Front-Side Microfabricated Tiny-Size Thermopile-Based Gas Flow Sensor With Integrated Pressure–Temperature Sensing on a Non-SOI (111) Silicon Wafer

In this brief, a monolithic gas mass flow, pressure, and temperature multifunctional composite sensor, fabricated on a non-SOI (111) silicon using a single-sided bulk micromachining technique, is first proposed and developed. Unlike traditional counterparts, the gas flow sensor utilizes a single-crystal silicon thermopile, which offers a higher Seebeck coefficient and lower noise compared to traditional polysilicon, thereby achieving higher sensitivity within a more compact footprint. For ambient temperature detection and compensation, a platinum thin-film temperature sensor is positioned adjacent to the flow sensor and is driven by a constant-temperature-difference circuit. In addition, an absolute pressure sensor with an embedded reference cavity in the silicon substrate is integrated on the opposite side of the flow sensor. The entire fabrication process is conducted solely from the front side, eliminating the need for double-sided alignment exposure, cavity-SOI, wafer bonding, and double-sided-polished wafer. This simplified approach results in a tiny chip size of $0.9\times 0.9$ mm, enabling low-cost, high-yield batch production in standard IC foundries. The experimental results show that the 200-sccm-ranged gas flow sensor achieves a high normalized sensitivity of 0.56 mV/sccm/W without any amplification, and a response time of 3.8 ms. The 700-kPa-ranged pressure sensor exhibits a sensitivity of 0.10 mV/kPa with a nonlinearity of 0.06% FS. The temperature sensor shows a highly linear response over the temperature range of $- 30~^{\circ }$ C to $60~^{\circ }$ C, with a temperature coefficient of resistance (TCR) of 0.0023/°C.