Chris-

I strongly suspect what is happening is that the nitride mask
on the "bad" channels has broken off during the etch.  As the
channel etches it severely undercuts the very thin mask, which
can break at any time.  Once the overhanging mask breaks off a
channel- particularly if it happens late in the process- the
etch rate increases significantly due to diffusion effects,
resulting in a wider and somewhat deeper channel.  And because
it could happen at any time, you end up with channels of
various widths.

I have seen exactly what you report on my wafers in the past,
and it stumped me for awhile because I thought that if the mask
broke off it would leave the edges of the channel jagged and
the width varying along its length, rather than the very
uniform albeit wider channel that I (and you) saw instead.
 Then I noticed that my nitride mask was under tremendous
tensile stress, and when I broke one with a probe tip it
snapped off from the entire channel and curled up at the ends,
just like a string breaking on a guitar.   The fractured edge
of the mask followed the edge of the channel almost perfectly,
which is why the channel ends up uniformly wider.

To verify this on your wafers, try agitating more vigorously to
"break" even more channels.

If you must etch such long channels, about the only way I have
found to control this problem is to lower the stress of the
mask film.  It also helps some if you make your mask feature
(which I assume is just a long thin slit) with rounded ends, so
you aren't making matters worse by concentrating the stress in
the sharp corners.  If you can work with shorter channels (ie
dashed lines), they will be less likely to break, and you can
at least increase your yield since one "blowout" won't ruin the
entire row.

I hope this helps you, and good luck.  If you want to discuss
it further, drop me a line.

--
Marc Straub
Visteon Automotive Systems, Ford Motor Company
Dearborn, MI
mstraub1@ford.com

-----------------------------------------

On Mar 11,  5:10pm, Chris Turner wrote:
> Subject: Isotropic etch problem
>      Hello,
>
>      We are producing a micro-chemical reactor module that
requires
>      isotropic etching of 100 micron wide channels in
silicon.
>
>      We do this using a standard HF/Nitric/Acetic acid
mixture with a
>      silicon nitride masking layer. This results in several
of the channels
>      etching differently to the rest. The different channels
are slightly
>      wider by about 5-10 microns and have a much rougher,
almost
>      crystalline, surface finish. There are 120 channels on a
wafer and
>      between 1 and 20 per cent can be different. This effect
runs the whole
>      length of the channel, but neighbouring channels can be
unaffected.
>
>      Has anyone seen this sort of thing before and if so is
there a way of
>      preventing it?
>
>      Any help gratefully received.
>
>      Thanks
>
>      Chris Turner
>      Senior Research Engineer
>
>      =========================
>      Central Research Labs
>      Dawley Road
>      Hayes
>      Middlesex
>      United Kingdom
>
>      Tel. +44 (0)181 848 6465
>      Fax. +44 (0)181 848 6442
>      e-mail cturner@crl.co.uk
>      Web. www.crl.co.uk
>
>
>-- End of excerpt from Chris Turner