Device Packaging 2019

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Suppressing Temperature Induced Voltage Offset Drifts in Silicon Carbide Pressure Sensors
Keywords: Voltage Offset Drift, Silicon Carbide , Pressure Sensors
We report the advancement towards the suppression of temperature induced drifts in the zero offset voltage in silicon carbide (SiC) piezoresistive pressure sensors when operating at high temperature. High temperature (> 600 C) pressure sensors are needed in closer proximity to the jet engine combustion chamber to provide real-time detection of the onset of thermoacoustic instabilities. If such instabilities are not mitigated, they could expand and potentially damage critical engine components. Silicon carbide pressure sensors were previously demonstrated to measure thermoacoustic instability during a combustor ground test, thus offering promise for its eventual permanent application. However, reliability challenges, particularly the observed zero pressure offset voltage runaway, have largely limited its application to only short duration engine ground tests. In this work, two batches of SiC pressure sensors having different metallization schemes were evaluated to determine the long term stability of the offset voltage at 600 C. The sensors were initially burned in for 24 hours at 600 C and cooled down, after which the bridge resistances were recorded and used as reference. The temperature was raised to 600 C and held over time while the bridge resistances were intermittently measured. Baring any failure, the goal was to soak the sensors for 1000 hours. Finally, the sensors were cooled down and the resistances measured and compared to the room temperature values after burn in. Compliance to within +/- 20 % qualified as a passing sensor. Only two sensors in Batch-1 fully met compliance. In the case of Batch-2 sensors, the voltage offset drift bands were seen to remain within +/- 5 mV while the bridge resistance elements were in compliance. The 100 % passing qualification of the Batch-2 sensors represents significant progress made in improving the reliability of SiC pressure sensors. Optimization of the contact metallization is in progress to further tighten the compliance specification. The results offer increasing promise in the application of these devices for longer term use at 600 C.
Robert Okojie, Research Electronics Engineer
NASA Glenn Research Center
Cleveland, OH

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