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

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     Central Research Labs
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