Investigation of Gold-Silicon Eutectic Bonding to Realize the Symmetric Sensing Element of MEMS Piezoresistive Accelerometer

In this article, we investigate the intermixing of gold-silicon (Au-Si) across the Chromium (Cr) adhesion layer for Au-Si eutectic wafer bonding process development. In the first experiment, a stack of Cr and Au film was deposited on a silicon wafer, annealed in nitrogen gas ambient followed by the etching of unreacted Au, and investigated using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. The study confirms the intermixing and formation of Au-Si alloy. With that understanding, the Au-Si eutectic wafer bonding process was developed to realize a new symmetric sensing element for a micro electro mechanical systems (MEMS) piezoresistive accelerometer. This structure is realized by bonding two silicon wafers containing complementary halves of the structure and Au-Au facing bond interface. Each half was formed by deep reactive ion etching (DRIE) in silicon-on-insulator (SOI) wafers. The DRIE-based method results in a smaller die size and eliminates the need for corner compensation, unlike potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH)-based wet etching of silicon, and it enables the fabrication of even a cylindrical seismic mass. Post bonding, the interface was investigated using SEM, high-resolution X-ray imaging, and MIL-STD-883-based die shear test. The structure was successfully realized, achieving a 100% dicing yield of bonded wafers. For the 12 tested samples, the minimum, maximum, average, and standard deviation of die shear strength are 18.3, 40.1, 28.9, and 6.9 kgf, respectively. The average shear strength per unit of bond area is 27 MPa.