GigaBurst

Increasing the efficiency of USP deduction

Aim of the development

Ultra-short pulse lasers allow very precise material processing with structure sizes in the micrometer range (lateral) and ablation depths below micrometers (axial) largely independent of the material to be processed. The processing is very gentle, requires no auxiliary materials such as etchants and hardly heats the workpiece. A disadvantage of the process, however, is the low removal rate, i.e. cubic meters of material removed per minute. Depending on the material, typical values in fine micromachining are in the range of 0.1 to 5 cubic meters per minute. There is some theoretical work on the fact that these values represent material-specific maxima in each case and can basically no longer be improved with single pulses. Therefore, the ablation behavior of the materials was investigated with so-called bursts. These are laser pulse trains within which the pulse repetition rate is very high (here 5 GHz). These pulse trains can only recently be generated by laser technology at energies sufficient for material processing. In essence, therefore, the question is whether the productivity of the laser tool increases if, for example, instead of a single pulse with 1 mJ pulse energy, a pulse train consisting of 1,000 pulses with 5 GHz is used, which has a total energy of 1mJ. Thus, the same laser energy input is compared in each case, but distributed differently in time. Experiments on very different materials (steel, copper, silicon, glass, and ceramics) were conducted to determine where the ablation rate, and thus productivity, could be increased by these GHz bursts. In each case, the three typical processes of drilling, cutting and surface ablation were investigated.
Advantages and solutions

The overall goal, an increase in material removal efficiency compared to classical machining with kHz repetition rates, was achieved. In particular for silicon and copper, and to some extent also for steel and ceramics, it was shown that the use of GHz bursts can increase the specific material removal rate by up to an order of magnitude without significantly reducing the machining quality. In addition, this type of processing allows the use of higher average powers in a single laser beam without having to use beam splitting techniques. Depending on the material, these advantages could be achieved for drilling only or for line and surface ablation as well. Advantageous parameter ranges for silicon machining were also found, which were previously unknown in the literature. On the way to these results, the materials were also observed in their thermals and a numerical model of the heating was created. The partial goal of numerically describing the laser ablation process in its thermals could not be achieved. this was mainly due to the complexity of the physical processes involved. These could not be abstracted or simplified sufficiently to describe the process correctly within an adequate computing time. However, the work on thermal behavior was informative in that it was possible to observe with a thermal imaging camera in each case how strongly the workpieces heat up and for which type of processing. The project results allow a significant cost reduction in the machining of the materials investigated, since the machine times for the removal itself can be reduced accordingly.

Advantages and solutions

The overall goal, an increase in material removal efficiency compared to classical machining with kHz repetition rates, could be achieved. In particular for silicon and copper, and to some extent also for steel and ceramics, it was shown that the use of GHz bursts can increase the specific ablation rate by up to an order of magnitude without significantly reducing the processing quality. In addition, this type of processing allows the use of higher average powers in a single laser beam without having to use beam splitting techniques. Depending on the material, these advantages could be achieved for drilling only or for line and surface ablation as well. Advantageous parameter ranges for silicon machining were also found, which were previously unknown in the literature. On the way to these results, the materials were also observed in their thermals and a numerical model of the heating was created. The partial goal of numerically describing the laser ablation process in its thermals could not be achieved. this was mainly due to the complexity of the physical processes involved. These could not be abstracted or simplified sufficiently to describe the process correctly within an adequate computing time. However, the work on thermal behavior was informative in that it was possible to observe with a thermal imaging camera in each case how strongly the workpieces heat up and for which type of processing. The project results allow a significant cost reduction in the machining of the investigated materials, since the machine times for the removal itself can be reduced accordingly.

Target market

According to industry association Spectaris, the photonics market has developed very well in recent years: Domestic business could grow by 15% in 2021 and foreign business by 19% compared to 2020, which is in line with the growth of previous years. The target markets for the project results include, on the one hand, all application areas in which conventional laser drilling processes are used. Examples include the drilling of injection nozzles, back-contact solar cells, needles for medical technology, filters, etc. In addition, in view of the project results achieved, the development of new markets is also conceivable. These markets include, above all, those for which laser drilling or structuring by means of UKP has so far been too inefficient in terms of time/cost expenditure, especially for macromaterial processing. These include the generation of cooling air and contact holes, the processing of guide plates in the electronics and semiconductor industries, and the generation of tribulogical structures and holes on large surfaces (> m), e.g. for wings and ship propellers. As transfer companies, small and medium-sized companies benefit on the one hand by increasing their productivity while maintaining or even improving quality. However, the greater potential is in the industrial sectors where laser drilling processes are already used on a large scale (e.g. for drilling injection nozzles). A direct cost comparison with other processes is only possible to a limited extent, since different processes perform differently depending on the target geometry. Laminar technology on aircraft wings and vertical stabilizers can be used as an illustrative application and calculation example. As a technology for reducing flow resistance and thereby saving resources and costs, it plays a central role in current aerospace research and development. The aim of the technology is to maintain the laminar flow of the boundary layer surrounding the wing by means of so-called Hybrid Laminar Flow Control (HLFC). Laminar retention is achieved by extracting a defined amount of the boundary layer via a micro-perforated outer skin. The outer skin then consists of a uniform micro-perforation in stainless steel or titanium with hole diameters of 50 µm and a hole spacing of 500 µm. The material thickness depends on the design of the outer skin and is 600-800 µm for a monolithic design and approx. 100 µm for a hybrid design. The laser drilling, electron beam drilling and (fine) etching processes are cited as particularly suitable for large-area perforation. Thus, depending on the material thickness, type and quality, drilling rates of up to 400 holes per second were achieved for monolithic construction with laser drilling and up to 200 holes per second with electron beam drilling. In etching for hybrid construction, it was shown that a large number of holes can be formed simultaneously. In the studies to date, the best results in laser drilling were achieved by using a fiber laser with 200 W average power, a repetition rate of 200 kHz, a pulse duration of 120 ns, and a pulse energy of 1 mJ. In conjunction with a galvanoscanner and F-theta optics, 500 pulses were required to drill 800 µm thick titanium, achieving a drilling rate of 400 holes per second. The biggest problems in optimizing the process were the thermal influences that occurred. It should also be noted that the holes have a quality typical of nanosecond machining, i.e. fusion edges. Due to the continuous development of UKP laser systems towards high average powers, established laser systems with > 100 W average power, pulse durations below picoseconds, high repetition rates > 100 kHz and pulse energies > 500 µJ are available. If the project results are fully implemented, the ablation rates of the UKP process would be in the order of magnitude of the ablation rates of the nanosecond processes and similar drilling rates could be achieved with significantly higher quality. In addition, the material would be subjected to less thermal stress, so that problems such as distortion of the sheets would be less of an issue. The machining time required to drill all the necessary surfaces on an A320 would thus be reduced to less than a day, and the process would be economical. This example can be used to illustrate the possibilities of the technology investigated in this project and the associated cost reductions. Thus, the use of UKP technology is becoming increasingly interesting, especially for macro applications, as it acquires the potential to compete with or even surpass other processes.