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Micromachining of Metals Using Ultrashort Laser Pulses

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Micromachining of Metals Using Ultrashort Laser Pulses C[~ Renew International Journal of Electrical Machining, No. 4, January 1999 Micromachining of Metals Using Ultrashort Laser Pulses Hans Kurt TONSHOFF, Ferdinand VON ALVENSLEBEN, Andreas OSTENDORF, Giinter KAMLAGE, Stefan NOL TE (Received on Oct.7, 1998) Laser Zentrum Hannover, Germany Abstract This paper will discuss the potential of ultrashort pulse laser sources for micromachining applications of metals. It will be demonstrated that by using ultrashort pulse lasers very high precision, comparable to those of EDM or other micromachining technique, is achievable. The application of this novel laser micromachining technology significantly improves the efficiency of the ablation process reducing processing times. The pulse duration requirements for melting- and burr-free ablation of metals will also be discussed. Some examples of microstructures in different metals will be shown. Key words: Ultrashort laser pulses, Laser micromachining 1. INTRODUCTION the tool. Moreover, the processing time generally rises for smaller electrode dimensions. Among all different techniques of micro­ processing the application of electrical discharge The problems of twist drilling and free machining (EDM) is one of the most reliable and cutting in the field of micromaching appear effective method of manufacturing structures in similar to those of EDM. Both techniques are metallic materials. This technology features high characterized by high tool wear and frequently accuracy and high quality of the machined breakage of the tools, which influences the costs surface. The ablation process is burr-free, and operating times negatively. making additional finishing procedures unnecessary. Since several years, e.g. the Since several years, lasers have been used for performance of micro-drilling with diameters materials processing, not only for cutting, smaller than 0,5 mm is established within the welding or hardening macroscopic parts, but also industrial environment. for creating micro-parts and structures with The increasing necessity of very fine dimensions of some microns in various types of structures «lOOflm), however, causes problems materials such as metals, ceramics, glasses, and for nowadays conventional machining semiconductors. techniques, such as EDM, free-cutting and twist In general, laser machining provides short drilling. These traditional methods have nearly processing times, low thermal stresses to the run to their limits, where further steps of workpieces and good possibilities of automatic miniaturization are hardly practicable, or at least control. Due to the fact that there is no economical solutions are lacking. mechanical contact between the tool and the workpiece laser machining is free of tool wear In a number of publications it is reported that and breakage. especially in EDM the wear of the electrodes In contrast to some other machining during the process is the main factor limiting the techniques no additives, such as lubricants or achievable resolution 1)-4). The use of complex dielectric liquids (EDM), are required. electrode geometries in micro-EDM leads to Furthermore, the laser head had not to be quick changes of the former electrode shape close to the workpiece, so machining can be resulting in a loss of accuracy, in particular when performed in hardly accessible places. the electrode has sharp edges. Generally, in Finally, due to the high power intensities of micro-EDM another detrimental feature has to be the laser radiation, most materials can be considered: electrodes with complex geometries machined regardless of their mechanical are difficult to fabricate, increasing the costs of properties. -1- Although, a lot of studies have been 2. ULTRASHORT PULSE LASER performed and much experimental emphasis has been placed on maximizing the precision of the Since the early nineties the development of ablation processes, laser micromachining of ultrashort pulse laser systems has shown metals is still only in some cases a real enormous success driven by scientific goals, competitor for EDM or other techniques within such as the generation of high intensities and the industrial world. The main problems coming high time resolutions. The milestone of this along with conventional laser micro material development is the "chirped pulse amplification" processing are the presence of widely heat (CPA) technique, which allows to extract high affected zones and the undesired appearance of energy pulses out of solid-state gain mediaS}. molten material. The melting and re­ Nowadays, commercially available systems solidification of the material is an inhomogenous are able to produce pulses shorter than 50 fs with process, decreasing the precision and of course peak powers in the terawatt range. the reproducibility of the process. Due to the Although, there are many scientific ultrafast large heat affected zones the resolution for laser systems installed worldwide, there is still creating fine structures is limited. Another rio ultrafast laser integrated in any industrial drawback is the necessity of further finishing production line. procedures increasing the processing times and In general, industrial user's most important costs. considerations are the reliability, equipment For today's industrial purposes conventional costs and operating costs of any system. As long pulsed lasers, such as CO 2- and Nd:YAG-lasers, as there is no proof for the short term and long are employed, which typically work with pulse term stability of ultrafast laser systems, no duration in the range of some nanoseconds to system will be employed for industrial purposes. microseconds. To make ultrafast technology more attractive Developments over the past few years, to industrial companies the new laser sources however, have lead to a new generation of pulsed have to be insensitive to mechanical vibrations laser sources, generating extremely short pulses or to changes in temperature or humidity of the in the range of femtoseconds and picoseconds surrounding air. and high pulse peak powers (ultrashort pulse Within the past few years a large number of lasers or ultra/ast lasers). new and exciting "real world" applications for The main advantage of ultrashort pulse laser short pulse lasers emerged which have spurred ablation is the possibility of rapid evaporation of the development leading to more compact, more materials due to extremely short and intensive reliable and user friendly laser sources. Some laser pulses, allowing melting-free and burr-free laser manufactures already offer hermetic sealed­ processing. off systems with climate control. Newest By means of ultrashort laser pulses it is developments like diode-pumped all-solid-state possible to machine almost every solid material, systems significantly improve the reliability of independent of its hardness, ductility and thermal the laser sources reducing the operating costs properties, e. g. all types of metals and glasses as due to less frequent service. well as organic materials. A number of Although, there are still some improvements promising investigations have shown the high necessary, the highly sophisticated short pulse precision that can be achieved with this new laser technology has found its way to ruggedized technology. The results are comparable to those and reliable systems and is now ready to break ofEDM. into the harsh industrial environment. Therefore, the ultrashort pulse laser represents a unique tool, combining all 3. ABLATION WITH ULTRASHORT advantages of laser material processing and the PULSES possibility of creating high resolution structures in nearly all kinds of materials. The main characteristic of ultrashort pulse Because of the great importance of metals for ablation is the deposition of energy in a very a variety of industrial applications, this paper short time period leading to rapid plasma exclusively concentrates on the laser formation and evaporation of the irradiated micromachining of metals using ultrafast laser material. Furthermore, heat diffusion into the sources. surrounding material can be minimized and no -2- distortion and shielding of the laser beam by a generated plasma takes place. When a short (femtosecond) pulse hits the material surface the absorbed energy is first deposited into kinetic (thermal) energy of free electrons. The major part of the energy is then transferred to the lattice leading to the ablation of the material in a vapor and plasma phase. This energy transfer from the heated electrons to the lattice occurs on a picosecond time scale. Therefore, energy deposition and material ablation are temporally separated processes. During the ablation process thermal diffusion into the material is negligible, even in metals. Fig. 1 Nanosecond laser ablation, stainless steel When a long (nanosecond) laser pulse interacts with a metal target, the deposited energy first heats the surface to its melting point and then to the vaporization temperature. Because there is enough time for the energy to dissipate into the material, a huge bulk of liquefied material is created leading to an undesired growth of burr (Fig. I ). Another detrimental effect of "long" pulse ablation is the interaction of the laser pulse with the plasma, detaching from the target surface. This hinders the precise deposition of the laser radiation into the workpiece, due to the absorbtion in the plasma followed by the distortion of the beam spatial profile. 6 In several investigations )-'2) it has been demonstrated that only by using extremely short Fig.2 Femtosecond laser ablation,
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