Energy Dispersive X-Ray Spectrometry with the Transition Edge Sensor Microcalorimeter: A Revolutionary Advance in Materi
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INTRODUCTION Materials microanalysis by electron-excited x-ray spectrometry currently depends upon semiconductor (usually Si) energy dispersive x-ray spectrometry (Si-EDS) and diffraction-based wavelength dispersive spectrometry (WDS). EDS and WDS are highly complementary. Si EDS provides a continuous view of the entire photon energy range of analytical interest (0.1 keV to 15 keV) that is critical for rapid qualitative and quantitative analysis, but the modest spectral resolution (130 eV at MnKct) leads to frequent peak interferences and poor detectability. WDS offers high spectral resolution (2 eV - 12 eV) that is critical to overcoming peak interferences and enhancing detectability, but WDS requires mechanical scanning to view the spectrum and its quantum efficiency is poor. Both factors impose a significant time penalty when a wide range of the spectrum must be viewed. The combination of EDS and WDS has proven to be highly effective for conventional microanalysis practice with incident beam energies E0 ->10 keV and beam currents (for WDS) in the 10 nA - 500 nA range [1]. Since the electron range decreases sharply as E01.7,the development of the high performance, low-voltage field-emission scanning electron microscope (FEG-SEM) has led to increased interest in analysis with beam energies below 5 keV. For such beam energies, the lateral and depth dimensions of x-ray excitation lie in the range 10 nmi - 100 nm, depending on E0 and the material composition. Such analytical spatial resolution approaches that previously achievable with analytical transmission electron microscopy, but with the enormous additional benefit that the specimen does not need to be thinned to achieve electron transparency, permitting "as-received" or process stream samples to be directly analyzed with the low-voltage FEG-SEM. However, low-voltage operation of the FEG-SEM is not without cost. Typical probe currents achieved under low-voltage conditions are in the pA to low nA range for nanometer probe sizes with cold field emission. Thermally-assisted field emission can achieve higher total 75 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
beam currents but at the expense of substantially larger probe size because of the lower brightness of such sources. Low beam current effectively precludes using conventional WDS because the time penalty becomes too great for practical applications. A second inevitable consequence of low-voltage operation is the restriction in the atomic binding energies that can be probed. The overvoltage, U, which is the ratio of the incident energy to the binding or critical excitation energy, U = EO/Ec, must exceed unity to begin to ionize a given shell. The ionization efficiency is a complex function of overvoltage, but for a bulk target, the ionization is proportional to the overvoltage to a power n, I - U", where n is in the range 1.3 to 1.7, depending on atomic number. The background of the spectrum is, ideally, due only to the xray bremsstrahlung. This continuous radiation has an intensity
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