Editing MEMS (section)

== 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 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=978-0-8194-9786-4|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 |first1=Xiaoqing |last1=Shi |first2=Stuart A. |last2=Boden |editor-first=Alex |editor-last=Robinson |editor2-first=Richard |editor2-last=Lawson  |chapter=17. Scanning helium ion beam lithography |chapter-url=  |title=Frontiers of Nanoscience |publisher=Elsevier |date=2016 |isbn=978-0-08-100354-1 |doi=10.1016/B978-0-08-100354-1.00017-X |pages=563–594 |volume=11 }}</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.

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=978-1-4398-9524-5|series=Fundamentals of Microfabrication and Nanotechnology|volume=3 |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=1536–51|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]].

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=978-0-7803-8265-7|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. {{citation needed|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=978-0-7803-8994-6|volume=2|pages=1118–21|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]].