CHARACTERISATION OF INTERACTION PHENOMENA IN HIGH REPETITION RATE FEMTOSECOND LASER ABLATION OF METALS Paper M1003

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1 CHARACTERISATION OF INTERACTION PHENOMENA IN HIGH REPETITION RATE FEMTOSECOND LASER ABLATION OF METALS Paper M1003 Joerg Schille 1,2, Lutz Schneider 1, Lars Hartwig 1, Udo Loeschner 1, Robby Ebert 1, Patricia Scully 2, Nicholas Goddard 2, Horst Exner 1 1 Laser Institute at the University of Applied Sciences Mittweida, Technikumplatz 17, Mittweida, Germany 2 The Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK Abstract The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-topulse distance on the ablation process was investigated using repetition rates ranging between 25.8 khz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermophysical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion. Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the surface temperature. Additionally, ultra high speed camera images give evidence of more voluminous ablation plumes at shorter pulse-to-pulse distances, induced by intense laser matter interaction. In contrast, for copper only a marginal impact of the repetition rate on the material removal was found. Thus for highly heat-conductive materials the ablation rate is assumed almost independent from the temporal pulse-to-pulse distance. Even high speed camera images show minor impact of the repetition rate on the ablation process. Finally the application of the laser micro machining technology in micro-mould manufacturing is presented. As a result micro-featured plastic demonstrators were produced by micro injection moulding, offering a wide range of sensor applications, for example in microfluidic systems. Introduction The commercial availability of high average power, high repetition rate ultrashort pulse lasers recently brings together the industrial need of high machining throughputs with high machining qualities. Thus highly repetitive laser micro processing is regarded as the key enabling technology for innovative production processes and will substitute standard manufacturing technologies in many micro machining applications. However, results published using that promising technologies revealed a considerable impact of the repetition rate. Heat accumulation and particle shielding were identified as the mainly influencing effects in laser matter interaction. Particle shielding, for example, was indicated in laser micro processing of stainless steel, and it was shown that irradiation of pulses at repetition rates of several hundred kilohertz the removal rate decreased [1-3]. Because of the short temporal pulse-to-pulse distance of only some micro seconds, the incoming laser pulse is shielded by interaction with particles and droplets, ablated by preceding incident pulses. By contrast, heat accumulation was assumed as another distinctive effect in highly-repetitive laser processing. Even in case of ultrashort pulse laser ablation a significant fraction of the absorbed energy remains in the material, and in multi-pulse ablation enhanced residual thermal energy deposition takes place [4]. Considering high repetition rates up to the megahertz range, the time between consecutive incident pulses is too short for complete heat dissipation into the bulk. In particular highly repetitive laser irradiation of low-heat conductive metals such as stainless steel causes accumulation of the remaining energy close to the processing area. As a result the surface temperature rises strongly, accompanied by both higher laser beam absorption and lower ablation thresholds. Further, under the assumption that the temperature increases to the melting point, even more

2 changes of the thermo-physical material properties influence the ablation process. However, along with the enhancement of the ablation efficiency, heat accumulation causes strong material melting, that affect detrimentally the processing quality. On the other hand, in case of highly-heat conductive materials such as copper, the influence of the temporal pulse-to-pulse distance to the ablation process was found as negligible. Heat accumulative effects are almost absent due to the fast dissipation of heat into the bulk, and the surface temperature increases only marginally. Thus the ablation process seems to be similar, even in case of high repetition rates up to the megahertz range. Furthermore, in comparison to stainless steel, laser beam shielding can be neglected for copper because of the reduced superheated layer that originates particles ejection by homogeneous nucleation [1, 5]. It has been shown already that the laser processing parameters influence the ablation efficiency considerably and shielding losses can be overbalanced by heat accumulation [6, 7]. Further laser micro structuring in the low fluence regime resulted in promising machining outcomes with respect to machining quality, accuracy and throughput. However, up to now the phenomena and mechanism occurring in highly repetitive laser processing using ultrashort laser pulses are insufficiently investigated and understood. Particularly in micro structuring there exists still a lack of knowledge about the interplay between the laser parameters and the resulting impact on the ablation process. In this work material removal rates obtained under various experimental processing conditions are related to ablation rates achieved in theoretical calculations. Thus an improved model to calculate the ablation volume per pulse is taken into account. Moreover a detailed characterisation of heat accumulation and particle shielding is presented by either temperature calculation using a simplified model or ablation plume images obtained using an ultra high speed camera. Finally micro-featured plastic parts made by injection moulding are presented to demonstrate three-dimensional laser micro structuring as a powerful technology in micro-mould fabrication. Experimental In the study a high repetition rate femtosecond fibre laser (IMPULSE, Clark-MXR) was applied, emitting a linearly polarised Gaussian beam of 1030 nm central wavelength, 180 fs pulse duration (sech²), and 25 MHz maximum repetition rate. The maximum average laser power was 13 W with the maximum pulse energy of 7.2 µj, obtainable up to 1.78 MHz. The pulse energy decreased with higher repetition rates, respectively. The laser was implemented in a micro machining work station schematically shown in Figure 1. A galvanometer scan system (intelliscan, Scanlab) was utilised to deflect the laser beam across the sample surface. The laser beam was focused by a telecentric f- theta objective with a focal length of 56 mm, resulting in a focal spot diameter of 30 µm. Thus the maximum laser peak fluence was 2.0 J/cm². The machining setup was completed by a pulse picker for discrete pulse separation, monitoring systems to control the process parameters, a confocal point sensor for depth measurement, and a polariser unit to change the direction of beam polarisation. Figure 1: Schematic of the experimental setup. In the study three different types of industrial grade metals sheets with various thermo-physical material properties were investigated: 0.5 mm thick stainless steel metal sheet (AISI 304), polished 99.9% pure copper metal sheet of 0.45 mm thickness, and aluminium alloy block material (Aluminium 6082-T6). For depth measurements cavities were made in the metal sheets using the line-by-line and layer-bylayer raster scan regime as shown in Figure 2. From the cavity depth the averaged ablated volume per single pulse V SP can be estimated accordingly Equation (1), taken from [6], were d P is the lateral pulse spacing between two consecutive incident pulses, d H is the hatch distance between the lines, d Z is the cavity depth, and n S is the scan number describing the quantity of repeatedly processed layers (number of over scans) as Equation (1).

3 diffusivity D, in case of stainless steel significant shorter diffusion lengths are clearly recognisable. Table 1: Thermal diffusion length l d calculated for copper and stainless steel according to Equation 3 for different time intervals: 20 µs 50 khz, 5 µs 200 khz, 1 µs 1 MHz. Figure 2: Sketch of the raster scan regime using line-byline and layer-by-layer strategy, where d P is lateral pulse spacing, d H is hatch distance and v S is scan speed. The lateral pulse spacing d P is determined by the relation between the scan speed v S and the repetition rate f rep as Equation (2). Results and discussion Simplified temperature calculation model A simplified temperature calculation model has taken into discussion to identify heat accumulation in highly repetitive ultrashort pulse laser processing. In the first stage of the model the following assumptions were included: (i) laser energy input is considered as uniform surface heat source, (ii) that fraction of remaining energy which not contributes to material ablation transfers into the bulk, the penetration depth is related to the thermal diffusion length l d as given in Equation 3, (iii) the two-temperature model is not taken into account, (iv) all other heat losses, such as convection, heat radiation, plasma/particle shielding, etc. are neglected, (V) temperature dependency of the thermo-physical material properties are excluded. Equation (3) The temperature rise induced by a single laser pulse is determined by the heat affected volume, the fraction of residual energy and the material. In case of a higher number of laser pulses impinging the same area, in the model the thermal impact of each preceding incident laser pulse is taken into calculation (Figure 3). Surface temperature behaviour is studied on both stainless steel and copper as materials with completely different thermo-physical characteristics. Table 1 presents the thermal diffusion lengths obtained in copper and stainless steel for different times. The time interval corresponds to the period between two consecutive incident laser pulses and depends on the repetition rate. Because of the lower thermal material D mm²/s l d Δt = 20 µs l d Δt = 5 µs l d Δt = 1 µs copper µm 46.3 µm 20.7 µm stainless steel µm 8.8 µm 3.9 µm Figure 3 illustrates schematically the energy distribution in copper (left) and stainless steel (right) reached after 5 following incident laser pulses. For calculation the repetition rate was considered of 1 MHz. It becomes clear that the energy of the first incident laser pulse affect the surface, even at this time when the fifth laser pulse irradiates. As a result each following laser pulse hits a warmer surface area and, particularly in case of low heat-conductive materials such as stainless steel, the surface temperature increases pulse by pulse. copper 1MHz repetition rate stainless steel Figure 3: Heat distribution after 5 following incident laser pulses reached for copper (left) and stainless steel (right). However surface temperature was calculated for 100 subsequently incident laser pulses using the simplified temperature calculation model described above. Calculations were carried out for three different time intervals between consecutive incident laser pulses, 20 µs, 5 µs, and 1 µs. The time intervals correspond to repetition rates of 50 khz, 200 khz, and 1 MHz, respectively. As shown in Figure 4 (top), for copper even in case of a short pulse-to-pulse distance of 1 µs only a marginal surface temperature increase of 11 C was obtained. According to that heat accumulation can be completely ignored in laser processing of copper in the range up to 1 MHz, prevented by high thermal diffusion and high melting temperature. By the way, copper is highly reflective for infrared wavelengths, thus, the lower fraction of 0.7 µj remaining energy per pulse was taken into calculation, and correlates to 10% absorptivity of a technical copper surface.

4 But a significant higher temperature rise up to 450 C was calculated for stainless steel, 0.85 µj remaining energy per pulse, and 1 MHz repetition rate. Thus heat accumulation can be expected as a considerably influencing effect in laser ablation in case of low heatconductive materials. optical penetration depth with the mean free path length of electrons to ( ) Equation (4). In case of ignoble metals the optical penetration depth is similar to the mean free path length of electrons, and the model is still valid. As a result the theoretical curve for stainless steel presented previously does not change. For noble metals indeed, the mean free path length of electrons is much longer than the optical penetration depth and was given in [8] for aluminium and copper of 46 nm and 70 nm, respectively. In our previous work a good agreement between theoretical and experimental results was supposed for aluminium and the penetration depth of 50 nm. That value is almost similar to the mean free path length of electrons value of 46 nm as reported elsewhere and the enhanced model seems to be valid. However, as suggested for copper as a material with specific characteristics [9] good agreements between theoretical and experimental data were obtained using the effective penetration depth of 42 nm instead of the mean free path length of electrons. In Figure 5 experimental data of copper are plotted against the ablation volumes calculated using the enhanced model. Figure 4: Surface temperature vs. pulse number, estimated for copper (top) and stainless steel (bottom) using a simplified temperature calculation model. The increase of the surface temperature is shown for time intervals of 20 µs, 5 µs, and 1 µs, corresponding to repetition rates of 50 khz, 200 khz, and 1 MHz, respectively. Calculation of ablation crater volume In a previous work we presented a model to calculate the ablation volume per laser pulse based on the ablation crater profile [6]. In the model the crater depth-limit was estimated by Beer s law considering the optical penetration depth. In case of stainless steel comparison of the calculated volumes with empirically determined values shows a good agreement. For aluminium and copper a difference between theoretical and experimental results existed, but no sufficient explanation was given at that time. However, it has been found that the energy transfer into the material is not only driven by the optical penetration of the beam. In [8] the mean free path length of electrons λ e - is assumed as depth-limit of the energy distribution. Accordingly the crater volume calculation model is enhanced by substitution of the Figure 5: Ablation crater volumes of copper, obtained in experimental and theoretical investigations, the enhanced ablation volume model was used for calculation, taking the effective penetration depth of 42 nm into account. Ablation depth vs. repetition rate The ablation depth has been investigated as a function of the repetition rate in the range between 25.8 khz and MHz. Aluminium, copper and stainless steel were irradiated with optimised parameters for minimised surface roughness, chosen from the outcomes of the parameter study presented previously [6]. The ablation depths were determined using

5 ablation depth [%] (related to 20 khz value) ablation depth [µm] ablation depth [µm] ablation depth [µm] Aluminium Aluminium repetition rate [khz] Copper Copper cavities were produced and the depth was averaged across the whole ablated surface. Further the relative change of the ablation depth with the repetition rate is analysed. The mean values of the depths achieved at higher repetition rates are related to the depth obtained at the lower repetition rate of 25.8 khz. The depth achieved at the lowest repetition rate is expected as almost unaffected by heat accumulation and particle shielding. As presented in Figure 6, the investigated materials show completely different ablation characteristics. Whereas in multi scan processing for copper no influence of the repetition rate on the ablation depth was observed, the highest increase of the ablation depth was obtained for aluminium Stainless steel repetition rate [khz] Stainless steel Aluminium Copper StSt repetition rate [khz] repetition rate [khz] Figure 6: Ablation depth vs. repetition rate obtained with multiple scans, the relative increase of the ablation depth is shown at the bottom, ablation depths are related to values achieved at the lowest repetition rate. a non-contact surface measurement system (µsurf explorer, nanofocus). With each parameter set, three Experimental data achieved for copper support the conclusion of the temperature calculation, that heat accumulation can be ignored in case of highly repetitive laser processing of high heat-conductive materials. Further no evidence of shielding losses is recognisable, and corresponds to the discussion above. In case of stainless the ablation depth varies with the repetition rate. The decrease of ablation rates obtained at repetition rates up to two hundred kilohertz can be explained by particle shielding, that is expected as most influencing effect. Irradiation of laser pulses with shorter time intervals due to higher repetition rates induce heat accumulation, that overbalance the shielding losses and the ablation depths increase. The highest relative increase of the ablation depth was estimated of almost 10 %, achieved using MHz. But no sufficient explanation can be given up to now for the strong increase of the ablation depth in aluminium. It can be seen, the ablation depth obtained at 1 MHz is approximately 60% higher, related to the depth achieved using 25.8 khz. However, aluminium is a high heat-conductive material, specified by low melting temperature and high evaporation temperature. Accordingly it can be assumed despite of the high heat conductivity the melting temperature can be reached very easily at higher repetition rates. Further the thermo-physical properties of the aluminium melting differ completely from the aluminium bulk material, thus the ablation characteristic changes considerably with the repetition rate. Ablation rate vs. lateral pulse spacing For a more detailed clarification of the interplay between heat accumulation and particle shielding, the impact of the lateral pulse spacing on the ablation volume was studied on copper and stainless steel. The

6 averaged material volumes removed per laser pulse were estimated from the cavity depths accordingly Equation (1). The cavities were produced with both different repetition rates (102 khz, MHz) and pulse energies (3.3 µj, 6.6 µj). As shown in Figure 7, laser irradiation of stainless steel with low pulse energies of 3.3 µj at low repetition rates led to constant ablated volumes. Thus the ablation process seems to be unaffected by the repetition rate over the entire investigated range of lateral pulse spacing. But shorter pulse-to-pulse distances caused a higher material ablation and heat accumulation is expected as influencing effect. Further in that processing regime no particle shielding takes place, and because of the moderate impinging laser fluence evaporation can be assumed as dominant removal process. much lower. But the curve shape gives evidence that shielding is dominant, and the long-distance effect of heat accumulation reduces with wider spacing. As mentioned above, the isotherm of the temperature field distributes by thermal diffusion, and for 200 khz repetition rate a diffusion lengths of 8.8 µm was calculated. Considering that length laterally, the ablated volume dropped significantly, because of shielding losses were not overbalanced due to less heat accumulation. However, with the higher repetition rate the removal rates increased considerably, although energy shielding is still apparent. Thus the results verify that in high repetitive laser processing of low heat-conductive materials energy losses caused by particle shielding will be overbalanced by accumulative effects. On the other hand, for copper neither significant effects of heat accumulation nor particle shielding were observed, as shown in Figure 7 (bottom). Only the wider pulse spacing caused a slightly change of the ablation rate. As a result in laser processing of highly heat-conductive materials the impact of the repetition rate on the ablation rate is almost negligible. Material removal vs. repetition rate Figure 7: Ablation volume vs. lateral pulse spacing for stainless steel (top) and copper (bottom); the pulse energy and the repetition rate were varied. For the higher pulse energies of 6.6 µj and the lower repetition rate the ablation volume decreased with wider spacing. In that regime a much stronger particle ejection took place, induced by the higher laser fluence. Thus initially a higher material ablation was assumed with wider pulse spacing, because of the energy losses caused by shielding were expected as In addition the impact of the repetition rate on the material removal was investigated by means of both stainless steel and copper considering ultra high speed camera images. In the study a four channel MCPimage intensifier ultra high speed camera (hsfc pro, PCO) was utilised. Using the double shutter exposure mode, eight images were taken from a single laser pulse impinging on the sample surface. To study the entire interesting time domain up to two microseconds, images taken from a second single pulse event were taken into account. The time between the single images was varied between 20 ns and 500 ns, the exposure time of each single image was 240 ns. The time frame used in the experiments is schematically shown in Figure 8 (top), considering the images of two different pulse events. Figure 8 illustrates the chronological sequence of the ablation process, starting from laser irradiation. Along with the ablation plumes ejected particles can be seen even two micro seconds later that time the laser pulse irradiates the metal surface. In case of stainless steel the impact of the repetition rate on the ablation process is clearly recognisable. Irradiation of pulses at high repetition rates induces a significantly brighter and more voluminous ablation plume, indicating a more intensive material removal.

7 COPPER STAINLESS STEEL MHz MHz khz khz MHz MHz khz khz Figure 8: Chronological sequence of the ablation process is shown by means of ultra high speed camera images, taken from ablation plumes on stainless steel (centre) and copper (bottom). On the top right the time frame used in double shutter ultra high speed camera photographing is schematically shown. The exposure time of 240 ns is illustrated by the rectangle. Exposure to the four cameras is termed by C1-A to C4-A for the first exposure series and C1-B to C4-B for the second exposure series. Images taken from the second pulse event are labeled by *. Further the sequence of incident laser pulses is shown within the time scale of two microseconds. Depending on the repetition rate a various number of pulses irradiate the processing zone. Each impinging laser pulse is highlighted by a black bar. 20 ns 80 ns 160 ns 260 ns 520 ns 760 ns 820 ns 900 ns 1000 ns 1260 ns 1500 ns 1740 ns 20 ns 80 ns 160 ns 260 ns 520 ns 760 ns 820 ns 900 ns 1000 ns 1260 ns 1500 ns 1740 ns

8 2.048 MHz MHz khz khz 32.0 khz Furthermore the increase of particle shielding can be supposed occurring at higher repetition rates, because of the considerably higher amount of ablated particles travelling around closely the processing zone. In laser irradiation of copper using different repetition rates the ablation plumes appears almost identical in terms of brightness and dimension. Only at the highest repetition rate of MHz a slightly higher amount of ablated particles can be estimated from the images. Thus for laser processing of copper at repetition rates higher than 2 MHz, particle shielding might be affect the ablation process, but for that time regime no any other experimental data are available yet. In Figure 9 the quantity and size of resolidified particles deposited close to the processing area were evaluated. In line-scan laser processing groove-like ALUMINIUM COPPER STAINLESS STEEL Figure 9: SEM images of groove-like structures processed in aluminium, copper and stainless steel in order to evaluate the quantity and size of resolidified particles, deposited close to the processing areas. Structures were achieved with 10 scans, 5.5 µj pulse energy and constant lateral pulse spacing but varied temporal pulse-to pulse distances.

9 structures were produces by irradiation of laser pulses of 5.5 µj pulse energy and different repetition rates between 32.0 khz and MHz. In each case both the lateral pulse-to-pulse spacing and the scan number were kept constant. Laser processing of aluminium at lower repetition rates showed a voluminous material bulging along the line edges. With higher repetition rates the number of large particles increased, but material bulging disappeared. For copper, the characteristics of ablated particles seem to be similar over the broad range of investigated temporal pulse distances. Only at the highest repetition rate the increase of particles in both size and quantity is observable. In case of stainless steel the resolidified particles formed much smaller in comparison to those originated in copper and aluminium. Further, whereas the particle size is almost similar up to MHz, much bigger particles were detected at Hz. However the impact of the particle size and quantity is on interest in further investigations. Figure 10 presents a micro structured aluminium mould with the approximate dimension of 15 mm in length by 11 mm in width by 80 µm in height. The width of the bridge was 58 µm at the surface and increases in the depth. In case of the horizontally polarised laser beam, the wall angle formed differently depending on the relation between the polarisation direction versus the plane of incidence. As a result the width of the bridges appeared differently at the bottom of the ablated structure between 110 µm and 116 µm in horizontal and vertical direction, respectively. But, as shown in the figure 10 (bottom), irradiation of a circular polarised laser beam resulted in a homogeneous bridge width of 112 µm at the bottom of the structure. The processing time of that demonstrator was roughly one hour. Micro-mould fabrication Micro-moulds were fabricated using the laser micro machining technology in order to produce microfeatured plastic demonstrators by micro-injection moulding. 2 mm 50 µm 50 µm width of the bridge (surface): 58 µm 500 µm 100 µm width of the bridge (bottom): 112 µm 5 mm Figure 10: Micro featured mould processed in aluminium block material; the drawing of the demonstrator and the manufactured aluminium mould are shown on top, a detailed view of the micro features is given by means of both SEM images (centre) and optical microscope images (bottom). Figure 11: Plastic replica of the microfluidic demonstrator mould; the channel was 80 µm deep with a width of 60 µm at the bottom (left) and 116 µm at the surface, respectively. The plastic replica of the micro featured aluminium demonstrator mould is shown in Figure 11. The size of the microfluidic channel is 116 µm (surface) respectively 60 µm (bottom) by 80 µm² in width and depth, and fits very well to the bridge dimension of the mould. The optical microscope images give evidence that laser processed micro structures can be highly detailed moulded in plastic. In another approach, a ripple structured mould processed on steel was injection moulded in plastic. The ripple structure was replicated, as presented in Figure 12. The image shows the white-light illumination of the replicated ripple structure and diffraction of light is clearly recognisable. Further the long-period substructure that appeared perpendicular to the ripple structure is observable in the plastic part. The replicated ripple structure is shown by digital optical microscope image of 4000X magnification.

10 Figure 12: Plastic replica of a ripple structured steel mould; structure size was 20 by 20 mm² with the ripple period of approx. 1 µm, diffraction of light is clearly recognisable, induced by white light illumination, the small picture (left) shows a digital optical microscope image of the replicated ripple pattern of 4000X magnification. Summary Highly repetitive laser processing of metals with different thermo-physical material properties was investigated in order to characterise the interaction phenomena such as heat accumulation and particle shielding. To discuss the impact of different temporal pulse-to-pulse distances on the ablation process, result obtained using a simplified temperature calculation model was taken into discussion. For laser irradiation of stainless steel at 1 MHz, a significant surface temperature rise up to 450 C was calculated. That verifies heat accumulation as influencing effect in highly repetitive laser processing of low heatconductive materials. For copper the calculations showed, that the surface temperature increases only marginally with the repetition rate, thus heat accumulation is negligible for highly heat-conductive materials. Additionally it was shown that the lateral pulse spacing influences the ablation process in case of low heatconductive materials. Whereas for small lateral spacing heat accumulation overbalances that losses induced by particle shielding, the ablation rate decreased at low repetition rates and wider lateral spacing between consecutive incident laser pulses. Induced by heat accumulative effects, almost 60% respectively 10% higher ablation rates were obtained for aluminium and stainless steel at MHz, compared to the values achieved at 25.8 khz. As proven theoretically by temperature calculation, for copper no impact of the repetition rate on the ablation rate was detected empirically. Furthermore the ablation characteristics were evaluated by means of ultra high speed camera images. For stainless steel, at shorter temporal pulse-to-pulse distances much brighter and more voluminous ablation plumes were recorded, indicating intense laser matter interaction. By contrast, the ablation plumes obtained for copper seemed to be almost similar, supporting the assumption the ablation process is almost unaffected by the repetition rate. Furthermore it was found that the quantity and size of resolidified particle did not change significantly, thus particle shielding can be ignored in case of copper. Finally micro-mould manufacturing was presented. In aluminium a micro-featured demonstrator mould was produced. Highly detailed microfluidic plastic replicas were obtained using the micro-injection moulding technology. Further a ripple structure fabricated on a steel mould was replicated. White light illumination of the plastic part showed diffraction of light. As a result high repetition rate laser micro processing seems to be a high-potential technology in micro-injection moulding. References 1. Ancona, A., et al., High speed laser drilling of metals using a high rep. rate, high average power ultrafast fiber CPA system. Opt. Express, (12): p Döring, S., et al., Microdrilling of metals using femtosecond laser pulses and high average powers at 515 nm and 1030 nm. Applied Physics A: Materials Science & Processing, (1): p Schille, J., et al., Micro structuring with highly repetitive ultra short laser pulses, in LPM2008-9th Symp.on Laser Precision Micromachining. 2008: Quebec, Canada. 4. Vorobyev, A.Y. and C. Guo, Direct observation of enhanced residual thermal energy coupling to solids in femtosecond laser ablation. Applied Physics Letters, (1): p König, J., S. Nolte, and A. Tünnermann, Plasma evolution during metal ablation with ultrashort laser pulses. Opt. Express, (26): p Schille, J., et al. Micro processing of metals using a high repetition rate femto second laser: from laser process parameter study to machining examples. in Proc. of 30th Int. Congress on Appl. of Lasers and Electro-Optics (ICALEO 2011) Orlando FL, USA. 7. Nolte, S., et al. High Repetition Rate Ultrashort Pulse Micromachining with Fiber Lasers. in Fiber Laser Applications: Optical Society of America. 8. Wellershoff, S.-S., Untersuchungen zur Energierelaxationsdynamik in Metallen nach Anregung mit ultrakurzen Laserpulsen (written in German), in Fachbereich Physik. 2000, Freie Universität Berlin: Berlin. 9. Byskov-Nielsen, J., Short-pulse laser ablation of metals:fundamentals and applications for micromechanical interlocking, in Department of Physics and Astronomy. 2010, University of Aarhus: Aarhus, Denmark.