Continued device scaling requires the formation of ever-shallower junctions with low resistance. A desirable option to form these junctions is still the use of conventional ion implantation. However in order to meet the junction depth/sheet resistance goals, a strong reduction in implant energy and increase in implant dose is required. Earlier work for B and BF2-implants, has suggested that during low energy ion implantation self-sputtering may become an important factor influencing/limiting the retained dose. The basic mechanism for the selfsputtering with increasing dose is the increasing dopant concentration at the surface leading to an increased probability for re-emission by the sputtering process. Simple models describing ion retention in combination with sputtering are based on this concept and indeed predict a selfsputtering process limiting the final retained dose. Unfortunately the theoretical calculations only predict a significant sputtering at doses >51016 at/cm2 whereas experimental results already show a limit in retained dose at 5 1015at/cm2.
In order to confirm the experimental data, low energy B, BF2, As and Sb implants have been made. Dose retention was monitored using nuclear reaction analysis and RBS whereby details of the dopant profile (redistribution) were studied using high resolution SIMS. For As and Sb no self-sputtering up to a dose of 1 1016at/cm2 can be found. For B a small dose loss (<10%) is seen significantly below the literature data. For BF2 a 20 % dose loss is observed. None of the SIMS profiles provide sufficient evidence for enhanced B-surface migration as required to explain the enhanced self-sputtering. On the other hand such a surface migration is reminiscent of the observations in SIMS whereby also an enhanced mobility of B during ion irradiation is required to explain the anomalous B-surface peak in many SIMS profiles. Based on the SIMS profiles a component sputter yield for B can be derived which is significantly higher than the matrix sputter yield suggesting a weak bonding of the segregated species leading to a reduced surface binding energy and thus enhanced sputtering yield.