> I've had some SOI wafers coated with Si3N4 and patterned on one face.
> Unfortunately, the wrong face has been patterned. I don't want to
> dispose of the wafers because of their cost. I've been told that I can
> have the nitride striped off both faces (with RIE), then
> re-applied and
> patterned on the correct face. What I need to know, does the stripping
> damage the silicon in anyway?
> Regards
> Richard Haigh

I have looked at the selectivity of a lot of plasma/RIE etches,
including mixtures of the gases SF6, CF4, CHF3, Cl2, HBr, and O2.
None of these is selective enough to etch through the Si3N4 and stop on
silicon;
the silicon surface surface will be etched somewhat.

Concentrated HF will strip off the Si3N4, but if the silicon is left in for
awhile,
the surface can get roughened.

The only etchant I know of that will etch through the Si3N4 and stop on Si
is hot phosphoric acid (85% by weight, from the bottle).
At 150 C I measured an etch rate of 25 A/min
and at 160 C 28 A/min for stoichiometric silicon nitride.
The etch rate varies with nitride composition.

While glass is also slowly etched, a heated glass bath can be used.
Ideally a reflux system would be used to condense the water that evaporates
and let it drip back into the bath.
If you don't have one, at least put a lid on the bath.
The etch rate drops as water is lost.

        --Kirt Williams Agilent Technologies