Ion Implantation and Synthesis of Materials - Studium

208 14 Application of Ion Implantation Techniques in CMOS FabricationI⎛ Z ⎞∝ ⎜ ⎟⎝m⎠1/2E3/2(14.10)where I is the beam current extracted from the source. Since the beam currentscales as beam energy to the power of 3/2, the device fabrication throughput willbe significantly reduced for low energy implantations. Due to the space chargeeffect, the drift mode, in which the extraction voltage of ions from the sourceequals the final beam energy, is being replaced by the deceleration mode, in whicha reverse bias is applied to decelerate the high energy ions down to the finaldesired energy.14.4.2 Energy ContaminationOne issue associated with the deceleration mode approach is charge exchange andbeam neutralization, which occur when the ion beam interacts with residual gasesin the beam lines. If ion neutralization happens during the deceleration process,neutralized ions will not undergo any further deceleration. As a consequence, aportion of the beam will have higher energies due to incomplete deceleration. Theextra radiation damage that comes from high energy ions increases the TED ofdopant implants (Shao et al. 2004). Reducing the beam line pressure can minimizethe charge exchange process and ion energy contamination. Lowering injection(initial acceleration) beam energies can further reduce energy contaminationbecause the charge exchange cross section between energetic ions and residualgases decreases with ion energy.14.4.3 Beam Shadowing EffectCoulomb repulsion between charged ions in the ion implanter can cause the beamto blow up. Furthermore, when the beam is not perfectly vertical to the mask,asymmetrical source and drain doping will occur. Figure 14.12 shows a schematicof beam blow-up and also of the doping effect from beam shadowing effect. Beamblow-up by the Coulomb explosion process can be reduced by adding an electronflood gun to the implanter. The electron shower results in a charge neutralizationby forming a mixture of positively charged dopant ions and negatively chargedelectrons, thereby reducing beam divergence. The beam shadowing effect can befurther reduced by decreasing the mask thickness.14.5 The Role of Ion Implantations in Device FabricationsAs this chapter has shown, ion implantation is an essential technology in theproduction of semiconductor devices. In addition, it plays a key role in sustainingthe rapid pace of development required by the ever evolving semiconductorindustry.