Investigation of surface mass transport in Al–Si–Ca–oxide glasses via the thermal induced decay of submicron surface gratings

Abstract

In metal crystals, annealing induced decay of micron sized surface features occurs via surface rather than bulk diffusion. Surface transport also dominates the decay of micron sized patterns etched into semiconductors and initial stages in the sintering of ceramics. In oxide glasses, surface transport is expected to take over at a sufficiently small feature size as well, but this transition has not yet been identified, possibly because of the difficulties of preparing periodic gratings <10μm in glass surfaces with chemical etching. We therefore extended these investigations to submicron patterned glass surfaces using advanced lithography and dry etching to generate submicron sized patterns. The glasses selected have compositions along the center line of the SiO2–Al2O3–CaO phase diagram and hence are pseudobinaries, in which SiO4 units are increasingly replaced by AlCa0.5O4 units while maintaining the fourfold coordination of SiO2. The flow properties of these glasses can be studied at lower temperatures than SiO2 but extrapolation can provide insight into pure SiO2. Results obtained indicate that gratings with periodicities between 6 and 0.4μm decay predominantly by viscous flow. However, small but systematic deviations suggest that surface diffusion may contribute in a minor way to near surface mass transport in the submicron regime. In general, the surface viscosity extracted from grating decay agreed with the viscosity measured in bulk samples by three-point bending. Deviations between the two were traced to the in- or outdiffusion of water vapor from the near surface in response to changes in humidity of the laboratory air.