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Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems is the technology of microscopic devices, particularly those with moving parts. It merges at the nanoscale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines in Japan and microsystem technology (MST) in Europe.

MEMS are made up of components between 1 and 100 micrometers 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] 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 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), electro discharge machining (EDM), and other technologies capable of manufacturing small devices.

History

MEMS technology has roots in the silicon revolution, which can be traced back to two important silicon semiconductor inventions from 1959: the monolithic integrated circuit (IC) chip by Robert Noyce at Fairchild Semiconductor, and the MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor) by Mohamed M. Atalla and Dawon Kahng at Bell Labs. MOSFET scaling, the miniaturisation of MOSFETs on IC chips, led to the miniaturisation of electronics (as predicted by Moore's law and Dennard scaling). This laid the foundations for the miniaturisation of mechanical systems, with the development of micromachining technology based on silicon semiconductor technology, as engineers began realizing that silicon chips and MOSFETs could interact and communicate with the surroundings and process things such as chemicals, motions and light. One of the first silicon pressure sensors was isotropically micromachined by Honeywell in 1962.[4]

An early example of a MEMS device is the resonant-gate transistor, an adaptation of the MOSFET, developed by Harvey C. Nathanson in 1965.[5] Another early example is the resonistor, an electromechanical monolithic resonator patented by Raymond J. Wilfinger between 1966 and 1971.[6][7] During the 1970s to early 1980s, a number of MOSFET microsensors were developed for measuring physical, chemical, biological and environmental parameters.[8]

Types

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.[9] Ohmic switches are controlled by electrostatically controlled cantilevers.[10] Ohmic MEMS switches can fail from metal fatigue of the MEMS actuator (cantilever) and contact wear, since cantilevers can deform over time.[11]

Materials for MEMS manufacturing

The fabrication of MEMS evolved from the process technology in semiconductor device fabrication, i.e. the basic techniques are deposition of material layers, patterning by photolithography and etching to produce the required shapes.[12]

Silicon

Silicon is the material used to create most integrated circuits used in consumer electronics in the modern industry. The economies of scale, ready availability of inexpensive 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. Semiconductor nanostructures based on silicon are gaining increasing importance in the field of microelectronics and MEMS in particular. Silicon nanowires, fabricated through the thermal oxidation of silicon, are of further interest in electrochemical conversion and storage, including nanowire batteries and photovoltaic systems.

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 molding, 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, aluminium, copper, chromium, titanium, tungsten, platinum, and silver.

Ceramics

The nitrides of silicon, aluminium and titanium as well as silicon carbide and other ceramics are increasingly applied in MEMS fabrication due to advantageous combinations of material properties. AlN crystallizes in the wurtzite structure and thus shows pyroelectric and piezoelectric properties enabling sensors, for instance, with sensitivity to normal and shear forces.[13] TiN, on the other hand, exhibits a high electrical conductivity and large elastic modulus, making it possible to implement electrostatic MEMS actuation schemes with ultrathin beams. Moreover, the high resistance of TiN against biocorrosion qualifies the material for applications in biogenic environments. The figure shows an electron-microscopic picture of a MEMS biosensor with a 50 nm thin bendable TiN beam above a TiN ground plate. Both can be driven as opposite electrodes of a capacitor, since the beam is fixed in electrically isolating side walls. When a fluid is suspended in the cavity its viscosity may be derived from bending the beam by electrical attraction to the ground plate and measuring the bending velocity. [14]

MEMS manufacturing technologies

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.[15] 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 changed the sensor industry in the 1980s and 90's.

Surface micromachining

Main article: Surface micromachining

Surface micromachining uses layers deposited on the surface of a substrate as the structural materials, rather than using the substrate itself.[16] 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.

Thermal oxidation

Main article: thermal oxidation

To control the size of micro and nano-scale components, the use of so-called etchless processes is often applied. This approach to MEMS fabrication relies mostly on the oxidation of silicon, as described by the Deal-Grove model. Thermal oxidation processes are used to produced diverse silicon structures with highly precise dimensional control. Devices including optical frequency combs,[17] and silicon MEMS pressure sensors,[18] have been produced through the use of thermal oxidation processes to fine-tune silicon structures in one or two dimensions. Thermal oxidation is of particular value in the fabrication of silicon nanowires, which are widely employed in MEMS systems as both mechanical and electrical components.

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.

File:Labonachip20017-300.jpg

Microelectromechanical systems chip, sometimes called "lab on a chip"

Applications

File:DLP CINEMA. A Texas Instruments Technology - Photo Philippe Binant.jpg

A Texas Instruments DMD chip for cinema projection

File:Gold stripe testing with MEMS.webm

Measuring mechanical properties of a gold stripe (width ~1 µm) using MEMS inside a transmission electron microscope.[19]

Some common commercial applications of MEMS include:

Industry structure

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 Developpement and is forecasted to reach $72 billion by 2011.[28]

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.

References

  1. ↑ Gabriel K, Jarvis J, Trimmer W (1988). Small Machines, Large Opportunities: A Report on the Emerging Field of Microdynamics: Report of the Workshop on Microelectromechanical Systems Research. National Science Foundation (sponsor). AT&T Bell Laboratories.
  2. ↑ Waldner JB (2008). Nanocomputers and Swarm Intelligence. London: ISTE John Wiley & Sons. p. 205. ISBN 9781848210097.
  3. ↑ Angell JB, Terry SC, Barth PW (1983). "Silicon Micromechanical Devices". Sci. Am. 248 (4): 44–55. Bibcode:1983SciAm.248d..44A. doi:10.1038/scientificamerican0483-44.
  4. ↑ Rai-Choudhury, P. (2000). MEMS and MOEMS Technology and Applications. SPIE Press. pp. ix, 3. ISBN 9780819437167.
  5. ↑ Nathanson HC, Wickstrom RA (1965). "A Resonant-Gate Silicon Surface Transistor with High-Q Band-Pass Properties". Appl. Phys. Lett. 7 (4): 84–86. Bibcode:1965ApPhL...7...84N. doi:10.1063/1.1754323.
  6. ↑ US patent 3614677A, Wilfinger RJ, "Electromechanical monolithic resonator", issued Oct 1971, assigned to International Business Machines Corp 
  7. ↑ Wilfinger RJ, Bardell PH, Chhabra DS (1968). "The Resonistor: A Frequency Selective Device Utilizing the Mechanical Resonance of a Silicon Substrate". IBM J. Res. Dev. 12 (1): 113–8. doi:10.1147/rd.121.0113.
  8. ↑ Bergveld, Piet (October 1985). "The impact of MOSFET-based sensors" (PDF). Sensors and Actuators. 8 (2): 109–127. doi:10.1016/0250-6874(85)87009-8. ISSN 0250-6874.
  9. ↑ "Evaluation of MEMS capacitive accelerometers". ieee.org. 1999-12-01. Retrieved 2019-08-06.
  10. ↑ "Introduction to MEMS and RF-MEMS: From the early days of microsystems to modern RF-MEMS passives". iop.org. 2017-11-01. Retrieved 2019-08-06.
  11. ↑ "MEMS technology is transforming high-density switch matrices". evaluationengineering.com. 2019-06-24. Retrieved 2019-08-06.
  12. ↑ Ghodssi R, Lin P (2011). MEMS Materials and Processes Handbook. Berlin: Springer. ISBN 9780387473161.
  13. ↑ Polster T, Hoffmann M (2009). "Aluminium nitride based 3D, piezoelectric, tactile sensors". Procedia Chemistry. 1 (1): 144–7. doi:10.1016/j.proche.2009.07.036.
  14. ↑ M. Birkholz; K.-E. Ehwald; T. Basmer; et al. (2013). "Sensing glucose concentrations at GHz frequencies with a fully embedded Biomicro-electromechanical system (BioMEMS)". J. Appl. Phys. 113: 244904. doi:10.1063/1.4811351.
  15. ↑ Kovacs GT, Maluf NI, Petersen KE (1998). "Bulk micromachining of silicon" (PDF). Proc. IEEE. 86 (8): 1536–1551. doi:10.1109/5.704259. Archived from the original (PDF) on 27 Oct 2017.
  16. ↑ Bustillo JM, Howe RT, Muller RS (1998). "Surface Micromachining for Microelectromechanical Systems" (PDF). Proc. IEEE. 86 (8): 1552–1574. CiteSeerX 10.1.1.120.4059. doi:10.1109/5.704260.
  17. ↑ Silicon-chip mid-infrared frequency comb generation Nature, 2015.
  18. ↑ Singh, Kulwant; Joyce, Robin; Varghese, Soney; Akhtar, J. (2015). "Fabrication of electron beam physical vapor deposited polysilicon piezoresistive MEMS pressure sensor". Sensors and Actuators A: Physical. 223: 151–158. doi:10.1016/j.sna.2014.12.033.
  19. ↑ Hosseinian E, Pierron ON (2013). "Quantitative in situ TEM tensile fatigue testing on nanocrystalline metallic ultrathin films". Nanoscale. 5 (24): 12532–41. Bibcode:2013Nanos...512532H. doi:10.1039/C3NR04035F. PMID 24173603.
  20. ↑ Acar C, Shkel AM (2008). MEMS Vibratory Gyroscopes: Structural Approaches to Improve Robustness. Springer Science. p. 111. ISBN 9780387095363.
  21. ↑ Johnson RC (2007). "There's more to MEMS than meets the iPhone". EE Times. https://www.eetimes.com/document.asp?doc_id=1305409. 
  22. ↑ Clarke P (2016). "Smart MEMS microphones market emerges". EE News Analog. https://www.eenewsanalog.com/news/smart-mems-microphones-market-emerges. 
  23. ↑ "DS3231m RTC" (PDF). DS3231m RTC Datasheet. Maxim Inc. 2015. Retrieved 26 Mar 2019.
  24. ↑ Louizos LA, Athanasopoulos PG, Varty K (2012). "Microelectromechanical Systems and Nanotechnology. A Platform for the Next Stent Technological Era". Vasc. Endovasc. Surg. 46 (8): 605–609. doi:10.1177/1538574412462637. PMID 23047818.
  25. ↑ Hajati A, Kim SG (2011). "Ultra-wide bandwidth piezoelectric energy harvesting". Appl. Phys. Lett. 99 (8): 083105. Bibcode:2011ApPhL..99h3105H. doi:10.1063/1.3629551. hdl:1721.1/75264.
  26. ↑ Hajati A (2012). "Three-dimensional micro electromechanical system piezoelectric ultrasound transducer". Appl. Phys. Lett. 101 (25): 253101. Bibcode:2012ApPhL.101y3101H. doi:10.1063/1.4772469.
  27. ↑ Hajati A (2013). "Monolithic ultrasonic integrated circuits based on micromachined semi-ellipsoidal piezoelectric domes". Appl. Phys. Lett. 103 (20): 202906. Bibcode:2013ApPhL.103t2906H. doi:10.1063/1.4831988.
  28. ↑ "Worldwide MEMS Systems Market Forecasted to Reach $72 Billion by 2011". AZoNano. 2007. https://www.azonano.com/news.aspx?newsID=4479. 

Further reading

External links