PLDlecture3 for engineering students.ppt

PLD Lecture

1. Pulsed Laser Deposition (PLD)
Anne Reilly
College of William and Mary
Department of Physics

2. 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

3. Thin Film Deposition
Transfer atoms from a target to a vapor (or plasma) to a substrate

4. 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=D
o
exp(-
D
/kT)

D 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.

5. target
substrate
Evaporation
(Molecular beam
epitaxy-MBE)
Ways to deposit thin films
target
substrate
Chemical
vapor
deposition-
CVD
Ar
+
substrate
gas
Sputtering

6. 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

7. 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

8. CCD /PMT
spectrometer
Target
Substrates
or Faraday
cup
laser beam
Pulsed Laser Deposition

9. 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

11. 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)

12. 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

13. Processes in PLD
Laser pulse

14. Processes in PLD
e-
e-
e-
e-e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
Electronic excitation

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

16. Processes in PLD
Heat diffusion (over microseconds)
lattice

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

18. Processes in PLD
lattice
If laser pulse is long (ns) or
repetition rate is high, laser may
continue interactions

19. Processes in Pulsed Laser Deposition
1. Absorption of laser pulse in material
Q
ab
=(1-R)I
o
e
-L

(metals, absorption depths ~ 10 nm,

depends on )
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)
abp
QTK
t
T
C 


)(

20. 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)
pulse
threshold
t
cmWsx
I
22/14
104











kTmm
mm
Q
QQ
n
nn
ion
ie
ie
n
ie
n
ie
exp

21. Incredibly Non-Equilibrium!!!
At peak of laser pulse, temperatures on target can
reach >10
5
K (> 40 eV!)
Electric Fields > 10
5
V/cm, also high magnetic fields
Plasma Temperatures 3000-5000 K
Ablated Species with energies 1 –100 eV

22. 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)

23. 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 ps
Conventional melting, boiling and fracture
Threshold fluence for ablation scales as 
1/2

 < 10 ps
Electrons photoionized, collisional
and multiphoton ionization
Plasma formation with no melting
Deviation from 
1/2
scaling

24. T
A
R
G
E
T
F
I
L
M

(
d
e
p
o
s
i
t
e
d

o
n

s
i
l
i
c
o
n
)
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 J/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

25. Opportunities
Ultrahigh quality films
Circuit writing
Isotope Enrichment
New Materials
Nanoparticle production

26. 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

27. 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

28. 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
B
10
/B
11
, Ga
69
/Ga
71
in BN and GaN films
Plasma centrifuge by toroidal and axial
magnetic fields of 0.6MG!

29. 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

30. http://www.ornl.gov/~odg/#nanotubes
New 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

32. 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)