Modified Microgrinding Process

Abstract

A method of forming a substrate is performed by grinding a substrate using abrasives so that both major surfaces of the substrate achieve desired flatness, smoothness, or both. In an embodiment, a coarser abrasive is used to grind one major surface, while a finer abrasive is simultaneously used to grind the other major surface. A single grinding step can used to produce a substrate having opposing surfaces of different surface roughnesses. This may help to eliminate a typical second downstream fine polishing step used in the prior art. Embodiments can be used with a wide variety of substrates, including sapphire, silicon carbide and gallium nitride single crystal structures grown by various techniques.

What is claimed is: 
     
         1 . A method of machining a wafer having first and second opposing major surfaces, the method comprising:
 grinding a first major surface of a wafer using a first fixed abrasive; and   grinding a second major surface of the wafer using a second fixed abrasive, the second fixed abrasive having a grit size that is coarser than the grit size of the first fixed abrasive, wherein at least a portion of the grinding of the first and second major surfaces of the wafer occurs simultaneously.   
     
     
         2 . The method of  claim 1 , wherein the wafer is a sapphire substrate. 
     
     
         3 . The method of  claim 1 , wherein the first fixed abrasive has a mean abrasive particle size of no more than 5 microns, no more than 20 microns, no more than 35 microns, or no more than 75 microns. 
     
     
         4 . The method of  claim 1 , wherein the second fixed abrasive has a mean abrasive particle size of at least 60 microns, at least 80 microns, at least 100 microns, or at least 200 microns. 
     
     
         5 . The method of  claim 1 , wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns. 
     
     
         6 . The method of  claim 1 , wherein grinding a first major surface of a wafer and grinding a second major surface of the wafer comprises grinding the wafer between a first abrasive plate and a second abrasive plate, the second abrasive plate having a coarser abrasive than the first abrasive plate, wherein the first abrasive plate grinds the first major surface of the wafer and the second abrasive plate grinds the second major surface of the wafer. 
     
     
         7 . The method of  claim 6 , wherein the second abrasive plate is located underneath the first abrasive plate so that the second abrasive plate grinds the bottom surface of the wafer and the first abrasive plate grinds the top surface of the wafer. 
     
     
         8 . The method of  claim 6 , further comprising applying a grinding fluid to cool the grinding surfaces and to remove loose abrasive material or swarf. 
     
     
         9 . The method of  claim 8 , further comprising recirculating the grinding fluid after it has been used to cool the grinding surfaces and to remove loose abrasive material or swarf and filtering the used grinding fluid before it is reintroduced to prevent loose coarse abrasive particles in the recirculated grinding fluid from damaging the surface of the wafer during grinding. 
     
     
         10 . The method of  claim 1 , wherein grinding a first major surface and grinding a second major surface of the wafer comprises:
 placing a wafer between first and second abrasive plates so that the top face of the wafer is in flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer is in flat contact with the abrading surface of the second abrasive plate; and   rotating the abrasive plates, the wafer, or any combination thereof to grind the top and bottom faces of the wafer.   
     
     
         11 . The method of  claim 1 , wherein grinding a first major surface and grinding a second major surface of the wafer comprises:
 placing at least one wafer into a circular carrier located between first and second abrasive plates;   bringing the top face of the wafer into flat contact with the abrading surface of the first abrasive plate and the bottom face of the wafer into flat contact with the abrading surface of the second abrasive plate;   rotating the abrasive plates; and   rotating the carrier to rotate the wafer between the rotating abrasive plates.   
     
     
         12 . The method of  claim 1 , wherein grinding the wafer with the second fixed abrasive removes 30 to 50 microns of material during the grinding process. 
     
     
         13 . The method of  claim 1 , wherein grinding the wafer with the first fixed abrasive removes 10 to 15 microns of material during the grinding process. 
     
     
         14 . The method of  claim 1 , wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the second fixed abrasive is at least 4000 Å, at least 5000 Å, or at least 7000 Å. 
     
     
         15 . The method of  claim 1 , wherein, when the grinding process is completed, the surface roughness on side of the wafer ground by the first fixed abrasive is no more than 1000 Å, no more than 500 Å, or no more than 100 Å. 
     
     
         16 . The method of  claim 1 , wherein the wafer comprises a single crystal substrate. 
     
     
         17 . The method of  claim 1 , wherein the wafer comprises a polycrystalline material. 
     
     
         18 . An apparatus for the double-sided grinding of a flat substrate, the apparatus comprising:
 an upper and a lower grinding plate, the two grinding plates being coaxially mounted so that a substrate can be disposed between the two grinding plates and the two grinding plates being rotatable about their coaxial central axis by a grinding plate driving mechanism;   a substrate carrier disposed between the two grinding plates, the carrier including a carrier driving mechanism for rotating the carrier about its own central axis and about the coaxial central axis of the upper and lower grinding plates; and   an upper fixed abrasive disk mounted to the inner surface of the upper grinding plate and a lower fixed abrasive disk mounted to the inner surface of the lower grinding plate, wherein the lower fixed abrasive disk has a coarser abrasive grit than the upper fixed abrasive disk so that double-sided substrate grinding of a substrate removes material from the opposing substrate surfaces at different rates and so that double-sided substrate grinding produces opposing substrate surfaces having different surface roughness.   
     
     
         19 . The apparatus of  claim 18 , wherein the difference between the average abrasive particle size in the upper fixed abrasive disk and the average abrasive particle size in the lower fixed abrasive disk is at least 20 microns, at least 50 microns, or at least 100 microns. 
     
     
         20 . The apparatus of  claim 18 , wherein the upper fixed abrasive disk, the lower fixed abrasive disk, or both comprise a bonded fixed abrasive.