Mechanical properties of plasma-deposited silicon-based inhomogeneous optical coatings

Amorphous hydrogenated silicon nitride, oxide and oxynitride films are depo sited by plasma-enhanced chemical vapour deposition (PECVD) using a dual-mo de microwave/radio-frequency (MW/RF) plasma system. Optical filters are pre pared by varying the film composition either abruptly (discrete, homogeneou s multilayer structure) or continuously (graded-index, inhomogeneous struct ure). The coatings are characterised both optically, by spectrophotometry a nd spectroscopic ellipsometry, and mechanically, with depth-sensing indenta tion and low-load microscratch testing. A comparison is made between the pr operties of the homogeneous multilayer and the inhomogeneous multilayer str ucture with the corresponding optical performance. This multiple technique approach for characterisation was proved to be efficient for analysis of th e optical and mechanical behaviour of coatings, and it provides a possibili ty for optimising the deposition process. It is demonstrated that the grade d system exhibits a higher mechanical strength and a better toughness than the discrete structure. (C) 1999 Elsevier Science S.A. All rights reserved.