Tough negative temperature coefficient diamond thermistors comprising tungsten carbide ohmic contacts

This work presents a novel methodology for the fabrication of planar diamond film NTC (Negative Temperature Coefficient) thermistors on sintered silicon nitride (Si3N4) ceramic substrates. Such devices comprise a temperature sensitive diamond coating on one of the substrate surfaces and tungsten carbide ohmic contacts on the opposite side. Hot filament chemical vapor deposition (HFCVD) technique was used for the fabrication of both temperature sensitive flat diamond surface and WC ohmic contacts. The doping source for the semiconducting diamond layers was boron oxide dispersed in ethanol, and was added to the hydrogen/methane gas mixture using argon gas. The fabrication of WC contacts was done by first evaporating the oxide layer of the tungsten filaments used for activating the gaseous species. Hydrogenation and carburization steps were then used to produce the carbide layer. Using this approach, NTC diamond thermistors were obtained and their electrical resistance dependence with temperature was plotted and fitted to the Steinhart-Hart equation, giving beta values between 1200 and 2000 K, from room temperature up to 420 degrees C. The WC-diamond contacts ohmic character was confirmed within the same temperature range, which highlights the potential application of such devices in precise in situ temperature measurements, at high temperature and under mechanically tough environment conditions.