Abstract
The 3-gamma annihilation of orthopositronium and the Doppler broadening of the positron annihilation line have been measured by implanting low energy positrons in low dielectric constant (low-k) SiOCH films. The evolution and stability of film porosity with thermal treatments in the 400–900 °C temperature range has been studied. The films have been produced by plasma enhanced chemical vapor deposition and after annealing in N2 atmospheres at 480 °C have been treated in N2 + He plasma. The minimum free volume of the pores in the as-produced samples has been estimated to correspond to that of a sphere with radius r = 0.6 nm. The treatment in the N2 plasma was found to seal the pores up to 45 nm depth. Both the composition of the films (as obtained by Rutherford backscattering spectroscopy and elastic recoil detection analysis) and the chemical environment of the pores probed by positrons were found to be very stable up to 600 °C thermal treatment. Above such a temperature a reduction of the hydrogen content accompanied by a change in the structure and in the chemical environment of the pores has been observed. In the samples thermal treated at 800–900 °C, the positronium formation is reduced by one-third respect with the as produced sample. In the annealed and as-produced films, a natural aging of 30 days in air was enough to contaminate the porosity, as pointed out by a strong reduction of the 3 annihilations. The effect of contamination and the distribution of the pores were completely recovered after a thermal treatment at 400 °C. Artificial aging of SiOCH films in controlled atmospheres of H2, O2, H2O has shown that H2O is the more efficient contaminant in reducing the effective volume of the pores.