You can etch away the buried oxide with vapor-phase HF+H2O or anhydrous HF
(no water).
FSI International makes the Excalibur.
The good news is that for prototyping, you can simply etch by holding the
wafer over a beaker of 49% HF.
After about 30 seconds, droplets of HF+H2O will condense on  the wafer,
which you don't want.
Instead, hold the wafer over the beaker for 15 s, remove (and maybe set on a
hot plate) for 15 s, and repeat.
Depending on the etch rate you get and the thickness of the oxide, release
can take over an hour.
You can see the color of oxide changing as you etch, indicating progress.
When done, I put the wafer on a hot plate for awhile to desorb any HF still
on the wafer.
It's easier to put the wafer in a holder, rather than try to hold it with
tweezer and get a cramped hand.
Of course, do all of this under a fume hood with protective equipment.
See papers on etching in JMEMS for more information.
   --Kirt Williams

----- Original Message -----
From: "Oudalov, A. (EL)" 
To: 
Sent: Sunday, October 24, 2004 9:49 AM
Subject: [mems-talk] Sticking of fragile MEMS structures


> Hi there,
>
> I am trying to release a comb drive structure etched by deep reactive ion
> etching (DRIE) on a SOI silicon wafer. I try to make very thin gaps
between
> the comb drives to have a large electrostatic force. Since the spring
> constant of my supporting structure is small I get problems when I release
> the chips in buffered hydro-fluoric acid (BHF). The adjacent combs stick
> permanently and the device is broken. I also tried critical point drying,
> the yield is little higher but not satisfactory. Who knows about another,
> more gentle method to release my combs?
>
> Thanks a lot,
> Alexander
>
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