NANOFABRICATION -1
CRYSTAL GROWTH
EEE5425 Introduction to Nanotechnology
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Crystal Growth
Bulk crystal growth
Growth of semiconductors crystals (e.g. Si, GaAs, Ge)
as standalone pieces.
•Czochralski method
•Bridgman method
•Various floating zone methods
Epitaxial crystal growth
Growth of a thin crystal layer on a wafer of a compatible
crystal.
•Molecular beam Epitaxy (MBE)
•Metal organic chemical vapor
deposition (MOCVD)
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Purity
Purity is very important for semiconductor crystals to fabricate
high quality electronic devices.
Today's Si wafers have the impurity level of parts per billion or
ppb. 1ppb = 5 x 1013 cm-3.
What is ppb?
Imagine the world
population is 6 billion.
And there are 6 aliens.
They would feel very lonely…
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Defects
a) Interstitial impurity atom;
b) Edge dislocation;
c) Self interstitial atom;
d) Vacancy;
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e) Precipitate of impurity atoms;
f) Vacancy type dislocation loop;
g) Interstitial type dislocation loop;
h) Substitutional impurity atom
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Crystal Growth (Si)
Starting
Material
Silica
Very impure
Silicon
SiCl4
(liquid)
Ultrapure
SiCl4
Ultrapure
polycrystalline Si
Ultrapure
Polycrystalline
Si
Czochralski method
Single Crystal
Growth
Grinding
Polished
Wafer
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Sawing
Chemical mechanical
Polishing (CMP)
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Purification
Low-grade (or metallurgical grade MGS) Si (or ferrosilicon) with a purity
of about 95 ~ 98% is firstly produced by placing silica (SiO2, sand,
quartzite) in a furnace (at about 1,800 oC) with various forms of carbon
to get rid of oxygen.
SiO2 (s) + SiC (s) → Si (s) + SiO (g) + CO (g)
or
SiO2 (s) + 2C (s) → Si (s) + 2CO (g)
The ferrosilicon is chlorinated to yield SiHCl3 (or SiCl4), both of which
are liquids at room temperature (BP ≈ 32 oC).
or
Si (s) + 3HCl → SiHCl3 (l) + H2 (g)
Si (s) + 4HCl → SiCl4 (l) + 2H2 (g)
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Purification
Multiple distillation of the liquids removes the unwanted impurities
(99.9999% pure). The purified SiHCl3 (or SiCl4) is then used in a
hydrogen reduction reaction to prepare the electronic-grade Si (EGS).
or
SiHCl3 (g) + H2 (g) → Si (s) + 3HCl (g)
SiCl4 (g) + 2H2 (g) → Si (s) + 4HCl (g)
The EGS, a polycrystalline material of high purity, is the raw material
used to prepare device-quality, single-crystal Si. Pure EGS generally
has impurity concentrations in the parts-per-billion range.
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Czochralski Method
Jan Czochralski (cho-HRAL-skee) (1885 1953) was a Polish chemist who invented the
Czochralski process, which is used to grow
single crystals and is used in the production of
semiconductor wafers.
He discovered the Czochralski method in 1916, when he
accidentally dipped his pen into a crucible of molten tin
rather than his inkwell. He immediately pulled his pen
out to discover that a thin thread of solidified metal was
hanging from the nib. The nib was replaced by a
capillary, and Czochralski verified that the crystallized
metal was a single crystal.
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Czochralski Method
The ultrapure poly-Si is placed in a quartz crucible and heated in an inert
atmosphere to form a melt (1412 oC). A small single crystal (or Si seed crystal),
with the normal to its bottom face carefully aligned along a predetermined
direction (typically a <111> or <100> direction), is then lamped to a metal rod and
dipped into the melt. The molten Si may be doped n-type or p-type in the melt to
produce a doped Si substrate. As the seed crystal is slowly pulled out, the Si in the
melt near the seed cools off and crystallizes onto the seed, extending the crystal
downward and outward. The diameter (or radius r ) of the cylindrically shaped
single crystal of Si (ingot) increases, depending on the rate at which the seed
pulls (typically, the growth rate ~1/√r ).
Recently, 300-/400-mm (12-/16-inch) wafers are in production.
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Czochralski Method
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Czochralski Method
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Bridgman Growth Method
•Mainly used for GaAs growth and other new crystals (eg. Bi2TeO5 , LiB4O7,
CdTe) for research.
•Typical wafer diameter is 2”.
•Growth of larger crystals requires very accurate control of the stoichiometry
of axial and radial temperature gradients to control dislocation density.
•Allows very small thermal gradients and, therefore, low dislocation densities.
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Float-Zone Crystal Growth
•Used to form single crystal semiconductor
substrates as an alternative to Czochralski
growth process;
•Polycrystalline material (typically in the form of
a circular rod) is converted into single-crystal by
zone heating (zone melting) starting at the plane
where single crystal seed is contacting
polycrystalline material;
•Used to grow Si wafers with very high purity (i.e.
very high resistivity) single crystal Si;
•Does not allow as large Si wafers as CZ does (200
mm and 300 mm) and radial distribution of
dopant in FZ wafer not as uniform as in CZ wafer;
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Sawing and Polishing
Grinding → X-ray crystallography → Sawing →Lapping → Chamfering → Polishing
Sawing
Chamfering
Chemical mechanical polishing
Using slurry of fine
SiO2 particles in a
basic NaOH solution.
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Economical Value
From sand
$35 / ton
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to
ultrapure Si single crystal wafers
$200 / 300mm wafer
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CPUs
$300 / CPU
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Doping
doping : process of intentionally introducing impurities into an extremely
pure (also referred to as intrinsic) semiconductor in order to change its
electrical properties.
For any material, there is a different affinity for impurities for the
liquid phase and the solid phase. This characteristic is described by
the distribution coefficient kd.
kd = CS/CL
CS = impurity concentration in the solid
CL = impurity concentration in the liquid
kd = f( material, impurity, temperature)
Example:
if kd=1/2, there are twice the impurities in the liquid as in the solid.
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Distribution Coefficient
Example Find the concentration of phosphorous (P) atoms in the melt to obtain
Si doped with 1016 atoms/cm3 (Czochralski growth) kd = 0.35 for P in Si.
cS
1016 atoms / cm 3
k d   cL 
 2.86 1016 atoms / cm 3
cL
0.35
How many grams of P should be added if the initial load in the crucible is 5 kg
of Si? (density of Si = 2.33g/cm3 )
v
m
5000 g
3


2145
.
9
cm
d 2.3g / cm3
In the total melt volume, we want 2.86X1016 atoms/cm3. The number of atoms is:

2.86 1016 atoms / cm3 VSi  VP

but VP  VSi, so VC  Vsi

2.86 1016 atoms / cm3 2145.0cm3  6.135 1019
P atoms =
ZP=31g

6.135 1019 atoms  31g / mole
 3.159 103 g  3.16mg
23
6.023 10 atoms / mole
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Epitaxial Growth
Epitaxial growth technique, a process whereby a thin, single crystal layer
of material is grown (or deposited) on the substrate of a single-crystal
substrate, which is used extensively in a device and integrate circuit
fabrication. The single-crystal substrate acts as the seed. Epitaxial
growth can be performed at temperatures considerably below the
melting point of the substrate crystal.
•Homoepitaxy: an epitaxial layer grown on the same substrate material
(e.g., Si/Si-sub)
•Heteroepitaxy: an epitaxial layer grown on the different substrate
material (e.g., AlGaAs/GaAs-sub)
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Lattice Matching
x=0.53
In creating heterojunctions, one must start with an available substrate material
that is lattice matched (i.e., the lattice constants of the two materials must be
as nearly equal as possible)
Defects will occur or there is strain on the lattice near the junction unless the
lattice constants of the two materials are very well matched.
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Vegard's Law
a(AxB1-x )= x.a(A)+ (1-x).a(B)
aInxGa1-xAs= x.aInAs+ (1-x) aGaAs
aInAs=6.06Å
aGaAs=5.65Å
aInP=5.87Å
5.87= x6.06+ (1-x) 5.65
5.87=5.65 +0.41x
0.22=0.41x
x=0.53
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Vegard's Law – Example
aAlN=3.112Å
aInN=3.533Å
aGaN=3.189 Å
AlxIn1-xN on GaN
x=???
aAlxIn1-xN= x.aAlN+ (1-x) aInN
3.189 = 3.112 x + (1-x) 3.533
3.189 = 3.533 – 0.421x
x = 0.82
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Liquid-Phase Epitaxy (LPE)
•One of the earliest way to grow epitaxial layers (primarily for compound
semiconductors)
•The wafers are on a tray that slides
Advantages:
Near-equilibrium growth,
excellent crystal quality
Inexpensive;
Fast
Disadvantages:
Difficult to scale up for production
Dimensional control poor
Structure complexity limited
Current status: Used for LEDs and laser
diodes in well established processes. Rarely
used in new installations.
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Liquid-Phase Epitaxy (LPE)
Koyo Thermo Systems Co., Ltd
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Molecular Beam Epitaxy (MBE)
• A highly versatile technique to put down monolayers for extremely precise control
of material growth.
• Sort of a solid-phase epitaxy (SPE).
• The individual elements (and dopants) are heated in their separate crucibles under
high vacuum. The gates to the individual crucibles can be opened and closed to vary
composition of the layers.
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Molecular Beam Epitaxy (MBE)
Single filament effusion cell
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Molecular Beam Epitaxy (MBE)
Advantages:
Extremely flexible, simple chemistry
Insitu monitoring;
Atomic layer control
Non-equilibrium technique
Disadvantages:
No in situ cleaning or purifying reactions
Expensive (to assemble and operate)
Non-equilibrium technique
Current status:
A research workhorse; heavily used in
production
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SEM of GaAs (dark) AlGaAs (light)
layers.
Each layer is 4 monolayers (11.3 A)
thick.
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Chemical Vapor Deposition (CVD) or
Vapor-Phase Epitaxy (VPE)
A process whereby an epitaxial layer is formed by a chemical reaction between
gaseous compounds:
SiCl4 (g) + 2H2 (g) → Si (s) + 4HCl (g)
•Performed at atmospheric pressure (APCVD) or at low pressure (LPCVD)
•A technique commonly used to Si layers on Si
•Also used grow III-V compounds (e.g., a GaAs substrate is exposed to an
atmosphere of
AsH3 + PH3 + GaCl to obtain GaAsP film
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Metal-Organic Chemical Vapor Deposition (MOCVD)
In obtaining the compounds containing Al,
VPE does not work ideally because the Al
does not diffuse well on the surface and
because of its high activity.
•In MOCVD, the Ga and Al metals are
introduced in organic compounds such as
Ga(CH3)3 and Al(CH3)3.
•MOCVD is capable of growing monolayers
(layers one atom thick), which makes
possible abrupt changes in composition and
highly precise control.
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Metal-Organic Chemical Vapor Deposition (MOCVD)
Advantages:
All sources gaseous
Precise composition and dimension
control
Disadvantages:
Involves complex chemistry
Uses toxic gases (AsH3, PH3)
Current status:
Viewed as the standard production
process for many epitaxial
heterostructures. Especially in GaN
based HEMT and LED wafer production.
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