Device Characterization

5.3.1 Effects of Doping Abruptness on DC & AC

As compared to the traditional I/I approach that utilized heavy ion PAI prior to the S/D-ext IMP (hereafter named as reference approach), our new approach exhibits a weaker reverse-short-channel-effect (RSCE), lower DIBL and lower Vth roll-off slope as shown in Fig. 5.4. Gamma-Lmask curves (Fig. 5.5) indicate the fact that the effective substrate doping concentration is lowered by using new approach especially for short channel devices for which significant overlap of halo I/I dose takes place. Fig. 5.6 compares the junction leakage currents of the short channel devices with the same Vth_sat as shown in Fig. 5.4. The new approach provides an obviously lower junction leakage than the reference approach. It reflects the truth that our new approach reduces the residual defects caused by I/I.

Id-Vg characteristics are shown in both linear-scale and logarithmic-scale in Fig. 5.7. In order to match Vth_lin between the new approach and the reference approach, about 30% lower halo I/I dose was used in the new approach as compared to that used in the reference approach. Sub-threshold swing shows no significant difference between two approaches while Vth_gm - Vth_lin is obviously lower, namely a faster on-off transition, for the new

approach. The difference in the abruptness of on-off transition is statistically proven and shown in Fig. 5.8.

The key device parameters are summarized in Table 1. Junction capacitance, junction leakage and Gamma are reduced obviously because of a significantly reduced halo I/I dose in the new approach. The obvious improvement in Gm/Gds (gain factor measured at Lmask=1m) is also beneficial to analog applications. With a shallower and more abrupt S/D-ext doping profile resulting from our new approach, a similar Rsd (S/D serious resistance extracted using method introduced in [5.23]) is achieved as compared to the reference approach. It reflects that truth that an improved doping activation of S/D-ext dopant compensates for the effects of a shallower junction.

The reduction of halo I/I dose also leads to better carrier mobility in addition to higher gate voltage overdrive. Higher driving force and higher trans-conductance are achieved by using our new approach as illustrated in Fig. 5.9. The obviously improved trans-conductance is the best evidence of improved carrier mobility. Fig. 5.10 compares the Ion-Ioff performance between the reference approach and the new approach. A significant Ion-Ioff

improvement of more than 7% was achieved by using our new approach. In terms of Ieff [5.18] performance, IL (drain current measured at Vg= 0.5Vdd, Vd = Vdd) was further improved because of a higher gate overdrive voltage as shown in Fig. 5.11. Therefore, 10% Ieff-Ioff improvement is achieved as shown in Fig. 5.12. It is evident that the improvement in Ieff-Ioff is more profound than that in Ion-Ioff when the gate overdrive voltage is increased.

The comparisons of inverter ring-oscillator time-delay (Td) are illustrated in Fig. 5.13 and Fig. 5.14. Evident improvement was demonstrated even

though this new approach was applied for only PMOSFETs in this work. By utilizing the new approach, the improvement in Td is more profound than what Ieff predicts (Fig. 5.13). The obviously different Td-Ieff characteristics between these two approaches strongly suggests different capacitive loading during the ring-oscillator switching. The 6% faster Td at the same Ieff is believed to be the combinational results of lower channel depletion capacitance and lower junction capacitance as shown in Table 5.1. As depicted in Fig. 5.14, 8% faster Td is observed at the same standby leakage.

5.3.2 Effects of Doping Abruptness on V

-sensitivity

With a higher Vg overdrive resulting from fast on-off transition by utilizing new approach, a lower Vdd-sensitivity is expected for the driving force as well as the circuit performance. In order to compare Vdd-sensitivity, Ion and Ion-Ioff performance are normalized to the values at Vdd=1V for the reference device and the one using new approach, respectively. Fig. 5.15 clearly indicates a lower sensitivity of Ion and Ion-Ioff performance against Vdd

variation by using new approach. The same behavior is also evident for Ieff

and Ieff-Ioff performance as demonstrated in Fig. 5.16. This guarantees a lower sensitivity of circuit speed variation against Vdd variation as shown in Fig.

5.17. Therefore, our new approach renders the circuit more applicable for low Vdd operation.

5.3.3 Effects of Doping Abruptness on Reliability

Driving force degradation caused by hot-carrier-injection (HCI) stress is one of the critical concerns for abrupt junction. However, this new approach

forms shallow junction and maintain short-channel electrostatic control by improving the intrinsic S/D-ext doping abruptness instead of using heavy halo I/I dose. In fact, the halo I/I dose is reduced by 30% in the new approach as compared to the reference approach. A lower electrical field is hence expected in the p/n junction between p-type S/D-ext and n-type halo region.

Therefore, no degradation in HCI lifetime is observed as clearly illustrated in Fig. 5.18.

5.4 Conclusion

A critical index, Vth_gm - Vth_lin, describing MOSFET on-off transition characteristics is presented and experimentally discussed in this work. Unlike the conventional emphasis on the only deep sub-threshold region, this index is increasingly important to the advanced nano-scale MOSFETs for the low-Vdd and low-Vth operation because this on-off transition region comprises an increasing portion of gate voltage consumed before turn-on when both Vdd

and Vth are scaled down. A novel USJ formation technique, featuring fast PAI using light I/I species at a high dose-rate, is introduced in this work to successfully reduce the gap between Vth_gm and Vth_lin by creating abrupt S/D-ext doping profile and reducing halo I/I dose. The improvement in gate voltage overdrive and carrier mobility leads to 10% higher driving force.

Finally, low Vdd-sensitivities in both DC performance and circuit speed are successfully demonstrated.

The fundamental concept of this novel technique is applicable for both n-type and p-type USJ formation although it was utilized for boron S/D-ext in this work. We believe this concept can be easily applied to phosphorus

S/D-ext. Arsenic has been used as the major n-type S/D-ext doping species because of its lower diffusivity as compared to phosphorus. However, the phosphorus doping activation level is about twice as compared to that of arsenic. Therefore, phosphorus S/D-ext is highly promising as long as its doping abruptness can be as good as arsenic. The results in this work should be very important to the investigation of using phosphorus as S/D-ext.

Table 5.1 A summary table of key DC and AC parameters. New approach (Abrupt S/D-ext + low halo I/I dose) gives a better short channel control and analog performance without degrading S/D series resistance.

PMOS key parameters