Ion Implantation and Synthesis of Materials - Studium

226 15 Ion Implantation in CMOS Technology: Machine Challenges15.3.3 Molecular ImplantationAnother way to take advantage of improved transport at higher beam energies is toimplant a molecule containing the desired dopant atom, e.g., BF 2 (borondifluoride). With a molecular mass of 49, compared with 11 for atomic B, theBF + 2 ion can be implanted at 4.5× the energy while still achieving the sameprojected range for the boron atom. Of course, one also gets an implant of theother atom in the molecule, in this case F at twice the boron dose. The extraspecies might offer some process advantage, but at the least it must not lead to aprocess disadvantage.The list of molecular species in common use at this time includes borondifluoride, BF + 2 , the phosphorus and arsenic dimer ions P +2 and As + 2 , andtetramers P + 4and As + 4. Additional molecular ion species currently underconsideration include decaborane, B 10H + 14, icosaborane (a decaborane dimer),B20H + 28, and octadecaborane B18H + 22. The molecular ions containing more thanone dopant atom per ion offer an additional advantage in that there is effectivelymore atoms implanted per unit of ion current. A figure of merit for the use ofmolecular implants is the relative atomic mass ratio which gives directly thehigher energies needed for molecular implants (Fig. 15.8). These higher energiesresult in more efficient beam transport because of lower space charge effects.15.4 Low Energy Productivity: Beam UtilizationIncreasing beam currents through improvements in beam transport is only part ofthe solution for improving productivity. The other part is to minimize the time thebeam spends off the water, characterized as beam utilization. We present here atreatment of utilization, developed by Brown et al. (2004). We then include acategorization of implanters commercialized over the last 35 years, in terms ofbeam type and scanning mechanism and the implication of each implanter’sarchitecture on beam utilization.atom ormoleculerelativeperveanceB BF 2 B 10 H 14 B 18 H 22 P P 2 As As 21 0.22 0.09 0.05 1 0.5 1 0.5Fig. 15.8. Relative mass ratios of atomic and molecular dopant ions