Top-Down Fabrication of Gate-All-Around Vertically-Stacked Silicon Nanowire FETs with Controllable Polarity

Asthe currentMOSFET scaling trend is facing strong limitations, technologies exploiting novel degrees of freedom at physical and architecture level are promising candidates to enable the continuation of Moore's predictions. In this paper, we report on the fabrication of novel ambipolar Silicon nanowire (SiNW) Schottky-barrier (SB) FET transistors featuring two independent gate-all-around electrodes and vertically stacked SiNW channels. A top-down approach was employed for the nanowire fabrication, using an e-beam lithography defined design pattern. In these transistors, one gate electrode enables the dynamic configuration of the device polarity (n- or p-type) by electrostatic doping of the channel in proximity of the source and drain SBs. The other gate electrode, acting on the center region of the channel switches ON or OFF the device. Measurement results on silicon show I-on/I-off > 10(6) and subthreshold slopes approaching the thermal limit, SS approximate to 64 mV/dec (70 mV/dec) for p(n)-type operation in the same physical device. Finally, we show that the XOR logic operation is embedded in the device characteristic, and we demonstrate for the first time a fully functional two-transistor XOR gate.

Multiple-Independent-Gate Field-Effect Transistors for High Computational Density and Low Power Consumption

Jian Zhang

Transistors are the fundamental elements in Integrated Circuits (IC). The development of transistors significantly improves the circuit performance. Numerous technology innovations have been adopted to maintain the continuous scaling down of transistors. With all these innovations and efforts, the transistor size is approaching the natural limitations of materials in the near future. The circuits are expected to compute in a more efficient way. From this perspective, new device concepts are desirable to exploit additional functionality. On the other hand, with the continuously increased device density on the chips, reducing the power consumption has become a key concern in IC design. To overcome the limitations of Complementary Metal-Oxide-Semiconductor (CMOS) technology in computing efficiency and power reduction, this thesis introduces the multiple- independent-gate Field-Effect Transistors (FETs) with silicon nanowires and FinFET structures. The device not only has the capability of polarity control, but also provides dual-threshold- voltage and steep-subthreshold-slope operations for power reduction in circuit design. By independently modulating the Schottky junctions between metallic source/drain and semiconductor channel, the dual-threshold-voltage characteristics with controllable polarity are achieved in a single device. This property is demonstrated in both experiments and simulations. Thanks to the compact implementation of logic functions, circuit-level benchmarking shows promising performance with a configurable dual-threshold-voltage physical design, which is suitable for low-power applications. This thesis also experimentally demonstrates the steep-subthreshold-slope operation in the multiple-independent-gate FETs. Based on a positive feedback induced by weak impact ionization, the measured characteristics of the device achieve a steep subthreshold slope of 6 mV/dec over 5 decades of current. High Ion/Ioff ratio and low leakage current are also simultaneously obtained with a good reliability. Based on a physical analysis of the device operation, feasible improvements are suggested to further enhance the performance. A physics-based surface potential and drain current model is also derived for the polarity-controllable Silicon Nanowire FETs (SiNWFETs). By solving the carrier transport at Schottky junctions and in the channel, the core model captures the operation with independent gate control. It can serve as the core framework for developing a complete compact model by integrating advanced physical effects. To summarize, multiple-independent-gate SiNWFETs and FinFETs are extensively studied in terms of fabrication, modeling, and simulation. The proposed device concept expands the family of polarity-controllable FETs. In addition to the enhanced logic functionality, the polarity-controllable SiNWFETs and FinFETs with the dual-threshold-voltage and steep-subthreshold-slope operation can be promising candidates for future IC design towards low-power applications.

EPFL2016

Fabrication and Characterization of Gate-All-Around Silicon Nanowires on Bulk Silicon

Didier Bouvet, Luca De Michielis, Mihai Adrian Ionescu, Kirsten Emilie Moselund, Vincent Pott

This paper reports on the top-down fabrication and electrical performance of silicon nanowire (SiNW) gate-all-around (GAA) n-type and p-type MOSFET devices integrated on bulk silicon using a local-silicon-on-insulator (SOI) process. The proposed local-SOI fabrication provides various nanowire cross sections: Omega-like, pentagonal, triangular, and circular, all controlled by isotropic etching using nitride spacers and silicon sacrificial oxidation. The reported top-down SiNW fabrication offers excellent control of wire doping and placement, as well as ohmic source and drain contacts. A particular feature of the process is the buildup of a tensile strain in all suspended nanowires, attaining values of few percents, reflected in stress values higher than 2-3 GPa. A very high yield (>90%) is obtained in terms of functionality of long-channel SiNW GAA mosfet. Device characteristics are reported from cryogenic temperature (T = 5 K) up to 150 degC, and promising characteristics in terms of low-field electron mobility, threshold voltage control, and subthreshold slope are demonstrated. Low field mobility for electrons up to 850 cm2 /Vmiddots is reported at room temperature in suspended devices with triangular cross sections; this mobility enhancement is explained by the process-induced tensile strain. In short, suspended SiNW GAA with small triangular cross sections, a single-electron transistor (SET) operation regime is highlighted at T = 5 K. This is attributed to a combined effect of strain and corner conduction in triangular channel cross sections, suggesting the possibility to hybridize CMOS and SET functions by a unique nanowire fabrication platform.

2008

Multiple-Independent-Gate Field-Effect Transistors for High Computational Density and Low Power Consumption

Jian Zhang

EPFL2016