Dense layers can be deposited by electron-beam evaporation or sputtering. Tantala can be used in combination with silicon dioxide layers to form high index-contrast multilayer structures. A particular advantage over titanium dioxide layers for near-IR laser and bandpass coatings is the absence of absorption above 900 nm. Evaporation causes some amount of dissociation and oxygen loss and requires a partial pressure of oxygen during deposition.
Adhesion is excellent to glass and to most other oxides.
The films generally grow with a crystalline microstructure. Under some evaporation conditions, such as low energy resistance-heated evaporation, low substrate temperature, or excessive background pressure, the films grow with low packing density and can exhibit index changes when vented to moist air. Deposition by sputtering, ion assist, or at very high substrate temperatures, improves the packing density and eliminates spectral shifting due to moist air-vacuum cycling, producing very stable layers. The refractive index responds to high energy deposition techniques and to substrate temperature because both parameters decrease the void volume by increasing the packing density of the microstructure. The refractive indices are dependent on the degree of oxidation and the film density achieved.