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 SinOm 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 SiOm (m = 1, 2) and Si2Om (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.