Hi Aaron,
   For super uniform shallow etches I use 6M KOH which is about 33% w/w at
75-80C with 5:1 KOH:IPA. The isopropyl acts as a surfactant to prevent hydrogen
bubbles from micromasking the surface which can cause surface defects. I think
some groups use commercial surfactants such as Triton X-100 in their bulk etches
for the same purpose but without having to worry about evaporation. At 75C we
get an etch rate of 1 um/min so you might want to lower the temperature for a
lower etch rate and better process control when doing a shallow etch.
  When preparing the substrate I prefer to solvate the resist in acetone, then
rinse in methanol and IPA without letting the acetone dry as this will leave
residue. Then plasma clean in O2. As an added precaution you might try a brief
HF dip to remove any native oxide just before going into the KOH.  By the way I
got the etch recipe from Marc Madou's book.

Good luck,
Michael

On Jul 2, 2014, at 4:14 PM, "Aaron Glatzer" 
wrote:

> We are etching some 300 micron deep trenches using KOH, and we are
> occasionally getting "craters" in the trench bottom surface.  These are 2-4
> micron deep 50-75 micron wide spots on the bottom of the trench.
>
> Is this typical for KOH etch?  Has anyone else seen this kind of thing?  Any
> idea what causes it?
>
>
> Technical details:
>
> <100> silicon.
>
> Masked with 2kA low stress LPCVD nitride.
>
> Nitride etched with SF6 plasma RIE; we guesstimate we etch 2000A into the Si
> (based on known-etchrate, and observed overetch time on endpoint signal).
>
> Resist stripped in O2-ash/piranha/O2-ash sequence.
>
> KOH 45%, etched at 85C, for about 5.5 hours.
>
>
> Typically we get a mildly textured surface (per optical inspection), but as
> indicated above occasionally we get what look in microscope like blisters,
> but optical profilometer indicates is actually a "crater".
>
> Any insight into this is appreciated.
>
> thank you,
> Aaron G.
>
>
>
>
>
>
> --
>
>
> Aaron Glatzer                        phone (419) 241-6963 x13
> Lead Process Engineer                fax (419) 241-6966
> Midwest Microdevices, LLC            aglatzer@midwestmicrodevices.com
> 329 14th Street
> Toledo, OH 43604
>
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