Abstract
In this paper we study the wavelength selection process for optical monitoring of thin film filters. We first discuss the technical limitations of monitoring systems as well as the criteria defining the sensitivity of different wavelengths to thickness errors. We then present an approach that considers the best monitoring wavelength for each individual layer with a monitoring strategy selection process that can be fully automated. We finally validate experimentally the proposed approach on several optical filters of increasing complexity. Optical interference filters with close to theoretical performances are demonstrated.
Highlights
Increasing stability of deposition systems has boosted the complexity of optical filter designs and precise control of the deposited layer thickness is required
Another technique called level cut monitoring [4] relies on the detection of some predefined transmittance levels and can benefit from correction algorithms and offers a wide range of new opportunities since it becomes possible to define a larger number of possible monitoring wavelengths compared to turning point monitoring
It can be seen that the spectral performances of the filters with both strategies are similar at shorter wavelength, but that the automated strategy performs significantly better at longer wavelengths
Summary
Increasing stability of deposition systems has boosted the complexity of optical filter designs and precise control of the deposited layer thickness is required. Turning point monitoring that relies on the detection of zero derivative is known to allow error self-compensation [3] at the monitoring wavelength Another technique called level cut monitoring [4] relies on the detection of some predefined transmittance levels and can benefit from correction algorithms and offers a wide range of new opportunities since it becomes possible to define a larger number of possible monitoring wavelengths compared to turning point monitoring. Level cut monitoring can be successfully used for dielectric quarter wave mirror deposition where turning point monitoring is usually seen as go-to method [5] It has its limitations, interest in broadband optical monitoring has increased in recent years [6, 7]. It has been reported that broadband optical monitoring can benefit from error compensation [8] but to a lesser extent than monochromatic monitoring
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