Multi-pulse LIBDE of fused silica at different thicknesses of the organic absorber layer

Abstract

Laser-induced etching techniques feature several unique characteristics that enable ultraprecise machining of transparent materials. However, LIBDE (laser-induced back side dry etching) and LIBWE (laser-induced back side wet etching) are preferentially studied due to experimental feasibilities either using a very thin or a bulk absorber at the rear side of the transparent material. This study aims to fill the gap by examining the thickness dependence of the absorbing material. Multi-pulse-LIBDE (MP-LIBDE) of fused silica using different thick photoresist absorber layers (dL=0.2–11.7μm) was performed with a KrF excimer laser (λ=248nm, tp≈20ns). The influence of several experimental parameters, such as laser fluence, pulse number, film thickness, on the ablation morphology and the etching rate were investigated. Especially at moderate fluences (F=0.7–1.5J/cm2) MP-LIBDE and LIBWE show several similar process characteristics such as the etching rate dependence on the laser fluence and the pulse number with a typical etching rate of approx. 12nm at 1J/cm2. However, the specific etching rate values depend on the absorber layer thickness, for instance. The morphology of the etched surface is smooth with a roughness of below 5nm rms. Further, the modification of the surface has been observed and will be discussed in relation to the multi-pulse laser etching mechanism.