I have encountered this problem also.  It looks like a black
or grey "soot" on the bottom.  It is not stuck to the bottom
of the features, and is moved when an alpha-step tip is
dragged across it.  In the past, I have cleaned the "soot"
from my features in an ultrasonic tank with either water or
solvents like acetone.  It actually works pretty well.  On
some occasions, I have had to iterate between the RIE and the
ultrasonic agitation because the "soot" acts as a mask.  I
have attempted to change settings on the RIE in order to
reduce the effect but I did not have any luck.  Hope this
helps.

---- Original message ----
>Date: Mon, 24 May 2004 17:14:18 -0700
>From: "Honggang Jiang" 
>Subject: [mems-talk] Residue remover of RIE polyimide on Al
metallization
>To: "General MEMS discussion" 
>
>Hi all:
>I encountered a problem while doing RIE polyimide (85%O2+15%
CF4) to expose
>the underlying Al metallization. It was found that some
residues were
>generated on top of the exposed Al after RIE etching. Does
anybody know what
>chemicals can be used to remove these residues without
attacking the exposed
>Al metal. Thank you very much.
>
>========================================
>Honggang Jiang
>Materials Science Engineer
>Second Sight
>12744 San Fernando Road
>Building 3
>Sylmar, CA  91342
>Ph: 818 833 5058
>Fx: 818 833 5067
>========================================
>
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Nels Ostrom
ostrom@uiuc.edu
Graduate Research Assistant
Optical Physics and Engineering
Dept. of Electrical and Computer Engineering