Introduction
What is MEMS?
It's not nano technology , though this term is sometimes used as a "catch all" for all things small. MEMS (Micro Electro Mechanical Systems) or MST ( Microsystems Technology) or even Micro-machining is the integration of mechanical elements, actuators, sensors, and electronics on a common substrate through the use of micro-fabrication techniques.
What exactly are MEMS?
MEMS is a technology used to create micro-miniature mechanical devices (sometimes out of silicon, but also from other materials). These devices can respond to external stimuli - for example, in sensing applications, and can then, in response to the stimuli, move (or actuate) mechanical structures. MEMS technology is already used to build accelerometers in automobile airbags, pressure sensors, flow rate sensors etc.. Micro-mechanical mirror arrays have also been developed for projection display applications. MEMS may sometimes be based on integrated circuit fabrication technologies similar to CMOS, with the added ability to incorporate moving and mechanical structures. Not as small as nanotechnology (which you could think of as paint technology or surface film technology), and certainly larger than today's leading edge silicon process technology, which is at 0.13 to 0.18 microns. MEMS devices are relatively large - typically ranging from one micron to 100 microns, sometimes even one millimetre.
Perhaps its the integrated functionality that is the key, rather than the size.The term "MEMS" has actually evolved over the years to embrace not only chips that have moving structures, but a broader range of devices that are fabricated with micro-machining.
Accelerometers in car airbags were the first commercial step for the technology, which now embraces ink jet printer heads, hard disk drives and pressure sensors. Gyroscopes for cars are emerging, however for the "old" acceleometer an important parameter is the sensitivity and frequency performance, expressed in root hertz What is meant by root hertz ( see acclerometers )??
Trends in MEMS Technology
MEMS technology is extending and increasing the ability to both perceive and control the environment by merging the capabilities of sensors and actuators with information systems. Future MEMS applications will be driven by processes that enable greater functionality through higher levels of electronic-mechanical integration and greater numbers of mechanical components working either alone or together to enable a complex action. These process developments, in turn, will be paced by investments in the development of new materials, device and systems design, fabrication techniques, packaging/assembly methods, and test and characterization tools.
Design and Simulation
MEMS is more demanding of design aids than microelectronics production. Most industrial designs of physical sensors today are based on detailed finite-element modeling of the mechanical microstructures using software available for conventional mechanics.
MEMS requires new drawing and layout tools to generate the patterns that will be used to add or remove material during processing. In addition, MEMS requires a number of different modeling tools, including simulators for mechanical deformation, electrostatic fields, mechanical forces, electromagnetic fields, material properties, and electronic devices. MEMS also needs the connective algorithms to reconcile and blend results from all the different simulators.
As devices become more complex and multiple simulators are involved, the complexity of both the simulations and the coupling increases considerably. Traditional modeling techniques become impractical and may even fail. Radically new approaches to modeling and simulation for the many physical effects and different MEMS functions have to be developed.
Materials Issues
An extensive, well-documented materials database that meets the requirements of MEMS development is essential for continuing progress in the field. Many of the new material property simulators will need new models and data to relate process parameters to material properties relevant for MEMS design.
The accuracy of the existing microelectronic device simulators is built on historic and huge amounts of material and device measurements, coupled to carefully controlled process conditions. By knowing the relationship between processing conditions and the resulting material parameters, microelectronics manufacturers can control material properties, and hence, device yields. Circuit designers are typically interested in those material properties that relate to the electronic function of the devices, such as doping levels and dielectric constants.
The material needs of the MEMS field are well recognized but are at a preliminary stage. In addition to single-crystal Si, polysilicon, Si3N4, and SiO2, other materials are being explored for MEMS. Interesting examples include SiC, shape memory alloy (SMA) metals, permalloy, and high-temperature superconductive materials. All these materials possess certain unique properties that, when combined with MEMS technology, make them attractive for certain applications.
A thorough understanding of the material properties of existing MEMS materials is just as important as the development of new materials for MEMS. There are very few reliable measurements of material properties (for example, modulus, residual stress, or reflectivity) relevant to the production of MEMS. The goal of studying the material properties in MEMS, and in thin films generally, is to develop models that relate process parameters to the film microstructure, as well as to the corresponding mechanical, electrical, optical, and thermal properties. Chapter 3 elaborates on the material properties and the required tests to enable a valid database.
Integration with Microelectronics
Future MEMS products will demand yet higher levels of electrical-mechanical integration and more intimate interaction with the physical world. The full potential of MEMS technology will only be realized when microelectronics is merged with the electromechanical components. Integrated microelectronics provides the intelligence to MEMS and allows closed-loop feedback systems, localized signal conditioning, and control of massively parallel actuator arrays.
Although MEMS fabrication uses many of the materials and processes of semiconductor fabrication, there are important distinctions between the two technologies. The most significant distinctions 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 test). The fundamental challenge of using semiconductor processes for MEMS fabrication is not so much in the type of processes and materials used, but more in the way those processes and materials are used.
MEMS will need the development of operating conditions on standard semiconductor equipment suited and optimized to the requirements of MEMS. For other processing steps unique to MEMS, the development of new manufacturing equipment and associated processes will be required. Table 1.3 lists some of the specialized process equipment that is required to enhance the manufacturability of MEMS.
Surface micromachining Release and drying systems to realize free-standing microstructures
Nano technology
Nanotechnology is the art of manipulating matter at the atomic scale. Like MEMS, it unites many fields such as physics, chemistry and biology. The term comes from the word "nano" meaning 10-9,or a billionth part. A nanometer (abbreviated nm), for example, is one billionth of a meter. An atom measures about one-third of a nanometer. The diameter of a human hair is about 100,000 nm
During the early 1980s, the invention by IBM researchers of two new microscopy techniques:
Ø atomic force microscopy (AFM)
Ø scanning tunnelling microscopy (STM)
formed the practical basis for nanotechnology. Both these techniques were a radical departure from previous types of microscopy, which worked by reflecting either light (in the case of optical microscopes) or an electron beam (in the case of electron microscopes) off a surface and onto a lens. But no reflective microscope, not even the most powerful, could image an individual atom. To do so, the new techniques use a cantilever to "read" a surface directly, the way a record player's needle reads a record. Atomic force microscopy works by passing the cantilever within a few nanometers of a surface. The atomic forces exerting a pull on the cantilever are measured to create an atom-by-atom topographical map.
Scanning tunnelling microscopy is similar, but measures a quantum effect called tunnelling. STM's cantilever carries a tiny charge, and classical physics says that a wall of potential energy should prevent the charge from jumping to the surface. But when two atoms come close enough, quantum rules let electrons "tunnel" through that wall. By modulating the voltage at the cantilever's tip, these techniques can be used to push and pull atoms into place.
This, then is the crux of the difference between MEMS and nano technology. Nanotechnology is about atomic manipulation (say, as sheets of atoms in a thin film), whereas MEMS is about making small moving parts on the micron (or larger) scale. Therefore MEMS is at least 1,000 times bigger than nano technology
The scope of the MEMS includes the following
Development of advanced second-generation memsmirror devices fabricated in a four-level, planarized surface-memsmachined polycrystalline silicon process:-
This describes the design and characterization of several types of memsmirror devices to include process capabilities, device modeling, and test data resulting in deflection vs applied potential curves and surface contour measurements. These devices are the first to be fabricated in the state-of-the-art four-level planarized polysilicon process available at Sandia National Laboratories known as the Sandia Ultra-planar Multi-level MEMS Technology (
Micro-electro-mechanical-systems (MEMS) and fluid flows
The memsmachining technology that emerged in the late 1980s can provide memsn-sized sensors and actuators. These mems-transducers can be integrated with signal conditioning and processing circuitry to form MEMS that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in memsn-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research.
Development of memselectromechanical deformable mirrors for phase modulation of light
The development of a silicon-based memselectromechanical deformable mirror is reported, with emphasis placed on design, fabrication, device characterization, and system integration issues. The mirror parameters are derived from theoretical and empirical models of adaptive optics. Sufficient yield is demonstrated with a standard robust actuator. Processing issues for obtaining a planar mirror surface are currently being addressed. A 20-channel electronic control circuit is used for the high-speed control of a segmented mirror array. Applications for this MEMS deformable mirror include adaptive optical imaging and projection systems as well as optical correlators for pattern recognition systems.
An optical probe for memsmachine performance analysis
Understanding the mechanisms that impact the performance of memselectromechanical systems (MEMS) is essential to the development of optimized designs and fabrication processes, as well as the qualification of devices for commercial applications. Silicon memsmachines include engines that consist of orthogonally oriented linear comb drive actuators mechanically connected to a rotating gear. These gears are as small as 50 memsns in diameter and can be driven at rotation rates exceeding 300,000 rpm. Optical techniques offer the potential for measuring long term statistical performance data and transient responses needed to optimize designs and manufacturing techniques. We describe the development of memsmachine optical probe (MOP) technology for the evaluation of memsmachine performance. The MOP approach is based on the detection of optical signals scattered by the gear teeth or other physical structures. We present experimental results obtained with a prototype optical probe and memsmachines developed at Sandia National Laboratories.
Polymer guided wave integrated optics - An enabling technology for mems-opto-electro-mechanical systems
This paper explores issues confronting the integration of waveguide technologies within the surface memsmachined MEMS environment. Specific focus is placed on initial efforts developing processes for guided wave polymer optics cointegration with the Multi-User MEMS Process Service (MUMPS) surface memsmachining process. Efforts studying the cointegration of polyimide waveguides with MEMS for integrated optical metrology and state feedback applications are highlighted. (Author)
Microelectromechanical systems (MEMS)
A graphical presentation is given for the status and prospects of MEMS being developed under the aegis of the Defense Advanced Projects Agency; these devices, which can be either electrical or mechanical, or both, merge computation with sensing and actuation in memsdimensional packages. Attention is here given to MEMS fabrications technologies derived from memselectronics manufacturing, the MEMS industry and market structure, MEMS military applications, and such representative devices as inertial measurement units, multiple device chips, optomechanical displays, environmental monitors, optical components, and a monolithic free-space optical disk pickup head.
Microwave and mechanical considerations in the design of MEM switches for aerospace applications
Microelectromechanical systems (MEMS) technology is expected to impact such aerospace systems as phased-array antennas, frequency multiplexers, spacecraft guidance and navigation, onboard communications, autonomous health monitoring and safety, space structures, thermal control, and on-board system reconfigurability. One of the most ubiquitous components enabled by MEMS technology is the electrostatic memselectromechanical (MEM) switch; due to its simplicity and high performance potential, this is poised to become the pioneering MEMS component for memswave signal processing-related applications in space-based communications systems. This paper discusses the impact of memswave performance specifications of the MEM switch on its mechanical structure and design. A quantitative discussion of switch parameters (actuation voltage, actuation frequency, insertion loss, and isolation) is presented. (Author)
Macro aerodynamic devices controlled by mems systems
During the past few years, memselectromchanical systems (MEMS) have emerged as a major enabling technology across the engineering disciplines. The possibility of applying MEMS to the aerodynamics field is explored. We demonstrate that memstransducers can be used to control the motion of a delta wing in a wind tunnel and can even maneuver a scaled aircraft in flight tests. The main advantage of using memsactuators to replace the traditional control surface is the significant reduction of radar cross-sections. At a high angle of attack, a large portion of the suction loading on a delta wing is contributed by the leading edge separation vortices which originate from thin boundary layers at the leading edge. We used memsactuators with a thickness comparable to that of the boundary layer in order to alter the separation process and thus achieved control of the global motion by means of minute perturbations.
MEMS - A technology for advancements in aerospace engineering
A technology, Micro-Electro-Mechanical Systems (MEMS), emerged in the late 1980s which enables us to fabricate mechanical parts on the order of memsns. Micromachining technology is suitable for developing new transducers or improving existing transducer designs. Due to the dramatic reduction in size, mems transducers can outperform traditional ones by orders of magnitude. Furthermore, MEMS is a fundamental technology which has the potential to influence advancements in many fields. In the automobile, electronics, bio-medical and television industries, MEMS products have already made appreciable impacts. In this paper, the applications of MEMS for aerodynamic control are presented. (Author)
Micro mirrors relieve communications bottlenecks
The most pervasive of the bottlenecks for communications carriers are the switching and cross-connect fabrics that switch, route, multiplex, demultiplex, and restore traffic in optical networks. Transmission systems move information as photons, but switching and cross-connect fabrics until now have largely been electronic, requiring costly time-consuming bandwidth-limiting optical-to-electronic-to-optical conversions at every network connection and crosspoint. This article discusses the recently developed MicroStar cross-connect fabric, unveiled by Bell Labs, which is based on mems-optoelectromechanical system device fabrication. This process enables the construction of memsmachines that are finding increasing acceptance in such application categories as communications, automotive, aerospace, and consumer electronics. MicroStar comprises 256 mirrors, each one 0.5 mm in diameter, spaced 1-mm apart, and covering a total of less than 1 sq in. of silicon. The mirrors sit within the router so that only one wavelength can illuminate any one mirror. Each mirror can tilt independently to pass its wavelength to any of 256 input and output fibers. The mirror arrays are made using a self-assembly process that causes a frame around each mirror to lift from the silicon surface and lock in place, positioning the mirrors high enough to allow a range of movement. MicroStar is part of Lucent Technology's LambdaRouter, a cross-connect system aimed at helping carriers deliver vast amounts of data unimpeded by conventional bottlenecks. The all-optical-format LambdaRouter is commercially available in June 2000 and offers a total switching capacity of more than 10 Tb/s, enabling communications providers to route more than 10 times the current Internet traffic in one second. (AIAA)
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