Isotropic Silicon Wet etch

Yahong Yao yy7343 at
Tue Mar 21 13:21:23 PST 2006

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.

>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 
>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.

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