A compact and efficient angle-resolved X-ray fluorescence spectrometer for elemental depth profiling

Highlights

• Description of compact angle-resolved X-ray fluorescence spectrometer.

• Exploiting scanning-free grazing emission X-ray fluorescence approach.

• Energy dispersive charge-coupled device with single-photon detection.

• Enabling non-destructive, quantitative elemental depth profiling.

• Depth profiling of nano-structured stratified specimen, here CIGS solar cells.

Abstract

An angle resolved X-ray fluorescence spectrometer based on the concept of scanning-free shallow detection with energy-dispersive area detectors is presented. The instrument is characterized with respect to energy resolution, linearity, angular discrimination and repeatability and the necessary data evaluation strategies are presented in detail. As demonstration of its capabilities and showcase for potential applications, two different copper indium gallium (di)selenide (CIGS) absorber layers, typically applied in thin film solar cells, are analyzed. In combination with quantitative (but integral) results for layer composition and layer thickness from a commercial XRF spectrometer, depth profiles of the Ga concentration of both ≈ 2 μm thin samples are obtained. The results of the novel spectrometer compare well with quantitative depth-profiles obtained from glow-discharge optical emission spectrometry. The combination of simplicity, stability and efficiency of the spectrometer concept makes it a potential candidate for industrial applications.

Description of compact angle-resolved X-ray fluorescence spectrometer. Exploiting scanning-free grazing emission X-ray fluorescence approach. Energy dispersive charge-coupled device with single-photon detection. Enabling non-destructive, quantitative elemental depth profiling. Depth profiling of nano-structured stratified specimen, here CIGS solar cells.

Introduction

While X-ray fluorescence (XRF) is a widely applied technique for non-destructive elemental analysis, there are only few means to achieve depth resolution. One possibility is the usage of two polycapillary lenses in an XRF setup, one in the excitation and one in the detection channel. By overlapping the two foci, the detected X-ray fluorescence signal is restricted to a probing volume with typical diameters of some tens of micrometers. By scanning the sample with this probing volume, depth resolved elemental information can be obtained with depth resolutions in the micrometer range. Depth resolution below 1 μm can be achieved in an XRF setup by scanning either the incidence angle of a well-collimated excitation beam or the detection angle, thereby adjusting the information depth. The angle-resolved (AR-) XRF method is further differentiated to grazing incidence (GI-) and grazing emission (GE-) XRF if interference effects appear. Those effects can lead to enhanced depth resolution and a boost in sensitivity with respect to elemental mass deposition.

In GIXRF and the related total reflection XRF (TXRF) method, the incident X-ray beam must be well collimated and monochromatized (and thus sufficiently coherent) to allow the formation of an X-ray standing wave field within the sample. In GEXRF, the restrictions with respect to collimation and monochromaticity apply to the detection channel rather than to the excitation beam to exploit the interference effects. Therefore, the method is well-suited to be applied with various (laboratory) ionizing sources as e.g. electron guns [[1], [2], [3]], proton accelerators [[4], [5], [6]], laser-produced plasmas [[7], [8], [9]] and X-ray tubes [[10], [11], [12], [13]], the latter being the most common. Depending on the target application, different excitation and detection schemes can be pursued and are realized in spectrometer concepts described hereafter: For quantification of light elements and superior background signal, the application of wavelength-dispersive (WD) detectors is suitable. Indeed, both techniques, GEXRF and WDXRF, require a small solid angle of detection to preserve angle and energy resolution, respectively. Thus, the combination of both methods is a natural one [[14], [15], [16]]. To compensate for the low detection efficiency inherent with the WDXRF concept, high-power X-ray tubes are applied to achieve reasonable measurement times [[17], [18], [19]]. Another approach to increase the simplicity of a GEXRF setup applied to trace element analysis is the usage of energy dispersive detectors. The increase in detection efficiency allows for a compact spectrometer being operated with low-power X-ray tubes [20,13].

We report on a spectrometer design making full use of the recently developed scanning-free (SF-) GEXRF concept [21,22,8,12]. In SF-GEXRF, a two-dimensional, ideally energy-resolved detector is applied. This enables to simultaneously record a large angular range instead of scanning the emission angles successively, thus maximizing the solid angle of detection by avoiding the necessity of small slits in the detection channel. Furthermore, the absence of (motorized) stages drastically simplifies the setup, while the static nature of the measurement and the usage of highly developed commercial X-ray tubes guarantee temporal stability. All three aspects, efficiency, stability (and thus repeatability) and simplicity are of major importance for using the ARXRF method in industry. The target application of the spectrometer is the investigation of complex layered systems or elemental gradients within the sub-μm range of samples with rather rough interfaces (roughly root-mean-squared roughness > 10 nm, see section Roughness estimation in Supplemental Data linked in the Appendix), preventing the formation of interference effects. This is also the reason why the spectrometer is not optimized for GEXRF measurements with its high demands on angular resolution, but has to be classified as ARXRF spectrometer. The low demands on angular resolution come with the benefit that the sample-to-detector distance (leading to lower angular resolutions) can be minimized to increase the solid angle of detection and thus detection efficiency. Here, the spectrometer is described, thoroughly characterized and as a first application elemental Ga gradients in copper indium gallium (di)selenide (CIGS) absorber layers for solar cells are presented.