Low energy ion beam transport

The need for ultra-shallow junction formation in advanced devices makes the development of high throughput ion implantation solutions at very low (sub-keV) energies increasingly more important. The fundamental challenges confronting the implant tool designer tasked with delivering these high throughput solutions are examined in this paper. A discussion of space charge and its implications for low energy beam transport is presented. The origins behind the shape of the classic beam current versus energy curve are detailed and the historical evolution of this curve is shown. Demonstration of the effects of space charge is made via consideration of beam current density and beam potential profiles under a variety of space charge conditions and highlights the importance of efficient space charge neutralization in the generation and transport of low energy beams. Issues resulting from space charge effects and related to the control of beam size, shape, and stability are outlined in the context of their importance to high productivity high current tool design. Improvements to ion source and beam extraction efficiency, and to overall beamline acceptance, have been the dominant historical paths leading to incremental improvements in low energy beam current performance. The adoption into production-worthy tools of deceleration mode and, more recently, molecular implantation for n-type dopants has further expanded the usable energy range of these leading edge tools. Most recently, significant developments to actively neutralize space charge have enabled even more substantial low energy beam current improvements. Performance details underlying this newest technology are presented.