Microstructure of vapor-deposited optical coatings

The microstructure of thin films applied by conventional physical vapor deposition for use as optical coatings is columnar for most of the materials commonly used. This has been established for about a decade through numerous experimental observations employing microfractographical replication for use with high resolution transmission electron microscopes. Scanning electron microscopes are more useful investigating coating defects, the most remarkable of these defects being known as nodules. From fundamental considerations of nucleation and growth of thin films, the origin of both columns and nodules and the dependence of their appearance on the deposition conditions are discussed in some detail. A simple 2-D simulation model assuming limited surface mobility of adatoms or admolecules shows striking similarities to peculiar properties of both columnar and nodular growth seen in actual investigations. Conclusions are drawn as to how the two types of microstructure influence general film properties and, in particular, how they influence possible laser damage mechanisms.