Links in this section:

Introduction
Fundamentals of the TEM technique
Beam-sample interaction
The Analytical TEM
Detector Protection
Qualitative Analysis
Quantitative Analysis
Microanalysis Examples (1)
Microanalysis Examples (2)
Microanalysis Examples (3)
Summary

The Analytical TEM (AEM)

Analytical TEMs are commonly equipped with a variety of detectors for sample analysis. The most important tools are bright field and dark field imaging, electron diffraction, electron energy loss spectroscopy (EELS), and EDS. Due to the confined space in the sample – polepiece region, EDS detector positioning and collimation are of utmost importance. The electron column must be designed to minimize stray radiation passing down the column and generating X-rays from microscope components that may cause artefacts in the EDS spectrum. Analytical TEMs are equipped with special apertures to eliminate hard X-rays being generated in the column and polepiece.

The presence of stray X-rays can be determined by performing a “hole count test”. This is performed by positioning the focussed electron beam in such a way that it is transmitted through a hole in the specimen. The specimen itself may scatter electrons and increase the stray radiation so the hole count test doesn’t represent the true conditions during analysis. Therefore, an even better test is to use a specially prepared standard sample (e.g. NiOx) and check the level of spurious contributions to the spectrum.

Detector geometry for TEMs presents a special challenge for designers due to the inherent space limitations in the objective polepiece area where the sample resides (Figure 3). Ideally, the detector should be able to view X-rays from above the specimen so that the sample can be analyzed in a horizontal or near horizontal position. The position of the detector is usually described by the take off angle (TOA) relative to the horizontal specimen plane. Care should be taken to limit the entry of backscattered electrons, as magnetic electron traps cannot be used in the TEM, due to the fact that they would cause astigmatism in the objective lens. Consequently, design of the collimator is very important. The solid angle of collection can be improved by increasing detector area although this tends to worsen the resolution of the detector. Alternatively the detector can be moved closer to the specimen but this presents severe challenges for a design that also has to minimize the pickup of stray radiation. TEM detectors are often fitted with a 30mm² area detector that gives reasonable resolution performance and 0.1-0.3sr solid angle when positioned 10-20mm from the specimen. This is significantly higher than the solid angle in a typical SEM. The solid angle is most important in AEMs as the volume of the material being analyzed is much smaller than that in a typical SEM analysis, hence far less X-rays will be generated. This is especially important in the analysis of biological materials. Peaks from the light elements B,C,O,N will often be overlapped by the L and M peaks of heavier elements. This overlap can be corrected by software if the peak shapes are well known. However, incomplete charge collection (ICC) in the detector can distort low energy peak shapes and it is therefore vital to determine what performance the detector will achieve in this low energy range if light elements are to be successfully detected. This is typically measured using the ISO 15632:2002 recommendation of measuring resolution at FKα and CKα.

Back | Next