ulsed laser deposition and characterization of pb(zr, ti)o3 and lanio3 thin films

2.5.2.1.2 Double crystal diffraction topography 382.5.2.2.3 Macroscopically elastic anisotropy 43 2.5.2.3 Residual stress measurement of ferroelectric films 46 2.5.3 Effect of Residual S

YU YONGHE (B Eng., M Eng.)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE

2006

The author is very grateful to Prof Lai Man On and Prof Lu Li, the project supervisors, for their constant guidance, flexible arrangements, constructive suggestions and critical discussions throughout this project.

Many friends offered help during this project, their credits should be cited Thanks should be given to Mr Thomas Tan, Mdm Zhong Xiangli, Mr Jurami Madon, Mr Maung Aye Thein, Mr Ng Hong Wei, Mr Abdul Khalim of the Materials Science Laboratory for their helpful technical support Special thanks are given to Dr Zhu Tiejun, Dr Tang Songbai, Mr Xia Hui, Mr Zhang Zhen, Mr Lacassin Lionel Alexandre Philippe, Mr Doan Vien Duyen Oanh, fellows of Thin Film Group for their help and invaluable discussions.

This work was partly performed at Singapore Synchrotron Light Source (SSLS) under NUS Core Support C-380-003-003-001, A*STAR/MOE RP 397908M and A*STAR

12 105 0038 grants The author would like to thank Dr Yang Ping and Dr Liu Tao from SSLC for their assistance in the synchrotron experiment.

Thanks are also extended to the Management of TÜV SÜD PSB Corporation, especially to Dr Huang Xianya for his encouragement and support.

Finally, the author would like to express his gratitude to his family for their understanding, concern, support and encouragement.

2.1.2 Ferroelectric Thin Film Synthesis Techniques 9 2.1.3 Application of Ferroelectric Thin Films 12

2.1.3.2 Non-volatile random access memory 13

2.2 Pulsed Laser Depositing Ferroelectric Thin Films 15

2.5.2.1.2 Double crystal diffraction topography 38

2.5.2.2.3 Macroscopically elastic anisotropy 43

2.5.2.3 Residual stress measurement of ferroelectric films 46 2.5.3 Effect of Residual Stresses on Ferroelectric Performances of The Films

47 2.6 Measurement on Thin Film Piezoelectric Constants 49

2.6.2 Substrate Effect on Piezoelectric Constants 50

3.7 Chemical Analysis 62

AMORPHOUS SUBSTRATES WITH LNO UNDERLAYER ELECTRODE

4.2 Microstructure and Orientation of The Films 65

4.3 Electrical Properties of PZT Films on SiO 2 /Si and Glass Substrates 73

ON AMORPHOUS SURFACE USING PULSED LASER DEPOSITION

6.2 Geometries of Pulsed Laser Deposition on Tilt Substrate(TSPLD) 92 6.3 Orientations of LNO Films Grown by TSPLD 94

6.6 Summary 107

CONSTANTS OF FILMS BY X-RAY DIFFRACTION INCORPORATED WITH FOUR-POINT BENDING

7.2 Stress Analysis Of Composite Film-Substrate Beam 109

7.3 Evaluation of Thin Film Elastic Constants 113

7.4 Measurement on Elastic Constants of PZT Film on Aluminum

CRYSTAL SUBSTRATES MEASURED BY HIGH RESOLUTION ROCKING CURVE TECHNIQUE

8.3 Measuring Residual Stresses in LNO and PZT Films 126

CONSTANTS OF PZT AND LNO FILMS

9.3 Examples of XECs Measurement 139

FILMS BY X-RAY DIFFRACTION

10.2.1 Strain measurement using X-ray diffraction 150 10.2.2 Measurement of strain induced by electricity 151 10.2.3 Determination of piezoelectric constants Experimental details 153

Following papers directly related to the research project were published during the study:

1 Y.H Yu, M.O Lai and L Lu, Determination on X-ray elastic constants of solid

thin films, Smart Structure Materials, 16(2007)487-492.

2 Y.H Yu, M.O Lai, L Lu and P Yang, Measuring residual stress of PZT thin film

on Si(100) by synchrotron x-ray rocking curve technique, J of Alloys and

Compounds, (2007) doi:10.1016/j.jallcom.2006.02.10.

3 Y.H Yu, M.O Lai, L Lu and G.Y Zheng, Measurement of in-plane elastic constants of crystalline solid films by X-ray diffraction coupled with four point

bending, Surface and Coating Technology, 200(2006)4006-4010.

4 Y.H Yu, M.O Lai and L Lu, Highly (100) oriented Pb(Zr 0.52 Ti 0.48 )O 3 /LaNiO 3

films grown on amorphous substrates by pulsed laser deposition, J of Applied

Physics A, Materials Science and Processing (2007)

doi:10.1007/s00339-007-3968-y.

5 Y.H Yu, M.O Lai and L Lu, Distribution in orientation axis of thin film grown

by pulsed laser deposition, Thin Solid Films, (2007) in press.

Ferroelectrics, and Frequency Control, (2007).

7 Y.H Yu, M.O Lai and L Lu, Biaxially aligned films grown on amorphous surface using tilted substrate pulsed laser deposition, to be submitted (2007).

8 Y.H Yu, M.O Lai and L Lu, Measurement on elastic constants of thin films by

synchrotron X-ray diffraction, The 4 th International Conference on Materials for Advanced Technologies (ICMAT 2007), Conf116a2326, 1-6 July, 2007,

Singapore.

9 Y.H Yu, M.O Lai and L Lu, P Yang, Residual stresses of PZT films grown on

single crystal substrate by high resolution rocking curve technique, Proc of The

6-10 Dec., 2005, Kuala Lumpur, Malaysia.

10 Y.H Yu, M.O Lai and L Lu and G.Y Zheng, Measuring in-plane elastic

constants by X-ray diffraction coupled four-point bending technique, Proc of The

34-GEN-951, 13-17 July, 2004, Singapore.

Growth of highly oriented and biaxially aligned Pb(Zr 0 52 Ti 0.48 )O 3 (PZT) and LaNiO 3 (LNO) thin films on amorphous surfaces by pulsed laser deposition (PLD) has been explored New techniques using X-ray diffraction (XRD) to characterise elastic constants, residual stresses, and piezoelectric constants of the ferroelectric films have been developed.

The deposition conditions were optimized to achieve the desired oriented PZT/LNO films on SiO 2 /Si (100) and alkaline earth aluminosilicate glass substrates The microstructure and crystal orientation, chemical and electrical characteristics of the films were examined The mechanisms of growth of the out-of-plane oriented films grown on amorphous substrates were discussed.

Distribution of the orientation axes of LNO and PZT films grown by PLD was evaluated They were found to incline from the normal of the substrate and the extent

of inclination of the orientation axis was location dependent The mechanism of distribution in the orientation axes of the films produced by PLD was explored The effect of distribution of orientation axes on ferroelectric properties of the films was also investigated.

Biaxially aligned LNO films on amorphous surface have been explored using tilt substrate PLD In-plane orientation of LNO films was achieved even when the nominal tilt angle of the substrate was only 30fl Mechanism of biaxial alignment

obtained even without knowing the material properties and geometries of both the substrate and the film while only the elastic modulus of the substrate and thickness of both substrate and film are required in determining the Young’s Modulus of the film.

The curvature of a crystal plane of single crystal substrate was assessed using resolution X-ray rocking curve technique (HRRC) with synchrotron radiation The residual stress of the film was then calculated from the change in curvature of the substrate based on the well-known modified Stoney’s equation More reliable results could be obtained because they are not affected by surface morphology and reflectivity of the films A new technique was developed to determine the X-ray

high-elastic constants (S 1 and ½S 2) of the thin films based on the residual stresses measured

by HRRC ½S 2 of the films can be obtained from the measured residual stress and the

slope of the d{ vs sin2{ curve without loading With the addition of d{ vs sin2{

curve after an external mechanical loading, S 1 can be obtained while the magnitude of the external loading is unknown.

XRD was explored to measure piezoelectric constants of the film Both d33f and d31f were obtained from the measurement of changes in the intercept and slope of dh{ vs.

sin2{ curves caused by applying electric field over the film This method improves the accuracy by directly measuring the strains in the films induced by the externally applied electric field instead of the surface displacement that could easily be interfered by the environment/vibration and surface morphology of the thin films.

Fig.2-1 Schematic of polarization hysteresis curve.

Fig.2-2 Crystal structure of ABO 3.

Fig.2-3 Phase diagram of PbZrO 3 -PbTiO 3.

Fig.2-4 Schematic of X-ray diffraction vector on specimen.

Fig.2-5 Schematic of ferroelectric film capacitor structure.

Fig.3-1 Schematic of PLD set-up.

Fig.3-2 Set-up for synchrotron radiation rocking curve measurement.

Fig.3-3 Photograph showing in-situ four-point bending fixture mounted in

XRD sample stage.

Fig.4-1 Effects of oxygen partial pressure on orientations of LNO films on

SiO 2 /Si(100) substrate.

Fig.4-2 XRD patterns of s-2s scan of PZT films deposited on SiO 2 /Si(100)

substrate with LNO bottom electrodes.

Fig.4-3 XRD patterns of LNO and PZT films deposited on glass substrate

(T sub = 600 flC, P O 2 = 50 and 300 mTorr for depositing LNO and

PZT respectively).

Fig.4-4 Rocking curves of PZT(100) for the film grown on SiO 2 /Si(100)

substrate with LNO underlayer (Tsub = 600 flC, P O2 = 50 and 300

mTorr for depositing LNO and PZT respectively).

Fig.4-5 Pole figure of PZT (110) diffraction for the film grown on

SiO 2 /Si(100) substrate with LNO underlayer (2s = 30.89fl; T sub =

600 flC, P O 2 = 50 and 300 mTorr for depositing LNO and PZT

respectively).

Fig.4-6 SEM micrograph showing cross sectional image of

PZT/LNO/SiO 2 /Si(100) multilayer structure(T sub = 600 flC, P O2 =

50 and 300 mTorr for depositing LNO and PZT respectively).

Fig.4-7 SIMS depth profile of multilayer structure of

PZT/LNO/SiO 2 /Si(100) (T sub = 600 flC, P O2 = 50 and 300 mTorr

for depositing LNO and PZT respectively).

6 7 8 31 51 57 60 61

Fig.4-9 Hysteresis loop of PZT/LNO films on amorphous substrates (T sub =

600 flC, P O 2 = 50 and 300 mTorr for depositing LNO and PZT

respectively).

Fig.5-1 Schematic drawing showing the geometry of pulsed laser deposition.

Fig.5-2 Pole figures of PZT film at r = 10mm,l=0fl.

Fig.5-3 Pole figures of PZT film at r = 21.6 mm,l=213fl.

Fig.5-4 (001) pole figure of LNO film at r = 10 mm,l=0fl.

Fig.5-5 Plane views of PZT film at (a) plume centre, (b) 10 mm from plume

centre.

Fig.5-6 Cross-sectional views of PZT/LNO/SiO2/Si(001) multilayer

structures by PLD at (a) plume centre, (b) 10 mm from plume

centre.

Fig.5-7 Typical P-E hysteresis loop of (001)PZT/LNO/SiO 2 /Si(001) at

different locations of the film.

Fig.6-1 Schematic of experimental set-up and geometry of TSPLD.

Fig.6-2 XRD spectra of LNO films grown on SiO 2 /Si(100) substrate.

(a)PD film (Fig.4-1(b)) (b) TD film measured at X 0= -18mm

(c)TD film measured at X 0 = 0 (d)TD film measured at X 0= 10mm

Fig.6-3 (110) pole figure of LNO film grown on parallel substrate.

Fig.6-4 (100) pole figure of LNO film grown on tilted substrate (n =30fl) at

location X 0=10mm.

Fig.6-5 (110) pole figure measured at X 0= 10mm of the LNO film grown on

tilted substrate (30fl) at (same plume direction as Fig.6-4).

Fig.6-6 (111) pole figure measured at X 0= 10mm of the LNO film grown on

tilted substrate (n =30fl) at (same plume direction as Fig.6-4).

Fig.6-7 hscan spectra of the (110) planes of the LNO film grown on tilted

substrate (n =30fl) measured at (a) X 0 = -18mm, (b) X 0= 10mm and

(c)X 0 = 18mm.

Fig.6-8 FWHM ofhscan on (110) planes and inclined angled of (100) axis

of TD film at different locations of tilted substrate (n=30fl).

73

81 81 82 83 84

85

86

93 95

96 97

98

98

99

100

substrate by PLD at (a) X 0 = -18mm, (b) X 0= 10mm.

Fig.6-11 Schematic diagram of crystal growth in LNO film by TSPLD.

Fig.7-1 Schematic drawing of four-point bending beam.

Fig.7-2 Strain and stress distribution along the beam thickness.

Fig.7-3 Definition of relationship between the diffraction vector g(hkl) and

the principal stress directions.

Fig.7-4 Lattice spacing d{-sin 2{ curves of PZT film under different loading.

Fig.7-5 Changes in intercept and slope of lattice spacing d-sin 2{ curves of

PZT film with maximum deflection of specimen.

Fig.8-1 Schematic of rocking curve technique for curvature measurement.

Fig.8-2 Rocking curves of Si(400) planes at different locations of

TiN/Ti/Si(100) substrate.

Fig.8-3 Rocking curves of Si(400) planes at different locations after

depositing LNO on TiN/Ti/Si(100) substrate.

Fig.8-4 Rocking curves of Si(400) planes at different locations after

depositing PZT on LNO/TiN/Ti/Si(100) substrate.

Fig.8-5 Rocking curve peak positions vs transit distance of the sample.

Fig.9-1 Schematic experimental set-up for X-ray diffraction with in-situ

four-point bending.

Fig.9-2 Lattice spacing of (330) plane from LNO film vs sin2{ curve before

and after loading.

Fig.9-3 Lattice spacing of (321) plane from PZT film vs sin2{ curve without

loading.

Fig.10-1 Schematic set-up for piezoelectric constant measurement.

Fig.10-2 XRD diffractogram of PZT on Ni substrate (Cr anode of X-ray

source).

Fig.10-3 dh{ vs sin 2{ curves of PZT film before and after applying electric

field.

106 110 111 114

117 118

124 127

127

128

129 136

139

140

150 156

157

Table 5-1 Directional anglesc and measured incline anglesd at

different spots of the films Table 6-1 Geometrical parameters at different locations of substrate

after tilt Table 9-1 Elastic properties of PZT materials

88

94

144

E c coercive field strength

u applied stresses of the film along X 2 axis

{ tilt angle of the specimen during XRD measurement

T s substrate temperature

c directional angle of the PLD plume

d incline angle of orientation axis

BST barium strontium titanate

CVD chemical vapour deposition

DCDT double crystal diffraction topography

FESEM Field emission scanning electron microscope

FET field-effect transistor

HRRC high resolution rocking curve technique

IBAD ion beam assisted deposition

ISD inclined substrate deposition

LST laser scanning technique

MEMS microelectromechanical system

MPB morphotropic phase boundary

MOCVD metal organic chemical vapour deposition

NVRAM non-volatile random memory

ODF Orientation Distribution Function

PFM piezo-response force microscopy

PNZT Nd modified Pb(Zr,Ti)O 3

PVD physical vapour deposition

PLD pulsed laser deposition

PMN lead magnesium niobate

RABiTS Rolling Assisted Biaxially Textured Substrates

STM scanning tunneling microscopy

ToF-SIMS time of flight secondary ion mass spectrometry

TSPLD tilted substrate pulsed laser deposition

CHAPTER 1 INTRODUCTION

1.1 BACKGROUND

Ferroelectricity is a phenomenon discovered in 1921 when J Valasek studiedRochelle Salt [1] A huge leap in the research on ferroelectric materials came in the

capacitor applications and piezoelectric transducer devices Since then, many other

lead lanthanum zirconate titanate (PLZT), barium strontium titanate (BST) andrelaxor ferroelectrics like lead magnesium niobate (PMN) have been developed andutilised in a variety of applications [2-9]

With the development of ceramic processing and thin film technology, many newapplications have emerged A renewed interest in ferroelectric films had arisen in the1990s since a number of techniques became available to produce high quality thinfilms of ferroelectrics Plasma sputtering, pulsed laser deposition (PLD), molecularbeam epitaxy (MBE), chemical vapour deposition (CVD), metal organic chemicalvapour deposition (MOCVD), and sol-gel techniques have been extensively explored

in the syntheses of high quality ferroelectric films and great advances have since beenachieved [10-21]

There are a lot of potential applications of ferroelectric films which take advantage oftheir various responses to applied electric field, mechanical stresses, heat and light [4,8-12] The remnant polarisation, piezoelectricity and electric-optical properties in

ferroelectric materials result in a number of unique and/or superior properties forelectric device applications The combination of ferroelectric materials with integratedsemiconductor technology can potentially offer a host of applications in the fields ofdisplays, sensors, memories and optical switches and modulations These attractiveapplications stimulated intensive research on ferroelectric films in the last decades [3-

8, 22]

As stated in the literature review in Chapter 2, the properties of ferroelectric filmsdepend mainly on the microstructure, composition and film stresses, andcharacterizing the properties of the films is very important and challenging

Pulsed laser deposition (PLD) as a novel technique to synthesise ferroelectric thinfilms with near stoichiometric composition is extensively studied Many fundamentaltheories are well established The distribution of thickness, composition, particulateshave also been widely explored However, whether there exists a distribution in thecrystallographic orientation of the grains of the film has not been addressed

Lead zirconate titanate (PZT) is one of the promising candidates used in high-densitymemory devices, advanced actuators and sensors, and microelectromechanicalsystems (MEMS) applications Highly oriented and biaxially aligned films exhibitexcellent performance and are strongly desired Some success has been achieved ingrowth of (001) oriented and epitaxial PZT films on single crystal substrate surfaces

optical-electronics, and their surfaces are amorphous No publication on growing

highly oriented and biaxially aligned PZT/LaNiO3(LNO) films using PLD directly onamorphous surfaces is reported yet.

The residual stresses in ferroelectric films play an important role in the performance

of ferroelectric Although X-ray diffraction is widely used in measuring residualstresses of thin films, lack of reliable X-ray elastic constants (XECs) impedesapplication of XRD in measuring residual stresses of ferroelectric films

Piezoelectric constants of ferroelectric films are their key properties in applications.Currently available inverse measurement methods are all based on detecting thedisplacement of the film surface that may be easily interfered by the environment andsample surface conditions (noise, vibration, surface roughness etc.)

The objectives of this thesis are to grow highly oriented and biaxially alignedPZT/LNO films on amorphous surfaces using PLD, to study the crystallographicorientation distribution of the grains of the films, and to explore new techniques formeasuring elastic constants, residual stresses and piezoelectric constants of theferroelectric films

The scope of the thesis is as follows:

using pulsed laser deposition (PLD) The deposition parameters have beenoptimized to achieve highly oriented and biaxially aligned films

b The distribution in crystallographic orientation axis of the films grown by

PLD is studied The mechanism of the tilt in orientation axis is explored

c New techniques using X-ray diffraction to measure elastic constants andpiezoelectric coefficients of the ferroelectric films are proposed

d Residual stresses in ferroelectric films are measured with high resolutionrocking curve technique (HRRC) A new method for obtaining X-ray stresselastic constants (XEC) of thin films is explored by making use of the residualstress of the films measured by HRRC

This thesis consists mainly of three parts The first part includes Chapters 1 and 2where introduction and literature review on the advancements in synthesis andcharacterization of ferroelectric thin films are presented respectively The second part(Chapters 3 to 6) describes the experimental details, growth of highly oriented andbiaxially aligned PZT/LNO thin films by PLD, and studies on the distribution oforientation axes of these films The third part (Chapters 7 to 10) presents the newtechniques developed for characterizing elastic constants, residual stresses, X-raystress elastic constants and piezoelectric constants The final chapters deal with theconclusions of the present works (Chapter 11) and possibilities of future work(Chapter 12)

CHAPTER 2 LITERATURE REVIEW

2.1.1 Ferroelectric Materials

Ferroelectric materials may be classified as non-linear dielectrics that exhibit an built electric dipole moment in the absence of an external electric field They may bederived from non-centrosymmetric crystal-lattice structure in which spontaneouspolarization is observed Applying a sufficiently large electric field can switch thedirection of polarization, leaving the crystal with a net polarization when the electricfield is removed The spontaneous polarization is generated from noncentrosymmetricarrangement of ions in the unit cell, which produces an electric dispole momentrelated to the unit cell The adjacent unit cells are inclined to polarize in the samedirection forming a region called ferroelectric domain [1, 9] There may be manydomains in a crystal separated by interfaces called domain walls A very strong fieldcould lead to the reversal of the polarization in the domain, known as domainswitching

in-The polarization (P) of the ferroelectric materials changes with the applied electric

field (E) as typically shown by ferroelectric hysteresis in Fig.2-1 At very high field

zero when the external field is removed At zero external field, some of the domains

remain aligned in the positive direction, hence the crystal will show a remnant

magnitude OF is applied in the negative direction The external field needed to reduce

Fig.2-1 Schematic of polarization hysteresis curve [1].

One of the most important crystalline structure of ferroelectric materials is perovskite

suitable metallic elements The most widely used ferroelectric materials such as

niobate (PMN) all have perovskite structure

P

Fig.2-2 Crystal structure of ABO 3.

undergoes a phase transition from a non-ferroelectric phase to a ferroelectric phase

Lead zirconate titanate (PZT) is one of the most widely used perovskite ferroelectricmaterials with excellent performance Its high remanent polarization, piezoelectricconstants and dielectric constant, strong electro-optic effect, and goodpyroelectricicity offer great opportunities in applications in MEMS, microelectronics

symmetrical positions At high temperatures, PZT has a cubic structure that isparaelectric When the temperature falls below the Curie point, the structureundergoes a phase transition to form a ferroelectric tetragonal or rhombohedral

structure The ions are at their equilibrium positions at which the free energy of thecrystal is minimum The centre of positive charge does not coincide with the centre ofnegative charge, thus resulting in a permanent electric dipole or spontaneouspolarization In the tetragonal phase, the spontaneous polarization is along the <100>set of directions while in the rhombohedral phase the polarization is along <111> set

of directions The morphotropic phase boundary (MPB) separating the tetragonal andorthorhombic phases has a composition with a Zr/Ti ratio of about 52/48 at roomtemperature as shown in Fig.2-3 Most ferroelectric properties such as dielectric andpiezoelectric constants show an anomalous behavior at MPB PZT ceramics with theMPB composition show excellent piezoelectric properties The poling is easier at thiscomposition because the spontaneous polarization within each grain can be switched

to one of the 14 possible orientations, i.e eight <111> directions for the rhombohedralphase and six <100> directions for the tetragonal phase [1]

Fig.2-3 Phase diagram of PbZrO 3 -PbTiO 3 [1].

To suit some specific requirements for certain applications, piezoelectric ceramics can

be modified by doping with some dopants which have valence difference from theions in the lattice [1] The dopants can be divided into “hard” and “soft” ones formingthe so-called "hard" and "soft" PZT's Hard PZT's are doped with acceptor ions such

the lattice Hard PZT's usually have lower permittivities, smaller electrical losses andlower piezoelectric coefficients They are more difficult to pole and depole, thusmaking them ideal for rugged applications On the other hand, soft PZT's are doped

creation of A site vacancies in the lattice Soft PZT's have higher permittivity, largerlosses, higher piezoelectric coefficient and are easy to pole and depole They may beused in applications requiring very high piezoelectric properties

2.1.2 Ferroelectric Thin Film Synthesis Techniques

Synthesis of ferroelectric thin films can be divided into two main categories, namelyphysical vapour deposition (PVD) and chemical methods PVD includes plasmasputtering [10, 12, 13], pulsed laser deposition (PLD) [10, 11, 13, 20, 23], molecularbeam epitaxy (MBE) [10, 13] and ion beam deposition (IBD) [10] Chemical methodsinclude chemical vapour deposition (CVD) [10, 13], metal organic chemical vapourdeposition (MOCVD) [14-16] and sol-gel [17, 18]

PVD involves the atomising of material from a solid, transporting and depositingthem onto a substrate to form a film Currently, PVD is the major depositiontechnique for depositing ferroelectric thin films

CVD films are produced by the chemical reaction of vapour phase precursors at thesubstrate surface A great advantage of such film growth is the ability to coat largearea and complex shape with good coverage.

Sol-gel is a fast developing technique and a promising method for growingferroelectric films It involves the hydrolysis and condensation of organometallicprecursors on the substrate The deposited sol is then annealed at high temperatures tocrystallise and densify the film

The deposition of the films generally consists of several sequential steps: adsorption,surface diffusion, reaction, initial aggregation (nucleation) and growth to develop themicrostructures of the films with certain morphology and crystallography

The nucleation and growth of the films are regulated by kinetic energy of theatoms/species and substrate temperature, surface energy and conditions of thesubstrates There are three modes of growth Layer-to-layer growth mode, also known

as Frank-van der Merve growth, occurs when a new layer formation starts after theprevious layer has been completed This mode is driven by the surface energy of thesubstrate, which exceeds the total energy of the epilayer surface energy and interfaceenergy It is often refereed to as 2D growth mode Another mode is island growthmode, or Volmer-weber growth mode The bonding between the film atomsthemselves in the deposited layer is stronger than that between them and the substrate.Such mode often produces three-dimensional islands An intermediate growth mode islayer-and-island growth mode, also called Stranski-Krastanow growth mode After amonolayer is completely formed, 3-D clusters nucleate on these layers due to the

change in surface energy of the initial layers The microstructures of the film aremainly determined by the growth mode.

The properties of ferroelectric films depend mainly on the microstructure,composition and film stresses Biaxially aligned and highly oriented films are stronglydesired In depositing ferroelectric thin film, film stoichiometry and processcompatibility to semiconductor manufacturing are two very important issues Besides,homogenous thickness over large area should be taken into consideration

affect the crystallization of the films Sol-gel method generally deposits films withamorphous structure at room temperature They need to be annealed at hightemperature to crystallize However, some ferroelectric materials such as PZT, PLZT,

by PVD or CVD To grow high quality ferroelectric thin films, the substratetemperatures are generally limited to some small temperature window If substratetemperature is too low, pyrochlore non-ferroelectric phase will form If substratetemperature is too high, the volatile elements in the ferroelectric materials will easily

be evaporated thus losing stoichiometry Energetic fluxes in some PVD methods(such as PLD, IBD) can produce effects similar to those of raising homologous

sometimes can be deposited at lower substrate temperature [11]

Microstructures of the films strongly depend on deposition methods Themicrostructure of the films produced by sol-gel is mainly determined by the initial

conditions Due to evaporation of organic components during high temperaturecrystallization, the films by sol-gel often contain porosity to a certain extent Thus, thechallenges in sol-gel deposition are to reduce porosity and lower annealingtemperature as much as possible.

Stoichiometry is another important issue in depositing ferroelectric thin films In gel, a layer of liquid with homogenous composition covers the substrate by spincoating Therefore, stoichiometry is relatively easy to achieve by sol-gel However,many factors in PVD, such as ambient pressure, substrate temperature and energy,could affect stoichiometry of the films They should be carefully controlled

sol-In all, the requirements on ferroelectric thin films depend on their applications.Understanding the relationship between microstructure, properties and depositionconditions is very important Optimizing deposition conditions, achieving preferredfilm orientation or even biaxial alignment, and obtaining desirable ferroelectricperformance are an imminent and challenging task

2.1.3 Application of Ferroelectric Thin Films

Ferroelectric thin films have been explored for applications in sensors and actuators,memory devices, medical ultrasound imaging, data storage, displays,microelectromechanical system (MEMS), and microelectronics [3-22]

2.1.3.1 Sensors and Actuators [4, 9, 22-27]

In microelectromechanical system (MEMS), sensors and actuators are constructed inmicro scale using lithographic techniques developed for integrated-circuit fabrication.The devices are able to response to external stimulus and environment They takeadvantage of piezoelectric, electro-optical, electrical and pyroelectric properties offerroelectric materials MEMS using ferroelectric films are capable of high frequencyresponse

The large piezoelectric properties of ferroelectric thin films such as PZT can be usedfor the fabrication of surface acoustic wave (SAW) devices [3] Ferroelectric actuatorshave been developed in MEMS for precise position control and for small pulse-driven

or ultrasonic motors [9] The strong electro-optical coupling of many ferroelectricmaterials provides for a number of potential applications of their thin films Opticalwaveguides fabricated with ferroelectric thin films could be used in directionalcouplers [9]

2.1.3.2 Non-volatile Random Access Memory [3, 10, 12, 28]

Ferroelectric thin films have attracted much attention due to their possible use in volatile random memory (NVRAM) applications The main advantages offered byferroelectric random access memories (FRAM's) include non-volatile and radiationhardened compatibility with CMOS and GaAs circuitry, high speed and high density

non-Among ferroelectric materials, PbZrxTi1-xO3 (PZT) and SrBi2Ta2O9 (SBT) are themost promising candidates The application in non-volatile random access memoryuses the switchable remnant polarisation of a ferroelectric thin film to storeinformation without requiring application of continuous power In fact, this provides asolid-state replacement to the current magnetic or optical disc storage media, but withmuch faster access speed.

One NVRAM approach is based on non-destructive readout It avoids repolarization

of the ferroelectric after a read operation For example, a ferroelectric film isdeposited over the gate of a field-effect transistor (FET) The polarisation state of theferroelectric film controls the source to drain current through the FET

Another destructive NVRAM approach uses the remnant polarisation of theferroelectric capacitor to store information The polarisation state is read bymeasuring the current drawn through the capacitor by an applied voltage pulse Sincethis read operation could reverse the polarisation state, it is known as a destructivereadout (DRO) device

2.1.3.3 Microelectronic Devices [3, 28]

Besides NVRAM, ferroelectric material is one of good candidates for gate dielectricmaterial As integrated circuit devices continue scaling down to deep sub-micro andeven nano-scale, gate oxide thickness and capacitor area become so small that muchhigher dielectric properties are required The dielectric properties of silicon oxide andnitride gate dielectrics used traditionally are too low to meet such requirements

Ferroelectric materials have much higher dielectric constant comparing to siliconoxide and nitride and are considered as one of the alternatives in future microelectricapplications.

As briefed in Section 2.1.2, there are several techniques available for thin filmdeposition Among these techniques, pulsed laser deposition (PLD) is one of thetechniques to prepare ferroelectric thin films with near stoichiometric composition

Using intense laser radiation to deposit thin film was first demonstrated by Smith andTurner [29] in 1965 During the next two decades, however, further investigationswere sporadic and slow The first major breakthrough came in middle of 1970s whenelectronic Q-switch was developed to deliver short pulses with very high powerdensity It was the second breakthrough, which was triggered by the successfulgrowth of high Tc superconducting films in 1987 [30] In the recent two decades,PLD was widely employed to deposit high quality ferroelectric, superconductive,dielectric, magnetic thin films [31-40] The high output power and an appropriateoperational wavelength make Excimer laser to be the most common type of the lasersuitable for the laser ablation

2.2.1 Advantages of PLD

PLD lodges atoms from the target via an ultraviolet (UV) laser pulse, unlike ions andelectrons, laser beam are much easier to transport and manipulate Since the

interaction between laser and gas phase species is relatively weak, the dynamic range

of deposition pressure is the largest compared to virtually all other depositionprocesses At wavelength of about 250 nm and below, all materials absorb the laserbeam either via linear or non-linear processes whereby coupling of energy is possible

to most surfaces, making the process very ubiquitous The technique can reproducethe target composition with relative ease under the appropriate deposition conditions

A variety of multi-layers can be easily produced by positioning different targets underthe laser beam The spatial confinement of the laser-target interaction and thesubsequent confined evaporant make PLD an inherently clean process [11] Allmaterials can be deposited by PLD since the PLD processes are independent of theirelectrical conductivity

2.2.2 Pulse Laser Deposition Process

Pulsed laser deposition is often described as a three-step process includingvaporisation of target material, transport of the vapour plume and film nucleation andgrowth of the film on a substrate These steps are continuously repeated for each laserpulse during the process

The PLD process consists of three stages [11]:

a) The laser beam strikes a solid target of some known general compositions andproduces a highly forward-directed plume of gas phase material;

b) The plume interacts (both chemically and physically) with a background ambient;c) The ablated material condenses onto a suitable substrate where a thin film

nucleates and grows

Laser of high power densities and short pulse cause short interaction cycles resulting

in congruent ablation of small volume of materials The heat load on the targetmaterial is small with no material segregation taking place Thus, the ablationmaterials have the same chemical compositions as those of the targets, which iscrucial in depositing multi-component materials thin films from single target

2.2.3 Deposition Parameters

PZT is one of the typical and most widely used ferroelectric materials Here PZT istaken as an example of ferroelectric material to elucidate the various parameters onthe properties and quality of the deposited thin films

A number of researchers found that using PLD, the perovskite PZT films could only

be obtained in a narrow window of the substrate temperature Safari et al reportedthat the window was around 575flC [41] When the substrate temperature was below500flC, the pyrochlore phase became dominant As the substrate temperature

film at about 575 - 600flC Further increase in temperature caused the Pb content inthe films to decrease, resulting in a pyrochlore structure.

2.2.3.2 Background gas [39-48]

Ambient gases present during PLD scatter, attenuate and thermalize the plume Theychange the important film growth parameters such as the spatial distribution,deposition rate and kinetic energy distribution of the deposition species Reactivescattering can result in the formation of molecules or clusters that can aid in theincorporation of the gas into the growing film

In depositing Pb containing ferroelectric films, oxygen partial pressure is critical inmaintaining the Pb content in the film [45-50] The ambient gas must not only keepthe Pb atoms on the surface once they arrive but must also be reactive in order toincorporate the Pb atoms into the films The observation of Pb deficiency in the filmsprepared from fully reacted targets compared to films from pressed-oxide targets may

be attributed to the lower concentration of PbO in the plume The generalunreactiveness of Pb toward oxygen can be compensated by either increasing theoxygen pressures or using more reactive oxidants [11]

At low pressure, the energetic particles might travel out of the plume and impinge onthe substrate This may lead to preferential sputtering of atoms already on thesubstrate causing their removal via kinetic collisions, thus resulting in a loss ofstoichiometry of the films and producing more metastable pyrochlore phases The gasambient pressure controls this slight deficiency of the volatile elements Therefore at a

given substrate temperature, the incorporation of a volatile element into the film isgreatly dependent on the pressure of the oxidising background gas At elevatedtemperature, the deposition pressure must be sufficient to retain the volatileconstituents and nucleate the correct crystallographic phase.

2.2.3.3 Laser fluence [11]

The chemical composition of the deposited films strongly depends on the laserfluence At the low fluence, thermal evaporation takes place and the volatilecomponents of the target are preferentially re-evaporated, resulting in a change intarget composition At high fluence, an ablation process dominates, and most atomicspecies are produced This results in the production of a large amount of material in asmall volume of the target surface which rapidly expands adiabatically into thevacuum and a more forward-directed and translationally energetic component of theplume It was found that the content of Pb decreases when laser fluence increases

of the volatile components could thus be much less Therefore, more excess volatilecomponents shall be added in the fully reacted targets than that in the pressed oxidetargets.

In actual applications, excess amount of PbO is added in the preparation of targets for

Pb contained ferroelectric materials to compensate for the loss of the volatilecomponent In this way, better quality of PZT films with a higher percentage ofperovskite phase can be obtained [39, 49, 50]

2.2.3.5 Underlayer electrodes

Ferroelectric thin films nucleate and grow on the underlayer electrodes The crystalstructure and surface conditions of the underlayer material/substrate are thereforecritical in order to obtain high quality thin films, especially to deposit oriented or evenepitaxial films [31, 33, 35, 39]

The most ideal conditions for growing epitaxial films are that the crystal structure ofthe underlayer material/substrate is the same as that of the films and the difference inlattice constants between them is very small It is energetically favorable for the filmmaterial to crystallographically align itself with such underlayer material/substrate so

as to match their bonding symmetry and periodicity [10]

Traditionally, noble metals such as Pt [51, 52] have been used as bottom electrode forferroelectric films, but they suffer severe degradation in performance with electriccycling Conductive oxides have been extensively explored as the bottom electrode to