EBB 323 Semiconductor Fabrication Technology

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EBB 323 Semiconductor Fabrication Technology
Epitaxy
Dr Khairunisak Abdul Razak Room 2.16 School of
Material and Mineral Resources Engineering Univers
iti Sains Malaysia khairunisak_at_eng.usm.my
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Topic outcomes

• At the end of this topic, students should be able
  to
• Name 2 types of epitaxy
• Describe the applications of epitaxial layers
• Explain techniques to produce epitaxial layers
• Describe structure and defects in epitaxial
  layers

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Introduction

• Epitaxy comes from Greek words
• Epi upon
• Taxis ordered
• Epitaxial growth single crystal growth of a
  material in which a substrate serve as a seed
• 2 types of epitaxy
• Homoepitaxy material is grown epitaxially on a
  substrate of the same material. E.g. grow of Si
  on Si substrate
• Heteroepitaxy a layer grown on a chemically
  different substrate. E.g. Si growth on sapphire
• Similar crystal structures of the layer and the
  substrate, BUT
• The shift of composition causes difference in
  lattice parameters
• Limit the ability to produce epitaxial layers of
  dissimilar materials

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Film deposited on a lt111gt oriented wafer ?lt111gt
orientation
The presence of SiO2 Layer cause depositing atoms
have no structure?polysilicon
Epitaxial and polysilicon film growth
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Applications of epitaxial layers

• Discrete and power devices
• Integrated circuits
• Epitaxy for MOS devices

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1. Discrete and power devices

• Technology change junction transistors ?
  diffused planar structure
• Requires a material structure that are not
  achieved by diffusion of dopants from the surface
• Si epitaxy was developed to enhance the
  electrical performance of discrete bipolar
  transistors
• Breakdown voltage of the discrete transistor was
  limited by the field avalanche breakdown of the
  substrate material
• Use higher resistivity substrates produced higher
  breakdown voltages but increased collector series
  resistance
• Structure needed thin, lightly doped and single
  crystal layer of high perfection upon more
  heavily doped Si substrate
• But, the use of a more heavily doped substrate
  reduces the collector series resistance while the
  base-collector breakdown voltage is governed by
  the lighter doping in the near surface region

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• Epitaxial deposition of a lightly doped P
  epitaxial layer on a N substrate make the
  desired properties are achievable
• Epitaxial grows also allows accurate control of
  doping levels and advantages which arises from a
  generally low oxygen and carbon levels in
  epitaxial layer
• Epitaxial technique was developed to 2 and 3
  layers epitaxial structure
• For lightly doped area of collector
• Based region was also grown epitaxially
• E.g. of multilayer structures Si-Controlled
  Rectifier (SCR), Triac, high voltage or high
  power discrete products

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Mesa discrete transistor fabricated in an
epitaxial layer on a heavily doped N substrate
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Transistors
Diodes
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2. Integrated circuit (IC)

• Development of planar bipolar IC caused the
  requirement for devices built on the same
  substrate to be electrically isolated
• The use of opposite typed substrate and epitaxial
  layer met part of the requirement
• Device isolation was completed by the diffusion
  of isolation region through the epitaxial layer
  to contact the substrate between active areas
• In planar bipolar circuits, common to employ a
  heavily doped diffused (or implanted) region
  under the transistor
• Usually called buried layer or DUF for
  diffusion under film
• The buried layer
• serves to lower the lateral series resistance
  between collector area below the emitter and the
  collector contact
• produce uniform planar operation of the emitter,
  avoiding current crowding which leads to hot
  spots near edges of the emitter

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Integrated circuits
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(a) A junction isolated bipolar device fabricated
as part of an integrated circuit using a buried
layer subcollector and a lightly doped
n-epitaxial layer
(b) An N-Well CMOS structure fabricated in a
lightly doped p-epitaxial layer
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3. Epitaxy for MOS devices

• Unipolar devices such as junction field-effect
  transistors (JFETs), VMOS, DRAMs technology also
  use epitaxial structures
• VLSI CMOS (complimentary metal-oxide-semiconductor
  ) devices have been built in thin (3-8 micron)
  lightly doped epitaxial layers on heavily doped
  substrates of the same type (N or P)
• That epitaxial structure reduces the latch up
  of high density CMOS IC by reducing the unwanted
  interaction of closely spaced devices

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Advantages of epitaxy

• Ability to place a lightly oppositely doped
  region over a heavily doped region
• Ability to contour and tailor the doping profile
  in ways not possible using diffusion or
  implantation alone
• Provide a layer of oxygen free material that is
  also contained low carbon

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Techniques for silicon epitaxy

• Chemical Vapour Deposition (CVD)
• Molecular Beam Epitaxy (MEB)
• Liquid Phase Epitaxy (LPE)
• Solid phase regrowth

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1. Chemical Vapour Deposition (CVD)

• The most common technique in Si epitaxy
• In the CVD technique
• Si substrate is heated in a chamber sufficient
  heat to allow the depositing Si atoms to move
  into position to
• Reactive Si containing gaseous compounds are
  introduced
• Gaseous react on the hot surface of the substrate
  and deposit a Si layer
• The deposit will take on Si substrate structure
  if the substrate is atomically clean and the
  temperature is sufficient for atoms to have
  surface mobility

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Schematic drawing of a simple horizontal flow,
cold wall, CVD reactor
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• Schematic CVD reactor geometries for
• True vertical reactor
• Classic horizontal flow reactor
• Modified vertical (or pancake) reactor
• Downflow cylinder reactor

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CVD processes and products
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CVD for silicon devices
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CVD reactions

• Pyrolysis chemical reaction is driven by heat
  alone, e.g. silane decomposes with heating
• SiH4 ? Si 2H2
• Reduction chemical reaction by reacting a
  molecule with hydrogen, e.g. silicon
  tetrachloride- reduction in hydrogen ambient to
  form solid silicon
• SiCl4 2H2 ? Si 4HCl
• Oxidation chemical reaction of an atom or
  molecule with oxygen, e.g. SiH4 decomposes at
  lower temperature
• SiH4 O2 ? SiO2 2H2
• Nitridation chemical process of forming silicon
  nitride by exposing Si wafer to nitrogen at high
  temperature e.g. SiH2Cl2 readily decomposes at
  1050?C
• 3SiH2Cl2 4NH3 ? Si3N4 pH 6H2

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CVD
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CVD film growth steps

• Nucleation
• Dependent on substrate quality
• Occurs at first few atoms or molecules deposit on
  a surface
• Nuclei growth
• Atoms or molecules form islands that grow into
  larger islands
• Island coalescence
• The islands spread , and coalescing into a
  continuous film
• This is the transition stage of the film growth,
  thickness several hundreds Angstroms
• Transition region film possesses different
  chemical and physical properties for thicker bulk
  film
• Bulk growth
• Bulk growth begins after transition film is formed

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CVD film growth steps
Types of film structure
Amorphous
Polycrystalline
Single crystal
Basic CVD subsystem
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• CVD Process steps
• Pre-clean remove particulates and mobile ionic
  contaminants
• Deposition
• Evaluation thickness, step coverage, purity,
  cleanliness and composition

Pre-clean
Deposition
Evaluation
Load wafer into chamber, inert atmosphere
Introduce chemical vapour
Remove vapour
Heat
Flush excess chemical vapour source
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2. Molecular Bean Epitaxy (MBE)

• Uses an evaporation method
• MBE is carried out at a lower temperature than
  1000-1200?C (typical CVD temperature)
• Reduces outdiffusion of local areas of dopant
  diffused into substrates and reduce autodoping
  which is unintentionally transfer of dopant into
  epitaxial layer
• MBE is favourable
• preparation of sub-micron thickness epitaxial
  layers or
• high frequency devices requiring hyper-abrupt
  transition in the doping concentration between
  the epitaxial layer and the substrate

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• In MBE,
• Si and dopant(s) are evaporated in an ultra high
  vacuum (UHV) chamber
• The evaporated atoms are transported at
  relatively high velocity in a straight line from
  the source to the substrate
• They condense on the low temperature substrate
• The condensed atoms of Si or dopant will diffuse
  on the surface until they reach a low energy site
  that they fit well the atomic structure of the
  surface
• The adatom then bonds in that low energy site,
  extending the underlying crystal by a vapour to
  solid phase crystal growth
• Usual temperature range of the substrate is
  400-800?C. Higher than 800?C is possible but it
  will increase outdiffusion or lateral diffusion
  of dopants in the substrate

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Schematic drawing of a molecular beam epitaxial
system
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• Insitu cleaning of the substrate
• Can be done by high temperature bake at
  1000-1250?C for several minutes under high vacuum
  to decompose the native surface oxide and to
  remove other surface contaminants
• Other technique is by using a low energy beam of
  inert gas to sputter clean the substrate
• Difficult to remove carbon but will decrease at
  the surface by diffusion into the substrate
  during short anneal at 800-900?C
• Wider range of dopants for MBE than CVD epitaxy
• Typical dopants Antimony, Sb (N-type), aluminum,
  Al or gallium (Ga) for P-type
• N-type dopant As and P, evaporate rapidly even
  at 200?C. Difficult to control
• P-type dopant Boron, evaporate slowly even at
  1300?C

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Schematic drawing of a multiple chamber MBE system
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MBE Equipment
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Liquid Phase Epitaxy (LPE)

• LPE technique is widely used for preparation of
  epitaxial layers on compound semiconductors and
  for magnetic bubble memory films on garnet
  substrate
• In films growth by LPE from solution melts, low
  cooling rates, when the surface reaction (growth)
• Kinetics are rapid compare to the mass transport
  of Si to the seed, epitaxial layer thickness will
  vary in proportion to the temperature drop
• Increase cooling rates, mass transport rate will
  increase and the growth rate will increase with
  cooling rate until growth rate becomes limited by
  surface reaction kinetics

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• Growth rate increases with cooling rate up to
  about 1 degree/min while growth rate above 2
  degree/min occurred under kinetically limited
  conditions

LPE growth rate increasing with cooling rate up
to about 1 micron per minute
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Schematic drawing of a typical silicon liquid
phase epitaxy (LPE)
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• Fabrication sequence for a vertical channel field
  effect transistor
• N and N epitaxial structure can be built using
  liquid or vapour phase epitaxial growth
• Preferential etching can be used to open areas
  part way through the N type epitaxial layer
• In this figure, LPE is used to fill the etched
  out gate areas which control current flow
  vertically from the top side source to the N
  substrate drain region

Schematic of fabrication steps in the fabrication
vertical field effect transistors by etch and LPE
refill techniques
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4. Solid Phase Re-growth

• Re-growth of amorphous layers
• Surface layers subjected to high dose ion
  implants are in amorphous structure due to the
  heavy damage inflicted on the lattice as the
  energetic ions are absorbed
• Annealing above 600?C ? amorphous layer
  re-crystallize
• Re-crystallisation occurs from interface moves
  toward the surface and results in solid phase
  epitaxial re-growth

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• Re-crystallisation of thin films
• Involves re-crystallisation of a deposited
  amorphous or polysilicon film
• Si film is deposited on a Si substrate or more
  commonly SiO2 ? heated using a strip heater
  passed over the surface or by a scanned pulsed
  laser to crystallise the film to single crystal
  or large grain polysilicon
• This fabrication technique is used to produce a
  stacked n-channel device in re-crystallised
  polysilicon on a thermally grown or deposited
  oxide
• Oriented epitaxial growth can be obtained by
  making series of holes in the oxide to allow
  points of contact between the underlying
  substrate and the deposited polysilicon
• The contact points become seeds areas for
  establishing re-growth orientation

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Re-crystallisation solid phase epitaxy using a
moving strip heater
A stacked MOS structure over an insulating oxide
fabricated in a re-crystallised polysilicon layer
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Structure and defects in epitaxial layer

• Surface morphology of Silicon epitaxial deposits
  is affected by growth and substrate parameters
• Growth parameters
• Temperature
• Pressure
• Concentration of Si containing gas
• Cl H2 ratio
• Substrates parameters
• Substrate orientation
• Defects in the substrate
• Contaminants on the surface of the substrate

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Typical defects in epitaxial layers

• Substrate orientation effects
• Spikes and epitaxial stacking faults
• Hillocks and pyramids in epitaxial layers
• Dislocations and slip
• Microprecipitates (S-pits)

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1. Substrate orientation effect

• Growth of smooth epitaxial films can be obtained
  on (100) and (110) oriented Si substrates
• Epitaxial growth on substrate surface on oriented
  on (111) plane results in facetted alligator
  skin surface
• (111) surfaces contain no atomic steps to provide
  a density of growth sites
• Without atomic steps, the growth produces
  pyramids and terraces
• Misorientation of the surface by ? 0.5 degree
  introduces a sufficient density of steps for
  growth of smooth planar films

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2. Spikes and epitaxial stacking faults

• Growth spike
• Originate from Si particle on the surface not
  removed by the pre-epitaxial cleaning process
• Si Chips may expose faster growing crystal planes
  than the plane of the substrate
• Chips nucleate and produce polysilicon nodule.
  The chips then protrude above the substrates
  surface into a region of richer supply of gaseous
  reactants
• Results in nodule grows at 2-10 times the rate of
  epitaxial film on the substrate.
• May be removed mechanically before the next step
  but will leave a region unusable for functional
  materials

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• Epitaxial stacking faults
• Crystallographic in nature and arise from defects
  in atomic arrangement during film growth
• Could result from an extra atomic layer
  (extrinsic fault) or a missing atomic layer
  (intrinsic fault) along 111 type plane

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Epitaxial growth spike
Stacking fault on lt111gt Si
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3. Hillocks and pyramids in epitaxial layers

• Hillocks Small oval mounds on the surface of the
  epitaxial
• Pyramids Faceted regions on the epitaxial
  surface
• Density of hillocks and pyramids is dependent on
  growth parameters such as type and concentration
  of Si source and deposition temperature

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4. Dislocations and slip

• Non-uniform heating of a substrate results in
  non-uniform thermal expansion of the substrate
  which produces elastic stresses
• The thermal stress can cause bowing which may
  lift the edge of the substrate away from the
  substrate in response to the thermal stress
• At lower temperature (lt 900?C) the yield point of
  the Si lattice is sufficiently high that the
  substrate behaves elastically. During cooling,
  the thermal stress is removed and the substrate
  returns to its original shape
• If the stress exceeds a critical values, the
  substrate will yield plastically ? occurs due to
  generation and motion of dislocations which are
  atomic level line defects which glide along slip
  planes of the crystal

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• The passage of one dislocation offsets the
  material above and below the slip plane by a unit
  known as Bergers vector of the dislocations
• Dislocations normally propagate from near the
  edge of the substrate (highest stress), and glide
  towards the centre of the substrate and produce
  plastic deformation of the substrate which
  relieves the thermal stress
• Dislocation motions is slow because dislocation
  moves to a region of lower shear stress
• The continuous slow motion of the dislocations
  produces creep deformation of the crystal
• Device impact from slip normally comes from rapid
  pipe diffusion of dopant along the core of the
  dislocations

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Typical wafer edge slip as a result of excessive
within wafer temperature gradients during heating
or during epitaxial film growth
Crystal slip
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5. Microprecipitates (S-pits)

• Microprecipitates may come from metallic
  elements such as copper, nickel, iron and
  chromium
• This is due to their solubility in Si at high
  temperatures and fast diffusion rates through the
  Si
• The metal contaminants may exist in the starting
  substrates or being pick up during handling in
  the loading operation or from metal parts or
  susceptors within the epitaxial reactor itself

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