Researchers from the Institute of Intense Lasers and Applications (CELIA) at the University of Bordeaux have developed a glass micro-drilling method using a femtosecond laser operating in gigahertz-burst (GHz burst) mode. The researchers used the method to achieve taper-free, elongated holes with smooth inner walls without creating cracks in the glass.
Typically, laser drilling with standard single femtosecond pulses results in tapered holes of strongly limited length and rough inner surface. The laser-matter interaction regime permits the drilling of holes with high aspect ratio in a single step without any chemical etching.
The operating regime of the femtosecond laser in gigahertz-bursts instead of repetitive single pulses enables interaction with the glass material combining two advantages, according to the researchers. The laser energy deposition in the material is distributed over a longer time, resulting in a highly efficient, controlled cumulative regime, whereas the outstanding micromachining quality of femtosecond laser pulses is conserved.
Further, the resulting top-down percussion drilling gives taper-free, elongated, and crack-free holes with glossy inner walls.
“Originally, our research on glassing by femtosecond laser micromachining was driven by the application of TGV (through glass via) for Fan-Out-Wafer Level Packaging,” University of Bordeaux professor Inka Manek-Hönninger said in an email to Photonics Media.
Within the frame of a collaborative project with ultrafast laser manufacturer Amplitude, which supplied the laser used in the study, and ALpHANOV, an optics and lasers technology R&D center located at the University of Bordeaux, the team investigated different glass drilling techniques using femtosecond lasers operating with various parameters and beam engineering strategies. The comparative study, based on the influence of different laser parameters on the depth as well as on the diameter, allowed the researchers to optimize the drilling conditions and achieve the reported high aspect ratios of the drilled holes.
Femtosecond laser GHz-burst drilling in glass. Courtesy of Pierre Balage et al.
In sodalime glass, for example, the researchers obtained crack-free holes of aspect ratios up to 37. In fused silica, the aspect ratios reached 73.
“For some materials like fused silica, the inner walls of the hole are glossy and almost transparent,” Manek-Hönninger said.
Through its study of the geometries of the holes, the researchers developed a hypothesis of a beam propagation by multiple internal reflections within the hole during the drilling, where the reachable hole depth at a fixed burst fluence is only limited by refraction losses.
“It turned out that the GHz-burst approach allows for drilling much more elongated holes than the standard approach of repetitive single femtosecond laser pulses,” Manek-Hönninger said.
The results hold implications for industrial purposes, although the process speed is limited by the thermal properties of the target material to avoid a heat affected zone. A parallelizing strategy, Manek-Hönninger said, must be implemented in order to meet industrial requirements in terms of throughput. Potential solutions could implement spatial light modulators, diffractive optical elements, or multiplane light conversion.
The femtosecond laser process using the GHz-burst mode may also hold promise for precise positioning of optical fibers or fiber arrays in photonics, or astrophysics, and could be used for drilling micro-probe handling tools used for printed chip circuit board control in microelectronics manufacturing.
The research was published in International Journal of Extreme Manufacturing (www.doi.org/10.1088/2631-7990/acaa14).