Optimization of fabrication parameters of barium doped pb(zr0 52ti0 48)o3 thin films on tisi substrates using pulsed laser deposition

material...2 Figure 1.2: The formation of 180○ and 90○ ferroelectric domain walls in a tetragonal perovskite ferroelectric; Ed: depolarizing field, Ps: spontaneous polarization...3 Figur

(Khoa học và kỹ thuật Vật liệu Điện tử)

NGƯỜI HƯỚNG DẪN KHOA HỌC :

1 TS Nguyễn Đức Minh

2 PGS TS Vũ Ngọc Hùng

Hà Nội – 2013

The work has been carried out in the internship program at Solutions in

September, 2013 Except where specific references are made, this thesis is entirely the result of my own work and includes nothing that is the outcome of work done in collaboration No part of this work has been or being submitted for other degree, diploma or qualification at this or other university

Enschede, September 2013

Pham Ngoc Thao

This work is done in following the internship program at Solutions in

I would like to express my gratitude to my supervisor Assoc Prof Vu Ngoc Hung, who offered me the invaluable guidance, supports in my two years study and research at International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), Vietnam

I am deeply indebted to my supervisor Dr Nguyen Duc Minh (ITIMS & SolMateS), who gave me a precious opportunity to the beautiful city-Enschede, The Netherlands-to join this internship program at SolMateS company I especially wish to thank him about taking professional guidance, and sharing experiences in practical work, giving constructive advices throughout this research and thesis writing

I am very grateful to Dr Matthijn Dekkers (SolMateS) for the long support, encouragement and his suggestions for this thesis With his help, I have

an opportunity to understand about working in a research enviroment of the commerical company, like SolMateS

Special acknowledgments to all members of SolMateS company who created friendly work environment, and gave me encouraging supports Their interest, and hard working to the work impress me so much It is my honor to work with all of them Dear Nicolas, thanks for your great support and kindness Shared office with you is my pleasure Dear Saskia and Francis, I want to say thank to both of you for administration assistance Dear Jan, I have really enjoyed time we spent together in talking about the ships and Dutch culture Dear Steven, thanks for your warm friendship

Khiem, Dr Nguyen Van Quy Many thanks to ITIMS employees for always supporting me such Dr Thanh, Dr Toan, Dr Ngoc Anh, Dr Ha, Ms Loan, Ms Lan, Dr Le, Dr Xuan And thanks go to all members of MEMS group such Dr Thong, Dr Hoang, Dr Hien, PhD student Chi, Eng Tai

I would also like to thank all friends in The Netherlands: Minh-Giang’s

family, Tuan-Hieu’s family, Chung (UvA), Bay (UvA), big cat Tom Aarnink

(UT), Boota (UT), Nirupam (UT), Kenan (UT) because of your warm and wonderful encouragement to me

Last but not least, I would like to thank to my parents and my sister for their endless love, support, motivations; all of my friends in Viet Nam for their friendship

This work was financially supported by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant number 103.02-2011.43, and by the Interreg project "Unihealth"

Enschede, September 2013

Pham Ngoc Thao

ACKNOWLEDGEMENTS ii

TABLE OF CONTENTS iv

LIST OF FIGURES viii

LIST OF TABLES xi

CHAPTER 1 1

THEORETICAL BACKGROUND 1

1.1 Introduction 1

1.2 Ferroelectricity 1

1.3 Lead Zirconate Titanate Pb(ZrxTi1-x)O3 (PZT) 5

1.3.1 Crystal structure 5

1.3.2 Phase diagram 6

1.3.3 Physical properties of PZT thin film 7

1.3.3.1 Ferroelectric properties 7

1.3.3.2 Dielectric properties 8

1.3.3.3 Piezoelectric properties 10

1.4 Approaches to improve the properties of PZT thin films 13

1.4.1 Doping 13

1.4.2 Electrode 15

1.6 Summary 19

CHAPTER 2 20

EXPERIMENTAL PRODUCES 20

2.1 Introduction 20

2.2 Thin film growth 20

2.2.1 General techniques for fabrication 20

2.2.2 Pulsed laser deposition (PLD) 22

2.2.2.1 Mechanisms of PLD 23

2.2.2.2 Experimental setup 24

2.3 Patterning process of PBZT thin film capacitors 26

2.4 Characterization techniques 28

2.4.1 Structural analysis 28

2.4.2 Morphological analysis 30

2.4.3 Electrical characterization 30

2.4.3.1 Ferroelectric properties 30

2.4.3.2 Dielectric properties 33

2.4.4 Mechanical characterization 33

2.5 Summary 34

3.1 Introduction 36

3.2 Structure and morphology 36

3.3 Electrical properties 39

3.3.1 Ferroelectric properties 39

3.3.1.1 Hysteresis loops 39

3.3.1.2 Fatigue behavior 40

3.3.1.3 Effect of applied field 43

3.3.2 Dielectric properties 44

3.4 Mechanical properties 45

3.5 Effect of poling process 48

3.6 Summary 50

CHAPTER 4 52

OPTIMIZATION OF ELECTRODE THICKNESS 52

4.1 Introduction 52

4.2 Structure and morphology 53

4.3 Electrical properties 56

4.3.1 Ferroelectric properties 57

4.3.1.1.Hysteresis loops 57

4.3.1.2 Fatigue behavior 58

4.5 Summary 62

CHAPTER 5 64

CONCLUSION AND SUGGESTION FOR FUTURE WORK 64

5.1 Conclusion 64

5.2 Suggestions for future works 65

REFERENCE 66

material 2

Figure 1.2: The formation of 180○ and 90○ ferroelectric domain walls in a tetragonal perovskite ferroelectric; Ed: depolarizing field, Ps: spontaneous polarization 3

Figure 1.3: Hysteresis loop and domain switching 3

Figure 1.4: Schematic illustration of the poling process 5

Figure 1.5: Schematic of cubic ABO3 perovskite 5

Figure 1.6: Phase diagram PZT solid solution 6

Figure 1.7 : Axes including normal (1-3) and shear directions (4-6) 10

Figure 1.8 : (a) Capacitor and cantilever structures; 3D-upward displacements of (b) capacitor and (c) cantilever The LDV measurements were performed 11

Figure 1.9: The example of the relationship between dielectric constant, d33 coefficient and Zr/Ti ratio of PZT films 12

Figure 1.10: (a) The dependence of 2Pr values of PZT films as a function of the thicknesses of LNO buffer layers; (b) The d33values of PZT films as a function of the thicknesses of LNO buffer layers 17

Figure 1.11: The chapter structure of thesis The main achievements of each chapter are summarized below the titles 18

Figure 2.1: (a) Flow diagram for the PZT thin film was deposited by Sol-gel processing; (b) The HRSEM of PZT thin film 21

Figure 2.2: A schematic construction of PLD system 25

Netherlands 29

Figure 2.5: A construction of SEM 30

Figure 2.6: Ferroelectric polarization (P–E) hysteresis loop of a PBZT thin film

capacitor 31

Figure 2.7: The typical signal of fatigue excitation 32

Figure 2.8: A Polytec MSA-400 micro–scanning laser Doppler vibrometer

system at IMS Group-Mesa+, University of Twente, Netherlands 33

Figure 2.9: Schematic view of the measurement set-up for the d33

Figure 3.3: XRD patterns of PBZT thin films at different deposition

temperatures: (a) full scale and (b) zoom scale of (111) peak 39

Figure 3.4: PBZT films deposited on TiN/Ti/SiO2/Si susbtrates by PLD technique: (a) Hysteresis loops at different temperatures; and (b) The temperature

Figure 3.5: (a) The SEM image of external failure; (b) The fatigue behavior of

insets show the electric field as a funtion of switching cycles 41

Figure 3.7: The hysteresis loops of PBZT thin films at 535 ○C deposition temperature under different applied field 43

Figure 3.8: The PBZT film capactiors: (a) Dielectric constant–electric field (ε– E) curves; and (b) Dielectric constant and dielectric loss as a function of

deposition temperature 44

Figure 3.9: Upward displacement versus different deposition temperature of

Figure 3.10: The effective piezoelectric constant versus deposition temperature 47

Figure 3.11: (a) The SEM cross-section, (b) Polarization hysteresis (P-E) loops

film by Sol-gel 49

Figure 4.1: The images of PBZT films on different thicknesses of TiN/Si

Figure 4.2: The micrographs of the PBZT film on 150 nm thickness of TiN

Figure 4.3: The XRD patterns of the PZT films with different susbtrates within

and (d-g) zoom scale of (110)-peak respect to each sample 55

Figure 4.4: The XRD patterns of the PBZT films with 150 nm thickness of

Figure 4.6: The relationship between Pr and Ec values and temperatures on

different thicknesses of electrodes 57

Figure 4.7: Remanent polarization Pr of Pt/PBZT/TiN capacitors on various electrode thicknesses as a function of cumulative switching cycles 59

Figure 4.8: The ε and tanδvalues versus temperatures on different thicknesses

of electrodes 60

Figure 4.9: Piezoelectric constant d33,f as a function of temperatures on different thicknesses of electrodes 61

respect to the compostions (around the morphotropic phase) of PZT films fabricated on SRO/STO substrates 8

Table 1.2: The dielectric constant (ε) values of PZT at various compositions using different deposition techniques 9

Table 1.3 : The dielectric constant value of some materials 10

Table 1.4: Ferroelectric and piezoelectric properties of the undoped and doped PZT thin films 14

Table 2.1: PLD parameters to obtain PBZT thin films with LNO as layer on TiN/Ti/SiO 2 /Si substrates 26

buffer-Table 2.2: Details for investigation steps to optimum the growth film on TiN electrode 26

Table 2.3: The information of steps in patterning process 28

Table 3.1: The list summary about these experimental results of PBZT film on

150 nm thickness of TiN electrode 50

Table 4.1: The list summary about these experimental results of PBZT films on various thicknesses of TiN electrodes 63

Moreover, the requirements of PZT properties on each particular application is discussed in this section Combination of good properties and stability in a wide range of operating conditions has led to the increasing amount of study on PZT material for oriented applications However, this material still remains many problems that must be overcome before viable commercial products can be produced The solutions for these drawbacks can be found in section 1.4 Finally, the research scopes and objectives are shown in the last section of this chapter

1.2 Ferroelectricity

The history of ferroelectricity have began since the year of 1920 when Pierre and Jacquez Curie found piezoelectricity in materials such as quartz, Rochelle salt, etc They discovered that these materials could generated voltage from mechanical stress and later on confirmed the opposite phenomenon:

mechanical deformation by applied electric field According Halasyamani et al

[25], ferroelectricity belongs to non-centrosymmetric materials which are of special interests because their symmetry – dependent properties Non-centrosymmetric can be divided into polar and non-polar crystal class The term

“polar” is more correctly used for the non-centrosymmteric containing a unique anisotropic axis In the polar class, ferroelectric materials possess a spontaneous

electric polarization that can be reserved under an external electric field [12]

Figure 1.1: Schematic diagram of the phase transition in a ferroelectric material

Phase transition of ferroelectricity can be changed by controlling temperature In materials science, this temperature is called the Curie temperature

which shows a linear funtion between polarization and applied electric field

the ferroelectricity observes a spontaneous nonzero polarization without applied field In this case, the non-linear between polarization and electric field is called

a hysteresis loop In order to have a better understanding about the behavior of this loops, domains and domain walls of the crystals into this material need to comprehend

Within ferroelectricity, a ferroelectric domain is a region where the spontaneous polarization is uniformly oriented For example, the six possible directions of spontaneous polarization can be found in tetragonal phase The spontaneous polarization can switch to any of six directions, during the phase transition

The boundaries separating domains are refered to as domain walls The

Paraelectric phase above T c Ferrolectric phase below T c

Tetragonal

Rhombohedral

wall between oppositely oriented domain, the separation between perpendicular

Figure 1.2: The formation of 180○ and 90○ ferroelectric domain walls in a

polarization [19]

The hysteresis loop and domain switching in ferroelectric materials can be shown in Fig.1.3 Initially, the net polarization is small

Figure 1.3: Hysteresis loop and domain switching [30].

opposite to the field, starts to switch along the direction of the field It leads to a

180 ○

Ferroelectric Domain Walls

90 ○

Alignment

Domain boundary movements

Orignal domains

Remanent polarization

No net polarization

Reverse alignment

non-linear measurement of charge intensity This switch still continue, until the result of the polarization measurement returns to be linear (saturation) It means all the domains align with the applied electric field direction With decreasing field, polarization decreases linearly And when the field returns to zero, the

nucleation of reserved polarization domain starts This process can be repeated Hence, this relationship between the polarization and electric field in ferroelectrics is often non-linear and its hysteresis due to domain wall motion and

switching

Poling process

As previous discussion, a ferroelectric crystal includes multiple domains

So, a single domain within the crystal can be obtained by domain wall motion It

is possible by the application of a sufficiently high elelectric field, the process is known as poling [60] Before poling, polycrystal ferroelectric materials do not possess any properties due to the random orientations of the ferroelectric

domains During poling, a dc electric field is applied on the ferroelectric sample

to be oriented or “poled” for domains Because the domains in the crystal is coincidentally oriented, they can’t be aligned perfectly with the applied field However, their polarization vectors can be still aligned to the maximum component that they can follow the direction of electric field In general, more complete alignment of domain polarization can be obtained by higher poling field, longer poling duration and higher poling temperature At these optimum parameters, the domains will move easy that is known as domain switching

After poling, the electric field is removed and a remanent polarization and remanent strain are still maintained in the ferroelectric material A simple illustration of the poling process is shown in Fig.1.4 Therefore, it should be noted that the poling process is very necessary for the bulk ferroelectric ceramics

since they are not naturally polarized [28]

Figure 1.4: Schematic illustration of the poling process [60]

1.3 Lead Zirconate Titanate Pb(Zr x Ti 1-x )O 3 (PZT)

Among ferroelectric materials, Lead Zirconate Titanate (PZT) is one of the most potential researches because of its superior properties This material has

(iii) a wide range of dielectric constants [23] Hence, until now, there are still

many interesting ideas and studies being carried out for PZT material

1.3.1 Crystal structure

PZT material belongs to the perovskite family, and exhibits a generally

Figure 1.5: Schematic of cubic ABO3 perovskite

Six oxygen atoms are arranged into an octahedron with Zr4+/Ti4+ at the

Fig.1.5

1.3.2 Phase diagram

range between 0.48 and 1.0, the symmetry of PZT is tetragonal Higher-level

Figure 1.6: Phase diagram PZT solid solution [19]

P C : cubic paraelectric phase, F T : tetragonal ferroelectric phase; F R(HT) : high temperature rhombohedral ferroelectric phase, F R(LT) : low temperature rhombohedral ferroelectric phase, A O : orthorhombic paraelectric phase, A T : orthorhombic paraelectric phase

Especially, this rhombohedral phase is divided into two phases, including

Note that in Fig.1.6 the phase boundary between the tetragonal and rhombohedral phases is nearly independent of temperature and called

morphotrophic phase boundary (denoted MPB) A morphotropic is used as term

to exhibit an abrupt structure change in a solid solution with various compostions [20]

1.3.3 Physical properties of PZT thin film

material with their interesting applications In addition, with many breakthroughs

in the fabrication of PZT film, the researches on this material has gathered greater momentum In comparison to bulk materials, the advantages of

ferroelectric films can be shown such as: (i) simple fabrication process with fewer processing steps, (ii) lower voltage requirement for polarization with thinner thickness, (ii) ideally suit for applications in integration, (v) larger areas

possible with competition cost [24]

1.3.3.1 Ferroelectric properties

random access memories) applications, PZT material has become a potential

properties of PZT strongly depend on its composition, it is desired to determine

the optimal composition for these applications In literature, many researches

also be confirmed by the studies of Foster et al [18] Furthermore, the researches

also shown that the PZT material at x = 0.52 composition seemed to be a

Table 1.1: The remanent polarization (P r ) and coercive field (E c ) values respect

to the compostions (around the morphotropic phase) of PZT films fabricated on SRO/STO substrates [36]

1.3.3.2 Dielectric properties

Generally, ferroelectrics are dielectric materials Dielectric constant and dielectric loss are also important paramters for their electrical properties In recent years, due to increasing capacitance or charge storage ability by polarization of molecules [6,31], dielectric materials are utilized widely in capacitors A significant coefficient for representing the charge storing capacity

between two plates of the capacitor can be defined by dielectric constant (ε) The

ε value of capacitor structure is given by the following Eq.1-1 [43]:

ε = (1-1)

where ε is the dielectric constant (permittivity) of material between the plates; C

Under an applied electric field, the dipoles in this material will change their orientations along the direction of the applied electric field But this process requires some finite time This delay in dielectric response towards the electric

field is called as dielectric relaxation or dielectric loss (tan δ) The equation

determination for this factor of capacitor structure is depicted in Eq.1-2 [43]

reported as function of Zirconium concentration Tab.1.2 shows the dielectric constant values of PZT at various compositions using different fabrication

techniques Although the reported values of dielectric constant (ε) are distinctly different for each method, almost results can be proved the maximum dielectric constant (ε) of PZT at MPB composition It can be explained by the

coexistence of tetragonal and rhombohedral phase that increases the number of

alternative crystallographic directions for polarization to 14 (eight from rhombohedral structure and six from tetragonal structure), so, the domains can switch easily [19,56]

Table 1.3 : The dielectric constant value of some materials

this comparison, The PZT with the higher ε value is the desired material for

DRAM (high-density planar density random access memories) applications

1.3.3.3 Piezoelectric properties

potential from applied mechanical stress and vice versa [14] To otain a better understanding about piezoelectric properties of the PZT material, piezoelectric coefficients and electromechanical coupling factor need to observe

Figure 1.7 : Axes including normal (1-3) and shear directions (4-6) [16]

generated in a material per unit mechanical stress applied to it Alternatively, it

is the mechanical strain generated in a material per unit electric field applied to it [48] The directions of deformation in PZT materials can be visualized from Fig.1.7

piezoelectric coefficient

Figure 1.8 : (a) Capacitor and cantilever structures; 3D-upward displacements of

(b) capacitor and (c) cantilever The LDV measurements were performed in [44]

value can be significantly performed, although these coefficients exist

value can be ignored The example about the observation of the displacement in this structure can be shown in Fig.1.8(b) With using cantilever structure in Fig.1.8(c), the substrate clamp effects is removed, the in-plane piezoelectric

measure the displacement in the devices, the laser Doppler vibrometer (LDV) measurement was performed More details of this measurement will be discussed

(a)

(c) (b)

in the chapter 2

properties is well matched with optimal composition (Zr/Ti ratio) in PZT material Many studies report that the composition of highest electromechanical activity (maximum piezoelectric coefficient) can be depicted at the morphotropic phase boundary (MPB) [34,43,59]

Figure 1.9: The example of the relationship between dielectric constant, d33

coefficient and Zr/Ti ratio of PZT films [43]

electromechanical coupling factor, k This factor can be defined by Eq.1.3 It

demonstrates the amount of electrical energy converts to mechanial energy, and vice versa

The value of k is always less than one because no material can convert

factor still can be found at MPB phase Thus, PZT material at MPB phase has been exploited as a promising candidate for transducer and actuator applications

The first type is called donor dopant, or known as soft dopant This type is

separated in able two occupations: (i) the ions which exhibit larger ionic radii,

higher piezoelectric coupling coefficient, and higher dielectric constant These advantages making it useful for actuation and sensing applications

The second additivies are acceptor dopants, or known as hard dopants Lower dielectric constant, lower dielectric loss (tan δ), and higher coercive field

improvement on the properties have received considerable attention to be applied

piezoelectric coefficient are not as larger as that of other dopants However, they have been strongly interested in the commercial products with their optimum breakdown voltage Due to this enhancement, they may be preferentially used in

piezoelectric MEMS accelerometers With higher breakdown voltage, the higher

applied electric voltage can be obtained Hence, the sensitivity of MEMS accelerometers can be significantly improved whereas output-noise density become to be minimized

The effect of three types of substitutions on the electrical and mechanical

properties of PZT films can be shown clearly by the research of Nguyen et al

a potential explanation why their properties aren’t as high as other doped-PZT

films However, Ba-doped PZT material is still utilized widely as a hopeful candidate for oriented applications with requiring higher applied voltage

1.4.2 Electrode

The excellent properties of PZT material has led to the increasing amount

of study on it [5,9] However, one of the main drawback of PZT material before producing the viable commercial product which is hight cost of electrode fabrication, still remains In addition, the compatibility between ferroelectric and electrode materials makes the constraint in the integration of the ferroelectric devices (ferroelectric capacitors) Therefore, the choice of the electrode and ferroelectric materials is an important consideration

For applications owning on PZT thin films, metal electrodes such as Au,

Pt, and Ag are being widely used Or other chooses, conductive-oxide materials,

materials [4,53] but also play an important role in buffer technology for improving the quality of the device application in multi-layers systems [11] However, the manufacturing processes of these electrodes have the high cost These investigations have been done to promote the development of new electrode generation with lower price Moreover, these materials still satisfy the complex role of electrodes [2]:

the two superposed materials

From this thesis, we suggest that one of the possible candidates in the new generation can be Titanium Nitride (TiN) TiN has found an increasing interest

because of its excellent properties such as: good mechanical properties, high conductivity, high corrosion resistance, low friction coefficient [38] And, TiN

also can utilize in CMOS process combination

1.4.3 Buffer layer

attracted the considerable attentions In literature, there are many investigations about the advantages of buffer layer [3,16], such as enhance the nucleation and growth of the perovskite phase; prevent the diffusion between the electrode and PZT film; and improve the properties of PZT films; etc

buffer layer, the optimum fatigue resistance of PZT film is shown In this case,

entrapment of oxygen vacancies and prevent charge injection from bottom electrode

Yoon et al [64] reported on the decrease of the crystallization temperature

layer can prevent the formation of the rosette structure, and decrease the leakage current of the film

Figure 1.10: (a) The dependence of 2Pr values of PZT films as a function of the

the thicknesses of LNO buffer layers [35]

(LNO) layer can be developed between the PZT film and bottom electrode With using LNO buffer layer, the orientation of PZT film can be controlled In fact, the textures of PZT films to be randomly oriented or preferentially oriented in (100) texture depend on the thickness of LNO layers (in Fig.1.10) Whereas, the ferroelectric and piezoelectric properties of PZT thin films are exhibited in conjunction with different preferred orientation Thus, by the change of the LNO thickness, the properties of PZT film can be controlled

1.5 Research scopes and Objectives

temperature, and relatively low processing temperatures, it remains as one of the leading materials for piezoelectric and ferroelectric applications However, there

is also much improvement that is needed for several demanding applications, as discussed in Section 1.4 With the surge of interest in the ferroelectric films in commercial products, it is of interest to focus this research project on the

electrodes (buffferd Si substrates) as a hopeful solution to improve breakdown voltage and best low-cost of products The optimum deposition parameter and electrode parameter that can be achieved to obtain better quality film Because the suitable techniques is very important to deposit the desired ferroelectric films,

we suggest to use Pulsed Laser Deposition (PLD) technique Overcome the drawbacks of traditional technniques, such as Sol-gel, Sputtering, PLD technique was utilized as the potential technique to deposit quality PBZT films More details about this fabrication technique will be discuss in the next chapter

 To understand the deposition process of PBZT films by PLD technique; the patterning process of capacitor structures with lift-off technique for Pt-top electrode, wet-chemical etching for PBZT films; and the measurement characterizations

 To optimize the properties of PBZT films on TiN electrodes by (i) optimum fabrication parameter: deposition temperature; and (ii) optimum

electrode parameter: electrode thickness

 To obtain a better understanding about the influence of measuring factor, such as applied field and poling process to ferroelectric films

schematically presented in Fig.1.11 Each chapter will be devoted to a core topic, and support materials will be discussed around this topic

Chapter 2 Experimental Produces

Patterning process Characterization techniques

Optimization of Deposition Temperature Optimization of Electrode Thickness

electrode

Conclusions and Suggestions for future works

Figure 1.11: The chapter structure of thesis

1.6 Summary

This chapter presents the theoretical background on the PZT material, and its promising properties: high electrical and mechanical properties Furthermore, its challenges and solutions to utilize broadly in producing commercial products still are examined From these researches, we expect to give promotions on the choice Ba-doped PZT (PBZT) as ferroelectric thin film and TiN as bottom electrode to focus the oriented applications With using Ba dopant and TiN electrode, the minimal disadvantage can be obtained: high breakdown voltage and best low-cost in the manufacturing process

CHAPTER 2

EXPERIMENTAL PRODUCES

2.1 Introduction

Several common methods with their advantages and disadvantages used

to deposit ferroelectric thin films, such as PZT films or PBZT films, are introduced in section 2.2 In comparison with other techniques, Pulsed Laser Deposition (PLD) is regarded as the most promising technique for depositing the PBZT films in our research In section 2.3, more details about each step in patterning process of PBZT thin film capacitors can be found And then, the characterization techniques, including: XRD, SEM, electrical characterization, and mechanical characterization, are presented in section 2.4

2.2 Thin film growth

2.2.1 General techniques for thin-film fabrication

In general, the fabrication methods for PZT films in general or PBZT film

in particular can be divided into two major categories: physical method and chemical method Physical method such Sputtering, Pulsed Laser Deposition Chemical method includes Sol-gel, PV-CVD

In Vietnam, although Sol-gel and Sputtering are one of the most techniques which have been successfully used to fabricate the ferroelectric thin films, they revealed some disadvantages:

because of the crack phenomenons on their surfaces

- Difficult to control the exactly component of multi-oxides thin film

21

- Loss of the volatile elements in the heat treatment

- High contamination

- Restrict to deposit on the large-area (wafer)

Sputtering: - Different sputtering yield leading compositional variation

- Geometry constrains of the experimental assembly

The example about the process by Sol-gel method and the homogeneous PZT film with 250nm thickness which was fabricated by this processing can be shown in Fig.2.1

Figure 2.1: (a) Flow diagram for the PZT thin film was deposited by Sol-gel

processing; (b) The SEM cross-section of PZT thin film [55]

Overcome their drawbacks, Pulsed Laser Deposition (PLD), or Pulsed Laser Ablation (PLA), has been widely utilized as a promising technique to deposit the quality films Follow the cooperative program between ITIMS (International Training Institute for Materials Science – Hanoi University of Science and Technology, Vietnam) and SolMateS company (Solutions in Material Science company – University of Twente, The Netherlands), we have an opportunity to investigate these optimizations of PBZT film growth on TiN/Si substrate using PLD technique It is hopeful that the solutions for commercial

(a)

(b)

products owing on ferroelectric thin films, whereby the higher breakdown voltage can obtained underlying Barium dopant into PZT film and best low-cost

in manufacture process relies on TiN electrode

2.2.2 Pulsed laser deposition (PLD)

Pulsed Laser Deposition (PLD) technique which uses laser beam with high power density to vaporize the hardest and most heat resistant materials Although studites concerning laser and deposition plume dynamics were conducted as early

as the 1960s, it wasn’t applied until about the late 1970 At that time, the laser pulses in the nanosecond regime (ns) became available, and the first films were deposited via PLD technique [49] PLD has garnered significant interest due to its various advantages over other deposition techniques One of the major advantages is that the stoichiometry of the target can be retained in the deposition films [29] All elements or compounds evaporation at the same time can be obtained because of the high rate of deposition Moreover, the another key feature of this technique is deposition of multilayers by deposition of multiple targets with using a laser beam To achieve this, the mulitarget holder is rotated, thus deposited material can be switched easily This advantage of PLD is expected as a solution to develop the new material generation, as well as the fabrication techique of novel device structure Thereout, compare with other processes, PLD allows for easy control, since the laser source is placed outside of reaction chamber

Consequently, PLD seems to be a preferred techniques for solid solution of binary oxide systems, a relevant example of which being the PZT or doped-PZT thin film [58] Using PLD technique, PBZT films can be fabricated with

volatile lead (Pb) material from a multicomponent target It is very important effect because the physical properties in PBZT films strongly depend on the precise control of the chemical composition [63] Considering the advantages, it

23

is no surprise that PLD is regarded as the most promising technique for

deposition the PBZT films in this thesis

Otherwise, with all that in mind, it should be noted that PLD does have some drawbacks One of the major problems is the droplets or the particulates deposition on the film These droplets originate from the fast heating and cooling

processes of the target, so, cannot completely be avoided Jeffrey et al [21]

report that there are some methods can be developed to reduce droplet size and

density: (i) use a shutter as a particle filter to remove the particulates which have slow velocity; (ii) polish the target surface before each run to obtain the quality target of high density and smooth surface; (iii) is use lower deposition rate

Another problem due on the narrow angular distribution of the plume is the lack

of uniformity over a large area of the plume This can be solved by controlling the laser beam with translation in large area scale onto the substrate At present, some PLD systems of high-tech companies can deposit thin films on the big wafers with 6 or 8 inches in diameter Depending on these drawbacks can be overcome or avoided, leaving the advantages of PLD to outweigh the disadvantages

2.2.2.1 Mechanisms of PLD

A solid target is irradiated with an intense laser beam, a small amount of material on the surface is vaporized and ejected away from the target The collection of laser parameters, such as intensity, frequence, pulse width, are necessary to vapor the desired material This vapor comes in contact with substrate surface, it will recondense on the surface Repeated pulses of laser can build up material on the substrate surface A thin film on substrate is formed The thin-film formation process is referred to as pulsed laser deposition, so, known as Pulsed Laser Deposition (PLD) technique In general, this process in PLD can be divided into the three stages [27,32]

(i) Laser radiation interaction with the target

In this stage, the laser beam is focused onto the target surface At sufficiently high densities and short pulse duration of laser beam, all elements in the target are rapidly heated up to their evaporation temperature Materials are come out of the target surface with same stoichiometry in the target The deposition rate is highly dependent on the fluence of the laser beam on the target

(ii) Dynamic of the deposition materials

During the second stage, the emitted materials tend to move towards the substrate surface The spot size of the laser and plasma temperature has significant influences on the uniformity of the deposited film In addition, the target-to-substrate distance is another parameter that controls the angular spread

of the deposited materials

(iii) Deposition of the ablation materials with the substrate, nucleation and growth of a thin film on the substrate surface

The third stage influences on the determination of the quality film The ejected high-energy species deposit onto the substrate surface and may induce various type of damage to the substrate One of these damages is droplet phenomenon on the substrate surface It can be explained by the condensation rate is higher than the rate of particles supplied by the sputtering, thermal equilibrium condition can be reached quickly

2.2.2.2 Experimental setup

A schematic of a typical PLD system is shown in Fig.2.2 A Lambda Physik KrF (Krypton fluoride) excimer laser with 248 nm wavelength and a pulse duration of 25 ns (full width at half maxima-FWHM of pulse) is used for all experiments Firstly, the target is polished to remove contaminants on the surface The substrate is attrached to a heater, the target is placed in front of the substrate, and then they is placed inside a chamber of the system

25

Before begining the deposition process, the PLD chamber must be

chamber can be controlled by change in the flow rate of deposition gas (oxygen)

using mass-flow controllers (0–40 ml/min) In addition, to set temperature on

substrate from room temperature to desire temperature, derivative (PID) temperature controller is utilized Especially, deposition energy after len always have to measure before experiments to reduce errors originating from loss caused by lenses in the beam path

Figure 2.2: A schematic construction of PLD system

During deposition, the laser beam is focused by a lens, passed through the chamber window, then coming in at an angle of 45º with the target A small amount of material on the surface is vaporized and ejected away from the target Deposited material on the substrate surface can be built by repeated pulses of laser Finally, after the three stages in previous discussion, the PBZT films were formed on TiN/Si substrates PLD parameters have used to obtain PBZT thin films in this thesis can be found in Tab.2.1, while the more details for investigation steps following the aim of this research can be shown in Tab.2.2

Incident laser beam

mirror

Target holder Substrate holder

substrate

Gas inlet Rotating target

Table 2.1: PLD parameters to obtain PBZT thin films with LNO as buffer-layer

on TiN/Ti/SiO 2 /Si substrates

2.3 Patterning process of PBZT thin film capacitors

Silicon (Si) is one of the most popular substrates and widely developed in MEMS applications The advantages of Si can be listed such as: low price with a

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very high surface quality necessary for the subsequent thin film processing, good thermal conductivity, etc In this thesis, we present our investigations on integration of ferroelectric films on TiN electrode with using the Si substrate Note that during storage in air the Si substrate will oxidize inevitably, called native oxide, and this substrate can be etched by a hydrogen fluoride solution to remove native oxide However, during heating at low pressures inside the PLD chamber, re-oxidation after etching is a possibility This oxide layer can prevent epitaxial growth of layer material on PBZT films

Fig.2.3 shows photolithography, lift-off technique and wet-chemical

NanoLab, University of Twente, The Netherlands

Figure 2.3: Flow diagram for process of PZT film capacitors

The whole process consists of two main steps namely patterning the Pt top

electrode (a–d) and patterning the PBZT layer (e–f) More details about each step

in this process can be found in Tab.2.3