Effect of heat treatment on the nanoporosity of silica PECVD films elucidated by low-energy positron annihilation and ellipsometric porosimetry

Abstract

The nanoporosity of silica thin films, prepared through plasma enhanced chemical vapor deposition with different flow rates of tetraethyl orthosilicate, was investigated by means of vapor-adsorption ellipsometric porosimetry (EP) and pulsed, low-energy positron annihilation lifetime spectroscopy (PALS). The effect of heat treatment on the subnano-scaled pore structure was observed via changes in the porosity and the pore dimension, obtained from EP and PALS, respectively. The open porosity of the as-deposited films, elucidated from the methanol (MeOH) adsorption isotherms at 26 °C, was found to range up to 5%. After annealing, the open porosity was reduced to almost zero, whereas the film total porosity was not significantly changed. This suggests that bottlenecks connecting pores near the film surface were developed by heat treatment, so that those bottlenecks prevented the MeOH molecules from diffusing into the open pores. On the other hand, the PALS results indicated that subnano-scaled pores of the films were enlarged along with the heat treatment; as a result, their sizes, evaluated from the lifetimes of ortho-positronium, were in the range of 0.31 nm to 0.37 nm in radius. These results from EP and PALS signified that the heat treatment enhanced the structural non-uniformity with respect to nanoporosity of the silica films.