Thin Film Theory & Coatings

Surface coatings comprise the second part of Omega’s deposition technology. These coatings are used when light must interact with the coating prior to passing through the substrate. Several examples of these depositions are anti-reflection coatings, dichroic beamsplitters, beam combiners, as well as, filters requiring thin substrates in place of thicker, protected coating assemblies. Another advantage is the ability to reduce the amount of energy absorbed by the substrate in products such as hot and cold mirrors.

Surface Coatings:

Both ion-assist refractory oxide and dual magnetron reactive sputtering produce surface coatings. As these coatings are designed to remain exposed to the effects of environmental stress, fluorides, metals and refractory oxide materials are chosen for their durability. In fact, many optical components are protected by durable surface coatings. The filters created by Omega undergo extensive testing simulating many years of environmental influences and stress such as heat, humidity and the cycling of stress through the process of constantly changing conditions. These filters demonstrate no observable signs of cosmetic or spectral degradation and can therefore be employed in the most rigorous environments. Another distinct advantage of surface coatings is the use of ion-assisted deposition. Through the process of accelerating the deposition materials and increasing their density on the substrate, much higher levels of incident power and heat can be applied with little effect on performance.

Vapor deposition Coatings or protected dielectric coatings
These optical filter types offer deep out-of-band blocking and very high phase thickness coatings with low residual stress and stable spectral performance. Deposition/coating cycles are short as compared to sputtered coatings resulting in affordable cost.

They are manufactured as multiple coated substrates cemented together using an optical epoxy. Through the removal of the coating material around the edge of the substrate (scribing, etching), a strong glass-to-glass bond is formed protecting the internal coatings. This assembly is then mounted and sealed in a protective ring to further enhance the longevity and stability of the coatings. The result is an extremely robust assembly fit for many environments and operational conditions.

The Coating Process

There are several deposition technologies employed to control the reflective and transmissive properties of light. In common, all methods utilize materials of different refractive indices to control portions of the electromagnetic spectrum. Not only are these materials chosen for their optical properties, but for their evaporation and condensation properties as well. Through proper selection of materials, their layer sequence and precise thickness, optical filters become a reality.

Physical Vapor Deposition:

Physical vapor deposition is the process in which a solid material is heated and passes directly from a solid to gas phase (sublimation) and then back again to a solid state as it condenses on a given substrate. Using a resistive heating source or an electron beam, the vaporization process is carefully controlled in temperature and vaporization rate, and then augmented by specific chamber geometry. As the materials enter the gas phase, its cloud is directed towards the desired substrate that is rotating in a planetary configuration. This configuration controls the uniformity of the condensed material on the finished product. Through an optical monitoring process, the physical thickness of each alternating layer of high and low refractive index material is precisely measured as it grows on to a substrate. A single coating may contain upwards of several hundred layers to achieve the desired spectral response.

Ion Assist:

Ion assist, or Ion Assisted Deposition (IAD), utilizes the physical vapor deposition method. During the gas phase of the coating process, the vapor plume is subjected to a beam of energetic ions directed towards the substrate thereby accelerating the particles and increasing their adherence and density. This method also acts upon the deposited film to enhance microstructure and fill voids. Ion assisted electron beam evaporation demonstrates greater environmental stability when compared to non-IAD depositions.

Dual Magnetron Reactive Sputtering:

Dual Magnetron Reactive Sputtering or DMRS is the latest addition to Omega’s coating portfolio. Unlike traditional physical vapor depositions that use refractory oxide materials, DMRS bombards a purified target using a magnetically accelerated Argon plasma. This plasma creates a momentum transfer to the target material that ejects the material towards the deposition substrates. At the same time, the introduction of an oxygen plasma to the vacuum environment completes the reactive process by oxidizing the deposited materials. The resulting thin-film layers have specific optical properties that are then used to selectively control transmission and attenuation across given regions of the electromagnetic spectrum.

Omega’s DMRS employs state-of-the-art programmable logic controls capable of making in-process corrections. Such control of the coating process yields exceptionally precise spectral performance and batch-to-batch repeatability.