Improved MEMS structure for stress-free flip-chip packaging

We present a new method for stress-free microelectromechanical systems (MEMS) flip-chip packaging. Residual stress, which is mainly generated during the reflow process, is a notorious problem in flip-chip packaging. The residual stress reduces the bump fatigue life and device performance owing to the deformation. Underfill encapsulation is a common way to reduce the residual stress in a flip-chip integrated circuit (IC) packaging process. However, it cannot be applied to MEMS packaging because MEMS devices usually include moving microstructures. We intend to resolve this problem by improving the MEMS structure, designing spring beams to introduce electrical pads. The residual stress, which is caused by the mismatch of the coefficient of thermal expansion (CTE) between the MEMS device and the package substrate, can be absorbed through the deflection of spring beams. By using this idea, a quartz MEMS-based capacitive tilt sensor, which was bulk etched and composed of vertical comb electrodes in wafer thickness for achieving large initial capacitance, was successfully packaged. A high melting point alloy Au80Sn20 was used as the solder joint material. The thermal cycling test and sensitivity evaluation experimental results demonstrated the effectiveness of the proposed method.