Isotropic Silicon Wet etch

Jim McVittie mcvittie at
Wed Mar 29 13:19:02 PST 2006


XeF2 and most so called isotropic plasma etches are not truely isotropic. They
usually show a concentration dependence. The problem shows up inside confined
structures where one sees that the etch rate is faster near the opening where
the F spices enters the structure. Back in the early 80s we developed a true
isotrope etch on the Dryteks. The conditions are:
100 CF4/ 10 O2, 750W and 300 mT.  The poly etch rate was 3700 A/m and thermalk
ox rate was ~ 125A/min. The resist ER was about 5 X faster than ox. We usually
did 1 min O2 descum step before the poly etch using  100 O2, 500 w, 150 mT  to
remove any organics from the poly or Si surface. The results where very
isotropic in that the Si removal was the same everywhere in the structure. Note
that the poly ER should show  some loading effects in that the rates will be
lower with larger exposed areas.


Yahong Yao wrote:

> Hi Rohan,
> I would think Drytek2 is the most convenient way to accomplish your goal.
> 1.6um photoresist can serve as the mask.  You can find the recipe and etch
> rate at:
> If for some reason, you need to use wet etch, then TMAH (tetramethylammonium
> Hydroxide) would be a good candidate.  Oxide is a very good mask in TMAH
> (1000A is way enoug for your purpose, but thicker oxide has less pinholes).
> 20% TMAH in water gives about 4000A/min at about 80C.  But the native oxide
> on open areas will influence the result.  Be sure to do a short HF dip
> (50:1) before you put wafers in TMAH.  If you use clean labwares for
> etching, then your wafers are still clean afterwards.
> Hope it helps.
> Yahong
> >From: "Rohan D. Kekatpure" <rohank at>
> >To: labmembers at
> >Subject: Isotropic Silicon Wet etch
> >Date: Mon, 20 Mar 2006 13:39:31 -0800
> >
> >
> >Dear Labmembers,
> >
> >I need to etch ~1um of silicon isotropically. In literature there seem to
> >be
> >3 ways of doing it: (a) Wet etching in HNA solution (HF + HNO3 + acetic
> >acid) (b) Wet etching in EDP solution (Ethylene diamine pyrochatechol) and
> >(c) dry etching in XeF2. I am flexible as far as requirements of roughness
> >and dimensional tolerences are concerned. I am just interested in knowing
> >what options are available to me within our SNF clean room.
> >
> >My preferred mask would be SiO2. I expect the HF in the HNA solution to
> >attack the SiO2 mask. So if anybody has an idea on the selectivity between
> >SiO2 and Si for an HNA etch, I would appreciate the input.
> >
> >Thank you very much in advance.
> >
> >-Rohan

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