Optical characterizations of nanoporous anodic alumina for thickness measurements using interference oscillations

Highly-ordered nanoporous anodic alumina (NAA) with varying thicknesses were prepared by changing the anodization time from 10 min to 10 h, using a two-step electrochemical oxidation in oxalic acid. Three optical characterizations, namely, Ultraviolet–Visible–Near Infrared (UV–VIS–NIR), Photoluminescence (PL), and Reflective interference spectra were used to measure NAA's thicknesses. All three types of spectra displayed oscillations due to the optical interferences which were used for the extraction of NAA thickness using two approaches (i) Fabry–Perot fringe equation and (ii) Bragg's interference condition equation. Both these approaches yielded the matching thicknesses. The results are coincident for all three measurement techniques. The obtained thicknesses are in good agreement with the cross-sectional SEM images showcasing the feasibility of such approaches. Measurements were carried out on NAA up to ∼10μm thickness beyond which no oscillations were observed. Out of the three, reflective interference spectroscopy turns out to be an inexpensive alternative for the NAA thickness measurement. Such nondestructive optical characterization methods are important from the device development point of view for chemical, optical and biosensing applications.