According to Z. Li, et al (Surface Science 529 (2003), 410-418), Pt
oxidizes under RIE O2. Perhaps that's what you saw. You can remove it with
Argon in your RIE, after O2. (or switch to an ion mill). It may also be
that the Parylene left something behind that is impervious to O2.

rakesh babu 

Subject [mems-talk] Parylene residue

Folks,

  I was trying to etch 18um parylene using O2 plasma
with Al as the mask and after 2 hrs of etching i found
some black residue which could only be removed by
using a swab. Can anyone please tell me the reason for
this . The metal under parylene was Platinum.