ANALYSIS: MICROMACHINING
“Laser micromachining has previously been limited in many applications due to the low available average power”
Figure 2: Silicon wafer structured with ultrashort laser pulses
Figure 3: Machining strategies and resulting surface structure in the micromachining of silicon with ultrashort laser pulses and 950W average laser power
Sa = 3.6µm, but nanocracks and solidified melt were still visible on the surface (see figure 3b). Due to the increased beam diameter at a constant scanning speed of 10m/s over the workpiece, increased pulse- to-pulse heat accumulation and overheating of the surface occurred. By increasing the scanning speed to 24m/s, the roughness was further reduced to Sa = 0.4µm and surface defects such as nanocracks and melt formation were completely avoided. The resulting surface with a fine ripple structure is shown in figure 3c. More detailed investigations and correlations on the influence of machining parameters on surface quality and removal rate in the micromachining of silicon have also been published5,6.
Figure 4: High-quality 3D micromachining and large-area surface structuring of silicon with ultrashort laser pulses and 1.01kW average laser power
Figure 5: High-quality micromachining of a) stainless steel, b) copper and c) aluminium with simultaneous high throughputs using ultrashort laser pulses and 1.01kW average laser power
the ablation threshold of 0.1J/ cm². Machining with high laser fluences caused a high roughness Sa = 16.6µm, as well as nanocracks and melt formation on the surface (see figure 3a). By shifting the focal position about four Rayleigh
lengths (17mm) below the sample surface, the beam diameter was increased to about 370µm and the laser fluence was reduced to about 0.7J/cm². Machining with low laser fluence caused a significantly reduced roughness
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Demonstration At an average laser power of 1.01kW, the machining strategy of low laser fluence and high scanning speed demonstrated high-quality 3D micromachining using a wedge-shaped geometry with an area of 5 x 5mm² and continuous depth increase to over 300µm cavity depth (see left of figure 4). The low average roughness of Sa = 0.6µm and low peak- to-valley roughness of 5.7µm over the entire wedge surface demonstrate a high machining quality, thus a possible use in applications with high demands on surface quality, such as THz optics. At the same time, a record-breaking throughput for micromachining was achieved with a very high material removal rate of 3.8mm³/s or 230mm³/min, corresponding to an increase of about a factor of 440 for comparable surface finishes. Due to the large machining field of 300 x 300mm², it was possible to produce large-area and high- quality surface structuring on silicon wafers (right of figure 4).
The compatibility of high
quality and simultaneously high throughput in micromachining with USP lasers in the kW range is not limited to silicon with the developed machining strategy. The application on metals such as stainless steel, copper and aluminium (see figure 5) showed comparable results to silicon, with a fine ripple structure on the surface and roughness Sa in the range of 1µm at removal rates of 2 to 3mm³/s.
Future work These results show the great potential of high average power USP lasers in high- quality micromachining of semiconductors and metals with high throughputs. Future research topics are the elaboration of strategies and development of technologies for the implementation of the high powers on the workpiece, for applications with smaller target geometries, for example by means of beam shaping and beam splitting. This project received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 687880.l
Daniel Holder is a research associate for laser material processing at the IFSW
References 1.
2.
D. Holder, A. Leis, M. Buser, R. Weber, and T. Graf, ‘High-quality net shape geometries from additively manufactured parts using closed- loop controlled ablation with ultrashort laser pulses,’ Advanced Optical Technologies 9, 101–110 (2020).
3.
D. Holder, M. Buser, S. Boley, R. Weber, and T. Graf, ‘Image processing based detection of the fibre orientation during depth-controlled laser ablation of CFRP monitored by optical coherence tomography,’ Materials & Design 203, 109567 (2021).
4.
A. Peter, D. Brinkmeier, M. Buser, V. Onuseit, and T. Graf, ‘Automated free-space beam delivery system for ultrafast laser beams in the kW regime,’ Procedia CIRP 94, 951–956 (2020).
5.
C. Röcker, A. Loescher, M. Delaigue, C. Hönninger, E. Mottay, T. Graf, and M. A. Ahmed, ‘Flexible Sub-1 ps Ultrafast Laser Exceeding 1 kW of Output Power for High-Throughput Surface Structuring,’ in Laser Congress 2019 (ASSL, LAC, LS&C) (OSA), AM4A.2.
6.
D. Holder, R. Weber, C. Röcker, G. Kunz, D. Bruneel, M. Delaigue, T. Graf, and M. A. Ahmed, ‘High-quality high-throughput silicon laser milling using a 1kW sub-picosecond laser,’ Optics letters 46, 384–387 (2021).
D. Holder, R. Weber, C. Röcker, G. Kunz, D. Bruneel, M. Delaigue, T. Graf, and M. A. Ahmed, ‘Scaling the throughput of high-quality silicon laser micromachining,’ Lasers in Manufacturing Conference (2021).
AUTUMN 2021 LASER SYSTEMS EUROPE 19
Holder et al.
Holder et al.
Holder et al.
Holder et al.
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