Monolithic Integration of Pressure Plus Acceleration Composite TPMS Sensors With a Single-Sided Micromachining Technology

This paper concerns the development of a single-side micromachined tire-pressure monitoring system (TPMS) sensor for automobiles, which is with a piezoresistive pressure sensor and a cantilever-mass piezoresistive accelerometer monolithically integrated in a 1.6 mm × 1.5 mm sized (111)-silicon chip. Single-wafer-based front-side silicon micromachining and metal electroplating technologies are employed to fabricate the device. Specially designed releasing trenches along (111) orientation are constructed to form the hexagonal pressure-sensitive diaphragm and the postsealed vacuum reference cavity. The fabrication of the accelerometer is also based on a hexagonal diaphragm that is latterly cut into suspended cantilevers and seismic mass. To achieve a high sensitivity, a high-density copper thick film is selectively electroplated to significantly increase the mass. The performance of the 115-g-ranged accelerometer is measured, exhibiting a sensitivity of 99.9 μV/g (under 3.3-V supply), nonlinearity of ±0.45% FS, and the noise floor of better than 0.2 g. The 750-kPa-ranged pressure-sensor sensitivity is measured as 0.108 mV/kPa (under 3.3-V supply), with the nonlinearity error smaller than ±0.1% FS and the temperature coefficient of sensitivity as -0.19%/°C FS before compensation. The noise floor of the pressure-sensor out- put signal is 0.15 kPa. The zero-point temperature coefficient is tested as -0.11%/°C FS and -0.024%/°C FS for the accelerometer and the pressure sensor, respectively. Fabricated with the low-cost front-side micromachining technique, the small-sized TPMS sensors are promising in practical applications and volume production.