Comparative study of different indirect laser-based methods developed for microprocessing of transparent materials

Abstract

Micro-optics and micro-fluidics are dynamically developing research areas. However, the preparation of micro-optical elements, channels and structures is a rather complicated process. The indirect laser etching methods, i.e. the backside wet and dry etching techniques (LIBWE, LIBDE) and the plasma assisted ablation (LIPAA) developed for micromachining of transparent materials are promising techniques. In this paper our aim was to accomplish a reliable comparison of these procedures. We applied same absorber material, tin in liquid and solid states, for wet-, dry- and plasma assisted etchings, respectively. The metal-fused silica interfaces were irradiated by a KrF excimer laser beam, the fluence was varied between 350 and 2000 mJ/cm2. Measuring the depth of the etched holes we found that LIBWE method had the highest etch rate, while the etch rates of LIBDE and LIPAA were similar in the investigated fluence range. On the basis of the etch rate-fluence graphs we estimated the threshold values which were 340, 360 and 500 mJ/cm2 for wet-, dry- and plasma assisted etchings, respectively. EDX analysis of the etched hole bottoms demonstrated that significant number of tin atoms were built into the surface layer of the fused silica developing a mixture layer which can play an important role in the etching processes.