Hi,
I am fabricating a micro-PCR chamber using SU-8. Here is my process below:
1. Patterned Gold/Titanium heaters and thermometers with spacing of 100microns
line/space on SiO2 wafer by evaporation. The heaters are serpentine in shape
and about 300nm thick.
2. Spin coat 4 layers of SU-8 2002 at 3000rpm each layer on top of the heaters
and soft-baked at 95'C for 3mins. The total thickness of this SU-8 2002 layer
is about 2.3 microns. This layer is then patterned with 365nm UV (i-line) to a
dose of 140mJ/cm2 and post-exposure baked.
3. Spin coat 1 layer of SU-8 2150 at 3000rpm on the previous SU-8 layer and
soft-baked at 65'C for 7mins and 95'C for 45mins. This layer was aligned to the
SU-8 2002 and exposed at 365nm to a dose of 260mJ/cm2. Then the whole stack was
post-exposure baked at 65'C for 5mins and 95'C for 15mins. This SU-8 2150 layer
forms a 150micron high wall surrounding the micro-PCR chamber.
4. The whole stack was then developed in SU-8 developer for 15mins with
agitation.
5. After development the stack was hard-baked at 150'C for 15mins.
After the device was fabricated, inspecting under the microscope, I noticed that
there were streaks of SU-8 formed on top of the SU-8 2002 floor. I believe
these streaks are the remains of the SU-8 2150 layer and they preferentially
aligned themselves along the 100micron space between the heaters. Examining
these streaks with the surface profiler, I discovered that these streaks were
actually SU-8 humps as tall as 30-40microns formed in the chamber itself on top
of the SU-8 2002 layer, which is the floor of the micro-PCR chamber.
These streaks were first noticed when I was soft-baking the SU-8 2150 cum SU-8
2002 stack, but at that time they appeared as notches at the edges of the
device.
Has anyone noticed this interesting phenomenon before? How do I get rid of
these humps? Are these humps formed due to the surface stress between the Gold
conductors and the SU-8 2002 during heating and cooling? If so, then I may need
to use an adhesion layer such as Omnicoat?
Can anyone please advise. Thanks!
Jeffrey