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, stainless steel pulses melting- and burr-free structures can be created as shown in Fig.2. The maximum output energy of the pulses The following experimental results will amounts to about 2 mJ at a repetition of 1 kHz. demonstrate the efficiency and the high The system is operating at a wavelength of 780 precision, achievable with the ultrashort pulse nm. laser. Especially when applying short laser pulses to ablate metals, which typically have 4. EXPERIMENTAL RESULTS comparatively low melting temperatures and high heat conductivities, the absence of thermal A number of extensive investigations aimed diffusion results in very precise structures with on the evaluation of optimum laser and practically no heat affected zones, as mentioned processing parameters and its influence on the before. precision have been performed. The laser source used for these investigations One of the main interesting questions is a commercial Ti:sapphire laser and amplifier, scientific investigators and industrial users dealt based on the "chirped-pulse amplification"(CPA) with in the past is: what optimum (minimum) technique with a continuously adjustable pulse pulse width is required to take the advantages of width between 120 fs and 30 ps. short pulse ablation?

-3- 150 fs 1 ps 10 ps

Fig.3 Influence of the pulse duration

In fact, also material removal with ultrashort compared to nanosecond ablation. In our pulses is highly material specific due to experiments it has been demonstrated that for different optical and thermal properties of each stainless steel the critical pulse duration at material. Drilling tests in stainless steel have which the efficiency of the ablation process shown that only by applying laser pulses in the drops significantly is again roughly around 1 ps sub-picosecond time scale sharp and melting­ (Fig.4). The application of laser pulses in the free edges are possible. By using pulses with a sub-picosecond time scale, therefore, does not duration of some picoseconds the proce'ss is only increase the precision, but also reduces the accompanied by the formation of burr. processing time. Drilling tests through 200 Ilm Fig.3 shows the comparison of three holes thick stainless steel plates have demonstrated drilled in stainless steel (1.4301). Even when the rapid rise of the processing time when operating with relatively low pulse energy and increasing the pulse duration. Comparing the low density of the radiation burr is created on processes using a few hundred femtoseconds top of the target as long as the pulse width is and 30 ps pulses have shown an increase of the around 1 ps or longer. processing time of around 85%. Therefore, femtosecond laser machining is Further benefit of using ultrashort pulses is characterized by both, high efficiency and high the high efficiency of the ablation process precision.

90 -+- 160 fs IJm ...c 70 +-' Cl. Q) 60 t-----t-----+--.",.:----4~.",c;.-+__--_+____t ...... 1. 2 ps 0 50 c 4- 4.3 ps 0 40 +-' CO t----.....---+-~,.-;..--+-----+_= __~~ ..... -.- 9.6 ps ..0 30 « 20 I----~...,_:;;.-~ __=--+-----+--~I___t ___ 22.8 ps 10 0 0 1000 1500 2000 2500 3000 3500 Number of Pulses

FigA Efficiency of short pulse laser ablation

-4- a) Copper b) Titanium

c) Stainless steel d) Shape memory alloy

Fig.5 Microstructures in metals

Fig.5 shows four examples of structures case a mllllmum mechanical and thermal created by ultrashort laser pulses in different damage and the lack of burr are very important. types of metals. Fig.5a shows the LZH-Iogo The creation of small holes in the sub­ created in copper. This material features very IOOllm range is known to be quite difficult for high heat conductivity and comparatively low all conventional micromachining techniques. melting temperature and is therefore very The ultrashort pulse technology, however, difficult to machine with conventional laser offers the possibility of very high resolution sources. By using the imaging technique, where ablation due to very small heat affected zones. the laser beam illuminates a mask which is Potential applications for this new drilling tool imaged and demagnified onto the surface of the will be the machining of components for the workpiece, nearly any shape of structure can be automotive industry (e.g. injection nozzles) or generated without the necessity to move the some hydraulic or pneumatic components target with respect to the laser beam. (Fig.5c).

Fig.5b shows an example of a 3-dimensional Another interesting material process where application. This structured titanium tube is a the advantages of ultrashort laser pulses is prototype of a medical implant (stent) used in obvious, is the machining of shape memory cardiac surgical procedures. Especially in this alloys (NiTi, Fig.5d). This type of alloy is

-5- known to be very sensitive to thermal stress, 2) Uno Y., Okada A., Nakanishi H., Guo C., which can lead to changes of the alloys Okamoto Y., Takagi T.: EDM properties and therefore to the loss of their Characteristics of CVD-Carbone Elektrode, specific characteristics, such as superelasticity IJEM, No. 3, January 1989, pp19-24 and memory effects. 3) Zuyuan Yu, Takahisa Masuzawa, Masatoshi The examples discussed above have to be Fujino: 3D Micro-EDM with Simple Shape regarded as only a small selection of the great Electrode, Part 2, IJFM Paper, No. 3, variety of possible machining applications. January 1998, pp71-78 However, these examples obviously show the 4) Masanori Kunieda, Masakzu Kijohara: potential of this novel machining method. Simulation of Die-Sinking E~M by Discharge Location Searching Algorithm, 5. CONCLUSIONS IJFM Paper, No. 3, January 1998, pp79-85 5) D. Strickland, G. Mourou, Opt. Commun. In this paper it has been demonstrated that 56,219 (1985) ultrashort pulse laser machining is a promising 6) S. Nolte, C. Momma, H. Jacobs, A. technology, which will certainly become a Tunnermann, B.N. Chichkov, B. serious competitor to nowadays conventional Wellegehausen, H. Welling: Ablation of micromachining techniques. Compared to metals by ultrashort laser pulses, J. Opt. conventional laser-based systems the Soc. Am. B 14,2716 (1997) application of short laser pulses offers the 7) C. Momma, S. Nolte, B. N. Chichkov, F. v. possibility of creating very precise and me~ting­ Alvensleben, A. Tunnermann: Precise laser free structures in metals, with precision ablation with ultrashort pulses, Applied comparable to those achievable with EDM. Surface Science 109/110(1997) 15-19 In addition, it has been demonstrated that 8) C. Momma, B. N. Chichkov, S. Nolte, F. v. ablation with laser pulses in the sub-picosecond Alvensleben, A. Tunnerm ann , H. Welling time scale leads to very efficient processes, B. Wellegehausen: Short-pulse laser which significantly decreases the operating ablation of solid targets, Optics times. Communications 129 (1996) 134-142 The ultrashort pulse laser represents a 9) B.N. Chichkov, C. Momma, S. Nolte, F. unique tool for micro material processing, von Alvensleben, A. Tilnnermann: where all advantages of laser material Femtosecond, picosecond and nanosecond processing, in general, come across with the laser ablation of solids, Appl. Phys. A 63, possibility to create high resolution structures in 109 (1996) nearly all kinds of materials. 10) C. Momma, S. Nolte, B. N. Chichkov, F. v. Because of new improvements of ultrafast Alvensleben, A. Tunnermann: Precise laser sources, which have lead to ruggedized Micromachining with Femtosecond Laser and reliable systems, this novel technology is Pulses, Laser und Optoelektronik, 29 (3)/ ready to get involved in the industrial 1997 environment. 11) C. Momma, S. Nolte, G. Kamlage, F. von Alvensleben, A. Tunnermann: Beam ACKNOWLEDGEMENTS Delivery of Femtosecond Laser Radiation by Diffractive Optical Elements, Appl. This work was financed by the German Phys. A (in press) Ministry of Science, Education, Research and 12) S. Nolte, C. Momma, G. Kamlage, A. Technology (BMBF, 13N7053). Ostendorf, C. Fallnich, F. v. Alvensleben, H. Welling: Polarization Effects in REFERENCES Ultra short Pulse Laser Drilling, (to be published) 1) Zuyuan Yu, Takahisa Masuzawa, Masatoshi Fujino: 3D Micro-EDM with Simple Shape Electrode, Part 1, IJFM Paper, No. 3, January 1998, pp7-12

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