Gate-all-around silicon nanowires for hybrid single electron transistor/CMOS applications

Scaling of semiconductor devices has pushed CMOS devices close to fundamental limits. The remarkable success story of Moore's law during the last 40 years, predicting the evolution of electronic device performances related to miniaturization, has always been respected. However, electron device challenges are now much more complex. In order to keep Moore's law living, device scaling-down is not enough. So called material and geometry technology boosters have been introduced. High-k dielectrics, silicon-on-insulator substrates or strained silicon are some booster examples, used at sub-micron scale. This work proposes an investigation of a hybrid CMOS/SET technology, based on gate-all-around silicon nanowires. It is shown that a silicon nanowire can serve two device purposes: first as innovative 3D metal-oxide-semiconductor field effect transistor (MOSFET), and second as single electron transistor (SET). The SET is a solid-state electronic device, which controls the transport of a unique or a few electrons. SET functions are not limited by its nano-scaled structure. The smaller is the better. SET consists of a small conductive quantum dot, called island, connected to two reservoirs acting as drain and source by tunnel junctions. The size of the island should be as small as possible – typical values ranging from 1 to 4nm – in order to have room temperature operation. Electron transport from source to island, and from island to drain is controlled by a capacitively coupled gate. The nano-scale size of the island makes the carrier electrostatic repulsion efficient. This effect is called Coulomb Blockade. SET advantages are an ultra-reduced size and a very low power consumption, while SET challenges are variability related to dimension control and background charge effect, room temperature operation and a reduced fan-out. The first chapter of this thesis introduces ideas of up-to-date MOS and SET devices and shows how to combine MOS and SET to obtain original electronic functions. The second chapter is a discussion of the nanowire as a technology platform for the integration of SET devices and also presents TCAD simulation results. Key contributions are reported in chapter three. This is the original and complete description of the different top-down processes used for the integration of a gate-all-around MOS/SET platform. Samples integrated at the EPFL Center of Microtechnology (CMI) are presented. Different approaches have been studied and used in order to overcome lithographic limitations, and to have a fast and reliable integration at moderate cost. This includes auto-aligned techniques, focused ion beam prototyping, and top-down local-SOI and true-SOI nanowires. The local-SOI technique based on silicon nano-channel wires has given the best results and offers a lot of flexibility in term of wire cross-sections and shapes. Gate-all-around silicon nanowires, obtained by sacrificial etching and self-limited oxidation, with a circular 5nm diameter cross-section have been characterized. Chapter four is the validation by measurements of both SET and MOS devices. Excellent room temperature characteristics have been observed in MOS structures, while both ID-VG Coulomb oscillations and ID-VD Coulomb gap are observed on smaller structures at cryogenic temperature (T<20K). The process described has many advantages in comparison with bottom-up grown wires, such an excellent crystallinity of the channel and ohmic source-drain contacts. Finite elements simulations have also highlighted the influence of carrier mobility, channel length and cross-section shape in our defined nanowire structures. The last chapter of this report concludes this work and gives perspectives on the near future. The hybrid combination of silicon nanowire MOSFET and SET can definitely be an appealing approach in order to bridge the gap between emerging nanoelectronic devices and more traditional CMOS.


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