Page 283 - An Introduction to Microelectromechanical Systems Engineering

262 Glossary

Bragg grating A periodic variation in the index of refraction within the core of an
optical fiber or a waveguide. It reflects light at its resonant wavelength that is pro-
portional to the periodic spacing of the grating.
Brownian noise see noise.
Bulk micromachining A class of micromachining processes that remove material
from a substrate, yielding micromechanical structures with thicknesses in the tens or
hundreds of micrometers. It usually refers to the orientation-dependent etching or
deep reactive ion etching of silicon.
Burn-in A methodology in reliability testing that involves operating a device
under extreme operating and environmental conditions in order to accelerate the life
of the product and precipitate an early failure.
Butterfly package A metal package that consists of a tub-like housing with elec-
trical leads on two sides. A butterfly package may also include a feed-through tube
to allow for a fiber optical connection.
CERDIP An acronym for ceramic dual-in-line package for integrated circuits. It
consists of a rectangular pressed ceramic body with pins on two opposites. A
ceramic cap is glass sealed to the body.

CERQUAD An acronym for ceramic quad-flat pack. Similar to the CERDIP but
has pins on all sides.

Characteristic impedance The input impedance of a transmission line. Intuitively,
it is the impedance of the load that, when attached to the terminal ends of a trans-
mission line, makes the line appear infinitely long. Characteristic impedance is of
great importance for good low-loss transmission. If the characteristic impedance of
a transmission line is mismatched to the load at the output terminals, then some of
the signal is reflected back at the junction causing loss and other undesirable effects.
It is a complex number with resistive and reactive components. It is independent of
the length of the transmission line but depends on frequency.
Check valve A valve that permits fluid flow only in one direction.
Chemical vapor deposition (CVD) A process based on the principle of initiating
a chemical reaction in a vacuum chamber, resulting in the deposition of a reacted
species on a heated substrate. Materials that can be deposited by CVD include poly-
silicon, silicon oxide, and silicon nitride.

Chromaticity A property of light that relates to the number of wavelengths (col-
ors) available within an optical beam. A laser is a highly chromatic source because it
contains essentially one main wavelength.
CMOS An acronym for complementary metal oxide semiconductor. A class of
electronic devices made of silicon and associated fabrication processes common in
integrated circuits.

Coefficient of thermal expansion (CTE) The rate of change in length of an object
as a function of temperature. In general, CTE =(∆L/L)/∆Twhere (∆L/L) is the frac-
tional change in length corresponding to a ∆T change in temperature. It is measured
in inverse units of temperature (/ºC or ppm/K).