Chip-Level Warp Control of SOI 3-Axis Accelerometer with the Zigzag-Shaped Z-Electrode☆

Abstract We developed a fully-differential 3-axis accelerometer with a Zigzag-Shaped Z-electrode (ZSZ) for motion controls of automobiles and robots. The ZSZ structure is composed of only two silicon layers of an SOI (Silicon-on-Insulator) wafer. The ZSZ structure consists of two kinds of capacitors in a Z-axis direction. The gaps of the capacitors were designed to be same and the sensor detected a capacitance change precisely. However, small gap differences at the capacitors in the Z-axis direction were appeared and it caused unequal sensitivities. This was because the fabricated sensor chip was warped. To overcome this drawback, we have proposed a novel method for chip-level warp control. The warp control is performed by forming a SiO2 frame on a backside of the chip. The frame works as a stress compensator of thermal expansion. The chip-level warp control demonstrated that gaps of the ZSZ were controlled to be equal and the sensor characteristics were also improved. The chip-level warp control is useful for sensors and actuators, which are sensitive to deformation in the Z-direction.