Effect of Stacked Work Function Metals on Gate Stress Reliability of n- and p-FinFETs

This research explored the impact of stacked work function metals (WFM) on gate stress reliability in n- and p-FinFETs. A study of four different gate stacks, utilizing p-type WFM (TiN) and n-type WFM (TiAl) in tri-gate FinFETs, resulted in four threshold voltages (
${V}_{\text {t}}{)}$
ranging from lowest to highest: n-type low
${V}_{\text {t}}$
, n-type standard
${V}_{\text {t}}$
, p-type standard
${V}_{\text {t}}$
, and p-type low
${V}_{\text {t}}$
. The research revealed that devices with the lowest (highest) WFM on the gates displayed the highest
${V}_{\text {t}}$
and subthreshold swing (S.S.) degradation under positive (negative) gate stress. Both energy diagram-based electric field (
${E}{)}$
simulations and device simulations confirmed the measurement results. Moreover, the study demonstrated that the
${E}$
monotonically decreased/increased with the increase of work function only when the absolute value of the gate voltage (
$\vert {V}_{\text {G}}\vert {)}$
was sufficiently large, while the
${E}$
reversed when the
$\vert {V}_{\text {G}}\vert $
became small. The work function of the reversing
${E}$
in the un-doped state approached the mid-gap energy level of silicon, as the
$\vert {V}_{\text {G}}\vert $
reached its minimum.