Time evolution of the residual stress of plasma-polymerized acetylene films in various ambients

The behavior of the residual stress with time of plasma-polymerized acetylene films in various ambients has been investigated. The plasma polymers were deposited on thin glass substrates in dc and rf discharges of acetylene. After deposition, the specimens were mounted in a chamber that could be evacuated or filled with air, oxygen, or nitrogen. Stress measurements were carried out by the bending beam method, using a He-Ne laser. A steady rise in the stress during time periods of several hours following film deposition was observed in most of the samples studied. Stress increments of more than one order of magnitude were detected in some of the films upon exposure to air. Pronounced stress increments were also observed during exposure to oxygen. Rises in the stress were detected even in a vacuum ambience. Nitrogen exposure was found to have little effect on the stress. The effect of the moisture content of air on the stress was also examined. The evolution of the stress was found to depend also on the film preparation conditions. The results strongly suggest that postplasma reactions involving active chemical species trapped in the film during deposition play an important role in the development of the stress.