Particulate formation in laser plasma
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Particulate Formation in Laser Plasma

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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

  • 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

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

  • 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:

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


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!)






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

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

Target : Al

Laser 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 ns

Follows ~1/t – Adiabatic


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

Below 10 mTorr the plume expands freely

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)

Plume behavior in weakly collisional transition regime

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

Instabilities appear at 1.3 Torr

  • Plume decelerates

  • Instability appears

  • Intensity peaks in slower component

Plume behavior in high pressure regime

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/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

  • 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)


Condensed Particulates

Contact Surface


Modeling and experiments on homogeneous nucleation and growth of clusters

Classical theory of aerosol nucleation and growth

Convective Diffusion

and Transport

Particle Growth Rates

Homogeneous Nucleation (Becker-Doring model)

Condensation Growth


where the coagulation kernel is given by

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

  • 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

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

High ambient pressure prevents interpenetration

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

The plume front is accelerated to hypersonic velocities

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

•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

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

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 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

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

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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