It is my first time posting to this whole thing. I think that it will be
pretty valuable and helpful, given that I am not very experienced. I am
an engineering co-op from Drexel working for a company that does a lot
of metrology and calibration stuff via lithography, but we also do
contract jobs for crazy stuff, defense stuff, all sorts of things. My
question is in regards to resist coating and spin coating specifically,
but I'll give some background because my question isn't all that
straightforward. We have a job to pattern a simple chrome cutoff ring on
a ground piece of glass in a holder. The ground glass is in the shape of
a contact lens mold, 3D and symmetrical about 2 axis, with a total
diameter of about 16 mm and diameter of the chrome ring at about
13.xxxxmm, to +/- 0.005mm. The chrome ring is solid from the 13 mm point
out to the edge of the glass, and interior of that is just the ground
glass lens. So, our exposure, development, and etch are set. They work
fine and the yield is about 80%, more than good enough in the R&D phase.
But we are having problems with the resist coating. What we are getting
is what my boss likes to call boogers, or defects in the resist coating,
or possibly on the surface of the chrome, but I have my doubts that it
is on the chrome because we clean them pretty well. The current cleaning
procedure just prior to coating involves cleaning the parts with
acetone. We are using Rohm and Haas (previously Shipley) 1818 positive
photoresist and spincoating the parts. We are not currently using any
pre-treat on the surface. The particle count in that part of the clean
room is very low, it's laminar flow and about a class 100 at worst. So
what we are getting are loads and loads of these defects, probably about
200-800? per part. Some are large, maybe 5 microns, and with rather
erratic shapes. Most are small and sub-micron. They all have some
transparency, using a microscope you can see that you can focus through
the defects. They are causing all sorts of problems, because even a
defect away from the line alters the resist and can affect the exposure
area.

I have a guess as to what it is. My thought is that they are air
bubbles. I need to go through a precise inspection when we run our next
set of parts, but that is my thought thus far. I think because the
resist is trying to go out, as well as downhill, it may be grabbing air
on the leading edge of the resist and dispersing it throughout the
coating. Does anyone have any suggestions as to another way to coat
these parts or a way to tune the spin coater to coat these better? I am
trying to get this project to work and am very receptive to any
suggestions, aside from electro deposition, because that is not
something the company would be interested in investing in at this point.


Any suggestions you can provide to help this work would be awesome.

David Casale
MLA