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We now have hyperpure poly-Si,
already doped to the desired level, and the next step must be to convert it to
a single crystal. There are essentially two
methods for crystal growth used in this case: |
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Czochralski or
crucible
grown crystals (CZ crystals). |
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Float
zone or
FZ crystals. |
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The latter method produces crystals
with the highest purity, but is not easily used at large diameters. 150
mm crystals are already quite difficult to make and nobody so far has made
a 300 mm crystal this way. Float zone crystal growth, while the main
method at the beginning of the Si age, is now only used for some
specialities and therefore will not be discussed here; some
details can be found in the
link. |
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The Czochralski method, invented by
the Polish scientist J.
Czochralski in 1916, is the method of choice for high
volume production of Si single crystals of exceptional quality and shall
be discussed briefly. Below is a schematic drawing of a crystal growth
apparatus employing the Czochralski method.
More details can be found in
the link. |
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Essentially, a crystal is
"pulled" out of a vessel containing liquid Si by dipping a
seed crystal
into the liquid which is subsequently slowly withdrawn at a surface temperature
of the melt just above the melting point. |
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The pulling
rate (usually a few mm/min) and the temperature profile determines the crystal diameter
(the problem is to get rid of the heat of crystallization). |
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Everything else determines the
quality and homogeneity - crystal growing is still as much an
art as a science! Some
interesting points are contained in the link. |
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Here we only look at
one major point, the segregation
coefficient
kseg of
impurity atoms. |
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The segregation coefficient in
thermodynamic equilibrium gives the relation between the concentration of
impurity atoms in the growing crystal and that of the melt. It is usually much
lower than 1 because impurity atoms "prefer" to stay in the
melt. This can be seen from the liquidus and solidus lines in the respective
phase diagrams. |
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In other words, the solubility of impurity atoms in the melt is larger
than in the solid. |
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"Equilibrium" refers to a
growth speed of 0 mm/min or, more practically, very low growth rates.
For finite growth rates, kseg becomes a function of the
growth rate (called kseff) and approximates 1 for high
growth rates (whatever comes to the rapidly moving interface gets
incorporated). |
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This has a positive and a negative
side to it: |
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On the positive side, the crystal
will be cleaner than the liquid, crystal
growing is simultaneously a purification method. Always provided that we
discard the last part of the crystal where all the impurities are now
concentrated. After all, what was in the melt must be in the solid after
solidification - only the distribution may now be different. |
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This defines the
negative side: The distribution of
impurities - and that includes the doping elements and oxygen -
will change along the length of a crystal -
a homogeneous doping etc. is difficult to achieve. |
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That segregation can be a large
effect with a sensitive dependence on the growth rate is shown below for the
possible doping elements; the segregation coefficients of the unwanted
impurities is given in a table. |
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| Atom |
Cu |
Ag |
Au |
C |
Ge |
Sn |
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| kseg |
4 ·
10–4 |
1 ·
10–6 |
2,5 ·
10–5 |
6 ·
10–2 |
3,3 ·
10–1 |
1,6 ·
10–2 |
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| Atom |
O |
S |
Mn |
Fe |
Co |
Ni |
Ta |
| kseg |
1,25 |
1 ·
10–5 |
1 ·
10–5 |
8 ·
10–6 |
8 ·
10–6 |
4 ·
10–4 |
1 ·
10–7 |
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We recognize one reason why practically only As, P,
and B is used for doping! Their segregation coefficient is close to
1 which assures half-way homogeneous distribution during crystal growth.
Achieving homogeneous doping with Bi, on the other hand, would be
exceedingly difficult or just impossible. |
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Present day single crystals of
silicon are the most perfect objects on this side of Pluto - remember that
perfection can be measured by using the second law of thermodynamics; this is
not an empty statement! A very interesting and readable article dealing with
the history and the
development of Si crystal growth from W. Zulehner (Wacker
Siltronic), who was working on this subject from the very beginning of
commercial Si crystal growth until today, can be found in the link. |
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What the
finished crystal looks like
can be seen in the link. What we cannot see is that there is no other crystal
of a different material that even comes close in size and perfection. |
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Our crystal does not contain
dislocations - a unique feature that only could be matched by Germanium
crystals at appreciable sizes (which nobody grows or needs)1). It also does not contain many other lattice
defects. With the exception of the doping atoms (and possible interstitial
oxygen, which often is wanted in a concentration of about 30 ppm),
substitutional and interstitial impurities are well below a ppb if not
ppt level (except for relatively harmless carbon at about 1 ppm)
- unmatched by most other "high purity" materials. |
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Our crystal is homogeneous. The
concentration of the doping atoms (and possibly interstitial oxygen) is
radially and laterally rather constant, a feat not easily achieved. |
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The crystal is now ready for cutting
into wafers. |
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Wafer Technology |
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It may appear rather trivial now to
cut the crystal into slices which, after some polishing, result in the
wafers used as the starting material for chip
production. |
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However, it is not trivial. While a wafer does not look like much,
its not easy to manufacture. Again, making wafers is a closely guarded secret
and it is possibly even more difficult to see a wafer production than a single
Si crystal production. |
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First, wafers must all be made to
exceedingly tight geometric specifications. Not only must the diameter and the
thickness be precisely what they ought to be, but the flatness is constrained
to about 1 µm. |
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This means that the polished surface
deviates at most about 1 µm from an ideally flat reference plane -
for surface areas of more than 1000 cm2 for a 300 mm
wafer! |
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And this is not just true for one
wafer, but for all 10.000 or so produced daily in one factory. The number of Si wafers sold in
2001 is about 100.000.000 or roughly 300.000 a day!
Only tightly controlled processes with plenty of know-how and expensive
equipment will assure these specifications. The following picture gives an
impression of the first step of a many-step polishing procedure. |
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© "Smithsonian", Jan 2000, Vol 30, No. 10
Reprinted with general permission |
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In contrast to e.g. polished metals,
polished Si wafers have a perfect
surface - the crystal just ends followed by less than two nm of "native oxide" which forms rather quickly in
air and protects the wafer from chemical attacks. |
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Polishing Si is not easy (but
fairly well understood) and so is keeping the surface clean of particles. The
final polishing and cleaning steps are done in a cleanroom where the wafers are packed for
shipping. |
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Since chip structures are always
aligned along crystallographic directions, it is important to indicate the
crystallography of a wafer. This is done by grinding
flats (or, for very large wafer - 200
mm and beyond notches) at
precisely defined positions. |
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The flats also encode the doping
types - mix ups are very expensive! The convention for flats is as
follows: |
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The main flat is always along a
<110> direction. However, many companies have special agreements
with wafer producers and have "customized" flats (most commonly no
secondary flat on {100} p-type material). |
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More about flats in the
link |
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Typical wafer specifications may
contain more than 30 topics, the most important ones are: |
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Doping
type: n or p-type (p-type is by far the most
common type) and dopant used (P,
As or B). Resistivity
(commonly between 100 Wcm to 0,001
Wcm with (5 - 1) Wcm defining the bulk of the business. All numbers
with error margins and homogeneity requirements |
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Impurity
concentrations for metals and other "life time killers"
(typically below 1012 cm–3), together with
the life time or diffusion length (which
should be several 100 µm). |
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Oxygen and carbon concentration (typically around 6
· 1017 cm–3 or 1 · 1016
cm–3, respectively. While the carbon concentration just has
to be low, the oxygen concentration often is specified within narrow limits
because the customer may use "internal
gettering", a process where oxygen precipitates are formed
intensionally in the bulk of the wafer with beneficial effects on the chips in
the surface near regions. |
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Microdefect
densities (after all, the point defects generated in thermal
equilibrium during crystal growth must still be there in the form of small
agglomerates). The specification here may simple be:
BMD ("bulk micro
defect") density = 0 cm–3. Which simply translates
into: Below the detection limit of the best analytical tools. |
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Geometry, especially several parameters relating to
flatness. Typical tolerances are always in the 1 µm regime. |
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Surface cleanliness: No particles and no atomic or molecular
impurities on the surface! |
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This link provides a
grapical overview of the
complete production process - from sand to Si wafers - and includes
a few steps not covered here. |
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Appreciate that the production of
wafers - at last several thousands per day - with specifications that are
always at the cutting edge of what is possible - is an extremely involved and
difficult process. |
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At present (Jan. 2004), there
are only a handfull of companies world wide that can do it. In fact, 4
companies control about 80% of the market. |
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This link leads to a recent
(1999) article covering
new developments in
Si CZ crystal growth and wafer technology (from A.P.
Mozer; Wacker
Siltronic) and gives an impression of the richness of complex issues behind the
production of the humble Si wafer. |
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This link shows
commercial wafer
specifications. |
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To give an idea of the size iof the
industry: In 2004 a grand totoal of about 4.000.000 m2
of polished Si wafers was produced, equivalent to about
1.25 · 108 200 mm wafers. |
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© H. Föll (Electronic Materials - Script)
