A Nion spherical-aberration (Cs) corrector was recently installed on Lehigh University's 300-keV cold field-emission gun (FEG) Vacuum Generators HB 603 dedicated scanning transmission electron microscope (STEM), optimized for X-ray analysis of thin specimens. In this article, the impact of the Cs-corrector on X-ray analysis is theoretically evaluated, in terms of expected improvements in spatial resolution and analytical sensitivity, and the calculations are compared with initial experimental results. Finally, the possibilities of atomic-column X-ray analysis in a Cs-corrected STEM are discussed.

Summary X‐ray detection by energy‐dispersive spectrometry in the analytical electron microscope (AEM) is often limited by low collected X‐ray intensity (P), modest peak‐to‐background (P/B) ratios, and limitations on total counting time (τ) due to specimen drift and contamination. A new AEM has been designed with maximization of P, P/B, and τ as the primary considerations. Maximization of P has been accomplished by employing a field‐emission electron gun, X‐ray detectors with high collection angles, high‐speed beam blanking to allow only one photon into the detector at a time, and simultaneous collection from two detectors. P/B has been maximized by reducing extraneous background signals generated at the specimen holder, the polepieces and the detector collimator. The maximum practical τ has been increased by reducing specimen contamination and employing electronic drift correction. Performance improvements have been measured using the NIST standard Cr thin film. The 0·3 steradian solid angle of X‐ray collection is the highest value available. The beam blanking scheme for X‐ray detection provides 3–4 times greater throughput of X‐rays at high count rates into a recorded spectrum than normal systems employing pulsepileup rejection circuits. Simultaneous X‐ray collection from two detectors allows the highest X‐ray intensity yet recorded to be collected from the NIST Cr thin film. The measured P/B of 6300 is the highest level recorded for an AEM. In addition to collected X‐ray intensity (cps/nA) and P/B measured on the standard Cr film, the product of these can be used as a figure‐of‐merit to evaluate instruments. Estimated minimum mass fraction (MMF) for Cr measured on the standard NIST Cr thin film is also proposed as a figure‐of‐merit for comparing X‐ray detection in AEMs. Determinations here of the MMF of Cr detectable show at least a threefold improvement over previous instruments.

Characterization on the atomic scale can be achieved by high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopes (STEMs) equipped with recently developed spherical-aberration correctors [e.g. 1]. In such instruments, electron energy-loss spectrometry (EELS) characterization of individual atomic columns can also be performed routinely [2]. Recently, a JEOL JEM-2200FS STEM / transmission electron microscope (TEM) integrated with a CEOS aberration corrector and an in-column Ω energy-filter was installed at Lehigh. This particular instrument has been optimized to perform atomic-resolution HAADF-STEM imaging and simultaneous EELS using relatively high probe currents [3].

One of the problems to perform x-ray microanalysis at elevated sample chamber pressure in a variable-pressure SEM is the electron beam skirting due to the presence of the gas 1 . The electrons in the skirt may strike the area millimeters away from the center probe area, generating x-ray fluorescence signals that interfere with the useful signal from the center area and cannot be distinguished by the energy dispersive (EDS) x-ray detector 2 . This will greatly reduce the detection sensitivity for very small samples, or degrade the image quality when performing an elemental mapping.The effect of electron skirting on x-ray microanalysis can be greatly reduced by using a polycapillary x-ray optic 3 between the specimen and the EDS detector. The optic is capable of collecting a large solid angle of x rays from a well-defined small area and redirecting them to the EDS detector. The x rays from the outer area of the specimen that is struck by the electron skirt will not be "seen" by the detector system. The approach will greatly improve the detection sensitivity and spatial resolution of x-ray microanalysis in a variable-pressure SEM system. The focusing feature of the polycapillary optic allows a reasonable working space between the optic and the specimen, which is critical for the sample protection and ease of operation.The polycapillary focusing optic used in our work has an input focal distance of 10 mm and a fieldof-view of about 0.2 mm for 1 keV x-rays. The optic was attached to a 30 mm 2 EDS detector in a FEI XL-30 environmental SEM system (Figure 1). The detector-mounting flange was modified to allow a fine adjustment of the detector position relative to the specimen. The optic was aligned so that the focus overlapped with the center of the electron beam at the working distance of 10 mm. X-ray analysis was performed on a specimen made of a 0.5 mm gold wire embedded in silver-loaded epoxy that filled a 3.5 mm hole of a brass disk 25 mm in diameter (Figure 2). X-ray spectra were taken from the center of the gold area with and without the optic attached to the detector. Figure 3 shows the comparison of the spectra at different chamber pressures. It is clearly seen that the silver signal excited by the spreading electrons outside the gold disc area was completely eliminated when using the optic. The higher the chamber pressure, the better improvement will be obtained by using the optic. The effective collecting solid angle when using the optic was equivalent to that when the detector was placed 40 mm from the sample (no optic), which was about 0.02 sr. Therefore the improvement of x-ray image quality is not at the cost of collection efficiency.

Recent theoretical calculations and practical experiments have proven that high-angle annular dark-field (HA-ADF) imaging is significantly improved due to the incident probe refined by a spherical aberration corrector (C s-corrector) in scanning transmission electron microscopes (STEMs) [1]. The Oak Ridge group has achieved the sub-Å image resolution using a 300 keV Cs-corrected STEM [2]. For microanalysis via electron energy-loss spectrometry (EELS) and/or X-ray energy dispersive spectrometry (XEDS), the major benefit due to the C s corrector