Introduction to Applications and Industries for Microelectromechanical Systems (MEMS)

Transcription

1 Introduction to Applications and Industries for Microelectromechanical Systems (MEMS) Jeremy A. Walraven Sandia National Laboratories. Albuquerque, NM USA Abstract Microelectromechanical Systems (MEMS) have gained acceptance as viable products for many commercial and government applications. MEMS are currently being used as displays for digital projection systems, sensors for airbag deployment systems, inkjet print head systems, and optical routers. This paper will discuss current and future MEMS applications. 1. Introduction What are MEMS? MEMS are typically defined as microscopic devices designed, processed, and used to interact or produce changes within a local environment. A mechanical, electrical, or chemical stimulus can be used to create a mechanical, electrical, or chemical response in a local environment. These smaller, more sophisticated devices that think, act, sense, and communicate are replacing their bulk counterparts in many traditional applications. What is your motivation for using MEMS technology? MEMS offer reduced power consumption, improved performance (application specific), reduced weight, and lower cost than their bulk device counterparts. Batch fabrication reduces manufacturing and assembly costs; reduced size and weight typically result in less power consumption and increased system design flexibility. Reducing the size and weight of a device allows multiple MEMS components can be used in serial or parallel to increase functionality, device capability, and reliability. MEMS components are categorized in one of six distinct applications. These include: Sensors Actuators RF MEMS Optical MEMS Microfluidic MEMS Bio MEMS Sensors are a class of MEMS that are designed to sense changes and interact with their environments. These classes of MEMS include chemical, motion, inertia, thermal, and optical sensors. Actuators are a group of devices designed to provide power or stimulus to other components or MEMS devices. In MEMS, actuators are either electrostatically or thermally driven. RF MEMS are a class of devices used to switch or transmit high frequency, RF signals. Typical devices include; metal contact switches, shunt switches, tunable capacitors, antennas, etc. Optical MEMS are devices designed to direct, reflect, filter, and/or amplify light. These components include optical switches and reflectors. Microfluidic MEMS are devices designed to interact with fluid-based environments. Devices such as pumps and valves have been designed to move, eject, and mix small volumes of fluid. Bio MEMS are devices that, much like microfluidic MEMS are designed to interact specifically with biological samples. Devices such as these are designed to interact with proteins, biological cells, medical reagents, etc. and can be used for drug delivery or other in-situ medical analysis. These six areas represent vastly different applications of MEMS devices currently being used or being developed for both commercial and government applications. 2. MEMS Applications 2.1 Sensors Sensors based on MEMS technology have a very wide range of applications. Applications range from motion and inertial sensors to chemical and biological sensors. MEMS technology is particularly well suited to this arena where traditional technology may fail. This may be due ITC INTERNATIONAL TEST CONFERENCE /03 $17.00 Copyright 2003 IEEE

2 to surrounding environments, system level limitations, or design/size constraints. resulting in low-power operation. Combustible gases react on the catalytic surface, releasing heat that changes the filament resistance. This device is proposed for use in exhaust gas analysis to indicate combustible gas hazards. Fig. 1. Single axis accelerometer used as a sensor for airbag deployment systems. Note the surrounding electronics on chip. Image courtesy of Analog Devices. One commercial success using MEMS technology involves the application of MEMS as a sensor for automobile airbag deployment systems. In this application, an accelerometer was designed and fabricated at Analog Devices to sense rapid changes in acceleration to deploy an airbag. As shown in Fig. 1, the MEMS device is surrounded by CMOS technology to process the output signals sensed from the accelerometer. Other devices have been fabricated to sense motion along both the x and y axis. Over 100 million airbag sensors have been sold over the past ten years, putting MEMS-based sensors solidly in the commercial marketplace. Efforts are currently underway to design and fabricate high sensitivity 3-axis (x, y and z) gyroscopes (Fig. 2). Analog Devices' family of gyroscopes (ADXRS), will be released soon and represent the first of a family of fully integrated, monolithic angular rate sensors to be released. The use of MEMS technology enables these gyros to be smaller, more accurate, more reliable, and more economical than their bulk counter parts, while exhibiting excellent resistance to both shock and vibration. Potential applications of these devices include automotive safety, commercial avionics, and industrial equipment [1]. Other sensor applications include air quality and trace chemical analysis. One example of a combustible gas sensor fabricated at Sandia National Laboratories is shown in Fig. 3. The combustible gas sensor consists of a conductive filament, coated with a catalytic layer, that is heated by an electrical current. Micromachining technology allows the filament to be made very small and suspended above the substrate for thermal isolation, Fig. 2. Three-axis gyroscope. Image courtesy of Analog Devices. Fig. 3. MEMS fabricated combustible gas sensor. The top sensor is a polysilicon fabricated filament, the bottom is the same filament coated with a catalytic film. 2.2 Actuators Actuators are a family of MEMS components designed to produce motion or provide the driving force for moving other MEMS fabricated components. Actuators are powered using an electronic-based drive signal. The drive signal can be in any waveform (square, sine, sawtooth, etc.) to produce motion. Two types of 675

3 actuators, electro-static and thermal, have been used to enable motion in various MEMS components. Electrostatic actuators operate using an applied electric field between fixed and moveable structures. One structure is fixed to the substrate and biased producing electric fields along the edges of the structure. The fixed components are typically the only biased structure in the device. This structure is typically a multi-fingered component where electric fields are present along the comb fingers. The moveable component of an electrostatic actuator is typically a multi-fingered device intermeshed with the fixed component to create an inter-digitated comb-like structure. The moveable comb fingers are attracted to the powered ones via the electric field fringes, pulling them towards the biased component. Restoring springs are fabricated with the comb fingers to provide the required restoring force to bring the moveable comb fingers back to their original position. As shown in Fig. 4, orthogonally positioned electrostatic actuators are used to rotate a gear. Linear motion can be translated into rotational or out-of-plane motion by coupling linear actuators [2]. shuttle gear and pin joint springs Fig. 4. Orthogonally positioned electrostatic actuators used to operate a gear. Note the moveable shuttle (upper left) and the gear (lower left). The electrostatic actuators are coupled together through a pin joint. Thermal actuators operate using current in an elongated structure fabricated slightly off-axis [3]. The off-axis fabrication creates the direction for the thermal actuator to move. When current passes through the horizontal arms of the thermal actuator, the arms heat and expand causing displacement, (or throw) in its designed direction. This results in a linear output as shown by the black arrow in Fig. 5. Electrostatically actuated components are available commercially. Most MEMS devices (microfluidic, optical, RF, etc.) operate by using some form of electrostatic actuation. One potential application for actuators may be in the hard drive/memory market [4-6]. Fig. 5. Thermal actuators with various arm widths. The arrow indicates the throw direction. 2.3 RF MEMS RF MEMS are becoming recognized as an excellent alternative to existing solid-state RF technologies. Although the field of RF MEMS is relatively new, results have shown that performance enhancements and manufacturing costs reductions make this technology a viable competitor with large-scale and solid-state counterparts [7]. In many cases, RF MEMS components can function as well as an equivalent solid-state circuit (depending upon its application). Research is currently underway to develop and commercialize RF products such as switches, tunable capacitors, high-q inductors, high-q mechanical resonators and filters, as well as microwave and millimeter-wave components. Industries may apply this technology to satellite and military communications, navigation, sensors, and avionics. Current state-of-the-art RF circuits use a combination of gallium arsenide FETs, PIN diodes, and/or varactor diodes to achieve required tuning, filtering, and/or switching functions. These devices typically require high power consumption, are expensive to fabricate, and have limited reliability. In some instances, poor RF performance (such as high insertion loss per switching cycle) serves as the impetus driving RF MEMS production. As has been discussed, several commercial and government applications exist for RF MEMS capabilities. Very few RF MEMS components are commercially available due to the relative infancy of the field, technological hurdles, and reliability issues that must be overcome for successful implementation. 676

4 One area of research is the RF signal switching arena. Two types of switches have been developed for this application. These include the metal-contact series switch and a shunt switch. Metal-contact series switches offer the ability to switch both AC and DC signals through metal-to-metal contact as shown in Fig. 6. Shunt switches allow AC switching through a thin dielectric and eliminate metallurgical contact issues caused by hot and cold switching, but have limited bandwidth. A metal contact RF MEMS switch is shown in Fig. 7. Signal Line Au Contact Au GaAs Substrate SiON Al or Au Part of Ground Line Polyimide Anchor Fig. 6. Cross section diagram of a metal contact switch revealing the contact and signal line metallization and the top and bottom electrodes. functions of the micromirrors are divided into three categories: 1-d, 2-d, and 3-d configurations. These configurations correspond to the degrees of motion of micromirror operation. The 1-d configuration allows the micromirror to control reflected light by tilting about a single axis, typically parallel to the plane of the micromirror array. The 2-d configuration allows tilting of the micromirror along orthogonal positions parallel to the plane of the micromirror array. The 3-d architectures steer light along orthogonal positions parallel to and perpendicular to the plane of the micromirror array. By rerouting optical signals directly, MEMS technology enables maintaining signal fidelity and continuity by creating an all-optical switching network. This technology may potentially replace existing electronics used for re-routing optical signals through an optoelectronic-opto conversion [8, 9]. Arrays of micromirrors have been fabricated and tested at 3x3, and 256x256 array sizes. An example of a 3-d MEMS based optical routing system is shown in Fig. 8. Note the size of each micromirror is smaller than the eye of a pin. Anchor Contact Switch Anchor Actuation Pads Fig. 7. Optical image of an RF MEMS metal contact switch developed at Sandia National Laboratories. 2.4 Optical MEMS Optical MEMS is a unique application where MEMS are used to direct, guide, filter, and, in some instances, amplify light. Well known applications of optical MEMS include optical switching and digital projection. In optical switching applications, micromirrors are used to steer light from an incoming fiber optic input to a fiber optic output. Many different types of MEMS micromirrors have been designed and fabricated. The Fig. 8. Optical micromirror array known as the Lambda waveguide developed by Lucent Technologies. Photo courtesy of Lucent Technologies. Other applications of optical MEMS include digital light projection systems for displays. Commercially available MEMS components such as the Digital Mirror Device (DMD ) fabricated by Texas Instruments improve image quality and resolution. Televisions, home theater systems, and business projectors using DLP (Digital Light Projection) technology rely on a single DMD chip configuration for light projection. DLP technologyenabled projectors for very high image quality or high 677

5 brightness applications such as cinema and large venue displays rely on a 3-DMD-chip configuration to produce static and dynamic images [10]. Two micromirrors used in the DMD display technology are shown in Fig. 9. Note the mechanical components are fabricated directly over the circuitry. a Pistons -10 o + 10 o Hinge Yoke Spring Tip CMOS b Nozzle Plate Fig. 9. DMD used for digital projection systems. Figure courtesy of Texas Instruments. 2.5 Microfluidic MEMS Microfluidic MEMS are devices that transport, dispense, combine and/or separate fluids at the microscopic level. Industries are now beginning to realize the potential of MEMS-based microfluidic ejection, transport, delivery, and detection systems. Applying MEMS in fluidic-based applications reduces the amount of fluid used, reduces waste, and can be processed in serial or parallel (multiple devices). c Hole Piston Typical applications of microfluidic MEMS include valves, pumps, and ink jet delivery systems [11, 12]. In ink jet delivery systems, MEMS are used to increase printing resolution (increased DPI) and reduce ink consumption, resulting in longer lifetimes of the ink cartridge. By increasing the density of ejection devices (using MEMS), and reducing droplet size, more ink drops can fit into a given area, thus increasing DPI. An example of an electrostatically actuated drop ejector is shown in Fig. 10. Other potential applications for microfluidic MEMS include water sampling and detection, insitu/non-invasive fluidic testing, etc. Multiple devices can be placed in strategic regions of water supplies or other fluidic systems to monitor toxicity and contaminant levels. Fig. 10. a) drop ejector array with the nozzle plate partially removed, revealing the pistons, b) close up of the nozzle plate hole and piston below it, c) strobe image of droplets travelling to the left (arrows). The droplets were ejected from a drop ejector in the array (right side). 2.6 Bio MEMS Like microfluidic MEMS, Bio MEMS are designed to transport, combine, and/or separate fluids at the microscopic level. One pertinent difference between these two devices is the ability to manipulate biological material contained within the fluid or manipulate biological fluid itself. Applications of biomedical MEMS such as microdialysis [13], biosensors [14], and laboratory analysis on a chip [15] are areas currently under research and development. Companies such as Molecular Devices [15] and i-stat [15] have produced 678

6 microphysiometers [16] and chemical laboratory analysis methods at the microscopic scale. Some benefits of microscale fluidic technology for medical applications include reduced fluid consumption, reduced waste products, decreased reaction times, and improved parallel chemical processing capability. The promise of packaging a variety of diagnostically relevant, otherwise expensive tests into a tiny amount of microfabricated real estate would represent a major advancement for many clinical situations [15]. Employing MEMS components for biological fluid analysis not only enables the benefits of a microfluidic component, but other mechanical components can be used to manipulate biological material, such as blood cells. contained within the biological fluid. An example of a Bio MEMS cell masher is shown in Fig. 11. This component uses microfluidic pumps to push biological fluid, or fluid with biological material, through a channel. An actuator with pointed edges is designed to push into the cell, perforating it, allowing the contents of the fluid to penetrate the cell. Fig. 11. Cell manipulator. Fluid containing cells can be pumped through and perforated using the attached actuator. 3. Conclusions MEMS have demonstrated their applicability and marketability over the past 10 years. MEMS components have been successfully integrated into commercial products ranging from inertial sensors for airbag deployment in the automobile industry to digital projection systems. The full breadth of MEMS applications has yet to be realized. 4. Acknowledgements The author would like to thank Jerry Soden, Brad Waterson of Analog Devices, Ingrid de Wolf of IMEC, and Rich Plass for discussions on MEMS components. The author would also like to thank the microelectronics development laboratory at Sandia National Laboratories for their processing efforts. Sandia National Laboratories is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Safety Administration for the United States Department of Energy under Contract DE-AC04-94AL For further information about MEMS technology at Sandia, please visit our website at: 5. References [1] [2] [3] R. Hickey, D. Sameoto, T. Hubbard, M. Kujeth, Time and frequency response of two-arm micromachined thermal actuators, J. Micromech. Microeng. 13 (January 2003) [4] [5] J. L. Griffin, S. W. Schlosser, G. R. Ganger, and D. F. Nagle. Modeling and performance of MEMSbased storage devices. Proc. of ACM SIGMET- RICS, pp , June [6] L. R.Curley, G. R. Ganger, and D. F. Nagle. MEMS based Integrated Circuit Mass Storage systems. Communications of ACM, 43(11); pp , Nov 2000 [7] J. J. Yao and M. F. Chang, A surface micromachined miniature switch for telecom. applications with signal frequencies from DC up to 4GHz, Transducers, June 1995, pp [8] Walker, S. J., Nagel, D. J., Report on Optics & MEMS, Naval Research Laboratories, Materials Science and Technology Division, May 15, 1999 [9] Arney, S., Aksyuk, V. A., Bishop, D. J., Bolle, C. A., Frahm, R. E., Gasparyan, A., Giles, C. R., Goyal, S., Pardo, F. L., Shea, H. R., Lin, M. T. and White, C. D., Design Reliability of MEMS /MOEMS for Lightwave Telecommunications, Proc. of the 27 th ISTFA, Nov. 2001, pp [10] [11] R. R. Allen, J. D. Meyer, W. R. Knight, Thermodynamics and Hydrodynamics of Thermal Ink Jets, Hewlett Packard Journal, May [12] P. Galambos, K. Zavadil, R. Givler, F. Peter, A. Gooray, G. Roller, and J. Crowley, A Surface Micromachined Electrostatic Drop Ejector, 11 th Intl. Conf. on Solid-State Sensors and Actuators, Munich, Germany, June 2001, pp

7 [13] S. Bohm, W. Olthuis, and P. Bergveld, A µtas Based on Microdialysis for On-line Monitoring of Clinically Relevant Substances, Proceedings of the µtas Workshop, 1998, pp [14] L. Bousse, Whole Cell Biosensors, Sensors and Actuators B (Chem), B34(1-3), 1996, pp [15] J. M. Ramsey, S. C. Jacobson, and M. R. Knapp, Microfabricated Chemical Measurement Systems, Nature Medicine, Vol. 1, 1995, pp [16] H. M. McConnell, et. al., The Cytosensor Microphysiometer: Biological Applications of Silicon Technology, Science 257, 1992, pp