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Laser Ablationablation LaserLaser AblationAblation FundamentalsFundamentals && ApplicationsApplications Samuel S. Mao Department of Mechanical Engineering Advanced Energy Technology Department University of California at Berkeley Lawrence Berkeley National Laboratory March 10, 2005 University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation What is “Laser Ablation”? Mass removal by coupling laser energy to a target material University of California at Berkeley Q Lawrence Berkeley National Laboratory ion lat ab er IsIs itit important?important? las ) Film deposition substrate * oxide/superconductor films material * nanocrystals/nanotubes plume target mass spectrometer ) Materials characterization * semiconductor doping profiling plasma lens * solid state chemical analysis target optical spectrometer ) Micro structuring * direct wave guide writing * 3D micro fabrication microstructure target transparent solid University of California at Berkeley Q Lawrence Berkeley National Laboratory ion lat ab er IsIs itit important?important? las ) Film deposition * oxide/superconductor films * nanocrystals/nanotubes ) Materials characterization * semiconductor doping profiling * solid state chemical analysis 100 µm ) Micro structuring * direct wave guide writing * 3D micro fabrication University of California at Berkeley Q Lawrence Berkeley National Laboratory ion lat ab er DoDo wewe reallyreally understand?understand? las laser beam “Laser ablation … is still largely unexplored at the plasma fundamental level.” J. C. Miller & R. F. Haglund, Laser Ablation and Desorption (Academic, New York, 1998) target University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation ) What is happening? laser pulse target femtosecond picosecond nanosecond microsecond 10-15 s10-12 s10-9 s10-6 s University of California at Berkeley Q Lawrence Berkeley National Laboratory ExperimentsExperiments - ultrafast imaging ) Pump-probe technique ablation laser beam pump beam probe beam delay time CCD ?? imaging laser beam target University of California at Berkeley Q Lawrence Berkeley National Laboratory 1 fs = 10 –15 s FemtosecondFemtosecond TimeTime ScaleScale ) Ultrafast imaging - time dependent energy transfer laser pulse air glass m µ 100 100 fs,800 nm E = 30 µJ C electronic excitation V self-focusing e-h plasma University of California at Berkeley Q Lawrence Berkeley National Laboratory FemtosecondFemtosecond TimeTime ScaleScale ) Electron number density ne – time/space dependence 5 19 -3 ne ~ 10 cm 0 fs • Transmittance (probe beam) ) 0 -3 5 333 fs Id −αd cm 19 0 = e laser pulse 5 I0 667 fs 0 • Absorption coefficient 5 0 2 1000 fs τ ω p 0 α = 5 nc 1+ω 2τ 2 1333 fs 0 5 • Plasma frequency z 1667 fs 0 5 n e2 2000 fs e electron number density (10 ω p = 0 meε 0 0 100 200 300 400 500 z (µ m) University of California at Berkeley Q Lawrence Berkeley National Laboratory FemtosecondFemtosecond TimeTime ScaleScale ) Peak electron number density ne - time dependence 19 5 6x10 0 fs ) 0 -3 5 19 333 fs 5x10 cm 19 0 5 667 fs ) -3 19 0 4x10 5 m 1000 fs (c 0 x 5 a 19 m 1333 fs e, 3x10 0 N 5 no breakdown 1667 fs 19 0 2x10 5 2000 fs electron number density (10 0 19 0 100 200 300 400 500 1x10 z (µ m) 0 500 1000 1500 2000 2500 time (fs) flattened peak electron number density University of California at Berkeley Q Lawrence Berkeley National Laboratory FemtosecondFemtosecond TimeTime ScaleScale ) Fundamental processes 9 Nonlinear optics Self-focusing - intensity dependence of refractive index laser pulse positive refractive index change 0 9 Nonlinear absorption Electronic excitation - interband absorption z C negative refractive index change suppress self-focusing V University of California at Berkeley Q Lawrence Berkeley National Laboratory FemtosecondFemtosecond TimeTime ScaleScale ) Propagation of electronic excitation in glass 500 450 250 µJ 120 µJ 400 60 µJ 350 30 µJ 300 12 µJ m) 250 µ z ( 200 v = 1.8x108 m/s 150 n = 1.65 100 50 0 -50 -500 0 500 1000 1500 2000 2500 100 fs,800 nm t (fs) E = 30 µJ [Appl. Phys. A 79, 1695–1709 (2004)] University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation ) What is happening? laser pulse target femtosecond picosecond nanosecond microsecond University of California at Berkeley Q Lawrence Berkeley National Laboratory 1ps= 10 –12 s PicosecondPicosecond TimeTime ScaleScale ) Picosecond imaging ablation laser t = 50 ps t = 50 ps pulse 50 µm (air) 50 µm target Cu (laser pulse: 35 ps, fluence: 60 J/cm2) (laser pulse: 35 ps, fluence: 90 J/cm2) University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Threshold behavior regime-1 plasma onset regime-2 25 1 2 20 m) µ h ( 15 50 µm t dep 10 tion la b a 5 40 J/cm2 85 J/cm2 110 J/cm2 0 0 100 200 300 400 500 (laser pulse length 35 ps; pictures taken at 20 ps) laser fluence (J/cm2) 9 Threshold for picosecond plasma formation – same as the threshold for ablation efficiency reduction: ~ 85 J/cm2 (laser fluence) ~ 1012 W/cm2 (power density) (threshold for direct laser-induced air breakdown: ~ 1013 W/cm2) University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Ultrafast interferometry t = 15 ps interference pattern z 50 µm target r University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Electron number density t = 15 ps 1.2x1020 z 1.0x1020 ) 19 -3 8.0x10 9 A large electron 19 (cm e 6.0x10 number density! N (close to target surface) 4.0x1019 air density 2.0x1019 0.0 0 50 100 150 200 250 Z (µm) University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Longitudinal (z) expansion longitudinal plasma extent vs. time 500 400 300 m) µ z m z ( µ 200 50 100 0 0 20406080100120 t (ps) (laser energy: 10 mJ) 9 longitudinal expansion is suppressed (t > 50 ps)! University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Lateral (r) expansion (t > 50 ps: expansion only in lateral direction) lateral plasma radius vs. time 50 40 30 m) µ r ( 20 10 r 0 0 500 1000 1500 2000 2500 t (ps) (laser energy: 10 mJ) 9 lateral expansion follows a power law! University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Energy deposition to picosecond plasma 100 t1/2 power law similarity relation (2D blast wave - line energy source) 1/2 1/ 4 t m) 10 µ E 1/ 2 r ( r ∝ t ρ 10.0 mJ o 7.5 mJ E: energy deposition density (laser axis) 1 10 100 1000 ρ0: ambient gas (air) density time (ps) 25 2 2 2 40 J/cm 85 J/cm 110 J/cm 20 m) µ Distribution of laser energyh ( 15 (100%): pt reduced efficiency 50 µm 10 on de ~ 50% absorbed by the picosecondi micro-plasma lat 5 b a ~ 50% reaching target0 surface 0 100 200 300 400 500 plasma onset laser fluence (J/cm2) University of California at Berkeley Q Lawrence Berkeley National Laboratory PicosecondPicosecond TimeTime ScaleScale ) Theoretical model (laser-solid-gas interaction) electron ion (gas) laser beam atom (gas) gas (1atm) photon Cu atom Cu electron beforeafter laser laser irradiation irradiation 9 laser heating of target (metal) • electron heating - absorption of laser energy • lattice heating - electron-phonon collisions 9 plasma development above target • surface electron emission (seed) • impact ionization of gas University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation ) What is happening? laser pulse target femtosecond picosecond nanosecond microsecond University of California at Berkeley Q Lawrence Berkeley National Laboratory 1 ns = 10 –9 s NanosecondNanosecond TimeTime ScaleScale ) Plasma evolution – picosecond to nanosecond z time dependence (35 ps, 7 mJ) z laser energy dependence (35 ps, 2 ns) University of California at Berkeley Q Lawrence Berkeley National Laboratory NanosecondNanosecond TimeTime ScaleScale ) Plasma development plasma advancement: ~ 106 cm/s ~ 10 µm every 1 ns (1000 ps) laser pulse shock wave plasma vapor target solid University of California at Berkeley Q Lawrence Berkeley National Laboratory NanosecondNanosecond TimeTime ScaleScale ) Plasma shielding – nanosecond laser Theory Experiment without plasma 10 2 20 GW/cm m) µ 1 h ( 30 GW/cm2 0.1 ablation dept 100 GW/cm2 0123456789101112 10 100 solid t (ns) laser fluence (J/cm2) 3 ns, 1064 nm laser ablation of Si 25 ns, 248 nm laser ablation of Cu (single pulse) (single pulse, in air, 100 µm spot diameter) University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation ) What is happening? laser pulse target femtosecond picosecond nanosecond microsecond University of California at Berkeley Q Lawrence Berkeley National Laboratory 1 µs = 10 –6 s MicrosecondMicrosecond TimeTime ScaleScale ) Plume evolution z below threshold 10 ns 64 ns 160 ns 760 ns 1.6 µs4.9 µs 100 µm (3 ns, 1.8x1010 W/cm2) z above threshold 5 ns 70 ns 200 ns 860 ns 1.3 µs 4.2 µs 100 µm (3 ns, 2.1x1010 W/cm2) University of California at Berkeley Q Lawrence Berkeley National Laboratory MicrosecondMicrosecond TimeTime ScaleScale ) Threshold behavior 25 18 GW/cm2 21 GW/cm2 +2.0 +2.0 m) m) 20 m) µ (µ (µ 0.0 0.0 -2.0 15 -2.0 -4.0 10 -4.0 ablation depth ablation depth -6.0 -6.0 4.9 µs ablation depth ( 5 4.2 µs 0 1010 1011 laser intensity (W/cm2) University of California at Berkeley Q Lawrence Berkeley National
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