Effects of the Chelating Agent on the Fabrication of SBT

Ferroelectrics, 335:35–43, 2006Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190600689209

Effects of the Chelating Agent on the Fabrication ofSBT Thin Films: Part II. Microstructural Properties

A. FRATTINI,1,∗ R. MACHADO,2 M. L. SANTIAGO,3

N. PELLEGRI,3 M. G. STACHIOTTI,2 R. BOLMARO,2

AND O. DE SANCTIS3

1Area Fısica, FCByF, UNR, Suipacha 531, 2000, Rosario2Instituto de Fısica Rosario, UNR, 27 de Febrero 210 Bis, 2000, Rosario3Lab. de Materiales Ceramicos, FCEIyA, IFIR, UNR, Av. Pellegrini 250, 2000,Rosario

The effects of the chelating agent on the micro structural properties of SBT thinfilms annealed at different temperatures are investigated. The films were prepared onPt/Ti/SiO2/Si substrates by a chemical solution deposition technique. Strontium acetate,bismuth nitrate and tantalum ethoxide were used as precursor materials, with methanoland glacial acetic as solvents. We make a comparative investigation of the surface mi-crostructure, grain size distribution, crystallinity, and degree of crystal orientation forfilms prepared using acetoin and alkanolamines as chelating agent.

Keywords SBT; CSD; thin films; microstructural properties

1. Introduction

During the last years, a significant amount of research and development has focused on ferro-electrics thin films for nonvolatile random access memory application (NVFERAM) [1, 2].Bismuth layer-structured ferroelectric (BLSF) materials such as SrBi2Ta2O9, SrBi2Nb2O9,SrBi4Ti4O15 have shown to be appropriate candidates for these devices [3]. Strontium Bis-muth Tantalate (SBT) is the most promising of them because this material has excellentresistance against fatigue even with simple Pt electrodes [4]. Meanwhile, it offers accept-able polarization and smaller coercive field, which is attractive for low voltage applicationsrequired by the next generation devices [5].

The main disadvantage of the ceramic SBT is the high temperatures necessary to trans-form the SBT material from the non-ferroelectric fluorite to the ferroelectric Aurivillius[6, 7] crystal phase. Only at processing temperatures as high as 800◦C in oxygen ambient,the phase transformation is completed and the film provides a saturated hysteresis loop.This process temperature is too high to be applied to semiconductor process, and impedeits integration into high density FeRAM structures, namely the stacked capacitor architec-ture. Therefore, it necessary to reduce the annealing temperature at which the SBT film is

Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005; received forpublication January 26, 2006.

∗Corresponding author. E-mail: [email protected]

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36/[620] A. Frattini et al.

transformed from the fluorite phase to the Aurivillius phase and to find a suitable tem-perature range of SBT crystallization at which both components of the stacked capacitormemory cell can work to satisfaction.

A variety of fabrication techniques, such as metal organic chemical vapor depositionMOCVD [8], pulsed laser deposition PLD [9], magnetron sputtering [10] and chemicalsolution deposition [11, 12] have been applied to the preparation of SBT thin films. Amongthem, solution deposition processes such as sol-gel are versatile methods that permit tofabricate films inside of a wide range of compositions, with a precise stoichiometry controland at low cost. Besides, the possibility of the control of the solution at molecular levelmakes possible to obtain films with controlled textures and microstructures and with variousthickness (from tens of nm to hundreds of nm).

It has been observed that the microstructure of sol-gel derived ferroelectric films ishighly dependent on the chemical precursors and solution preparation conditions [13].We have four different precursor solutions for the SBT thin films. We reported in thepart I of this article [14] detailed information for the preparation of SBT single alkoxideprecursors and powders. In this paper, we investigate the effect of different chelating agentson the crystallization behavior and microstructure of SBT films. The CSD method withspin coating technique is selected due to its low cost, easy control of stoichiometry and itsability to achieve uniform films over large areas. It is expected that these four precursorsmay induce different properties to the derived SBT films. The synthesis of the SBT filmsby chemical deposition process is reported. Crystal orientation and surface morphology ofSBT thin films are investigated.

2. Experimental

2.1 Precursor Solution Preparation

The precursor used for the deposition of the SBT thin films was an alcoholic solution. Inbrief, SBT precursor solutions were prepared by using strontium acetate, bismuth nitratepentahidrate and tantalum ethoxide as source materials, with methanol and glacial aceticas solvents. Acetoin (CH3COCH(OH)CH3), diethanolamine (CH2OHCH2)2NH) (DEA)and triethanolamine ((CH2OHCH2)3N) (TEA) were used as complexing or chelating agent[hereafter called chemical modifier (CM)]. Four precursor solutions were prepared, usingAcetoin, Acetoin-DEA, Acetoin-TEA y DEA, namely SBT-A, SBT-AD, SBT-AT and SBT-D, respectively. These SBT precursor solutions are suitable for the deposition of uniformlayers by spin coating with a concentration up to 0.05 M. As these solutions are capableof homogeneous gel formation upon evaporation of the solvent, it is expected that thechemical homogeneity present in the solution and gels, will also be maintained in the wetthin films after deposition and in the final product. The precursor solutions have been shownto remain stable against phase segregation by precipitation for at least three month at roomtemperature in ambient conditions.

2.2. Thin-Film Deposition

Platinized silicon wafers (Pt/Ti/SiO2/Si) were used as substrates. They were wet and cleanedwith (alcohol+nitrogen) prior to spin coating. The route for the preparation of the thin filmsis schematically represented in Fig. 1. To obtain the films, different SBT precursor solutionswith different CM were deposited by spin coating (3000 rpm, 30 s) in a Clean Bench. The

Fabrication of SBT Thin Films: Part II [621]/37

Figure 1. Flow diagram of the SBT films.

deposition of a layer is followed by a thermal treatment on two hot plates. It is based on thethermal decomposition pathway [14] of a precursor gel powder, prepared from the sameprecursor solution as used for thin-film deposition. The first hot plate (HP1) is set to atemperature of 260◦C, the second (HP2) to 450◦C, the temperature at which the SBT gelpowder is completely decomposed, as described in [14]. Each layer is then intermediatelycrystallized in a furnace at 700◦C (3 min). Four layers are deposited in this way, followed bya final crystallization step at 700 or 800◦C (60 min, flowing oxygen gas). A total thicknessof crack free film of approximately 150 nm was obtained in this manner. The thickness wasdetermined by atomic force microscopy (AFM), using a step made on the sample.

2.3. Characterization

The topographic images were observed by AFM. AFM images are obtained using aNanoTec ELECTRONICA equipment working in contact mode configuration, in air atroom temperature. A SiN tip supported by a silicon cantilever (0.76 Nm−1 spring constant)was used.

The XRD data were collected on a Phillips PW1700 diffractometer using CuKα radi-ation (1.5405

�

A) and a graphite monochromator (the step size being of 2θ = 0.02◦ and a10 seconds time per step). A grazing incidence configuration (GIXRD) was used to deter-mine the crystal structure of the oxide.

The crystallographic textures were measured in an X-ray Philips MPD diffractionmachine with Eulerian cradle and using Cu-Ka radiation monocromatized by a flat graphitecrystal, Xe–CH4 gas proportional detector and a parallel plate collimator in the diffractedbeam. The experimental textures were corrected by taking in account the thickness ofthe films because of varying absorption at different tilting angles. Pole figures allowed


calculating Orientation Distribution Functions reconstruct pole figures and check coherencyof experimental data. Three pole figures were measured: (006), (115) and (200). They area complete set for texture analysis and simultaneously provide the two principal directionsby which the alignment degree of the thin film is calculated

3. Results

We have prepared dense SBT thin films with an average thickness of 150 nm, using thespin-coating deposition of solutions synthesized by sol-gel. The films obtained from thedifferent precursor solutions presented a different morphology. Methanol was used as asolvent for the dilution of the stock solution. Physico-chemical characteristics of this alcoholallowed us to obtain solutions with suitable viscosity and surface tension for the wettingof the substrate. These properties, in combination with the very low concentration of thesolutions, make possible the fabrication of thin films by a repetitive deposition process witha substrate completely covered by the ferroelectric layer, without the formation of islands.As-annealed films were found to be dense, free of cracks and well adhered to the substrates.The films also showed very smooth surfaces as observed through an optical microscope.Microstructure examination using atomic force microscopy showed fine sized grains withgrain size between 50 to 300 nm. It was also found that the grain size obviously increasedwith increasing annealing temperature.

Figures 2–6 show the topographic images and the XRD patterns of the SBT thin filmsfabricated with different CM. All of the SBT thin films were polycrystalline, but possesseddifferent textures.

The topography and the surface roughness were analyzed by AFM technique. Figures2, 3, 4 and 5 shows the topography of different films of SBT measured at different stagesof phase transformation. Fig. 2a and b show two-dimensional microstructure SBT-AD at700 ◦C and 800◦C, respectively. At 700◦C the surface show signs of a little homogenouscrystallization and an irregular surface. When the heating treatment temperature wasincreased to 800◦C, the microstructure of the film presents clear grain boundaries, butreveals a rougher surface.

(a) (b)

Figure 2. AFM photographs of SBT-AD at (a) 700◦C and (b) 800◦C.


(a) (b)

Figure 3. AFM photographs of SBT-AT at (a) 700◦C and (b) 800◦C.

The morphology of the SBT-AT films annealed at 700◦C and 800◦C are showed in Fig.3a and b. Grains of around 140 nm were observed for 700◦C whereas at 800 ◦C the grainsgrows to 220 nm, but the surface roughness did not change significantly. In the SBT-A films,smaller grains are separated by clearly defined grain boundaries with smaller RMS, grainsand RMS grow with temperature (see Fig. 4a and b). In Fig. 4a AFM images of SBT-Dfilms presents small grains without clear grains boundary. At 800◦C (Fig. 5b) grains grewconsiderably and the grains boundaries are clear now. A fully information about grain sizeand RMS for SBT films is presented in a Table 1.

We reported in the Part I of this article that the use of alkanolamines produces thesegregation of metallic bismuth in the as-prepared powders which is responsible for the

(a) (b)

Figure 4. AFM photographs of SBT-A at (a) 700◦C and (b) 800◦C.


(a) (b)

Figure 5. AFM photographs of SBT-D at (a) 700◦C and (b) 800◦C.

formation of a multiphase system, where secondary phases such as Bi2O3 appeared in thecalcined powders [14]. However, the films observed by XRD do not present secondaryphases (see Fig. 6). The volatility of bismuth and bismuth oxide leads to the elimination ofthem during the heat treatment of the films.

The dependency of crystal orientation with the annealing temperature is evaluated usingthe intensities of the corresponding characteristic peaks in XRD patterns shown in Fig. 6.The peak intensity and sharpness in XRD patterns increase with annealing temperatureand reveal that the films are polycrystalline. Although the intensity of the (115) peak ispredominant, information of the a-axis orientation can be derived from the intensity of the(200) peak at 2θ = 33◦, while the c-axis orientation is characterized by the intensity ofthe (006) and (0010) peaks at 2θ = 21◦ and 36◦, respectively. It should be noted that thecrystallographic a-axis is designated as the only polar direction within the unit cell of theAurivillius phase. Consequently in c-axis oriented grains, the polar axis is oriented parallel

Table 1Grain size and RMS for SBT films

Grain size SD Number RMSSample (nm) (nm) of grains (nm)

SBT-AD (700◦C) 150 80 633 10SBT-AD (800◦C) 180 70 1573 15SBT-AT (700◦C) 140 65 3267 9SBT-AT (800◦C) 220 90 937 8SBT-A (700◦C) 110 40 3229 6SBT-A (800◦C) 160 70 955 11SBT-D (700◦C) 70 20 457 6SBT-D (800◦C) 120 50 2013 12


Figure 6. XRD patterns of SBT films at 700◦C and 800◦C.

to the substrate. Therefore, in Pt/SBT/Pt capacitors, c-axis oriented grains do not contributeto the remnant polarization. So, the degree of a-axis orientation can be represented by theratio (200)/(006). SBT-A films present the larger ratio indicating that these films are moresuitable for memory applications.

Figure 7. (006), (115) and (200) recalculated pole figures for SBT-A at 800◦C.


Figure 8. (006), (115) and (200) recalculated pole figures for SBT-D at 800◦C.

We performed a more detailed analysis of the preferred crystal orientations bytexture analysis using the measured (006), (115) and (200) pole figures. The resultsare shown in Figs. 7 and 8 and they confirm the raft results provided by simplediffractograms.

4. Conclusions

We have prepared SBT thin films by a chemical solution deposition technique using a singlealkoxide precursor solution. We have investigated the effects of acetoin, diethanolamine, andtriethanolamine as chemical modifier. Different morphologies of SBT films were obtainedby the variation of the CM. We have achieved a better ratio (200)/(006) for the SBTfilms prepared with acetoin. This was confirmed by a texture analysis which indicates thatthis sample present a preferred crystal orientation which is more convenient for nonvolatilerandom access memory applications. Furthermore, these films present a good microstructurewith grain size of about 70 nm (at 800◦C) and low RMS.

Acknowledgments

This work was supported by Agencia Nacional de Promocion Cientıfica y Tecnologica andConsejo Nacional de Investigaciones Cientıficas y Tecnicas (Argentina). M.G.S. thankssupport from CIUNR. R.M. thanks Fundacion Josefina Pratts.

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Effects of the Chelating Agent on the Fabrication of SBT