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Chem. Mater. 2004, 16, 1878-1883

Atomic Layer Deposition of Titanium Oxide on
Self-Assembled-Monolayer-Coated GoldEun K. Seo, Jung W. Lee, Hyung M. Sung-Suh, and Myung M. Sung*
Department of Chemistry, Kookmin University, Chongnung-dong,
Songbuk-ku, Seoul 136-702, Korea
Received November 8, 2003. Revised Manuscript Received February 17, 2004

We demonstrate an atomic layer deposition of TiO2 thin films on self-assembled monolayersof ω-functionalized alkanethiolates. The TiO2 thin films were grown on OH-terminatedalkanethiolate monolayer-coated gold by atomic layer deposition at 100 °C. The atomic layer
deposition of the TiO2 thin films is self-controlled and extremely linear relative to the numberof cycles. Selective deposition of the TiO2 thin film using atomic layer deposition wasaccomplished with patterned self-assembled monolayers as templates. Microcontact printing
was done to prepare the patterned monolayers of the alkanethiolates on gold substrates.
The selective atomic layer deposition is based on the fact that the TiO2 thin film is selectively
deposited only on the regions exposing OH-terminated alkanethiolate monolayers of the goldsubstrates, because the regions covered with CH3-terminated monolayers do not have any
functional group to react with precursors.

Introduction
Self-assembled monolayers (SAMs) of ω-functionalized alkanethiolates on gold have enormous application
prospects, because the surface properties are determined by the chemical nature of the terminal group of
the alkanethiols.1,2 Potential applications include thinfilm deposition, high-resolution patterning,3,4 biomolecule immobilization,5,6 molecular electronics,7,8 andchemical sensing.9,10 Recently, thin film deposition on
alkanethiolate SAM-coated gold has been investigatedin a number of publications.11-13 To successfully deposit
thin films on alkanethiolate SAM-coated gold, it isnecessary for the SAMs to withstand the temperatures
used in film formations. Since the alkanethiolate SAMs
on gold can withstand air (or vacuum) temperatures
only as high as 150 °C,14 a critical requirement for the* To whom correspondence should be addressed. E-mail: smm@
kookmin.ac.kr. Ph: 011-82-2-910-4767. Fax: 011-82-2-910-4415. Email. smm@kookmin.ac.kr.
(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991.
(2) Ulman, A. Chem. Rev. 1996, 96, 1533.
(3) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev.
1999, 99, 1823.
(4) Black, A. J.; Paul, K. E.; Aizenberg, J.; Whitesides, G. M. J.
Am. Chem. Soc. 1999, 121, 8356.
(5) Dong, Y.; Shannon, C. Anal. Chem. 2000, 72, 2371.
(6) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem.
1999, 71, 777.
(7) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999,286, 1550.
(8) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.;
Stoddart, J. F.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath,J. R. Science 1999, 286, 1550.
(9) Sirkar, K.; Revzin, A.; Pishko, M. V. Anal. Chem. 2000, 72, 2930.
(10) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988.
(11) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.;
Winograd, N. J. Phys. Chem. B 2000, 104, 3267.
(12) Xia, Y.; Zhao, X.; Whitesides, G. M. Microeletronic Eng. 1996,32, 255.
(13) Yu, H.; Rowe, A. W.; Waugh, D. M. Anal. Chem. 2002, 74, 5742.
(14) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys.
1996, 35, L799.

use of the deposition method with alkanethiolate SAMcoated gold as substrate is the temperature to deposit
thin films. Atomic layer deposition (ALD) is a gas-phasethin film deposition method working at low temperatures. During the past decade, ALD has attracted
considerable attention as a method for manufacturinghigh-quality thin films and producing tailored molecularstructures.15-17 Since the ALD is a low-temperatureprocess, it can be applied to thin film deposition on
alkanethiolate SAM-coated gold.
Among various patterning methods, there has beena growing interest in forming a patterned thin film bysite-selective deposition on a patterned monolayer as asimple low-cost fabrication process.12,18 Patterned alkanethiolate SAMs have been used as molecular resistsat the surface for selective deposition of thin films.12
Selective depositions of thin films using chemical vapor
deposition, electroplating, and electroless deposition
have been also accomplished with patterned SAMs astemplates.12 The ALD method relies on sequential
saturated surface reactions which result in the formation of a monolayer in each sequence. The successive
self-terminating growth mechanism in ALD inherently
eliminates gas-phase reactions. Elimination of the gasphase reaction results in emphasizing the importanceof the surface reaction. The ALD process offers an ideal
method for selective deposition of thin films on patterned SAMs, because it is very sensitive to the surface
conditions of the substrates.
Thin films of titanium dioxide (TiO2) have highrefractive index, excellent transmittance in the visible(15) Suntola, T. Mater. Sci. Rep. 1989, 4, 261.
(16) Leskela, M.; Ritala, M. Thin Solid Films 2002, 409, 138.
(17) Cameron, M. A.; Gartland, I. P.; Smith, J. A.; Diaz, S. F.;
George, S. M. Langmuir 2000, 16, 7435.
(18) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater.
2002, 14, 1236.

10.1021/cm035140x CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/15/2004

Deposition of TiO2 on SAM-Coated GoldFigure 1. Schematic outline of the procedure to fabricate
patterned TiO2 thin films by using microcontact printing andselective atomic layer deposition.

and near-IR frequency, and high chemical stability.19
They have been shown to be useful as antireflection
coating, sensors, and photocatalysts.20 The TiO2 thinfilm is also a promising material for the developmentof microelectronics including as an alternative to silicon
dioxide in the gate dielectric of metal oxide semiconductor field effect transistors (MOSFETS), due to its highdielectric constant.15 Here we report a patterning
method for TiO2 thin films using microcontact printingof alkanethiolate SAMs on gold, followed by selective
atomic layer deposition of the TiO2. Our approachconsists of three key steps, as shown in Figure 1. First,
patterned CH3-terminated alkanethiolate SAMs on gold
were formed by using microcontact printing. Second, theremaining regions of gold were coated with OHterminated alkanethiolate SAMs. Third, the TiO2 thinfilms were selectively deposited onto the SAM-patternedgold substrate by atomic layer deposition.
Experimental Section
Materials. 1-Octadecanethiol [CH3(CH2)17SH] (Aldrich; 98%),
11-mercapto-1-undecanol [HOCH2(CH2)10SH] (Aldrich; 97%),
ethanol (Aldrich; anhydrous, 99%), and [Ti(OPri)4] (STREM
Chemicals; 99.999%) were used as received. Poly(dimethylsi(19) Eckertova, L. Physics of Thin Films; Plenum Press: New York,1986.
(20) Gilmer, G. H.; Huang, H.; Roland, C. Comput. Mater. Sci. 1998,12, 354.

Chem. Mater., Vol. 16, No. 10, 2004 1879loxane) (Sylgard 184) was ordered from Dow Corning. Deionized water was purified with a Millipore Milli Q plus system,
distilled over KMnO4, and then passed through the Millipore
Simplicity system.
Preparation of Gold Substrates. Gold substrates usedin this research were prepared by evaporating 200 nm of gold
onto silicon (100) wafers that were precoated with 10 nm ofchromium to promote adhesion.
Preparation of SAMs. The freshly prepared gold substrates were immersed in a 2.5 mM solution of octadecanethiol(ODT) or mercaptoundecanol (MUO) dissolved in ethanol for
3 h. Monolayer deposition was carried out at room temperature. The samples were then ultrasonically washed in ethanol
to remove excess reactants and dried with nitrogen. The
quality of the monolayers was checked by XPS and watercontact angle.
Preparation of TiO2 Thin Films. The samples wereintroduced into the ALD system (Cyclic 4000, Genitech). TheTiO2 thin films were deposited onto the gold substrates using[Ti(OPri)4] and water as ALD precursors. Ar served as both acarrier and a purging gas. The [Ti(OPri)4] and water wereevaporated at 80 and 20 °C, respectively. The cycle consistedof 2 s exposure to [Ti(OPri)4], 5 s Ar purge, 2 s exposure towater, and 5 s Ar purge. The total flow rate of the Ar was 20sccm. The TiO2 thin films were grown at 100 °C under 2 Torr.
Preparation of Patterned SAMs. A patterned monolayerof CH3-terminated alkanethiolate on the gold substrate wasmade by using microcontact printing. Masters were fabricatedby using conventional photolithography or e-beam lithography
to pattern resists on silicon wafers. The masters have parallellines and spaces with dimensions from 5 to 0.5 µm. Poly(dimethylsiloxane) (PDMS) stamps were produced according
to a previously reported procedure.21-23 The PDMS stamps
were inked with a 10 mM ethanol solution of ODT and dried
with nitrogen. The stamp was placed in contact on the gold
substrate for 5 s. The stamp was carefully peeled off and thesubstrate was blown dry with nitrogen. The ODT-patternedgold substrates were immersed in a MUO solution for 30 min
to coat the remaining regions of the substrates with OHterminated alkanethiolate SAMs.
Analysis Techniques. Atomic force microscopy (AFM)
images of the 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 angles of the SAM samples weredetermined on a model A-100 Rame-Hart NRL goniometer inambient air by using the sessile drop method. X-ray diffraction(XRD, Rigaku RU-200BH) analysis was carried out to determine the crystallinity of the TiO2 films. The thickness of theTiO2 thin films was measured by scanning electron microscopy(SEM, JEOL) and UV spectrophotometer (Scinco S-3100).

Results and Discussion
1. Atomic Layer Deposition of TiO2 Thin Films.
The TiO2 thin films were grown on OH-terminatedalkanethiolate SAM-coated gold substrate by atomiclayer deposition using [Ti(OPri)4] and water precursors
at 100 °C. To verify that the growth mechanism is reallyself-controlled, the pulse time of the precursor wasvaried between 1 and 10 s. The film thickness per cycle
is saturated when the pulse time exceeds 2 s, whichindicates that the growth is self-controlled. Figure 2shows the thickness of the TiO2 thin films versus thenumber of ALD cycles. The growth of the TiO2 film
thickness is extremely linear relative to the number ofcycles, indicating that the ALD conditions were suf(21) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002.
(22) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994,10, 1498.
(23) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv.
Mater. 1994, 6, 600.

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Chem. Mater., Vol. 16, No. 10, 2004

Figure 2. Thickness of the TiO2 thin films versus the numberof ALD cycles.

Seo et al.

Figure 4. XRD pattern of TiO2 thin film grown at 100 °C.

Figure 5. XP spectra of TiO2 thin films deposited on (a) MUOcoated gold substrate, (b) ODT-coated gold substrate, and (c)clean gold substrate.

Figure 3. AFM images of clean gold substrate, MUO-coatedgold substrate, and TiO2 thin films deposited on MUO-coatedgold substrate.

ficient for complete reaction. The measured growth rate
was 0.51 Å per cycle. These results suggest that the TiO2
ALD is self-controlled and that the ALD conditions are
sufficient to complete reaction at 100 °C. The surface
morphology of the TiO2 films was examined by atomicforce microscopy. The root-mean-square (rms) roughnessof the TiO2 surface is as low as 10.2 Å, as shown inFigure 3. In comparison, the rms roughnesses of thecleaned and MUO-coated gold substrates are about 9.8and 9.9 Å, respectively. These results suggest that thesurface morphology of the TiO2 films grown at 100 °C
is as smooth as the clean gold substrate.
Figure 4 shows the XRD pattern for the TiO2 thin film
deposited on the MUO-coated gold substrate at 100 °C.
The thickness of the TiO2 film for XRD analysis wasabout 200 nm, which was thick enough to give us
detectable XRD line intensities. Only the gold substrate
peaks, however, were observed, indicating that an
amorphous TiO2 film was obtained at this temperature.
XPS analysis was performed to determine the composition of the TiO2 film deposited on the MUO-coated goldsubstrate at 100 °C. Figure 5a shows that the ratio of
the areas under the Ti 2p and O 1s peaks of this film isthe same as that of a TiO2 single crystal. For comparison, the TiO2 thin films were grown on the cleaned andODT-coated gold substrates under the same ALD conditions. The TiO2 thin films are not formed on thesesubstrates because the substrate surfaces do not haveany functional group to react with ALD precursors, asshown in Figure 5b,c. These observations indicate thatthe TiO2 ALD on the OH-terminated alkanethiolate
SAM-coated gold substrate at 100 °C is self-controlled
and extremely linear relative to the number of cycles
and that the TiO2 thin films are amorphous and assmooth as the clean gold substrate.

Deposition of TiO2 on SAM-Coated Gold

Chem. Mater., Vol. 16, No. 10, 2004 1881

Figure 6. AFM images and cross sections of the patterned ODT monolayers generated by using microcontact printing on gold
substrates: (a) 5.7 µm lines with 3.6 µm spaces, (b) 3.8 µm lines with 1.8 µm spaces, (c) 2.0 µm lines with 1.7 µm spaces, (d) 0.5µm lines with 0.4 µm spaces.

2. Formation of Patterned SAMs Using Microcontact Printing. Patterned SAMs of the octadecanethiolate on the gold substrate were made by using
microcontact printing. The masters we used for fabrication of stamps are silicon wafers with line-patterned
resists, on scales from 5 to 0.5 µm. The masters withlines larger than 1 µm were fabricated using photolithography on silicon wafers, whereas high-resolution
masters were fabricated using e-beam lithography. Thestamps were fabricated by casting poly(dimethylsiloxane) (PDMS) on the masters. After curing, the PDMS
stamps were peeled away from the masters. The PDMS
stamps were inked with an ODT solution and thenplaced in contact with the gold substrate. When thestamps were made using these masters, the raised linesof the master corresponded to the recessed spaces of thestamps. When the ODT-patterned SAMs were madeusing these stamps, a negative, hydrophobic pattern ofthe original master was produced on the gold surface.
Parts a and b of Figure 6 show AFM images and crosssections of patterned SAMs fabricated by using themasters having 3.8 µm parallel lines with 5.5 µm spaces
and 2.1 µm lines with 3.5 µm spaces, respectively. The
patterned SAMs in Figure 6a have 5.7 µm lines with
3.6 µm spaces, and the SAMs in Figure 6b have 3.8 µmlines with 1.8 µm spaces. Parts c and d of Figure 6 show
AFM images and cross sections of patterned SAMs
fabricated by using the masters having 1.9 µm lines

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Seo et al.

Figure 7. AFM images and cross sections of the patterned TiO2 thin films generated by using selective ALD on the SAMpatterned gold substrates: (a) 3.7 µm lines with 5.6 µm spaces, (b) 1.9 µm lines with 3.7 µm spaces, (c) 1.8 µm lines with 1.9 µmspaces, (d) 0.5 µm lines with 0.4 µm spaces.

with 1.8 µm spaces and 0.4 µm parallel lines with 0.4µm spaces, respectively. The patterned SAMs in Figure
6c have 2.0 µm lines with 1.7 µm spaces, and the SAMs
in Figure 6d have 0.5 µm lines with 0.4 µm spaces.
These images clearly show that the patterned monolayers retain the dimensions of the masters without
noticeable line spreading. Additionally, the height ofeach patterned monolayer is about 23 Å, which is close
to that of a densely packed ODT monolayer (∼24 Å).
The ODT-patterned gold substrates were then immersed in a MUO solution for 30 min to coat theremaining regions of the substrates with OH-terminatedalkanethiol SAMs. The regions covered by CH3-terminated SAMs formed from ODT are hydrophobic, low
free-energy surfaces, whereas the regions exposing theOH-terminated alkanethiolate monolayers of the gold
substrates are hydrophilic, high free-energy surfaces.
3. Selective Atomic Layer Deposition of TiO2Thin Films. The TiO2 thin films were selectively
deposited onto the SAM-patterned gold substrates byatomic layer deposition using [Ti(OPri)4] and water as
ALD precursors. The selective ALD of the TiO2 thinfilms was done at 100 °C and the number of ALD cycles
in a run was kept 300. The thickness of the TiO2 thinfilms, measured by AFM and SEM, is about 15 nm.
Figure 7 illustrates AFM images and cross sections ofmicropatterned TiO2 thin films, which were selectivelydeposited onto the monolayer-patterned gold substrateby ALD. The patterned SAMs showed high selectivityfor TiO2 ALD; hence, the patterns of the TiO2 thin films

Deposition of TiO2 on SAM-Coated Gold

were defined and directed by the patterned SAMs
generated with microcontact printing. The TiO2 thinfilms are selectively deposited only on the regions
exposing the OH groups of the MUO-coated gold substrates, because the regions covered with the ODTmonolayers do not have any functional group to reactwith ALD precursors. These AFM images clearly showthat the patterned TiO2 thin films retain the dimensionsof the patterned SAMs used as templates with nonoticeable line spreading.
Conclusions
We describe an atomic layer deposition of TiO2 thinfilms on OH-terminated alkanethiolate monolayercoated gold substrate at 100 °C. The ALD of the TiO2thin films is self-controlled and extremely linear relative
to the number of cycles, and the TiO2 thin films aresmooth and amorphous. We also demonstrate a selective
atomic layer deposition of TiO2 thin films on patterned

Chem. Mater., Vol. 16, No. 10, 2004 1883

alkanethiolate self-assembled monolayers formed bymicrocontact printing. The atomic layer deposition offers
an ideal method for selective deposition of thin films
on patterned SAM-coated gold substrate, because theALD process is very sensitive to the surface conditionsof the substrates. The TiO2 thin film is selectively
deposited only on the regions of the gold substrates
covered with the OH-terminated monolayers, because
the regions covered with CH3-terminated alkanethiolatemonolayers do not have any functional group to reactwith precursors.
Acknowledgment. This work was supported by aprogram of National Research Laboratory from theMinistry of Science and Technology, and a National
R&D Project for Nano Science and Technology from theMinistry of Science and Technology.
CM035140X