Single-Side Fabricated p + Si/Al Thermopile-Based Gas Flow Sensor for IC-Foundry-Compatible, High-Yield, and Low-Cost Volume Manufacturing

In this brief, a low-cost and high-yield bulk under thin-film (BUT) MEMS technique for the volume production of flow sensors is proposed and developed. The IC-foundry-compatible process is conducted only from the front side of (111) silicon wafers, without double-sided alignment exposure, wafer bonding, cavity silicon on insulator, and double-sided polished wafers needed. With the single-wafer-based BUT structure, the thermopile consisting of 21 pairs of single-crystalline Si/Al thermocouples that show significantly higher Seebeck coefficient and lower noise compared to the traditional poly-Si/Al thermocouples is employed to construct the gas flow sensor, and the fabricated sensor chip size is as small as ${0.65}\,\,\text {mm} \times {0.65}$ mm that facilitates low-cost high-throughput IC-foundry batch fabrication. The fabricated sensor shows a high normalized sensitivity of $199~\mu \text{V}$ /(SLM)/mW for nitrogen gas flow, which is 1 order of magnitude higher than that of the reported thermopile-based gas flow sensor, and a short response time of 3.5 ms could be achieved. With the new technique, the proposed sensors are promising in automotive and process control system applications.