Replication of Surface Structures with Polydimethylsiloxane
(PDMS Soft Lithography)
The procedure shown here was modified by T. Armbrister, G. Grigoriev, K. Hansgen, Z. Hess,
T. Ksander, X. Ma, J. Reid, A. Rini, and S. Rudisill from D. J. Campbell,
K. J. Beckman, C. E. Calderon, P. W. Doolan, R. M. Ottosen, A. B. Ellis, and
G. C. Lisensky, Journal
of Chemical Education, 76, 537-541, (1999).
PDMS is cured by an organometallic crosslinking
reaction to give an optically transparent polymer with the ability to reproduce
surface features. In the experiment the polymer is cured
in contact with optical transform slides or the features on a CD. The
surface features thus imprinted into the elastomer can be distorted mechanically
and their spacings monitored by Fraunhofer diffraction.
Procedure
Wear eye protection
Chemical gloves recommended
Never look directly into a laser
or shine a laser at another person.
PDMS monomer can be messy. Cover work surfaces with foil.
Preparation of PDMS
Dispensing the viscous liquid can be messy. Cover the work surface
and the balance with aluminum foil. Wear gloves.
Add 4.00 g of Sylgard polymer base to a large weighing
boat using a disposable plastic spoon. Add 0.40 g of curing agent using
a disposable dropper.
Thorough mixing of the PDMS components is essential for good
curing. Improper mixing can result in a polymer that is a sticky
mess. On
the order of 100 strokes with a stir stick are needed to mix the polymer
components so that they will yield an adequately cured sample.
Bubbles degrade the optical qualities of cured PDMS so bubbles
should be removed before curing. Most of the trapped bubbles from
mixing of the components will eventually rise to the top of the liquid
where they may be broken by blowing across the surface. Sonicating
the mixed liquid can help remove the bubbles more quickly. If an
ultrasonic cleaning bath is available, float a weighing boat containing
the mixed PDMS components on the surface of the water
in the sonicated bath.
Optical Transform Slide used as a lithography master
Identify the emulsion
side of an optical transform film (the side containing raised arrays). The
shiny side of the film will sharply reflect room lights on its smooth
surface; the matte emulsion side of the film will give more diffuse reflections. Peel
open the white plastic slide mount, pick up the film by its edges, and
place the film in a weighing boat with the emulsion side facing
upward.
PDMS can get caught in the sprocket holes. Either cut off the holes or make
a release layer by cutting out a rectangle to form a hole in a sheet of
photocopier or laser printer transparency. Place the plastic sheet with the
hole over the slide and place a rectangular metal frame on the plastic sheet.
Slowly pour about 4 g of the uncured PDMS mix into the mold assembly. Leave any
remaining PDMS sticking to the walls of the weighing boat; too many bubbles
are created during attemps to remove it. Let the assembly sit at room temperature
for a few minutes so that bubbles incorporated during pouring can rise
out of the PDMS. Gentle blowing over the surface may also eliminate bubbles.
Place the mold into the oven at 130 °C for 20 min.
Remove the mold from the oven and allow it to cool. Gently
remove the plastic sheet and film.
Recordable CD used as a lithography master
An alternative pattern with smaller features is a CD-R disk. Cut
out a section using scissors.
Carefully peel off the aluminum foil.
You may use either the foil or the polycarbonate support as the lithography
master.
Follow the procedure above, again using 4.00 g of Sylgard polymer base and
0.40 g of curing agent. You may reuse the same weighing boat for mixing.
Pour the mixture into the mold and place the mold into the oven at 130 °C
for 20 min.
Remove the mold from the oven and
allow it to cool. Gently
remove the foil or the CD portion from the cured PDMS.
Recordable DVD used as a lithography master
An alternative pattern with smaller features is a DVD-R disk. Cut
out a section using scissors.
Carefully peel apart the two polymer layers.
You may use either the foil or the polycarbonate support as the lithography
master.
Follow the procedure above, again using 4.00 g of Sylgard polymer base and
0.40 g of curing agent. You may reuse the same weighing boat for mixing.
Pour the mixture into the mold and place the mold into the oven at 130 °C
for 20 min.
Remove the mold from the oven and allow it to cool. Gently
remove the foil or the DVD portion from the cured PDMS.
Optical diffraction to measure feature sizes
Use the Fraunhofer equation, d sin φn = n λ,
to determine the feature spacing, d, by passing a laser with
wavelength λ through
the sample and measuring the nth diffraction angle, φn. This equation
assumes the incident laser beam is perpendicular to the surface and
that the beam passes through the sample. (For small angles sin φ ≈
X / L, where X is the diffraction
spot spacing and L is the PDMS slab-to-screen distance.)
How does the feature spacing compare with that of the original?
If you stretch the PDMS slab, how does the diffraction pattern change?
For the CD and DVD how do your values compare with the track spacing in the original?
Determine the feature spacing in the PDMS using the Fraunhoffer equation, d sin φn = n λ,
as above.
Determine the feature spacing, d, in the original CD or DVD
by shining a laser on the sample, measuring both the incident angle, θi and
the nth diffraction angle, θn, with respect
to the surface normal and using d (sin θn -
sin θi)
= n λ, where λ is the laser wavelength.
Optical
Transform Slides, Institute for Chemical Education, Madison, WI. A lower
quality alternative is to use a dithering pattern (from standard
computer art programs such as Canvas, Photoshop, or from the box shading function
in Microsoft Word) printed onto a transparency.
Molds cut from 1x1 inch 14 gauge (.083") square metal tubing
Oven (130 °C)
CD-R or DVD-R (Sony CD-R and Maxell DVD-R used successfully above) and scissors
