Volume 101, Issue 2

Scaling of complementary metal oxide semiconductor devices is critical to enhancing performance and reducing the production cost of transistors. Conventional gate stack film systems consisting of a SiO 2 dielectric layer between the Si substrate channel and a doped polycrystalline silicon (poly-Si) gate electrode exhibited excessively high gate current leakage when the physical thickness of this traditional dielectric was scaled to T phys  = ∼2 nm. The rate of scaling was initially preserved by incorporating nitrogen to form an SiO x N y insulator layer; however, this material soon experienced unacceptable levels of direct tunneling leakage current, which launched an industry-wide investigation of potential high dielectric constant (high-k) metal oxides as replacement materials for the SiO 2 based gate dielectric layer. Thermal stability requirements for the introduction of high-k dielectric materials necessitated the simultaneous replacement of poly-Si with a metal gate electrode due to several performance factors including unscalable threshold voltage. Although high-k/metal gate thermal stability has been demonstrated, significant challenges remain to be resolved for future technology nodes. This paper reviews the progress and challenges associated with the introduction of high-k/metal gate transistors, including threshold voltage tuning and gate dielectric thickness scaling, from a materials and process integration perspective.

Metallic hybrid structures comprizing superconducting and ferromagnetic materials are interesting objects for studying the mutual influence between superconductivity and ferromagnetic order. We focus on two particular systems that have been studied during the last few years. In superconducting spin-valves the transition temperature of a superconductor squeezed between two ferromagnetic layers depends on the relative orientation of the two magnetization directions. Results have been obtained for systems with in-plane and out-of-plane magnetizations. Furthermore, we report on the electronic transport through superconductor/ferromagnet nanocontacts that allow a determination of the current spin-polarization which is related to the bulk spin polarization.

The unusual electronic properties of graphene make it a prime candidate material for extending nanoelectronics and designing new types of switches. Graphene's unusual properties are a result of the unusual band structure associated with the hexagonal bonding pattern and the electron/hole transport through the pi orbitals. Graphene samples are frequently more than one layer, or few-layer graphene, and the change in electronic properties with each layer depends on the stacking configuration and the rotational misorientation between the layers. Transport measurements of single layer graphene (SLG) show that graphene exhibits the quantum Hall effect. In addition, Berry Phase corrections to carrier transport measurements are now widely recognized. Because graphene is a single layer of carbon atoms, it is difficult to find, manipulate, and measure. We review the status of physical and electrical characterization of graphene and discuss the remaining challenges. We discuss results from optical microscopy, transmission electron microscopy, low energy electron microscopy, nano-Raman, and several scanned probe methods. Issues such as determination of the number of layers of graphene and rotational stacking misorientation are emphasized.

This overview describes the present understanding of resistive switching phenomena encountered in chalcogenide-based cells which may be utilized in energy-efficient non-volatile memory devices and in array-based logic applications. We introduce the basic operation principle of the phase change mechanism, the thermochemical mechanism, and the valence change mechanism and we discuss the crucial role of structural defects in the switching processes. We show how this role is determined by the atomic structure of the defects, the electronic defect states, and/or the ion transport properties of the defects. The electronic structure of the systems in different resistance states is described in the light of the chemical bonds involved. While for phase change alloys the interplay of ionicity and hybridization in the crystalline and in the amorphous phase determine the resistances, the local redox reaction at the site of extended defects, the change in the oxygen stoichiometry, and the resulting change in the occupancy of relevant orbitals play the major role in transition metal oxides which switch by the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase change alloys but also for redox-related switching processes. The switching kinetics as well as the ultimate scalability of switching cells are related to structural defects in the materials.

Optical Data Storage has become the mainstream technology for distributing audio, video and computer software content as well as recording and archiving personal data during the past two and a half decades. The continuous demand for ever higher storage capacities and faster data transfer rates has led to the development of three disc format families Compact Discs, Digital Versatile Discs and Blu-ray Disc, which have become established in the market. Research and development for future high density formats exploits the already well established “bit-wise” recording and volume storage approaches like holographic and “page-wise” recording. Each existing and future technology imposes specific requirements on materials that can facilitate functioning and reliable products for the consumer. In this review several aspects of the interrelation between technological requirements and materials are highlighted.

Continuous scaling of Cu interconnect structures can significantly impact reliability-limiting processes such as electromigration and stress-induced voiding. Prior to the 65 nm technology node, mass transport under electromigration is dominated by diffusion along the Cu/dielectric cap interface and the electromigration lifetime will degrade by about half for every generation, even with the same current density. Beyond the 65 nm node, small grains were found to mix with bamboo grains in the 90 nm Cu damascene lines and the contribution of the grain boundary transport degraded the electromigration lifetime. The scaling effects on microstructure and electromigration reliability are examined in this paper with the objective of identifying the key issues and exploring potential solutions for sub-100 nm Cu interconnects. We discuss first the scaling effect on electromigration lifetime and the effect due to the small grains on electromigration lifetime and statistics. This is followed by a discussion on grain growth studies focusing on the formation of small grains and a recent simulation study on small grain effect on electromigration reliability. This paper concludes with a discussion of some recent developments in analytical techniques to investigate grain structure and electromigration reliability in sub-100 nm Cu lines.

Structural characterization techniques, including solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopies, were used in conjunction with mechanical testing to study the effects of thermally activated electron bombardment curing on organosilicate thin films. The electron beam process produced significant improvements in elastic modulus and fracture resistance while still preserving low dielectric constant. Detailed and quantitative analysis was used to elucidate fundamental curing effects on glass structure, including changes in film composition and local bond rearrangements. Enhancements in fracture properties with curing are shown to be due to increased network bond density resulting from changes in network connectivity coupled with moderate film densification.

Printing technology offers the potential for the rapid and inexpensive, large scale manufacture of electronic devices, especially organic and hybrid organic/inorganic devices. A variety of printing techniques, including gravure, flexography and ink-jet printing have been utilized for the fabrication of electronic devices. An overview is given of the types of printing used, the advantages of each technique, the materials requirements for successful printing, and the types of materials, including organic polymers and inorganic nanoparticles, that can be printed. Examples are given of how printing can be used to successfully print the active and inactive layers, including electrodes and dielectrics, of devices such as LEDs, solar cells and transistors, and the relative performance of devices made by different techniques is compared. The limitations and advantages of current technologies are discussed.

Transformations undergone by lead – calcium alloys are numerous and have different kinetics from a few minutes to a few years. Anisothermal calorimetric measurements were performed to be able to identify these transformations quickly. It was then possible to identify five transformations. Complementary measurements have enabled us to define transformations with an in-situ electrical resistance measurement to follow the evolution of the calcium in solid solution and with an in-situ hardness measurement to characterise the mechanical properties. The aim of these results is to simulate the ageing and overageing of the alloy in isothermal conditions.

Thermodynamic prediction based on a two-stage simulation has been carried out to investigate the composition range of Mg–Al–Ca alloys amenable to semi-solid forming. The results show that increasing the solute elements decreases the temperature sensitivity, but their effects on the solidification range are greatly different. In some Mg–Al–Ca alloys, the highest knee point in the liquid fraction vs. temperature curve is generated by the formation of Al 2 Ca phase. Several critical sections of the Mg–Al–Ca ternary system are also given. The criteria for semi-solid formability are identified and a range of compositions for Mg–Al–Ca alloys are evaluated in relation to these criteria. By comparing with A356 alloy and AZ91 alloy, a typical Mg–Al–Ca alloy AC74, is predicted to exhibit good semi-solid formability comparable to A356 and much better than AZ91.

The microstructural evolution of an Al-15.8 wt.% Cu slurry held for 80 min at 555 °C has been characterised in-situ by X-ray microtomography. The different parameters measured by segmentation of the 3D images are analysed by reference to a model describing capillary equilibrium in an aggregate of solid particles with uniform solid/liquid interface curvature. Excellent agreement is found between model predictions and experimental data, except for the fact that the solid/liquid interface area predicted by the model is lower by about 12.5 % than that measured experimentally. This suggests that, owing to the progressive grain coarsening, the slurry does not achieve uniform interface curvature. The model makes possible the conversion of experimental data into evolutions of the dihedral angle, average surface tensions ratio, and capillary pressure in the liquid.

Creep behavior of the tin-based lead-free Sn-0.7Cu, Sn-3.8Ag, Sn-9Zn, Sn-5Sb and Sn-2Bi binary alloys together with pure Sn, as the material for comparison, was studied by impression and indentation creep testing at room temperature. Stress exponent values in the range 3.5 to 14.7, obtained by the two methods, were in good agreement with each other and with those determined by room-temperature creep testing of the same materials reported in the literature. All of the tin-based alloys showed creep resistances higher than pure Sn. Among all tested materials, as indicated by their steady-state creep rates, Sn-2Bi showed the highest creep resistance followed by Sn-5Sb, Sn-9Zn, Sn-3.8Ag, Sn-0.7Cu and pure Sn. Different creep behaviors of the materials were discussed in terms of their microstructural features.

A mathematical model to describe secondary creep in a thick composite cylinder made of Al–SiC p and subjected to both internal and external pressures has been developed. The creep behavior of the composite has been described by a threshold stress based creep law with a stress exponent of 5. The model developed has been used to investigate the effect of varying size and content of the SiC p dispersoid on the stresses and strain rates in a composite cylinder. It is observed that the stress distributions in the cylinder do not have significant variation with varying size and content of the SiC p . Unlike stresses, the strain rates in the cylinder are reduced to a significant extent by decreasing the size of SiC p and increasing the content of SiC p .

In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO 2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO 2 -30 % H 2 O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO 2 and Ar–H 2 O. In the CO 2 - and/or H 2 O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H 2 O-containing gases and carburisation in the CO 2 gases. The oxide scales allow molecular transport of CO 2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H 2 O-free gas. This has been discussed in terms of the potential for H 2 O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO 2 –H 2 O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.

The effects of homogenization treatment and high-temperature extrusion on the hot working behavior of Mg-3Sn-1Ca alloy have been studied with a view to find an optimum processing route for the manufacture of wrought products. Processing maps and kinetic parameters have been obtained from the flow stress data recorded in hot compression experiments in the temperature range of 300 – 550 °C and strain rate range of 0.0003 – 10 s – 1 . These reveal that the homogenization treatment has only a marginal effect on the hot workability, the result attributable to the high thermal stability of CaMgSn particles in the microstructure. High temperature extrusion, however, lowers the hot working temperature significantly (by about 150 °C) and this has been attributed to grain refinement during primary processing. The mechanisms of hot deformation remained unchanged by the above treatments and the apparent activation energies for hot deformation are higher than those for self-diffusion suggesting that CaMgSn particles in the matrix cause large back-stress.

In the present paper, the structure and properties of a Al–Cr–Fe–Ti alloy produced by powder metallurgy are described. An initial rapidly solidified powder alloy was prepared by the pressure nitrogen melt atomisation. Subsequently, the granulometric powder fraction of less than 45 μm was hot-extruded. According to X-ray diffraction and transmission electron microscopy observation, the compact material is composed of α-Al grains and spheroids of intermetallic phases. The α-Al grains of the compacted material are recrystallized and neither residual stress nor texture were observed. The values of ultimate tensile strength and yield strength of the investigated material were similar to the values for conventional Al-based alloys. Dependences of ultimate tensile strength and yield strength on temperature were also measured. Excellent thermal stability of the powder metallurgy produced Al–Cr-based material was proven by room-temperature hardness measurement after long-term annealing at elevated temperature.

Ni 2 O 3 -modified TiO 2 – x N x visible-light photocatalysts have been prepared by the sol-gel method followed by calcining at 450 °C. The photocatalysts were characterized using scanning electron microscopy, thermogravimetry-differential thermal analysis, X-ray diffraction, Fourier transform infrared and X-ray photoelectron spectroscopy. The visible-light photocatalytic activity of the samples was evaluated by decomposition of methylene blue in air under visible-light irradiation (λ > 420 nm). Results revealed that the Ni 2 O 3 -modified TiO 2 – x N x samples exhibited irregular particle shapes. The crystal phase of Ni 2 O 3 -modified TiO 2 – x N x samples was anatase. Compared with pure TiO 2 , the absorption of the Ni 2 O 3 -modified TiO 2 – x N x samples extends significantly into the visible regions due to the doped N atom in the interstitial sites of the TiO 2 lattice. The visible-light photocatalytic activity of Ni 2 O 3 -modified TiO 2 – x N x samples was improved greatly since the loaded Ni 2 O 3 can facilitate charge transfer.

Thin films of zinc cadmium sulphide – a ternary compound intended to be used in solar cells – are prepared by co-evaporation. The films are prepared at room temperature, in a vacuum better than 10 –5 Torr. The composition of these films is varied from x = 0 to x = 1 in the compound Zn x Cd 1– x S. The structural, optical and electrical properties have been investigated as a function of x. The direct band gap varies from 2.415 eV to 3.41 eV for x = 0 to x = 1 respectively. The crystal structure is found to be hexagonal (Wurtzite) for 0 ≤ x ≤ 0.8 and cubic zinc-blende for x > 0.8. The lattice constants decrease and the optical bandgap increases with the increase in the value of x. The resistivity of the Zn x Cd 1– x S films is found to decrease from 3.6 × 10 4 Ω cm for x = 0 to 69.5 Ω cm for x = 0.3 and again increases beyond this range to a value of 6.3 × 10 3 Ω cm for x = 1.0.