Integrated ultra-thin-film gas sensors

Abstract

The fabrication and performance of an ultra-thin-film integrated gas sensor for detecting impurities in semiconductor process gases are described. Detector reponses are based on gas-induced resistance changes in an ultra-thin Ti-P film mounted on a thin dielectric window supported by a silicon rim. The window temperature can be shifted several hundred degrees Celsius in less than one second. Gas-induced resistance changes are composed of thermal and Chemical components. The gas concentration and its thermal conductivity determine the thermal contribution, whereas the chemical interactions between the gas and the sensor surface create a superimposed change of resistance, which can be observed when the detector is operated at constant temperature. Pretreament of the as-deposited Ti-Pt thin films under controlled environmental conditions allows tailoring of the electrical, sensing, and microstructural characteristics of the films. One such pretreatment scheme allows the creation of films that, when operated at 150 [deg]C, respond chemically to O2 from 1000 torr down to the 10-7 torr level without any evidence of hysteresis or long-term drift over hundreds of cycles of temperature and gas. There is no chemical response to CF4, which allows detection of sub-ppm levels of O2 in CF4. Temperature-programmed desorption (TPD) of O2 is shown to be a control requirement for dynamic sensing.