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ARTICLES IN THE BOOK

  1. Atomic force microscope
  2. Atomic nanoscope
  3. Atom probe
  4. Ballistic conduction
  5. Bingel reaction
  6. Biomimetic
  7. Bio-nano generator
  8. Bionanotechnology
  9. Break junction
  10. Brownian motor
  11. Bulk micromachining
  12. Cantilever
  13. Carbon nanotube
  14. Carbyne
  15. CeNTech
  16. Chemical Compound Microarray
  17. Cluster
  18. Colloid
  19. Comb drive
  20. Computronium
  21. Coulomb blockade
  22. Diamondoids
  23. Dielectrophoresis
  24. Dip Pen Nanolithography
  25. DNA machine
  26. Ecophagy
  27. Electrochemical scanning tunneling microscope
  28. Electron beam lithography
  29. Electrospinning
  30. Engines of Creation
  31. Exponential assembly
  32. Femtotechnology
  33. Fermi point
  34. Fluctuation dissipation theorem
  35. Fluorescence interference contrast microscopy
  36. Fullerene
  37. Fungimol
  38. Gas cluster ion beam
  39. Grey goo
  40. Hacking Matter
  41. History of nanotechnology
  42. Hydrogen microsensor
  43. Inorganic nanotube
  44. Ion-beam sculpting
  45. Kelvin probe force microscope
  46. Lab-on-a-chip
  47. Langmuir-Blodgett film
  48. LifeChips
  49. List of nanoengineering topics
  50. List of nanotechnology applications
  51. List of nanotechnology topics
  52. Lotus effect
  53. Magnetic force microscope
  54. Magnetic resonance force microscopy
  55. Mechanochemistry
  56. Mechanosynthesis
  57. MEMS thermal actuator
  58. Mesotechnology
  59. Micro Contact Printing
  60. Microelectromechanical systems
  61. Microfluidics
  62. Micromachinery
  63. Molecular assembler
  64. Molecular engineering
  65. Molecular logic gate
  66. Molecular manufacturing
  67. Molecular motors
  68. Molecular recognition
  69. Molecule
  70. Nano-abacus
  71. Nanoart
  72. Nanobiotechnology
  73. Nanocar
  74. Nanochemistry
  75. Nanocomputer
  76. Nanocrystal
  77. Nanocrystalline silicon
  78. Nanocrystal solar cell
  79. Nanoelectrochemistry
  80. Nanoelectrode
  81. Nanoelectromechanical systems
  82. Nanoelectronics
  83. Nano-emissive display
  84. Nanoengineering
  85. Nanoethics
  86. Nanofactory
  87. Nanoimprint lithography
  88. Nanoionics
  89. Nanolithography
  90. Nanomanufacturing
  91. Nanomaterial based catalyst
  92. Nanomedicine
  93. Nanomorph
  94. Nanomotor
  95. Nano-optics
  96. Nanoparticle
  97. Nanoparticle tracking analysis
  98. Nanophotonics
  99. Nanopore
  100. Nanoprobe
  101. Nanoring
  102. Nanorobot
  103. Nanorod
  104. Nanoscale
  105. Nano-Science Center
  106. Nanosensor
  107. Nanoshell
  108. Nanosight
  109. Nanosocialism
  110. Nanostructure
  111. Nanotechnology
  112. Nanotechnology education
  113. Nanotechnology in fiction
  114. Nanotoxicity
  115. Nanotube
  116. Nanovid microscopy
  117. Nanowire
  118. National Nanotechnology Initiative
  119. Neowater
  120. Niemeyer-Dolan technique
  121. Ormosil
  122. Photolithography
  123. Picotechnology
  124. Programmable matter
  125. Quantum dot
  126. Quantum heterostructure
  127. Quantum point contact
  128. Quantum solvent
  129. Quantum well
  130. Quantum wire
  131. Richard Feynman
  132. Royal Society's nanotech report
  133. Scanning gate microscopy
  134. Scanning probe lithography
  135. Scanning probe microscopy
  136. Scanning tunneling microscope
  137. Scanning voltage microscopy
  138. Self-assembled monolayer
  139. Self-assembly
  140. Self reconfigurable
  141. Self-Reconfiguring Modular Robotics
  142. Self-replication
  143. Smart dust
  144. Smart material
  145. Soft lithography
  146. Spent nuclear fuel
  147. Spin polarized scanning tunneling microscopy
  148. Stone Wales defect
  149. Supramolecular assembly
  150. Supramolecular chemistry
  151. Supramolecular electronics
  152. Surface micromachining
  153. Surface plasmon resonance
  154. Synthetic molecular motors
  155. Synthetic setae
  156. Tapping AFM
  157. There's Plenty of Room at the Bottom
  158. Transfersome
  159. Utility fog

 



NANOTECHNOLOGY
This article is from:
http://en.wikipedia.org/wiki/Microelectromechanical_systems

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License 

Microelectromechanical systems

From Wikipedia, the free encyclopedia

 
A mite next to a gear set produced using MEMS. Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www.mems.sandia.gov
A mite next to a gear set produced using MEMS. Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www.mems.sandia.gov

Microelectromechanical Systems (MEMS) is the technology of the very small, and merges at the nanoscale into "Nanoelectromechanical" Systems (NEMS) and Nanotechnology.

MEMS are also referred to as micromechanics, micro machines, or Micro Systems Technology (MST). MEMS are separate and distinct from the hypothetical vision of Molecular nanotechnology or Molecular Electronics.

MEMS generally range in size from a micrometer (a millionth of a meter) to a millimeter (thousandth of a meter). At these size scales, the standard constructs of classical physics do not always hold true. Due to MEMS' large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass.

They are fabricated using modified silicon fabrication technology (used to make electronics), molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing very small devices.

Companies with strong MEMS programs come in many sizes. The larger firms specialize in manufacturing high volume inexpensive components or packaged solutions for end markets such as automobiles, biomedical, and electronics. The successful small firms provide value in innovative solutions and absorb the expense of custom fabrication with high sales margins. In addition, both large and small companies work in R&D to explore MEMS technology. Complexity and performance of advanced MEMS based sensors are described by different MEMS sensor generations.

Common applications include:

  • inkjet printers, which use piezoelectrics or bubble ejection to deposit ink on paper.
  • accelerometers in modern cars for a large number of purposes including airbag deployment in collisions.
  • MEMS gyroscopes used in modern cars and other applications to detect yaw; e.g. to deploy a roll over bar or trigger dynamic stability control.
  • pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors.
  • Displays e.g the DMD chip in a projector based on DLP technology has on its surface several hundred thousand micromirrors.
  • Optical switching technology which is used for switching technology for data communications, and is part of the emerging technology of smartdust.

The motion-sensing controller in the Nintendo Wii video game system represents a popular consumer application of MEMS technology.

Finite element analysis is an important part of MEMS design.

MEMS materials

MEMS technology can be implemented using a number of different materials and manufacturing techniques; the choice of which will depend on the device being created and the market sector in which it has to operate.

Silicon

Silicon is the material used to create almost all integrated circuits used in consumer electronics in the modern world. The economies of scale, ready availability of highly accurate processing 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 lithography 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 injection moulding, embossing or stereolithography and are especially well suited to microfluidic 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 metals include Gold, Nickel, Aluminum, Chromium, Titanium, Tungsten, Platinum and Silver.

MEMS processes

Deposition processes

One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. In this text we assume a thin film to have a thickness anywhere between a few nanometers to about 100 micrometers. Commonly used deposition processes are: Electroplating, Sputtering, Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD).

Photolithography

Main article: Photolithography

Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. A photosensitive material is a material that experiences a change in its physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed and unexposed regions differs.

This exposed region can then be removed or treated providing a mask for the underlying substrate. Photolithography is typically used with metal or other thin film deposition, wet and dry etching.

Etching processes

There are two basic categories of etching processes: wet and dry etching. In the former, the material is dissolved when immersed in a chemical solution. In the latter, the material is sputtered or dissolved using reactive ions or a vapor phase etchant.

Wet etching

Main article: Wet etching

Wet chemical etching consists in a selective removal of material by dipping a substrate into a solution that can dissolve it. Due to the chemical nature of this etching process, a good selectivity can often be obtained, which means that the etching rate of the target material is considerably higher than that of the mask material if selected carefully.

Some single crystal materials, such as silicon, will have different etching rates depending on the crystallographic orientation of the substrate. One of the most common examples is the etching of silicon in KOH (potassium hydroxide), where Si <111> planes etch approximately 100 times slower than other planes (crystallographic orientations). Therefore, etching a rectangular hole in a (100)-Si wafer will result in a pyramid shaped etch pit, instead of a hole with curved sidewalls as it would be the case for isotropic etching, where etching progresses at the same speed in all directions. A long and narrow holes will produce v-shaped grooves. The surface of these grooves can be atomically smooth if the etch is carried out correctly with dimensions and angles being extremely accurate.

Another method to change the etchant selectivity of silicon is to heavily dope the desired pattern with boron. This makes the silicon/boron unetchable to silicon etches. This technique is termed an 'etchstop'. See footnote for further information. Footnote: http://www.techneglas.com/dopantpages/etch.htm

Reactive ion etching (RIE)

Main article: Reactive ion etching

In reactive ion etching (RIE), the substrate is placed inside a reactor in which several gases are introduced. A plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards, and reacts at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical. A schematic of a typical reactive ion etching system is shown in the figure below.

Deep reactive ion etching (DRIE)

Main article: Deep reactive ion etching

A special subclass of RIE which continues to grow rapidly in popularity is deep RIE (DRIE). In this process, etch depths of hundreds of micrometres can be achieved with almost vertical sidewalls. The primary technology is based on the so-called "Bosch process", named after the German company Robert Bosch which filed the original patent, where two different gas compositions are alternated in the reactor. The first gas composition creates a polymer on the surface of the substrate, and the second gas composition etches the substrate. The polymer is immediately sputtered away by the physical part of the etching, but only on the horizontal surfaces and not the sidewalls. Since the polymer only dissolves very slowly in the chemical part of the etching, it builds up on the sidewalls and protects them from etching. As a result, etching aspect ratios of 50 to 1 can be achieved. The process can easily be used to etch completely through a silicon substrate, and etch rates are 3-4 times higher than wet etching.

Silicon MEMS paradigms

Bulk micromachining

Main article: 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 wet etching. Anodic bonding of glass plates to silicon is used for adding features in the third dimension and for hermetical 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

Main article: Surface micromachining

Surface micromachining was created in the late 80's to render micromachining of silicon more planar, to make it resemble more the planar integrated circuit technology. The ultimate hope was that MEMS and integrated circuits could be combined on the same silicon wafer. The original surface micromachining concept was based on thin polycrystalline silicon layers used as movable mechanical structures and releasing them by sacrificial etching of the underlaying oxide layer. Interdigital comb electrodes were used to produce in-plane forces and to detect in-plane movement capacitively. This MEMS paradigm has enabled to manufacture 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) micromachining

Both bulk and surface micromachining are still used in industrial production of sensors, ink-jet nozzles and other devices. But in many cases the distinction between these two has diminished. New etching technology, deep reactive ion etching has made it possible to combine good performance typical to bulk micromachining with comb structures and in-plane operation typical to surface micromachining. While it is common in surface micromachining to have structural layer thickness in the range of 2 Ám, in HAR micromachining the thickness is from 10 to 100 Ám. The materials commonly used in HAR 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 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.

See also

  • NEMS, Nanoelectromechanical systems are similar to MEMS but smaller
  • MOEMS, Micro Opto-Electrical-Mechanical Systems, MEMS including optical elements
  • Micropower Hydrogen generators, gas turbines, and electrical generators made of etched silicon
  • IBM Millipede, a MEMS technology for non-volatile data storage of more than a terabit per square inch
  • STMicroelectronics for 2x and 3x accelerometers
  • Texas Instruments pioneers of the DMD chip
  • ADI one of the major early players in accelerometer development
  • Lucent who developed highly advanced optical telecommunications switches
  • Cantilever one of the most common forms of MEMS.
  • MEMS Thermal Actuator MEMS actuation created by thermal expansion
  • Electrostatic motors used where coils are difficult to fabricate

External links

  • The MEMS and Nanotechnology Clearinghouse (Information Source)
  • The MEMS and Nanotechnology Exchange (DARPA funded fabrication service)
  • MEMS Investor Journal
  • memx.com Image Gallery, Movie Gallery of Functioning MEMS
  • Tutorials on MEMS
  • Interactive MEMS Tutorials from Budapest University
  • Introduction and resources on MEMS and Microtechnology
  • A History of Silicon MEMS Product Development Through 1990
  • μFluids@Home (Distributed Computing Project)
  • memscyclopedia.org A (not so) short introduction to MEMS
  • VTI Technologies the pioneer and industry leader in accelerometers for low g-ranges
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