Chemical state analysis using Soft X ‐ray Emission Spectrometry in low voltage FE‐SEM

Abstract

The development of low voltage (LV) FE‐SEMs has been in progress, and spatial resolution for observation of less than 1.0 nm can now be achieved even at 1 kV. On the other hand, recent advanced nano materials are getting more complex and micronized. Therefore to understand details of nano materials precisely, it is necessary to utilize high spatial resolution images. However, chemical state analysis in FE‐SEM is also important as well as the high spatial resolution imaging. Recently, it becomes possible to analyse both elemental and chemical states in FE‐SEM by using newly developed Soft X‐ray Emission Spectrometer (SXES). The advantages of SEM‐SXES are high energy resolution spectra, analytical spatial resolution and applicability to bulk samples. In this report, we focus on high resolution imaging and chemical state analysis using SXES in low voltage FE‐SEM. The first commercial type SXES has been developed by Prof. M.Terauchi group of Tohoku University in collaboration with JEOL 1) . The SXES is possible to analyse chemical bonding states because it has high energy resolution at X‐ray energy of about 200 eV or even below such as 0.3 eV at the Al Fermi edge. In addition, this technique can detect X‐rays in an energy range from 50 to 210 eV, of which characteristic X‐ray energy range includes the emission spectra based on the valance band transitions in many elements 2) . The SXES is installed in the low voltage (LV) FE‐SEM JSM‐7800F Prime. This LVFE‐SEM can achieve large probe current even under low voltage condition such as 20 nA at 2 kV with a 30 μm diameter objective aperture, which is due to new design of high brightness Schottky emitter electron gun so called In lens Schottky plus. In addition, there is the decelerating method in the JSM‐7800F prime, called Gentle Beam Super High resolution (GBSH) mode. A negative voltage is applied to a sample in the GBSH mode. Therefore the incident voltage is reduced at low voltage to keep smaller probe size. Combination of the in‐lens Schottky plus electron gun and the GBSH mode is suitable for the SXES analysis. Usually the SXES is used at low kV to reduce background of continuous X‐rays. Figure 1 shows SEM images and spectra taken by SXES for the comparison between the incident electron energies of 2 keV and 15 keV. Sample is Prussian blue (PB, Fe III 4 [Fe II (CN) 6 ] 3 ) and these images were acquired by Everhart‐Thornley detector. The results show that the spectrum taken at 2 keV has less background than at 15 keV due to small electron interaction volume in the sample at 2 keV. Figure 2 shows SXES spectra of PB and Prussian white (PW: K 4 Fe II 4 [Fe II (CN) 6 ] 3 ) , in which the PW sample was reduced chemically from PB sample. The incident electron energy was 2 keV and the probe current was 22 nA. The PW is well known in that the CN bonding state is different from PB. The result shows that the peak energy and spectral shape are different between the N Kα spectra of PW and PB because of different chemical bonding states.