Pulsed Laser Deposition (PLD)

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Pulsed Laser Deposition (PLD)

• Anne Reilly
• College of William and Mary
• Department of Physics

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Outline
1. Thin film deposition 2. Pulsed Laser
Deposition a) Compared to other growth
techniques b) Experimental Setup c) Advantages
and Disadvantages 3. Basic Theory of PLD 4.
Opportunities
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Thin Film Deposition
Transfer atoms from a target to a vapor (or
plasma) to a substrate
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Thin Film Deposition
Transfer atoms from a target to a vapor (or
plasma) to a substrate
After an atom is on surface, it diffuses
according to DDoexp(-eD/kT) eD is the
activation energy for diffusion 2-3 eV kT is
energy of atomic species. Want sufficient
diffusion for atoms to find best sites. Either
use energetic atoms, or heat the substrate.
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Ways to deposit thin films
substrate
substrate
Chemical vapor deposition-CVD
Ar
target
target
Sputtering
Evaporation (Molecular beam epitaxy-MBE)
substrate
gas
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• Low energy deposition
• (MBE) 0.1 eV
• may get islanding unless
• you pick right substrate or
• heat substrate to high
• temperatures

High energy deposition (Sputtering 1
eV) smoother films at lower substrate
temperatures, but may get intermixing
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• Low energy deposition
• (MBE) 0.1 eV
• may get islanding unless
• you pick right substrate or
• heat substrate to high
• temperatures

High energy deposition (Sputtering 1
eV) smoother films at lower substrate
temperatures, but may get intermixing
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Pulsed Laser Deposition
CCD /PMT spectrometer
laser beam
Substrates or Faraday cup
Target
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Pulsed Laser Deposition
CCD /PMT spectrometer
laser beam
Substrates or Faraday cup
Target
Target Just about anything! (metals,
semiconductors) Laser Typically excimer (UV, 10
nanosecond pulses) Vacuum Atmospheres to
ultrahigh vacuum
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Advantages of PLD

• Flexible, easy to implement
• Growth in any environment
• Exact transfer of complicated materials (YBCO)
• Variable growth rate
• Epitaxy at low temperature
• Resonant interactions possible (i.e., plasmons in
  metals, absorption peaks in dielectrics and
  semiconductors)
• Atoms arrive in bunches, allowing for much more
  controlled deposition
• Greater control of growth (e.g., by varying laser
  parameters)

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Disadvantages of PLD

• Uneven coverage
• High defect or particulate concentration
• Not well suited for large-scale film growth
• Mechanisms and dependence on parameters not well
  understood

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Processes in PLD

Laser pulse
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Processes in PLD

Electronic excitation
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Processes in PLD

e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
lattice
e-
e-
e-
Energy relaxation to lattice (1 ps)
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Processes in PLD

lattice
Heat diffusion (over microseconds)
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Processes in PLD

lattice
Melting (tens of ns), Evaporation, Plasma
Formation (microseconds), Resolidification
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Processes in PLD

lattice
If laser pulse is long (ns) or repetition rate is
high, laser may continue interactions
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Processes in Pulsed Laser Deposition 1.
Absorption of laser pulse in material Qab(1-R)Ioe
-aL (metals, absorption depths 10 nm, depends
on l) 2. Relaxation of energy ( 1 ps)
(electron-phonon interaction) 3. Heat transfer,
Melting and Evaporation when electrons and
lattice at thermal equilibrium (long pulses) use
heat conduction equation (or heat diffusion
model)
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Processes in Pulsed Laser Deposition 4. Plasma
creation threshold intensity goverened by
Saha equation 5. Absorption of light by plasma,
ionization (inverse Bremsstrahlung) 6.
Interaction of target and ablated species with
plasma 7. Cooling between pulses (Resolidificatio
n between pulses)
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Incredibly Non-Equilibrium!!! At peak of laser
pulse, temperatures on target can reach gt105 K
(gt 40 eV!) Electric Fields gt 105 V/cm, also high
magnetic fields Plasma Temperatures 3000-5000
K Ablated Species with energies 1 100 eV
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PLD with Ultrafast Pulses (lt 1 picosecond) see
Stuart et al., Phys. Rev. B, 53 1749 (1996) A
new research area! If the pulse width lt electron
lattice-relaxation time, heat diffusion, melting
significantly reduced! Means cleaner holes and
cleaner ablation Direct conversion of solid to
vapor, less plasma formation Reactive chemistry
energetic ions, ionized nitrogen, high charge
states Leads to less target damage (cleaner
holes), and smoother films (less particulates)
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PLD with Ultrafast Pulses (lt 1 picosecond) see
Stuart et al., Phys. Rev. B, 53 1749 (1996) A
new research area! If the pulse width lt electron
lattice-relaxation time, heat diffusion, melting
significantly reduced! Means cleaner holes and
cleaner ablation Direct conversion of solid to
vapor, less plasma formation Reactive chemistry
energetic ions, ionized nitrogen, high charge
states Leads to less target damage (cleaner
holes), and smoother films (less particulates)
t lt 10 ps Electrons photoionized, collisional
and multiphoton ionization Plasma formation with
no melting Deviation from t1/2 scaling
tgt 50 ps Conventional melting, boiling and
fracture Threshold fluence for ablation scales as
t1/2
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20 ns EXCIMER versus 1 ps TJNAF-FEL
Cobalt 20 mJ/pulse, 20 ns, 308 nm, 25 Hz, 1 x
10-5 Torr
Steel, 20 mJ/pulse, 18 MHz, 3.1 micron 1 x 10-2
Torr, 60 Hz pulsed, rastered beam

Less melting!
TARGET
Few particulates! for Nb lt 1 per cm-2
FILM (deposited on silicon)
SEMs by B. Robertson, T. Wang, TJNAF
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Opportunities Ultrahigh quality films Circuit
writing Isotope Enrichment New
Materials Nanoparticle production
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Magnetic Moment of fcc Fe(111) Ultrathin Films
by Ultrafast Deposition on Cu(111) J. Shen et
al., Phys. Rev. Lett., 80, pp. 1980-1983
MBE
PLD
Higher quality films, better magnetic properties
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• MICE
• Direct writing of electronic components- in air!
• Rapid process refinement
• No masks, preforms, or long cycle times
• True 3-D structure fabrication possible
• Single laser does surface pretreatment, spatially
  selective material deposition, surface annealing
  ,component trimming, ablative micromachining,
  dicing and via-drilling

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Isotope Enrichment in Laser-Ablation Plumes and
Commensurately Deposited Thin Films P. P.
Pronko, et al. Phys Rev. Lett., 83, pp. 2596-2599

Over twice the natural enrichment of B10/B11,
Ga69/Ga71 in BN and GaN films Plasma centrifuge
by toroidal and axial magnetic fields of 0.6MG!
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Transient States of Matter during Short Pulse
Laser Ablation K. Sokolowski-Tinten et al., Phys.
Rev. Lett., 81, pp. 224-227
Fluid material state of high index of refraction,
optically flat surface
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New Materials and Nanoparticles D.B. Geohegan-ORNL
http//www.ornl.gov/odg/nanotubes
Study of plasma plume and deposition of carbon
materials
Carbon/carbon collisons-buckyballs
Fast carbon ions- diamond films
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References Pulsed Laser Vaporization and
Deposition, Wilmott and Huber, Reviews of Modern
Physics, Vol. 72, 315 (2000) Pulsed Laser
Deposition of Thin Films, Chrisey and Hubler
(Wiley, New York, 1994) Laser Ablation and
Desorption, Miller and Haglund (Academic Press,
San Diego, 1998)