On-Chip Integration of Acceleration, Pressure, and Temperature Composite Sensor With a Single-Sided Micromachining Technique

Abstract-This paper presents a novel composite sensor that is monolithically integrated with an accelerometer, a pressure sensor, and a thermometer by using a single-side-processed micromachining technique. Such composite sensors are highly demanded in tire pressure monitoring system (TPMS) applications. Based on heat convection, the accelerometer consists of a microheater and a pair of detector to measure the acceleration-induced temperature distribution change. Thermopile- and thermistor-based detectors both have been used to measure the temperature distribution change. Under a heating power of 10.9, 19.4, and 30.3 mW, the sensitivity of the thermopile-based accelerometer is measured as 37.92, 85.04, and 134.08 μV/g, respectively (while the thermistor based accelerometer shows a sensitivity of 25.96, 55.79, and 86.67 μV/g). The two detecting methods both feature enough frequency range that is higher than 100 Hz. In the pressure sensor, the low-stress silicon nitride diaphragm is designed into a narrow rectangular shape, on which four polysilicon piezoresistors are configured into a Wheatstone bridge. The sensitivity of the 450-KPa-ranged pressure sensor is measured as 45.9 mV/3 V FS, the nonlinearity is ±0.84% FS, and the over-range capability is ten times the measured range. The tire temperature monitoring thermometer is also made up of polysilicon and is measured with a linear resistance change versus temperature over the range of -40 °C-+100 °C. In the on-chip integrated sensors, a low-stress silicon nitride thin film is used as the electric isolation layer, the pressure diaphragm, and the heating-insulating freestanding beams. The boron-doped polysilicon film is used as the piezoresis tors, the heater, the thermopile, and the thermometer. After wafer level packaging with an aligned adhesive bonding technique, the 2.5 × 2.5 × 0.84 mm
³
microsensor chips are promising for TPMS low-cost fabrication and volume applications.