Silicon Crystal Structure and Growth

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Silicon Crystal Structure and Growth(Plummer -
Chapter 3)
2
Atomic Order of a Crystal Structure
Figure 4.2
3
Amorphous Atomic Structure
Figure 4.3
4
Unit Cell in 3-D Structure
Figure 4.4
5
Miller Indices of Crystal Planes
Figure 4.9
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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).

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Silicon has the basic diamond crystal structure
two merged FCC cells offset by a/4 in x, y and
z.
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Faced-centered Cubic (FCC) Unit Cell
Figure 4.5
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Silicon Unit Cell FCC Diamond Structure
Figure 4.6
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Basic FCC Cell
Merged FCC Cells
Omitting atoms outside Cell
Bonding of Atoms
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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)
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Point Defects
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Semiconductor-Grade Silicon
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Czochralski (CZ) crystal growing
Si is purified from SiO2 (sand) by refining,
distillation and CVD. It contains lt 1 ppb
impurities. Pulled crystals contain O (1018
cm-3) and C (1016 cm-3), plus dopants placed in
the melt.
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CZ Crystal Puller
Figure 4.10
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• 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

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Silicon Ingot Grown by CZ Method
Photograph courtesy of Kayex Corp., 300 mm Si
ingot
Photo 4.1
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An alternative process is the Float Zone
process which can be used for refining or single
crystal growth.
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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)
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Float Zone Crystal Growth
Figure 4.11
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Dopant Concentration Nomenclature
Table 4.2
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Segregation Fraction for FZ Refining
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Basic Process Steps for Wafer Preparation
Figure 4.19
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Ingot Diameter Grind
Figure 4.20
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Internal Diameter Saw
Figure 4.23
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After crystal pulling, the boule is shaped and
cut into wafers which are then polished on one
side.
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Wafer Notch and Laser Scribe
Figure 4.22
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Polished Wafer Edge
Figure 4.24
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Chemical Etch of Wafer Surface to Remove Sawing
Damage
Figure 4.25
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Wafer Dimensions Attributes
Table 4.3
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Increase in Number of Chips on Larger Wafer
Diameters(Assume large 1.5 x 1.5 cm
microprocessors)
Figure 4.13
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Developmental Specifications for 300-mm Wafer
Dimensions and Orientation
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
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Wafer Polishing
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Double-Sided Wafer Polish
Figure 4.26
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Improving Silicon Wafer Requirements
Adapted from K. M. Kim, Bigger and Better CZ
Silicon Crystals, Solid State Technology
(November 1996), p. 71.

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Quality Measures

• Physical dimensions
• Flatness
• Microroughness
• Oxygen content
• Crystal defects
• Particles
• Bulk resistivity

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Backside Gettering to Purify Silicon
Polished 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.
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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.

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Measurement of Wafer CharacteristicsDarkfield
and Brightfield Detection
Figure 7.15
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Schematic of Optical System
Figure 7.16
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Principle of Confocal Microscopy
Figure 7.17
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Particle Detection by Light Scattering
Figure 7.18
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Measurement of Wafer Characteristics
Hot Point Probe

• 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 25-100C 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.

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Basic principle of the hot probe, illustrated for
an N-type sample, for determining N- or P-type
behavior in semiconductors.
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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.

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Four-point probe measurement method. The outer
two probes force a current through the sample
the inner two probes measure the voltage drop.
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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.

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Four Point Probe
Figure 7.3
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Van der Pauw Sheet Resistivity(similar to
4-point probe, but uses shapes on wafer)
Figure 7.4
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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.

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Conceptual representation of Hall effect
measurement. The right sketch is a top view of a
more practical implementation.
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Fourier Transform Infrared Spectroscopy (FTIR)
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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.

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Schematic of TEM Transmission Electron
Microscope
Wavelength of 1 MeV Electron 1Angstrom
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Electron Microscopy (TEM) of SiO2 on Si
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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
  500C 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 400-500C 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.

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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 450C. There is also evidence that if C is
  present at 1 ppm, these donors may also form at
  higher temperatures (650-1000C).
• (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.

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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