Pulsed Plasma Enhanced Chemical Vapor Deposition of Alumina Thin Films: Influence of the Duty Cycle on Structure and Elastic Properties

Abstract

In this study, the duty cycle (Δ) has been systematically modified during the pulsed plasma enhanced chemical vapor deposition (PECVD) of alumina (Al2O3) coatings. This deposition method is prone to chlorine incorporation during growth as AlCl3 is used as precursor. Here, we show that the incorporation of chlorine can be reduced from 2.4 ± 0.1 to 1.2 ± 0.1 at.-% by increasing Δ from 62% to 91%. The film grown at Δ = 88% has a density of 3.85 ± 0.12 g cm−3 (96% of α-alumina bulk density) and an elastic modulus of 365 ± 50 GPa (∼83% of the bulk α-alumina value). Δ affects the normalized ion flux (NIF), which is the ratio between the flux of bombarding ions and the flux of deposited material. By increasing Δ, the formation of α-rich alumina is promoted at 560 °C, the chlorine content is reduced, and the film is densified. An increased Δ and hence NIF results in enhanced ion bombardment of the film surface during growth, promoting chlorine desorption. Furthermore, it is reasonable to assume that the deposition rate decrease measured is a consequence of increasing Δ and hence NIF. Both the density as well as the elastic modulus increase measured is consistent with the increased NIF caused by increasing the duty cycle.