Optically flat pellicles of parylene C (a chlorine-substituted member of the family of polymers based on paraxylylene) are attacked on both sides when exposed to fluorine vapor. The fluorinated surface layers so formed have higher specific gravity, lower refractive index, and significantly lower specific refraction than the original material. The demarcations between the two fluorinated surface layers and virgin parylene C are sharp as evidenced by the pronounced modulation of the channelled spectrum in transmission of the thin films. The interference patterns produced by the partially-fluorinated parylene films are shown to be consistent with theory for a symmetrical low-high-low multilayer structure. Measurements of the wavenumbers and relative amplitudes of the successive loops and nodes of the interference spectra (over the range 0.3 to 30 μm) on pellicles at various stages of fluorination have been used to derive the optical properties of the surface layers and to monitor the progress of the chemical reaction.
John N. Pike, John N. Pike,
"Optical Interference In Partially-Fluorinated Thin Films Of Parylene C", Proc. SPIE 0050, Optical Coatings: Applications and Utilization I, (1 March 1974); doi: 10.1117/12.954118; https://doi.org/10.1117/12.954118