Silicon Carbide Field-Effect Transistor (FET) Transducers for Harsh Environment Applications

The emerging field of harsh environment semiconductor devices has a high potential to improve efficiency and safety of combustion processes significantly. Above all, robust sensor and electronic devices enable system and exhaust gas monitoring, as well as combustion control in real-time. For these applications the exposure of the devices to the harsh environment is often unavoidable. Especially for pressure and exhaust gas sensing applications, the direct contact with the harsh environment is required for sensor operation. This imposes completely different requirements on the material and the device reliability compared to silicon microsystem technology standards. In particular, resistance against temperatures as high as 600 °C and highly corrosive air/moisture environments is required. Furthermore, thermal cycling is also a major concern. Silicon carbide (SiC) is a promising semiconductor material for harsh environment sensors and electronics due to its outstanding properties and the high level of maturity of the related process technology. Despite the excellent properties of the SiC substrate material, the lifetime of SiC devices in harsh environments is above all limited by the stability of the ohmic contacts and the gate dielectrics. This work aimed at the development of novel materials and processes for the realization of SiC field-effect transistor (FET) transducers capable of operating in exhaust gas environments at temperatures up to 500 °C. Highly-stable Pt-based metallizations, robust amorphous silicon carbide protective coatings, and highly reliable Ti ohmic contacts as well as oxide-nitride-oxide gate dielectrics were established. Extended environmental testing methods were established to assess the reliability of the device components, specifically addressing the requirement profile of harsh environment applications. These methods included thermal aging in different gas environments, thermal cycling, autoclave testing, storage in exhaust gas concentrate, and biasstress experiments. The single device components were initially optimized and tested by means of appropriate test structures processes. One of the key results was that the use of a stable protective coating is highly beneficial especially for the stability of the ohmic contacts. This allowed to demonstrate for the first time the long-term stability of a contact metallization to SiC in air/moisture environment at 600 °C. The optimized SiC-MISFET components were successfully integrated into a 'gate-first' SiC metal-insulator-semiconductor field-effect transistor (SiC-MISFET) fabrication process. The reliability of SiC-MISFET devices with encapsulated Pt gate metallizations was investigated. The devices showed excellent ruggedness in all environmental testing experiments. Outstanding stability was found also during 1000 h electrical operation at 500 °C. The absolute variation of the MISFET parameters after a first burn-in at 500 °C was less than 10 %. The origin of the drift of the MISFET parameters was identified by means of TEM analysis as the diffusion of Pt from the gate metallization into the gate toplayer of the gate dielectric stack. The operation of SiC-MISFET devices beyond 700 °C in air was shown for the first time, confirming the reliability enhancement allowed by the use of the advanced MISFET components developed in this work. A proof-of-principle for the application of SiC-MISFET transducers as gas sensors was finally given. The sensitivity of SiC-MISFET devices functionalized with a nanoporous Pt gate electrode was proved for NO, NO2 and NH3 down to a concentration of 2 ppm. Nearly-logarithmic sensor response and response times below 500 ms were found for all test gases.