Depth resolution studies in SiGe delta-doped multilayers using ultralow-energy Cs+ secondary ion mass spectrometry

It is known that depth resolution can be improved by lowering the primary ion impact energy (E,,) and/or increasing the impact angle (0) up to a critical 0, beyond which surface roughening ensues. However, lower E-p is accompanied by lower secondary ion yield, and for ultralow-energy Cs+ primary beam, a poorly focused beam. In this study, the authors subject a Ge delta-doped Si sample to ultralow-energy (< 1 keV) Cs+ bombardment over a wide range of impact angles (theta similar to 0 degrees-70 degrees). The authors demonstrated high depth resolution with full width at half maximum (FWHM) of similar to 1.9 and similar to 2.5 nm with E-p similar to 320 and similar to 500 eV, respectively, at theta similar to 50 degrees over a significant depth range (similar to 120 nm). At a higher energy of E-p similar to 1 keV, a FWHM of similar to 2.5 nm is achieved at theta similar to 60 degrees. The authors established that the relationship between improvements in depth resolution (FWHM) is linear and gradual with increasing theta. The decay lengths (lambda(d)) characterizing the trailing edge decreased with increasing theta up to 0 similar to 50 degrees-60 degrees throughout the depth evaluated, but lambda(d) decreased beyond theta similar to 60 degrees only near the surface (similar to 12 nm). The authors noted that good depth resolution is achievable with the following theta ranges; E-p similar to 320-500eV/theta similar to 30 degrees-50 degrees and E-p similar to 1 keV/theta similar to 40 degrees-60 degrees. Using the mixing-roughness-information depth model, the authors were able to differentiate the effect of atomic mixing and surface roughness on depth resolution of (5 layers. The impact of atomic mixing, surface roughness, and instrument conditions (poor focus) on depth resolution is also discussed. (c) 2007 American Vacuum Society.