Prospects for Trace Analysis in the Analytical Electron Microscope

The analytical electron microscope (AEM) uses a high energy (>100 kV) beam of electrons to generate a range of signals from a thin foil sample as shown in figure la [1,2]. Various detectors are configured in the AEM to pick up most of the generated signals (fig. lb). Microanalysis is usually performed using the characteristic x-ray signal, detected by an energy dispersive spectrometer (EDS) although occasionally the electron energy loss spectrum is also used. This paper will emphasize x-ray microanalysis only. The specific advantages that the AEM has for microanalysis are two-fold. First the instrument can be operated as a high resolution transmission electron microscope, thus permitting the analytical information to be related directly to the microstructure of the sample. Second, in the AEM most microanalysis is performed with a probe size <z 10 nm and a specimen thickness < = 100 nm. This results in an analyzed volume 10' of that commonly encountered in bulk microanalysis, for example, in the electron probe microanalyzer (EPMA). This small volume means that the spatial resolution of microanalysis is relatively good (routinely <50 nm) but generally trace analysis in the AEM is relatively difficult, because generated signal intensities are low.

One reasonable measure of analytical sensitivity used in the AEM field is the minimum mass fraction of one element that is detectable in the matrix of another. Using the criterion of Liebhafsky et al. [4], the peak is detectable if: (1)

Background
The analytical electron microscope (AEM) uses a high energy (>100 kV) beam of electrons to generate a range of signals from a thin foil sample as shown in figure la [1,2]. Various detectors are configured in the AEM to pick up most of the generated signals (fig. lb). Microanalysis is usually performed using the characteristic x-ray signal, detected by an energy dispersive spectrometer (EDS) although occasionally the electron energy loss spectrum is also used. This paper will emphasize x-ray microanalysis only. The specific advantages that the AEM has for microanalysis are two-fold. First the instrument can be operated as a high resolution transmission electron microscope, thus permitting the analytical information to be related directly to the microstructure of the sample. Second, in the AEM most microanalysis is performed with a probe size <z 10 nm and a specimen thickness < = 100 nm. This results in an analyzed volume 10' of that commonly encountered in bulk microanalysis, for example, in the electron probe microanalyzer (EPMA). This small volume means that the spatial resolution of microanalysis is relatively good (routinely <50 nm) but generally trace analysis in the AEM is relatively difficult, because generated signal intensities are low.

X-Ray Microanalysis in the AEM
The definition of "trace analysis" in this paper is assumed to be that commonly used in the EPMA, namely elemental concentrations <=0.5 wt% [3]. Under these conditions the average counts in This simple criterion can be combined with the Cliff-Lorimer equation [5] to give a minimum mass fraction of element B (Cu): A(24)' CA. kABA (2) where IAb and Ib are background intensities for elements A and B; IA is the integrated characteristic intensity from A; CA is the concentration of A (in wt%) and kAB-' is the reciprocal of the Cliff-Lorimer sensitivity k-factor kAu [5]. The equation can be rewritten [6] as: (3) Results using eqs (2) and (3) have been given by Romig and Goldstein [7] (=0.5% Ni in Fe), Michael [6] (=0.0 7 % Mn in Cu) and Lyman [81 (=0.1% Ni in Fe). The results of Michael [6] are shown in table 1. What is not apparent in these reported values is that since all the data were obtained from homogeneous samples, spatial resolution was of little consequence and was usually >50 nm which is the current limit for most thermionic source AEMs. The data in table 2 [9] are the first to compare the effect of spatial resolution on minimum detectability. These results show that a sensitivity <0.1 wt% Cr with moderate spatial resolution (<=50nm) can only be achieved with an AEM employing a field emission gun, such as the Vacuum Generators HB501. Thermionic source instruments such as the Philips EM430 can only demonstrate <0.1 wt% detectability with substantially poorer spatial resolution.

Future Prospects for X-Ray Analysis in the AEM
However, recent instrumental developments promise substantial improvement in trace analysis capability in the AEM. A combination of higher voltage beams (up to 400 kV), brighter (field emission) electron sources, improved microscope stage design [10] and x-ray spectrometry advances offer the prospect of extending the minimum mass fraction detectable by x-ray analysis down to =0.01 wt% [8]. If this can be achieved while maintaining spatial resolution at the 10 nm level or below, then the AEM will be close to detecting the presence of only a few atoms, as well as localizing them to within a few tens of unit cells.
From an experimental standpoint, Ziebold [11] has shown that C, depends on several factors, namely: will increase the value of IB (the peak intensity) and lu/IB (the peak to background ratio (P/B)) [8]. Unfortunately, there is no generally accepted definition of P/B. A recent attempt has been made to generate a "standard" sample from which to measure a "standard" P/B [12,13].
The standard sample is a 100 nm of evaporated Cr on a carbon film, supported on a Cu grid, and manufactured at the National Bureau of Standards.' The value of the P/B used is that originally suggested by Fiori et al. [14] and ratios the intensity in the full peak to the average background in a 10 eV channel. Thus the ratio is defined as P/B (10 eV).
Preliminary results (table 3) [13] indicate that modern AEMs show an enormous range in P/B (10 eV) at 100 kV and not all intermediate voltage instruments show the expected improvement at higher kVs. Nevertheless, an improved MDL of =0.05 wt% in a 10 nm probe is estimated at (4) where r is the counting time to acquire the peak. Going to an intermediate voltage such as 300 kV, 300 kV. However, if an FEG were added to a 300 kV AEM, a probe current of 5 X 10-' to 10'-A should be available in a 10 nm probe. This increase in probe current would result in an increase in P of 100 times and would improve the MDL by = 10 times to 0.01 wt% in a nominal 100 to 200 nm thick film at 300 kV [8]. Such an improvement of over an order of magnitude in analytical sensitivity brings x-ray analysis in the AEM into the 100 ppm range similar to that obtained in the electron probe microanalyzer. None of these calculations takes into account the possibility of increasing the value of r in eq (4). Typically r is limited by contamination, specimen drift and operator fatigue. Contamination can be virtually eliminated by careful specimen preparation and good (<10-8 Torr) vacuums.
Specimen drift can now be compensated electronically [15], effectively eliminating operator fatigue and permitting such experiments as overnight counting, long-term digital mapping and other techniques, hitherto the realm of classical bulk analysis using the EPMA at the micron level. Table 3. Peak to background (P/B (10 eV)) data for the CrKa peak obtained from a standard thin film sample in a range of AEMs  The transmission electron microscope can focus electrons onto a small region of a specimen, typically 1 nm to 1 JLm in diameter. If the specimen is suitably thin (preferably < 100 nm) and the transmitted electrons enter a high-resolution electron spectrometer, an electron energy-loss spectrum is produced. This spectrum ( fig. 1) contains a zeroloss peak, representing elastic scattering, one or more peaks in the 4-40 eV range (due to inelastic scattering from outer-shell electrons) and, at higher energy loss and lower intensity, characteristic edges due to ionization of inner atomic shells. These latter features are used in elemental microanalysis, usually by fitting a background in front of each edge and measuring the area Ie over an energy range A beyond each edge; see figure 1. The number of atoms (N per unit specimen area) of a particular element can be obtained from [1]: (1) The factor G makes allowance for any increase in detector gain between recording the low-loss region (area 1\) and the ionization edges; CT e is a crosS section for inner-shell scattering over the appropriate range of energy loss, which can be calculated from atomic theory or obtained experimentally. Energy-loss spectroscopy is therefore capable of providing absolute, standardless elemental analysis, although in practice it is usually the ratio of two elements which is of interest, in which case the quantities G and II cancel and need not be measured.
Energy-loss spectroscopy has been used to identify quantities of less than 10-20 g and concentrations of less than 100 ppm of elements such as phosphorus and calcium in an organic matrix [2,3]. However, the accuracy of quantitative analysis, using eq (I), is often no better than 20%. The main Sources of error, and possibilities for their removal, are discussed below.