Effects and mechanism of focusing condition on machining quality and bubble behaviors in femtosecond laser micro-grooving of silicon wafer in liquid

In this paper, the effect and mechanism of how varying focusing conditions would affect the machining quality of microgrooves and bubble behaviors (cavitation bubbles (CB) and persistent bubbles (PB)) in liquid-assisted femtosecond laser micro-grooving of silicon wafer were investigated. Firstly, the width and depth of the microgroove machined under negative defocusing condition (NDC) were considerably uniform, which showed an increase by 100 % and 125 % compared with that machined under positive defocusing condition (PDC). Secondly, the temporal evolution images of CB were captured by a time-resolved shadowgraph imaging system. Compared with PDC, the radius and lifetime of CB induced under NDC increased by 71 % and 100 %, and the time evolution curve of the radius of CB exhibited a high consistence with the Rayleigh distribution, while that induced under PDC fluctuated irregularly. Thirdly, the PB induced under NDC showed a backward flush and contributed to the formation of tendentious liquid flow, while that induced under PDC occupied the laser transmission path and caused disturbances on subsequent laser pulses. Finally, the differences in laser transmission mechanism under both the negative and positive defocusing conditions were discussed and the NDC was confirmed as the stable and efficient processing method. Through the comprehensive study of the bubble behaviors, the correlation between focusing conditions and machining quality of liquid-assisted laser cutting was established.