Ion Implantation and Synthesis of Materials - Studium

232 15 Ion Implantation in CMOS Technology: Machine Challengesalthough of significant technological interest for niche applications, has not yetachieved market adoption.15.5 Angle ControlOne of the enabling advantages ion implantation offers is precise spatialplacement of the dopant within the device. There are however a number ofsituations that might lead to an error in placement, either in depth or in the lateraldirection. Energy contamination, discussed in an earlier section, can result in anerror in depth. Similarly, an error in the angular alignment of the beam can resultin an error in ion placement in the lateral direction. Control over the angularalignment of the beam is critical in modern implanters. Angular alignment of thebeam can be influenced by a number of factors including beam steering, beamdivergence, as well as the method of beam scan or endstation design. In thesubsections below we describe a number of endstation and scan configurationswhich can impact angle control. It is worthwhile to note that the need forimproved angle control for smaller devices on larger wafers has been a significantdriving force behind implanter development, particularly during the transitionfrom 125/150 to 200 and 300 mm wafers.15.5.1 Impact of Beam Steering Errors on Device PerformanceThe most general case of angular misalignment is the tilting of an otherwiseperfect (collimated and nondivergent) ion beam relative to the wafer normal. If thetilt of an extension implant is in the plane of the length dimension of a transistor, itwill result in an undercut of the gate stack on one side and shadowing of theextension by the gate stack on the other. It is apparent that any deleterious effectsdue to the shadowing and undercut would only worsen as the transistordimensions and thermal budgets shrink further. Device simulations show that atthe 65 nm node angle tilt errors on the order of 1° can lead to large deviations inthe transistor on current (I on ) from the nominal value, see Fig. 15.13.One technique for reducing the impact of beam steering errors is to employ aquad-mode implant, i.e., an implant whereby a quarter of the dose is implanted atazimuthal rotations of 0°, 90°, 180°, and 270°. For a beam steering angle of −1°, aquad implant restores I on to nearly the maximum value, provided that the tilt-angleis sufficiently large (see Fig. 15.14). A quad-mode implant can be executedefficiently on any endstation with in situ wafer repositioning. In this case verylittle time is wasted between the segments.