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Synthesis of ZrO2 thin films by atomic layer deposition: growth kinetics, structural and electrical properties
Applied Surface Science 193 (2002) 120–128
Synthesis of ZrO2 thin films by atomic layer deposition: growth kinetics, structural and electrical properties Michel Cassira,*, Fabrice Goubina, Ce´cile Bernaya,b, Philippe Vernouxa, Daniel Lincota a
Ecole Nationale Supe´rieure de Chimie de Paris, Laboratoire d’Electrochimie et de Chimie Analytique (UMR 7575 CNRS), 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b Renault Technocentre, 1 Avenue du Golf, 78288 Guyancourt, France Received 21 February 2002; accepted 23 March 2002
Abstract Ultra thin films of ZrO2 were synthesized on soda lime glass and SnO2-coated glass, using ZrCl4 and H2O precursors by atomic layer deposition (ALD), a sequential CVD technique allowing the formation of dense and homogeneous films. The effect of temperature on the film growth kinetics shows a first temperature window for ALD processing between 280 and 350 8C and a second regime or ‘‘pseudo-window’’ between 380 and 400 8C, with a growth speed of about one monolayer per cycle. The structure and morphology of films of less than 1 mm were characterized by XRD and SEM. From 275 8C, the ZrO2 film is crystallized in a tetragonal form while a mixture of tetragonal and monoclinic phases appears at 375 8C. Impedance spectroscopy measurements confirmed the electrical properties of ZrO2 and the very low porosity of the deposited layer. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Atomic layer deposition; ZrO2; Thin film deposition
1. Introduction Zirconium dioxide is an interesting raw material in several fields because of its thermal, mechanical, optical and electric properties. In effect, thin films of ZrO2 are currently used as catalytic supports, buffer layers for superconductors and thermal barrier coatings [1–6]. Furthermore, pure zirconia has a monoclinic form at temperatures lower than 1000–1200 8C, but can be stabilized in a tetragonal or cubic form
*
Corresponding author. Tel.: þ33-1-55426387; fax: þ33-1-44276750. E-mail address: [email protected] (M. Cassir).
with different dopants, mainly CaO and Y2O3. The high ionic conductivity and stability of doped zirconia is ideal for application in oxygen sensors and fuel cells [7–11]. For example, yttria stabilized zirconia appears to be the best electrolyte compatible with the usual solid oxide fuel cell (SOFC) materials and conditions, due to its high chemical stability. Among the physical and chemical deposition techniques used to produce ZrO2 thin layers, chemical vapor deposition (CVD) of zirconium organometallic or inorganic precursors (i.e., zirconium alkoxides or chlorides) allows one to obtain high quality materials [12–15]. Atomic layer deposition (ALD), a relatively new and sophisticated technique derived from CVD, is a surface-controlled process allowing one to
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 4 7 - 7
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
produce dense films of less than 1 mm by building them up monoatomic by monoatomic layer [16–18]. The structure and growth of the material are controlled in one reaction sequence involving separated introduction of the precursors and regular purging. Thus, parasitic reactions of the precursors in the vapor phase are avoided. ZrO2 thin layers have already been deposited on different substrates as soda lime glass, silica, silicon, g-alumina and as interlayers on (Nb1x Tax )2O5 [1,19–27]. Ritala and Leskela¨ [1] obtained at 500 8C with glass substrates, using ZrCl4 and H2O reactants, amorphous 100–210 nm zirconia films with good thickness and uniformity. Rather long pulse and purge times were necessary to realize the ˚/ self-controlled growth (growth speed of 0.53 A cycle). Kukli et al. [21] succeeded in depositing ZrO2 films at lower temperatures (150–300 8C) using an alternate supply of Zr[OC(CH3)3]4 and H2O. The ˚ , with the growth rate increased from 0 to 1.9 A temperature decreasing from 325 to 200 8C; however, the films grown were very weakly crystallized and nearly unaffected by heat treatment. Kyto¨ kivi et al. [23,24] prepared zirconia-modified silica and gAl2O3 by ALD; in particular, the reaction mechanism between the precursor ZrCl4 and surface OH groups and the role of water treatment on the release of chlorine were analyzed. In a recent publication, Kukli et al. [22] deposited crystalline ZrO2 from ZrI4, H2O and H2O2 in the temperature range 250–500 8C, with ˚ at 275 8C. The a maximum growth rate of 1.25 A films contained a mixture of cubic and tetragonal phases; the cubic one was preferentially formed at lower temperature. The aim of the present work has been to optimize the deposition process of ZrO2 with ZrCl4 and H2O precursors on soda lime glass and SnO2-coated glass (allowing conductivity measurements on zirconia films) and in particular to analyze the growth kinetics as a function of temperature. Special attention was given to the identification of ALD windows. Nucleation and surface saturation processes are discussed. The general properties of the deposited film were determined by different techniques: structural analysis by X-ray spectroscopy (XRD), morphology by scanning electron microscopy (SEM) and Hg-porosimetry. They are compared to previous studies in the literature. ZrO2 electrical properties were determined by impedance spectroscopy.
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2. Experimental Film deposition was achieved with a commercial flow type-F-120 ALD reactor (ASM-Microchemistry, Finland) using nitrogen (99.999% from Air Liquide, France) as a carrier and purging gas. This computerized apparatus is equipped with a two-plate reaction chamber allowing one to process two 5 5 cm2 substrates. The reactants used are ZrCl4 (99.9% purity; Strem Chemicals) and de-ionized Millipore water, maintained at 17 8C. Formation of ZrO2 at high temperature proceeds from the following global reaction of the precursor with water [1]: ZrCl4 þ 2H2 O ! ZrO2 þ 4HCl Pulsing lengths varied between 0.5 and 2 s. The standard pulsing sequence is the following:
pulse pulse pulse pulse
1: 2: 3: 4:
ZrCl4 for 1.5 s; N2 purge for 2 s; H2O for 0.5 s; N2 for 2 s.
In the standard operating mode, both sequences are repeated during 2000 cycles. The temperature of the deposition chamber varied from 250 to 450 8C. The ZrCl4 precursor was evaporated at a temperature between 165 and 170 8C. Substrates were square samples 5 5 0:5 cm3 and constituted either of soda lime glass or SnO2-coated (500 nm) glass. Different techniques were used to analyze the deposited layers: (i) XRD with a CGR type Theˆ ta 60 diffractometer ˚ ), with y using a Cu Ka radiation (1.5418 A ˚) varying from 108 to 408. The grain size (in A was evaluated using the Sherrer formula: l D ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðb B2 Þ cos y with l the X-ray wavelength, b the width at half maximum of the sample peak, B the width at half maximum of the reference peak (monocrystalline silicon sample) and y the diffraction angle of the considered peak. In this study, the peak at 2y ¼ 30:2 relative to the tetragonal phase was used.
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(ii) SEM S440, from Leica. Deposit thickness was determined from cross-sectional SEM micrographs. (iii) Mercury porosimetry, commercialized y CE Instrument. (iv) The impedance measurements were carried out with an Autolab spectrometer (ECO Chemie BV) with a 50 mV AC signal amplitude and no DC bias for frequencies from 106 to 103 Hz. Thin layers of ZrO2 deposited on SnO2-coated glass were measured in 0.01 M KNO3 electrolyte. The counter electrode was a platinum wire and the reference was a saturated sulfate electrode (SSE). Impedance diagrams were analyzed using an equivalent circuit simulation software [28].
This so-called ALD window indicates surface-controlled reaction. Between 350 and 380 8C (part III), the growth rate increases with the temperature and the deposition is again under kinetic control. This tendency is followed by a second maximum between 380 and 400 8C (part IV). In this domain, the growth rate ˚ /cycle) than for the ALD is five times higher (5 A window, which corresponds, as we will show later, to approximately one monolayer/cycle. In the case of temperatures superior to 400 8C (part V), the growth rate decreases with the temperature. Beyond 430 8C, the curve slope decreases, which can be due to the desorption of reactants.
3. Results and discussion
Fig. 2 shows XRD spectra as a function of temperature. At 250 8C, films are possibly too thin for obtaining a diffraction signal. Alternatively, according to Kyto¨ kivi et al. [23,24], the deposit crystallinity increases with the temperature; therefore, it could be that the lack of diffraction signals is due to the fact that the deposit is not crystallized. From 275 to 350 8C, the ZrO2 film is crystallized in a tetragonal form with characteristic lines at 30.28, 518 and 60.38. The spectrum shows a preferential orientation with the (1 0 1) peak at 30.28. In the case of a ZrO2 powder, the intensity ratios of the 30.28 line with respect to 518 and 60.38 lines are about 5 and 4, respectively [29]. In our
3.1. Influence of temperature on ZrO2 deposition by ALD Fig. 1 shows the growth rate of ZrO2 as a function of temperature and temperature windows of ALD processing. Growth rates were calculated from a 2000-cycles deposit thickness. The rate function can be divided into five parts. Between 250 and 280 8C (part I), growth rate increases with the temperature and is under kinetic control. Between 280 and 350 8C (part II), a first plateau ˚ /cycle. depicts a stationary deposition rate of about 1 A
3.2. Structural and morphological characterization of ZrO2
Fig. 1. Growth rate and thickness for 2000 cycles of ZrO2 deposited on soda lime glass by ALD as a function of temperature.
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
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Fig. 2. XRD patterns of ZrO2 films deposited on soda lime glass by ALD at different temperatures; t and m refer to tetragonal and monoclinic structures, respectively.
case, these intensity ratios are significantly higher (about 8 and 10) and, therefore, it can be deduced that a preferential orientation of the (1 0 1) peak is involved. At 375 8C, the film structure is mostly tetragonal, but the intensity of 50.88 and 60.28 lines is increased with respect to 30.38 line, showing that the preferential orientation effect is decreased. A 24.58 line ascribed to the (1 1 0) peak of the monoclinic phase can also be observed [30]. The most intense peak for a powder diagram of monoclinic ZrO2 phase corresponds to the (1 1 1) peak at 28.38 [30]. In our case, this peak is not observed, which shows that the deposit is strongly oriented according to (1 1 0) peak at 24.58. At 400 8C, other lines corresponding to the monoclinic phase appear (28.28, 31.58, 35.28, 40.88 and a shoulder on 508 line), showing that this phase is disoriented. Beyond 400 8C, the crystallinity seems to decrease (peaks intensity decrease between 425 and 450 8C). This is in agreement with the results of Ritala and Leskela¨ [1], who obtained non-crystallized deposits at 500 8C. In contrast, Kyto¨ kivi et al. [23,24] obtained crystallized deposits at 600 8C. The evolution of the tetragonal/monoclinic ratio is due to the fact that at the temperatures involved in the deposition process, the stable phase is the monoclinic one. At the lowest temperatures, the tetragonal phase with a lower surface energy is favored. On the contrary, at higher temperatures under thermodynamic
equilibrium conditions, the monoclinic phase is favored. This phase appears at about 375 8C. Another explanation might be that the phase change occurs through a martensitic transformation that can be activated thermally or by a mechanical constraint [2]. In fact, during the quenching process, a constraint appears due to the difference between the thermal expansion coefficient between glass (about 33 107 K1) and zirconia (63 107 K). The resulting tension constraint on the film favors the formation of the monoclinic phase, because the transformation of tetragonal to monoclinic structure is accompanied by a volume increase of 2–7%. This constraint increases with the synthesis temperature. The crystal size was evaluated with (1 0 1) peak of the tetragonal form at 30.28 using the Sherrer equation. Between 300 and 450 8C, the value is about 5 nm. Fig. 3 depicts the temperature dependency of the morphology of ZrO2 on soda lime glass using the same deposition time (2000 cycles). SEM analysis at 250 8C (Fig. 3a) shows that the deposit is formed by small granules separated by a distance of about 1 mm. At 300 8C (Fig. 3b), the coalescence of the granules is not complete and the surface is relatively rough. As shown in Fig. 3c, the thickness of the deposited layer is slightly lower than 200 nm. It can also be observed that a film is growing between the granules. At higher temperatures, between 350 and 450 8C (Fig. 3d and e),
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the surface is relatively smooth and the film growth becomes uniform. The layers are dense and do not contain apparent porosity. At 400 8C (Fig. 3f and g), the morphology is relatively different: a dense and thicker deposit constituted of small sticks is formed. The morphology evolution seems to indicate that the film growth follows two modes. At low temperature, rapid nucleation occurs on a limited number of surface sites, i.e. surface defects. At higher temperature, the nuclei formed grow tri-dimensionally, according to a Volmer–Weber growth mode. But the film formation between the granules shows that the rest of the surface is also reactive even though nucleation is less rapid.
Nevertheless, nucleation becomes more homogeneous (more nucleation sites per surface unit) which shows that zirconia grows in a film form rather than in a granular form. Fig. 4 shows the time dependency of the soda lime glass surface after deposition, at 400 8C, with 50 and 100 cycles, respectively. The first nuclei formed can be clearly observed: they are more numerous at 400 8C with 100 cycles (in the pseudo-ALD window) than at 250 8C with 2000 cycles as shown in Fig. 3 (out of the ALD windows). The growth of ZrO2 on the zirconia already deposited on soda lime glass also seems to follow a nucleation
Fig. 3. SEM micrographs of ZrO2 films deposited on soda lime glass by ALD (2000 cycles) at different temperatures: (a) 250 8C; (b) and (c) 300 8C; (d) and (e) 450 8C; (f) and (g) 400 8C.
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
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Fig. 3. (Continued ).
process; nevertheless, the nucleation sites are smaller and more numerous on the zirconia layer (previously deposited on glass) than on glass. These smaller nuclei can be observed on the granules shown in Fig. 3b. At 400 8C, the deposit morphology varies (Fig. 3f), the growth mechanism changes, which can explain, at least partially, the increase in the growth rate at this temperature. At 275 8C, the XRD diagram indicates a preferential orientation of the tetragonal phase according to (1 0 1) plane. If we suppose that the film surface is parallel to this plane, the thickness of a monolayer is ˚ . The medium growth therefore dð1 0 1Þ ¼ 3:664 A ˚ /cycle, speed calculated from a deposit at 300 8C is 1 A which means a thickness less than the third part of a monolayer. At this temperature, the ALD window is
reached, which indicates that, at saturation, all the substrate surface sites are not reactive. Therefore, it is clear that we cannot obtain a monolayer per cycle. 3.3. Electrical properties of ZrO2 Typical impedance diagrams obtained at 25 8C on ZrO2 thin layers (deposited on a SnO2-coated glass at 400 8C) in an aqueous 0.01 mol l1 KNO3 electrolyte are represented in Fig. 5. The Nyquist plot (Fig. 5a) exhibits only one semi-circle, corresponding to the electrical behavior of ZrO2 layer. The associated resistance is 2:5 109 O (for a surface of 1.13 cm2 and a thickness of 475 nm) and the resistivity 6:25 1015 O cm. The resistivity value is in agreement with the value given for pure ZrO2 in the literature, about
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Fig. 5. Impedance diagrams obtained for ZrO2 (deposited on SnO2-coated glass) in KNO3 0.01 mol l1: (a) Nyquist plot; (b) Bode plot.
Fig. 4. SEM micrographs of ZrO2 films deposited on soda lime glass by ALD at 400 8C: (a) 50 cycles; (b) 100 cycles.
1015 O cm at room temperature. The conductivity is 1:6 1014 S cm1 is also relatively close to the literature, about 1012 S cm1 [31]. The capacity is 5:65 108 F, which means a capacitance of 3:7 108 F cm2. These results correspond to the behavior of a dielectric material. In effect, the capacity value allows one to deduce a film dielectric constant of 27, which is very close to the values given in the literature: between 20 and 31 [32].
An important fact is that no additional semi-circle associated with porosity is observed. The Bode plot (Fig. 5b) confirms this observation. The high frequencies (106–105 Hz) part of the diagram corresponds to the KNO3 electrolyte. The other part (between 105 and 103 Hz) is related to the ZrO2 film. A straight line with a slope of 1 can be observed. Thus, it can be assumed that the liquid electrolyte does not penetrate in the structure of the ZrO2 thin layer, even if the thickness of this layer is only about 450 nm. This observation is confirmed by Hg-porosity measurements. The total porosity measured is 1.14% in volume, with pores diameter between 6 and 22 nm. This low porosity can probably explain the slight slipping observed at low frequency on the Bode plot.
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
3.4. Analysis of the ZrO2 growth phenomena Ritala and Leskela¨ [1], using the same precursors (evaporation of ZrCl4 at 165 8C), determined a growth ˚ /cycle at a fixed speed of the ZrO2 layer of 0.53 A temperature of 500 8C with the pulse and purge times of 3 s (instead of 2 s in our case). This value corresponds well to the dependency we have found and can be set on the experimental curve obtained in Fig. 1. In fact, this curve is not classical in ALD, where in general there is no second plateau. A possible explanation could be the difference in the reactivity of two kinds of OH groups at the glass substrate surface. In effect, Kyto¨ kivi et al. [23,24] showed that isolated OH groups react identically with ZrCl4 in a temperature range 300–600 8C (according to our measurements, these groups react from 275 8C); therefore, a wide ALD window can be observed for these groups. In contrast, they showed that at 300 8C, OH groups involving hydrogen bonds are not very reactive. These groups provoke an ALD pseudowindow different than that of isolated groups. Thus, it would be possible to obtain two ALD windows, one corresponding to bound groups and the other to free groups. Combination of both phenomena would explain the general evolution of growth rate with temperature. Nevertheless, the most probable hypothesis to explain the existence of the ‘‘pseudo-plateau’’ shown in Fig. 1 is the progressive change in the atomic structure of the ZrO2 layer, from tetragonal to monoclinic. The increase in the growth rate from 350 to 400 8C is associated with a higher crystallinity of the deposited layer. The mechanism in the growth part, considering a ZrO2 layer covered with Zr(OH)4 superficial groups, can be described as follows. After a ZrCl4 pulse (s represents superficial groups): ðZrðOHÞ4 Þs þ ZrCl4 ! ZrO2 ðZrO2 Þs þ 4HCl " (1) After a H2O pulse: ZrO2 ðZrO2 Þs þ 2H2 O ! ZrO2 ðZrðOHÞ4 Þs
(2)
ZrO2 ðZrðOHÞ4 Þs þ ZrCl4 ! ZrO2 ZrO2 ðZrðOHÞ4 Þs þ 4HCl " ; etc:
(3)
ðZrO2 Þn ðZrðOHÞ4 Þs þ ZrCl4 ! ðZrO2 Þnþ1 ðZrO2 Þs þ 4HCl "
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(5)
For temperatures higher than 400 8C a rapid decrease of the growth rate is observed. The chemisorption, as described by reactions (2) and (4), is stopped and the surface hydroxyl groups are dehydrated at these temperatures. An olation reaction probably occurs, blocking the surface with respect to ZrO2 formation. ! ZrOZr þ H2 O This process, which is the reverse of reactions (2) and (4), may be attributed to the desorption of reaction products or reactive species from the surface [18]. 4. Conclusion The study of the zirconia deposit by ALD allowed us to determine the dependency of the growth rate versus the temperature. An ALD window was observed between 280 and 350 8C, which corresponds to the formation of a third monolayer per cycle. A pseudowindow appeared between 380 and 400 8C, with a ˚ /cycle, which maximum growth speed of about 5 A corresponds to one monolayer per cycle. The XRD spectra of the deposited layers as a function of the deposition temperature shows that, from 275 8C, the deposit phase is tetragonal and preferentially oriented. Then, the orientation becomes more random from 375 8C and the monoclinic structure appears. The highest crystallinity is obtained between 380 and 400 8C. The morphological study of the first deposition steps and of the thin layers, allowed one to determine two growth modes, depending on the deposition temperature: one in a granular form at the level of surface defects, and the other in the form of a homogeneous film. SEM analysis also showed that the films are dense and non-porous, which was confirmed by impedance spectroscopy and porosity measurements. Acknowledgements
It can be established in general: ðZrO2 Þn ðZrO2 Þs þ 2H2 O ! ðZrO2 Þn ðZrðOHÞ4 Þs (4)
We gratefully acknowledge the support of Renault Company and the advice of Jean-Pierre Buchel, Fabien Heurtaux and Marielle Marchand.
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References [1] M. Ritala, M. Leskela¨ , Appl. Surf. Sci. 75 (1994) 333. [2] K. Kukli, Atomic layer deposition of artificially structured dielectric materials, Ph.D. Thesis, Tartu University, Finland, 1999. [3] M. Atik, M.A. Aegerter, J. Non-Cryst. Solids 147–148 (1992) 813. [4] J. Shappir, A. Anis, I. Pinsky, IEEE Trans. Electron Dev. ED33 (1986) 442. [5] R. Taylor, J.R. Brandon, P. Morrell, Surf. Coat. Technol. 50 (1992) 141. [6] G. Johner, K.K. Schweitzer, Thin Solid Films 119 (1984) 301. [7] M.S. Khan, M.S. Islam, D.R. Bates, J. Mater. Chem. 8 (1998) 2299. [8] Y. Miyahara, K. Tsukada, H. Miyagi, J. Appl. Phys. 63 (1988) 2431. [9] L.S. Wang, E.S. Thiele, S.A. Barnett, Solid State Ionics 52 (1992) 261. [10] B. Tre´ millon, G. Picard, in: Proceedings of the First International Symposium on Molten Salt Chemistry and Technology, Kyoto, 1983, p. 93. [11] F. Goubin, C. Bernay, P. Vernoux, D. Lincot, M. Cassir, in: J. McEvoy (Ed.), Proceedings of the Fourth European Solid Oxide Fuel Cell Forum, Lucerne/Switzerland, 2000, p. 763. [12] E. Sipp, F. Langlais, R. Naslain, J. Less-Common Met. 132 (1987) 273. [13] Y. Takahashi, T. Kawae, M. Nasu, J. Cryst. Growth 74 (1986) 409. [14] J.-H. Choi, H.-G. Kim, S.-G. Yoon, J. Mater. Sci. Mater. Electron. 3 (1992) 87.
[15] Z. Xue, B.A. Vaartstra, K.G. Caulton, M.H. Chisholm, D.L. Jones, Eur. J. Solid State Inorg. Chem. 29 (1992) 213. [16] T. Suntola, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, Part B, Vol. 3, Elsevier, Amsterdam, 1994. [17] L. Niinisto¨ , M. Ritala, M. Leskela¨ , Mater. Sci. Eng. B 41 (1996) 23. [18] E.B. Yousfi, J. Fouache, D. Lincot, Appl. Surf. Sci. 153 (2000) 223. [19] K. Kukli, M. Ritala, M. Leskela¨ , Nanostruct. Mater. 8 (1997) 785. [20] K. Kukli, J. Ihanus, M. Ritala, M. Leskela¨ , J. Electrochem. Soc. 144 (1997) 300. [21] K. Kukli, M. Ritala, M. Leskela¨ , Chem. Vap. Deposition 6 (2000) 297. [22] K. Kukli, K. Forsgren, J. Aarik, T. Uustare, A. Aidla, A. Niskanen, M. Ritala, M. Leskela¨ , A. Ha˚ rsta, J. Cryst. Growth 231 (2001) 262. [23] A. Kyto¨ kivi, E.A. Lakomaa, A. Root, Langmuir 12 (1996) 4395. ¨ sterholm, J.-P. [24] A. Kyto¨ kivi, E.L. Lakomaa, A. Root, H. O Jacobs, H.H. Brongersma, Langmuir 13 (1997) 2717. [25] M. Copel, M. Gribelyuk, E. Gusev, Appl. Phys. Lett. 76 (2000) 436. [26] H. Zhang, R. Solanki, J. Electrochem. Soc. 148 (2001) F63. [27] H. Zhang, R. Solanki, B. Roberds, G. Bai, I. Banerjee, J. Appl. Phys. 87 (2000) 1921. [28] B.A. Boukamp, Solid State Ionics 20 (1996) 31. [29] J. Malek, L. Benes, T. Mitsuhashi, Powder Diffraction 12 (1997) 96. [30] C.J. Howard, J. Am. Ceram. Soc. 73 (1990) 2828. [31] D.K. Dawson, R.H. Creamer, Br. J. Appl. Phys. 16 (1965) 1643. [32] O. Gebhard, A. Herman, Electrochim. Acta 41 (1996) 1181.
Synthesis of ZrO2 thin films by atomic layer deposition: growth kinetics, structural and electrical properties Michel Cassira,*, Fabrice Goubina, Ce´cile Bernaya,b, Philippe Vernouxa, Daniel Lincota a
Ecole Nationale Supe´rieure de Chimie de Paris, Laboratoire d’Electrochimie et de Chimie Analytique (UMR 7575 CNRS), 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b Renault Technocentre, 1 Avenue du Golf, 78288 Guyancourt, France Received 21 February 2002; accepted 23 March 2002
Abstract Ultra thin films of ZrO2 were synthesized on soda lime glass and SnO2-coated glass, using ZrCl4 and H2O precursors by atomic layer deposition (ALD), a sequential CVD technique allowing the formation of dense and homogeneous films. The effect of temperature on the film growth kinetics shows a first temperature window for ALD processing between 280 and 350 8C and a second regime or ‘‘pseudo-window’’ between 380 and 400 8C, with a growth speed of about one monolayer per cycle. The structure and morphology of films of less than 1 mm were characterized by XRD and SEM. From 275 8C, the ZrO2 film is crystallized in a tetragonal form while a mixture of tetragonal and monoclinic phases appears at 375 8C. Impedance spectroscopy measurements confirmed the electrical properties of ZrO2 and the very low porosity of the deposited layer. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Atomic layer deposition; ZrO2; Thin film deposition
1. Introduction Zirconium dioxide is an interesting raw material in several fields because of its thermal, mechanical, optical and electric properties. In effect, thin films of ZrO2 are currently used as catalytic supports, buffer layers for superconductors and thermal barrier coatings [1–6]. Furthermore, pure zirconia has a monoclinic form at temperatures lower than 1000–1200 8C, but can be stabilized in a tetragonal or cubic form
*
Corresponding author. Tel.: þ33-1-55426387; fax: þ33-1-44276750. E-mail address: [email protected] (M. Cassir).
with different dopants, mainly CaO and Y2O3. The high ionic conductivity and stability of doped zirconia is ideal for application in oxygen sensors and fuel cells [7–11]. For example, yttria stabilized zirconia appears to be the best electrolyte compatible with the usual solid oxide fuel cell (SOFC) materials and conditions, due to its high chemical stability. Among the physical and chemical deposition techniques used to produce ZrO2 thin layers, chemical vapor deposition (CVD) of zirconium organometallic or inorganic precursors (i.e., zirconium alkoxides or chlorides) allows one to obtain high quality materials [12–15]. Atomic layer deposition (ALD), a relatively new and sophisticated technique derived from CVD, is a surface-controlled process allowing one to
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 4 7 - 7
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
produce dense films of less than 1 mm by building them up monoatomic by monoatomic layer [16–18]. The structure and growth of the material are controlled in one reaction sequence involving separated introduction of the precursors and regular purging. Thus, parasitic reactions of the precursors in the vapor phase are avoided. ZrO2 thin layers have already been deposited on different substrates as soda lime glass, silica, silicon, g-alumina and as interlayers on (Nb1x Tax )2O5 [1,19–27]. Ritala and Leskela¨ [1] obtained at 500 8C with glass substrates, using ZrCl4 and H2O reactants, amorphous 100–210 nm zirconia films with good thickness and uniformity. Rather long pulse and purge times were necessary to realize the ˚/ self-controlled growth (growth speed of 0.53 A cycle). Kukli et al. [21] succeeded in depositing ZrO2 films at lower temperatures (150–300 8C) using an alternate supply of Zr[OC(CH3)3]4 and H2O. The ˚ , with the growth rate increased from 0 to 1.9 A temperature decreasing from 325 to 200 8C; however, the films grown were very weakly crystallized and nearly unaffected by heat treatment. Kyto¨ kivi et al. [23,24] prepared zirconia-modified silica and gAl2O3 by ALD; in particular, the reaction mechanism between the precursor ZrCl4 and surface OH groups and the role of water treatment on the release of chlorine were analyzed. In a recent publication, Kukli et al. [22] deposited crystalline ZrO2 from ZrI4, H2O and H2O2 in the temperature range 250–500 8C, with ˚ at 275 8C. The a maximum growth rate of 1.25 A films contained a mixture of cubic and tetragonal phases; the cubic one was preferentially formed at lower temperature. The aim of the present work has been to optimize the deposition process of ZrO2 with ZrCl4 and H2O precursors on soda lime glass and SnO2-coated glass (allowing conductivity measurements on zirconia films) and in particular to analyze the growth kinetics as a function of temperature. Special attention was given to the identification of ALD windows. Nucleation and surface saturation processes are discussed. The general properties of the deposited film were determined by different techniques: structural analysis by X-ray spectroscopy (XRD), morphology by scanning electron microscopy (SEM) and Hg-porosimetry. They are compared to previous studies in the literature. ZrO2 electrical properties were determined by impedance spectroscopy.
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2. Experimental Film deposition was achieved with a commercial flow type-F-120 ALD reactor (ASM-Microchemistry, Finland) using nitrogen (99.999% from Air Liquide, France) as a carrier and purging gas. This computerized apparatus is equipped with a two-plate reaction chamber allowing one to process two 5 5 cm2 substrates. The reactants used are ZrCl4 (99.9% purity; Strem Chemicals) and de-ionized Millipore water, maintained at 17 8C. Formation of ZrO2 at high temperature proceeds from the following global reaction of the precursor with water [1]: ZrCl4 þ 2H2 O ! ZrO2 þ 4HCl Pulsing lengths varied between 0.5 and 2 s. The standard pulsing sequence is the following:
pulse pulse pulse pulse
1: 2: 3: 4:
ZrCl4 for 1.5 s; N2 purge for 2 s; H2O for 0.5 s; N2 for 2 s.
In the standard operating mode, both sequences are repeated during 2000 cycles. The temperature of the deposition chamber varied from 250 to 450 8C. The ZrCl4 precursor was evaporated at a temperature between 165 and 170 8C. Substrates were square samples 5 5 0:5 cm3 and constituted either of soda lime glass or SnO2-coated (500 nm) glass. Different techniques were used to analyze the deposited layers: (i) XRD with a CGR type Theˆ ta 60 diffractometer ˚ ), with y using a Cu Ka radiation (1.5418 A ˚) varying from 108 to 408. The grain size (in A was evaluated using the Sherrer formula: l D ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðb B2 Þ cos y with l the X-ray wavelength, b the width at half maximum of the sample peak, B the width at half maximum of the reference peak (monocrystalline silicon sample) and y the diffraction angle of the considered peak. In this study, the peak at 2y ¼ 30:2 relative to the tetragonal phase was used.
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(ii) SEM S440, from Leica. Deposit thickness was determined from cross-sectional SEM micrographs. (iii) Mercury porosimetry, commercialized y CE Instrument. (iv) The impedance measurements were carried out with an Autolab spectrometer (ECO Chemie BV) with a 50 mV AC signal amplitude and no DC bias for frequencies from 106 to 103 Hz. Thin layers of ZrO2 deposited on SnO2-coated glass were measured in 0.01 M KNO3 electrolyte. The counter electrode was a platinum wire and the reference was a saturated sulfate electrode (SSE). Impedance diagrams were analyzed using an equivalent circuit simulation software [28].
This so-called ALD window indicates surface-controlled reaction. Between 350 and 380 8C (part III), the growth rate increases with the temperature and the deposition is again under kinetic control. This tendency is followed by a second maximum between 380 and 400 8C (part IV). In this domain, the growth rate ˚ /cycle) than for the ALD is five times higher (5 A window, which corresponds, as we will show later, to approximately one monolayer/cycle. In the case of temperatures superior to 400 8C (part V), the growth rate decreases with the temperature. Beyond 430 8C, the curve slope decreases, which can be due to the desorption of reactants.
3. Results and discussion
Fig. 2 shows XRD spectra as a function of temperature. At 250 8C, films are possibly too thin for obtaining a diffraction signal. Alternatively, according to Kyto¨ kivi et al. [23,24], the deposit crystallinity increases with the temperature; therefore, it could be that the lack of diffraction signals is due to the fact that the deposit is not crystallized. From 275 to 350 8C, the ZrO2 film is crystallized in a tetragonal form with characteristic lines at 30.28, 518 and 60.38. The spectrum shows a preferential orientation with the (1 0 1) peak at 30.28. In the case of a ZrO2 powder, the intensity ratios of the 30.28 line with respect to 518 and 60.38 lines are about 5 and 4, respectively [29]. In our
3.1. Influence of temperature on ZrO2 deposition by ALD Fig. 1 shows the growth rate of ZrO2 as a function of temperature and temperature windows of ALD processing. Growth rates were calculated from a 2000-cycles deposit thickness. The rate function can be divided into five parts. Between 250 and 280 8C (part I), growth rate increases with the temperature and is under kinetic control. Between 280 and 350 8C (part II), a first plateau ˚ /cycle. depicts a stationary deposition rate of about 1 A
3.2. Structural and morphological characterization of ZrO2
Fig. 1. Growth rate and thickness for 2000 cycles of ZrO2 deposited on soda lime glass by ALD as a function of temperature.
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
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Fig. 2. XRD patterns of ZrO2 films deposited on soda lime glass by ALD at different temperatures; t and m refer to tetragonal and monoclinic structures, respectively.
case, these intensity ratios are significantly higher (about 8 and 10) and, therefore, it can be deduced that a preferential orientation of the (1 0 1) peak is involved. At 375 8C, the film structure is mostly tetragonal, but the intensity of 50.88 and 60.28 lines is increased with respect to 30.38 line, showing that the preferential orientation effect is decreased. A 24.58 line ascribed to the (1 1 0) peak of the monoclinic phase can also be observed [30]. The most intense peak for a powder diagram of monoclinic ZrO2 phase corresponds to the (1 1 1) peak at 28.38 [30]. In our case, this peak is not observed, which shows that the deposit is strongly oriented according to (1 1 0) peak at 24.58. At 400 8C, other lines corresponding to the monoclinic phase appear (28.28, 31.58, 35.28, 40.88 and a shoulder on 508 line), showing that this phase is disoriented. Beyond 400 8C, the crystallinity seems to decrease (peaks intensity decrease between 425 and 450 8C). This is in agreement with the results of Ritala and Leskela¨ [1], who obtained non-crystallized deposits at 500 8C. In contrast, Kyto¨ kivi et al. [23,24] obtained crystallized deposits at 600 8C. The evolution of the tetragonal/monoclinic ratio is due to the fact that at the temperatures involved in the deposition process, the stable phase is the monoclinic one. At the lowest temperatures, the tetragonal phase with a lower surface energy is favored. On the contrary, at higher temperatures under thermodynamic
equilibrium conditions, the monoclinic phase is favored. This phase appears at about 375 8C. Another explanation might be that the phase change occurs through a martensitic transformation that can be activated thermally or by a mechanical constraint [2]. In fact, during the quenching process, a constraint appears due to the difference between the thermal expansion coefficient between glass (about 33 107 K1) and zirconia (63 107 K). The resulting tension constraint on the film favors the formation of the monoclinic phase, because the transformation of tetragonal to monoclinic structure is accompanied by a volume increase of 2–7%. This constraint increases with the synthesis temperature. The crystal size was evaluated with (1 0 1) peak of the tetragonal form at 30.28 using the Sherrer equation. Between 300 and 450 8C, the value is about 5 nm. Fig. 3 depicts the temperature dependency of the morphology of ZrO2 on soda lime glass using the same deposition time (2000 cycles). SEM analysis at 250 8C (Fig. 3a) shows that the deposit is formed by small granules separated by a distance of about 1 mm. At 300 8C (Fig. 3b), the coalescence of the granules is not complete and the surface is relatively rough. As shown in Fig. 3c, the thickness of the deposited layer is slightly lower than 200 nm. It can also be observed that a film is growing between the granules. At higher temperatures, between 350 and 450 8C (Fig. 3d and e),
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the surface is relatively smooth and the film growth becomes uniform. The layers are dense and do not contain apparent porosity. At 400 8C (Fig. 3f and g), the morphology is relatively different: a dense and thicker deposit constituted of small sticks is formed. The morphology evolution seems to indicate that the film growth follows two modes. At low temperature, rapid nucleation occurs on a limited number of surface sites, i.e. surface defects. At higher temperature, the nuclei formed grow tri-dimensionally, according to a Volmer–Weber growth mode. But the film formation between the granules shows that the rest of the surface is also reactive even though nucleation is less rapid.
Nevertheless, nucleation becomes more homogeneous (more nucleation sites per surface unit) which shows that zirconia grows in a film form rather than in a granular form. Fig. 4 shows the time dependency of the soda lime glass surface after deposition, at 400 8C, with 50 and 100 cycles, respectively. The first nuclei formed can be clearly observed: they are more numerous at 400 8C with 100 cycles (in the pseudo-ALD window) than at 250 8C with 2000 cycles as shown in Fig. 3 (out of the ALD windows). The growth of ZrO2 on the zirconia already deposited on soda lime glass also seems to follow a nucleation
Fig. 3. SEM micrographs of ZrO2 films deposited on soda lime glass by ALD (2000 cycles) at different temperatures: (a) 250 8C; (b) and (c) 300 8C; (d) and (e) 450 8C; (f) and (g) 400 8C.
M. Cassir et al. / Applied Surface Science 193 (2002) 120–128
125
Fig. 3. (Continued ).
process; nevertheless, the nucleation sites are smaller and more numerous on the zirconia layer (previously deposited on glass) than on glass. These smaller nuclei can be observed on the granules shown in Fig. 3b. At 400 8C, the deposit morphology varies (Fig. 3f), the growth mechanism changes, which can explain, at least partially, the increase in the growth rate at this temperature. At 275 8C, the XRD diagram indicates a preferential orientation of the tetragonal phase according to (1 0 1) plane. If we suppose that the film surface is parallel to this plane, the thickness of a monolayer is ˚ . The medium growth therefore dð1 0 1Þ ¼ 3:664 A ˚ /cycle, speed calculated from a deposit at 300 8C is 1 A which means a thickness less than the third part of a monolayer. At this temperature, the ALD window is
reached, which indicates that, at saturation, all the substrate surface sites are not reactive. Therefore, it is clear that we cannot obtain a monolayer per cycle. 3.3. Electrical properties of ZrO2 Typical impedance diagrams obtained at 25 8C on ZrO2 thin layers (deposited on a SnO2-coated glass at 400 8C) in an aqueous 0.01 mol l1 KNO3 electrolyte are represented in Fig. 5. The Nyquist plot (Fig. 5a) exhibits only one semi-circle, corresponding to the electrical behavior of ZrO2 layer. The associated resistance is 2:5 109 O (for a surface of 1.13 cm2 and a thickness of 475 nm) and the resistivity 6:25 1015 O cm. The resistivity value is in agreement with the value given for pure ZrO2 in the literature, about
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Fig. 5. Impedance diagrams obtained for ZrO2 (deposited on SnO2-coated glass) in KNO3 0.01 mol l1: (a) Nyquist plot; (b) Bode plot.
Fig. 4. SEM micrographs of ZrO2 films deposited on soda lime glass by ALD at 400 8C: (a) 50 cycles; (b) 100 cycles.
1015 O cm at room temperature. The conductivity is 1:6 1014 S cm1 is also relatively close to the literature, about 1012 S cm1 [31]. The capacity is 5:65 108 F, which means a capacitance of 3:7 108 F cm2. These results correspond to the behavior of a dielectric material. In effect, the capacity value allows one to deduce a film dielectric constant of 27, which is very close to the values given in the literature: between 20 and 31 [32].
An important fact is that no additional semi-circle associated with porosity is observed. The Bode plot (Fig. 5b) confirms this observation. The high frequencies (106–105 Hz) part of the diagram corresponds to the KNO3 electrolyte. The other part (between 105 and 103 Hz) is related to the ZrO2 film. A straight line with a slope of 1 can be observed. Thus, it can be assumed that the liquid electrolyte does not penetrate in the structure of the ZrO2 thin layer, even if the thickness of this layer is only about 450 nm. This observation is confirmed by Hg-porosity measurements. The total porosity measured is 1.14% in volume, with pores diameter between 6 and 22 nm. This low porosity can probably explain the slight slipping observed at low frequency on the Bode plot.
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3.4. Analysis of the ZrO2 growth phenomena Ritala and Leskela¨ [1], using the same precursors (evaporation of ZrCl4 at 165 8C), determined a growth ˚ /cycle at a fixed speed of the ZrO2 layer of 0.53 A temperature of 500 8C with the pulse and purge times of 3 s (instead of 2 s in our case). This value corresponds well to the dependency we have found and can be set on the experimental curve obtained in Fig. 1. In fact, this curve is not classical in ALD, where in general there is no second plateau. A possible explanation could be the difference in the reactivity of two kinds of OH groups at the glass substrate surface. In effect, Kyto¨ kivi et al. [23,24] showed that isolated OH groups react identically with ZrCl4 in a temperature range 300–600 8C (according to our measurements, these groups react from 275 8C); therefore, a wide ALD window can be observed for these groups. In contrast, they showed that at 300 8C, OH groups involving hydrogen bonds are not very reactive. These groups provoke an ALD pseudowindow different than that of isolated groups. Thus, it would be possible to obtain two ALD windows, one corresponding to bound groups and the other to free groups. Combination of both phenomena would explain the general evolution of growth rate with temperature. Nevertheless, the most probable hypothesis to explain the existence of the ‘‘pseudo-plateau’’ shown in Fig. 1 is the progressive change in the atomic structure of the ZrO2 layer, from tetragonal to monoclinic. The increase in the growth rate from 350 to 400 8C is associated with a higher crystallinity of the deposited layer. The mechanism in the growth part, considering a ZrO2 layer covered with Zr(OH)4 superficial groups, can be described as follows. After a ZrCl4 pulse (s represents superficial groups): ðZrðOHÞ4 Þs þ ZrCl4 ! ZrO2 ðZrO2 Þs þ 4HCl " (1) After a H2O pulse: ZrO2 ðZrO2 Þs þ 2H2 O ! ZrO2 ðZrðOHÞ4 Þs
(2)
ZrO2 ðZrðOHÞ4 Þs þ ZrCl4 ! ZrO2 ZrO2 ðZrðOHÞ4 Þs þ 4HCl " ; etc:
(3)
ðZrO2 Þn ðZrðOHÞ4 Þs þ ZrCl4 ! ðZrO2 Þnþ1 ðZrO2 Þs þ 4HCl "
127
(5)
For temperatures higher than 400 8C a rapid decrease of the growth rate is observed. The chemisorption, as described by reactions (2) and (4), is stopped and the surface hydroxyl groups are dehydrated at these temperatures. An olation reaction probably occurs, blocking the surface with respect to ZrO2 formation. ! ZrOZr þ H2 O This process, which is the reverse of reactions (2) and (4), may be attributed to the desorption of reaction products or reactive species from the surface [18]. 4. Conclusion The study of the zirconia deposit by ALD allowed us to determine the dependency of the growth rate versus the temperature. An ALD window was observed between 280 and 350 8C, which corresponds to the formation of a third monolayer per cycle. A pseudowindow appeared between 380 and 400 8C, with a ˚ /cycle, which maximum growth speed of about 5 A corresponds to one monolayer per cycle. The XRD spectra of the deposited layers as a function of the deposition temperature shows that, from 275 8C, the deposit phase is tetragonal and preferentially oriented. Then, the orientation becomes more random from 375 8C and the monoclinic structure appears. The highest crystallinity is obtained between 380 and 400 8C. The morphological study of the first deposition steps and of the thin layers, allowed one to determine two growth modes, depending on the deposition temperature: one in a granular form at the level of surface defects, and the other in the form of a homogeneous film. SEM analysis also showed that the films are dense and non-porous, which was confirmed by impedance spectroscopy and porosity measurements. Acknowledgements
It can be established in general: ðZrO2 Þn ðZrO2 Þs þ 2H2 O ! ðZrO2 Þn ðZrðOHÞ4 Þs (4)
We gratefully acknowledge the support of Renault Company and the advice of Jean-Pierre Buchel, Fabien Heurtaux and Marielle Marchand.
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