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{{Short description|Very small devices that incorporate moving components}}
{{Other uses|MEMS (disambiguation)}}
[[File:MEMsfounding.jpg|thumb|Proposal submitted to [[DARPA]] in 1986 first introducing the term "microelectromechanical systems"]]
[[File:MEMS Microcantilever in Resonance.png|thumb|MEMS microcantilever resonating inside a [[scanning electron microscope]]]]

'''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&nbsp;mm), and MEMS devices generally range in size from 20 micrometres to a millimetre (i.e., 0.02 to 1.0&nbsp;mm), although components arranged in arrays (e.g., [[digital micromirror device]]s) can be more than 1000&nbsp;mm<sup>2</sup>.<ref>{{Cite book|url=https://books.google.com/books?id=7lKytgAACAAJ|title=Small Machines, Large Opportunities: A Report on the Emerging Field of Microdynamics: Report of the Workshop on Microelectromechanical Systems Research|vauthors=Gabriel K, Jarvis J, Trimmer W|publisher=AT&T Bell Laboratories|others=[[National Science Foundation]] (sponsor)|year=1988}}</ref> 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 [[microsensor]]s).<ref>{{cite book|title=Nanocomputers and Swarm Intelligence|vauthors=Waldner JB|publisher=[[ISTE Ltd|ISTE]] [[John Wiley & Sons]]|year=2008|isbn=9781848210097|place=London|pages=205|author-link=Jean-Baptiste Waldner}}</ref>

Because of the large surface area to volume ratio of MEMS, forces produced by ambient [[electromagnetism]] (e.g., electrostatic charges and [[magnetic moment]]s), 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]].<ref>{{Cite journal|vauthors=Angell JB, Terry SC, Barth PW|date=1983|title=Silicon Micromechanical Devices|journal=[[Scientific American|Sci. Am.]]|volume=248|issue=4|pages=44–55|doi=10.1038/scientificamerican0483-44|bibcode=1983SciAm.248d..44A}}</ref> These include molding and plating, [[Etching (microfabrication)|wet etching]] ([[Potassium hydroxide|KOH]], [[Tetramethylammonium hydroxide|TMAH]]) and [[dry etching]] ([[Reactive-ion 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 ==

An early example of a MEMS device is the resonant-gate transistor, an adaptation of the [[MOSFET]], developed by [[Harvey C. Nathanson]] in 1965.<ref>{{cite journal|vauthors=Nathanson HC, Wickstrom RA|date=1965|title=A Resonant-Gate Silicon Surface Transistor with High-Q Band-Pass Properties|journal=[[Applied Physics Letters|Appl. Phys. Lett.]]|volume=7|issue=4|pages=84–86|doi=10.1063/1.1754323|bibcode=1965ApPhL...7...84N}}</ref> Another early example is the resonistor, an electromechanical monolithic [[resonator]] patented by Raymond J. Wilfinger between 1966 and 1971.<ref>{{Cite patent|country=US|number=3614677A|title=Electromechanical monolithic resonator|status=patent|pubdate=|gdate=Oct 1971|invent1=Wilfinger RJ|inventor1-first=|assign1=International Business Machines Corp|url=https://patents.google.com/patent/US3614677A/en}}</ref><ref>{{cite journal|vauthors=Wilfinger RJ, Bardell PH, Chhabra DS|date=1968|title=The Resonistor: A Frequency Selective Device Utilizing the Mechanical Resonance of a Silicon Substrate|journal=[[IBM Journal of Research and Development|IBM J. Res. Dev.]]|volume=12|issue=1|pages=113–8|doi=10.1147/rd.121.0113}}</ref> During the 1970s to early 1980s, a number of MOSFET [[microsensor]]s were developed for measuring physical, chemical, biological and environmental parameters.<ref name="Bergveld">{{cite journal |last1=Bergveld |first1=Piet |author1-link=Piet Bergveld |title=The impact of MOSFET-based sensors |journal=Sensors and Actuators |date=October 1985 |volume=8 |issue=2 |pages=109–127 |doi=10.1016/0250-6874(85)87009-8 |bibcode=1985SeAc....8..109B |url=https://core.ac.uk/download/pdf/11473091.pdf |issn=0250-6874 |access-date=2019-10-16 |archive-date=2021-04-26 |archive-url=https://web.archive.org/web/20210426192332/https://core.ac.uk/download/pdf/11473091.pdf |url-status=dead }}</ref>

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.<ref>IEEE Catalog no. 87TH0204-8, Library of Congress no. 87-82657.  Reprinted in "Micromechanics and MEMS: Classic and Seminal Papers to 1990" (ed. Wm. S. Trimmer, {{ISBN|0-7803-1085-3}}), pgs. 231–236.</ref> CMOS transistors have been manufactured on top of MEMS structures.<ref>{{cite web | url=https://www.eetimes.com/mems-transistor-integrated-on-cmos/ | title=MEMS transistor integrated on CMOS | date=6 December 2011 }}</ref>

==Types==
There are two basic types of MEMS switch technology: [[Capacitor|capacitive]] and [[Ohmic contact|ohmic]]. A capacitive MEMS switch is developed using a moving plate or sensing element, which changes the capacitance.<ref>{{cite journal|title=Evaluation of MEMS capacitive accelerometers |date=1999-12-01 |doi=10.1109/54.808209 |last1=Beliveau |first1=A. |last2=Spencer |first2=G.T. |last3=Thomas |first3=K.A. |last4=Roberson |first4=S.L. |journal=IEEE Design & Test of Computers |volume=16 |issue=4 |pages=48–56 }}</ref> Ohmic switches are controlled by electrostatically controlled cantilevers.<ref>{{cite book|url=https://iopscience.iop.org/book/978-0-7503-1545-6/chapter/bk978-0-7503-1545-6ch1 |title=Introduction to MEMS and RF-MEMS: From the early days of microsystems to modern RF-MEMS passives |website=iop.org |date=2017-11-01 |doi=10.1088/978-0-7503-1545-6ch1 |access-date=2019-08-06|last1=Iannacci |first1=Jacopo |isbn=978-0-7503-1545-6 }}</ref> Ohmic MEMS switches can fail from metal fatigue of the MEMS [[actuator]] (cantilever) and contact wear, since cantilevers can deform over time.<ref>{{cite web|url=https://www.evaluationengineering.com/instrumentation/switching-systems/article/21082562/mems-technology-is-transforming-highdensity-switch-matrices |title=MEMS technology is transforming high-density switch matrices |website=evaluationengineering.com |date=2019-06-24 |access-date=2019-08-06}}</ref>

== Materials ==
[[File:BioMEMS with X-shpaed cantilever.png|thumb|Electron microscope pictures of X-shaped TiN beam above ground plate (height difference 2.5&nbsp;µm). Due to the clip in the middle, an increasing reset force develops when the beam bends downwards. The right figure shows a magnification of the clip.<ref name=JAP2013>{{cite journal | display-authors = 3 | author1 = M. Birkholz | author2 =  K.-E. Ehwald | author3 =  T. Basmer | author4 =  P. Kulse | author5=  C. Reich | author6 =  J. Drews | author7 =  D. Genschow | author8 =  U. Haak | author9 =  S. Marschmeyer | author10 =  E. Matthus | author11 =  K. Schulz | author12 =  D. Wolansky | author13 =  W. Winkler | author14 =  T. Guschauski | author15 =  R. Ehwald | title = Sensing glucose concentrations at GHz frequencies with a fully embedded Biomicro-electromechanical system (BioMEMS) | journal = J. Appl. Phys. | volume = 113 | issue = 24 | pages = 244904–244904–8 | year = 2013 | doi = 10.1063/1.4811351| pmid = 25332510 | pmc = 3977869 | bibcode = 2013JAP...113x4904B }}</ref>]]

The fabrication of MEMS evolved from the process technology in [[semiconductor device fabrication]], i.e. the basic techniques are [[Deposition (chemistry)|deposition]] of material layers, patterning by [[photolithography]] and etching to produce the required shapes.<ref>{{cite book|title=MEMS Materials and Processes Handbook|vauthors=Ghodssi R, Lin P|publisher=[[Springer Science+Business Media|Springer]]|year=2011|isbn=9780387473161|place=Berlin}}</ref>

; Silicon: Silicon is the material used to create most [[integrated circuit]]s 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 [[Hooke's law|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 (material)|fatigue]] and can have service lifetimes in the range of [[1000000000 (number)|billions]] to [[1000000000000 (number)|trillions]] of cycles without breaking. [[Semiconductor nanostructures]] based on silicon are gaining increasing importance in the field of microelectronics and MEMS in particular. [[Silicon nanowire]]s, fabricated through the [[thermal oxidation]] of silicon, are of further interest in [[electrochemistry|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 (manufacturing)|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 [[nitride]]s of silicon, aluminium and titanium as well as [[silicon carbide]] and other [[ceramic]]s are increasingly applied in MEMS fabrication due to advantageous combinations of material properties. [[aluminium nitride|AlN]] crystallizes in the [[wurtzite structure]] and thus shows [[pyroelectricity|pyroelectric]] and [[piezoelectricity|piezoelectric]] properties enabling sensors, for instance, with sensitivity to normal and shear forces.<ref name="PC2009">{{cite journal|vauthors=Polster T, Hoffmann M|date=2009|title=Aluminium nitride based 3D, piezoelectric, tactile sensors|journal=Procedia Chemistry|volume=1|issue=1|pages=144–7|doi=10.1016/j.proche.2009.07.036|doi-access=free}}</ref> [[titanium nitride|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&nbsp;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.<ref name=JAP2013/>

== Basic processes ==

=== Deposition processes ===

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.

==== Physical deposition ====

Physical vapor deposition ("PVD") consists of a process in which a material is removed from a target, and deposited on a surface. Techniques to do this include the process of [[sputtering]], in which an ion beam liberates atoms from a target, allowing them to move through the intervening space and deposit on the desired substrate, and [[Evaporation (deposition)|evaporation]], in which a material is evaporated from a target using either heat (thermal evaporation) or an electron beam (e-beam evaporation) in a vacuum system.

==== Chemical deposition ====

Chemical deposition techniques include [[chemical vapor deposition]] (CVD), in which a stream of source gas reacts on the substrate to grow the material desired. This can be further divided into categories depending on the details of the technique, for example LPCVD (low-pressure chemical vapor deposition) and PECVD ([[plasma-enhanced chemical vapor deposition]]). Oxide films can also be grown by the technique of [[thermal oxidation]], in which the (typically silicon) wafer is exposed to oxygen and/or steam, to grow a thin surface layer of [[silicon dioxide]].

=== Patterning ===

Patterning in MEMS is the transfer of a pattern into a material.

=== Lithography ===

Lithography in a MEMS context is typically the transfer of a pattern into 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 a photosensitive material is selectively exposed 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. Sometimes, photolithography is used to create structure without any kind of post etching. One example is SU8 based lens where SU8 based square blocks are generated. Then the photoresist is melted to form a semi-sphere which acts as a lens.

[[Electron beam lithography]] (often abbreviated as e-beam lithography) is the practice of scanning a beam of [[electron]]s in a patterned fashion across a surface covered with a film (called the [[resist]]),<ref name="mccord">{{cite book|vauthors=McCord MA, Rooks MJ|title=Handbook of Microlithography, Micromachining, and Microfabrication. Volume 1: Microlithography|publisher=[[SPIE]]|year=1997|isbn=9780819497864|veditors=Choudhury PR|volume=1|location=London|chapter=Electron Beam Lithography|doi=10.1117/3.2265070.ch2|chapter-url=http://www.cnf.cornell.edu/cnf_spietoc.html|access-date=2011-01-28|archive-date=2019-08-19|archive-url=https://web.archive.org/web/20190819183249/http://www.cnf.cornell.edu/cnf_spietoc.html|url-status=dead}}</ref> ("exposing" the resist) and of selectively removing either exposed or non-exposed regions of the resist ("developing"). The purpose, as with [[photolithography]], is to create very small structures in the resist that can subsequently be transferred to the substrate material, often by etching. It was developed for manufacturing [[integrated circuit]]s, and is also used for creating [[nanotechnology]] architectures. The primary advantage of electron beam lithography is that it is one of the ways to beat the [[diffraction limit]] of light and make features in the [[nanometer]] range. This form of [[maskless lithography]] has found wide usage in [[photomask]]-making used in [[photolithography]], low-volume production of semiconductor components, and research & development. The key limitation of electron beam lithography is throughput, i.e., the very long time it takes to expose an entire silicon wafer or glass substrate. A long exposure time leaves the user vulnerable to beam drift or instability which may occur during the exposure. Also, the turn-around time for reworking or re-design is lengthened unnecessarily if the pattern is not being changed the second time.

It is known that focused-[[ion beam lithography]] has the capability of writing extremely fine lines (less than 50&nbsp;nm line and space has been achieved) without proximity effect.<ref>{{cite book | chapter-url=https://www.sciencedirect.com/topics/engineering/beam-lithography | title=Materials and Processes for Next Generation Lithography | chapter=Scanning helium ion beam lithography | year=2016 | pages=563–594 | publisher=Elsevier }}</ref> However, because the writing field in ion-beam lithography is quite small, large area patterns must be created by stitching together the small fields.

[[Ion track technology]] is a deep cutting tool with a resolution limit around 8&nbsp;nm applicable to radiation resistant minerals, glasses and polymers. It is capable of generating holes in thin films without any development process. Structural depth can be defined either by ion range or by material thickness. Aspect ratios up to several 10<sup>4</sup> can be reached. The technique can shape and texture materials at a defined inclination angle. Random pattern, single-ion track structures and an aimed pattern consisting of individual single tracks can be generated.

[[X-ray lithography]] is a process used in the electronic industry to selectively remove parts of a thin film. It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist, or simply "resist", on the substrate. A series of chemical treatments then engraves the produced pattern into the material underneath the photoresist.

A simple way to carve or create patterns on the surface of nanodiamonds without damaging them could lead to a new generation of photonic devices.<ref>{{Cite web|title=Diamond Patterning Technique Could Transform Photonics|url=https://www.technologyreview.com/2014/03/26/112116/diamond-patterning-technique-could-transform-photonics/|access-date=2022-01-08|website=MIT Technology Review|language=en}}</ref> Diamond patterning is a method of forming diamond MEMS. It is achieved by the lithographic application of diamond films to a substrate such as silicon. The patterns can be formed by selective deposition through a silicon dioxide mask, or by deposition followed by micromachining or focused [[Ion milling machine|ion beam milling]].<ref>{{cite book|title=From MEMS to Bio-MEMS and Bio-NEMS: Manufacturing Techniques and Applications|vauthors=Madou MJ|publisher=CRC Press|year=2011|isbn=9781439895245|series=Fundamentals of Microfabrication and Nanotechnology|volume=3|location=Boca Raton|pages=252}}</ref>

=== Etching processes ===

There are two basic categories of etching processes: [[Etching (microfabrication)|wet etching]] 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.<ref>{{cite journal|vauthors=Williams KR, Muller RS|date=1996|title=Etch rates for micromachining processing|url=http://www-inst.cs.berkeley.edu/~ee245/fa07/lectures/WetEtchRates.WilliamsMuller.00546406.pdf|journal=Journal of Microelectromechanical Systems|volume=5|issue=4|pages=256–269|citeseerx=10.1.1.120.3130|doi=10.1109/84.546406|access-date=2017-10-26|archive-date=2017-08-09|archive-url=https://web.archive.org/web/20170809034445/http://www-inst.cs.berkeley.edu/~ee245/fa07/lectures/WetEtchRates.WilliamsMuller.00546406.pdf|url-status=dead}}</ref><ref name="bulk">{{cite journal|vauthors=Kovacs GT, Maluf NI, Petersen KE|date=1998|title=Bulk micromachining of silicon|url=http://www.ece.umd.edu/class/enee416.S2004/Bulk-Micromachining.pdf|url-status=dead|journal=[[Proceedings of the IEEE|Proc. IEEE]]|volume=86|issue=8|pages=1536–1551|doi=10.1109/5.704259|archive-url=https://web.archive.org/web/20171027074546/http://www.ece.umd.edu/class/enee416.S2004/Bulk-Micromachining.pdf|archive-date=27 Oct 2017}}</ref>

==== Wet etching ====
{{Main|Etching (microfabrication)}}

Wet chemical etching consists of the selective removal of material by dipping a substrate into a solution that dissolves it. The chemical nature of this etching process provides good selectivity, which means the etching rate of the target material is considerably higher than the mask material if selected carefully.  Wet etching can be performed using either isotropic wet etchants or anisotropic wet etchants.   Isotropic wet etchant etch in all directions of the crystalline silicon at approximately equal rates.   Anisotropic wet etchants preferably etch along certain crystal planes at faster rates than other planes, thereby allowing more complicated 3-D microstructures to be implemented. Wet anisotropic etchants are often used in conjunction with boron etch stops wherein the surface of the silicon is heavily doped with boron resulting in a silicon material layer that is resistant to the wet etchants.  This has been used in MEWS pressure sensor manufacturing for example.

Etching progresses at the same speed in all directions. Long and narrow holes in a mask will produce v-shaped grooves in the silicon. The surface of these grooves can be atomically smooth if the etch is carried out correctly, with dimensions and angles being extremely accurate.

Some single crystal materials, such as silicon, will have different etching rates depending on the crystallographic orientation of the substrate. This is known as anisotropic etching and 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 ([[crystallography|crystallographic orientations]]). Therefore, etching a rectangular hole in a (100)-Si wafer results in a pyramid shaped etch pit with 54.7° walls, instead of a hole with curved sidewalls as with isotropic etching.

[[Hydrofluoric acid]] is commonly used as an aqueous etchant for silicon dioxide ({{chem|SiO|2}}, also known as BOX for SOI), usually in 49% concentrated form, 5:1, 10:1 or 20:1 BOE ([[buffered oxide etch]]ant) or BHF (Buffered HF). They were first used in medieval times for glass etching. It was used in IC fabrication for patterning the gate oxide until the process step was replaced by RIE. Hydrofluoric acid is considered one of the more dangerous acids in the [[cleanroom]]. It penetrates the skin upon contact and it diffuses straight to the bone. Therefore, the damage is not felt until it is too late.

Electrochemical etching (ECE) for dopant-selective removal of silicon is a common method to automate and to selectively control etching. An active p–n [[diode]] junction is required, and either type of dopant can be the etch-resistant ("etch-stop") material. Boron is the most common etch-stop dopant. In combination with wet anisotropic etching as described above, ECE has been used successfully for controlling silicon diaphragm thickness in commercial piezoresistive silicon pressure sensors. Selectively doped regions can be created either by implantation, diffusion, or epitaxial deposition of silicon.

==== Dry etching ====
{{Main|Dry etching}}

[[Xenon difluoride]] ({{chem|XeF|2}}) is a dry vapor phase isotropic etch for silicon originally applied for MEMS in 1995 at University of California, Los Angeles.<ref>{{cite book|title=Microelectronic Structures and Microelectromechanical Devices for Optical Processing and Multimedia Applications|vauthors=Chang FI, Yeh R, Lin G, Chu PB, Hoffman EG, Kruglick EJ, Pister KS, Hecht MH|publisher=[[SPIE]]|year=1995|volume=2641|location=Austin, TX|pages=117|chapter=Gas-phase silicon micromachining with xenon difluoride|doi=10.1117/12.220933|s2cid=39522253|display-authors=3|editor1-last=Bailey|editor1-first=Wayne|editor2-last=Motamedi|editor2-first=M. Edward|editor3-last=Luo|editor3-first=Fang-Chen}}</ref><ref>{{Cite thesis|type=M.S.|title=Xenon difluoride etching of silicon for MEMS|last=Chang|first=Floy I-Jung|publisher=University of California|location=Los Angeles|oclc=34531873|date=1995}}</ref> Primarily used for releasing metal and dielectric structures by undercutting silicon, {{chem|XeF|2}} has the advantage of a [[stiction]]-free release unlike wet etchants. Its etch selectivity to silicon is very high, allowing it to work with photoresist, {{chem|SiO|2}}, silicon nitride, and various metals for masking. Its reaction to silicon is "plasmaless", is purely chemical and spontaneous and is often operated in pulsed mode. Models of the etching action are available,<ref>{{cite book|title=17th IEEE International Conference on Micro Electro Mechanical Systems. Maastricht MEMS 2004 Technical Digest|vauthors=Brazzle JD, Dokmeci MR, Mastrangelo CH|publisher=[[Institute of Electrical and Electronics Engineers|IEEE]]|year=2004|isbn=9780780382657|pages=737–740|chapter=Modeling and characterization of sacrificial polysilicon etching using vapor-phase xenon difluoride|doi=10.1109/MEMS.2004.1290690|s2cid=40417914}}</ref> and university laboratories and various commercial tools offer solutions using this approach.

Modern VLSI processes avoid wet etching, and use [[plasma etching]] instead. Plasma etchers can operate in several modes by adjusting the parameters of the plasma. Ordinary plasma etching operates between 0.1 and 5 Torr. (This unit of pressure, commonly used in vacuum engineering, equals approximately 133.3 pascals.) The plasma produces energetic free radicals, neutrally charged, that react at the surface of the wafer. Since neutral particles attack the wafer from all angles, this process is isotropic. Plasma etching can be isotropic, i.e., exhibiting a lateral undercut rate on a patterned surface approximately the same as its downward etch rate, or can be anisotropic, i.e., exhibiting a smaller lateral undercut rate than its downward etch rate. Such anisotropy is maximized in deep reactive ion etching. The use of the term anisotropy for plasma etching should not be conflated with the use of the same term when referring to orientation-dependent etching. The source gas for the plasma usually contains small molecules rich in chlorine or fluorine. For instance, carbon tetrachloride ({{Chem2|CCl4}}) etches silicon and aluminium, and trifluoromethane etches silicon dioxide and silicon nitride. A plasma containing oxygen is used to oxidize ("ash") photoresist and facilitate its removal.

Ion milling, or [[sputtering|sputter etching]], uses lower pressures, often as low as 10<sup>−4</sup>&nbsp;Torr (10&nbsp;mPa). It bombards the wafer with energetic ions of noble gases, often Ar+, which knock atoms from the substrate by transferring momentum. Because the etching is performed by ions, which approach the wafer approximately from one direction, this process is highly anisotropic. On the other hand, it tends to display poor selectivity. Reactive-ion etching (RIE) operates under conditions intermediate between sputter and plasma etching (between 10<sup>–3</sup> and 10<sup>−1</sup>&nbsp;Torr). Deep reactive-ion etching (DRIE) modifies the RIE technique to produce deep, narrow features. {{cn|date=January 2023}}

In reactive-ion etching (RIE), the substrate is placed inside a reactor, and several gases are introduced. A plasma is struck in the gas mixture using an RF power source, which breaks the gas molecules into ions. The ions accelerate towards, and react with, 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 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.

[[Deep reactive ion etching]] (DRIE) is a special subclass of RIE that is growing in popularity. In this process, etch depths of hundreds of micrometers are achieved with almost vertical sidewalls. The primary technology is based on the so-called "Bosch process",<ref>{{cite book|title=The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, 2005. Digest of Technical Papers. TRANSDUCERS '05|vauthors=Laermer F, Urban A|publisher=[[IEEE]]|year=2005|isbn=9780780389946|volume=2|pages=1118–1121|chapter=Milestones in deep reactive ion etching|doi=10.1109/SENSOR.2005.1497272|s2cid=28068644}}</ref> named after the German company Robert Bosch, which filed the original patent, where two different gas compositions alternate in the reactor. Currently, there are two variations of the DRIE. The first variation consists of three distinct steps (the original Bosch process) while the second variation only consists of two steps.

In the first variation, the etch cycle is as follows:
:(i) {{chem|SF|6}} isotropic etch;
:(ii) {{chem|C|4|F|8}} passivation;
:(iii) {{chem|SF|6}} anisotropic etch for floor cleaning.

In the 2nd variation, steps (i) and (iii) are combined.

Both variations operate similarly. The {{chem|C|4|F|8}} creates a polymer on the surface of the substrate, and the second gas composition ({{chem|SF|6}} and {{chem|O|2}}) 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 3–6 times higher than wet etching.

After preparing a large number of MEMS devices on a [[wafer (electronics)|silicon wafer]], individual [[die (integrated circuit)|dies]] have to be separated, which is called [[die preparation]] in semiconductor technology. For some applications, the separation is preceded by [[wafer backgrinding]] in order to reduce the wafer thickness. [[Wafer dicing]] may then be performed either by sawing using a cooling liquid or a dry laser process called [[wafer dicing#Stealth dicing|stealth dicing]].

== Manufacturing technologies ==

[[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.<ref name="bulk"/> Silicon is machined using various [[#Etching processes|etching processes]].  Bulk micromachining has been essential in enabling high performance [[pressure sensor]]s and [[accelerometer]]s 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.<ref name="surface">{{Cite journal|vauthors=Bustillo JM, Howe RT, Muller RS|date=1998|title=Surface Micromachining for Microelectromechanical Systems|url=http://www.ee.nthu.edu.tw/sclu/surface_micromachining.pdf|journal=[[Proceedings of the IEEE|Proc. IEEE]]|volume=86|issue=8|pages=1552–1574|citeseerx=10.1.1.120.4059|doi=10.1109/5.704260}}</ref> 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 circuit]]s 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 [[accelerometer]]s 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&nbsp;µm, in HAR silicon micromachining the thickness can be from 10 to 100&nbsp;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.

== Applications ==
[[File:DLP CINEMA. A Texas Instruments Technology - Photo Philippe Binant.jpg|thumb|A [[Texas Instruments]] DMD chip for cinema projection]]
[[File:Gold stripe testing with MEMS.webm|thumb|Measuring mechanical properties of a gold stripe (width ~1&nbsp;µm) using MEMS inside a [[transmission electron microscope]].<ref>{{cite journal|vauthors=Hosseinian E, Pierron ON|date=2013|title=Quantitative in situ TEM tensile fatigue testing on nanocrystalline metallic ultrathin films|journal=[[Nanoscale (journal)|Nanoscale]]|volume=5|issue=24|pages=12532–41|doi=10.1039/C3NR04035F|pmid=24173603|bibcode=2013Nanos...512532H|s2cid=17970529}}</ref>]]
Some common commercial applications of MEMS include:
* [[Inkjet printer]]s, which use [[piezoelectric]]s or thermal bubble ejection to deposit ink on paper.
* [[Accelerometer]]s in modern cars for a large number of purposes including [[airbag]] deployment and [[electronic stability control]].
* [[Inertial measurement unit]]s (IMUs): 
** MEMS [[accelerometer]]s
** [[MEMS gyroscope]]s in remote controlled, or autonomous, helicopters, planes and multirotors (also known as drones), used for automatically sensing and balancing flying characteristics of roll, pitch and yaw.
** [[MEMS magnetic field sensor]] ([[magnetometer]]) may also be incorporated in such devices to provide directional heading.
** MEMS [[inertial navigation system]]s (INSs) of modern cars, airplanes, submarines and other vehicles to detect [[yaw, pitch, and roll]]; for example, the [[autopilot]] of an airplane.<ref>{{cite book|url=https://books.google.com/books?id=WgFPvZyApd0C&pg=PA111|title=MEMS Vibratory Gyroscopes: Structural Approaches to Improve Robustness|vauthors=Acar C, Shkel AM|publisher=[[Springer Science+Business Media|Springer Science]]|year=2008|isbn=9780387095363|pages=111}}</ref>
* Accelerometers in consumer electronics devices such as game controllers (Nintendo [[Wii]]), personal media players / cell phones (virtually all smartphones, various HTC PDA models),<ref>{{Cite news|url=https://www.eetimes.com/document.asp?doc_id=1305409|title=There's more to MEMS than meets the iPhone|last=Johnson RC|date=2007|work=[[EE Times]]|access-date=14 Jun 2019}}</ref> [[augmented reality]] (AR) and [[virtual reality]] (VR) devices,and a number of digital cameras (various [[Canon Digital IXUS]] models). Also used in PCs to park the hard disk head when free-fall is detected, to prevent damage and data loss.
* [[MEMS barometer]]s
* MEMS microphones in portable devices, e.g., mobile phones, head sets and laptops. The market for smart microphones includes smartphones, wearable devices, smart home and automotive applications.<ref>{{Cite news|url=https://www.eenewsanalog.com/news/smart-mems-microphones-market-emerges|title=Smart MEMS microphones market emerges|last=Clarke P|date=2016|work=[[EE Times|EE News Analog]]|access-date=14 Jun 2019}}</ref>
* Precision temperature-compensated resonators in [[real-time clock]]s.<ref>{{cite web|url=https://datasheets.maximintegrated.com/en/ds/DS3231M.pdf|title=DS3231m RTC|date=2015|website=DS3231m RTC Datasheet|publisher=Maxim Inc.|access-date=26 Mar 2019}}</ref>
* Silicon [[pressure sensor]]s e.g., car [[tire]] pressure [[sensor]]s, and disposable [[blood pressure]] [[sensor]]s
* [[Display device|Displays]] e.g., the [[digital micromirror device]] (DMD) chip in a projector based on [[Digital Light Processing|DLP]] technology, which has a surface with several hundred thousand micromirrors or single micro-scanning-mirrors also called [[microscanner]]s
* [[Optical switch]]ing technology, which is used for switching technology and alignment for [[data communications]]
* RF switches and relays<ref>{{cite web | url=https://www.electronicdesign.com/technologies/power/article/21808737/mems-relays-push-power-limits | title=MEMS Relays Push Power Limits | date=22 October 2019 }}</ref><ref>{{cite book | chapter-url=https://link.springer.com/referenceworkentry/10.1007/978-981-10-5945-2_34 | doi=10.1007/978-981-10-5945-2_34 | chapter=RF MEMS Switch | title=Micro Electro Mechanical Systems | series=Micro/Nano Technologies | date=2018 | last1=Wang | first1=Li-Feng | last2=Huang | first2=Qing-An | last3=Han | first3=Lei | pages=1039–1076 | isbn=978-981-10-5944-5 }}</ref>
* [[Bio-MEMS]] applications in medical and health related technologies including [[lab-on-a-chip]] (taking advantage of [[microfluidics]] and [[Micropump|micropumps]]), [[biosensor|biosensors]], [[Chemoreceptor|chemosensors]] as well as embedded components of medical devices e.g. stents.<ref>{{cite journal|vauthors=Louizos LA, Athanasopoulos PG, Varty K|date=2012|title=Microelectromechanical Systems and Nanotechnology. A Platform for the Next Stent Technological Era|journal=[[Vascular and Endovascular Surgery|Vasc. Endovasc. Surg.]]|volume=46|issue=8|pages=605–609|doi=10.1177/1538574412462637|pmid=23047818|s2cid=27563384}}</ref>
* [[Interferometric modulator display]] (IMOD) applications in consumer electronics (primarily displays for mobile devices), used to create interferometric modulation − reflective display technology as found in mirasol displays
* Fluid acceleration, such as for micro-cooling
* Micro-scale [[energy harvesting]] including piezoelectric,<ref>{{cite journal|vauthors=Hajati A, Kim SG|date=2011|title=Ultra-wide bandwidth piezoelectric energy harvesting|journal=[[Applied Physics Letters|Appl. Phys. Lett.]]|volume=99|issue=8|pages=083105|doi=10.1063/1.3629551|bibcode=2011ApPhL..99h3105H|hdl=1721.1/75264|s2cid=85547220 |hdl-access=free}}</ref> electrostatic and electromagnetic micro harvesters.
* Micromachined [[ultrasound transducer]]s.<ref>{{cite journal|vauthors=Hajati A|date=2012|title=Three-dimensional micro electromechanical system piezoelectric ultrasound transducer|journal=[[Applied Physics Letters|Appl. Phys. Lett.]]|volume=101|issue=25|pages=253101|doi=10.1063/1.4772469|bibcode=2012ApPhL.101y3101H|s2cid=46718269}}</ref><ref>{{cite journal|vauthors=Hajati A|date=2013|title=Monolithic ultrasonic integrated circuits based on micromachined semi-ellipsoidal piezoelectric domes|journal=[[Applied Physics Letters|Appl. Phys. Lett.]]|volume=103|issue=20|pages=202906|doi=10.1063/1.4831988|bibcode=2013ApPhL.103t2906H}}</ref>
* MEMS-based loudspeakers focusing on applications such as in-ear headphones and hearing aids
* [[Microelectromechanical system oscillator|MEMS oscillators]]
* MEMS-based [[scanning probe microscopy|scanning probe microscopes]] including [[Atomic Force Microscopy|atomic force microscopes]]
* [[Lidar|LiDAR]] (light detection and ranging)

== 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 Development and is forecasted to reach $72 billion by 2011.<ref>{{Cite news|url=https://www.azonano.com/news.aspx?newsID=4479|title=Worldwide MEMS Systems Market Forecasted to Reach $72 Billion by 2011|date=2007|work=AZoNano|access-date=5 Oct 2015}}</ref>

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&nbsp;mm lines and select new tools, including etch and bonding for certain MEMS applications.

== See also ==
* [[MEMS sensor generations]]
* [[Microoptoelectromechanical systems]]
* [[Microoptomechanical systems]]
* [[Nanoelectromechanical systems]]

== References ==
{{Reflist|30em}}

== Further reading ==
* [http://www.microsystems.ru/eng/conf_news.php?id_table=1&file=102.htm Journal of Micro and Nanotechnique]
* ''[[Microsystem Technologies]]'', published by [[Springer Publishing]], [https://www.springer.com/east/home?SGWID=5-102-70-1113744-0&changeHeader=true&referer=www.springeronline.com&SHORTCUT=www.springer.com/journal/542 Journal homepage]
* {{cite book |year=2004 |editor1-last=Geschke |editor1-first=O. |editor2-last=Klank |editor2-first=H. |editor3-last=Telleman |editor3-first=P. |title=Microsystem Engineering of Lab-on-a-chip Devices |publisher=Wiley |isbn=3-527-30733-8 |url=https://books.google.com/books?id=EMyGfH5IgkkC}}

== External links ==
{{Commons category|MEMS}}
* {{cite book |first1=F. |last1=Chollet |first2=HB. |last2=Liu |date=10 August 2018 |id=5.4 |url=http://memscyclopedia.org/introMEMS.html |title=A (not so) short introduction to MEMS |isbn=9782954201504}}

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