Intrinsic transistor performance (ITP) characteristics show that GAA-NWFETs clearly outperform finFETs when normalizing ION-IOFF per footprint. Furthermore, an evaluation of several doping strategies for both type of devices, using ion implantation (I/I), allowed a comparison of inversion-mode (IM) FETs built with conventional junctions or an extensionless (Extless) scheme [4,5] vs. junctionless (JL) transistors [4,6]. To note that the latter are particularly advantageous in their process simplicity (no junction formation requirements) and compatibility with lower thermal budget flows. Reliability wise, optimized JL and Extless can be very attractive options thanks to a lower oxide field (Eox) at operating conditions. This is indeed confirmed by the GAA-NWFETs in Fig.2. Control of the lateral BOX recess during the fins release process in the GAA flow, and hence of the lateral bottom-gate overlap, is important not only for parasitic reasons but also for reliability purposes. Indeed, TCAD predicts a higher Eox at the bottom-gate edges in case of excessive lateral BOX recess which can lead to degraded BTI behavior for GAA-NWFET vs. finFET. The impact is however considerably less for Extless and JL, increasing the robustness of these devices against process variations in the lateral BOX recess. JL devices were also seen to have improved on and off state hot carrier (HC) reliability behavior as compared to other IM GAA-NWFETs [7]. Interestingly, improved subthreshold slope (SS) values after HC stress were also measured in some JL GAA-NWFETs as a result of the improved electrostatic control and the generation and location of acceptor type of interface traps in the wires of slightly concave sidewalls shape. Low-frequency (LF) noise analysis results suggest to a first order no significant impact of the device architecture on the gate stack integrity in regards to traps/defects. In addition, in agreement with BTI and HC results, JL GAA-NWFETs also show reduced noise. These characteristics, together with the devices smaller IOFF values yielding ring oscillators with substantially lower power dissipation, indicate JL can be an attractive option for low power circuits. Improvements in noise, reliability and mobility performance were also obtained in GAA-NWFETs by introduction of a TiAl-based EWF-metal [4,8], in line with the results previously reported in [9] on finFETs using Al diffusion mechanisms for EWF modulation.
References
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