Pulsed Laser Deposition (PLD)
Anne Reilly
College of William and Mary
Department of Physics
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
Thin Film Deposition
Transfer atoms from a target to a vapor (or plasma) to a substrate
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: D=Doexp(-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.
Ways to deposit thin films
substrate
target
substrate
Ar+
Chemical
vapor
depositionCVD
target
Sputtering
Evaporation
(Molecular beam
epitaxy-MBE)
substrate
gas
Low energy deposition
(MBE): ~0.1 eV
High energy deposition
(Sputtering ~ 1 eV)
may get islanding unless
you pick right substrate or
heat substrate to high
temperatures
smoother films at lower
substrate temperatures, but
may get intermixing
Low energy deposition
(MBE): ~0.1 eV
High energy deposition
(Sputtering ~ 1 eV)
may get islanding unless
you pick right substrate or
heat substrate to high
temperatures
smoother films at lower
substrate temperatures, but
may get intermixing
Pulsed Laser Deposition
CCD /PMT
spectrometer
laser beam
Substrates
or Faraday
Target
cup
Pulsed Laser Deposition
CCD /PMT
spectrometer
laser beam
Substrates
or Faraday
Target
cup
Target: Just about anything! (metals, semiconductors…)
Laser: Typically excimer (UV, 10 nanosecond pulses)
Vacuum: Atmospheres to ultrahigh vacuum
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)
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
Processes in PLD
Laser pulse
Processes in PLD
eee- e- ee- ee-e- e-eeee-
Electronic excitation
Processes in PLD
lattice
eee- e- ee- ee-e- e-eeee-
Energy relaxation to lattice (~1 ps)
Processes in PLD
lattice
Heat diffusion (over microseconds)
Processes in PLD
lattice
Melting (tens of ns), Evaporation, Plasma
Formation (microseconds), Resolidification
Processes in PLD
lattice
If laser pulse is long (ns) or
repetition rate is high, laser may
continue interactions
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)
T
C p
   ( KT )  Qab
t
Processes in Pulsed Laser Deposition
4. Plasma creation
threshold intensity:
4 x 104Ws1 / 2cm 2
I threshold 
t pulse
goverened by Saha equation: ne ni  QeQi memi exp   ion 


nn
Qn me  mi

kT 
5. Absorption of light by plasma, ionization
(inverse Bremsstrahlung)
6. Interaction of target and ablated species with plasma
7. Cooling between pulses
(Resolidification between pulses)
Incredibly Non-Equilibrium!!!
At peak of laser pulse, temperatures on target can
reach >105 K (> 40 eV!)
Electric Fields > 105 V/cm, also high magnetic fields
Plasma Temperatures 3000-5000 K
Ablated Species with energies 1 –100 eV
PLD with Ultrafast Pulses (< 1 picosecond)
see Stuart et al., Phys. Rev. B, 53 1749 (1996)
A new research area!
If the pulse width < 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)
PLD with Ultrafast Pulses (< 1 picosecond)
see Stuart et al., Phys. Rev. B, 53 1749 (1996)
A new research area!
If the pulse width < 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> 50 ps
Conventional melting, boiling and fracture
Threshold fluence for ablation scales as t1/2
t < 10 ps
Electrons photoionized, collisional
and multiphoton ionization
Plasma formation with no melting
Deviation from t1/2 scaling
20 ns EXCIMER
Cobalt ~20 mJ/pulse, 20 ns, 308 nm,
25 Hz, 1 x 10-5 Torr
versus 1 ps TJNAF-FEL
Steel, ~20 mJ/pulse, 18 MHz, 3.1 micron
1 x 10-2 Torr, 60 Hz pulsed, rastered beam
Less melting!
Few
particulates!
for Nb: < 1 per cm-2
SEMs by B. Robertson, T. Wang, TJNAF
Opportunities
Ultrahigh quality films
Circuit writing
Isotope Enrichment
New Materials
Nanoparticle production
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
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
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!
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
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 collisonsbuckyballs
Fast carbon ionsdiamond films
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)