DC-plasma polymerization of hexamethyldisiloxane .2. Surface and interface characterization of films deposited on stainless-steel substrates

Thin films of plasma-polymerized hexamethyldisiloxane (PP-HMDS) were deposited on stainless-steel substrates from a parallel-plate DC reactor. The parameters were varied between 6.7 and 66.7 Pa for the monomer pressure and 0.4 and 20 W for the discharge power. The deposition times ranged from 5 to 600 s, thus enabling an analysis of both the film/metal interface region and the bulk of the films. Characterization of the deposits was performed using XPS, reflection-absorption Fourier transform infrared spectroscopy, time-of-flight SIMS and scanning tunneling microscopy (STM). The results strongly suggest that the structure of the plasma polymer close to the interface is different from that of the bulk. In the case of low power and high pressure (LW/HP) deposition conditions, a non-crosslinked film is formed initially which appears to be covalently bonded to the substrate via Me-C bonds (Me = Fe and/or Cr). Film growth starts at nucleation sites on the steel surface. After approximately 1 min of deposition, the film structure gradually changes and becomes more crosslinked. In high power/low pressure (HW/LP) conditions, the surface coverage of the steel is initially higher and the film has a considerably higher degree of crosslinking, which does not change much during film growth. Time-of-flight SIMS analysis of the thick films confirmed the difference between the degree of crosslinking of the films deposited under different conditions. Remarkably, the elemental composition of the thick films formed at HW/LP and LW/HP conditions was approximately the same, as was concluded from XPS analyses. The STM images of the various films corroborated the initially more homogeneous coverage by the HW/LP film as compared with the LW/HP film. The images of the thick films suggested a strong effect of the concurrent ion bombardment in the case of the HW/LP film. A model for the mechanism of film growth and the effect of deposition conditions is proposed.