My experience with the ferric chloride etch is that it is very dependent
on agitation of the solution and the relative dimension of etch
patterns.

If the flow of solution is uneven, the etch rate will be correspondingly
uneven across a substrate. Small and large un-masked areas for etching
etch at different rates.

None of this matters much if you are patterning 4 mil features, but with
2 mil and smaller, these effects result in relatively extreme undercut
in some areas while you may have not etched through the copper in other
areas.

I think the solution is ensuring uniform solution flow over the
substrate (this is difficult with a manual process). Masks should be
designed so that as much as possible, areas that are to be etched are of
similar dimension. If you have 1cm squares and 1 mils squares to etch
out, you will have lots of trouble.

Best of luck.

L.

-----Original Message-----
From: Leidong Mao [mailto:leidongmao@yahoo.com]
Sent: Tuesday, August 24, 2004 1:36 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Etching copper in ferric chloride


Hello all,
     I have a problem in etching copper with ferric
chloride. The copper(20um) is deposited on a glass
epoxy substrate. Shipley 1813 resist was used to
define the patter and as etching mask, feature size
~60um. The copper substrate was etched in ferric
chloride solution @50C, under microscope it is shown
that some of the pattern was over-etched while others
are untouched.

     The photoresist is around 3um thick(S1813),
exposure dosage ~40mJ/cm2, soft bake for 8 mins@95C,
developed for 100sec, and post develop bake for 30mins
before being put into etchant.

     Any suggestions and help are appreciated. Thanks.

leidong



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