Hi Dave,

try cerium(IV)-sulphate / nitric acid. I can find the exact parameters
if you are interested in. The etch rate is about 20 nm/min. I used a
negative resist (Eagle 2100 ED/PR, Shipley) as etch mask which wasn't
attacked at all.

Regards,
Matthias

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Matthias Heschel
Microelectronics Center
Technical University of Denmark, Bldg. 345 east
DK-2800 Lyngby