Plasma-polymerized hexamethyldisiloxane films characterized by variable-energy positron lifetime spectroscopy

Abstract

Nanometer-size holes in plasma-polymerized thin films were characterized by variable-energy positron lifetime spectroscopy for the first time. Hexamethyldisiloxane (HMDSiO) was plasma-polymerized at different discharge powers (30–105 W) and monomer pressures (1.0–4.9 Pa). The positron lifetime spectra of deposited films were collected at positron energies of 1 and 5 keV. All films showed a well-defined long-lived component due to pick-off annihilation of ortho-positronium (o-Ps). The o-Ps lifetime τ3, reflecting the average size of free-volume holes in the film, increased with an increasing ratio of plasma discharge power, W, and monomer flow rate, F. Based on the empirical relationship between the o-Ps lifetime and the cavity radius, hole volumes were estimated to be 0.19–0.36 nm3. We also found that the o-Ps intensity, I3, depends strongly on the same parameter, W/F. Comparison with infrared (IR) absorption spectroscopy data showed that Ps formation is suppressed in films with fewer organic bonds and higher disorder, i.e., those increasingly inorganic in nature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2522–2528, 1999