Using a new technology known as a GHz pulsed femtosecond laser, scientists have increased the rate of silicon ablation by 23 times

Scientists at the RIKEN Center for Advanced Photonics in Japan have announced a 23-fold increase in silicon ablative processing using a new laser technology called "BiBurst".

The technique is achieved by using GHz femtosecond laser pulses grouped in a MHz envelope. The researchers demonstrated that the "BiBurst" mode was able to etch silicon at an ablative rate 23 times faster than the monopulse mode, without affecting the ablative quality, while avoiding air ionization. The team's research has been published in the International Journal of Extreme Manufacturing (IJEM).

It is necessary to increase the ablation rate in wafer semiconductor processing. In principle, the more intense the laser, the faster the ablation rate -- however, higher intensity femtosecond laser pulses are often accidentally damaged by ionizing the air and generating too much heat. BiBurst mode femtosecond laser processing has changed the common sense of femtosecond laser processing, and will help overcome a series of bottlenecks in industrial applications.

In a GHz pulse, the incident femtosecond laser pulse sequence has an extremely short pulse interval of several hundred picoseconds (ps), controlling the timing of energy deposition on the silicon and improving ablation efficiency and quality. The team then further explored the ability to perform high-speed, finely processed silicon at significantly higher energies of the BiBurst pulses, which correspond to the combined energy of each pulse in the BiBurst pulses.

It is worth noting that the energy of each femtosecond laser pulse (inner pulse) in the BiBurst pulse is significantly less than the energy of the pulse in the monopulse mode when delivering the same total laser energy. In monopulse mode, once the intensity exceeds the critical value, air ionization will cause serious damage to the ablative surface. In contrast, the BiBurst mode, due to its lower in-pulse intensity, provides a higher total energy to ablate silicon without causing air ionization.

Therefore, the BiBurst mode can improve the ablation rate by 23 times compared with the monopulse mode under the synergistic effect of total energy and ablation efficiency, while avoiding air ionization. In addition, BiBurst's timing control of energy deposition allows it to maintain a high ablative quality even at such a high total energy.

However, in monopulse mode, when the energy within the pulse exceeds the ablation threshold energy, the ablation efficiency will gradually decrease, because ablation can be directly induced within a single pulse. In this case, the ablative efficiency can no longer be improved depending on the synergistic contribution of successive internal pulses.

In conclusion, the lower energy in the pulse under BiBurst mode can not only avoid air ionization, but also maintain a higher ablation efficiency, thus obtaining a higher ablation rate than the GHz burst monopulse or monopulse mode. Thus, laser ablation in BiBurst mode has the potential to achieve higher processing throughput while maintaining high quality silicon micromachining applications.

Source: OFweek