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--> Executive Summary As the twenty-first century approaches, the capacity to shrink electronic devices while multiplying their capabilities has profoundly changed both technology and society. Beginning in 1948, the vacuum tube gave way to the transistor, which was followed by a series of major strides leading to integrated circuits (ICs), which led to on-chip electronic systems, such as large-scale memories and microprocessors. Present silicon very-large-scale-integrated (VLSI) chip technology seems destined to continue developing for at least another 20 years based on smaller and smaller electronic devices that can operate faster and do more. In the late 1980s, the design and manufacturing tool set developed for VLSI was adapted for use in a field called microelectromechanical systems (MEMS). These systems interface with both electronic and nonelectronic signals and interact with the nonelectrical physical world as well as the electronic world by merging signal processing with sensing and/or actuation. Instead of handling only electrical signals, MEMS also bring into play mechanical elements, some with moving parts, making possible systems such as miniature fluid-pressure and flow sensors, accelerometers, gyroscopes, and micro-optical devices. MEMS are designed using computer-aided design (CAD) techniques based on VLSI and mechanical CAD systems and are typically batch-fabricated using VLSI-based fabrication tools. Like ICs, MEMS are progressing toward smaller sizes, higher speeds, and greater functionality. MEMS already have a track record of commercial success that provides a compelling case for further development (e.g., pressure-sensing, acceleration sensing, and ink-jet printing). Like any developing field, however, commercial successes in the MEMS field coexist with products still under development that have not yet established a large customer base (e.g., MEMS display systems and integrated chemical-analysis systems). The U.S. Department of Defense and the National Aeronautics and Space Administration requested that the National Research Council conduct a study (1) to review current and projected MEMS needs based on projected applications, (2) to identify shortcomings in present and developing MEMS technologies, (3) to recommend how MEMS can best use advanced materials and fabrication processes to overcome these shortcomings, and (4) to recommend research and development areas that would lead to the necessary advances in materials and fabrication processes for MEMS. The Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems, under the auspices of the National Materials Advisory Board, was convened to undertake this study and write this report. The committee concluded that the MEMS field faces a number of challenges to the establishment of an environment that promotes healthy and vigorous growth. These challenges are presented in this Executive Summary along with recommendations for meeting them. Because of the broad perspective with which the MEMS field is viewed in the report, the findings and recommendations are not prioritized. LEVERAGING AND EXTENDING THE INTEGRATED CIRCUITS FOUNDATION A great deal of the excitement and promise of MEMS has arisen from the demonstrated ability to produce three-dimensional fixed or moving mechanical structures using lithography-based processing techniques derived from the established IC field. Conventional IC materials can continue to be used in new ways in MEMS, and much of the needed MEMS-specific hardware can still be leveraged from IC technology. Such MEMS developments are most likely to be accepted in traditional IC-fabrication facilities and therefore most likely to succeed commercially. In the microelectronics world, major steps forward have sometimes resulted from inspired looks backward at technologies and materials that were already known and well categorized. For MEMS, this "cleverness research" can take on a special character by posing mechanical problems to technologies that originally responded only to the demands of electrical design. A wide field of opportunity for creative work in MEMS could be based on what is already known about IC processing, particularly in the re-evaluation of the vast knowledge compiled during the history of IC development (e.g., transistor-transistor logic; integrated-injection logic; analog; bipolar; n-channel metal oxide semiconductors). Conclusion. The expertise and advanced state of the current microelectronics industry provides an enormous advantage for the development of MEMS. Leveraging and extending existing IC tools, materials, processes, and fabrication techniques is an excellent strategy for producing MEMS with
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--> comparable levels of manufacturability, performance, cost, and reliability to those of modern VLSI circuits. Recommendation. Efforts to stimulate solutions to the challenges of producing MEMS should capitalize on the families of relatively well understood and well documented IC materials and processes. These solutions may be found in current IC practices but may also result from creatively re-establishing older IC technologies. This recommendation calls for continuing strategic investment. ENLARGING THE SUITE OF MATERIALS SUITABLE FOR INTEGRATED-CIRCUIT-LIKE PROCESSING Although there may be commercial advantages to leveraging the present suite of IC-process materials, they will not be able to meet all of the demands that a growing number of users and applications will place on MEMS. Easily foreseen requirements (e.g., higher forces, stability in harsh and high-temperature environments, and robust high-aspect-ratio structures) will compel the application of new materials and extend the MEMS field beyond the boundaries of the IC world. Materials that are not usually used in IC processes include magnetic, piezoelectric, ferroelectric, and shape-memory materials. Actuating-force requirements for valve closures and motor drives, for example, are already drawing attention to the advantages these materials would bring to MEMS. Other developments, such as MEMS for optics, biological purposes, chemical-process controls, high-temperature applications, and other hostile environments, will inevitably draw attention to the need for an even broader range of materials. In the IC world, new materials are typically incorporated as thin-films and are produced by a limited number of techniques (e.g., low-pressure chemical-vapor deposition or sputtering). Many of these materials either do not show optimal mechanical properties in thin-film form or are difficult to deposit by typical IC-fabrication methods or are incompatible with the microelectronic IC process. For some MEMS designs, it is possible to apply these specialized materials either by incorporating them in a step prior to more-conventional processing or by adding them as a final step. Either option raises the possibility that the technology will be substantially different from better known processing techniques. Materials that are incompatible with the IC-processes might have to be handled by a specialized foundry. Conclusion. Extending the list of materials that have useful MEMS properties and can be processed using lithography-based, IC-compatible techniques will be beneficial to MEMS development. Recommendation. Research and development should be encouraged to develop new materials that extend the capabilities of MEMS. The new materials should be integrable, at some level, with conventional IC-based processing. This recommendation calls for continuing strategic investment. Recommendation. Research should be encouraged to develop techniques to produce repeatable, high-quality, batch processed thin-films of specialized materials and to determine the dependence of their properties on film-preparation techniques. For some materials, it may be advisable to establish ''foundries" that are available to the entire MEMS community and can serve as repositories for equipment and know-how. This recommendation calls for new strategic investment. CHARACTERIZING MEMS MATERIALS The IC industry has been built on an extensive, constantly expanding body of knowledge about the behavior of silicon and related materials as they are scaled down in size. No comparable resource has been established for MEMS, however. For example, although a great deal is known about the electrical properties of polysilicon thin-films, not much is known about their micromechanical properties or about specific details of the long-term reliability of mechanically stressed polysilicon or the surface mechanics related to friction, wear, and stress-related failure. There is a similar lack of fundamental knowledge about other thin-film materials borrowed from the electrical domain that are now exercised mechanically (e.g., silicon nitride, silicon dioxide, and thin-film metals). Many thin-film materials that are used in the IC industry (e.g., aluminum, silicon dioxide, amorphous silicon, porous silicon, various other deposited and plated metals, and polyimide) have still not been extensively studied and evaluated for their applicability to MEMS. Conclusion. A thorough understanding of the micromechanical properties of the materials to be used in MEMS at appropriate scales is not available. Recommendation. The characterization and testing of MEMS materials should be an area of major emphasis. Studies that address fundamental mechanical properties (e.g., Young's modulus, fatigue strength, residual stress, internal friction) and the engineering physics of long-term reliability, friction, and wear are vitally needed. It is important that these studies take into account fabrication processes, scaling, temperature, operational environment (i.e., vacuum, gaseous, or liquid), and size dependencies. Studies of the size effects of physical elements, on a scale comparable to the crystallite regions in a polycrystalline material, are required. This recommendation calls for continuing strategic investment.
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--> UNDERSTANDING SURFACE AND INTERFACE EFFECTS The properties of materials can differ at the small scales at which individual MEMS devices are configured, causing effects that can influence their behavior. At these tiny scales, material behavior is more influenced by surface-driven effects than by volume or bulk effects. For example, frictional effects take on overwhelming importance, in contrast to inertial effects, in small mechanical systems. If the interfaces act as electrical contacts (e.g., in MEMS microrelays), additional wear, corrosion, frictional effects, and contact forces are present. Surface-to-surface sticking (stiction) is also likely to be important in surface-driven processes. During the drying process and after the final cleaning of MEMS devices, the surface tension of the meniscus of liquids can pull suspended mechanical structures toward nearby surfaces, causing the structures to become stuck. Stiction can also occur during the operation of actuated MEMS if shock, electrostatic discharge, or other stimuli cause moving components to touch either each other or to touch another surface. The MEMS operating environment and the interfaces of this environment on individual MEMS devices can influence performance. Signals admitted to the MEMS package may have electrical, thermal, inertial, fluid, chemical, optical, and possibly other origins. Output can be electrical, optical, mechanical, chemical, hydraulic, or magnetic signals. MEMS applications to liquid systems, for example, would raise interface questions about the use of wetting and dewetting agents and the nature of fluids in micrometer-sized channels and cavities. The high-precision of some MEMS sensing devices also makes them sensitive to gas/solid interactions. Conclusion. Further development of moving elements in MEMS demands a more complete understanding of (1) the effects of internal friction, Coulomb friction, and wear at solid/solid interfaces and (2) the influence of interfaces on performance and reliability. This understanding should lead to the development of suitable coatings, lubricants, and wetting agents, as well as improved designs that take these effects into account. Recommendation. Surface and interface studies should be pursued to address questions associated with contact forces, stiction, friction, corrosion, wear, lubrication, electrical effects, and microstructural interactions at solid, liquid, and gaseous interfaces. Engineering design and manufacturing solutions to the problems associated with MEMS surfaces and interfaces should also be pursued. This recommendation calls for continuing strategic investment. ETCHING TECHNOLOGIES At the heart of MEMS is the ability to construct extremely small mechanical devices, preferably using batch processing. Wet etching has historically dominated the MEMS field because (1) structures can be micromachined from silicon in a short time and (2) chemical-etch equipment is well established, simple, and inexpensive. The disadvantages of wet-chemical processing are its inability to achieve vertical-sidewalls and nonorthogonal linear geometries in d silicon and its reaction with films on the wafer surface. Because of the lateral spread of etching, patterned features must also be spaced relatively far apart so that adjacent features do not merge, and the features on the mask and pattern-transfer layer must be biased or reduced (and sometimes even distorted) to achieve the desired size and shape at the completion of the wet-etch process. Although dry etching is a mainstay of IC processing and gas-phase dry-etching techniques are currently a subject of research for MEMS production, the etch depths for MEMS are often significantly greater than those commonly employed in IC-fabrication. Therefore, etching for MEMS may present different or additional challenges. Conclusion. Because controlled etching is so important to the fabrication of three-dimensional structures and, therefore, to progress in MEMS, methods of etching in a controlled fashion and ways of tailoring the isotropic or anisotropic etch-rates of various materials are of great value. Recommendation. Further research and development should be undertaken to improve etches, etching, and etching controls for MEMS. This work should take into account the status, potential development, and limitations of manufacturing-process equipment. This recommendation calls for continuing strategic investment. ESTABLISHING STANDARD TEST DEVICES AND METHODS Standard test devices and methods are required to determine the mechanical properties of MEMS devices, to demonstrate the repeatability and reliability of mechanical devices, and to facilitate quality-control practices. Package-level testing is currently the most common way to measure MEMS performance, but the development of in-process wafer-level testing will be necessary for low cost manufacturing. Wafer-level testing of MEMS presents special challenges that are often product dependent. Nevertheless, generic test structures that indicate basic mechanical properties of MEMS materials at the wafer-level should be developed and characterized. As more and more industries, universities, and other research groups enter the MEMS field, it is also becoming increasingly
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--> important to provide accepted standards that can be used for comparison. Conclusion. Test-and-characterization methods and metrologies are required to (1) help fabrication facilities define MEMS materials for potential users, (2) facilitate consistent evaluations of material and process properties at the required scales, and (3) provide a basis for comparisons among materials fabricated at different facilities. Recommendation. Standard test methods, characterization methods, and test devices should be developed and disseminated that are suitable for the range of materials and processes of MEMS. Ideally, metrology structures will be physically small, simply designed, easily replicated, and conveniently and definitively interrogated. MEMS engineering standards should be similar to those already established for materials and devices in conventional sizes by organizations such as the National Institute of Standards and Technology (NIST), the American Society for Testing and Materials (ASTM), and the Institute of Electrical and Electronics Engineers (IEEE). This recommendation calls for new strategic investment. MEMS PACKAGING Packaging a device, interfacing it to its operating domain, and assembling it as a part of a larger system are critical final production steps and can easily represent up to 80 percent of the cost of a component. Although considerable attention continues to be paid to innovative applications of MEMS processing techniques and devices, "back-end" processes have historically been approached on a specialized, case-by-case basis. The lack of publicly available technology or information to support packaging has meant that each organization has essentially had to invent and reinvent solutions to common problems. Possible extensions of batch processing to back-end processes could substantially reduce costs. Conclusion. Packaging, which has traditionally attracted little interest compared to device and process development, represents a critical stumbling block to the development and manufacture of commercial and military MEMS. The imbalance between the ease with which batch-fabricated MEMS can be produced and the difficulty and cost of packaging them limits the speed with which new MEMS can be introduced into the market. Expanding the small knowledge base in the packaging field and disseminating advances aggressively to workers in MEMS could have a profound influence on the rapid growth of MEMS. Recommendation. Research and development should be pursued on MEMS packaging and assembly into useful engineering systems. The goal should be to define, insofar as possible, generic, modular approaches and methodologies and to extend batch-processing techniques into the various back-end steps of production. This recommendation calls for new strategic investment. FOUNDRY AND COMPUTER-AIDED DESIGN INFRASTRUCTURE FOR MEMS Rapid development in the IC industry has been aided by the establishment of a foundry infrastructure that ensures that industry and government users will be able to manufacture IC products at competitive rates and enables companies that do not have wafer-processing capabilities to enter the field. One of the key factors in the development of the IC foundry infrastructure was the development of a CAD infrastructure that became the backbone of foundry operations. Design methods were implemented that allowed IC designers to develop systems independently and have them manufactured by submitting only a design-language file. The MEMS field is more complicated because of the broad range of electrical and mechanical applications, including consumer, automotive, aerospace, and medical products. Thus, several standard process MEMS foundries would have to be available and accessible, as well as custom, flexible fabrication facilities for users who require access and manipulation of the process to produce and optimize their products. The committee recognizes that realizing the concept of MEMS foundries may be difficult because many commercial companies have difficulty seeing "what's in it for them." Besides the danger of compromising proprietary know-how, companies offering a foundry service will have to commit to specific processes and reasonable turnaround schedules. In the instances where small industries have tried to accommodate MEMS foundry runs so far, the results have not been warmly received. A more feasible road to at least moderate success at the present juncture appears to be using academic and government laboratories to provide foundry services. The recent expansion of the National Nanofabrication Laboratory to sites at several universities and the capabilities of national laboratories, like Sandia and Livermore, may provide opportunities for MEMS foundries of a different nature, where direct hands-on work can be done by the MEMS researcher. This kind of operation could not be as widely extended as the more traditional foundry approach of MCNC, which interacts with users only through exchanges of software, but it may provide an interim avenue until specific areas in the MEMS field are further developed. Conclusion. Establishing standard CAD and foundry infrastructures for MEMS is essential in the near future to support the growth of MEMS from the prototype and low-volume commercial level to the volume-driven, low-cost commercial level. The development of a MEMS foundry-technology
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--> base, similar to the base that supports ICs, would ensure that MEMS products could be manufactured at competitive rates and would enable more small companies and research organizations to enter the field. Recommendation. A MEMS CAD-infrastructure that extends from the processing and basic modeling areas to full system-design capabilities should be established. A process technology infrastructure (e.g., supporting electrical, mechanical, fluid, chemical, and other steps and their integration to form complete systems) that is widely available to MEMS designers and product engineers should be developed. This recommendation calls for new strategic investment. ACADEMIC STRUCTURE TO SUPPORT MEMS The field of MEMS rests on multidisciplinary foundations. Practitioners who are poised to advance MEMS must have knowledge and skills in several fields of engineering and applied sciences. The participation of motivated, well trained young researchers is probably the single most important driver for success in MEMS. Some of these researchers will come from the ranks of trained IC engineers, who are already familiar with tools, materials, and procedures that are useful for MEMS. In general, however, these practicing engineers will have to learn new aspects of mechanical design, materials behavior, computing techniques, and systems design. Providing learning opportunities and educational materials for practicing engineers is important. But for future engineering students, effective instruction in MEMS will require major changes in curricula. A high priority should be placed on establishing an academic infrastructure that conveys the excitement and promise of the field, offers a sound and thorough education for MEMS researchers, and facilitates development of and access to new and innovative ideas across and among various disciplines. Conclusion. Contributors to MEMS can be recruited both from practitioners already active in the IC field and from newly trained engineers. To facilitate the entry of practicing engineers into the field, opportunities to learn material that is special to MEMS should be encouraged through stimulating short courses and specialized text materials. For engineering undergraduates entering MEMS, programs and industrial procedures should be encouraged that stimulate multidisciplinary university education and enhance the skill and knowledge base of those training for or contributing to the development of MEMS. New MEMS engineers will require a broad understanding of several fields (e.g., electrical, mechanical, materials, and chemical engineering). Recommendation. MEMS short courses and instructive materials that introduce practicing IC engineers to MEMS should be encouraged. Teaching institutions should be encouraged to see the benefits to their students and to their programs of emphasizing a broad, basic foundation in materials, production techniques, and engineering needed for MEMS. This recommendation calls for new strategic investment.
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