Coating Technologies

Since around 1930, four fundamental methods have been established for the industrial production of optical coatings. In addition to sol-gel deposition (dip coating) and atomic layer deposition (ALD), two technologies currently dominate the market:

1. Vacuum-based evaporation (thermal evaporation, electron beam evaporation)
2. Sputtering processes (magnetron sputtering, ion beam sputtering)

LAYERTEC selects the most technically and economically suitable coating technology for each application – in close coordination with the customer.

Thermal and Electron Beam Evaporation

Principle of Operation

Thermal and electron beam evaporation are established standard methods for optical coatings. LAYERTEC primarily uses these technologies for UV applications.

The deposition takes place in vacuum coating systems. The evaporation sources, containing the coating material, are positioned at the bottom of the chamber. Evaporation is achieved either by resistive heating (thermal evaporation) or by electron beam bombardment (electron beam evaporation).

The choice of method depends on the thermophysical properties of the coating material – particularly melting point, sublimation behavior, and chemical stability. Substrates are mounted on rotating holders above the source to ensure uniform coating thickness. Substrate temperatures typically range from 150 °C to 400 °C, depending on the substrate and coating material. This ensures low absorption and strong adhesion of the coating.

To produce dense and environmentally stable layers, ion-assisted deposition (IAD) is used. LAYERTEC employs ion sources such as APS pro® and LION® (Bühler Alzenau GmbH). A total of seven vacuum coating systems are available. Large-format substrates such as cylindrical lenses up to 1.15 m in length can also be coated.

Properties of Evaporation Coatings

The kinetic energy of the coating particles during conventional evaporation is approximately 1 eV. To enhance particle mobility on the substrate surface, substrate heating is required. Standard evaporated coatings tend to have relatively low packing density and often contain microcrystalline structures. This results in increased light scattering – typically from a few tenths of a percent up to a few percent, depending on the wavelength.

Porous layers may also be sensitive to environmental factors: water vapor can diffuse into the coating and cause a spectral shift in the reflection bands (typically ≈ 1.5 % of the nominal wavelength). This effect can be avoided using dense coatings produced via ion-assisted deposition (IAD).

IAD coatings offer excellent optical stability, low absorption, and high laser-induced damage thresholds. They are well suited for use in high-power laser systems and demanding optical applications.

Sputtering

Principle of Operation

In sputtering, coating atoms are ejected from a solid target by bombardment with energetic noble gas ions – typically argon. The ions create a collision cascade in the target material, resulting in the emission of atoms from its surface.

The efficiency of the sputtering process depends on various parameters, including the binding energy, atomic mass of the target, and the angle of ion incidence. The process is highly controllable and ideal for producing dense, uniform coatings with excellent reproducibility.

Properties of Sputtered Coatings

Sputtered coatings are characterized by the high kinetic energy of the arriving particles (~10 eV), which enhances their surface mobility. This results in amorphous, dense films with excellent structural uniformity.

Key optical properties include:

• Low scatter losses
• Long-term stability of spectral properties under varying environmental conditions (no moisture uptake)
• High laser-induced damage thresholds
• Mechanical durability

LAYERTEC operates more than 36 sputtering systems (magnetron and ion beam sputtering) and provides the optimal coating process for each application. Substrates up to 60 cm in diameter (magnetron) or 30 cm (IBS) can currently be coated.

Findings from the SPIE Laser Damage Conference demonstrate that each coating technology offers specific advantages, depending on the application and technical requirements.

Magnetron Sputtering

In magnetron sputtering, a gas discharge is generated in front of the target to produce ionized noble gas atoms (typically argon). These ions transfer momentum to the target atoms, ejecting them from the surface.

The discharge can be operated with direct current (DC), for conductive targets such as titanium, or with radio frequency (RF), for dielectric materials such as titanium dioxide.

Reactive gases like oxygen enable the formation of oxides directly from metallic targets (reactive sputtering).

LAYERTEC has developed magnetron sputtering into a robust industrial process. The technology offers excellent coating properties, particularly in the VIS and NIR spectral regions. The largest systems allow substrate diameters up to 600 mm.

Ion Beam Sputtering

In ion beam sputtering (IBS), ions are generated in a separate ion source and precisely directed onto the target. This setup enables tight control over process parameters and minimizes contamination through spatial separation of the process zones.

In advanced systems, reactive gases – typically oxygen – are introduced via a second ion source. This increases reactivity and results in dense, homogeneous coatings.

Unlike magnetron sputtering, the three key components – ion source, target, and substrate – are physically separated in IBS. This allows the production of optically superior coatings with extremely smooth surfaces and high reproducibility.