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Secondary Ion Mass Spectrometry characterization of thin films with nanometer and subnanometer depth resolution

Mardi 26 janvier 2021 à 11:00 - Zoom link
Publié le 26 janvier 2021

​By Dr.  Paweł Michałowski 

Secondary Ion Mass Spectrometry (SIMS) is a very precise surface sensitive analytical technique. A sample is bombarded with a primary ion beam which leads to the sputtering of the matter from its surface. A small part of the sputtered particles are charged (secondary ions). They are collected and undergo spectral analysis which provides information about their mass to charge ratio. A proper interpretation allows to determine the elemental and/or isotopic composition of the sample. Subsequent layers of the sample are removed during the analysis and thus it is possible to determine how the composition changes as a function of depth, creating so called depth profiles. The lateral analysis of the signal allows to create 3D images and cross-section views of the sample.

Application of ultra-low impact energy (90-150eV) in SIMS techniques practically eliminates the mixing effect and allows to reach sub-nanometer depth resolution. However, typical ion yield for such experiments is very low and an average intensity of signals is in a range of a few hundreds counts per second at best. To overcome this problem dedicated measurement procedures have been proposed and established: primary beam and extraction parameters are optimized for the detection of a single type of element in a specific material system. In this way ion yield is significantly increased (up to three orders of magnitude) and detailed characterization of a material maintaining the sub-nanometer depth resolution becomes possible. This approach, while certainly time-consuming, is invaluable for characterization of ultra-thin films and 2D materials. Similar approach can also be applied to full device structures, i.e. about 10 µm thick with more than 50 layers. They can be measured without any deterioration of the depth resolution even if the thinnest layer is only about 3-4 nm thick. The most recent experiments show that the technique can also be applied to organic materials.

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