I am trying to deposit Cobalt film (> 100 nm) on Si/SiO2 wafer first (by sputter), do some treatment on the thick film, and etch back the Co film down to sub 10-nm thickness.

Maintaining the initial smooth surface is much important than a precise thickness controlling. 

Is there any recommendable method to get a smooth and thin (sub-10 nm) Co film by etch back?

I wonder wet etching will be able to preserve the original smoothness.

(If possible, which kind of etchant is effective to Co?)

Thank you.