1

course:

Micro- and Nanoprocessing Technologies

Pulsed laser deposition of thin films of

functional materials

Graduate School at MC2 2016

lecture:

Course responsible: Ulf Södervall

Lecturer: Andrei Vorobiev

2

• concept of pulsed laser deposition PLD of thin films

• functional materials

You will learn about:

• features of PLD of functional materials

Objectives

2) Pulsed laser deposition of thin films: applications-led growth of functional materials /

edited by Robert Eason, N.J., Wiley, cop. 2007

Literature:

3) Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler,

New York, Wiley, cop. 1994

1) H. M. Christen and G. Eres, Recent advances in pulsed-laser deposition of complex oxides,

J. Phys.: Condens. Matter 20 (2008) 264005 (16pp)

3

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

4

Definitions

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.

Laser

is an electronic-optical device that produces coherent radiation.

The term is acronym for Light Amplification by Stimulated Emission of Radiation.

Functional materials

are materials having physical properties sensitive to the external effects

(temperature, electric and magnetic fields, pressure etc.).

5

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.

6

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.

7

Functional materials

Functional material

properties:

• permittivity

• permeability

• resistivity

• refractivity

• sound velocity

• …

External parameters:

• temperature

• pressure

• electric field

• magnetic field

• optical wavelength

• absorbed gas

• pH value

• …

function of

• Utilizing a functional material offers higher functionality of a system.

• Science and technology rely heavily on the development of functional materials.

8

Gaming

Mobile

Imaging

Portable

Media

Cellular

Phones

PDA

DSC

Video

MP3

Smart Phone

Convergent devices

Adapted from Philips

Mobile Convergence

9

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.

10

PLD of Functional materials

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.

x

11

Concept of PLD

• 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.

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 3.

PLD stages:

• laser ablation of target

• dynamic of plasma

• film nucleation and growth

12

Calas PLD System -

MC2ProcessLab, Chalmers

Growth of BSTO films by PLD

1 Hz

BSTO target

Heater at 650C

Laser0.4 mbar O2

Vacuum chamber

13

Advantages-Disadvantages of PLD

Advantages

• versatile method (any material)

• congruent evaporation

• high deposition rates (10s nm/min)

• clean process

• plume at high energy

• reactive gases (oxygen)

• broad range of gas pressures

Disadvantages

• splashing of micron-sized particulates

• small area of uniformity (1 cm2)

• non-conformal coverage

• extremely complex models hinder

theory based improvements

• To a large extent the two first problems have been solved.

• Congruent (stoichiometric) evaporation – main advantage for PLD of films of

functional materials.

14

Laser basics

• When perturbed by a photon matter may create another photon.

• The first photon is not destroyed (no absorption) - light amplification.

• The second photon has the same direction, frequency, phase and polarization.

Light Amplification by Stimulated Emission of Radiation

15

Laser basics (cont.)laser major parts

Gain medium

Pump source

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.

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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)

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

0.543

0.568

0.570

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

Wavelength (mm)Laser Type

Wavelengths of common lasers

• The PLD range is 200 - 400 nm (absorption by matter is strong enough).

• Excimer laser uses “excited dimer” pseudo-molecules for the gain media.

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Excimer laser basics

Ar+ + 2Ar Ar2+ + Ar

Ar2+ + Kr Kr+ + 2Ar

Kr* + eˉKr + eˉ excitation

Kr+ + 2 eˉ

Kr* + F2 (KrF)* + F harpooning

Kr+ + Fˉ + He (KrF)* + He recombination

charge-transfer

excimers formation reactions

• 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

KrF electronic potential

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Commercial excimer laser designcross sectional view of excimer laser tube

• KrF Lambda Physik, Inc. typical parameters: =248 nm, =30 ns, r=10 Hz, E0=1.5 J/cm2

• Pulsed mode provides non-equilibrium vaporization (10000 K in 10 nm surface layer).

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Advantages

• versatile method (any material)

• congruent evaporation

• high deposition rates (10s nm/min)

• clean process

• plume at high energy

• reactive gases (oxygen)

• broad range of gas pressures

Φ – power density (109 W/cm2)

c – velocity of light

0 – permittivity of vacuum

n – refractive index (1.5)

E – electric field (106 V/cm)

Advantages of PLD

E = (2Φ/c0n)1/2

Versatile method

• 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.

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Philosophy of multicomponent film deposition

adequate transport

of elements

location 1 location 2

source of

elements

(target)

energy

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)

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• The vapour and deposited films initially are almost pure Ba.

• The composition of deposited films would slowly drift enriching by Ti.

1700 C

PeTi=110-2 torr

PeBa=5102 torr

BaTiOx source

Ba

Ti

equilibrium heater

• Equilibrium heaters: resistive, e-beam, inductive systems.

kTm

PJ e

2

2

4105

Ti

e

Ba

e

Ti

Ba

P

P

J

J

Decomposition by equilibrium

22

Congruent evaporation by PLD

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.

Criterion for congruent evaporation

- heated thickness21

)(2 DL

th

sev

F

Fld ln

2

L ≤ dev

- evaporated thickness

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

laser beam

target

dev

Lnonequilibrium heating

evaporants

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Experimental data

40

50

30

20

10

0

No

rma

lize

d y

ield

300 400 450 500 550350

Channel

1.8 2.2 2.4 2.6 2.82.0 3.0Energy (meV)

Bi2Sr2Ca1Cu2Ox HTS grown by PLD

• Rutherford back scattering: solid line – expected yield, dots – measured composition.

T. Venkatesan and Steven M. Green, in The Industrial Physicist, 1996, p. 23

• PLD replicates the composition of the source in the film.

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Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Collisional sputtering cannot occur with laser pulses because photons transfer

energy (0.004%) less than displacement threshold Ed≈25 eV.

• Emitted particles with sufficiently high density interact, lose memory of primary

mechanism and therefore described by secondary mechanisms.

25

Thermal Sputteringtemperatures 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.

26

Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Collisional sputtering cannot occur with laser pulses because photons transfer

energy (0.004%) less than displacement threshold Ed≈25 eV.

• Emitted particles with sufficiently high density interact, lose memory of primary

mechanism and therefore described by secondary mechanisms.

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Electronic Sputteringlow laser-pulse energieshigh laser-pulse energies

• 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.

SiO2 sputtering:

excited electrons energy:N

nEE g

el

N ≈ 51022 atoms/cm3

n ≈1022 atoms/cm3

Eg = 1 eV

Eel ≈ 210-1 eV Teff ≈ 3000 K

Tm ≈ 1687 K

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Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Collisional sputtering cannot occur with laser pulses because photons transfer

energy (0.004%) less than displacement threshold Ed≈25 eV.

• Emitted particles with sufficiently high density interact, lose memory of primary

mechanism and therefore described by secondary mechanisms.

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Exfoliation Sputtering

• Exfoliation Sputtering does not contribute to film growth but creates defects.

exfoliation of W target

• The thermal shocks occurred repeatedly and if not released by melting result in

cracking and exfoliation.

thermal stress evaluation

convenient measure of thermal shock:

30

Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Collisional sputtering cannot occur with laser pulses because photons transfer

energy (0.004%) less than displacement threshold Ed≈25 eV.

• Emitted particles with sufficiently high density interact, lose memory of primary

mechanism and therefore described by secondary mechanisms.

31

Hydrodynamic Sputteringthermal expansion modelasperities formation in Au target

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.

32

Film Nucleation and Growth

• Layer-by-layer – potentially high quality epitaxial films

• 3D islanding – potentially polycrystalline films.

by Dietrich R. T. Zahn

33

• 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.

Expected effects of PLD conditions

V

vscsvc

Ga

aaar

3

221

3

)(2*

)ln(ln

kT

P

PkTG

e

V

critical cluster radius:

volume free energy:

P – pressure of the arriving atoms

Pe – vapor pressure of the film atoms

typical deposition rates:

1 nm/min - sputtering; 100 nm/min - PLD

range of PLD repetition rate R:

1-100 Hz

small clusters (high )

large clusters (low )

cluster dissociation/nucleation (low R)

34

Splashing

• splashing of micron-sized particulates

• small area of uniformity (1 cm2)

• non-conformal coverage

• extremely complex models hinder

theory based improvements

Drawbacks of PLD SEM image of YBCO film

• Splashing is a major drawback of PLD.

• 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.

• Splashing is an intrinsic problem, therefore it is difficult to overcome.

35

Mechanisms of SplashingSubsurface Boiling Shock Wave Recoil

Pressure ExpulsionExfoliation

by Jonathan Dickinson

molten

globulesmolten

globules

randomly

shaped

particulates

• 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.

36

Reducing of Splashingeffects of processing parameters

rev tHLD max

• the laser power at the expense of decreasing in deposition rate.

• the frequency of radiation at the expense of non-congruent evaporation.

The splashing decreases with:

maximum laser power density without splashing:SEM of YBCO films

=533 nm

=1064 nm

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 184.

21

)(

252

mKfL

L- the range of surface penetration of the light

- mass density

- electrical conductivity

f – frequency of the radiation

Subsurface Boiling

37

Reducing of Splashing (cont.)plume manipulationmechanical particle filter

• 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.

nflVc

38

Reducing of Splashing (cont.)off-axis PLD target surface improvement

H. Sakur et al., J. Appl. Pjys 65 (1989) 2475

smooth Ge film deposited from molten target

rev tHLD max

• Target surface improvement:

High density and smooth surfaces are desirable (polish target before each run).

• off-axis PLD:

The light species undergo scattering by ambient gas and deposit on substrate

facing 180 to the plume direction.

- mass density

no splashing < Dmax

39

Small Area of Uniformity

• 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.

T. Venkatesan et al., Appl. Phys. Lett. 52 (1988) 1193.

YBCO

coscos11

t cos11 - sharp angular dependence

50% over 20 mm

40

Models of Angular Distributioncalculated deposit profiles exact solution

approximationspolynomial

strongly supersonic Kelly

cosine-power

• The forward peaking phenomenon arises from collisions between plume species.

• The collision effect is important when a monolayer is removed in tens of ns.

f()=cos @ u=0

41

Angular Dependence of Compositionschematic of experimental geometry composition ratios

Y1Ba2Cu3Ox

Z. Trajanovic et al., Appl. Phys. Lett. 66 (1995) 2418

MY=89 MBa=137 MCu=64

RCu=135 pmRBu=215 pmRY=180 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.

42

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

43

Large-Area PLD Films

thickness distribution of Y2O3 films YBCO film composition uniformity

• 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.

Pulsed laser deposition of thin films: applications-led growth of functional materials / ed. by Robert Eason, N.J., Wiley, 2007

pp. 193-194.

44

Ferroelectric Films Made by PLD

0

200

400

600

800

1000

1200

1400

0

0.01

0.02

0.03

0.04

0.05

-400 -200 0 200 400

perm

itti

vit

y

tan

E (kV/cm)

Ba0.75

Sr0.25

TiO3

P=0E

P=0(-1)E

m=qΔ

P=Nm

ε

E

P

E

Nonlinear polarization

Field dependent permittivity

• Polarization due to ionic displacement Δ

• Field dependent permittivity – voltage tunable capacitor

(varactor) d

AC 0

permittivity and loss tangent vs field

45

Shown are also Q-factors of:

Si abrupt junction varactor

(Metelics, MSV34,060-C12,

Q=6500 @ 50 MHz, V=-4V)

GaAs HBV

(Darmstadt University of

Technology, fcut-off=370 GHz)

GaAs dual Schottky diode

(United Monolithic Simiconductors,

DBES105a,

fcut-off=2.4 THz).

A. Vorobiev, P. Rundqvist, K. Khamchane, and S. Gevorgian, Appl. Phys. Lett. 83, 3144 (2003)

10

100

1 10

Q-f

act

or

Frequency (GHz)

200

GaAs-HBV

Si

GaAs-Schottky

BST/Pt/Au (PLD)

BST/Pt (PLD)

Q=1/tan

Ferroelectric Varactors

Ferroelectric varactors may compete with the semiconductor analogous.

46

Tunable Delay Line

D. Kuylenstierna, A. Vorobiev, P. Linnér, and S. Gevorgian,

IEEE Trans. Microwave Theory and Techn., 53 (2005) 2164.

0 V

20 V

Ferroelectric Microwave Devices

100 µm

Applying dc voltage between 2 ports the delay time can be tuned.

delay time vs frequency

47

HTS films made by PLD

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, pp. 359.

Design for bicrystal junction DC SQUID

• 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.

48

Grain-boundary YBCO junctions

XRD /2-scan of a PLD YBCO filmThe bolometer response of grain boundary

YBCO Josephson oscillator

1.7 THz

E. Stepansov et al., J. Appl. Phys. 96, 3357 (2004)

YBCO/YSZ/sapphire

14 bicrystal substrate

• /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.)

49

Summary

• basic principles of PLD of thin films

• functional materials- offer higher functionality of a system

- multicomponent oxides (stoichiometry required)

You have learned:

• 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