Effect of humidity on the residual stress in silicon-containing plasma polymeric coatings

Residual stress measurements of thin films are common practice in device technology and are extremely important in particular for the characterization of thin film coatings. A largely ignored stress contribution is the difference in coefficient of hygroscopic expansion between the coating and substrate. This paper presents a rather novel approach to accurately evaluate the residual stress and coefficient of hygroscopic expansion of strongly curved specimens. Silicon-containing plasma polymer coatings with different carbon contents were deposited using hollow cathode arc discharge based PECVD. Samples of different layer composition were produced comprising silicon-containing plasma-polymer layers with a high carbon concentration and more “inorganic” SiO2 like layers with lower carbon concentration. All coatings show a compressive stress state. The highest stress was measured in the coating with the highest carbon content (239 ± 6 MPa) and decreases to 94 ± 31 MPa at lower carbon contents. Variation of the humidity showed that all coatings expand under influence of increasing relative humidity. The most inorganic coatings exhibits the highest expansion coefficient of 29.2 ± 2·10−6 (% r.h.)−1. The results obtained were compared with the results from contact angle measurements. An increase in the hygroscopic expansion corresponds with an increasing hydrophilicity of the coatings.