How do we apply the coatings to your lenses?

The technology of putting thin solid layers of material onto bulk solid components to modify the properties of the surface is used in many industries.

Machine tool tips are coated with a hard layer to improve their longevity; silicon chips are coated with metal to make surface electrical connections; even polythene foil is coated with material that stops air getting into food packaging and accelerating the process of decay.

In simple terms AR coatings are applied by depositing the material in a vacuum chamber using an electron beam gun. Lenses are held at the top of a steel chamber from which all the air has been evacuated using high-vacuum pumps. The coating material is then heated at the bottom of the chamber with electron beam guns. As it evaporates the coating material starts to condenses on the lenses. It is essential to do the evaporation in a vacuum chamber as, at atmospheric pressure, the evaporated molecules of the material cannot travel far enough or fast enough to reach the lens and bond to it. Furthermore, impurities would be present in the coating structure due to the presence of residual gas molecules within the chamber. This change in structure will reduce the transmission within the layers. As the quality of the vacuum rises so does the quality of the coating and its reproducability.

It is essential to start the coating process with a clean and dry lens. So, before the lenses reach the vacuum chamber they need to be thoroughly cleaned with ultrasound and specialist detergents.

Our system uses seven stages of detergent tanks and water rinsing followed by a hot air drying system, The total washing and drying cycle in the baths is about 50 minutes. Plastic lenses are then dried out to remove internal moisture. By de-stressing and dehydrating the lenses in this way the evaporated materials bond more readily and adhere more firmly.

When the lenses are clean and dry they are transferred to a clean air cabinet which has a filtered positive air flow to keep dust out. The lenses are loaded into special jigs called calotte segments to fit into the vacuum chamber.

Inside the vacuum chamber there are various items of equipment which are necessary to the coating process.

At the top you will see the calotte holding the lenses. This rotates slowly so that the lenses pass through the molecular cloud of evaporated material in a uniform way.

In the bottom of the chamber is the electron beam gun which can create temperatures of up to 3,000 degrees C. as some of the materials have high vapourisation temperatures. There is a shutter over the electron beam gun to stop the materials being deposited once the desired thickness is reached.

Looking up at the top of the chamber you can see a quartz crystal that measures the layer thickness and communicates data to the process controller which in turn gives instructions to the electron beam gun about the power requirements needed to meet the designed layer thickness and the speed of deposition of the material. Different materials require different deposition speeds to optimise the final refractive index of the layer.

Also at the top of the chamber are powerful radiant heaters that can raise the temperature of glass lenses to over 300 degrees C. This is necessary to make sure that the material adheres to the lens . Plastic lenses would of course melt at such temperatures and different processes are used to ensure adhesion at about 70 degrees C.

The oscillating quartz crystal measures the thickness of the material building up on the lenses through links to the process controller - a sophisticated computer connected to the vacuum pumping systems, the electron beam gun, the temperature of the lenses and all the services used by the machine - 3 phase electric power, compressed air, hot water, cold water and a refrigerant that cools part of the inside of the machine to minus 140 degrees C.

All these aspects of machine performance are constantly monitored and adjusted to provide the optimum performance. The controller also contains all the appropriate safety features - for example, not allowing any evaporation of the coating material until the vacuum is at the correct level.

The process controller contains the standard design for each of the coatings that can be applied. The system can control the actual deposition to within plus or minus 1% of the design parameters - which in some cases can be only a few atoms thick.

APS process

Siltint are one of the few companies in the UK that are able to create a virtually scratch proof anti-reflection coating using Leybold's Advanced Plasma System of deposition - called Safire. One of the advantages of this system is that it is a cold process and the lenses do not have to be heated to the 70 degrees C. normally used for plastic and it does not risk stressing the lenses by applying heat.

By creating positively charged ions of argon gas around the lenses the evaporated molecules are accelerated through the vacuum. The ions transfer energy to the growing film making it denser and harder.

The effect of plasma deposition is to create a much denser coating surface as illustrated by the electron-beam microscope photographs showing the surface of coated lenses at high magnification.

The broadband coating without plasma deposition at the top has a columnar structure that can harbour dust and grease - although the application of a water repellent layer will fill in most of the troughs. Safire, on the other hand, has the flat, dense structure shown on the bottom that provides a high quality abrasion-resistant surface.

An additional benefit of this unique deposition system is that all three elements of a high quality AR coating are applied in the same vacuum cycle rather than as separate processes

• the scratch resistant layer
• the anti-reflection layers or stack
• the hydrophobic top coat