A characteristic feature of soft biological tissue is a low compression modulus at small strains. On the other hand, the compression modulus of artificial elastomers, such as vulcanized rubber, generally increases with decreasing strain, according to the theory of rubber elasticity. In this paper, I will consider that the low compression modulus of biological tissue is possibly brought about by the organic and inorganic FGM structure of the tissue. An example of a bivalve hinge ligament is given here.

The tissue of the hinge ligament of a bivalve (Spisula sachalinensis) is a hybrid material, composed of a soft cross-linked protein matrix and inorganic calcium carbonate crystals (aragonite). In this example, small rectangular strips were cut from the ligament. The aragonite can then be easily removed from the tissue by treating it with dilute acetic acid (to decalcify the sample). At this point, mechanical anisotropy and swelling were measured and discussed, based on the anisotropic, fine structure of the aragonite.

Figure 1 shows the coordinates for a bivalve hinge ligament. The x axis and y axis are the growth direction and the tangential direction against the growth line, respectively. The z axis is approximately normal in direction to the shell surface. X-ray diffraction of a long strip cut along the x axis showed a typical fiber pattern characteristic of the aragonite structure. The crystallographic c axis of the aragonite coincided with the x direction.

Problems with mechanical stress in very large-scale integration (VLSI) interconnections appeared quite early in the history of the industry. Failures were observed in the dielectric itself, in the die, and in metal lines. Various process and design changes were made to fix the problems, but the continuing shrinking of dimensions and increased complexity of interconnection structures have resulted in recurrence of old problems, the appearance of new ones, and increasing difficulty in developing fixes. The choices for interconnection materials and processes have become increasingly constrained. The shift from hermetic to plastic packaging reduces the choices for passivation materials. The reduced dimensions for the active devices and the corresponding shortening of diffusion distances limit the “thermal budget”; interconnection processing must be carried out quickly and at relatively low temperatures. The reduced space available for interconnections has resulted in the use of high aspect ratios, multiple layers of metal, and the need for planarization. These all lead to further constraints on materials and processes. An additional complicating factor is the continuing effort to reduce the time from initial design to marketing the product. Since many mechanical-stress-induced problems are wear-out reliability problems, they may require extensive testing for detection. If a problem is not found until late in the product cycle, the economic consequences may be severe. To avoid costly late detection of problems and frantic scrambles for fixes, it is essential to have a clear understanding of the various origins of mechanical stress, the behavior of various materials and interfaces under stress, and the potential failure mechanisms.

A detailed knowledge of the micromagnetic structure of thin films is essential for our understanding of the magnetic processes in modern magnetic materials. Many technological applications in the magnetic-recording industry involve thin-film materials including magnetic and magneto-optic recording media and magneto-resistive sensors. Electron microscopy allows imaging of the magnetic structure of such thin films with very high spatial resolution. It is also possible in certain instruments to study the behavior of the materials in the presence of a magnetic field while the specimen is still in the microscope. This facility offers the prospect of full magnetic characterization of thinned materials using electron microscopy.

Magnetic recording has been the dominant recording technology for information storage since the invention of the computer. Currently, 1-Gbit/in. longitudinal magnetic recording (LMR) systems have been realized and densities of 10 Gbit/in., having bit areas less than 0.1 μm, are being developed. To reach this goal, a drastic scaling down of the track pitch, bit-cell length, head gap, medium thickness, and head-medium spacing is required. If this trend of increasing densities continues, an areal density of more than 300 Gbit/in. is predicted in the 21st century, based on computer simulation using the perpendicular magnetic recording (PMR) mode instead of the current LMR scheme. Magnetic-recording technologies and related materials have already been discussed in another issue of the MRS Bulletin, and hence in this paper, we concentrate on Co-Cr-X material used as a medium for PMR.

The PMR mode has been studied since 1975 and at present, Co-Cr-X (e.g., X = Ta) films with perpendicular anisotropy are the most promising media material. In general, such media should have the following properties: easy axis of magnetization perpendicular to the film plane, suitable coercivity (Hc) and remanent magnetization (Mr) for storing the information and reading it at a high signal-to-noise (S/N) level, uniform columnar size with a small diameter, magnetically uncoupled columns having a magnetization reversal based on rotation instead of a domain-wall motion, chemical stability under various environmental conditions, and a small surface roughness. In order to achieve the desired magnetic anisotropy and coercivity, a columnar morphology (small diameter) with an hep [0001] texture and exchange-decoupled, columnar boundaries—to create a magnetic microstructure for single domain switching columns with high coercivity—should be obtained. An overview of the preparation, microstructure, and magnetic properties of Co-Cr thin films is given in Reference 8. Depending on the deposition parameters, a so-called initial layer (with an in-plane magnetization) can be present.

This article focuses exclusively on the application of functionally gradient materials (FGMs) as structural components in high-heat-flux environments. Preliminary data suggest that FGMs can equal or surpass the properties of fiber-reinforced composites (FRCs) in these applications at a fraction of the material costs. An optimistic value of $1 per foot for 100-μm-diameter Saphikon alumina fiber yields a per-pound cost of $48,000. In contrast, a costly alumina powder used for FGMs might be in the range of .$20 per pound. While many counter-arguments can be advanced, this price differential cannot be easily overcome.

One relative difficulty with FGMs is their inherent compositional flexibility which, ironically, is normally considered an advantage. Determination of the optimum material combination for a given application should precede any effort directed toward determining the “best” processing route. This combination will then specify the processing technique. This would “short circuit” the expensive learning curve historically associated with FRCs.

“Processing for processing's sake” is no longer a viable option in today's composite research market. Without necessary advances in the predictive modeling of structure, composition (and, concurrently, cost) and behavior, FGMs may remain an “on-the-horizon” advanced composite.

Open interconnections on integrated circuits (ICs) are a serious and ubiquitous problem throughout the micro-electronics industry. The efforts to understand the mechanisms responsible for producing open interconnections and to develop analytical methods to localize them demonstrate the concern manufacturers have for this problem. Multiple layers of metallization not only increase the probability that an open conductor or via will occur because of the increased number of interconnections and vias but also increase the difficulty in localizing the site of the failure because upper layers may mask the failure site.

Rapid failure analysis of open-conductor defects is critical in new product development and reliability assessment of ICs where manufacturing and product development delays can cost millions of dollars a day. In this article, we briefly review some standard failure analysis approaches and then concentrate on new techniques to rapidly locate open-conductor defects that would have been difficult or impossible to identify using earlier methods. Each method is described in terms of the physics of signal generation, application, and advantages and disadvantages when compared to existing methods.