Dielectric Coatings

Dielectric coatings, also called thin-film coatings or interference coatings, consist of thin (typically sub-micron) layers of transparent dielectric materials, which are deposited on a substrate. Their function is essentially to modify the reflective properties of the surface by exploiting the interference of reflections from multiple optical interfaces. They can be used for highly reflecting laser mirrors or partially transmissive output couplers, for dichroic mirrors (treating different wavelengths differently), for anti-reflection coatings, for various kinds of optical filters (e.g. for attenuation of certain wavelength regions), beam splitters, heat reflectors, solar cell covers, and thin-film polarizers. While simple single-layer coatings are often used as anti-reflection coatings, dielectric mirrors normally use dozens of thin-film layers, sometimes even more than 100. A typical mirror design is the simple Bragg mirror, but there are many more sophisticated mirror designs.

The typical kind of dielectric coating consists of discrete layers with substantially different refractive indices. However, there are also gradient-index coatings for rugate filters, where the refractive index is varied continuously. That can be achieved, for example, by gradually varying the chemical composition of the material during growth.

In many cases, the coating substrate is some kind of glass, with a wide transparency range and high optical quality (low bubble content), a very smooth surface (after proper polishing), and high durability. However, dielectric coatings can also be applied to crystalline materials, e.g., as anti-reflection coatings on nonlinear crystals for nonlinear frequency conversion and Pockels cells, or on semiconductor devices such as edge-emitting laser diodes, vertical cavity surface-emitting lasers, and photodiodes. A further area of increasing importance is the fabrication of dielectric coatings on polymers (plastic materials), as plastic optics are increasingly used due to their competitive properties, e.g. in terms of price and the ease of fabricating non-spherical surfaces (e.g. for aspheric lenses).

The material properties of the dielectric films can substantially depend on the used fabrication method (see below) and fabrication parameters such as the substrate temperature or partial pressures of various substances. Compared with the ordinary bulk materials of nominally the same chemical composition, thin-film layers often have a reduced packing density and a reduced refractive index. When designing a dielectric coating for fabrication on a certain coating machine, one should have the refractive index data for that machine under the used conditions, as the differences can be substantial.

Dielectric coatings are often applied to optical glasses, but they can also be used for plastic optics.

Designing Multilayer Coatings

The reflection properties of dielectric multilayer coatings can be adapted to many different applications by using suitable coating designs. A coating design involves the choice of dielectric materials (typically, two different ones) and of all the layer thickness values. The article on dielectric mirrors contains some details on design methods.

Fabrication and Choice of Materials

This section applies mostly to coatings for glasses and crystalline materials, even though some aspects also apply to polymer optics. The fabrication of dielectric mirrors is usually based on one of the following techniques:

• Electron beam deposition involves the evaporation of material in a crucible by heating with an electron beam, which is generated from a hot filament and focused with a magnetic field. In the vacuum chamber, the evaporated material moves to the substrate, which can be covered with a mechanical shutter as soon as the right amount of material has been deposited. The target substrate is heated to improve the quality. For typical coating materials, the obtained thin films tend to be somewhat porous, leading to a reduced density and subsequently to a reduced refractive index. The optical properties can then exhibit a significantly increased temperature dependence, as water may fill the pores, and may be driven out of the coating at elevated temperatures. This can be a problem for some sensitive steep-edge filter designs, for example.
• A similar method uses evaporation by resistive heating of the crucible.
• Ion-assisted deposition (IAD) essentially works like e-beam evaporation, but involves an additional ion beam (consisting of oxygen and/or argon ions) which hits the target substrate. The comparatively high energy of the ions allows a reordering of the deposited material, leading to denser coatings, even without heating the substrate. The method works well for oxide coatings (e.g. SiO₂ or TiO₂), which can have a similar quality as those obtained with IBS (see below). IAD is not suitable, however, for fluoride materials, which tend to disassociate.
• Ion beam sputtering (IBS) uses an ion beam which, after neutralization with a second filament, hits a metal or metal oxide target to sputter material to the substrate. The flux and energy of the ions can be controlled independently and precisely. IBS generates fairly uniform, non-porous coatings with good adhesion and very low surface roughness (possibly below 1 Å), and is well reproducible. However, it is relatively slow, requires expensive equipment and materials, and is less flexible than other methods.
• Advanced plasma reactive sputtering (APRS) involves the sputtering of thin metal films, which are subsequently oxidized in a separate oxygen plasma region of the chamber. Separate magnetron sources are used for the different coating materials. APRS combines a high precision and high density of the coatings (similar to that of IBS) with a high speed (comparable to that of evaporation techniques).

In any case, one starts with some homogeneous substrate material such as BK7 glass, fused silica, or CaF₂, which is often pre-processed with method for preparing a very clean surface. Common coating materials are oxides such as SiO₂, TiO₂, Al₂O₃ and Ta₂O₅, and fluorides such as MgF₂, LaF₃ and AlF₃. The layers obtained are usually amorphous, with a density which can (depending on the fabrication technique) deviate from that of bulk material by more than 10%. Electron-beam deposition typically generates materials with lower densities, and thus also a lower refractive index. Such porous coatings have microvoids which can fill up with water when exposed to humid air; in effect, the refractive index and thus the whole properties of the coating depend on the humidity. Ion-assisted deposition and particularly ion beam sputtering achieve a higher density and accordingly a lower dependence on humidity. The optical damage threshold can also depend on the fabrication method.

Materials with a high refractive index contrast need to be used for high reflectivity mirrors, and particularly when a large reflection bandwidth is required. However, the chosen materials should also allow for fabrication with high optical quality and should have high stability under given environmental conditions (concerning laser wavelength and intensity, operation temperature, humidity, etc.).

Important aspects for the selection of a fabrication technique are

• the suitability for given coating materials
• the precision of the layer thickness values (which may be increased with automatic control involving in-situ growth monitoring)
• the optical quality of the deposited layers (influencing e.g. the scattering losses)
• the ability of the coatings to withstand high optical intensities
• the uniformity of layer thickness values over a larger area
• the consistency (reproducibility) and stability of obtained refractive indices
• the required substrate temperature
• the time required for the growth

For example, ion-assisted deposition produces TiO₂ films which are more compact and thus more stable and homogeneous and have a higher refractive index, compared with e-beam evaporation, which however is a faster process. The reason is essentially that TiO₂ has a tendency to grow in low-density nanostructures, which can be destroyed (compacted) by irradiation with high-energy ions.

In-situ growth monitoring is crucial for obtaining precisely controlled layer thickness values. One uses the fact that the optical reflection or transmission properties can be used during the process to monitor the thickness of the currently grown layer, so that the growth process for a layer can be stopped at exactly the right time. A challenge is that the growth temperature usually differs considerably from the intended operating temperature of the coating, and accurate temperature-dependent refractive index data are often not available.

Apart from the basic fabrication method, the process parameters such as the substrate temperature and growth rate can also be important for the quality. The details are often confidential proprietary information of the fabricators.

The requirements on the substrate material vary, depending on the application. For highly reflective mirrors, important technical aspects can be the surface roughness, but also the thermal expansion coefficient (which is ideally similar to that of the coating materials) and the thermal conductivity. (In high-power lasers, residual absorption in the coating can cause some bulging of the mirror surface, inducing thermal lensing.) For mirrors with partial transmission of the light (e.g. for output couplers of lasers, or dichroic mirrors), it is also important to have good transparency of the substrate in the relevant wavelength range in addition to high optical quality. The back side may have to be given an anti-reflection coating.

For anti-reflection coatings on laser crystals and particularly on nonlinear crystals with anisotropic thermal expansion, but also for substrates with small curvature radii or for optical devices exposed to some chemicals, it can be a challenge to obtain coatings which are sufficiently stable. Optical damage, often occurring at microscopic defects, can also be a problem for devices operating with high optical intensities.

Bragg mirrors can also be made of crystalline semiconductor materials, grown e.g. with molecular beam epitaxy (MBE) or with metal–organic chemical vapor deposition (MOCVD) (→ crystalline mirrors). Such mirrors are often parts of some larger structures, such as vertical cavity surface-emitting lasers.

The article on dielectric mirrors explains in some detail how the optical properties of dielectric coatings can be calculated, and which aspects are important for designing such structures.

See also: dielectric mirrors, dichroic mirrors, anti-reflection coatings, rugate filters, beam splitters, thin-film polarizers, crystalline mirrors, spotlight 2007-03-09