Ion Implantation and Synthesis of Materials - Studium

222 15 Ion Implantation in CMOS Technology: Machine ChallengesHigh Voltage Power SuppliesSince commercial ion implanters for semiconductor applications were derived andadapted from machines designed in the 1940s–1960s for nuclear researchprograms (including both the study of nuclear interactions over a wide energyrange and collection of isotopically enriched material), the development andcapability of power supplies cannot be underestimated. Today’s DC supplies thatdrive beamline electrodes and other components must resist arcing whileproviding voltages up to 80 keV with an accuracy of 0.5% or better. Linac powersupplies drive resonators up to 80 keV at 13.56 MHz with accurate phase control.In tandem accelerators, power supplies must maintain voltages up to 750 keV.High Vacuum SystemsSince ions propagate effectively only in high vacuum (pressures of 10 −4 Torr orlower), vacuum technology is a critical part of all ion implanters. Turbomolecularpumps are typically used on the source side of the machine to continuouslyremove residual material from the source plasma. On the endstation side,cryogenic pumps are often used since they are effective at removing hydrogen andother species that are evolved from photoresist during ion bombardment. Ifendstation pressure becomes too high, dose control can be affected as increasingfractions of ions are neutralized by residual gas. On the other hand, residual gasspecies in the appropriate amount are often helpful for space charge neutralization.In some high current systems, for instance, beamline water bleed has beeneffective at improving low-energy transport. In high vacuum, OH-species from thewater are attracted to the positive charge of the beam – the heavier OH-moleculescan be more effective neutralizers than electrons since they possess a higherresidence time in the beam.Control SystemsBecause ion implanters are incredibly versatile, high-speed machines running inhighly automated manufacturing facilities, the control systems are very complex.At a basic level, control systems must automatically sense component positions(everything from door interlocks to source electrode tilt), vacuum levels, waferlocations, ion current, ion beam scanning parameters, etc. Rapid beam tuning is amust to lower nonproductive time, so source plasma pressure and power,extraction settings, mass-analysis magnetic field, and uniformity compensationmust occur automatically. Implanters must also manage user recipes; acceptinformation downloaded from the wafer fab manufacturing execution system(MES), and upload information collected during process to the MES so that thewafers can be logged to the next operation.