Have a look at "Study of self-limiting etching behavior in wet isotropic
etching of silicon", C.-H. Han and E.-S. Kim, Jpn. J. Appl. Phys. Vol 37
(1998) pp. 6939-6941. They use HF:HNO3:CH3COOH to produce a spherical radius
of up to 2.5 mm in silicon. Good luck! Tomas Lindstrom

Message: 1
Date: Thu, 16 Aug 2001 09:34:41 -0700 (PDT)
From: Imad Yousef 
To: [email protected]
Subject: [mems-talk] Spherical shapes in Si

Dear colleagues,

I am trying to etch silicon in spherical shapes. KOH
etching tends to create a flat bottom based on the
mask opening. Do you know of any other technique?

Regards,
Imad Yousef