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Micropatterning of TiO 2 Thin Film in an Aqueous

Micropatterning of TiO2Thin Film in an Aqueous

Peroxotitanate Solution

Yanfeng Gao,Yoshitake Masuda,and Kunihito Koumoto* Department of Applied Chemistry,Graduate School of Engineering,Nagoya University,


Received August28,2003.Revised Manuscript Received January13,2004

We report micropatterning of TiO2thin film by a peroxotitanate complex deposition method, which was a method we developed recently.The deposition solution was obtained by dissolving titanic acid(H2TiO3)into a mixture solvent containing ammonia and hydrogen peroxide aqueous solution.The amorphous TiO2micropattern with a high resolution was achieved by using a template of photomodified self-assembled monolayers at room temper-ature.The film showed high purity,flatness,and crack-free characteristics.The fundamental dielectric characteristics suggest that the micropatterned TiO2thin films prepared by the present method are promising for application to dielectric layers.


Site-selective fabrication of materials with micro-/ nanofeatures in requisite areas is a bottom-up low-cost technique with good potential for application to bio-medicine1and microelectronics.2Although production of small parts,now in the realm of nanoscopic dimen-sions,has been accomplished,integration into more complex structures and devices represents a major challenge.With evolution,however,nature has inge-niously succeeded in producing an impressive variety of inorganic functional structures with designed shape and size on specific sites through the biologically controlled biomineralization process,usually at near room temperature and in aqueous solutions.3Inspired by this process,micropatterning of thin films such as TiO2,4-6ZrO2,7SrTiO3,8,9and SnO210has been achieved through chemical solution routes using the self-as-sembled monolayer(SAM)technique.This paper de-scribes our progress in achieving site-selective deposi-tion of crack-free TiO2thin films in an aqueous peroxotitanate solution at room temperature. Titanium dioxide thin films have attracted both industrial and academic attention for such potential applications as a gate dielectric in metal oxide semi-conductor field-effect transistors(MOSFETs),11a sur-face for solar energy conversion,12and high-efficiency photocatalysis13because of their well-known properties that include high refractive index,high permittivity,and transmittance in a visible region.To overcome some of the drawbacks of traditional methods,novel low-tem-perature processes for the one-step deposition of TiO2 thin films have been developed through controlled hydrolysis of titanium species either with use of an organic solvent6or in an aqueous solution.4,5,14,15The hydrolysis of(NH4)2TiF6,4TiCl4,14or TiF415can yield either amorphous or crystalline TiO2thin films depend-ing on the synthetic conditions employed.A complex peroxo precursor of titanium can also be employed for the deposition of TiO2thin films by several methods, such as electrochemical deposition16a and self-assembled monolayer technique.16b The peroxotitanium-type solu-tion is usually prepared by adding droplets of pure TiCl4 to an ice-cooled aqueous solution containing H2O2with or without excess acid.16However,all of these methods use starting materials containing chlorine or fluorine, which would be present in the produced thin film, resulting in poor dielectric properties and environmental

*Corresponding author.E-mail:g44233a@http://www.wendangku.net/doc/cc6bb772b14e852459fb575a.html.nagoya-u.ac.jp. Fax:+81-52-789-3201.Tel:+81-52-789-3327.

(1)(a)Chen,C.S.;Mrksich,M.;Huang,S.;Whitesides,G.M.; Ingber,D.E.Science1997,276,1425.(b)Flemming,R.G.;Murphy, C.J.;Abrams,G.A.;Goodman,S.L.;Nealey,P.F.Biomaterials1999, 20,573.(c)Bai,J.;Snively,C.M.;Delgass,W.N.Adv.Mater.2002, 14,1546.(d)Ward,J.H.;Bashir,R.;Peppas,N.A.J.Biomed.Mater. Res.2001,56,351.

(2)(a)Rogers,J.A.;Mirkin,C.Mater.Res.Bull.2001,26,506.(b) Chou,S.Y.;Krauss,P.R.;Renstrom,P.J.Science1996,272,85.(c) Chou,S.Y.;Keimel,C.;Gu,J.Nature2002,417,835.

(3)(a)Ba¨uerlein,E.Angew.Chem.,Int.Ed.2003,42,614.(b)Mann, S.Biomimetic Materials Chemistry;VCH:New York,1996;pp1-40.

(c)Aizenberg,J.;Muller,D.A.;Grazul,J.L.;Hamann,D.R.Science 2003,299,1205.

(4)Koumoto,K.;Seo,S.;Sugiyama,T.;Seo,W.S.;Dressick,W.J. Chem.Mater.1999,11,2305.



(6)Bartz,M.;Terfort,A.;Knoll,W.;Tremel,W.Chem.-Eur.J.2000, 6,4149.

(7)Gao,Y.-F.;Masuda,Y.;Yonezawa,T.;Koumoto,K.J.Ceram. Soc.Jpn.2002,110,379.

(8)Gao,Y.-F.;Masuda,Y.;Yonezawa,T.;Koumoto,K.Chem.Mater. 2002,14,5006.

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(15)Shimizu,K.;Imai,H.;Hirashima,H.;Tsukuma,K.Thin Solid Films1999,351,220.

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10.1021/cm030543i CCC:$27.50?2004American Chemical Society

Published on Web02/20/2004

concerns.Direct dissolution of TiO2into an aqueous solution avoids such problems.

We developed a novel solution system and succeeded in achieving direct deposition of a transparent,high-purity TiO2thin film in an aqueous peroxotitanate solution,which was prepared by dissolving metatitanic acid(H2TiO3)in a solvent mixture of concentrated H2O2 and NH3?H2O.17Although various peroxotitanate spe-cies were formed at different pH levels,they remained stable long enough to allow the deposition of a thin film. The precipitate collected after deposition of TiO2thin film could be dissolved in the mixture of H2O2and NH3?H2O aqueous solution and employed again for TiO2 deposition.The present method is simple,inexpensive, and environmentally friendly,showing the potential for industrial application.However,issues such as the preparation of crack-free thin films and micropatterning must still be addressed.

Experimental Section

Sample Preparation through Peroxotitanate-Complex Deposition Process.The preparation of samples has been described in our previous paper.17Briefly,3g of H2TiO3(80%, Mitsuwa)was added to an ice-cooled solvent consisting of25

Micropatterning of TiO 2 Thin Film in an Aqueous

Micropatterning of TiO 2 Thin Film in an Aqueous

3of H2O2(30%in H2O,Mitsubishi)and5cm3of ammonia

in H2O,Kishida).After the mixture was stirred for90

a homogeneous pale yellow-green solution was obtained.

homogeneous solution was then diluted with deionized

(>18M?cm)to5mM Ti4+at pH1.9-2.0(adjusted by

addition of an appropriate amount of HNO3).SAMs of

(OTS,Acros,New Jersey)were

prepared on the p-Si substrate(Shinetsu;resistivity:4-6

?cm)and UV modified by the method described in other

papers.4,7-10The substrate was then floated on the surface of

the diluted solution with the SAM surface upside down at room

temperature(~24or37°C)to deposit a thin film.After soaking

for1-120h,the substrate was taken out of the solution,

carefully rinsed with distilled water,and dried at50°C for24

h.After deposition,the precipitate was also filtrated,rinsed

with distilled water,and dried at50°C.

Characterization Techniques.A scanning electron mi-

croscope(SEM;model S-3000N,Hitachi)was used to observe

the deposits on the substrate.A scanning probe microscope

(SPI3800N,Seiko Instruments Inc.)was operated in AFM

contact mode using a triangular-shaped Si3N4cantilever to

observe the topography of the films;scans were carried out at

room temperature under ambient air with a frequency of1-2

kHz.The structure and phase composition were characterized

by X-ray diffraction(XRD;model RAD-1C,Rigaku;40kV,30

mA)with Cu K R radiation(λ)0.15418nm)at a scanning

speed of1°/min.The thickness and the refractive index of films

were measured by a laser ellipsometer(PZ2000,Philips)with

an incidence angle of70°and wavelength of632.8nm.The

zeta potential of the as-obtained powders was measured

(ZETASIZER3000HS A,Malvern Instruments,U.K.)by dis-

solving a trace of powder in50mL of deionized water.The

chemical composition of the deposited film was analyzed by

X-ray photoelectron spectroscopy(XPS;Escalab210,VG Sci-

entific Ltd.)with Mg K R as the X-ray source operated at a

constant pass energy of18eV.All spectra were referenced to

the C1s signal at284.6eV.Gold was evaporated onto both the

as-deposited film(0.785×10-6m2)and the rear side of an Si

wafer(approximately1.225×10-5m2)to form electrodes for

an MOS device.A source measurement unit(KEITHLEY236)

and an impedance analyzer(HP4192A)were employed to

measure the dielectric properties of the MOS device.

Results and Discussion

Micropatterning of TiO2Thin Film.SAMs pos-

sessing various functional groups have been employed

to tailor the morphology of thin films or to control

nucleation.SAMs themselves can be selectively modified

by X-ray,electron/ion beam,and UV irradiation,pro-

ducing an active surfactant with different physical and/

or chemical properties.9Figure1shows a schematic

outline of regio-selective surface modification of an

octadecyltrichlorosilane(OTS)-SAM and micropattern-

ing of TiO2thin films.UV irradiation through a photo-

mask caused a photocleavage reaction on the exposed



Figure1.Schematic outline of the preparation of OTS self-assembled monolayers and region-selective modification by UV light (a);the as-deposited micropatterns of TiO2thin film prepared in a5mM aqueous peroxotitanate solution under pH2.0at37°C for1h(b);enlarged SEM photographs of silanol regions(c)and octadecyl regions(d).

Micropatterning of TiO2Thin Film Chem.Mater.,Vol.16,No.6,20041063

areas,leaving unexposed areas unchanged.Hence,a template was formed,composed of the hydrophilic silanol region next to the hydrophobic octadecyl regions (Figure1a).This surface could then be used as a template for fabrication of micropatterned thin films. Figure1b shows the as-deposited TiO2thin films;the films were discrete TiO2areas of20×20μm2with intervals of5μm.The large contrast between octadecyl regions and silanol regions suggested successful selec-tive deposition of TiO2film on the silanol regions.The selectivity was further confirmed on the basis of en-larged photographs of the silanol region(Figure1c)and octadecyl region(Figure1d);a dense film was formed

on the silanol regions by coalescence of closely packed particles,compared to some fragmentary white grains that formed on the octadecyl regions.The resolution of the micropattern barely changed after cleaning by ultrasonication(150W)for2min.The variation,2.8%, was estimated by practical measurement of line widths and comparison to the line edge roughness,which is consistent with that of the photomask employed,indi-cating that a high resolution was obtained.Although further efforts must be made to miniaturize the dimen-sional features of our micropatterns considering the realization of nanosized devices,the present result is an inspiring first step toward a deeper understanding of solution-interface interaction and the self-assembly process.

Fabrication of colloid assemblies onto patterned SAMs that rely on either the electrostatic interaction between particles and surface or surfaces capillary forces have been reported.18-20Direct control over colloid self-assembly onto patterned SAMs offers several advan-tages over conventional methods,such as application of the external electric fields or manipulation of the interaction potential.Although the pK a value of our as-deposited solid is not available due to its complex composition(discussed later in this paper),the practical measurement of the collected precipitate gave an iso-electric point at about5.7(result is not shown),which strongly suggests that the particles carry a positive charge under the present conditions(pH)1.9).The silanol surface was positively charged or close to neutral at pH1.9.9Hence,the electrostatic interaction plays a minor role and cannot explain the site-selective deposi-tion on the silanol areas.In fact,the selective deposition of TiO2may be dominated by polar interactions,that is,chemical bond or hydrogen bond formation.In our case,even though the film was as thin as19nm,100% coverage(observed by AFM)was still obtained by direct adsorption of reaction-produced colloid particles.There-fore,we deduce that repulsive interparticle interaction was not significant during deposition in this case. Growth Rate,Refractive Index,and Roughness. Figure2shows the changes in film thickness,refractive index,and roughness with the deposition time.The thickness of the films with100%coverage(confirmed by AFM)ranged from19to590nm,which could be easily controlled by deposition time(2-120h).The change in the film thickness with time comprised three stages.During the initial13h,the film grew at a rate of5-10nm?h-1,which obviously increased to about22 nm?h-1during the period of13-24h.After24h,the film thickness continued to increase,whereas the growth rate significantly decreased to approximately 2-3nm?h-1.The growth rate was closely related to the solution conditions and surface functions.The classical theory for nucleation and crystal growth indicates that the formation of particles begins with the generation of tiny nuclei in a supersaturated medium followed by growth.The latter process is controlled by mass trans-port and by the kinetics of the addition and removal of individual species such as atoms,ions,or molecules to and from the particle surfaces.Hereby,the driving force for the removal(dissolution)of those species increases with decreasing particle size.Thus,when there is a slight difference in the particle size,the large particles will grow at the expense of the smaller ones.This mechanism is called Ostwald ripening and is generally believed to be the major mechanism of crystal growth. Although it is difficult to clearly conclude whether the film was deposited by the surface-promoted heteroge-neous nucleation or by the adsorption of particles generated through homogeneous nucleation to the sur-face in the very beginning of deposition,a certain induction period indeed exists,and hence the film growth rate in the initial stage was relatively low.With the reaction proceeding,the homogeneous nucleation dominated the formation of a solid phase and the solution became turbid.Film grew through the attach-ment of nuclei or clusters homogeneously formed in solution,resulting in a high growth rate.However,the solution became transparent again after about24h of soaking,suggesting that the film growth might have been governed by heterogeneous nucleation;that is growth through surface-promoted nucleation,resulting in a low growth rate.When a fresh substrate was immersed into a solution after aging for13h corre-sponding to the induction period shown in Figure2,a similar change in growth rate was observed;a growth rate was high in the beginning,but it slowed after solution became transparent again.We therefore believe that the attachment of nuclei generated in the solution resulted in the accelerated growth rate we observed in the second period of time(13-24h).In this stage,the supersaturation of the solution was much higher than that in the initial stage,which contributed to the higher

(18)(a)Zhu,P.-X.;Masuda,Y.;Koumoto,K.J.Colloid Interface Sci.2001,243,31.(b)Kru¨ger,C.;Jonas,U.J.Colloid Interface Sci. 2002,252,331.(c)Ye,Y.-H.;Badilescu,S.;Truong,V.-V.;Rochon,P.; Natansohn,A.Appl.Phys.Lett.2001,79,872.

(19)(a)Aizenberg,J.;Braun,P.V.;Wiltzius,P.Phys.Rev.Lett. 2000,84,2997.(b)Demers,L.M.;Mirkin,C.A.Angew.Chem.,Int. Ed.2001,40,3069.(c)Lee,I.;Zheng,H.-P.;Rubner,M.-F.;Hammond, P.T.Adv.Mater.2002,14,572.

(20)Tieke,B.;Fulda,K.-F.;Kampes,A.Nano-surface Chemistry; Rosoff,M.,Ed.;Marcel-Dekker:New York,2002;pp213-

Micropatterning of TiO 2 Thin Film in an Aqueous

242.Figure2.Relationship of film thickness,refractive index,and roughness dependent on the soaking time;the film was prepared in a5mM aqueous peroxotitanate solution under pH2.0at24°C.

1064Chem.Mater.,Vol.16,No.6,2004Gao et al.

growth rate and the large amount of precipitates ac-cumulating at the bottom of the beaker.In the following stage,the growth rate decreased with the decreasing degree of supersaturation.

The surface morphologies of the as-deposited thin films were almost the same;the films were formed by coalescence of nanosized particles 10-20nm in diam-eter,but the particles aggregated into large grains of 50-100nm.The precipitates were preferential for filling the voids,resulting in the formation of a dense particu-late film.All of the films demonstrated flat surfaces with root-mean-square (RMS)roughness of 5-8nm for the measured areas of 500×500nm 2(Figure 2).However,the relative roughness (defined as RMS/thickness)obvi-ously decreased with an increase in the film thickness,typically from 27%to 1%,by prolonging the soaking time from 2to 120h.The refractive index ranged from 1.5to 1.8.Although the refractive index of the film increased with increasing soaking time,it is still low compared to that of the crystalline TiO 2thin films (anatase:2.3-2.54,212.7521b ),which may be attributed to the amorphous characteristics and low density of the present as-deposited thin film.

Morphology and Topography of TiO 2Thin Film after Annealing.The as-deposited thin film was amorphous,but it crystallized into anatase after an-nealing at a temperature as low as 300°C (profiles are not shown).The energy needed for the phase transfor-mation is relatively low,although we failed to achieve direct crystallization of thin films in the solution even after a series of attempts,including increasing the soaking temperature (from room temperature to 50or 80°C),changing the pH (2.0-7.0),and adding different anions (Cl -or SO 42-from HCl or H 2SO 4,respectively).

After annealing at 700°C for 2h in air (Figure 3),the film shrank from 306to 280nm in thickness (measured by both SEM and ellipsometer,Figure 3a),a shrinkage of 8.5%.The particle size observed by SEM (Figure 3b)and AFM increased slightly compared to that of the as-deposited one (data is not shown);however,no obvious cracks were observed in the SEM photograph (Figure 3b).The dark groove in the AFM image (Figure 3c)suggested the appearance of cracks,but the height profile showed that the deepest section (~40nm)was still much smaller than the film thickness (~280nm)(Figure 3a),suggesting that the cracks were superficial.Hence,we successfully deposited a virtually crack-free TiO 2thin film with a thickness of 280nm after anneal-ing at 700°C.

Chemical Composition.For determination of the chemical composition of the as-deposited amorphous thin film,XPS analysis was conducted (data is not shown).Only Ti,O,and C were present in the as-deposited thin film.No other elements,such as N,were detected.The carbon was presumably incorporated as a result of contamination during storage in air.There-fore,the as-deposited thin film was of high purity.The binding energies for Ti 2p3/2were 458.8eV.Peak separa-tion of the O 1s spectrum clearly showed three kinds of oxygen with binding energies of 530.4,532.3,and 533.6eV,assigning to the Ti(IV)-O bonds,22OH groups,23and peroxo groups,24respectively.The quantitative analysis suggested a molar ratio of Ti/O )1/2.4-1/2.6for different samples,which was much lower than 1/2for the stoichiometric TiO 2.The FT-IR result definitely confirmed the presence of Ti -O bond,peroxo groups,and OH groups.17These findings,along with those from

(21)(a)Zhang,J.-Y.;Boyd,I.W.;O’Sullivan,B.J.;Hurley,P.K.;Kelly,P.V.;Se ′nateur,J.-P.J.Non-Cryst.Solids 2002,303,134.(b)Martinu,L.;Poitras,D.J.Vac.Sci.Technol.A 2000,18,2619.(c)Wang,Z.-C.;Helmersson,U.;Ka ¨ll,P.-O.Thin Solid Films 2002,405,50.




Micropatterning of TiO 2 Thin Film in an Aqueous


Figure 3.Cross-sectional SEM photograph (a),surface morphology (b),and topography (c)along with height profile along the corresponding line shown in the image for the TiO 2thin film (corresponding thin film obtained after soaking for 24h as shown in Figure 2)after annealing at 700°C for 2h in air.

Micropatterning of TiO 2Thin Film Chem.Mater.,Vol.16,No.6,20041065

XPS and TG-DTA,suggest a possible chemical composi-tion of TiO 2-x -0.5y (O 2)x (OH)y ?z H 2O,which is similar to that deposited on a glass substrate.17After annealing at 700°C,the molar ratio of Ti/O changed to 1/2.1,suggesting the decomposition of unstable groups such as OH and peroxo.

In the FT-IR spectrum (Figure 4)of the collected precipitate,a broad peak appearing at 3100-3600cm -1was assigned to fundamental stretching vibration of O -H hydroxyl groups (free or bonded),25which was further confirmed by a weak band at about 1620cm -1.The absorption band at 1620cm -1was caused by a bending vibration of coordinated H 2O as well as from Ti -OH.The bending vibrational mode of water may appear as shoulders on the spectrum at 3240cm -1.Peaks located at 500and 430cm -1were likely due to the vibration of the Ti -O bonds in the TiO 2lattice.26The peak centered at 900cm -1may be assigned to characteristic O -O stretching vibration,26a and the shoulder observed at 690cm -1may have been due to the vibration of the Ti -O -O bond.Although the peak detected at 1409cm -1could not be assigned,the FT-IR measurement firmly suggested the presence of Ti -O bonds,peroxo groups,and OH groups in the as-prepared precipitate.

After annealing at 200°C (Figure 4b),the shoulder assigned to peroxo groups became weak,suggesting decomposition taking place.Peaks attributed to ad-sorbed water almost disappeared,while vibration of hydroxyl groups still could be observed at about 3140cm -1,which was not detected after annealing at 300°C (Figure 4c).After annealing at 400°C (Figure 4d),only peaks for Ti -O vibration located at 400-700cm -1were observed.

Dielectric Properties.The basic dielectric charac-teristics showed that the as-deposited thin film dem-onstrates typical capacitance -voltage (C -V,Figure 5a)and current -voltage properties (I -V,Figure 5b).For these measurements,a metal-oxide-semiconductor (MOS)

capacitor (Au/TiO x /SiO 2/Si/Au)was fabricated using Au as both top and bottom electrodes;this capacitor was formed by a parallel plate capacitor of TiO 2,SAM (about 2nm according to the molecular structure of OTS),and an SiO 2layer.The high-frequency (100kHz)capaci-tance -voltage curve exhibited typical accumulation,depletion,and inversion areas as a result of the ac-cumulation of major carriers (holes for p-type silicon)in the oxide -semiconductor interface or depletion of them when the bias was swept from a negative voltage to a positive one.The dielectric constant of 63at 100kHz for an as-deposited thin film (306nm)was esti-mated by assuming the native silica layer to be 2nm (the electrode area:0.785cm 2).Such a result is much larger than the reported value for biomimetically de-posited amorphous TiO 2(21.6,2724-5728).This may be due to the specific composition of our thin film and/or existence of minor crystallized particles,although no crystalline phase was detected by XRD.

The leakage current (Figure 5b)at 1V (~35kV/cm)was about 4.3×10-8A cm -2,which is relatively high considering the thickness of our as-deposited thin film (306nm).The dielectric constant decreased almost linearly from 160at 1kHz to 23at 1MHz (bias voltage )-1V),and the corresponding dispersion factor increased at above 100kHz,reaching 0.9at 1MHz.All of these results may be associated with interface states and impurities such as OH -and H 2O.The film surface,as clearly seen in Figure 3b,exhibits discrete particulate characteristics,which should have a significant effect on the leakage current.However,the flat band shift in the C -V curve is not obvious.Further efforts should be made to improve the film properties before it can be applied as a dielectric.

(25)Zhang,R.;Gao,L.Key Eng.Mater.2002,224-226,573.

(26)(a)Yoko,T.;Kamiya,K.;Tanaka,K.J.Mater.Sci.1990,25,3922.(b)Zhang,J.;Boyd,I.;O’Sullivan,B.J.;Hurley,P.K.;Kelly,P.V.;Senateur,J.-P.J.Non-Cryst.Solid 2002,303,134.

(27)(a)Koumoto,K.;Masuda,Y.;Wang,D.J.Int.J.Soc.Mater.Eng.Resour.2002,10,49.(b)Wang,D.-J.;Masuda,Y.;Koumoto,K.Key Eng.Mater.2002,214,163.

(28)Shin,H.;De Guire,M.R.;Heuer,A.H.J.Appl.Phys.1998,83,

Micropatterning of TiO 2 Thin Film in an Aqueous


Figure 4.FT-IR spectra of the as-prepared powder (a)and those after annealing at different temperatures:(b)200°C;(c)300°C;(d)400

Micropatterning of TiO 2 Thin Film in an Aqueous


Figure 5.C -V and I -V characteristics of the as-deposited thin film;curves were obtained at room temperature.

1066Chem.Mater.,Vol.16,No.6,2004Gao et al.


We reported a novel solution system for site-selective deposition and micropatterning of TiO2thin films onto the self-assembled monolayers by the peroxotitanate-complex deposition(PCD)method.This method enables direct preparation of amorphous TiO2micropatterns in an aqueous peroxotitanate solution at room tempera-ture.The selectivity,film morphology,thickness,fun-damental dielectric properties,and growth mechanism were investigated.The results suggested that the present method is a promising alternative for site-selective deposition of TiO2thin films through a low-temperature,environmentally friendly process.


Micropatterning of TiO2Thin Film Chem.Mater.,Vol.16,No.6,20041067