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MANUFACTURING with short pulse lasers
home-made laser systems rendering systematic and comparative studies difficult. However, the benefit of heat accumulation was proven and could be explained by the gentle interaction of the laser with the glass material making it more ductile, and thus, less prone to crack formation.
Nowadays, MHz-bursts are widely used for ablation-free glass cutting with femtosecond lasers for example to cut screens of smartphones and tablets. The time spacing between subpulses is typically several tens of ns and the number of pulses within a burst is not necessarily very high, for example 4 to 20, for MHz-bursts of commercially available femtosecond laser sources based on Ytterbiumdoped active media emitting at a wavelength of around 1030 nm and meeting industrial requirements.
The ablation-free cutting process [ 4 ] is realized in two distinct steps. First, the glass is pre-cut by defined, oriented cracks arising from the interaction with a needle-formed laser beam, a socalled Bessel-Gauss beams. The spatial beam-shaping into such Bessel-Gauss beams is achieved using an axicon and subsequent imaging to adjust the desired size of the Bessel-beam. Second, the parts are mechanically separated. The cutting process is schematically illustrated in Figure 4. The cracks can be oriented along the laser trajectory using a phase mask.
More recently, percussion drilling of different glasses using GHzbursts was investigated. Here, the time spacing between the sub-pulses is on the order of one nanosecond,
Figure 4. Schematic illustration of ablation-free glass pre-cutting with a Bessel-beam.( a) Top view of oriented cracks along the glass.( b) Side view of successive shots forming the cutting plane along the laser trajectory. On the right, microscope images. and the bursts contain several tens to hundreds of sub-pulses. The energy of the individual pulses is typically much lower than in the case of MHz-bursts.
Modern, commercially available laser systems offer the possibility of switching between operation modes of repetitive single pulses, MHz-bursts, and GHz-bursts. This allows for very accurate studies comparing the three operation regimes as the beam is delivered by the same laser source avoiding fluctuations due to system change or realignment of the beam path.
The GHz-burst approach offers the possibility of drilling holes with a rather different morphology than with standard repetitive single pulses as shown on Figure 5a. The holes produced by GHz-bursts are almost cylindrical and of outstanding quality with smooth and glossy inner walls. Moreover, much deeper drillings are possible compared to machining with repetitive single pulses at near-infrared wavelengths. In fused silica, crack-free holes of very high aspect-ratios of up to 150:1 could be demonstrated. The drilling mechanism has been described as a three-stage process [ 5 ]. First, there is surface ablation with free expansion of the plume into the ambient air. Secondly, there is deep ablation and plume confinement due to an interaction of the plume with the inner walls. The confinement induces a saturation of the drilling speed. Finally, termination of the drilling occurs when the fluence within the holes decreases below the ablation threshold due to losses caused by refraction and scattering.
Drilling of through holes( Through Glass Via or TGV) is of increasing interest for the production of glass interposers for applications in microelectronics. Figure 5b shows microscope images of a matrix of through holes in fused silica drilled in GHzburst operation mode. The holes are very regular and the surrounding material is completely crack-free. The GHz-burst is an interesting and innovative approach for single-step
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