Abrupt initiation of material removal by focusing continuous-wave fiber laser on glass

Glass with high-aspect-ratio micro holes is used in system-in-package technologies and microfluidic devices. In this study, to investigate the feasibility of the process, we used a continuous-wave (CW) fiber laser to drill a hole in glass without an absorbent layer. To understand the process of removal, we conducted observations over a wide range of time scales from 1 kfps to 1 Mfps. A CW laser beam with a wavelength of 1070 nm was focused on the front surface of an aluminosilicate glass sample. The real-time observations revealed that the initiation of the material removal occurred tens or hundreds of milliseconds after the exposure of the laser beam. Once the removal of the material started, the depth of the hole rapidly increased at a rate of 3–4 $$\mu $$ m/ $$\mu $$ s. Although the time required for the initiation of the material removal varied with the laser power, the rate at which the depth of the hole increased was approximately constant. The model of the transient absorption and thermal diffusion showed that the abrupt material removal was caused by the dependence of absorption coefficient on temperature. The threshold temperature was calculated as 900–1200 $$^\circ $$ C. In this study, we demonstrated that a CW fiber laser can process high-aspect-ratio micro holes in glass without using an absorbent layer through precisely controlled exposure time.