Hafnium and zirconium tetramethylnonanedionates as new MOCVD precursors for oxide films

Materials Letters 59 (2005) 261 – 265 www.elsevier.com/locate/matlet

Hafnium and zirconium tetramethylnonanedionates as new MOCVD precursors for oxide films S.V. Paskoa,b, A. Abrutisa,*, L.G. Hubert-Pfalzgraf b,* a

Department of Chemistry, Vilnius University, Naugarduko 24, Vilnius, Lithuania b Universite´ Lyon 1, IRC, 2 av. A. Einstein, 69626 Villeurbanne Ce´dex, France

Received 7 April 2004; received in revised form 10 July 2004; accepted 11 July 2004 Available online 14 October 2004

Abstract New bulky Zr and Hf h-diketonates (2,2,8,8-tetramethyl-4,6-nonanedionates, tmnd) were synthesized and characterized by elemental analyses, 1H NMR, FT-IR and mass spectrometry. A volatile copper compound Cu(tmnd)2, an intermediate product of ligand synthesis, was isolated and characterized as well. The M(tmnd)4 (M=Zr, Hf) compounds were tested as precursors for MOCVD of ZrO2 and HfO2 films. Preferentially (001)/(010)/(100) textured and in-plane oriented films of monoclinic oxides have been deposited by pulsed liquid injection MOCVD on R plane sapphire. Smooth films could be grown, especially on sapphire and at low temperature (500 8C). The films on Si(100) were polycrystalline and had rougher surface. XPS study showed 3–4 and 7–8 at.% of carbon in HfO2 and ZrO2 films, respectively. Zr(tmnd)4 and Hf(tmnd)4 lead to significantly higher growth rates of ZrO2 and HfO2 films at low temperature than conventional Zr(thd)4 and Hf(thd)4 precursors (thd=2,2,6,6-tetramethyl-3,5-heptanedionate) and are attractive precursors for oxide films. D 2004 Elsevier B.V. All rights reserved. Keywords: Zirconium; Hafnium; Oxide; Thin films; h-diketonates; Chemical vapor deposition

1. Introduction Hafnia and zirconia are among the most promising alternatives to silicon oxide in microelectronics applications due to their high chemical and thermal stability [1] and elevated dielectric constants [2]. Several orders of magnitude lower leakage currents were observed when HfO2, ZrO2 [3], or Al2O3-coated HfO2 [4] films were used as insulating layers instead of SiO2 films. Other applications of thin films of zirconia and hafnia are optical [5], protective [6] or thermal barrier [7,8] coatings, tunnel junctions [9], in solid oxide fuel cells [7] and in gas sensors [5–7,10]. Both physical methods (PVD)—e-beam evaporation [11], sputtering [9], and chemical ones—anodization [12], atomic layer deposition [5,13], chemical vapor deposition

* Corresponding authors. Tel.: +3705 2331004; fax: +3705 2330987. E-mail address: [email protected] (A. Abrutis). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.07.061

(CVD) [2–4,6,7,14–16]—have been used for the deposition of ZrO2 and HfO2 thin films. The latter is an advantageous technique if compared with physical methods due to possibility to grow uniform layers on large, different-shaped substrates, and easy composition control in multimetal oxide films, especially for liquid injection MOCVD [2]. The metal organic precursors used for CVD of MO2 (M=Zr, Hf) are [2,16]: alkoxides M(OR)4 [OR=OtBu [4,7]; ONep (neopentoxide, M=Zr) [17]; mmp (1-methoxy-2methyl-2-propanolate) [6]], M(OtBu)2(mmp)2 [6]; Hf3O(ONep)10 [18]; amides—M(NEt2)4 [15]; h-diketonates such as acetylacetonate (acac), trifluoroacetylacetonate, hexafluoroacetylacetonate [14], 2,2,6,6-tetramethyl-3,5-heptanedionate (thd) [14,19]; 2,7,7-trimethyl-3,5-octanedionate (tod) [20], and mixed-ligand species, e.g., [Zr(OR)4 x Xx ]m, (X=acac, R=CH(CF3)2, x=1 [2]; X=thd, R=iPr, tBu, x=1,2 [21]). h-diketonates are among the most often used precursors due to their air stability and volatility. One of their drawbacks is a too high thermal stability implying that oxide films can be

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only deposited at relatively high temperatures. Sufficient deposition rates at low temperatures can be achieved by the use of other precursors, such as alkoxides [22]; however, their drawback is air sensitivity. An alternative is the use of hdiketonates with substituents allowing to achieve higher deposition rates compared to conventional precursors, such as M(thd)n [20]. Thus, we focused here on the synthesis of new sterically hindered h-diketonates RC(O)CHC(O)R containing bulky neopentyl moieties (R=Nep=CH2tBu) and performed preliminary studies of their applicability as precursors for liquid injection MOCVD.

2. Experimental 2.1. Synthesis and characterization Syntheses were performed under argon using Schlenk tubes and vacuum line techniques. 2,2,8,8-tetramethyl-4,6nonanedione (tmndH) and Cu(tmnd)2 were synthesized by a procedure similar to that described [20,23]. The copper complex, an intermediate used for purification of the ligand, was isolated (74% yield) and characterised. 2,2,6,6-tetramethyl-3,5-heptanedione (thdH), methylneopentylketone and 3,3-dimethylbutyrylchloride were used as received. Phenyl 3,3-dimethylbutyrate was synthesized as reported [23]. Solvents were purified by standard methods. FT-IR spectra were recorded with a Perkin-Elmer Paragon 500 Spectrometer as Nujol mulls. 1H NMR experiments were performed using a Bruker 250 MHz spectrometer. The ESR spectra (X-band, 9.44 GHz microwave power 10 mW, modulation frequency 100 kHz at rt) were obtained with a Varian E9 ESR spectrometer; the spectra were referenced to dpph (2,2-diphenyl-1-picrylhydrazyl, g=2.0035, H=3314 G). Electron impact mass spectra were obtained using a Thermofinnigan MAT95XL spectrometer at 70 eV. Elemental analyses were performed at Vilnius University. 2.1.1. Characterization of tmndH and Cu(tmnd)2 TmndH: 1H NMR (CDCl3 d (ppm)): 1.00 (s, 17.7 H, t Bu), 2.13 (s, 3.9 H, CH2), 5.39 (s, 0.98 H, CH), 9.67 (s, 0.98 H, OH enol). The diketone form (~2%) was characterized by the peaks at 1.10 (s, 0.31H, But), 2.23 (s, 0.07 H, CH2 (Nep)), and 3.49 ppm (s, 0.04 H, CH2 (dik)). IR, cm 1: 3180 w (mOH); 1606 vs (mC=O, mC=C). Cu(tmnd)2: IR, cm 1: 1598 w, 1561 vs., 1522 s (mC=O, mC=C); 488 m (mCu–O). ESR: (0.041 M hexane, rt): bgN=2.124, bAN=79.0 G; (0.009 M hexane, 77 K): g t=2.225, g 81=2.034, g 82=2.026, A t=175 G, A 81=32.7 G, A 82=29.7 G. Cu(tmnd)2 sublimes at 120–130 8C/10 4 Torr. 2.1.2. Synthesis and characterization of Zr(tmnd)4 (1) A solution of 0.42 g (1.98 mmol) tmndH in 3 ml toluene was added to 0.15 g (0.46 mmol) of Zr(OPrn)4 in 4 ml toluene. The reaction mixture was evaporated after stirring for 23 h at rt and sublimed at 220–230 8C/3.8d 10 2 Torr

(0.37 g, 86%). Anal. (1) Found: C, 67.28; H, 10.08. Calc. for C52H92O8Zr: C, 66.69; H, 9.90. 1H NMR (CDCl3 d (ppm)): 0.95 (s, 72H, tBu), 1.91 (s, 16H, CH2), 5.27 (s, 4H, CH). IR, cm 1: 1586 vs, 1520 vs. (mC=O, mC=C); 559 m (mZr–O). EI-MS: (M=Zr(tmnd)4), M+S (0.078%), M+S– Me2C=CH2 (0.038%), M+S–tBu–CH=C=O (0.35%), M+S– tmndd (100%), M+S–tmndd –NepCZCH (54%). Zr(thd)4 (81%) was synthesized by the same procedure. 2.1.3. Synthesis and characterization of Hf(tmnd)4 (2) A solution of 0.42 g (2.00 mmol) of tmndH in 5 ml toluene was added to a suspension of 0.16 g (0.50 mmol) of HfCl4 in 5 ml toluene. Anhydrous NH3 was bubbled through the reaction mixture three times for 15 min. After stirring at rt for 24 h, NH4Cl was filtered off, the filtrate evaporated and sublimed at 200–210 8C/3d 10 2 Torr (0.25 g, 49%). Anal. (2) Found: C, 61.71; H, 9.22. Calc. for C52H92HfO8: C, 61.01; H, 9.06. 1H NMR (CDCl3 d (ppm)): 0.95 (s, 72H, tBu), 1.92 (s, 16H, CH2), 5.25 (s, 4H, CH). IR, cm 1: 1588 vs, 1522 vs (mC=O, mC=C); 562 m (mHf–O). Hf(thd)4 (88%) was synthesized by the same procedure. All h-diketonates were soluble in hydrocarbons (except Cu(tmnd)2), ethers, less in alcohols and acetonitrile. 2.2. CVD experiments and films characterization Pulsed injection MOCVD technique [24,25], elaborated commonly at LMGP-Grenoble and Vilnius University, was applied for the study of the new compounds as possible precursors for oxide film deposition. This technique is based on repetitive injections of precise microdoses of solution of a volatile metal–organic precursor to a heated evaporator, flash evaporation, vapor transport by a carrier gas (Ar+O2), and vapor decomposition on a hot substrate. Its advantage over conventional MOCVD is easy and reproducible control of precursor feeding rate (and consequently growth rate and film thickness) by changing injection parameters (injection frequency and drop size, concentration of the solution). Detailed description of this technique is given elsewhere [25,26]. The deposition conditions are presented in Table 1. The substrates (R plane sapphire and Si (100)) were cleaned in Table 1 Deposition conditions of ZrO2 and HfO2 films Precursors Substrates Evaporator temperature Substrate temperature Ar flow O2 flow Total pressure Solvent Precursor concentration Injection duration Injection frequency Microdose mass

M(tmnd)4, M(thd)4 (M=Zr, Hf) Sapphire (R plane) and Si (100) 280 8C 500 and 700 8C 54 l/h 36 l/h 5 Torr Toluene 0.05 M 3 ms 2 Hz 3 mg

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boiling toluene before use (sapphire was also cleaned with an ultrasound bath for 15 min before wash with toluene). After deposition, the films were cooled down to room temperature in an O2 atmosphere (~760 Torr). The crystallization and texture of the films were studied by X-ray diffraction (XRD) with a Siemens D500 diffractometer (CuKa radiation). The film in-plane orientation was studied by XRD in Schultz geometry. The morphology of the films surface was examined by atomic force microscopy (AFM; Digital Instruments Multimode Scanning Probe Microscope, tapping mode). Growth rates were calculated from the thickness of the films on silicon determined by profilometry (Taylor-Hobson) on steps formed by lithography. X-ray photoelectron spectroscopy (XPS; RIBER LAS-3000, MgKa radiation) was used for the evaluation of film composition. The in-depth film 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.

fragments, the molecular ion peak being of low intensity. The major peak correspond to the loss of a diketonate radical, as observed for Zr(tod)4 [20]. Other fragments result from loss of diketonate moieties such as CH2=CMe2, NepCZCH. The paramagnetic copper complex (isolated as intermediate in the ligand synthesis) was characterized by ESR. The solid state spectrum indicated axial symmetry around the Cu(II) ion. The spectrum of Cu(tmnd)2 in hexane at rt presented a four-line pattern [63Cu, I=3/2, N=69%, 65Cu, I=3/2, N=31%] with bgN=2.124 and bAN=79.0 G. At least two series of hyperfine splitting patterns were observed on the spectrum of the solution frozen at 77 K. They most probably account for a slight tetrahedral distortion of the copper environment due to the bulky neopentyl substituents of the tmnd ligands.

3. Results and discussion

New complexes were tested as possible precursors for the deposition of HfO2 and ZrO2 films. Deposition experiments were performed by pulsed injection MOCVD at 500 and 700 8C. Films from standard precursors M(thd)4 (M=Zr,Hf) were also deposited for comparison.

3.1. Synthesis and characterization Metal 2,2,8,8-tetramethyl-4,6-nonanedionates have been synthesized either from the halides in basic conditions (Hf(tmnd) 4 ) or by ligand exchange from alkoxides (Zr(tmnd)4). The complexes can be sublimed without residue under vacuum: Cu(tmnd)2 at 120–130 8C/10 4 Torr, M(tmnd)4 at 200–230 8C/310 2 Torr. The M(thd)4 derivatives are more volatile (t subl=170–190 8C/310 2 Torr).

3.2. MOCVD deposition and characterization of HfO2 and ZrO2 films

1

H NMR data indicate that tmndH exists predominantly as enol (CH at 5.39 ppm) in CDCl3 solutions at rt. The presence of the tautomeric diketone form (~2%) is evidenced by the presence of a CH2 peak at 3.49 ppm as well as neopentyl peaks [singlets at 1.10 (Me) and 2.23 ppm (CH2)]. The elemental analyses, 1H NMR and mass spectra confirm the formulas of the compounds. Shifts of the absorption bands of the carbonyls in the FT-IR spectra to the 1598–1586 cm 1 region (compared to free ligands) as well as the apparition of metal–oxygen bands (488–562 cm 1) confirmed the formation of hdiketonic complexes. The mass spectrum of Zr(tmnd)4 in standard electron impact conditions showed only a limited number of

Fig. 1. XRD h/2h and u-scans of ZrO2 (a,b) and HfO2 (c,d) films on sapphire deposited at 700 8C from M(tmnd)4. The u-scans were measured for ( 111) reflection.

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Crystalline films (50–200 nm thick) of thermodynamically stable monoclinic ZrO2 and HfO2 were grown on R plane sapphire at 500 and 700 8C using M(tmnd)4 and M(thd)4 precursors. Films grown at 500 8C were still poorly crystallized, while those grown at 700 8C were well crystallized and exhibited prevailing (010) and (100) texture (Fig. 1a and c). Study by XRD in Schultz geometry of films grown at 700 8C revealed fourfold peaks in u-scans thus showing that films were highly preferentially in-plane oriented (Fig. 1b and d). Films on silicon deposited at low temperature (500 8C) were X-ray amorphous, while higher deposition temperatures (700 8C) lead to polycrystalline monoclinic films. In general, we did not find any clear difference between the crystalline quality of ZrO2 and HfO2 films grown from M(tmnd)4 and M(thd)4 precursors. The growth rates of MO2 films from M(tmnd)4 precursors were higher than those obtained using standard M(thd)4 precursors. At 700 8C, they did not differ significantly (17 and 19 nm/min for Zr-tmnd and Hf-tmnd, 15 and 16 nm/min for Zr-thd and Hf-thd). However, the growth rates difference became considerable at 500 8C [4–6 nm/min from M(tmnd)4 and 0.3–0.7 nm/min from M(thd)4]. Higher growth rates at low temperature might account for easier decomposition of M-tmnd precursors compared to Mthd due to the presence of the neopentyl group. Higher growth rates at low temperatures become important when deposition at low temperature is essential (e.g., for reducing interaction between substrate and film). The surface of deposited films was studied by AFM. Several AFM images of ZrO2 and HfO2 films obtained from M(tmnd)4 are presented in Fig. 2. Films grown at lower temperature (500 8C) had smoother surface than those deposited at 700 8C (compare the AFM images in Fig. 2a and b). Films on silicon were rougher compared to those on sapphire (see Fig. 2c and d). Comparison of film roughness obtained with M(tmnd)4 and M(thd)4 is in favor of the former in the case of sapphire substrate: roughness average

Fig. 2. AFM images (22 Am) of ZrO2 (a,b) and HfO2 (c,d) films deposited from M(tmnd)4. Substrate, deposition temperature, film thickness and roughness average (Ra) values are indicated on the figure.

Fig. 3. XPS spectra of ZrO2 (a) and HfO2 (b) films grown at 700 8C on sapphire. The spectra numbers corresponds to film surface (1) and film depth after 15 min (2) and 30 min (3) etching by Ar+ ions (1.5 keV).

(Ra) relative to thickness was 1–3% and 3.5–8%, respectively. The roughness of films on silicon was similar for both tmnd and thd precursors. ZrO2 and HfO2 films deposited on sapphire at 700 8C from M(tmnd)4 precursors were studied by XPS. Fig. 3 shows XPS spectra measured from film surface and after 15 and 30 min cleaning of the surface with Ar+ ions. The O/Zr and O/Hf ratios on film surface were 2.0 and 2.2, respectively, while after 30 min sputtering, the films were found to be substoichiometric (O/M ratios 1.9 and 1.8, respectively). The decrease of the O/M ratio might be due to selective sputtering of lighter O atoms [27]. The carbon content on the film surface was high (26 and 24 at.% for ZrO2 and HfO2). It significantly decreased after surface etching with Ar+ ions (see insets in Fig. 3). In the case of HfO2 films, the carbon content (3–4%) became close to detection limit. Hafnia films deposited at similar conditions from Hf(thd)4 were reported to be more contaminated with carbon in the bulk (~12%) despite lower number of carbon atoms in thd ligand [20]. However, 7–8% of carbon was found in the depth of ZrO2 films. This result probably does not reflect real carbon content in ZrO2 films and might be explained by rather rough surface (Ra=5.9 nm) of ZrO2 films (compare with Ra=2.2 nm for HfO2 films). Rougher

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film surface might lead to less efficient surface etching by Ar+ ions.

[5] [6]

4. Conclusions New bulky Zr and Hf h-diketonates (2,2,8,8-tetramethyl4,6-nonanedionates, tmnd) were synthesized and tested as MOCVD precursors for ZrO2 and HfO2 films. Using these complexes, textured and in-plane oriented films of monoclinic ZrO2 and HfO2 were deposited by pulsed liquid injection MOCVD on sapphire. The carbon content in deposited HfO2 films (3–4%) is close to the detection limit of XPS. M(tmnd)4 precursors (M=Zr and Hf) lead to significantly higher film growth rates at low temperatures and give smoother films on sapphire compared to conventional zirconium and hafnium 2,2,6,6-tetramethyl-3,5-heptanedionates, showing that they are attractive MOCVD precursors for oxide films.

Acknowledgements S. Pasko is grateful to CNOUS (Centre National des Oeuvres Universitaires et Scolaires) and Vilnius University for the fellowships.

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