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Journal of Sol-Gel Science and Technology 13, 877–883 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. °
Sol-Gel Derived Barium-Strontium Titanate Films V.A. VASILJEV AND K.A. VOROTILOV Moscow State Institute of Radioengineering, Electronics and Automation (Technical University), 78 Vernadsky prosp., 117454, Moscow, Russia M.I. YANOVSKAYA AND L.I. SOLOVJEVA Karpov Institute of Physical Chemistry, 10 Vorontsovo pole, 103064, Moscow, Russia A.S. SIGOV Moscow State Institute of Radioengineering, Electronics and Automation (Technical University), 78 Vernadsky prosp., 117454, Moscow, Russia
Abstract. Sol-gel techniques for the preparation of barium-strontium titanate (BST) films are discussed. The evolution of film microstructure during heat treatment, and the dielectric properties of BST films prepared from alkoxide solutions and from alkoxide solutions modified by 2-ethylhexanoic acid were studied. It is shown that the extent of the modification of the precursors by 2-ethylhexanoic acid changes the precursor molecular complexity governing the microstructure of the films and their electrical properties. Keywords:
1.
sol-gel process, ferroelectric thin films, barium-strontium titanate, microstructure, electrical properties
Introduction
Barium-strontium titanate (BST) films have been intensively investigated during recent years as highdielectric constant materials for high density dynamic random access memory devices (256 Mbit/1 Gbit), as well as for millimeter microwave integrated circuits (ICs). The sol-gel method is one of the approaches which is being extensively used for the preparation of complex oxide thin films in IC processing. A key issue of any sol-gel thin film processing is the chemistry of the precursor solution which governs the properties of the final oxide layer. Metal 2-ethylhexanoates are frequently used together with metal alkoxides for the preparation of the precursor solutions for thin film applications [1, 2]. In this project we studied the effect of 2-ethylhexanoic acid modification on the microstructural evolution in the film formation process and dielectric properties of the films.
2.
Experimental
Titanium isopropoxide, Sr and Ba were dissolved in methoxyethanol to give a 0.48 M solution with stoichiometry corresponding to Ba0.7 Sr0.3 TiO3 . After the metals had dissolved completely, an excess of 2ethylhexanoic acid was added, the solution was then heated on stirring, and the solvent was gradually distilled off. The amount of the solvent distilled varied from 0 to 52% of the initial volume. Distillation at ambient pressure allowed elimination of ∼40% of the initial volume. In order to increase the extent of distillation, the process was performed under reduced pressure, which allowed elimination of 52% of the initial volume. After cooling of the solution, xylene was added to the residue in an amount equal to the volume of the distilled solvent. It has been previously shown [3] that the reaction of titanium isopropoxide with 2-ethylhexanoic acid in methoxyethanol performed under distillation of the
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solvent results in elimination of ester and formation of oxoisopropoxy-2-ethylhexanoate: Ti(OPri )4 + MeOC2 H4 OH + C7 H15 COOH → TiOx (OPri ) y (C7 H15 COO)4−2x−y + C7 H15 COOC2 H4 OMe + iPrOH According to the chromatomasspectroscopy and NMR data only the ester of 2-ethylhexanoic acid with methoxyethanol, rather than with i PrOH was produced and isopropoxide groups are partially preserved in the product. Methoxyethoxide groups which substitute isopropoxide groups on dissolution of Ti(OPri )4 in methoxyethanol [4] are readily eliminated in reaction with 2-ethylhexanoic acid leaving an oxo-atom in the product. Oxoproducts thus formed are oligomers with varying molecular complexity. Distillation under rigid
conditions results in enhancement of molecular complexity. In the case of the trimetallic solutions similar reactions presumably occur. The viscosity of the solutions gradually decreases in the course of distillation, which is due to the consumption of the most viscous component, 2-ethylhexanoic acid; only distillation under vacuum results in increase of viscosity due to enhancement of molecular complexity of the species. The extent of distillation presumably is the main parameter of the process regulating the nature of the species formed in the complex trimetallic solution and thus influencing the film-forming process. For comparison a metal alkoxide solution containing no 2-ethylhexanoic acid was also prepared by dissolution of the necessary amount of Sr and Ba (Ba/(Ba + Sr) = 0.7) metals in the methoxyethoxide solution of Ti(OPri )4 .
Figure 1. IR spectra of the alkoxide precursor for BST films applied to the KBr single-crystal substrate: as prepared (a); cured at 200 (b), 400 (c), and 600◦ C, 30 min (d).
P1: SYD Journal of Sol-Gel Science and Technology
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November 9, 1998
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Barium-Strontium Titanate Films
The films were prepared by spin-on coating onto Si for optical characterization, Si/SiO2 /Ti/Pt substrates for dielectric and X-ray measurements, and KBr singlecrystal plates for IR spectroscopy studies. The film thickness and refractive indexes were measured by multiangle ellipsometry at 632.8 nm and the incident irradiation angles from 45◦ to 70◦ [5]. The shrinkage was determined as the percent change in thickness of the films after the heat treatment: Shrinkage = (1 − h T / h 200 ) × 100%, where h 200 is the thickness of the films after drying at 200◦ C, and h T is the thickness of the film annealed at the temperature T .
3.
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IR Spectra of BST Films
Figures 1(a)–(d) show the IR spectra of the BST alkoxide precursor (prepared without addition of the 2ethylhexanoic acid) cured at various temperatures in air for 30 min. The broad band around 3400 cm−1 (Fig. 1(a)) is due to O H stretching vibrations of the hydroxyls present in the system; it decreases in intensity with increase of the annealing temperature, and disappears after annealing at 400◦ C. The extremely low intensity of the bands due to C H stretching vibrations at 2800–3000 cm−1 testifies to the very low content of organic radicals in these samples, thus alkoxide groups are mostly eliminated in the course of spinning. These bands completely disappear in the samples annealed
Figure 2. IR spectra of the alkoxide precursor for BST films, modified with 2-ethylhexanoic acid, applied to the KBr single-crystal substrate: as prepared (a); cured at 200 (b), 300 (c), 400 (d), and 600◦ C, 30 min (e).
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at 400◦ C. It is also interesting to note a quite intense band at 1580 cm−1 . It has been previously shown by us that this band frequently appears in the spectra of alkaline and alkaline-earth alkoxides and is due to formation of carboxylate (mostly formate) during oxidative decomposition of the alkoxide radicals by oxygen dissolved in the alcoholic solutions [6], which is especially pronounced in the case of alkaline-earth methoxyethoxides. The broad band in the region of 1400 cm−1 registered in the spectra of the samples annealed at 400◦ C (organic residues by this temperature are already eliminated) needs further investigation, although Kamalasanan et al. [2] attributed it to formation of Ba O Ti bond. This band disappears on further annealing and is substituted by the band at 530 cm−1 , characteristic of the crystalline MII TiO3 [7]. Figures 2(a)–(e) show the IR spectra of the films coated by the BST precursor solution prepared with addition of 2-ethylhexanoic acid and subsequent elimination of 41% of the solvent by distillation. The spectra in this case have much lower intensity of the O H stretching vibration bands around 3400 cm−1 , but the C H stretching vibration bands in the range of 2800– 3000 cm−1 are considerably more intense corresponding to the much higher content of organics in these
films. The intensity of the latter bands practically does not change on annealing at 200◦ C, but decreases drastically in the samples annealed at 300◦ C and disappears completely in the films annealed at 400◦ C. Thus, the main loss of organics occurs on thermal treatment of these films in the temperature region of 200–300◦ C, rather than during spinning. The band at 1700 cm−1 in the films which did not undergo any thermal treatment corresponds to the stretching vibrations of the free 2-ethylhexanoic acid. The C H deformation vibration bands (1300–1500 cm−1 ), as well as the stretching vibrations of the carboxylate group (1400–1600 cm−1 ), gradually disappear on heating. However, just as in the previous case, a broad intense band in the region of 1400 cm−1 is observed and is especially pronounced in the films annealed at 400◦ C; it disappears in the crystalline films annealed at 600◦ C.
4.
Evolution of Film Thickness and Refractive Index during Heat Treatment
Evolution of the film microstructure during heat treatment was characterized by the change in film thickness (shrinkage) and refractive index (see Figs. 3(a)–(c)).
(a) Figure 3. Thickness (a), shrinkage (b) and refractive index (c) of BST films prepared from alkoxide precursors and modified alkoxide precursors with different extent of solvent distillation as a function of annealing temperature.
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Barium-Strontium Titanate Films
(b)
(c) Figure 3.
(Continued).
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The films prepared from metal alkoxides typically had a thickness of about 60 nm (after drying at 500◦ C). Preparation of thicker films is hindered by their cracking during drying. Modified metal alkoxide precursors permit preparation of films with greater thickness (140– 250 nm) in one application step without cracking. To obtain films with similar thicknesses from metal alkoxides, a four fold spin-on coating with intermediate bake was used. Film thickness decreased with the increase of extent of solvent distillation mainly due to reduction of solution viscosity as mentioned previously. The films prepared from the metal alkoxide precursor had significantly lower shrinkage (about 30% in the range of 200–500◦ C), in comparison with that prepared from the modified precursors (70–80%). In the latter case the most pronounced shrinkage occurs in the temperature range 200–300◦ C and corresponds to the loss of organics as shown by the IR spectra. The higher the extent of distillation and the consequent substitution of the alkoxide groups by 2-ethylhexanoate groups, the greater the shrinkage. Pyrolysis of organics is complete by 400◦ C; further shrinkage is due to formation of the metal-oxide network, structural relaxation and crystallization. The films prepared from the metal alkoxide solution without addition of 2ethylhexanoic acid contain no organic residue and the metal-oxide network is formed in the course of spin-on
Figure 4.
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deposition with a subsequent branching during further heating. Films prepared from metal alkoxide precursors have a higher refractive index than films obtained from the modified solutions (see Fig. 3(c)) testifying to their higher density and smaller interatomic spacing. This is also the reason for the lower cracking resistance of the films prepared without the modification process. It should be noted that thicker alkoxide films (prepared by four fold deposition) had lower shrinkage and a higher value of refractive index than those prepared by the one-step application process where structural relaxation is probably hindered by the influence of the substrate. 5.
Dielectric Properties
Figure 4 shows the dielectric constant of BST films (annealed at 700◦ C for 20 min) as a function of voltage bias. The films prepared from alkoxide solutions had relatively low dielectric constants (about 100) with a weak dependence from voltage. The reason for this lies in the formation of a metal-oxide network during the early stages of film formation in the case of the alkoxide precursor. This hinders structural relaxation and causes high mechanical stresses in the films. As a result crystallization starts at lower temperatures, but results in a structure with defects and distortions.
The dependence of dielectric constant of BST films (1 MHz, 20◦ C) as a function of voltage bias.
P1: SYD Journal of Sol-Gel Science and Technology
KL648-154-Vasiljev
November 9, 1998
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Barium-Strontium Titanate Films
By contrast, in the case of modified metal alkoxides precursors the metal-oxide network is created after pyrolysis of organics at higher temperatures when atoms with higher energy and mobility give better conditions for crystallite growth with a porous nonstressed structure. In general, greater solvent distillation led to a higher value of dielectric constant and more pronounced ferroelectric behavior (the peak dielectric constant increased). The correlation between dielectric properties and crystalline structure of BST films will be discussed in detail elsewhere. Acknowledgments This work was partially supported by Russian Foundation for Basic Research grants 96-33553 and
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97-02-17822, and State Scientific and Technical Program “Advanced Technologies and Devices of Microand Nanoelectronics” grant 218/68/1-2. References 1. M. Klee, R. Eusmann, R. Waser, W. Brand, and H. Hal, J. Appl. Phys. 72, 1566 (1992). 2. M.N. Kamalasanan, N.D. Kumar, and S. Chandra, J. Appl. Phys. 78, 4803 (1994). 3. T.V. Rogova, M.I. Yanovskaya, D.U. Grudtsyna, Yu.D. Grudtsyn, and E.P. Kovsman, Russ. J. General Chem. 67, (1997). 4. D. Ramamurthi and D.A. Payne, J. Amer. Ceram. Soc. 73, 2547 (1990). 5. Grigor´ev, V.I. Petrovsky, and I.A. Shapova, Izmeritelnaja Technicka 1, 15 (1991) (in Russian). 6. N.Ya. Turova, E.P. Turevskaya, M.I. Yanovskaya, N.M. Kotova, and R.R. Shifrina, Russ. J. Inorg. Chem. 38, 1055 (1993). 7. J.T. Last, Phys. Rev. 105, 1740 (1957).
