ABSTRACT
The science and technology of processing thin film silicides have been continuesly studied to understand their properties and solve technological problems.12-28 The more we understand the silicide formation, the better we can monitor the semiconductor
device performance.29-37 In the history of the evolution of semiconductor technology, dimensional scaling is always a crucial step in every device generation. Moore’s Law predicts that the numbers of transistors being placed in the integrated circuits will double every two years, accompanying the shrinkage of transistor sizes. As the trend approaches the end of the semiconductor roadmap for microelectronic VLSI technology based on Si fieldeffect transistors, device engineers and scientists have been seeking new methods and materials for nanoscale transistors fabrication in recent years. For this reason, the properties and synthesis of metal silicide in nanoscale has become a field of intensive interests.38-66 Silicon nanowire is a potential material to follow the device-scaling owing to their morphology, size, and electrical properties, which are potentially suitable for electronics assembling.67-68 Nanostructures in Si nanowires have been studied for basic components in electronic and optoelectronic devices, especially biosensors, and hence electrical contacts to Si nanowires are issues of both scientific and technological interests.69-72 The nucleation and growth of epitaxial silicide in Si nanowires is basic to a better understanding of the kinetics of silicide formation and essential to a better control of the building blocks in the future nanoelectronics. In this chapter, the reactions of Ni and Co silicides formation in Si nanowires by point contacts will be proposed. The study of nucleation and growth of silicide formation on Si wafers and in Si nanowires will be discussed; where homogeneous and heterogeneous nucleation in nanoscale silicide formation will be compared in nanoscale silicide formation. 5.1.2 Introduction to Contacts in Nanoscale Electronics
Silicide films are employed in integrated semiconductor devices as part of the contact that joins an interconnecting line to the Si substrate in a contact window and for interconnections. In the first application, the electric current flows mainly perpendicular to the silicide layer and across the silicide-Si interface. So the electrical characteristics of the contact between silicide and Si are of primary interest, especially parameters such as barrier heights, ideality factor and contact resistivity. These parameters are highly related to the morphology and microstructures of the contact interface. People would like to adjust these parameters and control
their reproducibility and stability particular for rectifying contacts because their properties enter directly into the final performance of a device. In the second application, electric current flows mainly along the plane of the silicide thin film. So the parameters of interests now are carrier concentrations, motilities and electromigration. A concern for the behavior of silicide layers under high-temperature annealing is impurities. The reasons silicides are of interests as contact materials and as interconnection from VLSI is not only their good electrical conduction but also the ability of many transition-metal films to form a uniform silicide layer on a Si substrate by a solid-phase-reaction. The reaction temperatures are low so that the silicide contacting process can be applied without affecting preceding processing step. Moreover, silicides also offer a choice in electronic barrier heights to Si. Interest in silicides for interconnection lines arose with the needs to reduce the resistivity of interconnections as the line widths were reduced with the scaling down the device size. Most silicides are good electrical conductors and can withstand the high temperature in a conventional self-aligned technology, which refractory metals cannot. Furthermore, silicides have small grain sizes which are desirable for good line definition upon etching. Finally, they are compatible with poly-Si gate technology and thus adapt readily to existing process technology. 5.1.2.2 One-dimensional nanostructures
Nanostructures are defined as structures with at least one dimension between 1 to 100 nm. One-dimensional (1-D) semiconductor nanostructures are of particular interest because of their potential applications in nanoscale electronic and optoelectronic devices, especially bio-sensors. However, quantum effects play an increasingly prominent role while dimension shrinking to nanoscale.73 Nanowires have demonstrated interesting electrical transport properties which are different from bulk materials and thin films. This is because, in nanowires, electrons could be quantum-confined laterally and thus could occupy discrete energy levels that are different from the energy bands found in bulk materials and thin films.74 Due to low electron density and low effective mass, the quantized conductivity is more easily observed in semiconductors, for example, Si and GaAs, than in metals.75 In order to understand more about the new
physics demonstrated by nanowires, much effort has been dedicated to fabricating high-quality semiconductor nanowires because of the importance role of semiconductor materials to the electronics industry. In particular, the 1-D nanomaterials with perfect atomic structures and their assembly into functional devices are the major challenges and goals for nanomaterials studies. Nanowire based sensing devices can be configured from highperformance field-effect transistors (FETs) by linking specific receptor groups to the surface of the nanowire. A macromolecule species such as a protein, can specifically bind to the receptor when the devices exposed to a solution, as shown in Fig.5.1a41 This will affect device conductance depending on the net charge of the biomolecule and the semiconductor type (p or n). Different nanowire elements with distinct surface receptors opens up the potential for multiplexed, real-time sensors, such as the simultaneous detection of proteins, DNA, viruses, and small molecules, as shown in Fig. 5.1b.41
