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

Nanometer-size holes in plasma-polymerized thin films were characterized by variable-energy positron lifetime spectroscopy for the first time. Hexamet hyldisiloxane (HMDSiO) was plasma-polymerized at different discharge powers (30-105 W) and monomer pressures (1.0-4.9 Pa). The positron lifetime spect ra of deposited films were collected at positron energies of 1 and 5 keV. A ll films showed a well-defined long-lived component due to pick-off annihil ation of ortho-positronium (o-Ps). The o-Ps lifetime tau(3), reflecting the average size of free-volume holes in the film, increased with an increasin g ratio of plasma discharge power, W, and monomer flow rate, F. Based on th e empirical relationship between the o-Ps lifetime and the cavity radius, h ole volumes were estimated to be 0.19-0.36 nm(3). We also found that the o- Ps intensity, I-3, 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., th ose increasingly inorganic in nature. (C) 1999 John Wiley & Sons, Ltd.