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RBS Theory: Metal Silicide Example
An example of a sample well-suited for RBS analysis is a metal silicide film. They are commonly used as interconnections between semiconductor devices because they conduct better than aluminum or silicon. The conductivity depends on the ratio of the silicon to the metal and on the thickness of the film. Both parameters can be easily determined by RBS.
The figure illustrates the interaction between the kinematic factor and the scattering cross section. The two spectra come from two TaSi films of different Ta/Si compositions on Si substrates. In this example, one of the films is 230 nm thick, while the other film is 590 nm thick. The experiment uses an ion beam of He++ at 2.2 MeV.
In both spectra, the high energy peak arises by scattering from tantalum in the TaSi film layer. The peak at lower energy is from silicon, which appears in both the TaSi film on the surface and in the Si substrate. Silicon is much less likely to cause scattering events than tantalum due to its smaller scattering cross section. To make the features of the silicon signal in these two spectra easily discernible, the silicon peaks have been multiplied by five.
For scattering at the sample surface, the only energy loss is due to momentum transfer to the target atom. The high energy edge of the tantalum peaks near 2.1 MeV corresponds to backscattering from Ta at the surface. The high energy edge of the silicon peaks near 1.3 MeV corresponds to backscattering from Si at the surface.
