The Optical Coating Process
There are several deposition technologies employed to manufacture filters. At Omega, these include evaporative methods and sputtering. 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 also for their ease of use in the manufacturing process.
Protected Optical Coatings
Physical vapor deposition (PVD) 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. 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 to oxides, a much thinner coating stack can achieve similar results. This reduces stress on the substrate, and can minimize 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.
Surface Optical Coatings
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 processes produce very dense films with physical properties that are nearly identical to those found in the bulk materials.
Using a beam of electrons as a heat source, the PVD process is carefully controlled in temperature and rate, and augmented by specific chamber geometry. The physical thickness of each alternating layer of high and low refractive index material is precisely measured as it grows on the substrate. A single optical filter may contain upwards of several hundred layers to achieve the desired spectral response.
Ion assist, or Ion Assisted Deposition (IAD), is used to densify films prepared by PVD. While in the gas phase, molecules are accelerated by a beam of energetic ions (typically Argon and Oxygen) directed towards the substrate. The IAD-prepared filters are denser and demonstrate greater environmental stability than traditional PVD deposited films.
Plasma Assisted Reactive Magnetron Sputtering (PARMS)
Plasma Assisted Reactive Magnetron Sputtering or PARMS is the latest addition to Omega's coating portfolio. Unlike traditional PVD that typically uses 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 element from the surface onto the substrates, forming a very thin (sub-monolayer) layer. In a different part of the chamber, an oxygen plasma oxidizes this 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, high batch-to-batch repeatability and environmental durability.