Ion Implantation and Synthesis of Materials - Studium

202 14 Application of Ion Implantation Techniques in CMOS Fabrication14.3.1 Retrograde Well ImplantRetrograde wells are formed by high energy dopant implantations, usually in theenergy region of 0.1–2 MeV. Such wells are characterized by a peak concentrationat a depth about 1 µm below the surface. Previously, well structures were formedusing conventional well formation, in which low energy dopant implantation isperformed and followed by a long time thermal process for dopant drive-indiffusion. Conventional wells are characterized by a peak concentration at thesurface. Figure 14.8 shows the typical doping profiles in retrograde wells andconventional wells. Compared with conventional wells, the retrograde wellsprovide some advantages (1) only a very small thermal budget is needed for deepwell formation and (2) the peak of the dopant concentration is buried at a deepdepth instead of at the surface. Therefore carrier scattering by dopants is reducedand the carrier mobility at the channel region (surface) is increased.The use of high-energy ion implantation makes alternative well formationschemes possible. In the so-called twin well process, two separate wells for n- andp-channel transistors are formed in a lightly doped substrate. The twin wellstructure, as illustrated in Fig. 14.9, has the advantages of independent adjustmentand optimization of n-channel or p-channel devices. Devices fabricated using atwin well processes are independent of the original substrate type. Moreimportant, twin well structures reduce the resistance across the base and theemitter of both npn and pnp bipolar transistors (RS and RW in Fig. 14.2.3).Therefore, the latchup is greatly reduced.Conventional well (drive-in diffusion)Dopant ConcentrationRetrograde well(high energy ion implanation)SurfaceDepthFig. 14.8. Schematic dopant distributions in a conventional well formed by the drive-indiffusion and in a retrograde well formed by high energy implantation