MEMS | CMU | Overview

Systems Laboratory
Gary K. Fedder
FAX: (412)268-9687
Kaigham J. Gabriel
FAX: (412)268-9687
Research Goals
As information systems increasingly leave fixed locations and appear in our pockets and palms, they are getting closer to the physical world, creating new opportunities for perceiving and controlling our machines, structures and environments. To exploit these opportunities, information systems will need to sense and act as well as compute. Investing engineered systems with superior capabilities to sense and act is the focus for our research activities in microelectromechanical systems (MEMS).

Carnegie Mellon University’s MEMS Laboratory is housed in the Electrical and Computer Engineering Department and is part of The Robotics Institute and the Institute for Complex Engineered Systems. The MEMS Laboratory is engaged in research on sensor and actuator systems with performance derived from mechanical features measured in microns and components numbering from a few to millions. Our research activities are focused on design of batch-fabricated electromechanical devices and development of related processes that leverage the enormous investment in mature VLSI process technologies. The beneficial attributes of batch-fabricated MEMS are low manufacturing cost, miniaturization, and integration.

Our primary research thrust is toward highly integrated microsystems that interact with the environment and push the boundaries of electrical-mechanical functional integration. To achieve this vision, we are developing two core technologies: MEMS integrated with conventional CMOS (Complementary Metal-Oxide-Semiconductor) electronics; and computer-aided design (CAD) methodologies and tools for MEMS. Current projects include inertial measurement systems, magnetic probe-based data storage, biologically inspired MEMS, ultrasonic imaging arrays, lumped-parameter modeling of electrostatic gaps and damping, schematic capture and synthesis tools for MEMS, and process development and device characterization of microelectromechanical structures made using CMOS processes.

MEMS Fabrication and Applications
Our group has developed a unique process to create electrostatically actuated microstructures with high-aspect-ratio composite-beam suspensions using conventional CMOS processing, followed by a sequence of maskless dry-etching steps. Devices being made with the CMOS-MEMS process include x-z microstages for a probe-based mass-data-storage system on a chip, biologically inspired microsensors, ultrasonic sensor arrays, and accelerometers and gyroscopes for an on-chip inertial measurement unit. Supportive efforts include design and analysis of metrology test structures, modeling of device physics, and development of process enhancements.

An example device, the two-axis lateral x-y microresonator shown in the figure, has a central plate mass suspended by spring flexures and actuated by electrostatic comb-finger drives. High-performance CMOS may be placed within 15 µm of a micromechanical device, thereby providing ultra-sensitive capacitive motion sensing with parasitic capacitance as low as 13 fF. The ability to place multiple conductors within a single mechanical structure is an extra degree of design freedom allowing novel electrostatic actuators to be fabricated. We are exploiting these and other features of CMOS micromachining to create low-cost, high-performance sensor and actuator systems.

(click on image below for larger version)

log-log plot


Log-log plot of number of transistors merged with number of mechanical components for existing and future MEMS devices and systems.


Modeling and Design of MEMS
High-performance sensor and actuators systems require integration of micromechanical sensors and actuators with control, communications, data processing, test, and calibration functions. With increasing functionality comes a subsequent increase in complexity and a need for advances in coupled-energy-domain system-level design. In the NODAS project, we are creating a circuit-level tool for nodal simulation of suspended microelectromechanical systems with an underlying hierarchical representation which is intuitive to the MEMS designer. At the high-end of the design spectrum, we are developing optimization-based synthesis tools for microresonators and inertial sensors. Our design research encompasses a range of activities: control of sensor arrays, understanding micromechanical system dynamics (e.g., gyroscopes), and parametric modeling and experimental verification of physical devices.



Scanning electron micrograph of a released two-axis electrostatic-comb-drive resonator made using the CMU CMOS micromachining process.


Laboratory Resources
Microelectromechanical testing is primarily accomplished on our Wentworth MP-901 wafer probestation, fitted with eight precision manipulators and a high-resolution camera. Researchers have access to a 4000 square-foot, Class 100 cleanroom within the ECE department housing state-of-the-art fabrication equipment. We use several CAD tools including MATLAB Simulink, Cadence and Saber simulators; and Ansoft Maxwell, MEMCAD, SDRC I-DEAS and ABAQUS finite-element packages.

Future Directions
The future and real promise of MEMS will be in our ability to design systems of components with thousands to millions of electromechanical parts integrated with electronics to create MEMS arrays with a systems function greater than the sum of the individual parts. The figure to the right illustrates MEMS electromechanical integration. The vertical axis is increasing number of transistors and the horizontal axis is increasing number of mechanical components. To first order, the number of transistors are a measure of information processing capability and similarly, the number of mechanical components are a measure of the ability to sense and act. Mapped on the chart are not only past MEMS devices and selected commercial MEMS devices and systems, but also regions of electrical-mechanical integration that will enable future MEMS applications.

This next stage in the evolution and maturity of MEMS will be driven less by captive fabrication facilities and process development and more by innovative, aggressive electromechanical systems design. MEMS is poised to take full advantage of advances in information technology and couple them to advances in robotics and control theory to drive a fundamentally new approach to electromechanical system design and fabrication. For the first time, approaches akin to VLSI electronics can be taken to usher in an equally exciting and productive era of VLSI electromechanics. By merging sensing and actuation with computation, MEMS will not only invest existing systems with enhanced capabilities and reliability, but will make possible radically new devices and systems designs that exploit the miniaturization, multiplicity and microelectronics of MEMS.

Representative Publications
A. Berlin and K.J. Gabriel, “Distributed MEMS: New Challenges for Computation,” IEEE Computational Science and Engineering Journal, vol. 4, no. 1, pp. 12-16, 1997.

J. E. Vandemeer, M. S. Kranz, and G. K. Fedder, “Nodal Simulation of Suspended MEMS with Multiple Degrees of Freedom,” in Proc. 1997 IMECE: The Winter Annual Meeting of ASME in the 8th Symposium on MEMS, Dallas, TX, November 16-21, 1997.

M. S. Kranz and G. K. Fedder, “Micromechanical Vibratory Rate Gyroscopes Fabricated in Conventional CMOS,” in Proc. of the Symposium Gyro Technology, Stuttgart, Germany, September 16-17, 1997.

G. K. Fedder, S. Iyer, and T. Mukherjee, “Automated Optimal Synthesis of Microresonators,” in Tech. Dig. of the IEEE Int. Conf. on Solid-State Sensors and Actuators (Transducers ‘97), Chicago, IL, June 16-19, 1997, v. 2, pp. 1109-1112.

G. K. Fedder, S. Santhanam, M. L. Reed, S. C. Eagle, D. F. Guillou, M. S.-C. Lu, and L. R. Carley, “Laminated High-Aspect-Ratio Microstructures in a Conventional CMOS Process,” Sensors & Actuators A, vol. 57, no. 2, pp. 103-110, 1996.

G. K. Fedder and R. T. Howe, “Multimode Digital Control of a Suspended Polysilicon Microstructure,” IEEE Journal on MEMS, vol. 5, no. 4, pp. 283-297, 1996.

K.J. Gabriel, “Engineering Microscopic Machines”, 150th Anniversary Issue, Scientific American: Technology in the 21st Century, vol. 273, no. 3, pp. 150-153, September 1995.

K.J. Gabriel, O. Tabata, and S. Sugiyama, “Surface-Normal Electrostatic Pneumatic Actuators,” Technical Proceedings of MEMS ‘92, Travemunde, Germany, pp. 110-114, February 1992.

M. Mehregany, K.J. Gabriel, and W.S. Trimmer, “Integrated Fabrication of Polysilicon Mechanisms,” IEEE Transactions on Electron Devices, vol. 35, no. 6, pp. 719-723, 1988.

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