STABILITY OF W AS ELECTRICAL CONTACT ON 6H-SIC - PHASE-RELATIONS AND INTERFACE REACTIONS IN THE TERNARY-SYSTEM W-SI-C

In this study the system W-Si-C was investigated under two aspects, me tallurgical and electrical, in order to understand the formation and t he properties of W electrical contacts on 6H-SiC. We combined two diff erent approaches. For the examination of the phase relations in the te rnary system we prepared bulk diffusion couples of W and monocrystalli ne SiC which were annealed and investigated using an SEM (secondary el ectron images, backscattered electron images. energy dispersive X-ray analysis). Secondly are furnace molten powder samples, annealed at dif ferent temperatures, were analysed by X-ray diffraction. To investigat e the electrical properties of a W/SiC junction transmission line cont act patterns were sputter deposited onto wafer strips. These samples w ere subjected to similar heal treatments and the current,voltage chara cteristics were measured with a source measure unit. Individual contac t resistivities could be evaluated using a special contact geometry. A s a result we discovered a four-phase equilibrium in the W-Si-C system at 1400 +/- 100 degrees C: 3W(5)Si(3) + 7SiCr reversible arrow 8WSi(2 ) + 7WC. This is in qualitative agreement with thermodynamic calculati ons. At 1300 degrees C the equilibrium WSi2 + WC exists. At 1000 degre es C the reaction kinetics are too slow to be detected in a bulk sampl e. The phase sequence developing in a bulk W:SiC diffusion couple at 1 300 degrees C is W/W5Si3/WC/SiC. W forms ohmic contacts on n-type 6H-S iC which are stable up to 1000 degrees C for at least several hours. F rom 1200 degrees C upwards a reaction between W and SiC leads to the f ormation of tungsten silicides and carbides and hence a deterioration of the electrical properties. The films disintegrate into small crysta ls of WC and W5Si3 leading to a large spread of the resistances of the individual contacts.