Analysis of intensities of positive and negative ion species from silicon dioxide films using time-of-flight secondary ion mass spectrometry and electronegativity of fragments

Intensities of positive and negative ion species emitted from thermally oxidized and plasma-enhanced chemical vapor deposited (PECVD) SiO₂ films were analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and the Saha-Boltzmann equation. Intensities of positive and negative secondary ion species were normalized to those of ²⁸Si⁺ and ²⁸Si⁻ ions, respectively, and an effective temperature of approximately (7.2 ± 0.1) × 10³ K of the sputtered region bombarded with pulsed 22 kV Au₃⁺ primary ions was determined. Intensity spectra showed polarity dependence on both n and m values of Si_nO_m fragments, and a slight shift to negative polarity for PECVD SiO₂ compared to thermally oxidized SiO₂ films. By dividing the intensity ratios of negative-to-positive ions for PECVD SiO₂ by those for thermally oxidized SiO₂ films to cancel statistical factors, the difference in absolute electronegativity (half the sum of ionization potential and electron affinity of fragments) between both films was obtained. An increase in electronegativity for SiO_m (m = 1, 2) and Si₂O_m (m = 1-4) fragments for PECVD SiO₂ films compared to thermally oxidized films was obtained to be 0.1-0.2 Pauling units, indicating a more covalent nature of Si-O bonds for PECVD SiO₂ films compared to the thermally oxidized SiO₂ films.