Download - Pld Lecture 2
Pulsed Laser Deposition (PLD)
Outline1. Thin film deposition
2. Pulsed Laser Depositiona) Compared to other growth techniquesb) Experimental Setupc) Advantages and Disadvantages
3. Basic Theory of PLD
4. Opportunities
Thin Film DepositionTransfer atoms from a target to a vapor (or plasma) to a substrate
Thin Film DepositionTransfer atoms from a target to a vapor (or plasma) to a substrate
After an atom is on surface, it diffuses according to: D=Doexp(-D/kT)D is the activation energy for diffusion ~ 2-3 eVkT is energy of atomic species.
Want sufficient diffusion for atoms to find best sites. Either use energetic atoms, or heat the substrate.
target
substrate
Evaporation
(Molecular beam epitaxy-MBE)
Ways to deposit thin films
target
substrate
Chemical vapor deposition-CVD
Ar+
substrate
gas
Sputtering
Low energy deposition(MBE): ~0.1 eV
may get islanding unlessyou pick right substrate orheat substrate to hightemperatures
High energy deposition (Sputtering ~ 1 eV)
smoother films at lower substrate temperatures, but may get intermixing
Low energy deposition(MBE): ~0.1 eV
may get islanding unlessyou pick right substrate orheat substrate to hightemperatures
High energy deposition (Sputtering ~ 1 eV)
smoother films at lower substrate temperatures, but may get intermixing
CCD /PMT
spectrometer
Target
Substrates or Faraday
cup
laser beam
Pulsed Laser Deposition
CCD /PMT
spectrometer
Target
Substrates or Faraday
cup
laser beam
Pulsed Laser Deposition
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
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Electronic excitation
Processes in PLD
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Energy relaxation to lattice (~1 ps)
lattice
Processes in PLD
Heat diffusion (over microseconds)
lattice
Processes in PLD
Melting (tens of ns), Evaporation, Plasma Formation (microseconds), Resolidification
lattice
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 materialQab=(1-R)Ioe-L
(metals, absorption depths ~ 10 nm, depends on )
2. Relaxation of energy (~ 1 ps) (electron-phonon interaction)
3. Heat transfer, Melting and Evaporationwhen electrons and lattice at thermal equilibrium (long pulses)use heat conduction equation:
(or heat diffusion model)abp QTK
tTC )(
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(Resolidification between pulses)
pulsethreshold t
cmWsxI22/14104
kTmmmm
QQQ
nnn ion
ie
ie
n
ie
n
ie exp
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)
> 50 psConventional melting, boiling and fractureThreshold fluence for ablation scales as 1/2
< 10 psElectrons photoionized, collisional and multiphoton ionization Plasma formation with no melting Deviation from 1/2 scaling
TAR
GET
FILM
(d
epos
ited
on si
licon
)20 ns EXCIMER versus 1 ps TJNAF-FELCobalt ~20 mJ/pulse, 20 ns, 308 nm,25 Hz, 1 x 10-5 Torr
Steel, ~20 J/pulse, 18 MHz, 3.1 micron1 x 10-2 Torr, 60 Hz pulsed, rastered beam
Less melting!
Fewparticulates!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 AblationK. Sokolowski-Tinten et al., Phys. Rev. Lett., 81, pp. 224-227
Fluid material state of high index of refraction, optically flat surface
http://www.ornl.gov/~odg/#nanotubesNew Materials and Nanoparticles
D.B. Geohegan-ORNL
Carbon/carbon collisons-buckyballs
Fast carbon ions- diamond films
Study of plasma plume and deposition of carbon materials
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)