Accessible determination of die-to-wafer bond strength with the Schwickerath test

Die-to-wafer or wafer-to-wafer direct bonding has been drawing significant attention and undergoing rapid development for its various applications in three-dimensional integrated circuits (3D-IC), such as image sensors, micro-electro-mechanical system (MEMS) sensors, and stacked memory products. The bond strength is one of the most considerable factors that affect the reliability of such stacked devices. Measurement of wafer-to-wafer bond strength is normally performed by the razor blade method, but there was no such well-established technique for die-to-wafer direct bond. To characterize the die-to-wafer bond strength accurately and conveniently, this work introduced the Schwickerath three-point bending test and derived an analytical solution of bond energy, which does not require initial crack preparation. To examine the correctness of applying this method in a novel area, finite element method (FEM) and razor blade experiments on equivalent samples were conducted. Furthermore, the annealing effect on die-to-wafer bond strength was studied. Top die thickness, loading rate in three-point bending test, and the compensation factor of analytical solution are discussed and summarized in this study.