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
=7nm. 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 25nm gate length MOSFETs is achieved (A
Vt
=0.95mV.µm), demonstrating the effectiveness of the undoped ultra-thin FDSOI architecture in terms of V
t
variability control.