Materials Science and Engineering B 118 (2005) 253–258

Reaction mechanism of a lanthanum precursor in liquid sourcemetalorganic chemical vapor deposition

Toshihiro Nakamura∗, Takuro Nishimura, Ryusuke Tai, Kunihide TachibanaDepartment of Electronic Science and Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Abstract

The reaction mechanism of a lanthanum precursor, tris(dipivaloylmethanato)lanthanum (La(DPM)3), was investigated in liquid sourcemetalorganic chemical vapor deposition (MOCVD) of lanthanum oxide films. The behavior of La(DPM)3 in the gas phase was analyzedunder actual CVD conditions by in situ infrared absorption spectroscopy. The infrared band identification was performed using densityfunctional theory calculations. We confirmed that the liquid source delivery using tetrahydrofuran as a solvent supplies a La(DPM)3 moleculemaintaining almost the whole molecular structure. The spectroscopic data on the gas-phase reactions were correlated with the characteristicsof the deposited oxide films. The temperature dependence of the infrared absorption suggests that the thermal decomposition of La(DPM)3

in the gas phase may trigger the film deposition. La(DPM)3 was only partially decomposed in the gas phase under the actual depositionc©

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onditions, which indicates that surface reactions rather than gas-phase reactions control dominantly the film deposition.2005 Elsevier B.V. All rights reserved.

eywords:Lanthanum; Chemical vapor deposition; Infrared absorption spectroscopy

. Introduction

A thinner gate insulator is strongly required for thecaling-down of the metal-oxide-semiconductor field-effectransistors (MOSFETs) in future ultra-large scale integratedircuits. Since a leakage current through the ultrathin SiO2nsulator places serious obstacles to future device reliabil-ty, high-dielectric constant (high-k) materials, such as oxidend/or silicates of hafnium and zirconium, were proposed aslternative gate insulators[1]. Recently, rare earth oxides alsoave been examined for next generation high-kgate insulatorpplications. In particular, a lanthanum oxide was reported

o show the excellent electrical properties and smooth sur-ace with the amorphous phase, since La2O3 has the largestandgap and the lowest lattice energy among rare earth ox-

des[2]. The lanthanum oxide is also an important compo-ent of a ferroelectric material, such as (Pb,La)(Zr,Ti)O3 [3]nd (Bi,La)4Ti3O12 [4,5], used in non-volatile ferroelectricandom access memories (FeRAMs), and a ferromagnetic

∗

material, such as (La,Sr)MnO3 [6–8], that exhibits colossmagnetoresistance (CMR) properties.

From the viewpoint of practical use in device proceswe adopted a liquid source metalorganic chemical vapoposition (MOCVD) technique because of its good steperage. The liquid source MOCVD technique has the adtage of providing a stable and efficient supply of sourceterials. However, the deposition chemistry is still not wunderstood in the liquid source MOCVD of lanthanucontaining oxide films. In view of a lack of established guing principles to control film quality, attempts to accumlate experimental data on gas-phase and surface reaunder actual CVD conditions are necessary to understhe film deposition mechanism. Recently, we have demstrated that spectroscopic techniques such as in situ infabsorption spectroscopy[9–13] and microdischarge opticemission spectroscopy[14,15] are useful for studying gaphase reactions of various MOCVD source moleculesthis work, in situ infrared absorption spectroscopy wasployed under actual liquid source CVD conditions to unstand gas phase reactions of a lanthanum CVD precurs

Corresponding author. Tel.: +81 75 383 8510; fax: +81 75 383 2290.

E-mail address:tosihiro@kuee.kyoto-u.ac.jp (T. Nakamura). solution of tris(dipivaloylmethanato)lanthanum (La(DPM)3,

921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2004.12.038

254 T. Nakamura et al. / Materials Science and Engineering B 118 (2005) 253–258

La(C11H19O2)3) in tetrahydrofuran (THF, C4H8O) was usedas a liquid lanthanum source material. We observed the tem-perature dependence of the infrared absorbance and measuredthe deposition rate and atomic composition of the depositedfilms. On the basis of the obtained experimental data, we dis-cussed the correlation between the thermal decomposition inthe gas phase and film deposition of lanthanum oxides.

2. Experimental

Fig. 1 shows a schematic diagram of the liquid-sourceCVD apparatus. The details of the experimental MOCVDapparatus were described previously[9–16]. A lanthanumsource material, La(DPM)3, was dissolved in THF at a con-centration of 0.1 mol/l. After the dissolved source was intro-duced into a vaporizer by N2 carrier gas at 200 ml/min, the va-porized source was transported into the MOCVD reactor andsubsequently mixed with O2 oxidant gas. The vaporizationtemperature was 250◦C. The flow rate of O2 was 500 ml/min.The pressure in the reactor was maintained at 1.3 kPa.

We observed infrared absorption spectra under actualCVD conditions. Details of the in situ infrared absorptionmeasurements have been described elsewhere[9–13]. Asolid sample of La(DPM)3 was prepared by mixing withpotassium bromide (KBr) powder and then pressing into as rm erepD pro-g eter

exchange functional[19] with the Lee–Yang–Parr correla-tional functional (B3LYP)[20] was employed. In both thecalculations, we used a composed basis set using an effec-tive core potential basis set of the second type on third orhigher row atoms[21–23] and Dunning’s double zeta D95basis set for the other atoms (LANL2DZ)[24]. In order todiminish the number of atoms that must be considered in thecalculations,tert-butyl groups of La(DPM)3 were replacedby methyl groups. By this simplification, DPM ligands werechanged into acetylacetone (ACAC) ligands. We carried outthe DFT calculations at the B3LYP/LANL2DZ level for thecorresponding ACAC chelates, La(ACAC)3.

Lanthanum oxide films were deposited on a 6 in. diameterSi(1 0 0) substrate. In order to evaluate the averaged deposi-tion rate, the thickness of the deposited films was measuredby cross-sectional scanning electron microscopy (SEM). Theatomic composition of the films was measured by X-ray pho-toelectron spectroscopy (XPS) after etching of the film sur-face.

3. Results and discussion

Fig. 2(a) indicates a typical infrared absorption spectrumof La(DPM)3 obtained by in situ spectroscopy. This spec-trum was obtained by subtracting the THF absorption fromt m ofLC thes iveryu -t

F us

elf-supporting disk[17]. This method is called the KBethod. Density functional theory (DFT) calculations werformed for the infrared band identification[12,17]. All theFT calculations were carried out using the Gaussian03ram[18]. In the DFT calculations, Becke’s three-param

Fig. 1. Schematic diagram of the liquid-source CVD apparatus.

he observed spectrum. The infrared absorption spectrua(DPM)3 obtained by the KBr method is shown inFig. 2(b).omparingFig. 2(a) with (b), both spectra show almostame structure, which means that the liquid source delsing THF as a solvent supplies a La(DPM)3 molecule main

aining the molecular structure.

ig. 2. Infrared absorption spectra of La(DPM)3 obtained (a) by an in sitpectroscopic measurement and (b) by the KBr method.

T. Nakamura et al. / Materials Science and Engineering B 118 (2005) 253–258 255

Using a DFT calculation, geometry optimization ofLa(ACAC)3 was performed under the partial constraint ofC3symmetry of the free-rotating CH3 groups.Fig. 3shows theoptimized configuration of La(ACAC)3, which belongs to aD3 symmetry group. Harmonic vibrational frequencies werecomputed through the analytic calculation of the energy sec-ond derivatives. Most intense peaks were found at the calcu-lated wavenumbers of 1560 and 1565 cm−1. These wavenum-bers were obtained using the scaling factor of 0.9978[25]in the present work. The vibrational mode at 1565 cm−1

is doubly degenerated. A vector diagram of the vibrationalmode of La(ACAC)3 for the absorption peak at the calculatedwavenumber of 1560 cm−1 is shown inFig. 4(a). The vibra-tional mode for the peak at 1560 cm−1 is assigned to theout-of-plane ring deformation mainly comprising the CCstretching mode. Vector diagrams of the doubly degeneratedmode for the peak at 1565 cm−1 are also indicated inFig. 4(b)and (c). The vibrational mode for the peak at 1565 cm−1 isassigned to the in-plane ring deformation mainly comprisingthe C O stretching mode. These two calculated modes ofLa(ACAC)3 are considered to correspond to the experimen-

tal absorption band of La(DPM)3 around the most intensepeak at 1567 cm−1 (seeFig. 2).

From the temperature dependence of the infrared absorp-tion spectrum, we investigated the behavior of La(DPM)3 inthe gas phase.Fig. 5 shows the temperature dependence ofthe infrared absorption of La(DPM)3 at the substrate temper-ature between 240 and 640◦C. As the substrate temperatureincreased, the absorbance derived from La(DPM)3 moleculesdecreased. This decrease is due to the thermal decomposi-tion of La(DPM)3 molecules. When the substrate tempera-ture was above 480◦C, the weak absorption peak at 889 cm−1

appeared. This 889 cm−1 peak is assigned to the CC H out-of-plane bending mode ofiso-butene[11]. The generationof the iso-butene as a thermally decomposed product corre-sponds to the dissociation of thetert-butyl group from theDPM chelate ring.

We investigated the correlation between the thermal de-composition in the gas phase and the characteristics of thedeposited lanthanum oxide films.Fig. 6shows the tempera-ture dependence of the infrared absorption at 1567 cm−1. Forcomparison, the temperature dependence of the averaged de-

Fig. 3. Optimized configuration of La(ACAC3

)by the B3LYP/LANL2DZ calculation.

256 T. Nakamura et al. / Materials Science and Engineering B 118 (2005) 253–258

Fig. 4. Vector diagrams of the vibrational modes for the most intense ab-sorption peaks of La(ACAC)3; (a) the mode for the absorption peak at thecalculated wavenumber of 1560 cm−1; (b) and (c) the doubly degeneratedmodes for the absorption peak at the calculated wavenumber of 1565 cm−1.

position rate of the thin films is also indicated inFig. 6. Atthe substrate temperature of 400◦C, the film deposition syn-chronized with the decrease of the infrared absorption byLa(DPM)3 in the gas phase. This experimental fact suggeststhat the thermal decomposition of La(DPM)3 in the gas phase

Fig. 5. Temperature dependence of the infrared absorption spectra ofLa(DPM)3.

may trigger the film deposition. The film deposition occurs atlow temperature before the infrared absorbance of La(DPM)3was completely quenched, which indicates that surface reac-tions rather than gas-phase reactions make a dominant con-tribution to the film growth. When the substrate tempera-ture was raised above 560◦C, the deposition rate decreaseddrastically. In order to clarify the origin of the behavior ofthe deposition rate, we evaluated the atomic composition ofthe deposited films by XPS measurements.Fig. 7shows thetemperature dependence of the atomic composition of de-posited lanthanum oxide films. When the substrate temper-

F1

ig. 6. Correlation between the infrared absorption of La(DPM)3 at567 cm−1 and the deposition rate of lanthanum oxide films.

T. Nakamura et al. / Materials Science and Engineering B 118 (2005) 253–258 257

Fig. 7. Temperature dependence of the atomic composition of depositedlanthanum oxide films.

ature was raised from 560 to 640◦C, the carbon concentra-tion was reduced to zero. Below 560◦C carbon-containingmaterials are deposited, while at higher temperatures the car-bon content of the deposited materials decreases, resulting ina decreased total amount of deposited materials. A carbon-free lanthanum oxide film can be obtained at the substratetemperature of 640◦C, while a carbon-containing film suchas carbonate species grows with higher deposition rate be-low 560◦C. The La to (La + O) ratio in the carbon-free filmsgrown at 640◦C is 0.32, although the stoichiometric ratio is0.40. This discrepancy indicates an excess of oxygen in thefilms.

4. Conclusions

In this study, the gas-phase reaction mechanism ofLa(DPM)3 under actual CVD conditions was investigated byin situ infrared absorption spectroscopy. DFT calculationswere carried out for the infrared band identification of theexperimental spectra. We confirmed that the liquid source de-livery using THF as a solvent supplies a La(DPM)3 moleculemaintaining almost the whole molecular structure. The tem-perature dependence of the infrared absorption suggests thathe thermal decomposition of La(DPM)3 in the gas phase mayt -c ondi-t gas-p hent el ainp bet inceL osi-t mayb wth

mechanism of other lanthanum-containing oxide films suchas (Pb,La)(Zr,Ti)O3, (Bi,La)4Ti3O12, and (La,Sr)MnO3films.

Acknowledgements

We would like to thank the Kyoto University Venture Busi-ness Laboratory (KU-VBL) for the support. We are indebtedto Dr. F. Yamamoto and Ms. K. Abe of the Department ofChemistry at Kyoto University for allowing the use of theirmachine for making KBr disk samples. This work is partlysupported by the Nippon Sheet Glass Foundation for Mate-rials Science and Engineering.

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