LIMITING FACTORS FOR SECONDARY-ION MASS-SPECTROMETRY PROFILING

Understanding the limitations of depth profiling with ion sputtering i s essential for accurate measurements of atomically abrupt interfaces and ultra-shallow doping profiles. The effects of cascade mixing, sput tering statistics, ion-induced roughness, the inhomogeneity of ion bea ms, and sample rotation on the depth resolution of Si 8-doped, AlAs, a nd InAs monolayers in GaAs and an AlGaAs(5 nm)/GaAs(5 nm) superlattice were investigated. Atomic force microscopy (AFM) investigation of the ion-induced surface ripple formation on a GaAs substrate sputtered wi th 3 keV O2+ at angle of incidence theta=40-degrees showed that ripple s form rapidly below 200 nm depth. AFM measured root mean square rough ness of Si delta-doped GaAs sputtered with 2 keV O2+ was 0.8 and 2.6 n m with and without sample rotation showing that ripples play a dominan t role in depth resolution degradation at shallow depth under these co nditions of bombardment. Sample rotation yielded the lowest full width at half-maximum, 4.1 nm for a Si delta layer at 120 nm depth correspo nding to a depth resolution DELTAZ=3.5 nm. Use of AFM enabled determin ation of the atomic mixing DELTAZ(m) and sputtering statistics DELTAZ( SS) components of depth resolution to be identified directly for the f irst time. These components were 3.1 and 1.5 nm, respectively.