SHORT-PULSE LASERS WITH THE HIGH POWER AND REPETITION RATE FOR MICROFABRICATION

Paper M1104

Mikhail Grishin1,2, Saulius Jacinavicius1, Giedrius Andriukaitis1, Marijus Brikas2, Gediminas Raciukaitis2

1Ekspla Ltd, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania, fax. +370-5-2641809

2Laboratory for Applied Research, Institute of Physics, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania

Abstract

Employment of short-pulse lasers in microfabrication opens up a lot of opportunities in tailoring and shaping variety of materials. The main drawback in a wide use of the laser technology on micro- and nanoscale today is the low fabrication speed. An increase in the pulse repetition rate is a way to reach the desired efficiency. New high-power and high repetition-rate lasers based on the diode-pumped Nd:YVO4 crystals with the pulse duration in picosecond and nanosecond time range were developed and applied in processing of different engineering materials. Pulsing up to 100 kHz allowed flexible machining of bulky and thin-film materials. Results of microfabrication and their relation with laser parameters are presented.

Introduction

Laser micromachining is the material processing technology on small and large areas with the precision of micrometer and sub-micrometer dimensions. It offers versatile methods in material processing: cutting, drilling, ablation, internal modification, etc. Flexible and environment-friendly fabrication of variety engineering materials can be realized according to requirements of modern micro- and nanotechnologies. The technology is finding applications in drilling precise holes for fuel injection [1] or thin film patterning for solar cells [2]. Further realization of the opportunities in the real-world applications is tightly related with the progress in development of the diode-pumped solid-state (DPSS) lasers with short and ultra-short pulses.

Ultra-short pulse lasers are welcome in material processing because of “cold” ablation with the minimal lateral thermal damage of a specimen. Light is absorbed by electrons and increases rapidly their energy which may cause nonlinear effects or direct emission of electrons. The material itself (lattice) is

heated with delay of the electron-phonon relaxation time. This time is material-dependent on the time scale of 0.1-20 ps [3, 4]. Electron-phonon coupling is a driving force in redistribution of absorbed energy, and response of the 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 [5-8]. Laser radiation with the pulse duration shorter than the characteristic time is desirable for processing with the minimal thermal load to the material.

On the other hand, precision of ultra-short laser fabrication is tightly related to the removal of material in small portions. Use of pulses with the duration of a few nanoseconds is well balanced in some cases between the high removal rate with long pulses and high quality of ultra-short-pulse fabrication. Affordable machining efficiency could be maintained with the appropriate repetition rate and the mean power of lasers should overcome the minimal value of a few watts. The solution was in development of high repetition rate lasers that generated short pulses.

The high repetition-rate, high output-power lasers with the pulse duration of a few nanoseconds and picoseconds have been designed in Ekspla Ltd. 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.

We present here technical specifications of the new high repetition rate short-pulse lasers, running at picosecond or nanosecond pulse durations. A variety of materials was processed by using their radiation, and the results are presented.

Laser sources for microfabrication

Picosecond laser PL10100

PL10100 – 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. PL10100 is compact laser with the 10 W output power at 1064 nm. It features high pulse energy (up to 200 µJ), high beam quality (M² < 1.5) and a high repetition rate (up to 100 kHz) of typically less than 10 ps pulses. Standard versions of the laser include output at wavelength of 532 nm or 355 nm with an intelligent pulsing control.

PL10100 consists of a diode-pumped passively mode-locked Nd:YVO4 oscillator, and a diode-pumped regenerative amplifier. The master oscillator and regenerative amplifier schema of the laser possess innovations in maintaining pump radiation and reliable, hands-free operation. High stability of the pumping and original synchronization design allows getting good pulse to pulse stability of regenerative amplifier output (typically about 0.7% rms). The optical part of the PL10100 laser is placed in a robust, precisely machined monolithic aluminum alloy block. The system is sealed and its output parameters remain stable in a wide range of environmental conditions over a long period of time.

Fig. 1 Optical head of the PL10100 laser.

Tthe PL10100 offers a maximum reliability due to optimized layout, PC-controlled operation, a built-in self-diagnostics and advanced status reporting. The high 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.

The average output power and pulse energy of the laser versus pulse repetition rate are shown in Fig. 2. As it is typical for the vanadate-based lasers, the power stabilizes at the repetition rate above 20 kHz.

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Fig. 2 Output power and pulse energy of the PL10100 laser versus on pulse repetition rate.

The laser shows a good beam quality at the output power of 10 W (Fig. 3).

Fig. 3 Beam profile of the PL10100 laser at 10W average

power.

The main parameters of the PL10100 laser are shown in Table 1.

Table 1. Parameters of the PL10100 laser.

Wavelength, nm 1064 Average output power, W >10 Pulse energy, µJ 200 µJ @ 50 kHz Repetition rate, kHz 50–100 Pulse duration, ps < 10 Polarization Vertical; 100:1 M2 <1.5

“Green“ nanosecond lasers of JAZZ series

A growing demand on the short pulse lasers for micromachining application stimulates development of the lasers of new generation. The JAZZ series Q-switched laser is solution where high peak power is required for material processing (Fig. 4). The innovative electro-optical Q-switch is a key technology

for generation of the record short pulses between all high power nanosecond lasers.

Fig. 4 Optical heads of the JAZZ lasers.

Possessing a specialized design, the diode pumped solid state lasers offer high output power of second harmonic (532 nm) radiation together with the high repetition rate. Typical dependence of the output power on pulse repetition rate for JAZZ 55 and JAZZ 20 lasers is shown in Fig. 5.

Fig. 5 Average power of Jazz series lasers as a function of

pulse repetition rate.

The highest mean power is at 20 kHz as it is typical of frequency converted Nd:YVO4 lasers. Short pulse duration remains over the whole range of laser operation up to 100 kHz (Fig. 6). Rugged body made of machined aluminum as well as sealed cavity ensure stable and reliable operation under diverse conditions of laboratory and factory working place. The Jazz series lasers are equipped with the intelligent control system. Output power as well as other parameters of the laser are monitored continuously in order to ensure long-term repeatability of performance and easy

adaptation of the laser into high throughput material processing systems.

Fig. 6 Pulse duration of Jazz series lasers as a function of

pulse repetition rate.

High beam quality together with short pulse duration and high output power make the JAZZ series laser aso high brilliance source (tool) for processing most of engineering materials used in semiconductor and electronics industry.

Table 2. Specifications of the JAZZ series lasers radiating at the 532 nm wavelength.

Model JAZZ 55 JAZZ 20 Wavelength 532 nm Output power, W 5.5 2 Pulse to pulse energy stability 2.5 % (rms) at 20 kHzPower stability ± 2 % over 8 hours Pulse duration, ns 6-25 Repetition rate, kHz up to 100 Beam diameter, mm ~0.8 Beam profile TEM00 M2 <1.5 Beam divergence, mrad < 1.3 Beam elipticity >0.85 at 20 kHz Polarization Horizontal >200:1 Timing jitter <0.5 ns at 1-50 kHz

New nanosecond laser Baltic100

Demand for frequency-converted lasers in electronics industry, especially in production of flat panel displays, stimulated development of a high pulse energy “green” laser. Parameters of the laser are shown in Table 3.

Table 3. Parameters of the Baltic100 laser.

Wavelength 532 nm Output power, W @ 10 kHz >10 Pulse energy, mJ @ 10 kHz >1 Repetition rate, kHz 5-50 Pulse duration, ns <10 Pulse energy stability @ 532 nm <3 % (rms) Power stability ± 2 % over 8 hoursM2 <1.5 The pulse energy in the mJ-range together with the excellent beam quality meet requirements in the peak power for the high-speed solar cell scribing, the intra-volume glass marking, diamond cutting, etc.

Applications in micromachining

The picosecond laser PL10100 and nanosecond lasers of JAZZ and Baltic series were examined in processing on a variety of materials. The flat-panel-display industry permanently stimulates 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 photovoltaic and flat panel displays. Metals and silicon remain important engineering materials in micro-technology. Typical results of machining the materials with the high repetition rate picosecond and nanosecond lasers are presented below.

Cutting of glass

Full body cutting of glass requires considerable fluence or high absorption. The LCD filter glass was cut using the picosecond laser PL10100. Excellent quality of the cut was achieved when UV radiation at 355 nm was applied with fluence of 10 J/cm2. A picture of the segment cut out of 1.1 mm thick LCD glass is shown in Fig. 7. Ashkenasi et al. [9] used nanosecond lasers emitting at 532 nm for glass cutting. Laser fluence of 120 J/cm2 was required to cut through 0.3 mm thick borofloat glass. Typical chipping depth by cutting with nanosecond lasers exceeded 50-100 µm. No chipping was observed after cutting the glass with the UV picosecond laser. Roughness of the laser cut surface was less than 10 µm. Lasers generating nanosecond pulses at 355 nm with the duration of around 10 ns were also applied to cut the LCD glass. A picture of a hole is shown in Fig. 8. Gentle removal of material facilitated the high quality of drilling without chipping.

Fig. 7 LCD glass of 1.1 mm thick, cut with PL10100 laser,

generating at 355 nm

Fig. 8 Top (left) an bottom (right) view of a hole cut by nanosecond laser generating at 355 nm.

The UV laser radiation with the nanosecond pulse duration was found to be effective in machining of glass. 3D structures were made in bulky glass for medical applications (Fig.9).

Fig. 9 Machining of 3D structures in glass.

Marking of transparent materials

A new wave in industrial marking of transparent materials for identification in production line is a use of intra-volume marking. Alignment marks and identification marks including the 2D datamatrix bar code can be made inside the bulk without any contamination or damage of surfaces. Identification mark made inside the Eagle2000 glass wafer is shown in Fig. 10. As small as10 µm point defects were created in the glass by local breakdown. Contrast of the marks was controlled by a number of scans or layers. The technology using nanosecond JAZZ lasers was approved for the glass thickness of 0.7-1.1 mm,

while the picosecond laser was able to mark the wafer with the thickness of 0.4 mm.

Fig. 10 Marking of glass wafer.

The high speed marking was achieved using a galvoscanner. A logo consisting of more than 2000 points was marked inside glass within 0.3 s at the 22 kHz repetition rate using the JAZZ 55 laser (Fig. 11).

Fig. 11 Intra-volume glass marking with the JAZZ 55 laser and galvoscanner.

Wafers of sapphire are often used as substrates in production of GaN electronic devices. The nanosecond laser at 355 nm was applied to mark a 2D datamatrix code on the surface. The dimension of the bar code was only 2.2 mm by 1.1 mm with the cell dimension less than 0.2 mm.

Fig. 12 2D datamatrix bar code marked on sapphire using

the 355 nm laser radiation. Thin film patterning on glass and polymers

The flat-panel-display and photovoltaic industry permanently presses on the development of new laser

technologies. Patterning of thin films on various substrates is a top one in the time of booming market for LCD, PDP and OLED displays and large investments into photovoltaics. The picosecond laser PL10100 was examined in processing on a variety thin films and multi-layers on rigid and flexible substrates. New “green” Q-switched lasers are under test in the application laboratory.

SiO2 removal from Si wafer

Local removal of silicon oxide from the wafer surface is required in production of solar cells of crystalline silicon. The laser power of 100 mW was high enough to make openings in the silicon oxide layer with PL10100 laser at 355 nm, but the process window was very narrow. A high processing speed can be easy achieved using full laser power.

Patterning of ITO on glass

The indium-tin oxide (ITO) film with the thickness of 150 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 the low energy density using the UV radiation, which is well absorbed by the oxide film.

Fig. 13 Contact structure in ITO on glass, formed by laser

ablation. The processing speed of 0.5 m/s was reached with the output power of 300 mW. The obtained minimal width of grooves at the top of the ITO film was 7 µm. The trench had a regular shape with smooth both walls and the bottom. The width at the bottom was only 2.87 µm [10]. The dimensions are reasonable for a high-density packing of the connector lines in OLED or RFID (Fig. 13). High quality processing of ITO without ridges is only possible using the laser with both a short wavelength and a short pulse duration.

Formation of series interconnections in the p-Si thin film solar cells

The thin film solar cells include series interconnections every 1-2 cm to reduce ohmic losses. Laser scribing is one of the processes to pattern the films. Three processes are typically required for back contact on the substrate (P1), semiconducting absorber layer (P2) and front contact on the top (P3). Selective removal of the layers is sensitive to laser parameters, especially when the contact material is transparent conducting oxide like ITO or ZnO.

The PL10100 laser was tested in patterning of the p-Si thin film solar cell deposited on the glass substrate with top and back contacts made of ZnO. The backside illumination through the glass substrate was used, and the fundamental radiation at 1064 nm was applied in the process P1. Fig. 14 shows a test pattern made in the ZnO back-contact, and a close view of the trench is shown in Fig. 15.

Fig. 14 Test-pattern in ZnO film on glass produced with the

picosecond laser working at 1064 nm.

Fig. 15 Scribe of the process P1 in ZnO layer made with the

PL10100 laser (λ=1064 nm). Process speed 500 mm/s.

The process window was found to be very narrow and special attempts were required to avoid thermally induced break of the glass.

Laser radiation with a wavelength of 532 nm was used in processes P2 and P3. Selective removal of single layers of p-Si and the ZnO top contact were achieved at a high speed (900 mm/s). A proper laser power facilitated clean removal of the films without chipping and crack formation. SEM pictures of the trenches are shown in Fig. 16 and Fig. 17.

Fig. 16 Scribe of the process P2 in the p-Si layer made with

the PL10100 laser (λ=532 nm). Process speed 900 mm/s.

Fig. 17 Scribe of the process P3 in ZnO layer made with the

PL10100 laser (λ=532 nm). Process speed 900 mm/s.

The edge isolation is a final process in the thin film solar cell production where laser radiation might be applied. Quality requirements for the edge isolation process are less important than the process efficiency. Therefore, picosecond as well as nanosecond lasers are in line to produce complete ablation of the films from the substrate.

Patterning of ITO and ZnO on polymer substrates

Polymer substrates replace glass where flexibility of the device is very important. OLED and solar cells are examples. Use of lasers to pattern the TCO films on polymer substrates is a more complicated task. The polymer is thermo sensitive and both the film and substrate are absorbing in the whole spectrum.

The green radiation of 532 nm was able to remove all the films, but the polyimide substrate was damaged in case of the ZnO film (Fig. 18). Damage of the polyimide substrate appeared at a twice lower laser power in comparison to the PET substrate using the laser radiation with the 532 nm wavelength.

Fig. 18 Pictures of the trenches etched with the laser in

ZnO/PI (top) and ITO/PI (bottom). Laser: PL10100; λ=532 nm, f=100 kHz; Translation

speed v=100 mm/s. The conducting films of ITO and ZnO were easily ablated from the substrates with the UV radiation, and the laser power was about 10 times lower than that for infrared radiation. Both active films of ITO and ZnO have high absorption for the UV radiation of 266 nm. The best results of the ITO/PET patterning with 266 nm radiation without the substrate damage were achieved when the translation speed of 100 mm/s and the average laser power of P=33 mW were applied [11]. The trench had a flat bottom of the clean polyimide surface.

Laser pattering processes in CIGS solar cells

When the CuInxGa(1-x)Se2 (CIGS) solar cells are formed on polymer substrates, they are prepared to be illuminated from the top of the CIGS structure. The backcontact is made of molybdenum, directly

deposited on the substrate. Structuring of the backcontact is the only laser process established in the production of CIGS solar cells on the glass substrate nowadays [12]. The main limiting factor for laser processing of the multilayer CuInSe2 structures is deposition of molybdenum on walls of channels scribed in the films, and the phase transition of semi-conducting CuInSe2 to metallic state close to the ablation area due to the thermal effect [13]. Both effects shunt the photo-electric device and reduce its conversion efficiency.

The picosecond laser PL10100 working at 355 nm was used to estimate the proper regimes for P1-P3 processes in CIGS on polyimide: • to evaporate the upper electro-conducting layer of

ITO; • to remove the semiconducting film and expose the

molybdenum backcontact; • to make an isolation trench by laser ablation of all

films down to the polyimide substrate; • to cut a complete structure of the CIGS solar cell

together with the substrate.

The picosecond pulse duration was short enough to prevent extensive formation of melt and no mixing of Mo with CIGS was found with the X-ray energy dispersion spectrometer (EDS).

Cutting and drilling of metals

Cutting stents of Nitinol with the 1064 nm radiation

The memory shape alloy Nitinol consisting of Ni and Ti is a perfect material for medical implants such as stents. Its mechanical properties depend on thermal treatments. That is why picoseconds lasers were applied to cut stents in order to reduce the thermal load to the material.

Fig. 19 Stents cut from the Nitinol. PL10100 laser, 4 W, 100

kHz. Courtesy of Cortronik GmbH & Co. KG.

Deep drilling in steel with the 532 nm radiation

Production holes of well defined shape for fuel injection in economically acceptable time remains a topic for both laser developers and applications engineers.

Holes of the 80 μm diameter were drilled in the SCM420 steel with the thickness of 1.2 mm using “green” radiation of the PL10100 laser. Rotation of the workpiece was used to make the holes circular (Fig. 20).

Fig. 20 Entry and exit sides of the hole drilled helically in

steel with the picosecond laser working at 532 nm. Entry diameter is 80 µm.

The holes were drilled in the 1.2 mm thickness steel with the picosecond laser PL10100 in the time range of 60-180 seconds (Fig. 21). Fast piercing through the workpiece appeared in a few tens of seconds, but finishing of the shape lasted as long as 150 s. The holes fabricated with second harmonic had smooth walls, without heat affected zones or melt ridges. The outlet/inlet ratio did not exceed 0.7 as simple rotation of the workpiece was applied.

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Fig. 21 Outlet/inlet ratio versus drilling time for helical

drilling. PL10100 laser, 50 kHz, 532 nm. 1) 2 W, 200 µm focus offset; 2) 2 W, 0 µm focus offset; 3) 3.32 W, 200 µm focus offset; 4) 3.32 W, 400 µm

focus offset.

Cutting of metals with the 355 nm radiation

Fine cutting of microcomponents from metals was performed with the UV picosecond laser. Two examples are shown in Fig.22. They were produced from the X2 NiCoMo 18-9-5 steel with tolerance: ± 2 μm. Cutting speed of the 0.22 mm thickness sheet metal was 0.5 mm/s with the 2.8 W mean laser power.

Fig. 22 Microcomponents cut in the X2 NiCoMo 18-9-5

steel.

Conclusions

New high output power diode-pumped laser sources were developed for industrial applications. They possess a high repetition rate (up to 100 kHz), short pulse durations (~10 ns or 10 ps) and the high average power (up to 10 W) together with efficient and reliable harmonic conversion.

The 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 with picosecond lasers compared with the conventional Q-switched nanosecond lasers. Unique processing abilities were revealed in the modern multilayered materials. Short pulse nanosecond lasers show perfect machining results in applications such as glass marking and polymer film ablation.

Acknowledgment

The work was partially supported by the Lithuanian State Science and Studies Foundation under projects No G-12/2008 and No.B-31/2008.

References

[1] Kraus M., Collmer S., Sommer S., Dausinger F. (2007) Microdrilling in steel with frequency-doubled ultrashort pulsed laser radiation, Proc. the 8th Int. Symp. on Laser Precision Microfabrication, Vienna, Austria.

[2] Haas S., Gordijn A., Stiebig H. (2008) High speed laser processing for monolithical series connection of silicon thin-film modules, Prog. Photovolt: Res. Appl. 16, 195-203.

[3] 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-S107.

[4] Groeneveld R.H.M., Sprik R., Lagendijk A., emtosecond spectroscopy of electron-electron and electron-phonon energy relaxation in Ag an Au, Phys. Rev. B.,51, 11433-11445 (1995).

[5] Chichkov B.N., Momma C., Nolte S. (1996) Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A, 63, 134-142.

[6] Nolte S., Momma C., Jacobs H., Tünnermann A., Chichkov B.N., Wellegehausen B., Welling H. (1997) Ablation of metals by ultra-short laser pulses, J. Opt. Soc. Am. B 14, 2716-2722.

[7] Breitling D., Ruf A., Dausinger F. (2004) Fundamental aspects in machining of metals with short and ultrashort laser pulses, Proc. SPIE 5339, 49–63.

[8] Drogoff L., Vidal F., Laville S., Chaker M., Johnston T., Barthelemy O., Margot J., Sabasi M. (2005) Laser ablated volume and depth as a function of pulse duration in aluminum targets, Appl. Optics, 44, 278.

[9] Ashkenasi D., Schwagmeier M. (2007) Laser ablative machining o glass: micro-drilling and contour cutting, Proc. of the forth international WLT-conference on Laser in manufacturing 2007, Munich, 18-22 June 2007

[10] Račiukaitis G., Brikas M., Gedvilas M., Rakickas T. (2007) Patterning of indium-tin oxide on glass with picosecond lasers, Appl. Surface Science, 253, 6584-6587.

[11] G. Račiukaitis, M. Brikas, G. Darčianovas, D. Ruthe, K. Zimmer (2007) Laser structuring of conducting films on transparent substrates, Proc. of SPIE 6732, 67320C.

[12] Rull G., Solar market offers new opportunities for lasers (2008) Optics Lasers Europe, July/August 2008, 20-21.

[13] Hermann J., Benfarah M., Bruneau S., Axente E., Coustillier G., Itina T., Guillemoles J-F., Alloncle P. (2006) Comparative investigation of solar cell thin film

processing using nanosecond and femtosecond lasers, J. Phys. D, 39, 453-460.

Meet the Author(s)

Mikhail Grishin graduated from Moscow Engineering Physical Institute, Solid -State Physics and Quantum Electronics Department in 1983. He has been a R&D program manager of Ekspla Ltd. since 2000 and is dealing with solid-state diode pumped picosecond lasers since 2003. He is a second year PhD student at the Institute of Physics.

Saulius Jacinavičius graduated from Vilnius University, the Department of Physics in 1980 and is working on new laser development at Ekspla Ltd. since establishing the company in 1992.

Giedrius Andriukaitis graduated from Vilnius University in 2008 and is a new product development engineer at Ekspla Ltd.

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 1985. 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 2003. He is head of Laboratory for Applied Research, Institute of Physics, Vilnius since 2004 and a consultant on laser technology with Ekspla Ltd.

Marijus Brikas graduated from Vilnius University in 2005 and is a last year PhD student at the Institute of Physics.