A CONVENIENT METHOD FOR X-RAY ANALYSIS IN TEM THAT MEASURES MASS THICKNESS AND COMPOSITION

摘要

Quantitative analysis of bulk specimens using energy-dispersive X-ray spectrometry (EDS) in a scanning electron microscope (SEM-EDS) with equivalent accuracy to a full standards analysis can be achieved with only a single measurement on a bulk pure element reference standard [1]. The measurement provides an indirect calibration of beam current and enables quantification without normalisation including a diagnostic analytical total. For analysis of thin specimens in a transmission electron microscope (TEM), X-ray intensity depends not only on beam current and composition, but also on density p and thickness which govern the atoms per unit area in the direction of the electron beam. Therefore, in TEM a diagnostic analytical total cannot be obtained, but if the beam current is known, the mass thickness, ρ,t, and relative elemental compositions can be determined. A bulk reference standard can in principle be used in TEM and Dijkstra et al. [2] attempted to obtain k-ratios from a thin sample and a bulk standard using the same beam current; however, the X-ray yield from the standard was too high at currents suitable for analysis of a thin sample. Boon [3] therefore devised a beam current monitor linear over 3-4 decades so that a bulk target could be measured at much lower current than the specimen and a beam current correction applied. Nevertheless, at TEM voltages (e.g., 200 kV) absorption corrections are severe for a bulk material and this method still requires the use of many standards and a special beam current monitor. Watanabe's zeta factor method [4] avoids any bulk standards and uses thin film standards of known mass thickness and composition, but still requires a method of measuring the beam current. However, beam current measurement is not always available, convenient, or of sufficient accuracy and suitable thin film standards are difficult to obtain. Here we consider a new approach that offers the same convenience as single-standard quantitative analysis in SEM. Instead of a bulk standard, a thin film of known mass thickness that is uniform over a large area is used as a reference. The procedure involves recording an X-ray spectrum from the reference film for each session of acquisitions on real specimens. There is no need to measure the beam current; the current only needs to be stable for the duration of the session. If the mass thickness for one element, (ρ t)_(ref). C_(ref), is known for the reference standard, and the X-ray detector has been characterised so that conversion efficiency is known as a function of energy, then the product of electron dose and detector solid angle can be determined from an X-ray spectrum obtained from the reference standard. Provided the same beam current and kV is used to record a spectrum from the specimen, the mass thickness can be determined and used to make corrections for specimen self-absorption when determining relative elemental concentrations using the Cliff-Lorimer k-factor method. Recording an X-ray spectrum from the reference standard for each session thus replaces explicit measurement of beam current and can be regarded as a "beam measurement". For the reference standard we have used a self-supporting 100 nm thick silicon nitride TEM support film covering a 1 mm x 1 mm aperture within a standard 3 mm diameter disk. To determine the mass thickness for Si, we modelled X-ray emission from bulk Si and thin Si_3N_4 films at 10 kV and 30 kV and used a transmission holder for the standard and a pure bulk Si wafer to measure Si k-ratios and thus obtain (ρt)Si. C_(Si) values at a series of points all over the support film to confirm uniformity over at least the central 0.6 mm region. For testing analysis in the TEM, we have used a JEOL JEM-2200-MCO-FEGTEM and a low background holder, tilted to 7.2°, with a cut-out section to reduce occlusion of the line of sight to the X-ray detector. The reference film was first used to obtain a beam measurement and this was repeated at many stage positions to check for occlusion of the detector by the specimen holder. Without altering microscope conditions, the reference was replaced by a wedge test specimen of Inconel 600 and a series of spectra obtained at different positions along the wedge. Using the new X-ray method, measurements of mass thickness were converted to thickness in nm by assuming a density of 8.43 g/cm~3. The results were in a range of 50 to 80 nm and all agreed with those calculated from the known wedge angle and the annular dark field signal to within experimental error. We also took electron energy loss (EELS) measurements and found thickness estimates to be very dependent on the choice of method for calculation of mean free path. The new X-ray method should allow mass thickness and composition measurements to be obtained conveniently in any TEM instrument with stable beam current, with no need for a beam current meter.