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http://aries.ucsd.edu/LASERLAB. Particulate Formation in Laser Plasma. M. 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. 2003 Simposio en Fisica de Materiales

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Particulate formation in laser plasma

http://aries.ucsd.edu/LASERLAB

Particulate Formation in Laser Plasma

M. 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

2003 Simposio en Fisica de Materiales

Centro de Ciencias de Materia Condensada

Universidad National Autónoma de México

24 January 2003


Laser plasmas have numerous applications in science and industry
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


Problems in micromachining are caused by workpiece equipment contamination
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
Use of lasers in thin film deposition (PLD) is also limited by a lack of control over particulate

  • Advantages

  • Almost any material

  • Doesn’t require very low pressures

  • Reactive deposition possible

  • Multilayer epitaxial films

  • Problems to be addressed

  • Particulates

  • Quality of films depends on deposition conditions

    – Detailed study of plume dynamics is necessary

  • Lack of adequate theory


Control of nanotube and nanoparticle fabrication requires a better understanding of the production mechanisms

Schematic of nanotube synthesis (D. B. Geohegan, ORNL)

  • 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?


A wide range of physics is involved
A wide range of physics is involved: better understanding of the production mechanisms

Understanding the mechanisms of particulate formation and methods to control them will enable greater use of lasers

  • 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


Subtopics
Subtopics better understanding of the production mechanisms

Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases

Modeling and experiments on homogeneous nucleation and growth of clusters

(surface ejection is another topic of interest to us!)


0.01Torr better understanding of the production mechanisms

0.1Torr

1Torr

10Torr

100Torr

Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases


Lasers used in ucsd laser plasma and laser matter interactions laboratory
Lasers used in UCSD Laser Plasma and Laser-Matter Interactions Laboratory

Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm

Lambda Physik 420 mJ, 20 ns multi-gas excimer

laser (248 nm with KrF)


Experimental setup for studies of ablation plume dynamics
Experimental setup for studies of ablation plume dynamics Interactions Laboratory

Target : Al

Laser Intensity : 5 GW cm-2

Ambient : 10-8 Torr – 100 Torr air


Approximate plasma parameters
Approximate plasma parameters Interactions Laboratory

Electron Density:

Measured using Stark broadening

Initial ~ 1019cm-3

Falls very rapidly within 200 ns

Follows ~1/t – Adiabatic

Temperature:

Measured from line intensity ratios

Initial ~8 eV

falls very rapidly

(Experiment Parameters: 5 GW cm-2, 150 mTorr air)


Plume behavior at low pressure
Plume behavior at low pressure Interactions Laboratory


Below 10 mtorr the plume expands freely
Below 10 mTorr the plume expands freely Interactions Laboratory

P = 10-6 Torr

P = 10-2 Torr

Laser intensity = 5 GWcm-2, Intensification time = 2 ns

Each image is obtained from a single laser pulse

Plume edge maintains a constant velocity (~ 107cm/s)



The plume splits and sharpens at 150 mtorr
The plume splits and sharpens at 150 mTorr Interactions Laboratory

  • 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



Instabilities appear at 1 3 torr
Instabilities appear at 1.3 Torr Interactions Laboratory

  • Plume decelerates

  • Instability appears

  • Intensity peaks in slower component



Above 10 torr the plume remains confined
Above 10 Torr the plume remains confined Interactions Laboratory

P = 10 Torr air

P = 100 Torr Air


Summary of plume dynamics vs pressure
Summary of plume dynamics vs. pressure Interactions Laboratory

Fitting Models:

  • Free expansion:

    R ~ t

  • Shock Model:

    R ~ (Eo/ro)1/5 t2/5

  • Drag Model:

    R = Ro(1–e-bt)

  • Best fit at 150 mTorr

    R ~ t0.445

(Harilal et. al, Journal of Physics D, 35, 2935, 2002)


Using both spectroscopy and imaging a triple fold plume structure was observed
Using both spectroscopy and imaging, a triple-fold plume structure was observed

  • 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

Al (396nm) at 18 mm

in 150 mTorr air

(Harilal et. al, J. Applied Physics, in press, 2003)


Shock structure was observed

Condensed Particulates

Contact Surface

Target

Modeling and experiments on homogeneous nucleation and growth of clusters


Classical theory of aerosol nucleation and growth
Classical theory of aerosol nucleation and growth structure was observed

Convective Diffusion

and Transport

Particle Growth Rates

Homogeneous Nucleation (Becker-Doring model)

Condensation Growth

Coagulation

where the coagulation kernel is given by


Dependence of homogeneous nucleation rate and critical radius on saturation ratio
Dependence of homogeneous nucleation rate and critical radius on saturation ratio

Reduction in S due to condensation shuts down HNR quickly; Competition between homogeneous and heterogeneous condensation determines final size and density distribution

  • High saturation ratios result from rapid cooling from adiabatic plume expansion

  • Extremely small critical radius results


Effect of ionization on cluster nucleation
Effect of ionization on cluster nucleation radius on saturation ratio

  • 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
A 1-D multi-physics model was developed radius on saturation ratio

Target : Si

Laser Intensity : 107–109 W cm-2

Ambient : 500 mTorr He

  • Laser absorption

  • Thermal response

  • Evaporation flux

  • Transient gasdynamics

  • Radiation transport

  • Condensation

  • Ionization/recombination

absorption Ioe–lx, 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


Model prediction of expansion dynamics
Model prediction of expansion dynamics radius on saturation ratio

High ambient pressure prevents interpenetration

(in any case, the 2-fluid model lacks kinetic effects)


The plume front is accelerated to hypersonic velocities
The plume front is accelerated to hypersonic velocities radius on saturation ratio

Thermal energy is converted into kinetic energy; collisions also appear to transfer energy from the bulk of the plume to the plume front

~62 eV

~2 eV

Surface temperature and laser irradiance vs. time

Spatial distribution of nucleation and growth rates at 500 ns


Model prediction of cluster birth and growth
Model prediction of cluster birth and growth radius on saturation ratio

• Clusters are born at the contact surface and grow behind it• Nucleation shuts down rapidly as the plume expands

Spatial distribution of nucleation (*) and growth (o) rates at 500 ns

Time-dependence of growth rate/birth rate


Besides spectroscopy and langmuir probes witness plates served as our primary diagnostic
Besides spectroscopy and Langmuir probes, witness plates served as our primary diagnostic

Witness plate preparation technique:

  • Start with single crystal Si

  • HF acid dip to strip native oxide

  • Spin, rinse, dry

  • Controlled thermal oxide growth at 1350 K to ~1mm, 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


Measurement of final condensate size
Measurement of final condensate size served as our primary diagnostic

500 mTorr He

5x108 W/cm2

5x109 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?

5x108 W/cm2

5x109 W/cm2


Saturation ratio and charge state derived from experimental measurements
Saturation ratio and charge state derived from experimental measurements

• Saturation ratio is inversely related to laser intensity!

Saturation ratio derived from spectroscopy, assuming LTE

Maximum ionization state derived from spectroscopy, assuming LTE


Cluster size distribution comparison of theory experiment
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
Summary and future work experiment

  • 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

research supported by the US Department of Energy, Office of Fusion Energy Sciences and the Hewlett Packard Company, Printing and Imaging Group


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