Patent Review: AR coatings: doing more than reflecting less

The concept of using a coating to reduce surface reflection has been known for many years. Yet the technology continues to advance, and patent activity shows no sign of slowing down. An additional focus is films that do more than just reduce reflection.Lord J.S. Rayleigh demonstrated in 1879 that a graded index-of-refraction layer could reduce reflection over a broad range of wavelengths (On the reflection of vibrations at the confines of two media between which the transition is gradual, Proc. London Math. Soc., 11 (1879) pp. 51-56). However in Rayleigh's day, there were relatively few applications for this effect.

In the modern world there is a strong impetus to push the technology to its limits because of the need for higher efficiency in solar cells and LED lighting, as well as the need to reduce glare from display screens. Patents in this technology continue to grow especially in countries other than the United States. The number of U.S. patent titles that refer to antireflection coatings in the first 7-1/2 years of this century is almost twice that of the 1990s. Even larger increases have occurred for European patents. And the Japanese patent office has been publishing significantly more patent applications in this area than the United States and Europe combined.

In practical implementations, it is common to use a single layer of constant index to reduce the reflectivity to zero at a chosen wavelength at normal incidence. Multi-layers or graded layers can provide low reflectance over a broader bandwidth, but usually with greater cost. In the case of single layers on silicon solar cells, for example, the chosen wavelength is optimized to maximize efficiency, while taking into account the solar spectrum, the silicon band gap and electrical loss mechanisms. The optimum coating thickness gives silicon cells the deep blue color that can be seen on solar farms and rooftops in ever-increasing numbers. Yet we know that if the coating stopped all reflection, the cells would appear jet black from any angle. Achieving this in a cost-effective manner has been elusive, but progress continues.

While a great deal of attention has rightly been placed on improving the reflectance properties, simultaneously achieving other technical benefits has been receiving attention too. Examples include improving physical hardness and abrasion resistance, as well as providing electrical conduction and even providing defogging properties. And lower cost processes will always attract attention. What else is being patented today? Here are some examples from filings with the U.S. Patent and Trademark Office:

US07374812: Low refractive index coating composition for use in antireflection polymer film coatings and manufacturing method.This patent by 3M describes a silicone-modified fluoropolymer that is suitable for displays. It uses higher fluorine content to achieve lower refractive index while also improving adhesion.

US07332213: Hardcoat film, antireflection film and equipment for display.This patent by Torray Industries describes a hard-coat anti-reflecting film for displays that has a mean reflectance between 400 and 600 nm of 1 percent or less.

US07153584: Hybrid film, antireflection film comprising it, optical product, and method for restoring the defogging property of hybrid film.This patent by Hoya Corporation describes films for lenses that provide both defogging and antireflection properties.

US07283303: Conductive anti-reflection coating.This patent by JDS Uniphase breaks up a quarter wave layer into smaller portions using thin layers of conductive material to provide a solid electrical contact with low contact resistance.

US20060099407: Antireflective coating composition, antireflection film, and fabrication method thereof.This application describes an anti-reflection coating with better mechanical strength. It achieves low index of refraction by using cross-linked colloidal particles with distributed nanopores.

US20060074172: Antiglare and antireflection coatings of surface active nanoparticles.This application from Optimax Technology Corporation describes a process for preparing durable antireflection coatings that use low and high refractive index layers with a self-assembling gradient layer in between.

David Waechter, Ph.D., is a Nerac analyst with 20 years of experience in engineering and research and development. His primary focus is on electrical engineering as well as energy and optics. Early in his career he was involved with developing screen-printed anti-reflection coatings for use in silicon and polycrystalline silicon solar cells. He has also worked on opto-electronic detectors and x-ray imaging technology. He holds a Ph.D. in electrical engineering from Carleton University in Ottawa, Canada. In addition to having been an invited lecturer, Dr. Waechter has published 16 journal articles and 41 conference papers. Nerac analysts work with clients in the following critical areas:

• Patentability and Invalidity
• Patent Portfolio Analysis
• Commercialization Strategy
• White Space Analysis