Impact of the temperature on laser ablation of residues from APPJ-etched N-BK7 optical glass

Atmospheric pressure plasma jet (APPJ) machining of optical materials, e.g. fused silica and silicon carbide is capable of generating free-form optical surfaces with excellent precision and low surface roughness. However, APPJ etching of the commonly used optical glass N-BK7®, which contains various metal oxides, with fluorine-containing gases results in the formation of surface residues consisting of metal fluorides. The accumulated residues form a layer on the glass substrate that deteriorates the etching process in terms of etching rate and surface roughness but can be ablated by pulsed laser irradiation. The aim of this study is to reduce the residual layer thickness required for its laser removal as a prerequisite to increased shape accuracy and reduced process-induced roughness.

N-BK7® samples were etched with APPJ while varying the surface temperature. Subsequently the APPJ-etched area was irradiated by excimer laser at different wavelengths (λ = 193; 248; 308 nm; t_Pulse = 20 ns). The etched and laser-exposed sample surfaces were thereafter analyzed by WLI and SEM.

It can be shown that residual layers resulting from etching at different temperatures can be removed at wavelengths of 193 nm and 248 nm. The thinnest layer for complete removal demonstrated is 95 nm at a wavelength of 193 nm. The minimum layer thicknesses for λ = 248 nm are in the range of 190–320 nm. At a wavelength of 308 nm, neither removal of the residual layer nor damage to the glass could be detected for the selected fluence range.