A Telescope Mirror Coating Primer

By James Mulherin of Optical Mechanics, Inc.

James Mulherin Ready to Unload Three 20 Inch Mirrors

In October of 2007 we installed a vacuum coating machine at Optical Mechanics, Inc. (OMI). I’ve studied coating processes for a while now, mainly in preparation for the addition of the mirror coating machine to our operation. Our machine was custom built for us based on what I learned. We took delivery of the machine in September of 2007 and to date have run over 300 mirrors through the process. When I started looking into acquiring a mirror coating machine I knew little about the details of the process. Ever since we started offering mirror coating services I hear all of the same questions from our customers that I once had myself. I wrote the following is a mirror coating primer to share what I’ve learned and to answer some of the most common questions I’m asked by our customers.

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.

Red-Bare, Black-Protected, Blue-Semi Enhanced, Green-Enhanced Aluminum

All of the above are subject to fiddling and fine tuning depending on several factors; evaporation method, process temperature, ion beam assisted deposition (IBAD) and the addition of process gas. This is where it gets complicated. I’ll start with the most basic process.

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

The Electron-beam Evaporation Source with Aluminum Crucible in Pocket


The Deposition Monitor and Controller

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.

When integrated with a deposition rate monitoring system, such as an oscillating crystal monitor, the e-beam can be made to deliver a very accurate deposition rate and thickness for each material in the coating design. The deposition monitor consists of quartz crystal oscillator that is coated along with the mirror. As material deposits on the crystal, the crustal rate of oscillation changes. The deposition controller monitors the change of oscillation rate and uses it to calculate the material deposition rate and accumulated thickness. For example, the controller can be programmed to evaporate aluminum at 10 nm per second to a thickness of 100 nm. The controller adjusts the e-beam power to achieve the desired rate then automatically turns the e-beam off when the desired thickness has been deposited. The controller will perform this operation automatically for each layer in the coating design.

Ion Beam Assist Deposition (IBAD)

The Ion-beam Source

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.

Process Gas

As mentioned earlier, some dissociation of the SiO2 and Ta2O5 occurs during the evaporation process. This happens regardless of the evaporation method used. This is where a slight partial pressure of process gas comes in. In this case, oxygen is injected into the process. The background of oxygen gas helps to keep the SiO2 and Ta2O5 at stabile equilibrium during the deposition process, resulting in more pure oxide layers.

A more effective method used to create pure SiO2 and Ta2O5 layers is to run oxygen through the ion beam source during IBAD. This provides a beam of high energy O2 molecules that drive the oxidation reaction to completion on the mirror surface as the SiO2 and Ta2O5 layers are deposited. This chemically reactive process is provided in addition to the physical impaction mentioned above, resulting in pure and dense oxide layers.

Coating Uniformity

A 30 inch Mirror Coated in Single Rotation Fixture


Three Mirrors of Various Sizes in the 3-Station Planetary Holder

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.

Will an enhanced coating change a mirrors figure?

With regard to the coating changing the figure of the mirror, consider the application of each layer of material. The layer most likely to change the mirrors figure is the aluminum layer as it forms the surface from which light is reflected. The SiO2 and Ta2O5 layers are transparent. They act only to enhance the reflectivity. If they lack uniformity their primary effect will be a variation in the reflectivity across the mirror.

The aluminum layer is about 90 nm thick, or approximately 1/6-wave at 550 nm. A loose tolerance on coating uniformity is 5%. This is equivalent to 1/120-wave at 550 nm. A more realistic target for uniformity is 2% or 1/300-wave. A properly applied coating will not change the figure of the mirror by any significant amount.

Coating deterioration

Coating deterioration involves two main elements: deterioration of the aluminum base layer and deterioration of the over-coat layers. One function of the over-coat layers is to protect the underlying aluminum. All over-coat layers are porous on the microscopic level. The number of over-coat layers is not as important as density and porosity. Density is more a function of process method than anything else. Coatings produced with IBAD are dense and provide better protection to the underlying aluminum than non-IBAD coatings.

A Failed Coating Scatters Light as See in the Ronchi Test

I've worked with many different coating vendors. The ones we've had longevity issues with did not use ion assist. They all look fine out of the box but non-ion assist failures exhibit signs of moisture absorption in the over-coat layers early in their useable life. When a porous coating absorbs moisture it starts to look hazy. As time goes on the moisture in the over-coat layers breaks them down further and accelerates the deterioration process. This allows the elements to get through to the aluminum. Eventually, you'll see blotches in the aluminum. If you clean the mirror, the spots where the blotches are will eventually wash away leaving bare glass. By this time, the rest of the coating will look very hazy. It will have deteriorated to the point where it scatters a significant amount of like.

It's not that all non-ion assist coatings will inevitably fail this way. Failures occur primarily due to something going awry in the process. (For example, low substrate temperature, moisture or other contamination on the mirror surface or insufficient vacuum.) There are other factors in the process that can drift and affect the quality of the coating. It is difficult to detect problems right out of the chamber because even a marginal coating may look good to the eye. That's why it's important to measure witness samples from every run. The instruments used to measure samples include the spectral reflectometer for measuring reflectance as a function of wavelength, the profilometer to measure layer thickness and the ellipsometer, used to measure thickness and refractive index. In addition to the reflectivity and index measurements you can do destructive testing on the samples such as: the tape peel test, eraser rub test and the salt spray test. If you see a problem developing in your witness samples this information will help pinpoint the problem so you can set things right and keep the coating within QC parameters.

Summary

OMI's Temescal FCE 4800 Automated Vacuum Coating Machine

There are many details that go into producing a high quality mirror coating. I haven’t listed all of them here. There are many factors on the input side of the coating equation that will change the output. The trick is to monitor the output and develop an understanding of which inputs to adjust to achieve and maintain process goals. At the same time good process monitoring and real time control of the process are required. Modern coating machines typically include e-beam evaporation, ion assist with process gas and deposition rate monitors with all of the machines functions controlled by a computer connected a programmable logic controller (PLC) for each of the systems on the coating machine. The machines are complex, fully automated and self monitoring and they can reliably and repeatably run a well designed and de-bugged process to produce a desired result. Much of this technology was developed by the semi-conductor industry where coating designs can include hundreds of layers with very tight tolerances. The coatings we use on telescope mirrors are exceedingly simple compared to the capability of modern coating machines.

James Mulherin
Lead Optician
Optical Mechanics, Inc.