We built a transmissometer to determine the thickness of thin films of NbN that are deposited as the starting material for SNSPDs. The transmissometer measures the transmittance of light through NbN thin films, and the thickness of the NbN is found by fitting the measurements to analytical values calculated with the transfer matrix method. This simple, low-cost and non-destructive tool can give the thickness of several nanometer thick films with a precision of 0.2 nm.
The paper by Kristen Sunter, Andrew Dane, Christopher Lang and Karl K. Berggren has been published in Applied Optics and is currently available on arXiv and online from Applied Optics.

Kristen A. Sunter, Andrew E. Dane, Christopher I. Lang, and Karl K. Berggren. “Infrared transmissometer to measure the thickness of NbN thin films,” Applied Optics 54(18), 5743-5749 (2015).