A high-temperature thermopile fabrication process for thermal flow sensors

A new high-temperature fabrication process for thermopile-based flow sensors is presented. The high-temperature passivation of LPCVD silicon nitride leads to an improvement for liquid applications because of the low tendency towards pinholes and the good step coverage. The thermopiles are made of p-doped polysilicon and titanium–tungsten (WTi) showing a thermopower of 287 μV K−1 for a single thermocouple. Besides, the nitride membrane was released by a DRIE process to achieve a reduced chip size and higher yield compared to established thermal flow sensors. Devices for different flow rates were fabricated and characterised. A very short reaction time of 2.6 ms could be achieved and the sensors show good sensitivity of 9.5 mV mm−1 s in agreement to the theoretical expectations based on an analytical model.