Surface-micromachined thermal conductivity detectors for gas sensing

This paper presents the design, analysis, fabrication and a special approach for readout of surface-micromachined thermal conductivity (TCD) gas sensors. IC-compatible surface-micromachining MEMS technology has been applied for the fabrication of high-sensitivity thermal sensors. The devices have been fabricated on thin SiN bridges with thermoelectric detectors as temperature sensors and poly-silicon resistors as heaters. A general thermal analysis of planar MEMS structures is included for calculating both the static and dynamic response of micro-TCD's. The analysis demonstrates that the performance of surface-micromachined devices can be superior to that of the more conventional bulk-micromachined devices. Methods for DC and AC readout of the device are discussed and have been implemented. The sensitivity for hydrogen in air is about 60μV per % H₂ at 1mW heater dissipation. Measurements on pure gases and gas mixtures have indicated a high sensitivity of 4.4 μV per Wm^-1K^-1 and a typical stability of 0.05 μV when using AC modulation.