Optical Coatings
By anand; Published 23 Mar 2009
Aluminum Coating Types
First, I offer a quick review of the basics for those not familiar with the coating process. A standard coating consists of just enough aluminum to achieve the maximum reflection that the material can provide plus a thin overcoat of Silicon dioxide (SiO2). As the aluminum is applied the reflectivity increases to a maximum of ~92% in the visible spectrum. This happens when the aluminum thickness is about 80 nm. At this thickness 91% of the light is reflected while the other 8% is either absorbed or transmitted. It is common to apply a bit more than 80 nm to ensure maximum reflectivity from the aluminum layer and to reduce transmission.
The standard protected aluminum coating includes one layer of SiO2 on top of the aluminum. The basic protective SIO2 overcoat is only thick enough to seal the aluminum from the elements and provide a hard, scratch resistant layer. It is not intended to enhance the reflectivity. In fact, it actually reduces the reflectivity to about 88%. This is due to destructive interference of light in the thin protective layer.
If you can measure and control the thickness of the SiO2 layer you can enhance the reflectivity by applying 1/2-wave optical thickness of the wavelength you are most interested in. For the visible spectrum centered on 550 nm, 1/2-wave optical thickness is 275 nm divided by 1.46, the refractive index of SiO2 or 188 nm. By applying the proper thickness, the reflectivity can be increased to about 91% at 550 nm. The region of 91% reflectivity is fairly broad. In other words, as you move away from 550 nm on the spectrum the reflectivity drops but it remains near 91% across most of the visible spectrum. Many coating labs refer to this single 1/2-wave layer as Semi-enhanced Aluminum.
An enhanced aluminum coating consists of 1/4-wave of SiO2 with an additional 1/4-wave of a high index material like tantalum pentoxide (Ta2O5). This produces a coating with 96% reflectivity across the visible spectrum. The 1/4-wave of SiO2 and 1/4-wave of Ta2O5 is referred to a High/Low or HL stack. The increased reflectivity of the enhanced coating is due to constructive interference of light in the HL stack. Most labs refer to this single HL stack coating as Enhanced Aluminum. To continue to increase the reflectivity you add additional HL stacks. You can increase the reflectivity to about 98% with two HL stacks. These are typically branded as proprietary enhanced aluminum coatings.
Thermal Evaporation:
Thermal evaporation of aluminum is accomplished by wrapping aluminum around a tungsten coil. Current is applied to the coil to melt the aluminum (wet the coil) then the current is increased to evaporate the aluminum. Evaporation of SiO2 is more of a challenge. SiO2 does not melt and evaporate from the liquid state, it sublimates. In other words it goes straight from solid to vapor. Sublimation from the crystalline state tends to produce an irregular plume of evaporant. This makes it difficult to control the rate of evaporation/deposition and the uniformity of the deposit on the mirror. To combat this problem, SiO2 is evaporated from a container with internal baffles and an opening in the top from which the evaporant escapes. This provides a more uniform plume and produces a uniform coating. Ta2O5 is typically evaporated from a crucible that is heated by a tungsten coil. It melts and evaporates like aluminum but at a higher temperature.
Each of the materials evaporated above condenses onto the relatively cool mirror surface. It is important that the material have some energy when it arrives at the mirror surface. This allows the material to settle into its nature crystalline structure before “freezing”. This is accomplished by evaporating/condensing the materials in a vacuum chamber. By avoiding collisions with air molecules in the coating chamber the evaporant maintains practically all of the energy it had when it left the evaporation source. If needed, additional energy can be provided at the mirror surface by heating the mirror.
During the evaporation process some chemical dissociation occurs. For example, some of the SiO2 breaks down to become SiO and Si. What is deposited on the glass is a mixture of these elements. Similar dissociation occurs during evaporation of Ta2O5. These impurities in the SiO2 and Ta2O5 result in a shift in refractive index away from the theoretical value for pure materials. In this case the refractive index of the deposits should be measured with an ellipsometer. With this information the coating design can be adjusted according to the measured indices.
The physical structure of the coating layers is an important factor in coating quality. If the substrate is cold or if the pressure in the chamber is too high the materials accumulate like frost with high porosity in their crystal structure. It is desirable to produce a crystalline structure with maximum density. As the material condenses on the mirror it has to maintain some of its energy. This residual energy allows mobility so the material can settle into the crystal matrix. In thermal evaporation this extra energy is provided by heating the mirror. The goal is to produce layers that are dense with low porosity, like freezing rain. Such a coating has a more predictable and repeatable refractive index and it is moisture stabile. A porous, frost-like coating absorbs moisture which causes a shift in the refractive index of the coating layers and scatters light. Absorption of moisture also results in premature deterioration of all of the layers, including the aluminum.
Electron Beam Evaporation:
In an electron beam (e-beam) system the material is evaporated by focusing a beam of high energy electrons onto a crucible containing the material. The path of the e-beam is controlled be a combination of permanent and variable electro magnets which can impart a sweep or spiral pattern to more evenly heat the material. The e-beam controller also varies the power in the beam to control the rate of evaporation and hence the rate of deposition. A computerized e-beam system with a deposition monitor can generate a very uniform evaporant plume, even when evaporating SiO2. The typical e-beam system has several indexable pockets. This allows the system to apply different material layers in a single coating run.
Ion Beam Assist Deposition (IBAD):
Ion assist provides several benefits to the coating process. The ion beam provides a final cleaning (ion scrub) of the mirror surface before the start of deposition. Immediately after cleaning, prior to loading in the coating chamber, a mirror it begins to accumulate water molecules on it surface. The energetic gas molecules produced by the ion beam strip away adsorbed moisture from the mirror surface so it can be pumped out of the coating chamber. The ion beam also removes residual organic molecules to produce a very clean glass surface which promotes adhesion of the aluminum layer.
The ion source also provides a plume of high energy gas molecules that impinge upon the coating layers as they are deposited on the glass. They literally hammer the coating molecules into place to provide a dense, moisture stabile matrix. Unlike the non-IDAB process, this is accomplished without additional heating of the mirror.
Coating Uniformity:
Coating uniformity is achieved by rotating the mirror above the evaporant source or rotating three or four mirrors in a planetary holder system. A single rotating mirror will typically require a uniformity mask. The mask blocks some of the evaporant such that the mirror receives a uniform thickness of coating from center to edge. Finding the right shape for the mask is non-trivial and requires several trial runs for each evaporant material. With care however, thickness uniformity of a few percent can be achieved over a large mirror. The better alternative is a planetary rotation system where the mirror rotates around its axis as well as around the center of the chamber. Uniformity as low as 1% can be achieved with a planetary rotation system.