A New Method to Increase the Doping Efficiency of Proton Implantation in a High-Dose Regime
The achievable doping concentration of the hydrogen-related donors induced by the proton implantation and successive annealing is, however, typically limited to a few 1015 cm‑3. Earlier studies, based mainly on co-implantations of helium and hydrogen, have led us to propose this to be due to an over-decoration effect of the hydrogen-related donors by excess hydrogen. Based on this assumption, we present a pre-conditioning method based on vacancy-related gettering sites in order to reduce the concentration of excess hydrogen. The procedure comprises a pre-implant with protons and an annealing step at elevated temperatures above the range typically used for proton-implantation doping and is thus fully applicable to a commercial manufacturing environment. During this pre-conditioning, thermally stable higher-order defect complexes are created that are for the most part electrically inactive. These defect complexes act as gettering centers and may in effect reduce the concentration of free hydrogen after a second implant. Experimental results based on spreading resistance measurements are presented in this report that clearly show the beneficial effect of the pre-conditioning and its ability to help overcome several limitations of proton-implantation doping of high-purity silicon.
The figure illustrates the effect of the new pre-conditioning method on two magnetic Czochralski-grown silicon wafers. The dashed line represents the carrier concentration profile after a 3-MeV implantation with a proton dose of 4×1014 cm-2 and subsequent annealing at 490 °C for 5 h. The solid line depicts a profile resulting from the same main process steps where the wafer has been pre-conditioned before. It appears that the peak concentration could be increased by a factor of 3 to approximately 2×1015 cm-3. For consecutive studies it may also be of interest that the pre-conditioning results in a steadily increasing carrier concentration up to a depth of 80 µm from the surface.