7. MEMS DESIGN TECHNIQUES, APPLICATIONS, AND INFRASTRUCTURE
(Joseph M. Giachino)

MEMS have been used to describe microminiature systems that are constructed with both IC-based fabrication techniques and other mechanical fabrication techniques. In most cases, an emphasis has been placed on having the techniques compatible with IC techniques to ensure the availability of related electronics close by. Most researchers require that the MEMS be contained within the same package, while some require that a MEMS be contained on a single chip.

To date, the integrated circuit industry has been the technology base that has driven MEMS. This is shown in both the bulk silicon and polysilicon efforts that have been the mainstay of MEMS devices. The MEMS community has made significant advances in the area of deep etching bulk silicon and in surface (sacrificial etching) micromachining with polysilicon. MEMS have driven the silicon community into understanding the mechanical and electrical properties of silicon structures. MEMS have driven researchers to investigate fabrication methods other than IC-based techniques to obtain microdevices. These techniques include LIGA, laser-assisted CVD, electroplating, and electroless plating.

The advantages of the MEMS technology include small size, low power, very high precision in manufacture, and the potential for low cost through batch fabrication. MEMS does offer a challenge in the area of how to effectively package devices that require more than an electrical contact to the outside. Pressure sensors are the most commercially successful MEMS-type sensors to use nonintegrated circuit-type packaging. Hall sensors, magnetoresistive sensors, and silicon accelerometers have all used IC-based packaging. The IC packaging is viable with these devices since the measurand can be introduced without violating the package integrity. Some optical systems use IC-type packages with windows. MEMS will require the development of an extensive capability in packaging to allow the interfacing of sensors to the environment. The very advantage of small size becomes a liability when a device is open to the environment. The general area of MEMS durability is also one that has to be improved. There is not a good understanding of all the mechanisms that lead to wear in moving devices on this scale; however, work to understand these mechanisms is being done by various groups. Proven durability is a major need before MEMS technology can be extended to high reliability, long-term (greater than five years) applications. Proven durability has been shown in pressure sensors and accelerometers. For rotating devices (micromotors), there is still a concern.

The greatest impact of MEMS is likely to be in the medical field. A true MEMS (sensor, actuator, and control) should allow the treatment of patients to improve substantially. The ability to monitor and dispense medicine as required by the patient will improve the treatment of both chronic (i.e., diabetes) and acute (i.e., infectious) conditions.

The key advantages of medical MEMS devices are small size, low power demand, and low cost per function. Small size allows in vivo implants and surgical procedures to be performed in confined spaces. There is extensive work now being done in dispensing systems for drugs that can be delivered as required by the patient. Experimental work has also been done using silicon scissors for surgery and silicon neural probes to study neuron activity. The small size of MEMS structures also makes them ideal for manipulating biological samples at the microscopic level. With scanning probe microscopes, it is possible to watch living cells at the molecular level. In the future, MEMS systems that manipulate genes routinely could be built. MEMS can be fabricated to create a complete sampling system, including sensors to determine what the item is, actuators to move the items about, and electronics to analyze and transmit information and, where appropriate, to do control functions.

MEMS applications to automotive, robotics, and consumer products will also have an impact in the next decade. MEMS systems will allow improved analytical capabilities on the molecular and atomic levels. This means a better understanding of such phenomena as wear. The capability of analyzing, combined with that of manipulating on the atomic level, means that very unique and specialized devices could be fabricated. Vision systems that emulate the human eye are not unreasonable goals.

Within the next ten years, MEMS systems will provide applications in a variety of areas, including:

To support the development of these products, an infrastructure has been established in the United States to exchange information, develop generic techniques for MEMS, and train students.

The structure in the United States does not have a central agency that coordinates and controls funding and/or research priorities. Individual government agencies have recognized the importance of MEMS and have supported the development of an infrastructure. The National Science Foundation was instrumental in advancing MEMS by establishing a Center of Excellence for Micromachining and supporting student attendance at conferences. Other government agencies, including the Advanced Research Projects Agency, the National Institutes of Health, the Department of Energy, the National Aeronautics & Space Administration, and the Department of Defense have all supported MEMS work at universities to advance their specific needs, and have underwritten student attendance at conferences. There has been extensive work at the national laboratories in MEMS for the environmental sensors. Sensors have been developed for water, air, and soil monitoring.

The professional engineering societies have been extremely supportive of MEMS work. The IEEE established the first workshop on solid-state sensors and actuators in 1984 and the MEMS conference in 1987. Today there are conferences supported by the ASME on MEMS as well as a joint IEEE/ASME journal on MEMS. These are both ongoing activities that allow for exchange of information among those active in the field of MEMS. Many of those first students are now active professionals in the MEMS field.

The government agencies have been very active in promoting cooperative research between industry and universities. The Center of Excellence requires that most of the funding come from outside the NSF. Conversion monies, those redirected from national defense, require that a commercial goal be established. MEMS in the United States has been very product oriented, with both large companies and small companies making significant advances. To expedite the availability of facilities to more universities and companies, the concept of a foundry system is receiving government support. Foundries are being established to allow proven integrated circuit structures to be manufactured, and then micromachining is performed at another facility. Institutions with minimal facilities can now do development in the MEMS area since they only need facilities for micromachining. Since proven integrated circuits will be used for the development, this should allow for faster development of concepts and devices.

The other area being supported by the government funding to universities is the CAD area for MEMS. To be able to do MEMS modeling requires the ability to do concurrent mechanical and electrical simulation. The present quality of MEMS modeling is one of the hindrances to fast commercialization of MEMS devices. To make modeling viable will require a data base of MEMS materials. There have been attempts by various organizations to compile data for general use. Since no central, recognized organization exists to perform this function, it has been done in an informal manner at conferences, workshops, and personal contact.

MEMS technology continues to grow in the United States, with expanded efforts in universities and research facilities. The commercialization of the technology is proceeding at a slower pace due to the need for companies to perceive the value to be gained by MEMS products. To date this has been in the high-volume sensor markets of the automotive and medical industries.

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A SELECTED BIBLIOGRAPHY OF RECENT U.S. RESEARCH ON MICROACTUATORS

Comb Drives

Fan, L.-S., and L. Crawforth. 1992. "`Spring-Softening' Effect in MEMS Microstructures." Technical Digest, Transducers '93. Pp. 767-770.

MacDonald, N.C. 1992. "Single Crystal Silicon Nanomechanisms for Scanned-Probe Device Arrays." Technical Digest, IEEE Solid-State Sensor and Actuator Workshop. Tang, W.C., M.G. Lim, and R.T. Howe. 1992. "Electrostatic Comb Drive Levitation and Control Method." J. Microelectromechanical Systems. 1, 4: 170-178.

Resonators

Boustra, S., H.A.C. Tilmans, A. Selvakumar, and K. Najafi. 1992. "Base Driven Micromechanical Resonators." Technical Digest, IEEE Solid-State Sensor and Actuator Workshop. Pp. 148-152.

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Nguyen, T.-C.H., and R.T. Howe. 1993. "Microresonator Frequency Control and Stabilization Using an Integrated Micro Oven." Technical Digest, Transducers '93. Pp. 1040-1043.

Rotating Micromotors

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Dhuler, V.R., M. Mehregany, and S.M. Phillips. 1992. "Micromotor Operation in a Liquid Environment." Technical Digest, IEEE Solid-State Sensor and Actuator Workshop. Pp. 10-13.

Huang, J.B., P.S. Mao, Q.Y. Tong, and R.Q. Zhang. 1993. "Study on Si Electrostatic and Electro-Quasi-Static Micromotors." Sensors and Actuators. 35, 3: 171-174.

Kumar, S., and D. Cho. "Electrostatically-Levitated Microactuators." 1992. In Micromechanical Systems. Pp. 53-68. (Paper presented at the Winter Annual Meeting of the ASME, 8-13 November, Anaheim, CA.)

Mehregany, M., M.P. Omar, and R.L. Mullen. 1992. "Analysis of Motive Force, Axial Torque, and Viscous Drag Torque in Side-Drive Micromotors." In Micromechanical Systems. Pp. 133-148. (Paper presented at the Winter Annual Meeting of the ASME, 8-13 November, Anaheim, CA.)

Friction and Sticking

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Deng, K., W.H. Ko, V.R. Dhuler, M. Mehregany, S.M. Phillips, and J.H. Lang. 1992. " A Comparative Study of Bearing Designs and Operational Environments for Harmonic Side-Drive Micromotors." Proc. IEEE Microelectromechanical Systems Workshop. 35, 1: 171-176.

Tavrow, L.S., S.F. Bart, and J.H. Lang. 1992. "Operational Characteristics of Microfabricated Electric Motors." Sensors and Actuators. 35, 1: 33-44.

Magnetic Drive

Ahn, C.H., and M.G. Allen. 1992. "A Fully-Integrated Micromagnetic Actuator With a Multilevel Meander Magnetic Core." Technical Digest, IEEE Solid-State Sensor and Actuator Workshop. Pp. 14-18.

Allen, M.G. 1993. "Polyimide-Based Processes for the Fabrication of Thick Electroplated Microstructures." Technical Digest, Transducers '93. Pp. 60-65.

Busch-Vishniac, I.J. 1992. "The Case for Magnetically Driven Microactuators." Sensors and Actuators. 33, 3: 207-220.

Guckel, H., T.R. Christenson, K.J. Skrobis, T.S. Jung, J. Klein, K.V. Hartojo, and I. Widjaja. 1993. "A First Functional Current Excited Planar Rotational Magnetic Micromotor." Proc. IEEE Microelectromechanical Systems Workshop. Pp. 7-11.

Ultrasonic Drive

Moroney, R.M., R.M. White, and R.T. Howe. 1991. "Ultrasonically Induced Microtransport With Cylindrical Geometry." Micromechanical Sensors, Actuators, and Systems. Pp. 181-190.

Piezoelectric Drive

Flynn, A.M., L.S. Tavrow, S.F. Bart, R.A. Brooks, D.J. Ehrlich, K.R. Udayakumar,and L.E. Cross. 1992. "Piezoelectric Micromotors for Microbots." J. of Microelectromechanical Systems. 1, 1:44-51.

Lal, A., and R.M. White. 1993. "Micro-Fabricated Acoustic and Ultrasonic Source/Receiver." Technical Digest, Transducers '93. Pp. 712-715.

Schiller, P., and D.L. Polla. 1993. "Integrated Piezoelectric Microactuators Based on PZT Thin Films." Technical Digest, Transducers '93. Pp. 154-157.

Microactuated Electrooptic Devices

Hung, C.-Y., R. Burton, T.E. Schlesinger, M.L. Reed, S.C. Smith, D.J. Holmgren, and R.D. Burnham. 1992. "Microelectromechanical Tuning of Electrooptic Devices." Proc. IEEE Microelectromechanical Systems Workshop. Pp. 154-157.

Sandejas, F.S.A., R.B. Apte, W.C. Banyai, and D.M. Bloom. 1993. "Surface Microfabrication of Deformable Grating Light Valves for High Resolution Displays." Technical Digest, Transducers '93. Pp. 6-7.


Published: September 1994; WTEC Hyper-Librarian