Impact of highly-doped s/d extension on the current drivability and reliability in 0.15μm CMOS

Abstract

The saturation drain current (drain voltage: 2V) at fixed gate overdrive (Vg-Vth=lV or 1.5V) was higher in higher S/D extension concentration devices at gate length of less than 0.3pm for nMOS. The extracted effective channel length (using the method proposed by ref.l) of high S/D extension concentration (dose: 6~10'~cm-~) device has shorter by about 45nm than lower SD extension concentration (dose: 2~10'~cm-~) device at Vg-Vth of 1V. This difference increases to about 72nm at Vg-Vth of 1.5V. Higher saturation drain current with higher S/D extension concentration is, therefore, attributed to the suppressed effective channel length modulation. This effect is remarkable in short channel devices because the modulation of effective channel length is less sensitive in longer channel devices. The SP extrinsic resistance in higher S/D extension concentration device has higher value and smaller gate voltage dependence. However, the reduction of effective channel length has major effect on the saturation drain current than the increase of S/D extrinsic resistance. For PMOS, saturation drain current with high S/D extension concentration (dose: 6x 1014cm-2) increased by more than 10% for 0.15pm device than that with low S/D extension concentration (dose: 4~10'~cm-~). This increase is attributed to shorter effective channel length in high S/D extension concentration device. The modulation of effective channel length is smaller in PMOS than in nMOS because the carrier concentration at the extension region may be increased by the impurity diffusion from S/D regions. Propagation delay time (Tpd) of the 0.15pm CMOS inverters was calculated using extracted SPICE parameters. The load capacitance dependence of Tpd was 0.488ps/fF and 0.416pslfF for low (dose: 4~1O'~cm-~) and high (dose: 6x10 14cm-2) S/D extension concentration, respectively, showing that about 15% higher load drivability is obtained in high SD extension concentration device. Hot-carrier reliability We examined the degradation of drain current (AId/Ido) in nMOS under DC stress. The stress gate voltage was chosen for substrate current to be maximum for each stress drain voltage. We defined the hot-carrier life time as the stress time for 10% drain current degradation. We found that the life time was longer with higher S/D extension concentration and that the allowable drain voltage to 10 years DC life time was 1.9Y If we adjust the stress condition as same substrate current (same impact ionization rate), the surface state generation estimated from the increase in charge pumping current is almost the same for high and low S/D extension concentration devices. However, AWdo is lower with higher SD extension concentration. We speculate that the increase in S/D extension resistance caused by the surface state generation is relaxed by screening effect of increased carrier concentration in higher S/D extension concentration device. Summary We have demonstrated that higher-concentration SD extension devices have superior characteristics in saturation drain current, load capacitance drivability, and hot-camer reliability, which seem to be promising for high-performance sub-quarter micron CMOS circuits. Reference