A precise spacing-control method in MEMS packaging for capacitive accelerometer applications

Abstract

Capacitive micro accelerometers with high sensitivity have found wide applications in geophysics. Reducing the interelectrode spacing, which is determined by the thickness difference between the electrodes and the solder bumps in flip-chip eutectic bonding, is an efficient way to improve the sensitivity of area-varying capacitive transducers in micro accelerometers. Traditional methods require extra materials and processes, and precise control of the thickness of both the solder pumps and the electrodes is necessary. This work introduces a novel method for the precise control of the interelectrode spacing using a three dimensional (3D) electroplating process. Standoff pillars and electrodes are deposited by a single electroplating process with a constant thickness difference, which is only determined by the gap of the seed features that can be precisely determined by photolithography. The standoff pillars are used to define the thickness of the solder bumps in the packaging process. The 3D electroplating process is studied, characterized and applied to a typical high-precision micro capacitive area-varying accelerometer. Experimental results show that the variation of the interelectrode spacing is decreased by more than 6.5 times, when compared to that without the 3D electroplating process. Benefitting from the reduced interelectrode spacing, the sensitivity is increased by more than 3 times, while the resolution is 10 ng (√Hz)−1, which is 2.5 times better. It is believed that such a method can be applied to MEMS devices where interelectrode spacing needs to be precisely controlled.