Thermal damage free material processing using femtosecond laser pulses for fabricating fine metal masks: Influences of laser fluence and pulse repetition rate on processing quality

We demonstrate thermal damage free processing by using femtosecond (fs) laser pulses on thin 64FeNi alloy (Invar) sheets for making fine metal masks (FMMs). To achieve high throughput as an essential prerequisite for the industrial application, high-fluence pulses can be exploited to increase ablation rates. However, the processing quality can be significantly degraded due to thermal damage induced by the high-effective-penetration-depth. To avoid the detrimental effect on the sample, a low fluence is an advantageous condition to realize cold processing. Meanwhile, the low fluence causes low productivity, and which can be compensated by applying high-repetitive pulses. But the accumulative heat on the surface induced by megahertz-repetition-rates should be suppressed. To investigate the influence of the repetition rates on the heat accumulation, the surfaces irradiated by the pulse trains were inspected by the scanning electron microscopy (SEM). A numerical estimation for the heat accumulation was also conducted by a simple toy model, and the calculation well coincided with the experimental observations.