Student Modeling of Transient Selective Laser Processing of GlassWednesday (24.06.2020) 10:00 - 10:20 Room 3
Micro-holes of high aspect ratio are required in glasses used for System-in-Package technologies or micro-fluidic devices. It has been reported that deep micro-holes can be drilled by a laser of a wavelength that corresponds to the high transmittance of the material. Ultrashort-pulse lasers can directly deliver energy into the material by multiphoton or tunneling ionization, even if the transmittance of the material is high. When continuous-wave (CW) lasers are used, absorbent layers are normally essential to facilitate the absorption of the light because the intensity of the light is not sufficiently high for a nonlinear optical effect. Transient selective laser processing (TSL) has realized high-speed high-aspect-ratio drilling by making a transient absorbent with an ultrashort-pulse laser. In TSL, a single shot of the ultrashort-pulse laser makes filament, which selectively absorbs the CW laser that is barely absorbed into glass at a normal state.
To reveal the TSL mechanism, we observed the process and constructed a numerical model. The result of the observation indicates that this process starts with the selective heating of filaments and advances mainly from the top surface after 5 μs. This suggests that an increase in the absorption coefficient dependent on temperature increases in glass is the main cause of the TSL. Based on this hypothesis, the transient laser absorption model was constructed. We estimated the dependence of the absorption coefficient on temperature using the model of thermal ionization. The depth of the hole calculated by an integrated model of a transient absorption based on this absorption coefficient model and a heat conduction model is consistent with the experimental result, especially for more than 5 μs after the start of the processing. Therefore, the thermal ionization of glass is thought to be the main cause of progressing in the TSL.
This research reveals the progressing mechanism in the TSL, especially for more than 5 s after the start of the process. In the TSL, thermal ionization at high-temperature glass enables continuous laser absorption and continuous material removal.
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