New Biaxial Flexure Test Device for Ultrathin Single Crystal Wafers

Abstract

The demand for germanium (Ge) wafers is rapidly increasing due to the importance of multijunction concentrator solar cells. Because Ge is expensive, the Ge wafers tend to be thin so as to maximize the number of wafers from a given crystal boule. However, their consequent susceptibility to microcracking and fracture during processing reduces the yield and drives up the cost. Wafer manufacturers therefore desire a simple and improved wafer strength testing. In this research, an innovative and inexpensive biaxial flexure testing device was developed. The device provides for a uniform application of very small loads and continuous load‐deflection measurements on full wafers. Experimental data closely matched predicted values. Compared with a traditional three‐point bend test, this method yielded an average flexure strength value of 120 MPa (closer to the predicted values) vs 146–152 MPa from the three‐point method. This result, as well as the fine fracture pattern, suggests that a much larger wafer area is being subjected to uniform loading compared with three‐point bend tests.