I was wondering why UV ellipsometry is better than visible ellipsometry?
Better for what?
I meant, for certain materials, it is better to use UV light source to measure thin film thickness, and sometimes it is better to use a visible or NIR light source. Can someone please give me a couple of examples of materials that are determined using UV, VIS, and NIR? Why is it good to have the whole UV-VIS-NIR spectrum in an ellipsometer?
If you only want to measure the thickness of a film, I believe that a single color i.e. laser is good enough as long as the medium has normal dispersion at that wavelength (negligible absorption). But, if you have access to a broader spectrum you can do spectroscopic ellipsometry, which allows one to more fully characterize a sample. For example, you can measure an absorption spectrum in conjuction with ellipsometry, etc. See this article https://en.wikipedia.org/wiki/Ellipsometry
The subection on single-wavelength vs spectroscopic ellipsometry directly addresses your question.
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