The best way is to measure the underetch after the mask-strip. If you
measure the line-width before etching and after mask-strip, then your
underetch is half of the difference between the two values.
With a resist-mask you can also often see the underetch after etching as a
somewhat different coloured edge at the border of the mask-pattern. You
might need a intefference-microscope for this though. With a metal etch-mask
this will not work, since the metal is  not transperant (at least, not for
the metal-thicknesses that are normally used as etch-mask). You could try
and see if you can see/measure the underetch from the back-side of the wafer
though, since your using glass substrates. So if  you place the wafer with
the backside up, you could the look through the glass and focus on the
front-side. Never tried it myself, but it might be something worth trying.
If you try this, just be carefull when focussing and keep track of the
disctance of the objective to the substrate, since you could end up damaging
your sample.
Succes,
Jason

___________________________________________________________
Jason Viotty
Senior Process Engineer
C2V
http://www.c2v.nl



-----Original Message-----
From: Tom Fan [mailto:[email protected]]
Sent: donderdag 4 september 2003 2:49
To: [email protected]
Subject: [mems-talk] Underetch Measurement



Hi All,

I am wondering how people measure the magnitude of underetch when doing an
isotropic wet etch of glass substrates (whether using PR or metals as etch
masks)? Is it by using the SEM?

Thanks,
Tom Fan

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