Lexicon
B
BOW
–
The deviation of the center point of the median surface of a free,
unclamped wafer from a median-surface reference plane established
by three points equally spaced on a circle with diameter a
specified amount less than the nominal diameter of the wafer.
C
Carrier Concentration
In contrast to metallic conductors such as aluminum copper,
semiconductors have two independent charge carriers: electrons and
holes. The behavior of diodes, transistors, and other
semiconductor devices often hinges upon the relative
concentrations of holes (p) and electrons (n),
expressed in cm3. In pure silicon, they are thermally generated in pairs. In this
intrinsic state, n and p remain equal. Their product
is
np = n2 ≈
10E10/cm3 at room temperature. In a
p-type substrate, however, the substitution of acceptor
atoms (like boron) into the silicon lattice results in an excess
of holes. Then pp>>np indicating an excess of
majority-carrier holes over minority-carrier electrons. The
product
np = ni2
will remain constant under thermal equilibrium (no applied bias or
other perturbations).
Conductivity Type
An n-type (negative-type) extrinsic silicon semiconductor is a semiconducting material that was produced by doping silicon with an n-type element of Group V A, such as P, As, or Sb. Consequently, electrons are the majority charge carriers of the material.
A p-type (positive-type) extrinsic silicon semiconductor is a semiconducting material that was produced by doping silicon with an p-type element of group III A, such as B, Al, or Ga. Since the dopants are acceptor atoms, holes are the majority charge carriers of the material.
E
Effect of impurities
–
An idea of the effect of particular atoms, when present as
impurities in semiconductors, may be obtained by looking at the
periodic table of elements. A part of this table is reproduced as
Figure:
If an impurity atom comes from a group higher than that of the semi- conductor element, it has an extra electron in its unfilled shell. Material with a majority of electrons (which are negatively charged) is called N-type.
This material with a majority of positively charged carriers is called P-type.
Take arsenic in germanium. The five electrons of this atom are distributed around the available germanium bonds and there is one left over. This electron is bound to the arsenic atom, but the binding is not by way of a 'linkage bond' and is easily broken. A germanium crystal containing arsenic as an impurity will have many extra free electrons at room temperature. It will have no extra holes though, because the extra broken bonds are not linkage bonds so their effect cannot move through the crystal. The crystal will still be electrically neutral, because it is made up of a collection of previously neutral atoms. A positive charge will be left behind by each free electron which has left an arsenic atom; this charge will reside on that atom, which of course is not free to move. These impurity atoms are known as donors because they donate free electrons to the crystal. Such a crystal will then have more free electrons than holes, so the former are called majority carriers and the latter minority carriers.
Now consider the effect of a gallium atom in a crystal of germanium. Having only three outer electrons the result is formation of a hole, in contrast to the free electron in the former case. Now the majority carriers are holes and the minority carriers are electrons. This material is called P-type and has a majority of positively charged carriers. Gallium is called an acceptor in this case because it accepts bound electrons from neighboring atoms.
Gallium arsenide (GaAs) or other combinations of group 3 and group 5 atoms have similar doping rules. If the impurity group is higher than the highest of the host atoms, the atom is a donor (e.g. selenium in GaAs). If the impurity's group is lower than the lowest in the host crystal it is an acceptor (e.g. zinc in gallium arsenide). If it is in between, as is germanium in gallium arsenide, it may be either. If a germanium atom replaces a gallium, it has too many outer electrons and is a donor. If it replaces an arsenic, it has too few and is an acceptor.
EPD
–
Crystal Etch Pit Density. EPD is a measure of the quality of
semiconductor wafer. Etch solution is applied and
"attacks" the surface of the wafer, resulting in pits
caused by the presence of the crystal's dislocations because the
dislocations increase the etch rate. Typical guaranteed value of
EPD for Si is <100/cm2. In practice, a good crystal
wafer shows zero EPD when EPD <100/cm2 condition is
met.
F
FM – Flatness Measurements
–
Describe the deviation of the front wafer surface, expressed in
TIR or maximum FPD, relative to specified reference plane when the
back surface of the wafer is ideally flat, as when pulled down by
a vacuum onto ideally clean flat chuck. (ADE, Massachusetts)
Focal plane
–
The plane perpendicular to the optical axis of an imaging system
which contains the focal point on the imaging system.
(Massachusetts)
FPD – Focal Plane Deviation
The distance parallel to the optical axis from a point on the wafer surface to the focal plane of the optical system. (Massachusetts)
The greatest positive or negative deviation from a reference plane which approximates the focal plane, when the wafer is mounted on a flat vacuum chuck. (ADE)
The focal plane deviation is the greatest distance above or below the chosen focal plane. (Motorola)
G
GTIR – Global Total Indicated Reading
–
Maximum peak to valley deviation of a wafer from a given reference
plane.
GTIR = A + B
H
Hard Back Side Damage (HBSD)
–
Is a technique of improving properties of Si wafers by bombarding
the back sides of the wafers with a wet stream of tiny quartz
beads and extra clean wafers after that. HBSD is applied very
early in the production process of Si wafers. The back-side,
physically damaged layer, is removed later, in subsequent
operations. In the HBSD result the damaged areas act as the
impurities (remnants from CZ crystal growth process) attractors
and denude zones are created in the material bulk, improving
purity of the silicon (front-side especially). The technique is
applied by all the best wafers producers. We think that all
heavily doped wafers, made by the best world producers, undergo
HBSD or similar techniques, like for example brushing. Simply
certificates are silent about that. The damage is not visible to
the human eye and special techniques of Si bulk analysis are
necessary to detect that wafer passed HBSD. We are sure to
recommend HBSD material for any application.
Haze Free
–
A silicon wafer having the best possible surface finish and
micro-roughness on the order of less than 10A.
L
Local Thickness Variation
–
The local thickness variation to each point is the vector sum the
elevation difference of pairs of immediately adjacent points,
expressed in micrometers per millimeter. (Motorola)
LPD
–
Light Point Defects.
N
Non-Linear Thickness Variation
–
Describe the thickness variation on a wafer defined by a center
thickness value and four edge thickness values obtained
⅛″ from the edge of the wafer. (Guidici)
O
Orientation
–
The growth plane of the crystalline silicon. Orientations are
described using Miller Indices such as (100), (111),
(110), etc. Different growth planes and orientations have
different arrangements of the atoms or lattice as viewed from a
particular angle.
P
Peak to Valley flatness (P/V)
–
The sum of the greatest positive & negative deviations for a
reference plane which approximates the median wafer surface plane
when the wafer is mounted on a flat vacuum chuck. (ADE)
Prime Grade
–
The highest grade of a silicon wafer. SEMI indicates the bulk,
surface, and physical properties required to label silicon wafers
as "Prime Wafers".
Primary flat
–
The flat of longest length located in the circumference of the
wafer. The primary flat has a specific crystal orientation
relative to the wafer surface; major flat.
Q
Quality Area
–
The portion of a wafer within the specified parameter is
determined. (ASTM)
R
Reclaim Grade
–
A lower quality wafer that has been used in manufacturing and then
reclaimed, etched or polished, and then used a second time in
manufacturing.
Resistivity
–
The resistance that a unit volume of a material offers to the
passage of electricity, the electric current being perpendicular
to two parallel faces. More generally, the volume resistivity is
the ratio of the potential gradient parallel with the current in
the material to the current density.
S
Secondary Flat
–
Indicates the crystal orientation and doping of the wafer.
SEMI
–
SEMI Organisation (https://www.semi.org/) is the global industry association representing the electronics
manufacturing and design industry, connecting more than 2,500
members and 1.3 million professionals worldwide (data for March,
2023). SEMI members are responsible for the innovations in
materials, design, equipment, software, devices, and services that
enable smarter, faster, more powerful, and more affordable
electronic products. Since 1970, SEMI has built connections that
have helped its members prosper, create new markets, and address
common industry challenges.
Slice Orientation
–
The crystallographic orientation of the surface of a wafer. The
primary and most common slice orientations are (100), (111) and
(110).
Striations (dopant rings)
–
SEMI standard says that striations are “helical features of Si
wafer are ascribed to periodic dopant incorporation differences
occurring at the rotating solid-liquid interface during crystal
growth.” They are visible to the unaided eye after preferential
etching. From the experience I know that skilled and trained
operators can see them without any etching on some heavily doped
wafers. The striations are not a wafer defect, but a residual
feature originating from dopant distribution during the crystal
growth process. They do not affect electronic properties of the
wafers. For many years it was believed that growing ingots in the
outer space, with no gravity, prevents forming the striations.
However NASA experiments (NP-119 Science in Orbit: The Shuttle &
Spacelab Experience, 1981-1986) tells us that “the space-grown
crystals had the same marked dopant striations seen in Earth-grown
crystals, confirming that Marangoni convection (flow driven by
surface tension) may be a dominant cause of the defects on Earth
and in space.”
Striations
T
Taper
–
Taper is the lack of parallelism between the back surface of the
wafer and the selected focal plane. The numeric value provided as
a sort criterion is the maximum difference between these two
planes, not the slope of the surface, and is thus reported in
micrometers over the diameter not microns per millimeter.
(Motorola)
Test Grade
–
A virgin silicon wafer of lower quality than Prime, and used
primarily for testing processes. SEMI indicates the bulk, surface,
and physical properties required to label silicon wafers as "Test
Wafers".
Thickness
–
The normal distance through a slice or wafer in a direction normal
to the surface at a given point.
TIR ≡ PV
Difference in elevation between the highest and lowest points on the surface of a wafer and the value is presented as a magnitude. (Guidici)
The smallest perpendicular distance between two planes, both parallel with the reference plane, which enclose all points on the front surface of a wafer within the flatness quality area or the site, depending on which is specified. (ADE)
TTV – Total Thickness Variation
–
Absolute difference in thickness between the thickest and thinnest
parts of wafer.
TTV = A - B
