1D stack scaning electron micrograph (SEM).
For the stop band starting at 1.7µm it was found that
the appropriate thicknesses are 170nm and 390nm for Si
and SiO2, respectively. The preliminary
design simulations for a ten layer filter showed a stop
band from 1.75µm to 3.2µm.
Initially, a stack was fabricated
using the e-beam evaporation technique, within MIT’s
Technology Research Laboratory. It was concluded that
the layer uniformity and thickness precision produced
by this technique were not satisfactory. A different
approach at producing the filter using low pressure
chemical vapor deposition (LPCVD) techniques yielded
significantly improved fabrication results.
A broadband spectrophotometer
was used to characterize the filters’ spectral performance. The
optical properties of the deposited materials were evaluated
using a spectroscopic ellipsometer. Effective filter fabrication
is a complex and iterative process requiring careful
experimental validation of theoretical predictions, and
evaluation of various techniques with a view to identifying the
optimal technique, in terms of cost, complexity and, of course,