Hi Andrea,

Have you ever tried to lower the etch bath temperature?  Although the etch rate
will decrease rapidly (every 10 ° by half), the roughness and also the
uniformity over the substrate will be much better. We always etch at 60°C in
a 10 l bath (30w%) supported by magnetic stirring. The etch rate is quite
constant at 0.35-0.36 microns/min over the etch time.
You could also try to put some Isopropanol in the bath.

Greetz,
Thomas

Thomas Lemke, Researcher
University of Freiburg
IMTEK - Department of Microsystems Engineering
Germany
homepage: www.imtek.de/konstruktion