Recent advances in secondary ion mass spectrometry to characterize ultralow energy ion implants

Abstract

The need to measure depth profiles of ultralow energy (ULE) ion implants in silicon, required for ⩽180 nm IC device technology, has placed unprecedented requirements of high depth resolution and depth accuracy for the technique of secondary ion mass spectrometry (SIMS). The classic SIMS approaches to depth profiling ion implants employed in ⩾250 nm device technologies are not valid for characterizing ULE implants. One reason is that the SIMS artifacts, typically observed at ⩽30 nm, now occur in the depth range of the ULE implant. Two approaches have been proposed to overcome this. They are (i) oblique incidence bombardment, at less than 60° to the surface normal, with oxygen flooding, and (ii) normal incidence bombardment without oxygen flooding. The principle of both these approaches is the same, and requires the analytical surface to be modified to promote consistent secondary ion yields. Studies show the need to reduce the bombarding angle to <60° when using oxygen flooding. Depth profiling with this analytical condition is 3× faster than by normal incidence bombardment. When using normal incidence bombardment, a greater shift towards the surface is observed due to a differential sputtering rate in the very near-surface region. With either approach, the depth resolution is the same after this initial sputtering rate increase. Oblique incidence bombardment appears to be the best approach to characterize both “as-implanted” and annealed ULE ion implants under ONE instrumental condition.