Page 288 - An Introduction to Microelectromechanical Systems Engineering

Glossary 267

that of all equipment tested, one failure occurred after 500,000 hours of testing.
MTTF is usually predicted using statistical methods.
MTBF An acronym for mean time between failures. It is used to measure the
mean time between successive failures, assuming the system can be repaired to the
same state prior to the failure.

Multi-chip modules (MCM) A type of high-density packaging approach com-
mon in the integrated circuit industry that involves electrically connecting a number
of dice on the same substrate.
n-type doping see doping.
Noise A random disturbance in an electrical, optical, acoustic, or mechanical sig-
nal. It frequently determines the resolution of a sensor. It is characterized by its spec-
tral density, which represents the magnitude in an infinitesimally small frequency
band. The power in the noise signal is proportional to frequency bandwidth; hence,
the voltage magnitude is proportional to the square root of bandwidth. Noise origi-
nates from a multitude of sources. The most common is electrical interference noise
and may be filtered. Thermal—or Brownian—and 1/f noise are fundamental physi-
cal entities. Thermal noise originates from physical mechanisms where energy is
converted to heat (e.g., electrical resistance or mechanical friction). It is white in
nature, meaning its spectral energy density is constant over frequency. Thermal
noise increases with temperature. In contrast, 1/f noise, as the name implies, has a
spectral density that decreases at higher frequencies. It is common in electronic cir-
cuits, and originates from crystal imperfections that momentarily trap electrons
(hence the frequency dependence). The corner frequency in a noise spectrum is the
frequency where 1/f and thermal noise are equal. A common measure of electrical
noise is the magnitude of the spectral density at a particular frequency, given in
V Hz.
Nucleotide The building block of deoxyribonucleic acid (DNA). It can be any of
adenine (A), cytosine (C), guanine (G), or thymine (T). The sequence of nucleotides
in DNA defines the genetic code. An oligonucleotide is a molecule composed of 25
or fewer nucleotides.
Numerical aperture (NA) In optics, it is the sine of the angle of the largest cone of
meridional rays (rays that cross the optic axis) that can enter or leave an optical sys-
tem or element multiplied by the index of refraction. For NA<0.25, it is approxi-
mately equal to one-half the inverse of the f-number and a measure of the aperture
size. The numerical aperture is always less than one in air.
Oxidation Chemical process by which the atoms of an element lose electrons. In
an aqueous solution, neutral atoms become positive ions.
Optoelectronic Family of devices and components that merge both optical and
electronic functions. They include lasers, optical detectors, modulators, optical
amplifiers, and integrated assemblies merging all such building blocks.
PECVD An acronym of plasma-enhanced chemical-vapor deposition.

Phase quadrature see quadrature.