The spectrum below shows a SEM-EDS result of a cross-section of a CPU containing the elements Si, Ta, O, N, F, and Cu. The accelerating voltage is pre-set at 3 kV to achieve high spatial resolution in EDS.

1. Explain why such a low accelerating voltage can improve the spatial resolution in EDS.

2. What kind of window do you need to select for the EDS detector?

3. If your supervisor pushes you to further increase the spatial resolution in EDS by decreasing the accelerating voltage, how low can the accelerating voltage be set for the CPU sample? (To simplify the case, we don't need to care about the signal-to-noise ratio.) Explain your answer.

(Please refer to the periodic table with characteristic X-ray energies as below.)

1. Low accelerating voltage improves spatial resolution in EDS due to two main reasons. Firstly, it reduces the depth of penetration of the incident electron beam into the sample and therefore restricts the volume of the sample that is excited and emits X-rays. The thinner the excited volume, the higher the spatial resolution. Secondly, the generation of X-rays is relatively shallow with low-energy electron beams, with electrons of lower energy being more affected by matter and with shorter penetration depths, which means the X-rays generated are closer to the surface of the sample, making the collection of emitted X-rays more efficient and improving the detection sensitivity.

2. To select the EDS detector window, we must choose an element whose characteristic X-ray energy is within the energy range of the detector window. It should also be narrow enough to minimize interference from nearby energy lines, but broad enough to collect sufficient counts for good accuracy. In this case, we have several elements to choose from: Si, Ta, O, N, F, and Cu. It is better to select the window that covers most of the elements (e.g. 0-10 keV).

3. If the supervisor insists on lowering the acceleration voltage further to increase the spatial resolution, it can be lowered up to 1-2 kV, as low-energy electron beams will have the greatest impact on the topmost atomic layers of the sample, resulting in higher spatial resolution. However, a lower acceleration voltage also leads to lower X-ray generation efficiency, which in turn results in a low signal-to-noise ratio. Therefore, one must maintain a balance between high spatial resolution and a good signal-to-noise ratio.

Learn more about one must maintain a balance between high spatial resolution and a good signal-to-noise ratio.

Learn more about EDS here,INPUT DEVICES

(sensory memory)

B-EDS 122_Examination_S1_2023

CPU

PROCESSOR / RAM

(working memory)

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HARD DRIVE STORAGE...

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