General question: In order to obtain a vertical sidewall profile, should I
increase the anisotropic etching or isotropic etching?

In detail:
I have used CHF3 to etch my SiON (1.72), the polysi is on the top of SiON as an
etching mask.
The recipe: CHF3 :40sccm , chamber pressure:38mTorr, RF power: 200W.

The diameter of air hole in polySi is around 170nm. After 14 minuters etching
the hole diameter is increased to around 260nm. From the cross-section of SEM
picture, the sidewall seems to be vertical. Is this possible to be true?  The
estimate etching rate :to polysi 4.83nm/nm,to SiON 92.43nm/min.

If I want to decrease the hole increase in horizontal direction, should I
increase the physical sputter (increase power?) Or add some H2?


Any suggestion will be appreciate. Thank you very much in advance!