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Hafnium oxoneopentoxide as a new MOCVD precursor for hafnium oxide films
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Journal of Crystal Growth 267 (2004) 529–537
Hafnium oxoneopentoxide as a new MOCVD precursor for hafnium oxide films A. Abrutisa,*, L.G. Hubert-Pfalzgrafb, S.V. Paskoa,b, A. Bartasytea, F. Weissc, V. Janickisd a Department of Chemistry, Vilnius University, Naugarduko 24, 01513 Vilnius, Lithuania Universit!e Lyon 1, IRC-UPR5401, 2 avenue A.Einstein, 69626 Villeurbanne C!edex, France c Laboratoire des Mat!eriaux et du G!enie Physique (LMGP), INPG-ENSPG, UMR 5628, BP 46, 38402 St Martin d’Heres, France d Faculty of Chemical Technology, Kaunas Technological University, Radvil’enu) 19, 3028 Kaunas, Lithuania b
Received 20 February 2004; accepted 6 April 2004
Communicated by R. Kern
Abstract The reaction between hafnium tetrachloride and neopentyl alcohol in the presence of ammonia lead to hafnium oxoneopentoxide (1). 1 was characterized by FT-IR, 1H and 13C NMR. Single-crystal X-ray diffraction identified it as a trinuclear oxo species Hf3(m3-O)(m3-ONep)(m-ONep)3(ONep)6. The oxoneopentoxide has been used as precursor for the growth of HfO2 films by pulsed liquid injection MOCVD. The influence of deposition temperature (320–750 C) on film growth rate, roughness and microstructure was studied and compared with the conventional Hf(thd)4 (thd=2,2,6,6tetramethyl-3,5-heptanedionate) precursor. Depending on the deposition temperature, amorphous, polycrystalline or epitaxial films can be obtained on sapphire (R-plane) substrates. The film growth rate was independent of the deposition temperature in the range 350–750 C. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Zx; 68.55.Jk Keywords: A3. Metalorganic chemical vapor deposition; B1. Hafnium; B1. Metalorganic compounds; B1. Oxides
1. Introduction Hafnia thin films display a number of attractive properties and can be used as insulating layers in microelectronics (alternative to SiO2) [1,2], as *Corresponding author. Tel.: +37052331004; fax: +37052330987. E-mail address: [email protected] (A. Abrutis).
protective [3] or optical [4] coatings, tunnel junctions [5], and in gas sensors [4]. These have been prepared by sol–gel [6], ALD [4,7] or MOCVD techniques [3], as well as by physical methods (magnetron sputtering [8], e-beam evaporation [4]). Hafnium alkoxides have been used for sol–gel preparation of piezoelectric (PbHf0.5Ti0.5O3 [9]) or refractory ceramics [10] and of hafnium carbide gel
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.012
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fibers [11]. Besides conventional b-diketonates such as Hf(thd)4 (thd=2,2,6,6-tetramethyl-3,5heptanedionate), alkoxides-Hf(OtBu)4 [1,12,13], Hf(mmp)4 [3], Hf(OtBu)2(mmp)2 [3] (mmp=1methoxy-2-methyl-2-propanolate) have also been used as precursors for MOCVD deposition of HfO2 thin films. Alkoxides are versatile precursors for tailoring properties at a molecular level [14]. Their advantage over b-diketonates as MOCVD precursors is lower film deposition temperature, but they are moisture-sensitive. Their hydrolytic susceptibility can be reduced by the use of sterically hindered alkoxide ligands or of functional alkoxides having additional N- or O-donor sites [3]. The neopentoxide group OCH2CMe3 (ONep) was chosen as a ligand since its bulky tBu moiety might prevent oligomerization and thus increases solubility properties [15]. Moreover, neopentoxide derivatives were reported to be less moisture sensitive and more volatile than isopropoxide analogs [16]. We wish to report herein the synthesis, characterization and application of hafnium oxoneopentoxide Hf3O(ONep)10. It was investigated as MOCVD precursor for Hf oxide films and compared with the conventional Hf(thd)4 precursor. HfO2 films were deposited by innovative pulsed injection MOCVD (PIMOCVD) technique and the influence of deposition temperature on film growth rate, roughness and microstructure was studied.
2. Experimental section 2.1. Synthesis Synthesis was performed under argon using Schlenk tubes and vacuum line techniques. Solvents were purified by standard methods. HfCl4, neopentyl alcohol and thdH were used as received. FT-IR spectra were recorded with a Perkin–Elmer Paragon 500 Spectrometer as Nujol mulls. 1H and 13 C/DEPT NMR experiments were performed using a Bruker 250 MHz spectrometer and CDCl3 as a solvent, the peaks were referenced to the protio impurity of the solvent. Elemental analysis
was performed at the Centre de microanalyses du CNRS. Synthesis of Hf3O(ONep)10 (1) A solution of neopentyl alcohol (18.19 g, 0.206 mol) in 20 ml of toluene was added to a suspension of HfCl4 (16.52 g, 0.0516 mol) in 40 ml of toluene. A slightly exothermic reaction occurred with dissolution of the halide. Anhydrous ammonia was bubbled through the reaction medium for 5 min., a very exothermic reaction occurred and NH4Cl precipitated. The NH3 treatment was repeated twice for 5 min. After filtration and washing of the precipitate several times with toluene, the solution was evaporated to dryness. The crude product was recrystallized in hexane. Two crops of 1 were obtained at 18 C (22.06 g, 90%). 1 sublimes at 230–240 C/2.3 104 Torr, 87%. 1 H NMR (CDCl3, 25 C, d (ppm): 0.84s (Met, 27H), 0.90s (Met, 27H), 0.95s (Mem, 27H), 1.01s (Mem3, 9H), 3.66s (CHt2, 6H), 3.73s (CHt2, 6H), m 13 3.92s (CHm3 C NMR 2 , 2H), 3.97s (CH2 , 6H). m3 (CDCl3, 25 C, d (ppm)): 26.45 (Me , 3C), 26.57 (Met, 9C), 26.68 (Met, 9C), 26.84 (Mem, 9C), 33.13 (Cm3, 1C), 33.21 (Cm, 3C), 33.65 (Ct, 6C), 77.11 m t (CHm3 2 , 1C), 80.18 (CH2 , 3C), 80.51 (CH2, 6C) (indices t, m, m3 refer to terminal, m- and m3-ONep ligands, respectively). IR, cm1: 1478m, 1394m, 1363m, 1259m; 1154s, 1128s (nC–O); 1051w, 1038m, 1014m, 935w, 900w, 750w, 726w; 629m; 531s, 445m, 386w (nHf–OR). Hf3O(ONep)10 is soluble in usual organic solvents including
Table 1 Deposition conditions of HfO2 films Precursors
Hf3O(ONep)10 and Hf(thd)4
Substrates Evaporator temperature Substrate temperature Ar flow O2 flow Total pressure Solvent Precursor concentration Injection duration Injection frequency Microdose mass
Sapphire (R-plane) and Si (1 0 0) 280 C 320–750 C 900 cm3/min 600 cm3/min 5 Torr Toluene 0.05 M (Hf) 3.0 ms 2 Hz B3 mg
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aliphatic and aromatic hydrocarbons. Crystals for single-crystal X-ray diffraction were obtained by recrystallization in hexane at 40 C and slow crystallization at RT. Hf(thd)4 (88%) was synthesized by the same procedure.
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2.2. CVD experiments and films characterization Pulsed injection MOCVD (PI-MOCVD) technique elaborated commonly at LMGP-Grenoble and Vilnius University [17–19] was applied for the evaluation of Hf3O(ONep)10 as a precursor
Fig. 1. XRD y=2y scans of HfO2 films deposited from Hf3O(ONep)10 at various temperatures on sapphire (left) and silicon (right). Similar scans were obtained for films grown from M(thd)4.
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for HfO2 film deposition. The principle of PIMOCVD is based on repetitive injections of microdoses of solution of a volatile metalorganic precursor, flash evaporation, vapor transport by a carrier gas (Ar and O2), and decomposition on a hot substrate. Its advantages over classical MOCVD are better reproducibility of precursor feeding rate and easy control of film thickness and growth rate by changing injection parameters (injection frequency and injected drop size, concentration of the solution). The deposition conditions are presented in Table 1. Solutions of Hf3O(ONep)10 (0.05 M) in toluene were used for films deposition. They were stable under argon for several days thus demonstrating their suitability for pulsed liquid injection MOCVD applications. Solutions exposed to air underwent precipitation after B5 h. Hf2(OiPr)8(iPrOH)2 solutions were less stable towards hydrolysis (precipitation occurred after B2 h). The substrates (R-plane sapphire and Si(1 0 0)) were cleaned in boiling toluene and its vapor before use. After deposition, films were cooled down to RT in an O2 atmosphere (B1 bar). The films crystallization and texture were studied by Xray diffraction (XRD) with a Siemens D500 diffractometer (CuKa radiation). The film in-plane orientation was studied by XRD in Schultz geometry. X-ray photoelectron spectroscopy (XPS; RIBER LAS-3000, MgKa radiation) was also used; the in-depth films purity was evaluated after Ar+ sputtering with 1.5 keV. The data were referenced to the carbon peak (284.6 eV) for charge effects correction. The composition of the films was determined by Energy-Dispersive X-ray Spectroscopy (EDS) coupled with a scanning electron microscope (SEM) (Philips). The morphology of the films surface was examined by atomic force microscopy (AFM) (Digital Instruments Multimode Scanning Probe Microscope, tapping mode). Thickness of the films on Si(1 0 0) substrates was determined by profilometry (Taylor–Hobson) on steps formed by lithography. The measured thickness was used for the calculation of the film growth rate.
3. Results and discussion 3.1. Synthesis and characterization The reaction between hafnium chloride and neopentyl alcohol was carried out in conditions similar to those of the synthesis of Hf2(OiPr)8(iPrOH)2 [20]. Compound 1 was obtained reproducibly and in high yield (>90%). 1 is volatile (87% sublimes at 230–240 C/2.3 104 Torr), but less than liquid hafnium alkoxides such as [Hf(OR)4]m (R=Et, iPr, tBu, tAmyl) [21]. It was characterized by FT-IR, 1H and 13C NMR and single-crystal Xray diffraction. Its FT-IR spectrum displayed C–O and Hf–O stretching bands at 1128–1154 and 386– 629 cm1, respectively. The 1H and 13C NMR spectra of 1 showed, already at RT, the presence of four magnetically non-equivalent neopentoxide ligands. This indicates that 1 is different from Hf(ONep)4 species. The identity of compound 1 was established by single crystal X-ray diffraction. It corresponds to a trinuclear species Hf3O(ONep)10. It is based on a triangular Hf3 framework capped by m3-oxo and m3-neopentoxide ligands, the metals bearing two terminal neopentoxide ligands being also connected by bridging m-ONep ligands. The structure could not be solved completely due to disorder problems and no metric parameters can thus be given nor discussed. However, the overall formula and standard M3(m3-X)2(m-X)3X6 framework were confirmed by elemental analysis and NMR (see below).
The NMR spectra of 1 in CDCl3 at RT displayed four types of methylene and methyl
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groups in the 1:3:3:3 integration ratio for the alkoxide ligands. These observations account for the retention of the solid state structure in solution as well as for a slow exchange, on the NMR time scale, between the various bridging m-, m3- and terminal neopentoxides. Several processes such as hydrolysis, C–O bond cleavage reactions can account for the origin of the oxo ligand. 1 has been obtained in reproducible high yields making hydrolysis unlikely. Spectroscopic data (1H NMR, FT-IR) were similar for the crude product and after recrystallization indicating than formation of the oxo species was not due to the work-up. Formation of the oxo ligands results most probably from C–O bond cleavage promoted by acidic conditions on the first stages of the reaction. The literature reports the formation of oxo and non-oxo Ti(IV)and Zr(IV) neopentoxides depending on the synthetic route [15,16]. Trinuclear oxo species M3(m3-O)(m3-Cl)(m-ONep)3(ONep)6, whose structures are similar to that of 1, were obtained either by metathesis of MCl4 (M=Ti, Zr) and sodium or potassium neopentoxide (1:4 stoichiometry) [15,22]. Non-oxo species, namely the M2(mONep)2(ONep)6 dimers were isolated by alcoholysis of Ti(OiPr)4 or Zr(NMe2)4 with NepOH, respectively. The sterically hindered neopentoxide ligand precludes the formation of solvates [23]. The formation of the oxo species is thus unlikely to result from desolvation as reported for alkoxides of large elements [24]. Hf2(OiPr)8(iPrOH)2 for instance is converted into Hf3(m3-O)(m3-OiPr)(mOiPr)3(OiPr)6 [25] by desolvation.
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2yB35:3 : At higher temperature highly crystalline films were obtained. Films were composed of a mixture of (1 0 0), (0 1 0) and (0 0 1) textured crystallites (Fig. 1). The j-scans made for 1% 1 1 reflection contained four well-defined peaks demonstrating highly preferential in-plane orientation of HfO2 crystallites. Films deposited from Hf(thd)4 showed similar behavior. Films on sapphire grown at low temperatures were annealed for 2 h at 750 C under vacuum. During annealing the amorphous films became polycrystalline. Similar result was obtained after annealing in air. The films deposited on Si(1 0 0) at temperatures up to 400 C are amorphous as observed on sapphire. At higher temperatures, polycrystalline slightly textured films of monoclinic HfO2 were obtained (Fig. 1). High deposition temperature results in the apparition of a XRD peak at 2y ¼ 44:3 ; corresponding probably to a product of an interaction between substrate and film. Increase of the peak intensity with temperature reflects the increasing interaction rate (at 600 C the additional peak is small, but at 750 C it becomes predominant). All HfO2 films were crack free and well adherent on the substrates (scotch tape test). Films on Si were etched with HF in order to form a step for thickness determination. Calculated growth rates expressed as nm/mmol of injected precursor are presented in Fig. 2. One can see that the film growth rate using Hf3O(ONep)10 was independent
3.2. MOCVD growth and characterization of the films HfO2 films (thickness 200–300 nm) were deposited by PIMOCVD on silicon and sapphire Rplane substrates at various temperatures in the range 320–750 C. For comparison, HfO2 films were also grown (under similar conditions) using the conventional Hf(thd)4 precursor. Films of HfO2 grown from Hf3O(ONep)10 on Rplane sapphire at low temperatures (320–400 C) were amorphous according to XRD. At 500 C, a broad peak of monoclinic HfO2 appears at
Fig. 2. Growth rate (nm/mmol of Hf) vs. temperature of HfO2 films from Hf3O(ONep)10 and Hf(thd)4 precursors.
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Fig. 3. AFM images (2 2 mm2) of HfO2 films deposited from Hf3O(ONep)10 at various temperatures on sapphire (left) and silicon (right) substrates.
of the deposition temperature in the range of 350– 750 C. Similar results were also obtained in the case of Ti and Zr neopentoxides [16]. Such a behavior corresponds to the diffusion controlled growth. On the contrary, film growth from Hf(thd)4 was kinetically limited in all the temperature range studied up to 750 C. We can conclude that Hf3O(ONep)10 leads to significantly higher growth rates of HfO2 films at low temperatures (o600 C) compared to the conventional Hf(thd)4 precursor. It is worth noting that the growth of HfO2 films from Hf(tod)4 (tod=2,7,7-trimethyloc-
tane-3,5-dionate), a b-diketonate with bulky neopentyl substituents, is an intermediate case—it enters a diffusion controlled range at B650 C [26]. The deposited HfO2 films were studied by AFM. Some AFM scans in a 2 2 mm2 area are presented in Fig. 3. The roughness average (Ra, nm) values as well as Ra/thickness ratios (%) of films deposited on both sapphire and silicon substrates increased with the increase of the deposition temperature (Fig. 4). Very smooth HfO2 films can be deposited from Hf3O(ONep)10 at low temperature (o400 C). For example, the Ra value
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Fig. 4. Surface roughness vs. deposition temperature of HfO2 films grown from Hf3O(ONep)10 on sapphire and silicon substrates. Ra(%)—relative to thickness roughness average.
of 260 nm thick HfO2 films grown at 350 C was only B0.5 nm. Comparison of film roughness obtained with Hf3O(ONep)10 and Hf(thd)4 is clearly in favor of the former—relative to thickness Ra values of films deposited from Hf(thd)4 at 500–750 C varied from 3% to 8.5%. The EDS study of HfO2 films deposited on silicon from Hf3O(ONep)10 showed that films were mainly sub-stoichiometric (Fig. 5a) and contained relatively small amounts of carbon (Fig. 5b). More carbon (4–6%) was detected in films deposited at low temperatures (o500 C), while at higher temperature the carbon content came down to the detection limit (2–3%). Oxygen-hafnium ratios in the films obtained from Hf(thd)4 were more dependent on the growth temperature and overstoichiometric or sub-stoichiometric films can be obtained at 500–700 C (Fig. 5a). Moreover, the carbon content in the films grown at 500 C from Hf(thd)4 was higher compared to Hf3O(ONep)10 (Fig. 5b). Both over-stoichiometric and substoichiometric HfO2 films have already been reported in the literature [3]. The HfO2 films deposited on sapphire at low (400 C) and high (700 C) temperature from Hf3O(ONep)10 were studied by XPS (Fig. 6). In both cases over-stoichiometry (O/Hf=2.7 and 2.2, respectively) was found on films’ surface. However, after surface etching with 1.5 keV Ar ions for
Fig. 5. Composition (by EDS) vs. deposition temperature of HfO2 films grown from Hf3O(ONep)10 and Hf(thd)4 on silicon. (a) film stoichiometry (O/Hf ratio) and (b) film purity (carbon content).
60 min, the O/Hf ratio changed to sub-stoichiometric (1.9 and 1.8, respectively) and this result is in accordance with EDX analysis of the overall film composition. The carbon content on film surface was very high (37 and 30 at%, respectively). After surface etching with Ar ions, the carbon content became close to the detection limit for the films grown at 400 C, while films grown at high temperature still showed a rather high carbon content (B10%). This surprising result probably can be explained by high crystallinity and rough surface of the films deposited at high temperatures leading to less efficient etching by Ar ions compared to smooth amorphous films.
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Acknowledgements SVP is grateful to Centre National des Oeuvres Universitaires et Scolaires (CNOUS) for a fellowship. Financial support to LHP by MINEFI (RECEMES 03) is also acknowledged. We thank Dr. P. Richard (Universite! de Bourgogne) for his efforts in solving the X-ray structure of hafnium oxoneopentoxide.
References
Fig. 6. XPS spectra of HfO2 films on sapphire deposited from Hf3O(ONep)10 at (a) 400 C and (b) 700 C. Spectra were measured on film surface (1) and after its etching (2) with Ar ions (1.5 keV, 60 min).
4. Conclusions Synthesized hafnium oxoneopentoxide Hf3(m3O)(m3-ONep)(m-ONep)3(ONep)6 is a promising precursor for the deposition of hafnia films. Depending on deposition conditions, amorphous, polycrystalline or epitaxial HfO2 films can be deposited on sapphire by pulsed liquid injection MOCVD. The advantages of the new precursor over the conventional Hf(thd)4 precursor in terms of MOCVD applications are significantly higher deposition rates at low temperatures, smoother film surface and higher film purity.
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ARTICLE IN PRESS A. Abrutis et al. / Journal of Crystal Growth 267 (2004) 529–537 [18] J.P. Senateur, C. Dubourdieu, V. Galindo, F. Weiss, A. Abrutis, in: K.-L. Choy (Ed.), Innovative Processing of Films and Nanocrystalline Powders, Imperial College Press, London, 2002, p. 71. [19] A. Abrutis, V. Plausinaitiene, V. Kubilius, A. Teiserskis, Z. Saltyte, R. Butkute, J.P. Senateur, Thin Solid Films 413 (2002) 32. [20] M. Veith, S. Mathur, C. Mathur, V. Huch, J. Chem. Soc., Dalton Trans. (1997) 2101. [21] N.Ya. Turova, The Chemistry of Metal Alkoxides, Kluwer Academic Publishers, Boston, 2002. [22] W.J. Evans, M.A. Ansari, J.W. Ziller, Polyhedron 17 (1999) 869.
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[23] B.A. Vaartstra, J.C. Huffman, P.S. Gradeff, L.G. HubertPfalzgraf, J.-C. Daran, S. Parraud, K. Yunlu, K.G. Caulton, Inorg. Chem. 29 (1990) 3126. [24] E.P. Turevskaya, N.I. Kozlova, N.Ya. Turova, A.I. Belokon, D.V. Berdyev, V.G. Kessler, Yu.K. Grishin, Polyhedron 18 (1999) 941. [25] S. Daniele, L.G. Hubert-Pfalzgraf, P.B. Hitchcock, M.F. Lappert, Inorg. Chem. Comm. 3 (2000) 218 and references therein. [26] S.V. Pasko, L.G. Hubert-Pfalzgraf, A. Abrutis, P. Richard, A. Bartasyte, V. Kazlauskiene, J. Mater. Chem. 14 (2004) 1245.
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Hafnium oxoneopentoxide as a new MOCVD precursor for hafnium oxide films A. Abrutisa,*, L.G. Hubert-Pfalzgrafb, S.V. Paskoa,b, A. Bartasytea, F. Weissc, V. Janickisd a Department of Chemistry, Vilnius University, Naugarduko 24, 01513 Vilnius, Lithuania Universit!e Lyon 1, IRC-UPR5401, 2 avenue A.Einstein, 69626 Villeurbanne C!edex, France c Laboratoire des Mat!eriaux et du G!enie Physique (LMGP), INPG-ENSPG, UMR 5628, BP 46, 38402 St Martin d’Heres, France d Faculty of Chemical Technology, Kaunas Technological University, Radvil’enu) 19, 3028 Kaunas, Lithuania b
Received 20 February 2004; accepted 6 April 2004
Communicated by R. Kern
Abstract The reaction between hafnium tetrachloride and neopentyl alcohol in the presence of ammonia lead to hafnium oxoneopentoxide (1). 1 was characterized by FT-IR, 1H and 13C NMR. Single-crystal X-ray diffraction identified it as a trinuclear oxo species Hf3(m3-O)(m3-ONep)(m-ONep)3(ONep)6. The oxoneopentoxide has been used as precursor for the growth of HfO2 films by pulsed liquid injection MOCVD. The influence of deposition temperature (320–750 C) on film growth rate, roughness and microstructure was studied and compared with the conventional Hf(thd)4 (thd=2,2,6,6tetramethyl-3,5-heptanedionate) precursor. Depending on the deposition temperature, amorphous, polycrystalline or epitaxial films can be obtained on sapphire (R-plane) substrates. The film growth rate was independent of the deposition temperature in the range 350–750 C. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Zx; 68.55.Jk Keywords: A3. Metalorganic chemical vapor deposition; B1. Hafnium; B1. Metalorganic compounds; B1. Oxides
1. Introduction Hafnia thin films display a number of attractive properties and can be used as insulating layers in microelectronics (alternative to SiO2) [1,2], as *Corresponding author. Tel.: +37052331004; fax: +37052330987. E-mail address: [email protected] (A. Abrutis).
protective [3] or optical [4] coatings, tunnel junctions [5], and in gas sensors [4]. These have been prepared by sol–gel [6], ALD [4,7] or MOCVD techniques [3], as well as by physical methods (magnetron sputtering [8], e-beam evaporation [4]). Hafnium alkoxides have been used for sol–gel preparation of piezoelectric (PbHf0.5Ti0.5O3 [9]) or refractory ceramics [10] and of hafnium carbide gel
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.012
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fibers [11]. Besides conventional b-diketonates such as Hf(thd)4 (thd=2,2,6,6-tetramethyl-3,5heptanedionate), alkoxides-Hf(OtBu)4 [1,12,13], Hf(mmp)4 [3], Hf(OtBu)2(mmp)2 [3] (mmp=1methoxy-2-methyl-2-propanolate) have also been used as precursors for MOCVD deposition of HfO2 thin films. Alkoxides are versatile precursors for tailoring properties at a molecular level [14]. Their advantage over b-diketonates as MOCVD precursors is lower film deposition temperature, but they are moisture-sensitive. Their hydrolytic susceptibility can be reduced by the use of sterically hindered alkoxide ligands or of functional alkoxides having additional N- or O-donor sites [3]. The neopentoxide group OCH2CMe3 (ONep) was chosen as a ligand since its bulky tBu moiety might prevent oligomerization and thus increases solubility properties [15]. Moreover, neopentoxide derivatives were reported to be less moisture sensitive and more volatile than isopropoxide analogs [16]. We wish to report herein the synthesis, characterization and application of hafnium oxoneopentoxide Hf3O(ONep)10. It was investigated as MOCVD precursor for Hf oxide films and compared with the conventional Hf(thd)4 precursor. HfO2 films were deposited by innovative pulsed injection MOCVD (PIMOCVD) technique and the influence of deposition temperature on film growth rate, roughness and microstructure was studied.
2. Experimental section 2.1. Synthesis Synthesis was performed under argon using Schlenk tubes and vacuum line techniques. Solvents were purified by standard methods. HfCl4, neopentyl alcohol and thdH were used as received. FT-IR spectra were recorded with a Perkin–Elmer Paragon 500 Spectrometer as Nujol mulls. 1H and 13 C/DEPT NMR experiments were performed using a Bruker 250 MHz spectrometer and CDCl3 as a solvent, the peaks were referenced to the protio impurity of the solvent. Elemental analysis
was performed at the Centre de microanalyses du CNRS. Synthesis of Hf3O(ONep)10 (1) A solution of neopentyl alcohol (18.19 g, 0.206 mol) in 20 ml of toluene was added to a suspension of HfCl4 (16.52 g, 0.0516 mol) in 40 ml of toluene. A slightly exothermic reaction occurred with dissolution of the halide. Anhydrous ammonia was bubbled through the reaction medium for 5 min., a very exothermic reaction occurred and NH4Cl precipitated. The NH3 treatment was repeated twice for 5 min. After filtration and washing of the precipitate several times with toluene, the solution was evaporated to dryness. The crude product was recrystallized in hexane. Two crops of 1 were obtained at 18 C (22.06 g, 90%). 1 sublimes at 230–240 C/2.3 104 Torr, 87%. 1 H NMR (CDCl3, 25 C, d (ppm): 0.84s (Met, 27H), 0.90s (Met, 27H), 0.95s (Mem, 27H), 1.01s (Mem3, 9H), 3.66s (CHt2, 6H), 3.73s (CHt2, 6H), m 13 3.92s (CHm3 C NMR 2 , 2H), 3.97s (CH2 , 6H). m3 (CDCl3, 25 C, d (ppm)): 26.45 (Me , 3C), 26.57 (Met, 9C), 26.68 (Met, 9C), 26.84 (Mem, 9C), 33.13 (Cm3, 1C), 33.21 (Cm, 3C), 33.65 (Ct, 6C), 77.11 m t (CHm3 2 , 1C), 80.18 (CH2 , 3C), 80.51 (CH2, 6C) (indices t, m, m3 refer to terminal, m- and m3-ONep ligands, respectively). IR, cm1: 1478m, 1394m, 1363m, 1259m; 1154s, 1128s (nC–O); 1051w, 1038m, 1014m, 935w, 900w, 750w, 726w; 629m; 531s, 445m, 386w (nHf–OR). Hf3O(ONep)10 is soluble in usual organic solvents including
Table 1 Deposition conditions of HfO2 films Precursors
Hf3O(ONep)10 and Hf(thd)4
Substrates Evaporator temperature Substrate temperature Ar flow O2 flow Total pressure Solvent Precursor concentration Injection duration Injection frequency Microdose mass
Sapphire (R-plane) and Si (1 0 0) 280 C 320–750 C 900 cm3/min 600 cm3/min 5 Torr Toluene 0.05 M (Hf) 3.0 ms 2 Hz B3 mg
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aliphatic and aromatic hydrocarbons. Crystals for single-crystal X-ray diffraction were obtained by recrystallization in hexane at 40 C and slow crystallization at RT. Hf(thd)4 (88%) was synthesized by the same procedure.
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2.2. CVD experiments and films characterization Pulsed injection MOCVD (PI-MOCVD) technique elaborated commonly at LMGP-Grenoble and Vilnius University [17–19] was applied for the evaluation of Hf3O(ONep)10 as a precursor
Fig. 1. XRD y=2y scans of HfO2 films deposited from Hf3O(ONep)10 at various temperatures on sapphire (left) and silicon (right). Similar scans were obtained for films grown from M(thd)4.
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for HfO2 film deposition. The principle of PIMOCVD is based on repetitive injections of microdoses of solution of a volatile metalorganic precursor, flash evaporation, vapor transport by a carrier gas (Ar and O2), and decomposition on a hot substrate. Its advantages over classical MOCVD are better reproducibility of precursor feeding rate and easy control of film thickness and growth rate by changing injection parameters (injection frequency and injected drop size, concentration of the solution). The deposition conditions are presented in Table 1. Solutions of Hf3O(ONep)10 (0.05 M) in toluene were used for films deposition. They were stable under argon for several days thus demonstrating their suitability for pulsed liquid injection MOCVD applications. Solutions exposed to air underwent precipitation after B5 h. Hf2(OiPr)8(iPrOH)2 solutions were less stable towards hydrolysis (precipitation occurred after B2 h). The substrates (R-plane sapphire and Si(1 0 0)) were cleaned in boiling toluene and its vapor before use. After deposition, films were cooled down to RT in an O2 atmosphere (B1 bar). The films crystallization and texture were studied by Xray diffraction (XRD) with a Siemens D500 diffractometer (CuKa radiation). The film in-plane orientation was studied by XRD in Schultz geometry. X-ray photoelectron spectroscopy (XPS; RIBER LAS-3000, MgKa radiation) was also used; the in-depth films purity was evaluated after Ar+ sputtering with 1.5 keV. The data were referenced to the carbon peak (284.6 eV) for charge effects correction. The composition of the films was determined by Energy-Dispersive X-ray Spectroscopy (EDS) coupled with a scanning electron microscope (SEM) (Philips). The morphology of the films surface was examined by atomic force microscopy (AFM) (Digital Instruments Multimode Scanning Probe Microscope, tapping mode). Thickness of the films on Si(1 0 0) substrates was determined by profilometry (Taylor–Hobson) on steps formed by lithography. The measured thickness was used for the calculation of the film growth rate.
3. Results and discussion 3.1. Synthesis and characterization The reaction between hafnium chloride and neopentyl alcohol was carried out in conditions similar to those of the synthesis of Hf2(OiPr)8(iPrOH)2 [20]. Compound 1 was obtained reproducibly and in high yield (>90%). 1 is volatile (87% sublimes at 230–240 C/2.3 104 Torr), but less than liquid hafnium alkoxides such as [Hf(OR)4]m (R=Et, iPr, tBu, tAmyl) [21]. It was characterized by FT-IR, 1H and 13C NMR and single-crystal Xray diffraction. Its FT-IR spectrum displayed C–O and Hf–O stretching bands at 1128–1154 and 386– 629 cm1, respectively. The 1H and 13C NMR spectra of 1 showed, already at RT, the presence of four magnetically non-equivalent neopentoxide ligands. This indicates that 1 is different from Hf(ONep)4 species. The identity of compound 1 was established by single crystal X-ray diffraction. It corresponds to a trinuclear species Hf3O(ONep)10. It is based on a triangular Hf3 framework capped by m3-oxo and m3-neopentoxide ligands, the metals bearing two terminal neopentoxide ligands being also connected by bridging m-ONep ligands. The structure could not be solved completely due to disorder problems and no metric parameters can thus be given nor discussed. However, the overall formula and standard M3(m3-X)2(m-X)3X6 framework were confirmed by elemental analysis and NMR (see below).
The NMR spectra of 1 in CDCl3 at RT displayed four types of methylene and methyl
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groups in the 1:3:3:3 integration ratio for the alkoxide ligands. These observations account for the retention of the solid state structure in solution as well as for a slow exchange, on the NMR time scale, between the various bridging m-, m3- and terminal neopentoxides. Several processes such as hydrolysis, C–O bond cleavage reactions can account for the origin of the oxo ligand. 1 has been obtained in reproducible high yields making hydrolysis unlikely. Spectroscopic data (1H NMR, FT-IR) were similar for the crude product and after recrystallization indicating than formation of the oxo species was not due to the work-up. Formation of the oxo ligands results most probably from C–O bond cleavage promoted by acidic conditions on the first stages of the reaction. The literature reports the formation of oxo and non-oxo Ti(IV)and Zr(IV) neopentoxides depending on the synthetic route [15,16]. Trinuclear oxo species M3(m3-O)(m3-Cl)(m-ONep)3(ONep)6, whose structures are similar to that of 1, were obtained either by metathesis of MCl4 (M=Ti, Zr) and sodium or potassium neopentoxide (1:4 stoichiometry) [15,22]. Non-oxo species, namely the M2(mONep)2(ONep)6 dimers were isolated by alcoholysis of Ti(OiPr)4 or Zr(NMe2)4 with NepOH, respectively. The sterically hindered neopentoxide ligand precludes the formation of solvates [23]. The formation of the oxo species is thus unlikely to result from desolvation as reported for alkoxides of large elements [24]. Hf2(OiPr)8(iPrOH)2 for instance is converted into Hf3(m3-O)(m3-OiPr)(mOiPr)3(OiPr)6 [25] by desolvation.
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2yB35:3 : At higher temperature highly crystalline films were obtained. Films were composed of a mixture of (1 0 0), (0 1 0) and (0 0 1) textured crystallites (Fig. 1). The j-scans made for 1% 1 1 reflection contained four well-defined peaks demonstrating highly preferential in-plane orientation of HfO2 crystallites. Films deposited from Hf(thd)4 showed similar behavior. Films on sapphire grown at low temperatures were annealed for 2 h at 750 C under vacuum. During annealing the amorphous films became polycrystalline. Similar result was obtained after annealing in air. The films deposited on Si(1 0 0) at temperatures up to 400 C are amorphous as observed on sapphire. At higher temperatures, polycrystalline slightly textured films of monoclinic HfO2 were obtained (Fig. 1). High deposition temperature results in the apparition of a XRD peak at 2y ¼ 44:3 ; corresponding probably to a product of an interaction between substrate and film. Increase of the peak intensity with temperature reflects the increasing interaction rate (at 600 C the additional peak is small, but at 750 C it becomes predominant). All HfO2 films were crack free and well adherent on the substrates (scotch tape test). Films on Si were etched with HF in order to form a step for thickness determination. Calculated growth rates expressed as nm/mmol of injected precursor are presented in Fig. 2. One can see that the film growth rate using Hf3O(ONep)10 was independent
3.2. MOCVD growth and characterization of the films HfO2 films (thickness 200–300 nm) were deposited by PIMOCVD on silicon and sapphire Rplane substrates at various temperatures in the range 320–750 C. For comparison, HfO2 films were also grown (under similar conditions) using the conventional Hf(thd)4 precursor. Films of HfO2 grown from Hf3O(ONep)10 on Rplane sapphire at low temperatures (320–400 C) were amorphous according to XRD. At 500 C, a broad peak of monoclinic HfO2 appears at
Fig. 2. Growth rate (nm/mmol of Hf) vs. temperature of HfO2 films from Hf3O(ONep)10 and Hf(thd)4 precursors.
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Fig. 3. AFM images (2 2 mm2) of HfO2 films deposited from Hf3O(ONep)10 at various temperatures on sapphire (left) and silicon (right) substrates.
of the deposition temperature in the range of 350– 750 C. Similar results were also obtained in the case of Ti and Zr neopentoxides [16]. Such a behavior corresponds to the diffusion controlled growth. On the contrary, film growth from Hf(thd)4 was kinetically limited in all the temperature range studied up to 750 C. We can conclude that Hf3O(ONep)10 leads to significantly higher growth rates of HfO2 films at low temperatures (o600 C) compared to the conventional Hf(thd)4 precursor. It is worth noting that the growth of HfO2 films from Hf(tod)4 (tod=2,7,7-trimethyloc-
tane-3,5-dionate), a b-diketonate with bulky neopentyl substituents, is an intermediate case—it enters a diffusion controlled range at B650 C [26]. The deposited HfO2 films were studied by AFM. Some AFM scans in a 2 2 mm2 area are presented in Fig. 3. The roughness average (Ra, nm) values as well as Ra/thickness ratios (%) of films deposited on both sapphire and silicon substrates increased with the increase of the deposition temperature (Fig. 4). Very smooth HfO2 films can be deposited from Hf3O(ONep)10 at low temperature (o400 C). For example, the Ra value
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Fig. 4. Surface roughness vs. deposition temperature of HfO2 films grown from Hf3O(ONep)10 on sapphire and silicon substrates. Ra(%)—relative to thickness roughness average.
of 260 nm thick HfO2 films grown at 350 C was only B0.5 nm. Comparison of film roughness obtained with Hf3O(ONep)10 and Hf(thd)4 is clearly in favor of the former—relative to thickness Ra values of films deposited from Hf(thd)4 at 500–750 C varied from 3% to 8.5%. The EDS study of HfO2 films deposited on silicon from Hf3O(ONep)10 showed that films were mainly sub-stoichiometric (Fig. 5a) and contained relatively small amounts of carbon (Fig. 5b). More carbon (4–6%) was detected in films deposited at low temperatures (o500 C), while at higher temperature the carbon content came down to the detection limit (2–3%). Oxygen-hafnium ratios in the films obtained from Hf(thd)4 were more dependent on the growth temperature and overstoichiometric or sub-stoichiometric films can be obtained at 500–700 C (Fig. 5a). Moreover, the carbon content in the films grown at 500 C from Hf(thd)4 was higher compared to Hf3O(ONep)10 (Fig. 5b). Both over-stoichiometric and substoichiometric HfO2 films have already been reported in the literature [3]. The HfO2 films deposited on sapphire at low (400 C) and high (700 C) temperature from Hf3O(ONep)10 were studied by XPS (Fig. 6). In both cases over-stoichiometry (O/Hf=2.7 and 2.2, respectively) was found on films’ surface. However, after surface etching with 1.5 keV Ar ions for
Fig. 5. Composition (by EDS) vs. deposition temperature of HfO2 films grown from Hf3O(ONep)10 and Hf(thd)4 on silicon. (a) film stoichiometry (O/Hf ratio) and (b) film purity (carbon content).
60 min, the O/Hf ratio changed to sub-stoichiometric (1.9 and 1.8, respectively) and this result is in accordance with EDX analysis of the overall film composition. The carbon content on film surface was very high (37 and 30 at%, respectively). After surface etching with Ar ions, the carbon content became close to the detection limit for the films grown at 400 C, while films grown at high temperature still showed a rather high carbon content (B10%). This surprising result probably can be explained by high crystallinity and rough surface of the films deposited at high temperatures leading to less efficient etching by Ar ions compared to smooth amorphous films.
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Acknowledgements SVP is grateful to Centre National des Oeuvres Universitaires et Scolaires (CNOUS) for a fellowship. Financial support to LHP by MINEFI (RECEMES 03) is also acknowledged. We thank Dr. P. Richard (Universite! de Bourgogne) for his efforts in solving the X-ray structure of hafnium oxoneopentoxide.
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Fig. 6. XPS spectra of HfO2 films on sapphire deposited from Hf3O(ONep)10 at (a) 400 C and (b) 700 C. Spectra were measured on film surface (1) and after its etching (2) with Ar ions (1.5 keV, 60 min).
4. Conclusions Synthesized hafnium oxoneopentoxide Hf3(m3O)(m3-ONep)(m-ONep)3(ONep)6 is a promising precursor for the deposition of hafnia films. Depending on deposition conditions, amorphous, polycrystalline or epitaxial HfO2 films can be deposited on sapphire by pulsed liquid injection MOCVD. The advantages of the new precursor over the conventional Hf(thd)4 precursor in terms of MOCVD applications are significantly higher deposition rates at low temperatures, smoother film surface and higher film purity.
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