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International Journal of Refractory Metals & Hard Materials 27 (2009) 33–36

Thermal stability of iridium coating prepared by MOCVD

Wenbin Yang, Litong Zhang *, Yunfeng Hua, Laifei Cheng

National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China

Received 8 October 2007; accepted 20 February 2008

Abstract

A multilayer Ir coating with equiaxed grains was prepared using metalorganic chemical vapor deposition on Nb substrates by thermaldecomposition of Ir(acac)3 precursor. Thermal stability of the coating was investigated at 1200 �C and 1300 �C in Ar. The Ir coatingexhibited different thermal stabilities depending on heating temperatures. After heat treatment, the prepared nanometer and sub-microm-eter grains grow obviously and grain size increased with rising heating temperature. Pores diffused outward and their amount reduced.After heat treatment, the coating gradually transformed into a compact layer. Nb diffused into Ir coating during heat treatment, andsolid solutions were formed at 1300 �C.� 2008 Published by Elsevier Ltd.

Keywords: MOCVD; Ir coating; Multilayer structure; Thermal stability; Diffusion; Nb

1. Introduction

Iridium (Ir) coatings are considered as ideal electrodematerials and oxidation resistance materials for futuremicroelectronic and ultra-high temperature structuraldevices [1,2]. Their preparation has received great concern.Magnetron sputtering, electro-chemical deposition andmetalorganic chemical vapor deposition (MOCVD) pro-cesses have been successfully employed to prepare Ir coat-ings [1,3–7]. The application of Ir coatings depends on theirthermal stability. The thermal stabilities of Ir coatings pre-pared by electrodepositing and magnetron sputtering havebeen studied, which depend on the grain structure and size[8,9]. It is well known that the Ir coatings prepared byMOCVD consist of nanometer or/and sub-micrometergrains [2,6,10–12]. Compared with the coatings preparedby electrodepositing and magnetron sputtering, the coat-ings prepared by MOCVD have smaller grain size and dif-ferent grain structure. However, little information isavailable in literature on the thermal stability of Ir coatings

0263-4368/$ - see front matter � 2008 Published by Elsevier Ltd.

doi:10.1016/j.ijrmhm.2008.02.001

* Corresponding author. Tel.: +86 29 88486068x828; fax: +86 2988494620.

E-mail address: Zhanglt@nwpu.edu.cn (L. Zhang).

prepared by MOCVD. In the present work, a multilayer Ircoating with equiaxed structure was prepared on Nb sub-strates by chemical vapor deposition with Ir(acac)3 precur-sor, and its thermal stability in 1200 �C and 1300 �C inargon was investigated.

2. Experimental

A multilayer Ir coating was prepared in a vertical, hot-wall MOCVD reactor. The Ir(acac)3 precursors was refilledfor five times through a pulling-drawing device. The pre-cursors was contained in a stainless steel crucible and trans-ported through vapor diffusion from high to low pressure.The reactor was cleaned and subjected to several cycles ofvacuum followed by Ar pressurization. The chamber pres-sure was maintained between 0.1 Torr and 2 Torr duringdeposition. Nb substrates were placed into the low pressurezone of the reactor. The typical growth conditions of eachlayer are listed in Table 1. Heat treatment was performed at1200 �C and 1300 �C for 30 min in Ar atmosphere, respec-tively, to investigate the thermal stability of the coating.

Surface and cross-section micro-morphologies of themultilayer Ir coating before and after heat treatment wereobserved with a scanning electron microscope (SEM,

Table 1Growth conditions for the five-layer Ir coatinga

Ir layer (close to Nb substrates) Process parameters

TP (�C) TD (�C) TT (min)

The first and the second 210 470 30The third and the forth 185 470 30The fifth 185 420 30

a TP means the volatilization temperature of Ir(acac)3; TD means thedeposition temperature; TT means the deposition time.

34 W. Yang et al. / International Journal of Refractory Metals & Hard Materials 27 (2009) 33–36

JSM-6700F). The composition of the coating was analyzedby energy dispersive spectra (EDS). An energy dispersiveX-ray spectrum (EDS, EDXA) line scan was performedto identify the composition of the coating. Phase identifica-tion and crystallization evaluation was obtained with an X-ray diffraction device (XRD, Rigaku D/MAX-2400 withCuKa radiation) in grazing incidence (GI) mode.

Fig. 1. Cross-section of as-deposited five-layered Ir coating on Nbsubstrate.

Fig. 2. Cross-section of five-layered Ir coating on Nb substrate heat treatedscanning analysis of EDS of the cross-section.)

3. Results and discussion

Fig. 1 shows the cross-section micro-morphology of thefive-layered coating deposited on Nb substrate. There weredistinct interfaces from the second to the fifth Ir layer. Theinterface between the first and the second layer was not dis-tinct. The first two layers consisted of dense packing spher-ical crystals with grain size of about sub-micrometer. Thethird and the forth layers were ideally compact polycrystal-line which showed a grain size of nanometer [12]. The out-most layer was the thinnest one among them, which mightbe associated with the lower growth rate at lower deposi-tion temperature [1,6,13,14]. EDS analysis has not foundany carbon and oxygen impurities in coating.

Fig. 2 shows the cross-section morphology of multilayercoating heat treated at 1200 �C and 1300 �C for 30 min inAr, respectively. Fig. 2a shows that the first two layers werenot stable at 1200 �C and recrystallized into one layer(defined as layer I). In layer I, the grain size increased toabout 0.3 lm. Layer I contained little amount of sphericalpores with diameter of 0.1 lm, and pores at the triple-junc-tion pockets of nanometer grains have not affected therecrystallization behavior significantly. The third and theforth Ir layer (defined as layer II) were relatively thermalstable at 1200 �C, their microstructure did not changemuch. While, the outmost Ir layer (defined as layer III)was not stable at 1200 �C and recrystallized into a uniformand compact layer.

Fig. 2b shows that all layers are not stable at 1300 �C.The multilayered coating recrystallized into one layerthrough atom diffusion. Compared with structure thatheated at 1200 �C, the typical structures of layer I, II andIII did not changed completely, but their scales were differ-ent, and the interfaces were fuzzed. It can be seen fromFig. 2b that layer I devoured Ir atoms from layer II

at 1200 �C (a) and 1300 �C (b) for 30 min. (Beneath the figure is the line

Table 2The positions and FWHM values of Ir peaks (111)

As received 1200 �C 1300 �C

Position 40.69 40.63 41.07FWHM value (�2Th.) 0.3936 0.2460 0.1948

Table 3The positions and FWHM values of Nb peaks (200)

As received 1200 �C 1300 �C

Position 55.49 55.49 55.28FWHM value (�2Th.) 0.2460 0.162 0.1299

W. Yang et al. / International Journal of Refractory Metals & Hard Materials 27 (2009) 33–36 35

through diffusion during recrystallization. The thickness oflayer I increased, vice verse that of layer II decreased.Simultaneously, Heat treatment resulted in Ir grain coars-ening by devouring the neighboring fine grains to exhibitstability towards normal grain growth [9]. The grain sizeof layer I increased to about 0.7 lm. The amount of poresdecreased, but their diameter increased to about 0.2 lmdue to the coalescence of smaller pores into large pores.It is also shown that pores diffused outward from therecrystallized coating.

Fig. 2 also shows the EDS line scanning analysis of thecross-section of the five-layered coating heat treated at1200 �C and 1300 �C, respectively. The EDS line profileshows that the Nb substrate diffused into Ir coating whentreated at 1200 �C and 1300 �C. While, the diffusion at1200 �C was weak and only reach the interface of layer Iand layer II. The diffusion of Nb into Ir at 1300 �C wasmuch more intensive than that at 1200 �C and almostreached the interface of layer II and layer III, seen inFig. 2b. It seems that the content of Nb in Ir coating fromNb–Ir interface to Ir surface after heat treated at 1200 �Cand 1300 �C all showed abrupt drops, and the drops alwayshappened near interfaces. It can be deduced that interfaceswere formed at the beginning and ending of depositions,the unstable depositing conditions, especially the degreeof supersaturation of the precursor, resulted in differentcompositions and microstructures, which made the inter-faces act as threshold for the diffusion of Nb. It was diffi-cult to evaluate the effect of Nb diffusion on therecrystallization of Ir coatings.

Fig. 3 shows the XRD patterns of the multilayer Ir coat-ing before and after heat treatments. It is seen that afterheat treatment, no signals corresponding to Ir–Nb solidsolutions and compounds are detected except Ir and Nb.The diffraction peak of Ir coatings heat treated at

(a)

(b)

(c)

25 35 45 55 65 75

Ir (

111)

Ir (

200)

Nb

(200

)

Ir (

220)

2θ ( ˚ )

Fig. 3. XRD patterns of the multilayer Ir coating on Nb substrate. (a) Asreceived, (b) heat treated at 1200 �C for 30 min, and (c) heat treated at1300 �C for 30 min.

1200 �C shows no shift, seen in Table 2, but there is a shiftof 0.6� towards lower direction after heat treated at1300 �C. Like the Ir peak, the diffraction peak of Nb showsno shift after heat treated at 1200 �C, seen in Table 3, whilethere was a shift of 0.2� toward lower direction after heattreated at 1300 �C. The shift of diffraction peaks werealways due to the distortion of lattice caused by mutual dif-fusion. According to the Ir–Nb phase diagram, the solidsolution of Nb into Ir can reach about 12 at.% at1600 �C [15]. It can be deduced that the solid solution ofNb into Ir at 1200 �C and 1300 �C is possible. So, the shiftsof diffraction peaks in Fig. 3 were due to the formation ofsolid solutions starts at 1300 �C. From the XRD result,solid solution was not formed at 1200 �C, maybe the tem-perature and the time is no enough.

Seen from Tables 2 and 3, both peaks of Ir and Nbbecame narrower for specimens heated at higher tempera-ture, which indicating recrystallization and grain growthin the coatings.

4. Conclusions

Five-layered Ir coating was prepared by MOCVD usingIr(acac)3 as precursor. The coating consisted of nanometerand sub-micrometer equiaxed grains. There were deposi-tion interfaces among Ir layers.

The thermal stability of each layer depended on thegrain structure and size. The layers with nanometer struc-ture were stable at 1200 �C. All layers were not stable,and the interfaces were annihilated and the multilayer coat-ings began to recrystallize into one compact structure at1300 �C. Pores diffused outward and their amount wasreduced. Grain size increased with rising heating tempera-ture. Cracks were not observed on the coating surfacebefore and after heat treatment. Ir coatings diffused mutu-ally with Nb substrates at 1200 �C, and formed solid solu-tions at 1300 �C during the recrystallization of nanometerstructure.

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