Mathematical model and design of a CMOS-MEMS infrared thermopile

Abstract

The development of a non-invasive technique to measure blood glucose concentration has led to numerous research efforts. Pulse Glucometry, a novel technique based on differential near infrared radiation absorption spectroscopy, has shown promising results in achieving this goal. However, in order for this technique to have a direct impact in the diabetic population, it must be implemented in a portable device. Because of this, the development of an infrared micro sensor, among other devices, is needed. Thermopiles have several advantages for working as infrared sensors and have already been used for applications in environmental monitoring and biomedical diagnostics. When designing thermopiles, it is of great importance to acknowledge the changes in the output voltage that the varying of each design variable may generate. This thesis has developed an accurate mathematical model for a thermopile with a bridge or cantilever structure, with a maximum error of 4.69% when compared with a finite element analysis simulation. The parameters that govern the behavior of the sensor were classified as fabrication process, material and design dependent parameters. Using the proposed analytical model, several analyses were made in order to find parameter-specific design optimization rules. The most relevant results were found for the design dependent parameters, the ones with the most ease of handling. It was shown for a previously fabricated thermopile, that by applying the proposed design optimization rules, the sensitivity would increase by a factor of 28. It was also shown that a thermopile with an area 147 times smaller could be fabricated without any loss of sensitivity. This thesis also presents a viable step by step fabrication process for such device. The process is divided in two stages a fabrications process compatible with the ones used in the integrated circuits industry and a post-processing stage for depositing materials not used in the first process and releasing the structure. Future work concerning this thesis is mainly focused in the integration of this device with a diffraction system and an infrared radiation source.