Chris Turner wrote:
>      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?

Chris:  We use HNA to do a fillet etch on our silicon mechanical devices
(valve membranes).  We do not see the effects you are describing;
however, neither are we etching channels as you describe.  It may be a
function of the etch stoichiometry/temperature.  Or, there may be an
alignment 'problem' between the mask and the crystal orientation (even
though HNA is supposed to be isotropic).  Or, there may be a surface
issue (what is the initial state of the surface, prior to exposure to
HNA?)

I'd be interested in hearing what response you get from others.

Cheers, Al Henning
--
Albert K. Henning, Ph.D.    650/617-0854 (Office)
Redwood Microsystems        650/326-9217 (FAX)
1020 Hamilton Avenue        henning@redwoodmicro.com
Menlo Park, CA  94025       http://www.redwoodmicro.com