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As next-generation packaging substrates, the use of glass materials with low dielectric-loss characteristics is a promising approach. However, producing micro-vias in glass is still challenging because it is a brittle material that is susceptible to breaking. One potential method for fabricating high-quality holes with high aspect ratios is ablation using a high-power KrF excimer laser. In this process, the glass surface is irradiated by a high-fluence laser beam formed using a focusing lens. To produce a small-diameter focused laser beam, a low beam quality factor (M2) of close to unity is required. However, this is generally only possible for an ideal single-mode Gaussian beam. For an excimer laser beam, the M2 value is larger due to the fact that the beam is produced by multi- mode oscillation. In the present study, we used a cylindrical off-axis unstable resonator with a KrF laser and obtained a high-quality beam (M2: V:7.1, H:5.4) with a high output power (58.4 mJ/pulse). With this light source, it is a feasible approach to fabricate fine holes by laser ablation with a diffraction-limited focused diameter of 20μm when an imaging lens with a focal length of 100mm was used. The laser output power was found to be similar to that for a conventional stable resonator. Gigaphoton demonstrates that a combination of a KrF excimer laser and an unstable resonator provides high-productivity fabrication of fine micro-via holes for the High-performance Advanced packaging.
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