Characteristic resonance reflection spectra of nanoporous alumina films and its application to precise thickness measurement

In this article, we investigate the thicknesses effect on the characteristic resonance reflection spectra of nanoporous anodic aluminum oxide (AAO) films and its potential application to nondestructive precise thickness measurement. The resonance spectrum of AAO was linked to the film thickness and pore diameter. Different thicknesses of AAO were prepared by changing anodization time. As a result, the amplitude of reflection peaks reduced at short wavelength and decreased with increasing thickness over a wavelength of 350∼850 nm. The pore diameter was also enlarged by pore widening to examine the evolution of reflection spectra. The relationship between the reflection peak and thickness was identified through equal inclination interference formula and potentially used for the nondestructive inspection of film thickness. The thickness of AAO films measured by the low-cost UV-vis reflection spectrophotometer was in a wide range from sub-μm to about 7 μm compared to that less than 1.3 μm by a complex and expensive ellipsometric method. The accuracy of thickness measured by resonance spectrum method was about 1.0 ± 0.4% uncertainty that was verified by scanning electron microscope measurement.