RBS Theory: Elemental Ratios

By measuring the height of the Ta and Si peaks and normalizing by the scattering cross section for the respective element, the ratio of Ta to Si can be obtained at any given depth in the film. The stopping cross section for TaSi is significantly higher than for pure Si. This means that a backscattered particle will lose more energy per unit volume in TaSi than in pure Si. An implication of this fact is that, for a given energy loss (DE), there are fewer atoms contained in a volume of TaSi than for the same volume of pure Si. This results in fewer backscattering events, and that means the peak for silicon will be lower in the TaSi than in the pure Si layer. In the spectrum illustrated below, the silicon peak has a step at its high-energy end: the lower peak is the TaSi; the higher peak is the pure silicon.

The height of a backscattering peak for a given layer is inversely proportional to the stopping cross section for that layer. The stopping cross section of TaSi is known to be only 1.37 times that of Si. This explains why the height of the peak corresponding to Si in the TaSi layer is less than one-half the height of the peak corresponding to Si in the substrate, even for a film with a Si:Ta ratio of 2:3.

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