Time-resolved Spectroscopy of Glass Ablation during Micro-via Processing using 248 nm Excimer Laser for Semiconductor Interposer Packaging

A glass interposer with micro-vias is promising for next-generation semiconductor packaging for post-5G high performance computing. Glass is the most promising material due to its electrical and mechanical characteristics; however, it is difficult to fabricate micro-vias in glass due to its high brittleness. The fabrication speed should also be improved to realize the commercialization of glass interposers. We have researched improvements in the production of micro-vias in glass, and optical systems with glass micro-vias. Processing glass material a 248 nm excimer laser was found to produce micro-vias (<15 m diameter) with very high aspect ratios (>33) and very high quality (no microcracks). Furthermore, long laser pulse widths (74 ns time-integrated square (TIS)) resulted in a higher drilling rate than short pulse widths (32 ns TIS). Herein, we report the results of spectroscopy measurements for the ablation of glass using a 248 nm excimer laser, together with the results of time-resolved spectroscopy. We also report the pulse width dependence of the time-resolved spectroscopy results for short and long laser pulses to determine the reason for the high drilling rate for a long pulse width. The measurement results indicated that the wavelengths of light emitted during ablation were mainly 280, 394, 422 nm. This emission began 30 ns after laser irradiation was started. We consider that absorption increases after laser irradiation, and that it takes approximately 30 ns for the laser energy to be absorbed and the ablation threshold to be reached. The time at which ablation occurred was almost constant, regardless of the excimer laser pulse width, even though the intensity of the long laser pulses was lower than that of the short pulses. The results indicate that a larger amount of laser energy is wasted during short-pulse laser drilling until ablation begins, so that a higher drilling rate is achieved with long pulses.