DEGRADATION MECHANISMS OF PASSIVATED AND UNPASSIVATED DIAMOND THERMISTORS

The purpose of this investigation was to ''amine the failure mechanisms of prototype thermistors as a function of temperature in air. A hot stage Raman microprobe was used to examined device degradation mechanisms at elevated temperatures by monitoring the diamond Raman signal and simultaneously measuring the resistance of the thermistor. An unpassivated device became non-conducting (resistance greater than 20 MOMEGA) at approximately 610-degrees-C in laboratory air. Although the diamond film remained intact under these conditions, the contacts were found to degrade via a combination of oxidation and carburization. Formation of an insulating titania phase caused the device to fail. A silica device passivation scheme was investigated for the protection of the contacts and diamond film from oxidation at elevated temperatures. Oxidation tests made on silica-coated diamond films showed a beneficial corrosion protection effect at 650-degrees-C in laboratory air. The passivated thermistor operated up to marginally higher temperatures of 670-degrees-C. At this temperature the passivation cracked and some spalling occurred. Post-failure analysis indicated that the contacts had oxidised and also that the insulating diamond had been etched. In contrast, the boron-doped resistive element of the thermistor was relatively unaffected, suggesting different susceptibilities to oxidation between the doped and insulating diamond.