Hi Xiaodong,
   Wow, very small features! One trick that has worked for us in the
past is to run the etch and deposition gases simultaneously. Also, you
may have problems with a notch appearing immediately below the oxide.
This seems to occur due to local substrate charging. I believe you may
have to switch to a resist and control its profile near the etch window
to mitigate this effect.

-Mike Martin
University of Louisville