soft X-ray emission spectrometer, SXES
soft X-ray emission spectrometer
A soft X-ray emission spectrometer, which obtains spectra of characteristic X-rays of low energies less than a few keV (hereinafter, called "soft X-rays") emitted from a specimen by electron-beam illumination. The spectrometer analyses the spectra with a high energy-resolution using a diffraction grating, and acquires the spectra using a CCD detector. The soft X-ray emission spectrometer (SXES) provided by JEOL covers an energy range of 50 eV to 2.3 keV. Its energy resolution is better than 0.3 eV at 73 eV of Al L-emission, and better than 5.0 eV at 704 eV of Fe Lα-emission. The high energy-resolution soft X-ray emission spectrometer enables us to perform the analysis of the bonding state of constituent elements (chemical state analysis). It is noted that the CCD detector is designed to remove an anti-reflection film coated for visible light use, so as to enhance the detection efficiency of soft X-rays.
Fig. 1. Schematic of a soft X-ray emission spectrometer using a diffraction grating attached to a scanning electron microscope.
Typical X-ray spectrometers are classified into two types: 1) Energy-dispersive X-ray spectrometer (EDS), which resolves the X-ray energies (E) of the spectra of the constituent elements of a specimen using a semiconductor detector, and 2) Wavelength-dispersive X-ray spectrometer (WDS), which resolves the X-ray wavelengths of the spectra using analyzing crystals. The soft X-ray emission spectrometer (SXES) is a type of WDS, which uses a diffraction grating instead of analyzing crystals.
Comparing to EDS, conventional WDS provides a high energy resolution, more than one order of magnitude higher than EDS. However, the conventional WDS instrument requires a large installation space because the spectrometer is composed of a large driving system. This makes it hard to install WDS onto the SEM because the SEM needs to attach various detectors. Thus for the SEM, a compact EDS for elemental analysis (energy resolution: ~140 eV at 5.9 keV of Mn Kα-emission) is normally used. Until recently, the Electron Probe Micro Analyzer (EPMA) has been a dedicated instrument for WDS.
In recent years, the SXES which uses a diffraction grating, instead of analyzing crystals, has successfully been developed. The SXES is composed only of the diffraction grating and a CCD detector, and is compactly designed with no driving system, thus requiring no large space for its installation. This advantage has made it possible for the compact SXES to attach to SEM like the EDS detector. As a result, nowadays SEM enables high energy-resolution X-ray spectroscopy for analysis of chemical bonding states of the constituent elements in the specimen.
Both of SXES and ordinary WDS can analyze soft X-rays with energies up to a few keV. SXES is distinguished from WDS because SXES can obtain X-ray spectra with the particularly low-energy down to 50 eV. The ordinary WDS can detect spectra from an energy range of a few 100 eV to about 13 keV. It is noted that WDS uses analyzing crystals, such as TAP (phthalic acid thallium (100)), PET (pentaerythritol (200)) and LIF (lithium fluoride (200)).
WDS uses Bragg reflections of the X-rays from the analyzing crystal, which travel only at specific angles. To serially detect the X-ray spectra, both the analyzing crystal and detector need to be driven serially. Thus, WDS spectra are serially acquired with time. On the other hand, as shown in Fig. 1, SXES continuously disperses the X-rays by means of the diffraction grating, and using a multi-channel CCD detector enables one-time acquisition of the X-ray spectra along the dispersion direction (energy axis). This detection is called "parallel detection" because of a simultaneous detection of the different spectra.
The features of SXES are listed below.
- 1) The energy resolution is greatly high, more than two orders of magnitude higher than that provided by EDS.
- 2) SXES can simultaneously acquire spectra from various elements, like EDS can.
Table. Comparison of X-ray spectrometers attached to SEM
| X-ray analyzer | |||
|---|---|---|---|
| Spectroscopic method | Wavelength-dispersive X-ray spectrometer (WDS) | Energy-dispersive X-ray spectrometer (EDS) | |
| Soft X-ray emission spectrometer (SXES) | Ordinary WDS | ||
| Analyzing crystal | Diffraction grating | Analyzing crystal | Semiconductor element |
| Energy range | ~50 eV to ~2 keV | A few 100 eV to ~13 keV | A few keV to 30 keV* *Depending on the highest accelerating voltage of an SEM instrument |
| Energy resolution | 0.3 eV | ~ 20 eV | ~ 130 eV |
| Detection | Parallel detection (simultaneous acquisition of spectra) |
Serial detection (serial acquisition of each spectral point) |
Parallel detection |
| Size |
|
||
Development of SXES
In WDS for analysis of soft X-rays, a plural number of analyzing crystals with different lattice spacings has to be used, depending on the wavelength range to be analyzed. Furthermore, the spectrometer had to be driven according to the wavelength of the X-rays to be detected.
In contrast, the recent advancement of processing technology utilizing lithography technology has led to the production of an aberration-corrected diffraction grating on a glass substrate with groove-fabricated to a precision better than a few μm.
Furthermore, the diffraction grating is so designed that the X-rays dispersed by the grating are focused at a single point for each energy, and that the focus point for every energy is created on a planar CCD detector. These features achieve a compact analyzing system with no driving.
(Proofread by Professor Masami Terauchi, Tohoku University)
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