Pressure sensing method based on the transient response of a thermally actuated micro-wire

This paper presents a novel kind of measurement method for MEMS pressure sensor based on the well known approach of the thermally actuated micro-wire. The originality of this work relies on the monitoring of the time-based transient response, particularly the step response of the heated wire - this is not a Pirani gauge. The sensor is investigated in the 1-11 bar pressure range as well as in the 20-80 °C ambient temperature range. The sensing part is a 1 mm long, 4 kΩ resistive micro-wire made of polysilicon and coated with silicon nitride. Its fabrication relies on a fully standardized industrial process, followed by a frontside anisotropic wet etching to release the micro-wire from the silicon substrate. The measurement relies on the time needed to reach a specified voltage and uses only a 1-bit ADC (i.e. a single voltage comparator), and a digital clock. Thus, in case of a readout circuitry monolithically integrated with the sensor, this measurement method will be very effective in terms of extra-power consumption, silicon area and design time. Performance were measured in nitrogen and gave: a sensitivity of approximately 100 ns/mbar, at atmospheric pressure a precision of 0.2% and at 11 bar a precision of 0.05%, a power consumption of approximately 1.5 mW with 600 μA, and showed highly linear characteristics with low drift with respect to ambient temperature. Those results are very encouraging given the proof feature of this work.