TABLE OF CONTENTS One of the enabling advantages ion implantation offers is precise spatial placement of the dopant within the device. There are however a number of situations that might lead to an error in placement, either in depth or in the lateral direction. Energy contamination, discussed in an earlier section, can result in an error in depth. Similarly, an error in the angular alignment of the beam can result in an error in ion placement in the lateral direction. Control over the angular alignment of the beam is critical in modern implanters. Angular alignment of the beam can be influenced by a number of factors including beam steering, beam divergence, as well as the method of beam scan or endstation design. In the subsections below we describe a number of endstation and scan configurations which can impact angle control. It is worthwhile to note that the need for improved angle control for smaller devices on larger wafers has been a significant driving force behind implanter development, particularly during the transition from 125/150 to 200 and 300 mm wafers.
The most general case of angular misalignment is the tilting of an otherwise perfect (collimated and nondivergent) ion beam relative to the wafer normal. If the tilt of an extension implant is in the plane of the length dimension of a transistor, it will result in an undercut of the gate stack on one side and shadowing of the extension by the...
