M Straub (Marc) wrote:
>
> 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, resulting in a wider and somewhat
> deeper channel.  And because it could happen at any time, you
> end up with channels of various widths.
>
> A standard nitride mask is under tremendous tensile stress;
> when
> the overhanging portion breaks it snaps off the entire channel
> and curls up at the ends, just like a string breaking on a
> guitar.
> The fractured edge of the mask follows the edge of the channel
> almost perfectly, which is why the channel ends up uniformly
> wider.
>
> I have seen this happen on my wafers. To verify it on your
> wafers,
> try agitating more vigorously to "break" even more channels.
>
> About the only way I have found to control this problem is to
> lower the stress of the mask film.  It may also help some to
> round
> the ends of your mask features so as not to  concentrate the
> stress
> in sharp corners.
>
> 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
>
_________________________________________
When we were doing some isotropic etching here we did not have a silicon
nitride deposition here and had to use an alternative mask. We came up
with a fairly thick oxide layer (2um or so) and Cr/Au on top.
Individually, these layers are not very good, oxide etches too fast and
the metals have pinholes. But together they worked really well. And this
mask never breakes off like nitride.
--
Alexander Hoelke

graduate student

University of Cincinnati
ECE - CMSM

Phone:  (513)556-1997 / (513)556-4795
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