Editing Surface micromachining

{{more footnotes|date=February 2018}}

'''Surface micromachining''' builds [[microstructure]]s by [[Nanoparticle deposition|deposition]] and etching structural layers over a [[substrate (materials science)|substrate]].<ref>{{cite journal |last=Bustillo |first=J.M. |author2=R.T. Howe |author3=R.S. Muller |date=August 1998 |title=Surface micromachining for microelectromechanical systems |journal=Proceedings of the IEEE |volume=86 |issue=8 |pages=1552–1574 |doi=10.1109/5.704260|citeseerx=10.1.1.120.4059 }}</ref> This is different from [[Bulk micromachining]], in which a [[silicon]] substrate [[Wafer (electronics)|wafer]] is selectively etched to produce structures.

== Layers ==
Generally, [[polysilicon]] is used as one of the substrate layers while [[silicon dioxide]] is used as a ''sacrificial layer.'' The sacrificial layer is removed or etched out to create any necessary void in the thickness direction. Added layers tend to vary in size from 2-5 micrometres. The main advantage of this machining process is the ability to build electronic and mechanical components (functions) on the same substrate. Surface micro-machined components are smaller compared to their bulk micro-machined counterparts.

As the structures are built on top of the substrate and not inside it, the substrate's properties are not as important as in bulk micro-machining. Expensive [[Silicon Wafer|silicon wafers]] can be replaced by cheaper substrates, such as [[glass]] or [[plastic]]. The size of the substrates may be larger than a silicon wafer, and surface micro-machining is used to produce [[thin-film transistor]]s on large area glass substrates for flat panel displays. This technology can also be used for the manufacture of [[thin film solar cell]]s, which can be deposited on glass, [[Polyethylene terephthalate|polyethylene terepthalate]] substrates or other non-rigid materials.

==Fabrication process==
Micro-machining starts with a silicon wafer or other substrate upon which new layers are grown. These layers are selectively etched by [[photo-lithography]]; either a wet etch involving an [[acid]], or a dry etch involving an [[ionized gas]] (or [[Plasma (physics)|plasma]]). Dry etching can combine chemical etching with physical etching or [[ion]] bombardment. Surface micro-machining involves as many layers as are needed with a different mask (producing a different pattern) on each layer. Modern [[integrated circuit]] [[Semiconductor device fabrication|fabrication]] uses this technique and can use as many as 100 layers. Micro-machining is a younger technology and usually uses no more than 5 or 6 layers. Surface micro-machining uses developed technology (although sometimes not enough for demanding applications) which is easily repeatable for volume production.

===Sacrificial layers===
A sacrificial layer is used to build complicated components, such as movable parts. For example, a suspended [[cantilever]] can be built by depositing and structuring a sacrificial layer, which is then selectively removed at the locations where the future beams must be attached to the substrate (i.e. the anchor points). A structural layer is then deposited on top of the [[polymer]] and structured to define the beams. Finally, the sacrificial layer is removed to release the beams, using a selective etch process that does not damage the structural layer.

Many combinations of structural and sacrificial layers are possible. The combination chosen depends on the process. For example, it is important for the structural layer not to be damaged by the process used to remove the sacrificial layer.

== Examples ==
Surface Micro-machining can be seen in action in the following MEMS (Microelectromechanical) products:

* Surface Micro-machined [[accelerometer]]s<ref>{{cite journal |last=Boser |first=B.E. |author2=R.T. Howe  |date=March 1996 |title=Surface Micro-machined Accelerometers |journal=IEEE Journal of Solid-State Circuits |volume=31 |issue=3 |pages=366–375 |doi=10.1109/4.494198|bibcode=1996IJSSC..31..366B }}</ref>
* 3D Flexible Multichannel Neural Probe [[array]]<ref>{{cite journal |last=Takeuchi |first=Shoji |author2=Takafumi Suzuki |author3=Kunihiko Mabuchi |author4=Hiroyuki Fujita|date=October 2003 |title=3D Flexible Multi-channel Neural Probe Array |journal=Journal of Micro-machines and Micro-engineering}}</ref>
* [[Nanoelectromechanical relay]]s

== See also ==
* [[Bulk micromachining]]
* [[Magnetic nanoparticles]]
* [[MEMS]]
* [[Semiconductor device fabrication]]

==References==
{{reflist}}

[[Category:Nanotechnology]]
[[Category:Microtechnology]]
[[Category:Microelectronic and microelectromechanical systems]]