Thin (90 nm) a-Si:H films have been crystallized on Corning 7059 glass substrates by 120 fs pulses of Ti:sapphire laser.
Initial films were deposited using low-temperature plasma enhanced deposition technique. The pulses with wavelength
of 800 nm, pulse energy up to 0.8 mJ, and repetition rate of 1 kHz were employed. The focused to 280 micron laser
beam was raster scanned, using x-y sample translation by computer-controlled motors. The structural properties of the
films were characterized by the spectroscopy of Raman scattering, excited by the argon laser (line 514,5 nm). The
ablation threshold was found to be of about 65 mJ/cm2. When pulse energy density was lower than ~30 mJ/cm2 no
structural changes were observed. In optimal regimes the films were found to be fully crystallized with fine grain
structure, according to the Raman scattering data. Numerical estimates show the pulse energy density was lower than the
Si melting threshold, so non-thermal "explosive" impacts may play some role. The possibility of the femtosecond laser
pulses to crystallize Si films on glass substrates is shown for the first time. The results obtained are of great importance
for manufacturing of polysilicon layers on non-refractory substrates for thin film microelectronics.
Femtosecond laser systems offer a good solution for the creation of straight microcuts and grooves on macroscopic workpieces, as they are becoming more established in industrial applications. Although such linear ablation processes have been investigated and improved before, the main obstacle is still the long processing time. Increasing the processing speed by applying high pulse energies usually leads to a significant quality loss. Using high pulse repetition rates at low pulse energies would lead to the best results, but the repetition rate of commercially available laser sources is mostly restricted to one to several kilohertz. However, a systematic investigation of further relevant parameters enables the processing quality and speed to be optimized. To demonstrate these relations, cuts and grooves using different motion parameters and focusing strategies are presented at the example of metal and silicon samples. With regard to the focusing strategy, it is shown that by using linear focus shapes in the direction of the cut, cutting speeds can be increased while maintaining high edge qualities of the cuts and grooves. The presented results prove the potential of femtosecond lasers for high quality cuts in different industrially relevant materials.
There is a proven potential of ultra-short laser pulses for precise cutting of all kinds of materials. Especially with regard to miniaturizations in the semiconductor industry, even industrial high-speed cutting processes nowadays aim for the precision of femtosecond laser systems. However, when working with a typical spot-focused laser beam on current standard systems, the processing speed is too low for an industrial cutting of larger parts as, for instance, silicon wafers. Apart from improving the laser systems with regard to pulse energies and repetition rates, it is therefore crucial to realize that material ablation can also be influenced by other process parameters. In case of straight microcuts e.g. in silicon wafers, a new approach is a beam shaping strategy using certain arrangements of cylindrical lenses. This can significantly contribute to an increase of the achievable cutting speed, and at the same time reduce the minimal kerf width while using the highest available laser power. We present examples of such kerfs in thin silicon wafers using a system of cylindrical lenses in comparison to a customary achromatic lens, and provide information about the focusing process and the chances and challenges entailed.
Semiconductor materials are still the material of choice also in many microsystems applications. This is mainly justified by the availability of mature processes and equipment originally developed for microelectronics fabrication. However, for microsystems more flexible requirements have to be fulfilled and new tools have to be developed especially with regard to smaller part numbers than in microelectronics. But also in microelectronics, conventional machines have often reached their limits in semiconductor processing which leads to the requirement of new processes. Lasers are generally able to ablate semiconductor materials. Especially ultrafast lasers are able to perform processes with high efficiency and accuracy. One of the most challenging conditions is not to influence the bulk material. Within this paper, the general interaction of ultrafast lasers with semiconductors is investigated. The ablation process is outlined and beam parameters influencing the removal and especially the cutting process are described, and potential applications are shown.