Flip-chip bonding of MEMS scanner for laser display using electroplated AuSn solder bump

Abstract

A MEMS scanner has been flip-chip bonded by using electroplated AuSn solder, bumps. The microelectromechanical systems (MEMS) scanner is mainly composed of two structures having vertical comb fingers. To optimize the bonding condition, the MEMS scanner was flip-chip bonded with various bonding temperatures. Scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) spectroscopic system was used to observe the microstructures of the joints and analyze the element compositions of them. The die shear strength increased as the bonding temperature increased. During the thermal aging test, the delamination occurred at the interconnection of the MEMS scanner bonded at 340 degrees C. It is inferred that the Au layer serving as pad metallization has been dissolved in the molten AuSn solder totally, and subsequently the Cr layer was directly exposed to the AuSn solder. Judging by the results of both die shear test and thermal aging test, the optimal bonding temperature was found to be approximately 320 C. Finally, using this MEMS scanner, we obtained an optical scanning angle of 32 when driven by the ac control voltage of the resonant frequency in the range of 22.1-24.5 kHz with the 100-V do bias voltages.