Ion Implantation and Synthesis of Materials - Studium

234 15 Ion Implantation in CMOS Technology: Machine Challenges15.5.2 Impact of Endstation Design and Beam Scan MechanismThe choice of endstation design and the mechanism of beam scan can alsocompromise angle control. We consider several examples below.Wafer Rotation Effect on Multiwafer EndstationAll multiwafer implanters with adjustable disk tilt angle induce a systematic errorin ion beam angle due to geometric effects. When the disk is tilted to achieve thedesired tilt and/or twist, the resulting misalignment between the ion beam and theaxis of disk rotation results (under most circumstances) in a rotation of the waferas it is passed under the stationary ion beam. Additionally, the wafers are mountedon pedestals angled up a few degrees from the disk face in the direction of theincoming ion beam (to ensure adequate thermal contact for wafer cooling). Thenet result is that the wafer tilt and twist become functions of positions on the wafersurface. It is the disk rotation about an axis not parallel to the ion beam that causesthe tilt and twist angles to vary; the slow scan motion has no effect because it islinear. Since the tilt and twist angles are functions of positions only in the fast scandirection, the set of points on any disk radius will all have the same tilt and twistangles.Travel Path Length Effect for a Ribbon Beam on a Single-WaferEndstationIf a beamline utilizes symmetrical scanning, the distance traveled by the ion beamfrom the source to the wafer will be nearly independent of scan position.However, some modern implanters have highly asymmetric parallelizing magnets(see Fig. 15.15). As suggested by the figure, this architecture makes the distancetraveled by the beam vary considerably across the wafer. Were it not for the beamdivergence (see Sect. 15.3.1), this might not matter. We have seen that divergencecauses not only the beam diameter but also the angle of the ions at the edge of thebeam envelope to be functions of beam travel distance. Consequently, this type ofarchitecture causes beam density and potentially beam angle to vary horizontallyacross the wafer. This may result in across-wafer variations in deviceperformance, especially for implants near the amorphization threshold. The effectwill be most severe for the most divergent (i.e., lowest energy) ion beams.Implanters utilizing a fast-scanned beam with a one-dimensional mechanicalscan can be further divided into a vertical scan and a plane-of-wafer scan, seeFig. 15.16. In the former, the mechanical scan direction is always perpendicular tothe ion beam vector. In the latter, the mechanical scan direction is adjusted withthe wafer tilt, so that it is always in the plane of the wafer. It is clear that, for thecase of a zero-degree implant, the two methods are identical. However, for tiltedimplants, the vertical scan method results in the beam travel distance (and hencethe beam size in the case of highly divergent beams) becoming a function ofvertical position on the wafer. The degree of across-wafer variation increases with the