Miniaturization process technology
2nd lecture: Silicon wafers
Prof. Yosi Shacham-Diamand
Fall 2004
Silicon Crystal Structure and Growth
(Plummer - Chapter 3)
Atomic Order of a Crystal Structure
Figure 4.2
Amorphous Atomic Structure
Figure 4.3
Unit Cell in 3-D Structure
Unit cell
Figure 4.4
Miller Indices of Crystal Planes
Z
Z
Z
Y
X
Y
X
(100)
Y
X
(110)
Figure 4.9
(111)
Silicon Crystal Structure
Crystals are characterized by a unit cell which repeats in the x, y, z directions.
• Planes and directions are defined using x, y, z coordinates.
• [111] direction is defined by a vector of 1 unit in x, y and z.
• Planes defined by “Miller indices” – Their normal direction
(reciprocals of intercepts of plane with the x, y and z axes).
Silicon has the basic diamond crystal structure –
two merged FCC cells offset by a/4 in x, y and z.
Faced-centered Cubic (FCC) Unit Cell
Figure 4.5
Silicon Unit Cell: FCC Diamond Structure
Figure 4.6
Basic FCC Cell
Merged FCC Cells
Omitting atoms
outside Cell
Bonding of Atoms
(Extra line of atoms)
Various types of defects can exist in a
crystal (or can be created by processing
steps). In general, these cause electrical
leakage and are result in poorer devices.
Point Defects
Vacancy defect
Interstitial defect
Frenkel defect
Semiconductor-Grade Silicon
Steps to Obtaining Semiconductor Grade Silicon (SGS)
Step
1
2
3
Description of Process
Produce metallurgical grade
silicon (MGS) by heating
silica with carbon
Purify MG silicon through a
chemical reaction to produce
a silicon-bearing gas of
trichlorosilane (SiHCl3)
SiHCl3 and hydrogen react in
a process called Siemens to
obtain pure semiconductorgrade silicon (SGS)
Reaction
SiC (s) + SiO2 (s) Æ Si (l) + SiO(g) + CO (g)
Si (s) + 3HCl (g) Æ SiHCl3 (g) + H2 (g) + heat
2SiHCl3 (g) + 2H2 (g) Æ 2Si (s) + 6HCl (g)
Czochralski (CZ)
crystal growing
• Si is purified from SiO2 (sand) by refining, distillation and CVD.
• It contains < 1 ppb impurities. Pulled crystals contain O (~1018
cm-3) and C (~1016 cm-3), plus dopants placed in the melt.
CZ Crystal Puller
Crystal puller
and rotation
mechanism
Crystal seed
Single crystal
silicon
Molten
polysilicon
Quartz
crucible
Heat shield
Carbon heating
element
Water jacket
Figure 4.10
• All Si wafers come
from “Czochralski”
grown crystals.
• Polysilicon is melted,
then held just below
1417 °C, and a
single crystal seed
starts the growth.
• Pull rate, melt
temperature and
rotation rate control
the growth
Silicon Ingot Grown by CZ Method
Photograph courtesy of Kayex Corp., 300 mm Si ingot
Photo 4.1
An alternative process is the “Float
Zone” process which can be used for
refining or single crystal growth.
• In the float zone process, dopants and other
impurities are rejected by the regrowing silicon
crystal. Impurities tend to stay in the liquid and
refining can be accomplished, especially with
multiple passes.
Float Zone Crystal Growth
Gas inlet (inert)
Chuck
Polycrystalline
rod (silicon)
Molten zone
Traveling
RF coil
RF
Seed crystal
Chuck
Inert gas out
Figure 4.11
Dopant Concentration Nomenclature
Concentration (Atoms/cm3)
Dopant
Pentavalent
Trivalent
Material
Type
n
p
< 1014
(Very Lightly Doped)
1014 to 1016
1016 to 1019
(Lightly Doped)
(Doped)
--
-
n
n
--
-
p
p
Table 4.2
n
p
>1019
(Heavily Doped)
+
n
+
p
Segregation Fraction for FZ Refining
Basic Process Steps for Wafer
Preparation
Crystal
Crystal Growth
Growth
Wafer
Wafer Lapping
Lapping
and
and Edge
Edge Grind
Grind
Cleaning
Cleaning
Shaping
Shaping
Etching
Etching
Inspection
Inspection
Wafer
Wafer Slicing
Slicing
Polishing
Polishing
Packaging
Packaging
Figure 4.19
Ingot Diameter Grind
Preparing crystal ingot for grinding
Diameter
grind
Flat grind
Figure 4.20
Internal Diameter Saw
Internal diameter
wafer saw
Figure 4.23
After crystal pulling, the boule is shaped
and cut into wafers which are then polished
on one side.
Wafer Notch and Laser Scribe
1234567890
Notch
Scribed identification number
Figure 4.22
Polished Wafer Edge
Figure 4.24
Chemical Etch of Wafer Surface
to Remove Sawing Damage
Figure 4.25
Wafer Dimensions & Attributes
Diameter
(mm)
Thickness
(µm)
Area
(cm2)
Weight
(grams/lbs)
Weight/25
Wafers (lbs)
150
200
300
400
675 ± 20
176.71
28 / 0.06
725 ± 20
314.16
53.08 / 0.12
775 ± 20
706.86
127.64 / 0.28
825 ± 20
1256.64
241.56 / 0.53
1.5
3
7
13
Table 4.3
Increase in Number of Chips
on Larger Wafer Diameters
(Assume large 1.5 x 1.5 cm microprocessors)
88 die
200-mm wafer
232 die
300-mm wafer
Figure 4.13
Developmental Specifications for 300mm Wafer Dimensions and Orientation
Parameter
Units
Nominal
Some Typical
Tolerances
Diameter
mm
300.00
± 0.20
Thickness
(center point)
µm
775
± 25
Warp (max)
µm
100
Nine-Point Thickness
Variation (max)
µm
10
Notch Depth
mm
1.00
+ 0.25, -0.00
Notch Angle
Degree
90
+5, -1
Back Surface Finish
Bright Etched/Polished
Edge Profile Surface Finish
Polished
FQA (Fixed Quality Area –
radius permitted on the
wafer surface)
mm
147
From H. Huff, R. Foodall, R. Nilson, and S. Griffiths, “Thermal Processing Issues for 300-mm Silicon Wafers:
Challenges and Opportunities,” ULSI Science and Technology (New Jersey: The Electrochemical Society, 1997), p. 139.
Table 4.4
Wafer Polishing
Double-Sided Wafer Polish
Upper polishing pad
Wafer
Slurry
Lower polishing pad
Figure 4.26
Improving Silicon Wafer
Requirements
1995
(0.35 µ m)
Wafer diameter
(mm)
Site flatnessA (µm)
Site size (mm x mm)
MicroroughnessB of front
surface (RMS)C (nm)
Oxygen content
(ppm)D
Bulk microdefectsE
(defects/cm2)
Particles per unit area
(#/cm2)
EpilayerF thickness
(± % uniformity) (µm)
Year
(Critical Dimension)
1998
2000
(0.25 µ m)
(0.18 µ m)
2004
(0.13 µ m)
200
200
300
300
0.23
(22 x 22)
0.17
(26 x 32)
0.12
26 x 32
0.08
26 x 36
0.2
0.15
0.1
0.1
≤ 24 ± 2
≤ 23 ± 2
≤ 23 ± 1.5
≤ 22 ± 1.5
≤ 5000
≤ 1000
≤ 500
≤ 100
0.17
0.13
0.075
0.055
3.0 (± 5%)
2.0 (± 3%)
1.4 (± 2%)
1.0 (± 2%)
Adapted from K. M. Kim, “Bigger and Better CZ Silicon Crystals,” Solid State Technology (November 1996), p. 71.
Quality Measures
•
•
•
•
•
•
•
Physical dimensions
Flatness
Microroughness
Oxygen content
Crystal defects
Particles
Bulk resistivity
“Backside Gettering” to Purify Silicon
Polished Surface
Backside Implant: Ar (50 keV, 1015/cm2)
The argon amorphizes the back side of the
silicon.
“Backside Gettering” to Purify Silicon
Backside Implant: Ar (50 keV, 1015/cm2)
9The argon amorphizes the back side of the silicon.
9The wafer is heated to 550oC, which regrows the
silicon, however, the argon can not be absorbed by the
silicon crystal so it precipitates into micro-bubbles and
prevents some damage from annealing.
9The wafer is held at 550oC for several hours, and all
mobile metal contaminants are attracted to and then
captured by the argon stabilized damage. Once
captured, they never leave these sites.
Chapter Review (Wafer Fabrication)
• Raw materials (SiO2) are refined to produce electronic
grade silicon with a purity unmatched by any other
available material on earth.
• CZ crystal growth produces structurally perfect Si single
crystals which are cut into wafers and polished.
• Starting wafers contain only dopants, and trace amounts of
contaminants O and C in measurable quantities.
• Dopants can be incorporated during crystal growth
• Point, line, and volume (1D, 2D, and 3D) defects can be
present in crystals, particularly after high temperature
processing.
• Point defects are "fundamental" and their concentration
depends on temperature (exponentially), on doping level
and on other processes like ion implantation which can
create non-equilibrium transient concentrations of these
defects.
Chapter Review (Wafer Fabrication)
• Raw materials (SiO2) are refined to
produce electronic grade silicon
with a purity unmatched by any
other available material on earth.
• CZ crystal growth produces
structurally perfect Si single
crystals which are cut into wafers
and polished.
Chapter Review (Wafer Fabrication)
• Starting wafers contain only
dopants, and trace amounts
of contaminants O and C in
measurable quantities.
• Dopants can be incorporated
during crystal growth
Chapter Review (Wafer Fabrication)
• Point, line, and volume (1D, 2D, and
3D) defects can be present in crystals,
particularly after high temperature
processing.
• Point defects are "fundamental" and
their concentration depends on
temperature (exponentially), on doping
level and on other processes like ion
implantation which can create nonequilibrium transient concentrations of
these defects.
Measurement of Wafer Characteristics
Dark-field and Bright-field Detection
Brightfield imaging
Darkfield imaging
Viewing
optics
Viewing
optics
Two-way mirror
Light source
Light reflected by
surface irregularities
Lens
Lens
Figure 7.15
ts
h
g
Li
rce
u
o
Schematic of Optical System
Phase and
and intensity
intensity
Phase
detection
detection
Photo detector array
Data generation,
generation, processing,
processing,
Data
display are
are networked
networked with
with
display
factory management
management software
software
factory
Split mirror
Video camera
Lens
CRT
Light source
Viewing optics
Objective lens assembly
Three-axis
piezo substage
Wafer positioning stage
Vibration isolation pad
Figure 7.16
Principle of Confocal Microscopy
Detector
Pinhole
Beam splitter
Laser
Pinhole
Objective lens
Wafer is driven up and
down along Z-axis
Center of focus
+Z
0
-Z
Figure 7.17
Particle Detection by Light Scattering
Photo detector
Detection of
scattered light
Reflected light
Incident light
Beam scanning
Wafer motion
Particle
Scattered light
Figure 7.18
Measurement of Wafer Characteristics
Hot Point Probe
9Hot point probe is a simple method to determine
whether a semiconductor is N or P type.
9 Principle of operation:
9Two probes touches the wafer, one is wormer than
the other
9A voltmeter reads the potential between the probes
9 If the warmer probe is more positive than the
colder probe than it is a semiconductor type N
9 If the warmer probe is more negative than the
colder probe than it is a semiconductor type P
Hot point probe
Basic principle of the hot probe, illustrated
for an N-type sample, for determining N- or
P-type behavior in semiconductors.
Four point probe
Four point probe
“Four-point probe” measurement method.
The outer two probes force a current
through the sample; the inner two probes
measure the voltage drop.
Four point probe
Thin samples
The area of a cylinder with
radius x and height t
ρ
V
Rs = = 4.532
t
I
Four Point Probe
Constant current source
V
ρs =
I
R
I
x 2πs (ohms-cm)
V
Voltmeter
Wafer
Figure 7.3
Hall Effect Measurements
The Hall effect was discovered more than 100 years
ago when Hall observed a transverse voltage across
a conductor subjected to a magnetic field.
Hall effect is used to determine the
material type, carrier concentration
and carrier mobility separately.
Lorentz force
Hall voltage
IB
VHall =
qnd
I - current,
B - magnetic field,
d - the sample thickness, and
q = 1.602 x 10-19 Cb is the elementary charge.
Conceptual representation of Hall effect
measurement. The right sketch is a top
view of a more practical implementation.
The van der Pauw Technique
exp(-pRA/RS) + exp(-pRB/RS) = 1
The van der Pauw Technique
VH = V24 Æ VH=IB/qnd
“Van der Pauw” Sheet Resistivity
(similar to 4-point probe, but uses shapes on wafer)
I
(b)
V
(a)
Contact
Conductive material
(c)
(d)
Figure 7.4
Fourier Transform Infrared
Spectroscopy (FTIR)
FTIR (Oxygen and Carbon Detection)
The CZ crystal growth process introduces oxygen and carbon into the silicon. These elements
are not inert in the crystal. It is important is to be able to measure them and to control them.
The method is Fourier Transform Infrared Spectroscopy. FTIR measures the
absorption of infrared energy by the molecules in a sample. Many molecules have vibrational
modes that absorb specific wavelengths when they are excited. By sweeping the wavelength
of the incident energy and detecting which wavelengths are absorbed, a characteristic
signature of the molecules present is obtained. Oxygen in CZ crystals is located in interstitial
sites in the silicon lattice, bonded to two silicon atoms. Low concentrations of carbon are
substitutional in silicon since carbon is located in the same column of the periodic table as
silicon and easily replaces a silicon atom. Oxygen exhibits a vibrational mode that absorbs
energy at 1106 cm-1 (wavenumber), that is at a wavelength of about 9 microns; carbon
absorbs energy at 607 cm-1.There are other wavelengths of IR light that are absorbed by the
silicon atoms themselves. By measuring the absorption of a particular wafer at 1106 or 607
cm-1, and comparing this absorption with an oxygen or carbon free reference, the FTIR
technique can be made quantitative.
An IR beam is split by a partially reflecting mirror and then follows two separate paths to the
sample and the detector. For pure silicon, if the movable mirror is translated back and forth at
constant speed, the detected signal will be sinusoidal as the two beams go in and out of phase.
The Fourier transform of this signal will simply be a delta function proportional to the
incident intensity. If the frequency of the source is swept, the Fourier transform of the
resulting signal will produce an intensity spectrum. If we now insert the sample, the resulting
intensity spectrum will change because of absorption of specific wavelengths by the sample.
The benefit of using the Fourier transform method as opposed to simply directly measuring
the intensity spectrum is simply that the signal to noise ratio is improved and as a result, the
detection limit is reduced. With modern instruments, the detection limit for interstitial oxygen
in silicon is about 2x1015/cm3. Carbon can be detected down to about 5x1015/cm3. Oxygen
precipitated into small SiO2 clusters can be detected by FTIR because in the SiO2 form, the
oxygen does not absorb at 1106 cm-1. As the precipitation occurs, the IR absorption at this
wavenumber decreases.
FTIR (Oxygen and Carbon Detection)
An IR beam is split by a partially reflecting mirror and then
follows two separate paths to the sample and the detector. For
pure silicon, if the movable mirror is translated back and forth at
constant speed, the detected signal will be sinusoidal as the two
beams go in and out of phase. The Fourier transform of this
signal will simply be a delta function proportional to the incident
intensity. If the frequency of the source is swept, the Fourier
transform of the resulting signal will produce an intensity
spectrum. If we now insert the sample, the resulting intensity
spectrum will change because of absorption of specific
wavelengths by the sample. The benefit of using the Fourier
transform method as opposed to simply directly measuring the
intensity spectrum is simply that the signal to noise ratio is
improved and as a result, the detection limit is reduced. With
modern instruments, the detection limit for interstitial oxygen in
silicon is about 2x1015/cm3. Carbon can be detected down to
about 5x1015/cm3. Oxygen precipitated into small SiO2 clusters
can be detected by FTIR because in the SiO2 form, the oxygen
does not absorb at 1106 cm-1. As the precipitation occurs, the IR
absorption at this wavenumber decreases.
FTIR (Oxygen and Carbon Detection)
•The CZ crystal growth process introduces oxygen
and carbon into the silicon. These elements are not
inert in the crystal. It is important is to be able to
measure them and to control them. The method is
Fourier Transform Infrared
Spectroscopy or FTIR
• Oxygen in CZ crystals is located in interstitial sites
in the silicon lattice, bonded to two silicon atoms.
• Low concentrations of carbon are substitutional in
silicon since carbon is located in the same column
of the periodic table as silicon and easily replaces a
silicon atom.
FTIR (Oxygen and Carbon Detection)
• Oxygen exhibits a vibrational mode that absorbs
energy at 1106 cm-1 (wavenumber), (~ 9 microns;
• Carbon absorbs energy at 607 cm-1.
• There are other wavelengths of IR light that are
absorbed by the silicon atoms themselves.
• By measuring the absorption of a particular wafer
at 1106 or 607 cm-1, and comparing this absorption
with an oxygen or carbon free reference, the FTIR
technique can be made quantitative.
Schematic of “TEM”
Transmission Electron Microscope
Electron gun
Anode
Liquid N2
Dewar
Condenser lens
X-ray detector
Objective
aperture
Aperture
Sample stage
}
Lenses
Displayed
sample image
CRT
CCD video camera
Fluorescent screen
Detector
Energy-loss
spectrometer
Wavelength of 1 MeV
Electron ~ 1Angstrom
Electron Microscopy (TEM) of SiO2 on Si
Oxygen Contamination in Silicon
Oxygen is the most important impurity found in
silicon. It is incorporated in silicon during the CZ
growth process as a result of dissolution of the quartz
crucible in which the molten silicon is contained.
The oxygen is typically at a level of about 1018 /cm3.
It has recently become possible to use a magnetic
field during CZ growth to control thermal convection
currents in the melt. This slows down the transport of
oxygen from the crucible walls to the growing
silicon interface and reduces the oxygen
concentration in the resulting crystal.
Oxygen in silicon is always present at concentrations
of ~10-20 ppm (5x1017- 1018/cm3) in CZ silicon. The
oxygen can affect processes used in wafer
fabrication such as impurity diffusion.
Oxygen Contamination in Silicon
Oxygen has three principal effects in
the silicon crystal.
(1) In an as-grown crystal, the oxygen is believed to
be incorporated primarily as dispersed single atoms
designated OI occupying interstitial positions in the
silicon lattice, but covalently bonded to two silicon
atoms. The oxygen atoms thus replace one of the
normal Si-Si covalent bonds with a Si-O-Si structure.
The oxygen atom is neutral in this configuration
and can be detected with the FTIR method. Such
interstitial oxygen atoms improve the yield strength
of silicon by as much as 25%, making silicon wafers
more robust in a manufacturing facility.
Oxygen Contamination in Silicon
(2) The formation of oxygen donors. A small amount of
oxygen in the crystal forms SiO4 complexes which act as
donors. They can be detected by changes in the silicon
resistivity corresponding to the free electrons donated by the
oxygen complexes. As many as 1016/cm3 donors can be
formed, which is sufficient to significantly increase the
resistivity of lightly doped P-type wafers. During the CZ
growth process, the crystal cools slowly through ~500oC
temperature and oxygen donors form. The SiO4 complexes
are unstable at temperatures above 500°C and so usually
wafer manufacturers anneal the grown crystal or the wafers
themselves after sawing and polishing, to remove the
oxygen complexes. These donors can reform, however,
during normal IC manufacturing, if a thermal step around
400-500°C is used. Such steps are not uncommon,
particularly at the end of a process flow.
Oxygen Contamination in Silicon
(3) The tendency of the oxygen to
precipitate under normal device processing
conditions, forming SiO2 regions inside the
wafer. The precipitation arises because the
oxygen was incorporated at the melt
temperature and is therefore supersaturated
in the silicon at process temperatures.
Carbon Contamination in Silicon
Carbon is normally present in CZ grown silicon crystals at
concentrations on the order of 1016/cm3.The carbon
comes from the graphite components in the crystal pulling
machine. The melt contains silicon and modest
concentrations of oxygen. This results in the formation of
SiO that evaporates from the melt surface. Generally, the
ambient in the crystal puller is Ar flowing at reduced
pressure, and the SiO can be transported in the gas phase
to the graphite crucible and other support fixtures. SiO
reacts with graphite (carbon) to produce CO that again
transports through the gas phase back to the melt. From
the melt, the carbon is incorporated into the growing
crystal.
Carbon Contamination in Silicon
Four Effects of Carbon on Silicon
(1) Carbon is mostly substitutional in the silicon lattice.
Since it is a column IV element, it does not act as a donor or
acceptor in silicon. Carbon is known to affect the
precipitation kinetics of oxygen in silicon. This is likely
because there is a volume expansion when oxygen
precipitates and a volume contraction when carbon
precipitates because of the relative sizes of O and C. There
is thus a tendency for precipitates that are complexes of C
and O to form at minimum stresses in the crystal. Since
precipitated SiO2 is crucial in intrinsic gettering, this can
have an effect on gettering efficiency.
Carbon Contamination in Silicon
(2) Carbon is also known to interact with
point defects in silicon. Silicon interstitials
tend to displace carbon atoms from lattice
sites, presumably because this can help to
compensate the volume contraction present
when there is carbon in the crystal.
Carbon Contamination in Silicon
(3) Thermal donors (Oxygen Effects) normally
form around 450°C. There is also evidence that if
C is present at ~1 ppm, these donors may also
form at higher temperatures (650-1000°C).
(4) Higher concentrations of C to Si (levels of a
few percent) can change the bandgap of the silicon
and may allow the fabrication of new types of
semiconductor devices in the future.
Carbon Contamination in Silicon
Carbon is normally present in CZ grown silicon crystals at concentrations on the order of
1016/cm3.The carbon comes from the graphite components in the crystal pulling machine. The
melt contains silicon and modest concentrations of oxygen. This results in the formation of SiO
that evaporates from the melt surface. Generally, the ambient in the crystal puller is Ar flowing at
reduced pressure, and the SiO can be transported in the gas phase to the graphite crucible and
other support fixtures. SiO reacts with graphite (carbon) to produce CO that again transports
through the gas phase back to the melt. From the melt, the carbon is incorporated into the
growing crystal.
Four Effects of Carbon on Silicon
(1) Carbon is mostly substitutional in the silicon lattice. Since it is a column IV element, it does
not act as a donor or acceptor in silicon. Carbon is known to affect the precipitation kinetics of
oxygen in silicon. This is likely because there is a volume expansion when oxygen precipitates
and a volume contraction when carbon precipitates because of the relative sizes of O and C.
There is thus a tendency for precipitates that are complexes of C and O to form at minimum
stresses in the crystal. Since precipitated SiO2 is crucial in intrinsic gettering, this can have an
effect on gettering efficiency.
(2) Carbon is also known to interact with point defects in silicon. Silicon interstitials tend to
displace carbon atoms from lattice sites, presumably because this can help to compensate the
volume contraction present when there is carbon in the crystal.
(3) Thermal donors (Oxygen Effects) normally form around 450°C. There is also evidence that
if C is present at ~1 ppm, these donors may also form at higher temperatures (650-1000°C).
(4) Higher concentrations of C to Si (levels of a few percent) can change the bandgap of the
silicon and may allow the fabrication of new types of semiconductor devices in the future.
Chapter Review (Wafer Metrology)
•
•
•
•
•
•
•
•
Microscopic examination for particulates.
Hot Point Probe (wafer doping)
Four Point Probe (wafer resistivity)
Hall Effect (carrier mobility)
FBIR (oxygen and carbon detection)
TEM (atomic resolution of defects / surface)
Effects of Oxygen on IC fabrication
Effects of Carbon on IC fabrication