Secondary ion mass spectrometry depth profiling of ultrashallow phosphorous in silicon

Abstract

High-precision quantitative secondary ion mass spectrometry (SIMS) trace analyses of ultrashallow P31 distributions in Si (i.e., junction depths of 50 nm or less) require the ability to eliminate the Si130H mass interference while simultaneously minimizing primary ion impact energy and maximizing sensitivity. Elimination of Si130H requires a relatively high mass resolution SIMS instrument such as the Cameca IMS-6f used in this work. A range of Cs+ primary ion energies ranging from 9.5 to 1.6 keV was investigated to determine which provided the best depth resolution as measured by decay length for ultrashallow depth profiles of 2 keV P in Si. Improvements (or lack thereof) in decay length as the primary ion impact energy was reduced were correlated with crater bottom roughness measurements. Changes in the ion yields of P and Si resulting from both the appreciable fraction of the analyzed depth made up of the surface native oxide and also from the depth required for the primary ion yield enhancing Cs+ to reach a constant level were also investigated utilizing bulk-doped P in Si. The resulting ion yield transients obtained were then used to generate an empirical correction function with the aim of improving the quantitative accuracy of the ultrashallow depth profile selected as having the minimum decay length obtained in this work. Finally, improvements in the P detection limit provided by optimization of the secondary ion postacceleration system are discussed.