Laser-induced front side and back side etching of fused silica with KrF and XeF excimer lasers using metallic absorber layers: A comparison

Abstract

Laser-induced front side (LIFE) and back side etching (LIBDE) are methods for nanometer-precision laser etching of transparent materials using thin absorber layers. The etching behaviour of fused silica at a laser wavelength of 248nm (KrF) and 351nm (XeF excimer laser) with a pulse duration of 25ns using a chromium absorber layer was analysed and compared for front and back side etching geometry. For both wavelengths as well as for both processes the etching depth d increases almost linearly in dependence on the laser fluence (it is: d≈δ*(Φ−Φth)). The etching depth at the same laser fluence is higher for 248nm compared to 351nm as well as for back side etching compared to the front side etching process (LIFE: δ(248nm)=20nm/(J/cm2), δ(351nm)=15nm/(J/cm2), LIBDE: δ(248nm)=38nm/(J/cm2), δ(351nm)=8nm/(J/cm2) with Φth,m from 0.3 to 2.65J/cm2). Furthermore, the measured depths were evaluated with the estimated etching depth calculated by a thermal model. The simple thermodynamic model allows a good qualitative description of the etching depth behaviour; however, the model does not allow the quantitative calculation of the etching depth.