3D nonlinear modeling of microhotplates in CMOS technology for use as metal-oxide-based gas sensors

A modeling approach has been developed to support CMOS microhotplate optimization and to allow for sensor system simulations. All steps are detailed that are necessary to arrive at a geometric hotplate representation for nonlinear 3D-FEM simulations starting from a physical microhotplate layout. A lumped-model description of the microhotplate is discussed, which forms the basis for combined simulations of sensor and circuitry. FEM simulations were performed for two different microhotplate designs. Both types of microhotplates were fabricated in a 0.8 mum industrial CMOS process by post-CMOS micromachining. The first design includes a circular microhotplate with a Si-island underneath-the heated area of the microhotplate. The characteristic figures such as thermal resistance and thermal time constant were extracted from the model, which showed an agreement within 5-10% with experimental values for temperatures up to 325 degreesC. The second design without Si-island featured an array of temperature sensors for assessment of the temperature distribution. Experimental data from the different sensor locations were compared to simulation results, and, again, showed excellent agreement with a maximum deviation of 5%. The influence of the nanocrystalline tin oxide thick-film layer on the temperature distribution was also experimentally investigated: better temperature homogeneity in the heated area and somewhat slower temperature dynamics.