In situ optical sub-wavelength thickness control of porous anodic aluminum oxide

Porous anodic aluminum oxide (PAAO), sometimes referred to as nanoporous anodic alumina, serves as a cost-effective template for nanofabrication in many fields of science and engineering. However, production of ultrathin PAAO membranes with precise thickness in the optical sub-wavelength range remains challenging because of difficulties regarding process control at the initial stage of anodic oxidation. In this study, we demonstrate a technique for consistently manufacturing PAAO with the targeted thickness. An electrochemical cell with an optical window was designed for reflectance spectroscopy of PAAO during anodization. Real-time fitting of spectra to a transfer-matrix model enabled continuous monitoring of the thickness growth of the PAAO layer. Automation software was designed to terminate the anodization process at preset PAAO thickness values. While the concept was illustrated using the widely used method of anodization in a 0.3 M oxalic acid electrolyte with a 40 V potential, it can be readily customized for other protocols. PAAO layers with effective thickness below 300 nm could be produced with a few nanometers accuracy using single-crystal aluminum substrates. The results were confirmed using spectroscopic ellipsometry. The method for controlling the thickness during anodization eliminates the necessity of sample sectioning for electron microscopy and is particularly valuable for the small-scale production of PAAO-based functional optical coatings.