CHARACTERIZATION OF DEFECT GEOMETRIES IN MULTILAYER OPTICAL COATINGS

Laser-induced damage in optical coatings is generally associated with micrometer-scale defects. A simple geometric model for nodule-shaped d efects is commonly used to describe defects in optical coatings. No sy stematic study has been done, however, to prove the applicability of t hat model to an optical coating deposition process. Not all defects ha ve a classic nodule geometry. The present study uses atomic force micr oscopy (AFM) and scanning electron microscopy to characterize the topo graphy of coating defects in a HfO2/SiO2 multilayer mirror system. Foc used ion-beam cross sectioning is then used to study the underlying de fect structure. This work develops a model for defect shape such that the overall geometry of a coating defect, particularly the seed size a nd depth, can be inferred from nondestructive evaluation measurements such as AFM. The relative mechanical stabilities of nodular defects ca n be deduced based on the nodule's geometry. Auger analysis showed tha t the seed material that causes nodular defects in HfO2/SiO2 multilaye rs is a hafnia oxide. Such characterization capabilities are needed fo r understanding the enhanced susceptibility of particular defects to l aser damage and for developing improved techniques for depositing low- defect density coatings.