High immunity to threshold voltage variability in undoped ultra-thin FDSOI MOSFETs and its physical understanding

Sources responsible for local and inter-die threshold voltage (V_t) variability in undoped ultra-thin FDSOI MOSFETs with a high-k/metal gate stack are experimentally discriminated for the first time. Charges in the gate dielectric and/or TiN gate workfunction fluctuations are determined as major contributors to the local V_t variability and it is found that SOI thickness (T_Si) variations have a negligible impact down to T_Si=7 nm. Moreover, T_Si scaling is shown to limit both local and inter-die V_t variability induced by gate length fluctuations. The highest matching performance ever reported for 25 nm gate length MOSFETs is achieved (A_Vt=0.95 mV.mum), demonstrating the effectiveness of the undoped ultra-thin FDSOI architecture in terms of V_t variability control.