Experimental investigation, modeling, and simulations for MEMS-based gas sensor used for monitoring process chambers in semiconductor manufacturing

There is a growing demand from the semiconductor industry for multi-component gas sensing for advanced process control applications. Microelectromechanical systems (MEMS) based integrated gas sensors present several advantages for this application such as ease of array fabrication, small size, and unique thermal manipulation capabilities. MEMS based gas sensors that are produced using a standard CMOS (Complimentary Metal Oxide Semiconductor) process have the additional advantages of being readily realized by commercial foundries and amenable to the inclusion of on-chip electronics. In order to speed the design and optimization of such integrated gas sensors, a commercial software package IntelliSuiteTM was used to model the coupled thermo-electro-mechanical responses of devices known as microhotplates. Models were built based on the GDSII formatted mask layout, process sequences, and layer thicknesses. During these simulations, key parameters such as device design and structure were investigated, as well as their effect on the resultant device temperature distribution and mechanical deflection. Detailed analyses were conducted to study the resonance modes for different sensor configurations, such as fixed-end and springboard arrangements. These analyses also included a study of the effect of absorbed material on device natural frequency. The modeling results from this study predict that the first three resonant frequency modes for these devices are in the 612 to 1530 kHz range for an all pinned device, and 134 to 676 kHz for a springboard arrangement. Furthermore, the modeling suggests that the resonant frequencies will decrease linearly as a function of increasing absorbed mass, as expected for a simple spring model. The change in resonant frequency due to mass absorption is higher for an all-pinned arrangement, compared to a springboard arrangement, with the second and third (twisting mode) showing the largest change. Thermo-electro-mechanical simulations were also performed for these devices, and the predicted mechanical deformations resulting from applied voltage compare favorably with experimental observations.