Use EDF or low conc. KOH;
read:

H. Seidel, L. Csepregi, A. Heuberger, H. Baumgärtel, "Anisotropic
Etching of Crystalline Silicon in Alkaline Solutions 1&2", Journal of
the Electrochemical Society, Vol. 137, November 1990
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Alexander D. Hoelke, Graduate Student Electrical Engineering
        University of Cincinnati, Center for Microelectronic
        Sensors and MEMS (CMSM)    Cincinnati, Ohio 45221
Phone 513-556-4774 (work) 972-470-9735(home)
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