Backside Etching at the Interface to Diluted Medium with Nanometer Etch Rates

Abstract

The laser etching using a surface adsorbed layer (LESAL) is a new and promising method for precise etching of transparent materials such as fused silica. The etching process is based on the laser irradiation of the interface between a transparent solid and a diluted (vaporized) backside medium that forms an adsorbed layer. A KrF excimer laser (λ = 248 nm) with pulse duration of 25 ns is employed to study LESAL of fused silica. LESAL by means of toluene as adsorbate features outstanding attributes, e.g., low threshold fluences (< 1 J/cm²), an almost constant etch rate on the order of 1 nm/pulse over a wide fluence region, and very low, nearly optical grade surface roughness (~ 1 nm rms). As LESAL mechanism, the thermal ablation of a modified surface layer is discussed. The modification is caused chiefly by the deposition of decomposition products from organic molecules photolysis. The usage of halogenated adsorbate fluorobenzene (C6H5F), difluorobenzene (C6H4F2), chlorobenzene (C6H5Cl), and dichlorobenzene (C6H4Cl2) as diluted backside media reduces significantly the etching threshold and causes an additional characteristic fluence region with Angstrom/pulse etch rate and homogenous and smooth surface topography. The generation of halogen radicals close to the heated surface due to the decomposition of the solvent seems to be the most probably mechanism for a chemical erosion of the fused silica surface.