Charge exchange and neutral transport contributions to energy contamination in decel mode, sub-keV ion implantation

The use of a deceleration lens close to the wafer to obtain high currents at sub-keV energies carries with it the risk of energy contamination. The extent to which this contamination affects the implant depth profile depends on a number of factors related both to hardware design and choice of deceleration conditions. While the beamline length and pressure have a linear effect on the extent of beam neutralization, the choice of pre-accel energy and the ratio of this energy to the final energy are also significant due to (a) the energy dependence of the charge exchange cross-sections and (b) the energy dependence of the probability that the fast neutrals formed in charge exchange collisions will reach the wafer. In this paper, measurements of the σ₁₀ charge exchange cross-sections of B⁺ ions in argon, a common plasma flood feed gas, over the energy range from 200 eV to 10 keV, have been combined with beam trajectory calculations in the section of beamline between the magnet exit and the final section of the decel lens. By this means, the dependence of the energy contamination on the pre-accel energy was determined. The results have been compared with secondary ion mass spectrometry (SIMS) measurements of the high-energy neutral content of the relevant decelerated ion implants. The fact that good agreement can be obtained when appropriate currents are used in the beam trajectory calculations confirms the importance of a knowledge of both the charge exchange cross-sections and the neutral transport efficiency.