Behind the scenes in optical coatings

Optical coatings are diverse and complex; our previous blogpost with sales representative, Mike Vergo, covered introductory coating topics and costs. In this article, we aim to provide more detail on metallic coatings from the initial request to a finished piece. To do that, we have asked our coating partner David Yoo of Colorado Thin Films to provide insight and get a real look behind the chamber door.

Understanding the Material, Application, and Environment
Optical coatings by nature are formulaic so you start with the material to be coated. Since every glass substrate has different properties, you have to understand the foundation before one can adhere the coating. Next, you have to know what are the customers intended performance specifications. Specifically, what wavelength or perimeters are you trying to achieve? For example, in mirror application, you want to reflect light so metallic coatings of aluminum, silver, and gold are the most popular choices. Meanwhile, HR (high-reflector) coatings allow for more precision in certain wavelengths and have a greater hardness and overall durability than bare metal coatings. Alternatively, transmissive optical coatings allow only a certain wavelength of light through the substrate. The third piece a coating vendor must understand is the environment for the coated optic/mirror, and finally, the durability specifications that are required (e.g. MIL-spec, ISO, etc.).

Designing the coating
Depending on the above required specifications, the fabricator can dictate what metal to deposit. Aluminum, silver and gold all have unique reflection ratios at different wavelengths. If a specific band requires a High Reflection, an HR coating will be designed to meet spec. If a customer has a laser application, we work with them to define the laser damage threshold (LDT) requirement which also impacts the coating design. Another critical factor in the design stage is the angle of incidence (AOI). The design process is usually optimized for NAOI (Normal AOI, 0 deg). A coating optimized for NAOI should perform almost the same, for 0~9 deg AOI. The reflection/transmission curve or graph alters as the AOI goes higher. 

Deposition
Once we have the substrate in hand, an initial inspection within a controlled environment under magnification assures the surface is prepped for coating. Namely, the surface must be absolutely free of flakes, dust, particles, or other residues. If there are any contaminants, we carefully clean each piece with either methanol or acetone. For these high-end applications, water is not an option as it can leave stains and spots. 

Once prepped, the uncoated optics are loaded into fixtures to ensure a uniform surface and maximum clear aperture to be coated. Tooling is designed in all shapes and sizes to best fit the customer’s needs. When depositing metal coatings, the temperature of the chamber is raised to apply thermal energy to the optic and lay the binding layer which ensures the deposited material adheres to the optic. Once the binding layer is applied, the temperature is lowered to room temperature and the metal layer is deposited using a thermal beam, (e-beam). The design of the metal coating is determined by the thickness of the thin film applied to the optic. Meanwhile, for high reflector coatings, the deposition process is a bit different. Namely, since HR coatings use a dielectric coating, the thermal environment in the chamber can be as high as 210~220C. Deposition can include multiple stacks of coatings with a combination of high and low index coating materials and designed thicknesses to behave a certain way to meet the required specifications. 

Inspection
When performing any coating run, we include a witness sample that is made out of the same material as the optic. The same optical coating recipe gets deposited onto both the witness sample and substrates. After coating and while the substrates cool down, we use the witness sample to test the reflectivity in our spectrophotometer using a universal reflectance accessory. We can test the reflection at different angles to determine if the coating meets specifications. For metal coatings, we apply Universal Photonic’s Blue Cote #33 evenly so the coated surface remains contaminant-free and protected during shipment. 

Summation
For customers and buyers who are working with engineering teams, the world of coatings can be daunting. When you are ready to discuss a thin-film coating, knowing the material, wavelength, end-use application, and specification standards are all of the key pieces for seamless results. At Colorado Thin Films, our business model is fast and reliable coatings, and after just three years we, continue to grow and build customer relationships and as well as accurate coatings. Feel free to learn more about Colorado Thin Films (CTF) and should you have any questions, reach out to CTF or your friendly Esco Optics sales engineer.