Interesting question. We would need more analysis on the grains and the steps in your process to know whether they are from the CMP. You may not be able to give that much information here, and I'm not an expert. So I'll give you a couple of ideas.
First find out when the particles started appearing. Then compare that date and time to all your process parameters and ingredient lots to help determine if something changed in the process or the ingredients to cause the problem.
Regarding the CMP slurry: Colloidal silica should be amorphous --> not crystalline. Another possibility is slurry contamination - which could be amorphous, but is more likely crystalline. Meanwhile, check your process steps to see if any parameters have changed since the particles started appearing.
You can also contact your CMP slurry supplier and ask them to help determine the identity and source of the particles.
Good luck.
