Interface Broadening in Sputter Depth Profiling Through Alternating Layers of Isotopically Purified Silicon.

Samples composed of alternating layers of 30Si and 28Si were sputter depth profiled at near normal incidence using a variety of primary ion beams (Ne+, Ar+, Xe+, N2+, O2+ and Cs+) at energies between 2 and 10 keV. The profiles exhibit the well-known asymmetric interface broadening, i.e. a relatively sharp rise of the signal at the leading edge (described by a characteristic width δ) and a comparatively slow exponential fall-off at the trailing edge (defined by a decay length λ). Both δ and λ increase with increasing beam energy. For inert gas and cesium ion bombardment at a fixed energy, the width δ reveals the qualitatively expected decrease with increasing primary ion mass. By contrast, the decay length λ does not show any dependence on the projectile mass or on the matrix sputtering yield. These findings are discussed in terms of ion ranges, cascade mixing, and possible shapes of the stationary internal mixing profiles. The interface broadening observed under N2+ and O2+ bombardment is much (up to a factor of more than two) smaller than expected on the basis of the data for inert gas and cesium ion bombardment. This effect is attributed to beam induced nitride and oxide formation which results in a reduced depth of mixing in the Si subsystem. Comparison of the present data with previous results shows that the decay length for several impurities in Si is shorter than for Si in Si