Infrared and Raman Studies on Films of Organosiloxane Networks Produced by PECVD

The effect of the incorporation of oxygen and nitrogen on the structure of films obtained by PECVD of hexamethyldisiloxane (HMDSO)−He−N2 and HMDSO−He−O2 mixtures is investigated using infrared and Raman spectroscopies. From transmittance spectra of films deposited onto single-crystal KBr disks, the transverse optical (TO) and longitudinal optical (LO) functions in the mid-infrared region were calculated. To correlate structural aspects with the observed LO−TO splittings, an identification analysis of functional group based on the infrared and Raman literature was made. It was concluded that the structure of the films deposited from HMDSO−He−O2 discharges was strongly dependent on the proportion of oxygen in the gas feed. In the absence of oxygen, i.e., for a discharge of a HMDSO−He mixture, the resulting film consisted of a network of interconnected siloxane and carbosilane units. Addition of O2 precluded the formation of methylene bridges and induced the formation of a material enriched with Si−O−Si groups. Films formed from the HMDSO−He−N2 plasmas, on the other hand, consisted mainly of interconnected siloxane and carbosilane units in addition to a small quantity of silazane units. On the basis of these results, we propose an interpretation for the variation of the LO−TO splitting amplitude for the asymmetrical stretching mode (AS1) of Si−O in Si−O−Si groups as a function of the oxygen or nitrogen incorporation into the films.