Low surface damage laser processing of silicon by laser-induced plasma etching (LIPE)

The precise surface machining of silicon by pulsed laser processing is challenging. Laser ablation enables direct patterning but causes numerous modifications of the surface and sub-surface region. Due to the increasing demand of precisely structured silicon surfaces in various fields, such as optics and micro-electronics and micro-fluidics, new laser-based ultra-precise surface machining techniques are required. Therefore, the recently developed laser-induced plasma etching process (LIPE) was studied in relation to chemical as well as structural modifications after the etching of single-crystalline silicon. For the studies, a fs-laser (775 nm, 150 fs, 1 kHz) with a pulse energy of maximum 750 µJ was focused to a CF4/O2 gas mixture at atmospheric pressure igniting a laser-induced plasma in front of the 〈1 0 0〉 Si sample. For comparison, a silicon surface was also structured by direct laser ablation. The LIP etched surface, that is characterized by SEM, TEM, XPS- and Raman-spectroscopy, shows no melting features, no structural surface or subsurface defects and almost no chemical contamination from etching besides a 2 nm thick silicon oxyfluoride on top of the atomically ordered silicon. The comparison shows clearly that the severe structural and chemical modifications at laser ablation of silicon can be avoided enabling ultraprecise surface machining.