14 Optical Coatings: Reflection and Anti reflection

Introduction

Optical coatings are composed of combination of thin film layers which can create interference effect. This is interference is used to enhance the transmission or reflection properties within an optical system. The performance of an optical coating is dependent upon the number of layers, the thickness of the individual layers, and the refractive index difference at the layer interfaces. Many common types of coatings are used on precision optics, including Anti-Reflection (AR) Coatings, High Reflective (Mirror) Coatings, Beamsplitter coatings, and Filter coatings such as shortpass, longpass, and notch filters. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce unwanted reflections. High Reflective coatings are designed to maximize reflectance at either a single wavelength or across a broad range of wavelengths and are most often used to create mirrors. Beamsplitter coatings are used to divide incident light into known transmitted and reflected light outputs. Filters are found in a large number of life science and medical applications and are used to transmit, reflect, absorb, or attenuate light at specific wavelengths. Edmund Optics also offers a variety of Custom Coatings to meet any application need.

Materials for Optical coating

Optical Coatings are designed for a specific incident angle and a specific polarization of light such as S-polarization, P-polarization, or random polarization. Using the coating at a different angle of incidence than what it is designed for will result in a significant degradation in performance, and sufficiently large deviations in incidence angle can result in a complete loss of coating function. Similarly, using a different polarization than the design polarization will generally yield undesirable results.

In Table 1, the chemical categories of the materials that are used for optical coating are segregated by spectral region; overlap exists between all regions. Table 2 lists the materials typically used in those spectral regions. These pure (unmixed) compounds represent selections distilled from dozens of potential materials according to required thin-film layer properties such as transparency, mechanical properties, environmental durability and ease of deposition.

With the exception of the fluoride compounds and lanthanum titanate, these materials are typically deposited by e-beam evaporation or by sputtering from targets. Fluoride compounds and many of the oxide compounds can, alternatively, be evaporated from resistance-heated sources. Because of chemical, stress and process incompatibilities, layers from the oxide and fluoride chemical classes are generally not combined in a coating. The exception is with UV coatings where layers are thin and the material selection of transparent materials is small. Multicomponent materials have been developed to achieve improvements in one or more thin-film layer properties over the single-element precursor. Examples are doped fluorides and oxides and mixtures of two or more similar materials. Mixed and doped starting materials can be obtained, or modifications can be accomplished during evaporation or sputtering from separate sources of materials. Layers of fluorides and sulfides (selenides) are combined in LWIR coatings.

Table 2 also segregates materials according to their mean high-, intermediate- and low-refractive indices within the spectral ranges of greatest usefulness, as limited by absorption. Refractive index is a property of key importance in multilayer coating designs for AR, edge filters, dichroic reflectors, polarizers, laser reflectors and spectral filters.

Deposition and growth of thin films of these materials to build optical coatings is accomplished by one of the many variations of two main physical vapor deposition (PVD) processes: evaporation and sputtering. Deposition of films of metal oxide compounds by evaporation or by sputtering can proceed from a preparation of the compound as the starting material or from the base metal and subsequent reactive oxidization to the final desired composition. The nonoxide materials in Table 1 generally start and end with similar compositions. The data tables that follow this article provide suggested deposition parameters.

Optical Coating Theory

Coatings control the reflection and transmission of light through the mechanism of optical interference. When two beams propagate along coincident paths and their phases match, the spatial location of the wave peaks also match and will combine to create a larger total amplitude. When the beams are out of phase (180° shift) their overlay will result in a subtractive effect at all the peaks, causing the combined amplitude to decrease. These effects are known as constructive and destructive interference respectively.

The relations that dictate the total reflectance of a multi-layer thin film structure are given in Equations 1 – 4:

Where, q is number of layers, δ is phase term, η is optical admittance of the layers, Np is Complex Refractive Index, tp is physical thickness of the layer, λ is wavelength, θp is angle of incidence, Y is the optical admittance of the stack and R is Reflectance of the stack.

While the wavelength of light and angle of incidence are usually specified, the index of refraction and thickness of layers can be varied to optimize performance. Changes in any of these will have an effect on the path length of the light rays within the coating, which in turn will alter the phase values as the light travels. This effect can most simply be explained through the example of a single-layer anti-reflection coating. As light propagates through the system, reflections will occur at the two interfaces of index change on either side of the coating. In order to minimize reflection, we would ideally have 180° phase shift between these two reflected portions when they recombine at the first interface. This phase difference directly corresponds to a λ/2 shift of the sinusoid wave, which can best be accomplished by setting the optical thickness of the layer to λ/4. Refer to Figure 1 for an illustration of this concept.

Index of refraction not only influences optical path length (and thus, phase), but also the reflection characteristics at each interface. The reflection is defined through Fresnel’s Equation (Equation 5), which provides the amount of reflection that will occur from the refractive index change at an interface at normal incidence.

The final parameter that must be taken into account is the incident angle of the light the coating is to be designed for. If the incident angle of the light is altered, the internal angles and optical path lengths within each layer will be affected; this will influence the amount of phase change in the reflected beams. When a non-normal incidence is used, S-polarized and P- polarized light will reflect differently from one another at each interface, which will cause different optical performances at the two polarizations. It is this phenomenon that allows for the design of polarizing beamsplitters.

Coating deposition process technology

The components of the deposition process, from performance requirement to finished product, are diagrammed in Figure 2. It is evident from the descriptions of the process components that material science and deposition technology play complex interdependent roles in successfully producing optical coatings.

•  Design and engineering considerations:

A thin-film design is generated that includes the wavelength dependencies of the optical constants, refractive index and extinction coefficient, determined for the materials and deposition process parameters. Predicted performance is evaluated with respect to spectral coverage, incidence angle, surface shape, substrate temperature constraints, mechanical durability and verification measurement. The starting form of the coating material and its preparation are selected based on deposition technique and thin-film design. Some materials require preconditioning to prepare for smooth deposition at a constant rate and consistent composition. The deposition process’ operational parameters, previously determined for each material, are programmed into the deposition controller along with the coating design. Deposition parameters coupled with material behavior determine the physical microstructural properties of the thin-film layer. Those film properties in turn determine the optical, mechanical and environmental properties of each deposited layer.

•  Materials selections: The materials that will be used in the design are selected according to the spectral criteria of Tables 1 and 2, as well as durability requirements and deposition process. Coating materials are preprocessed and formed to provide controlled vaporization behavior and to insure that the physical and optical properties of the deposited film are consistent from run to run. The optimum film composition, physical structure, and the vaporization behavior throughout the deposition process are strongly dependent on the preparation of the starting material. Furthermore, the different chemical compositions outlined in Table 1 and the specific compounds listed in Table 2 require individual preparations and forms.

•  Oxide compounds: Oxide films can be deposited from an evaporant source or sputtered using oxide or metallic targets. Evaporable metal oxide compounds are supplied in several forms and preparations depending on the particular compound. Typical physical forms are sintered or melted pieces, broken crystals, hot-or cold-pressed tablets, or pre-melted, preformed e-beam pocket cones. Cold-pressed and sintered preparations of tablets and pellets of various sizes are made from powder that might be combined with a binder and hot pressed in a vacuum or in an inert atmosphere. Compressed density (>95 %) is desired to minimize volume porosity from which water vapor or gas can be released upon heating. Some oxides adsorb surface water; others form hydrates that dissociate with heat. Both types of inclusions can result in the release of water vapor or other trapped gas and generate particulates or cause pressure variations. Fluoride compounds adsorb atmospheric water and experience the same problems and must be gently outgassed.

The chemical state of the starting material might be fully oxidized or be reduced to a suboxide. The reduced state often melts and is conductive; both properties are desirable for e-gun vaporization. Materials that evaporate from the melt have a lower tendency of ejecting microparticulates and spatter (large projectiles) that can result in optical scatter and pinhole creation. When the melt is electrically conducting, e-beam deflection and defocus due to localized charging are eliminated. The reduced forms are gray or black in color vs. white for the fully oxidized insulating material. Reoxidization is achieved in several ways. The first is deposition onto a substrate held at temperature >200 °C, providing an excess of oxygen. The second method is to produce oxygen ions in a reactive plasma process. The third popular technique is to use an ion assist (IAD) source that produces energetic Ar and O ions. If the activation/reactive energies are insufficient to produce a nonabsorbing film, postbaking in air at a high temperature (>300 °C) might be required.

Referring to the tables, we see that the low-index oxide common to all oxide coating combinations is SiO2. The companion high-index materials for visible through SW wavelengths are titania or tantala. A successful replacement for tantala, Ta2O5, is lanthanum titanate LaTiO3. As prepared, this material is actually a complex chemical compound. Compared to pure tantala, LaTiO3has the desirable properties of requiring lower evaporation temperature producing denser film layers of lower stress with nearly the same index. Its IR transparency extends to ~8 µm.

Titanium dioxide, TiO2, is a favorite visible-range material because of its high refractive index. It finds wide application in AR coatings in the ophthalmic industry. Numerous deposition techniques involving different oxidation states of the starting preparations have been studied. To avoid the presence of multiple and unstable crystalline states in the deposited coating, and evaporation difficulties, the starting material composition should be Ti3O5. This suboxide composition melts, enabling smooth, reproducible deposition. Another high-index alternative to titania and tantala is niobia. Oxides of the three materials can be deposited by sputtering the metal.

Some oxide compounds, and all fluoride compounds, melt before reaching evaporation temperature; others, such as silica, alumina and refractory oxide compounds (hafnia and zirconia), evaporate from a fused vitreous surface or sublimate (SiO). If the material retains its granular form rather than forming a melt, heat transfer among the grains or pieces is inefficient, and the concentration of heat at sharp boundaries such as dust, voids or trapped water sites can result in the explosive emanation of particles and pressure bursts. Extended preconditioning with a sweeping electron beam can reduce these problems.

•    Avoiding problems: Particulate showering and spatter and pressure bursts are common problems encountered when evaporating oxide compounds. When the starting material is not properly preconditioned or is of nonoptimum composition, particle emanation can range in intensity from occasional bursts to continuous showering. Particles ranging in size from nanometer to multiple-micrometer can be embedded in the layer and cause light scatter or voids (pinholes) when removed by abrasion. Particulates are often the initiation sites for laser damage and water penetration, and that failure mechanism is as important as absorption in the film in limiting the damage threshold. Included particulates become points of mechanical stress concentration and initiation sites for failures such as crazing and cracking. With underdense starting materials, pressure variation from the release of trapped gases during film layer growth can contribute to inhomogeneity of optical properties as well as upsetting the crystal monitor.

A technique that coating technicians apply to reduce these problems is to create a dense melt by repeated charging of the crucible and melting down to a dense mass. This procedure is wasteful of time and resources, and an improved preparation of many oxide compounds has been introduced: premelted cones that fit the e-beam pocket.

•  Controlling nanostructural growth: A growth structure problem common to oxide and fluoride films is that of low packing density. Apart from a small number of exceptions (SiO and alumina), thin films tend to grow with a columnar nanostructure unless special measures are adopted. The energy and chemical environments present during growth determine the nature and scale of the structure. When low energy is present, for example at low substrate temperature, the arriving adatoms do not have sufficient mobility energy to nucleate continuous coverage on the surface. Lacking surface mobility, isolated islands nucleate nanocrystallites that then grow in thickness. Subsequent self-shadowing results in the growth of an open structure of columns that have greater diameters at the substrate surface and taper with height. Vapor incidence at large angles can also increase columnar spacing and exaggerate these problems. The porous structure is not only mechanically weak and soft, but is permeable to moisture. Adsorption of water on the walls of the columns increases the effective optical index and also changes intrinsic stress; both properties are unstable to humid and arid exposure. Spectral shifting and stress-induced failure under varying humidity conditions are evidence of underdense layers.

Film density can increase to near bulklike values by increasing the growth energy of the adatoms or by changing the material composition to discourage singular growth dynamics. In preference to resorting to very high substrate temperatures to eliminate optical and mechanical instabilities, IAD is used to apply high energy to the film. In this technique, ions of oxygen and argon impact the growing film transferring high kinetic energies, ~100s eV. Greater surface mobility and high momentum transfer result in high packing density.

The second method for discouraging low packing density is to introduce an “impurity” content whose species occupy the spaces and the unsatisfied bonds normally accompanying nanocrystallites, thereby interfering with crystalline growth and resulting in an amorphous, dense structure. The additive should be mutually solvent with the host material and possess other favorable chemical properties. A number of oxide (and fluoride) mixtures have been engineered and are available commercially.

•  Multicomponent materials: The higher evaporation temperature oxide compounds, zirconia, hafnia and scandia, are transparent into the UV, as noted, but these materials present evaporation and optical problems. A refractive index gradient that varies with film thickness can exist in the film layer. Other oxide compounds, in pure form, suffer inconsistent evaporation behavior. The film itself might contain multiple crystalline states that can transform at high temperature, causing the optical and mechanical properties of the film to be unstable.

Mixed materials composed of a host oxide and small percentage of a similar oxide compound have been introduced to improve on evaporation consistency. In addition, the mixed materials promote the growth of dense film layer structures because the incorporation of the additive discourages the growth of multiple crystalline phases. Instead, a matrix of fine crystallites is assumed to form that is effectively “pseudo amorphous”. The result is stable properties under exposure to high temperature or high energy laser irradiation. Binary and tertiary mixtures have been engineered that exhibit improved hardness and tribological properties along with transparency.Lanthanum titanate is a material that provides improved properties, as discussed above.

High mutual solubility exists among refractory oxide compounds namely Al2O3, In2O3, MgO, Y2O3, ZrO2, TiO2and Sc2O3. The additive proportion is ~10 wt percent. Some examples of available mixtures are TiO2-ZrO2, ZrO2-MgO, ZnS-CeF3. Zirconia is used in laser coatings as the high-index layer. However it suffers from high mechanical stress, inhomogeneous index profile, and low laser damage tolerance. Mixing with 25 percent yttria results in a threefold increase in damage threshold and improved physical properties. Similar results have been reported with hafnia. Adding MgO to zirconia stabilizes the crystalline state to cubic; adding alumina to zirconia constrains the tetragonal state. In these cases, the nanostructure is not amorphous, but is a stable crystalline state. Composing a ternary combination of these compounds results in an amorphous state. The index of mixtures is lower than that for the pure high index component alone.

By varying the relative proportions of a compatible pair of high- and low-index materials, any intermediate index can be deposited to produce an index value that is not available in a single material. Coevaporation, cosputtering and alternating deposition of two sources are techniques used.

•  Fluoride coating materials: The evaporation or sputtering of nonabsorbing film layers of oxide compounds requires oxidation to the desired composition by introducing activated oxygen in a reactive process. Fluoride and semiconductor layer deposition proceed without the need to supply components; in fact high vacuum is required to limit gas incorporation in fluoride film layers. Fluorides retain their chemical composition upon evaporation unless excessively high power is applied. It is advisable to operate at a minimum e-beam voltage.

•  II-VI compounds: Optical coatings that operate to wavelengths ~5 µm can be built from the oxide and fluoride materials that we have discussed. Beyond ~5 µm, oxide compounds begin to absorb so sulfide and selenide compositions must be substituted along with semiconductors, as shown in Table 2. A handful of fluorides are also useable up to the limit imposed by thickness-dependent tensile stress level.

The low-index components for MWIR to LWIR coatings, ZnS and ZnSe, sublime and dissociate when evaporated. The starting materials are produced in a CVD process from the reaction of gases. The condensed solids are crushed and sized or pressed into desired working shapes. If substrate surface and chamber atmosphere conditions are favorable, the dissociated components will recombine with correct chemical composition at the substrate and produce nonabsorbing dense films. While e-beam is a useable evaporation technique, a heated baffled box source is preferred because in the nearly isothermal evaporation environment created, there are fewer hot spots that can produce unrecoverable composition deviations.

The high-index component of a LWIR multilayer AR, bandpass, or edge filter is usually germanium, with index = 4, and is evaporated from an e-gun melt. Procedure to obtain nonabsorbing films includes the elimination of oxygen and operation at a deposition rate and a substrate temperature that prevents the formation of an oxide. Silicon can be used for SWIR films, but it is difficult to avoid the formation of a silicon oxide whose evaporation temperature is much higher than that of the metal, and consequently produces spatter.

•  Semiconductor materials: Thin-film photovoltaic solar cells using materials alternative to silicon are being produced. Power-generating efficiency is available that is competitive with that achieved by amorphous thin-film silicon. Thin-film layers of direct bandgap semiconductors such as CdTe, CdS and ZnS and multicompositions, Cu(In,Ga)Se2 (CIGS) and CuInSe2 (CIS) or CuInSSe (CISS) are used to construct the absorber layers in PV cells on glass or flexible substrates. Multijunction cells can be constructed by varying the bandgaps of the semiconductors to extend their spectral sensitivity. Evaporation of CdS and ZnS layers is well established technology. Coevaporation or sputtering is used to deposit the CIGS layer (for example). Layers of PV CdS, ZnTe or CdTe can also be grown from a chemical bath in another construction process. The transparent conducting layer is applied over or under the absorber semiconducting layer within the stack that makes up the cell, depending on its character, i.e., its donor function.

•  Transparent conducting oxides: Solar cells of all compositions and constructions require a transparent conducting layer as one electrode. Transparent conductors (TCO) are based on the addition to a transparent oxide of a few percent of a cation or anion donor carrier that is responsible for electrical conduction. A variety of TCO materials have been developed; ITO, indium-doped tin oxide, has the longest history and widest application. ITO is being replaced by compositions that are less expensive than indium-containing material. Spray pyrolized SnO2 and SnO2:F are alternates to ITO for solar cell electrodes, as are Cd2SnO4 and Zn2SnO4. Zinc oxide-based formulations and reacted targets of these materials are receiving attention currently. Al:doped ZnO (AZO) is among the TCOs being developed, along with IZO (indium-doped) and GZO (gallium-doped). AZO is deposited by DC sputtering from a target that contains 2 to 4 percent aluminum. Another formulation is Al2O3:ZnO. AZO films are more transparent than ITO films, but less conductive, and material costs are one-third to one-fifth of those for ITO. Lower conductivity is not an issue with transparent electrodes on solar cells. Deposition on cold substrates and easier patterning procedures are further advantages of AZO over ITO.

•  Sputtering materials: Sputter deposition has the advantages of reproducibility of results and simple process automation. Optical films can be deposited from a metal target by oxidizing/nitriding sputtered metal ions to deposit an oxide or nitride film layer of the desired composition. DC magnetron sputtering is the technique used with metal targets that are electrically conducting. Oxide (insulating) targets can be sputtered by RF, but the rate is lower. Many variations in the sputter technique have been developed. Practically any material – alloy, mixture, pure metal, ceramic, oxide, nitride, boride, carbide, etc., – can be supplied as a target. Fluorides and sulfides (selenides) are rarely traditionally sputtered because of the hazardous   nature    of    the   components    and    difficulty   in    maintaining     composition.

•  Metals: Metal oxide and nitride compounds for optical applications can be deposited by admitting the appropriate reactive gas to the sputtering plasma. Similarly, metal carbide and boride compounds used in tribological coatings can be sputter deposited. High deposition rates are achieved by DC magnetron sputtering. Aluminum, gold and silver can be thermally or e-beam evaporated.

References

1. S. F. Pellicori, CERAC Coating Materials news articles, 1991 to present, and Technical Data Sheets. http://www.cerac.com/ – Technical Publications.
2. http://www.cerac.com/.