The thickness of TEM/STEM thin films can be obtained based on contamination spots with a high beam current and fine probe, especially, in STEM mode. In this method, the probe in a few nanometers in diameter is focused on the specimen and thus, hydrocarbon (HC) contamination spots are formed on the top and bottom surfaces of the foil as shown in Figure 3771a. And then, if the foil is tilted at an angle (θ) the separation of the contamination spots (W) can be used to calculate the thickness, given by,
θ -- The tilted angle between the top and bottom surfaces and the horizontal line. In modern EM systems, this angle can be read out from the TEM user-interface.
M – The magnification in the imaging condition
Figure 3771a. Schematic illustration of foil thickness evaluation based on contamination spots.
The advantage of this technique is that the analysis can be done rapidly and easily and at any small locations on the specimen. However, the value of the specimen thickness can be seriously overestimated or underestimated as indicated in Figure 3771b, depending on the size of the contamination spots and the judgment of the operator. In this figure, the obtained value W1 is actually corresponding to t1, while W2 is corresponding to t2. Clearly, the evaluation error for foil thickness will be smaller if the contamination spots are smaller and shorter as indicated by W0 and W in Figure 3771a.. But, we also need to pay attention to their visibility when the contamination spots are too small and short. Furthermore, the background contamination also affects the accuracy of the measurements .
Figue 3771b. Thick and wide contamination spots inducing evaluation error for foil thickness.
On the other hand, elemental techniques such as EELS and EDS mapping and linescan also can be employed to evaluate the foil thickness based on the same geometry discussed above.
 Rae, D. A., Scott, V. D. and Love, G. (1981) In Quantitative
Microanalysis with High Spatial Resolution (G. W.
Lorimer, M. H. Jacobs and P. Doig, eds.) The Metals