RBS Theory: Channeling

In addition to elemental compositional information, RBS can also be used to study the structure of single crystal samples. When a sample is channeled, the rows of atoms in the lattice are aligned parallel to the incident He ion beam. The bombarding He will backscatter from the first few monolayers of material at the same rate as a non-aligned sample, but backscattering from buried atoms in the lattice will be drastically reduced since these atoms are shielded from the incident He atoms by the atoms in the surface layers. For example, the back- scattering signal from a single crystal Si sample which is in channeling alignment along the <1-0-0> axis will be approximately 3% of the backscattering signal from a non-aligned crystal, or amorphous or poly-crystalline Si. By measuring the reduction in backscattering when a sample is channeled, it is possible to quantitatively measure and profile the crystal perfection of a sample, and to determine its crystal orientation.

Channeling can also be used to help improve the RBS sensitivity for light elements. For example, it is difficult to accurately measure N concentrations in TiN films deposited on Si substrates due to the interfering signal from the Si substrate. By channeling the substrate, its signal is reduced, thus improving the sensitivity for the N peak which is superimposed on the Si peak. Since TiN layers are typically polycrystalline, the channeling does not affect the backscattering signals from the Ti or N. Care must be taken, however, to avoid channeling in single crystal layers when performing compositional analysis, since the channeling effects can result in erroneously low concentrations for elements in these layers. In order to avoid channeling affects the orientation of the sample is continually manipulated in order to present a variety of crystal orientations during the analysis. Spectra acquired in this manner are frequently referred to as "Rotating Random" spectra, since the most common randomization routine involves tilting the sample 7 degrees off of the channeling axis, and then rotating the sample.

[Previous Topic] [Next Topic]