Advanced Chemical Analysis Using an Annular Four-Channel Silicon Drift Detector
Ralf Terborg , 1 * Andi Kaeppel , 1 Baojun Yu , 2 Max Patzschke , 1 Tobias Salge , 3 and Meiken Falke 1 1 Bruker Nano GmbH , Am Studio 2D , 10317 Berlin , Germany 2 Bruker Nano Analytics Division , 19F, Xinyuan Technology Building , No.418 Guiping Rd , Caohejing Development Zone ,
Shanghai 200233 , China 3 Natural History Museum , Cromwell Road , London SW7 5BD , UK
*
ralf.terborg@bruker.com
Abstract: Successful energy-dispersive analysis with silicon drift detectors (SDDs) spurred further instrumentation development, leading to multi-channel SDDs with four separate detector segments integrated into a single SDD chip. The use of multiple channels provides greater geometrical X-ray collection effi ciency and allows higher throughput. We demonstrate the use of an annular multi-channel detector in the scanning electron microscope. The detector is placed between the pole piece and the sample. Optimization of acquisition parameters for various problems of materials and life science is explained. The advantages for analyses at low beam current, high speed, and for highly topographic samples are shown.
Introduction Peltier-cooled silicon driſt detectors (SDDs) have become standard instrumentation for energy-dispersive X-ray spectrometry (EDS) in electron microscopes [ 1 , 2 ]. State-of-the-art SDDs reach an energy resolution of 121 eV at Mn-Kα , show good low-energy performance below 100 eV, and produce Gaussian peak shapes undistorted by detector artifacts such as shelf and tail eff ects. T is is benefi cial for peak separation over the entire available energy range. But peak separation is particularly important for overlaps of low-energy lines, such as the K-lines of light elements (Be - F) and the L-lines (K - Ni), M-lines (Mo – La), and N-lines (Hf - U) of heavier elements. T e use of multiple detectors allows a substantially higher acquisition speed and higher throughput because of greater geometrical collection effi ciency from a larger solid angle for X-ray collection. Such multiple detector arrangements need multiple accessible EDS ports, which may not be possible because of geometric limitations inside and outside the microscope. Thus, further increasing the solid angle and throughput by using more than two individual detectors becomes impractical. Fortunately, thanks to advances in SDD technology, other alternatives are available.
Multi-channel SDDs with four separate detector segments integrated into one single SDD chip have been developed to deliver improved EDS performance. The combined parallel readout of the detector segments provides the increased count rate that would be expected in the case of several individual detectors. The detection geometry can be optimized using an annular arrangement of the SDD segments, as implemented in the annular Bruker XFlash
The present article shows this detector optimized for diff erent tasks. We show results of this dealing with
® FlatQUAD system. 30
the challenges of meteorite exploration, polymer research, and low-yield life science samples.
Materials and Methods T e annular detector and its setup . T e four integrated SDD segments of the XFlash
symmetry around a hole through which the electron beam passes. T e detector is inserted horizontally and placed like a backscat- tered electron (BSE) detector between sample and pole piece. T us the annular active SDD area resides directly above and very close to the sample, achieving both a high take-off angle between 40° and 70°, which minimizes shadowing eff ects, and a relatively large solid angle of collection. Each of the four SDD segments has an active area of 15 mm², resulting in a total of 60 mm². T e respective annular geometry ( Figure 1 ) delivers a solid angle of more than 1 steradian (sr) [ 3 – 6 ]. To avoid alteration of X-ray spectroscopy results by electrons coming from the sample and to maintain low-energy X-ray sensitivity, three polymer windows of diff erent thickness can be placed semiautomatically in front of the chip depending on the acceleration voltage used. Because this annular SDD array is closer to the specimen, the solid angle value is 100 times higher than that typically available in a 10 mm² detector with a 35° takeoff angle and over 10 times higher than that available from two 30 mm 2 detectors or a single 100 mm 2 detector in conventional setups. High count rates can be obtained even with low probe currents. T is is achieved by processing the counts from the annular detector through four separate electronic channels in parallel, leading to a maximum output count rate of up to 2,400 kilo-counts per second (kcps) [ 3 , 7 ].
® FlatQUAD detector are arranged in radial
These advantages invite challenging analysis approaches such as the use of low electron accelerating voltages, ultra-low
Figure 1 : Left: Solid angle Ω and X-ray output count rate (OCR) versus detector-to-sample distance. Note that the optimum detector-sample distance d of 2.8 mm delivers a solid angle of >1.1 sr. Right: Detector (blue) arrangement between SEM pole piece and sample.
doi: 10.1017/S1551929517000141
www.microscopy-today.com • 2017 March
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76