The thickness and the refractive index of thin polymer films were meas
ured in situ during plasma polymerization of hexamethyldisiloxane (HMD
SO) by ellipsometry. The actual gas composition resulting from the pla
sma chemical conversion of organic vapour was analysed by mass spectro
metry. The gas composition influences both the deposition process and
the film properties. The optical dispersion and transmittance of the d
eposited films were characterized by spectroscopic ellipsometry (visib
le range) and spectrometry (UV-visible-IR range) respectively. It was
found that especially the refractive index can be varied over a wide r
ange depending on the plasma conditions, and that films with graded re
fractive index can be produced-