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An Assessment of the National Institute of Standards and Technology Materials Science and Engineering Laboratory: Fiscal Year 2008 Materials Reliability Division SUMMARY The theme of the Materials Reliability Division, located in Boulder, Colorado, is the reliability of structures—from bridges to single cells. The organization of the division lines up well with its work and helps focus on the emerging areas of nanoscale reliability and cell and tissue mechanics. The division has effectively built on its core competency of mechanical property measurements and extended the applications to small sizes (nano) and biological areas. The projects are clearly focused on the mission of the MSEL, and the division has engaged its NIST colleagues in Gaithersburg, Maryland, in several projects. The quality of the technical staff is generally excellent, and the division’s laboratories are well equipped, with a few exceptions. The division programs should add enhanced modeling and theory expertise. The recent transfer of staff to augment the theoretical and modeling capability is good. Reliability testing is only part, albeit an essential part, of the science and application of reliability engineering needed by U.S. industry. The division has made excellent progress in recruiting staff by transferring two individuals from Gaithersburg, but there remains an imbalance between experimental expertise and the modeling commonly used in industry. A more visible program of three-dimensional finite-element modeling of thermal and electrical stresses is desirable. The division should increase its efforts to access expertise in other divisions at NIST and/or to add staff. In general, the physical infrastructure facilities of the Materials Reliability Division are dated, but overall the laboratories are well equipped. The ratio of technician support to professional support seems to be very low for laboratories of this caliber and size (a situation not unique at NIST to this division). The productivity, as measured by publications and products, for example, Charpy test specimens, is very good. Staff members have won awards, and an appropriate number of meetings were hosted. The division is a good example of executing the MSEL project evaluation process to focus its effort on projects that fulfill the mission of the MSEL. The division has done a good job of developing new and important areas of research beyond its traditional focus. It has reached out to establish a customer base in areas such as biomaterials and has worked to reinvigorate its traditional core competency in large-structure reliability programs. TECHNICAL MERIT RELATIVE TO STATE OF THE ART The technical merit of the work in the Materials Reliability Division is high, whether in the historic role of supplying Charpy test specimens or at the new frontier of exploring the application of stress measurements to biological systems. The ability to address a diversity of projects attests to the quality of the staff and the management of the division. Generally, the staff has augmented its capability by forming partnerships in the areas in which it needs help. The area of medical device reliability would benefit from added expertise in reliability analysis. In general, the technical work reviewed was excellent and in some cases unique. Theory and modeling could enrich the experimental program. Steps have been taken to augment the staff; continued additions are warranted.
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An Assessment of the National Institute of Standards and Technology Materials Science and Engineering Laboratory: Fiscal Year 2008 ADEQUACY OF INFRASTRUCTURE The equipment associated with the structural materials projects of the division is often old, but it has been updated with modern electronics. The basic test equipment has some unique capabilities that were designed and constructed by NIST staff—for example, very low temperature test cells. A hydrogen test facility under construction, while not reviewed in detail, appears to be very versatile and will be a significant contribution to the work relating to the technologies targeted by the ACI and the America COMPETES Act (see the chapter below on this funding). The Nanoscale Reliability Group has constructed a unique atomic force microscope for the measurement of mechanical properties at the nanoscale and has established good teaming with industry and universities. ACHIEVEMENT OF OBJECTIVES AND IMPACT The technical work of the division is grouped in three topical areas: structural materials, cell and tissue mechanics, and nanoscale reliability, as discussed below. Structural Materials Group The Structural Materials Group has core competencies in mechanical testing on a macroscale and in developing standard measurement techniques for materials and properties that are critical to the nation’s infrastructure. This group manages the Charpy Standard Reference Material and Verification Program, which sells several thousand units a year. All of the structural materials work is clearly focused, and the payoffs from success are clear. Good customer ties were shown—for example, in joint planning with DOE and the U.S. Department of Transportation (DOT) on hydrogen and on pipeline safety. The mature (historic) Charpy Test Sample Program has an element of science in computational models and crack tip measurements. The impact of the work is high and clear for the specific projects and more generally as it relates to understanding the failure of large, complex structures. It is a vital role for NIST to play. This group maintains equipment capable of tensile, fatigue, and fracture impact analysis and crack tip opening analysis (CTOA) in the range of 1 N to 4.4 MN over a temperature range of liquid helium to 1000 °C. The equipment has been installed in the laboratory for an extended period of time, in some cases having been in service since the 1960s, although periodic electronic and other upgrades have kept most of the equipment in state-of-the-art condition. Some areas, such as nondestructive evaluation, have been diminished through attrition of staff. However, this laboratory and its personnel remain fully capable as a valuable, unbiased resource for responding to critical national infrastructure testing needs, such as the investigation of the collapse of the World Trade Center’s Twin Towers and now the hydrogen pipeline safety effort as one of the ACI-related programs. The Pipeline Safety project is a basic data-gathering and measurement activity that has as a clear customer the pipeline industry. It is an indispensable standards activity. A strength of the activity is the continued development of the fracture toughness test, based on the optical measurement of the crack tip angle, as a measurement tool for evaluating pipeline safety. This laboratory operates with a relatively small permanent staff, but it has done a good job of using postdoctoral, cooperative, and other guest researchers to supplement labor needs. In addition, the laboratory is highly collaborative in much of its work, leveraging its mechanical testing
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An Assessment of the National Institute of Standards and Technology Materials Science and Engineering Laboratory: Fiscal Year 2008 expertise with others laboratories in the MSEL and other agencies, such as DOT, for funding. There is a need for extending the strain-rate capabilities for the high-speed fracture mechanics testing of pipelines to remain competitive. Data from these tests are essential for those who accurately model pipeline rupture under burst conditions. Supplying Charpy test SRM samples is a large and important component of the work of the Structural Materials Group. This group has established itself as providing SRMs for Charpy testing to meet the ASTM (formerly known as the American Society for Testing and Materials) standards. This is the most successful SRM program at the MSEL. The Structural Materials Group supported the revision of two International Organization for Standardization (ISO) standards in FY 2007. Cell and Tissue Mechanics Group One of the reasons why NIST has a history of success is that it has maintained a research program at the leading edge of developing fields where measurement methods and, eventually, standards will be central. In the rapidly developing field of tissue and cell engineering, NIST has a vital role in indentifying the crucial parameters that will need to be measured. The Bioreactors project has established a strong and promising collaboration with a University of Colorado research group for investigating the effect of different types of forces on the histology, growth morphologies, and various tissue expressions under realistic in vitro tissue growth environments. The research being performed is elegant, conceptually simple, and likely to provide important insights into tissue growth under conditions pertinent to actual body strains. This is excellent basic research, although it is premature to judge whether it will also lead to a set of instruments that can measure tissue response characteristics that will be of essential value to the burgeoning field of tissue culture for biomedical replacements. Bringing in a biomedical industry partner could add value. The Single Cell Mechanics BioMEMS project is one among the worldwide activities working to establish methods for the in vitro measurement of cell deformation and mechanics. Many of these activities are focused on using the mechanical response of cells as a tool for assessing disease progression and the viability of cells. The more sophisticated projects are those that combine the two- and three-dimensional response of cells to deformation with complementary mechanics-based models of the deformation to elucidate the deformation mechanisms. The current force-displacement measurements at the Materials Reliability Division using a uniaxial straining MEMS device designed and fabricated at NIST facilities is a promising approach. The reliability is limited by the lack of strain measurements in other directions as well as by any mechanics modeling to relate the measurements to the underlying histological structure of the cells. Complementary efforts and tools reside in the Ceramics Division. The Materials Reliability Division should expand interdivisional collaborations. The single-cell tester used in this research was conceived by NIST and was fabricated by the NIST in-house MEMS facilities. The first of its kind, this tester has the potential to play a key role in increasing the understanding of tissue formation, disease progression, and disease treatment and may impact drug discovery. Medical Device Reliability is one of the projects in the biomaterials area. Medical applications represent a significant sector of the U.S. health industry. Current unacceptably high failure rates of implantable medical devices such as pacemakers, cardiac defibrillators, and neural transmitters point to the urgent need to establish standards and to develop measurement
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An Assessment of the National Institute of Standards and Technology Materials Science and Engineering Laboratory: Fiscal Year 2008 methods to ensure quality, reliability, and consistency and so to minimize the need for repetitive procedures to remove recalled devices from the human body. NIST is working with the International Electronics Manufacturing Initiative Medical Electronics team to address short- and long-term reliability issues with medical devices. The division has established programs to develop new measuring tools, explore methods to improve reliabilities, and develop accelerated testing standards. Teamed with companies and with the National Institutes of Health (NIH), the Cell and Tissue Mechanics Group has gathered critical data on the next generation of devices and identified key exposures of failure mechanisms. The target of this work is to reduce the failure rate to less than 0.1 percent (current levels are an order-of-magnitude higher). Although the group has organized workshops with important industry players, the division researchers would benefit from collaborating more directly with experts who can help establish more direct links between the current technical characteristics of devices and their reliability and performance. The division should apply a more focused and specific plan to address reliability issues and to include experts well versed in reliability. The Cell and Tissue Mechanics Group has noted that parylene (coatings used in implanted probes) degrades in the presence of electrical fields and has communicated this important observation to the Food and Drug Administration. Overall the Cell and Tissue Mechanics Group has excellent staff who are employing innovative approaches. Nanoscale Reliability Group Strain engineering is an important new area of semiconductor technology. As devices have scaled down from micrometers (1,000 nm) to 45 nm today and 20 nm in the next few years, perfectly induced localized strains in device structures can enhance electron and hole mobility by more than 30 percent, leading to faster devices. Strain metrology, therefore, is indispensable for the U.S. semiconductor industry.9 The metrology of strains at the nanoscale is extraordinarily difficult, and no group has yet measured strains in microprocessors. The Nanoscale Reliability Group has developed its techniques using geometrically simpler structures (doped nanowires) where measurement of the stress state is less complex. The group used transmission electron microscopy (TEM) and scanning electron microscopy (SEM) together with commercially available software tools to acquire diffraction data and lattice images with high spatial resolution (10 nm for diffraction, 0.2 nm for imaging), and then measured the changes in lattice parameters across nanowires. These data were used to produce two-dimensional strain maps in gallium nitride/indium gallium nitride (GaN/InGaN) nanowires. A goal of the work is to develop three-dimensional strain mapping, which would be a significant advance. A challenge using this approach is to interpolate the results of these measurements to actual devices. One of the successes of the Nanoscale Reliability Group has been the use of atomic force microscopy to study localized strain on the nanoscale. The quality of the research is comparable with that of leading groups in this area, and the contributions in the area of contact resonance AFM imaging are at the forefront of such work. Complementary work using tip enhanced Raman spectroscopy provides a suite of capabilities for developing metrology at the nanoscale. 9 A. Diebold et al., 2008, “Update Presentation on Metrology Roadmap,” International Technology Roadmap for Semiconductors, Spring 2008, U.S. Metrology Technical Working Group.
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An Assessment of the National Institute of Standards and Technology Materials Science and Engineering Laboratory: Fiscal Year 2008 There is considerable overlap with research on nanomechanics at the NIST Gaithersburg site. The establishment of Communication Working Groups within the MSEL is a good step toward strengthening the collaborations between the different AFM groups. Another of the strengths of the Nanoscale Reliability Group has been the evaluation of material properties at small length scales. This is a crucial technical and intellectual endeavor because many of the mechanical properties of metals and polymers are different at small lengths. For example, there is the well-known indentation size effect, wherein the hardness of many metals and polymers increases as the indentation load, and hence the size of the hardness impression, is decreased. The question arises as to whether other related mechanical properties of concern to the microelectronics industry, such as fatigue strength, are also different at small length scales and, if so, why. Few groups other than the NIST group at Boulder are tackling these important questions. One of the difficulties is that it is necessary to do both the testing and the evaluation at the same dimensions. One approach, taken both at NIST and at the Max Planck Institute in Germany (by a NIST researcher working there), is to use cyclic Joule heating to differentially strain and hence fatigue the interconnect lines while monitoring their microstructural changes with complementary electrical measurements. This work has quantified the combined effect of mechanical constraint and dimensional scaling, which were shown to dramatically alter the fatigue failure resistance of aluminum lines. Although aluminum lines are increasingly being replaced in microelectronics by copper, it is expected that this measurement approach can be readily transferred to the investigation of the fatigue of copper interconnects in devices. The focus on nanoscale strain metrology is understandable, given the huge importance of the microelectronics industry. The group may also wish to consider the strain metrology associated with polymer electronics devices and displays. There are two distinct challenges. On the one hand, polymer-based electronics will be much larger than current silicon-based electronics, and so the strain metrology has to be performed over much larger dimensions without loss of precision. On the other hand, even though polymers generally exhibit larger compliances than those of metals and semiconductors, the strain incompatibilities with nonpolymeric components, such as interconnects and chips, are larger, and so the local strains that have to be quantified will be larger. In addition, the recent use of Green’s function calculations to perform lattice displacement mapping around a germanium (Ge) quantum dot in silicon (Si) provides an opportunity for the group to develop capability in finite-element modeling, plasticity theory, and mechanics analysis. An active recruiting program to add staff and backfill for anticipated retirements is desirable. CONCLUSIONS The projects reviewed are focused on the mission of the MSEL and build effectively on the historic strength of the Materials Reliability Division in mechanical testing for reliability. Augmenting the experimental capability with modeling and simulation expertise by the transfer of staff and hiring will greatly enhance the strength of the program. The division has developed several unique measurement tools and devices that have potential to strengthen the competitive position of U.S. industry. The recently enhanced focus at NIST on obtaining patents may be beneficial and should be used to explore the best approach for commercializing these developments.