Analysis of spurious effects in membrane-based micromachined thermal conductivity sensors

Utilizing the distinctive features associated with microtechnology such as the small thermal masses of micromachined membranes, highly sensitive thermal sensors can be realized. Of particular interest are thermal conductivity sensors, which can, e.g., be used in liquid monitoring applications. In this contribution, the optimized design of membrane-based thermal conductivity sensors for liquids is investigated. The device considered features a heater structure and a spatially separated thermistor located on a silicon nitride membrane. Due to the unavoidable heat conduction in the membrane an unwanted spurious heat path is introduced. Depending on the thermal conductivity of the membrane material and the membrane thickness, this effect can reduce the performance of the sensor and needs therefore to be investigated. For this purpose a 2D finite element model has been introduced. The simulation results are compared to measurements that have been obtained from a device prototype