Selective MOCVD of titanium oxide and zirconium oxide thin films ...

ARTICLE IN PRESSJournal of Physics and Chemistry of Solids 69 (2008) 128–132www.elsevier.com/locate/jpcsSelective MOCVD of titanium oxide and zirconium oxide thin filmsusing single molecular precursors on Si(1 0 0) substratesB.-C. Kang a, , D.-Y. Jung a , R.A. Fischer b , J.-H. Boo aa Department of Chemistry, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Koreab Organometallics and Materials Chemistry, Ruhr-University Bochum, Universitässtr. 150, Bochum 44780, GermanyReceived 23 September 2006; received in revised form 23 June 2007; accepted 10 August 2007AbstractTitanium oxide (TiO 2 ) and zirconium oxide (ZrO 2 ) thin films have been deposited on modified Si(1 0 0) substrates selectively by metalorganicchemical vapor deposition (MOCVD) method using new single molecular precursor of [M(O i Pr) 2 (tbaoac) 2 ](M¼ Ti, Zr;tbaoac ¼ tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (mCP) method wasadapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has –CH 3 terminalgroup. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) bymodifying standard synthetic procedures. Selective depositions of TiO 2 and ZrO 2 were achieved in a home-built horizontal MOCVDreactor in the temperature range of 300–500 1C and deposition pressure of 1 10 3 –3 10 2 Torr. N 2 gas (5 sccm) was used as a carriergas during film depositions. TiO 2 and ZrO 2 thin films were able to deposit on the hydrophilic area selectively. The difference in surfacecharacteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO 2 or Si–OH layer on the Si(1 0 0) substrate led to thesite-selectivity of oxide thin film growth.r 2007 Elsevier Ltd. All rights reserved.Keywords: A. Oxides; A. Thin film; B. Vapor deposition; D. Surface properties1. IntroductionOxide materials have a wealth of interesting physicalproperties that can be used for various technologicalapplications. Many oxide materials have been usedextensively in the form of thin films because the applicationsinvolve micro-devices that require materials fabricatedon micron or submicron scales [1,2]. Titaniumdioxide (TiO 2 ) films have found widespread applicationas materials for optical coatings and protective layers forvery large-scale integrated circuits because of their wideenergy band gap, high refractive index, and good insulatingproperties. Moreover, it is useful in fabricating dielectriccapacitors in microelectronic devices because of its highdielectric constant. Up to now, TiO 2 has been extensivelystudied for its interesting electric, magnetic, catalytic, Corresponding author. Tel.: +82 31 290 5972; fax: +82 31 290 7075.E-mail address: bchkang@empal.com (B.-C. Kang).and electrochemical properties [3–6]. Based upon theseproperties, a variety of technological applications of TiO 2thin film are possible. Also, zirconium oxide (ZrO 2 )has been extensively studied for use as a silicon dioxidereplacement in the gate-oxide insulating layer in complementarymetal oxide semiconductor (CMOS) devices.ZrO 2 has high dielectric constants and a high-energyband gap [7].As these good properties and applications, metal oxidethin films have been deposited by several physical andchemical deposition techniques [8–11]. Among them,chemical vapor deposition (CVD) technique using metalorganiccompound as a precursor (MOCVD) has manyadvantages, such as good conformal coverage, the possibilityof epitaxial growth and selective deposition and theapplication to large-area deposition, etc. Also this methodis low cost and is easy to control the deposition/growthparameters. Thus, the MOCVD method is well known asthe one of the most powerful techniques and is suitable for0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.jpcs.2007.08.008

ARTICLE IN PRESSB.-C. Kang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 128–132 129stoichiometric and micro-structured thin film deposition.The patterning of thin film is of considerable scientific andtechnological interest. Soft lithography is a method tomake micro/nano size patterns and structures simply usingorganic materials without involving high energy. Inparticular, micro-contact printing (mCP) is a very convenientand nonphotolithographic technique that cangenerate patterned features of self-assembled monolayers(SAMs) on various surfaces [12]. The ability to control thewettability of a solid surface is important and useful in arange of technological applications [13–15]. In addition,the mCP technique shows that hydrophobic patterns withmicron dimensions can be formed on hydrophilic surfacesavoiding the use of photolithographic-type procedures. Xiaand Whitesides [13] and Folch and Schmidt [16] presentedthe current state of development of soft lithographictechniques and areas in which these techniques findapplications. The authors suggested the mCP techniquecould apply to micro-electro-mechanical system (MEMS),sensors, and microelectronics with large patterning featuresover 100 mm scale, because small-area wafer and largelinewidths are perfectly suited for most those applications.Earlier, we reported deposition of patterned TiO 2 thinfilms using titanium(IV) isopropoxide [Ti(O i Pr) 4 ] byMOCVD on Si(1 0 0) substrates where the surface was firstmodified by an octadecyltrichlorosilane (OTS) [17]. Theaim of the work presented here is to fabricate thin films andmicro/nano patterning of TiO 2 and ZrO 2 thin films byusing of new single molecular precursors. Herein, we reportthat site-selective MOCVD of TiO 2 and ZrO 2 thin filmsusing bis-isopropoxy-bis-tertiarybutyl-acetoacetate titanium[Ti(O i Pr) 2 (tbaoac) 2 ] as precursors on OTS patternedSi(1 0 0) substrates.2. ExperimentalThe polydimethylsiloxane [–Si(CH 3 ) 2 O–] n (PDMS)stamps were used for mCP and OTS [CH 3 (CH 2 ) 17 SiCl 3 )]SAMs were fabricated according to a previously reportedprocedure [13]. A solution of OTS in dry hexane was usedas the ‘‘ink.’’ The OTS solution was applied to the PDMSstamp using a spin coater. Then, the stamp was broughtinto contact with the Si(1 0 0) substrate whose surfacecontains a native oxide layer. The thermal stability of theOTS SAMs on the Si(1 0 0) substrate is so strong that itcan endure around 450 1C in a vacuum condition. Afterprinting, thin films were deposited on these OTS patternedSi(1 0 0) substrates by MOCVD using new metal-organicprecursors, [M(O i Pr) 2 (tbaoac) 2 ] (M ¼ Ti, Zr; tbaoac ¼tertiarybutyl-acetoacetate) as a single molecular precursor.Ti(O i Pr) 2 (tbaoac) 2 exists as pale-yellow crystalline solids atroom temperature, having a melting point 58 1C andZr(O i Pr) 2 (tbaoac) 2 is a white crystalline solid with 45 1Cmelting point. The details of the synthesis and characterizationof the compounds is reported in the literature[18,19]. The OTS patterned Si(1 0 0) substrate was pretreatedwith ethanol and de-ionized (DI) water in anultrasonic cleaner without acid treatment to protect theOTS patterning from being destroyed. The CVD experimentswere performed in a homebuilt MOCVD system.Nitrogen was used as the carrier gas (5 sccm). The basepressure of the MOCVD apparatus was 1 10 3 Torr, andthe working pressure was kept in the range of 30–50 mTorr.The precursor bottle was heated at 80 1C. The deposition ofTiO 2 thin films was carried out at 300–400 1C for 0.5–1 h.And the depositions of ZrO 2 thin films were performed at450–500 1C for a period of 2–4 h. The as-grown films werecharacterized by optical microscopy (OM), scanningelectron microscopy (SEM), energy dispersive X-ray(EDX), atomic force microscopy (AFM) and micro-Raman spectroscopy.3. Results and discussionSelectively deposited TiO 2 thin films can be identified byoptical microscopy (OM). Fig. 1 shows an OM image ofthe deposited TiO 2 thin film that was selectively grown onthe OTS patterned Si(1 0 0) substrate at 400 1C for 1 h. Therelatively dark area corresponds to the TiO 2 deposited areaand the bright area is OTS SAMs area.SEM and AFM analyses were performed to see thesurface morphologies of the selectively deposited TiO 2 thinfilms. Fig. 2 shows the SEM image and EDX data(see inset) of the selectively grown TiO 2 thin film on theOTS patterned Si(1 0 0) substrate. It was observed that theimages (which consist of two different contrasts shown inFig. 2) match well with the OM data. Moreover, it is seenthat boundary shape between the OTS SAMs area and theTiO 2 deposited area is also very sharp and clear. In order toidentify the composition of both dark (TiO 2 film) andbright areas (OTS SAMs), we utilized the EDX analysisshown in the inset in Fig. 2. In the case of bright area, theatomic ratio (%) of Ti and O was nearly 1:2, while inthe dark area there was no evidence of Ti and O elements.Fig. 1. Optical microscope image of a TiO 2 thin film deposited on theOTS patterned Si(1 0 0) substrate. The dark image is TiO 2 deposited areaand the bright image is OTS SAMs area.

130ARTICLE IN PRESSB.-C. Kang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 128–132Fig. 2. SEM image of a TiO 2 thin film deposited on the OTS patternedSi(1 0 0) substrate. Inset shows EDX spectra obtained from the selectivelygrown TiO 2 film (A) and OTS SAMs surface (B).Fig. 4. Micro-Raman spectra of (a) Si(1 0 0) substrate, (b) OTS SAMsarea, and (c) TiO 2 deposited area (Raman laser radiation: 514.5 nm,20 mW).Fig. 3. (a) 3D AFM image of selective growth of TiO 2 thin film onSi(1 0 0) surface. (b) 2D AFM image of the same film as (a), and heightprofile obtained from the direction of black line area.This is a good agreement with reported data of selectivegrowth of TiO 2 thin film with Ti(O i Pr) 2 (tbaoac) 2 [18] andof the CVD grown TiO 2 thin films with Ti(O i Pr) 4 [20].Fig. 3(a) shows the 3D AFM image of the selectivelygrown TiO 2 thin film on OTS SAMs patterned Si(1 0 0)substrate at 400 1C. The selective deposition of thin filmand the sharp boundary between TiO 2 thin film area andOTS SAMs area was clearly seen. Fig. 3(b) also shows the2D AFM image obtained from the same film as Fig. 3(a).From the images of Fig. 3(a) and (b), also we can see twodifferent contrasts similar to SEM images, indicating goodselectivity. With the height profile (see inset) of the 2DAFM image, we can reconfirm the selective growth of TiO 2thin film and the TiO 2 thin film was able to selectivelydeposit on 10 mm area with thickness of about 120 nm.The clear and convincing evidence of selective growthwas obtained by micro-Raman analysis with a 514 nm laserand 20 mW power. Fig. 4 shows the micro-Raman spectraof the Si(1 0 0) substrate (a), OTS SAMs printed area(b) and TiO 2 deposited area (c), respectively. In the case of(a) and (b), there were no characteristic bands except forthe peaks originated from the Si(1 0 0) substrate. However,in Fig. 4(c), we clearly found that the three bandscorresponding to the anatase TiO 2 Raman active fundamentalmodes, are recorded at 144 cm 1 (E g ), 399 cm 1(B 1g ), and 639 cm 1 (E g ). The band positions were in very agood agreement with published data on either polycrystallinepowders or monocrystals [21,22]. Moreover, the bandsdue to rutile and brookite phase of TiO 2 were not observed.With these data, we were able to conclude that the anatasephase TiO 2 thin film was able to mainly grow on theSi(1 0 0) substrate on which OTS SAMs were not covered,strictly speaking, a native oxide layer on Si(1 0 0) surface.This result proved the selectivity of TiO 2 thin film on theOTS patterned Si(1 0 0) substrate.Selectivity of deposited ZrO 2 film can be identified byoptical microscopy (OM) images. Fig. 5 shows an OMimage of the deposited ZrO 2 films on OTS patternedSi(1 0 0) substrate at 450 1C for 2 h. It shows contrastbetween OTS SAMs area and ZrO 2 thin films depositedregion. The relatively dark area corresponds to the ZrO 2deposited area and the bright area is OTS SAMs area.Fig. 6 shows the SEM images of selectivity grown onOTS patterned Si(1 0 0) substrate. In the case of (a), grownat 450 1C for 2 h, it shows that boundary shape between the

ARTICLE IN PRESSB.-C. Kang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 128–132 131OTS SAMs area and ZrO 2 deposited area is clear.However, as the deposition time was increased up to 4 h,the boundary became very unclear. It means the ZrO 2 wasable to deposit both hydrophilic area and hydrophobicarea. We can suggest that the terminal group of OTS wasoxidized and changed its character from hydrophobic tohydrophilic or removed from the substrate surface becausethe high deposition temperature and long deposition time[25]. The morphology of ZrO 2 thin film grown on Si(1 0 0)substrate was denser and smoother than that of ZrO 2 thinfilm grown on OTS SAMs surface.XPS survey spectrum analysis was carried out of the asdepositedZrO 2 film grown on Si(1 0 0) substrate at 450 1C(not shown here). The strong Zr 3d and O 1s peaks wereclearly indicated. However, C 1s peak clearly appearedaround 300 eV binding energy, we can guess that thecarbon comes from the precursor during depositions and/or surface contamination from the air. Also, the O 1s andZr 3d high-resolution XPS spectra were performed(not shown here). In the case of O 1s , there are twooxidation states attributed to O 2 and OH species at thebinding energy of about 532 and 530 eV. We can ensurethat the OH peak is just a surface contamination peakprobably originated from H 2 O in the air. The Zr 3d5/2 peakobserved as a single peak at binding energy of 182.2 eV,which is a good agreement to Zr 4+ .Fig. 7 shows the 3D AFM image of selectively grownZrO 2 thin film on OTS SAMs patterned Si(1 0 0) substrateat 450 1C for 2 h. The selectivity of thin film and the sharpboundary between ZrO 2 thin film area and OTS SAMs areclearly distinguished. The thickness of deposited ZrO 2 thinfilm was approximately 30–40 nm. The growth rate of ZrO 2thin films using Zr(O i Pr) 2 (tbaoac) 2 was much slower thanthat of TiO 2 thin films.OTS SAMs have been demonstrated as a promisingcandidate for a MOCVD resist layer which can block thesurface functional groups that are necessary for nucleationFig. 5. OM images of a ZrO 2 thin film deposited on the OTS patternedSi(1 0 0) substrate.Fig. 7. 3D and 2D AFM images of selective growth of a ZrO 2 thin film onSi(1 0 0) substrate.Fig. 6. SEM image of a ZrO 2 thin film deposited on the OTS patterned Si(1 0 0) substrate at (a) 450 1C for 2 h and (b) 450 1C for 4 h.

132ARTICLE IN PRESSB.-C. Kang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 128–132and growth during the deposition [23]. In this experiment,OTS can prevent nucleation for TiO 2 and ZrO 2 thin film onthe SAMs surfaces because it has hydrophobic terminalgroup (–CH 3 ), i.e., there are no reaction factors forchemisorption between the metal-organic (MO) precursorsand –CH 3 groups. But the MO precursors can obtainelectrons from the ‘‘base’’ originated from native oxidelayers on the Si(1 0 0) surface and they can makechemisorptions with substrate very strongly. After all, thinfilm is able to be formed on it. However, OTS SAMs areweak in respect of thermal stability over 450 1C [25], theyshould change their identities or make some defects on theOTS SAMs area. Therefore, it is difficult to make thin filmsselectively on substrates by combination of mCP andMOCVD methods at high temperatures. Thomas et al.showed that Ti(O i Pr) 2 (tbaoac) 2 precursor showed slightlysuperior properties in terms of evaporation behavior,deposition temperature, and smaller amount of residuematerial than those of Zr(O i Pr) 2 (tbaoac) 2 precursor [24].And Zr(O i Pr) 2 (tbaoac) 2 precursor could induce high vander Waals force with OTS SAMs surface and lowmolecular mobility on the OTS SAMs area owing to itshigh molecular weight compared to Ti(O i Pr) 2 (tbaoac) 2 .These disadvantage factors should require a long nucleationtime on the Si(1 0 0) substrate and cause less siteselectivedeposition than that of TiO 2 in our experiment.4. ConclusionsWe successfully deposited patterned TiO 2 and ZrO 2 thinfilms directly using a combination method of mCP andMOCVD methods. New developed precursors of[M(O i Pr) 2 (tbaoac) 2 ,M¼ Ti, Zr] were used as precursorsfor TiO 2 and ZrO 2 thin films. In the case of TiO 2deposition, it showed a good selectivity growth on thepatterned Si(1 0 0) substrate. The boundaries between theOTS SAMs area and TiO 2 deposited area were very cleancut and sharp without any breaks. The micro-Ramanspectroscopy and the EDX data showed that the depositedTiO 2 thin films were stoichiometric with anatase phase. Inthe case of ZrO 2 thin films, they showed a selectivedeposition on the substrate as deposition time below 2 h;however, the selectivity disappeared above 2 h. It meansthat OTS should change or lose its identity due to hightemperature and/or long deposition time. Even though thecombination of MOCVD and mCP is a very convenienttechnique to make micro-size structures without involvingphotolithographic-type procedures, there are some limitationssuch as deposition temperature and deposition time.However, it is expected that the combination of mCPof SAMs and MOCVD is a better method for fabricatingmicro-size, various functional thin films at lowtemperatures.AcknowledgmentsThe authors gratefully acknowledge the support by theBK21 project of Ministry of Education, Korea.References[1] K.S. Goto, Solid State Electrochemistry and its Applications toSensors and Electronics Devices, Elsevier, New York, 1988.[2] L.A. Ryabova, Current Topics in Materials Science, vol. 7, North-Holland, Amsterdam, 1981 (Chapter 5).[3] M.F. Yan, W.W. Rhodes, Grain Boundaries in Semiconductors,North-Holland, New York, 1981.[4] J. Reintjes, M.B. Schultz, J. Appl. Phys. 39 (1968) 5254.[5] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978)170.[6] D.J. Dwyer, S.D. 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