Coating Technologies

There are four basic processes that have been developed since around 1930. Besides layer deposition by dipping (sol-gel process) and layer deposition from the gas phase (atomic layer deposition), only two technologies currently dominate the industrial production:

1. Layer deposition by evaporation of the layer forming materials in a vacuum (thermal and electron beam evaporation)
2. Layer deposition by sputtering (Magnetron, IBS)

LAYERTEC strives to offer the most effective, cost-efficient coating method for the respective application in direct customer contact.

Thermal and E-Beam Evaporation

Working Principle

Thermal and electron beam evaporation are the most common techniques for the production of optical coatings. LAYERTEC uses these techniques mainly for UV coatings. The evaporation sources are mounted at the bottom of the evaporation chamber. They contain the coating material which is heated by resistive heating (thermal evaporation) or by an electron gun (e-beam evaporation).

The method of heating depends on the material properties (e.g. the melting point) and the optical specifications. The substrates are mounted on a rotating substrate holder at the top of the evaporation chamber. Rotation of the substrates is necessary to ensure coating homogeneity. The substrates must be heated to a temperature of 150 – 400°C, depending on the substrate and coating materials. This provides low absorption losses and good adhesion of the coating to the substrates.

To achieve more compact layers, ion sources are used. LAYERTEC is equipped with several evaporation machines covering the whole bandwidth of the above-mentioned techniques from simple thermal evaporation to ion assisted deposition (IAD) using the APS pro^® and LION^® ion sources.

APS pro^® and LION^® are trademarks of Bühler Alzenau GmbH.

Properties of Evaporation Coatings

The energy of the film forming particles is very low (≈ 1 eV). That is why the mobility of the particles must be enhanced by heating the substrates. The packing density of standard evaporated coatings is relatively low and the layers often contain micro-crystallites. This results in relatively high scattering losses (some tenth of a percent to some percent, depending on the wavelength). Moreover, atmospheric water vapor can diffuse in and out of the coating depending on temperature and humidity, resulting in a shift of the reflectance bands by ≈ 1.5 % of the wavelength. Shift-free, i.e. dense, evaporated coatings can be produced by IAD using the APS pro^® and LION^® ion sources which provide very high ion current densities. Nevertheless, evaporated coatings have also high laser damage thresholds and low absorption. They are widely used in lasers and other optical devices.

Sputtering

Working Principle

In general, the term “sputtering” refers to the extraction of particles (atoms, ions, or molecules) from a solid material through the impact with noble gas atoms or ions. A cascade of collisions within the target material ultimately releases the atoms that form the coating. The efficiency of this process depends on factors such as binding energies, atomic masses, and the incident angle of the impacting atoms.

Properties of Sputtered Coatings

• Because of the high kinetic energy (≈ 10 eV), i.e. high mobility of the film forming particles, sputtered layers exhibit:
  • An amorphous microstructure
  • A high packing density (which is close to that of bulk materials)

• These structural characteristics result in very advantageous optical properties such as:
  • Low losses due to scattered light
  • High stability of the optical parameters under various environmental conditions due to the blocking of water diffusion
  • High laser-induced damage thresholds
  • High mechanical stability

LAYERTEC currently operates over 40 magnetron and IBS systems, so that the optimum deposition technology can be selected for each customer application. The maximum substrate diameter for IBS sputtering is currently 30 cm, for magnetron sputtering 60 cm. The competitions of the SPIE Laser Damage conference of the last years show that each coating technology offers advantages for certain applications. Suitable for your specifications, LAYERTEC’s coating engineers select the appropriate method.

Magnetron Sputtering

Ions are delivered by a gas discharge which burns in front of the target. It may be excited either by a direct voltage (DC-sputtering) or by an alternating voltage (RF-sputtering). In the case of DC-sputtering the target is a disk of a high purity metal (e.g. titanium). Adding a reactive gas to the gas discharge (e.g. oxygen) results in the formation of the corresponding compounds (e.g. oxides). For RF-sputtering, dielectric compounds (e.g. titanium dioxide) can also be used as targets.

Developments at LAYERTEC have taken magnetron sputtering from a laboratory technique to a very efficient industrial process, which yields coatings with outstanding properties especially in the VIS and NIR spectral range. The largest magnetron sputtering machine can coat substrates up to a diameter of 600 mm.

Ion Beam Sputtering

This technique uses a separate ion source to generate the ions. To avoid contaminations, RF-generators are used in modern IBS machines. The reactive gas (oxygen) is in most cases provided by a second ion source. This results in a better reactivity of the particles and in more compact layers.

The main difference between magnetron sputtering and ion beam sputtering is that ion generation, target and substrates are completely separated in the IBS process while they are very close to each other in the magnetron sputter process.