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, metallurgical and electrical, in order to understand the formation and the properties of W electrical contacts on 6H-SiC. We combined two different approaches. For the examination of the phase relations in the ternary system we prepared bulk diffusion couples of W and monocrystalline SiC which were annealed and investigated using an SEM (secondary electron images, backscattered electron images. energy dispersive X-ray analysis). Secondly are furnace molten powder samples, annealed at different temperatures, were analysed by X-ray diffraction. To investigate the electrical properties of a W/SiC junction transmission line contact patterns were sputter deposited onto wafer strips. These samples were subjected to similar heal treatments and the current,voltage characteristics were measured with a source measure unit. Individual contact resistivities could be evaluated using a special contact geometry. As 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 calculations. At 1300 degrees C the equilibrium WSi2 + WC exists. At 1000 degrees C the reaction kinetics are too slow to be detected in a bulk sample. The phase sequence developing in a bulk W:SiC diffusion couple at 1300 degrees C is W/W5Si3/WC/SiC. W forms ohmic contacts on n-type 6H-SiC which are stable up to 1000 degrees C for at least several hours. From 1200 degrees C upwards a reaction between W and SiC leads to the formation of tungsten silicides and carbides and hence a deterioration of the electrical properties. The films disintegrate into small crystals of WC and W5Si3 leading to a large spread of the resistances of the individual contacts.