroxide, mineral acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, perchloric acid, hydrofluoric acid, liquid, linear or branched organic acids containing 1 to 8 carbon atoms, such as acetic acid, formic acid, butyric acid, etc.
All these oxidizing substances can also be used, insofar as it is technically possible, in any desired mixtures, which may also contain hydrogen peroxide in addition. Furthermore, surfactants may also be present, such as, anionic surfactants preferably, ammonium lauryl sulfate or sodium alkyl sulfate.
An aqueous oxidizing cleaning solution is preferred which contains 4 to 20% w/w hydrochloric acid, 1 to 10% w/w hydrofluoric acid and 1 to 5% w/w hydrogen peroxide as a mixture. The balance is 65 to 94% w/w of water. The percentages by weight are based upon the total weight of the cleaning solution. This preliminary cleaning is preferably followed by a rinsing step using fully demineralized water having a resistivity of 15-18 Mohm.
This is followed by the main cleaning step, in which an aqueous mixture of nitric acid and hydrofluoric acid is used. This aqueous mixture preferably contains 60 to 70% w/w nitric acid, 1 to 5% w/w hydrofluoric acid and 25 to 39% by weight of water. The percentages by weight are based upon the total weight of the cleaning solution.
The main cleaning step may also preferably be divided up by first washing with an aqueous cleaning solution containing hydrofluoric acid and nitric acid, in which the content of nitric acid is less than 70% w/w, said mixture preferably containing 60 to 70% w/w nitric acid, 1 to 5% w/w hydrofluoric acid and 25% to 39% by weight of water. Then washing is carried out in a further step with a cleaning solution which contains hydrofluoric acid and nitric acid. Here the content of nitric acid is greater than 70% w/w, and the mixt
Bauer Theresia
Dietl Josef
Ott Werner
Pichler Herbert
Schmidbauer Wilhelm
Chaudhry Saeed
Collard & Roe P.C.
Gulakowski Randy
Wacker-Chemie GmbH
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