High-throughput, high-quality micromachining of semiconductors with a 1kW sub-picosecond laser

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Daniel Holder, of the IFSW at the University of Stuttgart, shares how new USP laser technologies could facilitate the rapid micromachining of silicon wafers

Co-authored by: Christoph Röcker, Rudolf Weber, Marwan Abdou Ahmed and Thomas Graf, of the IFSW at the University of Stuttgart; David Bruneel of Lasea; Gerhard Kunz of Bosch; and Martin Delaigue of Amplitude

For the high-power micromachining of silicon wafers, a novel ultrashort pulse (USP) laser beam source with an average power exceeding 1kW, and a process for machining high-quality surfaces at high throughput, were developed in the EU project Hiperdias, which concluded in 2019.

The project not only achieved a record-breaking material removal rate of 3.8mm³/s at low roughness Sa < 0.6μm, but also saw high-power micromachining with over 1kW average laser power at sub-picosecond pulse duration demonstrated for the first time.

Micromachining with USP lasers enables the creation of technical, high-quality surfaces with low roughness and minimised melt formation, due to the low and local heat input. A fast and local removal of the material is necessary for the application of laser micromachining in processes with larger machining volumes. Examples include post-processing of additively manufactured metal components to improve their shape accuracy and surface quality1, or the removal of damaged areas in the repair of carbon fibre-reinforced plastic (CFRP) components2.

An emerging topic with applications in medical imaging, security and communication is the manufacture of optics for terahertz (THz) radiation. The optics can be manufactured from silicon wafers by micromachining with USP lasers. In the past, laser micromachining has been limited in many potential applications due to the low available average laser power and resulting low throughputs. With the recent development of beam sources in the kW range, new challenges are now emerging in converting the high laser power into high quality at high throughputs.

Equipment setup

To address these challenges, an application laboratory for high-power USP lasers in the kW range was established at the IFSW at the University of Stuttgart3. The beam source developed in Hiperdias was built at the IFSW4 and emitted laser pulses with duration <600fs at a wavelength of 1,030nm and a beam quality of M² < 1.5. The linearly polarised laser beam was guided into a Lasea LS 5-1 processing station (see figure 1) by means of deflection mirrors.

Figure 1: Processing station from Lasea for micromachining with a 1kW sub-picosecond laser from the IFSW. (Image: Lasea)

After the focusing optics with a focal length of 580mm and a galvanometer scanner for fast beam deflection, a maximum available average power of 1,010W could be measured on the workpiece. The laser was operated at a repetition rate of 500kHz, which corresponds to a maximum available single pulse energy of 2.02mJ. The single pulse energy could be evenly distributed over five sub-pulses in a burst train, each with a time interval of 23ns. The minimum beam diameter on the workpiece surface with the focus position on the workpiece surface was about 90μm. The process development was carried out in an ambient atmosphere on the polished side of 100mm diameter, 1mm-thick silicon wafers. Squares of 5 x 5mm² were scanned along parallel offset lines to create cavities. Figure 2 shows an example of the resulting surfaces after laser micromachining with different processing parameters. To increase the depth of the cavities, several scans were performed.

Figure 2: Silicon wafer structured with ultrashort laser pulses. (Image: Holder et al.)

Machining strategy

Machining with the focal position on the sample surface resulted in a laser fluence (energy density) per sub-pulse of 12J/cm² on the workpiece at 950W average laser power, which is a factor of 120 above 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 Sa = 3.6μm, but nanocracks and solidified melt were still visible on the surface (see figure 3b).

Figure 3: Machining strategies and resulting surface structure in the micromachining of silicon with ultrashort laser pulses and 950W average laser power. (Image: Holder et al.)

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.

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).

Figure 4: High-quality 3D micromachining and large-area surface structuring of silicon with ultrashort laser pulses and 1.01kW average laser power. (Image: Holder et al.)

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.

 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. (Image: Holder et al.)

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.

Daniel Holder is a research associate for laser material processing at the IFSW

References

  1. 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).
  2. 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).
  3. 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).
  4. 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.
  5. 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).
  6. 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).

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