Tungsten Carbide as a Diffusion Barrier on Silicon Nitride Active- Metal-Brazed Substrates for Silicon Carbide Power Devices

Recently, silicon nitride (Si3N4) has been receiving renewed attention because of its potential use as a substrate material for packaging of silicon carbide (SiC) power devices for high temperature applications. It is an attractive material for this application because it has moderate thermal conductivity and a low coefficient of thermal expansion, which is close to that of SiC. Materials that show promise for use as a diffusion barrier on Si3N4 substrate for bonding SiC devices to a Si3N4 substrate are refractory metals such as titanium (Ti), molybdenum (Mo), tungsten (W), and their alloys. Tungsten carbide (WC) shows promise as a diffusion barrier for bonding these devices to copper metallization on Si3N4 substrates. This paper presents the results of an investigation of a metallization stack (Si3N4/Cu/WC/Ti/Pt/Ti/Au) used to bond SiC dice to Si3N4 substrates. The dice were bonded using transient liquid phase bonding. Samples were characterized using X-ray diffraction for phase identification and Auger electron spectroscopy for depth profiling of the elemental composition of the metallization stack in the as-deposited state, and immediately following annealing. The metallization remained stable following subjection to a temperature of 400°C for 100 h in air.