MICROSTRUCTURES OF PARYLENE-N THIN-FILMS AND THE EFFECT ON COPPER DIFFUSION

Using scanning electron microscopy, the microstructure of annealed N-t ype parylene films on silicon substrate was observed and compared to t he as-deposited film. The diffusion of copper through the parylene-N f ilm was studied and correlated to the microstructure. A web-like micro structure was observed on annealing parylene-N to a temperature of 300 degrees C and higher. This microstructure differed from the as-deposi ted homogeneous and continuous structure at room temperature. The web- like structure observed is proposed to be a fibrillar crystalline stru cture embedded in an amorphous matrix. X-ray diffraction studies suppo rted this view and showed that the crytsalline structure was the beta phase. Also, when the film was annealed at 300 and 350 degrees C, a th in continuous layer was formed at the surface of the web-like parylene -N film. In contrast, no such thin layer was observed when the anneali ng was performed with a copper overlayer. Based on this observation, a two-stage annealing process was carried out to reduce the copper diff usion into parylene-N, pre-annealing, before copper deposition and pos t-annealing after copper deposition. The results, as judged from Ruthe rford backscattering spectroscopy indicate that the thermal stability for copper diffusion into parylene-N films can be increased by 50 degr ees C (from 300 to 350 degrees C) using pre-annealing. Experimental da ta shows that a minimum pre-anneal temperature of 250 degrees C for 1 h is required for this purpose.