Comparative study of the residual stress development in HMDSN-based organosilicon and silicon oxide coatings

Abstract

To investigate the stress formation mechanisms in thin plasma polymers, a comparative study of organosilicon (SiNOCH) and silicon oxide (SiOx) coatings in dependence of power input for deposition was conducted. Both coating types were produced in a low-pressure (15 Pa) microwave excited hexamethyldilisazane (HMDSN) plasma. Residual stress values were obtained using a high-throughput, time resolved and in-situ measurement method, including a CCD-camera, a line laser and micro-machined cantilever sensor chips. Both plasma polymer types were shown to form residual stresses with opposite signs. The stress evolution in the coatings revealed a strong dependency on the variation of power input for deposition. The SiOx coating exhibits mostly compressive stresses. Higher power inputs constitute higher ion momentums as well as a higher degree of fragmentation of the monomer. The SiOx coatings were deposited with a high oxygen flow and with a higher average energy of the plasma for all investigated parameter sets than the SiNOCH coating. Therefore, it is conceivable that ion peening is mostly responsible for the compressive stress formation in the SiOx coatings. In contrast to the SiOx coating, the SiNOCH coating can be applied without residual stress. For higher excitation powers, tensile stresses are predominant, most likely due to attractive forces between island or column boundaries and crosslinking.