-IJapaneseJoumal of Applied Physics
Vol. 43, No. 9A, 2004,pp. 63lH3l8
O20O4The JapanSocicty of Applied Physics

Preparation of Sol4el Derived Titanium Oxide thin Films Using Vacuum Ultraviolet Irradiation
with a Xenon Excimer Lamp
Yasuhiro NAGANUMA', Satomi TANAKA and Chihiro KATo
Ebina,Kanagawa
28-435, Japan
Kana|awaIndustrialTechnology
Reseorch
Institute,705-IShino-inwizumi,(Received
September
Jurc 10,2004;published9, 2004)
March16,2m4; r€visedJune4 2004;accepted
Titanium oxide thin tlms are preparedby the sol-gel techniquewith l72nm vacuumultraviolet (VUV) inadiation using aXq excimçr lamp. The effect of VUV iradiation on spin-coatedtlms was investigatedby tlV-vidible absorption,X-ray
photoelectroû,Fouder transform inftared and Ramanspe{troscopies.The results showedthat VUV irradiation causesihe
removal of hydroxyl and organic functional groups ftom the hanspareûtcoating îlm without causing the reduction oftitanium, so that the flLn beginsto transforminto an oxide from a hydroxide.It was found that VUV iradiation onto a driedcoating film is effective in acceleratingthe formation of Ti-o netwotks and crystallizatiot.
IDOI: 10.1143/JJAP.43.63l5l
excimerlamp
KEYWORDS:titaniumoxide,thinfilm,sol-!el, vacuumultraviolet,l.

Introduction

chemical statesand crystal structuresof the titanium oxide
films prepared by the sol-gel process using excimer lamps
becausefew reports are avai\ab\e.22)
In this study, titanium oxide films were synthesizedby
combining the sol-gel processand photoirradiation using aXej excimer lamp. The formation of the films was investigated by optical, chemical and structural measurements.

Titanium oxide (TiO2) has been of interest for its
excellent properties such as electrochemicalbehavior, highrefractive index, and good chemical stability. Thus, TiOz
thin films have recently attractedconsiderableattention due
to their various applications, such as microelectronicdevices, electrochromic displays, superhydrophilic films
2. Experimental
and antireflection coatings. A sol-gel process has been
Thin films of TiOz were prepared by a spin-coating
adopted for the formation of TiOz filmsl-5) with various
methods, such as electron beam evaporation,6)ion sputter- method. To prepare the coating solution, 5 m/ of titanium
isopropoxide (Ti(O-iC3Hr)+) was dissolved in 30 m/ ofingzi und anodic oxidation of titanium.8)
The sol-gel processis an industrially promising technique dehydratedethanol (CzHsOH). The solution was mixed with
continuous stirring at room temperature.Then, 0.26m1 offor the preparation of thin films, because it offers many
60Vo nitnc acid and 0.31m/ of distilled water were added
advantages in terms of low energy cost, low materialand stirred for 3 h.
consumption rate, simplicity, and speedy deposition onsubstrateswith good homogeneity as well as its nonrequireQuartz glass plates were used as substrates.They werement of expensivevacuum equipment. However, to densify photochemically cleaned by irradiating VUV light (wavelength l72nm) of 100W l^2 for 2min using aXe) excimer
and remove the remaining hydroxyl and organic functionalgroups in the films, heat treatment at several hundred lamp (Quark SystemsCo., Ltd.) in air. The coating solution
was spin-coatedonto the substrateat 500 rpm for 10 s and atdegrees is required after coating. The heat treatment at ahigh temperature will limit a substrate to a thermostable 2500rpm for 40s. After drying at 60"C for 10min using ahot plate, the coatings were inadiated with VUV light usingmaterial, and also has the problem of thermal damage.
Recently, the photochemical reaction using an excimer the sameXej excimer lamp as that used during the cleaningof the substrate in N2 gas with an oxygen concentration oflampe'to)as a vacuum ultraviolet (VUV) light source with
l07o with a flow rate of 1l/min, followed by annealingin aneasyhandling has been used for the cleaning of liquid crystal
electric furnace for t h at various temperaturesbetween 200panels, or the surface modification of polymeric materiThese applicationsutilize the cleavageof chemical and 500'C. The distance between the sample and the lamp
"1r.11-13)
window was set at 2mm. In addition, the thin films heatbonds by photon energy, and the oxidation reaction due toactive oxygen species(O(tD)) and/or ozone (O3) generated treated at various temperatures between 200 and 600"C
without irradiation were also produced. The thickness of thevia the absorptionof VUV tight of oxygen molecules.la)Thedried film was determinedto be about 150nm using a Sloan
process using photoirradiation is expected as a new
Dektak-3O30sT stylus profilometer. A scheme for thetechnique without thermal damage for fabricating a sol-gel
thin film on thermolabile substratessuch as polymer sheets. preparation of coatings to obtain the titanium oxide films
is shown in Fig. 1.
If the processing temperature for sol-gel thin film formation
The optical transmittance spectra of the films weredecreased,it would be possible to extend an application
obtained with a Hitachi U-3000 spectrophotometer in thesphere or to find a new usage. Moreover, what irradiation
200-700 nm wavelength range. X-ray photoelectron speconto a specific region is also possible by a masking method
troscopy (XPS) was carried out using a PHl-Perkin-Elmer
using light and is considered an advantage. The photomodel 5500 small-areaspectrometersystem,consisting of ainadiation solution-phase processing of such inorganic
materials,such as silicon o^i6.1s-18)and tantalum oxidele-2l) concentric hemispherical analyzer (CHA) and a MgKct
X-ray source (1253.6eV) at 350W. Photoelectronbinding
has been reported. However, there is little information on theenergies were calibrated at the Cls binding energy of thehydrocarbon at 285 eV. Fourier transform infrared (FI-IR)
6315

Jpn.J. Appl. Phys.,Vol.43, No.9A (2004)

oY. NAcANUva et al.

6317

to.E
yg

J=dLltL.

(E$oocvotro.È,cococ

G'cr
Loo.cl470

465

460455

Binding energy (eV)536 534 532 530 528

Binding energy(eV)Fig. 4. Ti2p and O1s XPS spectra from spin-coatedtitanium oxide films(a) dried, heat-treated at (b) 200'C, (c) 600'C and (d) VUV-irradiated.

observed. The titanium peaks for irradiated films are locatedat 458.7eV (Ti2pt/ù and 464.4eV (Ti2prp), and are inagreement with the value of the Tia+ valence state ofTiO2.28)This indicates that VUV irradiation under these
irradiation conditions will not induce a significant changein
Ti chemical stateshaving,different oxidation states,such as
In the dried film, the O1s peak position isTi3+ and 7i2+.23'2D
centeredat 530.5eV in Fig. 4(a) and this is mainly attributedto hydroxide. Another small component of high binding
energy due to the presence of adsorbed water or OH2e)wasdetected. The intensity in this component attenuateswith
heat treatment and VUV irradiation. Then the binding
energy of the Ols peak shifted to low binding energies,
indicating the formation of an oxide.28)
Figure 5 shows the FT-IR spectra of the titanium oxide
films prepared by drying, heat treatment and VI"IV inadiation. The absorption bands at approximately 3400 andl610cm-l are assigned to the vibrations of the O-H
stretching and H-O-H bending, respectively, while thebands at 1350-1420cm-t are assignedto the C-H bending
vibrations in organic starting materials.With the increasein
heat treatment temperature, the intensity of these absorption
bands decreased, and they disappeared at 400"C. The
disappearance of these absorption bands suggested theremoval of hydroxyl groups and H2O molecules, and thedecomposition of the organic compoundsfrom the film wasachieved. On the other hand, the absorption band coresponding to the Ti-O bond at approximately 840cm-1
sharpensas temperatureincreases,suggestingthe densification of the films. As shown in Fig. 5(d), the inadiation of
the VUV light onto the films decreasesthe absorptiondue to
C-H and O-H. To examine the impact of irradiation on solgel reactionssuch as a condensationreaction, we performed
the VUV irradiation before the heat treatment. The FI-IR
spectraof the films heat-treatedat 300'C before and after theVUV irradiation are shown in Figs. 5(e) and 5(f), respectively. In the films heat-treatedafter the VUV irradiation, theband of O-H has almost disappeared and the band of Ti-O(a) /\Hzo4000(crnt)
Wavenumber
Fig. 5. FI-IR spectra of spin-coated titanium oxide lilms (a) dried, heattreated at (b) 200'C, (c) 400'C, (d) VUV-inadiated, and heat-treated at300'C (e) before and (O after VUV irradiation.

bond is sharp compared with that of the merely heat-treatedone. The FT-IR spectrum of the film heat-treatedat 300'C
after the VUV irradiation is comparable to that of the film
heat-treatedat 400'C. This result suggeststhat many more
Ti-O networks are formed owing to progress of thedehydration and dealcoholization when VUV irradiation isperformed. This, in addition, is probably ascribed to theeffect of the incorporation and oxidation of active oxygen
speciesin the films by VUV irradiation.L3'20'2r)
To evaluate the crystallinity of the prepared films, theRaman spectra were measured. The changes in the Raman
spectra of the films heat-treated at 300"C be,fore and after
VUV inadiation are shown in Fig. 6. The anatasephase ofTiOz has Raman bands at approximately t45,396, 517 and-t.30)
When the film was heat-treated at 300'C
637 "without VUV irradiation, the intensity of the Raman bands
originating from an anataseis small and the film is almost anamorphousstructure. Neverthelessit should be noted that theVUV irradiation to the film prior to heat treatment increases
the intensity of the Raman band at 145cm-r, indicating thatthe amorphous film transforms to its anatase.In irradiationfrom a light source, the influence of heat on the sample isnegligible, because infrared radiation is not generated.
Consequently,these observationssuggestthat VUV inadiation before the heat treatment of the films advances
crystallization at low temperatures. This is so because thereaction in the subsequent heat treatment was assisted
becauseonly the VUV irradiation without the heat treatment
already removes hydroxyl and organic functional groups,
as shown in Fig. 5(d). Futthermore, it is likely that theJpn.J. Appl. Phys.,Vol. 43, No. 9A (2004)

6316

Y. NAcANUvtt et al.

ocË=dL(E=ocoE,

1000

400
Binding energy(eV)

800600200Fig. 3. XPS spectra of spin-coated titanium oxide films (a) before and(b) after VUV irradiation. The inset shows the Nls XPS spectra.

TitaniumFig. 1.

oxide frlm

Schematic diagram for preparation of titanium oxide thin films.

absorption spectra were recorded over a range of wave
numbers from 4000 to 400cm-l on a Shimadzu F-[-IR8200PC spectrometerusing the diffuse reflection method at aresolution of 4 cm-t with a TGS (Triglycine Sulfate)detector. Raman spectra were recorded using a JASCO
NR-1800 spectrometer.An argon ion laser with a wavelength of 488 nm was used as the light source. Theacquisition time per scan was set at 200 s. Gold-deposited
quafrz glass plates were used as substratesfor FT-IR andRaman measurements.
3.

I - Ioexp(-ecd),

Results and Discussion

Figure 2 shows the change in the optical transmittancespectra of the dried films after VUV irradiation and heat
treatment. These transmittances are more than 707o in thevisible range. The spectrum of the inadiated film has a sharp

so()
c(!

Ë,Eotrot-absorption edge similarly to that of the film heat-treated at400'C and an interference fringe due to multiple reflections
was observed.The transmittance of the VUV-inadiated film
was higher than that of the dried one near the absorption
edge. This suggeststhat the decomposition of the starting
materials occurred by irradiation.
The composition and chemical states of titanium oxide
films were investigated by XPS. Since Ar+ ion sputtering
causes a reduction of the TiOz surtace,23'24)the XPS
measurementswere performed without Ar+ ion sputtering
for removing surface contamination. Figure 3 shows widescan XPS spectra obtained from dried titanium oxide
coatings before and after the VUV irradiation. For thenonirradiated film the peaks of carbon, oxygen, nitrogen andtitanium were observed, whereas for the VUV-inadiatedfilm the peak of nitrogen was absent in the observed spectra.
This result can be explained as follows: The intensity of theVUV light inadiated from the lamp window under atmospheric pressure is given by2s'zs1'---'-

Wavelengthdried
VUVirradiatedheat-treatedat 400'C(nm)Fig. 2. Optical transmittance spectra of spin-coated titanium oxide films
dried. VUV-inadiated and heat-treated at 400'C.

(1)

where /s is the intensity of incident light, e is the absorption
coeffrcient of an oxygen molecule in VUV light with awavelengthof l72nm, c is the oxygen concentrationand d isthe distance between the sample and the lamp window. By
calculating eq. (1) using the absorption coefficient reported
by Inoue et al.,ra)we noted that about 74Voof the photon in
the incident light had reached the sample surface. Thephoton energy of the VUV light, i.e., 698 kJ/mol of the Xej
excimer lamp, is sufficiently high as compared with thebinding energy of the chemical bonds, such as C-C(347.7kJlmol), C-O (351.5kJ/mol) and C-N (291.6k1/mol).27)Thus VUV irradiation can break chemical bonds.
Furthermore, oxygen molecules generate active species byabsorbing the VIfV hght, and the conversion to the volatile
products of nitrogen due to the strong oxidation of the active
speciesresults in nitrogen elimination.
The Ti2p and Ols XPS spectra from titanium oxide f,lms
after drying, heat treatment and VUV irradiation are shownin Fig. 4. The XPS measurementsshowed no explicit change
in T12p spectra due to the heat treatment and VUVirradiation although a slight shift of the peak position wasIpn. J. Appl. Phys.,Vol. 43, No. 9A (2004)

6318

Y. NAcANUMI et al.

145

Quark SystemsCo., Ltd. for allowing us to use the excimer
lamp and for fruitful discussion.

o.Ë
tr=.ct(b)G.Ë
oçotr(a)

700

600

500

400300200

Ramanshift (crnl)
Fig. 6. Raman spectra of spin-coated titanium oxide films heat-treated at
300'C (a) before and (b) after VUV inadiation.

incorporationof the active oxygen speciesto the films brings
about the decrease in the density of defects,20'2t':t)undaccele.ratescrystallization during the heat treatment. The
films prepared using the VUV irradiation are expected tohave a lower defect density than the films which were only
heat-treated.This may have brought about the increasein thehardnessof the films, and the decreasein the leakagecurrent
density in I-V characteristics.
4.

Conclusions

The preparationof titanium oxide thin f,lms by the sol-gel
technique using l72nm VUV irradiation with an excimer
lamp was investigated.VUV irradiation causesthe removalof hydroxyl and organic functional groups from the transparent coating film without causing the reduction oftitanium. Then the film begins to transform into an oxidefrom a hydroxide. It was found that the VUV inadiation
before the heat treatment promotes the formation of theTi-O networks and crystallization at comparatively lower
temperaturesin subsequentheat treatmentas comparedwiththat under the nonirradiationcondition. Theseresultssuggestthat the photoirradiation sol-gel process is promising for
preparing titanium oxide films on the thermolabile substrates
used in optical and electronic applications.It is conceivablethat this technique is applicable to the preparation of othermetal oxides.
Acknowledgment
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