Dear MEMS community,
  We did isotropic etching on Boron-doped p++ single crystal silicon,
the concentration is 1*10^20/cm3, the etchant is HF:HNO3:CH3COOH=3:25:10. I
found the depth is 1.5um after about 1min of etching, but the undercut is
5um under a 14*14um SiO2 mask.
  Does anyone have similar experience? How much is the etch rate? Why is
the undercut so quick?



Thanks very much.
Regards
Junhua Zhu
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Junhua Zhu
MEME Research Group
Department of Precision Instruments
Tsinghua Univ.
Beijing
P.R. China
100084