Pulsed plasma-enhanced chemical vapor deposition from hexafluoropropylene oxide: Film composition study

Abstract

Films deposited using pulsed plasma-enhanced chemical vapor deposition (PECVD) from hexafluoropropylene oxide (HFPO) were investigated by X-ray photoelectron spectroscopy (XPS). As compared to continuous rf PECVD, pulsed excitation increases the CF2 fraction in the film. Film composition was determined as a function of plasma processing conditions including on-time, off-time, pressure, flow rate, substrate temperature, electrode spacing, substrate potential, and input power. Varying the on–off pulsing cycle resulted in compositional control of the deposited films. At a low duty cycle [ton/(ton + toff)], up to 70% CF2 could be incorporated into the film. The input gas, HFPO, may facilitate greater CF2 incorporation into the films as this gas thermally decomposes into a difluorocarbene. Both absolute on-time and off-time, rather than simply duty cycle, are important parameters for determining film composition. A simple model was developed to describe the experimentally determined variation %CF2 as a function of substrate temperature and off-time. This model accounts for changes in film composition due to plasma-surface modification and differences in gas-phase chemistry. The model suggests that surface modification by the plasma is the dominant factor only for long on-times or for low deposition rates. However, the gas-phase concentration of CF2 relative to other film-forming species is typically the controlling factor under conditions which achieve the high %CF2 in the film. The gas-phase composition will depend on both abslute on-time and off-time, rather than simply on the duty cycle. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 67:1489–1502, 1998