Analysis of a B₄C/Mo/Si multilayer interferential mirror by SIMS: Influence of the sputtering ion

We present the analysis of a B₄C/Mo/Si multilayer interferential mirror (MIM) by secondary ion mass spectrometry (SIMS). The sample is prepared by magnetron cathodic sputtering and consists of a succession of 20 B ₄C (4.0 nm)/Mo (2.2 nm)/Si (12.0 nm) tri-layers deposited on a silicon substrate. Two types of ions have been used for bombardment: Cs ⁺ at an impact energy of 2 keV and O₂ ⁺ at 0.5 keV. An artefact is clearly observed on the BCs⁺ profile which presents two maxima and a pronounced minimum at the maximum of the C profile, while the B profile obtained upon oxygen sputtering has its maximum in coincidence with that of the C profile. It is clearly seen under Cs and O sputtering that the B₄-on-Mo and Si-on-B₄C interfaces are more abrupt than the Mo-on-Si interfaces, evidencing the role of the boron carbide layer as a diffusion barrier. It is also observed that the B ₄C layers are inhomogeneous, some boron being present in excess with respect to C at the B₄C-on-Mo and Si-on-B₄C interfaces. These results evidence the importance of the interaction between the primary ions and some elements (boron in our case) present in the samples.