P b (Z r,T i)03 THIN FILMS SPUTTERED FROM A M ULTIELEM ENT METALLIC TARGET

VNU. 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

32 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

RF- power

200 ^w

Base pressure ~10‘6 m bar

S u b s tra te

sity si

P t/T i/S i0 2/Si

T arg et Ti

Pt Pb/Zr/Ti

W orking pressure 3 x103 m bar 1x102 m bar

Ambience gas Ar 0 2/Ar:40/60

S u b s tra te tem p eratu re T_{A 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)

Preparation 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

T = 6 5 0 c 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

1 .« P b O

ss

•

I _ □ t = • _{7 f\} n 120 mill._•

As-dcpositcd

20 30 40

2 0 ( d c g .)

50 60

Fig. 3. The XRD p a tte rn of PZT thin films annealed for various a n n ealin g tim es

34 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 , which is norm ally caused by a deficiency in lead due to a n evaporation at hi;gh 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 one and AB3O7 phas e seemed to be an intermediate ph as e before the PZT fi lm s 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)

P 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 to 40 50

20 (deg.)

Fig. 4. The XRD p a tte rn of PZT thin films annealed in various annealing ambiences.

6C 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 IU11

2 11 I I I

Fig. 5. The AFM micrograph of PZT thin film annealed at 650°c for 60 min. in the

air

36 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 m ethod u sin g a m u lti-elem e n t 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