Stress Considerations in Thin Films for CMOS-Integrated Gas Sensors

The integration of gas sensing elements into hand-held electronics will provide individuals the ability to detect harmful chemicals and pollutants in the environment in real time. Metal oxide gas sensors rely on changes in their electrical conductance due to the interaction of the oxide with a surrounding gas at an elevated temperature. The intrinsic stress in the metal oxide films during the low-temperature, high-pressure, and low oxygen content sputter deposition of tin oxide (SnO2) and indium-tin-oxide (ITO) is examined in this work. The surface free energy for the two films is found to be 1.69J/m and 1.85J/m, respectively, and the intrinsic stress during the early stages of film growth is plotted. The spray pyrolysis deposition technique is implemented to grow a tin oxide film at 400oC, which is able to detect the presence of multiple gases in the environment. Deposition at this temperature leads to a thermo-mechanical stress of 380MPa.