Sensitivity enhancement of a silicon micro-machined thermal flow sensor

A micro-machined thermal flow sensor to measure a minute amount of gas flow rate is developed, and tested to quantify the flow measuring characteristics over the variation of flow rate. A bifilar-Evanohm-R alloy heater and chromel-constantan thermopiles were formed on a sandwich membrane, Si3N4/SiO2/Si3N4, which consist a core part of the sensor. In performance test of the sensor, range of gas flow rate was regulated from 0.2 to 1.0 slm and the heater power was variated from 0.72 to 5.63 mW. In the present experiment, the Reynolds number based on the diameter of a flow passage was ranged from 81 to 408. It is confirmed from the experiment that the sensitivity (S = ∂ΔV/∂Q; ΔV, voltage difference between thermopiles; Q, flow rate) of the sensor is enhanced with increasing the heater power and decreasing the measuring flow rate range. Also, the sensor response time required to a stable state is reduced with increasing the flow rate range.