Nondestructive characterization of nanoporous alumina films using terahertz scattering imaging

Abstract

Ordered nanoporous (NP) alumina (Al 2 O 3 ) films were grown on Al substrates through a two-step electrochemical anodization process. The morphology as well as the roughness are characterized by terahertz (THz) reflectometry and scattering imaging. In particular, we show that before the second anodization, irregular surface morphology of the NP Al 2 O 3 films leads to significant THz scattering, whereas the far more homogeneous films following the second anodization show far less scattering. The regularity of the surface morphology is not readily ascertained using visible-light optical microscopy. The THz results are corroborated by field-emission scanning electron microscopy (SEM) and atomic-force microscopy (AFM), both of which are time-consuming, not easy to operate, and can only provide the local characterization of NP Al 2 O 3 films on Al substrate. THz-based techniques allow for the nondestructive characterization of the surface morphology of the entire NP Al 2 O 3 films on the 10–100 μm length scale important for a range of applications while providing information over macroscopic scan areas, and obviate the need for sample preparation required by other common surface-morphology characterization techniques. • Terahertz scattering imaging is applied to characterize the homogeneity of nanoporous alumina films after anodization. • Irregular surface morphology of nanoporous alumina films observed in terahertz images is corroborated by FE-SEM and AFM. • The stratigraphy and thickness of nanoporous alumina films are obtained nondestructively based on terahertz images. • Terahertz imaging may provide a reliable nondestructive and contactless modality to evaluate the quality of nanoporous films.