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 io n o f Pb(Z r,T i)03 THIN FILMS SPUTTERED FROM A MULTIELEM ENT METALLIC TARGET N g u y e n T h a n h H u y a, Vu N goc H u n g a b, N g u y e n P h u T h u y a,c ữ International Training Institute for Materials Science (ITIMS) b Institute o f Engineering Physics, Hanoi University o f Technology c F aculty o f Technology, Hanoi N ational University Abstract: 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 AB0 perovskite structure in the thin films sputtered at a substrate temperature of 250 °c was formed through various intermediate phases by annealing at 650 °c for 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 I n t r o d u c t io 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 Systems (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 the n g e 00-700 ° c for hour or more, are necessary 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 Diffraction (XRD) The surface morphology and electrical properties ỔĨ the films were also reported 31 Nguyen Thanh Huy, Vu Ngoc Hung, Nguyen Phu Thuy 32 E x p e r im e n t a l p r o c e d u r e An Alcatel SCM-400 13.56MHz RFm ag n etro n sp u tte rin g system was used to deposit th e PZT th in film& on P t/T i/S i0 2/Si m u ltip lay er su b s tra te s During sputterin g process, the substrates, as bottom electrodes, w ere h e a te d a t 250 ° c T he s p u tte rin g ta r g e t w as m ulti-elem ent m etallic, the design of which is shown in Fig This was composed of individual sectors of Pb, Zr, and Ti m etals with Fig The multi-element Pb/Zr/Ti:2/7.5/8.5 compositional ratio Prior to PZT m etal target (after the deposition, the sp u tterin g cham ber was sputtering) pum ped down to a base pressure of mbar Subsequently, gas m ixture of Ar and with A r /0 2:60/40 ratio was introduced to a p ressu re of x l '2 m bar An RF-power was fixed at 200 w The selected sp u tterin g conditions for the deposition of bottom electrodes and PZT th in films are su m m arize d in Table T a b le T h e c o n d itio n s for p r e p a r a tio n of b ottom e le ctro d es a n d th e PZT(51/49) th in films Bottom electrodes 200 w RF- power ~ 10‘6 m bar Base pressure S ubstrate T arget P Z T thin film s sitysi Ti Pt Pt/T i/Si02/Si Pb/Zr/Ti W orking pressure 3x10 mbar 1x 10 m bar Ambience gas Ar 2/Ar:40/60 S u b strate tem perature TA Room Deposition time Film thickness Ti: Pt: 30 Ti: 20 nm Pt: 100 nm 250 °c hours 500 nm The PZT film th ickn ess was determ ined about 500 nm by the grazing incident X-Ray Reflection (XRR), as illu stra te d in Fig The Zr/Ti:51/49 compositional ratio of the PZT film s was analyzed using Electron Probe Micro-Analyzer (EPMA) In order to crystallize perovskite phase, the as-deposited films were a n n ea le d using Preparation of Pb(Zr, Ti)03 thin films sputtered from conventional furnace at 650°c fixed tem p era tu re in the a ir and an oxygen ambience for 30, 60, 90, 120 min., respectively Phase and crystalline behavior analyses of PZT films were performed using X-Ray Diffractometer with 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 easured using RT 66 A standardized hysteresis tester (R adiant Technology) 33 20 (deg.) Fig The XRR p a tte rn of the PZT thin film annealed at 650°c for 60 R e s u lts an d d i s c u s s i o n C ry s ta llo g r a p h ic s tr u c t u r e The crystal stru c tu re of the as-deposited and an nealed PZT thin films was exam ined by XRD p a tte rn s in comparison with crystallographic inform ation reported earlier on PZT films and ceramics This p a rt may shed light on th e optim um a nnealing time and ambience required for the perovskite PZT formation The XRD p a tte rn s of the films annealed for th e various tim es in the air are given in Fig It can be seen from the figure th a t all of films alw ays show two peaks at 2Ớ=40° and 46.55° corresponding to the Pt polycrystalline phase • P erovskite T =650c □ P latinu m * P y ro c h lo re ryiuuiiiuic ô P b O ss ^ -p I tn= •7_ □ f\ 120 mill • >» As-dcpositcd 20 30 40 (d c g ) 50 60 Fig The XRD p attern of PZT thin films annealed for various annealing times In case of the as-deposited film, th ere are two peaks a t 29.6° and 34.15° depicting non-ferroelectric pyrochlore, Pb,T i 20 cubic oxygen deficiency phase at low tem p era tu re A p eak a t 56.24° is identified ct-PbO, stru c tu re Mean while it is possible th a t Z r 2, T i a re presen ted in a n am orphous form because whole Pb, Zr, Ti were oxidized in s p u tte rin g process sim ultaneously 34 Nguyen Thanh Huy , Vu Ngoc Hung , Nguyen Phu Th.uy When the film was annealed for 30 min., a - P b tra n s fe rre d to the lead-riich phases PbOss t h a t are observed at 29 of 23.2°, 25.45°, 32.1°, an d 42.4° Their relati ve high intensity may show the excess contents of Pb and Oọ in the th in film[3] T h e peaks at 29.05° and 33.94° m atch with the pyrochlore monoclinic AB 3O p h a se, which is norm ally caused by a deficiency in lead due to an evaporation at hi;gh tem peratu re In th is work, the pyrochlore phase ap pears even when lead-rich PbOgg compound presented in accordance with [4] It has revealed th a t th e formation of AB 30 7-type was favored over A B 3-type even when the Pb/Ti ratio was g reater th.an one and AB3O7 phas e seemed to be an intermediate ph as e before the PZT fi lm s crystallized into the ABO3 perovskite phase The peaks at 21.75° and 38.55° perfoirm the perovskite stru tu re It is confirmed th a t the PZT films deposited at t h e su b strate te m p e tu re 250°c can be converted into the perovskite crystalline stru ctu re at the a nnealing te m p e tu re 650 °c with a sufficient a nn ealin g time Increasing the a nnealing time to 60 min., complete perovskite crystallization occurs Besides the form er peaks, the others at 30.77°, 44.15°, and 54.85° according to the perovskite phase appears The ratio of intensity betw een the peaks show t h a t the stru c tu re of the PZT th in film is polycrystalline (like PZT ceramic)[5] However, the (110) intensity peak at 30.77° is largely superior to the others, thus it is considered th a t the th in films has a preferred-(llO ) orientation The tren d suggests th a t these films may be converted into the single perovskite phase by increasing the annealing time or a n n ealin g tem p era tu re ■Nevertheless, the annealed films for more th a n 60 are diphase w ith pyrochlore and perovskite coexistence The pyrochlore peaks re a p p e a r t h a t proves Pb loss T he deficiency of Pb is u n d e rstan d a b le by considering its high volatility for a long annealing time at a high tem p era tu re After the annealing tim e reaching 120 min., the non-ferroelectric pyrochlore phase is predom inating with strong and sh a rp peaks To investigate the effect of annealing ambiences, the PZT th in films were annealed for 60 a t 650 °c in the air, an oxygen and an argon gas ambience and the results are depicted in Fig As above discussion, the thin film, which was heated in the air, has the polycrystalline perovskite s tru c tu re with predom inated (110) orientation The film annealed in the oxygen ambience has still the correct perovskite stru c tu re b u t (100) orientation is preferred It may be caused by the influence'of oxygen content on PZT orientations It has been reported th a t the oxygen partial pressu re is an im p o rtan t factor in determ inin g the Pb valence sta te in th e PZT film s, w hich d ic ta te s th e k in e tic s of th e p y ro ch lo re-p ero v sk ite p h ase transformation^] A low Pb valency state enhances the kinetics of transformation, whereas a high Pb valence state suppresses the kinetics of tran sfo rm a tio n leading to incomplete tran sfo rm a tio n of the pyrochlore to perovskite phase The growth of PZT(100) on P t/T i/S i0 2/Si wafers a ttrib u te d to the form ation of crystalline PbO(OOl) P r e p a r a t ° n ° f Pb(Zr, Ti)03 thin films sputtered from d u rin g p rolysis PbO has good lattice notching w ith PZT(IOO) orientatioi; this lowers the in terfacia energy and promotes the nucliation of PZT(IOO) On the contiiry, for the PZT film a n n e a le d in the argon ambience the pyro