Ultrahigh-speed laser processing of alkali-free glass enabled by wide-angle Bessel TSL processing

Bessel transient and selective laser (Bessel TSL) processing enables high-aspect-ratio drilling in transparent materials. However, its application to alkali-free glass looks distorted compared to the fused silica. Here, we reveal that plasma-induced thermal loading near the hole entrance expands the electron-excited region and prevents the laser from propagating deeper into the material, resulting in instability. By reshaping the Bessel beam into a wider-angle configuration, the beam path avoids the plasma-affected region while increasing energy density. This enables stable, high-quality drilling in alkalif-ree glass, providing a viable approach for applying Bessel TSL to practical semiconductor packaging substrates.