Silicon Crystal Structure and Growth
(Plummer - Chapter 3)
Amorphous Atomic Structure
Figure 4.3
Atomic Order of a Crystal Structure
Figure 4.2
Unit Cell in 3-D Structure
Unit cell
Figure 4.4
Unit cell forms
Faced-centered Cubic (FCC) Unit Cell
Figure 4.5
Silicon Crystal Structure
•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).
Crystals are characterized by a unit cell which repeats in the x, y, z directions.
Miller Indices of Crystal Planes
Z
X
Y
(100)
Z
X
Y
(110)
Z
X
Y
(111)
Figure 4.9
Silicon Unit Cell: FCC Diamond Structure
Figure 4.6
Basic FCC Cell Merged FCC Cells
Omitting atoms outside Cell Bonding of Atoms
Silicon has the basic diamond crystal structure – two merged FCC cells offset by a/4 in x, y and z.
Crystal Orientations in IC Fab.
There are two major principal Silicon crystal orientations that are used in manufacturing IC , (1 1 1) and ( 1 0 0 ).
The surface terminations on can make a difference in the surface electrical and physical properties.
CMOS-Based Tech. crystals uses ( 1 0 0 ).Bipolar-Based Tech. crystals historically
used ( 1 1 0 ) but recently uses ( 1 0 0 ).
Crystal Orientation Benchmark
<1 1 1> < 1 0 0 > Orientation
Largest Lowest Atom`s density
Fastest Slow Oxidation Rate
Easier -- Growing facility
Most -- Defects
111 Orientation
100Orientation
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.
(Extra line of atoms)
Dislocations
Misplacement of the unit cells in a single crystal.Caused by: Growth conditions, Lattice strain and
physical abuse during fabrication.Typical value: 200-1000 per square centimeters.
Major Defects
Vacancy: atom missing from a location in the structure.
A natural phenomenon in every crystal.
Occurs when a crystal or wafer is heated and cooled suddenly.
Minimization of vacancies: desire for low temperature processing.
Vacancy defect
Interstitial defect
Contaminants jammed in the crystal structure .
Semiconductor-Grade Silicon
Steps to Obtaining Semiconductor Grade Silicon (SGS)
Step Description of Process Reaction
1 Produce metallurgical grade silicon (MGS) by heating silica with carbon
SiC (s) + SiO2 (s) Si (l) + SiO(g) + CO (g)
2
Purify MG silicon through a chemical reaction to produce a silicon-bearing gas of trichlorosilane (SiHCl3)
Si (s) + 3HCl (g) SiHCl3 (g) + H2 (g) + heat
3
SiHCl3 and hydrogen react in a process called Siemens to obtain pure semiconductor- grade silicon (SGS)
2SiHCl3 (g) + 2H2 (g) 2Si (s) + 6HCl (g)
• 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.
Czochralski (CZ) crystal growing
Crystal seed
Molten polysilicon
Heat shield
Water jacket
Single crystal silicon
Quartz crucible
Carbon heating element
Crystal puller and
rotation mechanism
CZ Crystal Puller
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.(See the Plummer for models of this process)
Float Zone Crystal Growth
RF
Gas inlet (inert)
Molten zone
Traveling RF coil
Polycrystalline rod (silicon)
Seed crystal
Inert gas out
Chuck
Chuck
Figure 4.11
Dopant Concentration Nomenclature
Concentration (Atoms/cm3)
Dopant Material
Type < 1014
(Very Lightly Doped)
1014 to 1016
(Lightly Doped) 1016 to 1019
(Doped) >1019
(Heavily Doped)
Pentavalent n n-- n- n n+ Trivalent p p-- p- p p+
Table 4.2
Segregation Fraction for FZ Refining
Crystal GrowthCrystal Growth
ShapingShaping
Wafer SlicingWafer Slicing
Wafer Lapping and Edge
Grind
Wafer Lapping and Edge
Grind
EtchingEtching
PolishingPolishing
CleaningCleaning
InspectionInspection
PackagingPackaging
Basic Process Steps for Wafer Preparation
Figure 4.19
Flat grind
Diameter grind
Preparing crystal ingot for grinding
Ingot Diameter Grind
Figure 4.20
Internal diameter
wafer saw
Internal Diameter 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
Table 4.3
Diameter (mm)
Thickness (m)
Area (cm2)
Weight (grams/lbs)
Weight/25 Wafers (lbs)
150 675 20 176.71 28 / 0.06 1.5 200 725 20 314.16 53.08 / 0.12 3 300 775 20 706.86 127.64 / 0.28 7 400 825 20 1256.64 241.56 / 0.53 13
88 die200-mm wafer
232 die300-mm wafer
Increase in Number of Chips on Larger Wafer Diameters(Assume large 1.5 x 1.5 cm microprocessors)
Figure 4.13
Developmental Specifications for 300-mm Wafer Dimensions and Orientation
Parameter Units NominalSome Typical
Tolerances
Diameter mm 300.00 0.20
Thickness(center point)
m 775 25
Warp (max) m 100
Nine-Point ThicknessVariation (max)
m 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
Table 4.4
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.
Wafer Polishing
Double-Sided Wafer Polish
Upper polishing pad
Lower polishing pad
Wafer
Slurry
Figure 4.26
Improving Silicon Wafer Requirements
Year(Critical Dimension)
1995(0.35 m)
1998(0.25 m)
2000(0.18 m)
2004(0.13 m)
Wafer diameter(mm)
200 200 300 300
Site flatnessA (m)Site size (mm x mm)
0.23(22 x 22)
0.17(26 x 32)
0.1226 x 32
0.0826 x 36
MicroroughnessB of frontsurface (RMS)C (nm)
0.2 0.15 0.1 0.1
Oxygen content(ppm)D
24 2 23 2 23 1.5 22 1.5
Bulk microdefectsE
(defects/cm2) 5000 1000 500 100
Particles per unit area(#/cm2)
0.17 0.13 0.075 0.055
EpilayerF thickness( % uniformity) (m) 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 dimensionsFlatnessMicroroughnessOxygen content Crystal defectsParticlesBulk resistivity
“Backside Gettering” to Purify SiliconPolished Surface
Backside Implant: Ar (50 keV, 1015/cm2)
The argon amorphizes the back side of the silicon. The 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. The 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.
Measurement of Wafer CharacteristicsDarkfield and Brightfield Detection
Brightfield imaging
Two-way mirror
Light source
Lens
Viewing optics
Viewing optics
Darkfield imaging
Light s
ourceLens
Light reflected by surface irregularities
Figure 7.15
Schematic of Optical System
Phase and intensity detection
Phase and intensity detection
Data generation, processing, display are networked with factory management software
Data generation, processing, display are networked with factory management software
Lens
Light source
Video camera
CRT
Photo detector array
Objective lens assembly
Viewing optics
Split mirror
Vibration isolation pad
Wafer positioning stage
Three-axis piezo substage
Figure 7.16
DetectorPinhole
Wafer is driven up and down along Z-axis
Laser
Pinhole
Beam splitter
Objective lens
Center of focus+Z
-Z0
Principle of Confocal Microscopy
Figure 7.17
Particle Detection by Light Scattering
Incident lightBeam
scanning
Photo detector
Particle
Wafer motion Scattered light
Reflected light
Detection of scattered
light
Figure 7.18
Measurement of Wafer Characteristics
The hot point probe is a simple and reliable means to determine whether a wafer is N or P
type is the Hot Point Probe. The basic operation of this probe is illustrated in the
next slide. Two probes make ohmic contact with the wafer surface. One is heated 25-100°C hotter than the other. A voltmeter placed across the probes will measure a
potential difference whose polarity indicates whether the material is N or P type.
Hot Point Probe
Basic principle of the hot probe, illustrated for an N-type sample, for determining N- or P-type behavior in semiconductors.
Hot Point Probe
Consider an N-type sample. The majority carriers are electrons. At the hot probe, the thermal energy of the
electrons is higher than at the cold probe so the electrons will tend to diffuse away from the hot probe, driven by the
temperature gradient. If a wire were connected between the hot and cold probes, this would result in a measurable
current, whose direction would correspond to the electrons moving right to left. (The current by definition would be in
the opposite direction.) If we place a high-impedance voltmeter between the probes, no current flows, but a potential difference is measured, as illustrated. As the
electrons diffuse away from the hot probe, they leave behind the positively charged, immobile donor atoms that provided the electrons. The negatively charged mobile electrons tend to build up near the cold probe. This results in the hot probe
becoming positive with respect to the cold probe. By a similar set of arguments, if the material were P type, positively
charged holes would be the majority carriers and the polarity of the induced voltage would be reversed. The direction of
the current between the two probes would also be reversed in P-type material, if they were shorted with a wire. Thus a
measurement of either the short-circuit current or the open circuit voltage tells us the type of the material.
“Four-point probe” measurement method. The outer two probes force a current through the sample; the inner two probes measure the voltage drop.
Measurement of Sheet Resistance
The most common method of measuring the wafer resistivity is with the four-point
probe. We measure the sample resistance by measuring the current that flows for a given applied voltage. This could be done
with just two probes. However, in that case, contact resistances associated with
the probes and current spreading problems around the probes are important and are not easily accounted for in the analysis. Using four probes allows us to force the
current through the two outer probes, where there will still be contact resistance
and current spreading problems, but we measure the voltage drop with the two
inner probes using a high-impedance voltmeter. Problems with probe contacts
are thus eliminated in the voltage measurement since no current flows
through these contacts.
Four Point Probe
Figure 7.3
Wafer
R
Voltmeter
Constant current source
V
Irs =
V
Ix 2ps (ohms-cm)
“Van der Pauw” Sheet Resistivity(similar to 4-point probe, but uses shapes on wafer)
I
(a)
(c) (d)
ContactConductive material
V(b)
Figure 7.4
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 .
The technique is more powerful than the sheet resistance method described above because it
can determine the material type, carrier concentration and carrier mobility separately.
The basic method is illustrated in the next slide. The left part of the figure defines the
reference directions and the various currents, fields and voltages; the right part of the figure
illustrates a top view of a practical geometry that is often used in semiconductor
applications.
Conceptual representation of Hall effect measurement. The right sketch is a top view of a more practical implementation.
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.
Schematic of “TEM” Transmission Electron Microscope
}
Energy-loss spectrometer
Aperture
Sample stage
Detector
CCD video camera
Fluorescent screen
CRT
Condenser lens
Anode
Lenses
Electron gun
X-ray detector
Objective aperture
Displayed sample image
Liquid N2 Dewar
Wavelength of 1 MeVElectron ~ 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 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 .)2( The formation of oxygen donors. A small amount of the 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 .)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 .
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 fabricationEffects of Carbon on IC fabrication
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