Editing MEMS (section)

== 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=1552–74|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.