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 electron number density 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
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 Laboratory MicrosecondMicrosecond TimeTime ScaleScale
) Theoretical model
superheated liquid layer
~ 1 ns ~ 100 ns ~ 1 µs solid solid solid
• Normal vaporization (Hertz-Knudsen equations) 20 experiment theory ∂x m −1/ 2 Lev m 1 1 = βpb (2πmkBT) exp[ ( − )] m) 15 ∂t x=0 ρ kB Tb T µ h ( ablation below threshold: normal evaporation 10 on dept i
• Explosive boiling (heat diffusion – Tmax~ Tc) ablat ∂T ∂ ∂T 5 ρC = (k ) +αI exp(−αx) ∂t ∂x ∂x laser 109 1010 1011 ablation above threshold: normal evaporation and explosive boiling laser intensity (W/cm2)
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 LaserLaser AblationAblation
) Fundamental processes
µs µs boiling psnsfs laser radiation droplets ionization (shock wave) vaporization electronicplasma ns plasma convection melting vapor ionization (photon) electronicliquid ps conduction excitation heated zone
solid fs absorption/excitation
University of California at Berkeley Q Lawrence Berkeley National Laboratory LaserLaser AblationAblation Applications
1.0 ns laser, 266 nm fs laser, 266 nm o i t a r
u 0.5 C / Zn
0.0 20 40 60 80 100 120 140 160 180 time (s)
micro-analysis nano-material
University of California at Berkeley Q Lawrence Berkeley National Laboratory ApplicationsApplications ofof UltrafastUltrafast LaserLaser AblationAblation
) Ultrafast laser ablation (τpulse < tthermal) – FEL capability! • Non-thermal ablation regime - fs laser E + ion
electron target
9 Reduced dependence on thermal properties 9 Reduced larger cluster particles generation
University of California at Berkeley Q Lawrence Berkeley National Laboratory ApplicationsApplications ofof UltrafastUltrafast LaserLaser AblationAblation
) Micro Analysis The problem of nanosecond laser ablation
1.0 ns laser, 266 nm fs laser, 266 nm ns u ratio /C n 0.5 fs signal: Z S M
0.0 20 40 60 80 100 120 140 160 180 time(s) Mass Spectrometry - Laser ablation of brass (CuZn alloy)
University of California at Berkeley Q Lawrence Berkeley National Laboratory MicroMicro--AnalysisAnalysis ApplicationApplication
New magnetic film material (data storage applications) 1010 laser 109 depth profile (bulk material)
.) 8 u 10 Fe 107
106 sputtering target Cu 105 ent counts (a. 104 em
el 3 depth profiling 10 102 0.20 0 1020304050 time (s) laser surface profiling (film material) 0.15
0.10 deposited film e ratio /F u C 0.05 surface profiling 0.00 0 5 10 15 time (s)
University of California at Berkeley Q Lawrence Berkeley National Laboratory ApplicationsApplications ofof UltrafastUltrafast LaserLaser AblationAblation
) Nano Material The problem of nanosecond laser ablation
50 µm
50 µm
1 µm
Nanowires with large particles
University of California at Berkeley Q Lawrence Berkeley National Laboratory NanoNano--MaterialMaterial ApplicationApplication
) Pulsed laser deposition – ZnO nanowire growth
fabrication chamber Setup gas target ultrafast laser substrate gas temperature control
University of California at Berkeley Q Lawrence Berkeley National Laboratory NanoNano--MaterialMaterial ApplicationApplication
) Nanowire nanolaser ) Nanolaser spectra
3000 UV lasing above threshold excitation 2500 below threshold .) source 2000 .u (a 266 nm (Nd:YAG) (room temperature) 1500
ity s
n 1000 te in 500
0 ZnO nanowire: 370 375 380 385 390 395 400 wavelength (nm) natural laser cavity 106 .) .u (a ity s n te
~ 100 nm in 5 n 10 lasing o i
s spontaneous is m sapphire e ZnO nanowire -3 -2 -1 0 1 substrate 10 10 10 10 10 excitation energy (mJ) [Science 292 (2001) 1897]
University of California at Berkeley Q Lawrence Berkeley National Laboratory AcknowledgementsAcknowledgements
U. S. Department of Energy
Yanfeng Zhang Quanming Lu Peidong Yang Richard Russo Xianglei Mao
University of California at Berkeley Q Lawrence Berkeley National Laboratory