Product miniaturization accompanied by high-precision and high-reliable material removal is of great interest in the automotive, optics, microelectronics, and biomedical device industries. Cost-effective processes and products require highly flexible manufacturing technologies and short processing times. However, even on a microscale, conventional material removal techniques (such as electric discharge machining, turning, drilling, or milling) cannot meet these ambitious aims.
Laser technology makes it possible to machine a wide variety of materials and structures by non-tactile heating and a highly defined, localized laser energy input. Ultra-short-pulse laser technology also offers significantly enhanced precision and machining qualities as well as a minimized heat load of the surroundings. At present, commercially available laser powers and repetition rates are too low for widespread industrial use. However, recent developments in high-power, high-repetition-rate ultra-short-pulse laser systems mean that laser micromachining looks set to become an increasingly important technology in modern micromanufacturing and engineering.
My colleague and I studied laser microprocessing using high-repetition-rate femtosecond lasers to describe and understand the fundamental phenomena and interacting effects. We combined a high-repetition-rate femtosecond fiber laser with fast laser-beam deflection systems, a galvanometer scanner, and our own design of resonant scanner.1–3 Our primary interest was the impact of the laser repetition rate on the ablation process as well as applications in 3D laser microstructuring.
Our initial results indicate new phenomena in laser-matter interaction, such as heat accumulation and particle shielding.1,2,4,5 We found that, together, highly-repetitive laser pulses and the low thermal conductivity of the irradiated materials induce cumulative effects. Thus, a considerable temperature rise around the irradiated area enhances the laser beam absorption and results in lowered ablation thresholds and higher ablation rates. By contrast, a slower repetition rate of hundreds of kilohertz leads to lowered ablation rates. In this case, subsequent incident laser pulses interact with ablated particles and ejected clusters, induced by preceded laser pulses, and energy losses due to particle and plasma shielding occur. Further, irradiation of high average laser power induces microformations. In addition to well-known ripple structures, we observed conical microformations.1,5
The effect of the repetition rate on the ablation threshold can be seen using a simple technique6 that plots D2, the square of the ablated diameter against the irradiated laser fluence, so that the ablation threshold can be estimated by extrapolating D2→0. Considerably lowered ablation thresholds at higher repetition rates due to cumulative heat effects can be clearly recognized (for the effect on aluminum, see Figure 1: top). In addition, material properties, such as the thermal conductivity and melting/evaporation temperature, influence the ablation behavior (see Figure 1: bottom). Highly repetitive laser irradiation of materials with low thermal conductivities, such as stainless steel, results in a lower ablation threshold. By contrast, the ablation rate of materials with a high thermal conductivity, like copper, is not significantly affected by the repetition rate. (It seems that particle shielding is the reason for the increase in ablation rate for copper at 100kHz.)
Figure 1.Impact of the repetition rate onto the ablation threshold. (top) Liu plot for aluminum to estimate the ablation threshold depending on the repetition rate. (bottom) Ablation threshold as a function of the repetition rate for stainless steel, aluminum, and copper (normalised onto 20kHz value).
So-called laser-induced periodic surface structures (LIPSS) are another interesting phenomenon in femtosecond laser processing (see Figure 2: top). Ripple structures form mostly orthogonal to the electrical field of the incident laser beam with a periodicity close to the laser wavelength. Given sufficient laser energy input and laser fluence in the range of the ablation threshold, conical microstructures form. We found that the microcone properties can be influenced by the processing parameters1,5 and verified the effect of microcones on surface properties by measuring the wetting and the reflectivity.5Depending on the microcone properties, we found hydrophobic stainless steel surfaces with contact angles of up to 150° (see Figure 2: bottom) and reflectivities of less than 5%.
Figure 2.(top) Laser-induced periodic surface structures on stainless steel. Note the ripple and microcone formations. (bottom) Variations in wetting of micro-cone-covered surface. (a) Plot of wetting by Young's law. (b) Hydrophobic surface due to high-volume microcones. (c) Hydrophilic surface due to smallest cone sizes. (d) Wetting of untreated technical stainless steel surface. α: The ‘contact angle,’ at which the liquid-vapor interface meets the solid-liquid interface. σl: Liquid-vapor surface tension. σls: Liquid-solid surface tension. σs: Solid-vapor surface tension.
In general, high repetition rates result in a fast processing speed for high-average-power femtosecond laser ablation. We obtained high machining throughputs with ablation rates up to 1.8mm3/min.1In 3D laser microstructuring, we achieved laser processing times up to 40 times shorter than kilohertz femtosecond systems.2 The processing quality was also much higher (see Figure 3 for machining examples of line-scan ablation, 2.5D laser ablation, and 3D laser microstructuring3).
Figure 3.Machining examples obtained in high-repetition-rate femtosecond laser processing. (top) Line-scan ablation (cross section, 1000 scans). (center) 2.5D laser ablation. (bottom) 3D laser microstructuring.
A future challenge is to implement ultrafast scan systems, such as resonant scanners rather than galvanometer scanners, to achieve much higher processing speeds. This would result in even higher machining throughputs and, accompanied by advantageous processing qualities, attract great industrial interest in high-repetition-rate femtosecond laser technology for many micromachining applications.
Laser Application Center, University of Applied Sciences Mittweida
Joerg Schille graduated in physical engineering/laser material processing from the University of Applied Sciences Mittweida, Germany, in 2003. He worked as a research and development engineer with medical and industrial enterprises before returning to work on laser microprocessing with short and ultra-short laser technologies in 2006. His expertise is in short/ultra-short pulse laser micro processing, high-rate laser processing, laser surface texturing, and laser safety. He has been a PhD student with the School of Chemical Engineering and Analytical Sciences (CEAS) at the University of Manchester (UK), since 2009.
1. J. Schille, R. Ebert, U. Loeschner, P. Regenfuß, T. Suess, H. Exner, Micro structuring with highly repetitive ultra short laser pulses, Proc. 9th Int'l Symp. Laser Prec. Microfab. (LMP2008), pp. 08-57, 2008.
2. J. Schille, R. Ebert, U. Loeschner, P. Scully, N. Goddard, H. Exner, High repetition rate femtosecond laser processing of metals, Proc. SPIE
7589, pp. 758915, 2010. doi:10.1117/12.842600
3. J. Schille, R. Ebert, L. Hartwig, U. Loeschner, P. Scully, N. Goddard, H. Exner, Rapid micro processing of metals with a high repetition rate femto second fibre laser, Proc. 11th Int'l Symp. Laser Prec. Microfab. (LPM2010), 2010.
4. J. Schille, R. Ebert, H. Exner, U. Loeschner, L. Schneider, N. Walther, P. Scully, N. Goddard, 3D micro machining with a high repetition rate ultra short fibre laser, Proc. 5th Int'l WLT Conf. Lasers Manuf., pp. 549-554, 2009.
5. J. Schille, R. Ebert, U. Loeschner, L. Schneider, N. Walther, P. Regenfuss, P. Scully, N. Goddard, H. Exner, An ultrafast femtosecond fibre laser as a new tool in rapid microtooling, Proc. 5th Int'l Cong. Laser Adv. Mater. Process., pp. 09-043, 2009.
6. J. M. Liu, Simple technique for measurements of pulsed Gaussian-beam spot sizes, Opt. Lett.
7, no. 5pp. 196-198, 1982. doi:10.1364/OL.7.000196