A
wafer is a thin slice of semiconductor material, such as a
silicon crystalMonocrystalline silicon or single-crystal Si, or mono-Si is the base material of the electronic industry. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges. It can be prepared intrinsic, i.e...
, used in the
fabricationSemiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
of integrated circuits and other microdevices. The wafer serves as the
substrateSubstrate is a solid substance onto which a layer of another substance is applied, and to which that second substance adheres. In solid-state electronics, this term refers to a thin slice of material such as silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide , an alloy...
for microelectronic devices built in and over the wafer and undergoes many
microfabricationMicrofabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
process steps such as doping or
ion implantationIon implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
,
etchingEtching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
, deposition of various materials, and
photolithographicPhotolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
patterning. Finally the individual microcircuits are separated (
dicingWafer dicing is the process by which die are separated from a wafer of semiconductor following the processing of the wafer. The dicing process can be accomplished by scribing and breaking, by mechanical sawing or by laser cutting...
) and
packagedIntegrated circuit packaging is the final stage of semiconductor device fabrication per se, followed by IC testing.Packaging in ceramic or plastic prevents physical damage and corrosion and supports the electrical contacts required to assemble the integrated circuit into a system.In the integrated...
.
Several types of
solar cellA solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
are also made from such wafers. On a
solar wafer a solar cell (usually square) is made from the entire wafer.
Formation
Wafers are formed of highly pure (99.9999% purity),
nearly defect-free single crystalline material. One process for forming crystalline wafers is known as
Czochralski growthThe Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...
invented by the Polish chemist
Jan CzochralskiJan Czochralski was a Polish chemist who invented the Czochralski process, which is used to grow single crystals and is used in the production of semiconductor wafers....
. In this process, a cylindrical
ingotAn ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
of high purity
monocrystalline siliconMonocrystalline silicon or single-crystal Si, or mono-Si is the base material of the electronic industry. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges. It can be prepared intrinsic, i.e...
is formed by pulling a
seed crystalA seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...
from a '
meltZone melting is a group of similar methods of purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal...
'. Dopant impurity atoms such as
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
or
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
can be added to the molten intrinsic silicon in precise amounts in order to
dopeIn semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
the silicon, thus changing it into
n-typeN-type semiconductors are a type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material . This creates an excess of negative electron charge carriers....
or
p-typeA P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....
extrinsic silicon.
The
ingotAn ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
is then sliced with a wafer saw (
wire sawA wire saw is a machine using a metal wire or cable for cutting. There are two types of wire saw machines: continuous and oscillating . Sometimes the wire itself is referred to as a "blade"....
) and
polishedPolishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...
to form wafers. The size of wafers for photovoltaics is 100–200 mm square and the thickness is 200–300 μm. In the future, 160 μm will be the standard. Electronics use wafer sizes from 100–300 mm diameter. (The largest wafer made has a diameter of 450 mm but is not in production yet.)
Cleaning, texturing and etching
Wafers are cleaned with
weak acidA weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
s to remove unwanted particles, or repair damage caused during the sawing process. When used for
solar cellA solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s, the wafers are textured to create a rough surface to increase their efficiency. The generated PSG (
phosphosilicate glassPhosphosilicate glass, commonly referred to by the acronym PSG, is a silicate glass commonly used in semiconductor device fabrication for intermetal layers, i.e., insulating layers deposited between succeedingly higher metal or conducting layers...
) is removed from the edge of the wafer in the
etchingEtching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal...
.
Standard wafer sizes
Silicon wafers are available in a variety of sizes from 25.4 mm (1 inch) to 300 mm (11.8 inches). Semiconductor fabrication plants (also known as
fabs) are defined by the size of wafers that they are tooled to produce. The size has gradually increased to improve throughput and reduce cost with the current state-of-the-art fab considered to be (12 inch), with the next standard projected to be (18 inch). Intel,
TSMCTaiwan Semiconductor Manufacturing Company, Limited or TSMC is the world's largest dedicated independent semiconductor foundry, with its headquarters and main operations located in the Hsinchu Science Park in Hsinchu, Taiwan.-Overview:...
and
SamsungThe Samsung Group is a South Korean multinational conglomerate corporation headquartered in Samsung Town, Seoul, South Korea...
are separately conducting research to the advent of "
prototypeA prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
" (research) fabs by 2012, though serious hurdles remain. Dean Freeman, an analyst with Gartner Inc., predicted that production fabs could emerge sometime between the 2017 and 2019 timeframe, a lot of that will depend on a plethora of new technological breakthroughs and not simply extending current technology.
. Thickness 275 µm.. Thickness 375 µm.. Thickness 525 µm. or 125 mm (4.9 inch). Thickness 625 µm.
- 150 mm (5.9 inch, usually referred to as "6 inch"). Thickness 675 µm.
- 200 mm (7.9 inch, usually referred to as "8 inch"). Thickness 725 µm.
- 300 mm (11.8 inch, usually referred to as "12 inch"). Thickness 775 µm. ("18 inch"). Thickness 925 µm (expected).
Wafers grown using materials other than silicon will have different thicknesses than a silicon wafer of the same diameter. Wafer thickness is determined by the mechanical strength of the material used; the wafer must be thick enough to support its own weight without cracking during handling.
A unit wafer fabrication step, such as an etch step or a lithography step, can be performed on more chips per wafer as roughly the square of the increase in wafer diameter, while the cost of the unit fabrication step goes up more slowly than the square of the wafer diameter. This is the cost basis for shifting to larger and larger wafer sizes. Conversion to 300 mm wafers from 200 mm wafers began in earnest in 2000, and reduced the price per die about 30-40%.
However, this was not without significant problems for the industry.
The next step to 450 mm should accomplish similar productivity gains as the previous size increase.
However, machinery needed to handle and process larger wafers results in increased investment costs to build a single factory. There is considerable resistance to moving up to 450 mm by 2012 despite the obvious productivity enhancements, mainly because companies feel it would take too long to recoup their investment. The difficult and costly 300 mm process only accounted for approximately 20% of worldwide capacity on a square inches basis by the end of 2005. The step up to 300 mm required a major change from the past, with fully automated factories using 300 mm wafers versus barely automated factories for the 200 mm wafers. These major investments were undertaken in the economic downturn following the
dot-com bubbleThe dot-com bubble was a speculative bubble covering roughly 1995–2000 during which stock markets in industrialized nations saw their equity value rise rapidly from growth in the more...
, resulting in huge resistance to upgrading to 450 mm by the original timeframe.
Other initial technical problems in the ramp up to 300 mm included vibrational effects, gravitational bending (sag), and problems with flatness. Among the new problems in the ramp up to 450 mm are that the crystal ingots will be 3 times heavier (total weight a metric ton) and take 2-4 times longer to cool, and the process time will be double. All told, the development of 450 mm wafers require significant engineering, time, and cost to overcome.
Analytical die count estimation
For any given wafer diameter [
d,
mm] and target IC size [
S,
mm2], there is an exact number of integral die pieces that can be sliced out of the wafer. The gross Die Per Wafer [
DPW] can be estimated by the following expression:
Note, that the gross die count does not take into account the die defect loss, various alignment markings and test sites on the wafer.
Crystalline orientation
Wafers are grown from crystal having a regular
crystal structureIn mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
, with silicon having a
diamond cubicThe diamond cubic crystal structure is a repeating pattern of 8 atoms that certain materials may adopt as they solidify. While the first known example was diamond, other elements in group IV also adopt this structure, including tin, the semiconductors silicon and germanium, and silicon/germanium...
structure with a lattice spacing of 5.430710 Å (0.5430710 nm). When cut into wafers, the surface is aligned in one of several relative directions known as crystal orientations. Orientation is defined by the
Miller indexMiller indices form a notation system in crystallography for planes and directions in crystal lattices.In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written , and each index denotes a plane orthogonal to a direction in the...
with [100] or [111] faces being the most common for silicon.
Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic.
Ion implantationIon implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
depths depend on the wafer's crystal orientation, since each direction offers distinct paths for transport.
Wafer
cleavageCleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("
dieA die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...
s") so that the billions of individual
circuit elementsAn electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...
on an average wafer can be separated into many individual circuits.
Wafer flats and crystallographic orientation notches
Wafers under 200 mm diameter have
flats cut into one or more sides indicating the
crystallographicCrystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
planes of the wafer (usually the {110} face). In earlier-generation wafers a pair of flats at different angles additionally conveyed the doping type (see illustration for conventions). Wafers of 200 mm diameter and above use a single small notch to convey wafer orientation, with no visual indication of doping type.
Impurity doping
Silicon wafers are generally not 100% pure silicon, but are instead formed with an initial impurity doping concentration between 10
13 and 10
16 per cm
3 of
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
,
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
,
arsenicArsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, or
antimonyAntimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
which is added to the melt and defines the wafer as either bulk n-type or p-type. However, compared with single-crystal silicon's atomic density of 5×10
22 atoms per cm
3, this still gives a purity greater than 99.9999%. The wafers can also be initially provided with some
interstitialInterstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites.They are generally...
oxygen concentration. Carbon and metallic contamination are kept to a minimum.
Transition metalThe term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s, in particular, must be kept below parts per billion concentrations for electronic applications.
Compound semiconductors
While silicon is the prevalent material for wafers used in the electronics industry, other
compoundA compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
III-V or II-VI materials have also been employed. Gallium arsenide (GaAs), a III-V semiconductor produced via the Czochralski process, is also a common wafer material.
See also
- Monocrystalline silicon
Monocrystalline silicon or single-crystal Si, or mono-Si is the base material of the electronic industry. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges. It can be prepared intrinsic, i.e...
- Epilayer
- Epitaxy
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where the overlayer is in registry with the substrate. In other words, there must be one or more preferred orientations of the overlayer with respect to the substrate for this to be termed epitaxial growth. The...
- Junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
- Layer (electronics)
A layer is the deposition of molecules on a substrate or base .High temperature substrates includes stainless steel and polyimide film and PET ....
- Low-cost solar cell
- Rapid thermal processing
Rapid Thermal Processing refers to a semiconductor manufacturing process which heats silicon wafers to high temperatures on a timescale of several seconds or less. During cooling, however, wafer temperatures must be brought down slowly so they do not break due to thermal shock...
- Refining
Refining is the process of purification of a substance or a form. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly...
- Screen printing
- SEMI font
SEMI Font, also known as SEMI OCR font, is used for marking silicon wafers in the semi-conductor industry.The SEMI font character set include 26 uppercase letters, 10 numbers, dash and period....
- Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...
(SOI) wafers
- Solar panel
- RCA clean
The RCA clean is a standard set of wafer cleaning steps which needs to be performed before high temp processing steps of silicon wafers in semiconductor manufacturing. RCA cleaning includes RCA-1 and RCA-2 cleaning procedures...
- Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
- Klaiber's law
Simply stated, Klaiber's law proposes that "the silicon wafer size will dictate the largest diameter of ultrapure water supply piping needed within a semiconductor wafer factory."...
- Wafer bonding
Wafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems , nanoelectromechanical systems , microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation...
External links
- Everything Wafers - A guide to semiconductor substrates type, property, cleaving, etching, and fabrication.