Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
1999-05-06
2001-11-06
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C051S308000, C106S003000, C438S692000, C438S693000, C451S036000, C451S037000, C252S079200, C252S079400, 43
Reexamination Certificate
active
06312487
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a polishing compound useful for edge polishing of a semiconductor substrate composed of a silicon wafer or a wafer of compound semiconductor whose surface is covered by thin layer of silicon oxide or silicon nitride, a method for preparation of the polishing compound and a polishing method using the polishing compound. More specifically, this invention relates to such polishing compound having buffer action and large electrical conductivity.
DESCRIPTION OF THE PRIOR ART
Electric parts such as IC, LSI, or VLSI, which are made from semiconductor materials e.g. silicon mono crystalline, are produced as follows. That is, an ingot of silicon mono crystalline compound semiconductor is sliced to a thin wafer, precise electrical circuit pattern is formed on the surface of the sliced wafer and divided into chips. Then, IC, LSI, or VLSI are prepared based on the chip. The as cut wafer which is prepared by slicing an ingot is lapped, etched and polished to a mirror finish and at least one side of the wafer is mirror finished. On the mirror finished surface, fine and precise electrical pattern is formed at the device procedure. The wafer is processed while maintaining the original disk shape until it is divided to unit semiconductor chips. Between above mentioned main procedures, there are washing, drying and transporting procedures. If the outermost periphery of the wafer has steep and sharp edge and coarser surface roughness during above mentioned series of procedures, the sharp edge contacts with an apparatus or other substance and very fine particles are generated by very small breaking. Further, very fine contamination particles are caught in the coarse surface of the edge, and the caught fine contamination particles are released at the latter procedure and cause a contamination problem of the precision finished surface, thereby deteriorating the yield of products and the surface quality. Usually, to prevent the above mentioned problem, the outermost periphery edge of the wafer is chamfered (beveling) and mirror finished (edge polishing) at the early stage of the wafer processing.
In general, the above mentioned edge polishing is carried out by the following method. Namely, on the surface of a rotatable drum of an edge polishing machine a polishing pad such as resin foam, synthetic leather or unwoven clothe are wound and stuck, the edge part of silicon wafer whose edge is chamfered is pressed to the rotating drum with necessary angle by rotating, with constant supply of aqueous solution of polishing compound composed mainly by colloidal silica. Thus, the polishing of the outermost peripheral edge of wafer is carried out. The polishing compound used here is the same one used for the surface polishing of the wafer.
As the polishing compound, an aqueous dispersion prepared by dispersing fine particles of colloidal silica in an alkaline solution is commonly used. The pre-polishing and polishing process are different from the previous process based on so-called mechanical processing which uses for instance a diamond grinding stone or hard abrasives of aluminum oxide. The feature of the pre-polishing and polishing process is to utilize the chemical effect of alkali to a silicon wafer, concretely utilize a corroding effect of alkali to a silicon wafer. That is, by the corroding effect of alkali, a thin soft corrosion layer is formed on the surface of a silicon wafer. The formed corrosion layer is removed by a mechanical action of fine particles of colloidal silica, thus the polishing progresses. As mentioned above, since the polishing progresses by the chemical action of alkaline component contained in the solution, it is necessary to maintain pH of the solution of polishing compound within a region greater than 7. That is, when pH of the solution becomes closer to 7, which indicates the neutral region, the chemical action becomes weak and the removing rate falls down. Further, when pH of the solution becomes closer to 14, which indicates strong alkali region, the chemical action becomes strong and the removing rate is improved.
Therefore, in this kind of polishing, the chemical and physical features of the solution of polishing compound is very important. That is, the surface of the silicon wafer is corroded by an alkaline component and a thin corrosion layer is formed. The thickness and the hardness of the thin corrosion layer is largely effected by features of the solution of polishing compound. Since the electrochemical features of the solution of polishing compound have strong influence to the thickness and the hardness, it is very important to keep pH of the solution in a stabilized region. If pH of the solution is easily affected by a change of external conditions such as heat, contact with outside air or contamination from outside, thickness of the layer, speed for corroding, consistency of the layer and removing speed of the layer fluctuate delicately, and it is difficult to expect precise and homogeneous polishing results. Further, since the corrosion layer is removed by the mechanical action of the particles of colloidal silicon oxide abrasives contained in polishing compound, it is necessary that these particles have adequate diameter and are not easily broken or do not cohere and form gel. That is, particles of silicon oxide remove the corrosion layer formed by an alkaline component effectively by a mechanical action. Therefore, the polishing compound must not be a compound which gives bad influence to the mirror finished surface after the corrosion layer is removed.
Up to the present, many kinds of compounds have been proposed as a polishing compound of a silicone wafer. For instance, silica sol and silica gel are proposed as a polishing compound in U.S. Pat. No. 3,170,273. Further, the technique for improving the removing speed by adjusting pH of the colloidal solution within the range from 10.5 to 12.5 is disclosed in U.S. Pat. No. 3,328,141. In U.S. Pat. No. 4,169,337, the technique to add amines in a polishing compound is disclosed. In Japanese Patent Application 2-158684, a polishing compound composed by water, colloidal silica, water soluble polymer of higher than 100,000 molecular weight and water soluble salts is disclosed. Further, in Japanese Patent Application 2-158684, a method for polishing by the use of a polishing compound containing 10 to 80 wt. % of piperazine, a kind of water soluble amine, to silica of silica gel or silica sol is disclosed.
In the meanwhile, the technique of semiconductor devices has remarkably progressed, and the wafer to which surface thin layer of oxide film is provided is becoming more popular. For instance, to prevent the contamination from the reverse side, for insulation or to prevent the diffusion of contamination, usually a thin layer of oxide or nitride is prepared on the surface of mirror finished silicon wafer. The hardness of the surface of wafer which is covered by oxide or nitride layer is remarkably harder than that of bare wafer. The oxide or nitride layer is prepared on the surface of wafer whose surface is mirror finished and the edge part of outermost periphery is ground by #1000 beveling grinding stone. Therefore, the surface roughness of the edge part of outermost periphery is coarser compared with the surface of wafer. The oxide or nitride layer of 2000 Å thickness is provided on the coarse edge surface. As mentioned above, the surface of wafer whose surface is covered by oxide or nitride is remarkably harder compared with the surface of bare wafer. And when the surface is polished by alkaline colloidal silica, the polishing speed drops down to 1/3 level and is disadvantageous from the economical view point. Especially, in the case of edge part polishing which has rougher surface, it is necessary to polish the hard layer and silicon simultaneously. As the method to process the hard surface, the method using the polishing compound which contains fine particles of metal oxide, nitride or carbide as abrasive is usually used. However, the polishing ability is larg
Marcheschi Michael
SpeedFam Co Ltd
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