preparation & characterization of ruo2 thin films from ru(co)2(tmhd)2 by metalorganic cvd

7/16/2019 Preparation & Characterization of RuO2 Thin Films From Ru(CO)2(Tmhd)2 by Metalorganic CVD

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Thin Solid Films 413 (2002) 85–91

0040-6090/02/$ - see front matter ᮊ 2002 Elsevier Science B.V. All rights reserved.PII: S 0 040- 6090 Ž0 2.0 0 3 4 3 - 7

Preparation and characterization of RuO thin films from2

Ru(CO) (tmhd) by metalorganic chemical vapor deposition2 2

Reui-san Chen , Ying-sheng Huang *, Yao-lun Chen , Yun Chia a , b b

Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC a

Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b

Received 3 December 2001; received in revised form 22 March 2002; accepted 27 March 2002

Abstract

A new metalorganic ruthenium compound which contained two b-diketonate and two CO ligands arranged in cis-dispositionwas used in preparation of high quality ruthenium dioxide (RuO ) thin films by cold-wall metalorganic chemical vapor deposition.2

A detailed characterization of the films including scanning electron microscopy (SEM), electrical resistivity, Raman scatteringand X-ray diffraction measurements were carried out. The surface morphology of the films was investigated by SEM, from whicha columnar growth pattern was observed using a cross-sectional scanning electron micrograph analysis. The resistivity measurementshows a metallic conducting characteristic, while Raman study indicates the formation of a high quality, nearly stress-free RuO2

film. In addition, changes of structural and electrical properties after thermal annealing are discussed.ᮊ 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ruthenium dioxide; Chemical vapor deposition; Thin film; Scanning electron microscopy; Resistivity; Raman scattering; X-raydiffraction

1. Introduction

In recent years, the physical properties of rutheniumdioxide (RuO ) have been extensively investigated for2

various applications w1– 8x. Its high electrical conductiv-ity, high thermal and chemical stability, low etchingdamage and effective inter-diffusion barrier have con-tributed to the uses in very large-scale integration w1–3x thick film resistors w4– 6x, a buffer layer for YBCOsuperconducting film w7x and an electrode material inferroelectric random-access memory w8x.

High quality RuO thin films have been prepared2

from several techniques, such as metalorganic chemicalvapor deposition (MOCVD) w9–11x, reactive sputteringw12–14x, molecular beam epitaxy w15x, pulsed laserdeposition w16x and solution growth techniques w17x. Inthis work, MOCVD was adopted for the growth of required RuO thin films because of its good step2

coverage ability and popular usage in the fabrication of integrated circuits.

*Corresponding author. Tel.: q886-2-27376424. E-mail address: [email protected] (Y.-s. Huang).

In order to improve the thin film quality of theMOCVD processes, a low melting, highly volatile andrelatively stable source reagent is demanded. Variousorganometallic ruthenium compounds, such asRu(acac) , acacsacetylacetonate, Ru(C H ) ,3 5 5 2

Ru (CO) w9x and Ru(OD) w10x, ODs2,4-octanedion-3 12 3

ate, etc, were used as the CVD precursors in the pastseveral years. Although some of these source reagentsare either liquid at ambient or consistent of relativelylow-melting solids, most of them either need excessivehigh temperature for vaporization or are thermally and

chemically unstable under typical CVD processing con-ditions, making them less suitable to serve as the CVDsource reagents. Recently, a new metalorganic com-pound, Ru(CO) (tmhd) , tmhds2,2,6,6-tetramethyl-2 2

3,5-heptanedionate, which possesses the desired physicalproperties of good source reagents, was prepared andhad been successfully used in depositing pure Ru thinfilms on Si substrates w18x.

In this article, we report the preparation of RuO thin2

films on Si substrates using this newly synthesizedprecursor Ru(CO) (tmhd) by cold-wall MOCVD meth-2 2

od. A detailed characterization of the films including



86 R.-s. Chen et al. / Thin Solid Films 413 (2002) 85–91

Fig. 1. Schematic diagram of the cold-wall MOCVD apparatus.

scanning electron microscopy (SEM), electrical resistiv-ity, Raman scattering and X-ray diffraction (XRD)

measurements were carried out. Their surface morphol-ogy as well as the compositional, electrical and opticalproperties for the as-deposited and annealed thin filmsare studied. In addition, the changes of structural andphysical properties upon thermal annealing under oxy-gen ambient (1 atm, 450 8C, 12 h) are discussed.

2. Experimental

2.1. Deposition of RuO films2

Cold-wall MOCVD is chosen to prepare the thin filmsamples. A schematic diagram of the cold-wall MOCVDsystem is shown in Fig. 1. There are two different flowpaths for oxygen carrier gas, connecting to the growthchamber. The first one is a by-pass flow path, which is

designed for controlling the chamber pressure, while thesecond is heated to the designated temperature and isused for conveying the source vapor to the growthchamber. Two independent temperature controllers arealso mounted to the system, monitoring the temperatureof the heated source path and the precursor container.Both the precursor container and the heated path arekept at a constant temperature of 100 8C to avoid thecondensation of the precursor during gas-phase trans-port. Oxygen flow rate and system pressure of CVDchamber are adjusted to 100 sccm and 4.5 torr,respectively.

Single crystal Si (1 0 0) wafer was chosen as the

substrate. A thin layer of indium metal was applied tothe back of the substrate, which would provide anexcellent physical contact to substrate heater and wouldgive a low temperature gradient from heater to siliconsubstrate. Dark blue RuO film was slowly observed on2

the substrate surfaces during CVD runs, and the exper-iments were stopped after approximately 4 h. For thedepositions conducted at 250–450 8C, the observed filmthickness and the calculated growth rate was in therange of 100–1000 nm and 25–250 nm h , respective-y1

ly. The thermal annealing was performed in O environ-2

ment at atmospheric pressure for 12 h.

2.2. Characterization of the films

A Hitachi S-4000 SEM system was used to observethe morphology and the thickness of the films grownunder different substrate temperatures. The crystallinityof the thin film samples was analyzed using XRDanalysis. Electrical conductivity measurements weremade on films by the four-point probes method using aKeithley model 182 nanovoltmeter, a Keithley model617 electrometer and a Keithley model 220 constantcurrent source at the temperature range 16–300 K. Theresidual resistivity ratio (RRR), which is typicallydefined as r(300 K)yr(4.2 K), is now changed tor(300 K)yr(16 K) in our study. This data would provideinformation on the electron mean free path and isextremely useful for the assessment of average grainsize of the thin film samples. A similar set of experi-ments was also repeated for all thin film samples afterheating at 450 8C under oxygen ambient at 1 atm for

12 h to measure the effect of thermal annealing.Raman measurements were recorded at room temper-

ature utilizing the back scattering mode on a RenishawRaman Microscope System 2000. The excitation laserbeam was focused onto the sample with an opticalmicroscope. The Raman spectra were recorded with anAr-ion laser excitation at 514.5 nm. The optical penetra-tion depth of the laser was estimated to be 40 nm forthe RuO sample w19x and the diameter of the circular2

light spot was approximately 200 mm. A charge coupledevice (CCD) camera was used to pick up the resultingRaman scattering signals. The resolution of the spec-

trometer was turned to 2 cm .

y1

3. Results and discussion

3.1. Surface morphology

A series of SEM images of RuO thin films with2

different substrate temperatures T taken from the tops

and a tilt angle 458 are presented in Fig. 2. Well-definedcubic and columnar RuO grains for all the thin film2

samples are observed. This observation suggests that thegrain growth process is controlled by the surface reac-tion, not by the mass-transported controlled process. It

is evident that the average grain size increases from 60to 300 nm with increasing the substrate temperaturefrom 250 to 400 8C. The average grain size observed inthis work is slightly larger than the data reported byLee et al. w10x, and the uniformity of grain size and thesurface coverage are also better than that reported byLiao et al. w11x. The comparison of the grain size of theas-deposited RuO films between this work and the2

other literature data are listed in Table 1. Moreover,from the surface morphology as shown in Fig. 2, thesample obtained at the highest temperature 450 8C showsa reduction in the grain size and appreciable coalescence




Fig. 2. SEM images of surface morphology of RuO thin films deposited at different substrate temperatures T taken from the top and a tilt angle2 s

458, respectively.




Table 1The comparison of the as-deposited RuO films between this work and the other literature data2

This work Green et al. w9x Lee et al. w10x Liao et al. w11x

Precursor Ru(CO) (tmhd)2 2 Ru(C H O ) yRu(C H )5 7 2 3 5 5 2 Ru(OD)3 Ru(C H )5 5 2

Substrate Si (1 0 0) Si (1 0 0) Si SiPrecursor temperature (8C) 100 – 160 85Deposition temperature (

8C) 250–450 600y575 250–550 500–550

Deposition pressure (torr) 4.5 1y5 1 –Growth rate (nm h )y1 25–250 – 120–600 600Grain size (nm) 60–300 10–50y500 50–120 6470Resistivity (mV cm) 45–210 643y89.9 48–120 60RRR 1.3–3.9 – – 6FWHM of E mode (cm )y1

g 12.6–14.8 – – 14

Fig. 3. Resistivity as a function of temperature for RuO films depos-2

ited at different substrate temperatures T .s

Fig. 4. Room temperature resistivity of the as-deposited and theannealed RuO films, showing a variation according to the substrate2

temperatures.

between grains, as compared with that at the lowertemperature T s400 8C. It is believed that the three-s

dimensional grain growth is controlled through a properchoice of substrate temperature. Higher temperaturewould provide a greater kinetic energy to surpass surface

migration and lattice incorporation on substrate duringcrystal growth processes. However, if the substratetemperature were getting too high, the excessive kineticenergy would increase the rate of surface reaction andproduce severe diffusion among the lattices and causingthe formation of unwanted lattice defects. Thus, thepacking of the grain structure could be severely dam-aged. As a result, for the sample prepared at the highesttemperature of 450 8C, this improperly selected substratetemperature may be the principle cause for the reductionand coalescence of the grain structures.

Two more deposition experiments were also carriedout at the temperatures of 200 and 500 8C, respectively.

At 200 8C, no effective RuO deposition was observed,2

for which the lower kinetic energy was insufficient to

trigger the activation process. As a result, most of thesource reagent remained intact and can be recovered atthe exit of our CVD system. On the other hand, theRuO thin film was also not observed for the experi-2

ments carried out at and above 500 8C. This could be

due to the formation of volatile RuO at this excessive4high temperature, although it synthesis was typicallycarried out at 700–900 8C under O atmosphere w20,21x.2

3.2. Electrical resistivity

Fig. 3 shows the resistivity as a function of tempera-ture for the RuO thin films deposited at the different2

substrate temperatures. The resistivity data obtained areconsistent with the behaviors of metallic conductors,where the lowest resistivity was obtained at room tem-perature with respect to those obtained at the highertemperature. Moreover, a decrease of room temperature

resistivity with an increase in substrate temperature wasobserved, and these results are shown in Fig. 4 togetherwith those for the annealed samples. The optimal room




Fig. 5. RRRs of the as-deposited and the annealed RuO films, show-2

ing a variation according to the substrate temperatures.

Fig. 6. Raman spectra of the RuO thin films prepared by MOCVD2

at different substrate temperatures T .s

temperature resistivity of the as-deposited RuO films is2

approximately 45"5 mV cm. This value is compatiblewith the result of Lee et al. w10x using the liquidprecursor (Ru(OD) ), and is slightly lower than that of 3

60 mV cm made by Liao et al. w11x and the data of 89.9 mV cm by Green et al. w9x using commerciallyavailable Ru(C H ) . A large jump in resistivity between5 5 2

300 and 350 8C is observed. The occurrence is quiteconsistent with the SEM images as shown in Fig. 2where the grain size of the RuO shows a noticeable2

increase as the deposition temperature is increased from300 to 350 8C.

The RRR was applied to investigate the influences of grain boundary on the resistivity of the thin film sam-ples. Mar et al. w22x have calculated the scatteringcoefficient of the RuO thin films using the grain2

boundary scattering model w23x. They have concludedthat the scattering coefficient S s0.785 is much higherthan that of the metallic conductors such as Cu and Al.Their investigation suggests that total resistivity of theRuO thin films is solely determined by electron scat-2

tering at the grain boundary. The RRRs as a function of

substrate temperature are depicted in Fig. 5, showing anincrease from 1.3 to 3.9 between the temperature range250–400 8C, and seem to flatten out at approximately400 8C. Generally speaking, the RRR values are pro-portional to the sizes of the crystallites that depositedon the substrate, and the greater RRR value implies theformation of larger crystallites and better contactbetween grains. This result agrees with the SEM pho-tographs where the largest RuO particulates were2

observed for the thin film deposited at 400 8C. Basedon these observations, we may conclude that grainboundary scattering is a major factor of carrier transport,

which has been documented in previous literaturesw14,22x.

3.3. Raman scattering

The first-order Raman spectra of the as-deposited andannealed RuO thin films deposited at different temper-2

ature are displayed in Fig. 6. Three major Raman modes,namely the E , A and B modes are observed atg 1g 2g

vicinity of 528, 646 and 716 cm , which are consistenty1

with the previous assignments w24x. From the curvefitting analysis, the peak positions and the full-width athalf maximum (FWHM) of these Raman signals can beaccurately determined. The FWHMs are found to varyas a function of substrate temperature, among which theFWHM of the E mode vs. the substrate temperature isg

plotted in Fig. 7. The optimal value of FWHM (;12cm ) is only slightly greater than that observed for they1

RuO single crystal (11 cm ) w25x, showing the for-y12

mation of a higher quality, nearly stress-free RuO film2

on Si substrate. Moreover, an obvious decrease of FWHM of the E modes was observed after heating theg

as-deposited thin films under oxygen ambient at 4508C. This result indicates that the micro-structure of theRuO films could be further improved through a thermal2

annealing process.According to the previous discussion on the results

of SEM and resistivity measurements, the grain sizeincreases with increasing substrate temperature. Thismeans that the coherence length, which is correlated tothe grain size will also be improved by the substrate

temperature. The FWHM of all Raman modes shouldbe lowered w25x. However, this behavior is not observed




Fig. 7. FWHM of the E mode for RuO thin films obtained at dif-g 2

ferent substrate temperatures.

Fig. 8. XRD patterns for RuO thin films obtained at different sub-2

strate temperatures T .s

in our Raman scattering measurements. This may resultfrom the relatively shallow probing depth of laser beam,which is approximately 40 nm for the RuO films.2

3.4. X-ray diffraction

The XRD patterns of the as-deposited RuO thin films2

are shown in Fig. 8. The XRD measurements indicatethat the most preferred orientations are (1 1 0), (1 0 1)

and (2 1 1) planes. The tetragonal rutile RuO structure2

is confirmed using the main diffraction peaks of the

RuO standard sample. In addition, two broad diffraction2

signals were located in the 2u ranges of 21–238 and

32–348. These extra diffraction signals were alsoobserved by simply heating the blanked Si substrate inair at 150 8C, suggesting that they are formed by oxygenincorporation into the Si substrate surface.

4. Conclusion

High quality RuO thin films were prepared from a2

new precursor Ru(CO) (tmhd) by cold-wall MOCVD2 2

method. A detailed characterization of the films wascarried out. Well-defined polycrystalline RuO grains2

and large variation of grain sizes are observed from theSEM analysis. The electrical conductivity study showslower resistivities for the films deposited at the temper-ature 250–450 8C. According to the result of our RRRanalysis, the dominant factor that influences the resistiv-ity is determined by the electron scattering at the grainboundary. The Raman scattering measurements indicatethat the micro-structure of the films could be improved

through a thermal annealing process under O atmos-2

phere at 450 8C. Finally, XRD patterns reveal theformation of tetragonal rutile structure for the RuO thin2

films.

Acknowledgments

R.S. Chen and Y.S. Huang wish to acknowledge thesupport of the National Science Council of TaiwanR.O.C. under projects: NSC 89-CPC-7-011-009 andNSC 90-2112-M-011-001. Y. Chi thanks the NationalSciences Council for support of the work (NSC 90-

2113-M007-051).

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preparation & characterization of ruo2 thin films from ru(co)2(tmhd)2 by metalorganic cvd

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