Spectral sharpening algorithm for a polychromatic reflectometer in the extreme ultraviolet

In this paper we present a practical approach to the analysis of spectra recorded from a table-top reflectometer operating in the extreme ultraviolet (UV) spectral range. Such in-lab tools, which utilize light from plasma based sources, are currently under investigation for a broad range of applications such as surface and thin-film analysis, near-edge X-ray absorption fine structure (NEXAFS) studies, or reflectivity measurements. In a polychromatic approach one is able to record surface-sensitive reflectometric spectra that are characteristic for each material or thin-film-layer stack. By monitoring the incident and emergent spectrum before and after the sample using two independent detectors, one can deduce its reflectivity. However, the analysis of the raw data, in terms of digitized spectrometric intensity values, can be difficult due to the nature of the quasi-continuous emission from such plasma sources, particularly with xenon, which is a good broad-band radiator in the 10-20 nm spectral range. The complexity of configurations involved in transitions of highly ionized xenon makes a line-by-line analysis very difficult as the real spectrum consists of thousands of unresolved transitions superimposed by distinctive lines. Additionally, sampling issues, detector geometry, and minor setup misalignments can distort the result. We propose a practical algorithm for spectral decomposition and sharpening of such data. Measurement results are presented that confirm the functionality of the algorithm.