I have the similiar problems as you. I guess it is because of the NH4F. To get
rid
of this problem, be sure that the container of the BHF ethant is good covered to
prevent the solution from evaporation.


>Hi all, i prepared 500ml of fresh BHF 1:4 and etched 50 nm of silicon
> nitride from the entire surface of a 4'' silicon wafer. At the end of the
> etch i noticed the formation of a salt which covered the entire silicon
> wafer. In previous etch in the same conditions it never appared. What could
> it be :o ?
>