Dear all,

I have a question about a - for us - surprising problem with using
chromium as an etch mask in DRIE (the Bosch process).

Short version: We believe that a thin layer of something (unknown to
us) was formed on top of the chromium during the etching. Have anyone
else on this list experienced this problem before (1) and would you
know what species might have been formed?

Longer version: Recently when we etched silicon (SOI wafer, the etch
was stopped when all trenches had reached the buried oxide below 50mu
device layer). The chromium seemed to be absolutely untouched after
the etch (as it has always been previous times). Except chromium,
silicon and buried oxide there was also CYTOP, (a polymer) gold and
ordinary positive resist on the wafer.

 But after the etch the chromium wouldn't go away in regular chrome
etchant. We then made the following two observations:

A. When gold pads on top of the chromium were etched the chromium
below these *could* be removed with Chromium etchant.

B. When the chromium was slightly and very shallowly scratched with a
tweezer these scratches *could* be etched by chromium etchant.

Because of these two points we draw the conclusions that a some
unknown has formed a thin layer on top of the chromium.

It should also be added that we have used chromium as an etch mask
before in the same machine without encountering this problem. But at
earlier times we have always used lower platen power shorter etch
times. This time - also a first - we used for a fraction of the total
etch the SOI option that our STS machine provide. I can't see why this
last would have an impact, but the etch overall changed quite
dramatically and seemed much more aggressive while using the SOI
option.

I would be thankful for any answers.

regards,

Sven Holmström