Studies of ultra shallow n+–p junctions formed by low-energy As-implantation

The generation and the evolution of extended defects in ultra-shallow n+–p junctions, formed by As ion implantation into silicon at low energies of 15, 10 and 5 keV and a dose of 1 × 1015 cm−2, and rapid thermal annealing (RTA) at temperatures of 650 °C ≤ T ≤ 950 °C have been studied using transmission electron microscopy (TEM) measurements. The generated defects in the end-of-range region are dislocation loops, which grew larger and their density decreased with increasing annealing temperature. Reduction in the implantation energy causes a decrease in defect size and density as well as in dissolution temperature. The loops dissolved at 950 °C for 15 and 10 keV, whereas for 5 keV they dissolved at 850 °C. Arsenic transient enhanced diffusion (TED) studied by ToF–SIMS measurements was observed at temperatures above 650 °C for all implantation energies, with markedly less TED for the 5 keV, although As segregates near the surface region. The results suggest that the surface plays a key role on the formation and the dissolution of the dislocation loops and the As TED, by acting as a perfect sink of point defects. A significant degradation in electrical activation efficiency and a sharp increase in sheet resistance were observed at the low energy of 5 keV. In addition, the increase of temperature causes a slight decrease in electrical activation efficiency. Out-diffusion of As (10–25%) plays a significant role in the electrically active fraction of the dopant, due to the extreme proximity to the surface of high As concentrations. Junctions shallower than 40 nm, with 50–40% of the implanted dose electrically active and sheet resistance of 370–320 ohm/square, were obtained for the 5 keV. Finally, the TED during RTA was correctly simulated using a RTA model implemented in SSUPREM4 of the process simulator, including the dislocation loops and the dose loss.