Lab-on-a-Chip for Studying Growing Pollen Tubes - Plant Cell Morphogenesis: Methods and Protocols - page 235

8. Low-resolution (low cost) photomasks can be easily obtained
by using high-resolution digital printing on a transparent fi lm
at 3,600 dpi. However, this method results in rough walls for
feature sizes in the range of 10-50 μ m and completely misses

features below 10 μ m. For smoother, well-defi ned features

m, a more precise (and expensive) alternative is
Direct Writing Laser Lithography on a glass/chrome mask.
9. The piranha bath is advised but optional depending on the
cleanliness state of the silicon wafer and the cleanroom. However,
we found that it is a must if the silicon wafer is being recycled.

down to 1 μ

10. The HF cleaning is used to eliminate any native oxide layer on
the silicon wafer. We have noticed that this native oxide layer
often prevents the SU-8 from adhering fi rmly to the wafer
during the developing process. It is advised to keep the silicon
wafers always inside a hermetic box in the cleanroom to avoid
contact with the ambient air as much as possible. HF cleaning
should be carried out if and only if adhesion problems between
silicon and SU-8 arise since HF needs to be handled with
extreme care due to its potentially lethal effects. Although not
advised, the Tefl on container can be replaced with a regular
plastic container if necessary.
11. We found that it is best to dispense the SU-8 directly from the
bottle to avoid the formation of air bubbles in the SU-8 layer.
Be particularly careful at the end of the dispensing when pulling
out the bottle since a narrow stream of photoresist might easily
create bubbles in the already dispensed SU-8. Stop the dispens-
ing with tissue if necessary. The spin speed is set to obtain a
thickness of 80 μ m since Camellia pollen grains vary from 50

to 60 m in diameter. However, the thickness can be changed

to accommodate different-sized specimens. A hot plate is pre-
ferred over a convective oven to ensure uniform temperature
across the wafer during soft-bake. Thicker layers require longer
soft-bake times (see manufacturer's SU-8 datasheet for a com-
plete table of suggested soft-bake times). After proper soft-bake
the SU-8 layer must be smooth; extend the duration of the
soft-bake should wrinkles, bumps, or bubbles appear on the
surface (an expired resist can also produce similar issues).

μ

12. UV light exposure is critical in the mold fabrication. Make
sure the SU-8 layer is completely fl at and hardened before
exposure. Any air bubble or dirt in the SU-8 layer will result in
a loss of features in the area. Although exact exposure energies
can be found in the SU-8 datasheet, an exposure matrix must
be carried out in order to determine the optimal exposure
parameters for the current setup of UV exposure system, sub-
strate, and SU-8 thickness. Enough exposure should produce
a latent image on the SU-8 layer within the fi rst minute of
postexposure bake. It is useful to realize that since SU-8 is a