Showing papers in "Materials Science & Engineering R-reports in 2002"

Diamond-like amorphous carbon

Abstract: Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

Plasma-surface modification of biomaterials

Abstract: Plasma-surface modification (PSM) is an effective and economical surface treatment technique for many materials and of growing interests in biomedical engineering This article reviews the various common plasma techniques and experimental methods as applied to biomedical materials research, such as plasma sputtering and etching, plasma implantation, plasma deposition, plasma polymerization, laser plasma deposition, plasma spraying, and so on The unique advantage of plasma modification is that the surface properties and biocompatibility can be enhanced selectively while the bulk attributes of the materials remain unchanged Existing materials can, thus, be used and needs for new classes of materials may be obviated thereby shortening the time to develop novel and better biomedical devices Recent work has spurred a number of very interesting applications in the biomedical field This review article concentrates upon the current status of these techniques, new applications, and achievements pertaining to biomedical materials research Examples described include hard tissue replacements, blood contacting prostheses, ophthalmic devices, and other products

Six cases of reliability study of Pb-free solder joints in electronic packaging technology

Abstract: Solder is widely used to connect chips to their packaging substrates in flip chip technology as well as in surface mount technology. At present, the electronic packaging industry is actively searching for Pb-free solders due to environmental concern of Pb-based solders. Concerning the reliability of Pb-free solders, some electronic companies are reluctant to adopt them into their high-end products. Hence, a review of the reliability behavior of Pb-free solders is timely. We use the format of “case study” to review six reliability problems of Pb-free solders in electronic packaging technology. We conducted analysis of these cases on the basis of thermodynamic driving force, time-dependent kinetic processes, and morphology and microstructure changes. We made a direct comparison to the similar problem in SnPb solder whenever it is available. Specifically, we reviewed: (1) interfacial reactions between Pb-free solder and thick metalliztion of bond-pad on the substrate-side, (2) interfacial reactions between Pb-free solder and thin-film under-bump metallization on the chip-side, (3) the growth of a layered intermetallic compound (IMC) by ripening in solid state aging of solder joints, (4) a long range interaction between chip-side and substrate-side metallizations across a solder joint, (5) electromigration in flip chip solder joints, and finally (6) Sn whisker growth on Pb-free finish on Cu leadframe. Perhaps, these cases may serve as helpful references to the understanding of other reliability behaviors of Pb-free solders.

Recent progress of molecular organic electroluminescent materials and devices

Abstract: Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the device construction and operating principles, a review is presented on recent progress in organic electroluminescent materials and devices. Small molecular materials are described with emphasis on their material issues pertaining to charge transport, color, and luminance efficiencies. The chemical nature of electrode/organic interfaces and its impact on device performance are then discussed. Particular attention is paid to recent advances in interface engineering that is of paramount importance to modify the chemical and electronic structure of the interface. The topics in this report also include recent development on the enhancement of electron transport capability in organic materials by doping and the increase in luminance efficiency by utilizing electrophosphorescent materials. Of particular interest for the subject of this review are device reliability and its relationship with material characteristics and interface structures. Important issues relating to display fabrication and the status of display development are briefly addressed as well.

Substrates for gallium nitride epitaxy

Abstract: In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Formation and application of porous silicon

Abstract: All manifestations of pores in silicon are reviewed and discussed with respect to possible applications. Particular emphasis is put on macropores, which are classified in detail and reviewed in the context of pore formation models. Applications of macro-, meso-, and micropores are discussed separately together with some consideration of specific experimental topics. A brief discussion of a stochastic model of Si electrochemistry that was found useful in guiding experimental design for specific pore formation concludes the paper.

Contact mechanics and the adhesion of soft solids

Abstract: The contact behavior of solid materials is determined by the system geometry, externally applied loads and internal adhesive forces, and by the materials properties. As the size and stiffness of a material decrease, adhesive forces become increasingly important in determining its contact behavior. The current trends toward the use of smaller features in many areas of technology, and the importance of ‘soft’ materials including biological tissues, elastomers and polymer gels, demand that the effects of adhesion on contact mechanics be properly understood. The focus of this review is on the mechanics of contact experiments that are commonly used to characterize adhesion, and on the materials science of adhesion of elastic and viscoelastic materials.

Measuring mechanical properties of coatings: a methodology applied to nano-particle-filled sol–gel coatings on glass

Abstract: The main aim of this paper is to demonstrate the practical use of nano-indentation and scratch testing in determining mechanical properties of thin coatings. We place our emphasis on how information obtained using both techniques can be combined to give a more complete representation of the properties of a coating–substrate system. Part I of the paper gives an overview of approaches to determine mechanical properties of thin coatings that have been proposed in the literature, and develops them further to be useful tools in the analysis of coatings. This results in methods for measuring the mechanical properties of thin coatings. We particularly emphasise the determination of the elastic modulus, hardness, coating and interfacial fracture toughness and residual stress using indentation and scratch testing. Part II of the paper illustrates the application of these methods to relatively soft coatings of methyltrimethoxysilane (MTMS) filled with colloidal silica or alumina particles on glass. The coatings were prepared using a sol–gel process. We report results of the dependence of the mechanical properties on the filler particle content, illustrating that microstructural changes can also be tracked using these techniques. The effects of the nature and volume fraction of the filler particles are discussed.

The metal-organic chemical vapor deposition and properties of III–V antimony-based semiconductor materials

Abstract: This article comprehensively reviews the growth of III–V antimony-based semiconductor materials using metal-organic chemical vapor deposition (MOCVD). It does this by first discussing the general trends found for the growth of these materials. Next the specific growth techniques are discussed for each of the antimony-based systems including the binaries InSb, GaSb, and AlSb. The growth techniques used for many of the ternaries and quaternaries of these materials are also discussed. Following this a brief description of the use of dopants, novel organometallic sources and superlattices is presented. Next, the use of common characterization techniques is presented for different types of materials. A variety of the types of devices is then presented followed by a short summary and forecast of future directions that are currently being pursued in these materials.

Contact bonding, including direct-bonding in a historical and recent context of materials science and technology, physics and chemistry historical review in a broader scope and comparative outlook

Abstract: Bonding is a subject matter, which on the one hand is at least as old as written history, and on the other hand is as modern as ultrahigh-vacuum (UHV) technology. In this paper, we present the main threads of its historical evolution and modern evaluation. Bonding has always been a high-tech technology, which used to be governed by an ‘object in view,’ and nowadays is governed by the ‘state-of-the-art.’ Direct-bonding, i.e. the glueless joining of two solid bodies, is more or less embodied in what we have called ‘contact bonding,’ i.e. a large variety of bonding and annealing techniques. Reasonably weak van der Waals attractions are transferred into strong chemical bonds by annealing. Sir Isaac Newton was the first to see direct-bonding, as testified by his famous central black spot surrounded by ‘Newton rings,’ established between an optical contact of a flat and a convex optical surface. Before World War II, direct-bonding was mainly applied in classical optical instruments (such as interferometers); after World War II it was primarily applied in semiconductor technology, optoelectronics, micromechanics and microelectromechanics. This leads to the need for the thinning of one of the wafers for appropriate applications, such as silicon-on-insulator (SOI). More recently, direct-bonding has been investigated for a large variety of materials, thus leading to significant upgrades in terms of flatness, smoothness and cleanliness. A polishing strategy is one consequence of this, which we will deal with in some detail. During the last decade of the 20th century, great progress was made in UHV-bonding, a technology comparable to lateral solid-phase epitaxial growth (SPEG). Bonding and crystal growing have, therefore, become united disciplines. Wafer thinning now has a new impact, for example, by dedicated ion implantation and low-temperature annealing, called ‘smart-cut.’ A great deal of effort has been exerted to master lattice mismatch in the form of dislocations, i.e. compliant layers. The outlook of these technologies is promising, to say the least, and might one day surpass the physical limits of those of bulk monocrystalline materials such as silicon. All these subject matters are treated step-by-step in this paper. We take a phenomenological approach, sometimes alone or in combination with other disciplines, but not specifically application-directed. The paper covers pragmatic issues and also treats know-how.

Structure–property relationships in metallocene polyethylenes

Abstract: The development of new polyolefins based on metallocene technologies represent a considerable advance in the performance of polyethylenes available for a wide range of applications. The ability to obtain a homogeneous short chain branching distribution and control molecular weight is remarkable. The added ability to introduce long chain branching (LCB) into what would otherwise be linear polyethylenes has opened up a broad range of processing possibilities. Through a review of this technology, it becomes apparent that significant elements of the fundamentals underlying the materials science of these new polyolefins are in need of clarification. The morphology of metallocene polyethylenes, particularly in the low-crystallinity range, has yet to be conclusively resolved. The occurrence of multiple melting endotherms for ethylene/α-olefin copolymers with, ostensibly, a homogeneous branching distribution is in need of explanation. LCB incorporated into the linear copolymers enhances shear thinning and melt elasticity. Yet to be fully determined, however, are the types of long chain branch architectures that most effectively promote these desirable rheological attributes, and to what degree these types are present in metallocene polyolefins.

Application of ion scattering techniques to characterize polymer surfaces and interfaces

Abstract: Ion beam analysis techniques, particularly elastic recoil detection (ERD) also known as forward recoil spectrometry (Frcs) has proven to be a value tool to investigate polymer surfaces and interfaces. A review of ERD, related techniques and their impact on the field of polymer science is presented. The physics of the technique is described as well as the underlying principles of the interaction of ions with matter. Methods for optimization of ERD for polymer systems are also introduced, specifically techniques to improve the depth resolution and sensitivity. Details of the experimental setup and requirements are also laid out. After a discussion of ERD, strategies for the subsequent data analysis are described. The review ends with the breakthroughs in polymer science that ERD enabled in polymer diffusion, surfaces, interfaces, critical phenomena, and polymer modification.

Optical conductivity of oxides

Abstract: Optical conductivity spectra of conducting oxides, cuprates, including high-Tc superconductors, nickelates, manganites, doped titanates, barium–lead–bismuth oxides, barium–potassium–bismuth–lead oxides, oxides showing metal–insulator phase transitions, and conducting polymers for the sake of comparison, are reviewed in the spectral range from the milli-electron-volt up to a few electron-volts. The optical conductivity profiles evolve from Drude-like to incoherent scattering and even pseudo-gap-like, depending on doping and temperature. When Drude or Drude-like models do not apply, the screening of phonons by charge carrier is generally incomplete and the polaron concept is considered. Spectra are fairly well modeled within a general approach compatible with the Drude framework and extended by factorizing over all complex poles and zeroes of the dielectric response, allowing discrimination of trapped and mobile polarons. Other models are reviewed: extended Drude with either frequency-dependent relaxation time or non-persistent electron velocity, marginal Fermi liquid, Luttinger liquid, localization, Anderson disordered metal, and those implying opening of gaps or pseudo-gaps in the conductivity. The signatures of stripes resulting from ordering of charges and spins—commensurate or incommensurate with the period of the crystal lattice—observed in certain nickelates, cuprates and manganites are discussed.

Silicide-induced stress in Si: origin and consequences for MOS technologies

Abstract: Metal/silicides have been introduced in CMOS technology, some years ago, the main goal being to reduce the sheet resistance of highly doped regions. Research and development has been focussing mainly on minimizing specific resistivity and controlling Si consumption during thin film formation in confined areas. This has lead to a shift from TiSi 2 as the most commonly used silicide to CoSi 2 , which is now the silicide of record in the advanced technologies. Others such as NiSi or silicidation technologies based on the reaction of metal alloys are under investigation. In this review, the silicide technology is approached from a completely different viewing angle. The local growth of metal/silicides gives rise to high stress levels in the neighboring Si. The characterization of silicide-induced stress in the Si due to dense silicide structures is reported based on a systematic study with micro-Raman spectroscopy (μRS), complemented with finite element simulations. The formation of defects in the Si attributed to the stress is demonstrated with transmission electron microscopy (TEM). The effect of the silicide-induced stress is studied through the electrical performance of diodes and transistors. It has been observed that the stress introduced in the Si, adjacent and underneath the silicided regions, impacts the electrical performance of the devices. In this paper, it is explained how the electrical performance is related on the one hand, to the silicide materials properties and on the other hand, to the structure of the transistor, it is implemented in. It is clear that the control of the stress in Si devices, in general, is an important issue for transistor performance and is significantly influenced by the presence of silicided regions.

Progress in the room-temperature optical functions of semiconductors

Abstract: Optical functions of a specific semiconductor in a particular wavelength region are often needed in optics and optoelectronics research. Basic optical properties of some materials are available in handbooks, but recent advances in the experimental techniques for growth, sample preparation and determination of optical functions, as well as theoretical advances in modeling the optical functions, prompted demand for a review of the state of the art of the optical functions of some important semiconductor materials. This paper gives an overview of the experimental methods for the determination of optical functions in the 1–6 eV spectral range and the available models for calculating the optical functions. Also, the paper summarizes the progress in the study of optical functions of several important semiconductors such as GaP, InP, InAs, GaSb, InSb, AlSb, AlxGa1−xAs, HgxCd1−xTe, SixGe1−x, GaN, InN, AlN, 6H–SiC, and ZnO. Besides discussing the available data and expected positions of optical transitions, the paper focuses on the model of the dielectric function which allows reproducing accurately the experimental dielectric function values for all the materials considered here.