MeV ion irradiation-induced creation and relaxation of mechanical stress in silica

In situ wafer curvature measurements were performed to study mechanical stress in amorphous SiO₂ during Xe, Ne, and Er ion irradiation at energies in the 0.27-4.0 MeV range. Three phenomena are observed: network compaction, radiation-induced viscous flow, and a nonsaturating anisotropic deformation phenomenon. The radiation-induced viscosity is shown to be inversely proportional to the energy density deposited into atomic displacements. The relation between radiation-induced flow and diffusion is discussed in the context of the Stokes-Einstein relation. Viscous flow serves to relax stress, yet a continuous nonsaturating anisotropic deformation effect causes the stress in the irradiated layer to saturate at nonzero values: Xe irradiation at an energy below 3.6 MeV results in a tensile saturation stress; for higher energies a compressive stress builds up. These effects are explained in terms of competing bulk and surface deformation processes resulting from local heating of the SiO₂ around the ion tracks. The macroscopic effect of deformation phenomena is illustrated by showing the surface morphology after 4.9 MeV Er irradiation of silica through a contact implantation mask. Finally, an in situ stress study of an alkali borosilicate glass is presented. In this case a fourth radiation induced effect is observed, namely, the generation and annihilation of volume occupying point defects. These defects are shown to anneal out at room temperature, following a broad spectrum of activation energies.