High temperature contacts to chemically vapour deposited diamond films—reliability issues

Refractory metals (Ti, Mo, W and Ta) with precious metal overlayers (Au and Pt) were used to form ohmic contacts to polycrystalline boron doped chemically vapour deposited diamond films. Refractory metals afford high resistance to thermal and environmental stresses. In addition, many refractory metals show a chemical affinity for carbon, resulting in the formation of a metal-carbide reaction layer with reduced Schottky barrier height at the contact interface. Low resistance ohmic contacts can be formed by heavy boron doping of the diamond film in the contact region. The viability and reliability of various refractory metal contact schemes were assessed to determine their upper operating temperatures and life expectancies in hostile environments. Reverse engineering of the contacts and detailed material analysis revealed likely failure mechanisms. It was found that MoAu gave the highest degree of thermal stability, while Ti gave the lowest contact resistance. The contact resistance was strongly dependent on the doping level of the diamond film.