The Coating Process
There are several deposition technologies employed to control the reflective and transmissive properties of a filter. 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 sequence and precise thickness, optical interference filters become a reality. Considerations such as image quality, system sensitivity, spectral isolation, and most importantly, cost, are all factors when choosing a filter. Often times, consultation at the initial stages of R&D greatly influences the proper choice of coating technology allowing a system to achieve the highest level of performance.
Physical vapor deposition is the process in which a solid material is heated and passes directly from a solid to a gas (sublimation) and then back again to a solid state as it condenses on a given substrate, usually while under vacuum. The traditional approach has been to use a resistive heating element to evaporate the materials. As described in a later section, there are several other methods in use today. The thermally-prepared coatings were Omega’s bread and butter business for over 25 years. To protect and preserve these filters for long-term stability, they are typically laminated with a coverslip after preparation. Although they have fallen out of favor recently, they still offer several advantages over surface coatings. Because the range in refractive index is much higher amongst these materials when compared with oxides, a much thinner coating stack can achieve similar results. This reduces stress on the substrate, and resulting wavefront distortions. This becomes particularly important as the wavelengths of interest move into the Infrared, where thicker layers are required. These optical filters offer deep out-of-band blocking with low residual stress and stable spectral performance, often at a lower cost.
Surface coatings are typically comprised of metals, fluorides and refractory oxides deposited with e-beam (often with ion-assist) or PARMS. In fact, many optical components are protected by durable surface coatings (for instance, plastic eyeglasses often have a protective layer of glass applied for scratch resistance). These more energetic deposition processes produce very dense films with physical properties that are nearly identical to those found in the bulk materials.
Using a resistive heating source (for lower temperature materials) or an electron beam (for refractory oxides), the vaporization process is carefully controlled in temperature and rate, and augmented by specific chamber geometry. As the materials enter the gas phase, the cloud is directed towards the substrate which is often rotating in a planetary configuration. This motion controls the uniformity of the condensed material on the finished product. 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. The lower temperature (resistively heated) materials form a more fragile coating that is later protected with an index-matched epoxy layer and coverslip.
Ion assist, or Ion Assisted Deposition (IAD), enhances the physical vapor deposition method. During the gas phase of the coating process, the vapor plume is subjected to a beam of energetic ions (typically Argon and Oxygen) 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.
Plasma Assisted Reactive Magnetron Sputtering (PARMS)
Plasma Assisted Reactive Magnetron Sputtering or PARMS is the latest addition to Omega's coating portfolio. Unlike traditional physical vapor depositions that typically use refractory oxide starting materials, PARMS bombards an elemental target (usually Si or Nb) using a magnetically accelerated Argon/Oxygen plasma. The accelerated atoms transfer momentum to the target material which in turn ejects the material from the surface onto the substrates, forming a very thin (sub-monolayer) layer. In a different part of the chamber, an oxygen plasma oxidizes the newly formed atomic layer, creating an oxide. The resulting thin-film layers are very dense and durable. State-of-the-art programmable logic enables in-process corrections to layer thicknesses. Such control of the coating process yields exceptionally precise spectral performance and batch-to-batch repeatability.