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Polysilicon to Wafers and Cells
Silicon
High purity silicon ("Polysilicon") is the key
feedstock for almost all solar cells and modules produced today.
Silicon-based PV cells and modules accounted for 91% of all PV
production in 2005, according to Photon Consulting.
In unpurified form, silicon is widely abundant,
making up more than 25% of the Earth's crust. While there is no
fundamental limitation on the availability of unpurified silicon
material, the standard "Siemens" purification process requires
several costly steps, including the processing of volatile chemicals
(Trichlorosilane) at 1,100 degrees Celsius.
The solar PV industry and semiconductor
manufacturers are the two main consumers of polysilicon. In 2000 the
solar industry consumed only 10% of the world's silicon supply. In
2006 the PV industry consumed more than half of the world's
available supply of polysilicon for the first time ever, according
to Photon Consulting. This historic shift illustrates the growing size
and importance of the solar PV industry, and polysilicon
manufacturers are expanding production capacity dramatically to meet
the growth in the PV industry. However, the time lag between
planning new polysilicon capacity and the actual production of
polysilicon is typically 1.5 to 2.5 years, thereby contributing to
the current shortage of silicon supply. The impact of this shortfall
is evident in the sharp rise in polysilicon prices since
2004. Ingot Silicon consumers in
the solar PV industry must convert silicon feedstock into
silicon ingots to enable further processing into wafers, cells
and modules. Silicon-based solar modules fall into two categories:
monocrystalline and multicrystalline. In each category, the
polysilicon must be converted into a crystalline structure. A
monocrystalline ingot is comprised of one large crystal structure,
which yields a uniform color and texture throughout the ingot. A
multicrystalline ingot contains numerous smaller silicon crystals
and often has a mottled or flecked appearance. Trina Solar uses
monocrystalline technology because this solution produces solar
cells and modules with a higher efficiency conversion rate of
sunlight to electricity. The most common technology used in the production of ingots for
monocrystalline solar cells is based on a technique called the
Czochralski Process. In this process a silicon seed
crystal at the end of a metal rod is lowered into a quartz crucible
of molten silicon liquid. As the rod and seed crystal are slowly
pulled out of the crucible, a single cylindrical silicon crystal
forms on the seed crystal. The production of monocrystalline ingot requires precise
specifications and careful monitoring to ensure uniform crystal
growth and contaminant-free ingots. Completing a single cylindrical
silicon crystal ingot takes between 36 and 40 hours and yields an
ingot of approximately 2 meters long and 6 to 8 inches in
diameter. Single Crystal Growing
for Wafer Production (Page 1 of 2)
Integrated circuits are built on single-crystal
silicon
substrates that possess a high level of purity and perfection. Single-crystal silicon
is used in VLSI fabrication instead of polycrystalline silicon since the
former does not have defects associated with grain boundaries found in
polysilicon. Such defects have been known to limit the lifetimes of
minority carriers.
Aside
from the need to be single-crystalline in nature, silicon substrates
must also have a high degree of chemical purity, a high degree of
crystalline perfection, and high structure uniformity. The acquisition of
such high-grade starting silicon material involves two major steps: 1)
refinement of raw material (such as quartzite, a type of sand) into
electronic
grade polycrystalline silicon (EGS) using a complex multi-stage process; and 2)
growing of single-crystal silicon from this EGS either by Czochralski or
Float Zone process.
Czochralski Crystal Growth Czochralski (CZ) crystal growth, so named in honor of
its inventor, involves the crystalline solidification of atoms from a
liquid phase at an interface. The basic CZ crystal growing process
is more or less still the same as what has been developed in the
1950's. CZ
crystal growing consists of the following steps. 1) A
fused silica crucible is loaded with a charge of undoped EGS together with a precise amount
of diluted silicon alloy. 2) The gases inside the growth chamber are then evacuated. 3)
The growth chamber is then back-filled with an inert gas to inhibit the entrance of atmospheric gases into the melt
during crystal growing. 4) The silicon charge inside the chamber is then
melted (Si melting point = 1421 deg C). 5) A slim seed of crystal silicon (5 mm dia. and 100-300 mm
long) with precise orientation tolerances is introduced into the molten silicon.
6) The seed crystal is then withdrawn at a very controlled rate. The seed crystal
and the crucible are rotated in opposite directions while this withdrawal
process occurs.
Fig. 1. Examples of Czochralski
Pullers Float Zone Crystal Growth The
float zone
(FZ) process
is another method for growing single-crystal silicon. It involves the
passing of a molten zone through a polysilicon rod that approximately has the same
dimensions as the final ingot. The purity of an ingot produced by
the FZ process is higher than that of an ingot produced by the CZ process.
As such, devices that require ultrapure starting silicon substrates should
use wafers produced using the FZ method.
The
FZ process consists of the following steps. 1. A polysilicon rod is mounted vertically inside a chamber, which
may be under vacuum or filled with an inert gas. 2. A needle-eye coil that can run through the rod is activated to
provide RF power to the rod, melting a 2-cm long zone in the rod. This molten zone
can be maintained in stable liquid form by the coil. 3.
The coil is then moved through the rod, and the molten zone moves along with
it. 4.
The movement of the molten zone through the entire length of the rod
purifies the rod and forms the near-perfect single crystal.
FZ
growing equipment can also use a stationary coil, coupled with a mechanism
that can move the silicon rod through it.
Fig. 2. Examples of
After
the single-crystal silicon ingot has been manufactured, it undergoes a
routine evaluation of its resistivity, impurity content, crystal
perfection, size and weight. It is then ground using diamond wheels to
make it a perfect cylinder that has the right diameter. It then undergoes
an etching process to remove the mechanical imperfections left by the
grinding process. Fig. 3. A Single-Crystal Silicon
Rod
The
cylindrical ingot is then given one or more 'flats' by another round of
grinding. The largest flat, called the primary flat, is used by automated
wafer handling systems for alignment. Flats (primary and secondary)
are also used to identify the crystallographic orientation and conductivity of the wafer. The
ingot is then sawn into thin wafer slices, each of which will be subjected to
further etching and polishing until it is ready for use as substrates for VLSI
fabrication. The above process of silicon growing, grinding,
shaping, sawing, etching, and polishing to produce input wafers is known
as wafering.
Fig. 4. An ingot slicer (left) and a wafer
grinder/polisher
(right) Wafer Wafer sawing is the process of cutting the
monocrystalline ingot into thin slices to enable the processing of silicon
into solar cells. Producing thinner wafers and reducing silicon waste is a
major area of focus in the solar industry's campaign to lower the cost of
module production and ensure more efficient use of silicon. A large source of lost silicon is "kerf", the silicon
dust produced during the sawing process. "Kerf loss" refers to the silicon
removed from the ingot in the sawing process used to produce the wafers.
Because the sawed grooves are approximately the same width as the produced
wafers, kerf loss can approach 50% of the total silicon in the ingot. Wire sawing is the standard technique used to slice
ingots into wafers. The raw ingot is first cooled and then the top and
tail of the ingot are cut off and can later be reused in the ingot
production process as reclaimable silicon. Next, the ingots are cut into
400-500 cm long sections and the cylindrical shape is 'squared' into four
equal sides, so as to be mounted safely in the wire saw machine. In the sawing process a single strand of stainless steel
wire hundreds of kilometers in length and 160 to 200 microns thick is
pulled over the ingot by grooved rollers. To complete this process
commonly, a mixture made up of oil and normally an abrasive material known
as slurry is pumped over the wires to provide the friction needed for the
cutting action. CROPPER SQUARER WIRE SAW
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