Hello Li Zhang,

I had such problem. I think the problem is in different physical properties
of oxide and silicon, herein especially the different thermal coefficient of
expansion.

Try to analyze your process. Maybe you can change some parameters to make
the grown or drop of the process temperature more gentle or lower.
Verify the process pressure. Check if there is nothing what could make the
undesired uncontrolled grown or drop of the temperature.
If you can use teh DRIE machine with the electrostatic clamping use it.
Analyze your etched patterns.
And be aware that if you don't control teh time etching your BOX layer also
will start to be etched by SF6.

Thin BOX layer of the SOI is very fragile from mechanical point of view
during the temperature changes.

I recommend you first to change some parameters during the process so that
to protect your SOI form teh rapid change of the temperature.
If you have still some questions. Don't hesitate to ask.

  Good luck!

 Karolina