Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-07-18
2003-07-22
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S406000, C428S446000, C428S447000, C451S040000, C451S050000
Reexamination Certificate
active
06596395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. (Field of the Invention)
The present invention relates to silicon balls used as substrates for semiconductor circuits and solar cells and a method of making such silicon balls.
2. (Description of the Prior Art)
As a semiconductor technology for the next generation, globular silicon semiconductors have recently come to be spotlighted. Globularizing of a conventional plane silicon wafer has an advantage in that the manufacturing cost that is brought about infinite plant and equipment investment can be reduced and that utilization of silicon material can be maximized as a result of the use of a relatively large spherical surface having a large specific surface area.
In order to use globular silicon as a semiconductor circuit substrate, a surface property comparable to the silicon wafer and a high shape precision are required.
However, the globular silicon satisfying such requirements and a practical method of making such globular silicon have not yet been made available.
Silicon balls of single crystal silicon are generally manufactured by a lapping process of lapping workpieces of single crystal silicon to produce lapped balls and a subsequent finishing process for finishing the lapped balls, that is, a polishing process of polishing the lapped balls to produce the polished balls.
A first problem inherent in the lapping process and associated with the lapped balls will be discussed.
To increase the shape precision of a globular body, a grinding is generally performed. However, processing of silicon balls with the use of a generally utilized metal grinder for cast iron or carbon steel, no surface roughness will be improved even though very finely divided abrasive particles are used. The reason therefor appears to result from that in the practice of the metal grinding process, silicon balls tend to receive a relatively large processing force from the grinder and the abrasive particles and that the silicon balls tend to be agglutinated.
To alleviate the foregoing problems, the inventors of the present invention have suggested a lapping process using an alkaline solution and a lapping process using a resinous material for a material of the surface plate. However, even with these suggested methods, a sufficient shape precision and a sufficient surface property have not yet been attained.
More specifically, although the processing with the use of the alkaline solution utilizes a dissolution, a chemical reaction takes place uniformly on the workpieces and; therefore, compensation for shape is limited. Also, the surface roughness to be controlled by the alkaline concentration is limited and, while the solution concentration has to be lowered to secure a minute surface roughness, virtually no processing proceeds if the concentration is lowered to a certain extent.
In the case of the use of the resinous surface plates, since the processing force which the silicone balls receive from the surface plate and/or the abrasive particles embedded therein is lessened as a result of elastic deformation of the resin, and since no agglutination occur between the workpieces and the surface plate without strongly impairing the surface of the balls, improvement in surface roughness can be expected. However, the extent to which the shape precision can be increased is limited because of the elastic deformation of the resin. Also, the use of very finely divided grinding particles for the purpose of securing a minute surface roughness will result in malfunction of the grinding particles during the processing and, as a result thereof, virtually no processing proceeds.
Also, in order to secure the surface property comparable to the silicon wafer, it is necessary that after the lapping, polishing is carried out to improve the surface roughness and, at the same time, to remove crystal strains and residue stresses brought about by the lapping process. However, with the previously described lapping method, the crystal strains and the residue stresses are so large that a relatively large machining allowance is required during the polishing process and this tends to constitute a cause of shape deterioration after the polishing process.
In other words, while an objective of the polishing process that is effected subsequent to the lapping process is to remove flaws and a residue stress layer resulting from the lapping process, to thereby provide a surface free from crystal flaws and residue stresses, the polishing process in which CMP (chemical machine polishing) is utilized involves a first problem in which because of difference in chemical action due to crystalline orientation, the shape precision such as a sphericity tends to degrade.
A second problem inherent in the polishing process and associated with the polished balls will now be discussed.
In the manufacture of the silicon wafer, polishing is generally performed as a final process so that the silicon wafer can be finished to have a surface having a surface roughness not greater than, for example, 5 angstrom and free from crystal strains. Where silicon balls are to be used as semiconductors, as discussed hereinbefore, the surface property comparable to the silicon wafer and the high shape precision are required.
In the case of the silicon balls, although the processing using the surface plates is most suited for the production of highly accurate balls, this method in which abrasive particles are used to achieve a mechanical grinding is ineffective to remove minute surface flaws and processing strains at a final stage and, therefore, the polishing is essential as a final process.
However, no polished balls of single crystal silicon that can be used as such semiconductor circuit substrates or the like have not yet been made available.
As a method of polishing the silicone balls, a method is known in which the workpieces are retained by a resinous retainer and are polished by a cloth of a kind generally used for polishing wafers. However, this method is susceptible to degradation of the shape, and the workpieces have to be supported in individually separated fashion, resulting in reduction in productivity.
The polishing is a method of grinding performed by the utilization of a chemical action without accompanying strains and residue stresses generated. However, where the workpieces to be processed is globular, processing of the workpieces while the latter are sandwiched between opposed flat polishing clothes tends to result in failure of a function of shape retention and, hence, the shape tends to considerably change. Also, the workpiece retainer is required to retain the workpieces and the workpieces are mounted one by one on the workpiece retained, resulting in reduction in productivity. These are a second problem.
SUMMARY OF THE INVENTION
A primary object of the present invention is to substantially solve the first problem discussed above to thereby provide lapped balls of single crystal silicon that can easily be manufactured so as to satisfy the requirements necessary for them to be used in semiconductor circuit substrate or the like, and also to thereby provide a method of easily making the lapped balls of single crystal silicon that have a required sphericity and a surface property.
Another important object of the present invention is to substantially solve the second problem discussed above to thereby provide polished balls of single crystal silicon of a high productivity that are highly accurate and free from strains and residue stresses in a processed surface and that can be used as semiconductor circuit substrates, solar cells or the like and also to thereby provide a method of making at a high productivity and highly accurately the polished balls of single crystal silicon that are free from strains and residue stresses in the processed surface.
In order to accomplish the foregoing objects, the present invention in one aspect provides a lapped ball of single crystal silicon having a sphericity of not greater than 0.08 &mgr;m and also having a residue stress layer of not greater than 5 &mgr;m in a depth from a processing
Kiliman Leszek
NTN Corporation
LandOfFree
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