12-01-2011, 12:16 PM
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DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGG.
KURUKSHETRA INSTITUTE OF TECHNOLOGY & MANAGEMENT
KURUKSHETRA- 136119
Microelectromechanical systems
Microelectromechanical systems (MEMS) (also written as micro-electro-mechanical, MicroElectroMechanical or microelectronic and microelectromechanical systems) is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines (in Japan), or Micro Systems Technology - MST (in Europe).
MEMS are separate and distinct from the hypothetical vision of molecular nanotechnology or molecular electronics. MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm) and MEMS devices generally range in size from 20 micrometres (20 millionths of a metre) to a millimetre. They usually consist of a central unit that processes data, the microprocessor and several components that interact with the outside such as microsensors[1]. At these size scales, the standard constructs of classical physics are not always useful. Because of the large surface area to volume ratio of MEMS, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass.
The potential of very small machines was appreciated before the technology existed that could make them—see, for example, Richard Feynman's famous 1959 lecture There's Plenty of Room at the Bottom. MEMS became practical once they could be fabricated using modified semiconductor device fabrication technologies, normally used to make electronics. These include molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing small devices. An early example of a MEMS device is the resonistor – an electromechanical monolithic resonator.
Why MEMS?
Those who don’t have a strong background in MEMS and want to get a global understanding of the technology, I speculate that they now have reached to the most difficult question of “Why MEMS?”
A finical consultant once said in a Business Awareness Workshop (I attended lately) that the main challenge for any new technology in the market is to offer one of three things:
• Enabling a new function
• Providing significant cost reduction
• Or both!!!
Actually, the answer to this question is "Both". "Applications Using MEMS-Based Components" section will illustrate that there is a lot features available in our world that could never be achieved without MEMS technology. The section after it shows how MEMS cost can usually be reduced by mass production
Applications Using MEMS-Based Components
As mentioned before, MEMS recently are more used as components or subsystems so we may not notice them in our life but this section is narrowing the scope on some of popular application that would never perform the way it’s without MEMS technology.
Nintendo’s Wii
Nintendo’s Wii is (the hottest computer game) is using MEMS technology which allows the Wii controller to respond to changes in direction (Using MEMS Gyroscope), speed and acceleration (Using MEMS Accelerometer), down to the most delicate movements of its players
MEMS Gyroscope
Invented in the nineteenth century, the gyroscope (shown in the Figure 5) is a critical navigational instrument used for maintaining a fixed orientation with great accuracy, regardless of Earth rotation. Before micromachining the gyroscope was a bulky component, although it was heavily used in airplanes so (obviously) its big size wasn’t an issue.
The Coriolis Effect can be understood by studying the next example which is illustrated in Figure 6. Assume that there is a airplane departed from England hitting to south France, a lot would think that the pilot should fix the flywheel to a certain direction which is "south" and he will arrive to France but if he does that he will find himself in north Italy!!! That happens because he didn't take into his consideration the rotation of the earth from east to west beneath him which generated a force perpendicular to the direction of the airplane and changed its course.
So after studying the previous example we conclude that if an object (airplane) is moving in X-direction while its bounded to frame (earth) is rotating around X-direction (neglecting the 23.5 tilt in earth's axis) the object will be affected by a force (Coriolis force) in Y-direction.
Back to the MEMS Gyroscope, the main goal is to sense the rotation of the frame (Wii's Controller) so we put a Proof Mass which is oscillated in certain direction, if any Coriolis forces (perpendicular direction) is sensed at the frame this will be an indication that the frame is rotated
Surface-micro machined vibratory rate gyroscope (only an example not necessary used in Wii)
It is a direct implementation of the lumped model presented in Figure 7. Standard comb drive actuators were used to excite the structure to oscillate along one in-plane axis (x-axis), which allows relatively large drive amplitudes. Any angular rate signal about the out-of-plane axis (z-axis) excites a secondary motion along the other in-plane axis (y-axis). The sensing element is shown in Figure 8 and consists of a 2-μm-thick polysilicon structure.
MEMS Cost Competitiveness
The key element to cost competitiveness is Mass Production (that is, the practice of simultaneously manufacturing hundreds or thousands of identical parts, thus decreasing the overall impact of fixed costs—including the cost of maintaining expensive clean room -used in semiconductor manufacturing - and assembly facilities) see Figure 11. This is precisely the same approach that has resulted over the last few decades in a great decrease in the price of computer chips. Unfortunately, the argument works in reverse too: Small manufacturing volumes will bear the full burden of overhead expenses, regardless of how “enabling” the technology may be.
Silicon
Silicon is the material used to create most integrated circuits used in consumer electronics in the modern world. The economies of scale, ready availability of cheap high-quality materials and ability to incorporate electronic functionality make silicon attractive for a wide variety of MEMS applications. Silicon also has significant advantages engendered through its material properties. In single crystal form, silicon is an almost perfect Hookean material, meaning that when it is flexed there is virtually no hysteresis and hence almost no energy dissipation. As well as making for highly repeatable motion, this also makes silicon very reliable as it suffers very little fatigue and can have service lifetimes in the range of billions to trillions of cycles without breaking. The basic techniques for producing all silicon based MEMS devices are deposition of material layers, patterning of these layers by photolithography and then etching to produce the required shapes.
Polymers
Even though the electronics industry provides an economy of scale for the silicon industry, crystalline silicon is still a complex and relatively expensive material to produce. Polymers on the other hand can be produced in huge volumes, with a great variety of material characteristics. MEMS devices can be made from polymers by processes such as molding, embossing or stereo lithography and are especially well suited to micro fluidic applications such as disposable blood testing cartridges.
Metals
Metals can also be used to create MEMS elements. While metals do not have some of the advantages displayed by silicon in terms of mechanical properties, when used within their limitations, metals can exhibit very high degrees of reliability.
Metals can be deposited by electroplating, evaporation, and sputtering processes.
Commonly used material are gold, nickel, aluminum, copper, chromium, titanium, tungsten, platinum, and silver.
MEMS manufacturing technologies
Bulk micromachining
Bulk micromachining is the oldest paradigm of silicon based MEMS. The whole thickness of a silicon wafer is used for building the micro-mechanical structures. Silicon is machined using various etching processes. Anodic bonding of glass plates or additional silicon wafers is used for adding features in the third dimension and for hermetic encapsulation. Bulk micromachining has been essential in enabling high performance pressure sensors and accelerometers that have changed the shape of the sensor industry in the 80's and 90's.
Surface micromachining
Surface micromachining uses layers deposited on the surface of a substrate as the structural materials, rather than using the substrate itself. Surface micromachining was created in the late 1980s to render micromachining of silicon more compatible with planar integrated circuit technology, with the goal of combining MEMS and integrated circuits on the same silicon wafer. The original surface micromachining concept was based on thin polycrystalline silicon layers patterned as movable mechanical structures and released by sacrificial etching of the underlying oxide layer. Interdigital comb electrodes were used to produce in-plane forces and to detect in-plane movement capacitively. This MEMS paradigm has enabled the manufacturing of low cost accelerometers for e.g. automotive air-bag systems and other applications where low performance and/or high g-ranges are sufficient. Analog Devices have pioneered the industrialization of surface micromachining and have realized the co-integration of MEMS and integrated circuits.
High aspect ratio (HAR) silicon micromachining
Both bulk and surface silicon micromachining are used in the industrial production of sensors, ink-jet nozzles, and other devices. But in many cases the distinction between these two has diminished. A new etching technology, deep reactive-ion etching, has made it possible to combine good performance typical of bulk micromachining with comb structures and in-plane operation typical of surface micromachining. While it is common in surface micromachining to have structural layer thickness in the range of 2 µm, in HAR silicon micromachining the thickness can be from 10 to 100 µm. The materials commonly used in HAR silicon micromachining are thick polycrystalline silicon, known as epi-poly, and bonded silicon-on-insulator (SOI) wafers although processes for bulk silicon wafer also have been created (SCREAM). Bonding a second wafer by glass frit bonding, anodic bonding or alloy bonding is used to protect the MEMS structures. Integrated circuits are typically not combined with HAR silicon micromachining. The consensus of the industry at the moment seems to be that the flexibility and reduced process complexity obtained by having the two functions separated far outweighs the small penalty in packaging. A comparison of different high-aspect-ratio microstructure technologies can be found in the HARMST article.
A forgotten history regarding surface micromachining revolved around the choice of polysilicon A or B. Fine grained (<300A grain size, US4897360), post strain annealed pure polysilicon was advocated by Prof Henry Guckel (U. Wisconsin); while a larger grain, doped stress controlled polysilicon was advocated by the UC Berkeley group.
