Spectrum Image Mapping with a Silicon Drift Detector (SDD) in an SEM

Abstract

The emerging class of silicon drift detectors (SDD) will significantly improve elemental x-ray mapping as performed in the scanning electron microscope (SEM) [1,2]. Several attributes of the SDD make it particularly useful for mapping. The SDD can operate with very short time constants, with the limiting value at 1 µs or less, so that the output count rate can exceed 500 kHz, which is at least an order of magnitude faster than the conventional monolithic crystal Si-EDS for similar resolution performance [3]. Moreover, individual SDDs can be made with a large active area, e.g., as large as 100 mm 2 , and arrays of SDD detectors have been demonstrated, giving solid angles approaching π steradians. With this combination of large solid angle and high pulse processing speed, the SDD becomes extremely attractive for performing elemental mapping when the x-ray flux can be made high. This is possible with SEMs capable of delivering focused probes carrying high beam current (e.g., 10 nA -500 nA), given that the specimen can withstand the high electron dose necessary to produce high x-ray count rates. Moreover, with such high x-ray flux, it becomes attractive to perform elemental mapping in the spectrum imaging mode, whereby a complete EDS spectrum, e.g., 2048 channels of 10 eV width with at least a 16-bit intensity range, is stored for each map pixel. The resulting spectrum imaging datacube thus contains all of the information possible within the performance constraints of the SDD detector. Such a database enables the user to pose any analytical spectrometry question during subsequent data evaluation with data mining.Spectrum imaging at 500 kHz with 10 ms pixels would yield approximately 5000 counts in the spectrum, which should be sufficient to detect all major constituents (concentration, C > 0.1 mass fraction) as well as many minor constituents (0.01 < C < 0.1). At 1 ms per spectrum sample, there would still be 500 counts per pixel, which could detect major constituents and be useful for surveying. A 128x128 map at 10 ms per pixel could be recorded in 164 seconds, not considering the time overhead for storing the data. At such extremely high x-ray count rates, properly binning the data stream to implement the spectrum imaging mode raises significant challenges. Spectral distortions are often detected at the shortest time constants, and deadtime correction may fail. Spectrum imaging has been explored with a 50 mm 2 SDD developed by Radiant Detectors LLC that was controlled with SAMx spectrum collection software [4,5]. The resulting spectrum image datacubes were evaluated with the SAMx software as well as the NIST LISPIX image processing software engine, which was also used for data mining. Raney nickel (which contains Ni and Al as major constituents) was used as a test material for spectrum image mapping because of its complex chemical microstructure, the differences in relative excitation the presence of minor and trace constituents both as alloying agents and as contamination. Figure 1 shows maps of AlK, NiK, and FeK ...