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