hello

I am in charge of poly etch back for trench, using cl2/HBr/He-O2 gases.
etch back short loop flow:

1 poly etch back  P5000
2 50:1 HF SCP 60 Second
3 SEM S-4700

   After dipping the wafer using 50:1 HF about 60 Sec. I send wafer to S-4700
for checking poly recess depth. It seems that there is a white spike residue on
the trench groove as showing in the attachment picture.
I have try to remove the white spike by using B-Clean, and other chemical acid.
But I am fail, Can you  give me some  advices? Thanks!