Crayton,

In my experience, the hard bake will reduce the bubbling.  I have
never proved it conclusively, but I believe that the bubbles may be
normal nitrogen evolution from the chemical reaction in the PR.  You
can look up the exact details, but when exposed, nitrogen is formed
and escapes through the surface of the resist.  Under some
circumstances, the nitrogen can agglomerate and form bubbles.  This
chemical reaction evolving nitrogen can also be triggered by vacuum
processes (i.e., evaporation).  I have even seen it when exposing
resist using vacuum contact mode.  It would probably be worse the
older the PR is, so if you are near the expiration date of your
resist, try a new bottle.  Different PR formulations behave
differently, so it is hard to pinpoint an exact cause.  For all these
reasons, I think that a flood exposure prior to hard bake should go a
long way eliminate the bubbles.  It is an easy experiment.  If it
doesn't eliminate them, then there is a different cause, possibly
related to surface preparation.

Brad Cantos
[email protected]
http://holage.com

LinkedIn: http://www.linkedin.com/in/bradcantos

On Mar 30, 2009, at 7:49 PM, Yang Fu wrote:

> Hi,
>
> Thank you for the advice. For Brad's question, I do not see noticeable
> rounding of the PR (using stylus profiler). I am going to
> fabrication more
> samples to get a better idea of the rounding. I will try both of your
> advices to solve this problem. Could someone throw light on where
> does these
> bubbles come from? They appear when I bake the second PR layer after
> exposure if the hard bake for the first PR layer is not long enough.
> Thank
> you.
>
> Sincerely
>
> Crayon Fu
> University of Virginia