Growth of chemical vapor deposition aluminum titanate films at different CO 2 / H 2 andaluminum butoxide inputsDong-Hau Kuo and Cheng-Nan Shueh Citation: Journal of Vacuum Science & Technology A 22, 151 (2004); doi: 10.1116/1.1632918 View online: http://dx.doi.org/10.1116/1.1632918 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/22/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Engineering titanium and aluminum oxide composites using atomic layer deposition J. Appl. Phys. 110, 123514 (2011); 10.1063/1.3667134 Growth process and properties of silicon nitride deposited by hot-wire chemical vapor deposition J. Appl. Phys. 93, 2618 (2003); 10.1063/1.1542658 Plasma-enhanced chemical-vapor deposition of titanium aluminum carbonitride/amorphous-carbonnanocomposite thin films J. Vac. Sci. Technol. A 20, 87 (2002); 10.1116/1.1424271 Process-dependent thermal transport properties of silicon-dioxide films deposited using low-pressure chemicalvapor deposition J. Appl. Phys. 85, 7130 (1999); 10.1063/1.370523 Properties of amorphous and crystalline Ta 2 O 5 thin films deposited on Si from a Ta(OC 2 H 5 ) 5 precursor J. Appl. Phys. 83, 4823 (1998); 10.1063/1.367277

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Growth of chemical vapor deposition aluminum titanate films at differentCO2 ÕH2 and aluminum butoxide inputs

Dong-Hau Kuoa) and Cheng-Nan ShuehDepartment of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien,Taiwan, Republic of China

~Received 10 December 2002; accepted 31 March 2003; published 8 January 2004!

Amorphous aluminum titanate films are prepared on silicon substrates by low-pressure chemicalvapor deposition ~CVD! using a mixture of aluminum tri-sec-butoxide~ATSB!, titaniumtetrachloride (TiCl4), CO2, and H2 as the reactants~the ATSB/TiCl4 /CO2 /H2 system!. The effectsof the CO2 /H2 and ATSB inputs and substrate temperature on the growth, microstructure, andcomposition of the CVD Al2O3– TiO2 films are discussed. The films have an increased growth rateand an increased Ti content at lower temperatures. The adsorption-controlled reaction is identified,which is attributed to the gas/solid reaction to weaken the film/substrate interface. The growth ratesare also higher at higher H2 and ATSB flows. The film thickness is 0.47–1.13mm for theCO2 /H2-varying system and of 0.34–1.37mm for the ATSB-varying system at depositiontemperatures of 350–500 °C. The proposed reactions are presented to explain the film growth. Thedetermined adsorption energy can explain the effect of temperature on composition. ©2004American Vacuum Society.@DOI: 10.1116/1.1632918#

I. INTRODUCTION

Aluminum oxide ~alumina, Al2O3) as an insulator hasseveral advantages for semiconductor device applications.1

Alumina films are a better barrier to mobile ionic species andhave high chemical stability and radiation resistance.2,3 Italso has dielectric constant twofold higher than silica. More-over, alumina films can be used for passivation of bipolardevices, as a diffusion mask, and as a buffer layer in silicon-on-insulator devices for three-dimensional integratedcircuits.4 Some precursors for metal–organic chemical vapordeposition ~MOCVD! alumina have been reviewed byRees.1,5–9

Aluminum tri-sec-butoxide, @C2H5CH(CH3)O#3Al,ATSB, is an inexpensive and safe precursor for the aluminadeposition. Both Haanappelet al.5–7 and Haet al.8 have uti-lized ATSB to deposit alumina films in a hot-wall MOCVDreactor, as protective coatings on AISI 304 and as inorganicmembranes on porous Vycor glass, respectively. Kuoet al.have deposited MOCVD alumina films in a cold-wall reactorby using ATSB as the precursor.9

Titanium oxide~titania, TiO2), with high refractive index,excellent transmittance in the visible-wavelength region, andhigh chemical stability, has been used in antireflection coat-ing, sensors, and photocatalysis. Recently, TiO2 films haveattracted attention for use in fabricating capacitors in dy-namic random access memory and gate oxide in transistors,due to their higher dielectric constants up to 100,10,11 but theproblem of leakage current needs to be solved. Atomic layerdeposition of TiO2 is developed by using TiCl4 /O2 ~Ref. 12!and TiCl4 /H2O ~Refs. 13 and 14! systems below 500 °C.Chemical vapor deposition~CVD! of TiO and Ti2O3 ob-tained from TiCl4 /H2 /CO2 gas mixtures at 1000 °C have

been reported.15 The research of TiCl4-containing CVD re-actions at lower deposition temperatures under the coexist-ence of CO2 and H2 draws our interest.

Composite films have been reported to modify film prop-erties, e.g., structure~crystalline versus amorphous!, density,porosity, thermal diffusivity, thin-film stress, optical scatter,etc.16 To reduce the leakage current, sputtered Zr–Si–O hasbeen suggested as an alternate gate dielectric.17 SputteredAl–Ti–O films deposited from a 10/90 (Al2O3 /TiO2) targetdisplay better dielectric properties and higher resistivities, ascompared with those of pure TiO2 .18

In this study, CVD Al2O3-TiO2 composite films are de-posited at temperatures below 500 °C by reacting ATSB andTiCl4 with different flows of CO2 and H2 instead of H2O.The purposes of this research are to investigate the deposi-tion of mixed oxides with precursors of metal halide andalkoxide and to study the effects of oxidizing conditions ondeposition by changing the CO2 /H2 inputs. The effects ofprocess parameters~substrate temperature, CO2 /H2 input,and ATSB amount! on growth rate, surface morphology, andfilm composition are studied experimentally and explainedby growth reactions involving CO2 and H2 and desorptionenergy.

II. EXPERIMENT

Aluminum titanate~alumina–titania, Al2O3– TiO2) filmswere prepared in a vertical cold-wall CVD reactor. The sche-matic setup of the CVD reactor for the Al2O3– TiO2 depos-iting system is shown in Fig. 1. The substrate was placed onthe heater with reactive gases injected from the aboveshower. The deposition parameters for the alumina–titaniafilms are summarized in Table I. Argon flowed in three lines:one for ATSB, one for TiCl4 , and one for keeping the totalflow rate constant. Flow rates of ATSB and TiCl4 were con-trolled by adjusting the amount of argon~Ar! carrier througha!Electronic mail: dhkuo@mail.ndhu.edu.tw

151 151J. Vac. Sci. Technol. A 22 „1…, Jan ÕFeb 2004 0734-2101Õ2004Õ22„1…Õ151Õ7Õ$19.00 ©2004 American Vacuum Society

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evaporators of 138 and 27 °C, respectively. The flow rates ofAr carrier for ATSB and TiCl4 were expressed as Ar~ATSB!and Ar(TiCl4) flow rates, respectively. Both of Ar~ATSB!and At(TiCl4) were kept at 50 sccm for studying theCO2 /H2 effect, while V(CO2)/V(H2) remained 50/50 forthe ATSB effect. To avoid the condensation of ATSB, all gaslines were heated to 155 °C. The pressures of the ATSBevaporator and chamber were kept at 1 Torr during deposi-tion, and 760 Torr for TiCl4 . CO2 and H2 were used to adjustthe oxidization conditions. The flow rates of CO2 and H2

were expressed as V(CO2) and V(H2), respectively. Ar,CO2, and H2 flow rates were measured with calibrated mass-flow controllers. The~100!-oriented silicon wafer was thechosen substrate, which was ultrasonic cleaned in acetone,washed with distilled water, and dried before deposition.

Film thickness was measured three times for each sampleto obtain average values by a Tencor P-1 Profiler~MountainView, CA!. Crystal structures were analyzed using an x-raydiffractometer~XRD, Bruker D8, Germany!. High-resolutionfield-emission scanning electron microscopy~HR FESEM,Hitachi S-4100, Japan! was used to observe the film mor-phology, while scanning electron microscopy~SEM! ~Hita-

chi S-3500H! equipped with energy dispersive spectroscopywas used to analyze the composition of the films.

III. RESULTS

A. Growth behavior of aluminum titanate films

The effect of temperature on the growth rate of aluminumtitanate films at different ratios of flow rates of CO2 and H2

is shown in Fig. 2~a!. The growth rate or film thickness de-creased with the increase in deposition temperature and theV(CO2)/V(H2) ratio, indicating an adsorption-controlled re-action. The data were fitted by least-mean-square regressionanalysis with an exponential equation in the form ofexp@Q/R(T1273)#, whereQ represents adsorption energy,Ris the gas constant, andT is the substrate temperature inCelsius. Growth rates decreased from 1.13 to 0.80, 1.05 to0.63, and 1.02 to 0.47mm/h for V(CO2)/V(H2) ratios of10/90, 50/50, and 90/10, respectively, as the deposition tem-perature increased from 350 to 500 °C. Figure 2~b! displaysthe variation of the growth rate with substrate temperature atdifferent Ar~ATSB! flow rates of 25, 50, and 75 sccm.Growth rates decreased from 1.37 to 0.93, 1.05 to 0.63, and0.88 to 0.34mm/h for Ar~ATSB! flow rates of 25, 50, and 75sccm, respectively, as the deposition temperature increasedfrom 350 to 500 °C.

Figure 3 shows the variations of apparent heat of adsorp-tion with ~a! the V(CO2)/V(H2) ratio and ~b! Ar~ATSB!flow rate. The apparent heat of adsorption increased withincreasing the V(CO2)/V(H2) ratio. They were 9.761.2,14.662.4, and 15.764.7 kJ/mol for V(CO2)/V(H2) ratios of

FIG. 1. Schematic setup of the CVD reactor for the Al2O3– TiO2 depositingsystem.

TABLE I. Deposition conditions for the CVD aluminum titanate filmsynthesis.

Substrate temperature,Tsub 350–500 °C

Chamber pressure,Ptotal 1 TorrATSB vaporizer pressure,PATSB 1 TorrATSB vaporizer temperature,TATSB 138 °CTiCl4 vaporizer pressure,PTiCl4

760 TorrTiCl4 vaporizer temperature,TTiCl4

27 °CAr flow rate of ATSB carrier, Ar~ATSB! flow rate 25, 50, 75 sccmAr flow rate of TiCl4 carrier, Ar(TiCl4) flow rate 50 sccmCO2 flow rate, V(CO2) 10–90 sccmH2 flow rate, V(H2) 10–90 sccmAr flow rate BalancedTotal flow rate 300 sccmDeposition time 1 h

FIG. 2. Effect of substrate temperature on growth rate of CVD aluminumtitanate films at different~a! V(CO2)/V(H2) ratios and~b! Ar~ATSB! flowrates.@Ptotal51 Torr, Vtotal5300 sccm,TTiCl4

527 °C, PTiCl451 atm, TATSB

5138 °C, PATSB51 Torr, Ar(TiCl4)550 sccm, Ar(ATSB)525– 75 sccm,VAr5remainder]. The data were fitted by the least-mean-square regressionanalysis by an exponential equation: exp@Q/R(T1273)#.

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10/90, 50/50, and 90/10, respectively. On the other hand, theapparent heat of adsorption decreased with the increase inthe Ar~ATSB! flow rate. They were 18.965.2, 14.662.4,and 10.562.5 kJ/mol for Ar~ATSB! flow rates of 25, 50, and75 sccm, respectively.

B. X-ray-determined structure and microstructuralobservations

The as-deposited aluminum titanate films formed in thetemperature range of 350–500 °C are amorphous, as deter-mined by XRD. Crystalline CVD–TiO2 films ~anatasephase! are obtained from the TiCl4 /CO2 /H2 system at350 °C.19 The difficulty in crystallization of aluminum titan-ate films can be attributed to the Al2O3 component, whichcannot crystallize at low deposition temperatures byMOCVD.20

HR FESEM micrographs of the CVD alumina–titaniafilms deposited at different V(CO2)/V(H2) ratios of ~a! 10/90, ~b! 50/50, and~b! 90/10 with substrate temperatures at350 and 500 °C are shown in Fig. 4. A smoother film wasobtained at 500 °C and a low V(CO2)/V(H2) ratio of 10/90.‘‘Splotchy’’ deposits, which consist of islands of depositsseparated by a nanosized area devoid of deposition, wereobserved at other process conditions. Substrate temperaturedid not have an apparent effect on microstructure. Similarmicrographs of the CVD alumina–titania films deposited atdifferent Ar~ATSB! flow rates also were observed by HRFESEM ~Fig. 5!. A smoother film was obtained at a higher

substrate temperature of 500 °C and a lower Ar~ATSB! flowrate of 25 sccm. ‘‘Splotchy’’ deposits with nanosized voidswere also observed.

C. Compositional analysis

The variations of the Al/(Al1Ti) ratio with depositiontemperature at different~a! V(CO2)/V(H2) ratios and~b!Ar~ATSB! flow rates are shown in Figs. 6~a! and 6~b!, re-spectively. The result demonstrated an apparent increase ofAl content with substrate temperature, ranging 0.49–0.74 fordifferent V(CO2)/V(H2) ratios, while varying 0.49–0.74 fordifferent Ar~ATSB! flow rates as the temperature increasedfrom 350 to 500 °C. The different CO2 /H2 and ATSB inputshad an effect on film composition. As compared with the Alcontent of 0.667 in a 1:1 stoichiometric Al2O3– TiO2 , thefilm compositions changed from an alumina-less content atlow temperatures to an alumina-rich one at high tempera-tures. A larger modification of composition can be executedby adjusting the substrate temperature and the Ar~ATSB!flow rate, while a smaller change can be obtained by varyingthe CO2 /H2 input.

IV. DISCUSSION

The growth rate and composition of Al2O3– TiO2 filmsdeposited by the ATSB/TiCl4 /CO2 /H2 system changed withthe CO2 /H2 input @Figs. 2~a! and 6~a!#. The films depositedwith the V(CO2)/V(H2) ratios of 10/90 and 90/10 showeddifferent growth rates and compositions, indicating CO2 /H2

takes part in the film growth. A growth mechanism is re-quired to explain the increases of growth rate and the Ticontent with decreasing the CO2 /H2 input. Conventionally,CO2 and H2 are added to form water vapor above 700 °C foroxidizing the metal chlorides for metal oxide in CVDreactions.21–24 The utilization of the CO2 /H2 combinationinstead of H2O is due to its moderate reaction and to avoidhomogeneous nucleation.25 However, the deposition in thisstudy was conducted below 500 °C, at which water vapor didnot form by reacting CO2 and H2.

In the TiN deposition using TiCl4 , H2 , and N2 at 1000–1273 K, the formation of TiCl3 was proposed by a gas-phasereaction:26

TiCl4~g!11/2H2~g!⇔TiCl3~g!1HCl~g! . ~1!

The occurrence of this gas-phase reaction was supported bythe thermodynamic equilibrium calculations. At equilibrium,TiCl3 becomes the most abundant titanium-containing spe-cies in the gas phase and the reaction~1! is enhanced at anincreased H2 flow. The gas-phase TiCl3 was adsorbed,TiCl3* , on the substrate to perform the TiN deposition.

The proposed reactions for forming aluminum titanatefilms are based on the formations of CVD TiN~Ref. 26! withthe TiCl4 /N2 /H2 system and MOCVD SiO2 from the ther-mal decomposition of tetraethyl orthosilicate~TEOS!.27,28

From the TiCl4 /N2 /H2 system, the formation of adsorbedhydroxyl-containing species is proposed below:

TiCl4~g!11/2H2~g!⇔TiCl3~g!1HCl~g! , ~1!

FIG. 3. Variation of the apparent heat of adsorption of CVD aluminumtitanate films with~a! V(CO2)/V(H2) ratio and~b! Ar~ATSB! flow rate.

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TiCl3~g!⇔TiCl3* , ~2!

TiCl3* 1CO2~g!⇔$Cl3TiuO2v

1CO%complex* ~3!

$Cl3TiuO2v

1CO%complex* 11/2H2~g!

⇔HO–TiCl3* 1CO~g! . ~4!

Combining Eqs. (1)1(2)1(3)1(4):

TiCl4~g!1H2~g!1CO2~g!⇔HO–TiCl3* 1CO~g!1HCl~g! ,~5!

TiCl4~g!1mH2~g!1mCO2~g!

⇔~HO!m– TiCl~42m!* 1mCO~g!1mHCl~g! , ~6!

wherem51, 2, 3, or 4.Similar to thermal decomposition of metal alkoxides via a

cyclic elimination mechanism, the formation of(HO)n– Al(O–R8)32n* is proposed below:

Al @O–C2H5CH~CH3!#3*

⇔~HO!n– Al~O–R8!32n* 1nC4H8~g! ,~7!

R8[C2H5CH~CH3!.

The depositing Al2O3– TiO2 films using theATSB/TiCl4 /CO2 /H2 system can be considered as the con-densation chain reactions between Al(O–R8)3* and(HO)m– TiCl(42m)* with a by-product of R8– OH or be-tween (HO)n– Al(O–R8)32n* and (HO)m– TiCl(42m)* witha by-product of H2O. From the proposed reactions, the effectof processing on composition of the Al2O3– TiO2 films isclear: higher H2 flow produces higher growth rate@Fig. 2~a!#and Ti content in films due to the formation of more Tiadsorbates.

Substrate temperature has presented a strong effect ongrowth rate. The initiation reaction~1! is an endothermicreaction, based on the thermodynamic calculation.29 Thermaldecomposition of ATSB@reaction ~7!# is accelerated athigher temperatures. The dissociation of Ti precursors andthermal decomposition of ATSB are favored at elevated tem-perature, therefore, the reaction rate should increase with theincrease in substrate temperature. However, the experimentalresults indicated a desorption-controlled reaction~Fig. 2!.Growth rate decreases due to much desorption instead ofdeposition at higher temperature, caused by the weak bond-ing to the substrate. The evidence of weak bonding is pro-

FIG. 4. SEM micrographs of the CVD Al2O3– TiO2

films deposited at different V(CO2)/V(H2) of ~a! 10/90, ~b! 50/50, and~c! 90/10 sccm with substrate tem-peratures at 350 and 500 °C.

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vided by the ‘‘splotchy’’ deposits on the substrate,30 as ob-served by HR FESEM in Fig. 4. The weak bonding isoriginated from the gas~ATSB and TiCl4)/solid ~reacted Aland Ti adsorbates! reactions and leads to the increased de-sorption at increased temperature. As higher temperatures fa-vor both reactions~1! and~7!, the higher Al content at highertemperatures indicate the easier desorption of Ti adsorbates,as compared with Al adsorbates.

The easier desorption of Ti adsorbates at higher tempera-tures can be revealed by comparing Figs. 3~a! and 6~a!. Fromthe experimental results shown in Fig. 3~a!, lower adsorptionenergy at a lower V(CO2)/V(H2) ratio corresponds to a Ti-rich film and the relatively easier desorption of Al adsor-bates. The relatively higher adsorption energy at a higherV(CO2)/V(H2) ratio corresponds to a higher Al content infilms and the relatively easier desorption of Ti adsorbates. Itis concluded that desorption of Ti adsorbates is relativelydifficult as compared with that of Al precursors. Therefore,desorption of Ti precursors is easier and Al-rich films can beobtained at higher temperatures. On the other hand, desorp-tion of Al precursors is easier and Ti-rich films are obtained

FIG. 5. SEM micrographs of the CVD Al2O3– TiO2

films deposited at different Ar~ATSB! flow rates of~a!25, ~b! 50, and~c! 75 sccm with substrate temperaturesat 350 and 500 °C.@Ptotal51 Torr, Vtotal5300 sccm,TTiCl4

527 °C, PTiCl451 atm, TATSB5138 °C, PATSB

51 Torr, Ar(TiCl4)550 sccm, Ar(ATSB)525– 75sccm,VAr5remainder].

FIG. 6. Composition dependence of the CVD Al2O3– TiO2 films on deposi-tion temperature at different~a! V(CO2)/V(H2) ratios and~b! Ar~ATSB!flow rates.

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at lower temperatures. This result explains the effect ofgrowth temperature on the film composition.

For considering the ATSB effect on the growth of CVDAl2O3– TiO2 films, it is found that the higher ATSB flowresults in higher growth rate@Fig. 2~b!# and Al content@Fig.6~b!#. The increased ATSB flow enhances the site occupationwith the ATSB adsorbates and the reaction probability withTi adsorbates. Therefore, the surface was covered with moreadsorbed Al and less Ti precursors at a higher ATSB flow,which results in a higher Al content in films. Furthermore,the Al content of the CVD Al2O3– TiO2 films increased only;20% as the amount of ATSB flows were doubled or tripled@Fig. 6~b!#. It is indicated that the more the ATSB is added,the more the ATSB adsorbates will desorb. This behavior canbe explained with Fig. 3~b!, where adsorption energy islower and more desorption occurs at higher ATSB flows.

The deposition of adsorption-controlled reactions also hasbeen observed for a plasma-enhanced CVD~PECVD! sys-tem containing oxidation-sensitive species such as hexam-ethyldisilazane@(CH3)3Si–NH–Si(CH3)3 , HMDSN# underthe condition of coexisting CO2 and H2 for silicon dioxidefilms, where Si–C and Si–N bonds are required to be oxi-dized. The apparent heats of adsorption are between 15 and25 kJ/mol.31 Comparing the ATSB/TiCl4 /CO2 /H2 andHMDSN/CO2 /H2 systems, both the oxidation-sensitive andreactive reactants of TiCl4 and HMDSN exist under theCO2 /H2 condition. The gas/solid reaction can originate fromthe attack of adsorbates by reactive species in gas or CO2 /H2

to pull them from the substrate, which results in weak bond-ing. By utilizing the less oxidation-sensitive precursor of tet-raethoxysilane to deposit PECVD SiO2 films, an adsorption-controlled reaction with a smaller desorption energy of 37.7~9!–54.4 kJ/mol~13 kcal/mol! was observed.32 Without theaid of plasma, the CVD reaction of the TEOS-derived SiO2

films is thermally activated. Plasma in CVD activates theformation of reactive radicals to enhance gas/solid reactions,which result in the adsorption-controlled process. The ex-perimental results in this study support the fact that gas/solidreactions occurring in CVD can lead to an adsorption-controlled instead of thermal-activated reaction.

H2 behaves as a catalyst in the formation of CVDAl2O3– TiO2 films deposited with the ATSB/TiCl4 /CO2 /H2

system. A catalyst is defined as an agent that facilitates achemical reaction without becoming incorporated in the re-action products. The idea of using a catalyst for CVD isnovel and has been reported by Klauset al.33 In this study,higher H2 flow can obtain higher growth rates and changecompositions. H2 initiates the formation of(HO)m– TiCl(42m)* for the subsequent condensation chainreactions, but it is not incorporated in films.

The CVD reactions for preparing the Al2O3– TiO2 filmsare attractive. Although a low temperature process by usingthe ATSB/TiCl4 /CO2 /H2 system has presented interestingkinetic behaviors, the deposits display a porous microstruc-ture and weak bonding with the substrate. Not for gate oxideand memory device, the mixed oxide films with nanosizedpores, instead, have a potential for sensor applications.

V. CONCLUSIONS

Al2O3– TiO2 films were successfully deposited by low-pressure chemical vapor deposition using aluminum tri-sec-butoxide, titanium tetrachloride, CO2, and H2 as reactants.Growth rates decreased from 1.13 to 0.80, 1.05 to 0.63, and1.02 to 0.47mm/h for V(CO2)/V(H2) ratios of 10/90, 50/50,and 90/10, respectively, as the deposition temperature in-creased from 350 to 500 °C. Higher ATSB flow also en-hanced growth rates. In this way, the adsorption-controlledreaction due to weak interface bonding is identified. The evi-dence of weak bonding is supported by the observation ofsplotchy deposits.

A growth mechanism was proposed to explain the effectof the CO2 /H2 input on film growth and properties. It ex-plains the higher growth rate and Ti content at the lowerCO2 /H2 input. Comparing the results of adsorption energyand film composition, it is concluded that desorption of Tiadsorbates is relatively difficult and favors occurring athigher temperatures. Higher ATSB flow results in highergrowth rate and Al content. The lower desorption energy athigher ATSB flow explains the low efficiency in Al incorpo-ration into films.

The weak interface bonding, which results in thedesorption-controlled reaction, originates from the gas/solidreaction. A pulling force produced during the reactions be-tween the reactive gas species and the adsorbates weakensthe film/substrate interface. Finally, H2 , initiating the TiCl4reaction and behaving as a catalyst, plays a crucial role in thedeposition of the ATSB/TiCl4 /CO2 /H2 system.

ACKNOWLEDGMENTS

Funding for this study was provided by the National Sci-ence Council of the Republic of China under Grant No. NSC91-2216-E-259-003.

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