Optical coherence tomography using broad-bandwidth XUV and soft X-ray radiation

S. Fuchs,A. Blinne,M. Wünsche,V. Hilbert,C. Rödel,E. Förster,U. Zastrau,G. G. Paulus,E. Frumker,J. Bierbach

doi:10.1007/s00340-012-4934-8

Abstract

We present a novel approach to extend optical coherence tomography (OCT) to the extreme ultraviolet (XUV) and soft X-ray (SXR) spectral range. With a simple setup based on Fourier-domain OCT and adapted for the application of XUV and SXR broadband radiation, cross-sectional images of semiconductors and organic samples becomes feasible with current synchrotron or laser-plasma sources. For this purpose, broadband XUV radiation is focused onto the sample surface, and the reflected spectrum is recorded by an XUV spectrometer. The proposed method has the particular advantage that the axial spatial resolution only depends on the spectral bandwidth. As a consequence, the theoretical resolution limit of XUV coherence tomography (XCT) is in the order of nanometers, e.g., 3 nm for wavelengths in the water window (280–530 eV). We proved the concept of XCT by calculating the reflectivity of one-dimensional silicon and boron carbide samples containing buried layers and found the expected properties with respect to resolution and penetration depth confirmed.

Highlights

Optical coherence tomography is a well-established method to retrieve three-dimensional, cross-sectional images of biological samples in a non-invasive way using near-infrared radiation [1]
The technique exploits the low temporal coherence of broadband radiation causing a rapid decay of interference structures in a Michelson-type interferometer
In traditional or time-domain optical coherence tomography (OCT), the sample reflectivity is scanned in axial directions while the depth of the layer to be imaged can be varied by changing the reference arm length

Summary

Introduction

Optical coherence tomography is a well-established method to retrieve three-dimensional, cross-sectional images of biological samples in a non-invasive way using near-infrared radiation [1]. The axial resolution of OCT is in the order of the coherence length lc which depends on the central wavelength λ0 and the spectral width (FWHM) ΔλFWHM of a light source with a, e.g., Gaussian shaped spectrum lc. Within the last decade and in conjunction with the quickly developing sector of advanced material design, the scale length of interest has dropped from micrometers to a few nanometers. Both the semiconductor circuit industry, aiming at fast and power-saving solutions, as well as structural biology and environmental chemistry with their enormous interest in nanostructures, call for resolutions in the nanometer regime. In the water window at 280–530 eV as defined by the K absorption edges of carbon and oxygen, respectively, a coherence length as short as 3 nm can be achieved and highlights possible applications of XCT for life sciences

Theory of XUV coherence tomography

Conclusion