1. MEMS
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical
elements, sensors, actuators, and electronics on a common silicon
substrate through microfabrication technology.
Microsystems: Engineering systems that could contain MEMS components
that are design to perform specific engineering functions
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2. Scale of MEMS & Microsystems
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3. Micromachining
Silicon micromachining is the process of fashioning microscopic mechanical
parts out of a silicon substrate or, indeed, on top of a silicon substrate.
It is used to fabricate a variety of mechanical microstructures including
beams, diaphragms, grooves, orifices, springs, gears, suspensions, and a great
diversity of other complex mechanical structures.
These mechanical structures have been used successfully to realise a wide
range of microsensors and microactuators.
Silicon micromachining comprises two technologies:
•Bulk micromachining
•surface micromachining.
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4. Bulk micromachining
This type of micromachining is used to realise micromechanical structures
within the bulk of a single-crystal silicon (SCS) wafer by selectively removing
the wafer material.
Etching is the key technological step for bulk micromachining. The etch
process employed in bulk micromachining comprises one or several of the
following techniques:
1.Wet isotropic etching
2.Wet anisotropic etching
3. Plasma isotropic etching
4. Reactive ion etching (RIE)
5. Etch-stop techniques
In addition to an etch process, bulk micromachining often utilises wafer
bonding and buried oxide-layer technologies.
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5. ISOTROPIC AND ORIENTATION-DEPENDENT
WET ETCHING
Wet chemical etching is widely used in semiconductor processing.
It is used for lapping and polishing to give an optically flat and damage-free
surface and to remove contamination that results from wafer handling and
storing.
Most importantly, it is used in the fabrication of discrete devices and
integrated circuits (ICs) of relatively large dimensions to delineate patterns
and to open windows in insulating materials.
most of the wet-etching processes are isotropic, that is, unaffected by
crystallographic orientation.
However, some wet etchants are orientation-dependent, that is, they have
the property of dissolving a given crystal plane of a semiconductor much
faster than other planes
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6. Anisotropic etching characteristics of different wet
etchants for single-crystalline silicon
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7. ISOTROPIC AND ORIENTATION-DEPENDENT
WET ETCHING
A commonly used orientation-dependent etch for silicon consists of a mixture
of potassium hydroxide (KOH) in water and isopropyl alcohol.
The etch-rate is about 2.1 um/min for the (110) plane, 1.4 um/min for the
(100) plane, and only 0.003 um/min for the (111) plane at 80 °C; therefore,
the ratio of the etch rates for the (100) and (110) planes to the (111) plane are
very high at 400:1 and 600:1, respectively.
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