Ion Implantation and Synthesis of Materials - Studium

220 15 Ion Implantation in CMOS Technology: Machine ChallengesFig. 15.3. Medium current beamline showing an electric scanner and electrostaticparallelizing lens. The ion source, analyzer magnet, and resolving aperture serve to injectthe unscanned beam from the left. The parallel scanned beam on the right is passed througha postacceleration column and an electrostatic deflector before reaching the waferelectrodes separated by quadrupole lenses. The electrodes, each connected to anRF resonator, bunch the beams into packets and accelerate the packets bymodulating the phase of the RF signal on each subsequent electrode. Theresonators serve to provide accelerating voltages of up to ~80 keV on eachelectrode.Following acceleration to the final energy in the linac stage of the beamline, thebeam passes through an electromagnet (typically referred to as the final energymagnet, or FEM), which deflects, disperses, and focuses the beam, ensuring thatonly ions of the desired momentum pass through to the wafer. Emerging from theion source and extraction optics, the DC beam is typically smaller than in highcurrent tools, both in the dispersive and nondispersive planes, by approximately afactor of two. The typical beam diameter passing through the linac and exiting theFEM is ~ 20 mm, and this beam arrives at the wafer ~ 30 mm in diameter. Theoverall beamline length in the linac-based high energy implanter is approximately2.5 m. Figure 15.4 features a rendered drawing of the beamline.As an alternative to the linac-based implanter, a DC-tandem accelerator is usedin some commercial high energy implanters. The basic concept of a tandemaccelerator relies on charge exchange to effectively double the acceleratingcapability of any given potential placed on the high voltage terminal of thebeamline. In operation, positive ions are extracted from the source, converted tonegative ions in a gas charge exchange cell, and then stripped of electrons to single,