Abstract
One of the critical steps in the fabrication of complex optical interference filters is the precise control of the thickness of the layers during the fabrication process. However, the definition of the optimal optical monitoring strategy remains a challenge as it relies on user experience and there is no reliable automatic determination of this strategy. Here, we propose a semi-automated method that allows the determination of the optimal strategy. It is based on the combination of trinary mappings to select spectral regions that are compatible with optical monitoring and the use of the reflected phase error at a single wavelength versus optical monitoring wavelength. We show how this procedure can be used for the determination of either a single optical monitoring wavelength or a multi-wavelength procedure of a complex filter and confirm these theoretical results with an experimental demonstration.
Highlights
Optical interference filters offer a very broad range of optical functions for the control of the spectral properties of light
While the filters structure used to show some periodicity due to the use of common formulae based on quarter wave layers, the filters that need to be deposited are no longer periodic and can exhibit a very broad range of thicknesses ranging from a few nanometers to a few hundreds of nanometers for filters in visible and near-IR range
The choice of the optimal optical monitoring wavelength is a critical step that will directly affect the final performances of the filter
Summary
Optical interference filters offer a very broad range of optical functions for the control of the spectral properties of light. The use of optical criteria such as turning point monitoring (TPM, determination of the moment the derivative of the optical monitoring signals cancels) with determined monitoring wavelength is less and less used and Level-cut (LC, determination of pre-defined optical monitoring signal levels) or a more advanced method such as optical monitoring by swing (or Percentage Of Extremum Monitoring (POEM)) are generally preferred [4,5] While such techniques can accurately determine the moment a deposition has to be stopped, the accuracy highly depends on the selected optical monitoring wavelength. We provide a thorough description of the combination of three methods that can be implemented in order to define an optimal strategy for optical monitoring of optical interference filters. A thorough description of the method applied to various kinds of filters can be found in [11]
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