The A B 03 perovskite structure in the thin films sputtered at a substrate tem perature of 250 °c was formed through various intermediate phases by annealing at 650 °c for 1 hour in the
Trang 1VNU j o u r n a l Of1 SCIENCE, Mathematics - Physics, T x x , N01 - 2004
p r e p a r a t i o n o f P b (Z r,T i)03 THIN FILMS SPUTTERED FROM
A M ULTIELEM ENT METALLIC TARGET
N g u y e n T h a n h H u y a, Vu N g o c H u n g a b, N g u y e n P h u T h u y a,c
ữ In tern a tio n a l Tra in in g I nstitute for M aterials Science (ITIMS)
b In stitu te o f Engineering Physics, Hanoi University o f Technology
c F a cu lty o f Technology, Hanoi N ational University
A bstract: PZT thin films have been grown on Pt/Ti/Si02/Si substrates by the
reactive RF-magnetron sputtering deposition method using a multi-element
metallic target The A B 03 perovskite structure in the thin films sputtered at a
substrate tem perature of 250 °c was formed through various intermediate
phases by annealing at 650 °c for 1 hour in the air or an oxygen gas ambience
The film compositions were estimated by Electron Probe Micro-Analyzer
(EPMA) The surface morphology observed by Atomic Force Microscopy (AFM)
showed a densely packed grain structure with no rosettes structure The
remanent polarization value of the thin film with a thickness of 500 nm was
1.37 nC/cm
1 I n t r o d u c t i o n
Over the last 20 years, thin films of lead zirconate titanate, Pb(Zr,Ti)03 (PZT), have been attracted much attention due to their applications in nonvolatile memory and MicroElectroMechanic System s (MEMS) devices[l,2] Most of works have been concentrated on the compositions of the Morphotropic Phase Boundary (MPB) at around Zr/Ti=50/50 compositional ratio, at which the piezoelectric and ferroelectric properties show their maximum values Various technical solutions proposed for obtaining the thin films of PZT such as sol-gel method, pulsed laser ablation, sputter deposition, and ion beam sputtering have been employed Among them, the sputtering method has been most widely used for obtaining high quality PZT films because of its relatively simple fabrication process.
One of the most important factors in the preparation of PZT thin films is the control of the correct perovskite phase formation in order to obtain the desired electrical properties Mostly, in the as-deposited films there have been no perovskite phase So post-deposition annealing treatments at high temperatures,
typically in th e r a n g e 6 0 0 -7 0 0 ° c for 1 h o u r or more, are n e ce ssa ry to crystallize the
film into perovskite structure.
In this study, PZT thin films were fabricated by the reactive RF-magnetron sputtering method The perovskite phase crystallization of PZT films was investigated in term of the effects of annealing time and heat treatment ambience
by X-Ray D iffraction (XRD) The surface morphology and electrical p roperties ỔĨ the
films were also reported.
31
Trang 232 Nguyen Thanh Huy, Vu Ngoc Hung, Nguyen Phu Thuy
2 E x p e r i m e n t a l p r o c e d u r e
An A lcatel SCM-400 13.56MHz
RF-m ag n e tro n s p u tte r in g system was used to
deposit th e PZT th in film& on P t/T i/S i0 2/Si
m u ltip la y e r su b s tr a te s D uring s p u tterin g
process, the substrates, as bottom electrodes,
w ere h e a te d a t 250 ° c T h e s p u tte r in g ta r g e t w as
m u lti-elem e n t m etallic, th e design of which is
shown in Fig 1 T his was composed of individual
sectors of Pb, Zr, and Ti m etals with
Pb/Zr/Ti:2/7.5/8.5 compositional ratio Prior to
th e deposition, th e s p u tte rin g cham ber was
pum ped down to a base p res su re of 1 0 6 mbar
S u b seq u e n tly , gas m ix tu re of Ar and 02 w ith A r / 0 2:60/40 ra tio was introduced to a
p re s s u re of 2 x l 0'2 m bar An RF-power was fixed a t 200 w The selected s p u tte rin g conditions for th e deposition of bottom electrodes a n d PZT th in films are
s u m m a riz e d in Table 1
T a b l e 1 T h e c o n d itio n s for p r e p a r a t i o n of b o tto m e le c tro d e s a n d t h e PZT(51/49)
th in films
Bottom electrodes P Z T thin film s
W orking pressure 3 x103 m bar 1x10 2 m bar
S u b s tra te tem p eratu re TA Room 250 °c
Deposition time Ti: 5 min
Pt: 30 min 4 hours
Film thickness Ti: 20 nm
Pt: 100 nm 500 nm The PZT film th ic k n es s was d e term in ed ab o u t 500 nm by th e grazing incident X-Ray R eflection (XRR), as illu s tra te d in Fig 2 The Zr/Ti:51/49 compositional ratio
of th e PZT film s was analyzed using Electron Probe M icro-Analyzer (EPMA) In
o rd er to cry s talliz e perovskite phase, th e as-deposited films were a n n e a le d using
Fig 1 The multi-element
PZT m etal targ et (after
sputtering)
Trang 3Preparation of Pb(Zr, T i) 0 3 thin films sputtered from. 33
20 (deg.)
Fig 2 The XRR p a tte rn of the PZT
thin film annealed at 650°c for 60
min
conventional furnace a t 650°c fixed
te m p e ra tu re in th e a ir and an oxygen
ambience for 30, 60, 90, 120 min.,
respectively P h a se and crystalline
behavior analyses of PZT films were
performed using X-Ray Diffractom eter
w ith the CuKa source (A = 1.5405 A)
Surface morphology and grain size were
investigated using Atomic Force
Microscopy (AFM) The ferroelectric
property of PZT thin films were also
m easu red using RT6 6A sta n d ard ize d
h y ste resis teste r (R ad ian t Technology)
3 R e s u lt s a n d d i s c u s s i o n
C r y s ta llo g r a p h ic s t r u c t u r e
The crystal s tru c tu re of the
as-deposited and a n n e a le d PZT
thin films was exam ined by XRD
p a tte r n s in com parison with
crystallographic inform ation
rep o rted earlier on PZT films and ^
ceram ics This p a rt may shed light
on th e optim um a n n e a lin g time
and ambience req u ire d for the
perovskite PZT form ation The
XRD p a tte r n s of th e films a n n ealed
for th e various tim es in th e air are
given in Fig 3
It can be seen from th e figure
th a t all of films alw ays show two
p eaks a t 2Ớ=40° and 46.55°
corresponding to the Pt
polycrystalline phase
In case of th e as-deposited
film, th e re are two p eaks a t 29.6° and 34.15° depicting non-ferroelectric pyrochlore,
P b ,T i206 cubic oxygen deficiency phase at low te m p e ra tu re A p e a k a t 56.24° is identified ct-PbO, s tru c tu re M ean while it is possible t h a t Z r 0 2, T i 02 a re p re s e n te d
in a n am orphous form because whole Pb, Zr, Ti were oxidized in s p u t te r in g process sim ultaneously
-p
1
>»
3
• P e r o v s k i t e
□ P l a t i n u m
* P y r o c h l o r e
P b O
ryiuuiiiuic
ss
•
I _ □ t = • 7 f\ n 120 mill.•
As-dcpositcd
2 0 ( d c g )
Fig 3 The XRD p a tte rn of PZT thin films
annealed for various a n n ealin g tim es
Trang 434 Nguyen Thanh Huy, Vu Ngoc Hung, Nguyen Phu Th.uy
When the film was an n ealed for 30 min., a - P b 02 tr a n s fe r r e d to the lead-riich
phases PbO ss t h a t are observed at 29 of 23.2°, 25.45°, 32.1°, a n d 42.4° Their relati ve
high in te n sity may show th e excess contents of Pb and Oọ in the th in film[3] T h e peaks a t 29.05° and 33.94° m atch with th e pyrochlore monoclinic AB3O7 p h a s e ,
tem p era tu re In th is work, the pyrochlore phase a p p ea rs even when lead-rich PbOgg compound p res en te d in accordance with [4] It h as revealed t h a t th e formation of
AB30 7-type was favored over A B 0 3-type even when th e Pb/Ti ratio w as g rea ter th.an
crystallized into th e ABO3 perovskite phase The peaks a t 21.75° and 38.55° perfoirm the perovskite s tr u tu r e It is confirmed t h a t th e PZT films deposited a t t h e
s u b s tra te te m p e r a tu re 250°c can be converted into th e perovskite cry stallin e
s tru c tu re a t the a n n e a lin g te m p e r a tu re 650 °c with a sufficient a n n e a lin g time
Increasing the a n n e a lin g time to 60 min., complete perovskite cry stallizatio n occurs Besides the form er peaks, th e o th ers a t 30.77°, 44.15°, and 54.85° according
to the perovskite p h ase ap p ears The ratio of in te n sity b etw een the p e ak s show t h a t the stru c tu re of th e PZT th in film is polycrystalline (like PZT ceramic)[5] However, the (110) in te n sity peak a t 30.77° is largely superior to the o th ers, th u s it is considered t h a t th e th in films h as a preferred-(llO ) orien tatio n
The tre n d suggests t h a t these films may be converted into the single perovskite p h ase by increasing th e an n ea lin g tim e or a n n e a lin g te m p e ra tu re
■Nevertheless, th e a n n e a le d films for more th a n 60 min are diphase w ith pyrochlore and perovskite coexistence The pyrochlore peaks r e a p p e a r t h a t proves Pb loss T h e deficiency of Pb is u n d e rs ta n d a b le by considering its high volatility for a long
an nealing tim e a t a high te m p e ra tu re After the an n ea lin g tim e reach in g 120 min., the non-ferroelectric pyrochlore phase is p red o m in atin g with stro n g and s h a r p peaks
To investigate th e effect of an n ea lin g ambiences, th e PZT th in films were
a nnealed for 60 min a t 650 °c in th e air, an oxygen a n d an argon gas am bience a n d the resu lts are depicted in Fig 4 As above discussion, th e th in film, which w as
h eated in th e air, h a s th e polycrystalline perovskite s tr u c tu r e with p red o m in ate d (110) orientation The film an n ealed in the oxygen am bience h as still th e correct perovskite s tr u c tu r e b u t (100) orien tatio n is preferred It m ay be c au sed by th e
in flu e n ce 'o f oxygen con ten t on PZT orientations It h as been rep o rte d t h a t th e oxygen p a rtial p re ssu re is an im p o rta n t factor in d e te rm in in g th e Pb valence sta te
in th e PZT film s, w h ich d ic ta te s th e k in e tic s of th e p y ro c h lo re -p e ro v sk ite p h a se
transformation^] A low Pb valency state enhances the kinetics of transformation,
w hereas a high Pb valence sta te sup p resses th e kinetics of tra n s fo rm a tio n leading
to incomplete tra n s fo rm a tio n of th e pyrochlore to perovskite phase The grow th of PZT(100) on P t/T i/S i0 2/Si wafers a ttr ib u te d to the form ation of c ry stallin e PbO(OOl)
Trang 5P r e p a r a t ° n ° f Pb(Zr, T i) 0 3 thin films sputtered from. 35
-P
*C/5 3
□
• P e r o v s k i t e
□ P l a t i n u m
❖ P y r o c h l o r e
T h e air
.
O x y g e n gas
A r g o n g as
ẳ I 1 1 1 I I ẵ ẫ 1 I i I I ẳ
20 (deg.)
Fig 4 The XRD p a tte rn of PZT thin films
annealed in various annealing ambiences
d u r in g p rolysis PbO h as good
lattice notching w ith PZT(IOO)
o rien tatio i; this lowers the
in te rfa c ia energy and prom otes
th e nucliation of PZT(IOO) On
the c o n tiiry , for th e PZT film
a n n e a le d in the argon ambience
th e pyro<hlore phase dom inates
complete}/
Altlough PZT films were
a n n e a le d for different tim es in
v ario u s anbiences, th e (110) or
(100) o’ientation is mainly
observed These re s u lts imply
th a t PZT essentially h a s n a tu r a l
preference to grow in (110) or
(100) directions a t te m p e r a tu re s less th a n 700 °C[7]
On he basic of th is inform ation, th e cry stallizatio n of s p u tte r d PZT th in films was studied The as-deposited film shows peaks due to a - P b 02 and A2B20 7.x crystalline while Z r 02 a n d T i 02 are am orphous After an n ea lin g , lead-rich PbO ss and lead deficient AB307 phases were found as the in te rm e d ia te p h ases and they reacted each o th er to form the perovskite s tr u c tu r e d u rin g post-deposition h e a t treatm en ts In order to obtain the correct perovskite crystal s tr u c tu r e th e PZT th in films need to be a n n e a le d for 60 mill, a t 650 °c in the a ir or th e oxygen ambience
S u r f a c e m o rp h o lo g y a n d g r a i n size
The surface morphological
investigation on the s p u tte re d PZT thin
film annealed a t 650 ° c for 60 min in the
air was shown in Fig 5 The film h as a
relative smooth surface w ith no
microcrack, a dense s tru c tu re , and fine
grain d istrib u tin g homogeneous Agreeing
well with AFM res u lt, th e average grain
size is also e stim a te d to be 50 nm by
using S h e rre r’s eq u atio n w ith the h a lf
width of the (110) diffractive peak
observed a t 2Ớ=30.77° in th e XRD p a tte rn
in Fig 4
Í
100 urn
5 0 m i l
Ọ IU 11
2 11 I I I
Fig 5 The AFM micrograph of PZT thin
film annealed at 650°c for 60 min in the
air
Trang 636 Nguyen Thanh Huy, Vu Ngoc Hung, Nguyen Phu Thuy
F e r r o e le c tric p r o p e r t y
F erro electric b eh av io r of th e PZT film a n n e a le d -a t 650 ° c for 60 min in th e
a ir was stu d ied by a p lo ttin g P-E h y ste resis loop an d is show n in Fig 6
The re m e n a n t p o larizatio n P r of the
PZT film is ab o u t 1.37 |iC/cm 2 The film
exhibits th e c h a ra c te ristic “elliptic”
shaped ferroelectric resp o n se of a h a rd
ferroelectric T his sh ap e is common for
film refe rred to as “lossy” or “leaky”,
which is not good in su la to r The loop has
shifted tow ard th e n egative side This
asym m etry also observed by m any
w orkers w as a ttrib u te d to th e difference
in in te rfac ia l s ta te s betw een top
electrode/PZT in terface an d PZT/bottom
electrode in te rfa c e [8] due to th e electrodes
w ere p rep a red in com pletely different
conditions
4 C o n c lu s io n s
PZT th in film s were p rep a red on p latin ized silicon s u b s tra te s by
RF-m agnetron s p u tte rin g RF-m ethod u sin g a RF-m u lti-eleRF-m e n t RF-m etallic ta rg e t Lead-rich
P b 0 8s and lead-deficient AB3O7 phases were discussed as im m e d ia te p h ases in th e ABO;i correct p ero v sk ite cry stallizatio n The com plete p e ro v sk ite s tru c tu re could be obtained a t th e optim ized condition being a t 650 °c a n n e a le d te m p e ra tu re for 60 min in th e a ir or th e oxygen am bience
References
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2 E Defay, c Millon, c Malhaire, D Barbier, Sensors a n d Actuators, 99(2002), 64.
3 S M Ha, D H Kim an d H H P ark , Thin S o lid Film s, 3 5 5 -3 5 6 (1999), 525.
4 c V R V a sa n t K u m ar an d R P ascual, J Appl Phys., 71(1992), 864.
5 D Czekaj, M J M Gomes, M V asilevskiy an d M P e ira, J Euro Ceram Soc,.
19(1999), 1489
6 S K alp at an d K U chino, J Appl Phys., 90(2001), 2703.
7 K Iijim a, Jpn J Appl Phys 30 (9)(1991), 2149.
8 S O k am u ra, s M iyata an d Y M izutani, Jpn J Appl Phys 38 (1999) 5364.
V (Volt)
Fig 6 The P-E p a tte rn of th e PZT th in
film a t 650 °c for 60 min in the air