Subnanopore structural change of time-elapsed silica PECVD films elucidated by slow positron annihilation and ellipsometric porosimetry

To examine the effect of elapsed time on the nanoporosity, subnanoporous silica thin films, fabricated by plasma-enhanced chemical vapor deposition (PECVD), were investigated by means of lowenergy positron annihilation lifetime spectroscopy and vapor-adsorption ellipsometric porosimetry. The structural change of the subnanoscaled pores was elucidated by comparison of the as deposited and 6-month-old films. It is expected that the change in the subnanoscaled pores of the present films, after exposure to air for half a year, is due to the adsorption of water molecules from air, followed by the filling up the nanoscaled pores as well as partial polycondensations between silanol groups at the silica grain boundaries of the films.