Ultra-high temperature (≫ 300°C) suspended thermodiode in SOI CMOS technology

This paper reports for the first time on the performance and long term continuous operation of a suspended silicon on insulator (SOI) thermodiode with tungsten metallisation at temperatures beyond 300degC. The thermodiode has been designed and fabricated with minute saturation currents (due to both small size and the use of SOI technology) to allow an ultra-high temperature range and minimal nonlinearity. It was found that the thermodiode forward voltage drop vs temperature plot remains linear upto 500degC, with a non-linearity error of less than 7%. Extensive experimental results on performance of the thermodiode, fabricated using a CMOS (complimentary metal oxide semiconductor) SOI process have been presented. These results are backed up by infra red measurements and a range of 2D and 3D simulations using ANSYS and ISE software. The on-chip electronics for thermodiode and micro-heater drive, as well as the transducing circuit for the sensor were placed adjacent to the membrane. Moreover, we demonstrate that the thermodiode is considerably more reliable in long-term direct current operation at high temperatures when compared to the more classical resistive temperature detectors (RTDs) using CMOS metallisation layers (Tungsten or Aluminum). Finally, we believe that the thermodiode suffers less of piezojunction/piezo-resistive effects when compared to silicon based RTDs. For this we compare a membrane thermodiode with a reference thermodiode placed on the silicon substrate and assess their relative performance at elevated temperatures.