Interconnects technology and back-end processing moved to the center stage of very large-scale integration (VLSI) technology in the mid-1980s. At that time, the critical dimensions dropped below 1 μm while the chip size and complexity increased to a level where interconnects were recognized to be a limiting factor. As dimensions decreased, the step coverage of sputtered aluminum inside contacts and via-contact holes decreased and alternative technologies were studied. The increasing cost of ownership (COO) of single-wafer Al sputtering processes also supported the search for alternative technologies, such as tungsten chemical vapor deposition (CVD) for via contacts and plugs (Figure 1). Only recently have all the W CVD process steps been optimized to lower cost without loss of reliability and/or performance. The development of cluster tool technology and multiwafer process modules also allowed reliable and cost-effective utilization of the W/Al technology.

Tungsten technology for VLSI circuits became complementary to that of aluminum. Tungsten thin-film resistivity ρw = 7–8 μΩ cm is much higher than that of aluminum ρAl = 3–4 μΩ cm, introducing large W interconnect resistance-capacitance (RC) delays compared to Al. Therefore, tungsten is not favorable for high-speed global-interconnect schemes. However, tungsten is suitable for local interconnects where the impedance of the driving transistors is dominant and the RC interconnect delay is less significant. Tungsten is also suitable for contact filling, in which the via resistance is negligible. For these applications, tungsten became a dominant technology and was integrated with the aluminumalloy-based technology used for global interconnects.

Try to imagine a world without incandescent lamps. We light our homes with 20-200-watt lamps, which are routinely purchased in local stores. These lamps are made at the rate of millions per week and their manufacture represents a large global industry. The consumer expects them to last approximately 1,000 hours and to be energy efficient and inexpensive.

Many types of incandescent lamps exist. Figure 1 shows several examples. The most common types are A-line (the general household lamp), automotive lamps, and reflector lamps for floodlight or projector applications. We also have many thousands of specialty lamps, which have a broad range of applications, such as audio-visual projectors, fiberoptic systems, video-camera lights, airport-runway markers, photoprinters, medical/scientific instruments, and stage/studio systems. They can have power ratings from a few watts to several thousand watts, but they all have at least one feature in common. They contain a filament made of tungsten wire. The diameter of the wire will vary, depending on the power rating and application of the lamp, but for most common household lamps it is less than 100 μm.

FGMs have been fabricated using the combustion synthesis (or self-propagating high-temperature synthesis (SHS)) process by exploiting a rapid and exothermic chemical reaction, in order to synthesize some (or all) of the constituents in an FGM to simultaneously increase its density. The thermal energy required to drive the process is derived from this internal, chemical source, rather than from an external and usually expensive source (e.g., a furnace). The combustion synthesis process is a powder-based process that has been used to synthesize over 300 compounds, and is particularly useful in preparing materials such as highly refractory ceramics and high-temperature intermetallics that are difficult to prepare by other synthesis methods. In addition, the process can be used to prepare ceramic-metal and ceramic-intermetallic composite materials. As a result, only slight modifications of the combustion synthesis are required to prepare functionally gradient materials from these same combinations of materials.

Sample preparation begins by the creation of a series of mixtures from the powders that will react to form the constituent materials of the FGM sample. Each of these mixtures contains a slightly different percentage of reactants, so that each mixture will yield its own (predetermined) volume fraction of each of its constituents, following the combustion synthesis process. Prior to the combustion step, the samples are assembled by stacking layers of each of the reactant powder mixtures in appropriate amounts, in such a way that the multilayered powder mixture will faithfully produce the composition gradient that is required in the resultant FGM.