Hi Bob,
I think the reason you are seeing an additional bead is the acetone.
Usually the preferred solvent is the thinner for your photoresist or a
specific edge-bead solvent sold by most PR suppliers.   Photoresist is
highly soluble in acetone and it is possible that the fumes  from the bowl
are enough to modify the edge profile.   If your PR is thin enough (below
0.75 um or so) then you can watch the color of the applied PR film during
the EBR step.  If it goes through many color changes, then the acetone is
affecting the entire film, which is not a good thing in any case.  Acetone
is also highly flammable and therefore dangerous to use in a spray mode
without other precautions.  Also, although I am not familiar specifically
with the Suss system, the nozzle for delivery should be at a low angle and
the flow of the solvent should not leave an excess of liquid on the edge of
the wafer.

Hope this helps.

Brad Cantos


> About my process:  I am spinning the wafer at 1250RPM whilst the solvent
> (acetone) is dispensed at pressure by an automated arm through a syringe.
> 1250RPM is the maximum speed the system will allow and high speeds are
> recommended to prevent backside contamination.
>
> Many thanks for any comments in advance,
>
> Bob Ibbotson
> Heriot-Watt University