Fabrication, Characterization, and Thermal Failure Analysis of a Micro Hot Plate Chemical Sensor Substrate

This paper describes the fabrication and characterization of a micromachined, micro hot plate (MHP) sensor substrate for chemical sensor applications. The MHPs were fabricated using a silicon-processing foundry followed by a novel combination of laser and chemical etching of individual die. An IR microscope was used for thermal imaging of the MHP and shows that the device is very efficient (11.5°C/mW) and has a fast response time (30 ms). The performance of the MHP is compared to thermal measurements on a commercial sensor substrate; the MHP consumes almost 30 times less power. The MHPs also have very good performance as compared to other similar micromachined devices described in the literature. From efficiency measurements of devices in air and in vacuum, heat loss by conduction in air is determined to be ∼30% larger than conduction through the membrane. The measured thermal efficiency in vacuum and in air is consistent with expectations based on simple analytical models, the thermal conductivities of the membrane and air, and the MHP geometry. Data are presented on the high-temperature, thermal failure mode of the devices using scanning electron microscopy and energy dispersive X-ray analysis. At temperatures close to the Si-Au eutectic point, we find that the gold metallization fails due to interaction with underlying polysilicon layer.