Structural changes of tungsten heating filaments during CVD of diamond

Hot tungsten filaments (>2000 °C) are used for the activation of the gas phase during hot filament chemical vapor deposition (CVD) of diamond. In the present investigation, technically pure and doped tungsten filaments were subjected to typical process conditions. The changes in the tungsten filaments microstructure were investigated. During the first 60 s, a rapid transformation of W into W2C occurs due to the high reactivity of the metallic surface. Afterwards, the W2C shell grows inward at a linear rate. The activation energy of this process is 30 kJ mol−1, which corresponds to the dissolution of methane on the filament surface which is the rate determining step. Once W has transformed completely into W2C, a WC shell starts to grow from the surface. In doped tungsten a parabolic behavior can be found for the growth of WC which occurs when the diffusion of carbon through the growing shell is the rate-determining step. Additionally, the temperatures of the filament and substrates drop considerably. Both filament and substrate temperature drops are discussed in detail with the help of a model describing the power balance.