Graduate School at MC2 2015
course:
Micro- and Nanoprocessing Technologies
lecture:
Pulsed laser deposition of thin films of
functional materials
Lecturer: Andrei Vorobiev
Course responsible: Ulf Södervall
1
Objectives
You will learn about:
• principles of pulsed laser deposition PLD of thin films
• functional materials
• features of PLD of functional materials
Literature:
1) H. M. Christen and G. Eres, Recent advances in pulsed-laser deposition of complex oxides,
J. Phys.: Condens. Matter 20 (2008) 264005 (16pp)
2) Pulsed laser deposition of thin films: applications-led growth of functional materials /
edited by Robert Eason, N.J., Wiley, cop. 2007
3) Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler,
New York, Wiley, cop. 1994
2
Outline
• History and fundamentals of PLD
• PLD of functional materials
• Equipment
• Advantages and limitations
• Mechanisms of pulsed Laser sputtering
• Film nucleation and growth in PLD
• Splashing and forward peaking
• PLD of HTS and ferroelectric thin films
• Summary
3
Definitions
Laser
is an electronic-optical device that produces coherent radiation.
The term is acronym for Light Amplification by Stimulated Emission of Radiation.
Pulsed laser deposition (PLD)
is a physical vapor deposition technique where a high power pulsed laser beam
is focused to strike a target of the desired composition. Material is then
vaporized and deposited as a thin film on a substrate facing the target. This
process can occur in ultra high vacuum or in the presence of a background gas,
such as oxygen when depositing films of oxides.
Functional materials
are materials having physical properties sensitive to the external effects
(temperature, electric and magnetic fields, pressure etc.).
4
Evolution of laser technology and its applications
Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 2.
5
Historical Development of PLD
1917 - Albert Einstein postulated the stimulated emission process.
1960 - Maiman constructed the first ruby laser.
1962 - Breech and Cross used ruby laser to vaporize and excite atoms from a solid.
1965 - Smith and Turner used ruby laser to deposit thin films.
1970s - i) reliable electronic Q-switches for generating very short pulses;
ii) high-efficiency second harmonic generators for shorter wavelength.
1987 - PLD in Bellcore group used successfully to grow HTS YBCO.
1990 - PLD growth of ferroelectric Bi based perovskite oxide films in Ramesh group.
1990s - PLD production related issues concerning reproducibility and large-area
scale up have begun to be addressed.
2000s - Numerous device applications based on PLD films of functional materials
(YBCO, BSTO etc.) are being explored.
6
Functional materials
function of
Functional material
properties:
• permittivity
• permeability
• resistivity
• refractivity
• sound velocity
•…
External parameters:
• temperature
• pressure
• electric field
• magnetic field
• optical wavelength
• absorbed gas
• pH value
•…
• Utilizing a functional material offers higher functionality of a system.
• Science and technology rely heavily on the development of functional materials.
7
Mobile Convergence
Portable
Media
MP3
PDA
Smart Phone
Cellular
Phones
Convergent devices
DSC
Gaming
Mobile
Imaging
Video
8
Adapted from Philips
Functional materials
• Ferroelectric………………………….. BaxSr1-xTiO3
• High temperature superconductor….YBa2Cu3O7
• Magnetic field sensor……………….. La1-xCaxMnO3
• Surface acoustic wave sensor………LiNbO3
• Liquid petroleum gas sensor………...Pd-doped SnO2
• High temperature piezoelectric………Ta2O5
• Fast-ion conductor…………………….Y2(SnyTi1-y)2O7
•…
• A wide range of functional materials are complex oxides.
• A key requirement in preparations is to control compositional evolution.
• A unique feature of PLD is stoichiometric preservation of composition.
9
PLD of Functional materials
x
Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler, New York, Wiley, 1994
• PLD and S are the most appropriate techniques for deposition of complex oxides.
• PLD reproduces target stoichiometry in an oxidizing ambient.
10
Concept of PLD
PLD stages:
• laser ablation of target
• dynamic of plasma
• film nucleation and growth
Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 3.
• The laser-target interaction: electromagnetic energy is converted into electronic
excitation and then into thermal/mechanical energy to cause ablation.
• A plume: atoms, molecules, electrons, ions, clusters, particles, and molten globules.
• The plume expands with hydrodynamic flow characteristics.
11
Growth of BSTO films by PLD
Vacuum chamber
0.4 mbar O2
Laser
BSTO target
Calas PLD System MC2ProcessLab, Chalmers
1 Hz
Heater at 650C
12
Advantages-Disadvantages of PLD
Advantages
Disadvantages
• versatile method (any material)
• splashing of micron-sized particulates
• congruent evaporation
• small area of uniformity (1 cm2)
• high deposition rates (10s nm/min)
• non-conformal coverage
• clean process
• extremely complex models hinder
• plume at high energy
theory based improvements
• reactive gases (oxygen)
• broad range of gas pressures
• To a large extent the two first problems have been solved.
• Congruent (stoichiometric) evaporation – main advantage for PLD of films of
functional materials.
13
Laser basics
Light Amplification by Stimulated Emission of Radiation
• When perturbed by a photon matter may create another photon.
• The second photon has the same direction, frequency, phase and polarization.
• The first photon is not destroyed (no absorption) - light amplification.
14
Laser basics (cont.)
laser major parts
Pump source
Gain medium
1) The pump source provides energy to the gain medium.
2) The gain medium transfers energy into the laser beam amplified by
stimulated emission.
3) The optical resonator provides a narrow, low-divergence beam.
15
Wavelengths of common lasers
Laser Type
Argon fluoride (Excimer-UV)
Krypton chloride (Excimer-UV)
Krypton fluoride (Excimer-UV)
Xenon chloride (Excimer-UV)
Xenon fluoride (Excimer-UV)
Helium cadmium (UV)
Nitrogen (UV)
Helium cadmium (violet)
Krypton (blue)
Argon (blue)
Copper vapor (green)
Argon (green)
Krypton (green)
Frequency doubled
Nd YAG (green)
Helium neon (green)
Krypton (yellow)
Copper vapor (yellow)
Wavelength (mm)
0.193
0.222
0.248
0.308
0.351
0.325
0.337
0.441
0.476
0.488
0.510
0.514
0.528
0.532
Helium neon (yellow)
Helium neon (orange)
Gold vapor (red)
Helium neon (red)
Krypton (red)
Rohodamine 6G dye (tunable)
Ruby (CrAlO3) (red)
Gallium arsenide (diode-NIR)
Nd:YAG (NIR)
Helium neon (NIR)
Erbium (NIR)
Helium neon (NIR)
Hydrogen fluoride (NIR)
Carbon dioxide (FIR)
Carbon dioxide (FIR)
0.594
0.610
0.627
0.633
0.647
0.570-0.650
0.694
0.840
1.064
1.15
1.504
3.39
2.70
9.6
10.6
0.543
0.568
0.570
• The PLD range is 200 - 400 nm (absorption by matter is strong enough).
• Excimer laser uses “excited dimer” pseudo-molecules for the gain media.
16
Excimer laser basics
KrF electronic potential
excimers formation reactions
Ar+ + 2Ar
Ar2+ + Kr
Ar2+ + Ar charge-transfer
Kr+ + 2Ar
Kr* + eˉ
Kr + eˉ
excitation
Kr+
Kr* + F2
+ 2 eˉ
(KrF)* + F harpooning
Kr+ + Fˉ + He
(KrF)* + He recombination
• Excimers are only stable in excited states.
• If excimers are generated, the medium is automatically in population inversion
with the unstable ground state.
• Technical implementation: gas mixtures in high-voltage gas discharge
17
Commercial excimer laser design
cross sectional view of excimer laser tube
• Pulsed mode provides non-equilibrium vaporization (10000 K in 10 nm surface layer).
• KrF Lambda Physik, Inc. typical parameters: =248 nm, =30 ns, r=10 Hz, E0=1.5 J/cm2
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Advantages of PLD
Advantages
• versatile method (any material)
• congruent evaporation
Versatile method
E = (2Φ/c0n)1/2
• high deposition rates (10s nm/min)
Φ – power density (109 W/cm2)
• clean process
c – velocity of light
• plume at high energy
0 – permittivity of vacuum
• reactive gases (oxygen)
n – refractive index (1.5)
• broad range of gas pressures
E – electric field (106 V/cm)
• The electric field inside the material (106 V/cm) is sufficiently high to cause
dielectric breakdown.
• Thus, any material will be transformed to form a plasma.
19
Philosophy of multicomponent film deposition
location 1
source of
elements
(target)
energy
location 2
adequate transport
of elements
coated
substrate
a pure film of
the correct composition
“…This process transports elements from one location to another by supplying energy
to elements in a source, causing them to be transported to a surface to be coated.
Ideally, such a process coats the surface with a pure film of the correct composition.”
T. Venkatesan and Steven M. Green, The Industrial Physicist, p. 22 (1996)
20
Decomposition by equilibrium
Ba
Ti
BaTiOx source
equilibrium heater
J
2
e
P
2kTm
1700 C
PeTi=110-2 torr
PeBa=5102 torr
J Ba PeBa
4


5
10
J Ti
PeTi
• Equilibrium heaters: resistive, e-beam, inductive systems.
• The vapour and deposited films initially are almost pure Ba.
• The composition of deposited films would slowly drift enriching by Ti.
21
Congruent evaporation by PLD
Criterion for congruent evaporation
laser beam
L ≤ dev
L  2( D )
dev
evaporants
L
nonequilibrium heating
target
d ev 
1
2
- heated thickness
ls
F
ln
- evaporated thickness
2 Fth
SrTiO3 ablation by pulse =30 ns:
L ≈ 0.3 µm
dev ≈ 0.2 µm
E. G. Gamaly et al., Physics of Plasmas, 9 (2002) 949
Nonequilibrium heating by pulsed laser beam produces a flash of evaporants that
deposit on a substrate as a film with composition identical to that of the target.
22
Experimental data
Bi2Sr2Ca1Cu2Ox HTS grown by PLD
1.8
2.0
Energy (meV)
2.2
2.4 2.6
2.8
3.0
50
Normalized yield
40
30
20
10
0
300
350
400
450
Channel
500
550
T. Venkatesan and Steven M. Green, in The Industrial Physicist, 1996, p. 23
• Rutherford back scattering: solid line – expected yield, dots – measured composition.
• PLD replicates the composition of the source in the film.
23
Mechanisms of Pulsed Laser Sputtering
primary mechanisms
secondary mechanisms
• Emitted particles with sufficiently high density interact, lose memory of primary
mechanism and therefore described by secondary mechanisms.
• Collisional sputtering cannot occur with laser pulses because photons transfer
energy (0.004%) less than displacement threshold Ed≈25 eV.
24
Thermal Sputtering
temperatures for vaporization
vaporization rate
Al2O3 TOF temperature < 1900 K
• Thermal sputtering, in the sense of vaporization from a transiently heated target,
may require temperatures well above the melting or boiling points.
• In the case of Al2O3 the particle emission by thermal sputtering is not possible
at such low temperatures.
25
Mechanisms of Pulsed Laser Sputtering
primary mechanisms
secondary mechanisms
• Emitted particles with sufficiently high density interact, lose memory of primary
mechanism and therefore described by secondary mechanisms.
• Collisional sputtering cannot occur with laser pulses because photons transfer
energy (0.004%) less than displacement threshold Ed≈25 eV.
26
Electronic Sputtering
high laser-pulse energies
excited electrons energy:
Eel 
low laser-pulse energies
Eg n
N
SiO2 sputtering:
N ≈ 51022 atoms/cm3
n ≈1022 atoms/cm3
Eg = 1 eV
Eel ≈ 210-1 eV
Teff ≈ 3000 K
Tm ≈ 1687 K
• High laser-pulse energies: dense electron excitation increases the total energy
of atoms and the vapor pressure by orders of magnitude.
• Low laser-pulse energies: defects form in and near the target surface, migrate
to the surface which leads to the energetic expulsion of individual atoms.
27
Mechanisms of Pulsed Laser Sputtering
primary mechanisms
secondary mechanisms
• Emitted particles with sufficiently high density interact, lose memory of primary
mechanism and therefore described by secondary mechanisms.
• Collisional sputtering cannot occur with laser pulses because photons transfer
energy (0.004%) less than displacement threshold Ed≈25 eV.
28
Exfoliation Sputtering
exfoliation of W target
thermal stress evaluation
convenient measure of thermal shock:
• The thermal shocks occurred repeatedly and if not released by melting result in
cracking and exfoliation.
• Exfoliation Sputtering does not contribute to film growth but creates defects.
29
Mechanisms of Pulsed Laser Sputtering
primary mechanisms
secondary mechanisms
• Emitted particles with sufficiently high density interact, lose memory of primary
mechanism and therefore described by secondary mechanisms.
• Collisional sputtering cannot occur with laser pulses because photons transfer
energy (0.004%) less than displacement threshold Ed≈25 eV.
30
Hydrodynamic Sputtering
asperities formation in Au target
thermal expansion model
minimum droplet size
• The asperities develop on the target surface due to thermal expansion and
accelerated away during cooling period.
• Hydrodynamic Sputtering does not contribute to film growth but creates defects.
31
Film Nucleation and Growth
by Dietrich R. T. Zahn
• Layer-by-layer – potentially high quality epitaxial films
• 3D islanding – potentially polycrystalline films.
32
Expected effects of PLD conditions
small clusters (high )
critical cluster radius:
r* 
 2(a1c v  a2s c  a2s v )
3a3GV
volume free energy:
large clusters (low )
GV  
kT  P 
kT
ln    
ln( )
  Pe 

P – pressure of the arriving atoms
Pe – vapor pressure of the film atoms
cluster dissociation/nucleation (low R)
typical deposition rates:
1 nm/min - sputtering; 100 nm/min - PLD
range of PLD repetition rate R:
1-100 Hz
• Small cluster size (PLD) promotes Layer-by-layer growth, since adatoms on
small clusters will be more likely to add to the edges.
• Repetition rate may control the nucleation and growth mode.
33
Splashing
Drawbacks of PLD
SEM image of YBCO film
• splashing of micron-sized particulates
• small area of uniformity (1 cm2)
• non-conformal coverage
• extremely complex models hinder
theory based improvements
• Splashing is a major drawback of PLD.
• Splashing is an intrinsic problem, therefore it is difficult to overcome.
• In an electronic device the particulates can induce the formation of defects
and scattering centers that lower carriers mobility, shorten the minority lifetime,
and downgrade the damage thresholds.
34
Mechanisms of Splashing
Subsurface Boiling
molten
globules
Shock Wave Recoil
Pressure Expulsion
molten
globules
Exfoliation
randomly
shaped
particulates
by Jonathan Dickinson
• Subsurface boiling:
Subsurface layer is superheated before surface reaches evaporation point.
• Shock Wave Recoil Pressure Expulsion:
Expansion of plume causes drop in pressure/shock wave just above surface.
• Exfoliation:
Repetitive laser ablation forms microdendrits carried toward by plume.
35
Reducing of Splashing
effects of processing parameters
SEM of YBCO films
maximum laser power density without splashing:
Dmax  LH ev tr
=533 nm
L
Subsurface Boiling
252
(  f  K m )
1
2
L- the range of surface penetration of the light
 - mass density
 - electrical conductivity
f – frequency of the radiation
=1064 nm
Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 184.
The splashing decreases with:
• the laser power at the expense of decreasing in deposition rate.
• the frequency of radiation at the expense of non-congruent evaporation.
36
Reducing of Splashing (cont.)
mechanical particle filter
plume manipulation
Vc  nfl
• Mechanical particle filter:
Larger particulates move slower (20 m/s) and are caught by rotating vanes.
• Plume manipulation:
Heavier particulates travel away from the substrate. Scattered species travel
along a bisecting trajectory.
37
Reducing of Splashing (cont.)
off-axis PLD
target surface improvement
Dmax  LH ev tr
no splashing < Dmax
 - mass density
smooth Ge film deposited from molten target
H. Sakur et al., J. Appl. Pjys 65 (1989) 2475
• off-axis PLD:
The light species undergo scattering by ambient gas and deposit on substrate
facing 180 to the plume direction.
• Target surface improvement:
High density and smooth surfaces are desirable (polish target before each run).
38
Small Area of Uniformity
YBCO
cos11
cos
t  cos11
- sharp angular dependence
50% over 20 mm
T. Venkatesan et al., Appl. Phys. Lett. 52 (1988) 1193.
• The flux is strongly forward peaked resulting in small area uniformity (1cm2).
• Application/commercialization requires large area films (4 inch or large) for
cost effective production.
39
Models of Angular Distribution
calculated deposit profiles
exact solution
approximations
polynomial
strongly supersonic
Kelly
cosine-power
f()=cos @ u=0
• The forward peaking phenomenon arises from collisions between plume species.
• The collision effect is important when a monolayer is removed in tens of ns.
40
Angular Dependence of Composition
schematic of experimental geometry
composition ratios
Y1Ba2Cu3Ox
Z. Trajanovic et al., Appl. Phys. Lett. 66 (1995) 2418
MY=89
MBa=137
MCu=64
RY=180 pm
RBu=215 pm
RCu=135 pm
• The Cu is lighter than Ba and Y and scatters more readily off the straight path.
• The Ba has large cross section for oxigen scattering than those of Y and Cu.
41
Large-Area PLD Approaches
schematic of a large-area PLD system
off-axis and rotational/translational PLD
Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, pp. 294-295.
Basic approaches to scaling-up PLD:
• rastering laser beam
• off-axis positioning
• rotating/translating substrate
42
Large-Area PLD Films
thickness distribution of Y2O3 films
YBCO film composition uniformity
Pulsed laser deposition of thin films: applications-led growth of functional materials / ed. by Robert Eason, N.J., Wiley, 2007
pp. 193-194.
• The films are obtained by the laser rastering technique on rotating substrates.
• The uniformity of thickness is ±4% over 8-inch area.
• The uniformity of composition is ±1.5% over 6-inch area.
43
Ferroelectric Films Made by PLD
permittivity and loss tangent vs field
1400
P
1200 Ba0.75Sr0.25TiO3
0.04
m=qΔ
800
0.03
P=Nm
600
0.02
1000
400
0.01
200
0
-400
0
-200
0
200
400
E (kV/cm)
tan
permittivity
Nonlinear polarization
0.05
E
Field dependent permittivity
P=0E
ε
P=0(-1)E
E
• Polarization due to ionic displacement Δ
• Field dependent permittivity – voltage tunable capacitor
(varactor)
C
 0 A
d
44
Ferroelectric Varactors
GaAs-Schottky
200
Shown are also Q-factors of:
BST/Pt/Au (PLD)
Si abrupt junction varactor
(Metelics, MSV34,060-C12,
Q=6500 @ 50 MHz, V=-4V)
Q-factor
100
GaAs HBV
(Darmstadt University of
Technology, fcut-off=370 GHz)
BST/Pt (PLD)
Si
Q=1/tan
10
1
GaAs-HBV
10
GaAs dual Schottky diode
(United Monolithic Simiconductors,
DBES105a,
fcut-off=2.4 THz).
Frequency (GHz)
A. Vorobiev, P. Rundqvist, K. Khamchane, and S. Gevorgian, Appl. Phys. Lett. 83, 3144 (2003)
Ferroelectric varactors may compete with the semiconductor analogous.
45
Ferroelectric Microwave Devices
Tunable Delay Line
100 µm
delay time vs frequency
0V
D. Kuylenstierna, A. Vorobiev, P. Linnér, and S. Gevorgian,
IEEE Trans. Microwave Theory and Techn., 53 (2005) 2164.
20 V
Applying dc voltage between 2 ports the delay time can be tuned.
46
HTS films made by PLD
Design for bicrystal junction DC SQUID
Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, pp. 359.
• Bicrystal grain-boundary junction exploits the weak-link behavior induced
by high-angle boundaries at bicrystal interface.
• Polycrystalline YBCO films are not good due to many grain boundaries
throughout the SQUID loop itself.
47
Grain-boundary YBCO junctions
XRD /2-scan of a PLD YBCO film
The bolometer response of grain boundary
YBCO Josephson oscillator
YBCO/YSZ/sapphire
14 bicrystal substrate
1.7 THz
E. Stepansov et al., J. Appl. Phys. 96, 3357 (2004)
• /2- and -scan reveal no additional peaks due to CuO and -particles or other
orientations of YBCO in the a-b plane.
• High characteristic frequency and low microwave loss allows terahertz
applications (direct Josephson detectors, oscillators, spectrometers etc.)
48
Summary
You have learned:
• basic principles of PLD of thin films
• functional materials
- offer higher functionality of a system
- multicomponent oxides (stoichiometry required)
• features of PLD of functional materials
- advantages/disadvantages of PLD of functional materials
- laser sputtering mechanisms (non-equilibrium heating preserves stoichiometry)
- effects on film growth (layer-by-layer growth is promoted)
- splashing (reducing approaches)
- plume forward peaking (large area films approaches are demonstrated)
- state-of-the-art YBCO and BSTO films/devices made by PLD at MC2
49