Activation energy of an asymmetrical, radio frequency discharge with methane

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 Ea 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 CH4 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 dact, which was examined using optical photography. Considering a geometrical scaling factor dact / dgas (dgas: 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.