Selective Atomic Layer Deposition of Titanium Oxide onPatterned Self-Assembled Monolayers Formed byMicrocontact Printing
Mi H. Park, Young J. Jang, Hyung M. Sung-Suh, and Myung M. Sung*
Department of Chemistry, Kookmin University, Chongnung-dong, Songbuk-ku,
Seoul 136-702, Korea
Received September 20, 2003. In Final Form: January 4, 2004
We demonstrate a selective atomic layer deposition of TiO2 thin films on patterned alkylsiloxane selfassembled monolayers. Microcontact printing was done to prepare patterned monolayers of the alkylsiloxane
on Si substrates. The patterned monolayers define and direct the selective deposition of the TiO2 thin filmusing atomic layer deposition. The selective atomic layer deposition is based on the fact that the TiO2 thinfilm is selectively deposited only on the regions exposing the silanol groups of the Si substrates because
the regions covered with the alkylsiloxane monolayers do not have any functional group to react withprecursors.

Introduction
Thin films of titanium dioxide (TiO2) have high refractiveindex, excellent transmittance in the visible and near-IR
frequency, and high chemical stability.1 They have beenshown to be useful as antireflection coating, sensors, andphotocatalysts.2 The TiO2 thin film is also a promisingmaterial for the development of microelectronics, including
as an alternative to silicon dioxide in the gate dielectricof metal oxide semiconductor field effect transistors(MOSFETS), due to its high dielectric constant.3 Moreover,titanium oxide is a constituent of important multicomponent oxides such as BaTiO3 and SrTiO3, which are wellknown to be ferroelectrics.4
Self-assembled monolayers (SAMs) are thin organicfilms which form spontaneously on solid surfaces. They
have been shown to be useful as passivating layers andalso for the modification of surface properties. Potential
applications include wetting, adhesion, friction, chemicalsensing, ultrafine scale lithography, and protection ofmetals against corrosion.5-7 Several different varieties ofSAMs have been investigated, including alkanethiols[CH3(CH2)n-1SH] on Au, Ag, and Cu, and alkyltrichlorosilanes [CH3(CH2)n-1SiCl3] on SiO2, Al2O3, and mica.5 A
key to utilizing SAMs in many advanced applications isthe ability to pattern the monolayers.
High-resolution patterns can be formed by printing
SAMs using soft lithography8-10 or by decomposing SAMs
* To whom correspondence should be addressed. E-mail:smm@kookmin.ac.kr. Phone: 011-82-2-910-4767. Fax: 011-82-2910-4415.
(1) Eckertova, L. Physics of Thin Films; Plenum Press: New York,1986.
(2) Gilmer, G. H.; Huang, H.; Roland, C. Comput. Mater. Sci. 1998,12, 354.
(3) Suntola, T. Mater. Sci. Rep. 1989, 4, 261.
(4) Leskela, M.; Ritala, M. Thin Solid Films 2002, 409, 138.
(5) Ulman, A. An Introduction to Ultrathin Organic Films; Academic
Press: Boston, MA, 1991.
(6) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.;
Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.;
Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932.
(7) Jennings, G. K.; Laibinis, P. E. Colloids Surf., A 1996, 116, 105.
(8) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J.
Am. Chem. Soc. 1992, 114, 9188.
(9) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002.
(10) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994,
10, 1498.

using electron beams,11 ion beams, photolithography,12 orscanning probe microscopy.13 A critical requirement for
the use of patterned SAMs in advanced applications is
the ability to generate them in the most economical,
practical manner possible. Among these techniques forgenerating patterned SAMs, microcontact printing is themost practical because it is simple and rugged. This
technique has been used to produce substrates for several
different applications.10,14,15
Among various fabrication methods, there has been agrowing interest in forming a patterned thin film by siteselective deposition on a patterned monolayer as a simple
low-cost fabrication process.16,17 Selective depositions ofthin films using chemical vapor deposition, electroplating,and electroless deposition have also been accomplishedwith patterned SAMs as templates. Atomic layer deposition (ALD) is a gas-phase thin film deposition method
using self-terminating surface reactions. During the past
decade, ALD has attracted considerable attention as amethod of manufacturing high-quality thin films andproducing tailored molecular structures.18-20 The ALD
method relies on sequential saturated surface reactions
which result in the formation of a monolayer in eachsequence. The successive self-terminating growth mechanism in the ALD inherently eliminates gas-phase reactions. Elimination of gas-phase reaction results in emphasizing the importance of surface reaction. Since theALD process is very sensitive to the surface conditions ofthe substrates, it offers an ideal method for selective
deposition of thin films on patterned SAMs.
Here we report a patterning method of TiO2 thin films
using microcontact printing of alkylsiloxane SAMs, fol(11) Carr, D. W.; Lercel, M. J.; Whelan, C. S.; Craighead, H. G.;
Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., A 1997, 15, 1446.
(12) Huang, J. Y.; Hemminger, D. A. Langmuir 1994, 10, 626.
(13) Xu, S.; Liu, G. Langmuir 1997, 13, 127.
(14) Jeon, N. L.; Nuzzo, R. G. Langmuir 1995, 11, 3024.
(15) Xia, Y.; Mirksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem.
Soc. 1995, 117, 9576.
(16) Xia, Y.; Zhao, X.; Whitesides, G. M. Microelectron. Eng. 1996,32, 255.
(17) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater.
2002, 14, 1236.
(18) Suntola, T. Mater. Sci. Rep. 1989, 4, 261.
(19) Leskela, M.; Ritala, M. Thin Solid Films 2002, 409, 138.
(20) Cameron, M. A.; Gartland, I. P.; Smith, J. A.; Diaz, S. F.; George,S. M. Langmuir 2000, 16, 7435.

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Park et al.

Figure 2. UV absorbance and thickness of the TiO2 thin films
versus the number of ALD cycles.

Figure 1. Schematic outline of the procedure to fabricate
patterned TiO2 thin films by using microcontact printing andselective atomic layer deposition.

lowed by selective atomic layer deposition of the TiO2.
Our approach consists of two key steps, as shown in Figure
1. First, the patterned alkylsiloxane SAMs were formedby using microcontact printing. Second, the TiO2 thin films
were selectively deposited onto the SAM-patterned Si
substrate by atomic layer deposition.
Experimental Section
Materials. Octadecyltrichlorosilane [CH3(CH2)17SiCl3] (Aldrich, 97%), hexane (Aldrich, anhydrous, 99%), hexadecane(Aldrich, anhydrous, 99%), chloroform (Aldrich, anhydrous, 99%),
and carbon tetrachloride (Aldrich, anhydrous, 99.5%) were used
as received. Poly(dimethylsiloxane) (Sylgard 184) was orderedfrom Dow Corning. Deionized water was purified with a Millipore
Milli Q plus system, distilled over KMnO4, and then passed
through the Millipore Simplicity system. [Ti(OPri)4] (STREM
Chemicals, 99.999%) was used as received.
Preparation of Si Substrates. The Si substrates used inthis research were cut from n-type (100) wafers with resistivity
in the range 1-5 Ω cm. The Si substrates were initially treatedwith a chemical cleaning process, which involves degreasing,HNO3 boiling, NH4OH boiling (alkali treatment), HCl boiling(acid treatment), rinsing in deionized water, and blow-drying
with nitrogen, proposed by Ishizaka and Shiraki21 to remove
contaminants. A thin protective oxide layer was grown on the Si
substrate by chemical oxidation with peroxysulfuric acid.
Preparation of TiO2 Thin Films. The samples wereintroduced into the ALD system Cyclic 4000 (Genitech). The TiO2
thin films were deposited onto Si(100) substrates using [Ti(OPri)4]
and water as ALD precursors. Ar served as both a carrier anda purging gas. The [Ti(OPri)4] and water were evaporated at 353
and 293 K, respectively. The cycle consisted of a 2 s exposure to(21) Ishizaka, A.; Shiraki, Y. J. Electrochem. Soc. 1986, 133, 666.

[Ti(OPri)4], a 5 s Ar purge, a 2 s exposure to water, and a 5 s Arpurge. The total flow rate of the Ar was 20 sccm. The TiO2 thinfilms were grown at 423 K under 2 Torr.
Preparation of Octadecylsiloxane SAMs. Alkylsiloxane
SAMs were formed by immersing the Si substrates in a 2.5 mmol
solution of octadecyltrichlorosilane (OTS) precursor dissolved inhexadecane-chloroform (4:1). The samples were then washedin carbon tetrachloride to remove excess reactants and dried
with nitrogen. The quality of the monolayers was checked by
XPS and water contact angle. The water contact angle is about
110°, and the 1sC/2pSi peak area ratio measured by XPS is about
1.7.
Preparation of Patterned SAMs. A patterned monolayerof OTS on the Si substrate was made by using microcontact
printing. Masters were fabricated by using conventional photolithography or e-beam lithography to pattern resists on siliconwafers. The masters have parallel lines and spaces withdimensions from 5 to 0.15 µm. Polydimethylsiloxane (PDMS)
stamps were produced according to a previously reported
procedure.9,10,22 The PDMS stamps were inked with a 10 mM
hexane solution of OTS and dried with nitrogen. The stamp wasplaced in contact on the Si substrate at 288 K for 30 s. The stamp
was carefully peeled off, and the substrate was blown dry withnitrogen. The relative humidity was kept fixed at 60% in theseexperiments.
Analysis Techniques. Atomic force microscopy images ofthe samples were obtained on a PSIA XE-100 operating in tapping
mode. The X-ray photoelectron spectroscopy (XPS) measurements
were conducted using the ESCALAB MKII. Water contact anglesof the SAMs samples were determined on a model A-100 RameHart NRL goniometer in ambient air by using the sessile drop
method. X-ray diffraction (XRD, Rigaku RU-200BH) was carriedout to determine the crystallinity of the TiO2 films. The thicknessof the TiO2 thin films was measured by transmission electron
microscopy (TEM, JEOL) and UV spectrometry (Scinco S-3100).

Results and Discussion
1. Atomic Layer Deposition of TiO2 Thin Films.
The TiO2 thin films were grown on a Si(100) substrate byatomic layer deposition using [Ti(OPri)4] and water at 423
K. To verify that the growth mechanism is really selfcontrolled, the pulse time of the precursor was varied
between 1 and 10 s. The film thickness per cycle issaturated when the pulse time exceeds 2 s, which indicatesthat the growth is self-controlled. The UV absorbance andthickness of the TiO2 thin films versus the number of ALD
cycles are shown in Figure 2. The growth of the TiO2 film
thickness is extremely linear relative to the number ofcycles, indicating that the ALD conditions were sufficientfor complete reaction. The measured growth rate was 0.68
Å per cycle. These results suggest that the TiO2 ALD is(22) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater.
1994, 6, 600.

Deposition of Titanium Oxide on Patterned SAMs

Langmuir C

Figure 3. AFM images of TiO2 thin films versus the numberof ALD cycles.

Figure 5. AFM images and cross sections of the patterned
octadecylsiloxane monolayers by using microcontact printingon Si substrates.

Figure 4. XRD patterns of TiO2 thin films grown at 423 K.

self-controlled and that the ALD conditions are sufficientfor complete reaction at 423 K.
Evolution of the surface morphology of the TiO2 films
was examined by atomic force microscopy (AFM) as theALD cycle increased (Figure 3). Regardless of the numberof the cycle, the surface morphology of the TiO2 films issmooth and uniform, and the root-mean-square (rms)roughness of the surface is as low as 3.0 Å. In comparison,
the surface roughness of the initial cleaned Si substrateis about 2.1 Å. Figure 4 shows the XRD pattern for theTiO2 thin film on the Si substrate at 423 K. Only the Sisubstrate peaks were observed, indicating that an amorphous TiO2 film was obtained at this temperature.
These observations indicate that the TiO2 ALD is selfcontrolled and extremely linear relative to the number ofcycles, and the TiO2 thin films are smooth and amorphousat 423 K. For comparison, the TiO2 thin films were grown
on OTS-coated Si substrates at the same ALD conditions.
The TiO2 thin films are not formed on the OTS-coated Si
substrate because the substrate surface does not haveany functional group to react with ALD precursors.
2. Formation of Patterned SAMs Using Microcontact Printing. Patterned SAMs of the octadecylsiloxane on the Si substrate were made by using microcontact printing. The masters we used for fabrication ofstamps are silicon wafers with line-patterned resists, onscales from 5 to 0.15 µm. The masters with lines larger
than 1 µm were fabricated using photolithography on

silicon wafers, whereas high-resolution masters werefabricated using e-beam lithography. The stamps werefabricated by casting polydimethylsiloxane (PDMS) on themasters. After curing, the PDMS stamps were peeled awayfrom the masters. The PDMS stamps were inked with an
OTS solution and were placed in contact on the Si substrate. When the stamps were made using these masters,
the raised lines of the master corresponded to the recessed
spaces of the stamps. When patterned SAMs of OTS weremade using these stamps, a negative, hydrophobic patternof the original master was produced on the Si surface.
Parts a and b of Figure 5 show AFM images and crosssections of patterned SAMs fabricated by using themasters having 4.2 µm parallel lines with 5.5 µm spaces
and 2.2 µm lines with 1.6 µm spaces, respectively. Theseimages clearly show that the patterned monolayers retain
the dimensions of the masters without noticeable linespreading. Additionally, the height of the patternedmonolayer is about 24 Å, which is close to that of a densely
packed octadecylsiloxane monolayer (∼25 Å). The regions
covered by SAMs formed from OTS are hydrophobic, lowfree-energy surfaces, whereas the regions exposing thesilanol groups of the Si substrates are hydrophilic, highfree-energy surfaces. Parts c and d of Figure 5 show AFM
images and cross sections of patterned SAMs fabricatedby using the masters having 400 nm lines with 550 nm
spaces and 170 nm parallel lines with 130 nm spaces,respectively. The patterned SAMs in Figure 5c have 570
nm lines with 380 nm spaces, and the SAMs in Figure 5dhave 180 nm lines with 120 nm spaces. These results
indicate that the patterned SAMs almost retain thedimensions of the masters with slight line spreading.
However, the height of the patterned monolayer is aboutDLangmuir

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Park et al.

was kept at 150. The thickness of the TiO2 thin films,
measured by AFM and TEM, is about 17 nm. Figure 6
illustrates AFM images and cross sections of micropatterned TiO2 thin films, which were selectively deposited
onto the monolayer-patterned Si substrate by ALD. Thepatterns of the TiO2 thin films were defined and directedby the patterned SAMs generated with microcontact
printing. The TiO2 thin films are selectively deposited
only on the regions exposing the silanol groups of the Sisubstrates because the regions covered with the octadecylsiloxane monolayers do not have any functional groupto react with ALD precursors. These AFM images clearlyshow that the patterned TiO2 thin films retain thedimensions of the patterned SAMs with no noticeable line
spreading. Additionally, the patterned TiO2 thin films on
the submicrometer scale were also formed on the SAMpatterned Si substrates by atomic layer deposition, eventhough the height of the patterned SAMs was lower thanthat of a densely packed octadecylsiloxane monolayer.
Conclusions

Figure 6. AFM images and cross sections of the patterned
TiO2 thin films by using selective ALD on the octadecylsiloxane
SAM-patterned Si substrates.

20 Å, which is lower than that of the patterned monolayeron the micrometer scale.
3. Selective Atomic Layer Deposition of TiO2 ThinFilms. The TiO2 thin films were selectively deposited onto
the octadecylsiloxane SAM-patterned Si substrates byatomic layer deposition using [Ti(OPri)4] and water as
ALD precursors. The selective ALD of the TiO2 thin films
was done at 423 K, and the number of ALD cycles in a run

We describe a selective atomic layer deposition of TiO2
thin films on patterned octadecylsiloxane self-assembled
monolayers formed by microcontact printing. The atomiclayer deposition offers an ideal method for selectivedeposition of thin films on patterned SAMs because theALD process is very sensitive to the surface conditions of
the substrates. The TiO2 thin film is selectively deposited
only on the regions exposing the silanol groups of the Si
substrates because the regions covered with the octadecylsiloxane monolayers do not have any function group toreact with ALD precursors. The ALD of the TiO2 thinfilms is self-controlled and extremely linear relative tothe number of cycles, and the TiO2 thin films depositedat 423 K are smooth and amorphous.
Acknowledgment. This work was supported by aprogram of National Research Laboratory from theMinistry of Science and Technology and by a National
R&D Project for Nano Science and Technology from theMinistry of Science and Technology.
LA035760C