Surface roughness optimization during femtosecond UV laser ablation

Abstract

We study the properties of glasses and crystals after an ultraviolet (UV) laser ablation process using 300 fs duration pulses, centered at 206 nm wavelength at the repetition rate of 50 kHz. We show that linear light absorption allows fabricating grooves with a surface roughness in the order of tens of nanometers and even sub-10 nm with low heat affected zones (HAZ) without any post-processing. We produced a single scan laser ablated microchannels’ depth, surface roughness and full width at half maximum maps based on the experimental results. These maps show relations with an overlap of laser pulses and a pulse energy and are useful when considering the working regime for specific applications. Our results depict the minimum channel bottom roughness of 8 nm in Sapphire after one scan. This value represents several times smaller roughness when compared to the nanosecond UV laser ablation. We have also studied a multi shot ablation and by evaluating our results using a scanning electron microscope (SEM) we noticed nanogratings on the walls of the craters of soda-lime and BK7 glasses with a period of 300 nm, that is bigger than the laser wavelength (206 nm). We also show that a precise surface micro patterning can be achieved with our experimental setup. Finally, our theoretical fluence distribution optimization provides a precise fabrication parameters estimate for a small surface roughness condition.