1.  Field of Invention
This invention concerns edge grinding methods and apparatus.  In particular it relates to methods and apparatus for edge grinding wafers of semi-conductor material so as to produce an accurate peripheral profile.                 2.  Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.         
The present invention seeks to provide a process which grinds the overall diameter of a wafer to the desired accuracy, typically to better than 10 microns, and apparatus for performing the grinding processes required and ensuring that the desired accuracy is maintained. 
It is an object of the invention that the method and apparatus should be applicable to both notch grinding and edge grinding processes, each of which are applicable to the grinding of semi-conductor wafers. 
The peripheral profile of a semi-conductor wafer is typically triangular in cross-section with the apex of the triangle accurately positioned relative to the two flat faces of the semi-conductor disc.  Typically the position of the apex of the triangular cross-section relative to the thickness of the wafer (which itself is very carefully controlled) has to be better than 10 microns. 
Edge grinding is performed using grooved grinding wheels which rough, semi-finish and finish grind the edge profile.  The profile is then polished as a final manufacturing step. 
A small notch is normally formed at one point around the circumference of such a semi-conductor wafer and typically this is formed using another grooved grinding wheel, albeit of very small diameter. 
Generally speaking it is also important that the edge profile is continued accurately around the inside of the notch. 
The overall process typically comprises the steps of rough grinding the periphery of a wafer so as to form two frusto-conical surfaces which converge to an annular ridge which defines the maximum overall diameter of the wafer, semi-finish grinding, to size, finish grinding to low tolerances, and finally polishing the ground surfaces. 
Since polishing removes virtually no material, and a finish grinding step is almost akin to polishing, it is important that the rough grinding and semi-finish grinding steps position the two frusto-conical surfaces as accurately as possible relative to the faces of the wafer and thereby position the annular ridge defined by the two converging surfaces to the required degree of accuracy relative to the thickness of the wafer. 
It is therefore an object of the present invention to provide a method and apparatus for forming a groove in a grinding wheel which will accurately form the desired profile around a wafer. 
It is another object of the invention to provide a method and apparatus for positioning a grinding wheel relative to a wafer so as to grind frusto-conical surfaces around the edge of the water to define an annular rim and to position the latter accurately relative to the thickness of the wafer. 
It is an object of the invention also to provide a process which will be applicable whether the grooved grinding wheels are formed and re-formed in situ, or have to be demounted from the machine to be re-formed.