Laser-induced reactive microplasma for etching of fused silica

The ultra-precise machining (UPM) of surfaces with contact-free, beam-based technologies enables the development of flexible and reliable fabrication methods by non-vacuum processes for future application in advanced industrial fields. Laser machining by laser ablation features limitations for ultra-precise machining due to the depth precision, the surface morphology, and laser-induced defect formation. Contrary to physically-based etching, chemical-based dry and wet processing offer high quality, low damage material removal. In order to take advantage of both principles, a combined laser-plasma process is introduced. Ultra-short laser pulses are used to induce a free-standing microplasma in a CF4 gas atmosphere due to an optical breakdown. CF4 gas, with a pressure of 800-900 mbar, is ionized only near the focal point and reactive species are generated therein. Reactive species of the laser-induced microplasma can interact with the surface atoms of the target material forming volatile products. The release of these products is enhanced by the pulsed, laser-induced plasma resulting in material etching. In the present study, SiO2 surfaces were etched with reactive species of CF4 microplasma generated by their laser-induced break down with 775 nm pulses of an fs-laser (150 fs) at a repetition rate of 1 kHz. The dependency of the depth, the width, and the morphology of the etching pits were analysed systematically against the process parameters used. In particular, a linear increase of the etching depth up to 10 mu m was achieved. The etched surface appears smooth without visible cracks, defects, or LIPSS (Laser-induced periodic surface structures).