On substrate dopant engineering for ET-SOI MOSFETs with UT-BOX

Abstract

The importance of substrate doping engineering for extremely thin SOI MOSFETs with ultra-thin buried oxide (ES-UB-MOSFETs) is demonstrated by simulation. A new substrate/backgate doping engineering, lateral non-uniform dopant distributions (LNDD) is investigated in ES-UB-MOSFETs. The effects of LNDD on device performance, Vt-roll-off, channel mobility and random dopant fluctuation (RDF) are studied and optimized. Fixing the long channel threshold voltage (Vt) at 0.3 V, ES-UB-MOSFETs with lateral uniform doping in the substrate and forward back bias can scale only to 35 nm, meanwhile LNDD enables ES-UB-MOSFETs to scale to a 20 nm gate length, which is 43% smaller. The LNDD degradation is 10% of the carrier mobility both for nMOS and pMOS, but it is canceled out by a good short channel effect controlled by the LNDD. Fixing Vt at 0.3 V, in long channel devices, due to more channel doping concentration for the LNDD technique, the RDF in LNDD controlled ES-UB-MOSFETs is worse than in back-bias controlled ES-UB-MOSFETs, but in the short channel, the RDF for LNDD controlled ES-UB-MOSFET is better due to its self-adaption of substrate doping engineering by using a fixed thickness inner-spacer. A novel process flow to form LNDD is proposed and simulated.