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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

Examples of Microanalysis in the TEM

Drift Correction using INCASiteLock™

 In a thin AEM sample, analytical resolution is governed by the size of the focussed probe which may be as low as 10nm. Therefore, X-ray maps can be collected at high magnifications with excellent spatial resolution. However, the X-ray signal is very weak and visibility of features will be totally masked by statistical noise unless enough counts are recorded in the map. Therefore, if small features or elements present in low concentrations need to be studied, then very long acquisition times will be required to obtain sufficiently good counting statistics. Signal strength can be improved by increasing the beam current with a field emission gun microscope but even so, long counting times are often required.

A major limitation of X-ray mapping at such high magnifications is image drift caused by movement of the sample or the AEM stage. This will either cause distortion of the image if long dwell times are used in a single scan over the field of view, or will cause blurring of the X-ray map data if multiple scans are performed with a short dwell time per pixel.

For this reason, it is useful to correct for drift using compensation software that continually moves the analysis point to track any movement in the specimen during acquisition. The software periodically obtains high resolution electron images and uses sophisticated image processing algorithms to detect movement between successive images, and thus generate an appropriate correction x-y deflection to be used during X-ray acquisition. If the drift between successive electron image acquisitions is less than the required analytical resolution, then the compensation technique can be used for point analysis, linescans and X-ray maps.

A typical application for X-ray mapping in the AEM is the study of semi-conductor layer crosssections for failure analysis. Figure 7a shows the results of mapping a semi-conductor crosssection when drift occurs. The X-ray maps (Al-red, Si-green and Ti-blue) are overlaid on the STEM image. Oxford Instruments SiteLock™ drift correction software has been used to correct for this drift (Figure 7b). The program first collects a digital STEM image of the site of interest and this can be used to define discrete points or lines or grids of points for analysis. During acquisition of the data, STEM images are acquired at predetermined intervals and compared with the original image of the site using a correlation algorithm. Even with a noisy STEM image, small shifts can be determined and used to correct the deflection voltages so that the site of interest is effectively “locked” to the same position on the specimen, even if the stage is drifting.

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