Patent ID: 7892861

Claim:
A method for selecting a wafer that reduces defects close to the periphery when fabricating a compound-material wafer from two or more initial wafers comprising: determining a second derivative of the profile of the wafers, determining an edge roll off (ERO) value of the wafers in dependence on the determined second derivative, selecting two wafers having ERO values of more than approximately 50 nm, and attaching the two selected wafers together, wherein at least one ERO value is determined according to the equation: ERO=Y ( a )− Y ( fga ), wherein “Y(a)” and “Y(fga)” correspond to the height of a wafer at positions “a” and “fga”, respectively, on a radius of the wafer, wherein “a” is selected so that “Y(a)” corresponds to the height of the wafer for which the second derivative, “d 2 Y(r)/d 2 r”, of the height profile, “Y(r)”, is zero, r being the radius of the wafer in that derivative, and wherein “fga” is selected at a radial edge position of approximately 0.5 mm to approximately 2.5 mm away from the outer periphery of the wafer.