Ablation, Drilling and Micro-Cutting (LiM 2019)

Hot Electron Plasma Temperatures and Soft X-Ray Emission During Laser Processing with Ultrafast Lasers
Rudolf Weber, Roswitha Giedl-Wagner, Daniel Förster, Thomas Graf, Anton Pauli

Soft X-ray emission during industrial materials processing with ultrafast lasers is of increasing interest. The soft X-rays mainly originate from the free, hot electrons in the plasma. Therefore, the spectrum of the emission approximately follows a Maxwell-Boltzmann distribution, which corresponds to the temperature of the hot electrons.

For the current work, the spectra of the soft X-ray emission were measured for intensities between about 1013 W/cm2 and 1015 W/cm2. The corresponding temperatures of the hot electrons were determined by fitting Maxwell-Boltzmann distributions to the spectra. Furthermore, the resulting H(0.07) dose rates at a distance of 20 cm from the plasma were measured and extrapolated to dose rates for an average laser power of 1 kW.

Keywords: Laser plasma, soft X-ray emission, dose rates, Ultrafast laser processing, hot-electron temperature temperature


Ultra-short pulse laser drilling: Requirements, constraints and strategies for upscaling
David Brinkmeier, Volkher Onuseit, Thomas Graf

The availability of kW-class USP lasers leads to new challenges in order to scale laser drilling processes beyond their thermal limit. In this work, we discuss the implications and solutions to utilize next generation laser sources in drilling applications, based on an analytical heat accumulation model combined with an estimation of the achievable borehole geometry. Using this model it is possible to choose the right processing strategy, such as spatial parallelization and / or temporal sequencing, for an efficient use of kW-class USP lasers depending on laser parameters and the borehole geometry. Additionally the model allows the definition of selection criteria for an appropriate laser source, as well as focusing optics, for a desired borehole.

Keywords: Laser drilling; Heat accumulation; Drilling strategies; High average power ultrafast laser


Deep drilling of metals with ultra-short laser pulses
Daniel J. Förster, Christian Freitag, Rudolf Weber, Thomas Graf

A simplified model of the percussion drilling process to predict the achievable drilling depth was derived and systematically investigated for varying process parameters. The model allowed to design a process in such a way that microholes with an aspect ratio of around 125:1 (with respect to the focal diameter) and a drilling depth of 10 mm could be achieved. For this process, a ps-laser with a pulse energy of 3 mJ and a repetition rate of 30 kHz was used. In this paper, the simplified drilling model to predict achievable hole depths of laser percussion drilling processes is introduced. The application of a high-energy laser to achieve high aspect ratios and hole depths is discussed.

Keywords: USP laser; drilling; modelling


Helical drilling of high-aspect-ratio microholes in stainless steel using ultrashort laser pulses
Chao He, Malte Weber, Arnold Gillner

Laser-beam helical drilling with ultrashort pulsed lasers is one of the few techniques for producing high precision microholes, which demonstrate excellent wall definition and small heat-affected zone. This study is aim to achieve a laser direct fabrication of high-aspect-ratio microholes in stainless steel with thickness of 1.2 mm and 3 mm. By using the helical drilling optics, the multi-reflection behavior in the bore channel can be adapted by changing the incident angle and helical path of laser beam. By taking advantage of this, the deep drilling with minimized hole-diameter is enabled. Influences on hole-profiles and hole-quality by processing parameters such as laser focal position and helical optics are presented and discussed. Quasi-cylindrical holes with diameter of approximately 60 µm and a depth-to-diameter ratio greater than 20:1 and 50:1 can be fabricated in a 1.2 mm and a 3 mm thick stainless steel respectively.

Keywords: ultrashort laser pulse; microdrilling; helical optics; high-aspect-ratio; stainless steel


High-speed manufacturing of HLFC structures by laser micro drilling
Andreas Stephen, Roberto Ocana, Joseba Esmoris, Christian Werner, Carlos Soriano, Frank Vollertsen, Rafael Sanchez

To reduce the fuel consumption of large passenger aircrafts the hybrid laminar flow control (HLFC) is a key technology. For drag reduction, several million holes with diameters around 50 µm at the leading edges of the aircraft wings and stabilizers are needed. In this paper, developed laser micro drilling strategies are analysed with respect to quality aspects and machining speed for the perforation of such titanium sheets. Applied methods are the “on-the-fly” single pulse and the percussion drilling technique both using pulsed fibre lasers in single mode operation. Differences can be found of course in the applied pulse length of 200 µs respectively 100 ns by q-switching. The strategies are focused to achieve holes with deviations in diameter of less than 3 µm at machining speeds of more than 300 holes per second, whereas less than 1% of the holes should be blocked by residual melt or particles.

Keywords: Micro processing; drilling; surface functionalization


Fiber-reinforced composite microdrilling with high-power Sirius XeCl excimer laser for aerospace applications
Dmitry Klyukin, Andrei G. Anisimov, Roger M. Groves

Over the last several years hybrid laminar flow control has gained a lot of interest for drag reduction by boundary layer suction, using small holes at the leading edges of aircraft wings and tail elements. A lot of effort has been spent on drilling of small holes into different materials with diameters around 100 µm. This paper focuses on practical aspects of bringing laser micro-drilling technology from the laboratory level closer to the real-life applications in carbon fibre reinforced composite materials. The fabrication of large suction inserts was performed with an XeCl Sirius excimer laser with a power of 1 kW at a pulse repetition rate up to 1 kHz. The percussion drilling approach require the adjustment of several parameters, such as number of pulses, pulse energy and repetition rate and resulted in the fabrication of round holes with diameter of 100 µm in carbon fibre reinforced thermoplastic with a thickness of 900 µm.

Keywords: laser drilling; composite materials; excimer laser; carbon fibre reinforce polymer,pulse energy modulation.


Fabrication of cutting edge microgeometries on PcBN tools using pulsed laser ablation
A. Krödel, B. Breidenstein, T. Grove

Polycrystalline cubic boron nitride (PcBN) is widely used in industry as a cutting tool material for the machining of hardened steels, cast iron or nickel-based super alloys. For the generation of geometrical features on such cutting tools, laser ablation is a novel and promising technology, which offers wear-free processing and high geometrical flexibility. However, the influences of different laser parameters and pulse durations on the surface integrity of PcBN tools is not known in literature. Therefore, at first PcBN materials are laser processed with different laser sources ranging from nanosecond to femtosecond pulse durations. Laser induced phase transformations and residual stresses are characterized. Optimum laser parameters for each laser source are derived. Afterwards, a method for laser preparation of defined cutting edge microgeometries is presented. Lastly, the fabricated cutting tools are used in machining of hardened steel and the cutting performance is compared to conventionally machined reference tools.

Keywords: Type your keywords here, separated by semicolons


Laser Cutting of PE Polymer Films with Adapted Fiber Laser Beam Sources
Maximilian Brosda, Phong Nguyen, Alexander Olowinsky, Arnold Gillner

In the field of plastics technology, films are used for many applications that consist of PE or at least have PE layers. For many applications it is necessary to separate or perforate these films. Conventionally, this is done by using knives or punching tools. The increasing trend towards format flexibility requires new format-independent processes such as the laser cutting of polymers. PE has a wavelength dependent absorption coefficient, which has around 2000 nm areas with higher intrinsic absorption. By using a thulium fiber beam source these areas can be addressed. In comparison to the co2 laser used up to now, the radiation can be flexibly guided via optical fibers. The influence of process parameters such as gas pressure and feed rate on the cutting quality is investigated. The cutting quality is evaluated by thin sections and reflected light microscopy. In addition, the process is monitored by a thermographic recording.

Keywords: Laser cutting; Polymer; Fiber Lasers; Film


Ultra-short pulsed laser machining of ultra-hard cutting tool materials
Melik Hajri, Paul Börner, Jodok Weixler, Konrad Wegener

Ultra-short pulsed (USP) laser technology has been successfully used in recent years for post-processing or even the complete fabrication of micro cutting tools from ultra-hard cutting tool materials. In this paper, polycrystalline diamond (PCD) and cemented carbide (WC-Co) are machined using an USP laser. Pockets are generated with different process parameters. The influence of the pulse duration on the ablation behavior is examined in the range between 0.3 ps and 10 ps. The energy-specific ablation volume is calculated out of the measured ablation volume. Finally, the experimental results are compared to an analytical model, showing the influence of the pulse duration on the threshold fluence and penetration depth. The findings can be used for productivity calculations and comparisons to other manufacturing technologies. Furthermore, the analytical model can serve for extrapolations and should help to choose the optimal laser system for the manufacturing of cutting tools.

Keywords: ultra-short pulsed laser machining; cemented carbide; polycristalline diamond; pulse duration; threshold fluence;
penetration depth; Neuenschwander model; tangential laser processing; parallel laser processing


GHz femtosecond processing
E. Audouard, G. Bonamis, K. Mishchik, E. Mottay, C. Hönninger, J. Lopez, I. Manek-Hönninger

Most current future industrial applications of ultrafast lasers require high processing quality, but also high throughput and productivity. While ultrafast lasers excel processing with an exquisite precision, each laser pulse only removes a small amount of material. This disadvantage can be counterbalanced by distributing the energy into a burst of pulses at a GHz-level repetition rate. Indeed, we conceptually show that GHz ablation efficiency is achieved through the balance of the pulses dedicated to heat accumulation and effective ablation. For that, several inter-related parameters, such as pulse energy, number of pulses per burst, intra-burst and inter-burst repetition rates must be optimized. A new highaverage power GHz-burst femtosecond laser source, delivering output powers from 20 W to 100 W allowed for reaching specific ablation rates up 2.5 mm3 /min/W for Si.

Keywords: femtosecond laser processing; GHz laser, ultrafast ablation


Interest of singlemode fibers in photonic jet sub-micron laser processing
Sylvain Lecler, Robin Pierron, Grégoire Chabrol, Djamila Bouaziz, Jean-Paul Yehouessi, Géraud Bouwmans

Laser is a flexible non-contact method, commonly used in industry, to process mechanical pieces, at scale down to 10 µm. Ultrashort pulse lasers make possible to reach smaller size tacking advantage of nonlinear phenomena. Nevertheless, these sources are still too expensive for many potential applications. Rather than concentrating energy in time, this can be done in space, without using microscope objective, but by generating a photonic jet. A photonic jet is a
propagative beam concentrated beyond the diffraction limit in the near field of a dielectric object. Initially obtained with microspheres on the sample to process, the phenomena has been recently demonstrated using shaped optical fiber tips, easier to manipulate and without contact with the sample. Whereas multimode fibers have been used until now, we show how singlemode fiber and especially large mode area fiber can achieve the same process with 8 times less power, maintaining a reasonable working distance.

Keywords: Micromachining; submicron laser ablation; photonic jet; shaped optical fiber tip


Ablation suppression of titanium optimizing the delay time by two-color femtosecond double-pulse laser
Naoki Shinohara, Masahiro Tsukamoto, Masaki Hashida , Keisuke Takenaka, Satoru Asai, Inoue shunsuke, Shuji Sakabe

Two-color femtosecond double-pulse laser beam has been used to discuss the suppression of ablation rates on titanium (Ti) surface in the delay time (Δt) from 0 to 700 ps. The double pulse beam consisted of 800 nm with 150 fs pulse and 400 nm with > 150 fs pulse in cross polarization. The ablation rate was clearly suppressed at the delay time of Δt ~ 80 ps. For first pulse of 400 nm case, the ablation rate was suppressed at the delay time of Δt ~ 150 ps. The suppressed delay time was approximately three times difference for both irradiation case. The difference of the suppression time might be suggested that the ablation rate was effectively suppressed when the expanding surface plasma produced by first laser pulse should be close to the critical density for the second laser pulse.

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High speed laser piercing of CFRP using 1W nanosecond UV laser pulses
Hiroharu Tamaru, Atsushi Kosuge, Takashi Hira, Masahiro Moriyama, Shuntaro Tani, Isao Ito, Zhigang Zhao

Laser piercing of 1.6 mm-thick Carbon Fiber Reinforced Plastics (CFRPs) was demonstrated for the first time, using a pulsed laser with a wavelength of 258 nm, generated by fourth harmonic generation of a 1030 nm laser. The laser power, pulse duration and repetition rate of the 258 nm laser pulses were 1 W, 7 ns and 10 kHz respectively. The linearly-polarized laser pulses of diameter 20 µm were focused on the surface of CFRP. A single piercing was completed in about 0.13 second. Piercing holes were observed by X-ray CT and the hole diameters of the input and output surfaces were about 30 µm and 5 µm, respectively. The aspect ratio of the pierced hole was greater than 50. The resin of the CFRP top surface irradiated by laser pulses evaporated with a diameter of 50 µm, but no heat-affected zone was observed at the surface of piercing holes by X-ray CT observation. 6x6 hole arrays with 200 µm interval was also demonstrated by laser piercing.

Keywords: CFRP; Piercing; high-aspect; HAZ-free


CAM solution for quasi-tangential laser ablation of complex 3D workpieces
Norbert Ackerl, Johannes Gysel, Maximilian Warhanek, Konrad Wegener

A fast forward computation of the laser ablation paths and hatches for complex 3D geometries enabled by a computeraided manufacturing (CAM) toolbox is presented. Specifically, the laser paths for quasi-tangential irradiance condition on a rotary specimen are calculated and the machine code for a laser manufacturing system with up to seven synchronously controllable axes generated. In order to set the parameters for the path determination, an empirical parameter study points to the material specific removal rate in accordance with pulse and line overlap. Therefore, a manifold use of this tool is possible for all laser sources from continuous wave to ultra-short pulses. This leads to a unique flexible way, hitherto not available for fast prototyping applications. Following, laser manufactured specimens with complex contour shapes reveal the high potential of this approach. High-precision laser manufactured dental implants and diamondgrinding tools with tolerances in the micrometer range serve as demonstrators.

Keywords: Computer aided manufacturing, Laser manufacturing, Ultra-short pulsed, Laser turning, Tool path planning


Depth and quality limit for percussion-drilled microholes with depth > 1 mm using ultrashort pulsed laser radiation
Anne Feuer, Daniel J. Förster, Rudolf Weber, Thomas Graf

Based on the assumption that laser-drilled microholes can be approximated by a cone the maximum depth as a function of the laser fluence can be predicted by a simple analytical model. In this contribution, it will be shown, that the calculated maximum depth agrees well to microholes in stainless steel with drilling depth > 1mm. A Ti: sapphire laser at a wavelength of 800 nm and a pulse duration of 1 ps was used to percussion drill microholes with pulse energies up to 5 mJ and the corresponding maximum drilling depth was determined. Due to the low repetition rate of 1 kHz heat accumulation effects could be excluded. Furthermore, the quality of the microhole exit was investigated as a function of the peak fluence in 1 mm stainless steel, including the formation of side channels and the development of the shape of the microhole exit.

Keywords: Laser micro processing; Drilling; Percussion-drilled microholes; Final drilling depth; Sidechannels


Use of bursts for femtosecond ablation efficiency increase
Amélie Letan, Eric Audouard, Konstantin Mishchik, Clemens Hönninger, Eric Mottay

The challenge of ultrafast processing is to increase throughput for applications in industrial production. The laser burst mode is identified as a method increasing the throughput in fs-processing applications by a higher ablation efficiency. In this context highly controlled burst is a key function for femtosecond lasers. The architecture of femtosecond laser allows to generate bursts with user defined number of sub pulses and energy shapes over a wide range. In this work, we show how to optimize processing parameters in order to maximize the ablation efficiency and avoid detrimental thermal
effects. In particular, the role of energy distribution in sub pulses will be highlighted, showing the possibility of increasing effective pulse energy by a factor up to 5, with a precise control of corresponding consequences for ablation results in cutting or surfacing. Simulation tool for a quick estimation of suited parameters will be also presented.

Keywords: Bursts of femtosecond pulses, femtosecond processing, ultrafast ablation


Modeling and experimental validation of single-pulse and multi-pulse picosecond laser beam ablation of cemented tungsten carbide
Juan Pablo Calderón Urbina, Claus Emmelmann

Ultra-short pulse laser beam processing of ultra-hard and compound materials represents a challenge in the determination of correct parameters and settings efficient ablation. Therefore, this paper presents an approach for the modeling and validation of the ablation process on cemented tungsten carbide to gain understanding of an effective application of the technology. The model is based on the identification of the ablation threshold of the material by heat
transfer fundamentals, the adaptation of the Beer-Lambert law to a picosecond laser pulse and the development of a multi-pulse ablation model supported by relevant known theories (e.g., Chichkov et Al., 1996; Darif and Semmar, 2008; Jaeggi et Al., 2011) and using single-pulse mode and partition of pulses into bursts. Experimental ablation depth shows a corresponding behaviour to the model in the analysis of a single pulse and multi-pulse irradiation with single-pulse mode, and an interesting phenomenon, both accurate and unexpected, with the application of multi-pulse irradiation with burst-mode in two levels.

Keywords: Laser ablation; Picosecond; burst-mode; cemented tungsten carbide; Modeling


n- and p-type laser doping of Si thin film transistors coated with chemical solution for CMOS circuit fabrication
Kaname Imokawa, Takayuki Kurashige, Nozomu Tanaka, Akira Suwa, Daisuke Nakamura, Taizoh Sadoh, Tetsuya Goto, Hiroshi Ikenoue

We demonstrated that p- and n- type activation layer can be formed on crystallized Si thin films by KrF excimer laser doping. H3PO4 (n-type) solution and Al2O3 (p-type) sol were coated on the surface of Si as dopant sources. Phosphorus and aluminum concentrations were found to be over 1019 on the surface, and their depth profiles were uniform in the Si films. Thus, the technique of laser doping is promising for contact formation in the source/drain of Si thin-filmtransistors (TFTs) at a low cost. In this study, the electric characteristics of n-MOS and p-MOS TFTs fabricated by laser doping are presented for CMOS circuit fabrication.

Keywords: Laser Doping; KrF Excimer Laser; Low-temperature-poly-Si; Flexible Display; Thin-Film Transistor;


Ultrafast Laser Ablation at 1035 nm, 517 nm and 345 nm as a Function of Pulse Duration and Fluence
Norman Hodgson, Sebastian Heming, Albrecht Steinkopff, Hatim Haloui, Tony S. Lee

Ablation rates for 25 materials (metals, semiconductors and dielectrics typically used in industrial manufacturing) were measured as a function of pulse duration (0.4 – 18 ps) and pulse fluence by using a 40 W modelocked, 1035 nm Yb Fiber MOPA operating at repetition rates of up to 1 MHz, with extra-cavity frequency doubling and frequency tripling. In addition to determining the maximum ablation rates and optimum pulse fluences, the heat affected zone and surface quality were analyzed. The data confirm the predictions by the two-temperature model. As long as the pulse fluence is close to the ~7.4 times the ablation threshold fluence, and the pulse duration is less than about 10 ps, the width of the heat affected zone is independent of pulse duration.

Keywords: Ultrafast laser ablation; ablation rate; femtosecond lasers; laser material processing; heat affected zone;


High speed UV femtosecond machining
C. Hönninger, M. Delaigue, K. Mishchik, E. Audouard, E. Mottay, David Bruneel, Anne Henrottin, Jose A. Ramos

High power femtosecond UV lasers up to 40 W contribute to increase the application range of ultrafast laser processing, by the use of new materials such as functionalized polymers or organic materials. From the user perspective, dealing efficiently with high average power levels and high pulse repetition rates, e.g. in high speed scanners, requires an increase in the pulse modulation speed as well as free triggering in order to synchronize the laser pulses with scanner or axes positioning, and ultimately with the application. The challenge in femtosecond lasers is then to maintain the inversion level constant through the entire amplifier chain, and hence the output pulse energy, for any user profile. With this issue solved, the user can adapt the laser pulse period or repetition rate to the variation of speed in case of complex movements in order to maintain a constant fluence on the sample.

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Investigations on material removal mechanisms of steel by means of laser processing for balancing processes
Peter Hellwig, Klaus Schricker, Jean Pierre Bergmann

Rotating parts as for example rotors for industrial application present small but undesirable unbalances after chipping. Rotors which are exposed to high rotational speeds require unbalances approaching zero. Mass corrections such as material removal has to be carried out in a short time, in order to hold a profitable cycle time. Ablation by means of laser beam allows a high quality removal, but a low removal rate. Laser-based material removal through a welding process represents a novel approach for the use in balancing processes. In this case sputter formation is purposely used, in order to enhance the material removal. In this contribution, a 1.5 kW fibre laser (cw-mode) was used for processing X5CrNi18-10 steel sheets. Spatter formation out of the molten pool was identified as primary removal mechanism. Further investigations regarding process velocity up to 10 m/s, various incidence angles of the laser beam and the number of passes were carried out. Thereby, precise weighings confirmed a sufficient removal rate for the application in balancing processes. Concluding studies were carried out regarding surface parameters, e.g. profile depth, to refer them to industrial requirements and conventional chipping methods.

Keywords: removal rate; cw-based remote welding; laser beam incidence angle; material removal of steel


The micro via processing for semiconductor package by excimer lasers
Junichi Fujimoto, Masakazu Kobayashi, Akira Suwa, Akira Mizutani, Yasufumi Kawasuji, Masaki Arakawa, Takashi Onose, Hakaru Mizoguchi

Recently infrared/UV lasers have faced resolution limit of finer (<10µm) via hole size micromachining requirement on semiconductor package process. It will be required for multi die semiconductor package substrates by Fan-Out Wafer/Panel Level Package. The substrate materials are currently using the silicon wafer, organic build-up film. The glass substrate will be used high frequency signal transfer application like 5G tele-communication use. The glass via holes are hard to process with less defect. In this study, we investigated via hole quality by DUV laser ablation process by using deep ultra violet excimer lasers. The results show the possibilities of micromachining on the glass substrates. We have succeeded <30 µm holes aspect ratio 10 on glass substrate without any significant defects. Excimer lasers could be expected on manufacturing process for next generation semiconductor packages.

Keywords: excimer laser, deep ultraviolet, 193nm, ArF laser, interposer, non-alkali glass


Water Jet Guided Laser Machining of Metal Matrix Composites
S. Marimuthu, J. Dunleavey, B. Smith

Laser cutting or drilling of monolithic materials like metals and alloys is a well-established process, used extensively in various applications including aerospace, medical and automotive. However, traditional laser processing (thermal based) of materials like metal matrix composites (MMC) is challenging due to the differences in the chemical and physical properties of the matrix and reinforcement particles. The main investigation on this paper concentrates on the water-jet guided nanosecond laser (WJG) cutting of aluminium metal matrix composite (Al MMC) reinforced with aluminium oxide fibre and WJG drilling of Al MMC reinforced with silicon carbide particles. The results of the WJG laser process was compared with the results from the conventional long pulse/continuous wave laser process.

Keywords: Laser; water; nanosecond; Al MMC; micro-machining; cutting; drilling;


Heat-affected zone analysis of fiber laser cut medical devices and its dependencies regarding laser and design parameters
Clara Böhm, Dr. Christian Kneis, Chris Bräuner, Dr. Philipp Hempel

Fiber laser cutting of Nitinol during the manufacturing of medical devices is very challenging due to many aspects. A change in the material’s mechanical properties, caused by the local heating effects, is one these. Depending on the material’s temperature during laser cutting, different types of microstructures can be observed: non-affected bulk material (TMaterial < TRecrystallization), heat-affected zone (TRecrystallization < TMaterial < TMelting) and recast/dross (TMaterial > TMelting). The focus of this study is a quantitative investigation of the formation of these microstructures during fiber laser cutting. For this purpose, laser and process parameters (e.g. pulse duration, cutting speed, repetition rate, beam caustic) as well as geometric features of the specimen such as tip radius or strut width are varied to reveal their influence on the material during cutting.

Keywords: Micro-Cutting; Heat-Affected-Zone; Process Parameters; Microstructure