Monitoring thin films of the fence post design and its advantages for narrow bandpass filters.

A fence post design, when viewed on a plot of index of refraction versus film thickness, has thin (usually of equal thickness) high-index posts that stand above a broad low-index ground. Monitoring fence post and related posthole designs offers error compensation and error reduction. There tend to be two or more extrema within the optical monitoring trace of each layer between the fence posts that aid in the calibration and control of film thickness. This also leads to a potentially improved control during deposition of narrow bandpass filters that have been designed with nonquarterwaves at the passband wavelength.