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PII: S0040-6090(97)00682-2
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Thin Solid Films 315 1998 912
Letter
Patterning of a polysiloxane precursor to silicate glasses by microcontact
printing
Christian Marzolin, Andreas Terfort, Joe Tien, George M. Whitesides
)
Department of Chemistry, Har</i>Õ<i>ard Uni</i>Õ<i>ersity, Cambridge, MA 02138, USA
Received 10 June 1997; accepted 23 July 1997
Abstract
A polysiloxane precursor to silicon dioxide was used to generate silica patterns on a flat surface. The conversion of a thin film of this
polymer to silica was done either by photolithography or by printing an organic acid on the film. The former technique allowed the
formation of structures with thickness as large as 2 mm. The latter method could generate patterns with sub-micron resolution and .
thicknesses smaller than 0.2 mm. These structures could be used as resists for O
reactive ion etching RIE . q 1998 Published by
2
Elsevier Science S.A.
Keywords: Glass photoresist; Microcontact printing; Polysiloxane
1. Introduction
This paper describes a new method to generate mm-scale
patterns of silicate glass on flat and non-planar substrates. The precursor material is a polysiloxane polymer spin-on
.
glass , which can be crosslinked into silica by exposure to
protons. Two different methods can lead to the formation
of silica structures. The first methodwhich has been
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described in the literature 1 involves the sensitization
of the polysiloxane by a UV acid generator and produces
patterns by photolithography. The second, non-photolitho-
graphic technique that we describe converts the polymer to
silica by bringing an elastomeric stamp bearing the desired
pattern and inked with a suitable acid into contact with it. .
This methodmicrocontact printing mCP of acidsis a
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useful variant of soft lithography 2 .
Layers of silicon dioxide or spin-on glass are commonly
used in VLSI fabrication. They have good planarization
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and dielectric properties 3 . They are also used as resists
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for etching metallic and organic layers 4 . Chemically . w x
amplified glass photoresist GP 1 has given conventional
spin-on glass the capability to accept patterning, and has
been used as a resist in multilevel processing. .
Microcontact printing
mCP has been developed to .
pattern self-assembled monolayers SAMs on gold and
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silver 5 , and, with more difficulties, on SirSiO
and
2
)
Corresponding author.
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other substrates 6 . These monolayers can act as resists for
wet etching and allow the fabrication of microstructures
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7 . The combination of acid-sensitive spin-on glass and
mCP provides a useful new capability to form silica mi-
crostructures.
Here, we present a method to fabricate a patterned glass
film. A film of acid-sensitive spin-on glass on a silicon
wafer is stamped with a suitable acid. The acid diffuses
into the film and induces the transformation of the polymer
into a precursor to silica. After removing the unreacted
polymer and curing, a silicate glass pattern is produced. .
Although films as thick as 2 mm after processing have
been patterned by UV exposure of the polysiloxane precur-
sor, the thicknesses of the films that can be patterned by
stamping did not exceed 0.2 mm, reflecting, we presume,
the requirement that the protons diffuse from the surface
into the bulk of the thin film. Nevertheless, films patterned
by stamping were sufficiently thick to act as resists for O
2 .
reactive ion etching RIE of an underlying polymer film.
2. Experimental
2.1. Materials .
Polished silicon wafers
Silicon Sense, MA
were
cleaned before use by sonication in trichloroethylene, ace-
tone and methanol. Aluminum-coated wafers were pre-
0040-6090r98r$19.00 q 1998 Published by Elsevier Science S.A. All rights reserved. .
PII S 0 0 4 0 - 6 0 9 0 9 7 0 0 6 8 2 - 2 (
)
C. Marzolin et al.r Thin Solid Films 315 1998 912
10 pared by e-beam evaporation of ; 100 nm of Al 99.99%,
.
Alfa . Diacetoxy-di-t</i>-butoxysilane and methyltriacetoxysi- .
lane United Chemicals were used as received. Tetrahy- . .
drofuran
THF , 4-methyl-2-pentanone
MIBK , t</i>-butyl .
methyl ether, 2-propanol IPA , and triethylamine were
used without further purification. Triphenylsulfonium hex- .
afluoroantimonate 50% in propylene carbonate was used
as received. The elastomeric molds used in mCP were prepared by casting a silicone precursor
Sylgard 184,
.
Dow Corning, NY on masters prepared by conventional
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photolithography 8 .
2.2. Formation of the n-alkylsulfonic acids .
The commercially available Aldrich sodium salt of the .
respective sulfonic acid 1 g was suspended in anhydrous .
Et O 50 ml , and HCl was bubbled in under cooling.
2
After 15 min, the volatiles were removed in vacuo, and the
residue was resuspended in anhydrous Et O and filtered.
2
The clear organic phase was evaporated to dryness, yield-
ing the corresponding sulfonic acids as colorless oils or
solids. Yields were generally above 90%.
2.3. Synthesis of glass photoresist
Synthesis of the glass photoresist followed the literature
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1 . Briefly, a solution of 46.2 g of diacetoxy-di-t</i>-buto-
xysilane in 1.5 l of MIBK was cooled to y108C under
nitrogen atmosphere, and 68 ml of triethylamine was slowly
added to it. A solution of deionized water in 80 ml of THF
was added dropwise, and the mixture was heated to 708C
for 4 h. The silane polymerized by hydrolysis and conden-
sation of the acetoxyl groups, the t</i>-butoxyl groups remain-
ing intact. We used a 3:1 molar ratio of water:silane,
which corresponds to six equivalents in the polymerization
reaction. After cooling and rinsing the solution with water,
the product was purified through a short silica column. .
The methyl-doped GP GP:Me was prepared with the
same procedure as the undoped polysiloxane, except that 4
g of methyltriacetoxysilane was added as a 10% solution
in MIBK at the end of the polymerization reaction and left
to react at 708C for another hour. This corresponded to a
10% molar ratio of methyl to silicon.
2.4. Priming
The silicon wafers were primed before spin-coating of
the GP films. Priming increased the adhesion between the
exposed GP and the wafer and prevented the film from
cracking. A freshly prepared 1:1:10 mixture of diacetoxy-
di-t</i>-butoxysilane, triethylamine, and MIBK was spin-
coated on the wafers at 3000 RPM. The wafers were then
rinsed with IPA and deionized water. The contact angle of
water with the primed surface was ca. 408 at this stage,
showing that the wafer was partly covered with t</i>-butoxyl
groups.
2.5. UV exposure
For UV exposure, the glass photoresist GP:Me was
sensitized with 2% molar of triphenylsulfonium hexafluo-
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roantimonate 1 . After exposure of the films through a chrome mask with a mercury lamp main wavelengths:
.
365 nm and 405 nm , the wafers were baked at 1008C for
2 min. They were then developed in anisole for 30 s with
strong agitation and blown dry with nitrogen.
2.6. Microcontact printing
The procedure used in patterning of GP films by mCP
is depicted on Fig. 1. An elastomeric stamp was inked by
soaking for 1 s in a 10-mM solution of hexadecanesulfonic
acid in a 1:1 mixture of t</i>-butyl methyl ether and IPA. The
stamp was blown dry for 1 min. It was then put in contact
with the GP-coated wafer for 1 min, and the wafer was
baked on a hot plate at 1308C for 20 min. Development
was carried out in anisole for 20 s.
2.7. Reacti</i>Õ<i>e ion etching A 1.4-mm thick film of polyimide Pyralin PI 2556, du
.
Pont was spun onto a wafer and cured at 3508C for 30
min. The surface of the film was then oxidized in an O
2
plasma for 20 s, and GP was spin-coated at 2500 RPM
from a 10% MIBK solution. Microcontact printing and
development of GP were performed as described above.
Following development, the wafer was etched in an O
2
Fig. 1. Schematic outline of the method used to fabricate patterns on a
glass photoresist film by microcontact printing. The silicone stamp is
briefly soaked in a 10 mM solution of hexadecanesulfonic acid in t</i>-butyl
methyl ether. It is then blown dry for 1 min and put in contact with a
wafer coated with 0.6-mm thick GP film for 30 s. The wafer is baked for
10 min at 1308C on a hot plate and developed for 20 s in anisole. (
)
C. Marzolin et al.r Thin Solid Films 315 1998 912
11 .
RIE 25 sccm, 150 mTorr, 100 W, y600 V
for 12
bias
min.
3. Results
3.1. UV exposure
A maximum thickness of ca. 1 mm before exposure and
0.3 mm after exposure was reached for conventional glass
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photoresist, as reported in the literature 1 . Thicker films
cracked either during spin-coating or during post-exposure
baking. Doping silica precursors with methyl groups is
known to increase the compliance and decrease the ten-
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dency of the silica structures to break during annealing 9 .
Commercial spin-on glass contain methyl groups for this
purpose. The methyl-doped glass photoresist that we used
allowed us to reach thicknesses of 6 mm by spin-coating a
.
Fig. 2. SEMs of patterned silica structures. a These structures were
generated by photolithography on a 6-mm thick GP:Me film using a
chrome mask patterned into 2-mm lines separated by 2 mm. After
development, the wafer was annealed at 4008C for 1 h in air. The
.
resulting lines were 2 mm high and 1.5 mm wide. b These structures
were generated by mCP on a 0.6-mm thick GP film with a silicone stamp
bearing 2-mm lines separated by 2 mm. The lines were annealed at 4008C
for 1 h in air.
Fig. 3. SEMs of silica patterns fabricated by stamping a 0.6-mm thick GP
film on an Al-coated wafer.