Investigation of cw and ultrashort pulse laser irradiation of powder surfaces a comparative study

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1 Investigation of cw and ultrashort pulse laser irradiation of powder surfaces a comparative study Robby Ebert, Frank Ullmann, Joerg Schille, Udo Loeschner, Horst Exner Laser Institute at the University of Applied Sciences Mittweida, Technikumplatz 17, Mittweida, Germany ABSTRACT The paper presents results obtained in a comparative study of laser irradiation of tungsten powder surfaces using a continuous wave fiber laser and a high repetition rate femtosecond laser. Depending on the energy input per unit length different melt structures have been produced. In general, if the same average laser power level was applied the structures show the same appearance independent from the laser source. But there was both a little higher degree of initial fusing and cross-linking along the processed path when the powder surface was irradiated with ultrashort pulses. Further, with increasing laser intensity a change in structure formation as well as a broadening of the laser processed path has been occurred, although the energy input per unit length remains constant. However, accumulation of slab-like structures, which was previously observed in high-intense ultrashort pulse laser irradiation, has been become more pronounced in cw laser irradiation above a certain number of consecutive scans. Moreover, characteristic effects, such as formation of ripples and nanomelt structures appearing in ultrashort pulse laser processing have been not detected in cw laser irradiation. Keywords: femtosecond laser, high repetition rate, tungsten, nanomelt structure, energy input per unit length 1. INTRODUCTION Laser micro sintering has been developed recently for fabricating of micro-featured components and devices made from metal and ceramic powders [1]. This method, therefore, is currently the only technical feasible method permitting direct fabrication of free-formed microstructures with undercuts. By using short and high-intense laser pulses even metals with a high melting point such as tungsten can be machined. The spatial resolution achieved in laser micro sintering was better than 30 μm. For a further increase of the resolution complementary studies have been conducted utilizing a femtosecond laser [2]. The feasibility of sintering processes using femtosecond laser radiation has been successfully demonstrated already in microelectronics, and track widths of 400 nm were reported [3, 4]. In this study results obtained in laser irradiation of tungsten powder surfaces using continuous wave (cw) and femtosecond (fs) laser were compared. The formation of the originated melt structures was characterized by means of SEM photographs with special regard to the energy per unit length, intensity of laser radiation and process regime. 2. EXPERIMENTAL PRINCIPLES In this study an existing laser micro sintering facility [5] was used, consisting of a femtosecond laser, an attenuator to adjust the pulse energy, a pulse divider, a galvanometer scan system for beam deflection and focussing, and a vacuum sintering chamber. In addition a continuous wave laser was implemented in the experimental setup. In order to ensure comparable processing conditions, in cw laser irradiation the powder surface was set in 0.3 mm defocused position relative to the focal plane. Thus the spot radii of the cw laser beam and the femtosecond laser have been in the same dimension of about 20 μm at the powder surface. The laser parameters are summarized in Table 1. Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVIII, edited by Xianfan Xu, Guido Hennig, Yoshiki Nakata, Stephan W. Roth, Proc. of SPIE Vol. 8607, 86070X 2013 SPIE CCC code: X/13/$18 doi: / Proc. of SPIE Vol X-1

2 Table 1: Laser parameter. processing regime ultrashort pulsed wavelength λ [nm] pulse duration (sech 2 ) t [fs] pulse repetition rate f [MHz] focus spot radius on powder surface w 86 [μm] and 1 18, 20, 23 continuous 1065 / / 20 In fs laser irradiation the processing parameters were varied as following: incident laser power P av between 0.05 W and 7.0 W, the pulse energy Q P between 0.05 μj and 7 μj, and the pulse repetition rate of MHz and 1 MHz. Accordingly, by taking the equations given below into account, the pulse peak power P P and the maximum intensity I max calculate in the range between 0.24 MW and MW as well as 0.048*10 12 W/cm 2 and 6.73*10 12 W/cm 2, respectively. The focus spot radius w 86 of the femtosecond laser beam enlarged in the course over time. Examples for a radius of 20 μm are given in Table 2. By contrast, because of the different processing regimes, the maximum intensity of 6.67 * 10 6 W/cm 2 of the cw laser beam was about 6 orders of magnitude smaller compared to the femtosecond laser beam (Table 3). fs laser: cw laser: Table 2: Femtosecond laser pulse parameter (w 86 = 20 μm). pulse energy Q P [μj] fluence H 0 [J/cm 2 ] pulse peak power P P [MW] max. intensity I max [W/cm 2 ] * * Table 3: cw laser parameter. laser power P av [W] max. intensity I max [W/cm 2 ] * * 10 6 The powder surface was irradiated using both single and multiple linear passes of the laser beam across the sample surface (scan) under a vacuum pressure of 0.05 Pa. In all basic experiments the scan offset distance between individual sintered tracks was 50 μm. The scan speed and the laser power were varied in order to investigate the impact of the process regime on the irradiation process. Thereby the energy per unit length, which is determined by the ratio of the impinging average laser power and scan velocity was chosen as one of the significant evaluation parameter. The most significant processing parameters applied in this study are presented in Table 4. Proc. of SPIE Vol X-2

3 Table 4: Significant processing parameters. processing regime laser power P av [W] pulse repetition rate f [MHz] scan speed v [m/s] energy per unit length Q S [J/m] ultrashort pulsed / continuous 1.8 / / / In the experiments tungsten powder supplied by Goodfellow was used. The particle size distribution was specified by the manufacturer with a grain size lower than 1 μm and the majority part of 300 nm. The bulk density was measured of 2.8 g/cm 3, which corresponds to a relative powder density of approximately 15%. The powder surface was normally produced by means of a single dispensing process using a special cylindrical dispenser [5]. The thickness of the powder layer was approximately 1000 μm. In order to prepare a defined powder surface, it was cleaned before every experiment through laser irradiation. To this end, the powder surface was irradiated line by line with a pulse energy of 0.7 μj and a scan velocity of 4.5 m/s. With these parameters, plasma was observed during the first pass of the laser beam across the surface. But the plasma was absent during the subsequent passes because of no plasma effects were observed visually on the powder surface. Based on scanning electron microscope (SEM) imaging, however, it became clear that the powder surface has been slightly affected by the irradiation. The irradiated regions appeared lighter in the SEM images. This is also discussed more detail in the paper. 3. EXPERIMENTAL RESULTS 3.1 Energy per unit length Q S > 3 J/m In laser irradiation of tungsten powder a similar intensive plasma formation was observed in both processing regimes using the fs and cw laser, but the impact of the irradiated energy per unit length is noteworthy. Accordingly it can be seen that the degree of crosslinking along femtosecond laser irradiated lines was slightly greater at the higher energy inputs per unit length (Figure 1). By contrast, in single pass cw laser irradiation primarily narrow ridges perpendicularly aligned to the scan direction and occasional welding beads were formed. The reason for this was the continuously acting laser radiation and the surface tension of the resulting melt. Due to the femtosecond laser radiation the locally melted regions could be better connected to each other. The width of the generated lines was somewhat less. Despite significantly lower intensity deep gaps were created at a high linear energy and also in the application of cw radiation, so that this cannot be associated with the high peak intensity of the femtosecond laser radiation, but rather with a high Proc. of SPIE Vol X-3

4 enough average intensity. Furthermore there were slight differences regarding the width of the melting lines. It has been found that the line width obtained with femtosecond laser radiation formed 10% smaller compared to continuous laser radiation. Q S = 12.0 J/m Q S = 18.0 J/m Q S = 28.0 J/m Q S = 42.0 J/m Figure 1: SEM images of laser irradiated powder surfaces, v = 0.1 m/s, w 86 = 20 μm, f = 1 MHz (femtosecond laser), 1 scan. Q S = 3.6 J/m Q S = 8.4 J/m Q S = 13.4 J/m cw laser femtosecond laser cw laser femtosecond laser Figure 2: SEM images of irradiated powder surface, v = 0.5 m/s, w 86 = 20 μm, f = 1 MHz (femtosecond laser), 1 scan. As illustrated in Figure 2, the differences between the two irradiation regimes can be seen even smaller by using a lower energy per unit length. The melt was concentrated at the edges of the laser processed tracks and singular molten substructures emerge, so that any crosslinking took hardly place. These enamel beads occurred more frequently with decreasing energy per unit length as a result of shorter exposure times, caused by increasing scan speeds. But from the results it can be concluded that formation of molten structures is more affected by the scan speed rather than the laser Proc. of SPIE Vol X-4

5 power. The tracks obtained with a similar energy input per unit length indicate periodic structure formation in case of longer exposure times and lower laser power, as presented in Figure 1 left. By contrast, irradiation of higher laser power at faster scan speeds caused origin of singular enamel beads, see Figure 2, right. Further, with regard to the processing regime, in femtosecond laser irradiation somewhat larger structures emerged with a higher degree of crosslinking, whereas in cw laser processing primarily smaller welding beads were formed. Here, too, a better connecting between the individual melting areas was obtained using the femtosecond laser radiation, but the crosslinking was interrupted at higher speeds. With a further increase in speed, the trend is continuing. Further, with both investigated laser processing methods the melting beads tended to form poly crystals (Figure 3). They occurred with almost all used process parameters and resembled in form and size. Therefore it is assumed that the heat dissipated slowly from the isolated melt beads, and thus there was still enough time for crystallization. Figure 3: SEM images of crystallized melt beads, obtained following a single laser processed line scan by using either the femtosecond laser, f = 1 MHz, Q S = 14.0 J/m, v = 0.3 m/s (left), or the cw laser, Q S = 16.0 J/m, v = 0.3 m/s (right). However, even with multiple laser irradiations the melting structures generated by cw laser radiation only slightly differ from those generated by femtosecond laser radiation. In both laser irradiation regimes the degree of fusion marginally increased with higher number of exposures. But after ten irradiations with a distinct laser power and scan speed the origin of slab-like structures was observed and thus in some cases the entire laser processed area raised, starting from the right and left edge (Figure 4). This long-distance effect of the laser radiation appeared similarly in femtosecond laser irradiation. The explanation therefore is difficult, because there was still unprocessed tungsten powder between the irradiated lines. However, the resulting tensions induced by multiple irradiations of molten lines can be suggested as one reason for the mentioned observations. Figure 4: SEM image of a cw laser irradiated tungsten powder surface, P = 1.2 W, v = 0.1 / 0.3 / 0.5 m/s, 10 scans. Proc. of SPIE Vol X-5

6 From the results obtained in the high energy per unit length process regime it can be derived, that the formation of molten structures is mainly driven by the total laser energy input. Specific effects induced by the femtosecond laser radiation have not been found, exceptionally the formation of ripple structures on melting beads as reported in a previous work [2]. Further, for small spatial pulse distance it can be proposed that melt formation is much more pronounced on a powder surface than on solid body surfaces. This assumption is based on the relatively large penetration depth of the radiation into the powder bed and the less heat conductivity of powder particles. 3.1 Energy per unit length Q S < 3 J/m In the low energy process regime, determined with an energy input per unit length of less than 3 J/m, variances between the individual effects of cw and fs laser radiation have been observed. Sinter-like structures with distinctive characteristics depending on the processing regime appeared particularly under multiple laser irradiations but also in the range of energy per unit length, where melt structure formation starts. Thereby a higher importance can be attributed to the femtosecond laser beam because of the possibility to irradiate high-intense laser pulses at comparably low energy per unit lengths. Figure 5: SEM images, cw laser, 1 scan, left: P = 0.4 W, v = 0.5 m/s, Q S = 0.8 J/m, I max = 0.63 * 10 6 W/cm 2, right: P = 3.8 W, v = 5 m/s, Q S = 0.76 J/m, I max = 6.03 * 10 6 W/cm 2. In cw laser irradiation the lower limit of the energy per unit length of about Q S = 0.7 J/m was found for the formation of molten structures. However, it has been not verified whether the energy input can be reduced with a further increase in the intensity, because of the scan speed was limited to v = 5 m/s. The comparison of results obtained with two different intensities of I max = 0.63 and 6.03 W/cm 2, shown in Figure 5 left and right, indicates that with a much more higher intensity at almost the same energy per unit length, there was just slightly more melt. This can be explained by the fact that at the higher intensity the mean exposure time decreases due to the higher scan speed, in this specific case from 8μs to 800 ns, respectively. By the way it can be assumed that an even higher intensity will not cause a significant reduction of the lower limit of the energy per unit length for melt formation. Figure 6: SEM images of multiple cw laser irradiated tungsten powder surfaces obtained with 2 scans (left) and 10 scans (right), the processing parameter were: P = 0.2 W, v = 0.1 m/s, Q S = 2 J/m, I max = 0.31 * 10 6 W/cm 2. Proc. of SPIE Vol X-6

7 In the cw laser processing regime the lower intensity threshold for melt formation has been determined of I max = 0.3 * 10 6 W/cm 2. For structure formation it was necessary to reduce the scan speed to v = 0.1 m/s and to increase the energy per unit length to Q S = 2 J/m. Further it was found that multiple irradiation of the powder surface was not enhancing the melt formation quantitatively, as indicated in Figure 6 for 2 (left) and 10 scans (right), respectively. Considering the same laser power and scan speed, in cw laser processing melting structures primarily emerged at the surface of the powder bed (Figure 7 left), whereas under femtosecond laser irradiation the structures are much more delicate and melting beads formed also in deeper regions (Figure 7 right). Figure 7: SEM images of laser irradiated tungsten powder surfaces obtained with different laser systems but almost similar processing parameters, such as P = 4.2 W, v = 5 m/s, Q S = 0.84 J/m, 1 scan; left: cw laser I max = 6.67 * 10 6 W/cm 2, right: femtosecond laser, f = 1 MHz, I max = 4.04 * W/cm 2. Figure 8: SEM image of femtosecond laser irradiated tungsten powder, w 86 = 18 μm, P = 2.2 W, f = 1 MHz, v = 5 m/s, Q S = 0.44 J/m, I max = 2.12 * W/cm 2, 1 scan. In comparison to continuous laser processing, the energy per unit length of only Q S = 0.4 J/m was required for melt structure formation using the femtosecond laser. Moreover, in addition to the considerably lower threshold value, the melting structures were much more finely organized in the femtosecond process regime (Figure 8). Here the widths of the melting structures have been determined in the range between the little 1 and 1.5 micrometers. In a previous work plasma formation appearing at the powder surface has been investigated with special regard to both the intensity and the energy per unit length. In this complementary approach, the laser beam was scanned across the powder surface for three times in order to delimit the side effects and to assess the reliance of the parameters. It was Proc. of SPIE Vol X-7

8 found that a very weak plasma was still appeared at a peak intensity of only I max = * W/cm 2 and the energy per unit length of Q S = 0.5 J/m. Furthermore, whereas the reduction of the energy input per unit length led to the extinction of the plasma plume. For all three laser passes plasma formation was observable for femtosecond laser irradiation of the powder surface with an intensity of I max = * W/cm 2 and the energy per unit length of Q S = 0.7 J/m, respectively. The further increase of the intensity resulted in a more stringent dependence of the plasma formation on the energy per unit length and the scan speed. Accordingly at I max = 0.29 * W/cm 2 and Q S = 0.5 J/m the plasma was only observed with the first laser pass. Therefore, it was assumed that the plasma occurred during the first pass of the laser beam across the sample surface was induced by laser decomposition of the residues located on the powder surface layer, such as organic substances. Consequently, these parameters were used to clean the powder surface. But for processing parameters above the lower limit values of I max = 0.19 * W/cm 2 and Q S = 1 J/m, first melting structures were observed, induced by the laser pretreatment process. During the next laser pass processed following laser pre-cleaning, no plasma occurred, even at the low I max = * W/cm 2 and Q S = 0.7 J/m. Therefore the reason became clear by our own investigations. In addition the comprehensive study of plasma formation at tungsten powder surfaces using high-intense femtosecond laser pulses indicated some other significant threshold values. Thus plasma formation has been detected for all three laser passes within the parameter range above I max = 0.48 * W/cm 2 and Q S = 0.35 J/m. Moreover, for intensities higher than I max = 0.96 * W/cm 2 the plasma appears independent from the energy per unit length, and plasma appearance was weakened at scan speeds faster than v = 3.8 m/s. Based on these findings I max = 0.67 * W/cm 2 and Q S = J/m (Q P = 0.7μJ, v = 4.5 m/s) were determined as appropriate parameters for powder cleaning. As a result the tolerance limits for single plasma formation were enlarged and the scan speed was significantly higher. Figure 9: SEM images of femtosecond laser irradiation using w 86 = 18 μm, P = 4 W, f = 1 MHz, v = 5 m/s, Q S = 0.8 J/m, I max = 3.85 * W/cm 2, but different number of scans, such as 2 scans (top left), 5 scans (top right), 10 scans (bottom left), and finally 20 scans (bottom right). Proc. of SPIE Vol X-8

9 But in contrast to cw laser irradiation as presented in Figure 6, multiple irradiations of femtosecond laser pulses applying the low energy per unit length processing regime caused the origin of micrometer-scaled structures, summarized in Figure 9. It can be seen that two passes induces the formation of the melt (Figure 9 top left). Further, with 5 laser beam crossings ripple structures appeared, which are typically in ultrashort pulse laser irradiation (Figure 9 top right). Moreover, with a higher number of passes (Figure 9 bottom left and right) the ripples formed in deeper regions. The melt was formed initially from the existing powder particles inducing network structures. But with further passes the intensity and energy per unit length were no longer sufficiently high to melt the formed structures again. The molten structures will have a greater mass compared to the nanoparticles in the powder bed, and thus it can be assumed that the parameter threshold for melting is higher. Therefore it was reported already that the thresholds for material melting or evaporation strongly depend on their size [5, 6]. Because of the formation of larger melt structures during subsequently processed beam crossings the laser intensity exceeded the threshold for ripple formation, which is lower than the melting threshold. Because of these findings the cleaning process was reevaluated. It is assumed that by the cleaning not only the surface of the particles was cleaned, but also very small nanoparticles were partially evaporated. Thereby, in turn, material vapor plasma was initiated. That means femtosecond laser irradiation of the powder surface with a certain relatively small intensity induces selective evaporation of particles in the powder bed up to a certain particle diameter. This is the only way to explain the fact that after powder cleaning with I max = 0.29 * W/cm 2 and Q S = 0.5 J/m at the lower impinging intensity of I max = * W/cm 2 and Q S = 0.7 J/m no plasma formation was observed. By contrast without the cleaning process, however, plasma was observed during each laser beam crossing. Because of the intensity was four times higher in the pre-cleaning process, small nanoparticles were selectively evaporated. As a result during the next following laser beam crossings the intensity was no longer sufficient high enough to obtain similar effects in terms of material melting as reported for tungsten powder containing nano-scaled particles. As a result it becomes clear that in the femtosecond laser irradiation of nano-powders the distribution of the particle size has a significant impact on the process, and thus at the threshold parameters in particular. However, multiple femtosecond laser irradiations of tungsten powder beds by using low energy per unit lengths and the maximum available intensity caused the formation of novel compacted structures. The formation of such structures can be attributed driven by the high intensity of the femtosecond laser pulses, because of similar structures have been not observed in continuous laser processing. Figure 10: SEM images of femtosecond laser irradiated tungsten powder, the processing parameter were: f = MHz, v = 2.5 m/s, 10 scans, at the top: Q S = 0.3 J/m, w 86 = 23 μm, I max = 3.57 * W/cm 2, bottom left: Q S = 0.35 J/m, w 86 = 18 μm, I max = 3.85 * W/cm 2, bottom right: Q S = 0.35 J/m, w 86 = 18 μm, I max =6.74 * W/cm 2. Proc. of SPIE Vol X-9

10 It was found that a minimum intensity of about I max = 4 * W/cm 2 was required for fabrication of compacted structures, and according to that structure formation depending on the intensity is presented in Figure 10. In that approach the energy per unit length was in the range between Q S = 0.35 and 1.4 J/m. A full development of the structures just occurred at intensities higher than I max = 5 * W/cm 2 (Figure 10, bottom right). As a result of the high intensity the ripple formation was inhibited, but instead a compaction of the melting structures took place. Figure 11: SEM images of femtosecond laser irradiated powder, v = 1.25 m/s, f = MHz, Q S = 0.7 J/m, w 86 = 18 μm, I max = 6.74 * W/cm 2, 10 scans. The most compact structures, such as shown in Figure 11, have been achieved by using an energy per unit length of Q S = 0.7 J/m and the highest available intensity of I max = 6.74 * W/cm 2. In that case the lateral pulse spacing was 10 micrometer. In a first view as given in Figure 11 (left), the structures appeared almost melt-free and thus comparable to structures generated using classical sintering methods. But the analyses using high-resolution SEM images (Figure 11, right) indicated nano-particles, nano-melt jets and layered melting structures at the structure surface. Thus it can be concluded that the structures have been originated based on melt. Moreover because of the high intensity of the laser pulses, the upper melt layers of a thickness in the range of a few nanometers evaporated accompanied by the compacting of the melt caused by the resulting recoil [7]. A similar process has been observed earlier in laser micro sintering using nanosecond pulses [8], but in that case the structural size of the resulting melt structures was in the range between a few up to some tens of micrometers. But in this work, by contrast, a local limitation of the melt within the nanometer/submicrometer range is demonstrated. Insofar, nanomelt structures with nanometer feature sizes can be suggested as the main distinction from classical sintered structures. 4. SUMMARY In this comparative study laser irradiation of tungsten powder using continuous and femtosecond laser radiation was investigated depending on the energy input per unit length. By irradiation of an energy per unit length greater than Q S = 3 J/m almost similar melt structures were obtained with both laser systems at comparable processing conditions. But the degree of partial melting as well as cross-linking along laser processed lines was slightly larger with femtosecond laser radiation. In contrast with cw laser radiation the tendency to form enamel pearls was more likely observed. Furthermore there were slight differences regarding the width of the melting lines. It has been found that the line width obtained with femtosecond laser radiation formed a little smaller compared to continuous laser radiation. Moreover formation of poly-crystals was detected in both processing regimes. But in case of multiple exposures of the powder bed slab-like structures originated within the laser processed area. In addition the threshold values were determined for material melting. In cw laser processing an energy per unit length greater than Q S = 0.7 J/m was required to form melt structures, but with fs laser radiation, however, melt formation has been observed at the lower energy per unit length of Q S = 0.4 J/m. Finally compacted nanomelt structures such as nanoparticles and nano-fusion jets induced by multiple femtosecond laser irradiations have been verified. Proc. of SPIE Vol X-10

11 ACKNOWLEDGEMENTS The authors would like to thank BMBF for sponsoring the Innoprofile projects Rapid microtooling using laser-based methods and Rapid Micro High rate laser machining (Ministry ref. no. 03IP506 and 03IP506X) and Dorena Fleischer for supporting the experimental work. REFERENCES [1] H. Exner, M. Horn, A. Streek, P. Regenfuß, F. Ullmann, R. Ebert, "Laser micro sintering a new method to generate metal and ceramic parts of high resolution with sub-micrometer powder", Proceedings of 3rd International Conference on Advanced Research in Virtual and Rapid Prototyping, Leiria (Portugal), (2007) [2] R. Ebert, F. Ullmann, D. Hildebrandt, J. Schille, L. Hartwig, S. Kloetzer, A. Streek, H. Exner, "Laser Processing of Tungsten Powder with Femtosecond Laser Radiation", JLMN-Journal of Laser Micro/Nanoengineering, Vol. 7, No. 1, (2012) [3] Y. Son, J. Yeo, H. Moon, T. W. Lim, S. Hong, K. H. Nam, S. Yoo, C. P. Grigoropoulos, D.-Y. Yang, S. H. Ko, "Nanoscale Electronics: Digital Fabrication by Direct Femtosecond Laser Processing of Metal Nanoparticles", Advanced Materials, Vol. 23, Issue 28, (2011) [4] Y. Son, T. W. Lim, J. Yeo, S. H. Ko, D.-Y. Yang, "Fabrication of Nano-scale Conductors by Selective Femtosecond Laser Sintering of Metal Nanoparticles", 10th IEEE Conference on Nanotechnology (IEEE- NANO), (2010) [5] R. Ebert, F. Ullmann, L. Hartwig, T. Suess, S. Kloetzer, A. Streek, J. Schille, P. Regenfuss, H. Exner, "Laser microsintering of tungsten in vacuum", Proc. SPIE, Vol. 7589, 75891G (2010) [6] J. Huang, Y. Zhang, J. K. Chen, "Size Effects During Femtosecond Laser Interaction With Nanosized Metal Particles", J. Heat Transfer, Vol. 134, Issue 1, (2012) [7] A. Hu, Y. Zhou and W.W. Duley, "Femtosecond Laser-Induced Nanowelding: Fundamentals and Applications", The Open Surface Science Journal, 3, (2011) [8] P. Regenfuss, A. Streek, L. Hartwig, S. Kloetzer, Th. Brabant, M. Horn, R. Ebert, H. Exner: Principles of Laser Micro Sintering, Proceedings of the 17th Annual SFF Symposium, Austin (Texas/USA), (2006) Proc. of SPIE Vol X-11