Lab-on-a-Chip for Studying Growing Pollen Tubes - Plant Cell Morphogenesis: Methods and Protocols - page 231

3. Draw the microfl uidic design in Fig. 1 in the CAD software
( see Note 7 ).
4. Reproduce the design on a photomask ( see Note 8 ).

1. The fabrication process of the silicon/SU-8 mold is illustrated
in Fig. 2a . Start by cleaning a silicon wafer with a piranha bath.
The solution is dangerously aggressive and corrosive; use pro-
tective gear. Carefully mix three parts of sulfuric acid (H 2 SO 4 )
and one part of peroxide (H 2 O 2 ) in a glass container. A total
volume of 200 ml is enough for cleaning one or two wafers.
Use tweezers to place the silicon wafer slowly inside the solu-
tion. The exothermic reaction of the solution is good for
cleaning only for about one hour. Afterwards, move the wafer
to deionized water and air-dry with fi ltered, pressurized air or
ideally with a N 2 gun ( see Note 9 ).
2. Perform an HF cleaning. Hydrofl uoric acid is a lethal solution;
handle with extreme care. Use protective gear: butyl rubber
gloves, face shield, safety glasses, leather closed shoes, lab coat,
and chemical apron. Perform the cleaning inside a fume hood.
Have a safety shower and HF antidote (calcium gluconate gel)
nearby in case of skin contact. Mix 10 ml of HF with 200 ml
of deionized water in a Tefl on container. Place the silicon
wafer slowly inside the solution with tweezers and leave for
3 min. Next move the wafer to deionized water and air-dry.
Neutralize the HF with copious amounts of diluted sodium
bicarbonate. HF should be disposed as a corrosive hazardous
waste ( see Note 10 ).
3. Spin-coat 4 ml of SU-8 on the 10 cm silicon wafer at 1,500 rpm
for 30 s. Soft-bake for 5 min at 65 °C and then 10 min at
95 °C on a hot plate to harden the photoresist (by evaporating
the photoresist solvent) and to increase adhesion to the sub-
strate. Next, leave the wafer to cool down at room tempera-
ture ( see Note 11 ).
4. Expose the negative photoresist SU-8 to UV light using the
photomask ( see Note 12 ).
5. Perform a postexposure bake (PEB) directly after exposure to
enhance the chemical linking induced by the UV light. Bake
for 5 min at 65 °C and then 10 min at 95 °C on a hot plate.
6. Develop the photoresist layer to obtain the fi nal SU-8 pattern.
Pour enough developer in a glass container to fully cover the
silicon/SU-8 mold. Immerse the silicon/SU-8 mold in the
SU-8 developer to dissolve the areas not exposed to UV light.
Agitate gently. The development time depends directly on the
thickness of the SU-8 layer. For an 80

3.2 Silicon/
SU-8 Mold

m thick SU-8 layer,
the development time is about 8 min. Next, rinse with IPA
and again with fresh developer. Air-dry. Monitor the state of

μ