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Experimental Solid Angle  of the X-ray Perimeter Array Detector (XPAD) on the Argonne PicoProbe Analytical Electron Microscope

Experimental Solid Angle  of the X-ray Perimeter Array Detector (XPAD) on the Argonne PicoProbe Analytical Electron Microscope

Session

Poster Session 2

Authors

Nestor J. Zaluzec (1)

Affiliations

1. Photon Science Directorate, Argonne National Laboratory

Keywords

AEM, TEM, STEM, Microanalysis, XEDS, EDS, Spectroscopy, SDD, SiLi, X-ray, Quantitation, 

Abstract text

For the last few decades research efforts at Argonne have been focused upon the optimization of hardware and software  for x-ray energy dispersive spectroscopy (XEDS) to improve the performance of microanalytical sensitivity in the Analytical Electron Microscope [1-3]. The Argonne X-ray Perimeter Array Detector (XPAD) is our latest innovation and is  interfaced to the prototype  ANL 30-300 kV PicoProbe Analytical Electron Microscope  engineered by ThermoFisher Scientific Instruments [4].  This  unique XEDS configuration consists of a linear array of detectors which judiciously circumscribes the specimen. It’s implementation is designed to  maximize the collection solid angle and minimizes the penumbra and system peak effects of the instrument [5].  

 

To measure and qualify the performance of this system we use  a commercially available test specimen of nanocrystalline Ge on microporous SiNx [6]. This uniformly  thick 20 nm Ge film on top of  20 nm of SiNx has an regular array of two micron holes  which also facilitates accurate measurement of the relevant parameters needed   to assess the functionality of the microanalytical system.  In figure 1 is plotted the penumbra optimized performance of this detector measured using the Ge/SiNx test specimen at 200 kV for a holder tilt range of + 45 degrees. The experimental variation in the angle corrected intensity is  ~ 10%.  Non-optimized commercial holders will nearly always have  greater variation. 

Figure 1. ) Experimental Penumbra Profile illustrating 10% variation due to geometrical shadowing of the detector as a function of primary holder tilt.  We note that the individual detectors of the array will show greater variation, however, the symmetry of the configuration mitigates this variation when all detectors are summed.

 

The modeled geometrical solid angle of the XPAD was calculated to be 4.47 sR.  In order to confirm this we have conducted experimental measurements  of the solid angle by quantitatively measuring the x-ray emission from the Ge/SiNx test specimen.  Using the microporous holes in the  test specimen we are able to measure the incident beam current using an in-situ Faraday cup without removing the specimen and only translating the specimen by ~ 1 micron.   Figure 2 compares the normalized intensity variation  for the Ge Ka emission  with theoretical ionization cross-section. As expected the beam normalized intensity increases with decreasing accelerating voltage and follows the expected dependence of  the  relativistically  corrected cross-section.  A nominal 40% increase from  300  to 60 kV has been confirmed. [7-8]

 

Figure 2, Comparison of variation in cross-section with x-ray emission as a function of accelerating voltage (values normalized at 200 kV) .  Solid line is the model, squares are experimental values. 

 

Using our custom/optimized specimen holder an experimental measurement of the absolute intensity as a function of beam current was performed to measure the solid angle to compare with the design.  To do such we utilize the relationship shown in following equation. 

here I_α  = measured x-ray intensity (Integral counts) per unit area,  σ_α(E_o, Z) =  α^th-shell ionization cross section,   E_o  = incident electron energy,   ω_a  = a^th-shell fluorescence yield, Γ_α =  α^th-shell radiative partition function, W_z  and C_Z the atomic weight and  composition, Z = atomic number,  N_o =  Avogadro’s number, ρ = local specimen density,  ξ_o  = the incident electron beam current, t = acquisition (live) time,   ε_α = the detector energy  efficiency and  Ω_α = the detector collection solid angle.  Of all these parameters,  the accuracy in our measurement of Ω is most affected by the value of the least known parameter, namely  the K-shell ionization cross-section [7-8]. 

 

In table 1 we list the range of values of Ω based upon our experimental measurements conducted at 60, 200 and 300 kV.    Ideally all the solid angle measurements in this table would be identical, however, owing to the uncertainity in the  values of σ_α (E_o, Z) this is not achieved.  Ω is thus evaluated for two different K-shell cross-section models to give a realistic estimate of the uncertainties.  In table 1, it is clear that the experimental values for Ω are well within the range expected by the design parameters for  XPAD of 4.47 sR.  

 

Accelerating Voltage  (kV)	Average  Ge Ka Intensity (Integrated counts/nA-sec)	Theoretical Cross-section (Barns)	Experimental   Ω (sR)
60	21370	232.2-261.6	4.28-3.80
200	15570	157.8-182.7	4.59-3.97
300	15160	144.0-167.5	4.90-4.21

 

Table 1: Experimentally determined solid angle for XPAD on the ANL PicoProbe. The modeled solid angle was 4.47 sR and is within the errors of the cross-section models used herein.

Further work is in progress to assess the functionality of XPAD. 

This work was supported by the Photon Science Directorate and Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, as well as the Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. It was also supported in part by  CRADA  #01300701 between Argonne National Laboratory and ThermoFisher Scientific Instruments

References

[1] Zaluzec N.J.; (2004), “XEDS Systems for the Next Generation Analytical Electron Microscope”,Micro. Microanal., 10, S2, 122

[2] Zaluzec, N.J. (2009). “Innovative instrumentation for analysis of nanoparticles: The π steradian detector”.  MicroscopyToday 17(4), 56. 

[3] Argonne National Laboratory. (2010). High collection efficiency X-ray spectrometer system with integrated electron beam stop, electron detector and X-ray detector for use on electron-optical beam lines and microscopes, US Patent 8,314,386, 

[4] Zaluzec, N.J.  Microscopy & Microanalysis (2021), Pittsburg Pa,  August 2021

[5] Zaluzec N.J., Wen J., Wang  J. , Miller D.J.; (2016) “ Quantitative Measurements of the Penumbra of XEDS Systems in an AEM “ Microsc. Microanal. 22 (Suppl 3), 278- 

[6] http://www.temwindows.com/category_s/55.htm

[7] Zaluzec N.J. , (1984)  “K and L Shell Cross Sections for X-ray Microanalysis in an AEM”, Analytical Electron Microscopy, ed.  D. Williams, San Francisco Press, 279-284, 

[8] Llovet, X, Powell, C.J.,  Salvat, F.  and Joblonski A, (2014) “Cross Sections for Inner-Shell Ionization by Electron Impact, Journal of Physical and Chemical Reference Data 43, 13102