Transcript Microcontact Printing - University of Oklahoma

Microcontact Printing
(Soft Lithography)
OU NanoLab/NSF NUE/Bumm & Johnson
Outline
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Motivation
History
Schematic
Procedure
– Photomask
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Snowflake
"I Love NanoLab"
OU interlock logo
Stamp Master
The stamp
Chemistry of the Process
Transferring the Pattern
Etching the Substrate
Examples
OU NanoLab/NSF NUE/Bumm & Johnson
Motivation
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Production of silicon
integrated circuits
Patterning biological
macromolecules,
organic and inorganic
salts, colloidal
materials, conducting
polymers, polymer
beads
The figure to the right
is a patterned rod
OU NanoLab/NSF NUE/Bumm & Johnson
Image courtesy of Jackman et al, Science 1998. vol. 280(5372) pp 2089-2091
Motivation: Novel
Example
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Transferring
micrometer size
pattern onto various
surfaces
– Surface does not have
to be flat (as opposed
to photolithography)
– The figure to the right
is an example of how
one would transfer
patterns to a cylindrical
rod
OU NanoLab/NSF NUE/Bumm & Johnson
Image courtesy of Jackman et al, Science 1998. vol. 280(5372) pp 2089-2091
History
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Both lithography and stamp printing have been
around for centuries
– It was the combination of the two that gave rise to
microcontact printing
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In 1993, Amit Kumar and George M. Whitesides
developed microcontact printing at Harvard
University
Subsequent methods of soft lithography have since
been explored
In 1996 IBM began research on improving optical
lithography in order to enhance the precision of the
printing process
OU NanoLab/NSF NUE/Bumm & Johnson
Process Schematic
A prepolymer (2)
covering the master
(1) is cured by heat
or light and
demolded to form
an elastomeric
stamp (3), which is
inked by immersion
(4) or with an ink
pad (5) and printed
onto the substrate
(6), forming a selfassembled
monolayer, which is
transferred into the
substrate by a
selective etch.
Scanning electron
micrographs show
the master, image
of the stamp, and
the printed and
etched pattern.
OU NanoLab/NSF NUE/Bumm & Johnson
Image taken from http://www.aip.org/tip/INPHFA/vol-8/iss-4/p16.pdf
Photomask
opaque
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Create desired pattern on computer
(AutoCAD,
Adobe Illustrator, or your favorite graphics program)
Print onto transparency
Further reduction of size can done
photographically
Pattern must be in solid black and white and
printed using an opaque ink, which is
determined by the photoresist
OU NanoLab/NSF NUE/Bumm & Johnson
About the Photomask:
Snowflake
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The snowflakes in the photomask
are made such that each
subsequent one was half the size
of the previous one
Measurements were made
regarding snowflake size as well
as distance between the
snowflakes
– These measurements were used
to evaluate how well the pattern
transfer occurred
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0.629 mm
11.3 mm
9.175 mm
The depth of the stamp was
determined (determined by the
master):
– If the stamp is too shallow, there
can be contact in between the
features
– If the stamp is too deep, the
features can be distorted
OU NanoLab/NSF NUE/Bumm & Johnson
From Whitesides et. al paper
About the Photomask:
"I Love Nanolab"
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In this photomask,
“I Love Nanolab”
the largest text
(#1) are 3mm by
21.3 mm. Each
subsequent set is
half the size of the
previous.
The smallest size
(#8) is 128 times
smaller than the
biggest size, and is
23.4µm X 166.4µm
3 mm
Don’t YOU love NanoLab?
OU NanoLab/NSF NUE/Bumm & Johnson
21.3 mm
About the Photomask:
OU Logo
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In this photomask,
the largest OU logo
is 10 mm wide and
13.7 mm long.
10
mm
13.7 mm
OU NanoLab/NSF NUE/Bumm & Johnson
Stamp Master (mold)
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Done on glass slide using the mask as the source
of the pattern
Apply photoresist (10-20 μm)
– (MicroChem SU-8 2010) Spin coat for even distribution
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Cover Sample with Mask and Expose to UV light
– The resist will harden upon exposure (negative resist)
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Soft-Baking
– Put on hotplate, or oven for 3 minutes
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Develop
– Unexposed resist will be dissolved, while the exposed
resist remains
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Wash Master and clean up
OU NanoLab/NSF NUE/Bumm & Johnson
The Stamp
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Clean master sample and place in walled
container (disposable petri dish).
Pour silicone resin evenly over master
– The silicone resin is liquid polydimethylsiloxane
[PDMS]
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Bake the PDMS to solidify (65C for 15-20h).
Remove cured stamp from master and wash
both in EtOH
OU NanoLab/NSF NUE/Bumm & Johnson
Chemistry of the Process
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Stamp is inked with a thiol (R-S-H)
The thiol ink pattern is stamped
onto the metal surface
The thiol ink works as an etch resist
Etching removes metal on uninked
areas
A thiol ink is ~2mM solution of
hexadecanethiol
The substrate is a Pd or Au thin film
evaporated on polished Si wafer
OU NanoLab/NSF NUE/Bumm & Johnson
Transferring the Pattern
to a Thin Metal Film
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Wash both substrate (the metal) and
the stamp in EtOH
Using Q-Tip, coat the stamp with the
thiol solution and dry it
dry under N2 stream for ~30sec
Bring stamp into contact with
substrate for 10 sec., remove stamp
and dry substrate under N2
OU NanoLab/NSF NUE/Bumm & Johnson
Useful Chemistry
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The sulfur atoms bonds to the metal (Pd or Au)
The carbon chains of the thiol will then align
with each other to create a hydrophobic
SAM (self-assembling monolayer)
The monolayer acts as a protective coating
against the etchant
– The best results are often obtained by the longest chains
because they create a larger and therefore better barrier
OU NanoLab/NSF NUE/Bumm & Johnson
Etching the Substrate
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Immerse substrate in diluted (1:3) etchant for
<30 sec (Pd Etchant, type TFP in CYANIDE!!! –
be careful, it’s highly toxic!)
The time here is very important!!! If you leave
it too long in the etch, you will most likely
remove ALL the Pd, if it’s not long enough,
then your pattern will not be fully developed.
Remove from etchant and immediately quench
it in deionized water, rinse and dry.
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Good and Bad Examples:
Black lines
drawn to
mark the
edge of the
snowflakes
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