HIGH REPETITION RATE ps- AND fs- LASERS FOR MICROMACHINING Paper #M1001

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1 HIGH REPETITION RATE ps- AND fs- LASERS FOR MICROMACHINING Paper #M11 Gediminas Raciukaitis 1, Mikhail Grishin 2, Romas Danielius 3,4, Jonas Pocius 4, Linas Giniunas 3,4 1 Institute of Physics, Savanoriu Ave. 231, LT-23, Vilnius, Lithuania 2 Ekspla Ltd, Savanoriu Ave. 231, LT-23, Vilnius, Lithuania 3 Light Conversion Ltd., Sauletekio Ave. 1, LT-1223 Vilnius, Lithuania 4 Laser Research Center, Vilnius University, Sauletekio Ave. 1, LT-1223 Vilnius, Lithuania Abstract Rising demand for efficient laser micro-processing has led to new developments in high repetition rate ultra-short pulse lasers. Laser companies in Lithuania cooperated with the Institute to Physics and Vilnius University to develop and demonstrate lasers offering 1 s khz, up to 1 Watts, and high beam quality, in addition to versatile control and rigid design. The outcome is the picosecond laser PL11 from Ekspla Ltd and the direct diode pumped femtosecond laser Pharos from Light Conversion Ltd, excellent tools for researchers in laser technology and variety of applications. Here we report results from machining of metals, glass and other materials, and discuss the cooperative development process. Introduction Laser micromachining as the material processing technology on a small and very small scale is finding numerous real-world applications. Together with the newest technologies, the diode-pumped solid-state (DPSS) lasers are moving from research laboratories to production lines. Future of the precise microfabrication with lasers is closely related to the use of pulses of laser radiation with durations shorter than the time of electronphonon relaxation. This time is material-dependant on the time scale of.1-2 ps [1-3]. Electron-phonon coupling is a driving force in redistribution of absorbed energy, and response of material might be affected by excitation. Therefore, for a given set of laser parameters, e.g. fluence and wavelength, the pulse duration has a significant effect on micromachining results [4-8]. Laser radiation with a pulse duration shorter than the characteristic time is desirable for processing with the minimal thermal load to material. On the other hand, precision of ultra-short laser fabrication is tightly related to the removal of material in small portions. Therefore, affordable machining efficiency could be maintained with the appropriate repetition rate of the lasers. Picosecond lasers with the repetition rate of hundreds of khz came up in the last few years but new laser sources with revised control parameters and output stability are still required. Lasers for industrial applications in micromachining should be robust and reliable. A proper mechanical design is necessary for good long-term stability of the system. Until recently Ti:sapphire lasers were the only sources of femtosecond pulses. However, these lasers typically run at a repetition rate in order of 1 khz and require sophisticated green lasers for their pumping. A number of new laser crystals have been investigated in recent years in search for those that would meet the requirements of a wide amplification bandwidth, good thermo-optical properties, and compatibility with direct diode pumping ideology. The ytterbium-doped potassium gadolinium tungstate crystal (Yb:KGW) possesses several distinctive features which make it an attractive laser medium for femtosecond applications [9,1]. An amplification bandwidth of Yb:KGW is sufficient for amplification of sub-15fs pulses. A highly intense absorption band at 98 nm allows relatively easy absorption saturation, which is important for both efficiency and net-gain bandwidth. An extremely small quantum defect combined with low thermo-optical coefficients reduces the thermal lensing to a manageable level, and high natural birefringence eliminates the thermally-induced birefringence. Lasers based on this material are able to generate ultra-short pulses with high average power at the high repetition rate. Here we present basic parameters of the new high repetition rate ultra-fast lasers, running at picosecond or femtosecond pulse durations. A variety of materials was processed by using their radiation, and the results are presented. Development of the lasers and their testing in micromachining were performed in close collaboration between laser companies (Ekspla and Light Conversion) and groups of

2 scientists at the Laser Research Center of Vilnius University and the Laboratory for Applied Research of the Institute of Physics. New laser sources for micromachining applications Picosecond laser PL11 PL11 a picosecond high power and high pulse energy laser from the very beginning was designed to be a versatile tool for variety of industrial material processing applications. The built-in oscillator and regenerative amplifier schema of the laser possess innovations in maintaining pump radiation and reliable, hands-free operation. PL11 is an OEM rugged, compact laser with the 1 W output power at 164 nm. It features high pulse energy (up to 2 µj), high beam quality (M² < 1.5) and a very high repetition rate (up to 1 khz) of typically 1 ps pulses. Optional harmonics are available at 532, 355 and 266 nm. PL11 consists of a diode-pumped passively mode-locked Nd:YVO 4 oscillator, and a diodepumped regenerative amplifier. High stability of the pumping and original synchronization design allows getting good pulse to pulse stability of regenerative amplifier output (typically about.7% rms). The optical part is placed in a robust, precisely machined monolithic aluminum alloy block, which could be used as a separate module for extremely compact OEM-solutions (Fig. 1). The system is sealed and its output parameters remain stable in a wide range of environmental conditions over a long period of time. PL11 has been designed as a low-maintenancecosts solution. All replacements of consumables can be performed at user facilities by trained technicians. The average output power and pulse energy of the laser versus pulse repetition rate at the pump power of 44 W are shown in Fig. 2. As it is typical for the vanadate-based lasers, the power stabilizes at the repetition rate above 2 khz. Output power, W Pulse repetition rate, khz 1 Fig. 2 Output power and pulse energy of the PL11 laser versus on pulse repetition rate. The laser shows a good beam quality at the output power of 1 W (Fig. 3) Pulse energy, µj Fig. 3 Beam profile of the PL11 laser at 1W average power. The main parameters of the PL11 laser (Ekspla Ltd.) are provided in Table.1. Fig. 1 Design of the PL11 laser. Designed for hands-free operation, the PL11 offers a maximum reliability due to optimized layout, PC-controlled operation, a built-in self-diagnostics and advanced status reporting. The superior beam quality allows easy focusing of the laser beam into the smallest spot size at various working distances and reaching laser fluences sufficient for processing virtually any material. Table 1 Specifications of the PL11 laser. Wavelength, nm 164 Average output power, W >1 Pulse energy, µj 2 5 khz Repetition rate, khz 5 1 Pulse duration, ps < 1 Polarization Vertical; 1:1 M 2 <1.5

3 Femtosecond PHAROS laser A growing demand on the ultrashort pulse lasers for micromachining application stimulates development of the lasers of new generation. PHAROS is a femtosecond laser (Fig. 4) with all essential components of the chirped-pulse-amplification (CPA) concept, with fundamental wavelength in the range of nm.. With the pulse duration approaching that of a regular Ti:Sapphire laser, direct diode pumping, robustness and compactness make PHAROS the system of choice for the most advanced research laboratories as well as for the real-world applications, particularly in micromachining. the system and time needed for switching between the two regimes. Two pump modules, each capable of >3W power, are used to pump the laser crystal of the regenerative amplifier. The regenerative amplifier is seeded and cavity dumped by means of a low-loss BBO Pockels cell at a repetition rate of up to 35 khz. The repetition rate is currently limited only by the Pockels cell and its driver (a 1MHz repetition rate amplifier is under development). The performance of the laser at different repetition rates can be optimized by adjusting two parameters: build-up time of the regenerative amplifier and the length of the compressor. For repetition rates above 5-6 khz virtually no optimization is needed Pharos output power, W Repetition rate, khz Pulse energy, µj Fig. 4 Femtosecond CPA laser system Pharos (external dimensions 95x5x25mm). The oscillator of PHAROS employs a Kerr lens based mode-locking and chirped mirrors for group velocity dispersion (GVD) compensation. Once started by cavity length variation, the mode-locking sustains throughout the day and is immune to external vibrations due to a compact and robust mechanical design. Typical average output power of the oscillator used to seed the regenerative amplifier is 6 mw with the pulse duration of ~1 fs. Its output power can be increased to the level of 1.5 W or the pulse duration can be reduced to < 5 fs. The stretcher and compressor are designed using high groove density transmission gratings, which are more environmentally stable, and ensure superior power handling capability, compressor throughput higher than 7% as well as compactness. The aberrations associated with high dispersion gratings are compensated by the use of custom aspheric optics. The ultra-compact design of the stretcher/compressor significantly contributes to high overall PHAROS operation stability. It is also important that the same set of gratings is used for femto- as well as picosecond operation, which reduces both the cost of Fig. 5 Output power (a) and pulse energy (b) of the Pharos laser. Dependence of the output power and pulse energy on the repetition rate is depicted in Fig. 5. An average power exceeding 6W after compression with excellent beam quality (Fig. 6) is easily achieved with the regenerative amplifier in the repetition rate range of 5-35 khz. Due to the low quantum defect and good thermo-optical properties of the Yb:KGW crystal the output power of the regenerative amplifier is limited only by the available pump power. The femtosecond regenerative amplifier with output power exceeding 1W after compression should become available in near future. 2 M<1.1 Fig. 6 Beam profile at the output of the laser (5W).

4 Typical output pulse duration of the PHAROS laser is about 27 fs (Fig. 7). At lower repetition rates (<5kHz) the pulse duration tends to increase due to spectrum narrowing. To sustain short pulses one needs to shape properly the seed spectrum, which can be done by simple optional shaper. Short pulse (SP) version of the Pharos provides 17fs pulse durations at the expense of some power loss and increased complexity. Intensity, a.u Delay, fs τ=274*1.41 fs Fig. 7 Autocorrelation curve of the Pharos output pulse. Although designed as a femtosecond laser, PHAROS can deliver a variety of pulse durations (Table 2). This feature may be especially useful at the research stage when optimal pulse parameters are to be determined for a particular process. Table 2 Oscillator Short pulse version Standard Detuned compressor Before compression Q-switch mode Pulse durations available from the PHAROS. down to 47 fs 15 fs 27 fs 27 fs-1 ps 2 ps 1 ns The base of PHAROS is a box-shaped structure divided into sealed sections for the seed oscillator, regenerative amplifier and stretcher/compressor. The top of the box provides enough space for installing two optical parametric amplifiers (OPAs), harmonic generators and the beam shaping optics. The repetition rate is defined either by the internal clock oscillator or by an external input. Microcontrollerbased units support programmable start of power supplies, closed loop power stabilization of both the oscillator and amplifier. The input of parameters is possible locally using keys and menus or by a computer via USB interface. An optional Pulse Picker allows switching on and off each laser pulse by an external device. The frequency converters that can be installed include highly efficient harmonic generators as well as broadly tunable OPAs of different types. Second to fourth harmonics can be produced in BBO with typical doubling efficiency exceeding 6%. Table 3 Specification of the Pharos laser Parameter Pharos Max. avearge power 7W Pulse width* <27 fs Repetition rate up to 35 khz Wavelength 13 nm Output stability <1% rms Beam quality TEM, M 2 <1.2 * at repetition rate 5-35 khz Applications in micromachining Femtosecond pulses The short output pulse duration (<27 fs), high average output power (>6 W), variable repetition rate (1-35 khz) and excellent beam quality (M 2 <1.1) makes the Pharos laser an ideal tool for micromachining applications. Fig. 8. represents the scanning electron microscopy image of the first attempt to cut a ~7 µm thick layer of copper deposited on a polymer. The cut was made using the PHAROS operating at the 3 W average output power and the 9 khz pulse repetition rate with cutting speed of 4 mm/sec. Fig. 8 SEM picture of a cut made using the Pharos laser. Upper layer is copper, bottom layer is polymer. The presumptive applications of the PHAROS laser are: o Micromachining: drilling, cutting, ablation; o Micro-and nano-structuring; o Writing Bragg gratings and waveguides; o Multi-photon polymerization; o Time-resolved spectroscopy.

5 Picosecond pulses The picosecond laser PL11 was examined in processing on a variety of materials used in semiconductor and electronics industry and biomedical applications. The flat-panel-display industry permanently presses on the development of new laser technologies. Machining of glass and thin films on it is a top one in the time of booming market for LCD, PDP and OLED displays. Cutting of glass The LCD filter glass was cut using picosecond laser PL11. Excellent quality of the cut was achieved when UV radiation was applied. Pictures of round and hexagonal items cut out from the glass are shown in Fig. 9 and Fig µm Fig. 11 Matrix of craters marked on glass. 266 nm, 3.6 µj; depth of the marks is 1 µm. Drilling of polymers Ultra-short pulses prevent heating and burning during fabrication with the laser in polymers. Drilling of holes was performed in the polymer film of 2 µm thickness. A matrix of holes with close separation between them was made with the PL11 laser with the 266 nm radiation (Fig. 12.). A hundred shots with pulse energy of 6 µj were required for complete drilling through the film. Use of nanosecond pulses or infrared radiation led to melting and burning of the material. Fig. 9 The round and hexagonal parts cut out from the LCD filter glass with thickness of.3 mm using the PL11 laser radiation at 266 nm. Fig. 12 Matrix of holes drilled in polymer film. Left: backside illuminated, right: sample illuminated from the top. Fig. 1 Smooth edge of the hexagonal part cut out from the LCD filter glass using the PL11 laser with fourth harmonics. Marking of the craters was performed on the glass substrate. Wavelength, pulse energy of laser radiation, focused spot size on a sample and a number of shots were changed during the experiments. Pictures of matrix of craters etched with the laser into the glass are shown in Fig. 11. Processing in PCB Printed circuit boards (PCB) are made of alternating layers of polymers and copper. Flexible polymer films and fibre-reinforced resins are usually applied as isolating and supporting material. Laser cutting and drilling of these electronic materials are emerging applications with further miniaturisation of electronics devices. Picosecond lasers were successfully examined in separation of the thin

6 flexible films and cutting and drilling of multilayered substrates. Cutting and drilling of metals Thin metal sheets are important materials for making masks, holders or fixtures, etc. Ultra-short lasers are tools of choice when cutting narrow slots or drilling microholes with perfect shape are required. We examined the PL11 laser in drilling holes with diameter from.5 mm to 2. mm in the sheet metal of stainless steel, tantalum and tungsten. IR laser radiation at 164 nm was used with the output power of 5 W. Table 4 Parameters used in laser cutting of the thin sheet metals. metal thickness, µm rep. rate, khz speed, mm/s # of scans Stainless steel Tantalum Tungsten Examples of holes are shown in Figs stainless steel, some recast and heating of material occurred because of low thermal conductivity of the material (Fig. 15). At cutting the tantalum, only a little recast was formed (Fig. 14). Fig. 15 Holes cut by laser in stainless steel (.5 and.4 mm diameters). Residual metal is left at entrance. In order to show advantages of ultra-short laser processing in metal, a comparative study of metal cutting with the PL11 laser and the nanosecond laser (9 ns) with nearly the same pulse energy and repetition rate was performed. For testing the 1x1 mm 2 square holes were cut in all tested materials with both lasers. The cut edges in the copper foil of the 34 µm thickness made with nanosecond (pulse duration 9 ns) and the PL11 lasers are shown in Fig.16. An extended heat affected zone with the ridge of melt was typical of processing with the nanosecond laser. After processing with the picosecond laser, the recast on edges was small and could be easily removed in the ultrasound bath. Fig. 13 Holes cut by laser in tungsten (left:.5,.4 and.3 mm diameters; right:.4 mm diameter). Fig. 14 Holes cut by laser in tantalum (.5,.4 and.3 mm diameters). The best quality of cut with PL11 was found in tungsten: clean cut, without recast (Fig. 13). In Fig. 16 The square holes cut in the copper foil of 34 µm thickness and cleaned in the ultrasonic bath. Left: cut with the PL11 laser (1 ps, 1 khz, 3.2 W, cutting speed 5 mm/s); right) cut with the NL64 laser (9 ns, 2 khz, 3.2 W, cutting speed.2 mm/s). The ablation rate was evaluated from experiments with nanosecond and picosecond lasers. The square holes with dimension of 1x1 mm 2 were ablated in

7 stainless steel with both lasers at various pulse energies. The depth of holes was measured using the optical microscope, and volume of the ablated material was estimated. The number of laser pulses used for the ablation of a hole was counted and the ablation rate (depth) in µm/pulse was calculated. The calculated ablation rate is shown in Fig.17. The ablation rate for picosecond pulses was about 5 times higher than that for the nanosecond laser at the same laser fluence ns 1 ps Fig.18 shows the grooves etched with the nanosecond laser (9 ns) and with the PL11 picosecond laser in aluminum. The grooves were made by multi-pass scanning with a laser beam. Processing with the picosecond laser provided sharp edges with a little debris around, though the overall processing speed was 2 times lower. Invar is widely used in production of masks for LCD filters. Equidistance slots are required for formation of pixels arrays in displays. We used various harmonics of the PL11 laser radiation for cutting the slots in the 34µm thick Invar sheet (Fig. 19). Ablation depth, nm/pulse Laser fluence, J/cm 2 Fig. 17 Ablation rate vs laser density for picosecond and nanosecond lasers. There could be a few reasons for the result. One is the lower ablation rate for shorter pulses. The metal is evaporated at lower energy densities, and part of material removed from a hole in a form of vapor is significantly larger using the picosecond laser radiation. The second reason might be that at laser ablation, plasma is formed from ionized air and metal vapor. The time required for the plasma formation is in a time scale of a few tens picoseconds. Plasma can effectively absorb the light, preventing the laser beam to reach the metal surface. Fig. 19 Slots cut in the Invar foil with 164 nm radiation. Their width is of 45 µm and the period is of 25 µm. Left: top illumination; (b) backside illumination. Laser power 2.3 W, repetition rate 5 khz, scan speed 5 mm/s. The period that can be achieved is limited by thermal load to the workpiece. Narrow separation lines can simply melt due to a decrease in head conduction of the line. Short-wavelength radiation introduced less heat into the Invar foil during the processing and 45µm slots with separation of 45 µm were cut by using 266nm radiation (Fig. 2), while for 164 nm radiation the minimal separation was 12 µm. Fig. 18 Top of the groove ablated in aluminum with 164 nm radiation. Left: with the nanosecond laser, 5W, 4 khz, 9 ns, depth 12 µm; right: with the PL11 laser, 2.4 W, 1 khz, 1 ps, depth 115 µm. Fig. 2 Slots cut in the Invar mask for OLED & LCD with the PL11 laser: 266 nm, 1 khz,.35 W; thickness 34 µm; cutting speed 5 mm/s.

8 Patterning of ITO The indium-tin oxide (ITO) film with the thickness of 15 nm on a glass substrate was patterned with the picosecond laser radiation. Low absorption of the material in the infrared and visible spectrum impeded clean removal of the material. On the other hand, the film was completely ablated with a low energy density using the 266 nm radiation, which is well absorbed by the oxide film. The high laser fluences initiated glass ablation. The working window was found to be the most appropriate for laser processing at this wavelength. The ultra-fast lasers with the high repetition rate were examined in processing a variety of engineering material. A better machining quality and in some cases higher efficiency can be achieved in comparison with the conventional Q-switched nanosecond lasers. Unique processing abilities were revealed in the modern multilayered materials. Acknowledgment The work was partially supported by the Lithuanian State Science and Studies Foundation under grants No B5/23 Diogenas and No B21/25 Matilda. This document has been produced with the financial assistance of the European Union for implementation of the project No.S-BPD4-ERPF /4. References Fig. 21 AFM profile of the trench ablated in the 15 nmthick ITO film on a glass substrate with the PL11 laser at 266 nm. The processing speed of.5 m/s was reached with output power of 3 mw. The obtained minimal width of grooves at the top of the ITO film was 7 µm. The AFM profile of the trench is shown in Fig.23. The trench had a shape of a regular trapezoid with smooth both walls and the bottom. The width at the bottom was only 2.87 µm. The dimensions are reasonable for a high-density packing of the connector lines in OLED or RFID. The height of spikes formed from the melted material nearby the trench did not exceed 2 nm. Conclusions New ultra-fast directly-diode-pumped laser sources were developed for industrial applications. We hope, that high repetition rates (up to 35 khz), ultra-short pulse durations (27 fs or 1 ps) and high average powers (up to 1 W) together with the robust mechanical design and reliable operation ensure faster adaptation of laser micromachining technologies from laboratories to a factory place. [1] Wellershoff S.S., Hohlfeld J., Gudde J., Matthias E. (1999) The role of electron-phonon coupling in femtosecond laser damage of metals, Appl. Phys. A 69, S99-S17. [2] Groeneveld R.H.M., Sprik R., Lagendijk A. (1995) Femtosecond spectroscopy of electronelectron and electron-phonon energy relaxation in Ag an Au, Phys. Rev. B.,51, [3] Mannion P.T., Favre S., Ivanov D.S., Connor G.M., Glynn T.J. (25) Experimental investigation of micromachining on metals with pulse durations in the range of the electronphonon relaxation time (pico- to subpicosecond), in Proc. of 3-rd Int. WLT- Conference on Lasers in Manufacturing Munich, June 25. [4] Chichkov B.N., Momma C., Nolte S. (1996) Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A, 63, [5] Nolte S., Momma C., Jacobs H., Tünnermann A., Chichkov B.N., Wellegehausen B., Welling H. (1997) Ablation of metals by ultrashort laser pulses, J. Opt. Soc. Am. B 14, [6] Breitling D., Ruf A., Dausinger F. (24) Fundamental aspects in machining of metals with short and ultrashort laser pulses, in Proc. Photon Processing in Microelectronics and Photonics III (Laser Applications in Microelectronic and Optoelectronic Manufacturing IX, San José, CA). Proc. SPIE 5339, [7] Salle B., Gobert O., Meynadier P., Pedrix M., Petite G., Semerok A. (1999) Femtosecond and picosecond laser microablation: ablation

9 efficiency and laser microplasma expansion, Appl. Phys. A 69, S381. [8] Drogoff L., Vidal F., Laville S., Chaker M., Johnston T., Barthelemy O., Margot J., Sabasi M. (25) Laser ablated volume and depth as a function of pulse duration in aluminium targets, Appl. Optics, 44, 278. [9] Liu H., Nees J., Mourou G. (22) Directly diode-pumped Yb:KY(WO 4 ) 2 regenerative amplifiers, Opt.Lett. 27, [1] Nickel D., Stolzenburg Ch., Giesen A., Butze F. (24) Ultrafast thin-disk Yb:KY(WO 4 ) 2 regenerative amplifier with a 2kHz repetition rate, Opt.Lett. 29, [11] Kuleshov N.V., Lagatsky A.A., Podlipensky A.V., Mikhailov V.P., Huber G. (1997) Pulsed laser operation of Yb-doped KY(WO 4 ) 2 and KGd(WO 4 ) 2, Opt. Lett., 22, [12] Hellstrom J.E., Bjurshagen S., Pasiskevicius V., Liu J., Petrov V., Griebner U. (26) Efficient Yb:KGW lasers end-pumped by high-power diode bars, Appl.Phys. B 83, leading manufacturer of femtosecond optical parametric amplifiers. Currently he holds position of R&D director at Light Conversion and is a chief researcher at Laser Research Centre of Vilnius University. Jonas Pocius graduated from Vilnius University, the Department of Physics in 1998 and received Master's degree in Laser Physics and Optical Technologies from Vilnius University in 24. Since 22 he has been involved in the development of directly diode pumped femtosecond lasers at Light Conversion Ltd., he has been a junior research assistant at Laser Research Center of Vilnius University since 23, and he has been Ph.D. student at the Department of Physics, Vilnius University since 24. Linas Giniūnas graduated from Moscow Engineering Physics Institute in 1987 and received Ph.D. degree in laser scanning microscopy from Vilnius University in He was a project manager at Light Conversion of few projects on industrial applications of low-coherence interferometry from 1995 till 2. He has been involved in development of directly diode pumped femtosecond lasers and micro-optics for laser diode bar beam shaping since 2. From 1999 he has been a senior researcher at Laser Research Center of Vilnius University. Currently he is also a manager of a group of femtosecond lasers at Light Conversion. Meet the Author(s) Gediminas Račiukaitis graduated from Vilnius University, the Department of Physics in 1978 and received Ph.D. degree in non-linear spectroscopy of semiconductors from Vilnius University in From 1995 he was a program manager at Ekspla Ltd. and worked on laser technology and its applications in industry. He is a member of LIA since 23. He is head of Laboratory for Applied Research, Institute of Physics, Vilnius since 24 and a consultant on laser technology with Ekspla Ltd. Mikhail Grishin graduated from Moscow Engineering Physical Institute, Solid -State Physics and Quantum Electronics Department in He has been a R&D program manager of Ekspla Ltd since 2 and is dealing with solid-state diode pumped picosecond lasers since 23. Romas Danielius graduated from Vilnius University in He completed his Ph.D. work on timeresolved spectroscopy of photosynthesis in1981. Besides that, his research interests included ultrashort pulse lasers and nonlinear optics. In 1994 he cofounded Light Conversion Ltd., which became the