Hello Prassana,

One reason why large areas can get etched slower then small ones is that
etchant in the volume close to the large area is used faster then at small
ones. There is much more material to be etched away so concentration of
fresh etchant drops faster there. If there is not sufficient etchant mixing
fresh developer will get to surface only by diffusion through concentration
gradient. Stirring the bath or doing multiple puddles or dynamic etch on
single wafer etcher is the way to avoid it.

Hope that helps.

Best regards,

Daniel Figura
smartfabgroup
process consulting . data processing . fab software
Phone: +44 20 3286 4342, Fax: +44 20 3286 4342 33
E-mail: daniel.figura@smartfabgroup.com, Web: www.smartfabgroup.com


-----Original Message-----
From: mems-talk-bounces+daniel.figura=smartfabgroup.com@memsnet.org
[mailto:mems-talk-bounces+daniel.figura=smartfabgroup.com@memsnet.org] On
Behalf Of Prasanna Srinivasan
Sent: Tuesday, March 09, 2010 18:41
To: General MEMS discussion
Subject: [mems-talk] Gold etching

Hello everybody,

I need some inputs on gold etching. I have an SOI wafer with evaporated
Cr/Au (40nm/500nm). The wafer was patterned using AZ9260 (6 micron thick).
When I etched the exposed gold layer using pottassium iodide etchant (KI3)
for about 55 seconds, I noticed large exposed areas (circles of 250 microns
radius) are etched slower than the small narrow crevices (2-10 microns
width) on a wafer. I was expecting exactly opposite behavior in the etch lag
because, it should take a relatively more time for the etchant to reach the
narrow crevices. Can anyone explain me why I notice this behavior? Also I
would appreciate if anyone can suggest a suitable way to minimise the etch
lag between different features.

Thanks,
Prasanna