Papers by Author: Laura J. Evans

Abstract: The fabrication and prolonged 500 °C electrical testing of 4H-SiC junction field effect transistor (JFET) integrated circuits (ICs) with two levels of metal interconnect is reported in another submission to this conference proceedings. While some circuits functioned more than 3000 hours at 500 °C, the majority of packaged ICs from this wafer electrically failed after less than 200 hours of operation in the same test conditions. This work examines the root physical degradation and failure mechanisms believed responsible for observed large discrepancies in 500 °C operating time. Evidence is presented for four distinct issues that significantly impacted 500 °C IC operational yield and lifetime for this wafer.

Abstract: We report on the initial demonstration of a tungsten-nickel (75:25 at. %) ohmic contact to silicon carbide (SiC) that performed for up to fifteen hours of heat treatment in argon at 1000 °C. The transfer length method (TLM) test structure was used to evaluate the contacts. Samples showed consistent ohmic behavior with specific contact resistance values averaging 5 x 10-4 Ω-cm². The development of this contact metallization should allow silicon carbide devices to operate more reliably at the present maximum operating temperature of 600 °C while potentially extending operations to 1000 °C.

Abstract: This paper updates the long-term 500 °C electrical testing results from 6H-SiC junction field effect transistors (JFETs) and small integrated circuits that were introduced at ICSCRM-2007. Two packaged JFETs have now been operated in excess of 7000 hours at 500 °C with less than 10% degradation in linear I-V characteristics. Several simple digital and analog demonstration integrated circuits successfully operated for 2000-6500 hours at 500 °C before failure.

Abstract: This paper reports on initial results from the first device tested of a “second generation” Pt-SiC Schottky diode hydrogen gas sensor that: 1) resides on the top of atomically flat 4H-SiC webbed cantilevers, 2) has integrated heater resistor, and 3) is bonded and packaged. With proper selection of heater resistor and sensor diode biases, rapid detection of H2 down to concentrations of 20 ppm was achieved. A stable sensor current gain of 125 ± 11 standard deviation was demonstrated during 250 hours of cyclic test exposures to 0.5% H2 and N2/air.

Abstract: This paper reports on the fabrication and testing of 6H-SiC junction field effect transistors (JFETs) and a simple differential amplifier integrated circuit that have demonstrated 2000 hours of electrical operation at 500 °C without degradation. The high-temperature ohmic contacts, dielectric passivation, and packaging technology that enabled such 500 °C durability are briefly described. Key JFET parameters of threshold voltage, on-state resistance, transconductance, and on-state current, as well as the gain of the differential amplifier integrated circuit, exhibited less than 7% change over the first 2000 hours of 500 °C operational testing.

Abstract: High aspect ratio silicon carbide (SiC) microstructures are needed for microengines and other harsh environment micro-electro-mechanical systems (MEMS). Previously, deep reactive ion etching (DRIE) of low aspect ratio (AR ≤1) deep (>100 *m) trenches in SiC has been reported. However, existing DRIE processes for SiC are not well-suited for definition of high aspect ratio features because such simple etch-only processes provide insufficient control over sidewall roughness and slope. Therefore, we have investigated the use of a time-multiplexed etch-passivate (TMEP) process, which alternates etching with polymer passivation of the etch sidewalls. An optimized TMEP process was used to etch high aspect ratio (AR up to 13) deep (>100 *m) trenches in 6H-SiC. Power MEMS structures (micro turbine blades) in 6H-SiC were also fabricated.