Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization

Adhesion of plasma-deposited optical and protective coatings, such as amorphous hydrogenated silicon nitride, SiN_1.3, on polymethyl-methacrylate (PMMA) substrates has been found to be limited by a cohesive failure inside the PMMA bulk. Using direct exposure to a low pressure plasma in helium or to vacuum ultraviolet (VUV) radiation generated from plasma, the adhesion of SiN_1.3 at high humidity and elevated temperature has been substantially increased. Using a multitechnique analytical approach, the enhanced adhesion was attributed to the initial etching of the weak boundary layer followed by formation of a crosslinked, graded, mechanically stabilized layer in the interfacial region (interphase), which possesses a physical thickness of 50 to 100 nm and a microhardness of about 2 GPa.