Stability and Repeatability of 2-Layer Anti-Reflection Coatings
The manufacture of a simple anti-reflection (AR) coating design can often prove to be more difficult than anticipated. The case described here required near zero reflectance (0.15%) at a single wavelength at two angles of incidence (0Ε & 30Ε), design criteria which can easily be met in theory with a simple 2-layer structure using materials such as ZrO2/SiO2. However, when manufacturing tolerances were brought into play, variations in film thickness and film index made it difficult to produce coatings which consistently met the requirement.
By changing to a different material system (MgF2/TiO2), we were able to use a different 2-layer design which is much less sensitive to these process variations and which improved the yield to 100%. The theoretical basis for this reduced sensitivity is given, together with confirming experimental results.