This chapter discusses the procedure used to perform a rigorous quantitative elemental microanalysis by SEM/EDS following the k-ratio/matrix correction protocol using the NIST DTSA-II software engine for bulk specimens. Bulk specimens have dimensions that are sufficiently large to contain the full range of the direct electron-excited X-ray production (typically 0.5–10 μm) as well as the range of secondary X-ray fluorescence induced by the propagation of the characteristic and continuum X-rays (typically 10–100 μm).

grinding and polishing. “Chemical polishing” should be avoided since chemical reactions may induce shallow, near-surface compositional changes that affect the very shallow region that is excited and sampled by the electron beam. Ion beam milling can be used to shape and finish the specimen, but it must be recognized that implantation of the primary ion and differential material removal caused by differences in the sputtering rates of the elements can modify the composition of a shallow surface layer.

20.1\ Requirements Imposed

on the Specimen and Standards

The k-ratio/matrix correction protocol for the analysis of bulk specimens has two basic underlying assumptions:

\1.\ The composition is homogeneous throughout the entire volume of the specimen in which primary characteristic X-rays are directly excited by the incident electron beam and in which secondary X-ray fluorescence is induced during the propagation of the primary characteristic and continuum X-rays. A compositionally heterogeneous specimen which does not satisfy this requirement cannot be analyzed by the conventional k-ratio/matrix correction protocol. Examples of such heterogeneous specimens include a horizontally layered specimen such as a thin film on a substrate or an inclusion with dimensions similar to the interaction volume embedded in a matrix. Such specimens must be analyzed with protocols that account for the effects of the particular specimen geometry.

\2.\ The X-ray intensities measured on the location of interest on the specimen and on the standard(s) differ only because the compositions are different. No other factors modify the measured intensities. In particular, geometric effects that arise from physical surface defects, such as scratches, pits, and so on, can modify the interaction of the electron beam (electron backscattering, beam penetration) with the specimen and can alter the subsequent X-ray absorption path length to the detector compared to an ideal flat bulk specimen. This requirement places strict conditions on the surface condition of the specimen and standards. A highly polished, flat surface must be created following the appropriate metallographic preparation protocol for each particular material. The surface should be finished to a surface roughness below 100 nm

root mean square (rms) with a typical final polish performed with 100-nm diamond, alumina, ceria or other polishing compound as appropriate. When the analysis involves measuring low energy photons below 1 keV (e.g., for the elements Be, B, C, N, O,

and F), the surface finish should be better than 50 nm rms. The preparation protocol should utilize physical

20.2\ Instrumentation Requirements

The basis of the k-ratio/matrix corrections protocol is measurement of the X-ray spectra of the specimen and standard

(s) under identical conditions of beam energy, known electron dose (the product of beam current and EDS live-time, with accurate dead-time correction), EDS parameters (detector solid angle, time constant, calibration, and window efficiency), target orientation (tilt angle, ideally 0° tilt, i.e., beam perpendicular to the target surface), and EDS take-off angle (i.e., the detector elevation angle above the flat sample surface).

20.2.1\ Choosing the EDS Parameters

Consistency in the choice of the EDS parameters is critical for establishing a robust analytical measurement environment, and this is especially important when archived standard spectra are used.

EDS Spectrum Channel Energy Width and Spectrum Energy Span

As shown in .  Fig. 20.1, when the energy axis is expanded sufficiently, the EDS spectrum is seen to be a histogram of energy channels of a specific width (e.g., 5 eV, 10 eV, 20 eV) and number (e.g., 1024, 2048, 4096). For accurate peak-­fitting purposes, it is desirable to have an adequate number of channels spanning the characteristic X-ray peaks. Because the EDS resolution is a function of photon energy, low photon energy peaks below 1 keV are substantially narrower than higher energy peaks. A choice of 5 eV for the channel energy width will provide a sufficient number of channels to adequately span all of the peaks of analytical interest, including the peaks that occur below 1 keV. C K-L2,3 is broadened in EDS to approximately 50 eV full width at half-maximum (FWHM), so a choice of

5-eV/channel will provide at least 10 channels to span the low photon energy peaks, which is important for accurate peak fitting. It is also desirable for the measured spectrum to span the full range of the excited X-ray energy, from an effective threshold of approximately 100 eV to the Duane– Hunt limit, which corresponds to the incident beam

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energy, E0. The energy span is given by the channel width multiplied by the number of channels. With a 5-eV channel width, a choice of 4096 channels will provide access to photon energies as high as 20.48 keV. For beam energy above 20 keV, the number of channels should be increased to retain the 5-eV channel width, or alternatively the channel width can be increased to 10 eV, but with the conse-

20 quence that fewer channels will describe each characteristic peak.

EDS Time Constant (Resolution and Throughput)

The EDS is only capable of processing one photon at a time. The basic measurement cycle is the photoelectric absorption of the photon in the detector active volume, measurement of the charge deposited by scattering of the

photoelectron to determine the photon energy, and incrementing the appropriate energy bin in the EDS histogram by one count. The EDS time constant (also known as the shaping time, the processing time, the maximum throughput, or other terms in different vendor EDS systems) effectively determines the amount of time spent on the measurement cycle. A short time constant enables more photons to be processed per unit of real (clock) time, but the trade-off of faster processing is poorer accuracy in assigning the photon energy. While the characteristic X-ray peak has a sharply defined energy, with a natural peak width of a few eV or less, the EDS measurement process inevitably substantially broadens the measured peak. For example, Mn K-L2,3 has a natural width of approximately 7 eV (determined as the FWHM) but as displayed in the EDS histogram, the Mn K-L2,3 peak is broadened to 122–150 eV