02.crystal growth
TRANSCRIPT
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Necking and dislocation free CZ crystalgrowth Maximum growth rate and diameter
control Segregation of impurities along length
and diameter Defects in CZ crystals FZ Crystal growth
CRYSTAL GROWTH
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Miniaturisation and Crystal Growth
As channel length decreases from 10m 1m 0.1m1. Silicon crystal needs to be purer.
Dopant impurity causing shallow levels - P, B, Al, As, SbIntrinsic carrier concentration ~ 1.5x1016/m3
Before crystal growth, silicon is chemically purified and used as
feedstock for crystal growth..Impure SiSiHCl3 Distill Pure SiHCl3CVD pure Si
Heavy metals causing deep level Cu, Fe, Ti, Ni, V, etc. C, N, O
Purification done to get dopant impurity level to 1018/m3
(eq. To 1000 ohm-cm silicon)
2. Defect concentration to be low.Planar Defects stacking faults and twins
Line Defects dislocationsPoint defects vacancies, interstitials and clusters
This is done by the Czochralski (CZ) Crystal Growth amethod named after J.Czochralski discovered in 1916.
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Single Crystal Growth
Homogeneous nucleation :As a liquid cools, several crystals will nucleate
and grow if nucleation is homogeneous.Ultimately a polycrystal will form.
Heterogeneous nucleation :
If a seed crystal is introduced into the melt andif the supercooling (Tm-T) is low, a singlecrystal will grow on the seed crystal with same
orientation. This will give a single crystal.
To grow a single crystal: One has to decrease the nucleation
rate.
Introduce a seed crystal to promote
heteregnenous nucleation and growth
seed
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Screw Dislocation & Crystal Growth
Presence of screw dislocation
provides an easy growth mechanismfor crystal growth since
atoms/molecules find a ready placewhere they can position themselves inthe crystal making it grow. No
nucleation is necessary.
Actual crystal facesSchematic
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Crystal GrowthBridgman Method: Melt is held inside a crucible. Crucible is lowered through a temp gradient. Nucleation starts from bottom & a favorable
nuclei outgrows others to give a crystal.
Problem is contamination and stress created oncrystal due to crucible.
Czochralski Method: Dip a seed crystal into the melt nad withdraw
Crystal will grow at the solid-liquid interface. No stress in the crystal
Some contamination from crucible
Float Zone Method: Melt part of the solid.
Molten part is moved slowly from the seed side
No contamination Stress in the crystal and slow process
furnaceL
S
L
seed
melt
Seed
Crystal
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Czochralski Silicon Crystal Growth
fused quartz
crucible
CZ Crystal Growth Process:
Silicon melt is held inside afused quartz crucible that issupported by a graphitecrucible.
Heating is done using agraphite heater or induction.
Crucible has to be raisedwhile pulling the crystal since
we want the thermalconditions at the growthinterface same throughout.
Both crucible and the crystalsare rotated in opposite
directions to stir the melt.graphiteheater
graphite
crucible
C
Z
SiCrystal
Si melt~1450C
Crystalrotation
s
Cruciblerotation
c
pull
raise
Ar
SiO
Ar+CO+SiO
CO
Ar
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Czochralski Silicon Crystal Growth
Method:A seed crystal is
dipped into a meltand withdrawn. The
crystal is grown on
the seed.
Advantages:It does not require acrucible to hold the
crystal. So very littlestress present in the
crystal.
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Czochralski Silicon Crystal Growth
Parameters to be controlled :1. Temperature of the melt (i.e., superheat T=T-Tmelting)2. Variation in temperature of the melt
3. Temperature gradient in the crucible4. Pull rate of the crystal/seed5. Rate of raising the crucible
6. Rotation of the crystal/seed7. Rotation of the crucible
8. Argon gas flow
9. Crucible material and dimensions
Diameter of the Crystal :The diameter is controlled by :
1. Pull rate of the crystal
2. Rate of rising of the crucible
3. Superheat T and temperature variations
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Czochralski Silicon Crystal Growth
Doping : By adding Si-P, Si-B master alloys
Orientation : By orienting the seed crystal properly
Resistivity : 10-5 1 ohm-m though dopant control
Dislocation Density : zero or 2.5 m
Weight : >100 kg
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Crucible Material
Properties of Crucible Material Required: Refractoriness Tm
Si = 1415C No reaction with liquid Si
No contamination into liquid Si high purity grade needed
Possible Crucible Materials : SiO2, SiC, Si3N4, Al2O3
SiC : Segregation coefficient for C is low.SiC forms at the liquid-solid interface.
Si3N4 : Same problem as SiC
Al2O3: Contamination with Al a p-type dopant
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Crucible Material
SiO2 : High purity fused quartz is mostly used to contain the liquid Si.
It gives O contamination through Si + SiO22SiO or [O]. It is structurally weak at 1450C and hence is supported by
a graphite crucible.
It devitrifies (i.e., crystallises into crystalline silica) andcracks.
Remnant Si after crystal growth solidifies inside the
crucible. Since silicon expands on solidification, thecrucible cracks. So silica crucible can be used only once.
It is difficult to remove B from silica and hence gives Bcontamination to CZ Si crystal (ko for B ~0.8)
SiO and CO gets released at the SiO2-liquid Si interface andgraphite-SiO2 interfaces. They can be removed by proper flow ofargon gas.
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Necking During CZ Growth
There are dislocation in the seed.
When crystal grows on these seeds, the
dislocations will grow and multiply.
To reduce the dislocation content,
Growth rate is increased to reduce thediameter of the seed so that a neck forms.
As the neck forms, some of thedislocations will move out of the crystal at
the neck.
Process can be repeated and dislocationfree crystal obtained.
The technique of growing dislocation free crystal is alsoknown as Dash technique after its discoverer W.C.Dash.
seed
Dislocationfree crystal
neck
dislocation
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Heat Flow and Convection
The convection currents inside silicon melt changes direction assilicon crystal grows since heat flow direction through the supporting
stem changes.This changes the curvature of the solid-liquid interface affecting
growth, impurity segregation.
Crystal
TETm
TETm
Tm
Heat flow
End of Growth
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Crystal Growth Rate
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The latent heat released on solidification has to dissipate throughthe surface of the crystalcrystal.
Heat flow
d = 2r
L
Diameter of crystal is inverselyproportional to pull rate.
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Crystal Growth Rate
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Maximum Growth Rate:
Heat flow
d = 2r
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Crystal Growth RateTemperature Control & Diameter of Crystal
Unless temp is controlledaccurately, diameter willvary with time.
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Impurity Segregation
Along Length of CZ Si Crystal :
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Impurity Segregation
Effective Segregation Coefficient : In practice there is an impurity build up at
the solid-liquid interface.
This build up diffuses gradually into the
liquid with diffusivity D.
Thickness of diffusion layer is .
This build up will reduce with stirringbetween solid and liquid.
Crystal
High impuritycontent
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As growth velocity increases, keff 1
0.001
0.01
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1
0 8 16
v (mm/min)
keff
BP
SbAl
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Impurity ko Max. solid solubility Diffusion coeff. in melt(at/m3) (m2/sec)
B 0.8 6x1026 2.4x10-8
P 0.35 1.3x1027
5x10-8
As 0.3 1.8x1027 5x10-8
Sb 0.023 7x1025 1.5x10-8
Al 2x10-3 5x1026 7x10-8
Cu 4x10-4 1.5x1024 8x10-8
Ag 10-6 2x1023 8x10-8
Fe 8x10-6 3x1022 -
Ni 3x10-5 8x1023 -Co 8x10-6 2.3x1022 -
Ti 9x10-6 - -Mn 10-5 3x1022 -
O 1.4 2x1024 3.3x10-8
C 0.07 3.5x1023 2x10-8
Impurity Segregation
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Impurity Segregation
Impurity content can beestimated from measurement ofresistivity of the crystal.
The relationship is given in thegraph on the right.
Higher the resistivity, lower is thedopant concentration.
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Impurity Segregation
Variation along Diameter :In absence of dislocations, solid nucleates at the edge of thecrystal and grows inwards.
grows in
Center part will be richer in impurity.The difference will depend upon:
Segregation coefficent Stirring of the melt
(i.e. crucible and crystal
rotation speeds)
B
P
center
x
x/
o
edge edge
25 rpm
5 rpm
centre
x
x
/o
edge edge
edge
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Impurity Segregation
Variation along Diameter :If the solid-liquid interface is not flat, there will beadditional variations since the wafer cut is flat.
Each layer deposited has a different resistivity withinner layer having higher resistivity.
center
x
x
/o
edge edge
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O & C in CZ Silicon
Oxygen & carbon content depends on theprocesses at various interfaces:
1. Liquid Si solid interface
segregation
2. Liquid Si gas interface evaporation of SiO from liquid Si
reaction with CO
CO + Si C + SiO or [O]Si3. Quartz Crucible liquid Si interface
Si + SiO2 2SiO or [O]Si4. Graphite Crucible Quartz Crucible
interface
SiO2 + C SiO + CO
Origins of Oxygen & carbon content in CZ Si crystal are silica and
graphite crucibles used to hold the silicon melt.
CZ
SiCrystal
Si melt~1450C
Ar
SiO
Ar+CO+SiO
CO
Ar
CO
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O & C in CZ Silicon
Oxygen in liquid stays dissolved in the
solid Si since ko>1. Carbon in liquid gets rejected into the
liquid since ko
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Oxygen in CZ Silicon
Normally we get 20ppma 1024/m3 of oxygen interstitiallydissolved in CZ Si crystal.
Interstitial oxygen acts as a n-type donor in silicon. On heating it starts to form SiO4 complexes at 300-500C
0
5
10
15
300 600 900 1200
Temp, degC
[O]x1023
/m3
0
5
10
Time at 450 degC
[O]x1020/m3
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Oxygen in CZ Silicon
along length of crystal
,ohm-m
asgrown
annealedat650C/2hr
P-doped Si
along length of crystal
,ohm-m
asgrown
annealedat650C/2hr
B-doped Si
Oxygen content affects the resistivity of silicon on heating.On heating dissolved interstitial oxygen (n-type dopant) precipitates out.
This can even change the type of silicon.
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Oxygen in CZ Silicon
400C [O], dissolved interstitial oxygen, forms donor states.
700C Homogeneous nucleation of SiO4 clusters
800C
900C
1000C
1100C
Oxide precipitate grows.
Oxide precipitates nucleate stacking faults anddislocations
Stacking fault generation
C & O goes into solid solution
Effect of Annealing:
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Point Defects in CZ Silicon
Vacancies and interstitials are present inside silicon.
CV = 1000 exp(-3.66eV/kBT) 1.3x10-8 at Tm
Si
CI = exp(6.11) exp(-3.04eV/kBT) 4x10-7 at Tm
Si
Temp(K) 1688 1500 1300 1100CV 1.3x10
-8 5x10-10 6.5x10-12 1.7x10-14
CI 4x10-7 2.7x10-8 8.6x10-10 7.8x10-12
So self interstitials are predominant point defects present in the CZ
Si crystal.Others are C, O, Fe, Cu, etc.
As silicon cools down, point defects like O and I cluster.
Since VSiO2 2.2 Vsi, such clustering results in strain in the crystal.This can result in 1) Prismatic dislocations
2) Emission of Si interstitials3) Absorption of vacancies
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Stacking Faults in CZ Silicon
Extrinsic
Due to condensation of
interstitials
IntrinsicDue to condensation ofvacancies
Due to creation of interstitials and vacancies, they can
condense and nucleate a stacking fault bounded by a
dislocation loop. These can be extrinsic or intrinsic types.
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Swirl Defects in CZ Silicon
The growth interface is convex at the start and
becomes concave at the end of crystal growth dueto heat flow conditions.
Since silicon grows layer by layer in absence of
dislocations, when a wafer is cut, one sees circulardefects in the wafer corresponding to each layergrown.
These defects are cluster of point defects.
Swirl defects
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Float Zone (FZ) Silicon Crystals
Method:A part of the silicon held vertically is melted using an
induction coil. The molten part (molten zone) holds
through surface tension if conditions are right. Aseed crystal is put at the bottom and the liquid zone
is moved up to grow the crystal.
Advantages: Completely crucibleless no contamination Silicon can be free of oxygen and carbon Purification during crystal growth possible
Disadvantages: High dislocation density due to sharp
temperature gradient present Limitation on largest diameter that can be
grown
L
seed
Si
InductionCoil forheating
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FZ Silicon Crystals
Si
l
L
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,'x'from'dx'bymoveszonetheAs
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FZ Silicon Crystals
19
20
21
22
23
0 5 10
x/L
at/m
3
10
10
10
10
10
Normal freezing
1 zone pass
23
4
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FZ Silicon Crystals
Impurity Segregation :
If ko < 1,impurity goes to the finishing end.
If ko > 1,impurity goes to the starting end.
FZ Silicon Crystals:upto 100 mm diameter
> 10 ohm-m (1000 ohm-cm)
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NTD Silicon
Neutron Transmuted Doped Silicon (NTD Si):
30Si + 1n 31P + -rays
Extremely high purity FZ silicon (1000 ohm-cm) is irradiated withneutrons in a reactor. This produces P atoms inside sil icon.
Penetration depth for neutrons is ~100cm. So crystal getsuniformly doped with P.
Half life of 31P is ~14 days. So crystal needs to be kept isolated for
about 30 days before use.
x
Rs
Rs/ Rs = 10%
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Ingot to wafer :Ingot Boule forming wafer slicing (ID saw)
Wafers Lapping & polishing
Boule forming, orientation measurement
old standard: flatperpendicular to direction;Wafer slicing using ID saw typically within 0.5
, 2 - 5 off axis
Silicon Wafer Fabrication
ID saw
Boule formationSi
Diamond
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[110]
{111}p-type
[110]
{111}n-type
45
[011]
{100}
p-type
[011]
{100}
n-type
Orientation Flats in Silicon Wafer
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lapping grind both sides, flatness ~2-3 m
~20 m per side removed
edge profiling
etching chemical etch to remove surface damaged layer
~20 m per side removed polishing
chemi-mechanical polish, SiO2/ NaOH slurry
~25 m per polished side removed gives wafers a mirror finish
cleaning and inspection
Silicon Wafer Fabrication
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Available Silicon Material
CZ/FZ Silicon Crystal :
O - 2 ppma, 100 kg
Silicon Wafer :
Diameter 200 0.2 mmThickness 625 10 mTaper - < 10 mGlobal flatness - < 3 mLocal flatness - < 1 m over
20x20mm
Bow - < 10 mWarpage - < 10 mOrientation - or Surface finish 10 nm for
polishedwafer