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Phase transformations in hafnium dioxide thin films grown by atomic layer deposition at high temperatures
Applied Surface Science 173 (2001) 15±21
Phase transformations in hafnium dioxide thin ®lms grown by atomic layer deposition at high temperatures Jaan Aarika,*, Aleks Aidlaa, Hugo MaÈndara, Teet Uustarea, Kaupo Kuklib, Mikael Schuiskyc a Institute of Materials Science, University of Tartu, TaÈhe 4, 51010 Tartu, Estonia Institute of Experimental Physics and Technology, University of Tartu, TaÈhe 4, 51010 Tartu, Estonia c Department of Inorganic Chemistry, The AÊngstroÈm Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden b
Received 3 March 2000; accepted 12 October 2000
Abstract High-temperature cubic phase of HfO2 was observed by re¯ection high-energy electron diffraction in nanocrystalline thin ®lms grown by atomic layer deposition from HfCl4 and H2O at substrate temperatures of 880±9408C. The phase was formed at properly chosen precursor doses and it was observed on the surface of ®lms, which according to X-ray diffraction data consisted of monoclinic HfO2. The thickness of the surface layer, in which the cubic phase appeared, was estimated to be 5±10 nm. According to Auger electron spectroscopy data, formation of the cubic phase was accompanied with an increase in the ionicity of O±Hf bonds. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.50K; 68.55 Keywords: Hafnium dioxide; Atomic layer deposition; Structure; Auger electron spectroscopy
1. Introduction Hafnium dioxide (HfO2) is a material forming several polymorphs. Some of those observed at high temperatures and/or high pressures, mainly, are of interest because of high density and hardness [1,2]. Although pure HfO2 tends to appear in the monoclinic phase at room temperature and atmospheric pressure, orthorhombic and tetragonal phases formed at high pressures can be partially quenched to atmospheric pressure [2,3]. Small amounts of metastable phases *
Corresponding author. Tel.: 372-7-375877; fax: 372-7-375540. E-mail address: [email protected] (J. Aarik).
have also been found in thin HfO2 ®lms and nanolaminates grown by atomic layer deposition (ALD) at low pressures and relatively low temperatures [4±6]. Nevertheless, contrary to ZrO2 [7±9] and TiO2 [10,11] thin ®lms, where metastable phases have been dominating, only some weak X-ray diffraction (XRD) lines of tetragonal [4,5] or orthorhombic [4,6] phases have been recorded in HfO2 ®lms grown at substrate temperatures 300±6008C [4±6]. Existing experimental data, however, indicate that the transition from the amorphous to the crystalline growth appears in the ALD process of HfO2 at signi®cantly higher temperature [6] than in the ZrO2 [9] and TiO2 [12] processes. Therefore, it was reasonable to suppose that transformations between different crystalline phases might
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J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
also appear at higher temperatures. This circumstance stimulated us to study the atomic layer growth and structure formation of HfO2 thin ®lms at higher temperatures than those used earlier. In the present work, we demonstrate that although the monoclinic phase is the dominating one in the ®lms, application of different precursor doses allows variation of the surface layer structure at the substrate temperatures of 880±9408C. 2. Experimental In the present work, the HfO2 thin ®lms were grown at substrate temperatures, TG, ranging from 500 to 9408C. The ®lms were deposited on single crystal (1 1 1)-oriented silicon substrates using HfCl4 and H2O as the precursors and N2 as the carrier gas. A low-pressure (250 Pa) hot-wall ALD reactor [6] was applied in the experiments. The set-up used in the previous studies [6] allowed thin ®lm growth at temperatures up to 6008C. Therefore an additional infrared heater was used in the present work to further increase the temperature of a graphite susceptor. As a result of such upgrade, substrate temperatures as high as 10008C were obtained in the reactor whereas the temperature stability was about 108C. In order to grow the ®lms, the substrates were alternately exposed to HfCl4 and H2O. Between the precursor pulses the reaction zone was purged with pure N2. This avoided overlap of precursor pulses and possible gas phase reactions. Thus, the deposition process consisted of cycles that included a reaction of HfCl4 with the surface, purge, reaction of H2O with the surface and another purge. The H2O pulse time and both purge times equaled 2 s. The HfCl4 pulse time was varied from 1 to 2 s, while the HfCl4 source temperature (TS) and ¯ow rate of H2O vapor ranged from 130 to 1548C and from 0.5 to 1 mPa m3/s, respectively. The number of cycles used in a growth run were varied from 1000 to 3000. The thickness of the ®lms was determined from ellipsometry measurements. Re¯ection high-energy electron diffraction (RHEED), XRD and grazing incidence X-ray diffraction (GIXRD) methods were applied for the structure studies. The electron energy of 75 keV was used in RHEED studies. In XRD and GIXRD measurements the Cu Ka radiation was
applied. The composition of ®lms was studied by Auger electron spectroscopy (AES). The energy of primary electron beam applied in the AES measurements equaled 3 keV. Argon-ion bombardment was used to clean the surfaces before AES measurements. 3. Results and discussion The RHEED and XRD measurements showed that the ®lms grown at substrate temperatures of 500± 6008C were polycrystalline and consisted mainly of monoclinic HfO2. Although the ®lms contained also some traces of another phase (see arrows in Fig. 1), the amount of that was not remarkable. A weak diffraction peak corresponding to interplanar distance d 0:2949 nm 2y 30:28 could be attributed to the orthorhombic [13], tetragonal [14] or cubic [15] phases, while the other peak at 0.1562 nm 2y 59:28 could belong to orthorhombic phase. The lattice parameters of the monoclinic phase, re®ned from the XRD data, were a 0:513, b 0:516, c 0:529 nm and b 99:3 . Both XRD and RHEED measurements revealed a well-developed texture in these ®lms. The lattice plane (0 0 1) of the crystallites with the monoclinic structure was preferentially par-
Fig. 1. XRD patterns of ®lms grown at 500 (upper curve) and 9408C (lower curve). The ®lms were grown at HfCl4 source temperature of 1508C and H2O ¯ow rate of 0.7 mPa m3/s. The precursor pulses were 2 s long. The number of ALD cycles was 1800. Arrows indicate re¯ections that do not belong to monoclinic HfO2.
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
allel to the substrate surface. As a result, the re¯ection ÿ1 1 1, which had to be the strongest one according to powder diffraction data [16], was weak in the XRD patterns of the ®lms grown at 5008C (Fig. 1). The apparent sizes of crystallites determined from XRD data depended on the orientation and ranged from 30 to 50 nm in the ®lms of 375 nm thickness. Considerable changes in the thin ®lm structure were observed by RHEED when the growth temperature was raised above 6008C. The background intensity increased indicating an increase in the amount of amorphous phase. Moreover, the texture started to depend on the precursor doses. In the ®lms grown at the HfCl4 source temperature of 1508C and H2O ¯ow of 0.7 mPa m3/s, the texture became signi®cantly weaker with the increase of TG up to 6908C. Furthermore, the preferential orientation recorded by RHEED disappeared when the growth temperature reached 7808C. It was revealed, however, that raising the H2O ¯ow up to 1 mPa m3/s allowed growth of textured ®lms at these substrate temperatures, as well. Even more remarkable changes in the RHEED patterns appeared at the growth temperature ranging from 880 to 9408C. The ®lms grown at HfCl4 source temperature of 150±1528C and at H2O ¯ow of 0.5± 0.7 mPa m3/s showed no preferential orientation of crystallites (Fig. 2(a)). Moreover, the most intense RHEED re¯ections of these ®lms (Table 1) did not belong to the monoclinic phase any more. The peaks, except one at d 0:294 nm, were not attributable to the orthorhombic and tetragonal phases, either. Instead, most of the measured re¯ections coincided with those calculated for the cubic phase [15]. The lattice parameter re®ned from the RHEED data equaled 0:513 0:003 nm (Table 1) and in the range 0:15 d 0:30 nm, all calculated interplanar distances, except the one at d 0:1622 nm, coincided with those recorded by RHEED. In the RHEED patterns, there was only one re¯ection at d 0:1838, which did not correspond to any d-value of the cubic phase. All re¯ections observed were broad (Fig. 2(a)) indicating small sizes of crystallites. According to rough estimations the sizes of crystallites did not exceed 6±9 nm. The cubic phase was observed in the ®lms grown at suf®ciently low H2O doses and high HfCl4 source temperatures (Table 2), only. RHEED re¯ections of the monoclinic phase became detectable when the
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Fig. 2. RHEED patterns of ®lms grown at TG 940 C and HfCl4 source temperatures of (a) 1508C and (b) 1308C. The patterns show (a) cubic structure without preferential orientation and (b) monoclinic structure with preferential orientation.
H2O ¯ow was raised from 0.5±0.7 to 1 mPa m3/s. These re¯ections were stronger at the trailing edges of substrates. Conversely, the intensity of re¯ections corresponding to the cubic structure was higher at the leading edges, i.e. close to the precursor inlets. Although the RHEED patterns depended on the precursor doses, the growth rate was invariant when the H2O ¯ow was varied from 0.5 to 1.0 mPa m3/s. Reduction of the HfCl4 source temperature from 152 to 1288C in¯uenced the thin ®lm structure observed by RHEED as well as the growth rate (Table 2). In the ®lms grown at TS 130 C, only the re¯ections of the monoclinic structure (Table 1) were observed and the RHEED patterns of these ®lms (Fig. 2(b)) showed preferential orientation, which was similar to that obtained at TG 600 C. In addition, a considerable decrease of the growth rate with decreasing TS appeared at TS 143 C. Contrary to RHEED, XRD revealed that monoclinic HfO2 was the dominating crystalline phase in all ®lms grown at TG 940 C, i.e. also in those grown at H2O ¯ow of 0.5±0.7 mPa m3/s and TS 150 C (Table 1). A re¯ection, which could belong to the cubic phase, appeared in the ®lms of 75 nm thickness at 2y 30:6 d 0:295 nm (Fig. 3) but it was very
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Table 1 Comparison of interplanar distances determined by XRD and RHEED in HfO2 ®lms grown at substrate temperature of 9408C with those calculated for monoclinic (a 0:511, b 0:516, c 0:529 nm and b 99:2 ) and cubic a 0:513 nm phases of HfO2 RHEED data (nm)
XRD data (nm)
Calculation (nm)
TS 150 C
TS 130 C
TS 150 C
TS 130 C
Cubic phase
Monoclinic phase
± 0:294 0:003 ± ± 0:257 0:002 ± 0:2301 0:0018 ± ± 0:2084 0:0016 ± ± 0:1838 0:0016 0:1815 0:0014 0:1706 0:0012 ± ± ± 0:1543 0:0008 ± ±
0:313 0:002 ± 0:281 0:002 0:2595 0:0016 ± 0:2480 0:0015 ± 0:2194 0:0012 ± ± 0:2003 0:0010 0:1984 0:0010 0:1844 0:0009 0:1811 0:0010 ± 0:1684 0:0008 0:1653 0:0008 0:1603 0:0007 ± 0:1527 0:0006 0:1502 0:0006
0.3143 ± 0.2825 0.2613 0.2580 0.2486 0.2314 0.2200 0.2182 ± 0.2010 0.1977 0.1836 0.1806 ± 0.1685 0.1649 0.1599 ± 0.1530 0.1502
0.3144 0.2953 0.2824 0.2610 ± 0.2486 0.2314 0.2193 0.2182 ± 0.2006 0.1977 0.1834 0.1805 ± 0.1682 0.1646 0.1598 ± 0.1530 0.1499
± 0.2961 ± ± 0.2564 ± 0.2294 ± ± 0.2094 ± ± ± 0.1813 0.1713 ± ± 0.1622 0.1546 ± ±
0.3147 ± 0.2823 0.2613 0.2581 0.2492 0.2314 0.2204 0.2182 ± 0.2010 0.1980 0.1836 0.1807 ± 0.1686 0.1649 0.1603 ± 0.1533 0.1502
weak indicating insigni®cant amount of this phase. Moreover, this re¯ection was not observed in thicker ®lms (Fig. 3) that according to RHEED data also contained the cubic phase. The XRD data showed somewhat weaker preferential orientation in the ®lms grown at TG 940 C than in those deposited at 5008C (Fig. 1). Nevertheless, the relative intensity of the ÿ1 1 1 re¯ection was still more than two times lower compared with the value given in the powder diffrac-
Table 2 In¯uence of HfCl4 source temperature on structure recorded by RHEED and on growth rate of thin ®lms grown at substrate temperature of 9408C and H2O ¯ow of 0.7 mPa m3/s TS (8C)
Structure
Growth rate (nm/cycle)
128 134 143 150 152
Monoclinic Cubic monoclinic Cubic monoclinic Cubic Cubic
0.034 0.041 0.060 0.065 0.067
Fig. 3. XRD patterns of ®lms with thickness of t 75, 125 and 220 nm grown at 9408C. HfCl4 source temperature of 1508C and H2O ¯ow rate of 0.7 mPa m3/s were used in growth process. Arrow indicates the re¯ection that does not belong to monoclinic HfO2.
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
tion database [16]. The lattice parameters of the monoclinic phase re®ned from the XRD data were a 0:511, b 0:516, c 0:529 nm and b 99:2 . The apparent sizes of crystallites preferentially oriented in the directions [0 0 1] and [1 1 1] reached 70 nm in the ®lms with the thickness of 75 nm grown at TG 940 C. The data presented above show that in case of ®lms grown at TG 940 C and low HfCl4 doses, the results of XRD and RHEED measurements are consistent with each other. Both con®rm formation of the monoclinic phase. The dependence of the growth rate on the HfCl4 source temperature (Table 2) demonstrates, however, that no self-limited growth was obtained at these HfCl4 doses. At higher HfCl4 doses, at which the growth rate was independent of TS (Table 2), an obvious disagreement between XRD and RHEED results appeared. An explanation for this disagreement could be that the cubic phase appeared at the outermost surface of the ®lms while inside the ®lms, monoclinic HfO2 was formed. Indeed, RHEED is known as a surface-sensitive method. Using ultrathin HfO2 ®lms deposited on silicon, we revealed that the RHEED re¯ections of substrate material disappeared when the ®lm thickness reached 5±10 nm. Therefore no RHEED re¯ections from the monoclinic phase could be expected, if there was a layer of cubic HfO2 of similar thickness on the top of the ®lm. According to our estimations the XRD and GIXRD measurements allowed reliable detection of re¯ections from the ®lms, which were thicker than about 20±30 and 10 nm, respectively, provided that the sizes of crystallites were comparable to the ®lm thickness. In case of smaller crystallites the sensitivity was presumably lower. Consequently, it is possible that the layer of cubic HfO2 was too thin and/or poorly ordered to be recorded by XRD and GIXRD. An alternative explanation for the disagreement between XRD and RHEED data is that the cubic phase was formed under electron beam (EB) excitation during the RHEED measurements. The idea of EB-induced phase transformation is supported by the fact that a similar process has been earlier observed in the experiments where TiO2 of rutile phase was converted into TiO2-II phase [17]. Our data still show that even if formed under EB excitation, cubic HfO2 could be obtained in the ®lms grown at certain process parameters, only. Evidently the cubic phase was not
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formed from the monoclinic phase because in the ®lms grown at TG 940 C and TS 130 C or at TG 780 C, RHEED detected the monoclinic phase only, and no phase transformations were observed during the RHEED studies. Therefore at properly chosen process parameters, a surface layer, which contained cubic phase or could be transformed into this phase, appeared on the outermost surface of the ®lms. XRD measurements demonstrated that the thickness of this layer could not increase signi®cantly with the increase of the total thin ®lm thickness. Indeed, the intensity of XRD re¯ections corresponding to the monoclinic phase monotonously increased with the increase of ®lm thickness from 75 to 220 nm (Fig. 3), although according to the RHEED data, all these ®lms were covered with a layer of the cubic structure. In order to reveal the possible contribution of the material composition to the structure changes observed by RHEED, AES measurements were performed on the ®lms grown under different conditions. These studies demonstrated that the shapes of Auger spectra correlated with the ®lm structure recorded by RHEED. The intensity ratio of hafnium NOO and oxygen KL1L2,3 peaks, IHf /IO, was 1:62 0:09 in case of monoclinic phase and 1:66 0:09 in case of cubic phase. The difference in the IHf /IO ratio was smaller than the experimental error but it was still two times larger than the difference observed for the ®lms of monoclinic structure grown at 500 and 9408C. A translation of Auger spectra was also observed with the change of surface structure (Fig. 4). However, no unambiguous conclusions could be made from this fact because the shift was rather small and besides other effects it could be due to different charging of the surfaces of the ®lms under EB excitation. A more interesting result is that the I1/I2 ratio, where I1 is the intensity of the oxygen KL1L2,3 peak and I2 is the intensity of the oxygen KL2,3L2,3 peak (Fig. 4), depended on the surface layer structure recorded by RHEED. The I1/I2 ratio equaled 0:235 0:002 and 0:228 0:002 in the ®lms with the monoclinic and cubic surface layer structure, respectively. Although the difference was about 3% only, it clearly exceeded the experimental uncertainty. Moreover, the I1/I2 ratio was independent of whether the ®lms showing the RHEED patterns of the mono-
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Fig. 4. Auger electron spectra showing oxygen KL1L2,3 and KL2,3L2,3 lines of HfO2 ®lms with monoclinic and cubic surface layer structure grown at TS 130 and 1508C, respectively. TG 940 C. The structure indicated in inset was determined by RHEED. According to XRD both ®lms contained monoclinic phase.
clinic structure were grown at 500 or 9408C. Comparison of the I1/I2 ratios enabled us to conclude that the localization probability of valence electrons at oxygen atoms, i.e. the ionicity of O±Hf bonds was higher in the ®lms where the cubic phase was formed than in the ®lms of monoclinic structure. In the former ®lms, however, the I1/I2 ratio changed when the surface layer was removed in the ion bombardment process. After seven cycles of bombardment the ratio reached 0:234 0:002, and became comparable to the value measured for the ®lms with the monoclinic surface layer structure. Very rough estimations showed that the etching depth of about 1 nm corresponded to each bombardment cycle. Consequently, the surface layer of different properties was not thicker than about 10 nm. This result well agreed with the conclusions based on the comparison of the RHEED and XRD data. Thus, the cubic phase observed by RHEED was evidently formed in a thin surface layer, in which the
crystallites were small and the O/Hf ratio was somewhat lower than that corresponding to monoclinic HfO2. This result and the fact that the cubic phase appeared in the ®lms grown at high hafnium precursor doses and low oxygen precursor doses correlate with the data of El-Shanshoury et al. [15]. These authors have observed cubic HfO2 by transmission electron diffraction in thin ®lms grown by oxidation of hafnium at 500±6008C. They have found that small crystallite sizes stabilize the cubic phase while higher concentration of oxygen at the surface accelerates the structure transformation to the monoclinic phase. Similarly, it has been shown that oxygen de®ciency and small crystallite sizes stabilize metastable tetragonal phase of ZrO2 [18,19]. Thus, it seems that such conditions are needed to obtain metastable phases of high density in both ZrO2 and HfO2. Instability of cubic HfO2 in larger crystallites could explain why the amount of this phase did not increase with the ®lm thickness, although it appeared on the surface of ®lms with very different thickness. Observation of the cubic phase with random orientation of crystallites on the surface of preferentially oriented monoclinic HfO2 indicates, in turn, that at TG 880ÿ940 C and high HfCl4 doses TS 150ÿ152 C, the ALD-type reactions do not enable direct growth of monoclinic HfO2. Under these conditions, the monoclinic phase is probably obtained in the phase transformation process, which starts when the surface intermediate layer of cubic and/or disordered structure with suf®cient thickness is formed. At low HfCl4 doses TS 130 C and suf®ciently high H2O doses, by contrast, surface reactions evidently result in the immediate growth of monoclinic HfO2. The latter conclusion is based on the fact that in this case both RHEED and XRD revealed existence of preferentially oriented monoclinic HfO2. It is possible that the crystallites of monoclinic structure, once formed, grow epitaxially in the further deposition process. This is because in the ®lms grown at TG 940 C, the crystallite sizes determined by XRD are rather close to the ®lm thickness. 4. Conclusions We have shown in this report that the surface structure of HfO2 thin ®lms grown by ALD from
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
HfCl4 and H2O can be varied when changing the growth temperature and precursor doses. Besides the monoclinic phase, the metastable cubic phase can be obtained in the surface layer of thin ®lms grown at substrate temperatures 880±9408C. The results of both AES and structure studies con®rm that the structure changes appear in the surface layer, which is thinner than about 10 nm. The possibility to vary the surface structure may be important in technical applications because the phase composition probably in¯uences the chemical activity and adsorption capability of the ®lm surface. The changes in the O±Hf bond ionicity observed in the AES measurements well support this hypothesis. Acknowledgements The authors like to thank A.-A. Kiisler and P. Ritslaid for technical assistance. The work was supported by Estonian Science Foundation (Research Grants No. 1878, 3999 and 4205). References [1] J. Wang, H.P. Li, R. Stevens, J. Mater. Sci. 27 (1992) 5397. [2] J.M. LeÂger, J. Haines, B. Blanzat, J. Mater. Sci. Lett. 13 (1994) 1688.
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[3] J.M. LeÂger, A. Atouf, P.E. Tomaszewski, A.S. Pereira, Phys. Rev. B 48 (1993) 93. [4] M. Ritala, M. LeskelaÈ, L. NiinistoÈ, T. Prohaska, G. Friedbacher, M. Grasserbauer, Thin Solid Films 250 (1994) 72. [5] K. Kukli, J. Ihanus, M. Ritala, M. LeskelaÈ, Appl. Phys. Lett. 68 (1996) 3737. [6] J. Aarik, A. Aidla, A.-A. Kiisler, T. Uustare, V. Sammelselg, Thin Solid Films 340 (1999) 110. È sterholm, J.-P. [7] A. KytoÈkivi, E.-L. Lakomaa, A. Root, H. O Jacobs, H.H. Brongersma, Langmuir 13 (1997) 2717. [8] K. Kukli, J. Ihanus, M. Ritala, M. LeskelaÈ, J. Electrochem. Soc. 144 (1997) 300. [9] K. Kukli, M. Ritala, M. LeskelaÈ, Nanostruc. Mater. 8 (1997) 785. [10] J. Aarik, A. Aidla, T. Uustare, Phil. Mag. Lett. 73 (1996) 115. [11] J. Aarik, A. Aidla, V. Sammelselg, T. Uustare, J. Cryst. Growth 181 (1997) 259. [12] J. Aarik, A. Aidla, T. Uustare, V. Sammelselg, J. Cryst. Growth 148 (1995) 268. [13] Joint Committee on Powder Diffraction Standards, Card 210904. [14] Joint Committee on Powder Diffraction Standards, File 8342. [15] A. El-Shanshoury, V.A Rudenko, I.A. Ibrahim, J. Am. Ceram. Soc. 53 (1969) 264. [16] Joint Committee on Powder Diffraction Standards, File 34104. [17] M.R. McCartney, D.J. Smith, Surf. Sci. 250 (1991) 169. [18] M. Bellotto, A. Caridi, E. Cereda, G. Gabetta, M. Scagliotti, Appl. Phys. Lett. 63 (1993) 2056. [19] R. Srinivasan, T.R. Watkins, C.R. Hubbard, B.H. Davis, Chem. Mater. 7 (1995) 725.
Phase transformations in hafnium dioxide thin ®lms grown by atomic layer deposition at high temperatures Jaan Aarika,*, Aleks Aidlaa, Hugo MaÈndara, Teet Uustarea, Kaupo Kuklib, Mikael Schuiskyc a Institute of Materials Science, University of Tartu, TaÈhe 4, 51010 Tartu, Estonia Institute of Experimental Physics and Technology, University of Tartu, TaÈhe 4, 51010 Tartu, Estonia c Department of Inorganic Chemistry, The AÊngstroÈm Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden b
Received 3 March 2000; accepted 12 October 2000
Abstract High-temperature cubic phase of HfO2 was observed by re¯ection high-energy electron diffraction in nanocrystalline thin ®lms grown by atomic layer deposition from HfCl4 and H2O at substrate temperatures of 880±9408C. The phase was formed at properly chosen precursor doses and it was observed on the surface of ®lms, which according to X-ray diffraction data consisted of monoclinic HfO2. The thickness of the surface layer, in which the cubic phase appeared, was estimated to be 5±10 nm. According to Auger electron spectroscopy data, formation of the cubic phase was accompanied with an increase in the ionicity of O±Hf bonds. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.50K; 68.55 Keywords: Hafnium dioxide; Atomic layer deposition; Structure; Auger electron spectroscopy
1. Introduction Hafnium dioxide (HfO2) is a material forming several polymorphs. Some of those observed at high temperatures and/or high pressures, mainly, are of interest because of high density and hardness [1,2]. Although pure HfO2 tends to appear in the monoclinic phase at room temperature and atmospheric pressure, orthorhombic and tetragonal phases formed at high pressures can be partially quenched to atmospheric pressure [2,3]. Small amounts of metastable phases *
Corresponding author. Tel.: 372-7-375877; fax: 372-7-375540. E-mail address: [email protected] (J. Aarik).
have also been found in thin HfO2 ®lms and nanolaminates grown by atomic layer deposition (ALD) at low pressures and relatively low temperatures [4±6]. Nevertheless, contrary to ZrO2 [7±9] and TiO2 [10,11] thin ®lms, where metastable phases have been dominating, only some weak X-ray diffraction (XRD) lines of tetragonal [4,5] or orthorhombic [4,6] phases have been recorded in HfO2 ®lms grown at substrate temperatures 300±6008C [4±6]. Existing experimental data, however, indicate that the transition from the amorphous to the crystalline growth appears in the ALD process of HfO2 at signi®cantly higher temperature [6] than in the ZrO2 [9] and TiO2 [12] processes. Therefore, it was reasonable to suppose that transformations between different crystalline phases might
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 5 9 - X
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J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
also appear at higher temperatures. This circumstance stimulated us to study the atomic layer growth and structure formation of HfO2 thin ®lms at higher temperatures than those used earlier. In the present work, we demonstrate that although the monoclinic phase is the dominating one in the ®lms, application of different precursor doses allows variation of the surface layer structure at the substrate temperatures of 880±9408C. 2. Experimental In the present work, the HfO2 thin ®lms were grown at substrate temperatures, TG, ranging from 500 to 9408C. The ®lms were deposited on single crystal (1 1 1)-oriented silicon substrates using HfCl4 and H2O as the precursors and N2 as the carrier gas. A low-pressure (250 Pa) hot-wall ALD reactor [6] was applied in the experiments. The set-up used in the previous studies [6] allowed thin ®lm growth at temperatures up to 6008C. Therefore an additional infrared heater was used in the present work to further increase the temperature of a graphite susceptor. As a result of such upgrade, substrate temperatures as high as 10008C were obtained in the reactor whereas the temperature stability was about 108C. In order to grow the ®lms, the substrates were alternately exposed to HfCl4 and H2O. Between the precursor pulses the reaction zone was purged with pure N2. This avoided overlap of precursor pulses and possible gas phase reactions. Thus, the deposition process consisted of cycles that included a reaction of HfCl4 with the surface, purge, reaction of H2O with the surface and another purge. The H2O pulse time and both purge times equaled 2 s. The HfCl4 pulse time was varied from 1 to 2 s, while the HfCl4 source temperature (TS) and ¯ow rate of H2O vapor ranged from 130 to 1548C and from 0.5 to 1 mPa m3/s, respectively. The number of cycles used in a growth run were varied from 1000 to 3000. The thickness of the ®lms was determined from ellipsometry measurements. Re¯ection high-energy electron diffraction (RHEED), XRD and grazing incidence X-ray diffraction (GIXRD) methods were applied for the structure studies. The electron energy of 75 keV was used in RHEED studies. In XRD and GIXRD measurements the Cu Ka radiation was
applied. The composition of ®lms was studied by Auger electron spectroscopy (AES). The energy of primary electron beam applied in the AES measurements equaled 3 keV. Argon-ion bombardment was used to clean the surfaces before AES measurements. 3. Results and discussion The RHEED and XRD measurements showed that the ®lms grown at substrate temperatures of 500± 6008C were polycrystalline and consisted mainly of monoclinic HfO2. Although the ®lms contained also some traces of another phase (see arrows in Fig. 1), the amount of that was not remarkable. A weak diffraction peak corresponding to interplanar distance d 0:2949 nm 2y 30:28 could be attributed to the orthorhombic [13], tetragonal [14] or cubic [15] phases, while the other peak at 0.1562 nm 2y 59:28 could belong to orthorhombic phase. The lattice parameters of the monoclinic phase, re®ned from the XRD data, were a 0:513, b 0:516, c 0:529 nm and b 99:3 . Both XRD and RHEED measurements revealed a well-developed texture in these ®lms. The lattice plane (0 0 1) of the crystallites with the monoclinic structure was preferentially par-
Fig. 1. XRD patterns of ®lms grown at 500 (upper curve) and 9408C (lower curve). The ®lms were grown at HfCl4 source temperature of 1508C and H2O ¯ow rate of 0.7 mPa m3/s. The precursor pulses were 2 s long. The number of ALD cycles was 1800. Arrows indicate re¯ections that do not belong to monoclinic HfO2.
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
allel to the substrate surface. As a result, the re¯ection ÿ1 1 1, which had to be the strongest one according to powder diffraction data [16], was weak in the XRD patterns of the ®lms grown at 5008C (Fig. 1). The apparent sizes of crystallites determined from XRD data depended on the orientation and ranged from 30 to 50 nm in the ®lms of 375 nm thickness. Considerable changes in the thin ®lm structure were observed by RHEED when the growth temperature was raised above 6008C. The background intensity increased indicating an increase in the amount of amorphous phase. Moreover, the texture started to depend on the precursor doses. In the ®lms grown at the HfCl4 source temperature of 1508C and H2O ¯ow of 0.7 mPa m3/s, the texture became signi®cantly weaker with the increase of TG up to 6908C. Furthermore, the preferential orientation recorded by RHEED disappeared when the growth temperature reached 7808C. It was revealed, however, that raising the H2O ¯ow up to 1 mPa m3/s allowed growth of textured ®lms at these substrate temperatures, as well. Even more remarkable changes in the RHEED patterns appeared at the growth temperature ranging from 880 to 9408C. The ®lms grown at HfCl4 source temperature of 150±1528C and at H2O ¯ow of 0.5± 0.7 mPa m3/s showed no preferential orientation of crystallites (Fig. 2(a)). Moreover, the most intense RHEED re¯ections of these ®lms (Table 1) did not belong to the monoclinic phase any more. The peaks, except one at d 0:294 nm, were not attributable to the orthorhombic and tetragonal phases, either. Instead, most of the measured re¯ections coincided with those calculated for the cubic phase [15]. The lattice parameter re®ned from the RHEED data equaled 0:513 0:003 nm (Table 1) and in the range 0:15 d 0:30 nm, all calculated interplanar distances, except the one at d 0:1622 nm, coincided with those recorded by RHEED. In the RHEED patterns, there was only one re¯ection at d 0:1838, which did not correspond to any d-value of the cubic phase. All re¯ections observed were broad (Fig. 2(a)) indicating small sizes of crystallites. According to rough estimations the sizes of crystallites did not exceed 6±9 nm. The cubic phase was observed in the ®lms grown at suf®ciently low H2O doses and high HfCl4 source temperatures (Table 2), only. RHEED re¯ections of the monoclinic phase became detectable when the
17
Fig. 2. RHEED patterns of ®lms grown at TG 940 C and HfCl4 source temperatures of (a) 1508C and (b) 1308C. The patterns show (a) cubic structure without preferential orientation and (b) monoclinic structure with preferential orientation.
H2O ¯ow was raised from 0.5±0.7 to 1 mPa m3/s. These re¯ections were stronger at the trailing edges of substrates. Conversely, the intensity of re¯ections corresponding to the cubic structure was higher at the leading edges, i.e. close to the precursor inlets. Although the RHEED patterns depended on the precursor doses, the growth rate was invariant when the H2O ¯ow was varied from 0.5 to 1.0 mPa m3/s. Reduction of the HfCl4 source temperature from 152 to 1288C in¯uenced the thin ®lm structure observed by RHEED as well as the growth rate (Table 2). In the ®lms grown at TS 130 C, only the re¯ections of the monoclinic structure (Table 1) were observed and the RHEED patterns of these ®lms (Fig. 2(b)) showed preferential orientation, which was similar to that obtained at TG 600 C. In addition, a considerable decrease of the growth rate with decreasing TS appeared at TS 143 C. Contrary to RHEED, XRD revealed that monoclinic HfO2 was the dominating crystalline phase in all ®lms grown at TG 940 C, i.e. also in those grown at H2O ¯ow of 0.5±0.7 mPa m3/s and TS 150 C (Table 1). A re¯ection, which could belong to the cubic phase, appeared in the ®lms of 75 nm thickness at 2y 30:6 d 0:295 nm (Fig. 3) but it was very
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J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
Table 1 Comparison of interplanar distances determined by XRD and RHEED in HfO2 ®lms grown at substrate temperature of 9408C with those calculated for monoclinic (a 0:511, b 0:516, c 0:529 nm and b 99:2 ) and cubic a 0:513 nm phases of HfO2 RHEED data (nm)
XRD data (nm)
Calculation (nm)
TS 150 C
TS 130 C
TS 150 C
TS 130 C
Cubic phase
Monoclinic phase
± 0:294 0:003 ± ± 0:257 0:002 ± 0:2301 0:0018 ± ± 0:2084 0:0016 ± ± 0:1838 0:0016 0:1815 0:0014 0:1706 0:0012 ± ± ± 0:1543 0:0008 ± ±
0:313 0:002 ± 0:281 0:002 0:2595 0:0016 ± 0:2480 0:0015 ± 0:2194 0:0012 ± ± 0:2003 0:0010 0:1984 0:0010 0:1844 0:0009 0:1811 0:0010 ± 0:1684 0:0008 0:1653 0:0008 0:1603 0:0007 ± 0:1527 0:0006 0:1502 0:0006
0.3143 ± 0.2825 0.2613 0.2580 0.2486 0.2314 0.2200 0.2182 ± 0.2010 0.1977 0.1836 0.1806 ± 0.1685 0.1649 0.1599 ± 0.1530 0.1502
0.3144 0.2953 0.2824 0.2610 ± 0.2486 0.2314 0.2193 0.2182 ± 0.2006 0.1977 0.1834 0.1805 ± 0.1682 0.1646 0.1598 ± 0.1530 0.1499
± 0.2961 ± ± 0.2564 ± 0.2294 ± ± 0.2094 ± ± ± 0.1813 0.1713 ± ± 0.1622 0.1546 ± ±
0.3147 ± 0.2823 0.2613 0.2581 0.2492 0.2314 0.2204 0.2182 ± 0.2010 0.1980 0.1836 0.1807 ± 0.1686 0.1649 0.1603 ± 0.1533 0.1502
weak indicating insigni®cant amount of this phase. Moreover, this re¯ection was not observed in thicker ®lms (Fig. 3) that according to RHEED data also contained the cubic phase. The XRD data showed somewhat weaker preferential orientation in the ®lms grown at TG 940 C than in those deposited at 5008C (Fig. 1). Nevertheless, the relative intensity of the ÿ1 1 1 re¯ection was still more than two times lower compared with the value given in the powder diffrac-
Table 2 In¯uence of HfCl4 source temperature on structure recorded by RHEED and on growth rate of thin ®lms grown at substrate temperature of 9408C and H2O ¯ow of 0.7 mPa m3/s TS (8C)
Structure
Growth rate (nm/cycle)
128 134 143 150 152
Monoclinic Cubic monoclinic Cubic monoclinic Cubic Cubic
0.034 0.041 0.060 0.065 0.067
Fig. 3. XRD patterns of ®lms with thickness of t 75, 125 and 220 nm grown at 9408C. HfCl4 source temperature of 1508C and H2O ¯ow rate of 0.7 mPa m3/s were used in growth process. Arrow indicates the re¯ection that does not belong to monoclinic HfO2.
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
tion database [16]. The lattice parameters of the monoclinic phase re®ned from the XRD data were a 0:511, b 0:516, c 0:529 nm and b 99:2 . The apparent sizes of crystallites preferentially oriented in the directions [0 0 1] and [1 1 1] reached 70 nm in the ®lms with the thickness of 75 nm grown at TG 940 C. The data presented above show that in case of ®lms grown at TG 940 C and low HfCl4 doses, the results of XRD and RHEED measurements are consistent with each other. Both con®rm formation of the monoclinic phase. The dependence of the growth rate on the HfCl4 source temperature (Table 2) demonstrates, however, that no self-limited growth was obtained at these HfCl4 doses. At higher HfCl4 doses, at which the growth rate was independent of TS (Table 2), an obvious disagreement between XRD and RHEED results appeared. An explanation for this disagreement could be that the cubic phase appeared at the outermost surface of the ®lms while inside the ®lms, monoclinic HfO2 was formed. Indeed, RHEED is known as a surface-sensitive method. Using ultrathin HfO2 ®lms deposited on silicon, we revealed that the RHEED re¯ections of substrate material disappeared when the ®lm thickness reached 5±10 nm. Therefore no RHEED re¯ections from the monoclinic phase could be expected, if there was a layer of cubic HfO2 of similar thickness on the top of the ®lm. According to our estimations the XRD and GIXRD measurements allowed reliable detection of re¯ections from the ®lms, which were thicker than about 20±30 and 10 nm, respectively, provided that the sizes of crystallites were comparable to the ®lm thickness. In case of smaller crystallites the sensitivity was presumably lower. Consequently, it is possible that the layer of cubic HfO2 was too thin and/or poorly ordered to be recorded by XRD and GIXRD. An alternative explanation for the disagreement between XRD and RHEED data is that the cubic phase was formed under electron beam (EB) excitation during the RHEED measurements. The idea of EB-induced phase transformation is supported by the fact that a similar process has been earlier observed in the experiments where TiO2 of rutile phase was converted into TiO2-II phase [17]. Our data still show that even if formed under EB excitation, cubic HfO2 could be obtained in the ®lms grown at certain process parameters, only. Evidently the cubic phase was not
19
formed from the monoclinic phase because in the ®lms grown at TG 940 C and TS 130 C or at TG 780 C, RHEED detected the monoclinic phase only, and no phase transformations were observed during the RHEED studies. Therefore at properly chosen process parameters, a surface layer, which contained cubic phase or could be transformed into this phase, appeared on the outermost surface of the ®lms. XRD measurements demonstrated that the thickness of this layer could not increase signi®cantly with the increase of the total thin ®lm thickness. Indeed, the intensity of XRD re¯ections corresponding to the monoclinic phase monotonously increased with the increase of ®lm thickness from 75 to 220 nm (Fig. 3), although according to the RHEED data, all these ®lms were covered with a layer of the cubic structure. In order to reveal the possible contribution of the material composition to the structure changes observed by RHEED, AES measurements were performed on the ®lms grown under different conditions. These studies demonstrated that the shapes of Auger spectra correlated with the ®lm structure recorded by RHEED. The intensity ratio of hafnium NOO and oxygen KL1L2,3 peaks, IHf /IO, was 1:62 0:09 in case of monoclinic phase and 1:66 0:09 in case of cubic phase. The difference in the IHf /IO ratio was smaller than the experimental error but it was still two times larger than the difference observed for the ®lms of monoclinic structure grown at 500 and 9408C. A translation of Auger spectra was also observed with the change of surface structure (Fig. 4). However, no unambiguous conclusions could be made from this fact because the shift was rather small and besides other effects it could be due to different charging of the surfaces of the ®lms under EB excitation. A more interesting result is that the I1/I2 ratio, where I1 is the intensity of the oxygen KL1L2,3 peak and I2 is the intensity of the oxygen KL2,3L2,3 peak (Fig. 4), depended on the surface layer structure recorded by RHEED. The I1/I2 ratio equaled 0:235 0:002 and 0:228 0:002 in the ®lms with the monoclinic and cubic surface layer structure, respectively. Although the difference was about 3% only, it clearly exceeded the experimental uncertainty. Moreover, the I1/I2 ratio was independent of whether the ®lms showing the RHEED patterns of the mono-
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J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
Fig. 4. Auger electron spectra showing oxygen KL1L2,3 and KL2,3L2,3 lines of HfO2 ®lms with monoclinic and cubic surface layer structure grown at TS 130 and 1508C, respectively. TG 940 C. The structure indicated in inset was determined by RHEED. According to XRD both ®lms contained monoclinic phase.
clinic structure were grown at 500 or 9408C. Comparison of the I1/I2 ratios enabled us to conclude that the localization probability of valence electrons at oxygen atoms, i.e. the ionicity of O±Hf bonds was higher in the ®lms where the cubic phase was formed than in the ®lms of monoclinic structure. In the former ®lms, however, the I1/I2 ratio changed when the surface layer was removed in the ion bombardment process. After seven cycles of bombardment the ratio reached 0:234 0:002, and became comparable to the value measured for the ®lms with the monoclinic surface layer structure. Very rough estimations showed that the etching depth of about 1 nm corresponded to each bombardment cycle. Consequently, the surface layer of different properties was not thicker than about 10 nm. This result well agreed with the conclusions based on the comparison of the RHEED and XRD data. Thus, the cubic phase observed by RHEED was evidently formed in a thin surface layer, in which the
crystallites were small and the O/Hf ratio was somewhat lower than that corresponding to monoclinic HfO2. This result and the fact that the cubic phase appeared in the ®lms grown at high hafnium precursor doses and low oxygen precursor doses correlate with the data of El-Shanshoury et al. [15]. These authors have observed cubic HfO2 by transmission electron diffraction in thin ®lms grown by oxidation of hafnium at 500±6008C. They have found that small crystallite sizes stabilize the cubic phase while higher concentration of oxygen at the surface accelerates the structure transformation to the monoclinic phase. Similarly, it has been shown that oxygen de®ciency and small crystallite sizes stabilize metastable tetragonal phase of ZrO2 [18,19]. Thus, it seems that such conditions are needed to obtain metastable phases of high density in both ZrO2 and HfO2. Instability of cubic HfO2 in larger crystallites could explain why the amount of this phase did not increase with the ®lm thickness, although it appeared on the surface of ®lms with very different thickness. Observation of the cubic phase with random orientation of crystallites on the surface of preferentially oriented monoclinic HfO2 indicates, in turn, that at TG 880ÿ940 C and high HfCl4 doses TS 150ÿ152 C, the ALD-type reactions do not enable direct growth of monoclinic HfO2. Under these conditions, the monoclinic phase is probably obtained in the phase transformation process, which starts when the surface intermediate layer of cubic and/or disordered structure with suf®cient thickness is formed. At low HfCl4 doses TS 130 C and suf®ciently high H2O doses, by contrast, surface reactions evidently result in the immediate growth of monoclinic HfO2. The latter conclusion is based on the fact that in this case both RHEED and XRD revealed existence of preferentially oriented monoclinic HfO2. It is possible that the crystallites of monoclinic structure, once formed, grow epitaxially in the further deposition process. This is because in the ®lms grown at TG 940 C, the crystallite sizes determined by XRD are rather close to the ®lm thickness. 4. Conclusions We have shown in this report that the surface structure of HfO2 thin ®lms grown by ALD from
J. Aarik et al. / Applied Surface Science 173 (2001) 15±21
HfCl4 and H2O can be varied when changing the growth temperature and precursor doses. Besides the monoclinic phase, the metastable cubic phase can be obtained in the surface layer of thin ®lms grown at substrate temperatures 880±9408C. The results of both AES and structure studies con®rm that the structure changes appear in the surface layer, which is thinner than about 10 nm. The possibility to vary the surface structure may be important in technical applications because the phase composition probably in¯uences the chemical activity and adsorption capability of the ®lm surface. The changes in the O±Hf bond ionicity observed in the AES measurements well support this hypothesis. Acknowledgements The authors like to thank A.-A. Kiisler and P. Ritslaid for technical assistance. The work was supported by Estonian Science Foundation (Research Grants No. 1878, 3999 and 4205). References [1] J. Wang, H.P. Li, R. Stevens, J. Mater. Sci. 27 (1992) 5397. [2] J.M. LeÂger, J. Haines, B. Blanzat, J. Mater. Sci. Lett. 13 (1994) 1688.
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