A microbridge heater for low power gas sensing based on the 3-Omega technique

Abstract

The 3-Omega measurement technique was applied to a microbridge heater for the purpose of low power gas sensing. The sensor performance was evaluated for mixtures of CO 2 , Ar, He and CH 4 in N 2 in an isothermal chamber where concentrations were precisely controlled by mass flow controllers. A custom 3-Omega conditioning circuit controls the AC heating current and detection of the 3-Omega voltage signal. The amplitude and phase lag, and the in-phase and outof-phase components of the 3-Omega signal are presented for each case and are related directly to gas concentrations. Advantageously, the phase lag can determine the gas concentration without calibration of an individual gas sensor. Using this technique and our current microbridge thermal conductivity detector (TCD), it is possible to resolve gas concentrations down to 0.07%. The frequency behavior of the 3-Omega signal as it pertains to the thermal environment and sensor sensitivity is discussed. The 3-Omega technique combined with the microbridge is capable of operating with power consumptions that are < 10 % of the power consumed by conventional Wheatstone bridge measurements at room temperature.

Metal-coated glass microfiber for concentration detection in gas mixtures using the 3-Omega excitation method

“…The phase lag, however is most sensitive to gas concentration at frequencies near 600 Hz, which is the characteristic frequency of the sensor-gas system and also exhibits a maximum at this frequency. As discussed in our previous work [7], this characteristic frequency is related to the geometry and the thermophysical properties of the sensor-gas system. The phase lag describes the thermal response of the system and thus, is dictated by the specific heat of the gas mixture.…”

“…The 3-Omega technique has been extensively used in thermal conductivity measurements [9] and is well described in literature [10], but it also offers an approach to low-power gas sensing that we have recently investigated. [7] In a standard 3-Omega measurement, a sinusoidal electrical current of amplitude I ω at frequency ω=2πf is driven through a metal heater line, resulting in a Joule heating at frequency 2ω. The periodic heating creates a thermal wave that penetrates the surrounding medium.…”

“…In our previous study on 3-Omega gas sensing, a polysilicon microbridge suspended on a silicon dioxide sacrificial layer was used which could readily resolve gas concentrations of 1,000 ppm in binary mixtures [7]. Since a significant portion of the heat was conducted within the microbridge and dissipated to the solid substrate, the sensitivity to the gas medium was limited.…”

“…The sensor was placed in an isothermal chamber whose temperature was uniformly increased from 20 ˚C to 50 ˚C in 5 ˚C increments and the resistance's temperature dependence dR/dT was measured to be 0.1155 ± 0.008 ohm/˚C (see Supplemental Information for details). An experimental set-up similar to the one described in our previous work was used [7]. An input current amplitude of 12 mA was used and frequencies were swept between 0.5 and 2000 Hz.…”

“…However some of these TCDs operate at high temperatures [6], which necessitate routine re-calibration and may be destructive to the detected sample. More recently, we have demonstrated the ability to perform low power gas sensing using the 3-Omega technique on a polysilicon microbridge geometry while still maintaining good sensitivity [7]. However, in the microbridge design a significant portion of the heat was conducted along the microbridge to the substrate, which reduced the heat input to the surrounding gas medium, thereby limiting the sensitivity of 3-Omega technique to 1,000 ppm.…”