Characterization of highly anisotropic three-dimensionally nanostructured surfaces

TL;DR

This paper demonstrates how generalized ellipsometry can accurately characterize the geometrical and optical properties of highly anisotropic, nanostructured thin films, including during growth processes, using advanced modeling approaches.

Contribution

It introduces and compares modeling approaches for ellipsometry analysis of nanostructured thin films, enabling precise determination of structural and optical parameters.

Findings

Models agree well with SEM images

Effective medium approximations provide additional parameters

In-situ growth monitoring is feasible with these models

Abstract

Generalized ellipsometry, a non-destructive optical characterization technique, is employed to determine geometrical structure parameters and anisotropic dielectric properties of highly spatially coherent three-dimensionally nanostructured thin films grown by glancing angle deposition. The (piecewise) homogeneous biaxial layer model approach is discussed, which can be universally applied to model the optical response of sculptured thin films with different geometries and from diverse materials, and structural parameters as well as effective optical properties of the nanostructured thin films are obtained. Alternative model approaches for slanted columnar thin films, anisotropic effective medium approximations based on the Bruggeman formalism, are presented, which deliver results comparable to the homogeneous biaxial layer approach and in addition provide film constituent volume fraction parameters as well as depolarization or shape factors. Advantages of these ellipsometry models are discussed on the example of metal slanted columnar thin films, which have been conformally coated with a thin passivating oxide layer by atomic layer deposition. Furthermore, the application of an effective medium approximation approach to in-situ growth monitoring of this anisotropic thin film functionalization process is presented. It was found that structural parameters determined with the presented optical model equivalents for slanted columnar thin films agree very well with scanning electron microscope image estimates.