Quantitative analysis of trace levels of surface contamination by X‐ray photoelectron spectroscopy. Part I: Statistical uncertainty near the detection limit

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We discuss analyses of trace levels of surface contamination using X-ray photoelectron spectroscopy (XPS). The problem of quantifying common sources of statistical and systematic uncertainties for these measurements is formulated in terms of the needs of extreme ultraviolet lithography, but the results and conclusions are applicable to a broad range of XPS applications.We quantify the systematic uncertainties introduced by particular cases of overlapping peaks on different substrate structures by simulating measured spectra with the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis (SESSA). One example demonstrates that the relative atomic concentrations of trace elements such as S, P, and halogens on a Ru surface could be dramatically overestimated if the fitting of the overlapping Ru 3d and C 1s peaks excludes the contribution from carbon. We also show how spectra generated by SESSA can be compared with measured spectra to determine absolute amounts of surface impurities on layered samples of the type used for extreme ultraviolet lithography. We provide estimates of the total uncertainty for such measurements by considering the systematic limitations of SESSA and the statistical uncertainties of the measurements. The same procedure can be employed for other multilayered materials. Finally, we describe two approaches for converting XPS detection limits for an elemental impurity in an elemental matrix to the corresponding detection limits for the impurity as a thin film on the surface of the matrix material.KEYWORDS detection limits, quantitative analysis, surface contamination, systematic uncertainties, x-ray photoelectron spectroscopy 1 | INTRODUCTION Detection and quantification of trace levels of surface contamination is a challenge for many modern analytical techniques. X-ray photoelectron spectroscopy (XPS) is widely used to obtain quantitative information about surface composition and the chemical states of elements in the near-surface region from which the signals originate.Industrial laboratories use XPS to evaluate the type and amount of surface contamination in applications for which the X-ray flux does not cause significant modification of the sample surface during analysis. Our work was motivated by one such industrial application that, until recently, was used to qualify resists for use in extreme ultraviolet (EUV) lithography scanners. 1 High fluxes of energetic EUV photons (13.5 nm) combined with the outgassing from photoresists can damage EUV optics. 2 As a result, every newly formulated EUV photoresist was assessed by exposing a witness sample to its outgas products. X-ray photoelectron spectroscopy was used in the final step of these tests to detect and quantify trace levels of metals, nonmetals, and halogens. This information could be used to estimate potentially irreversible reflection losses of EUV optics associated with the given resist. 3In Part 1 of this work, 4 we described the statistical uncertainties associated with mea...

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Quantitative analysis of trace levels of surface contamination by X‐ray photoelectron spectroscopy. Part II: Systematic uncertainties and absolute quantification

Faradzhev

Hill

Powell

2017

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“…Therefore, if the same background subtraction method is used, the uncertainty in the relative intensities should be dominated by noise in the data, which can be evaluated. 22,23 To assess the relative transmission, we assume that the intensity, I, of a subshell peak is described by proportionality (1).…”

Section: Relative Transmission Of the Spectrometer Between 2000 Ev mentioning

confidence: 99%

Intensity calibration and sensitivity factors for XPS instruments with monochromatic Ag Lα and Al Kα sources

Shard

Counsell²,

Cant

et al. 2019

This paper provides a description of the transmission function of an X‐ray photoelectron spectroscopy (XPS) instrument operating with exchangeable Al Kα (1486.6 eV) and Ag Lα (2984.3 eV) sources. Both sources use the same quartz crystal monochromator and illuminate the same area of the sample. The transmission‐function–corrected data from sputter cleaned gold provides a useful reference material to calibrate other instruments of the same type. Sensitivity factors for Ag Lα and Al Kα are calculated from photoionisation cross sections and electron effective attenuation lengths. These compare well with previous experimental values and data acquired from ionic liquids. The intensity of the Ag Lα source is found to be approximately 50 times lower than the Al Kα source. This, coupled with generally lower photoemission efficiencies, results in noisier data or extended acquisition times. However, there are clear advantages to using the Ag Lα source to analyse certain elements where additional core levels can be accessed and for many technologically important elements where interference from Auger electron peaks can be eliminated. The combination of calibrated data from both sources provides direct and easily interpreted insight into the depth distribution of chemical species. This could be particularly important for topographic samples, where angle resolved experiments are not always helpful. We also demonstrate, using thin coatings of chromium and carbon, that the inelastic background in Ag Lα wide‐scan spectra has a significantly increased information depth compared with Al Kα.

Summary of ISO/TC 201 standard: ISO 19668—Surface chemical analysis—X‐ray photoelectron spectroscopy—Estimating and reporting detection limits for elements in homogeneous materials

Shard

Clifford

2017

This International Standard specifies a procedure by which elemental detection limits in X-ray photoelectron spectroscopy (XPS) can be estimated from data for a particular sample in common analytical situations and reported. This document is applicable to homogeneous materials and is not applicable if the depth distribution of elements is inhomogeneous within the information depth of the technique. In many applications, it is used to either confirm or deny the presence of an elemental species at a surface. The XPS detection limit is of great practical importance because the technique is routinely used to measure the concentration of elements which are present in low concentrations at a material surface, and knowledge of the limit of detection provides a statement of confidence when no element can be detected. Furthermore, if a particular detection limit for a specified element is required, it permits the analyst to calculate the acquisition time needed to achieve the required limit of detection. This information is important to plan experimental work, or to assess whether XPS is sensitive enough for the work in a practical timescale.This document provides a straightforward approach to calculating detection limits in XPS from experimental data in common analytical situations. It specifies the minimum requirements for the data, which include XPS measurement of the sample composition, including all detectable elements, and 1 or more XPS spectra taken under the desired operating conditions. The latter spectrum or spectra must include the kinetic energy region where a peak from the specified element is expected, and the kinetic energy region containing a peak from a reference element in the sample. The relative sensitivity factors for all elements present in the sample and the specified element must be known.The standard also provides informative annexes which allow the uncertainty in the calculated detection limit to be determined 1 and describe how the XPS detection limit is defined. Example data and calculations are provided as well as useful conversions. The final annex explains how detection limits calculated in a previous publication 2 are related to those in ISO 19668.The detection limit, X D , for element j is estimated using data from a reference element x, present in the sample and from the background noise at the expected position of a peak from element j. Specifically, the atomic concentration, X x , measured by XPS and the summed intensity (integrated counts) in a peak, A x , of element x are required.Equation 1 describes the relationship.The sensitivity factors for the elemental peaks, S x and S j , are required, and A D is the minimal detectable summed intensity for the peak from element j. In which W j is the full width at half maximum of the peak for element j. If the element is absent, then W j is set equal to W x . The energy step size in the data is ε, and k is the coverage factor, the recommended value being 2.33. The standard deviation of background intensity at the position of peak j is given b...