ENGINEERING. Ferroelectric capacitors for Integrated Circuits ... The sol-gel processing of a number of ferroelectric films is discussed emphasizing the issues of.
MICROELECTRONIC ENGINEERING ELSEVIER
Ferroelectric
Microelectronic Engineering 29 (1995) 41-44
capacitors for Integrated
Circuits
K.A.Vorotilov a, M. I.Yanovskaya b, L.I. Solovjeva b, A. S.Valeev°, V.I. Petrovskya, V.A. Vasiljeva and I.E.Obvinzevab a Moscow Institute of Radioengineering, Electronics and Automation, 117454, Vernadsky prosp., 78, Moscow, Russia 1 b Karpov Institute of Physical Chemistry, 103064, Vorontsovo pole, 10, Moscow, Russia 1 c SRI Molecular Electronics, 103460, Moscow, Russia The sol-gel processing of a number of ferroelectric films is discussed emphasizing the issues of obtaining electrical properties critical for their applications. The effect of lead excess in solution prepared by the electrochemical techniques on structural and electrical properties of PZT films are discussed. The barium strontium titanate (BST), strontium titanate-zirconate (STZ) and zirconiumstannate titanate (ZST) films were prepared by sol-gel techniques for the use in dynamic random access memories (DRAMs) and millimeter microwave integrated circuits (MMICs).
1. PZT MEMORY ELEMENTS PROCESSING: THE EFFECT OF LEAD CONTENT The solutions prepared by mixing of the solutions of Zr and Ti methoxyethoxides in methoxyethanol obtained by direct anodic dissolution of corresponding metals with solution of lead acetate (dehydrated with the acetic acid anhydride) were used as filmforming materials [1,2]. Effects of different processing conditions (e.g. lead acetate dehydration techniques, annealing procedure etc.) were discussed in our previous works [3,4]. Here we discuss the effect of lead content in solutions on the properties of the films obtained. Lead content in the films is the most important factor which in many cases determines their crystal structure, the character of electrically active defects and ultimately the electrical properties of the films [5-7]. Volatility of lead oxide in the processes of temperature treatment which is usually
necessary for crystallization of films is a serious complication. In the conventional sol-gel techniques the excess of lead (5-15%) is as a rule introduced in the film-forming solutions, in the case of each preparation technique however the exact amount of lead required is to be specially determined. Here we demonstrate the role of lead in the crystallization process of alkoxy-derived PZT (53/47) films under changes of lead excess in initial solutions over a wide range from 0 to 50%. The films with the thickness of about 0.2!am were prepared on Si-SiO2-Ti-Pt substrates. All peaks in the X-ray patterns of the films annealed at 650°C correspond to the perovskite phase, whereas admixtures of other phases were registered in the films annealed at 600°C (pyrochlore at low lead excess from 0 to 5% and PbO at 50% excess of the lead). The films had mixed texture in (100) and (111) directions with gradually increase of (100) orientation (see Figure la). The films with low lead content have typical "rosette" structure,
1The research described in this publication was made possible in part by Grants NN MRQ300 (a) and MPR300 (b) from the International Science Foundation. 0167-9317/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00112-3
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K.A. Vorofilov et al. / Microelectronic Engineenng 29 (1995) 41-44
and microcrystalline uniform films were obtained only from solutions with more than 10% lead excess. At the same time, the crystallite sizes decreased with the increase of the lead content. The films obtained from solutions with the excess content of lead not exceeding 5% and annealed at 600°C demonstrated weak ferroelectric properties (Figure lb), which is due to the considerable content of the nonferroelectric pyrochlore p h a s e crystallized under these conditions. Increase in the annealing temperature leads to considerable enhancement of the value of the remanent polarization (2PR =53 I~C/cm 2 at 5% excess of lead in solutions), however the dielectric hysteresis loops were neither saturated nor symmetric. The reason for this presumably lies in formation of pyrochlore microcrystallites at the film surfaces [8]. Although the fraction of the latter in the volume is low (in accordance with the X-ray data) its effect on the electrical properties of the films may be considerable, as far as it alters the properties of the filmelectrode interface. The increase in the excess of lead in solutions up to 10% leads to the films with saturated dielectric hysteresis loops. The annealing temperature in these cases already is not so important for the properties of the films in comparison with the films with low Pb content. These films demonstrate the lowest values of the coercive field alongside with the highest values of capacity. Further increase in the lead content up to 30% practically does not affect the saturation of hysteresis loops, however slight decrease in the values of the remanent polarization and the breakdown voltage occurs (Figure lb,c). The reason for these phenomena presumably lies in the decrease of the crystallite sizes, and in the accumulation of PbO in amorphous or microcrystalline form. The role of the latter is especially pronounced in the case of films prepared from solutions with the 50% excess of lead, for which considerable decrease in the breakdown voltage and practically complete disapearance of the ferroelectric properties were observed.
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30
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C) Figure 1. The effect of the excess of lead in initial solution on orientation (a), r e m a n e n t polarization (b), and breakdown voltage (c) of PZT films. 2. FERROELECTRICS WITH HIGH DIELECTRIC CONSTANT 2.1. Barium-strontium titanate BST has been intensively investigated during the last years as a candidate for high-s material for 256Mbit/1Gbit DRAMs (e.g.[9]). We have studied two ways of BST films preparation: 1) alkoxide route: titanium isopropoxide was dissolved in methoxyethanol,
K.A. Vorotilov et al. / Microe/ectronic Engineeling 29 (1995) 41-44
barium and strontium metals were added in stoichiometric quantities to this solution; 2) modified alkoxide route: alkoxide solutions were modified by 2-ethylhexanoic acid. The films prepared from alkoxides had a rather low dielectric constant (s= 150, tanS=O.019) probably due to mechanical stresses as a result of high polycondensation shrinkage during baking. By contrast, modified solutions permitted to decrease mechanical stresses in the films (the film thickness without cracking after one deposition was increased to about 0.2 Fm in comparison with O.051~m from alkoxide solutions) and to improve electrical performance of the films (s=700-850, tanS=O.01-O.02). Annealing temperature has a strong impact on crystallization of BST films prepared from modified alkoxide precursors. The films annealed at 700°C demonstrated higher capacity than the films annealed at 600°C (see Figure 2) which seems to be due to increase in crystallite sizes. The capacitance-voltage (CV) characteristics were highly nonlinear with a slight hysteresis (despite the fact that this composition is to be in paraelectric phase at this temperature). --
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Figure 2. CV characteristics of Si-Si02-Ti-PtBao.7Sro.3TiO3-Pt capacitors, prepared at different annealing temperatures of BST film. Temperature dependence of dielectric constant of Bao.7Sr0.3Ti03 films (Figure 3) shows broad diffusive phase transition with the peak near 45-50°C, that is somewhat higher than in the case of bulk material with the same composition (Tc=20°C [10]). Similar behavior (diffuse transition and shift of Curie
43
temperature) of BST films prepared by RF magnetron sputtering (but at much lower values of s) was observed recently in [11], and possible reasons for this were discussed in [12]. 900 8 5 0
o
800
u
750
.............................................
.o 650 600 20
40 60 80 Ten,pera£ure,
i00 [°C]
120
Figure 3. Typical temperature dependencies of dielectric constant of Si-Si02-Ti-PtBao.7Sro.3TiO3-Pt capacitors prepared from different modified alkoxide solutions.
2 . 2 . Strontium titanate-zirconate and zirconium-stannate titanate For the first time we have prepared thin STZ and ZST films by sol-gel technique. SrZro.2Tio.e03 has a rather high value of dielectric constant (100-140) and good thermal stability, which is desired for capacitors of ICs and electroluminescent display devices [13]. STZ films were prepared from alkoxides solutions (titanium and zirconium isopropoxides were dissolved in methoxyethanol and added to solution of Sr-methoxyethoxide in methoxyethanol). Films were annealed for crystallization at 600°C. Figure 4a shows frequency dispersion of dielectric constant and loss tangent of STZ capacitor on Si-Si02-Ti-Pt and Si-Si02-1n203 substrates. It is interesting, that STZ films prepared on In203 substrates demonstrated higher crystallinity and higher value of dielectric constant with lower losses (s=123 and tanS=O.014) than the films prepared on platinized silicon (s=81 and tanS=O.027). The change in capacitance with applied voltage of STZ films was relatively low and demonstrated paraelectric behavior (there was no CV hysteresis) as it is shown in Figure 5.
K.A. Vorofilov et al. / Microelectronic Engineenng 29 (1995) 41-44
44
Ceramics of Zro.sSno.2TiO4 are commonly used as resonator components in microwave devices (e.g. [14]). ZST films were prepared from alkoxide precursors (Sn butoxide was added to solutions of Ti and Zr isopropoxides in methoxyethanol) and annealed at 600°C. The films had relatively low frequency dispersion of dielectric constant and loss tangent as it is shown in Figure 4b. The values of s and tan6 (s=40, tan~=0.007-0.012) are typical for this material [14]. The capacitance of ZST film was absolutely independent of voltage. All these data and also prelimenary measurements of these films at the frequencies up to 28 GHz [15] make this material a plausible candidate in MMICs.
150
125 u
i00 u
..............i................ ..
75
50 -8
-4
0
4
Voltage,
[V]
8
Figure 5. CV characteristic of STZ capacitor.
REFERENCES 180
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m 0
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b) Figure 4. Frequency dispersion of dielectric constant and loss tangent of STZ (a) and ZST (b) capacitors prepared on Si-SiO2-Ti-Pt (11) and Si-SiO2-1n203 (x) substrates.
1. L.l.Solov'eva et al., Metal Alkoxides Preparation Technique, Russian Patent No. 2017714 (1994). 2. M.I.Yanovskaya et al., Integrated Ferroelectrics, 4 (1994) 275. 3. K.A.Vorotilov et al., Integrated Ferroelectrics, 3 (1993) 33. 4. M.l.Yanovskaya et al., Integrated Ferroelectrics, in the press. 5. M.Klee et al., Philips J.Res., 47 (1993) 263. 6. M.Klee et al., Integrated Ferroelectrics, 4 (1994) 263. 7. H.Watanabe et al., Proc. of the 4" Symposium on Integrated Ferroelectrics, Monterey, March 1992, 346. 8. I.MReaney, this Journal. 9. R.E.Jones et al., this Journal. 10.F.Jona and G.Shirane, Ferroelectric Crystals, Pergamon Press, 1962. 11 .T.S.Kim et al., J. Appl. Phys., 75 (1994) 7998. 12.Z.Surowiak et al., Thin Solid Films, 176 (1989) 227. 13.T. Matsuoka et al., J. Appl. Phys., 64 (1988) 3512. 14.R.Kudesia et al., J. Am. Ceram. Soc., 77 (1994) 3215. 15.Y.M.Poplavko et al., Integrated Ferroelectrics, 5 (1994) 19.
