Thin film coatings, such as antireflective (AR) coatings, are made from various materials, such as metals, oxides, and compounds, and are deposited in layers onto a substrate. Thin film coatings can be deposited in both single and multiple layers, and the configuration you choose determines how it will manipulate different wavelengths of light.
Thin film coatings have many different characteristics, which are used to improve or alter some element of the substrate’s capabilities. The design and configuration of thin film coatings heavily depend on performance requirements, and the proper design is crucial to the functionality and overall success of your application.
Single Layer AR Coatings
Single-layer AR coatings can have different refractive indices depending on the material. For example, single-layer AR coatings of magnesium-fluoride have a refractive index of 1.38. Applying the coating to a substrate with a 1.9 refractive index provides 0% reflection.
Single-layer AR coatings of magnesium-fluoride can be adjusted to perform with various wavelengths and typically prevent reflection of 550 nm lasers. Single-layer AR coatings are prevalent, but complex applications may require multi-layer AR coatings.
Double & Triple Layer AR Coatings
Two or more AR coating layers can overcome the limitations of a single layer AR coating. Combining high and low index coatings, such as 2.3 and 1.38 produces a narrow bandwidth and close to 0% reflection. Three-layer coatings create a broadband AR coating using two high and a single low index coating, such as 2.1 and 1.38.
Some substrates cannot achieve the necessary refractive index with a single coating. Multi-layer coatings allow manufacturers to use more available materials to block a more diverse range of incident angles and wavelengths. It is vital to consider the ideal materials when selecting a two or three-layer AR coating, as the refractive indexes available are limited and deposition is imperfect.
When designing a configuration for thin film coatings, consider the following factors:
- Thin film coatings offer increased performance at lower angles of incidence.
- Longwave pass (LWP) filters allow for greater transmission and typically higher-performance than shortwave pass (SWP) filters. LWP filter substrates may be more expensive than those in SWP filters, but they enhance manufacturing tolerance and use more simple AR coatings.
- Designing a coating with a greater than 2:1 bandwidth ratio increases the difficulty. It requires more layers and increases the percentage of reflection, with a higher reflection penalty when coating 30° and 45° angles of incidence.
- Designs may not always translate well during manufacturing. When a design requires more than 12 layers, the deposition process can be unmanageable in practice. Coatings that require more than three materials or a 10 nm or less thickness can also result in unsuccessful manufacturing.
- Specify only necessary coating requirements. A range that is wider than necessary can be detrimental to performance.
It is also necessary to consider the substrate texture. Substrates with lithography or etching require an AR coating with an approximate profile with a smaller height and width than the shortest wavelength.
Single wavelength coatings are typically easier to manufacture compared to multiple wavelength coatings. Specific materials must be chosen for each wavelength, increasing the cost and complexity, especially when transmitting long and short wavelengths.
Thin Film Coatings From Evaporated Coatings, Inc.
When designing thin film coatings, such as AR coatings, films can be deposited in single and multiple layers to suit various substrates and applications. There are essential considerations when designing a thin film coating that will offer the performance you expect. At Evaporated Coatings, Inc., we are a leader in thin film coatings with over 50 years of experience in optical coating solutions. We can work with you to design and deposit custom AR coatings based on the needs of your application.
For more information, or for help with your thin film coating design, contact us today.