Void formation in silica glass induced by thermal oxidation after Zn ion implantation

Thermal annealing effects on Zn⁺ ion-implanted silica glass (a-SiO₂) have been studied in order to control void formation. Void formation in a-SiO₂ with Zn⁺ ion implantation and subsequent oxidation has been observed using transmission electron microscopy (TEM). Zn⁺ ions of 60 keV were implanted into a-SiO₂ to a fluence of 1.0 × 10¹⁷ ions/cm². After the implantation, thermal annealing at 600 or 700 °C for 1 h in oxygen gas was conducted. In as-implanted state, metal Zn nanoparticles (NPs) of 10-15 nm in diameter are formed in the depth region around the projected range. The size of the Zn nanoparticles increases after the annealing at 600 °C in oxygen gas. Annealing in oxygen gas at 700 °C for 1 h caused two processes: (1) the migration of Zn atoms which formed Zn NPs in as-implanted state to the surface of the a-SiO₂ substrate and (2) the transformation to the oxide phase on the substrate. The transportation of Zn NPs to the surface leaves voids of 10-25 nm in diameter inside the a-SiO₂. These results indicate that the oxidation at 700 °C for 1 h causes the migration of Zn atoms to the surface without diffusion and recombination of vacancies which form the voids.