Charge Exchange Cross Sections Relevant to Ion Implantation for Ultra Shallow Junctions

Charge exchange processes between beam ions and background gas molecules play an important role, both in the pre- and post-mass analysis regions of the beamline and affect the performance of an ion implanter in terms of transported current and energy purity. For significant parts of the energy range of relevance to ion implantation, especially the very low energies required for ultra shallow junction formation, the cross sections for charge exchange of common dopants are not known and this restricts our ability to understand and control the performance of the system. An apparatus constructed for the measurement of charge exchange cross sections using the growth curve method [1 2|, has been linked to the Salford ultra low energy ion implanter and used to measure the energy dependence of relevant charge exchange cross sections at low energies. The apparatus and measurement procedure will be briefly described. The operation of the system was validated through initial measurements of the symmetric σ₁₀ Ar⁺-Ar charge exchange cross sections, which are well documented. Earlier measurements above 10 keV [1, 2] were extended down to 100 eV. Cross sections for B⁺ in plasma flood species such as Ar and Xe were measured for energies between 200 eV and 20 keV. The measured asymmetric cross sections are similar, oscillating functions of the ion energy, having values between 0.2 and 1.3×10^-15 cm ² in the energy range considered, showing maxima at 1 keV and minima between 3-10 keV. B⁺-Ar cross sections are about a factor 2 lower than for B⁺-Xe. These observations indicate optimum conditions for the choice of beam transport energies and plasma bridge gas.