Effect of film thickness on structural, morphology, dielectric and electrical properties of parylene C films

Parylene C (C₈H₇Cl)_n thin films were deposited along the Si(100) index on a p-type Si substrate at room temperature by a vapor deposition polymerization method. The effects of film thickness on the crystalline structure, roughness, dielectric and electrical properties of parylene C films were investigated. X-ray diffraction has confirmed an amorphous structure in thinner films (d < 40 nm ≈ R_g: gyration radius) and a semicrystalline structure in thicker films (d < 40 nm). The roughness (Rms) varied according to a power law depending on the thickness Rms ∼ d^β with β = 0.22 ± 0.05. The effect of the diffusion monomers and chains relaxations in volume are the main factors limiting the evolution of the surface roughness according to film thickness for d < 40 nm and the crystalline size effect for d > 40 nm due to surface energy minimization when the film gets thicker. The dielectric constant increases as a power law according to film thickness ε′ = d^β with β = 0.042 ± 0.04 for d < 1000 nm and 0.0138 ± 0.02 for d > 1000. This is interpreted as caused by the decreasing of voids and discontinuities of the material with increasing film thickness. The leakage current density J suddenly increases with decreasing film thickness especially for d < 1000 nm, referring to a more pronounced amorphous content, but it is practically thickness independent from d > 10 000 nm and it shows a value of a bout 4 × 10⁻⁸ A/m for an applied field of 20 MV/m.