Spectral reflectance and transmittance are used on optical coatings and filters to measure thickness of individual layers in multilayer dielectric stacks. Multilayer dielectric stacks are used in a variety of applications where precise optical filtering is required, such as military, aerospace, architectural glass, and optical metrology equipment of all types, including Raman spectroscopy and 3D optical profilometry.
Multilayer dielectric stacks are used to selectively manage the reflectance and/or transmittance of light. By manipulating the thickness of the constituent film layers, individual high-pass (HP), low-pass (LP) and band-pass (BP) filters can be combined to create a very specific filter. Typically, 2-3 different materials are layered in an alternating fashion to produce the desired filter effect, but the Filmetrics FILMeasure® software algorithms can accommodate many more, including a recent example of an unknown but commercially-available stack of 53 layers on a fused silica substrate.
The Filmetrics® F10-RT was designed to provide a wide spectral range and optical transmission measurement capability to address the challenge to measure thickness of multilayer/multi-parameter film stacks.
In order to measure optical transmittance, there must be no opaque layers in the optical path; any backside opaque (scattering) layers should be removed in the area of interest prior to transmittance measurement.
In this example we show a measurement of a 53 layer repeating TiO2 and SiO2 dielectric filter stack on glass. Reflectance and transmittance data was collected using our F10-RT system with optional UVX upgrade, which provides a spectral range of 190 – 1700nm. For thick and/or complex film stacks that require modeling with many variables, a wider wavelength range provides the additional data necessary for determining a unique measurement solution. The F10-RT reflectometer captures reflectance and transmittance spectra with a single mouse-click.
Measurement Set Up