Dependence of plasma parameters on electric potential of electrode in microwave plasma

The authors have been studying the double-tubed coaxial line-type microwave plasma chemical vapor deposition (MPCVD). The discharge tube of the present MPCVD is a dual-tube structure made of a fused quartz outer discharge tube and stainless steel inner tube. The authors have discovered that the ion bombardment energy can be controlled without varying the electron densities and temperature by changing the potential of the substrate placed in a spatial after-glow plasma. In the present research, it is found that the ion bombardment energy can also be controlled by changing the potential of the inner tube placed in the discharge plasma. However, the electron densities and temperature have exhibited a tendency different from the one when the substrate potential is varied. Hence, a theoretical calculation has been carried out on the electron densities and temperature based on Maxwell-Boltzmann distribution. It is found that a different tendency can be explained from the fact that the electrons successively vanish on the deposition chamber wall from the higher-energy side as the plasma spatial potential is reduced from about 13 V to 4 V so that the electron velocity distribution is modified.