A New Patterning Method Using Photocatalytic Lithography and Selective Atomic Layer Deposition

Article abstract of DOI:10.1021/ja038769+

We report a new patterning method using photocatalytic lithography of alkylsiloxane self-assembled monolayers and selective atomic layer deposition of thin films. The photocatalytic lithography is based on the fact that the decomposition rate of the alkylsiloxane monolayers in contact with TiO₂ is much faster than that with SiO₂ under UV irradiation in air. The photocatalytic lithography, using a quartz plate coated with patterned TiO₂ thin films, was done to prepare patterned monolayers of the alkylsiloxane on Si substrates. A ZrO₂ thin film was selectively deposited onto the monolayer-patterned Si substrate by atomic layer deposition. Copyright

Full text of DOI:10.1021/ja038769+

Published on Web 12/12/2003
A New Patterning Method Using Photocatalytic Lithography and Selective
Atomic Layer Deposition
Jae P. Lee and Myung M. Sung*
Department of Chemistry, Kookmin UniVersity, Chongnung-dong, Songbuk-ku, Seoul 136-702, Korea
Received September 28, 2003; E-mail: smm@kookmin.ac.kr
Self-assembled monolayers (SAMs) are thin organic films which
form spontaneously on solid surfaces. They have been shown to
be useful as passivating layers and also for the modification of
surface properties. Potential applications of the SAMs include
wetting, adhesion, friction, chemical sensing, ultrafine scale lithog-
raphy, and protection of metals against corrosion.1-3 A key to
utilizing SAMs in many advanced applications is the ability to
pattern the monolayers.
High-resolution patterns can be formed by printing SAMs using
soft lithography,4-6 or by decomposing SAMs using electron
beams,7 ion beams, photolithography,8 or scanning probe micros-
copy.9 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 various patterning
methods, photolithography is the most practical, because it can
transfer an entire pattern on a photomask to a SAM at a single
time. Photolithography with alkanethiol SAMs has been done to
obtain submicrometer spatial resolution.10 However, there have been
a few reports on UV photopatterning of alkylsiloxane SAMs in
air. Previous studies have shown that the UV photopatterning of
the alkylsiloxane SAMs requires relatively large amounts of energy
because they are quite stable to UV irradiation of 254 nm, as
compared to alkanethiolate SAMs.11 Recently, we found that the
Figure 1. Schematic outline of the procedure to fabricate patterned thin
films by using photocatalytic lithography and ALD.
monolayers are rapidly and homogeneously decomposed on a TiO2
thin film under UV irradiation in air through the photooxidation of
the alkyl chains.12 Upon UV irradiation of 254 nm, the semiconduc-
conditions of the substrates, it is an ideal method for selective
tor TiO2 can generate electron-hole pairs, and subsequently produce
highly reactive oxygen species (e.g., OH, O2 radicals) via electron
scavenging by adsorbed O2 and hole trapping by the surface OH-
or adsorbed H2O.13 These oxygen radicals can oxidize and de-
compose most organic compounds and some inorganic compounds.
Considering its effectiveness for the photocatalytic decomposition
of organic compounds,14 TiO2 photocatalysis can be a promising
way to decompose the alkylsiloxane SAMs and can be ultimately
applicable to the UV photopatterning of them.15,16
Among various fabrication techniques, there has been a growing
interest in forming a patterned thin film by site-selective deposition
on a patterned monolayer as a simple low-cost fabrication pro-
cess.17,18 Selective depositions of thin films using chemical vapor
deposition, electroplating, and electroless deposition have also been
accomplished with patterned SAMs as templates. Atomic layer
deposition (ALD) is a gas-phase thin film deposition method that
uses self-terminating surface reactions. During the past decade, ALD
has attracted considerable attention as a method for manufacturing
high-quality thin films and producing tailored molecular structures.19
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 the ALD
inherently eliminates gas-phase reactions. Elimination of the gas-
phase reaction results in emphasis of the importance of the surface
reaction. Because the ALD process is very sensitive to surface
deposition of thin films on patterned SAMs.
Here, we report a new fabrication method using photocatalytic
lithography of alkylsiloxane SAMs, followed by selective deposition
of thin films using atomic layer deposition. Our approach consists
of three key steps, as shown in Figure 1. First, the alkylsiloxane
SAMs were formed by immersing Si substrate in alkyltrichlorosilane
solution. Second, photocatalytic lithography using a quartz plate
coated with patterned TiO2 thin films was done to prepare patterned
SAMs of alkylsiloxane on the Si substrate. Third, ZrO2 thin films
were selectively deposited onto the SAMs-patterned Si substrate
by atomic layer deposition.
The octadecylsiloxane SAMs were formed by placing the
oxidized Si substates in a 2.5 mmol solution of octadecyltrichlo-
rosilane (OTS) precursor dissolved in 4:1 hexadecane/chloroform
for 1 h. The samples were then washed in carbon tetrachloride to
remove excess reactants and dried with nitrogen. The quality of
the monolayers was checked by X-ray photoelectron spectroscopy
(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.
Photodecomposition of the octadecylsiloxane SAMs in contact
with TiO2 thin films deposited on the quartz plates was studied
using XPS and contact angle analysis. For comparison, photode-
composition of the SAMs in contact with SiO2 of the quartz plate
surface without the TiO2 was also studied. The decomposition rate
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10.1021/ja038769+ CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Three-dimensional AFM image (5 µm × 5 µm) for the patterned
octadecylsiloxane monolayer by using photocatalytic lithography.
of the monolayers in contact with the TiO2 thin films is 20 times
faster than that in contact with the SiO2 under UV irradiation in
air. The octadecylsiloxane SAMs in contact with the TiO2 are
completely decomposed after 2 min of UV irradiation, whereas the
monolayers in contact with the SiO2 remain intact.
Figure 3. Three-dimensional AFM images (5 µm × 5 µm) and cross
sections for the patterned ZrO2 thin films fabricated by using photocatalytic
lithography and ALD.
Patterned SAMs of the octadecylsiloxane on the Si substrate were
made by using photocatalytic lithography. The quartz plate coated
with patterned TiO2 thin films having 400 nm parallel lines and
580 nm spaces was placed in contact on the SAMs-coated Si
substrate. The SAMs-coated Si substrate was then exposed through
the quartz plate to a 254 nm UV lamp for 2 min. Figure 2 shows
an AFM image of patterned SAMs having 575 nm lines with 405
nm spaces, indicating that the patterned monolayer retains the
dimensions of the TiO2 pattern of the quartz plate without noticeable
line spreading. Additionally, the height of the patterned monolayer
is about 24 Å, which is close to that of a densely packed
octadecylsiloxane monolayer (∼25 Å). This observation indicates
that the monolayer regions contacted with the TiO2 thin films of
the quartz plate are completely decomposed during the first 2 min
of irradiation and expose the silanol groups of the Si substrate.
However, the monolayer regions not in contact with the TiO2 of
the quartz plate remain intact after 2 min of UV irradiation and
still expose the methyl groups of the SAMs.
cylsiloxane on the Si substrate were made by using the quartz plate
coated with the patterned TiO2 thin films under UV irradiation in
air. These patterned SAMs define and direct the selective deposition
of the ZrO2 thin films using ALD.
Acknowledgment. This work was supported by a program of
the National Research Laboratory from the Ministry of Science and
Technology, and grant no. R01-2001-000-00047-0 from the Korea
Science & Engineering Foundation.
Supporting Information Available: Details of materials, prepara-
tion of Si substrates, preparation of quartz plates coated with patterned
TiO2 films, photocatalytic lithography, preparation of patterned ZrO2
films, and analysis techniques (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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A ZrO2 thin film was selectively deposited onto the octadecyl-
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