Luận văn thạc sĩ optimization of fabrication parameters of barium doped pb zr0 52ti0 48 o3 thin films on ti si substrates using pulsed laser deposition

32 Figure 2.8 A Polytec MSA-400 micro scanning laser Doppler vibrometer system at IMS Group-Mesat, University of Twente, Netherlandi,...33 Figure 2.9 Schematic view of the measurunent sc

Pham Ngoc Thao

TỎI ỨU HÓA QUÁ TRÌNH LÁNG DỌNG MÀNG MÓNG Pb(Zrss;Th4ø)O;

TREN DE TiN/Si SU DUNG PHUONG PHAP BOC BAY XUNG LASER

Chuyên ngành : Khoa hoc va kỹ thuật Vật liệu Điện tử

LUẬN VĂN THẠC SĨ KHOA HỌC

‘The work has been carried out in the intemship program at Solutions in Material Science (SolMateS) company, the Netherlands from 1* April to 30" September, 2013 Except where specific references are made, this thesis is entirely the result of my own work and inchudes nothing that is the outcome of

work done in collaboratiun No part of this work has been or being submitted for

other degree, diploma or qualification at this or other university

Fuschede, Septemibor 2013 Pham Ngoc Thao

Material Science (SolMateS) company from 1" April to 30" September, 2013

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

I am deeply indebted to my supervisor Dr, Nguyen Due Minh (ITIMS & SolMates), who gave me a precious opportmnity to the beautiful city-Enschede, The Netherlands-to join this intemship program at SolMateS company I especially wish to (hank him about taking professional guidance, and sharing

cxporio practical work, giving constructive advi

throughout this ressarch 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 tw all members of SolMatcS company who created friendly work environment, and gave me encouraging supports Their interest, and hard working to the work impress me so utuch 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, ] want to say thank to both of you for administration assistance Dear Jan, T have really cujoyed tins we spant togetlior in talking about the ships and Duteh culture Doar 8ieven, thanks for your wanu frindship

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

I would also like to thank all ftiends in ‘The Netherlands: Minh-Giang’s family, Tuan-Iliew’s family, Chung (LvA) Bay (vA), big cat Tom Aarnink (UT), Boota (IT), Nirupam (IT), Kerem (UT) because of your warn 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 fiends in Viet Narn for their

friendship

This werk was financially supported by Vietnam National Foundation for Scions and Technology Development (NAPOSTRD) under Grant number 103,02-2011.43, and by the Interreg projcel "Unihealth”,

Fuschede, Septeribor 2013

Pham Ngọc Thao

tii

ACKNOWLEDGEMENTS ii TABLE OF CONTENTS àc teen ii

13 Lead Zizconate Titanate PO(Zr,Tiy JO; (PET) vsssenneninsnnenenesnmineenen

1.3.1 Cryst8l struftre seo

1.3.2 Phas đïggRam, à cọ nam ieeriririrririerrevssuỔ

1.3.3.2, Dielectric propertios sinsneeninmninnininnninnneee® 1.3.3.3, Piezoelectric properties sanesensnensnennensnnmnened

1⁄4 Approaches to improve the properics of PZT thửn ñlms L3

14.1 Doping, ào

2.2 Thin film growth:

2.2.1 General techniques for fabrication

2.2.2 Pulsed laser deposition (PLD) on ni i

2.4.4 Mechanical characterizalion cà chnihiieeeeirriee

3.1 Introduction

3.2, Structure and morphology u.ssnisneneninnenenascuveeenianeenianeie

3.3, Bleettical property sssssessessseesossssssrseseersesseassnersenreersenieeeneerseeisenensen

3.3.1 Ferroelectric properties

3.3.1.1 Ilysteresis loops

kh nh 3.3.1.3 Effect of applied field eccrine

3.3.2 Diclcctrie propcTfics co

4.3.1 Eerroelectrio pT0perliex à nan Hung

4.3.1.1.Hysferesis ÏoGPS, ào ninreiereiereierreer

4.3.1.3 Fatiguc bchavior cnniieierereriririrrie

Figure 1.2 The formation of 180° and 90° fauelectic domain walls in a

(clragonal perovskite ferroclectric, Ey depolarizing field, P,: spontancous

Figure 1.3: 1iystsresis loop and domain switcứng 3 Figure 1.4: Schematic illustration of the poling process 5 Figure 1.5; Schematic ofcubio ABO; perovskii 3 Figure 1.6: Phase diagram PZT solid soluion ccocveooceouỔ Figure 1.7: Axes including nonmal (1-3) and shear đirections (4-6) L0

Figure 18: (@) Capacitor and cantilever structures, 3D-upward displaeoruenls

of (b) capacitor and (c) cantilever, The LDV measurements were performed 11

Figure 1.9: ‘The exwupls of the relationship between dielectric constant, dss

ooeflicient and Z1/Tï ratto of PZT fiÌns - sold

Figure 1.10: (a) The dpondsnce of 2P, values af PZT filins as a function of the thicknesses of LNO bulfer layers; (0) The dysvalues of PZT films as # fimetion of the thicknesses of LNO buffer lay©Ti neo

Figure 1.11; The chapter structure of thesis The main achicvements of cach

chapter are summarized below the tifles ocsoocve 18

Figure 2.1: (@) Flow diagram for the P27 thin film was deposited by Sol-gel

Figure 2.2: A schematic construction of PLD system, 25

viii

Netherlands _— "¬—.-

Figure 2.5: A construction of SEM Tran re - 30

Figure 2.6: Ferroelectric polarization (PE) hysteresis loop ofa PBZT thin film

Figure 2.7: The typical sienal of ntigue exoilation 32

Figure 2.8 A Polytec MSA-400 micro scanning laser Doppler vibrometer system at IMS Group-Mesat, University of Twente, Netherlandi, 33

Figure 2.9 Schematic view of the measurunent sct-up for the dạ;

COPHECIENE es ceeesssesteesesseessesecssanieessanaeesse Ö34

Figure 3.4; Micrographs of PBZT films on TiN/TSiQ,/Si substrates at

Figure 3.5; (2) The SEM image of external failure; (b) The fatigue behavior of PUPBZT/LIN capacitor at 400 kV/om applied field at $65 °C temperature The insets show the electric field as a funtion of switching cyCles, cuseesnmendl

Figure 3.8 The PB2T film eapactions: (a) Diclectrie conslamt-oleotrie Rold @— F) carves, and (b) Di

eulrie constant and dicleulsic loss as a function of

deposition temperature essssssssssseessessnnersereriserimieniniianie evans dd

Figure 3.9: Upward displacement versus different deposition temperature of

PBZT films on TiX/Ti/SiO,/Si susbtrate at 3V electric field AG

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

substrates fom 535 to 575 “C tempcraturcs by microscopy 54

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

electrodes within range fiom 535 to 575 “C deposition temperature S6

Figure 49: Điezoelachic conslaml đạy; as a function of temperatures on

fabricated on SROISTO substrates _ 8 Table 1.2: The dielecric constant (@) values of P2T at various compositions

Fable 13: The dielectric constant value of some materiats de

fable 14: Berroelectric and piezoelectric properties of the undoped and

Table 2.3: PHÙ parameters to obtain PBZT thin films with LNO as buffer-

Table 2.2 Details for investigation steps to optinumm the growth film on TiN

Table 2.3: The information of steps in palierring provEs uc

Table 3.1: The list summary ahout these expertmental results of PRZT film on

1.2, while one of their most potential researches is that lead arconate titanate

Pb(Zr,Ti,,)03 (PZT) material and its properties are shown in section 1.3 Moreover, the requirements of P27’ propertiss on each particular application is discussed in this sevtion, Combination of good properties and stability in a wide range of operaling conditions has led to the increasing arnounl of sindy on P2T 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, Ferroclectricity

The history of ferrocleetricity have began since the year of 1920 when Pierre and Jacquez Curie found piezcelectricity 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: imechanical deformation by applied electric field According Malasyarnant ef al [25], ferrocleetricily belongs to non-centrosymmetric materials which ars of special iniaesis because thơm symmueny — dependent properties Non centrosynuuctric ¢an be divided into polar and non-polar crystal class The tum

“polar” is more correctly used for the non-centrosymmteric containing a unique

anisotropic axis In the polar class, feroelecttic materials possess a spontaneous

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

Rherbohedral Paraelericic plume shove T, Wevratectric phase đảm Ty

Figure 1.7; Schematic diagram of the phase transition in a ferrodkectric material

Phase transition of ferroslectricity can be changed by controlling

tmporature In materials science, this fermporstuse is called the Curie temperature

CT, or Curie point) where the pormanert polarization in the material changes to induced polarization Usually, the phase above T, is termed as paraclectric phase which shows a linear funtion between polarization and applied electric field, Below T, temperature, the phase is termed as tetragonal or hombohedral phase, the ferroclectricity 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

wall between oppositely oriented domain, the separation between perpendicular

domains is called 90° or a- domain wall

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

Figure 1.3: lysteresis loop and domain switching [30]

With increasing field, the domains which its polarization direction opposite to the field, starts to switch along the direction of the field It leads to a

von tincar measurcment of clurge intensity This switch still continne, uni) the result of the polarization mcasurcment retumns to be lincar (saturation) It means all the domains align with the applied electric field direction, With decreasing field, polarization decreases linearly And when the field retams to zero, the polarization remains a positive value which called the remanent polarization P, When a negative electtic field can be reached at coercive field (E,), the nucleation of reserved polarization domain starts, This process can be repeated Isnce, this relationship between the polarization and electric field in ferroclcetzies is often non-lincar and its hysteresis duc 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 ublained by domain wall motion Ti

is possible by the application of a sufficiently high clclcctric ficld, the process is

known as poling [G0] Before poling, polycrystal ferroelectric materials do not possess any properties due to the random orientations of the ferroelectric

domains, During poling, a de 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

Mowever, their polarization vectors can be still aligned to the maximum

component (hat they can follow the direction of clechie field In general, more complete alignment of domain polarization can be oblained by higher poling ficld, 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

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

1.3 Lead Zirconate Titanate Pb(Zr,Ti,.)O3 (PZT)

Among ferroelectric materials, Lead Zirconate Titanate (PZT) is one of

the most potential researches because of its superior properties This material has

optimum properties such as (i) high Curie temperature, Te, so, a high temperature

of operation (above 200 °C); (ii) high electromechanical coupling coefficients:

(ii 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

chemical formula ABO In this case, A site is occupied by Pb” cation; B site is

occupied by Zr*/Ti** cation and oxygen anions on the six centered faces

Six oxygen atoms are arranged into an octahedron with Zr**/Ti** at the

center, they can be called as a three-dimensional of BOs octahedra This octahedral are linked by their comers, whereas the Pb** in the interstices created

by the linked octahedron A unit cell of cubic perovskite ABO; is shown in Fig.1.5

1.3.2 Phase diagram

Fig.1.6 describes the phase diagram of PZT within a range from 0 °C to

500 °C temperature PZT is a solid solution of PbZrO; (PZ) and PbTiO; (PT) At

room temperature, they can be existed im a tetragonal ferroelectric phase (Fr) or a

rhombohedral ferroelectric phase (Fx) When the mole percentage of PT is in the range between 0.48 and 1.0, the symmetry of PZT is tetragonal Higher-level

replacement of Ti** with Zr** ion results is a rhombohedral phase

010203040 5.60 T79 006 5 Ø9

Figure 1.6: Phase diagram PZT solid solution [19]

Pe: cubic paraclectrie phase, Fr: tetragonal ferroelectric phase; Fram: high temperature thombohedral ferroelectric phase, Faq; low temperature rhombohedral ferroelectric

phase, Ao: orthorhombic paraelectric phase, Az: orthorhombic paraelectric phase

Fspecislly, this shombohedral phase is divided into two phases, inolulBng high temperature Fag and low temperature Fea phase, Both of them exhibit the rhombohedral symmetry, Further, the increase concentration of Zr ion also generates an paraelectic orthorhombic phase A, (%<0.05), Increasing

temperature close to the Curie temperature (T.), another paraelectrie tetragonal

phase (An) is observed in a very limited composition range

Note that in Fig.1.6 the phase boundary between the tetragonal and rhombohedral phases is neatly independent of temperature and called morphotrophic phase boundary (denoted MPB) A morphotropic is used as term

lo exhibit an abrupt structure change ina solid solution with various compostions

[20]

1.3.3 Physical properties of PZT thin film

Since the mid-1970's, ferroelectric films have developed as a promising material with their interesting applications, In addition, with many breakthroughs

in the fabrication of PZT film, the researches on thi

1.3.3.1 Ferroclectric properties

Noteworthy for two important parameters in ferroelectric properties are remanent polarization (P.) and coercive field (E,), For NV-RAM (nonvolatile random access memories) applications, PZT material has become a potential candidate in comparison with other materials such as BiyTis01; (BIT) [17], (Bar 251,)TIO; (BST) [41], or SrBiz(Mb,TR):O; (SBTN) [51] Because the ferroelectric properties of PZT strongly depend on its composition, il is desired to determine

the optinmt composition for these applications In literature, many rescarches reported that the highest P, value found at MPB composition while the decreasing trend of E, value reached with increasing Zr concentration, In range composition with x — 20 80, Chen ef ai, [8] indicated that the P, value increased from 13

36 j:Cvcm? and reached the highest value at MP3 phase, whereas the B, value decreased from 78 — 23 kV/om with increasing x ‘These trend of Ð, and B, can also be confirmed by the studies of Foster ef ai, [18] Furthermore, the researches also shown that the PZT material at x = 0.52 composition seemed to be a optimum composition, because this composition exhibited large P, and low B,

133, 36]

Composition

¥= 0.43 | x= 0.46 | x= 0.49 | v= 0.52 | ¥= 0.55 | x= 0.58 PhZr,Tiy Os

P.(uCyom*) 42 46 35 40 32 33

Ti, &Viem) 365 355 40 3⁄5 450 457 Table 11: The remanent polarization (P,) and coercive field (E,) values respect

to the compostions (around the morphowopic phase) of PZT films fabricated on

SROSTO substrates [36],

1.3.3.3 Dicketric properties

Gonerilly, ferrocleeisies wre diclectriy materials Dielectric comstanl 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 (2) The

¢ value of capacitor structure is given by the folowing Fig.1-1 [43]

cả

ET fA 1-1)

where ¢ is the dicleetrie constant (permittivity) of material between the plates, C

is the capacitance, [F]; dis the distance between the plates [im]; <p is the dicleetrie

constant of fiee space (8.854 x10“ [F/m)); 4 is the area of overlab of the two

plates, [m”]

Under an applied electric field, the dipoles in this matezral will change their orientations along the dircction of the applied electric ficld 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 6) The equation determination for this factor of capacitor structure is depicted in Eq.1-2 [43]

6 2ñ7C

fable 1.2: The dielectric constant (ci values of PZT at various compositions

using different deposition techniques

From several researchers, the dielecttic behavior of PZT material has been 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 dicloetric constant (6) are distinetly different for each method, almost results can be proved the maximum dielectric constant (2) of PZT at MPB composition It can be explained by the

coexistence of tetragonal and rhombohedral phase that increases the number of

allemalvc crystallographic directions for polarization to 14 (ight from rhombohedral structure and six from tetragonal structure}, s0, the domains can switch easily [19,56]

Table 1.3: The dielectric constant value of some materials

In addition, Tab.1.3 shows the dielectric constants of other materials In this comparison, The P?:T with the higher ¢ value is the desired material for

DRAM (high-densily planar density rmdom sceess memories) applications

1.3.3.3 Plezoclectric properties

The term “piezoelectricity” is defined as a material to generate an electrie potential from applied mechanical stress and vice versa [14] To otain a better understanding about piezoelectric properties of the PZT material, pievoelectrie coefficients and electromechanical coupling factor need to observe

Figure 1,7 : Axes including normal (1-3) and shear directions (4-6) [16]

The picaodlectric cucflicicnt (@) is defined as the dectie polarizaion

lô

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

Depending on the structure of the particular device, either the ds, or d33

value can be significantly performed, although these coefficients exist

simultaneously in this device In practise, in the capacitor structure, the d5; value

is much larger than d;; value because of the clamping effect between the film and

susbtrate Thus, this structure just only shows the strong value of d33 and its ds;

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

coefficient, d;;, can be detected, whereby the dj; is not significant effect To

measure the displacement in the devices, the laser Doppler vibrometer (LDV)

measurement was performed More details of this measurement will be discussed

11

in the chapter 2

Similar to ferroelectric properties, the improvement of piezoelectric properties is well malched with optimal composition @/Ti zalo) in PAT inaturiat, Many studios tsport that the composition af highwsl clectromechanical activity (maximum piezoelectric costficient) can be depicted at the morphotropic phasc boundary (MPB) [34,43,39]

vice versa

mechanical energy converted electrical energy corwerted

XỔ = Mechanical input electrical energy mergy converted 4p _ slectrical energy conwerted input mechanical energy a3)

The value of & is always less than one because no material can convert perfectly one form of energy to another Due to highest dss value, the highest k factor still ean be formd at MPB phase Thns, P2T material at MPR phase has Leen exptoiled as a promising candidate for transducer and actuator applications

1.4 Approaches to improve the properties of PZT thin films

show smaller ionic radii, such as such as Nb", Ta’, etc., occupy the B-site to replace Zr‘'/Ti* ions With doping this additive, the most notable changes are

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 8), and higher caereive field (B) values are one of the most significant influences of these dopants Their improvement on the properties have recived considerable allention to be applicd for ultrasonic applications Some cxamples can be listed in this case such as: K*

Na’, etc replaces Pb?! mi the A-sites or Fe”', Al’, ete., occupies the B-sites of

piesoolcclrid eucffioieml are nal as larger as that of olher dopants However, they have been strongly interested in the commercial products with their optimum breakdown voltage, Due to this enhancement, they may be prettrentially 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 significanily improved whereas output-noise density

become to be minimized

The effect of three types of substitutions on the zlectrical and mechanical properties of PZT films can be shown clearly by the research of Nguyen ef af [46], as shown in Tab.1.A

4 polential oxplanation why thoir propurtios arcn’t as high as olhor dopod-PZT:

fins However, Ra-doped PZT material is sill utilized widely as a hopeful

candidate for oricnted applications with requizing higher applicd 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

clectrode materials makes the constraint in the imlsgralion of thẻ fnroclacirie

devices (ferroelectric capacitors) Therefore, the choice of the electrode and

ferroelectric materials 1s an important consideration

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

Pt, and Ag are being widely used, Or other chooses, conductive-oxide materials, such as SiRuO; (SRO), LaNiO; (LNO), are not only favorable electrode materials [4,53] but also play an important role in buffer technology for improving the quality of the device application in rmilti-layers systems [11] Llowever, the mannfacturing processes of these electrodes have the high cost These investigations have been done to promote the development of now electrode generation with lower price, Morcover, these materials still satisfy the complex role of electrodes |2]:

fi) High conductivity

ii) Good adhesion to both faxroslectrie fin and underlying structure, iii) High stability under the conditions of fabrication process,

iv) Actas a diffusion barrier to prevent or io relard the imterdiffusion of

the two superposed malcrials

because of its excellent properties such as: good mechanical properties, high conductivity, high cozrosion resistance, low friction coefficient [38], And, TiN also can utitive in CMOS proce

s combination,

1.4.3 Buiter layer

In order t0 mect the improvement of PZT propertics, builer layers have

allravicd the considerable alicntions In literature, there are many i

ostigations

about the advantages of buiiér layer [3,16], such as enhance the nucleation and growth of the porovskite phase; prevent the diffusion between the electrode and P2T film; and improve the properties of PZT films; ete

According to the research of Alkoy ef af, [1], with using PbZ103 as the butter layer, the optinmun tatigue resistance of PZT film is shown In this case, the PbZrO; act as a interface layer which can be modified to reduce the entrapment of oxygen vacancies and prevent charge injection from botiom

electrode

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

of PZT by using PbTiO; buffer layer Moreover, it is also presented that PbTiO layer can prevent the formation of the rosette structure, and decrease the leakage

current of the film

Thickness of LO bur layer (eat THe of LNG buler layer den

Figure 1.10: (a) The dependence of 2P, values of PZT films as a function of the thicknesses of LNO buffer layers; (b) The da;vatues of PZT films as a fimetion of

the thicknesses of LNG buffer tayers [35]

Ta control the texture of PZT film, Kim e¢ af [35] reported that LaNiO,

(NO) layer can be developed between the PZT filtn and botlorm electrode With using LNO buffer layer, the orientation of PZT Glm can be controlled ti 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 fernoelectric 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

‘As mentioned in the afore sections, depending on the attractive properties

of PZT films such as a high P,, high domain switching speed, a high Curie

temperature, and relatively low processing temperahues, it remains as one of the

leading materials far piezoelectric and ferroelectric applications However, there

is also much improvement that is needed for severat demanding applications, ax

discussed in Scetion 1.4, With the surge of interest in the fesrodlaetrie films in coumnereial products, i is of interest to focus this rescarch project on the integration of Pb, sBay -(Zto5:Tigas)Os films (PBZT, 10%wt Ba-doped) on TiN electrodes (buttfard Si substrates) as a hopetl 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 dusired ferroelectric Fitts,

we suggest lo use Pulsed Taser Dupesition (PLD) technique Overcome the

drawbacks of traditional teclmmiques, such as Sol-zel, 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

The scopes and objectives of this project can therefore be summaried:

¢ To understand the deposition process of PR2T films by PLD technique; the pattcmning process of capacitor structures with lift-off technique for Pt- top electrode, wet-chemical etching for PBZT fins, and the measurement characterizations

To optimize the properties of PBZT films on TiN electrodes by (i)

optimum fabrication parameter: deposition temperature; and (1) optimum

electrode parameter: electrode thickness

¢ To obtain a better understanding about the influence of measuring factor, such as applicd ficld and poling process to ferroclectrie films

Conclusions and Suggestions for future works

Figure 1.11: The chapter structure of thesis.

CHAPTER 2

EXPERIMENTAL PRODUCES

2.1, Intreduction

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 PB#T films in our research In section 2.3, more details about each step in patterning process of PBZT thin Blm capacitors can be found, And then, the

chavavtcrization tedmiques, including: XRD, SEM, electrical characterization,

and mechanical characterization, are presented in scetion 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 methed and chemical method Physical imethod such Spullering, Pulsed Laser Deposition

Chemical method includes Sol-gel, PV-CVD

In Vietnam, although Svl-gel and Sputtering are onc of the most techniques which have been successfully used to fabricate the ferroelectric thin

films, they revealed some disadvantages:

Sol-ecl: - Difficult to achieve quality films with more 500 nm thickness

because of the crack phlenomenons on their surtaces

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

20

- 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

Add MOE Heat 124°C

Laser Ablation (PLA),

drawbacks, Pulsed Laser Deposition (PLD), or Pulsed has been widely utilized as a promising technique to

deposit the quality films Follow the cooperative program between ITIMS

(Intemational 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

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protkicts owing ơn fiaoclectria thin films, whereby the higher breakdown voltage can obtained underlying Barium dopant into PZT film and best low-cost

in manmfacture process relies on TiN electrode

2.2.2 Pulsed Laser deposition (PLD)

Pulsed Laser Deposition (PLD) technique which uses t

sor ban with high power density lo vaporire the hardest and most heal resistant inaterials, Allhough studites conccming lascr and deposition plume dynamics were conducted as carly

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 ñhns [29] All clements or compounds evaporation at the same time can be

obtained because of the high ratz of deposition Morcover, the another key feature of this technique is deposition of multilayers by deposition of multiple targets with sing, 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, PID allows for easy control, since the laser sources is placed outside of

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

deposition the PBZT films in this thesis

Otherwise, with all thal in mind, it should be noted that PLD docs 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 Jefftey 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 velovily; /#i) polish the target surface Lefore cach nm to obtain the quality target of Iigh density and smooth surface, fi) is use lower depositiom rate Another problem duc 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 im large area scale onto the substrate At present,

some PLD systems of high-tech companies can deposit thin films on the big waters 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

2.2.3.1 Mechanisms of PLD

A solid target is irradiated with an intense laser beam, a small amount of material on the surface is vaporized and gjactsd away from the taget 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 surfhec Repeated pulscs of laser can build up material on the substrate surface A thm film on substrate is formed The thin-tilm 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]

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(i) Laser radiation interaction with the target

[In this stage, the lascr beam is focuscd 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 fii) Dynamic of the deposition materials

During the second stage, the emitted materials tend to move towards the subsirato surfMoz The spot sive of the Taser and ylasma 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

fit) Deposition of the ablation materials with the ssbsivate, nucleation and

growth of a thin film on the substrate surface

The third stage iniluences 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

2.2.1.2 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-T'WIIM of pulse) is used for all experinents, Firstly, the target is polished to Temove contaminants on the surface The substrate is attrachod to ø henter, the target is placed in front of the substrate, and then they is placed inside a chamber of the system

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Before begining the deposition process, the PLD chamber must be

vacuum pumped to base pressure at 10°-10° mbar The pressure inside this

chamber can be controlled by change in the flow rate of deposition gas (oxygen) using mass-flow controllers (0-40 m//min) In addition, to set temperature on

substrate from room temperature to desire temperature, proportional-integral-

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

Incident laser beam

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 45° 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

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TANK©; — PhụyBaoi(Zms;Tio)O;

Parameters

‘Ambient pressure (mbar) 0.10 (03) 0.10 (0

Optimization of | Deposit on 150 nm thickness of TIN electrode (denoted 150) at

temperate different temperature:

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 etching are used for the structuring PBZT films on TiN/Ti/SiO./Si substrates (at

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 patteming the PBZT layer (ef) More details about each step

in this process can be found in Tab.2.3

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