Abrupt initiation of material removal by focusing continuous-wave fiber laser on glass
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Abrupt initiation of material removal by focusing continuous‑wave fiber laser on glass Reina Yoshizaki1 · Yusuke Ito1 · Naoyuki Miyamoto1 · Akihiro Shibata2 · Ikuo Nagasawa2 · Keisuke Nagato1 · Naohiko Sugita1 Received: 21 April 2020 / Accepted: 21 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 𝜇m/𝜇 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 ◦ 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. Keywords Laser drilling · Micro hole · Glass · In-situ observation
1 Introduction High-aspect-ratio micro holes in glass are used in a systemin-package technology [1] and microfluidic devices [2]. For drilling deep micro holes in glass, mechanical machining [3], chemical etching [4], electric discharge machining [5], and laser processing [1] have been reported. Laser processing is widely used as an efficient drilling process because of its high efficiency. It is broadly classified into two types: ultrashort pulse laser processing and continuous-wave (CW) laser processing. Ultrashort pulse lasers can directly transmit energy to the target material through multiphoton or tunneling ionization because of its significantly high intensity. Therefore, ultrashort pulse laser can initiate micro * Reina Yoshizaki [email protected]‑tokyo.ac.jp 1
Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Tokyo 113‑8656, Japan
Technology General Division, AGC Inc, Yokohama 230‑0045, Japan
2
processing in the material even if the transmittance of the material is high [6, 7]. However, this process has an inevitable problem; stress waves generated during the process damage the material [8]. When CW lasers are used, material is removed thermally without causing damage from the stress waves. High linear absorption coefficient of material
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