BiBurst mode, developed by laser application researchers at the RIKEN Centre for Advanced Photonics (RAP), uses GHz bursts of femtosecond laser pulses grouped in MHz envelopes to significantly speed up silicon ablation without sacrificing ablation quality.
The team from the Advanced Laser Processing Research Team has effectively boosted the throughput of silicon microfabrication by ablation for practical applications using the BiBurst mode, as reported in the International Journal of Extreme Manufacturing (IJEM).
The team has shown that, when air ionisation is avoided, the BiBurst mode can etch silicon at an ablation speed that is 23 times quicker than the single-pulse mode without degrading the ablation quality. It is highly desirable to increase ablation speed to increase throughput for real-world applications. Therefore, these discoveries may have a significant effect on both industrialists and basic scientists.
The team has previously stated that when compared to the single-pulse mode, the BiBurst mode femtosecond laser pulses improve the ablation efficiency and quality of crystalline silicon. By regulating the temporal energy deposition on silicon during the GHz bursts, femtosecond laser pulse trains with incredibly brief pulse-to-pulse intervals of several hundred picoseconds (ps) enhance the effectiveness and quality of ablation.
The team has since looked more closely at the potential for high-throughput silicon micromachining at substantially higher BiBurst pulse energies, which correspond to the combined energy of each pulse in the BiBurst pulse.
Importantly, when the same amount of total laser energy was given, the energy of each femtosecond laser pulse (intra-pulse) in the BiBurst pulse was considerably lower than the pulse energy for the single-pulse mode. When the intensity is over the threshold value for the single-pulse mode, air ionisation causes substantial damage to the ablated surface. Due to its lower intra-pulse intensities, BiBurst mode, in contrast, may deliver a significantly higher total energy to ablate silicon without causing air ionisation.
As a result, when compared to the single-pulse mode, the BiBurst mode achieves an ablation speed that is 23 times higher thanks to the synergetic effect of higher total energy and higher ablation efficiency. Additionally, the BiBurst may regulate the timing of energy deposition.
The team has suggested that the joint contribution of subsequent intra-pulses can help the GHz burst attain its enhanced ablation efficiency. Due to free electron creation, the burst’s preceding intra-pulses in particular create transitory absorption sites for the following intra-pulses, increasing the ablation efficiency.
On the other hand, because individual intra-pulses can directly induce ablation, the ablation efficiency rapidly declines as the intra-pulse energy surpasses the ablation threshold energy for the single-pulse mode. In this regime, it is no longer anticipated that the collaborative contribution of succeeding intra-pulses will increase ablation efficiency.
In contrast to a single GHz burst or single-pulse mode, the lower intra-pulse energy in the BiBurst mode can not only prevent air ionisation but also retain a greater ablation efficiency, resulting in a faster ablation rate.
Therefore, for realistic applications of silicon micromachining, BiBurst mode ablation has the potential to offer a substantially higher throughput while keeping good quality.
Corresponding author, Prof. Koji Sugioka, said that “The first demonstration of GHz burst mode femtosecond laser ablation was implemented by Ilday’s group in 2016. They showed that GHz burst mode ablation can improve ablation efficiency due to its efficient energy deposition (one order of magnitude higher). These results highly impacted the laser micro- and nano-processing community, and multiple research groups followed them to investigate GHz burst mode ablation.”
“Although the ablation efficiency is one of the important factors for the practical use, higher ablation rate per pulse is more critical to improving the throughput. In principle, increasing the laser intensity increases the ablation rate. However, the higher intensity of femtosecond laser pulses often suffers from incidental damage due to air ionization and excessive heat generation.”
“We have shown for the first time that GHz/MHz BiBurst mode of femtosecond laser has the possibility to significantly improve throughput without deterioration of ablation quality. We expect that the BiBurst mode femtosecond laser processing will rewrite the common sense of the femtosecond laser process to overcome bottlenecks in the industrial applications.”
