Lect 7Thermal oxidization

1
Lect 7 Thermal oxidization

• Upon exposure to oxygen, the surface of the
  silicon wafer is oxidized and a thin layer of
  silicon dioxide is formed.
• SiO2 works as a perfect insulator as well as a
  barrier layer.
• Thermal oxidization is a process where the
  silicon wafer is heated to high temperatures (900
  to 1200o) in the presence of oxygen or water.
• The interaction between O2 (H2O) and Si results
  in an oxide layerSi O2 ? SiO2 Si H2O ?
  SiO2 2H
• At these high temperatures O2 and H2O vapor
  diffuses easily through the silicon dioxide
  layer and interacts with the silicon beneath to
  further expand the oxide layer.
• The diffusion of different molecules is
  charac-terized by the diffusion coefficient, D,
  
• While growing the oxide layer, silicon is
  consumed and the SiO2 expands 54 above the
  original surface and 46 below it.

Original Si surface
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Thermal oxidization
Original layer of oxide

• Due to the exposure of the surface to oxygen
  andwater (oxidizers), there exists an initial
  layer of oxide of thickness Xi.
• When heating the wafer at high temperatures
  theoxidizers diffuse through the SiO2 and
  oxidize the silicon layer beneath
• As the oxide layer grows, the oxidizer needs to
  gothrough more and more of SiO2 layers causing
  theoxidization rate to reduce.

Silicon
Heating with the presence of O2 or H2O
Silicon
SiO2
Newly oxidized layer
Si
Heating with the presence of O2 or H2O
Silicon
3
Thermal oxidization

• To model this process we will assume that oxygen
  diffuses through SiO2.
• Ficks first law of diffusion Particles flow per
  unit area, J (referred to as flux) is
  proportional to the concentration gradient.
• N is the concentration of oxidizers inside the
  SiO2 number of oxidizer per unit volume.
• Assume that the flux is constant through the SiO2
  layer as no oxidizers are consumed. Then
• The boundary condition is that, at x0, NNO,
  where NO is the concentration of the oxidizers at
  the oxide surface. At the Si-SiO2 interface, xXO
  and the concentration is Ni, then
• At the Si-SiO2 interface, the oxidizers react
  with Si creating SiO2. Hence, the flux is assumed
  to be proportional to the concentration Ni
• ks is the reaction rate constant.

4
Thermal oxidization

• The flux, J, of the oxidizers through the SiO2 is
  defined as number of particles per unit area per
  unit time.
• Assume that M is the molecules density of the
  oxidizers which incorporated into a unit volume
  of the resulting oxide
• Then the total number of particles that oxidize a
  unit area A and thickness dX0 is
• The total number of particles per A per unit time
  dt is
• Replacing the flux with the expression from
  before
• Integrating both sides

No
Concentration N
Ni
SiO2 Si
Xo
Distance from surface, x
SiO2
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Lect8 Thermal oxidization

• The integration constant can be estimated
  applying the proper boundary conditions
• Boundary conditions t0, X0Xi
• Deal-Groove model
• The solution to this equation is the
• If then
  if
  and

6
Thermal oxidization

• For very short time, the thickness changes
  linearly with time, hence B/A is referred to as
  the linear rate costant
• For very long time, the thickness changes in
  quadratic form with time, hence B is referred to
  as the parabolic rate constant
• Both constant change with temperaturein an
  exponential form.
• D can be either B/A or BDo is a constant EA is
  referred to as the activation energyk is
  Boltzmanns constant (1.381x10-23 J/K)T is
  temperature in Kelvin
• The equation indicates a strong dependence of the
  oxide thickness on temperature

Xo(t)
Linear region
Parabolic region
Xi
t
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Thermal oxidization

• Typical values for Co and EA are in the table
  below
• The table shows a finite value for Xi for the dry
  O2 case. That indicates that a finite value of ??
  is needed.
• The large value of Xi (25 nm) indicates some sort
  of inaccuracy in the model.

8
Thermal oxidization

• The oxide growth rate is much higher in wet
  atmosphere.
• Hence, larger thickness when using wet O2
  compared to dry O2.
• Slow growth rate results in dense and high
  quality oxide and hence it is usually used for
  the MOS gate oxide region.
• Fast growth rate in wet oxygen results in a lower
  quality oxide region and it is used for as a
  masking or protection layer.
• From the equations both rate constants A and B
  depend on No which depends on the pressure of the
  oxidizing species.

9
Thermal oxidization

• The diffusion coefficients of several impurities
  (Boron, Phosphorous) in SiO2 are orders of
  magnitudes smaller than in Si.
• That qualifies SiO2 as a mask for impurities
  during high temperature diffusion.
• Relatively deep diffusion can take place in
  unprotected regions of silicon whereas no
  significant impurities penetrate through the
  covered regions.
• Typical values of Do for Boron and Phosphorous in
  SiO2 at 1100 oC are
• Typical values of Do for Boron and Phosphorous in
  Si at 1100 oC are
• The presence of ambient gases (impurities) in the
  substrate (Si or SiO2) influences diffusion.

10
Thermal oxidization

• Oxidation technology
• Most thermal oxidation is performed in furnaces,
  at temperatures between 800 and 1200C.
• A single furnace accepts many wafers at the same
  time, in a specially designed quartz rack (called
  a "boat").
• Historically, the boat entered the oxidation
  chamber from the side (this design is called
  "horizontal"), and held the wafers vertically,
  beside each other.
• Many modern designs hold the wafers horizontally,
  above and below each other, and load them into
  the oxidation chamber from below.
• Vertical furnaces stand higher than horizontal
  furnaces, so they may not fit into some
  microfabrication facilities. However, they help
  to prevent dust contamination. Unlike horizontal
  furnaces, in which falling dust can contaminate
  any wafer, vertical furnaces only allow it to
  fall on the top wafer in the boat.

11
Lect. 9 Thermal Oxidation

• Oxide quality
• Contamination metal ions such as sodium are
  highly mobile in SiO2 Mobile and can degrade the
  performance of the MOSFET.- Sodium-ions cause
  positive charges in the oxide and an excessive
  positive charge at the Si-SiO2 interface. -These
  charges attract electrons to the surface of the
  MOS causing a negative shift in the threshold
  voltages.-chlorine can immobilize sodium by
  forming sodium chloride. Chlorine is often
  introduced by adding hydrogen chloride or
  trichloroethylene to the oxidizing medium. Its
  presence also increases the rate of oxidation?
• Dirty interface Growing thick oxides using wet
  oxygen the higher growth rate leaves more
  dangling bonds at the silicon interface, which
  produce quantum states for electrons and allow
  current to leak along the interface.
• Density Wet oxidation also yields a
  lower-density oxide, with lower dielectric
  strength.
• Breakdown voltage Dry oxidization with higher
  density results in high breakdown voltage.
  However, the long time required to grow a thick
  oxide in dry oxygen makes this process
  impractical. Thick oxides are usually grown with
  a long wet oxidation bracketed by short dry ones
  (a dry-wet-dry cycle). The beginning and ending
  dry oxidations produce films of high-quality
  oxide at the outer and inner surfaces of the
  oxide layer, respectively.

12
Thermal oxidation

• Local oxidation
• The ability to selectively oxide specific
  locations on the silicon wafer is of great
  interest in high density transistor processes.
• The technique utilized for localized oxidation is
  referred to as LOCOS (Local Oxidation Of
  Silicon).
• To perform local oxidation, the areas not meant
  to be oxidized will be coated in a material that
  does not permit the diffusion of oxygen.
• Oxygen and water vapor do not diffuse well
  through silicon nitride, hence Si3N4 is used as
  an oxidization barrier.
• For local oxidization process -a thin layer of
  SiO2 is deposited to protect the wafer -A Si3N4
  is deposited over the oxide.-A photolithography
  step is applied and a pattern is etched through
  the two layers.

Si3N4 SiO2
Silicon wafer
13
Thermal oxidation

• Semirecessed oxide structure
  Fully-recessed oxide structure

Silicon wafer
Silicon wafer
Silicon etching
Oxidation
Oxidation
Nitride removal
Nitride removal
Almost flat oxide surface
14
Thermal oxidation

• Oxide thickness characterization
• The simplest way to characterize the thickness of
  an oxide is by comparing the color of the wafer
  with the reference color chart attached.
• When light transmits through SiO2, reflects from
  theSiO2-Si interface and leaves the oxide it
  gains a totalphase of and an
  amplitude of r.
• The total reflected field is a summation of the
  infinite reflections
• The total field forms a power series.
• The intensity is

Reflected light
Incident light
.
.
d
SiO2
Silicon
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Thermal oxidation

• The reflected light intensity is
• The peak of the intensity when??2? m, where m is
  integer.
• At a certain peak of wavelength ?? the reflected
  light will appear with a color that corresponds
  to that wavelength.
• Hence the color can be translated directly into
  a thickness shown by the equation above and the
  chart table attached.