Stress Effects in Semiconducting Metal Oxide (SMOx) Materials on MEMS Gas Sensors

All packaged MEMS sensors experience strain arising from CTE (Coefficient of Thermal Expansion) mismatches and from assembly in the final product when deployed in applications. The effect of these stresses on MEMS based SMOx gas sensor performance has not been studied before and can be a significant source of error in gas sensing applications. In this paper, we have studied the stress effect in MOx by - (a) quantifying strain seen in a packaged MOx gas sensor over typical operating temperature conditions and (b) measuring MOx materials under strain to quantify the effect. We have simulated the effect of thermomechanical stresses on a MEMS gas sensor arising from CTE mismatches in the package and MEMS layers using a well-known finite element software Comsol (R). While studying the effect of stress/strain on a typical SMOx material (SnO2) using a four-point flexure test over a temperature range of 200-300 degrees C, we observed a dependency of the relative change in resistance to the particle size of the MOx with applied strain. From these results we see that 1 degrees C change in ambient temperature, the equivalent amount of ethanol response due to stress can range from 0.026 ppm to 0.3 ppm for material like SnO2.