Crystal growth

• The starting point in any integrated circuit fabrication a single

crystal silicon wafer i,e. the substrate material.

•Silicon is one of the most abundant material in the world found

about 25.7% of the earths crust as SiO2 (in the form of quartezite).

•The first step in the refining process is to convert the quartezite to

Metlurgical Grade Silicon (MGS)

•It is purified further in order to get very high purity semiconductor

grade silicon that is 6N purity or 99.9999 % purity.

•The crystal growth can be broadly classified as Bridgman technique

and Czochralski technique.
•The most of the silicon that is commercially available today, are
grown by Czochralski technique
•We start off with the material silicon dioxide and then, we have to
do a a carbothermic reduction process, where the silicon dioxide
reacts with carbon in order to form silicon plus carbon mono oxide
at 2000 c

• To convert MGS to Electronic Grade Silicon (EGS) several steps are

required . The SiHCl3 is boiled along with the impurities it contains
and separated by its boiling point. After the , it is extremely pure
and is ready to be converted back to purified polysilicon in a large
Chemical Vapor Deposition (CVD) reactor using the following
reaction

.
•A thin silicon rod is used as the nucleation surface on which
the silicon deposits. As pollycrystal may be several meters
long and several hundred mm in diameter.

•The overall deposition can take place many days.

•The resulting polysilicicon is broken up in to pieces to load as

charge.

•This silicon is in polycrystalline form.

•Melt it and then solidify this.

•Instead of this carbothermic reduction process, a

metallothermic reduction process is also sometimes adopted
but It is not commercially very successful
• Bridgman technique
•The Bridgman system of crystal
growth is a very simple process.
•we have a quartz ampoule or a
quartz tube, and evacuated and
sealed.
•Inside this ampoule, you have a
quartz boat.
•In this boat, a seed crystal is
placed and the rest is filled with
the charge.
•Charge mean the high purity
silicon which is (99.999% purity)
or powdered form.
•Then the charge is heated up to its melting point, but make sure
that the seed crystal is not molten.
•Therefore, the temperature profile of the furnace has to be
something like - temperature versus x.
•The seed end is kept at a lower temperature, while the charge,
the melt is at high temperature and therefore it is in the molten
condition.
•The end of the melt which is in contact with the seed crystal is
now becoming progressively cooler. It is coming in the low
temperature zone of the furnace.
•Therefore, that end is going to solidify and since this is in contact
with the seed crystal,
•It will take the orientation of the seed crystal and the
solidification will take place as a single event.
•Silicon is a material which expands on solidification.
•So, in a Bridgman crystal growth that is the greatest drawback,
because the charge is kept in a quartz boat.
•The confining boundary of the quartz boat is going to create a lot
of stress during the solidification of the charge-dislocations will
be formed.
• This is one of the major reason, why, in spite of the inherent
simplicity of the Bridgman crystal growth technique, people go
for Czochralski system of crystal growth.
•Czochralski system of crystal
growth is actually a much more
complicated system, compared
to a Bridgman system.
• It was developed by a Polish
scientist Yan Czochralski in 1918.
It’s a generic method of making
any crystal ingot but is especially
popular for making silicon
•It is a much more sophisticated,
much more difficult system to
handle.
•To control a large number of
parameters, but you can grow
single crystals which have much
less dislocations, much less
defects.
•a Czochralski system is also known as a liquid solid
mono-component growth system, - commonly referred to as the Cz,
•There are basically four subsystems in this Czochralski crystal growth
system.
(i). Furnace
(ii). Crystal pulling mechanism
(iii). Ambient control
(iv). Control systems
•The system has big quartz chamber with gas inlets, gas outlets, may
be even some pumps to maintain the requisite pressure.
•This quartz envelope or chamber may be water cooled.
•The most important part in this furnace is the quartz crucible (silicon
dioxide) that is, a cup in which the charge is going to be placed
• So, there is less chance of contamination through a quartz crucible
because it is also comprised of silicon.
•Therefore some oxygen may come from the crucible to the single
crystal.
•This quartz crucible is usually placed inside a graphite susceptor. (a
bigger cup ) and placed on a graphite heater.
•The heating is done by, usually by RF.
• The things inside the furnace are a quartz crucible, a graphite
susceptor, a very high purity graphite heater and the cooling for the
outer quartz chamber.
•Then, we have the crystal pulling mechanism.
•Through this, a pull rod is passed and at the end of the pull rod, a
small seed crystal is fixed in a chuck.
•The pull rod is pulled up during the crystal growth with ambient
control.
•Otherwise, since we are using a graphite susceptor and graphite
heaters. Therefore, there must not be any oxygen inside the system.
The whole thing will just go up in smoke,- it just carbon dioxide and
there will be no susceptor, no heater, nothing left.
•it should not react with silicon otherwise react with silicon to form
silicon dioxide in the presence of oxygen.
•First of all evacuate the quartz chamber and then fill it up with an
inert ambient - argon or helium or nitrogen, and maintain
atmospheric pressure or sometimes even reduced pressure.
•The charge gets molten inside the crucible and uniformly in the
liquid state,
•Then the pull rod is gradually lowered till the seed crystal touches
the melt surface.
•Then, very slowly, the pull rod is pulled up
•The melt that is in contact with the seed crystal will get solidified
and as we pull up the pull rod.
•Very accurate control is necessary, so that instead of growing the
crystal you do not melt down the seed crystal itself.
•The pull rate must be carefully adjusted, so that get a single
crystal.
•If the pull rate as well as the thermal conditions are not carefully
adjusted, the reverse can also happen. The seed can also get
melted.
•If you draw it too fast you may not get a single crystal. If you
draw it too slow, you may even get the seed crystal melted.

•How the pull rate should be carefully adjusted. Let us use a heat
transfer equation as .

•where L is the latent heat of fusion or solidification,

•dm/dt is the mass solidification rate.
•KL is the thermal conductivity of he liquid,
•KS is the thermal conductivity of the solid,
•dT/dx1 is the thermal gradient in the melt,
•dT/dx2 is the thermal gradient in the solid that is in the crystal
•A1 and A2 are the area of the isotherms in the melt and in the
solid.
•If we assume that the thermal gradient in the melt is zero, that
is the entire melt is in the constant temperature that is at the
melting point, i.e

•The mass solidification rate can be written as

•i.e. the rate of change .

•than
•This is the maximum attainable pull rate when assuming thermal
gradient in the melt is going to be zero.
•In practice, the pull rate is kept less than the maximum pull rate.

•the pull rate is going to vary inversely with the area for a given
Hi (heat input) – Ho (heat output) .

•The defect in the crystal is depending quite significantly on the pull

rate.
•How?

•As the crystal is cooling, from its near melting point, the thermal
point defects will try to coalesce into dislocation loops. It will try to
agglomerate, all of them will come together and form a dislocation
loops and form an array agglomerate. This is happen at a
temperature around 950 c .
•Now, if the pull rate is larger than 2 mm/min, then we can
quench this defect formation.

•But again, if we want a large diameter crystal, then we cannot

afford to use a very large pull rate.

•The average pull rate actually varies 1.5 mm/min when the
diameter is about 75 mms .

•For measuring silicon wafer diameter we still use inches.

•it is relatively easier to grow a smaller diameter single crystal

with very low defect density, Because for a smaller diameter
crystal, you can afford to use a large pull rate.
•Dopant incorporation in the crystal : grow a p-type crystal or an
n-type crystal.
•Any impurity will have a solid solubility in the crystal and it will
also have an equilibrium solubility in the melt and these two are
not necessarily same means different solubility in the crystal and
in the melt.
•This difference is given by the segregation coefficient, usually
referred to as K0
•K0 is given as the ratio between CS and CL , where CS is the solid
solubility in solid silicon and CL is the equilibrium solubility in the
melt.
•usually for the most common impurities this K0 < 1 ; means the
solid solubility is less than the equilibrium solubility in melt.
•means as you grow the crystal, these impurities are preferentially
left inside the melt, they do not come into the solid.
•Means as you are drawing the crystal, the melt is becoming
progressively richer in the impurity. So, when we grow a crystal,
we will find that the doping concentration will vary from one end
to the other.
•The top end will have relatively less dopant concentration and the
bottom end will be relatively more doped. That is true for most
common impurities with the notable exception of oxygen.
•That is, oxygen will have a segregation coefficient greater than 1
and will come preferentially into the crystal,
•That is a cause for concern, because we know in the furnace itself
we have a source of oxygen. We are using quartz crucible, quartz
envelopes. Although the envelope is water cooled, but the crucible
is heated.
•So, there is a possibility of oxygen incorporation during the crystal
growth.
•What are the effects of oxygen in the single crystal silicon?
•Usually in a Czochralski grown crystal, the amount of oxygen will
vary from 5x1017 - 1018/cm3.
•this oxygen in the silicon will have three possible effects.
•(i). within a limit this oxygen, 95% will stay in the interstitial sites
i.e. will find the space in between the regular array of silicon
atoms and go and sit there. The rest 5%, however it can form a
complex like SiO4 , will be electronically active and that complex
will be donor-like. therefore it will affect the resistivity of the
grown crystal and the formation temperature of these complexes
are around 450 to 500C. So, when the crystal is being cooled, this
SiO4 complexes will be formed. They will make the silicon n-type,
donor like complexes.

•To reduce this, cut them into wafers and then heat to temperature
above 600C, complexes are dissolved, because the formation
temperature is only around 450 to 500C
•when you cool it down, a wafer is going to cool much faster than
the entire single crystal. the complexes have less time to form.
Therefore, their concentration is going to be reduced.
•So, this is a common practice in order to reduce the donor
complexes due to oxygen
•95% of the oxygen that is incorporated in the interstitial sites
improves the mechanical properties of silicon,
•If the oxygen concentration in silicon is greater than 6.4x10 17
/cm3, then we come to the third problem and that is the
problem of oxygen precipitate.
•Therefore, when the crystal is being cooled down, if the oxygen
concentration is greater than 6.4x1017 / cm3, this is going to be
precipitated in silicon and once it precipitates will give rise to
compressive strain, will start shooting off dislocations from that
site.
•So, oxygen concentration in the Czochralski grown crystal is quite
a problem.
•if you are making power devices where the breakdown voltage
needs to be very high, the leakage current has to be very low, then
this oxygen concentration in silicon may not be acceptable,
because you know, dislocations are generally going to deteriorate
the device performance i.e. lowering down the breakdown voltage
and increasing the leakage current.
CZ Silicon Characteristics
• Growth is easiest in the (111) direction
- Surface energy of (111) < surface energy (100) < surface
energy (110)

• Doping is accomplished by adding controlled amounts of dopant

- Typically crushed powder of doped wafers
- Elemental P and Si will react explosively

• Oxygen get incorporated in to the crystal

- 1016 to 1018 cm-3
- Mostly by etching of SiO2 crucible
- Makes the crystal stronger.

The silicon has an interpenetrated

FCC lattice and silicon atoms are
tetrahedrally bonded to other silicon atoms
Prob:- A boron-doped crystal pulled by the Czochralski technique is required
to have a resistivity of 10 Ω cm when half the crystal is grown. Assuming that a
100 gm pure silicon charge is used, how much 0.01 Ω cm boron doped silicon
must be added to the melt? Assume k0 = 0.8 and the hole mobility µp = 550 cm2
volt-1 sec-1.

Sol:-
 Prob-2:- A Czochralski crystal is pulled from a melt containing 1015
cm-3 boron and 2x1014 cm-3 phosphorus. Initially the crystal will be P
type but as it is pulled, more and more phosphorus will build up in the
liquid because of segregation. At some point the crystal will become N
type. Assuming kO = 0.32 for phosphorus and 0.8 for boron, calculate
the distance along the pulled crystal at which the transition from P to N
type takes place.

Sol.:-