Abstract

Backscattering spectrometry,¹ utilizing analysis beams of ⁴He ions in the energy range 1.5-5.0 MeV, is an underused technique for determining film thickness, stoichiometry, and impurity levels. The basic technique is illustrated in Figure 1. Analysis ions rebound from struck target (film) ions to impinge on the detector (subtends solid angle Ω) with energy characteristic of the mass of the struck particle; further, the He ions lose energy passing into and out of the film material. Energy analysis of the backscattered He ions from a film of uniform composition yields a nearly flat-topped "peak" for each element present in the film. The film elements may be identified by the energies (E_1C, E_1B) of the high-energy sides of the peaks. Areal density, (Nt)_i, in atoms/cm², may be determined for each element from knowledge of solid angle Ω, cross section σ_i, and integrated peak count A_i for Q incident analysis ions. Concentration variations appear as variations in the "flatness" of the peak tops. Note that the average stoichiometric ratio, N_c/N_b, depends only on the ratio of integrated peak counts, a_c /a _b , and knowledge of the scattering cross section ratio, σ_B/σ_C. Cross sections for scattering of 2 MeV ⁴He on all elements are nearly Rutherford (Coulomb's law holds), so Rutherford backscattering (RBS) is an absolute method that does not require the use of standards. Average stoichiometric ratios may be determined as accurately as 0.1% by acquisition of sufficient data, while areal densities are uncertain by about 3%. Conversion of areal density (in atoms/cm²) to physical film thickness (in Å) requires knowledge of film density. Conversely, if the physical thickness is known, areal density measurement will provide film density. Low-mass substrates (such as carbon) are preferred so that the substrate signal does not interfer with the film signals.

© 1988 Optical Society of America