Biology Reference
In-Depth Information
negative photoresist, the areas exposed to UV light are meant
to remain on the silicon wafer, whereas the areas covered by
the photomask will be dissolved during development.
Therefore, it is generally preferred to overexpose (ensure the
exposed SU-8 will remain at the expense of enlarging the fea-
tures due to light scattering) than to underexpose (lose parts
of the design).
13. If there are adhesion problems between the silicon wafer and
the SU-8 layer, the photoresist will peel off from the wafer
during development. Should this detachment occur, an HF
cleaning may be required.
14. Since the photoresist is a thermal resin, the hard-bake tem-
perature should be carried out at a temperature slightly higher
than the expected microdevice operating temperature in order
to ensure the mechanical properties of the photoresist. The
visual effect of the hard-bake is to “smooth” the SU-8 layer.
15. Direct visual inspection of the mold is required to determine
what step of the process went wrong or can be improved.
Verify the thickness of the mold to adjust the SU-8 spin-
coating speed accordingly. Verify the dimensions of the mold
to adjust the exposure time and possibly the PEB temperature.
Pay particular attention to the smoothness of the SU-8 walls
and any lost feature since it may indicate a photomask with a
resolution that is too low. Any undeveloped SU-8 can be
removed by extending the development time. Any cracks in
the mold bulk can be reduced by increasing the hard-bake
temperature or duration.
16. We found that a minimum of 65 °C is needed for proper
silanization. The level of silanization can be varied by dispens-
ing more or less drops or by using shorter or longer times.
Adjust if necessary. We found that glass can also be silanized in
the same way.
17. The total amount of poured PDMS depends on the size of the
container of the mold. In order to not waste PDMS, place the
silicon/SU-8 mold in a glass container of approximately the
same size. Aluminum foil can also be used as a container with
folded “walls.” The PDMS on top of the mold will determine
the thickness of the PDMS layer. This thickness of the PDMS
layer is not critical, usually 2-3 mm, as long as the inlet and
outlet drilling can be done properly. However, excessive thick-
ness may compromise the optical properties when using high-
resolution microscopy. Since the PDMS volume for a 100 mm
diameter mold is in the range of a few tens of milliliters
(usually between 40 and 60 ml), it is recommended to
use syringes to measure the PDMS base and curing agent
quantities. Use separate syringes for base and curing agent to
avoid cross-contamination.
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