Conventional coating is the thermal evaporation of the coating material under vacuum, using a resistance evaporator or an electron beam gun (hence the name “e-beam coating”).
By heating the substrates to about 280°C, the adhesive strength as well as the hardness and density and thus also the refractive index of the evaporated layers can be significantly increased. For this reason, dielectric layers are usually deposited “hot”. Fluorides and metal oxides are usually used as evaporation materials. Metal oxides are vapor-deposited reactively with the addition of oxygen.With this cost-effective technology, a variety of high-quality and efficient coatings can be produced.
Ion Plating (actually: Reactive Low Voltage Ion Plating, short RLVIP) is an ion and plasma assisted coating technology.
In this process, a low voltage arc is ignited by a hot cathode (plasma source) immersed in argon onto the evaporation material target from a specially modified electron beam evaporation device. Argon is used as the processing gas. For oxidic coatings, oxygen is added from a separate inlet as the reactive gas.Due to the positive potential at the target and the negative potential at the insulated substrate holder, the positive ions receive a high kinetic energy. This is about 30 eV to 50 eV and leads to maximum packing density (almost 100%, even without substrate heating) and thus to highest possible refractive indices. The coatings adhere excellently, are extremely hard and absorb neither gas nor water vapour.The optical properties of a coating produced by ion-plating technology are completely stable, independent of factors such as temperature or humidity. With their low absorption, IP coatings are suitable for the highest laser powers. Scattering losses are extremely small due to the smooth surfaces. The field of application of such coatings is also greatly extended by their high environmental resistance (abrasion-resistant, space-qualified and saltwater-resistant) and a possible maximum thermal load of up to 550°C (on quartz or sapphire).The high compression leads to greater mechanical compressive stress in the layers. This can be compensated on double-sided polished, plane-parallel substrates by a compensation layer on the backside with any optical function.For very demanding coating systems only special glasses and crystals are suitable as substrates, such as fused silica, Schott Borofloat, D263 T, sapphire, silicon, YAG, LiNbO3 or comparable materials.
High-quality optical filters are among others produced with sputtered hybrid plasma deposition. This process produces dense, thin coatings with exceptional hardness, abrasion resistance and adhesion to the substrate.
These layers have a higher packing density and a lower void ratio than unstabilized metal oxide layers and are therefore less affected by water absorption. In contrast to unstabilized metal oxide layers, which can experience spectral shifts in the range of 2-5% of wavelength, these layers typically show total shifts from wet to dry of less than 0.02% of wavelength. The layers are also typically 5 to 10 times less sensitive to thermal fluctuations than unstabilized metal oxide layers due to the densification. These coatings do not require additional protection such as hermetic sealing by lamination or other methods to achieve their exceptional durability.
