Running discharge for PECVD inner coating of metal tubes

Abstract

The breakdown of a magnetically enhanced microwave discharge in a metallic waveguide has been studied in argon plasmas depending on magnetic field, microwave power and gas pressure. A sharp transition between a localized discharge and a so-called running discharge has been observed on increasing the pressure in the pascal range at 100W microwave input. The structure of the running discharge has been determined as a spatially limited plasma package of some 10 cm in length. It was possible to distinguish a primary ignition zone of about 0.5 m and a running zone which was limited by the set-up at 3.0 m. The running velocity of the plasma package (about 10 4 m s −1) remains constant over all the running zone, while it increases with increasing microwave power and/or gas pressure. The running discharge has been employed as a novel thin film deposition process for inner coating of metal tubes by means of plasma-enhanced chemical vapor deposition (PECVD). Amorphous hydrogenated carbon (a-C:H) films were deposited over a length of 2.5 m on the inner wall of an assembled waveguide system. The average deposition rate from a running discharge in methane was 0.27 Å s −1. An increase in the average deposition rate to 2.6 Å s −1 was achieved using acetylene as process gas, while the H-to-C ratio in the a-C:H films remains nearly constant at one.