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 a wide range of optical devices; some examples:

• anti-reflection coatings
• highly reflecting laser mirrors or partially transmissive output couplers
• dichroic mirrors (treating different wavelengths differently)
• various kinds of optical filters (e.g. for attenuation of certain wavelength regions)
• beam splitters
• heat reflectors
• solar cell covers
• thin-film polarizers

While simple single-layer coatings are often used as anti-reflection coatings, dielectric mirrors typically use dozens of thin-film layers, sometimes more than 100. A typical mirror design is the simple Bragg mirror, but there are many more sophisticated mirror designs.

Typically, a dielectric coating consists of discrete layers of two different materials (frequently some oxides such as SiO₂ and TiO₂) with substantially different refractive indices. However, there are also gradient index coatings for rugate filters, where the refractive index is continuously varied. This can be achieved, for example, by gradually varying the chemical composition of the material during growth. Also, there are coatings using three or more different materials.

In many cases, the coating substrate is a type of optical glass with a wide range of transparency 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 antireflection 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. Another 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 ease of fabrication of 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. A common example is titanium dioxide (TiO₂); layers of that can be deposited with different techniques, and the resulting density and refractive index depends substantially on the fabrication method. Low-density coatings also have a higher tendency of incorporating water, which implies performance changes depending on the humidity of ambient air and the device temperature. 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 reflectivity of multilayer dielectric coatings can be tailored to many different applications by using appropriate coating designs. A coating design includes the choice of dielectric materials (typically two different ones) and all layer thickness values. The articles on anti-reflection coatings and 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:

This section applies primarily to coatings for glass and crystalline materials, although some aspects 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 generated from a hot filament and focused by a magnetic field. In the vacuum chamber, the vaporized material moves to the substrate, which can be covered with a mechanical shutter when the correct amount of material has been deposited. The target substrate is heated to a temperature as need to obtain high optical quality. (Thermal energy helps deposited atoms to find their proper location in the local lattice.) For typical coating materials, the resulting thin films tend to be somewhat porous, resulting in a reduced density and subsequently a reduced refractive index. The optical properties can then exhibit a significantly increased temperature dependence, as water can fill the pores and be driven out of the coating at elevated temperatures. This can be a problem for some sensitive steep-edge filter designs.
• A similar method uses evaporation by resistive heating of the crucible.
• Ion-assisted deposition (IAD) essentially works like e-beam evaporation, but with the addition of an ion beam (consisting of oxygen and/or argon ions) that strikes the target substrate. The relatively high energy of the ions allows rearrangement of the deposited material, resulting in denser coatings without heating the substrate. The method works well for oxide coatings (e.g. with SiO₂ and TiO₂), which can be of similar quality to those obtained with IBS (see below). However, IAD is not suitable for fluoride materials, which tend to dissociate.
• Ion Beam Sputtering (IBS) uses an ion beam that, after neutralization with a second filament, strikes a metal or metal oxide target to sputter material onto the substrate. The flux and energy of the ions can be independently and precisely controlled. IBS produces fairly uniform, non-porous coatings with good adhesion and very low surface roughness (possibly less than 1 Å), and is highly 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 then oxidized in a separate oxygen plasma region of the chamber. Separate magnetron sources are used for different coating materials. APRS combines high precision and high density of coatings (similar to IBS) with high speed (comparable to evaporation techniques).

In any case, one starts with a homogeneous substrate material. In case of dielectric mirrors and AR coatings, that may for example be BK7 glass, fused silica, or CaF₂. The substracte is often pre-processed by some method to produce 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 resulting layers are typically amorphous, with a density that can vary by more than 10% from the bulk density (depending on the deposition technique). Electron beam deposition typically produces materials with lower densities and thus lower refractive index. Such porous coatings have microvoids that can fill with water when exposed to humid air; in effect, the refractive index and thus the overall properties of the coating depend on humidity and temperature. Ion assisted deposition, and especially ion beam sputtering, achieves a higher density and thus a lower dependence on humidity. The optical damage threshold may also depend on the deposition method.

Materials with high refractive index contrast must be used for mirrors with high reflectivity, especially if a large reflection bandwidth is required. However, the chosen materials should also allow fabrication with high optical quality and high stability under given environmental conditions (regarding laser wavelength and intensity, operating temperature, humidity, etc.).

Important aspects for the selection of a processing technique are

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

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

In-situ growth monitoring is crucial to obtain precisely controlled film thickness values. One takes advantage of the fact that 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. One challenge is that the growth temperature is usually quite different from the intended operating temperature of the coating, and accurate temperature-dependent refractive index data are often not easily 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 for the substrate material vary depending on the application. For highly reflective mirrors, the surface roughness can be important, but also the coefficient of thermal expansion (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, leading to thermal lensing). For mirrors with partial light transmission (e.g. for output couplers of lasers, or dichroic mirrors), good transparency of the substrate in the relevant wavelength range is important in addition to high optical quality. The backside may have to be provided with an antireflection coating.

Dielectric coatings often exhibit a substantially lower threshold for laser-induced damage. That can also substantially depend on the fabrication method, including steps of preparing the substrate before deposition of the coating.

For antireflection coatings on laser crystals and especially on nonlinear crystals with anisotropic thermal expansion, but also for substrates with small radii of curvature or for optical devices exposed to some chemicals, it can be a challenge to obtain coatings that are sufficiently stable over a wide range of temperatures.

Bragg mirrors can also be made from crystalline semiconductor materials grown, for example, by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) (→ crystalline mirrors). Such mirrors are often part of larger structures such as vertical cavity surface emitting lasers.

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

More to Learn

Encyclopedia articles:

• dielectric mirrors
• dichroic mirrors
• anti-reflection coatings
• rugate filters
• beam splitters

Blog articles:

• The Photonics Spotlight 2007-03-09: “The Trouble with Crystal and Coating Damage”