007) 216–218www.elsevier.com/locate/matlet

Materials Letters 61 (2

New MOCVD precursor for iridium thin films deposition

Xin Yan ⁎, Qiuyu Zhang, Xiaodong Fan

Department of Chemical Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China

Received 31 October 2005; accepted 6 April 2006Available online 5 May 2006

Abstract

Thin films of metallic iridium were grown by metal organic chemical vapor deposition in a vertical hot-wall reactor. The new solid compound Ir(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptadione) was used as the iridium source. The iridium precursor was analyzed by elemental analysis, infraredspectroscopy, 1HNMR spectroscopy and thermogravimetry (TG). The results of TG showed that the iridium β-diketonate was found to vary with thenature of the β-diketonate group and the use of the thd led to a precursor with higher volatilities than the Ir(acac)3 (acac = acetylacetonate) source.Deposited iridium films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) in order to determine crystallinity andsurface morphology.© 2006 Elsevier B.V. All rights reserved.

Keywords: MOCVD; Thin films; Iridium; Precursor

1. Introduction

Depositions of noble metal thin films, including Pt, Pd, Rh, orIr, are of particular interest because of their unique physical andchemical properties. Generally, these metals have high meltingtemperature, high resistance towards oxidation and goodelectrical conductivity; therefore, they are considered as idealelectrode and barrier materials for future microelectronic de-vices. Iridium, due to the absence of a carbide state and theexcellent electrical properties of its oxide, is of great interest tothe scientific and technical communities [1,2]. Various physicaland chemical processes have been employed to prepare iridiumthin films, including chemical vapor deposition (CVD), electro-chemical vapor deposition (EVD), sputtering, and other wetchemical processes [3–6]. These vapor deposition methods aregenerally more expensive because they involved the use ofsophisticated reactors and vacuum systems and they are notviable for mass production. Wet processes are cost-effective butrather poor in reproducibility. MOCVD, the precursor based onmetal-organic precursors, is a suitable method for the fabricationof many noble thin films [7–9]. It possessed several advantages:simple equipment, high deposition rate, easy control of filmcomposition and availability for conformal coverage.

⁎ Corresponding author. Tel.: +86 29 88495304; fax: +86 29 88491000.E-mail address: yan_xin007@163.com (X. Yan).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.04.034

In a previous report [10], MOCVD technology was used toprepare iridium thin films on a glass substrate with an Ir(acac)3(acac = acetylacetonate) precursor. Thus resultant iridium filmswere dense, smooth and homogeneous, with an average grainsize of 10–40 nm. However its high melting point and lowvolatility limited its possible application field. In our work, wereport the synthesis of Ir(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate), in which two (CH3)3C substituents are intro-duced onto the acac ligand to improve the volatility of the final

Fig. 1. TG curves of Ir(thd)3 and Ir(acac)3 in N2 with a heating rate of 10 °C/min.

Fig. 2. The XRD pattern of the iridium film deposits at 400 °C on glasssubstrates.

Fig. 4. The AFM plane images of the iridium films.

217X. Yan et al. / Materials Letters 61 (2007) 216–218

metal complexes. Iridium thin films were prepared by MOCVDon glass substrates. The structure and morphology of the iridiumthin films were characterized by X-ray diffraction (XRD) andatomic force microscopy (AFM).

2. Experimental

The precursor, Ir(thd)3 was synthesized from the H2IrCl6reaction from aqueous ammonia and Hthd in ethanol/aqueoussolution followed by recrystallization from benzene–hexane.Themetal complex has been characterized by elemental analysis,infrared spectroscopy, 1H NMR spectroscopy, and thermogra-vimetry (TG) analysis. Ir(thd)3 was studied by thermogravimet-ric analysis to get information about volatility and thermalstability. TheMOCVD experiments were performed in a verticalhot-wall reactor. The deposition temperature used was at 350–500 °C. The iridium films were deposited on glass substrates.

3. Results and discussion

Fig. 1 shows the TG curves of the samples in N2 which show thesublimation features of Ir(thd)3. The Ir(thd)3 maintains its initial weight

Fig. 3. The EDS analysis of the iridium film

up to about 195 °C in N2 corresponding to its sublimation temperature,then starts to lose weight significantly above 230 °C, and no furtherweight loss appears up to 290 °C. The residue amount is about 1.76% ofthe initial weight. The Ir(acac)3 maintains its initial weight up to about220 °C in N2 corresponding to its sublimation temperature, then starts tolose weight significantly above 260 °C, and no further weight lossappears up to 300 °C. The residue amount is about 1.74% of the initialweight. The TG results show that the Ir(thd)3 is highly volatile than Ir(acac)3, which may serve as a precursor in MOCVD of iridium films.

Fig. 2 displays the XRD patterns of the Ir films on glass substratesat the growth temperature of 400 °C. The θ–2θ scan data of thefilms exhibited strong 2θ peaks at 40.8616°, 47.4554°, and 69.3365°,83.6825°, 88.1406° respectively, corresponding to the (111), (200),(220), (311) and (222) peaks of Ir, revealing that the Ir films were fullypolycrystalline and no evidence for a preferential orientation was found.

The EDS data of the iridium films is shown in Fig. 3. The spectrareveal the presence of the Si, Au, Ca, Cu, Ir elements in the films.Among these elements, Si, Ca, and Cu are from the glass substrates andthe equipment itself, and Au was deposited by sputtering in order toimprove the films conductivity. Thus, the presence of Ir in the films isfurther confirmed by the EDS data whereby the Ir peak is detected inthe EDS spectra.

The AFM plane and 3-D images of the iridium films were showed inFigs. 4 and 5. The surface had a roughness of about 1.006 nm. The AFMimages show that the Ir films consist of closely spaced particles withvarious shapes and sizes. The average particle size in the films is 40 nm.We speculate that growth starts probably with the isolated iridiumclusters, which grow three dimensionally. Thus the growth mechanism

deposits at 400 °C on glass substrates.

Fig. 5. The AFM stereo images of the iridium films.

218 X. Yan et al. / Materials Letters 61 (2007) 216–218

follows the Volmer–Weber model. Up to now it has not been possible torealize atomic resolution. More detailed investigations will be presentedelsewhere.

4. Conclusions

The new solid compound Ir(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptadione) was used as the iridium source. The propertiesof the iridium β-diketonate were found to vary with the natureof the β-diketonate group and the use of the thd led to aprecursor with higher volatilities than the Ir(acac)3 source. We

reasoned that the sterically hindered iridium β-diketonate Ir(thd)3 has increased volatility with respect to the Ir(acac)3. Theiridium thin films were successfully synthesized on glass sub-strates at 400 °C by the MOCVD method. Deposited films werefound to consist of islands grown on the glass substrate. Thegrowth mechanism follows the Volmer–Weber model.

Acknowledgements

This work was supported by the National Defence AviationFoundation of China under contract no 00G53066.

References

[1] Yoon J. Song, H.H. Kim, Sung Y. Lee, D.J. Jung, B.J. Koo, J.K. Lee, Y.S.Park, H.J. Cho, S.O. Park, Kinam Kim, Appl. Phys. Lett. 76 (2000) 451.

[2] Ju Cheol Shin, Cheol Seong Hwang, Hyeong Joon Kim, Appl. Phys. Lett.76 (2000) 1609.

[3] J.S. Cohan, Haojie Yuan, R.S. Williams, Appl. Phys. Lett. 60 (1992) 1402.[4] K. Nishio, T. Toshio, Sol. Energy Mater. 68 (2001) 279.[5] T. Pauporte, R. Durand, J. Appl. Electrochem. 30 (2000) 35.[6] K. Mumtaz, J. Echigoya, T. Hirai, J. Mater. Sci. Lett. 12 (1993) 1411.[7] J.P. Endle, Y.-M. Sun, N. Nguyen, Thin Solid Films 388 (2001) 126.[8] Philippe Serp, Roselyne Feurer, Philippe Kalck, Helder Gomes, Joaquin

Luis Faria, Jose Luis Figueiredo, Chem. Vap. Depos. 7 (2001) 59.[9] J.P. Endle, Y.-M. Sun, N. Nguyen, Thin Solid Films 388 (2001) 126.[10] Y.-M. Sun, J.P. Endle, K. Smith, Thin Solid Films 346 (1999) 100.