The evolution of the bicycle provides a good history of structural materials since the 1850s. The first machine that resembled a bicycle was made of a wooden beam with a wooden wagon wheel at each end. This presumably inspired the Hobby Horse, which had a forged iron beam for a frame, along with wooden wheels. The next step was the “boneshaker,” still with wooden wheels, but with the innovation of pedals, which allowed riders to keep their feet off the ground. Major reductions in weight came with the introduction of steel tubing for the frame and tensioned wire-spoke wheels, as in the high-wheelers of the 1880s. These breakthroughs came on the heels of the development of tube drawing and wire drawing in the new steel industry. With the introduction of the Humber “safety bicycle” in 1890, the evolution of the present-day diamond-frame configuration of the bicycle was completed. By this time, hardened steel components were available for the chain, cogs, bearings, and the chain wheel. In the previous year, John Dunlop had patented the pneumatic tire, and this invention took over the market for bicycle tires by 1895. The cotton-fiber-strengthened rubber-matrix tire was probably one of the earliest fabricated composite materials.

The use of brazing to join the cold-drawn steel tubes led to the development of the diamond frame. Lugs held the ends of the tubes in place. The brazing alloys were, and still are, based on Cu-40%Zn and on the Ag-Cu eutectic alloy. Wood and bamboo were tried as substitutes for steel tubes in diamond frames during the past century, but the lack of an available method for joining the pieces together defeated these attempts to save weight. The main materials developments in bicycles over the next 80 or 90 years after the Humber safety were the introduction of low-alloy steels for tubing, primarily the Cr-Mo-0.3%C 4130-type steel, the substitution of aluminum-alloy extrusions for steel tubing for wheel rims after the 1950s, and the use of stainless-steel spokes instead of carbon steel.

“Wafer bonding” refers to the phenomenon in which mirror-polished, flat, and clean wafers of almost any material—when brought into contact at room temperature—are locally attracted to each other by van der Waals forces and adhere or “bond” to each other. Wafer bonding is alternatively also known as “direct bonding” or “fusion bonding,” or more colloquially as “gluing without glue.” Although this is by no means required, in most cases, the wafers involved in actual applications are typical semiconductor wafers consisting of single-crystalline material used in microelectronics or optoelectronics such as silicon or gallium arsenide.

Although electrorheological (ER) fluids are presently being used in some commercial devices, their full potential has yet to be realized. The maximum shear stress (~2 kPa) of commercial ER fluids is not sufficient for most pertinent applications, especially in regard to the automotive industry. However, recent theoretical studies indicate that a shear stress of at least an order of magnitude greater than 2 kPa might be attained by the proper design of the ER suspension. This article reviews the status of the understanding of ER behavior which leads to this possibility.

Most of the research to date on ER fluids has been on suspensions of either inorganic or organic particles in dielectric host oils. Hence, the present review will deal mainly with such “heterogeneous” ER fluids. However, in recent years interesting results have been reported for all-liquid ER fluids, especially those containing liquid crystals. These “homogeneous” ER fluids are discussed in the article by Inoue et al. in this issue.

The need for integrated optical interconnects in electronic systems is derivedfrom the cost and performance of electronic systems. If we examine the cost of all interconnects, it becomes apparent that there is an exponential growth in cost per interconnect with the length of the interconnect. A remarkable feature of interconnect cost is that the exponential relation holds over all length scales—from the shortest interconnects on a chip to the longest interconnects in global telecommunications networks. Longer interconnects are drastically more expensive, and these costs are ultimately related to the labor cost associated with each interconnect. Given this economic pressure, it is not surprising that there is a driving force to condense more functions locally on the same chip, board, or system. In condensing these functions, the number of long interconnects are decreased and the overall cost of the electronic system decreases dramatically. A specific glaring example of this driving force is Si complementary-metal-oxide-semiconductor (CMOS) technology, especially the case of microprocessors. In the Si microprocessor case, the flood gates to interconnect condensation were opened and the miraculous trend of lower cost for exponentially increasing performance was revealed.

Over the past several years, the number of materials routinely employed for the fabrication of major bicycle components (e.g., frame, fork, and handlebar) has increased from one (steel) to at least five (steel, aluminum, titanium, fiber-reinforced polymers, and magnesium). Historically the primary driving force for implementation of new materials in bicycle manufacture has been the almost fanatical desire to reduce the weight of the bicycle. Although weight reduction of the bicycle will continue to be important, an old design paradigm—driven mostly by the recent popularity of bicycles designed to be ridden off-road (mountain bikes)—has re-emerged: the design of bicycles that minimize the amount of road/trail shock transmitted to the rider.

A growing interest exists in developing technologies that use silicon and other electronic materials as mechanical materials. Using standard processes of the integrated-circuit industry, researchers have successfully fabricated miniature mechanical components (micromachines) such as membranes, gears, motors, pumps, and valves. The integration of miniaturized mechanical components with microelectronic components has spawned a new technology known as microelectromechanical systems (MEMS). It promises to extend the benefits of microelectronic fabrication to sensing and actuating functions. Early applications of this technology include the digital mirror display, which has of the order of 106 aluminum thin-film micromirrors fabricated on top of a complementary-metal-oxide-semiconductor static random-access-memory integrated circuit. Other applications include integrated accelerometers for tasks such as air-bag deployment.

A number of fabrication techniques have been developed for this technology and have been reviewed elsewhere. In this review, I focus on surface-micromachining technology and adhesion and friction problems in surface-micromachined polycrystalline silicon (polysilicon) structures, though many of the principles discussed will also apply both to single-crystalline silicon and nonsilicon-based structures. Surface micromachining, defined as the fabrication of micromechanical structures from deposited thin films, is one of the core technological processes underlying MEMS. Surface microstructures have lateral dimensions of 50-500 μm with thicknesses of 0.1–2.5 μm and are offset 0.1–2 μm from the substrate. The basic steps in a surface-micromachining process appear in Figure 1. First the substrate is typically coated with an isolation layer (Figure la) that protects it during subsequent etching steps. A sacrificial layer is then deposited on the substrate and patterned. For simplicity, Figure 1b shows that the opening of the sacrificial layer is terminated on the isolation layer. The microstructural thin film is then deposited and etched (Figure 1c). Finally selective etching of the sacrificial layer creates the freestanding micromechanical structures such as the cantilever beam shown in cross section in Figure 1d. The technique can be extended to make multiple-layer microstructures.

From a commercialization standpoint, electronic applications have been particularly elusive for diamond. Market estimates of $560 million per year by the year 2000 indicate the original enthusiasm in this area. Now such projections seem unreasonably optimistic, and even a niche commercial application in the area of electronics would be considered a success. However when taken in a broader context, this extended time frame for commercialization is not at all unusual for new technologies, and many new advances have continued to bolster the enthusiasm of diamond electronics research groups. Diamond has such an extensive list of exceptional properties that it continues to be a candidate for numerous electronic applications from heat spreaders to detectors to microvacuum tubes.

A variety of theoretical calculations established diamond's potential in the early years of CVD-diamond research. As shown in Table I, figures of merit indicated that diamond's potential far exceeded the potential of more common semiconductors such as silicon and gallium arsenide for certain applications. Typically these applications revolved around high power or high temperature. However more accurate assessments of diamond's capability after further development uncovered several issues with many electronic applications of interest: (1) n-Type doping: Although scattered reports of n-type doping have been made a high-quality, low-resistance n-type material is not available, limiting the potential applications for diamond.

(2) Deep activation of p-type carriers: At reasonable dopant levels, the activation energy for p-type carriers is approximately 0.3 eV, causing diamond to be highly resistive and sensitive to temperature variations at normal operating temperatures.

Thin-film integrated optics is becoming more and more important in optical-communications technology. The fabrication of passive devices such as planar optical waveguides, splitters, and multiplexers is now quite well-developed. Devices based on this technology are now commercially available. One step to further improve this technology is to develop optical amplifiers that can be integrated with these devices. Such amplifiers can compensate for the losses in splitters or other optical components, and can also serve as pre-amplifiers for active devices such as detectors.

In optical-fiber technology, erbium-doped fiber amplifiers, are used in long-distance fiber-communications links. They use an optical transition in Er3+ at a wavelength of 1.54 μm for signal amplification, and their success has set a standard of optical communication at this wavelength. Using the same concept of Er doping, planar-waveguide amplifiers are now being developed. For these devices, silicon is often used as a substrate, so that optoelectronic integration with other devices in or on Si (electrical devices, or Si-based light sources, detectors, and modulators) may become possible. Figure 1 shows an example of a silicon-based optical integrated circuit5 in which a 1 × 4 splitter is combined with an amplifying section.

In a silicon-on-insulator (SOI) structure, a thin layer of monocrystalline silicon rests on a dielectric layer—generally amorphous—itself on a silicon wafer. Because such a structure cannot be achieved by conventional methods, different ways had to be imagined to facilitate its construction.

The basic physics phenomenon that led the author to invent the process generally known under the name of Smart-Cut® is blistering. Blistering (Figure 1), in addition to flaking and exfoliation, is a visible macroscopic effect that has been known for a long time and is induced by high-dose implantations of inert gas or hydrogen ions in materials. These macroscopic effects result from the cooperative result of the microscopic effects induced in depth by penetration of particles. The microscopic effects of hydrogen or rare-gas implantation such as creation of microcavities, microblisters, or microbubbles (close to the penetration depth Rp corresponding to the maximum concentration) have been known for a long time. These microcavities enhance propagation of intercavity fractures where their density (depending on statistical fluctuations) reaches a percolation threshold. This leads to formation of a local cluster where all the microcavities are joined by a fractured zone, resulting in a blister at the surface. The driving force of this mechanism is the gas pressure in the microcavities and the stresses in the layer.

Recent advances in nanofabrication technologies have enabled us to fabricate semiconductor quantum dots in which electrons are three-dimensionally confined. These quantum dots are often referred to as artificial atoms since their electronic properties—for example the ionization energy and discrete excitation spectrum—resemble those of real atoms. Electrons bound to a nucleus potential encounter sufficiently strong effects of quantum-mechanical confinement and mutual Coulomb interactions that they are well arranged in ordered states, and this leads to the arrangement of atoms in the periodic table. It is well known in atom physics that the threedimensional spherically symmetric potential around atoms gives rise to the shell structure 1s, 2s, 2p, 3s, 3p,…. The ionization energy has a large maximum for atomic numbers 2, 10, 18,…. Up to atomic number 23, these shells are filled sequentially. Hund's rule determines whether a spin-down or a spin-up electron is added. This article describes how closely we can approach the electronic properties of real atoms through the use of semiconductor quantum dots.

Both the effects of quantum confinement and Coulomb interaction become strong in quantum dots when the dot size is comparable to the electron wavelength and contains just a few electrons. The consequence of these factors on transport have only recently been studied in vertical-dot devices, which contain a dot located between source and drain contacts by means of heterostructure tunnel barriers because the few-electron regime is only accessible in the vertical-dot device. Studies include transport measurements through submicron resonant tunneling devices and submicron gated resonant-tunneling devices, and capacitance measurements on submicron double-barrier structures. However quantum-dot devices usually contain some disorder—for instance because of impurities or when the shape of the dot is irregular—which readily causes sample specific inhomogeneity in the electronic properties. Clean quantum dots, in the form of regular disks, have only recently been fabricated in a semiconductor heterostructure (Figure 1), and have been used to study the atomlike properties of artificial atoms.

Electrorheological (ER) fluids are most broadly classified as either particle dispersion or homogeneous types. The history of their development has been one of a search for fluids of either type that exhibit a large ER effect—an increase in viscosity or shear stress upon exposure to an electric field—together with longterm stability under various temperatures and conditions of use.

The first particle dispersion ER fluids were invented by W.M. Winslow in the late 1940s. Many fluids of this type have been investigated since then, and a few have been developed and adopted for practical applications.

Homogeneous ER fluids have been investigated even longer. The earliest reported observations of the ER effect were for homogeneous fluids composed of glycerin and paraffin oil in the 1890s. In the 1930s, the ER effect was also observed in polar liquids such as nitrobenzene and anirin and in colloids of superfine metal powders. In all cases however the viscosity in the electric field was only two or three times the zero-field viscosity, which is too small for useful applications.

Since the advent of low-molecular-weight liquid-crystalline (LC) materials in the 1960s, investigations have been made on the viscosity changes accompanying their inherent tendency to align in an electric field, but their maximum viscosity generally is only a few times their zero-field viscosity, and their shear stress is in any case small.

Photodetectors must provide fast and efficient conversion of photons to charge carriers. When considering semiconductor light sources, the indirect bandgap of silicon and germanium represents a serious obstacle to radiative electron-hole recombinations. Momentum conservation demands the simultaneous interaction of the electron-hole pair with a momentum-matching phonon. As a consequence, radiative recombinations are five orders of magnitude less probable in Si if compared to a direct semiconductor such as GaAs.

Although the absorption of a photon and the generation of an electron-hole pair may be considered as the inverse process to emission, photon absorption within indirect semiconductors is a highly probable process if the photon energy is sufficient to bridge the energy gap in a direct process. The resulting electronhole pair is created in an excited state and relaxes sequentially. The ubiquitous-silicon solar cells operate this way. In the visible spectral range, Si photodetectors have demonstrated fast and efficient performance, being readily adapted for opto electronic applications and being fully compatible to standard-silicon processing schemes.