Modeling wet chemical etching of surface flaws on fused silica

Fluoride-based wet chemical etching of fused silica optical components is useful to open up surface fractures for diagnostic purposes, to create surface topology, and as a possible mitigation technique to remove damaged material. To optimize the usefulness of etching, it is important to understand how the morphology of etched features changes as a function of the amount of material removed. In this study, we present two geometric etch models that describe the surface topology evolution as a function of the amount etched. The first model, referred to as the finite-difference etch model, represents the surface as an array of points in space where at each time-step the points move normal to the local surface. The second model, referred to as the surface area-volume model, more globally describes the surface evolution relating the volume of material removed to the exposed surface area. These etch models predict growth and coalescence of surface fractures such as those observed on scratches and ground surfaces. For typical surface fractures, simulations show that the transverse growth of the cracks at long etch times scales with the square root of etch time or the net material removed in agreement with experiment. The finite-difference etch model has also been applied to more complex structures such as the etching of a CO2 laser-mitigated laser damage site. The results indicate that etching has little effect on the initial morphology of this site implying little change in downstream scatter and modulation characteristics upon exposure to subsequent high fluence laser light. In the second part of the study, the geometric etch model is expanded to include fluid dynamics and mass transport. This later model serves as a foundation for understanding related processes such as the possibility of redeposition of etch reaction products during the etching, rinsing or drying processes.