Spectroellipsometric analysis of CHF3 plasma-polymerized fluorocarbon films

Abstract

Optical and morphological properties of fluorocarbon films (CFx) deposited on Si substrates from CHF3 in a reactive ion etcher were studied by spectroscopic ellipsometry (SE). The study has led for 0.3–1.5μm thick films to a thickness-independent, roughnessless, apparently nongraded two-layer model that requires the Cauchy dispersion and the Gaussian absorption for the film refractive index and extinction coefficient, and to an interface mixture of CFx, Si, and voids. The Si native oxide removal was ascertained from films formed on SiO2-sputtered Si substrate. The sample SE results were further verified by x-ray diffraction, atomic force microscopy, Fourier transform infrared, x-ray photoelectron and Auger electron spectroscopies, and capacitance studies. The film microstructure may thus be considered isotropic and homogeneous resulting from a random addition of CFx fragments including cross-links along the film depth. The films have low-dielectric constant(k≈2.2) quality index of 1.38(615nm) and are noticeably absorbent in UV while exhibiting strong transparency in visible. These films growing at a rate of 0.033μm∕min have 2%–3% thickness nonuniformity. The SE model decision making steps are described and the degree of confidence on the SE results are discussed with the support of literature results and those from and the above studies.