Improvements in SDD Effi ciency – From X-ray Counts to Data
Tara Nylese * and Jens Rafaelsen
EDAX Inc., Materials Analysis Division, AMETEK, 91 McKee Drive, Mahwah, NJ 07430 *
Tara.Nylese@Ametek.com
Abstract: Continuing advancements in window materials, detector modules, and electronics are leading to higher count rates, better light-element sensitivity, and improved energy-resolution stability over a wide range of count rates. In this article we will briefl y review how the different parts of the EDS system interact, from X-rays leaving the sample to the production of useful data and where recent changes have taken place. We then apply the gains offered by this new technology to three samples to illustrate the benefi ts that can be reaped.
Introduction T e Si(Li) energy-dispersive spectroscopy (EDS) X-ray detector was the industry standard from the inception of its use on electron microscopes [ 1 ]. Since then silicon driſt detectors (SDDs) have been introduced [ 2 ], and these are currently the X-ray detector of choice across most fi elds and industries. T e original advantage of SDDs was an increase in the speed of the sensor. T is “speed gain” was a loosely defi ned benefi t because it referred only to the X-ray charge collection at the chip and the increased rate of electron transfer to the collecting anode due to a voltage-biased driſt fi eld. In the earliest generations of SDDs these speed improvements were not fully realized because of limitations in electronic processing and other physical considerations. More recently there have been tremendous improvements in performance of leading-edge SDD technol- ogies at all steps of the signal chain. T is has led some companies
to take a whole system approach that provides enhancements in collection effi ciency as well as pulse processing speed. In general terms, the fi nal number of useful X-rays analyzed by a given EDS system depends on the number of X-rays generated in the sample, the fraction of the generated X-rays that reaches the detector chip, the detector chip response, and the effi ciency of the electronics processing the signal.
Materials and Methods Input X-ray count rate . T e input counts per second (ICPS) seen by a given EDS detector is highly dependent on the sample, electron accelerating voltage used, and the beam current at the sample. Keeping those parameters the same, the number that is usually considered to have the biggest impact on the ICPS is detector area. While detector area is an easy and intuitive number to understand, one has to be careful when trying to estimate the number of detected X-rays based on this number because the solid angle of the detector can vary signifi cantly depending on which microscope or microscope port the detector is mounted on and how far the detector can be inserted. Recent reports provide solid-angle calculations for diff erent detector geometries [ 3 ] and a helpful web-based calculation tool [ 4 ]. Window support grid area . A noteworthy parameter in the above-mentioned web-based calculation tool is the areal fraction covered by the window support grid. Most EDS
Figure 1 : SEM images showing the support grid structure for two X-ray windows. (a) Polymer window. Image width = 2.5 mm. (b) Silicon nitride window. Image width = 1.3 mm. The hexagonal pattern of the silicon nitride window support structure covers 18% of the window area, whereas the venetian blind support structure for the polymer window covers 23% of the window area.
46 doi: 10.1017/S1551929517000190
www.microscopy-today.com • 2017 March
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