Dear Kim

As Andrea suggested you can use HNA for making the circular grooves. In wet
etchants, it is not possible to etches circular masking shapes (or other shapes)
with vertical sidewalls due to crystallographic limitation of silicon. If the
sidewalls is not a issue, you can use surfactant (NC-200, PEG, Triton-X-100)
added 25 wt% TMAH  in order to etch any shapes of masking pattern with
negligible undercutting at the rounded and sharp corners, curved and <100>edges.
Recently some studies of TMAH+surfactant have been published  for the
fabrication of new shapes of structures. Some of the references are given below:

1. Various Shapes of Silicon Freestanding Microfluidic Channels and
Microstructures in One Step Lithography, Journal of Micromechanics and
Microengineering, vol. 19, no.5, pp. (055003)1-11, May
2009.http://www.iop.org/EJ/abstract/0960-1317/19/5/055003/

2. Study of rounded concave and sharp edge convex corners undercutting in CMOS
compatible anisotropic etchants, Journal of Micromechanics and Microengineering,
vol. 17, no.11, pp. 2299-2307, November 2007.
http://www.iop.org/EJ/abstract/-search=62827073.2/0960-1317/17/11/017

3. Study on anisotropic silicon etching characteristics in various surfactant-
added tetramethyl ammonium hydroxide water solutions" J. Micromech. Microeng.
vol.15, pp.2028-37, 2005.http://www.iop.org/EJ/abstract/0960-1317/15/11/006/

If you are not able to download these papers, please let me know. I will send
you by email as attachments.

Good luck

Prem Pal