α α′ α″ α‴ Poly-tetrafluoro-p-xylylene as an interlayer dielectric for thin film multichip modules and integrated circuits

Abstract

α α′ α″ α‴ Poly-tetrafluoro-p-xylylene (PA-f ) has been evaluated as an interlayer dielectric for multichip modules and integrated circuits, and its properties are reported. It has a lower dielectric constant and higher thermal stability than parylene-n (PA-n). The as-deposited films have very low crystallinity. The crystallinity increases as the film is annealed. Thermogravimetric analysis has shown that these films lose weight at temperatures higher than 480 °C. A shrinkage in the films of ∼10% was observed when annealed in a vacuum at a temperature of 425 °C. The as-deposited film has a low dielectric constant of 2.38 and a volume resistivity of 1.3×1016 Ω cm. The refractive index at optical wavelengths was 1.3 for as-deposited samples and it increased with annealing temperature. The stress levels observed after annealing are also lower (20 MPa) than for PA-n (40 MPa). The hardness, as determined by a microindentor, of PA-f was 0.56 GPa in the bulk, but the surface hardness was ∼1.0 GPa. The conformal nature of these films permitted successful coating of micron-sized gaps having an aspect ratio of 1.5–2. Diffusion of Cu into PA-f is reported here and is contrasted with Cu/PMDA-ODA polyimide. Scanning electron microscopy of film cross sections shows microstructure changes above 350 °C. The low dielectric constant and high thermal stability of PA-f makes it a good insulator for high-speed interconnection applications.