Ted,
I wholeheartedly agree with the suggestions given so far.  I've experienced
this very problem in production; it can be difficult to resolve.

The problem is caused by acid attack on the photoresist.  While I don't
usually see it unless normality is run very high (>2.8), it is possible for
the "scum" to build up over time.  All of the suggestions so far (reduce
antifoam in developer, maximize developer rinsing, dry layers between develop
and etch, carbon filtration) will help reduce the buildup.  If the problem is
severe, you may need to completely dump the etcher, clean the machine, and
start over with fresh solution.

It is also helpful to maximize your developing process.  Do some SEM studies
of the sidewall.  Soft, partially polymerized resist left on the sidewall is
susceptible to attack in the etcher.  Optimizing development can help make up
for less than perfect rinsing and drying.

One other thing to check is the ventilation system on the DES line.  In some
shops, I have seen vent pipes from the etcher and the stripper join together
at a main duct.  Sometimes, condensed vapors build up in the ducts, and
polymer laden material drips back down the sides and into the process
equipment.  You are then adding back the very material you want to remove.
Good luck.

Mark Dowding
Regional Technical Service Manager
INSULECTRO

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