MEMS
MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometres to a millimetre (i.e., 0.02 to 1.0 mm), although components arranged in arrays (e.g., digital micromirror devices) can be more than 1000 mm2.[1] They usually consist of a central unit that processes data (an integrated circuit chip such as microprocessor) and several components that interact with the surroundings (such as microsensors).[2]
For other uses, see MEMS (disambiguation).
Because of the large surface area to volume ratio of MEMS, forces produced by ambient electromagnetism (e.g., electrostatic charges and magnetic moments), and fluid dynamics (e.g., surface tension and viscosity) are more important design considerations than with larger scale mechanical devices. MEMS technology is distinguished from molecular nanotechnology or molecular electronics in that the latter two must also consider surface chemistry.
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.[3] These include molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electrical discharge machining (EDM), and other technologies capable of manufacturing small devices.
They merge at the nanoscale into nanoelectromechanical systems (NEMS) and nanotechnology.
History[edit]
An early example of a MEMS device is the resonant-gate transistor, an adaptation of the MOSFET, developed by Harvey C. Nathanson in 1965.[4] Another early example is the resonistor, an electromechanical monolithic resonator patented by Raymond J. Wilfinger between 1966 and 1971.[5][6] During the 1970s to early 1980s, a number of MOSFET microsensors were developed for measuring physical, chemical, biological and environmental parameters.[7]
The term "MEMS" was introduced in 1986. S.C. Jacobsen (PI) and J.E. Wood (Co-PI) introduced the term "MEMS" by way of a proposal to DARPA (15 July 1986), titled "Micro Electro-Mechanical Systems (MEMS)", granted to the University of Utah. The term "MEMS" was presented by way of an invited talk by S.C. Jacobsen, titled "Micro Electro-Mechanical Systems (MEMS)", at the IEEE Micro Robots and Teleoperators Workshop, Hyannis, MA Nov. 9–11, 1987. The term "MEMS" was published by way of a submitted paper by J.E. Wood, S.C. Jacobsen, and K.W. Grace, titled "SCOFSS: A Small Cantilevered Optical Fiber Servo System", in the IEEE Proceedings Micro Robots and Teleoperators Workshop, Hyannis, MA Nov. 9–11, 1987.[8] CMOS transistors have been manufactured on top of MEMS structures.[9]
Types[edit]
There are two basic types of MEMS switch technology: capacitive and ohmic. A capacitive MEMS switch is developed using a moving plate or sensing element, which changes the capacitance.[10] Ohmic switches are controlled by electrostatically controlled cantilevers.[11] Ohmic MEMS switches can fail from metal fatigue of the MEMS actuator (cantilever) and contact wear, since cantilevers can deform over time.[12]
Basic processes[edit]
Deposition processes[edit]
One of the basic building blocks in MEMS processing is the ability to deposit thin films of material with a thickness anywhere from one micrometre to about 100 micrometres. The NEMS process is the same, although the measurement of film deposition ranges from a few nanometres to one micrometre. There are two types of deposition processes, as follows.
Manufacturing technologies[edit]
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.[20] Silicon is machined using various etching processes. Bulk micromachining has been essential in enabling high performance pressure sensors and accelerometers that changed the sensor industry in the 1980s and 1990s.
Surface micromachining uses layers deposited on the surface of a substrate as the structural materials, rather than using the substrate itself.[25] 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 has pioneered the industrialization of surface micromachining and has realized the co-integration of MEMS and integrated circuits.
Wafer bonding involves joining two or more substrates (usually having the same diameter) to one another to form a composite structure. There are several types of wafer bonding processes that are used in microsystems fabrication including: direct or fusion wafer bonding, wherein two or more wafers are bonded together that are usually made of silicon or some other semiconductor material; anodic bonding wherein a boron-doped glass wafer is bonded to a semiconductor wafer, usually silicon; thermocompression bonding, wherein an intermediary thin-film material layer is used to facilitate wafer bonding; and eutectic bonding, wherein a thin-film layer of gold is used to bond two silicon wafers. Each of these methods have specific uses depending on the circumstances. Most wafer bonding processes rely on three basic criteria for successfully bonding: the wafers to be bonded are sufficiently flat; the wafer surfaces are sufficiently smooth; and the wafer surfaces are sufficiently clean. The most stringent criteria for wafer bonding is usually the direct fusion wafer bonding since even one or more small particulates can render the bonding unsuccessful. In comparison, wafer bonding methods that use intermediary layers are often far more forgiving.
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.
Some common commercial applications of MEMS include:
Industry structure[edit]
The global market for micro-electromechanical systems, which includes products such as automobile airbag systems, display systems and inkjet cartridges totaled $40 billion in 2006 according to Global MEMS/Microsystems Markets and Opportunities, a research report from SEMI and Yole Development and is forecasted to reach $72 billion by 2011.[39]
Companies with strong MEMS programs come in many sizes. Larger firms specialize in manufacturing high volume inexpensive components or packaged solutions for end markets such as automobiles, biomedical, and electronics. Smaller firms provide value in innovative solutions and absorb the expense of custom fabrication with high sales margins. Both large and small companies typically invest in R&D to explore new MEMS technology.
The market for materials and equipment used to manufacture MEMS devices topped $1 billion worldwide in 2006. Materials demand is driven by substrates, making up over 70 percent of the market, packaging coatings and increasing use of chemical mechanical planarization (CMP). While MEMS manufacturing continues to be dominated by used semiconductor equipment, there is a migration to 200 mm lines and select new tools, including etch and bonding for certain MEMS applications.