Stability of W as electrical contact on 6HSiC: phase relations and interface reactions in the ternary system WSiC
✍ Scribed by Fred Goesmann; Rainer Schmid-Fetzer
- Publisher
- Elsevier Science
- Year
- 1995
- Tongue
- English
- Weight
- 854 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0921-5107
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✦ Synopsis
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 arc 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 heat 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 °C: 3WsSi 3 + 7SiC~8WSi 2 + 7WC. This is in qualitative agreement with thermodynamic calculations. At 1300 °C the equilibrium WSi, + WC exists. At 1000 °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 °C is W/WsSi3/WC/SiC.
W forms ohmic contacts on n-type 6H SiC which are stable up to 1000 °C lbr at least several hours. From 1200 °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 WsSi, leading to a large spread of the resistances of the individual contacts.