Single-Side Fabrication of Multilevel 3-D Microstructures for Monolithic Dual Sensors

Monolithic integration of micromechanical composite sensors needs to fabricate multiple levels of three-dimensional (3-D) microstructures to satisfy individual requirements from individual on-chip sensing elements. Meanwhile, volume fabrication of the composite sensors is needed in many applications that prefer single-sided process in low-cost non-silicon-on-insulator single wafer. A novel single-sided micro-machining technique is herein proposed and developed to form such multilevel 3-D structures, where only integrated-circuit (IC) foundry available processes are used, i.e., neither double-sided process nor wafer-bonding is used. With the IC-foundry compatible micromachining process, a six-level 3-D microstructure has been successfully formed for tire-pressure monitoring system (TPMS) sensors. Benefited from the single-side process and the namely pressure-sensor in accelerometer (PinG) dual-sensor architecture, the single-wafer-based dual-sensor features a tiny chip size of 1.25 mm x 1.25 mm x 0.45 mm. Supplied with 3.3 V, 0.1-mV/kPa sensitivity for the 500-kPa-ranged pressure sensor and 0.05-mV/g sensitivity for the 120-g-ranged accelerometer are measured. By freely suspending the pressure-sensor structure from the stress-free mass end, the influence of acceleration to the pressure sensor is well eliminated, which was the main problem of the previous PinG sensors. Besides the achieved high-performance TPMS dual sensor, the IC-foundry manufacturable technique for multilevel 3-D microelectromechanical systems (MEMS) structures can be widely used in various monolithic MEMS devices.