Effects of gas type on the sensitivity and transition pressure of integrated thermal flow sensors

The effects caused on the response of integrated thermal flow sensors by operating pressure and gas type changes are investigated. The sensors are conventional differential temperature calorimeters fabricated by applying two post-processing steps to standard microelectronic chips. Differences in terms of sensitivity and linearity have been observed for Xe, Ar and N2 in the flow rate interval 0-200 sccm. The progressive sensitivity reduction caused by decreasing the pressure from 100 to 12 kPa is ascribed to the onset of the transition to free molecular heat exchange regime. Estimation of the transition pressure for the three gases under test is presented.