Introduction and Principle of Optical Coating
Optical coatings are used to enhance the transmission, reflection or polarization characteristics of optical components. For example, approximately 10% of the incident light will be reflected from each uncoated glass device surface. The reflectance of each surface can be reduced to less than 0.1% with a transmission-enhancing film, and increased to more than 99.99% with a highly reflective dielectric film. Optical coatings consist of thin layers of materials such as oxides, metals or rare earth materials. The performance of optical coatings depends on the number of layers, their thickness and the difference in refractive index between the layers.
To maximize or minimize interference, their optical thickness is typically λ/4 optical thickness (QWOT) or λ/2 optical thickness (HWOT) of the wavelength of the light used in the application. These films consist of alternating high and low refractive indices to induce the desired interference effect
Optical coatings are designed to improve the performance of optical components at specific angles of incidence and polarization (e.g., s-polarization, p-polarization, or random polarization). If the incidence angle or polarization angle of the coating is different from that for which it was designed, it will result in a significant degradation of performance. Large enough deviations in incidence and polarization can result in complete loss of coating functionality.
Optical Coating Theory
To understand optical coatings, it is necessary to understand the Fresnel equations for refraction and reflection. Refraction is the change in the direction of propagation of a wave as it travels from one optical medium to another, and is determined by Snells Law of Refraction as follows.
(1) n1sinθ1=n2sinθ2n1sinθ1=n2sinθ2
where n1 is the refractive index of the incident medium, θ1 is the angle of the incident light, n2 is the index of the refracted/reflected medium, and θ2 is the angle of the refracted/reflected light
Using Sneer's law, the angle of light at any position of a multilayer thin film coating consisting of plane-parallel surfaces with different refractive indices can be found. Since Snell's law applies to each interface, the inner angle of light within the film is independent of the film order or the position of the film in the stack
When light strikes an electron on the surface of the material it enters, it is reflected. The electrons absorb and re-emit the light that has lost some energy. Bright and highly reflective specular materials have more freely moving electrons for maximum reflection and minimum transmission.
Coating Techniques
There are several techniques for applying optical coatings, including evaporative deposition, plasma sputtering, ion beam sputtering, and atomic layer deposition
Evaporation Deposition
In evaporative deposition, the source material in a vacuum chamber is evaporated by heating or electron beam bombardment. During evaporation, the vapor condenses onto the optical surface and precisely controls the heating, vacuum pressure, substrate positioning and rotation to maintain a uniform optical coating of a specific design thickness. Evaporative deposition can accommodate larger coater sizes and is often more cost effective than the other techniques described in this section. The relatively mild nature of vaporization produces loose or porous coatings. These loose coatings suffer from water absorption, which can alter the effective refractive index of the layer and lead to degraded performance. The evaporation process cannot be precisely controlled during evaporation deposition, so the layer thickness cannot be controlled as precisely as with other techniques such as ion beam sputtering. However, the advantage of these loose coatings is that they are relatively stress-free. Evaporative coatings can be enhanced using ion beam assisted deposition (IBAD or IAD), where the ion beam acts directly on the substrate surface, increasing the adhesion energy of the source material to the surface and producing a denser, stronger coating.
Plasma Sputtering
Plasma sputtering includes a range of known techniques including advanced plasma sputtering and magnetron sputtering. The general concept comes from the generation of plasma. Ions in the plasma are then accelerated into the source material, impacting loose, high-energy ions that are then sputtered onto the target optical element. Although each plasma sputtering has its own unique characteristics, advantages and disadvantages, the techniques are combined because they share a common operating concept. The differences in this group are much less than in the other coating techniques discussed in this section. Plasma sputtering achieves a compromise between evaporative deposition and ion beam sputtering in terms of price and performance.
Ion Beam Sputtering (IBS)
During ion beam sputtering (IBS), a high-energy electric field is used to accelerate the ion beam (Figure 5). This acceleration provides significant kinetic energy (~10-100 eV) to the ions. When the source material is impacted, the source material ions are "sputtered" from the target and form a dense film upon contact with the optical surface.5 A major advantage of using IBS coating rather than evaporative deposition is the ability to more precisely monitor and control the growth rate, energy input, and oxidation level of individual coatings. This level of control enables highly reproducible coating batches and minimal layer thickness errors, thus ensuring that the coating performance is consistent with the designed spectral and phase parameters.5 IBS coatings are much smoother than those using other coating technologies, making IBS the only coating technology capable of producing "super mirror" surfaces with reflectivity in excess of 99.99%. "The high density of the IBS coating makes it robust and improves its chemical resistance, extending the life of the coating and allowing it to withstand harsher environments. The refractive index of the layers can also vary during the IBS process, which further improves process control.5 Known for its accuracy and repeatability, IBS is the preferred coating deposition technique for high-performance laser optical coatings.The disadvantage of IBS is that it is more costly than other technologies because of the longer cycle times and stresses created in the optical components, which can lead to distortion and optical aberrations.
Atomic Layer Deposition (ALD)
Unlike evaporative deposition, the source material for atomic layer deposition (ALD) does not need to be evaporated from a solid, but is provided directly as a gas. Despite the use of a gas, high temperatures are often used in the vacuum chamber. In the ALD process, precursors are delivered in the form of non-overlapping pulses, each of which is self-limiting. The chemistry of the process is designed so that only a single layer can adhere to each pulse and the geometry of the surface is not a limiting factor. This allows for extraordinary control over layer thickness and design. This results in slow deposition rates and high costs per coating run. However, the chambers used for ALD are typically quite large and can cover many optical elements in a single run. ALD is also line-of-sight independent, which means it can be used to coat optical elements with unusual geometries that would be difficult to coat by other methods.
