J Phys. IV France 8 (1998) Sol-gel processing of bismuth strontium tantalate thin films E.P. Turevskaya, K.A. Vorotilov*, V.B. Bergo, S.V. Sokolov, A.S. Sigov* and D. Benlian** Chemistry Department, Moscow State Lomonosov University, 119899 Moscow, Russia * Moscow State Institute of Radioengineering, Electronics, and Automation, Technical University, Vernadsky Prosp. 78, I1 7454 Moscow, Russia ** Universite de Provence, Laboratoire de Chimie de Coordination, Sainf Jer6me, 13397 Marseille cedex 20, France Abstract. The synthesis of bismuth strontium tantalate thin films by akoxide route and their ferroelectric properties were studied. It was found that single phase ferroelectric films with the thickness of 0.4-0.5 pm could be formed by four repeated cycles of deposition of Zethylhexanoic acid solution of metal akoxides on Si-Siq-Ti-Pt wafers followed by annealing in the temperature range of 700-750°C for 30 min after each deposition. The films showed remanent polarization ranging from 3.5 to 4.0 p ~ / c m 2 and coercieve voltage 1.5-2.0 V. 1. INTRODUCTION Intensive researches on FERAMs during the last time showed their growing potential on the memory market as devices combining features of a DRAM and an EEPROM [I]. SrBizTazO9 (SBTO) is an alternative to PZT ferroelectric material with superior fatigue behavior which was recently proposed for NVRAM applications [2,3]. Ferroelectric properties of SrBizTaz09 and SrBi2Mz09 ceramics have been investigated originally as early as in 1961 by G.A.Smolensky [4]. It was demonstrated recently that polarization fatigue in PtISBTOiPt capacitors was not observed up to lot2 cycles which is associated with a small domain wall depinning energy in SBTO [5]. During two past years SBTO thin film preparation process has been studied by different techniques - sol-gel (including its modifications: spin-on coating [2, 6-10], spin-on deposition of photo-sensitive solutions [I 11, liquid source misted chemical deposition [3,12] and electrostatic spray [13]), flash MOCVD [14], laser ablation [15, 161 and RF magnetron sputtering [17]. The major problem of SBTO films processing is a high crystallization temperature of perovski-te phase (typically about 800°C). Dierent processing strategies were proposed recently to decrease crystallization temperature. For the films prepared by sol-gel techniques a decrease in crystallization temperature was achieved by introduce of a seeding layer [8] and by annealing at reduced oxygen pressure [12]. In this work we have studied how solution chemistry effects the crystallization behavior of SBTO films. 2. PREPARATION OF SOLUTIONS AND DEPOSITION OF SBTO FILMS In order to choose the proper derivatives of Bi and Ta among the first members of the aliphatic homologous series, appropriate for preparation of the precursor solutions, we have studied the interaction in three-component systems Bi(OR)3-Ta(0R)s-ROH, where R= Me, Et or 'Pr. Bismuth and tantalum alkoxides were prepared according to previously described techniques [18,19]. It was found that metal ethoxides are the best of investigated precursors of bismuth tantalates via alkoxide route. Only in the ethoxide system the solutions of various concentrations (with Bi/Ta I 1.5) can be prepared. Vacuum evaporation of the solvent fiom these solutions results in formation of viscous pastes, which can be stored without noticeable decomposition as long as several months. Application of these pastes in preparation of precursor solutions was found to be the most convenient. Among other studied precursors of bismuth tantalates we also used Bi carboxylates (2 -ethyIhexanoic acid derivatives) and mixed - ligand derivatives of Bi and Ta which were prepared by partial substitution of alkoxy-groups in M(OR& by action of Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1998914