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CHAPTER FIVE Electronic and Photonic Materials CHAPTER SUMMARY The Panel on Electronic and Photonic Materials was charged with examining materials research needs for future defense systems in electronics, optoelectronics and photonics, and microsystems (including sensor systems). The tremendous vitality and innovation of the private sector allowed the panel to consider which future defense needs could be met by making use of commercial developments in industry and which were so specific to DoD as to require DoD investment. The panel began by examining the following military needs that would particularly benefit from advances in electronic and photonic materials: Detection, identification, and defense against or avoidance of threats; High-fidelity imaging signals; Communications systems; Compact systems to transmit at very high power and high frequency; Enemy identification and monitoring; Dynamic camouflage/stealth; and Health monitoring of equipment and personnel. While this panel considered a wide variety of military needs from several vantage points, ranging from individual devices or components to entire miniature systems, a number of common themes emerged that point to important areas for research:
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Improvements in the fundamental understanding of existing materials that have been identified as promising for military applications; The search for new materials with extreme properties, sometimes orders of magnitude better than what is currently available; New ways of combining materials, particularly at the nanoscale, in order to obtain new functionality, with particular attention to ways to routinely combine inorganic and organic or even biological materials; Consideration of packaging and thermal management issues, in parallel with research on materials for the devices themselves; Materials processing to effect the successful introduction of new materials and the optimization and manufacturability of existing materials (low-temperature processes are generally desirable and sometimes essential, particularly as device dimensions shrink and as increasingly dissimilar materials are used in close proximity to one another); and Theory and modeling as indispensable partners of experimental efforts in the search for and optimization of new materials, especially as nanoscale structure and control become paramount. INTRODUCTION The Panel on Electronic and Photonic Materials was charged with examining materials research needs for future DoD systems in three areas: (1) electronics; (2) optoelectronics and photonics; and (3) microsystems, including sensor systems. Progress in electronic and photonic materials and systems that are heavily dependent on them, such as microsystems and sensors, is occurring at an astonishing rate, fueled in large measure by tremendous investments by the private sector. This panel considered which future defense needs could be met by taking advantage of commercial developments and which were so specific to DoD as to require DoD investment. Each section of this chapter discusses (1) commercial drivers; (2) DoD drivers where materials have potentially high impact; and (3) priorities for DoD-funded materials research. DOD NEEDS FOR ELECTRONIC AND PHOTONIC MATERIALS The panel considered the following military needs, identified by DoD leaders,1 to be particularly ripe for resolution via advances in electronic and photonic materials:
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Detection, identification, and defense against—or avoidance of—threats. Low-cost, highly capable commercial technologies are increasingly enabling even nations with very limited resources to mount considerable regional threats based on precision strikes from their own territories. This development, coupled with the periodic U.S. need to mount operations in a wide variety of conditions, requires increased reliance on systems that can detect, identify, defend against, or avoid many types of threats. Because such systems must be deployed on military equipment, as distributed sensors, or on individual soldiers, they must be compact, light, energy-efficient systems with broad capabilities. High-fidelity imaging signals. Imaging systems that could be used to identify camouflaged soldiers and materiel by soldiers or on vehicles (manned or unmanned) would reduce casualties and materiel loss. Compact systems to transmit at very high power and high frequency. Compact high-power systems would allow operation of communications and surveillance equipment at greater distances. Size and weight will determine whether such systems are appropriate for use on unmanned vehicles or by individual soldiers. Operation in new high-frequency regimes allows for higher bandwidth as well as access to spectroscopic information that is not accessible in other frequency ranges. Communications systems. Increased coordination among forces, the need for global awareness, and increased reliance on information from remote sensors and unmanned vehicles all drive the need for compact, energy-efficient communications systems that can transmit and receive securely at high bandwidth. 1 Andrews, M., “Army Vision and S&T: Accelerating the Pace of Transformation,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000. Delaney, L., “Air Force Modernization,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000. Harwell, K., “Air Force Research Laboratory: Technology Vision,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 16, 2000. Henley, L., “The Revolution in Military Affairs After Next,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000. Marshall, A., “Overview of DoD Vision and System Needs,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000. Marshall, A., “Overview of DoD Vision and System Needs,” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, January 29, 2002. Vickers, M., “The Revolution in Military Affairs (RMA),” briefing presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000.
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Enemy identification and monitoring. Monitoring of enemy troop and materiel movement, as well as discrimination between enemy and friendly elements, will become increasingly important as future conflicts move to new battlefields, such as urban environments. Dynamic camouflage/stealth. The ability to dynamically change the spectroscopic signature of surfaces, either on soldiers or equipment, would greatly reduce the probability of detection by the enemy. Health monitoring of equipment and personnel. Monitoring of personnel for health or exposure to chemical or biological agents will become increasingly important. Monitoring the health of equipment in the field is also a longstanding need, as already detailed in Chapter 3. SPECIFIC AREAS OF OPPORTUNITY Below, DoD needs in three specific areas of opportunity are discussed: electronics, optoelectronics and photonics, and microsystems. Electronics Commercial Drivers The silicon-based microelectronics industry has been a tremendous force in the U.S. economy for at least two decades. The phenomenal ability of the industry to keep pace with, and even exceed, Moore’s Law (doubling the logic density every 18 months, with a concomitant reduction in cost/bit) has put unprecedented data processing and storage in the hands of every American. The trend is expected to continue for another decade, as documented in the 2001 edition of the International Technology Roadmap for Semiconductors (ITRS) (SIA, 2001). DoD can leverage this revolution by taking advantage of industrial developments. For the longer term, the conclusion of the ITRS is that “10-15 years in the future, it becomes evident that most of the known technological capabilities will approach or have reached their limits.” It projects that a more cost-effective alternative to planar silicon complementary metal oxide semiconductor (CMOS) technology will be required in this time frame. The solution may involve new materials, new cell architectures, and new processes. This forecast represents the first projection that scalable planar silicon CMOS is reaching a limit. Along with this singular discontinuity several
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other critical material needs are identified in this roadmap for the commercial semiconductor segment over 10-15 years: Mask materials and photoresists for <45-nm lithography, Wafer materials and crystal-growing technology for >300-mm wafers (450 mm), and Yield model development for new materials and integration. DoD needs to support research into novel materials for nanoscale integration beyond the limits of CMOS scaling. The commercial sector is driving the increase in information storage capability at the same rate as Moore’s Law, although the relatively high level of DoD investment has enabled this industry to take advantage of new materials developments more aggressively than has happened for semiconductors. DoD Drivers with Potentially High Materials Impact During this century U.S. effectiveness in battle will depend on the superiority of our ability to gather information that allows us to identify, locate, and destroy or at least avoid the threat (see Figure 5-1). In spite of very rapid progress in electronic materials and systems, it has not been possible to keep up with the increasing demand for compactness, power (high or low, depending on the application), simplicity in packaging, and signal purity. Since many future military systems will be space based, use lighter platforms, or be deployed on individual soldiers, these demands are becoming imperatives. Significant improvement over existing capabilities can be achieved only by inserting novel high-performance electronic materials into sensors, detectors, and components. The following military technologies are particularly ripe for improvement: Detection, identification, and defense against—or avoidance of—threats, Compact systems to transmit and receive very high or very low power at very high frequencies, and Embedded technologies that enable these systems. Certain components in these systems are in particular need of materials advances, including
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FIGURE 5-1 Relationships in the Future Combat System. SOURCE: Browning, V.M., “An Overview of Electronic Materials,” briefing presented to the Panel on Electronic and Photonic Materials of the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, March 29, 2001. RSTA = Reconnaissance, Surveillance, and Targeting Antenna. Transmitters, receivers, and detectors for high-frequency broadband systems; High-power electronic components; Analog/digital converters; Electronic beam steering; Radio frequency (RF) microsystem components; Acoustic components; and Ferroelectric materials. In addition, materials developments that are on the horizon or that may be envisioned could provide the basis for revolutionary technological advances; among them are superconducting materials and nanostructured materials.
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Transmitters, Receivers, and Detectors for High-Frequency Broadband Systems The military needs materials for transmitters, receivers, and detectors that can operate at frequencies from MHz to over 100 GHz frequencies. SiGe is seen as the next-generation material for high-speed, high-frequency applications, but it is not suitable at the highest frequencies; these require different materials, most likely the group III-nitride crystals and their derivatives. Materials research should concern itself with developing appropriate substrates, optimizing epitaxial growth (which will require better understanding of fundamental aspects of the growth of these polar materials), and improving the junctions and contact layers (which will entail identifying important defects and ways to eliminate or control them). High-Power Electronic Components The high-power transmitters and receivers in future systems cannot be made using traditional semiconductors like Si or GaAs. Materials for low-frequency and high-power applications like SiC and its derivatives are promising, but achieving the required signal purity must be rooted in a fundamental understanding of how to control their chemistry, crystal growth, epitaxy, and defect density.2 The group III-nitride materials are also promising, but meeting the operating requirements for high-power applications or in hostile ambients will require interconnects and overlayers that can function around 500°C. In addition to detailed understanding of crystal growth and the control of important defects in the materials, attention must be paid to high-temperature contact materials, overlayers, and joining materials. A hybrid of carbides and nitrides of gallium, aluminum, and boron is one promising approach, although the monumental challenges of design, fabrication, and processing are still to be solved. Analog/Digital Converters Electronic analog-to-digital converters (ADC) have continued to show steady but slow progress in resolution and speed (Walden, 1999). Performance limiters due to sampling aperture jitter and comparator ambiguity are directly related to device speed; electronic materials that yield faster devices will accelerate progress. Optical ADC and optically assisted ADC have shown promise for improving perfor 2 Petroski, K.J., “High Power SiC Microwave Devices for Amplifier Devices,” paper presented at GOMAC, San Antonio, TX, 2001.
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mance but they need new materials with 500 times higher electro-optical coefficients than lithium niobate or other oxide materials. Preliminary experiments suggest that semiconductor materials like rubidium hydrogen selenate class materials, solid solutions, or substituted II-VI, II-VI materials have potential (Guilbert et al., 1998). Electronic Beam Steering One of the biggest challenges facing defensive systems is replacement of the mechanical or optical (mirror) beam steering with all-electronic laser-beam steering. This could be achieved with materials whose damage threshold, modulation capabilities, and transparency properties are amenable to electronic steering. Tailoring the Bragg diffraction of materials is one promising approach. As a function of microwave frequency, the angle, power, and wavelength for steering could be controlled if a material with suitable diffraction characteristics could be produced. RF Microsystem Components Microelectromechanical microwave switches have a wide range of applications in military systems. They have extremely low insertion loss (0.2 dB) and crosstalk over a broad microwave frequency range (>40 GHz). Microwave phase shifters based on MEMS RF switches have shown much lower insertion loss than conventional electronic phase shifters. This reduces the number of amplifiers needed and also the size, weight, and power of phased-array antenna systems. A MEMS RF switch could also be used in frequency agile filters. Materials affect the reliability of these switches. Engineering of metal electrodes with low contact resistance and no stiction during separation is critical for metal-to-metal contact switches, especially for hot switching under high-power conditions. Prevention of dielectric charge-up is essential for capacitive switches. RF MEMS components will also play an important role in future wireless communication systems, as discussed below. Acoustic Components Improved materials are needed for space and underwater acoustics. The materials properties that are important are power capability, sensitivity, noise performance, size, weight, robustness, shock resilience, aging, manufacturability, cost, and bandwidth. Materials must meet stringent performance criteria, including high Q, low loss, and temperature-stable operation. Artificially structured ternary and quaternary materials are promising. Theory and modeling will be important in finding material solutions to the severe constraints of this problem.
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Ferroelectric Materials Ferroelectric materials have high dielectric constant, wide tunability, or both. Unfortunately, current ferroelectrics are limited to low dielectric relaxation frequency. Ferroelectrics with much higher dielectric relaxation frequency would allow miniaturization and tunability of high-frequency microwave circuits. New Materials For Tomorrow’s Technologies While high-transition-temperature materials have attracted most of the scientific interest in superconductivity in recent years, most commercial superconducting devices use lower transition-temperature materials composed only of metals. Industry has invested heavily in oxide superconductors for passive microwave components, SQUID magnetic-field sensors, high-current conductors for power transmission and high-field magnets, and Josephson-based circuits for mixed-signal (analog and digital) circuits and AC voltage standards. The recently discovered borides are also receiving considerable attention. While superconducting materials offer potential performance advantages over conventional materials, this has so far come at the cost of additional system complexity due to both the refrigeration requirement and the challenges posed by other properties of the materials, such as the brittleness and anisotropy of high-temperature superconductors. Unless refrigeration becomes efficient and reliable or higher temperature superconducting materials are discovered, superconducting devices will be relegated to niche military applications. Speculative concepts such as quantum devices and spintronics have the potential to dramatically change the way military information is processed and encrypted. These concepts require exquisite control of material structure and properties on the nanometer if not subnanometer scale—a tremendous growth and processing challenge. The development of materials in the form of arrays of nanowires instead of thin films is a promising approach. New materials may also be needed. While the risks of such research are high, the possible payoffs for the first nation to field systems based on the successful embodiment of some of these concepts may be immeasurable. In the limit of miniaturization, it can be expected that electronic devices will reach the scale of small numbers of atoms or molecules—the domain of molecular electronics. The challenges associated with realizing this dream are discussed in Chapter 6. The military will continue to have an insatiable appetite for data storage. The sorts of materials issues that arise in pushing storage density
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limit to the ultimate are essentially the same as for other nanostructured materials: the needs to control structure and composition on the atomic or nanometer scale, and to be able to read and write on that same scale. The specific materials required will also probably be different from today’s. Priorities for DoD Materials Research in Electronic Materials The priorities for DoD-funded research into electronic materials fall into only a few categories: Fundamental Understanding of Existing Materials Known materials, such as SiC and Group III-nitrides, are very promising for high-power and high-frequency applications. These materials are different from and in many respects more complex than the materials they would replace. Understanding the performance limits of the materials, as well as achieving them reproducibly, will require a multipronged scientific investigation incorporating experiment, theory, and modeling. New Materials with Extreme Properties The need for a material with an electro-optic coefficient 500 times higher than traditional oxides to meet new requirements for ADCs is an example of extreme materials properties that will require new approaches to materials structure and processing. The desire for electronic materials that can function around 500°C is also an extreme demand. Room-temperature superconducting materials and quantum devices would profoundly change system options. Theory and modeling will be key in guiding the experimental approaches to promising compositions and structures. Packaging and Thermal Management High-power devices that may function at temperatures around 500°C place severe requirements on interconnect, overlayer, and packaging materials and on the system components in close proximity to the device itself. Materials Processing Optimizing the performance of existing materials and successfully fabricating new ones with extreme properties will succeed or fail in large measure with the emphasis given first to understanding and then to controlling their fabrication and processing. As an example, improved materials and processing will enable production of components and subsystems (Figure 5-2) for frequency-agile communication and radar
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FIGURE 5-2 Generalized concept incorporating oscillators, filters, phase shifters, and circulators for a multilayer package with integrated circuits to improve quality and impedance matching. SOURCE: Courtesy of S. Wolf, DSO Office, DARPA. applications that take advantage of the field-variable properties of ferroelectrics, ferrites, and other new materials. Theory and Modeling Understanding the role of defects and the relative importance of defects in materials like SiC and GaN will depend heavily on theory. Elimination or control of the most detrimental defects is likely to be done experimentally, with significant guidance from growth models. In design of new materials with extreme properties, theory is likely to guide modelers to the most promising general structures and compositions. Modeling in turn will be required to narrow the list of most promising materials to a number that can be tested experimentally. Materials for Revolutionary Technologies The scaling of both Si planar CMOS technology and high-density storage using known approaches will reach fundamental limits within the next 15 years. Perhaps the most fruitful area in which to look for potentially revolutionary technologies is in materials and material architectures that would enable entirely new approaches to computation, encryption, and data storage. A first step in this direction would be to find materials and processes that could be used to demonstrate experimentally some of the current theoretical concepts in
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active mass transport. In liquid-phase systems employing aqueous media, this can be achieved with voltage biases at various points in a system. Gas systems relying on suction- or pressure-driven mass transport using microscale diaphragm pumps are being considered for microsystem applications but scale unfavorably with miniaturization.4 The flexural-plate-wave approach (Moroney et al., 1991; Meng et al., 2000), whereby gas flow could be achieved by acoustic streaming, has been suggested but not fully explored. Distributed transport mechanisms based on ciliary, peristaltic, or other biomimetic actuation could be options for gas transport. Efficient low-power valves and pumps for both gas and liquids require chemically inert thin-film materials. Preconcentration and focusing of samples, perhaps selectively, can be achieved with porous materials of adjustable surface area, pore size, and functionality. Common but proprietary materials used for this purpose are carbon molecular sieves, graphitized carbons, and rigid polymers, which are thermally stable to over 400°C. Adapting these or similar types of materials, to allow in situ deposition, patterning, and control of porosity within etched channels of a microsystem would be useful for gas-phase and liquid-phase analysis. Tailored organic/inorganic nanocomposites are an attractive class of nanoengineered materials for this application. Templating by incorporation of porogens of selected sizes or functionality can impart a crude level of selectivity in these supramolecular systems (Raman et al., 1996). Molecular separation based on controlled motion, competitive binding, or affinity would also benefit from nanoengineered materials and micromachined channels. Imparting structural features that would allow modulation of the binding or affinity with electric or photonic input would be a novel means of providing in situ control of separations. Clever channel construction methods and architectures that build upon anisotropically etched Si (Matzke et al., 1998), vapor-phase deposited polymers,5 and sacrificial-polymer processing (Mastrangelo et al., 1998; Bhusari et al., 2001) are examples of current research. Hybrid organic-inorganic nanocomposites are also promising (Lu et al., 2001). 4 Cabuz, C. 2001. Available online at <http://www.darpa.mil/dso/thrust/md/Mm/pump/honeywell/html>. Accessed March 29, 2001. 5 Noh, H.-S., C. Bonner, P.J. Hesketh, and G.C. Frye-Mason, “Fabrication of Parylene Column for Micro Gas Chromatograph,” paper presented at the International Symposium on Mechatronics, Atlanta, GA, 2000.
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Finally, heating and cooling elements are needed to control the temperature of certain systems components. Microfluidic systems may provide thermal management. Alternatively, thermoelectric (TE) materials offer a promising approach. Unfortunately, thermal desorption and temperature-programmed separations require heating and cooling rates and values that exceed the capabilities of current TE materials. Materials or approaches that reduce demands for high temperature or that allow high temperatures to be achieved at low power are required. Fluid motion could also be used for control of the optical or mechanical properties of devices. Other Novel Applications Important R&D activity is under way on MEMS devices and microsystems for optical devices, data processing, data storage, and displays, motivated by both defense and consumer applications such as telecommunications. MEMS-based systems for chemical and biological sensing are also being pursued vigorously. With the opportunities to develop new sensor and actuator microsystems come significant materials challenges. The same types of fluid control systems being developed for sensors may also be exploited in microsystems for controlled chemical reactions. These chemical reactor systems could be used to generate power, for example, through internal combustion or fuel-cell-type reactions. These power sources could integrate with microsystems or be used for other compact power applications, as discussed in Chapter 4. Miniaturized or highly parallel chemical reaction systems could also be used for combinatorial approaches to fabricating novel materials for study or for creating materials that might be difficult or impossible to fabricate in bulk. Highly miniaturized reaction systems even approaching the nanoscale may provide a path to effective nanomaterials fabrication and processing, a need identified in Chapter 6. Priorities for DoD Materials Research on Microsystems The panel envisions microsystems with more diverse materials than current electronic, photonic, sensor, or structural systems. Materials for signal processing, power transmission, and communication will need to be integrated with materials that provide structural, chemical, or mechanical functions. Compatibilities among diverse materials, particularly at interfaces, will be increasingly challenging. As component dimensions shrink and nanoscale materials are employed, surface/volume and interface/ volume ratios will be large. Methods to interface nanoscale (grown)
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materials with engineered devices will grow in importance. Issues like thermal expansion mismatch and interdiffusion must be mitigated. Exploitation of nanoscale phenomena and even connection of biological or bioderived materials may be a valuable direction, because biological systems have arrived at solutions to many of the functionality issues we hope to address. The effective incorporation of bioderived or bioinspired materials into durable engineered systems remains a challenge. Nanoscale elements, perhaps carbon nanotubes, or functional macromolecules with nanoengineered properties may provide new material options. This raises the need to integrate molecular components into functional systems. One of the fundamental areas for significant research will be on compatible materials for the control, metering, and flow of fluids. The panel envisions mass flow control with capabilities resembling those of electrical charge and light-flow control in electronic and photonic systems. Similarly important will be the ability to actuate and detect the motion of mechanical components in fluid and dirty environments. Chemical resistance and compatibility are likely to be critical, and there is strong motivation to incorporate into integrated systems materials with broad chemical resistance. Thermal management and temperature stability will also be important. Fabrication of Microsystems In fabricating microsystems, one can build on the processes already well developed for microelectronic and photonic device and substrate fabrication. Lithographic processes created for the semiconductor industry can be exploited with the addition of etching and growth processes specific to the broader range of microsystem materials. Hybrid arrays of discrete sensors have been reported (Haug et al., 1993) and even commercialized (Gardener and Bartlett, 1999), but have not yet been fully exploited. With the possible exception of acoustic-wave sensors requiring a piezoelectric transduction layer, current sensor substrate materials are not performance-limiting. Furthermore, they are compatible with CMOS processing (Hagleitner et al., 2001), permitting integration of heteroarray sensors with each other and with Si-based circuitry and MEMS components. Such a monolithically integrated heteroarray could provide a wide variety of sensors with electronics and a full range of MEMS capability, but fabricating such a complex microsystem presents tremendous materials processing challenges. The capability of controlling the location of polymers with different properties within microscopic tolerances throughout a microsystem needs
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to be developed for microanalytical systems. In situ thermal or photopolymerization from liquid- or, preferably, gas-phase precursors is one promising approach (Healey et al., 1995; Prucker et al., 1999; Hsieh and Zellers, 2002; Yu et al., 2001). Nanometer-resolution soft lithography (Huck et al., 2001), laser-induced manipulation and deposition of nanoscale particles (Niidome et al., 2001), and matrix-assisted laser desorption (Pique et al., 2000) are other novel approaches. Further technique developments and the design of materials for deposition, patterning, in situ modification, and adhesion could yield significant payoffs. Microsystem Packaging The packaging of electronics continues to be an area demanding new materials. For microelectronics, the drive to higher densities, higher speeds, and smaller sizes increases the need for thermal management. Military devices and sensor-based microsystems may be required to operate in hostile environments, placing even greater demands on electronics packaging. Materials are needed that are mechanically durable, thermally stable, and impervious to gas or chemicals that could alter the devices. The packages must also allow for low-loss electrical connections. Photonic devices must connect to guided-wave or free-space optics, requiring materials with transparency across various parts of the electromagnetic spectrum. Sensor-based microsystems create additional demands on materials; they must be nonmagnetic, chemically inert, and even biocompatible. These microsystems also require interconnections for mass transport and storage of biochemical materials. Low-temperature processing and joining methods must be developed that are compatible with the new materials. The demand for new materials and processes for electronic, photonic, and microsystem packaging is very strong; lack of appropriate materials may limit the functionality of future systems. DoD will need packaging methods that allow for systems to interact with the environment in ways—e.g., exchanging fluids, chemical communication, and optical coupling—that protect components that are environmentally sensitive. Packaging must also be robust, enabling device operation in environments as hostile as inside a human body, an operating engine, or a high-speed vehicle. Conclusions Microsystems are already showing their promise as a revolutionary technology that gives future U.S. military a significant advantage. The advantages of creating low-power, compact, lightweight, reliable systems
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for defense applications are compelling. They motivate addressing the significant materials issues involved. Although substantial R&D activity is under way, attention is particularly needed to materials processing, incorporation of bioderived and bioinspired components, materials compatibility, integration of nanoscale materials with engineered devices, and packaging. In the near term, emphasis is likely to be on integrating known materials into microsystems in a way that preserves the functionality of each material. In the future, new materials will be needed to increase system functionality. Power and thermal management are critical crosscutting issues, with ultimate links to the materials used in electronic and photonic engineered systems. Mass flow control and chemical compatibility are emerging needs for many microsystem applications. Rationally designed polymeric, nanocomposite, and anisotropic materials assembled or implemented in a way that allows response amplification, show significant promise as chemical microsensor interface materials and as preconcentration and separation media. Models to guide the design of such materials for multisensor heteroarrays are needed to optimize their information yield. Nanoscale devices that rely on quantum effects represent the most intriguing and potentially revolutionary avenues for research, with likely improvements in detection sensitivity and some promise for improved selectivity. Coupling materials design, synthesis, and film deposition strategies with MEMS and MOEMS device and system fabrication strategies is essential to mass production and to ensuring compatibilities during bonding, interconnecting, and packaging. RESEARCH AND DEVELOPMENT PRIORITIES This panel considered a wide variety of military needs from several vantage points, ranging from individual devices or components to entire (miniature) systems. Six common themes emerged that point to the most important areas of research. Fundamental Understanding of Existing Materials Important properties of many materials that are promising for defense applications are not well understood. While empirical engineering of known materials for a specific application can be expected to gradually advance the state of these materials, this approach has limits. The most
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dangerous possibility, perhaps, is that an undiscovered intrinsic property of a material may ultimately make it wholly unsuitable for the application. On a more optimistic note, fundamental scientific understanding is likely to give knowledge of the ultimate properties of a material and serve as a guide to its optimal processing. Truly fundamental understanding of complex real-world materials is only possible through a judicious combination of experiment, theory, and modeling, with intense feedback among the three. This will become even more important as we explore materials on the nanoscale at the interface between the discrete (quantum) and continuum (bulk) regimes of material behavior, where widely applicable modeling tools have not yet been developed. New Materials with Extreme Properties The increased functionality integrated into ever-decreasing volume drives the search for new materials that have properties (e.g., nonlinear optical properties) that are enhanced by an order of magnitude or more over known materials. Comparable improvements in material purity are also likely to be needed. Nanoengineered materials whose structure, composition, and morphology are varied on a nanometer scale in one, two, or all three dimensions are probably required. Another approach that is appropriate in some cases is to search for single-phase materials with more elements and crystallographic complexity than have historically been considered. Identifying such promising materials is daunting. Effective and efficient experimentation will require theoretical and especially computational tools that can deal with such complex systems reliably. Optimization and exquisite control of processing will be essential to the reproducibility of such complex structures, especially those that combine different classes of materials (e.g., organic and inorganic). New Ways of Combining Materials Another way of increasing functionality in a small volume is to combine materials in new ways. Routine incorporation of nanoengineered components into a system (especially a microsystem) would both add functionality and reduce volume and weight. Different classes of materials need to be combined in ways that are only beginning to be studied. These combinations often use inorganic
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materials that have high processing temperatures, structural rigidity, and known long-term stability with organic or even biological materials that have low temperature tolerances, relatively poor structural stability, sensitivity to the ambient (which is usually why they were chosen in the first place), and poor or unknown long-term stability. Processing such hybrid structures presents a tremendous challenge. The interfaces between materials must be understood, because the functionality often is derived from materials properties very close to the interface. As device and system dimensions shrink, interfaces become even more dominant because an increasingly higher percentage of atoms or molecules find themselves at one interface or another. Theory and modeling can be expected to serve as guides to selecting appropriate combinations of materials and processing methods. Packaging and Thermal Management Electronic and photonic materials will be integrated at increasingly greater scales in the coming decades. While the functionality that such systems provide will be staggering by today’s standards, it comes at a price. Packaging and thermal management will be a significant challenge. Research in these areas needs to be done hand in hand with research in the areas already outlined. Materials Processing Before they can be introduced into new systems, it will be necessary to understand, optimize, and control the processing of the materials discussed in this chapter. Fabricating such complex materials in the first place will be a formidable challenge, as will be the design of processes to incorporate these new materials into structures and devices for military systems. Low-temperature processes are desirable in most cases, indispensable in some. The complexity contemplated is too great to be able to rely on empirical process optimization alone. Processing models built on a solid theoretical underpinning will be key. These models will need to span length scales from nanometers to the scale of the device.
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Representative terms from entire chapter: