EBB 323 Semiconductor Fabrication Technology

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EBB 323 Semiconductor Fabrication Technology
Oxidation
Dr Khairunisak Abdul Razak Room 2.03 School of
Material and Mineral Resources Engineering Univers
iti Sains Malaysia khairunisak_at_eng.usm.my
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Outcomes

• By the end of this topic, students should be able
  to
• List principle uses of silicon dioxide (SiO2)
  layer in silicon devices
• Describe the mechanism of thermal oxidation
• Draw a flow diagram of a typical oxidation
  process
• Describe the relationship of process time,
  pressure, and temperature on the thickness of a
  thermally grown SiO2 layer
• Explain the kinetics of oxidation process
• Describe the principle uses of rapid thermal,
  high pressure and anodic oxidation

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Uses of dielectric films in Semiconductor
technology
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• What is oxidation?
• Formation of oxide layer on wafer
• High temperature
• O2 environment

• Principle uses of Si dioxide (SiO2) layer in Si
  wafer devices
• Surface passivation
• Doping barrier
• Surface dielectric
• Device dielectric

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1. Surface passivation

• SiO2 layer protect semiconductor devices from
  contamination
• Physical protection of the sample and underlying
  devices
• Dense and hard SiO2 layer act as contamination
  barrier Hardness of the SiO2 layer protect the
  surface from scratches during fabrication process

SiO2 passivation layer
Si
Si
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Cont..

• ii. Chemical in nature
• Avoid contamination from electrically active
  contaminants (mobile ionic contaminants) of the
  electrically active surface
• e.g. early days, MOS device fabrication was
  performed on oxidised Si? remove oxide layer to
  get rid of the unwanted ionic contamination
  surface before further processing

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2. Doping barrier

• In doping ? need to create holes in a surface
  layer in which specific dopants are introduced
  into the exposed wafer surface through diffusion
  or ion implantation
• SiO2 on Si wafer block the dopants from reaching
  Si surface
• All dopants have slower rate of movement in SiO2
  compared to Si
• Relatively thin layer of SiO2 is required to
  block the dopants from reaching SiO2

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Cont..

• SiO2 possesses a similar thermal expansion
  coefficient with Si
• At high temperature oxidation process, diffusion
  doping etc, wafer expands and contracts when it
  is heated and cooled
• ? close thermal expansion coefficient, wafer does
  not warp

Dopants
Si
SiO2 layer as dopant barrier
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3. Surface dielectric

• SiO2 is a dielectric ? does not conduct
  electricity under normal circumstances
• SiO2 layer prevents shorting of metal layer to
  underlying metal
• Oxide layer
• MUST BE continuous no holes or voids
• Thick enough to prevent induction
• If too thin SiO2 layer, electrical charge in
  metal layer cause a build-up charge in the wafer
  surface ? cause shorting!!
• Thick enough oxide layer to avoid induction
  called field oxide

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Metal layer
Oxide layer
Wafer
Dielectric use of SiO2 layer
source
Drain
S
D
MOS gate
Field oxide
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4. Device dielectric

• In MOS application
• Grow thin layer SiO2 in the gate region
• Oxide function as dielectric in which the
  thickness is chosen specifically to allow
  induction of a charge in the gate region under
  the oxide
• Thermally grown oxides is also used as dielectric
  layer in capacitors
• Between Si wafer and conduction layer

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Types of oxidation

• Thermal oxidation
• High pressure oxidation
• Anodic oxidation

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Device oxide thicknesses

• Most applications of semiconductor are dependent
  on their oxide thicknesses

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Thermal oxidation mechanisms

• Chemical reaction of thermal oxide growth
• Si (solid) O2 (gas) ? SiO2 (solid)
• ?
• Oxidation temperature 900-1200?C
• Oxidation Si wafer ? placed in a heated chamber
  ? exposed to oxygen gas

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SiO2 growth stages
Si wafer
Initial

• In a furnace with O2 gas environment

• Oxygen atoms combine readily with Si atoms
• Linear- oxide grows in equal amounts for each
  time
• Around 500Å thick

Si wafer
Linear

• Above 500Å, in order for oxide layer to keep
  growing, oxygen and Si atoms must be in contact
• SiO2 layer separate the oxygen in the chamber
  from the wafer surface
• Si must migrate through the grown oxide layer to
  the oxygen in the vapor
• oxygen must migrate to the wafer surface

Si wafer
Parabolic
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Three dimension view of SiO2 growth by thermal
oxidation
SiO2 surface
Original SiO2 surface
SiO2
Si substrate
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• Linear oxidation
• Parabolic oxidation of silicon
• where X oxide thickness, B parabolic rate
  constant, B/A linear rate constant, t
  oxidation time
• Parabolic relationship of SiO2 growth parameters
• where R SiO2 growth rate, X oxide thickness,
  t oxidation time

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Cont..

• Implication of parabolic relationship
• Thicker oxides need longer time to grow than
  thinner oxides
• 2000Å, 1200?C in dry O2 6 minutes
• 4000Å, 1200?C in dry O2 220 minutes (36 times
  longer)
• Long oxidation time required
• Dry O2
• Low temperature

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Dependence of silicon oxidation rate constants on
temperature
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Oxide thickness vs oxidation time for silicon
oxidation in dry oxygen at various temperatures
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Oxide thickness vs oxidation time for silicon
oxidation in pyrogenic steam ( 640 Torr) at
various temperatures
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Kinetics of growth

• Si is oxidised by oxygen or steam at high
  temperature according to the following chemical
  reactions
• Si (solid) O2 (gas) ? SiO2 (solid) (dry
  oxidation)
• Or
• Si (solid) 2H2O (gas) ? SiO2 (solid) 2H2(gas)
  (wet oxidation)
• Also called steam oxidation, wet oxidation,
  pyrogenic steam
• Faster oxidation OH- hydroxyl ions diffuses
  faster in oxide layer than dry oxygen
• 2H2 on the right side of the equation shows H2
  are trapped in SiO2 layer
• Layer less dense than oxide layer in dry
  oxidation
• Can be eliminated by heat treatment in an inert
  atmosphere e.g. N2

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• 2 mechanisms influence the growth rate of the
  oxide
• Actual chemical reaction rate between Si and O2
• Diffusion rate of the oxidising species through
  an already grown oxide layer
• No or little oxide on Si the oxidising agent
  easily reach the Si surface
• Factor that determine the growth rate is kinetics
  of the silicon-oxide chemical reaction
• Si is already covered by a sufficiently thick
  layer of oxide
• Oxidation process is mass-transport limited
• Diffusion rate of O2 and H2O through the oxide
  limit the growth rate is diffusion of O2 and H2O
  through the oxide
• A steam ambient is preferred for the growth of
  thick oxidesH2O molecules smaller than O2 thus,
  easier diffuse through SiO2 that cause high
  oxidation rates

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Si oxidation
Oxygen concentration profile during oxidation
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• Mass transport of O2 molecules from gas ambient
  towards the Si through a layer of already grown
  oxide
• Flux of O2 molecules is proportional to the
  differential in O2 concentration between the
  ambient (C) and oxide surface (C0)
• Where h is the mass transport coefficient for O2
  in the gas phase
• Diffusion of O2 through the oxide is proportional
  to the difference of oxygen concentration between
  the oxide surface and the Si/SiO2 interface. The
  flux of oxygen through the oxide, F2 becomes
• Where,
• Ci oxygen concentration at theSi/SiO2
  interface
• D diffusion coefficient of O2 or H2O in oxide
• tox oxide thickness

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• Kinetics of the chemical reaction between silicon
  and oxygen is characterised by reaction constant,
  k
• In steady state, all flux terms are equal F1
  F2 F3. Eliminating C0 from the flux equations,
  we can obtain

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• If N0x is a constant representing the number of
  oxidising gas molecules necessary to grow a unit
  thickness of oxide, one can write
• The solution to this differential equation is

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• If tox0 when t0, th eintegration yields
• Or
• Defining new constant A and B in terms of D, ks,
  Nox and C
• We can obtain
• From which we find tox

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• ? is introduced to take into account the possible
  presence of an oxide layer on the Si before
  thermal oxide growth being carry out
• Oxide layer can be a native oxide layer due to
  oxidation of bare Si by ambient air or thermally
  grown oxide produced during a prior oxidation
  step
• ?0 if the thickness of the initial oxide is
  equal to zero
• When thin oxides are formed the growth rate is
  limited by the kinetics of chemical reaction
  between Si and O2.
• Eq. 5.12 becomes
• Which is linear with time.
• The ratio is called linear growth
  coefficient, and is dependent on crystal
  orientation of Si

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• When thick oxides are formed, the growth rate is
  limited by the diffusion rate of oxygen through
  the oxide. Eq 5.12 becomes
• The coefficient B is called parabolic growth
  coefficient and is independent on crystal
  orientation of Si.
• The parabolic growth coefficient can be
  increased
• Increase the pressure of the ambient oxygen up
  to 10-20 atm (high pressure oxidation)
• The linear growth coefficient can be increased
• Si consists of high concentration of impurities
  e.g. phosphorous increase point defects in the
  crystal which increase the oxidation reaction
  rate at the Si/SiO2 interface
• Oxidation process also generate point defects in
  Si which enhance diffusion of dopants. Some
  dopants diffuse faster when annealed in oxidising
  ambient than in neutral gas such at N2

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Oxidation rate

• Controlled by
• Wafer orientation
• Wafer dopant
• Impurities
• Oxidation of polysilicon layers
• Wafer orientation
• Large no of atoms allows faster oxide growth
• lt111gt plane have more Si atoms than lt100gt plane
• Faster oxide growth in lt111gt Si
• More obvious in linear growth stage and at low
  temperature

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Crystal structure of silicon
lt100gt plane
lt111gt plane
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Dependence of oxidation linear rate constant and
oxide fixed charge density on silicon orientation
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• Wafer dopant(s) distribution
• Oxidised Si surface always has dopants N-type or
  P-type
• Dopant may also present on the Si surface from
  diffusion or ion implantation
• Oxidation growth rate is influenced by dopant
  element used and their concentration e.g.
• Phosphorus-doped oxide less dense and etch
  faster
• Higher doped region oxidise faster than lesser
  doped region e.g. high P doping can oxidise 2-5
  times the undoped oxidation region
• Doping induced oxidation effects are more obvious
  in the linear stage oxidation

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Schematic illustration of dopant distribution as
a function of position is the SiO2/Si structure
indicating the redistribution and segregation of
dopants during silicon thermal oxidation
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• Distribution of dopant atoms in Si after
  oxidation is completed
• During thermal oxidation, oxide layer grows down
  into Si wafer- behavior depends on conductivity
  type of dopant
• N-type higher solubility in Si than SiO2, move
  down to wafer. Interface consists of high
  concentration N-type doping
• P-type opposite effect occurs e.g Boron doping
  in Si move to SiO2 surface causes B pile up in
  SiO2 layer and depletion in Si wafer ? change
  electrical properties

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• Oxide impurities
• Certain impurities may influence oxidation rate
• e.g. chlorine from HCl from oxidation atmosphere
  ? increase growth rate 1-5

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• Oxidation of polysilicon
• Oxidation of polysilicon is essential for
  polysilicon conductors and gates in MOS devices
  and circuits
• Oxidation of polysilicon is dependent on
• Polisilicon deposition method
• Deposition temperature
• Deposition pressure
• The type and concentration of doping
• Grain structure of polysilicon

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Thermal oxidation method

• Thermal oxidation ? energy is supplied by heating
  a wafer
• SiO2 layer are grown
• Atmospheric pressure oxidation ? oxidation
  without intentional pressure control
  (auto-generated pressure) also called
  atmospheric technique
• High pressure oxidation ? high pressure is
  applied during oxidation
• 2 atmospheric techniques
• Tube furnace
• Rapid thermal system

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Oxidation methods
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Horizontal tube furnace

• Quartz reaction tube reaction chamber for
  oxidation
• Muffle heat sink, more even heat distributing
  along quartz tube
• Thermocouple placed close to quartz tube. Send
  temp to band controller
• Controller send power to coil to heat the
  reaction tube by radiation/conduction
• Source zone- heating zone

Place the sample
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Horizontal tube furnace

• Integrated system of a tube furnace consists of
  several sections
• Reaction chamber
• Temperature control system
• Furnace section
• Source cabinet
• Wafer cleaning station
• Wafer load station
• Process automation

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Vertical tube furnaces

• Small footprint
• Maybe placed outside the cleanroom with only a
  load station door opening into the cleanroom
• Disadvantage expensive

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Rapid Thermal Processing

• Based on radiation principle heating
• Useful for thin oxides used in MOS gates
• Trend in device miniaturisation requires
  reduction in thickness of thermally grown gate
  oxides
• lt 100Å thin gate oxide
• Hard to control thin film in conventional tube
  furnace
• Problem quick supply and remove O2 from the
  system

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• RTP system able to heat and cool the wafer
  temperature VERY rapidly
• RTP used for oxidation is known as Rapid Thermal
  Oxidation (RTO)
• Have O2 atmosphere
• Other processes use RTP system
• Wet oxide (steam) growth
• Localised oxide growth
• Source/ drain activation after ion implantation
• LPCVD polysilicon, amorphous silicon, tungsten,
  silicide contacts
• LPCVD nitrides
• LPCVD oxides

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RTP design
e.g. RTP time/temperature curve
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High Pressure Oxidation

• Problems in high temperature oxidation
• Growth of dislocations in the bulk of the wafer ?
  dislocations cause device performance problems
• Growth of hydrogen-induced dislocations along the
  edge of opening ? surface dislocations cause
  electrical leakage along the surface or the
  degradation of silicon layers grown on the wafer
  for bipolar circuits
• Solve low temperature oxidation BUT require a
  longer oxidation time

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• High pressure system ? similar to conventional
  horizontal tube furnace with several features
• Sealed tube
• Oxidant is pumped into the tube at pressure 10-25
  atm
• The use of a high pressure requires encasing the
  quartz tube in a stainless steel jacket to
  prevent it from cracking
• High pressure oxidation results in faster
  oxidation rate
• Rule of thumb 1 atm causes temperature drop of
  30?C
• In high pressure system, temperature drop of
  300-750?C
• ? This reduction is sufficient to minimise the
  growth of dislocations in and on the wafers

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• Advantage of high pressure oxidation
• Drop the oxidation temperature
• Reduce oxidation time
• Thin oxide produced using high pressure oxidation
  ? higher dielectric strength than oxides grown at
  atmospheric pressure

High pressure oxidation
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Oxidant sources

• Dry oxygen
• Water vapor sources
• Bubblers/ flash
• Dry oxidation
• Chlorine added oxidation

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1. Dry oxygen

• Oxygen gas must dry ? not contaminated by water
  vapor
• If water present in the oxygen
• Increase oxidation rate
• Oxide layer out of specification
• Dry oxygen is preferred for growing very thin
  gate oxides 1000Å

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2a. Bubblers

• Bubbler liquid DI water heated close to boiling
  point 98-99?C
• create a water vapor in the space above liquid
• When carrier gas is bubbled through the water and
  passes through the vapor ? saturated with water
• Influence of elevated temp inside tube ? water
  vapor becomes steam and results in oxidation of
  Si surface
• Problem contamination of tube and oxide layer
  from dirty water and flask

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2b. Dry oxidation (dryox)

• O2 and H2 are introduced directly into oxidation
  tube ? mixes
• High temperature in tube forms steam ? wet
  oxidation in steam
• Advantage
• Controllable gas flow can be controlled by flow
  controllers
• Clean can purchase gases in a very clean and dry
  state
• Disadvantage explosive property of H2 ? overcome
  by flow in excess O2

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2c. Chlorine added oxidation

• Chlorine addition
• Reduce mobile ionic charges in the oxide layer
• Reduce structural defects in oxide and Si surface
• Reduce charges at Si-SiO2 interface
• Chlorine sources
• Gas anhydrous chlorine (Cl2), anhydrous hydrogen
  chloride
• Liquid trichloroethylene (TCE), trichloroethane
  (TCA)
• TCA is preferred source for safety and ease of
  delivery

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Post-oxidation evaluation

• Surface inspection
• quick check of the wafer surface using UV light
  (surface particulates, irregularities, stains)
• Oxide thickness
• several techniques such as color comparison,
  fringe counting, interference, ellipsometers,
  stylus apparatus, scanning electron microscope
• Oxide and furnace cleanliness
• Ensure oxide consists of minimum number of mobile
  ionic contaminants. Use capacitance/voltage (C/V)
  evaluation detect total number of mobile ionic
  contaminants NOT the origin of contaminants

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Thermal nitridation

• lt 100Å SiO2 film possesses poor quality and
  difficult to control
• Silicon nitride (Si3N4)
• Denser than SiO2 ? less pin holes in thin film
  ranges
• Good diffusion barrier
• Growth control of thin film is enhanced by a flat
  growth mechanism (after an initial rapid growth)

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Nitridation of lt100gt silicon