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It is written somewhere that in the
beginning God created heaven and the earth. It is not written from what. |
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We do not know for sure what the heaven is made
of but we do know what the the earth is made of, at least as far as the upper
crust is concerned. Interestingly enough, he (or she) created mostly
Silicon and Oxygen with some dirt (in the form of
the other 90 elements) thrown in for added value. |
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Indeed, the outer crust of this planet (lets say
the first 100 km or so) consists of all kinds of silicates - Si +
O + something else - so there is no lack of Si as a raw material.
Si, in fact, accounts for about 26 % of the crust, while O
weighs in at about 49 %. |
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However, it took a while to discover
the element Si. Berzellius came up with some form of it in
1824 (probably amorphous), but it was Deville in 1854 who first obtained
regular crystalline Si. |
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This is simply due to the very high chemical
reactivity of Si. Pure Si (not protected by a thin layer of very
stable SiO2 as all Si crystals and wafers are) will
react with anything, and that creates one
of the problems in making it and keeping it clean. |
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Liquid
Si indeed does react with all substances known to man - it is an
universal solvent. This makes crystal growth from liquid Si somewhat
tricky, because how do you contain your liquid Si? Fortunately, some
materials - especially SiO2 - dissolve only very slowly, so
if you don't take too long in growing a crystal, they will do as a vessel for
the liquid Si. |
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But there will always be some dissolved
SiO2 and therefore oxygen in your liquid Si, and that
makes it hard to produce Si crystals with very low oxygen
concentrations. |
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What we need, of course, are Si
crystals - in the form of wafers - with
extreme degrees of perfection. |
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What we have
are
inexhaustible resources of
Silicondioxide, SiO2,
fairly clean, if obtained from the right source. Since there is no other
material with properties so precisely matched to the needs of the semiconductor
industry, and therefore of the utmost importance for our modern society, the
production process of Si wafers shall be covered in a cursory way. |
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Fortunately, the
steel
industry needs Si, too. And Si was already used as a crucial
alloying component of steel before it started its career as the paradigmatic
material of our times. |
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Most of the world production of
raw Si still goes to the steel industry and
only a small part is
diverted for the semiconductor
trade. This is why this stuff is commonly called "metallurgical grade" Si or
MG-Si for short. The
world production in 2006 was around 4 Mio tons per year. |
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How is MG- Si (meaning poly crystalline
material with a purity of about 99%) made? More or less like most of the
other metals: Reduce the oxide of the material in a furnace by providing some
reducing agent and sufficient energy to achieve the necessary high
temperatures.. |
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Like for most metals, the reducing
agent is carbon (in the form of coal or
coke (= very
clean coal)). The necessary energy is supplied electrically. |
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Essentially, you have a huge furnace (lined with
C which will turn into very hard and inert SiC anyway) with three
big graphite electrodes inside (carrying a few 10.000 A of current) that
is continuously filled with SiO2 (= quartz sand) and
carbon (= coal) in the right weight relation plus a few added secret
ingredients to avoid producing SiC. This looks like this |
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The chemical reaction that you want to take place
at about 2000 oC is |
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But there are plenty of other reactions that may
occur simultaneously, e.g. Si + C Þ
SiC. This will not only reduce your yield of Si, but
clog up your furnace because
SiC is not liquid at the reaction temperature and extremely hard - your
reactor ends up as a piece of junk if you make SiC. |
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Still, we do not have to worry about
MG-Si - a little bit of what is made for the steel industry will suffice
for all of Si electronics applications. |
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What we do have to do is to purify the MG-Si - about
109 fold! |
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This is essentially done in three
steps: |
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First,
Si is converted to SiHCl3 in a "fluid bed"
reactor via the reaction |
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This reaction (helped by a catalyst) takes place
at around 300 oC. The resulting Trichlorosilane is already much purer than
the raw Si; it is a liquid with a boiling point of 31.8
oC. |
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Second, the
SiHCl3 is distilled (like wodka), resulting in extremely pure
Trichlorosilane. |
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Third, high-purity Si is produced by the
Siemens process or, to use its modern
name, by a "Chemical Vapor
Deposition" (CVD) process - a process
which we will encounter more often in the following chapters. |
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The doped
poly-Si (not to be confused
with the poly-Si layers on chips) used for the growth of single
Si crystals is made in a principally simple way which we will discuss by
looking at a poly-Si CVD reactor |
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In principle, we have a vessel which can be
evacuated and that contains an "U" shaped arrangements of slim
Si rods which can be heated from an outside heating
source and, as soon as the temperature is high enough (roughly 1000
oC) to provide sufficient conductivity, by passing an electrical
current through it. |
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After the vessel has been evacuated and the
Si rods are at the reaction temperature, an optimized mix of
SiHCl3 (Trichlorosilane), H2 and
doping gases like
AsH3 or
PH3
are admitted into the reactor. In order to keep the pressure constant (at a
typical value of some mbar), the reaction products (and unreacted gases) are
pumped out at a suitable place. |
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On hot
surfaces - if everything is right this will only be the Si - a chemical
reaction takes place, reducing the SiHCl3 to Si and
forming HCl (hydrochloric acid) as a new compound: |
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Similar reactions provide very
small but precisely measured amounts of As, P or B that
will be incorporated into the growing polysilicon |
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The Si formed will adhere to the Si
already present - the thin rods will grow as fresh Si is produced. The
incorporation of the dopants will produce doped
polysilicon. |
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In principle this is a simple
process, like all CVD processes - but not in reality. Consider the
complications: |
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You have to keep the Si ultrapure - all
materials (including the gases) must be specially selected. |
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The chemistry is extremely dangerous:
AsH3 and PH3 are among the most poisonous
substances known to mankind; PH3 was actually used as a toxic
gas in world war II with disastrous effects. H2 and
SiHCl3 are easily combustible if not outright explosive, and
HCl (in gaseous form) is even more dangerous than the liquid acid and
extremely corrosive. Handling these chemicals, including the safe and
environmentally sound disposal, is neither easy nor cheap. |
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Precise control is not easy either. While the
flux of H2 may be in the 100 liter/min range, the
dopant gases only require ml/min. All flow values must be precisely
controlled and, moreover, the mix must be homogeneous at the Si where
the reaction takes place. |
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The process is slow (about 1 kg/hr) and
therefore expensive. You want to make sure
that your hyperpure (and therefore expensive) gases are completely consumed in
the reaction and not wasted in the exhaust - but you also want high throughput
and good homogeneity; essentially conflicting requirements. There is a large
amount of optimization required! |
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And from somewhere you need the slim rods -
already with the right doping. |
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Still, it works and
abut 10.000 tons of poly-Si are produced at present (2000)
with this technology, which was pioneered by Siemens
AG in the sixties for the microelectronic industry. (in 2007 it
is more like 21.000 to plus another 30.000 tons for the solar
industry). |
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Electronic grade
Si is not cheap, however, and has no obvious potential to become very
cheap either. The link provides todays specifications and some more
information for the product. Here is an example for the poly-crystalline rods
produced in the Siemens process: |
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While this is not extremely important for the
microelectronics industry (where the added value of the chip by far surpasses
the costs of the Si), it prevents other Si products, especially
cheap solar cells (in connection with all
the other expensive processes before and after the poly-Si process).
Starting with the first oil crisis in 1976, many projects in the USA and
Europe tried to come up with a cheaper source of high purity poly-Si, so
far without much success. |
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By now. i.e. in 2007, demand for
electronic grade Si si surging because of a booming solar cell industry.
A short ovberveiw of the current
Si crisis can be found in
the link. |
© H. Föll (Electronic Materials - Script)
