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Laser Plasma and Laser-Matter Interactions Laboratory
Particulate Formation in Laser
PlasmaM. S. Tillack, S. S. Harilal, C. V. Bindhu and D. Blair
Center for Energy Research and Mechanical and Aerospace Engineering Department
Jacobs School of Engineering
http://aries.ucsd.edu/LASERLAB
2003 Simposio en Fisica de MaterialesCentro de Ciencias de Materia CondensadaUniversidad National Autónoma de México
24 January 2003
Laser plasmas have numerous applications in science and industry
Micromachining Thin film deposition Cluster production Nanotube production Surface modification Surface cleaning Elemental analysis X-ray laser Photolithography Medicine Inertial fusion energy
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Problems in micromachining are caused by workpiece & equipment contamination
Contaminated surface After cleaning
Laser ink-jet printer head before and after cleaning (courtesy of HP)
Laser entrance window on our ablation chamber
Use of lasers in thin film deposition (PLD) is also limited by a lack of control over particulate
Problems to be addressed Particulates Quality of films depends on deposition conditions – Detailed study of plume dynamics is
necessary Lack of adequate theory
Advantages Almost any material Doesn’t require very low
pressures Reactive deposition possible Multilayer epitaxial films
Control of nanotube and nanoparticle fabrication requires a better understanding of the production mechanisms
Understanding why laser ablation produces such high nanotube yields is a high priority
Species responsible for growth? Spatial distribution and transport? Growth times and rates?
Schematic of nanotube synthesis (D. B. Geohegan, ORNL)
08 ns1000 nsSurface absorptionThermal conductionSurface meltingVaporizationMultiphoton ionization Plasma ignitionExplosive phase change
Plasma absorptionSelf-regulating heat transferAdiabatic expansion Ambient interpenetrationVapor coolingRapid condensationPlume stagnation
Absorption, reflection Heat transfer Thermodynamics (phase change) Plasma breakdown Shock waves (gas) Stress waves (solid) Laser-plasma interactions Gas dynamic expansion Atomic & molecular processes Others
Understanding the mechanisms of particulate formation and methods to control them will enable greater use of lasers
A wide range of physics is involved:
Subtopics
1. Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases
2. Modeling and experiments on homogeneous nucleation and growth of clusters
(surface ejection is another topic of interest to us!)
Surface absorptionThermal conductionSurface meltingVaporizationMultiphoton ionization Plasma ignitionExplosive phase change
Plasma absorptionSelf-regulating heat transferAdiabatic expansion Ambient interpenetrationVapor coolingRapid condensationPlume stagnation
1. Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases
0.01Torr
1Torr
0.1Torr
10Torr
100Torr
Lasers used in UCSD Laser Plasma and Laser-Matter Interactions Laboratory
Lambda Physik 420 mJ, 20 ns multi-gas excimerlaser (248 nm with KrF)
Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm
Experimental setup for studies of ablation plume dynamics
Target : AlLaser Intensity : 5 GW cm-2
Ambient : 10-8 Torr – 100 Torr air
Approximate plasma parameters
Electron Density:Measured using Stark broadening
Initial ~ 1019cm-3
Falls very rapidly within 200 nsFollows ~1/t – Adiabatic
Temperature: Measured from line intensity ratios
Initial ~8 eVfalls very rapidly
(Experiment Parameters: 5 GW cm-2, 150 mTorr air)
Plume behavior at low pressure
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Below 10 mTorr the plume expands freely
Laser intensity = 5 GWcm-2, Intensification time = 2 nsEach image is obtained from a single laser pulsePlume edge maintains a constant velocity (~ 107cm/s)
P = 10-6 Torr P = 10-2 Torr
Plume behavior in weakly collisional transition regime
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The plume splits and sharpens at 150 mTorr
• Strong interpenetration of the laser plasma and the ambient low density gas
• Observed plume splitting and sharpening.
• This pressure range falls in the region of transition from collisionless to collisional interaction of the plume species with the gas
• Enhanced emission from all species
Plume behavior in strongly collisional transition regime
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Instabilities appear at 1.3 Torr
Plume decelerates Instability appears Intensity peaks in
slower component
Plume behavior in high pressure regime
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Above 10 Torr the plume remains confined
P = 10 Torr air P = 100 Torr Air
Summary of plume dynamics vs. pressure
Fitting Models:
Free expansion: R ~ t
Shock Model:R ~ (Eo/o)1/5
t2/5
Drag Model: R = Ro(1–e-bt)
Best fit at 150 mTorrR ~ t0.445
(Harilal et. al, Journal of Physics D, 35, 2935, 2002)
Using both spectroscopy and imaging, a triple-fold plume structure was observed
Al (396nm) at 18 mmin 150 mTorr air
First peak in the TOF is not seen in the imaging studies
– low dynamic range of ICCD?
The delayed peak does not match well with a SMB fit
A convolution of two SMB fits matches well
Faster peak – a small group of high KE – suffer negligible background collisions
Slower peak – undergo numerous collisions with background and decelerate (Harilal et. al, J. Applied Physics, in press, 2003)
2. Modeling and experiments on homogeneous nucleation and growth of clusters
Target
Contact Surface
Shock
Condensed Particulates
Classical theory of aerosol nucleation and growth
Homogeneous Nucleation (Becker-Doring model)
∂n∂t[ ]growth,
homo=
Psat
kT
⎛ ⎝ ⎜
⎞ ⎠ ⎟
2 2σmπ
⎛ ⎝ ⎜
⎞ ⎠ ⎟
1/ 2 S2
ρl
exp−πσdcrit
2
3kT
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥ δ Vcrit( ),
#
m3s
1
m3
⎡ ⎣ ⎢
⎤ ⎦ ⎥ dcrit =
4σmρlkTlnS
, and Vcrit =π6dcrit
3
∂n∂t[ ]growth,
hetero=−∂I
∂ V( ) =−∂∂ V( ) n∂
∂t V( )( ), #
m3s
1
m3
⎡ ⎣ ⎢
⎤ ⎦ ⎥ ∂
∂t V( ) =2π π6( )
1/ 3 S−K( )PsatDdp
kTVmolF,
m3
s
⎡
⎣ ⎢
⎤
⎦ ⎥
Condensation Growth
Coagulation
∂n∂t[ ]coag
=12
β V*,V−V *( )n(V*)n(V−V*)dV*0
V
∫ − β V,V*( )n(V)n(V*)dV*0
∞
∫
β V,V*( ) =2π D+D*( ) dp +dp*
( )Fcoagwhere the coagulation kernel is given by
Convective Diffusionand Transport
∂n
∂t+∇ • nv v ( ) −∇ • D∇n( ) +∇ •
v c n= ∂n
∂t[ ]growth,homo
+ ∂n∂t[ ]growth,
hetero+ ∂n
∂t[ ]coag
Particle Growth Rates
Dependence of homogeneous nucleation rate and critical radius on saturation ratio
• High saturation ratios result from rapid cooling from adiabatic plume expansion
• Extremely small critical radius results
Reduction in S due to condensation shuts down HNR quickly; Competition between homogeneous and heterogeneous condensation determines final size and density distribution
ΔG =4πr3
3Vm
(μL −μv)+4πσr2
Effect of ionization on cluster nucleation
ΔG =4π3Vm
(r3 −ra3)(μL −μv)+4πσ (r2 −ra
2)+e2
2(1−ε−1)(r−1 −ra
−1)
• Ion jacketing results in an offset in free energy (toward larger r*)
• Dielectric constant of vapor reduces free energy
Cluster birthrate vs. saturation ratio(Si, 109 W/cm2, 1% ionization)
A 1-D multi-physics model was developed
Laser absorption Thermal response Evaporation flux
Transient gasdynamics Radiation transport Condensation Ionization/recombination
absorption Ioe–x, w/plasma shielding
cond., convection, heat of condensation
2-fluid Navier-Stokes
simple Stephan-Boltzmann model
modified Becker-Doring model
modified Saha, 3-body recombination
j =M2π
Γσ c
pv
RTv
−σ e
psat
RTf
⎛
⎝ ⎜
⎞
⎠ ⎟
Target : SiLaser Intensity : 107–109 W cm-2
Ambient : 500 mTorr He
Model prediction of expansion dynamics
High ambient pressure prevents interpenetration(in any case, the 2-fluid model lacks kinetic effects)
The plume front is accelerated to hypersonic velocities
Spatial distribution of nucleation and growth rates at 500 ns
~62 eV
Thermal energy is converted into kinetic energy; collisions also appear to transfer energy from the bulk of the plume to the
plume front
Surface temperature and laser irradiance vs. time
~2 eV
Model prediction of cluster birth and growth
Spatial distribution of nucleation (*) and growth (o) rates at 500 ns
Time-dependence of growth rate/birth rate
•Clusters are born at the contact surface and grow behind it
•Nucleation shuts down rapidly as the plume expands
Besides spectroscopy and Langmuir probes, witness plates served as our primary diagnostic
• Start with single crystal Si• HF acid dip to strip native
oxide• Spin, rinse, dry• Controlled thermal oxide
growth at 1350 K to ~1m, 4 Å roughness
• Ta/Au sputter coat for SEM• Locate witness plate near
plume stagnation point
Witness plate prior to exposure, showing a single defect in the native crystal structure
Witness plate preparation technique:
Measurement of final condensate size
500 mTorr He
5x108 W/cm2 5x109 W/cm2
5x109 W/cm25x108 W/cm2
5x107 W/cm2
• Good correlation between laser intensity and cluster size is observed.
• Is it due to increasing saturation ratio or charge state?
Saturation ratio and charge state derived from experimental measurements
Maximum ionization state derived from spectroscopy, assuming LTE
Saturation ratio derived from spectroscopy, assuming LTE
•Saturation ratio is inversely related to laser intensity!
Cluster size distribution – comparison of theory & experiment
The discrepancy at low irradiance is believed to be caused by anomolously high charge state induced by free electrons
Summary and future work
• We have obtained a better understanding of the mechanisms which form particulate in laser plasma– Clusters in the size range from 5-50 nm are routinely produced at
moderate laser intensity– Model predictions appear to match experimental data
• In-situ particle measurements (scattering, spectroscopy) would be very useful to further validate the mechanisms
• Better control of size distribution and enhanced yield are desired
• Model improvements are needed: 2-D, kinetic treatment• Applications of nanoclusters & quantum dots will be explored
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research supported by the US Department of Energy, Office of Fusion Energy Sciences and the Hewlett Packard Company, Printing and Imaging Group