Introduction to Anti-Reflective Coatings
An AR coating is an optical coating applied to a surface to reduce the amount of light reflected off the surface. It is typically used for optical applications where the coating is applied to the front of an interface between air and a lens, glass barrier, or mirror. AR coatings are designed to maximize the amount of light that transmits or enters the surface while minimizing the light lost to reflection. The coatings improve the efficiency of optical instruments, enhance contrast in imaging devices, and reduces scattered light that can interfere with the optical performance of telescopes, cameras, and binoculars, and decreases glare on eyeglasses.
The physics of how light travels through a medium and behaves at interfaces between two different mediums dictates how an AR coating works and behaves. AR coatings take advantage of the electromagnetic-wave properties of light to enhance transmittance. We’ll review the basics physics behind how AR coatings work, introduce several common types of AR coatings and their applications and discuss the characteristics of AR coatings.
How Anti-Reflective Coatings Work
When a light wave traveling through air encounters a new medium, some of the incident light transmits through the medium, while some of it reflects off the interface between the air and the medium. The amount of light that is transmitted and reflected is calculated using Fresnel’s Equations which are dependent on the indices of refraction for the air and the medium. Each medium has an index of refraction that is calculated as follows:
Where c is the speed of light in a vacuum, and v is the speed of light in the medium.
Fresnel’s Equation defines the fraction of light that is reflected as follows:
If a thin-film coating is applied to the front of the interface, there are two reflections – one at the interface between the air and the coating and another at the interface between the coating and the medium. Each of these reflections has a corresponding fraction of reflected light or Rair-coating and Rcoating-medium respectively.
If the thickness of the thin film is a quarter of a wavelength of the lightwave (λ/4), then the light reflected at the coating-medium interface travels half a wavelength further than the light reflected at the air-coating interface. The result is destructive interference of the reflected light or the elimination of reflection (see Figure 1). This is an ideal AR coating.
Take, for example, light traveling in air that encounters crown glass which is commonly used for lenses and optical components. Air has an index of refraction of 1.0003. Using the speed of light in crown glass which is ~1.97 x 108 meters/second, the index of refraction for crown glass is calculated as 1.52.
Using Fresnel’s Equations for reflection, approximately 4% of the incident light is reflected at the air-glass interface.
Fresnel’s equations can be used to find the index of refraction for the ideal AR coating as the geometric mean of the product of refraction indices for air and crown glass. So, an ideal AR coating has an index of refraction equal to 1.23. This ideal material would eliminate all reflection off of the crown glass.
Limitations of Anti-Reflective Coatings
The index of refraction depends on the Angle of Incidence (or AoI) while Fresnel’s equation is valid only for a normal angle of incidence. Suffice to say, a larger AoI will result in a higher index of refraction.
Many optical applications operate across a spectrum wavelength ranges, including infrared (700nm to 1mm), visible (400nm to 700nm), and ultraviolet (100nm to 400nm). The exception is lasers which are tuned to a narrow band of wavelength ranges.
An ideal AR coating with an index of refraction of 1.23 does not exist. As a result, various techniques and methods have been developed for selecting and designing AR coatings that aim to come as close to this ideal as possible for a given application.
Types of Anti-Reflective Coatings
Single-Layer AR Coatings
Magnesium Fluoride (MgF2) is often used as a Broadband AR (BBAR) coating suitable for visible light applications such as the crown glass example discussed previously. Magnesium fluoride has refractive index of 1.38 which is close to the anti-reflective ideal index of refraction of 1.23.
If Magnesium Fluoride is applied to the surface of crown glass at a thickness of 0.145μm – approximately one-quarter of a wavelength of green light in the middle of the visible band – the amount of light reflected drops from 4% to around 1%. The performance is even better for glasses with an index of refraction near 1.9.
AR lenses for eyeglasses, cameras, and other visible light optical applications use a Magnesium Fluoride coating. It is ideal for these applications because the coating is hard and relatively easy to apply; however, with improvements in manufacturing techniques, many of these applications have transitioned to multi-layer coatings (see next section). Magnesium Fluoride also has mild resistance to abrasion, good resistance to humidity and can be cleaned with mild solvents.
There are other fluoropolymers with indices of refraction closer to the ideal refractive index of 1.23, but they are harder to apply and less durable. They are better suited for highly specialized applications, but can be applied to plastic substrates such as polycarbonates.
For applications targeting wavelength ranges outside of the visible spectrum, other dielectric coating materials may be used as a single-layer coating. Silicon Nitride (Si3N4) and Titanium Dioxide (TiO2) are common AR coatings for solar cell photovoltaics operating in the near-infrared region (NIR).
The limitation of a single-layer AR coating is that the refractive index is inherent to the material and it is difficult to alter. Manufacturing techniques such as adding a nanostructure layer to the surface of the thin film or building nanostructures within the thin film can alter the index of refraction physically.
Multi-Layer AR Coatings
Multi-layer coatings are a common way to improve the optical performance of an AR coating. As the name implies, a multi-layer coating uses several layers of a thin film coating to successively reduce the reflected light. With a multi-layer coating, it is possible to reduce reflection to less than 0.1% of the incident light.
A multi-layer coating works on the same principles as demonstrated earlier with the air to crown glass example. In this case, there is a reflection between air and the coating (air-coating), at each interface between coating layers (coating-coating) and again between the coating and the substrate (coating-substrate). The material and thickness of each layer of the coating are designed to maximize the destructive interference of the reflected light to maximize transmission.
Although there are no specific combinations of layers, it is common to alternate between higher and lower indices of refraction. For a two-layer AR coating, first, a coating with an index of refraction of 2.3 is applied to the glass. The composite results in an index of refraction of 1.9. If a layer of Magnesium Fluoride is then applied on top of that higher index coating, the result is a near-ideal index of refraction of 1.23 (see Figure 3). The thickness of each layer is a function of the target wavelength.
For normal incident light and two layers, selecting the coating materials and determining the layer thicknesses is a somewhat straightforward application of Fresnel’s equations. However, arduous numerical models are used for multi-layer materials.
A multi-layer coating can be designed for combinations of substrate materials, angles of incidence and ranges of wavelengths to optimize optical performance. Examples of optical substrate materials include glass, sapphire, zinc selenide, germanium, zinc sulfide, calcium fluoride, and chalcogenides.
Multi-layer coatings are more costly to manufacture, require more care in designing and are typically reserved for high-performance optical instruments used for planetary astronomy, photolithography, and aerospace telemetry.
Generally, more AR coating layers offer a broader band of performance, but there are practical limitations to how many layers can be applied. Manufacturing techniques, cost, and mechanical properties tend to limit and determine the ideal number of layers. The bond between layers reduces the durability of multi-layer coatings so these coatings are more delicate than their single-layer counterparts.
Multi-layer coatings are often classified according to the wavelength spectrum for the application. Broadband Anti-Reflection (BBAR) coatings are typically characterized by the designed range of wavelengths such as near-infrared (NIR), short-wave infrared (SWIR), medium-wave infrared (MWIR), long-wave infrared (LWIR), and visible.
Figure 4 shows the index of refractions curves for two of EMF’s BBAR coatings.
A select category of AR coatings is called “V” coatings. These coatings use the same principles of light reflectance and transmission, but they are designed for maximum performance across a very small range of wavelengths.
“V” coats derive their name from the shape of the index of refraction curve over a range of wavelengths. “V” coatings are tuned so that the index of refraction is high except at designed wavelength (DWL). The resulting curve is a near “V” shaped curve centered around the DWL.
“V” coatings are designed for specialized applications such as lasers that use light sources tuned to a single frequency. Figure 5 shows the index of refraction curves for four such “V” coatings manufactured by EMF.
Here are some typical applications for each of these “V” coatings.
|UV AR coating centered at 353nm (A)||UV safety glasses, Eximer (XeF) lasers used for photolithography|
|Red VIS light coating centered at 683nm (B)||Red LEDs, digital microscopy, and imaging applications|
|NIR or SWIR coating centered at 1064nm (C)||Nd:YAG lasers used for cataract surgery, dental lasers, engraving and etching, weaponry, and optical|
|NIR or SWIR coating centered at 1550nm (D)||Optical communications devices and fiber optics|
Manufacturing Anti-Reflective Coatings
There are two broad categories of how AR coatings are fabricated. First are conventional techniques – bottom-up and top-down technologies.
Second is non-conventional techniques e.g. lithography, micro-replication, photo-aligning and photo-patterning. Non-conventional techniques will not be discussed here, but to learn more about these techniques see Anti-Reflective Coatings: A Critical, In-Depth Review by Hermant K. Raut, et al. (Energy and Environmental Science, August 2011).
Bottom-up technologies include Sol-gel processing techniques like dip coatings, spin coating, and meniscus coating, Physical Vapor Deposition (PVD), Glancing Angle Deposition (GLAD), Chemical Vapor Deposition (CVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD). Top-down technologies – not discussed here – include wet and dry etching typical of solar cell applications.
- Sol-gel techniques, developed in the 1960s, are not recommended for high-end optical applications. They tend to have poor thickness tolerance and unreacted solvents can impact performance.
- PVD techniques include electron beam, sputtering, pulsed laser, and cathodic arc deposition. PVD is often used for coating large flat substrates like computer monitors, televisions, architectural glass as well as protective goggles and eyeglasses.
- GLAD is a Physical Vapor Deposition (PVD) technique where the vaporized material is applied to an angled and rotating substrate to create a variable density, variable porosity, and textured coating. This technique is excellent for depositing TiO2 and SiO2 coatings for applications where nanostructures are desired.
- CVD, in particular PECVD, is the preferred method for applying many multipurpose AR coatings because of the excellent bond strength between the thin film and the substrate. The result is good abrasion resistance and superior durability. In addition, PECVD can be used for applying AR coatings to plastics.
Properties of Anti-Reflective Coatings
AR coatings are thin-film coatings applied to a substrate. Due to the mechanical and chemical differences between the thin film and the substrate, the durability of AR coatings is highly dependent on the bond between them as well as the bond between layers of coatings in multilayer coatings. As such, AR coatings are most susceptible to abrasion and adhesive pulls that peel away the coating, solvents that damage the bond, and thermal cycling that stresses the bond.
The hardness, strength, and durability of the coating itself plays a significant role in the longevity of an AR coating. The degree to which an AR coating is scratch and solvent resistant depends on the coating material. For example, an SiO2 coating has a slightly higher hardness than an MgF2 coating and this impacts how well they resist scratches and impact.
The most common damage to AR coatings on consumer products is scratching; however, with proper care and cleaning, these coatings can last several years.
One particular case of damage for AR coatings is the laser-induced damage threshold (LIDT) in laser applications. The beam intensity of high-powered lasers can damage coatings. The threshold is dependent on several factors including wavelength, pulse duration/repetition, spot size, angle of incidence and spatial effects. For this reason, it is essential to characterize laser applications to select coatings with high LIDTs.
AR coatings are an excellent way to reduce light reflection and increase light transmission for optical materials. They can be designed for specific applications to work over a broad range of wavelengths or, in the case of “V” coatings, they can be designed for a very narrow and specific target wavelength. AR coatings have applications in everyday items like eyeglasses and high tech applications like infrared imaging systems.
EMF offers a range of High-Efficiency AR or HEAR coatings that are compatible with a variety of substrates. AR coatings can be designed for infrared, ultraviolet or visible light ranges including customized coatings designed for specific applications.