@article{RungHaeckerHellmann2021, author = {Rung, Stefan and H{\"a}cker, Niklas and Hellmann, Ralf}, title = {Thermal imaging of high power ultrashort pulse laser ablation of alumina towards temperature optimized micro machining strategies}, series = {IOP Conference Series: Materials Science and Engineering}, volume = {1135}, journal = {IOP Conference Series: Materials Science and Engineering}, number = {012027}, doi = {0.1088/1757-899X/1135/1/012027}, pages = {1 -- 9}, year = {2021}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{LutzHelmTschirpkeetal.2023, author = {Lutz, Christian and Helm, Jonas and Tschirpke, Katrin and Esen, Cemal and Hellmann, Ralf}, title = {Drilling Sequence Optimization Using Evolutionary Algorithms to Reduce Heat Accumulation for Femtosecond Laser Drilling with Multi-Spot Beam Profiles}, series = {MDPI Materials}, volume = {2023}, journal = {MDPI Materials}, number = {16/5775}, doi = {10.3390/ma16175775}, pages = {1 -- 12}, year = {2023}, abstract = {We report on laser drilling borehole arrays using ultrashort pulsed lasers with a particular focus on reducing the inadvertent heat accumulation across the workpiece by optimizing the drilling sequence. For the optimization, evolutionary algorithms are used and their results are verified by thermal simulation using Comsol and experimentally evaluated using a thermal imaging camera. To enhance process efficiency in terms of boreholes drilled per second, multi-spot approaches are employed using a spatial light modulator. However, as higher temperatures occur across the workpiece when using simultaneous multi-spot drilling as compared to a single-spot process, a subtle spatial distribution and sequence of the multi-spot approach has to be selected in order to limit the resulting local heat input over the processing time. Different optimization approaches based on evolutionary algorithms aid to select those drilling sequences which allow for the combination of a high efficiency of multi-spot profiles, a low-generated process temperature and a high-component quality. In particular, using a 4 x 4 laser spot array allows for the drilling of 40,000 boreholes in less than 76 s (526 boreholes/s) with a reduced temperature increase by about 35\%, as compared to a single spot process when employing an optimized drilling sequence.}, subject = {Laserbohren}, language = {en} } @inproceedings{LutzSommerhuberKettneretal.2024, author = {Lutz, Christian and Sommerhuber, Ryan and Kettner, Matthias and Esen, Cemal and Hellmann, Ralf}, title = {Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000954}, year = {2024}, abstract = {We report on process sensing using a membrane-free optical microphone to monitor the acoustic emission during ultrashort pulsed laser ablation of multilayer materials. The acoustic emission during ablation is used to detect material transitions, with the specific signatures allowing to create a reliable process control for identifying individual layers. The outstanding properties of membrane-free optical microphones in terms of high bandwidth and high temporal resolution are ideally qualified for characterizing an ultrashort pulsed laser process, with its properties and capabilities being presented in this contribution. In particular, for layer- and material-selective ablation of multilayer printed circuit board components, copper and polyimide layers are ablated and the material transition is detected by analyzing the acoustic signal at different frequency levels, which is a novelty in the field of ultrashort pulsed laser process sensing. The investigations show, that the optical microphone can be used to resolve both the scanning paths and ablated layers by means of interruptions in a time-resolved acoustic spectrogram. Furthermore, as a result of a higher ablation rate of polyimide compared to copper and thus the increase of the emitted acoustic energy, the material transition between copper and polyimide layers can clearly be detected. The detection of this process event can be used for process control.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{LutzHelmEsenetal.2024, author = {Lutz, Christian and Helm, Jonas and Esen, Cemal and Hellmann, Ralf}, title = {Computational optimization of borehole sequences for the reduction of heat accumulation in drilling processes using ultrashort pulse lasers}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000930}, year = {2024}, abstract = {We report on laser drilling of borehole arrays using a high-power ultrashort pulse laser with particular focus on reducing heat accumulation in the workpiece by optimizing the drilling sequence, particularly for highly efficient multi-spot drilling. Different optimization approaches are chosen to improve the drilling sequence, also comparing a simplex algorithm and an evolutionary algorithm. From a laser application point of view, we also compare drilling sequences using a single spot and up to 16-fold multi-spots generated by a spatial light modulator, as to accelerate the drilling process in terms of the number of drilled holes per second. To evaluate the temperatures generated during drilling of up to 40,000 holes in less than 76 seconds in stainless steel foil, temperatures are measured by a thermal imaging camera and subsequently compared to a COMSOL-based simulation for all optimized drilling sequences. With respect to an average temperature of 706 °C without optimization, a reduction by 252 °C, i.e., a reduction by nearly 36 \% based on the Celsius scale, is achieved using a 4 × 4 beam splitter and an optimized drilling sequence with a drilling rate of 526 holes per second. In addition, using a 2 × 2 beam splitter, a temperature reduction of up to 40.5 \% is achieved for a drilling process with a rate of 129 holes per second using an optimized drilling sequence.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{LutzSchwarzMarxetal.2023, author = {Lutz, Christian and Schwarz, Simon and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Multi-Bessel Beams Generated by an Axicon and a Spatial Light Modulator for Drilling Applications}, series = {MDPI Photonics}, volume = {2023}, journal = {MDPI Photonics}, number = {10/413}, doi = {10.3390/photonics10040413}, pages = {1 -- 9}, year = {2023}, abstract = {We report on an optical setup to generate multi-Bessel beam profiles combining a refractive axicon and a spatial light modulator. Based on their particular beam profile, Bessel beams offer advantageous properties for micro drilling processes and internal volume processing, especially for transparent materials. In addition, the laser power of industrial, ultrashort pulsed lasers has increased significantly over the last few years, offering the possibility for highly efficient processes using multi-spot profiles. Our optical concept combines the dynamic possibilities of beam splitting using a spatial light modulator with the benefits of Bessel beams, which facilitates multi-Bessel beam processing. Beside the simulation and experimental evaluation of the generated multi-Bessel beams, we exemplify the applicability of the developed module for the perforation of thin metal foils by micro drilling.}, subject = {Bessel-B{\"u}ndel}, language = {en} } @article{MarxLutzHellmannetal.2022, author = {Marx, Jan and Lutz, Christian and Hellmann, Ralf and Esen, Cemal}, title = {Holographic multi-spot generation for ultra-short pulse Bessel beam processing of stainless steel}, series = {Procedia CIRP}, volume = {111}, journal = {Procedia CIRP}, publisher = {Elsevier BV}, issn = {2212-8271}, doi = {10.1016/j.procir.2022.08.003}, pages = {648 -- 652}, year = {2022}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{LutzSchwarzMarxetal.2024, author = {Lutz, Christian and Schwarz, Simon and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Combination of an axicon and a spatial light modulator for the generation of multiBessel beams for microdrilling using ultrashort pulsed laser}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000946}, year = {2024}, abstract = {Due to their special beam profile, Bessel beams offer advantageous properties for micro-drilling and internal volume processing, especially for transparent materials. In addition, the laser power of industrial ultrashort pulsed lasers hasnincreased significantly in recent years, offering the possibility of highly efficient processes using multi-spot profiles. We report on optical simulations and an experimental optical setup for generating multi-Bessel beams by combining a refractive axicon and a spatial light modulator. This setup offers a new possibility of multi-Bessel beam generation by functional division of the Bessel beam generation by the axicon and the possibility of separation flexible beam splitting by a spatial light modulator. The beam profiles generated with this setup are analyzed and compared in this study using optical simulations and experiments. In addition to the analysis of the Bessel beams in the machining plane, they are also analyzed in the propagation direction by means of intensity distributions measured by a camera in small-step z positions. Furthermore, the experimental applicability of the module-based beam shaping system is demonstrated by integrating into an existing laser system.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{HaefnerBischoffHelmetal.2023, author = {H{\"a}fner, Tom and Bischoff, Kay and Helm, Jonas and Kunz, Tim and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Picosecond laser microvia drilling of ABF material using MHz burst mode}, series = {Materials Research Express}, volume = {10}, journal = {Materials Research Express}, number = {9}, pages = {096301 -- 096311}, year = {2023}, abstract = {We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is being evaluated by the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are being discussed on the ablation threshold, penetration depth and laser microvia drilling quality. We find that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer. Therefore, the parameter clearance is introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5\% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7\% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{FranzYangMicheletal.2023, author = {Franz, Daniel and Yang, Yongting and Michel, Luis and Esen, Cemal and Hellmann, Ralf}, title = {Evaluation of an ultrashort pulsed laser robot system for flexible and large-area micromachining}, series = {Journal of Laser Applications}, volume = {35}, journal = {Journal of Laser Applications}, number = {4}, pages = {042057 -- 042065}, year = {2023}, abstract = {We report for the first time on the realization, characterization and application of an ultrashort pulsed laser robot system for flexible and large-area 2D and 3D laser micromachining with 6 articulated axes. To characterize the dynamic positioning of the laser beam during and after axes movement, CMOS image sensors were integrated into the beam path. A method introduced for the alignment of the optical axes allows a reduction of the deviations in laser beam positioning to less than 141.8 ± 92.9 μm within a 110° rotation range of axis 4. In addition, a high laser beam positioning repeatability of less than 102.2 μm is demonstrated over a total period of 14 h for a movement of axis 5 within a range of 0° to 90°. Initial laser cutting, laser structuring and laser marking applications on automotive dashboards and glass substrates are presented for flexible and large area 2D and 3D manufacturing. By applying a special laser cutting strategy for processing AF 32 eco thin glass, high cutting quality is achieved with a taper of up to 96.3\% without the generation of cracks, demonstrating the innovative potential of the high-precision laser robot system. Nonetheless, different identified inherent influences of each axis 1-5 during robot axis movement demand for an innovative beam stabilization concept to achieve high precision in laser beam positioning.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{FranzSchusterSchwarzetal.2025, author = {Franz, Daniel and Schuster, David and Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fabrication and analysis of through-glass vias for glass-based electronic packaging using an ultrashort pulsed laser}, series = {Optics and Lasers in Engineering}, volume = {193}, journal = {Optics and Lasers in Engineering}, publisher = {Elsevier BV}, issn = {0143-8166}, doi = {10.1016/j.optlaseng.2025.109106}, year = {2025}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{FranzYangEsenetal.2026, author = {Franz, Daniel and Yang, Yongting and Esen, Cemal and Hellmann, Ralf}, title = {Flexible 2D Laser Cutting of Ultra-Thin Glass Using an Ultrashort Pulsed Laser Robotic System}, series = {Journal of Laser Micro/Nanoengineering}, volume = {21}, journal = {Journal of Laser Micro/Nanoengineering}, number = {1}, editor = {Esen, Cemal}, publisher = {Japan Laser Processing Society}, issn = {1880-0688}, doi = {10.2961/jlmn.2026.01.2004}, year = {2026}, subject = {Laserschneiden}, language = {en} } @inproceedings{FranzSchusterSchwarzetal.2025, author = {Franz, Daniel and Schuster, David and Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Ultrashort pulsed laser drilling of through-glass vias for glass core packaging}, series = {Proceedings of LAMP2025 - the 9th International Congress on Laser Advanced Materials Processing, Ise-City, Japan, June 2025}, booktitle = {Proceedings of LAMP2025 - the 9th International Congress on Laser Advanced Materials Processing, Ise-City, Japan, June 2025}, year = {2025}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{BischoffKeferEsenetal.2024, author = {Bischoff, Kay and Kefer, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Towards entirely ultrashort pulsed laser based process chain to manufacture polymer lab-on-chip systems with electrical, photonic, and microfluidic functionalities}, series = {The 5th Smart Laser Processing Conference (SLPC)}, booktitle = {The 5th Smart Laser Processing Conference (SLPC)}, year = {2024}, subject = {Ultrakurzzeitlaser}, language = {de} } @article{BischoffEsenHellmann2024, author = {Bischoff, Kay and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond reductive Laser Sintering under multiple focus conditions for rapid production of conductive copper layers}, series = {Procedia CIRP}, volume = {124}, journal = {Procedia CIRP}, number = {124}, publisher = {Elsevier BV}, issn = {2212-8271}, doi = {10.1016/j.procir.2024.08.189}, pages = {629 -- 633}, year = {2024}, abstract = {Reductive laser sintering is an advantageous alternative to conventional precious metal- or mask-based processes for the electrification of polymers and has been demonstrated in a variety of sensor applications. Ultrashort pulsed lasers promise precise control of the energy applied during sintering and ablation. Besides fine tracks, microelectronic circuits include wide contact pads, which leads to time-consuming processing with small laser spot sizes. The use of larger focal diameters, which results in wider lines and thus a higher metallization rate, is accompanied by a loss of structure resolution. We show the influence of focusing on the femtosecond reductive laser sintering of copper(II) oxide on cyclic olefin copolymers. While small focus diameters down to 23 µm provide high accuracy, increasing the laser power and the focus diameter up to 400 µm significantly accelerates the process, first by enlarging the line width and second by increasing the scanning speed due to the exposure time. Two-dimensional copper electrodes for 23 µm and 200 µm focal diameters were generated by hatching individual lines and compared with respect to their morphology. The enlarged spot can be used for high-speed generation of electrodes with a surface conductivity of 117 mΩ/sq. Hybrid processes with production under combined focusing conditions are presented for the first time.}, subject = {Laserstrahlsintern}, language = {en} }