Anti-Reflective Coating Explained

 

If you’ve ever squinted reflexively after a bright sunbeam reflected off your windshield, you probably wished for a pair of sunglasses with an anti-reflective coating on the lenses to cut the glare. While light reflection is necessary for objects like mirrors, it causes absorption in glasses, telescopes, and lenses. However, depositing a special coating on the object’s surface (as in anti-reflective lenses) reduces reflections and glare, improving visual acuity. [1]

At Korvus Technology, we’re the UK’s premier source for thin film manufacturing, and over 25 organisations, universities, and brands trust our HEX Series deposition system. In this article, we’ll explain anti-reflection coatings, including different types, how they work, limitations, common uses, and more.

Anti Reflective Coating Explained

Properties of Anti-Reflective Coating

 

The mechanical and chemical properties of anti-reflective lens coatings make them invaluable for modern-day applications, including anti-glare glasses, lasers, display screens, optic lenses, and solar panels.

However, the inherent differences and bonds between the coating’s thin layer and the front and back surfaces of the substrate impact durability, hardness, strength, refraction, and reflectability. [1,3] Therefore, most anti-glare coatings are vulnerable to abrasion, which can pull off the coating on the lens surface. Thermal cycling and solvents can also cause stress or damage to the bond. [5]

Of course, the properties of an anti-reflection coating directly influence its useful lifespan. In particular, optoelectronic devices like camera lenses and touchscreens require the best anti-reflective coating possible. Ideally, the coating should have broadband, ultrathin thickness, and non-iridescent properties. [3]

How It Works

 

Through thin film and vacuum deposition technology, you can apply an AR coating to an object’s surface (like that of a standard lens), reducing light reflections and eye strain. [3] Anti-reflection coatings also depend on their refractive index to minimise light loss on lens surfaces. [1,4,5]

An anti-glare coating works by splitting light waves into two reflections. The split creates destructive interference, causing the light waves to cancel each other partially or entirely. [4] How the light waves travel and behave through mediums and interfaces determines how the AR coating will work. [5]

Once light passes through the air and meets a medium, the Fresnel equations can determine the amount of light reflected and transmitted, depending on the refractive indices. [1,3] The following equation defines the fraction of reflected light:

Rairmedium = [(nair – nmedium) / (nair + nmedium)](squared)

The equation calculates the index of refraction for an optimal AR coating that will reduce reflections off the surface. [1,5]

Different Types of Anti-Reflective Coatings

 

Single-layer AR Coatings

 

A single-layer AR coating may only become anti-reflective at a single wavelength, typically in the visible middle. [4] When depositing single-layer quarter-wavelength AR coatings, they can reduce surface reflectivity for incidence angle and limited wavelengths. [3]

The path length of the incident light will differ, reducing destructive interference. Many applications require single-layer anti-reflection coating, including photodiodes, lasers, and solar cells. However, the reflection dip in a single-layer anti-reflection coating makes it unfeasible for displays, lenses, and glasses. [3]

Multi-layer AR Coatings

 

A multi-layer AR coating contains multiple microscopic layers to improve performance and minimise reflection to less than 0.1% of incident light. Each thin layer is deposited onto the surface substrate to increase the destructive interference, maximising transmission. [3,5]

Most manufacturers switch between a low and high refractive index when depositing layers. Generally, anti-reflection coatings with multiple layers provide stronger broadband performance. However, the cost of manufacturing multi-layer anti-reflection coatings is prohibitive. [5] These coatings are more sophisticated than single-layer coatings and essential for optical applications, like lenses, astronomy, and aerospace telemetry. [1]

“V” Coatings

 

A “V” anti-reflection coating follows the same transmission and light reflectance principles as a single-layer coating. However, it undergoes optimisation to improve performance within a small niche of wavelengths. [1] The name derives from its high refractive index, creating a “V” shape that curves over multiple wavelengths. The centre arcs around each design wavelength (DWL). [5]

“V” AR coatings are for highly specialised applications that single- and multi-layer coatings are unsuitable for, like high-frequency lasers. Other applications include high index lenses, anti-reflective glasses with UV protection and less glare, digital microscopy, fibre optics, engraving, and more. [5]

Limitations

 

Anti-reflective coating and anti-glare lenses have dozens of practical uses for modern-day technology thanks to their unique properties. However, that doesn’t mean manufacturing AR coatings is easily accessible or affordable for the masses. As with any delicate and complex manufacturing process, there are certain limitations to consider.

The manufacturing process for anti-reflection coatings presents significant limitations. Most techniques cannot accommodate the deposition of AR coating on large-scale surfaces.

Furthermore, chemical vapour deposition or sol-gel chemistry creates a durable, strong AR coating. However, the process is prohibitively expensive, particularly for multi-layer stacks. Additionally, multi-layer filters are highly sensitive to variations in the refractive index and coating thickness. [3,4,5]

Another common limitation occurs with quarter-wavelength anti-reflection coatings. To lower the refractive index, manufacturers must use a porous coating material, which occurs in a single processing step. However, the coating’s porous nature reduces its strength and could make it more vulnerable to contamination. [3,4,5]

Nanostructured lenses with AR coatings that have a gradient to increase the refractive index have effective anti-reflection properties. However, the nanostructures in the topcoat are a double-edged sword as they decrease the mechanical strength of the coating. [3,4,5]

Manufacturing Process

 

Next, we’ll summarise the different manufacturing processes for anti-reflection coatings and lenses. These processes fall under two primary categories: conventional techniques and non-conventional techniques. [5] Of course, cutting-edge equipment – such as the HEX Series deposition system we manufacture – is necessary for creating anti-reflection coatings. Conventional techniques include top-down and bottom-up technologies. [3,5]

·         Top-down: Wet and dry etching, primarily for solar cell applications

·         Bottom-up: Sol-gel processing, physical vapour deposition, chemical vapour deposition

·         Non-conventional: Micro replication, lithography, photo-aligning

Conventional Techniques

 

Sol-gel chemistry processing is one of the most commonplace techniques for creating anti-reflection coatings and lenses. It uses metal oxides and organic solvents to condense the compounds into an inorganic polymer bond. [5] Standard sol-gel techniques include meniscus coating, dip coating, and spin coating.

Physical and chemical vapour deposition are two other common manufacturing methods and require using complex deposition systems like the HEX Series. Etching is another conventional technique, but it uses selective surface ablation to achieve the desired AR coating. [3,5]

Non-conventional Techniques

 

Some manufacturers use non-conventional techniques when creating an anti-reflective coating. Lithography falls under this category and consists of patterning the substrate surface with microscopic features. [5]

Micro-replication is another type of non-conventional manufacturing process. It involves a roll-to-roll process replicating nanostructures on a thermoplastic film surface, such as PVC. The photo-aligning technique is another method that minimises transmission to 99.1%. [5]

Common Uses of Anti-reflective Coatings

 

Generally, anti-reflection coating applications have two purposes (besides eliminating reflections): to improve an object’s aesthetic or efficiency. [2] Regarding aesthetics, applications include anti-glare glasses, picture glass, and electronic displays.

However, other applications like telephoto lens material, light-emitting diodes, and solar cell panels require AR coatings that maximise efficiency. [2] An anti-reflective lens coating that improves vision is also ideal for increasing available light transmission, enhancing contrast, eliminating ghost images, and sharpening visible focus.

The anti-reflective coating cost varies based on the manufacturing process, necessary equipment, intended use, surface substrate, etc. [2] However, we’re happy to answer questions regarding the cost of anti-reflection coatings and how they can add value to your business.

Final Thoughts

 

As you can see, anti-reflective coatings offer modern-day technology a world of opportunities for improving products, efficiency, and our quality of life. At Korvus Technology, we’re proud to be the leading source for deposition systems in the UK. To learn more, check out our blog or contact us online.

References

 

1. Bauer, G. (n.d.). Anti-reflection coatings. PVEducation. Retrieved August 25, 2022, from https://www.pveducation.org/pvcdrom/design-of-silicon-cells/anti-reflection-coatings

2. Burghoorn, M., et al. (2013). Single layer broadband anti-reflective coatings for plastic substrates produced by full wafer and roll-to-roll step-and-flash nano-imprint lithography. Materials, 6(9), 3710–3726. Retrieved August 25, 2022, from www.ncbi.nlm.nih.gov/pmc/articles/PMC5452668/, 10.3390/ma6093710.

3. Keshavarz Hedayati, M., & Elbahri, M. (2022). Antireflective coatings: Conventional stacking layers and ultrathin plasmonic metasurfaces, a mini-review.” Materials 9(6), 497. https://doi.org/10.3390/ma9060497

4. Nave, R. (n.d.). Anti-reflection coatings. HyperPhysics. Retrieved August 25, 2022, from http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/antiref.html

5. Raut, H. K., Ganesh, V. A., Nair, A. S., & Ramakrishna, S. (2011). Anti-reflective coatings: A critical, in-depth review. Energy & Environmental Science, 4(10), 3779–3804. https://doi.org/10.1039/c1ee01297e

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