Study of work-function variation in stacked multiple-channel-structure device

Abstract

For stacked multiple-channel-structure devices such as stacked nanowire FETs (stacked NWFETs), work-function variation induced (WFV-induced) threshold voltage (V T ) variation is quantitatively investigated using a three-dimensional technology computer aided design (TCAD) tool. In the stacked structure device, the more nanowires are stacked, the more the WFV-induced V T variation can be decreased. It is proposed that the ratio of average grain size to gate area (RGG) for stacked multiple channel structures can be used to estimate the WFV-induced V T variation. For accurate estimation, the effective gate area (GA eff ) should be considered in the stacked structure, as the variation is averaged out by stacking multiple channels. If two 3-stacked NWFETs have the same GA eff but different nanowire diameters (i.e. one with 10/10/10 nm and the other with 7/9.34/13 nm), the two would have identical WFV-induced V T variation. In this context, if a single NWFET and a 2-stacked NWFET have identical GA eff , the two would have the same V T variation. Using the RGG concept with GA eff enables the WFV-induced V T variation to be precisely estimated, regardless of nanowire diameter and the number of nanowires.