thanks for your explanation, yes, the bombardment of ion will decrease the
thickness of the phtoresist, but the problem confused me is why the surface of
remained photoresis is not smooth, but roughed? Is the residue byproduct of
etching responsible for this?


The basic premise of what you are seeing is that fact that RIE is not
a completely chemical process, it is partially physical, so there
will always be some etching of photoresist, or any material for that
matter, especially for high power etch recipes.

There is no need to worry unless you plan to do an etch that is long
enough to go completely through the photoresist (which you are
getting close to), at which point I would suggest just using a
thicker photoresist, but depending on the size of your features that
may cause it to be more difficult to define them.

Nicolas Duarte

I am using a RIE etcher to pattern polysilicon with SF6 & O2, and
i observed that
after a long time etching, the color of the photoresist changed
and under SEM, the smooth surface of photoresist is replaced by a rough surface,
would anybody give an explanation of this phenomenon?