Challenge to advanced laser materials processing in ... - CiteSeerX

RIKEN Review No. 50 (January, 2003): Focused on Laser Precision Microfabrication (LPM 2002)

Challenge to advanced laser materials processing in Japanese industry Isamu Miyamoto# Department of Manufacturing Science, Gradient School of Engineering, Osaka University

Japan is one of the most advanced countries in manufacturing technology, and lasers have played an important role in the advancement of manufacturing technology in a variety of industrial fields. The contribution of laser materials processing to Japanese industry is significant for both macroprocessing and microprocessing. The present paper describes the recent trend and topics of industrial applications in terms of the hardware and the software in order to show how the Japanese industry is challenging in advanced materials processing using lasers, and national projects related to laser materials processing are also briefly introduced.

1. Introduction Lasers have greatly contributed to major fields of science as well as technologies since the first success in laser oscillation in 1960. There is no doubt that the laser is one of the greatest inventions of the 20th century. Particularly in materials processing, lasers have played an important role. The advancement of the latest surgery, electronic devices, measuring instruments, automobiles, aircrafts, nuclear power generation and so on might have been vastly limited without laser materials processing. Entering the 21st century, manufacturing technologies have become more and more reliant upon lasers, and there is no doubt lasers will continue to play an important role and become more valuable. The 21st century is indeed “the age of light”. Japan is one of the most advanced countries in manufacturing technology and lasers have been playing an important role. This article describes how the Japanese industry challenges in advanced materials processing using lasers for macro- to microprocessing in manufacturing technologies.

2. Lasers and peripherals 2.1 Development of all-solid-state high-power lasers A 5-year R&D national project, “Advanced Photon Processing and Measurement Technology”, started in March, 1997 involving 13 private companies, a university, and four research groups at the National Institute of Advanced Industrial Science and Technology.1) This project has two R&D themes, “High-power all-solid-state laser” and “Tightly focusing allsolid-state laser” for developing solid-state lasers.

technologies have been transferred to industry for the commercial LD-pumped Nd:YAG laser system. In the second theme, the aim is to develop tightly focusing allsolid-state laser technology for high-accuracy processing, and different types of lasers were developed. A high-brightness high repetition rate UV (ultraviolet) all-solid-state laser with an output power of 1 kW was attained at an efficiency of η=23 % by 4 modules, and can be focused into a diameter of 40µm. In this system, advanced CLBO crystals having higher laser-induced damage threshold were also developed. By using these crystals, a UV output of 42 W at 266 nm was attained with a stability of 100 h at 20 W. A disk-type 1 kWfiber laser was also developed. 2.2 Development of line-narrowed F 2 lasers The Association of Super-advanced Electronic Technologies (ASET) started a government-supported two-year project (see Section 4.3), “The F2 Laser Lithography Development Project”, in 2000 to realizing F2 lithography technologies.2) In this project, an average power output as high as 25 W is required for high throughput. In the F2 laser system, the spectral width depends on the optical system used in F2 lithography; catadioptric requires 1.2 pm, and dioptiric requires 0.5 pm for CaF2 +BaF2 lenses and 0.2 pm for all CaF2 lenses. A spectral width of 0.2 pm is required for lens systems using only CaF2 , because BaF2 is not available now (Fig. 1).

The aim of the first theme is to develop high-power (10kW) LD-pumped lasers with a compact laser head of less than 0.05 m3 volume using both rod- and slab-type Nd:YAG crystals for cutting and welding applications of heavy sections. A rod-type Nd:YAG laser composed of 6 modules was developed with a CW (continuous wave) output of 12.0 kW at an optical conversion efficiency η=23%. A slab-type laser with an output of more than 10 kW was also developed. These # e-mail address: [email protected]

20

Fig. 1. Lens design and laser system for 157 nm lithography.2)

The main goal of the project is to develop a line-narrowed F2 laser with 25 W at a repetition rate of 5 kHz with wavelength stability of ±0.05 pm for lithography. Such a high power output with an ultranarrow spectral width is not easy to attain with a single oscillator, and therefore a F2 laser consisting of an oscillator and an amplifier was developed. An output power of 30 W was of an oscillator and an amplifier was developed. An output power of 30 W was attained with a spectral width of 0.12 pm (wavelength stability ±0.04 pm) and a repetition rate of 5 kHz by the end of the project by three laser manufacturers, who joined the project in 2002. 2.3 Peripheral equipment In laser materials processing of large constructions, an optical fiber system is essential for beam delivery. Figure 2 shows transmission loss of an optical fiber of 30 m length and 0.8 mm core diameter of SI type developed by Mitsubishi Heavy Industries.3) The transmission loss is less than 10%, which is mostly caused by reflection at the edges of the optical fiber. A laser power of at least up to 50 kW can be transmitted without causing any damage to the optical fiber. It should be noted, however, that in delivering 50 kW, for instance, a reflection of 5% at each edge corresponds to as high as 2.5 kW, which is sufficiently high to cause problems to the peripheral equipment and has never been experienced in existing laser systems.

Fig. 3. Optical fiber system of 1 mm core diameter SHG Nd: YAG laser with peak power of 10 MW can be transmitted through the optical fiber.4)

The development of fiber delivery systems for a high-peak pulsed laser is also a challenging attempt. An optical fiber system has been developed for transmitting a Q-switched SHG Nd:YAG laser with 15 ns pulse width.4) In order to prevent damage due to the high peak power, a homogenizer is used to flatten the intensity distribution of the laser beam at an incident face of the optical fiber of 1 mm diameter as shown in Fig. 3. This optical fiber system has been used in the laser peening technique to prevent SCC (stress corrosion cracking: see Section 3.1) in large constructions of nuclear power plants. A DOE (diffractive optical element) has a variety of applications for beam redistribution. A micro-structuring technique of polycrystal ZnSe for the DOE for a CO2 laser has been developed where the optical phase is directly controlled by microstructuring of the surface of the optical element designed by Fourier optical analysis. A surface roughness as

Fig. 4. RIE technique with chlorine gas provides ZnSe DOE with extremely fine surface.5)

fine as 5 nm Ra. was attained with ZnSe by RIE (reactive ion etching) using chlorine, gas as shown in Fig. 4.5) One of the applications that is currently being investigated is beam splitter for via hole drilling (see Section 4.2). They are also developing optical elements for beam shaping and homogenization.

3. High-power-laser processing 3.1 Heavy industry Recently, the output power and beam quality of Nd:YAG lasers have been enhanced rapidly, and their welding performance of heavy sections can surpass that of high-power CO2 lasers. Thus Nd:YAG lasers have been adopted primarily in production because of their simpler beam delivery system, higher absorption to metal and negligible laser-plasma interaction.

Fig. 2. Transmission loss of Nd: YAG laser in optical fiber of 30 m length.3)

Heavy industry requires high-power lasers capable of welding thicker plates with low thermal distortion. Thus PW (pulse wave) lasers have been developed in addition to CW lasers at the aforementioned national project,1) since PW

21

lasers provide approximately 1.5 times deeper penetration with parallel-sided weld bead than CW lasers, thereby causing less thermal distortion. Figure 5 (a) is an example of welding a plate of 20 mm thickness by using an averaged laser power of 7.6 kW having a peak power of 25 kW.3) It is estimated that laser welding technology can weld steel plates of thicknesses up to 20 mm, which requires approximately 20 kW of laser power. In shipyards, most applications involve steel plates of thicknesses up to 20 mm. The electron beam or other conventional welding technologies such as narrow-gap welding is used for welding thicker plates. Laser cutting thick plates is also applicable in heavy industry. One of the future applications of such a process is the dismantling of aged nuclear power plants, which will start in the next 5–10 years in Japan, and laser cutting will be adopted for this purpose, since cutting can be implemented with remote control with minimal secondary products. An example of laser cutting of a 90- mm-thick steel plate is shown in Fig. 5. For dismantling purposes, a laser power of 20 kW is again necessary for cutting steels of 200 mm thickness. Performance of deep penetration welding can be improved by increasing focusibility of lasers, which also causes the disadvantage of limited gap tolerance. Hybrid welding is addressed, since the gap tolerance of the welding joint is greatly improved while maintaining the performance level of deep penetration welding. A MIG-YAG hybrid head has been developed in which the TIG electrode is placed coaxially in the middle, and divided laser beams are converged again at a point under the end of the MIG electrode so that symmetrical welding can be accomplished in any welding direction. Undercut occurs at the prepared gap of 0.4 mm with normal Nd:YAG laser welding, whereas no undercut is produced with the coaxial MIG-YAG welding up to the prepared gap of 0.8 mm as shown in Fig. 6.6)

large ablation pressure of up to 2 GPa is produced at the surface by irradiating the pulsed laser beam underwater. It should be noted that compressive residual stress as large as 700 MPa is produced by using commercially available SHGNd:YAG lasers without preconditioning or coating unlike the case of using existing methods developed in France and US. Figure 7 shows distributions of residual stress with and without laserpeening. It is evident that the crack formation is suppressed due to the compressive residual stress. Minimal thermal distortion in laser welding is also attractive for applications. This advantage is pronounced in the welding of long structures made of materials having a large thermal

Fig. 6. Comparison of gap tolerance betwwen MIG-YAG hybrid welding and conventional YAG laser welding.6)

One of the serious problems in welded steel constructions is SCC (stress corrosion cracking) where susceptibility of cracking is enhanced at the weld bead due to residual tensile stress. A challenging technology called “laser peening” has been developed to provide compressive residual stress at the surface by irradiating the ns-pulse SHG-YAG laser underwater.4) A Fig. 7. Effect of residual stress induced by laser peening on SSC susceptibility.4)

Fig. 5. (a) Welding system using 10 kW-Nd: YAG laser with peak power of 30 kW, and examples of (b) welding and (c) cutting of heavy sections.6)

22

Fig. 8. Schematic illustration of canister. Double-paneled stainless streel tube of 3 m length is constructed by laser welding with deformation less than 1 mm.7)

Fig. 10. Pipe production line by using 20 kW-CO2 laser with induction preheating.10)

Fig. 9. Laser power vs year introduced to steel industry.

expansion coefficient. Figure 8 shows a thin double-paneled stainless-steel tube of 3 m length to which butt welding and fillet welding using lasers were applied with deformation of less than 1 mm7) . The support disks were also manufactured by laser cutting. 3.2 Steel industry In the steel industry, a variety of interesting applications of laser processing to steel production have been found as early as the late 1970s. Applications in the steel industry are summarized in terms of the year of introduction and laser power in Fig. 9. The applications are divided into two categories; one is continuation and automation of steel production line, and the other one is development of novel materials suitable for laser materials processing.

Fig. 11. Novel steel developed for preventing porosities by reducing carbon content and deoxidizing elements.13)

One of the earliest applications for the continuation and the automation of the steel production line is joining steel coils in built-up line. CO2 lasers up to 10 kW were used for welding coils in cold rolling, annealing and pickling lines.8) Pipes of high quality were also constructed using the 20 kW-class CO2 laser9, 10) and welding speed can be doubled by preheating the steel through induction up to 800◦ C (Fig. 10). Another challenging technique for continuation of the steel production line is welding hot slabs of 1000◦ C using two high-power 45 kWCO2 lasers developed by Nippon Steel.11) They developed a continuous finish-rolling system to join hot slabs. The development of novel materials suitable for laser materials processing is also desired to overcome the problems that cannot be solved by improving the process technique only. A new laser welding technique has been developed where as-cut plates are welded for manufacturing large steel constructions using a 20 kW-class CO2 laser.12) The advantages are cost reduction in edge preparation and removal of oxide film formed in the laser cutting process. Although the gap tolerance can be greatly improved by weaving and filler-wire feeding, weld defects such as porosities, solidification cracks and spatter still remain due to the existence of the oxide film. In order to solve these problems, a novel material with reduced carbon content and more deoxidizing elements has been developed to suppress the carbon-oxygen reaction in molten metal, and a sound bead without porosities and solidification cracks has been successfully obtained (Fig. 11).13)

Fig. 12. Laser processing in Japanese automobile industry (by I. Maruyama).

3.3 Automotive industry Figure 12 summarizes the techniques of laser materials processing that have been developed in the Japanese automobile industry since the 1970s. Nd:YAG and CO2 lasers have been used for processing body and power train including body cutting, roof rail welding and tailoring blanks. In addition to these applications, in-process monitoring systems of weld quality have also been introduced into production.14–16) Funding for production facilities has been considerably limited due to the long economical resession in Japan. The automobile industy is an example of this. For instance, 3dimensional laser welding systems, which are popular in Ger-

23

many, have not been installed until recently in this country. It is interesting, however, that there are more LD-pumped YAG lasers installed for production in Japan than there are in Germany at the moment. This is probably because the cost of electricity is much higher in Japan than overseas and therefore, the change of high-power Nd:YAG lasers for materials processing from lump-pumping to LD-pumping occurred quickly even though it started very recently in Japan. We can find several advanced technologies in the Japanese automobile industry. One of the interesting applications is laser cladding of the engine valve and the valve seat in Toyota (Fig. 13).16) The conventional press-fit structure of valve seats has been replaced by laser cladding because the wall temperature can be lowered to reduce knocks, and the diameter of the valve/seat can become smaller. Toyota introduced laser-cladding technology into production in 1997 for a type of sports car, and it is now being used for other popular cars. 8 units of 5 kW CO2 lasers yield a production rate of 30000 parts per month. The welding system of plastic intake manifold in which diode lasers are used with a 6-axis robot has been developed (Fig. 14).17) The space required for welding facilities that use diode lasers is much smaller than for those that use CO2 and Nd:YAG lasers. This technique is expected to be used for a variety of applications in which vibration for joining is

not allowed and the heat-affected zone must be minimized. Joining dissimilar materials including plastic-to-plastic and plastic-to-metal is also a potential application of LD direct processing.

4. Laser precision microfabrication 4.1 Trend of electronic devices Electronic devices such as computers, cellular phones, PDAs, and digital cameras are becoming smaller, lighter, and faster. For instance, the weight and the size of cellular phones have decreased considerably, while their function has markedly improved. As a result, the cost of PWB (printed wiring circuit boards) for cellular phones has rapidly decreased.18) Miniaturization, high-density packaging and high throughput are demands of manufacturing in the electronic industry. This trend is accompanied by the demands for 3-dimensionalization of the electronic devices and optoelectronic integration, and laser materials processing is expected to play an important role in production. 4.2 Via hole drilling One of the most successful applications of laser materials processing is via hole drilling of PWB. The world market for build-up PWB is increasing steadily due to continuous expansion of the market of electronic consumer products, and Japan’s share in the global micro-via market for PWB is more than 50% as seen in Fig. 15. The energy source for drilling via holes of 100–200 µm diameters is CO2 lasers with a pulse width of several tens of µs at the moment. The minimum diameter of the via hole that the CO2 laser can cover has become much smaller than expected due to the improvement of focusing optics and the laser performance, and via holes of 30–40 µm diameter have been realized by using the CO2 laser

Fig. 13. Laser cladding system for valve seat of 1ZZ-FE engine cylinder head.16)

Fig. 15.

Fig. 14. Laser welding system for joining plastic intake manifold combined with 6-axis robot.17)

24

World market for build-up PWB (by NEC).

Fig. 16. Micro via holes drilled by CO2 laser (35 µm in diam.) and THG-Nd: YAG (25 µm in diam.).19)

Table 1. Aims of laser micro via-hole drilling system.

Cost

Quality

Reliability

- Multibeam processing (multihead, division of beam by DOE) - Positioning of focus spot - Increasing table speed - Larger window of fθ lens Improvement of yield - Preciseness of positioning (scanner, reduction of temperature change) - Smaller hole diameter High density -Higher peak-short pulse CO2 laser -Lens with higher refractive index -THG-Nd:YAG laser - Direct drilling of Cu foil In-process - Detection of light emission or reflected laser monitoring beam &control - Control of pulse number for each hole Increasing throughput

Inspection of smear thickness (off-line)

Fig. 18. Key technologies to be developed in the F2 lithography project conducted by ASET.2) Fig. 17. Influences of pulse width and peak energy on copper direct processing with CO2 laser.19) Table 2. Target and achieved values in F2 lithography project by ASET.2)

(Fig. 16).19) For via holes of smaller diameters, the THGNd:YAG laser is expected to be used.

Theme

Item Repetition rate Spectral bandwidth Output power Resolution of spectral measurement Optical power loss

Target 5000 Hz 0.2 pm 25 W 0.01 pm

Achieved 5000 Hz 0.12 pm 30 W 0.008 pm

Table 1 summarizes the aims of laser micro-via hole processing for PWB, which are cost reduction, quality improvement and quality assurance. Throughput is essential for low cost. The drilling rate of a single laser spot depends on the scanning frequency of the galvanomirror, which is approximately 1100–1200 Hz. The drilling rate can be increased by dividing the laser beam into multiple spots. DOE (HOE) is one of the most effective ways to divide the beam spot and DOE of ZnSe has been realized by using RIE as aforementioned.5) The drilling rate can be increased in proportion to the number of split beam spots easily, although the drilling is limited to repeated drilling patterns. Drilling systems using beam splitters are also developed in which divided beams are independently scanned by a galvanomirror so that drilling with an independent drilling pattern becomes possible.

residual smear thickness for quality assurance; one system detects light emitted from the interaction point21) and the other detects the laser beam reflected from the inner copper foil.22)

The quality of the via hole is improved by using lasers with a high peak power and a short pulse. CO2 lasers with a higher peak and a shorter pulse can be used not only for reducing smear thickness,20) but also for direct drilling of copper foil while maintaining the quality and dimensions of the via hole as shown in Fig. 17.19) In-process monitoring systems have also been incorporated in the drilling machine to inspect the

4.3 Lithography and mask repair In lithography, the wavelength of lasers has become increasingly shorter to cope with the continuous requirement for smaller nodes. There is a two-year project, ASET, with the aim to develop the basic technologies of F2 lithography.2) As shown in Fig. 18, key technologies to be developed in this project are (1) high-power line-narrowed F2 laser, (2)

F2 laser

Optical coating

0.3% per surface Measurement repeatability 0.02% Chemically Optical transmittance 95%/m clean Light intensity uniformity ±0.2%

0.18% 0.01% 98%/m ±0.2%

25

Fig. 19.

Chromium film pattern laser-ablated with pulse width 120 fs (left) and 250 ps(right).25)

Table 3. Road map of mask repair using laser (by NEC). Year Node (nm) Repair accuracy (nm) Laser wave-length (nm) Pulse width (ns)

1981 2000 500 1064 20

1985 1000 300 10

1991 700 200 530 5

1995 350 100 0.9

2000 130 45 351 0.25

157 nm optical coating technologies and (3) gas-purging and chemically clean technologies. Table 2 shows the target and achieved values of the project. The high-power F2 laser was described in 2.2. It is seen that these target values have been successfully achieved in this project. Technology for repair of photomasks has also been improved to meet the design rule of photomask patterns. Table 3 shows the trend of mask repair; wavelength and pulse width of lasers used for mask repair have become shorter. Ns lasers were used until the 700 nm node, and ps lasers were used after the 350 nm node.23) One of the candidates after the 130 nm node for the near-future design rule is the etching technique. In this technique, laser beam is irradiated in etching atmosphere, and a Cr film pattern narrower than the diffraction limit was realized without any role-up, splash and damage of the quartz substrate.24) Another approach is ablation with an ultrashort pulse laser to minimize heat dispersion. As seen in Fig. 19, fs lasers of 800 nm wavelength provide better resolution than ps lasers without vacuum chamber, as well as a linewidth narrower than the diffraction limit due to their well-defined threshold of ablation.25) 4.4 Laser microbending Laser microbending provides new procedures for precise deformation without mechanical contact, and has a variety of applications in production such as for adjustments of magnetic head height26) and the clearance of lead switch.27) These applications utilize shrinkage stress exerted in the melted portion. A microbending technique without surface damage is realized by heating to temperatures just below the melting point where the yield strength is locally lowered. Microbending of a beryllium the melting point where the yield strength is locally lowered. Microbending of a beryllium sheet of 50 µm thickness has been developed for adjusting micromechanical relays. In this system, Q-switch lasers are used because a steep temperature gradient is required for bending thin plates by this method.28) Adjusting time is decreased by up to 100 ms by an

26

Fig. 20. Microbending for adjusting matrix-arrayed micro-mechanical relay by using Q-switch laser.28)

increment control of the beam path from the laser irradiation point. This system is soon to be applied to batch production of matrix-arrayed relays as shown in Fig. 20. A technique for bending ceramic parts has been developed for adjusting curvature of the slider in hard disks. Figure 21 shows an example of flattening of the slider of an Al2 O3 -TiC ceramic piece of 0.3 mm thickness.29) Laser beam spots are irradiated along the automatically detected ridge-line to attain a flatness less than 1 nm in the crown, camber and twist. No debris is produced unlike the case of the LCAT (Laser Curvature Adjust Technique) by IBM30) where bending is accomplished by ablation of a ceramic plate having residual stress. 4.5 Modification of transparent material An optical fiber network will soon be available for home use, and optical fiber closures can be observed in the metro area. For optical fiber network, low-cost, reliable, high-performance optical devices are required. A fiber bragg grating, which is used for the Add-Drop Module in the WDM (Wavelength Division Multiplexing) network, can be fabricated by irradiation of KrF or ArF excimer laser through the phase mask. Recently, this device has attracted attention because of its low production cost and long-term reliability (Fig. 22). The precise formation of gratings in the very small coupler is very important for high-quality ADMs. Gain flattening of the Erdoped fiber amplifier (EDFA) can be carried out using several long-period fiber gratings. A big national project, FESTA (Femto second Technology

Fig. 21. Microbending of AI2 O3 -TiC ceramic for hard disk slider.29)

Fig. 22. Low-cost devices for last mile using grating device written by UV laser.

Research Association), is currently under way in Japan. One of the goals of this project is to develop a very small waveguide using a photonic crystal structure as shown in Fig. 23.31) Researchers are attempting to fabricate devices using a fs laser for nano-machining technology. This project started in 2001 and is now in phase II, which will continue until 2006.

Fig. 23. Nanofabrication technique of photonic band-gap structures and ultrasmall optical waveguides using fs laser developed in FESTA.31)

ogy and cost reduction of laser system are required. The author wishes to thank the members of Japan Laser Processing Society who provided valuable information for the present article. References K. Matsuno: Proc. SPIE 4831 (2002), Paper No. 2302 (in press). 2) H. Komori et al.: Proc. SPIE 4426, 424 (2001); T. Ariga et al.: Proc. SPIE 4691 (2002), Paper No. 65 (in press). 3) T. Ishide et al.: Proc. SPIE 3888, 543 (1999). 4) S. Sano et al.: Proc. SPIE 4831 (2002), Paper No. 2306 (in press); S. Sano: Dissertation, Osaka University (2001), Study on improvement mechanism of residual stress at metal surface by laser irradiation underwater. 5) K. Kurisu et al.: Proc. SPIE 4831 (2002), Paper No. 1279 (in press). 6) T. Ishide, M. Nayama, M. Watanabe, and T. Nagashima: International Institute of Welding, IIW-Doc. IV-1708-02 1)

5. Conclusion Laser materials processing has significantly contributed to Japanese industry in macroprocessing as well as in microprocessing. The fact that approximately 25% of all industrial lasers worldwide are used for production in Japan indicates that Japan is one of the most important countries in terms of laser materials processing. To further increase the applications of laser materials processing for production, innovation of laser technology, peripheral equipment, new materials suitable for laser materials processing, quality assurance technol-

27

7) 8) 9)

10)

11) 12)

13)

14)

15)

16)

28

(2002); T. Ishide et al: Proc. SPIE 4831 (2002), Paper No. 2376 (in press). A. Kitagawa: Dissertation, Osaka University (2001), Fundamental study on laser applications to large constructions. M. Ito et al.: Proc. 1st Laser Advanced Materials Processing Congress (LAMP’87) (1987), p. 535. T. Hayashi, Y. Inaba, T. Kudo, and T. Tanaka: Proc. Laser Materials Processing Conference, ICALEO’96, LIA Vol. 81 (1996), P. D-132. M. Ono, T. Shiozaki, M. Ohmura, H. Nagahama, and K. Kohno: Proc. ECLAT’96, 6th European Conference on Laser Treatment of Materials (1996), p. 115. K. Minamida: Proc. SPIE 4831 (2002), Paper No. 2363 (in press). K. Ono, K. Adachi, Y. Matsumoto, I. Miyamoto, and T. Inoue: Proc. Laser Materials Processing Conference, ICALEO’99, LIA Vol. 87 (1999), p. D83. K. Ono: Dissertation, Osaka University (2001), Applications of high power CO2 laser to manufacturing process of steel structures. I. Miyamoto, H. Maruo, and Y. Arata: Proc. Laser Materials Processing Conference, ICALEO’84, LIA Vol.44 (1984), p. 68. I. Miyamoto, K. Kamimuki, and H. Maruo: Proc. Laser Materials Processing Conference, ICALEO’93, LIA Vol. 77 (1993), p. 413. K. Mikame: Proc. 2nd Laser Advanced Materials Processing Congress (LAMP’92) (1992), p. 947.

17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27)

28) 29)

30) 31)

M. Terada and H. Nakamura: Proc. SPIE 4831 (2002), Paper No. 2346 (in press). I. Miyamoto: Proc. SPIE 4426, 1 (2001). A. Fukishima et al.: Electronic packaging technology(in Japanese), Vol.16, No. 6, June (2000). S. Noguchi, E. Ohmura, and I. Miyamoto: Proc. SPIE 4830 (2002), Paper No. 2322 (in press). T. Nakayama, T. Sano, I. Miyamoto, K. Tanaka, and Y. Uchida: Proc. SPIE 3933, 379 (2000). K. Ichihashi et al: Proc. Laser Materials Processing Conference, ICALEO2000 LIA Vol. 90 (2000), p. D36. Y. Morishige: Proc. SPIE 4426, 416 (2001). Y. Morishige: Private communication. T. Okamoto, E. Ohmura, I. Miyamoto, and Y. Morishige: Proc. SPIE 4830 (2002), Paper No. 1320 (in press). K. Funami and T. Okada: J. Jpn. Laser Processing Society(in Japanese), 4, 253 (1997). C. M. Verhoeven, H. F. P. der Bie, and W. Hoving: Proc. Laser Microfabrication, ICALEO 2000, LIA Vol. 90 (2000), p. B21. K. Kitada and N. Asahi: Proc. SPIE 4830 (2002), Paper No. 1204 (in press). N. Matsushita et al: 65th Select Committee Meeting of Micro Joining, Japan Welding Society (in Japanese), Paper No. MJ390-2001 (2001). A. C. Tam: Proc. SPIE 4088, 380 (2000). http://www.festa.or.jp