Carsten,

I dont know, how the particles distribute on wafer surface?? how
many wafer have been etched already without cleaning the chamber?
It looks to me, that you have "dirty" chamber. Using CF-gasses to
etch dielectric you might need to clean the chamber frequently.
Try to use long O2-plasma to clean the chamber if you dont want to
open it. Repeat the process, if the particles become less - you need
to clean the chamber. If not, it could be the back-scattering.

br
Ngo