Microstructures of parylene-N thin films and the effect on copper diffusion

Abstract

Using scanning electron microscopy, the microstructure of annealed N-type parylene films on silicon substrate was observed and compared to the asdeposited film. The diffusion of copper through the parylene-N film was studied and correlated to the microstructure. A web-like microstructure was observed on annealing parylene-N to a temperature of 300°C and higher. This microstructure differed from the as-deposited homogeneous and continuous structure at room temperature. The web-like structure observed is proposed to be a fibrillar crystalline structure embedded in an amorphous matrix. X-ray diffraction studies supported this view and showed that the crystalline structure was thes phase. Also, when the film was annealed at 300 and 350°C, a thin continuous layer was formed at the surface of the web-like parylene-N film. In contrast, no such thin layer was observed when the annealing was performed with a copper overlayer. Based on this observation, a two-stage annealing process was carried out to reduce the copper diffusion into parylene-N, preannealing, before copper deposition and post-annealing after copper deposition. The results, as judged from Rutherford backscattering spectroscopy indicate that the thermal stability for copper diffusion into parylene-N films can be increased by 50°C (from 300 to 350°C) using pre-annealing. Experimental data shows that a minimum pre-anneal temperature of 250°C for 1 h is required for this purpose.