Determination of bending stress of Si wafer using concentrated load

Abstract

The technique of concentrated load with a simple O-ring supporter is used to measure the deflection of Si wafers. The load varies so that the ratio of the deflection to the wafer thickness changes from 0 to 1. For some samples, this ratio goes up to 1.4 at which the samples are fractured. It is observed in the experiment that the stress of the wafer can be described by the linear deflection theory for small deflection. However, when the deflection is larger than 1/5 of the wafer thickness, nonlinear deflection theory should be used and the stress in the middle plane cannot be ignored anymore. The total stress for large deflection is approximately equal to the sum of the stress of a linear elastic plate and the stress in the middle plane. The experiment also shows that the bending strength of the Si wafer strongly depends on the surface conditions. Mirror finishing surface gives high mechanical strength.