Activation energy of an asymmetrical, radio frequency discharge with methane

Abstract

The investigation of processing parameters in plasma polymerization is of great interest for the optimization of film properties as well as the comparison and up-scaling of different plasma reactors. For radical-dominated plasma processes, the plasma chemistry in the active plasma zone determines the activation energy E a required to obtain stable, functional coatings. Especially, asymmetrical, radio frequency discharges enable a high conversion of monomer flow into film deposition. However, within one experimental set-up, different activation energies depending on the actual gas flow seemed to appear for pure CH 4 discharges. The differences were found to depend on the different expansion of the plasma into the vacuum chamber, i.e., the width of the active plasma zone d act, which was examined using optical photography. Considering a geometrical scaling factor d act / d gas ( d gas: distance between gas inlet and deposition electrode at a vertical gas flow), a normalized activation energy can be derived determining the plasma polymerization process. The unconfined asymmetrical set-up is compared to a symmetrical reactor with confined geometry supporting the described concept for the up-scaling of plasma processes.