Fabrication, process optimization and electrical characterization of thermopile based CMOS-MEMS flow sensors in 180 nm CMOS technology

This paper presents details of fabrication process, its optimization and electrical characterization activities carried out for the development of thermal mass flow sensor. Realized sensor is thermopile based and fabricated in SCL’s 180 nm baseline CMOS process. The fabrication process flow features 13 CMOS masking steps followed by a DRIE-MEMS process. The developed process introduced no additional process layer or thermal cycle in the baseline thus preserving the thermal budget of CMOS process for simultaneous fabrication of CMOS ASICs. Major sensor elements like thermopiles, heater and RTDs are made up of doped polysilicon (gate poly-Si) thin film. Ion-implantation process (dose and energy) was optimized to meet specific design targets. Effect of ion-implantation dose on electrical properties of poly-Si including Seebeck coefficient was analysed. Seebeck coefficient was measured using planar CMOS and micromachined test structures. Calculated values of Seebeck coefficient of n-poly/Al found in the range of 90 to 165 µV/K (absolute values) at different implant dose. Post-fabrication, electrical test (ET) data of process monitoring test structures shows good process control. Initial characterization of the sensor with gas flow test conditions verified the functionality of the sensor. Behaviour of the developed sensor was measured upto flow of 100 sccm of with output voltage span of 25 mV at constant power of 10 mW supplied to heater.