Fundamentals and Application of Chemical Solution Techniques for Thin

Films Synthesis

Dr. Oscar de Sanctis

Laboratorio de materiales Cerámicos

IFIR – CONICET, FCEIyA - Universidad

Nacional de Rosario

ARGENTINA

Email: oski@fceia.unr.edu.ar

Thin Film Deposition (extrinsic)Physical Vapor Deposition (PVD) -Film is formed by atoms directly

transported from source to the substrate through gas phase

•Evaporation

•Thermal evaporation

•E-beam evaporation

•Sputtering•DC sputtering

•DC Magnetron sputterng

•RF sputtering

•Reactive PVD

Chemical Vapor Deposition (CVD) -Film is formed by chemical reaction on the surface of substrate

•Low-Pressure CVD (LPCVD)

•Plasma-Enhanced CVD (PECVD)•Atmosphere-Pressure CVD (APCVD)

•Metal-Organic CVD (MOCVD)

Chemical Solution Deposition (CSD) – Film is formed starting from a liquid film

Platting

General Characteristics of Thin Film Deposition•Deposition Rate

•Film Uniformity

•Across wafer uniformity

•Run-to-run uniformity

•Materials that can be deposited

•Metal•Ceramic•Polymer

•Quality of Film –Physical and Chemical Properties

•Stress. •Adhesion. •Stoichiometry. •Film density, pinhole density. •Grain size, boundary property, and orientation. •Breakdown voltage. •Impurity level

•Deposition Directionality

•Directional: good for lift-off, trench filling

•Non-directional: good for step coverage

•Cost of ownership and operation

CVD Reactors1.) Atmospheric Pressure CVD (APCVD)

Advantages: High deposition rates, simple, high throughput

Disadvantages: Poor uniformity, purity is less than LPCVDUsed mainly for thick oxides.

2.) Low Pressure CVD (LPCVD at ~0.2 to 20 torr)

Advantages: Excellent uniformity, purityDisadvantages: Lower (but reasonable) deposition rates than APCVD

Used for polysilicon deposition, dielectric layer deposition, and doped dielectric deposition.

3.) Metal Organic CVD (MOCVD)Advantages.: Highly flexible—> can deposit semiconductors, metals,

dielectrics

Disadvantages: HIGHLY TOXIC!, Very expensive source material. Environmental disposal, costs are high.

4.) Plasma Enhance CVD. PE-MOD (Metal Organic Decomposition)

Plasmas are used to force reactions that would not be possible at low temperature.

Advantages.: Uses low temperatures necessary for rear end processing.

Disadvantages: Plasma damage typically results.Used for dielectrics coatings.

Chemical Solution Deposition (CSD) Processes

Synthesis of the precursor solution Liquid solution RT

Deposition on a substrate Liquid film RT

Drying film Hybrid amorphous film 100-200ºC

Burning of organic species Porous amorphous film 280-390ºC

Densification and crystallization Dense crystalline film 450-850ºC

Five basic stepsS

HR

INK

AG

E

——

——

——

——

—→→ →→

CSD collects all the processes that the precursor and the deposited film are liquid

Precursor Solution

The Preparation of multioxides materials precursor solution, such as perovskites, involves the use ofcation compounds that are dissolved in a common solvent

Several processes can be used for precursor solution preparation

Each process gives one characteristic precursor that

exhibit dramatic effect on the film properties

Simple processes consist in mixing soluble metal compounds in a common solvent where the precursor species retain a strong

resemblance to the starting.

More complex process are similar to a engineering molecular, tendering to synthesize precursors with structures similar to the final crystal structure of the desired perovskite film

FUNDAMENTS OF THE SOL-GEL

HORM(OR)-HO OHM(OR) 1-n2n +→+

Hydrolysis

Condensation: (water elimination)

OHM(OR)-O-M(RO)M(OR)-HOOH-M(RO) 21-n1n1-n1n +→+ −−

Oligomerization

1-n

(OR)(OR)(OR)

1n M(OR)MOMOM-O-M(RO)2-n2-n

2-n

−−−−−−

FORMATION OF THE GEL NETWORK

Characteristics SG process

• High water sensitive starting material

• Chemical bonding

• Different reaction ratio of metal precursors

• Poor control of molecular structure

• From lineal chain to 3-dimensional network

• Porous films

• Unstable solution

• High shrinkage

Zirconium n-

Propoxide

+ 2-Methoxyethanol

Lead Acetate

Trihydrate

+ 2 Methoxiethanol

Titanium

Ethoxide

+ 2-Methoxyethanol

Lead

precursor

(X)

Zirconium

precursor

(Y)

Titanium

precursor

(Z)

PZT

PRECURSOR

(W)

Alcohol

Exchange

(5 h, 124 ºC)

Alcohol

Exchange

(4 hs, 124 ºC)

Dehiydration

and alcoholysis

(10 hs, 124 ºC)

Complexation

(124ºC, 3 h)

( )3x-222 b(OEtOMe) 3)( OOCCHPMeOEtOHOHOAcPb →+⋅

Lead Precursor, Alcoholisis

Zirconium and Titanium Precursors, Alcohol exchangenn HOMeOEtOHOZr Pr4Zr(OEtOMe) 4)Pr( 44 +→+

HOEtMeOEtOHOEtTi 4Ti(OEtOMe) 4)( 44 +→+

High water sensitive starting material

Multi-metallic alkoxide

Precursor structure resembles to

the PZT perovskite lattice

Sol-Gel Process: 2- metoxyethanol Route

Common alkoxide group

OHOAcPb 22 3)( ⋅Lead Precursor

Zirconium and Titanium Precursors

Modified Sol-Gel Process

( )xn OOCCHHOAcOZr 3x-4

n)Zr(OPr )Pr( →+4

( )xOOCCHHOAcOEtTi 3x-4n)Ti(OPr )( →+4

Chelating agents: HOOCCH3 (Acetic acid) OR 2-4-pentanedione (ACAC)

SIMILAR PROCESS

LESS REACTIVITY

LESS STRUCTURECONTROL

Metal-Organic Decomposition: MOD

Precursors Solvent Long chain caboxylate

M(OOCCnHn+1 Xilene

Short chain

M(OOCCnHn+1

Alcohol

NO REACTIVITY AND NO

STRUCTURE

Chelate route

Starting materials• Chelating agents: Diethanolamine, ,Triethanolamine,

3-Hydroxy-2-butanone (Acetoin). • (n-1) or (n-2) non hydrolyzing metal precursor soluble

in alcohol. Nitrates or acetates. • One or two metal alkoxides.• Short chain alcohol solvent

•Low temperature burning residual organic groups (<450 ºC).

•Very low content of hydroxyls residual

•Onset of crystallization at lower temperature

PROCESS Simplicity Reactivityreactants

Organic residue

Shrinkage

Structurecontrol

OtherParameter

control

Stableprecursor

Nitrates Very High

None Very low None None Very high

Citrate/Nitrate Very High

None Very low None None High

Pechini/Nitrate High Low None ThicknessTexture

High

MOD Medium Low Very High None None High

Chelate route Medium Low Very Low Low ThicknessTexture

High

Modified Sol-gel Low High High Medium ThicknessTexture

Low

Sol-gel Very low Very high Medium High Thickness Very low

PROCESSES FOR PRECURSOR SOLUTION PREPARATION

DIP COATING

n

lρg

Vµt

∝

SPIN COATING

Step 2: the substrate is accelerated up to its final rotation speed. Aggressive fluid expulsion from the wafer

Step 3: the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behaviour. Possible starting of evaporation for volatile solvent

Step 1: is the deposition of the coating fluid onto the wafer or substrate.

• Solution viscosity

• Solid content

• Angular speed

• Spin Time

SPIN COATING

PARAMETERS

SPIN COATING DEFECTS

Film too thinSpin speed too high. Select lower speedSpin time too long. Decrease time during high speed stepInappropriate precursor solution

Film too thickSpin speed too low. Select higher speedSpin time too short. Increase time during high speed stepInappropriate precursor solution. To dilute solution

• Air bubbles on wafer surface

• Air bubbles in dispensed fluid

• Dispense tip is cut unevenly or has burrs or defects

Comets, streaks or flaresFluid velocity (dispense rate) is too highSpin bowl exhaust rate is too highResist sits on wafer too long prior to spinSpin speed and acceleration setting is too highParticles exist on substrate surface prior to dispenseFluid is not being dispensed at the centerof the substrate surface

Swirl patternSpin bowl exhaust rate is too highFluid is striking substrate surface off centerSpin speed and acceleration setting is too highSpin time too short

Uncoated AreasInsufficient Dispense Volume.Non uniform wetting

PinholesAir bubblesParticles in fluid. Solution filtrationParticles exist on substrate surface prior to dispense. Substrate cleaning

As-prepared film

BURNING

ORGANIC

GROUPS

Low porous Amorphous film

High porous Amorphous film

Densification and Crystallization

The film microstructure and orientation

depends from:

�The substrate and Solution chemistry,

�Material composition and crystallization

path

�Sequence and temperatures of the thermal

treatment.

Densification

Porous and Amorphous

Crystalline film

Sintering Crystallization

Mechanisms of Sintering

Condensation Reaction

Structural relaxation

Viscous flow

Solid state reaction

→

→

→

→

TEM

PER

AT

UR

E

HEATING RATE Sintering Mechanics

Densification

Low Consecutive Lower

High Concurrent Higher

Phase transitionPhase transition

2

20

G

x

∂>

∂2

20

G

x

∂<

∂

CrystallizationFilm crystallization in solution-derived thin films performs by a nucleation-and-growth process. The theoretical picture is parallel to that of glasses crystallization.

Path A: FerroelAmorhCa

G→

∆

→

PZT amorph→Pyrochlore→PZT perovkite

Path B:

FerroelInterdAmorhCiia

GG→→

∆∆

→→

SrTiO3 amorph → SrTiO3 Feroel

Nucleation

Volume free energy change (∆Ga→c) drives

Surface energy, ∆GS counter-drives

γ: interfacial energy crystalline – amorphous),

γππ 23 43/4 rGrG ca +∆=∆ →

caGr

→∆−=

γ2*

caGG

→∆=∆

πγ

3

16*

Nucleus growth

r r CG n G T S∆ = ∆ − ∆ ( )0

0

!ln

! !

r

C

r

n nS k

n n

+∆ =

( )0

0

!ln

! !

r

r r

r

n nG n G kT

n n

+∆ = ∆ −

0 0

ln lnr rr

r

n nG kT kT

n n n

−∆ =

+ �0

r T

G

n

∂=

∂

0

expr rn G

n kT

∆ = −

ac

ScSa

γ

γγθ

−=cos

Homogeneous vs Heterogeneous Nucleation

θθθθ: contact angleγγγγSa: substrate – amorphous interfacial energy

γγγγSc: substrate – crystalline interfacial energy

γγγγSa: amorphous – crystalline interfacial energy

( )4

12 2)cos)(cos( θθθ

−+=f( ) **

HOMOHETE GfG ∆=∆ θ

0º≤θ≤≤θ≤≤θ≤≤θ≤180º 1 ≥≥≥≥cosθ≥θ≥θ≥θ≥-1 0 ≤≤≤≤ f(θθθθ)≤≤≤≤1

Full wetting θθθθ=0 f(θθθθ)=0 ∆∆∆∆G*HETER=0

No wetting θθθθ=180º f(θθθθ)=1 ∆∆∆∆G*HETER=∆∆∆∆G*HOMO

SUBSTRATE SUBSTRATE

Random Polycrystalline film Oriented columnar film

Nucleation rate

−=kT

Eaexp0νν Collision frequency at interface nucleus,

Ea: atomic movement activation energy

−

∆−=

kT

E

kT

GnnN aS expexp

*

00ν

RATE

FavorsNucleation

Conventional < 100 ºC/min At low

Temperature

RTA > 1000 ºC/min At high Temperature

Effects of the Heat Treatment on the Nucleation

PROBLEMIn ferroelectric capacitors built with films of BaTiO3 (BT), PbZr0.2Ti0.8O3 (PZT) and SrBi2Ta2O9 (SBT) and electrode Pt. What heat treatment would develop the appropriated microstructure that produce the maximum remanet polarization

→

→

→

→

→

→

→

→

←

→←

→

←

→←

→

Gro

wth

[[ [[0

01

]] ]]

Gro

wth

[[ [[001

]] ]]

Po

lari

zati

on

[[ [[00

1]] ]]

←→←→←→←→

Polarization [[[[110]]]]

Pt

BT and PZTSBT

Pt

BT Amorphous →→→→ BT crystalline

PATHS PZT Amorphous →→→→ pyrochlore →→→→ PZT crystalline

SBT Amorphous →→→→ Fluorite SBT →→→→ perovskite SBT

SUBSTRATE

FILM

XPS SiO2 film on AISI 304Thermal treatment 700 ºC for 1 hours

exp aE

RTν

∝ −

Interdiffusion Structural effect

INTERACCION FILM-SUBSTRATE

Thermal Activated Process

Lead less Ferroelectrics

(KXNa1-X)NbO3 (NKN)

Acetoin

+

Etanol

Etox. Nb(V)

Solución

precursora

de Niobio

Etox. K

+

Etox. Na

+

Etanol

Solución

(Na0.50 K0.50)NbO3

(NKN)

Solución

H2O

HCl

Acetoin

+

Etanol

Etox. Nb(V)

Acetoin

+

Etanol

Etox. Nb(V)

Solución

precursora

de Niobio

Solución

precursora

de Niobio

Etox. K

+

Etox. Na

+

Etanol

Solución

(Na0.50 K0.50)NbO3

(NKN)

Solución

H2O

HCl

Solución

H2O

HCl20 30 40 50 60

♦♦♦♦

♦♦♦♦∗∗∗

∗

••••

∗

∗

∗

••••

♦♦♦♦

♦♦♦♦♦♦♦♦

♦♦♦♦

600ºC

850ºC

800ºC

700ºC

2 θθθθ

750ºC

EFECTO DE LA TEMPERATURA

♦♦♦♦

NKN

Pt

Por indentificar

00

1

10

1

002200