Abstract

Level monitoring was described by Macleod and Pelletier¹ and elaborated on by Zhao² and has been shown to have error compensation capabilities of potentially great benefit to optical coatings of certain types. The insight into the principles can tend to be obscured by the mathematics for even those with advanced education. We review the principles of level monitoring using circle diagrams to aid in their visualization and show how to design the level monitoring scheme for various cases including the design of precoated monitoring chips. The film thickness sensitivity of various optical monitoring strategies varies considerably. We discuss ways to choose the more sensitive strategies in particular cases to increase the repeatability of the results. Zero level trigger point monitoring offers great sensitivity of change in reflectance vs change in thickness, and it should therefore give small optical thickness errors when optically monitored. We describe zero level trigger point monitoring and how and when it can be applied.

© 1986 Optical Society of America