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

e-e-

e-

e-e-e-

e-

e-

e-

e-e-

e-

e-

e-

Electronic excitation

Processes in PLD

e-e-

e-

e-e-e-

e-

e-

e-

e-e-

e-

e-

e-

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)