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 248 nm (KrF) and 351 nm (XeF excimer laser) with a pulse duration of 25 ns 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 approximate to delta*(Phi - Phi(th))). The etching depth at the same laser fluence is higher for 248 nm compared to 351 nm as well as for back side etching compared to the front side etching process (LIFE: delta(248 nm) = 20 nm/(J/cm(2)), delta(351 nm) = 15 nm/(J/cm(2)), LIBDE: delta(248 nm) = 38 nm/(J/cm(2)), delta(351 nm) = 8 nm/(J/cm(2)) with Phi(th,m) from 0.3 to 2.65 J/cm(2)). 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. (C) 2012 Elsevier B. V. All rights reserved.