Ion Implantation and Synthesis of Materials - Studium

14.4 Issues of Ion Implantations During Device Scaling 20714.3.6 Gate ImplantAt the same time as source/drain implantation, the polycrystalline silicon gates aresimultaneously doped by ion implantation. After the source/drain/gate doping, arapid thermal anneal is required to activate the dopants and repair lattice damagescreated by the implantation. Different from diffusion in monocrystalline bulksilicon, dopant diffusion in the polycrystalline silicon gate material is very fast.This is due to the grain boundaries in polycrystalline silicon, which providedopants with quick diffusion pathways (Wolf and Tauber 1986). This rapiddiffusion of dopants results in a more uniform doping distribution at shortannealing times. Phosphorus, boron, and arsenic are usually used for gate doping.It is critical that the dopants do not diffuse into the channel region. The presenceof a thin oxide layer between the polycrystalline silicon gate and the substrate issufficient to block most of the diffusion. However, if some of gate dopantspenetrate through the oxide layer, an undesired threshold voltage variation willoccur (Schaber 1985).Dopant penetration can be a serious issue for alternative high-permittivity (highk)gate dielectric materials. One typical example is HfO 2 . It has been reported thatthe dopant penetration through HfO 2 is accelerated by the presence of grainboundaries that form when an amorphous HfO 2 layer is transformed into apolycrystalline layer after high temperature annealing (Quevedo-Lopez et al.2002). To reduce dopant penetration, several different approaches have beenproposed, which include either incorporating other impurities into the dielectriclayer or by depositing a thin intermediate buffer layer between the polycrystallinesilicon gate and the gate dielectric layer (Park et al. 2000).14.4 Issues of Ion Implantations During Device ScalingShrinking devices demand a higher level of control of the ion implantation doseand ion energy purity. Extreme low energy ion beams with high beam currents areof crucial importance to achieve the production needs of CMOS devices withphysical gate lengths less than 100 nm. Both conditions require the production ofnew implant tools that can minimize space charge effects, energy contamination,and beam shadowing effects.14.4.1 Space Charge EffectsDuring ion beam transport along the beam line, the beam will experience a blowupas a consequence of the mutual repulsion of the charged particles. Themagnitude of this space-charge phenomenon is related to the charge-state, Z, thebeam energy, E, and the mass of particles, m. The space charge effect can beapproximated by Kanaya et al. (1972)