Page 290 - An Introduction to Microelectromechanical Systems Engineering

Glossary 269

Sacrificial etching A micromachining processing method in which an inter-
mediate layer sandwiched between two layers of a different material is preferentially
(sacrificially) etched and selectively removed. Usually, the etch selectivity is high
between the intermediate layer and the two sandwich layers. The purpose of the sac-
rificial layer is to mechanically release one or both of the sandwich layers. Silicon
oxide is a commonly used sacrificial layer.
Silicon fusion bonding Also known as silicon direct bonding, it is a process to
fuse or bond together two silicon substrates. The bond is strong, generally occurring
at the molecular level.
Silicon on insulator (SOI) Substrates consisting of a thin layer of silicon dioxide,
typically 0.5 to 2 µm thick, sandwiched between two crystalline silicon layers. The
silicon dioxide is known as buried oxide. One method to fabricate SOI substrates is
by silicon fusion bonding a silicon wafer with a thin layer of silicon dioxide on its
surface to a bare silicon wafer, then thinning one of the silicon substrates. SOI is a
well-proven technology for the fabrication of CMOS electronic circuits suitable for
high-temperature operation (up to 300ºC), as well as for high-voltage (>100V) and
high-frequency (>10 GHz) applications.
Sheet resistance The resistance of one square of material in units of Ω per square
(Ω/ ). It is equal to resistivity divided by the thickness of the material. For thick-
film resistors, it is generally implicit that the unit thickness is one mil (25.4 µm).
Solidus temperature see liquidus temperature.

Sound Power Level (SPL) Sound pressure, in decibels, measured in reference to a
base sound pressure of 20 µPa in air. The reference is usually 1 Pa in water.
Sputtering A process to deposit a thin film on the surface of a substrate. It
involves the removal of material from a target by ion bombardment and subsequent
redeposition on the substrate.
SRAM An acronym for static random access memory, a type of electronic mem-
ory that can be arbitrarily addressed. Unlike EEPROM, it cannot hold the data once
electrical power is turned off.

Stiction A well-known problem resulting from the geometrical scaling law when
surface adhesion forces are larger than the mechanical restoring forces of a sus-
pended micromachined structure.
Strain In mechanics, a deformation produced by stress. In a beam, it is equal to
the change in length divided by the original beam length.
Surface micromachining A class of fabrication processes yielding micromechani-
cal structures which are only a few micrometers thick, usually involving the removal
of a sacrificial layer.
Surface mount technology (SMT) An advanced electronic packaging technol-
ogy in which the type of packages are particularly small so that they can be soldered
in high density on the surface of a printed-circuit board.
SVGA see VGA.
SXGA see VGA.