Ion Implantation and Synthesis of Materials - Studium

216 15 Ion Implantation in CMOS Technology: Machine Challengeselements of the beamline. Today, the majority of new implant systems use anindirectly heated cathode (IHC) (Horsky 1998). The IHC is able to realize sourceoperatinglifetimes two to five times longer than could be achieved with the earlierFreeman- or Bernas-type sources, which consisted of a hot filament immersed in aplasma discharge. Virtually all implanter beamlines also require a mass analysisdevice – almost universally a dipole electromagnet – which provides momentumdispersion and transverse focusing of the ion beam.High Current BeamlinesThe primary objective of high current implanters is to deliver multi-mA beams inthe range from as low as sub-keV to as high as 180 keV. Recently, the maximumenergy requirement often has been relaxed to 60 keV. The increasingly lower lowenergyrequirements have driven the design of high current beamlines to berelatively short and to have large cross-sections. Each of these attributes isfavorable for delivering the highest possible usable beam current to the wafer.The primary challenges to delivering high beam currents at lower energiescenter around the effects of space charge forces on these beams. Ions in an ionbeam experience a repulsive force exerted by neighboring ions, causing the beamto expand in size as it propagates through the beamline. This beam size expansiontypically becomes worse as the beam current or ion mass is increased, or as theenergy of the beam is decreased (as a result of a lower energy beam moving moreslowly, thereby allowing more time for the expansion forces to act on the beam).Beam size expansion due to space charge is a problem primarily due to the loss ofion current (and hence productivity) whenever the beam passes through anaperture in the beamline which is smaller than the beam. Beam size expansion canalso affect angle control, depending on the design of the endstation and/or thebeam scanning mechanism, as discussed in Sect. 15.5.2.Most common high current beamlines are optically simple, consisting of onlyan ion source, an analyzer magnet with focusing elements, and a resolvingaperture. This allows the beam to travel through the entire beamline without anyexternally imposed electrostatic fields present (see Fig. 15.1). This mode ofoperation is called drift mode since the ions are given their final energy via the ionsource and extraction optics alone and are left to drift through the remainder of thebeamline at that energy.The analyzer magnet in high current tools typically bends the beam through~90° with a radius of ~300 mm. The total beamline length is in the 1.0–1.5 mrange, consisting of 200–300 mm from the ion source to the entrance of theanalyzer magnet, 200–300 mm from the exit of the analyzer magnet to theresolving aperture, and 400–700 mm from the resolving aperture to the wafer.Emerging from the ion source and extraction optics, the beam is approximately50 mm tall and converging slightly in the nondispersive plane and it isapproximately 5 mm wide and diverging in the dispersive plane. In the dispersiveplane, the beam is focused by the analyzer magnet to a waist at the resolvingaperture. The beam size passing through the resolving aperture is approximately5–25 mm, depending