Thus far, little has been mentioned about the actual properties of the more useful materials employed in thin-film work. The following list, though not exhaustive, highlights the key properties of some commonly used materials. It is important to note that thin-film properties are highly dependent on process conditions.

The same material may exhibit a range of properties, even when processed on different machines. Stability of these properties across production runs is crucial, and random fluctuations can only be mitigated through tighter process control. Published figures should be considered as general guidelines rather than precise indicators of film properties.

Magnesium Fluoride:

Magnesium fluoride is one of the most widely used materials in thin-film work. Its refractive index is approximately 1.38–1.39 in the visible region (see Figure 11.21), making it a common choice for lens blooming, often as a single layer.

Fluorite was used historically but was replaced by magnesium fluoride due to its superior hardness and durability. Magnesium fluoride can be evaporated from a tantalum or molybdenum boat, with the best results achieved at substrate temperatures of 200–300 °C.

However, during evaporation, issues such as spitting may arise due to magnesium oxide coatings around magnesium fluoride grains. To minimize these issues, pure grades of the material, specifically designed for thin-film deposition, should be used and stored to protect against atmospheric moisture.

Magnesium fluoride films often exhibit high tensile stress. While single films are usually safe, multilayers containing magnesium fluoride can experience spontaneous delamination due to accumulated shear stress. Therefore, magnesium fluoride is not recommended for structures with multiple layers.

Zinc Sulfide and Cryolite:

These materials are particularly easy to handle, with zinc sulfide having a refractive index of about 2.35 and cryolite around 1.35 in the visible range. Both materials sublime rather than melt and can be deposited from tantalum or molybdenum boats or howitzers (see Figure 11.6).

Although they are not very robust, their ease of use makes them suitable for multilayer filters in the visible and near-infrared regions, often protected by a cemented cover slip. Substrates generally do not require heating for visible applications.

Zinc sulfide is also valuable in the infrared region, with performance extending to about 25 μm. For optimal durability, substrates should be heated to approximately 150 °C and cleaned with glow discharge before deposition. Electron bombardment of growing zinc sulfide films has been shown to improve durability by altering the crystal structure, producing films with better stability and adhesion.

Refractory Oxide Layers:

A range of refractory oxides is available, especially for high-index layers. Cerium Dioxide was once widely used but is now less common due to challenges in achieving homogeneous layers and consistent refractive indices.

Titanium Dioxide is now preferred for visible and near-infrared applications, offering the highest refractive index among transparent high-index materials. It is robust but requires careful evaporation conditions due to its high melting point. Reactive deposition methods, such as evaporation in the presence of oxygen or sputtering, are commonly used to ensure complete oxidation and achieve high-quality films.

Silicon Dioxide (Silica):

Silicon dioxide is the standard low-index material used with titanium dioxide. Films are typically deposited using electron-beam sources. Silicon monoxide, a convenient starting material, can oxidize to silicon dioxide during deposition, producing amorphous layers with excellent durability and transmission. Ultraviolet irradiation of silicon dioxide films can further improve their transmission properties and refractive index.

Rare Earth Oxides and Fluorides:

Oxides and fluorides of lanthanides, such as cerium, lanthanum, and yttrium, form stable, high-quality layers for various applications. Electron-beam evaporation often enhances their optical properties, especially in the ultraviolet region.

Semiconductors (Silicon and Germanium):

Silicon and germanium are commonly used in infrared applications due to their high refractive indices (3.5 for silicon and 4.0 for germanium). Germanium is easier to handle, often deposited from tungsten or molybdenum boats or graphite crucibles. Silicon is more challenging due to its reactivity but can be deposited using electron guns with water-cooled crucibles.

Other Materials:

Materials such as tellurium and lead telluride offer high refractive indices and excellent transmission in the infrared. These materials must be carefully handled to avoid compositional changes during deposition. Zinc sulfide and silicon monoxide are common low-index counterparts in the infrared.

Mixtures and Inhomogeneous Layers:

Mixtures of materials can provide intermediate refractive indices, offering greater flexibility in coating design. Techniques such as co-evaporation or ion-assisted deposition allow precise control over mixing ratios, enabling the creation of homogeneous or gradient-index films. Examples include mixtures of zinc sulfide and magnesium fluoride, or cerium oxide and cerium fluoride, which can be tailored for specific applications.

Advanced Techniques and Developments:

Recent advancements, such as ion-assisted deposition and reactive sputtering, have expanded the range of materials available for thin-film applications. These methods allow for the deposition of hard, durable nitride and oxynitride films with tunable refractive indices, suitable for a variety of optical applications.