Nodular defects in dielectric multilayers and thick single layers

The investigation of nodular defects in both thick dielectric single layers and multilayer systems by scanning electron microscopy (SEM) reveals the nodules to be of essentially the same shape no matter how many layers were deposited and what materials were used. The SEM micrographs of a thick LaF(3) layer used in IR coatings and of TiO(2)/SiO(2) multilayer systems are presented. Whereas the nodules in the thick fluoride layer are clearly caused by small spherical particles in an underlying layer of Ge, the cause is less obvious with the multilayer nodules. However, micrographs of uncoated glass substrate surfaces obtained by replicating films used for transmission electron microscopy as well as investigations by SEM and electron micro-probe techniques indicated that particular surface defects or contaminations of submicroscopic size act as the nuclei necessary for the formation of nodules. A growth model and possible measures to prevent nodular defects are discussed from a general point of view.