Ultrafast laser microwelding for transparent and heterogeneous materials

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1 SPIE Commercial and Biomedical Applications of Ultrafast Lasers VIII, Conference 6881, January 2008 San Jose Convention Center, San Jose, CA Ultrafast laser microwelding for transparent and heterogeneous materials Kazuyoshi ITOH 1 and Takayuki TAMAKI 2 Osaka University 1, Nara National College of Technology 2 1

2 Outline - Introduction to filamentation - Ultra-fast laser micro-welding of glass with filaments - Ultra-fast laser micro-welding of heterogeneous materials 2

3 Filamentation Lens Nonlinear Medium High-Intensity Pulse Normal Focusing Self Focusing (Optical Kerr Effect) Filament Filamentation Plasma Formation 3

4 Filamentation - balancing between self-focusing and defocusing by plasma - Scattering or luminescence from filament Micrograph of the resultant index change Magnified micrograph 4

5 Formation of single filament NA of focusing lens : 0.1 Exposure time 5 min. ( 300,000 pulses ) Multiple filaments 100 µm Scattering damages 100 µm Core diameter : ~ 2 µm 5

6 Applications of filamentation - Waveguide writing Waveguides* 1, WG Couplers* 2 - Writing optical elements Mirrors, Lenses* 3, Optical devices* 4 - Welding transparent materials *1K. Yamada, W. Watanabe, T. Toma, K. Itoh, J. Nishii, Opt. Lett. 26, 19 (2001). * 2 W. Watanabe, T. Asano, K. Yamada, K. Itoh, J. Nishii, Opt. Lett. 28, 2491 (2003). * 3 K. Yamada, W. Watanabe, Y. Li, K. Itoh, and J. Nishii, Opt. Lett,. 29, 1846 (2004). * 4 Y. Li, W. Watanabe, T. TamakiI, J. Nishii, and K. Itoh,. J. Appl. Phys.,. 44, 5014 (2005). 6

7 - Ultra-fast laser micro-welding of glass with filaments Takayuki Tamaki, Wataru Watanabe, Junji Nishii, and Kazuyoshi Itoh, Jpn. J. Appl. Phys., Vol. 44, No. 22, L687-L689 (2005). Takayuki Tamaki, Wataru Watanabe, and Kazuyoshi Itoh, Optics Express, Vol. 14, Issue 22, (2006). W. Watanabe, S. Onda, T. Tamaki, and K. Itoh, Appl. Phys. B, Vol. 87, pp (2007). 7

8 Scanning the filament Low repetition source Fast scanning ~ Gap High repetition source ~ Accumulation of heat Low repetition Slow scanning ~ No gap 8

9 Optical setup Wavelength: 800 nm Pulse duration: 130 fs Repetition: 1 khz Incidence energy: 1.0 µj/pulse Numerical aperture: 0.30 Irradiation area: 100 µm x 100 µm Translation speed: 5.0 µm/s 9

10 Welding flat samples Samples Jig for welding 10

11 Micrographs Top view Side view 11

12 Joining strength (Same material) Pulse Energy [µj/pulse] Scanning Speed [mm/s] Borosilicate glass Fused silica glass 15 ~ 150 kgf/cm 2 Usual adhesive ~ 50 kgf/cm 2 (kgf: kilogram force) 12

13 Optical transmittance 81.4 % 85.9 % 87.0 % Pulse Energy [µj/pulse] % 87.1 % % % % 86.7 % 87.3 % 87.1 % Scanning Speed [mm/s] Borosilicate glass 81 ~ 87 % Theoretical limit: 92 % Fused silica glass 87 ~ 89 % Theoretical limit: 93 % 13

14 Effects of Annealing Welded part Optical Micrograph before annealing 100 µm 100 µm Optical Micrograph after annealing Annealing makes welded part invisible. (Implication of disappearance of defects or stress.) 14

15 Enhancement of joining strength & optical transmittance Joining strength Borosilicate glass Fused silica glass Before annealing Mpa After annealing Optical transmittance Before annealing 88 % 87 % After ( 336 kgf/cm 2 ) annealing 92 % 91 %: (Theoretical limit: 93 %) (Theoretical limit: 92 %) 15

16 Enhancement of optical transmittance by annealing Borosilicate glass Fused silica glass Before annealing 88 % 87 % After annealing 92 % 91 % (Theoretical limit: 93 %) (Theoretical limit: 92 %) 16

17 - Ultra-fast laser micro-welding of different glass 17

18 Heterogeneous welding: 10 dissimilar kinds of glass Laser Pulses Geometry Borosilicate glass 39* [ 10 / Pulse Energy [µj/pulse] 1 Joined Not joined 0.1 Scanning Speed [mm/s] 1 Fused silica glass 5.9* [ 10 / *Thermal expansion coefficient Wataru Watanabe, Satoshi Onda, Takayuki Tamaki, Kazuyoshi Itoh, and Junji Nishii, Appl. Phys. Lett., Vol. 89, No. 2, (2006). 18

19 Joining strength and transmittance % 73.2 % % 73.0 % Pulse Energy [µj/pulse] Pulse Energy [µj/pulse] 71.8 % 73.0 % 73.4 % % 73.6 % Scanning Speed [mm/s] 88.3 % Scanning Speed [mm/s] Joining strength Optical transmittance 19

20 Heterogeneous welding: Material *Thermal expansion coefficient Polymer Semiconductor Metal Alloy PMMA 700 [ 10 / Silicon 28 [ 10 / Cupper 183 [ 10 / Stainless steel 175 [ 10 / dissimilar kinds of materials Fused silica glass 5.9 [ 10 / Borosilicate glass 39 [ 10 / Geometry Wide range of heterogeneous welding 20

21 - Ultra-fast laser micro-welding of glass and metal 21

22 Ultra-fast Laser Micro-welding of Glass and Copper Realizing tight contact between glass and copper 22

23 Optical Setup 23

24 (a) Side view (b) Top view (c) Whole image Optical microscope images Laser source: Regenerative Ti:sapphire laser (Spectra Physics, Spitfire) Central wavelength: 800 nm Pulse duration: 130 fs Repetition rate: 1 khz Pulse energy: 4 µj/pulse Scan speed: 1 mm/s Joining strength: 23 24

25 Joining Strength Joint Strength [] Pulse Energy [µj] 0.38 µj Glass plate ( mm 3 ) Small glass plate ( mm 3 ) Average:

26 Electron Micrograph of an Interface Glass Copper 1 µm No crack nor gap was observed. But, some bumpy irregularity presents. 26

27 Summary -Ultra-fast laser micro-welding of glass with filaments -Ultra-fast laser micro-welding of homogeneous and heterogeneous welding, such as silica and borosilicate glass, silica glass and metals average joining strength: 21.5 Mpa ( ~220 kgf/cm 2 ) 27

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30 Welding with High Repetition Laser Pulses Wavelength: 1558 nm Pulse duration: 947 fs Repetition rate: 500 khz Input energy: 0.8 µj/pulse Translation speed: 20 µm/s Objective lens: 0.40-NA (Numerical aperture) Samples: Borosilicate glass Joint strength: 100 µm/s µm/s

31 Conventional and ultra-fast laser micro-welding Conventional AGlass Other B material Wide heating Cooling with cracks Femtosecond pulses Glass Other material Expansion due to temperature rise Local heating Contraction due to cooling Cooling without cracks 31

32 Application of Ultra-fast Laser Micro-welding to Metal Package (Glass & Kovar) (a) Birds-eye view (b) Top view 32

33 Coworkers: Dr. Junji NISHII National Institute of Advanced Industrial Science and Technology Dr. Yasuyuki OZEKI and Mr. Tomoyuki INOUE Osaka University Mr. Satoshi ONDA Yokogawa Electric Corporation Mr. Seiji SOWA Konica Minolta Opto, Inc. 33