Organic and mineral samples, containing light elements like oxygen, nitrogen or beryllium have been analyzed using an attachable X-ray optic with an SDD in SEM. Compared to the measurements without the additional optic, there is a significant increase of the detection sensitivity for X-rays in the energy range below 1 keV. In contrast the spectrum above 1 keV is not affected. The optic can provide similar intensities for low and high energy X-ray lines even at high accelerating voltage.
The silicon drift detector (SDD) technology has reached good energy resolution and exceeds the Si(Li) detector in particular at low X-ray energies. Below 600 eV the hardware-based pile-up rejector of EDS systems does not work and there is no applicable software-based pile-up correction so far. Because the SDDs pulse processing time is about twentyfold faster compared to the Si(Li) detector, the SDD is able to process much higher count rates in the low energy region without producing sum peaks . But the analysis of light elements is still disappointing due to the insufficient production of low energy X-rays and the absorption inside the sample matrix and by the X-ray window. To benefit from the SDDs excellent energy resolution and line separation in the region below 600 eV it is important to significantly increase the low energy detection sensitivity.
To get more signal from the light elements, the beam current might be increased or the detector could be moved closer to the sample to enlarge the covered solid angle. But both would increase also the high energy counts and the bremsstrahlung, thus leading to a rising dead time. Another way to become more sensitive for the low energy region is to reduce the accelerating voltage of the SEM from 15 or 20 kV down to 5 kV or less. But this is only possible if no high energy X-ray lines are to be stimulated.
In the following an X-ray optic is described, which significantly increases the detection sensitivity for low energy X-rays. The spectrum above 1 keV is not affected and the settings of the SEM will not change. The experiments are carried out with the LoMAX EDS optic from Parallax Research Inc., attached to a 10 mm2 e2v SDD equipped with an ultra-thin AP3.3 Moxtek window and a digital pulse processor from XIA LLC.
The optic works by total external reflection, similarly to a shaped mono-capillary .
It is a grazing incident X-ray optic, using grazing angles from about 6 down to about 2 degrees. X-rays diverging into a large solid angle were captured by reducing that angle and thus directed towards the detector. The optic that I use has three reflecting surfaces in the shape of cones of revolution with different opening angles. X-rays, going from the sample into a large angle with respect to the detector axis hit the optic near the small entrance while those emitted into smaller angles hit the optic near the detector. But due to the different grazing angles within the optic they all fall onto the surface of the detector. The inner surface of the optic is nickel, which is polished to about 1 nm rms smoothness. Nickel has very good reflecting properties for the examined energy range whereas the materials of glass capillaries are not as good. The lower the X-ray energy the higher the gain of the nickel-based optic. High energy X-rays pass directly through the optic without reflection, and the optic does not alter the spectrum above 1 keV. The ratio of peak intensities with and without optic shows a unity gain for energies above 1 keV and an approximately linearity of the gain below 750 eV with a maximum gain of about 10 times for the Be-K and the Si-L line. The optic body consists of non-magnetic copper and so does not influence the primary electron beam.
Alignment of the Optic
The optic is mounted by sliding the ring clamp adaptor over the end of the EDS detector tube and tightened by means of an Allen screw (fig. 1, 2). Alignment has to be done carefully so that the optics axis passes through the X-ray emitting point of the sample. A deviation of 200 µm leads to a drop down of the X-ray signal. The precise adjustment is done by a pointer which is screwed into the ring clamp adaptor instead of the optic and is adjusted by means of 4 tiny screws on the front of the split clamp. The pointer is imaged using an SEM magnification of about 200 times to see its position relative to the center and to find out the right working distance. The optic is designed for a distance of 4 mm from the X-ray emitting spot of the sample to the optic entrance aperture. Contacting the sample surface with the pointer, which is 4 mm longer than the optic, results in the exact working distance of the SEM. The adjustment process has to be repeated until the pointer is within 0.1 mm of the center (fig. 3, 4).
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