Comparison of profile tailing in SIMS analyses of various impurities in silicon using nitrogen, oxygen, and neon ion beams at near-normal incidence

Abstract

We have performed a systematic SIMS study into the effect of (i) the chemical nature and (ii) the energy of the primary ions on the decay length λ which characterizes the exponential fall-off of impurity sputter profiles. The samples consisted of low resistivity, p-type Si covered with thin metallic overlayers. Bombardment was carried out at 2° off normal. Aspect (i) was investigated for tracers of Cu and Ga using N2+, O2+, and Ne+ primary ions at an energy of 5 keV/atom. The effect of the beam energy, aspect (ii), was studied for eight different tracer species and N2+primary ions at energies between 2 and 5 keV/atom. In the case of Ga, λ was found to be shorter with N2+or O2+primary ions (λ=7.0 and 7.5 nm, respectively) than with Ne+ (λ=12 nm). This effect is attributed to beam induced formation of Si3N4 or SiO2 layers, whereby the effective width of the internal distribution of intermixed Ga impurities in the Si subsystem is reduced significantly. In contrast to Ga, the decay length for Cu is smallest under bombardment with Ne+ (λ=16 nm), quite large with N2+(26 nm) and extremely large with O2+(2.2 μm). Segregation of Cu atoms at the Si3N4/Si and the SiO2/Si interface, respectively, is responsible for this depressed impurity removal rate. Within experimental accuracy the observed variation of the decay length with N2+energy E [keV/atom] can be written in the form λ=kEp, where k and p are element specific parameters which range from k=1.2 nm for Pb to 10 nm for Cu and from p=0.6 for Cu and Ag to 1.0 for Pb. The results are discussed with reference to conceivable shapes of the distribution of intermixed impurity atoms.