Single (111)-Wafer Single-Side Microfabrication of Suspended P(+)si/n(+)si Thermopile for Tiny-Size and High-Sensitivity Thermal Gas Flow Sensors

This article presents a tiny-size and high-performance thermal gas flow sensor with suspended $\text{p}^{+}$ Si/ $\text{n}^{+}$ Si thermopile formed in an ordinary non-silicon-on-insulator (SOI) (111) silicon wafer. A front-side bulk micromachining technique is herein employed to form the proposed gas flow sensor without double-side fabrication process, wafer bonding, and cavity-SOI needed. To achieve high sensitivity and quick response time, the suspended single-crystalline $\text{p}^{+}$ Si/ $\text{n}^{+}$ Si thermocouple which shows significantly higher Seebeck coefficient and lower noise compared to that of the traditional $\text{p}^{+}$ polysilicon/ $\text{n}^{+}$ polysilicon thermocouple is first used to construct the flow sensor. And the fishbone-shaped suspended SiN dielectric membrane is designed to maximize the thermal resistance between the $\text{p}^{+}$ Si heater and the silicon substrate, reducing heat dissipation from the $\text{p}^{+}$ Si heater to the silicon substrate. In addition, using high heat-conductivity Au-film to connect the cold junction and the bulk silicon acting as a heat-sink not only increases the temperature difference between the hot junction and the cold junction but also reduces the complexity of the fabrication process. Thanks to the single-side micromachining process, the sensor chip size is as small as $0.65\times0.60$ mm. Finally, the characterization of the fabricated gas flow sensor was evaluated, showing an ultrahigh normalization sensitivity of 5 V/sccm/W, a quick response time of 1.44 ms, and a minimum detectable flow velocity (MDFV) of 0.0083 sccm.