Development and characterization of a dispersion-encoded method for low-coherence interferometry

TL;DR

This paper introduces a dispersion-encoded low-coherence interferometry method that achieves sub-nanometer surface profiling, high dynamic range, and spatially-resolved refractive index measurement, enhancing capabilities in surface and thin-film characterization.

Contribution

The work presents a novel dispersion-encoding approach for low-coherence interferometry, enabling tunable measurement range, high resolution, and simultaneous multilayer characterization.

Findings

Achieved 0.1 nm axial resolution and 79.91 μm measurement range.

Profiles up to 1.5 mm length without mechanical scanning.

Refractive index resolution of 3.36×10^{-5} with spatial resolution.

Abstract

This work discusses an extension to conventional low-coherence interferometry by the introduction of dispersion-encoding. The extension facilitates the measurement of surface height profiles with sub-nm resolution. The selection of a dispersive element for encoding allows for tuning of the axial measurement range and resolution of the setup. The approach is theoretically designed and implemented for applications such as surface profilometry, the characterization of polymeric cross-linking and as a tool for the determination of layer thicknesses in thin-film processing. During the characterization of the implemented setup, it was shown that an axial measurement range of 79.91 $\mu m$ with a resolution of 0.1 nm was achievable in the evaluation of surface profiles. Simultaneously, profiles of up to 1.5 mm length could be obtained without the need for mechanical scanning. This marked a significant improvement in relation to state-of-the-art technologies in terms of dynamic range. It was also shown that axial and lateral measurement range can be decoupled partially. Additionally, functional parameters such as surface roughness were characterized with the same tool. The characterization of the degree of polymeric cross-linking was performed as a function of the refractive index. Here, the refractive index could be acquired in a spatially-resolved manner with an index resolution of down to $3.36 \pm 10^{-5}$. This was achieved by the development of a novel mathematical analysis approach. For the acquisition of layer thicknesses of thin-films, an advanced setup was developed which could be used to characterize the thickness thin-films and its (flexible) substrate simultaneously.