MICROELECTROMECHANICAL SYSTEMS



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--> Microelectromechanical Systems (MEMS) Kaigham J. Gabriel Electronics Technology Office Defense Advanced Research Projects Agency Arlington, Virginia As information systems increasingly leave central control areas and appear in distributed systems, they are getting closer to the physical world, thus creating new opportunities for perceiving and controlling the physical environment. To exploit these opportunities, information systems will need to sense and act as well as compute. Filling this need is the driving force for the development of microelectromechanical systems (MEMS). Using both the fabrication techniques and materials of microelectronics as a basis, MEMS processes are used to construct both mechanical and electrical components. Mechanical components in MEMS, like transistors in microelectronics, have dimensions that are measured in microns and numbers measured from a few to millions (Figure 1). MEMS is not about any one single application or device, nor is it either defined by a single fabrication process or limited to a few materials. More than anything else, MEMS is a fabrication approach that conveys the advantages of miniaturization, multiple components, and microelectronics to the design and construction of integrated electromechanical systems. Potential applications include miniature inertial measurement units for competent munitions and personal navigation; distributed unattended sensors for asset tracking and environmental/security surveillance; mass data storage devices; miniature analytical instruments; a range of embedded pressure sensors for passenger car, truck, and aircraft tires; noninvasive biomedical sensors; fiberoptic components and networks; distributed aerodynamic controls; and on-demand structural strength sensors.

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--> Figure 1 Typical MEMS structures are very planar, but they can be several hundred  microns in area. This structure is about 2 microns thick in polysilicon.  Source: Analog Devices, Inc. MEMS Fabrication Technologies Although MEMS fabrication uses many of the materials and processes of bulk and surface micromachined semiconductor fabrication, there are important distinctions between the two technologies. The most significant distinctions between MEMS fabrication and semiconductor fabrication are in the process recipes (the number, sequence, and type of deposition, removal, and patterning steps used to fabricate devices) and in the end-stages of production (bonding of wafers, freeing of parts designed to move, packaging, and testing). The fundamental challenge of using semiconductor processes for MEMS fabrication is not in the type of processes and materials used but in the way those processes and materials are used. Wafer-to-wafer bonding, a versatile fabrication technique that yields high quality interfaces and bonds, is commonly employed to get around the restrictions in the type of structures that can be fabricated using bulk micromachining. Because anisotropic etching, by definition, only removes material, bonding of wafers allows for the addition of material to the bulk micromachining reper-

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--> toire. Thus, wafer-to-wafer bonding (bonding under pressure or a combination of pressure and a high voltage across the wafer) of two or more micromachined wafers is used to construct MEMS. Constituent wafers can be bulk micromachined wafers, wafers with prefabricated electronics, or wafers micromachined by other techniques. In many cases, the bonded wafers are silicon-to-silicon, but silicon-to-quartz and silicon-to-pyrex bonds are also common. Despite the usefulness of bulk micromachining and wafer-to-wafer bonding (and their continuing commercial importance), these micromachining techniques are limiting in the type of features that can be sculpted. Bulk micromachined structures and features are defined by the internal crystalline structure of the material. Fabricating multiple, interconnected electromechanical parts of free-from geometry using bulk micromachining is often difficult or impossible. Although wafer-to-wafer bonding gets around some of these limitations, truly free-form geometries and integrated multicomponent (multiple, interconnected, and cofabricated components) electromechanical structures presently are produced by a relatively new micromachining approach, surface micromachining, that is fundamentally different from bulk micromachining and wafer-to-wafer bonding. MEMS Program at Darpa The long-term goal of DARPA's MEMS program is to merge information processing with sensing and actuation in order to realize new systems and strategies for both perceiving and controlling systems, processes, and the environment (Department of Defense, 1995). There are many opportunities for insertion of MEMS devices into DOD systems across a number of technologies and products. These include: inertial navigational units on a chip for munitions guidance and personal navigation; distributed unattended sensors for asset tracking, border control, environmental monitoring, security surveillance, and process control; integrated fluidic systems for miniature chemical/biological analysis instrumentation, hydraulic and pneumatic systems, propellant and combustion control, and printing technology; weapons safing, arming, and fuzing to replace current warhead systems (to improve safety and reliability); low-power, high-resolution, small-area displays for tactical and personal information systems; embedded sensors and actuators for condition-based maintenance of machines and vehicles, and for on-demand amplified structural strength in lower-weight weapons systems/platforms and disaster-resistant buildings;

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--> mass data storage devices for storage densities of terabytes per square centimeter; integrated microoptomechanical components for identify-friend-or-foe (IFF) systems, displays, and fiberoptic switches/modulators; and active, conformal surfaces for distributed aerodynamic control of aircraft, adaptive optics, and precision parts and material handling. Chemical and Biological Agent Detection As the 1994 chemical-agent attacks on the Tokyo subway system demonstrate, chemical and biological agents are a continuing and pervasive threat. User-friendly miniature devices are needed that can be used to perform key missions, such as nuclear, biological, and chemical (NBC) operations; treaty verification; cargo inspections; and detection/identification of fuels, explosives, and illegal drugs. MEMS research and development progress in the next 5 years may result in a variety of small, low-cost, low-power portable analytical instruments having compact versatility and a built-in self-test/calibration feature. For example, an ideal MEMS NBC detector, with a small display, could be developed that would be an integral component of each gas mask. A detector of this type could be mounted as well on other items of military equipment. These MEMS devices would enable the quick detection, alarm, and identification of threat agents, and thus could also verify that decontamination efforts were effective. Such capabilities would eliminate the need for many specialized teams that currently must be dispatched to a reported contamination site. Mass Data Storage Mass data storage requirements continue to increase as the military moves toward increased digitization. Future tactical computing systems must be small, light, and often low power to be useful to highly mobile forces. For example, a dismounted reconnaissance team would need a system that could hold several digital maps, photographs, field manuals, and databases—potentially requiring 10 gigabytes or more of storage. No portable battery-powered data storage system exists that can support this need. Both MEMS-enhanced conventional magnetic disk drives and future atomic-resolution data storage systems fabricated on silicon substrates and integrated with signal processing electronics substantially will decrease the size, weight, power requirements, latency of access, failure rate, and cost of data storage. Advanced tunneling-based write-once, read-many-times (WORM) devices offer as much as 100,000 times the storage density of a current CDROM. Microdisks, when coupled with advances in low-power computing and displays, would enable major advances in portable electronic devices. If this technology were applied only to portable devices, one could easily envision one digital assistant (with an embedded MEMS disk drive) being issued to each service member. This would result in a minimum DOD market of 1.5 million units.

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--> Aircraft Performance Aircraft development requires continued efforts to squeeze every possible ounce of performance into a design enabling an aircraft to travel faster and farther, and with greater payload, greater maneuverability, and higher efficiency. These goals could be realized through use of distributed MEMS sensors and actuators on the wing flaps, controlling the separation of leading-edge vortices. Active deferrable surfaces also could be applied to rotor blades on helicopters to achieve greater lifting efficiency, on submarine surfaces to reduce noise, and as advanced sonar with multiple arrays. Structural Strength In weight-critical applications, increasing the strength-to-weight ratio of structural components offers improvements in performance. MEMS devices can be surface mounted or embedded into advanced and conventional structural members both to monitor static and dynamic loading conditions and then to react as required in order to provide localized strengthening. With distributed sensors and actuators injecting the right amount of energy at the critical time and place, structural components will become stronger for just those few necessary microseconds. The potential exists to create materials that are six to ten to theoretically a hundred times stronger than what the Euler buckling theory would predict. These materials then could be used in aircraft as well as earthquake-resistant buildings. MEMS R&D Strategy The MEMS research and development strategy at DARPA is as follows: Invest in advanced MEMS devices and systems, leading toward MEMS with higher levels of functional capability, higher levels of integrated electronics, and greater numbers of mechanical components. Activities in this area will accelerate both the development of actuator-enabled applications and the shift from discrete MEMS component manufacturing to the manufacturing of integrated MEMS devices. Focused thrusts include the development of new materials, devices, systems, fabrication processes, and interfacing/packaging techniques. Invest in the development of a MEMS infrastructure by developing support and access technologies, including electronic design aids and databases, shared fabrication services, and test/evaluation capabilities. Infrastructure development activities will increase and broaden the pool of MEMS designers; will enable rapid, timely, and affordable access to MEMS technologies for evolving needs; and will create a national mechanism for cost-effective MEMS prototyping and low-volume production. An ongoing project supported by DARPA offers regular, shared access to a single common MEMS fabrication process, which already has been employed by over 300 users at service/federal laboratories, domestic companies, and universities. More than half of the users (and all of the small businesses) are getting their first and only access to MEMS technology through this shared fabrication service.

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--> Invest in activities to accelerate the insertion of presently available or near-term commercial MEMS products into military systems and operations; examples include munitions safing and arming and condition-based maintenance. In this area investments focus on improved and affordable manufacturing resources, assembly/packaging techniques, and methods of assessing and qualifying device performance and reliability. Activities encourage and are aligned with industry-formed teams that speed the introduction and use of MEMS fabrication processes and products. Coordinate and complement federal programs by establishing a DOD and interagency MEMS specialists group that is chaired by a representative of DARPA. Examples of ongoing activities in this area include coordinated projects in fluid dynamics and integrated MEMS fluidic devices (Air Force Office of Scientific Research and DARPA) and in materials standards and databases (National Institute of Standards and Technology and DARPA), as well as a project to broaden MEMS education and training programs, to increase the number of qualified MEMS instructors, and to couple these instructors with shared fabrication services (National Science Foundation and DARPA). Market Trends Forecasts for MEMS products throughout the world show rapid growth for the foreseeable future. Early market studies projected an eightfold growth in the nearly $1 billion 1994 MEMS market by the turn of the century, but more recent estimates are forecasting growth of nearly 12 to 14 times today's market, reaching $12 to $14 billion by the year 2000. Whereas sensors (primarily pressure and acceleration) are the principal MEMS products produced today, no single product or application area is set to dominate the MEMS industry for the foreseeable future, since the MEMS market is growing both in the currently dominant sensor sector and in the actuator-enabled sectors. Furthermore, because MEMS products will be embedded in larger non-MEMS systems (e.g., automobiles, printers, displays, instruments, and controllers), they will enable new and improved systems with a projected market worth approaching $100 billion in the year 2000. Although MEMS devices will be a relatively small fraction of the cost, size, and weight of these systems, MEMS will be critical to their operation, reliability, and affordability. MEMS devices, and the smart products they enable, increasingly will be the performance differentiator for both commercial and defense systems. MEMS Roadmap Figure 2 illustrates the central concept of this technology, which is the merger of computation with sensing and actuation. As we view the breadth

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--> Figure 2 MEMS technology trends and roadmap. Log-log plot of number of transistors merged with number of mechanical components for MEMS devices and systems. Contours of equal transistors-to-mechanical-components ratios (T/M) are lines of 45° slope. Lines representing T/M ratios ranging from 10-4 to 106 are shown for reference. The resulting map represents a quantitative way to measure and track MEMS technology advances across different application areas. Source: Reprinted with permission from Scientific American (Gabriel, 1995). and spectrum of applications, it is important to understand the underlying similarities of all of these different application areas. Along the vertical axis is the number of transistors, which is a rough measure of the increasing ability to compute. Along the abscissa is the increase of the log plot of the number of mechanical components, which is the increasing ability to sense and act. The majority of existing MEMS devices are down in the lower left-hand corner, where devices have a few mechanical components with a few transistors. The real potential and application area is going to be as we move out from the corner and explore increasing levels of integration and increasing levels of sensing and actuation. Two representative examples are the ADXL-

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--> 50, an integrated surface micromachined accelerometer that is a single mechanical component with a few hundred transistors, and the digital micromirror display (DMD), which in its high-definition version has two million mirrors and about twelve million transistors. There is a lot of space left to explore. The DARPA program is pushing the edges of the envelope to explore the application possibilities. References Department of Defense. 1995. Microelectromechanical systems, A DoD dual use technology industrial assessment. Final Report. Washington, D.C.: U.S. Department of Defense. Gabriel, K. J. 1995. Engineering microscopic machines. Scientific American 273(3):118-121.