Study on morphology of high-aspect-ratio grooves fabricated by using femtosecond laser irradiation and wet etching

Morphologies of high-aspect-ratio silicon grooves fabricated by using femtosecond laser irradiation andselective chemical etching of hydrofluoric acid (HF) were studied. Oxygen was deeply doped into sili-con under femtosecond laser irradiation in air, and then the oxygen-doped regions were removed by HFetching to form high-aspect-ratio grooves. After HF etching, periodic nano-ripples which were inducedin silicon by femtosecond laser were observed on the groove sidewalls. The ripple orientation was per-pendicular or parallel to the laser propagation direction (z direction), which depended on the relativedirection between the laser polarization direction and the scanning direction. The formation of nano-ripples with orientations perpendicular to z direction could be attributed to the standing wave generatedby the interference of the incident light and the reflected light in z direction. The formation of nano-ripples with orientations parallel to z direction could be attributed to the formation of self-organizedperiodic nanoplanes (bulk nanogratings) induced by femtosecond laser inside silicon. Materials in thetail portion of laser-induced oxygen doping (LIOD) regions were difficult to be etched by HF solution dueto low oxygen concentration. The specimen was etched further in KOH solution to remove remainingmaterials in LIOD regions and all-silicon grooves were fabricated.