Reduction in diffusion of copper in parylene by thermal pretreatment

Abstract

A thermal preannealing technique for limiting diffusion of Cu into parylene-n (PA-n) in an interconnection application has been demonstrated. These thermal pretreatments enhance fabrication latitude for PA-n. The Cu was deposited by partially ionized beam technique and the PA-n was vapor deposited. Diffusion was investigated using the Rutherford backscattering (RBS) technique. Cu was found to diffuse into as-deposited PA-n at temperatures of about 623 K. It was also found that with certain thermal pretreatment of PA-n, this diffusion could be stopped for temperatures as high as 623 K. Pretreatment at temperatures of 523 and 623 K were successful in stopping diffusion. The microstructure has been investigated using scanning electron microscopy and RBS. The annealing causes formation of lengthy grainlike structures perpendicular to the surface. Under some preanneal conditions, the grain structure reaches the surface and diffusion is observed for these samples. The preannealing technique has been successful in reducing diffusion caused at soldering temperatures (623 K) in a Cu/PA-n interconnection scheme. It must be noted that pretreatments at temperatures of 573 K were generally not effective, except for one case where the temperature was ramped down over a 2 h period.