Ion Implantation and Synthesis of Materials - Studium

218 15 Ion Implantation in CMOS Technology: Machine Challengesbeam utilization efficiency. This takes into account the size and shape of thebeam, the number of passes of the beam across the wafer required to provideadequate dose uniformity, and the speed with which the mechanical system canaccelerate at the end of each scan. In general, due to the finite acceleration times atthe end of each scan (which become proportionately larger compared to the timespent with the beam on the wafer as the scan speed increases), the single-waferarchitectures typically suffer from lower beam utilization efficiencies than themultiwafer architectures.Most ion implanters with the simple fixed-spot beamline described above alsomake use of a multiwafer, mechanically scanned type endstation to provideadequate wafer cooling and improve overall tool productivity (discussed in moredetail in Sect. 15.4). A rendered drawing of a typical high current tool with amultiwafer batch endstation is shown in Fig. 15.2.High current beamlines can alternatively be designed using a fixed ribbon beamwith a single-wafer process chamber. The challenge here is in producing a fixedribbon beam with a uniform spatial extent that covers at least the entire diameterof the wafer, thus enabling a single direction of mechanical wafer scanning only.A significantly more complicated beamline is required to produce such a fixedribbon beam with sufficiently uniform ion flux and beam angle uniformity acrossthe wafer. In addition to the usual ion source, analyzer magnet, and resolvingaperture, an additional parallelizing magnet, as well as multiple independenttuning elements along the width of the ribbon, are required. The chief motivationfor a ribbon-beam-based architecture is to achieve the advantages associated withsingle-wafer processing.Medium Current BeamlinesThe primary objective of medium current implanters is to deliver a wide range ofbeam currents, from a few µA to several mA at energies in the range of a few keVto a few hundred keV. This broad set of expectations for medium current toolsmake them among the most versatile, with the capability to perform almost anyimplant required in typical processing, albeit sometimes at the expense ofoptimum productivity. There has been significant variety in the architecture ofmedium current beamlines, but with the onset of 200 mm processing in the early1990s, all commercially successful implementations involve the use of scannedion beams.Formation of the ion beam takes place in a manner similar to high current tools,at energies typically in the range of 40–80 keV. Following the analyzer magnetand resolving aperture, the beam is typically scanned in the dispersive plane viaeither an electric or a magnetic scanning element. The scanning takes place atfrequencies ranging from 100 to 1,000 Hz (slower for magnetic scanning, wherethe inductance of the electromagnet poses a limitation) and over an angular extentof 10–20°. Following scanning, the beam must then be made parallel. This istypically achieved with either an electrostatic or a magnetic optical element.