Shadow-mask evaporation for the fabrication of optical filters with spatially tailored thickness
Résumé
The fabrication of optical filters whose reflection/transmission response is spatially-graded has been the object of
numerous research studies over the past decades given their applications in areas including multi- and hyperspectral
imaging, structural colouring and even holographic encryption. In this context, the key enabling feature is the ability to
tailor the thickness profile of at least one layer of the optical coating multilayer stack. To-date, this 3-dimensional
structuration has been achieved either at the deposition stage or as an additional post-deposition process step. In the
former case, the technique relies on the shaping of the material deposition flux thanks to the insertion of a (moving)
mask inside the evaporation or sputtering machine. As such, the method is usually limited to the implementation of
centimetre-scale variations. A contrario, to reach sub-millimeter-scale features, the preferred approach is based on postdeposition
layer structuration, which is performed using grayscale lithography in the form of multi-(mask-)level optical
lithography, or using e-beam or laser lithography. All these approaches are nevertheless relatively complex since they
involve either multiple steps or need a very precise calibration of the exposition curve.
In this paper, we report that the evaporation through re-usable shadow masks can be used to create optical filters
whose spatial variations can be controlled with a ~70-μm-resolution. Using metal-mirror Fabry-Pérot interferometer
structures as representative optical filters, we demonstrate the ability to adjust the resonance wavelength, the filter
bandwidth and extinction ratio, and the coupling strength and splitting in cascaded resonators.
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