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Scanning electron microscopes are usually equipped with energy-dispersive X-ray detectors for electron probe microanalysis. This widespread analytical method allows investigators to determine the elemental composition of specimens with a spatial resolution of about 1 µm. However, owing to the electron-specimen interaction, the emitted spectra reveal, in addition to characteristic lines, also a high level of continuous bremsstrahlung background. As a result, elements with low concentrations cannot be identified. The minimum detection limit can be diminished by two orders of magnitude if the characteristic lines are excited as fluorescence by an additional x-ray source. In this case, the emergence of bremsstrahlung is considerably reduced. Combining a high-brilliance microfocus x-ray tube with efficient polycapillary optics enables one to realize an experimental arrangement for performing local fluorescence analysis at the same point where the electron beam hits the sample. The polycapillary optics under consideration focuses the emitted x-radiation onto focal spots between 30 and 100 µm in diameter. Count rates of several thousands cps have been achieved. Elemental maps have been obtained by means of the motorized specimen stage of the microscope. Copyright © 2005 John Wiley & Sons, Ltd.
An inter-laboratory comparison organized as a pilot study (PS) has been performed under the umbrella of the Consultative Committee for Amount of Substance – Metrology in Chemistry (CCQM) / Surface Analysis Working Group (SAWG) with the objective to compare k-values measured with Electron Probe X-ray Microanalysis (EPMA) for copper-gold alloys. The k-value, defined as the ratio of the Xray photon intensity of an X-ray characteristic line of the unknown sample to the corresponding one in the reference material, is a rough estimate of the mass fraction of the respective element. Basically, the mass fractions can be calculated from the k-values after applying a matrix correction procedure, which includes empirical approaches, so that traceability gets lost. Due to its physical, measurable nature, the k-value has been selected in the present PS as the primary measurand. The mass fraction has been set as the secondary (derived) measurand. Both EDX and WDX have been employed. The WDX results and the overall comparability between the k-values obtained by EDX and WDX are still in progress.
A pilot study (PS) has been performed under the Consultative Committee for Amount of Substance (CCQM) / Surface Analysis Working Group (SAWG) with the objective to compare the atomic fractions of Cu, In, Ga and Se in CIGS alloy films. Four polycrystalline CIGS films with different atomic fractions were fabricated by variation of the relative atomic fraction of Ga on 100 mm x 100 mm soda-lime glass (SLG) substrates. Similar to real solar cells the atomic fractions of the four elements (Cu, In, Ga, Se) are not homogeneous with depth. For the analysis of the CIGS layers of about 2 μm thickness depth profiling with surface analysis techniques such as XPS, AES and SIMS was recommended. A CIGS alloy reference sample with atomic fractions certified by isotope dilution ICP-MS at KRISS has been also put at disposal by the coordinator of the comparison. The certified values were close to the atomic fractions of the samples to be analyzed. Hence, the atomic fractions of Cu, In, Ga and Se in the CIGS films could be determined by the relative sensitivity factors (RSF) derived from the reference CIGS film. The total ion intensities of the constituent elements were obtained by the total number counting (TNC) method.
Standardization and metrology are two terms which are used rather rarely at major conferences as well as in scientific publications in the field of microbeam analysis. For laboratories operating under an accreditation scheme the operator of the microscope/microprobe must have available internal, national or international written standards which should be applicable to any quantitative analysis. Hence, requirements, specifications, guidelines or characteristics of methods, instruments or samples are provided with the final goal that these can be used consistently. In this way it is ensured that microbeam analyses results are reliable and meet quality-management requirements.
A short overview of thin film analysis by SEM/EDX is presented. Relevant steps such as optimization of instrumnetal parameters, extraction of net peak areas, use of standards, calculation of k-values and quantification with the dedicated thin film analysis software (Stratagem) are explained. Examples of different layered aystems are given together with validation of the results.
Quantitative analysis of a bulk sample requires that the
composition of the sample is homogeneous over the analyzed
volume. For inhomogenous samples the calculation of
the matrix effects is not correct and this can lead to wrong results in the element concentrations. For samples containing a layer structure a different quantitative evaluation has to be applied. This can be provided with the standard-based analysis in ESPRIT in combination with the STRATAGem software.
It was demonstrated in the past that the electron probe microanalysis (EPMA) can be applied to determine accurately both elemental composition and thickness of thin films by using the dedicated software package for thin film analysis Stratagem. A relatively small number of film materials such as pure metallic films of platinum and nickel, binary alloys of Fe-Ni, and Pt-Ni-Co ternary alloy films has been reported in literature as working successfully. Further, the software can be applied ‘inversely’, i.e., by feeding it with the thickness of the film and using the determined mass coating, one can easily calculate the film density, which for porous layers leads us to the true film porosity.
The present study repeats measurements on an already tested system of Fe-Ni thin films on silicon and reports for the first-time results of analysis on Si-Ge thin films deposited on a non-conductive aluminium oxide substrate. Standard-based and standardless EPMA (with EDS) results were used in combination with Stratagem for the quantification.
Further, X-ray fluorescence analysis (XRF) can be used for the determination of elemental composition and thickness of such films as well. In this case, XRF with a μ-focus X-ray source (μ-XRF) attached to a SEM was applied. For quantification, a fundamental parameter (FP) approach has been used to calculate standard-based and standardless results. Compared to EPMA, XRF has a larger information depth and a higher elemental sensitivity because of a generally lower background.
Both thin film systems have been chosen as samples of an international round robin test (RRT) organised in the frame of standardisation technical committee ISO/TC 201 ‘Surface chemical analysis’, under the lead of KRISS. The main objective of the RRT is to compare the results of atomic fractions of Fe1-xNix and Si1-xGex alloy films obtained by different surface Analysis techniques, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) applied in the depth-profiling operation mode.
Five samples of different atomic fractions of each thin film system, i.e., Fe1-xNix and Si1-xGex, have been grown by ion beam sputter deposition on silicon and Al2O3 wafers, respectively. Reference FeNi and SiGe films with well-known elemental composition and thickness have been also supplied for standard-based analysis. The atomic fractions of all the samples including the references have been certified by RBS (Rutherford backscattering spectrometry) and ICP-AES (inductively coupled plasma atomic emission spectroscopy).
Measurement of elemental composition of Fe1-xNix and Si1-xGex alloy thin films by EMPA and µ-XRF
(2019)
It was demonstrated in the past that the electron probe microanalysis (EPMA) can be applied to determine accurately both elemental composition and thickness of thin films by using the dedicated software package for thin film analysis Stratagem. A relatively small number of film materials such as pure metallic films of platinum and nickel, binary alloys of Fe-Ni, and Pt-Ni-Co ternary alloy films has been reported in literature as working successfully. Further, the software can be applied ‘inversely’, i.e., by feeding it with the thickness of the film and using the determined mass coating, one can easily calculate the film density, which for porous layers leads us to the true film porosity.
The present study repeats measurements on an already tested system of Fe-Ni thin films on silicon and reports for the first-time results of analysis on Si-Ge thin films deposited on a non-conductive aluminium oxide substrate. Standard-based and standardless EPMA (with EDS) results were used in combination with Stratagem for the quantification.
Further, X-ray fluorescence analysis (XRF) can be used for the determination of elemental composition and thickness of such films as well. In this case, XRF with a μ-focus X-ray source (μ-XRF) attached to a SEM was applied. For quantification, a fundamental parameter (FP) approach has been used to calculate standard-based and standardless results. Compared to EPMA, XRF has a larger information depth and a higher elemental sensitivity because of a generally lower background.
Both thin film systems have been chosen as samples of an international round robin test (RRT) organised in the frame of standardisation technical committee ISO/TC 201 ‘Surface chemical analysis’, under the lead of KRISS. The main objective of the RRT is to compare the results of atomic fractions of Fe1-xNix and Si1-xGex alloy films obtained by different surface Analysis techniques, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) applied in the depth-profiling operation mode.
Five samples of different atomic fractions of each thin film system, i.e., Fe1-xNix and Si1-xGex, have been grown by ion beam sputter deposition on silicon and Al2O3 wafers, respectively. Reference FeNi and SiGe films with well-known elemental composition and thickness have been also supplied for standard-based analysis. The atomic fractions of all the samples including the references have been certified by RBS (Rutherford backscattering spectrometry) and ICP-AES (inductively coupled plasma atomic emission spectroscopy).
CCQM (Consultative Committee for Amount of Substance - Metrology in Chemistry) is an international body of the Meter Convention (BIPM) established as a framework of National Metrology Institutes running inter-laboratory of comparisons to demonstrate the international comparability of chemical measurements, with traceability to international or national reference standards. In 2003 the surface analysis working group (SAWG) was established at CCQM and since then several inter-laboratory comparisons in the field of EPMA have been carried out. In many cases the EPMA results deviated significantly from each other and the expanded uncertainties were greater than expected. Both methods EDS and WDS were employed. However, light elements such as carbon and nitrogen were the major elements analyzed and it is known that the corresponding lowenergy characteristic X-ray lines are more challenging to quantify by EPMA than the higher energy lines of elements with higher atomic numbers. Moreover, the standards selected and their qualities were variable, the quantification models were different, and the acquisition parameters were not specified sufficiently in the analysis instructions.