Solid-state reaction of electroplated thin film Au/Sn couple at low temperatures

Au-Sn solid state reaction during storage can significantly affect the quality of Solid Liquid Interdiffusion (SLID) bonds for industrial usage in the hermetical encapsulation of Microelectromechanical Systems (MEMS). This work studies the growth of interfacial reaction products and their metallurgical evolution between pure Sn and base metal Au in a solid state at a low temperature. Au/Sn couples were electrodeposited on metallized Si wafers and were annealed at the temperatures of 298 K and 333 K. AuSn, AuSn2, and AuSn4 are sequentially distributed in the reaction region. The constancy of the thickness ratio, before the Sn layer was consumed, points to the same controlling mechanism for the growth of each layer. The exponent n for the fitting curve of the thickness with the annealing time is equal to 0.392 at T = 298 K, indicating a mechanism controlled by grain boundary diffusion, while the exponent, n = 1.08, at T = 333 K indicates a process controlled by interface reactions. A much slower IMC growth rate can be obtained with no obvious tin damage at 203 K. The results show that the solid-state reaction is rapid at room temperature and at 333 K, but the growth rate can be reduced by storing the samples at a lower temperature.