Rapid micro processing of metals with a high repetition rate femto second fibre laser

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1 Rapid micro processing of metals with a high repetition rate femto second fibre laser Joerg SCHILLE *1,*2, Robby EBERT *1, Lars HARTWIG *1, Udo LOESCHNER *1, Patricia SCULLY *2, Nicholas GODDARD *2 and Horst EXNER *1 *1 University of Applied Sciences Mittweida, Laser Application Centre, Technikumplatz 17, Mittweida, Germany *2 The University of Manchester, The Photon Sciences Institute & CEAS, Oxford Road, M13 9PL Manchester, Great Britain schille@hs-mitweida.de Innovative rapid micro processing technologies were investigated by implementation of a high repetition rate femto second fibre laser and novel scanning systems. Previous experiments in femto second laser machining with high repetition rates indicated new mechanisms in laser matter interaction with the repetition rate as one of the mainly influencing parameter. Depending on temporal distances between the laser pulses considerable changes of the ablation behaviour were detected, mainly caused by heat accumulation or particle shielding. Utilising high repetition rate laser technologies in 3d micro structuring, high ablation rates and short processing times were reported. A further reduction of processing times or laser irradiation with the maximal available laser power is limited by insufficient scan speed of commercial available galvanometer scanning systems. In this work high repetition rate laser micro processing of stainless steel was studied utilising both, a self-built resonant scanner and a fast galvanometer scanner system. Different laser processing regimes were investigated to demonstrate the possibilities and limits of the new technologies. The important laser processing parameters were varied and parameter dependencies discussed by means of the width and the depth of ablated gaps as soon as ablation depths and processing qualities of laser generated three dimensional micro structures. Keywords: femto second laser, high repetition rate, micro processing, micro structuring, heat accumulation, beam deflection, resonant scanner 1. Introduction Femtosecond laser technology was on big interest in high precision laser manufacturing since the commercial availability in the middle of the 1990 s. Up to now, lots of experiments demonstrated the advantages of femto second laser processing contrary laser machining with longer laser pulses. Improved processing qualities were obtained due to a precisely defined energy input, confined ablation thresholds, negligible heat affected zones, and small debris deposition. The plasma was absent during the laser irradiation and no plasma shielding occur. However, despite the advantageous laser processing qualities, a wide industrial application of the femtosecond laser technology was averted due to insufficient machining throughputs, caused by slow processing speeds in consequence of low average laser power and less repetition rates. Laser micro processing utilising high repetition rate femto second lasers with high average laser power exhibits new mechanisms in laser matter interaction [1-5]. Heat accumulation and particle shielding were indicated as mainly influencing phenomena. Thus, considerable differences of the ablation behaviour exist depending on temporal distances between consecutive laser pulses. Heat accumulation causes locally a rise in temperature encircled the laser working zone, accompanied by better absorption conditions and lowered ablation thresholds. For metals with either low thermal conductivities or low evaporation temperatures, such as stainless steel and aluminium, the influence of the repetition rate onto the ablation threshold was experimentally verified [5]. For laser processing with repetition rates in ranges of some 100 khz, phenomena of particle and plasma shielding were reported [1, 3]. Another important advantage of high repetition rate laser technology is a high machining throughput. Ablation rates up to 1.8 mm³/min and more than 40 times reduced processing times compared to 3D micro structuring using conventional femto second lasers were found [5]. Up to now, a further reduction of processing times or irradiation with the maximum laser energy was limited by insufficient scanning speed of commercial available galvo scan systems. In this work high repetition rate laser micro processing of stainless steel was studied utilising both, a self-built resonant scanner and a commercial available fast galvo scanner system. Different laser processing regimes were investigated to demonstrate the possibilities and limits of the new technologies in laser micro machining. The important laser processing parameters were varied, such as pulse energy, repetition rate, scan direction and scan number. Laser parameter dependencies will be discussed by means of the gap width and the gap depth of ablated line-scans as soon as ablation depths and processing qualities of laser processed 3D micro structures. 2. Experimental The experimental setup illustrates Figure 1. Main parts were a commercial high repetition rate femtosecond fibre 1

2 laser, a high precision three axis translation stage (Foehrenbach) operating with Aerotech motion control, and two different beam deflection systems. Synchronisation of the laser and the scanner systems was implemented by the enhanced pulse picker device (III edition). determined by a line with constant lateral pulse distances, obtained enclose the zero crossing position of the scan mirror (Figure 2d). The longest processing length of approximately 2 mm was reached at the maximal scan amplitude; a decrease of the scan amplitude caused reduced processing lengths. Figure 1: Experimental setup. 2.1 Laser system Experiments were conducted using a diode pumped, high repetition rate femto second fibre laser (IMPULSE; Clark-MXR). Maximum pulse energy of 7.3 µj was available up to 1.78 MHz and decreases with higher repetition rates. Pulse energy was lowered with half wave plate attenuator. For a fast beam switching an acousto optical modulator was implemented. The external pulse picker device provided discrete repetition rates lower than the maximum at constant laser performance. Significant laser parameters summarises Table 1. Table 1: Laser parameter. Parameter IMPULSE (Clark-MXR) central wave length λ 1030 nm max. repetition rate f P < 25 MHz max. av. laser power P av 14.3 W max. pulse energy Q SP 7.3 µj pulse duration τ H 180 fs 3 ps (sech 2 ) beam quality M 2 < Scanner technology The laser beam was deflected across the sample surface employing two different types of laser scanner technology; a commercial galvanometer scanner (Hurryscan II; Scanlab) and a self-built resonant scanner unit. The laser beam was focused onto the sample surface using a telecentrical f-theta lens of 56 mm focal distance. As shown in Figure 2a, the novel resonant scanner consists of a high speed resonant optical scanner (SC-30; EOPC) and a fixed beam deflection mirror. The scanner oscillates in a sinusoidal motion at a fixed frequency of 3.4 khz. The highest scan speed was reached at the zerocrossing point, and stationary at the turning points for a short time. The laser beam was deflected with permanent changing scan speeds accompanied by varying lateral pulse distances at constant laser repetition rates. Figure 2b exemplifies the lateral pulse distance of the laser pulses along a line-scan at a repetition rate of 500 khz. The maximal scan speed of the resonant scanner was 130 m/s. Modulation of the scan amplitude enabled the adjustment of the lateral pulse distance onto the application. The applicable laser processing length of the line-scan was 3 µm pulse distance: 18 µm Figure 2: Resonant scanner: (a) schematic of resonant scanner system; (b) lateral pulse distance along a line-scan; (c) lateral displacement between bi-directional line scans; (d) constant lateral pulse distances enclose the zero position. However, during a single scan period, a lateral displacement of 3 µm at least between the bi-directional line-scans was detected (Figure 2c). Consequently in laser line-scan ablation experiments only one scan direction was used. The laser system, resonant scanner and control software were synchronised by an enhanced pulse picker device (III edition). In contrast to the foregone pulse picker with a working range up to 333 khz, thenceforward the maximal laser repetition rate was usable. In the experiments the repetition rate was limited to 4.1 MHz, due to considerably reduced laser pulse energies below the ablation threshold at higher repetition rates. Table 2: Scanner parameter; 1) line-scan with constant lateral pulse distance d P, 2) d P = MHz, 3) d P = MHz, 4) onto sample surface. Parameter Galvo scanner Resonant scanner scan velocity v S 4.5 m/s 130 m/s 1) scan length l S 25 mm 2 mm 2) 0.75 mm 3) focal length f 56 mm 56 mm focus diameter d µm 27 µm av. laser power P av 7.3 W 13.0 W rep. rate f R 1.02 MHz 4.1 MHz 4) pulse energy Q SP 6.8 µj 3.2 µj 4) fluence H J/cm² 1.12 J/cm² Significant scanner parameters summarises Table 2. The main advantage of the self-built resonant scan system was a considerably faster scan speed of 130 m/s against 4.5 m/s using the galvo scanner. On other hand, the galvo scanner covered a scan field of 25 x 25 mm² compared to a scan length of 0.75 mm, using the resonant scanner at 2

3 adequate lateral pulse distances. Focal spot diameters were measured of 25 µm and 27 µm for the galvo scanner and the resonant scanner respectively. The marginally wider focus spot size can be related to the incorrect position of the resonant scan mirror to the focussing lens or misalignments. 2.3 Processing regime Multi-pulse laser ablation of stainless steel using high repetition rate femto second lasers joined together with high speed scanner technologies was evaluated by means of three different processing regimes: (I) line-scan ablation, (II) 2.5D laser ablation and (III) 3D laser micro structuring. The mainly influencing process parameters, laser pulse energy and repetition rate, ranging from 0.9 to 6.8 µj at 100 khz, 1.02 MHz and 4.1 MHz, were primarily on interest. Thus, the higher the pulse energy or the repetition rate, the higher the irradiated average laser power. On other hand, the lateral pulse distance remained constant between 4 µm and 4.5 µm during the experiments. Accordingly the scan speeds 0.45 m/s, 4.5 m/s and 18 m/s were applied at 100 khz, 1 MHz and 4 MHz repetition rates respectively. The pulse distance was determined as suitable in foregone experiments to obtain smooth ablation areas. The speed limit of the galvo scanner was 4.5 m/s, consequently the resonant scanner was implemented in line-scan ablation experiments using a higher scan speed of 18 m/s. Furthermore the scan number as soon as the lateral pulse distance between the single ablation lines were varied in the course of line-scan ablation experiments. The width and the depth of the ablated lines were evaluated by means of top-view and cross sections photographs, taken by a digital microscopy (VHX-100, Keyence). 2.5D and 3D laser ablation was investigated in terms of influence of pulse energy, repetition rate and scan regime onto the ablation behaviour. Ablated depths as soon as processing qualities were evaluated by digital microscopy photographs, a confocal displacement sensor (CF4, nanofocus) and SEM photographs (JSM-6510LV, JEOL). Laser fabrication of 3D micro structures can be drawn up within three significant processing steps: At first, drawing of an inverse 3D model of the ablation structure in CAD software. Followed by slicing of the CAD model in layers and enter the laser machining parameters; and finally, stepwise laser processing line by line and layer by layer. The number of layers depends on the model height and the ablation rate. Applying high repetition rate laser system together with galvo scanner, increased material ablation at the start and the end of the line-scans takes place. The higher material ablation was caused by a higher pulse overlapping due to the acceleration and deceleration movement of the scan mirror. The higher material ablation could be avoided by acceleration and deceleration sections according to the processing regime already discussed in [4]. 3. Results and discussion 3.1 Line-scan ablation Line-scan laser ablation was studied emphasised on the influence of pulse energy, repetition rate and scan number onto the widths and depths of laser etched gaps as soon as resolution between single ablation lines. Lateral pulse distances remained constant of 4.5 µm, and faster processing speeds were applied at higher repetition rates. A constant energy input per unit section was irradiated at similar laser pulse energy and higher repetition rates. The effect of processing parameters onto the width of the gaps summarises Figure 3. Generally, the width increases with the scan numbers and laser fluence. At the low pulse energy of 1 µj an insignificant influence of the repetition rate was observed. The smaller gap width at 4 MHz results from the lower irradiated laser fluence, due to a wider focus spot obtained with the resonant scanner. In contrast, the galvo scanner was used in 100 khz and 1 MHz experiments. However, at higher irradiated pulse energies, the gap width obtained at higher repetition rates exceeds the values at lower ones. gap width [µm] uJ_100kHz 0.92uJ_1MHz 0.92uJ_4MHz 2.76uJ_100kHz 2.76uJ_1MHz 2.76uJ_4MHz 6.81uJ_100kHz 6.81uJ_1MHz scan number Figure 3: Gap width vs. scan number at various pulse energies (0.92, 2.7, 6.81 µj) and repetition rates (100 khz, 1 MHz, 4 MHz). gap width [µm] W_1MHz 3.55W_4MHz 0.68W_100kHz 6.81W_1MHz 10.65W_4MHz scan number Figure 4: Gap width vs. scan number at various irradiated laser power and repetition rates. Figure 4 illustrates the influence of the repetition rate onto the gap width, referred to either, a constant or maximal irradiated average laser power. In case of similar irradiated laser power, a considerably lowered gap width can be seen at the higher repetition rate, shown for 3.7 W at 1 MHz and 3.6 W at 4 MHz, respectively. Otherwise, using the maximal available average laser power (10.4 W at 4 MHz, 7 W at 1 MHz and 0.7 W at 100 khz), the smallest width was achieved at the highest repetition rate. Therefore the laser pulse energy can be assumed as mainly influencing parameter; the laser fluence determines the width of the ablated zone. The lowest fluence was irradiated at the highest repetition rate due to the relation P av = Q sp * f rep, with the smallest ablated zones respectively. 3

4 Cross sections of the ablated groove structures exhibit Figure 5. Indeed, depths increase with the laser pulse energy, but with higher number of scans, ablation crater profile comparable to hollow waveguides was observable. During propagation of the laser beam through the crater structures, energy losses due to multiple beam absorption, reflection and scattering at the sidewalls takes place. At deeper crater structure sizes, the ablated material was redeposited on the sidewalls due to less laser energy hitting the structure bottom. In consequence the laser energy was insufficient to eject the ablated material out of the crater. Q SP = 0.92 µj Q SP = 2.76 µj Q SP = 6.81 µj Figure 6: Formation of melting structures at the bottom of laser ablated groove structures, after 100 scans with 4 MHz and pulse energies of a) 0.92 µj, b) 1.84 µj and c) 2.76 µj; figure d) illustrates a groove structure obtained with 250 scans and 1.84 µj. a) 100kHz uJ_100kHz 0.92uJ_1MHz 0.92uJ_4MHz 2.76uJ_100kHz 2.76uJ_1MHz 2.76uJ_4MHz 6.81uJ_100kHz 6.81uJ_1MHz b) 1MHz gap depth [µm] c) 4MHz 50 µm Figure 5: Cross section of groove structures achieved at various processing parameters (pulse energy: 0.92, 2.76, 6.81 µj; scan number: 100, 500, 1000; rep. rate: 100 khz, 1 MHz, 4 MHz) Furthermore, at higher irradiated average laser power, molten material residuals inside the craters were observed. Thus, heat accumulation accompanied by strong material melting can be assumed as dominant effect at higher repetition rates. Figure 6 illustrates the development of melting structures at the bottom of the ablated gap; processing parameters were: 100 scans, repetition rate of 4 MHz and laser pulse energies of either 0.92 µj (6a), 1.84 µj (6b), or 2.76 µj (6c), respectively. Next to the well known ripple structures, deep crater and melting formation in micro meter scales emerged at higher pulse energies. Figure 6d exhibits the groove structure, obtained at 250 scans and 2 µj pulse energy. Irregular formations, such as deep ablation craters and molten wall structures are clearly recognisable at the structure bottom. The achieved depths of the line scans summarises Figure 7. At lower pulse energies of either 1 µj or 3 µj increasing depths were determined up to 500 scan numbers. For application of higher scan numbers no further increase was detected, independently from the repetition rates. Thus, the ablation process was limited by material re-deposition and material melting. In case of 7 µj, a sufficient amount of laser energy was irradiated scan number Figure 7: Gap depth vs. scan number at various pulse energies (0.92, 2.76, 6.81 µj) and rep. rates (100 khz, 1 MHz, 4 MHz). The ablated material could be ejected out of the crater and deeper ablation depths were obtained. In consequence the effect of the irradiated average laser power and the repetition rate onto the ablation depth is negligible and the pulse energy determines the achievable structure depth. On other hand, high applied average laser power provided short processing times respectively high machining throughputs. However, laser micro processing applying high average laser power was accompanied by a reduced spatial resolution between single ablated line-scans forced by heat accumulation effects. Figure 8: Cross section of groove structures obtained using various laser processing conditions; parameters were: rep. rate, pulse energy, pitch distance. 4

5 Proceedings of LPM the 11th International Symposium on Laser Precision Microfabrication Table 4: Ablation rates in 2.5D laser structuring. Figure 8 illustrates cross sections of the gap structures at a line pitch of 25 µm and 35 µm. The maximal available pulse energies of either 6.81 µj at 100 khz and 1 MHz or 2.76 µj at 4 MHz was irradiated. The highest processing quality was obtained at the lowest repetition rate and the maximal pulse energy. Aspect ratios were determined up to 1:10. At a lateral line pitch of 25 µm, height defects occurred, caused by ablation of the inner wall formations due to overlapping laser-scan lines. The spatial resolution to reach alternating groove-wall formation without any height defects was found of 35 µm pitch distance. Ablation rate mm³/min Pulse energy unidirectional 100kHz 1MHz 1MHz µj µj µj The lowered ablation rates using unidirectional laser ablation were caused by scan mirror movement backwards without any laser irradiation. In contrast, during laser processing material ablation takes place at the mirror forward and backward movement. Principally, the maximum obtained ablation rates of 0.55 mm³/min seems to be lower than the rates of 1.8 mm³/min reported elsewhere [3], but the processing quality was on priority. Additionally the scanning direction influences the granularity of the laser processed structure. As clearly observable in Figure 10, laser processing caused rounded shapes at the laser turning point and soft edges. Application of unidirectional processing regime, more detailed and sharp structure features were obtained D laser ablation 2.5D laser ablation will be discussed by means of a trench-like test structure shown in Figure 9. The influence of the repetition rate and the scan direction was on interest. Pulse energy and scan numbers were varied in a way, that a constant amount of laser energy (accumulated laser energy) hit the sample surface. In view of that the scan number was decreased at higher pulse energies. The ablation depths obtained were evaluated by using a confocal displacement sensor, summarised in Table 3. Figure 9: Trench-like test structure; dimensions: 1 mm width, 1 mm length and 90 µm height. Figure 10: Detailed ablation structure obtained using unidirectional (left) and (right) processing regimes. Despite a constant laser energy input, at the smallest irradiated pulse energy the lowest ablation depth was reached. Furthermore a little increase of the ablation depth with the repetition rates is observable due to heat accumulation effects. Otherwise the scan direction influences the ablation depth insignificantly. In summary, 2.5D laser ablation using high pulse energies, high repetition rates and scan direction yields high ablation rates accompanied by short processing times. Further a significant effect of the pulse energy onto the surface behaviour was detected. As shown in figure 11, the higher the irradiated pulse energy the rougher the structure surface. At the sidewalls ablation residuals were found. Table 3: Ablation depths depending on laser process condition. laser power [W] energy per unit section [J/m] scan number accumulated laser energy [J/m] 100 khz unidirectional 1 MHz 1 MHz Ablation depth in µm Figure 11: Trench-like structures obtained applying lower (above, 2.5 µj) and higher pulse energies (below, 3.75 µj). Table 4 summarises the ablation rates, which were calculated from the ablation depths given in Table 3. 5

6 3.3 3D laser micro structuring In 3D laser micro structuring utilising a high repetition rate femto second laser, complex three dimensional test structures were fabricated. Beam deflection was carried out by galvo scanner system. Figure 12 illustrates a 3D test structures of 1 x 1 x 0.1 mm³ (width x length x depth), obtained after laser processing of 100 layers. The smooth ablation surface and detailed micro scaled features can be observed. The acceleration section processing regime as soon as focus position tracing of 1 µm from layer to layer were applied to avoid strong pulse overlapping and defocusing effects. Maximum processing speed was 1332 mm/s, respectively a repetition rate of 333 khz at 4 µm lateral pulse distance. Much higher scan speed was not useful due to the required very short scanner switching times between the micro scaled features and irregular scan patterns were obtained. Laser processing time was less than 6 minutes. In comparison to conventional femto second laser machining with repetition rates in the khz-range, improved processing qualities and much higher ablation rates were obtained, accompanied by much more than 40 times lowered processing times. In consequence of the high throughput, high repetition rate femto second laser processing seems to be applicable in Rapid Micro Tooling technologies, but further improvements of machining qualities is still needed. In line-scan ablation the width and the depth of the gaps as soon as the resolution of the distance between single ablated lines were on interest. At higher irradiated pulse energies, the gap width obtained at higher repetition rates exceeds the values at lower ones. In case of constant irradiated laser power, a considerably lowered gap width was achieved at the higher repetition rates. The laser fluence was assumed as the mainly influencing parameter of the width of the ablated zone. Furthermore the depths of the ablated lines were evaluated. At higher irradiated average laser power, molten material residuals inside the craters were observed due to heat accumulation and material melting. The effect of the repetition rate onto the ablation depth is negligible and the pulse energy determines the achievable structure depths. The highest resolution was obtained at low irradiated average power, the highest aspect ratios 1:10 at low repetition rates by irradiation of high pulse energies. In 2.5D and 3D laser processing high processing throughputs were obtained The maximal ablation rate in 2.5D laser ablation was 0.55 mm³/min. Ablation rate and processing quality were mainly influenced by scanning direction and irradiated laser power. Whilst unidirectional processing at lower laser pulse energy resulted in detailed and sharp structures, processing at higher pulse energies led to short processing times. In 3D laser micro structuring much more than 40 times lowered processing times accompanied by improved processing qualities was achieved compared to laser processing using conventional femto second laser systems. Joining together high repetition rate femto second laser and ultra fast scan systems result in high processing throughputs and high processing qualities. In future these innovative technologies should attract increasing interest in Rapid Micro Tooling processes. Acknowledgment The presented results were conducted in the course of the project INNOPROFILE Rapid Microtooling mit laserbasierten Verfahren, (pr. no. 03IP506), funded by German Bundesministerium für Bildung und Forschung. References Figure 12: 3D laser micro structuring test structure. 4. Summary In this study, a high repetition rate femto second laser was joined together with different types of laser scanner systems, a commercial galvanometer scanner and a selfbuilt resonant scanner unit. The novel resonant scanner reached a maximal scan speed of 130 m/s at a resonant frequency of 3,400 Hz. Rapid Micro Processing of stainless steel was investigated by means of three different processing regimes: (I) line-scan ablation, (II) 2.5D laser ablation and (III) 3D laser micro structuring. [1] A.Ancona, F.Roeser, K.Rademaker, J.Limpert, S.Nolte, and A.Tuennermann: Optics Express Vol. 16, No. 12 (2008). [2] B.Tan, S.Panchatsharam, and K.Venkatakrishnan: J. Phys. D - Appl. Phys. 42, (2009). [3] J.Schille, R.Ebert, U.Loeschner, P.Regenfuss, T.Suess, and H.Exner: Micro structuring with highly repetitive ultra short laser pulses ; Proc. of LPM the 9 th International Symposium on Laser Precision Microfabrication, Quebec (Canada), (2008). [4] J.Schille, R.Ebert, U.Loeschner, L.Schneider, N.Walther, P.Regenfuss, P.Scully, N.Goddard, and H.Exner: An ultrafast femtosecond fibre laser as a new tool in Rapid Microtooling ; Proc. of LAMP the 5 th International Congress on Laser Advanced Materials Processing, Kobe (Japan), (2009). [5] J.Schille, R.Ebert, U.Loeschner, P.Scully, N.Goddard, H.Exner: High repetition rate femtosecond laser processing of metals, Proc. of SPIE / LASE 2010, San Francisco (USA), (2010). 6