Vertically Integrated Double-Bridge Design for CMOS-MEMS Tri-Axial Piezo-Resistive Force Sensor

This study demonstrates a novel vertically integrated double-bridge design for tri-axial piezo-resistive force sensor using commercial available CMOS process (Fig. 1). The normal and shear force sensing capabilities are achieved by measuring the signal changes of three independent Wheatstone bridges embedded in two bridges (fixed-fixed beams). The proposed sensor design has three merits: (1) two orthogonal bridges enable the directional sensing for shear forces, (2) piezo-resistors and electrical routings embedded in two bridges are designed to form three independent Wheatstone bridges for tri-axial force sensing, (3) the footprint reduction of the sensing chip is achieved by the vertical integration of two bridges. The measurements demonstrate the feasibility of tri-axial force sensing, and indicate that the sensitivities of the proposed tri-axial force sensor are, respectively, normal force (Z-axis) -0.18 mV/N, shear force (X-axis) -7.46 mV/N, and shear force (Y-axis) 7.26 mV/N.