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.
Modern ion implanters typically use dc arc discharge Plasma Electron Floods (PEFs) to neutralize wafer charge. The arc discharge requires using at least some refractory metal hardware, e.g. a thermionically emitting filament, which can be undesirable in applications where no metallic contamination is critical. rf discharge PEFs have been proposed to mitigate contamination risks but the gas flows required can result in high process chamber pressures. Axcelis has developed a microwave electron cyclotron resonance (ECR) PEF to provide refractory metals contamination-free wafer neutralization with low gas flow requirement. Our PEF uses a custom, reentrant cusp magnet field providing ECR and superior electron confinement. Stable PEF operation with extraction slits sized for 300 mm wafers can be attained at Xe gas flows lower than 0.2 sccm. Electron extraction currents can be as high as 20 mA at absorbed microwave powers < 70 W. On Axcelis' new medium current implanter, plasma generation has proven robust against pressure transients caused by, for example, photoresist outgassing by high power ion beams. Charge monitor and floating potential measurements along the wafer surface corroborate adequate wafer charge neutralization for low energy, high current ion beams.
Measurements on the current-voltage-magnetic field characteristics of a space-charge-limited cylindrical cross-field diode below cutoff are presented. The measured current is found to be lower than predicted by simple cold-fluid theory. This reduction combined with observed oscillations in the current can be explained by secondary electron emission from the anode, leading to an increase of space charge in the diode.
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