Comparison study of ohmic contacts to 4H-silicon carbide in oxidizing ambient for harsh environment gas sensor applications

We report the results of the effect of three different stacks of ohmic contacts such as TiW (180 nm)/Ti (30 nm)/Pt (300 nm), Ni (100 nm)/TaSix (200 nm)/Pt (400 nm), and TaSix (200 nm)/Pt (400 nm) on a highly doped n-type 4H-silicon carbide for high temperature measurements in oxidizing ambient. The ohmic contacts in the transmission line method (TLM) structures were cut to 2 x 2 mm(2) chips, and glued on heater, which has a built-in Pt-100 element, for the temperature measurement. Finally the contacts on the heater were mounted on a 16-pin socket, which was placed in an Al-block with a gas flow channel. For this measurement, we used 20% O-2 in N-2 flowing at a rate of 80 ml/min at temperatures of 500 degrees C and 600 degrees C. The long-term reliability test of each ohmic contact in an oxidizing environment up to 520 h showed that TiW/Ti/Pt had the best stability and the lowest contact resistivity, while other contacts (Ni/TaSix/Pt and TaSix/Pt) had a severe degradation of contacts due to the oxidation. Therefore, we believe that our TiW based metallization schemes can be applied in harsh environment such as the automobile and aerospace exhaust systems. (c) 2005 Elsevier Ltd. All rights reserved.