Tamás Csizmadia

Laser-induced backside dry etching : wavelength dependence

The laser-induced backside dry etching (LIBDE) method has been developed by analogy with the well-known LIBWE technique for micromachining of transparent materials. In our experiments tin thin films were applied as absorbing layers and the dependence of the etch depth on the applied laser wavelength was investigated. Multipulse irradiation experiments at different wavelengths have also been carried out. A fused silica plate coated by a tin layer was used as a target. The metal film–transparent material interface was irradiated through the fused silica plate by the most frequently used nanosecond excimer laser beams: ArF (193 nm), KrF (248 nm) and XeCl (308 nm). It was found that the etch depth increased linearly with decreasing wavelength in investigated range. Multipulse investigations proved that the energy of the absorbed photons has a strong influence on the mechanism of LIBDE. The etch depth–pulse number curves showed saturation at 1500 mJ cm−2 applied laser fluence at around 2, 3 and 30 pulses for 308 nm, 248 nm and 193 nm wavelengths, respectively. This indicates that the effectiveness of multipulse LIBDE increases when the wavelength of the applied laser source is decreased in the UV range. XPS and spectroscopic ellipsometric analyses showed that formation and embedding of tin-oxides into the upper surface layer can be responsible for the shift of the saturation in the case of ArF as compared with the other studied wavelengths.

Dynamics of the laser-induced nanostructuring of thin metal layers: experiment and theory

Nanostructures are of increasing importance in manifold application fields such as electronics, optics and beyond. However, the fast and cost-effective production of nanostructures is a big technological challenge for laser machining. One promising approach is laser irradiation of thin metal layers, which allows the fabrication of metal nanostructures induced by a melting and transformation process. The influence of laser parameters (laser fluence, laser pulse number) on the morphology of the nanopatterned film and the dynamics of the nanostructure formation during excimer laser irradiation of a 20 nm chromium film on fused silica were studied. The dynamics of nanopatterning, comprising hole and droplet formation, were investigated by time-dependent reflection and transmission measurements as well as time-dependent optical microscopy. The resulting patterns were investigated by optical and scanning electron microscopy (SEM). However, for an optimization of this process a better understanding of the underlying physical phenomena is necessary. Therefore, experimental data of laser-induced nanopatterning were compared with results of physical simulations that consider the heat equation (laser–solid interaction including melting and evaporation) and the Navier–Stokes equation (transformation processes of the molten phase). The simulations, making use of laser fluence-dependent effective material parameters (surface tension and viscosity), are in good agreement with the experimental results.