Alexander Hoelke wrote:
>
> 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
> FAX:            (513)556-7326
>
DLC works very well for this purpose.