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

The need to measure depth profiles of ultralow energy (ULE) ion implants in silicon, required for less than or equal to 180 nm IC device technology, h as placed unprecedented requirements of high depth resolution and depth acc uracy for the technique of secondary ion mass spectrometry (SIMS). The clas sic SIMS approaches to depth profiling ion implants employed in greater tha n or equal to 250 nm device technologies are not valid for characterizing U LE implants. One reason is that the SIMS artifacts, typically observed at l ess than or equal to 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 degrees to the surface normal, with oxygen flooding, and (ii) normal incidence bombardment without oxygen flood ing. The principle of both these approaches is the same, and requires the a nalytical surface to be modified to promote consistent secondary ion yields . Studies show the need to reduce the bombarding angle to <60 degrees when using oxygen flooding. Depth profiling with this analytical condition is 3x faster than by normal incidence bombardment. When using normal incidence b ombardment, a greater shift towards the surface is observed due to a differ ential sputtering rate in the very near-surface region. With either approac h, the depth resolution is the same after this initial sputtering rate incr ease. Oblique incidence bombardment appears to be the best approach to char acterize both "as-implanted" and annealed ULE ion implants under ONE instru mental condition. (C) 1999 American Vacuum Society. [S0734-211X(99)04505-9] .