Rapid and complex dynamics of through glass via formation using a picosecond quasi-continuous wave laser as revealed by time-resolved absorptance measurements and multiphase modeling

Small through holes in glass, also called through glass vias (TGVs), find application in many technologies from microfluidics to interposers (i.e., connection layers between integrated circuits and printed circuit boards). We have recently described a novel high-throughput method for forming 10–20 (mu)m diameter TGVs in thin glass substrates that uses only a single laser source and requires no substrate modification and no chemical post-processing. Understanding the hole formation dynamics and the complex physical processes involved would aid greatly in extending this technique to larger vias, different material sets, and forming TGVs with better consistency and lower thermal stress. To this end, the dynamics and mechanism of the drilling process are elucidated in this study through time-resolved absorptance measurements in conjunction with multiphysical modeling and hole morphology measurements. It was found that the drilling dynamics vary drastically with the focus position of the beam: at the center of the process window, with the focus at the bottom surface of the glass, holes are drilled with a linear speed of 10 m/s, representing the fastest linear drill speed for TGV formation to date for applications in the integrated circuit substrate space; the formation dynamics are complex, and hole morphologies are consistent with material ejection from both sides of the glass. Near the edges of the process window, with the focus position ± 175 (mu)m from the bottom of the glass, the dynamics reveal incubation times of tens of microseconds prior to initiation of material removal, and much slower material ejection. The unique combination of time-resolved measurements and multiphysical modeling enables an in-depth understanding of the TGV formation mechanism that is unachievable with the individual techniques.