CMOS-MEMS capacitive tactile sensor with vertically integrated sensing electrode array for sensitivity enhancement

Abstract

This study presents the design, fabrication, and testing of a novel capacitive type CMOS-MEMS tactile sensor with vertically integrated sensing structures for enhancing sensitivity, and discretized sensing array design for inhibiting residual stress warpage. The proposed type tactile sensor utilizes polymer buffer above the sensor and polymer fill-in between the vertically integrated sensing structures as force transmission layers. This enables effective force loading onto the sensor, and more importantly, simultaneous deformation of the two vertically integrated sensing structures during force loading, thereby guaranteeing an increase in sensitivity. The enhancement in sensitivity of the proposed tactile sensor was demonstrated to be achieved under the same footprint area. To mitigate warpage issues that may occur due to thin film residual stresses, the sensor was discretized into arrays of separate sensing units. Sensing units are electrically connected in a parallel fashion to ensure maximum capacitance signal. As compared to a reference tactile sensor with conventional sensing electrodes, the proposed design showed a near 1.3-fold enhancement in sensitivity. Measurements also indicate the 5 × 5 array design could tolerate the influence of residual stress. Additionally, effect of operating temperature on initial capacitance (temperature coefficient of offset, TCO) and capacitance change signal during force loadings (temperature coefficient of sensitivity, TCS) were investigated for both the reference and proposed type tactile sensors.