Moisture diffusion in plasma-enhanced chemical vapor deposition dielectrics characterized with three techniques under clean room conditions

Abstract

Absorption of moisture by thin dielectric materials alters their properties and can cause several reliability issues. Even at standard room temperature and low humidity level, some dielectric materials are sensitive to moisture. In this study, moisture diffusion in two plasma-enhanced chemical vapor deposition (PECVD) films is investigated with three measurement methods to determine diffusion coefficients and saturated moisture concentrations: mass measurements, bending radius of curvature measurements and infrared spectroscopy. The two PECVD silicon dioxides are deposited at 200 °C and 400 °C. They were exposed to moisture in clean room environment (21 °C and 40% relative humidity) for about 800 h. The present results confirm that mass measurements, bending radius of curvature measurements and infrared spectroscopy can be used to monitor thin dielectric films in these environmental conditions. They lead to similar values for the diffusion coefficient. These values are in the range of [1.5–4.2] × 10−15 cm² s−1 for the 200 °C film and [2.3–3.6] × 10−15 cm² s−1 for the 400 °C one. Saturated moisture concentrations confirm that the two dielectrics are sensitive to moisture even at 21 °C, 40% relative humidity. Besides, the results show that standard fickean behavior does not provide the best fit to model water diffusion for some dielectric films. A dual stage model that appears to be more adapted is finally introduced.