Oxidation of Silicon components of silicon-based transistors, including our MOS devices:

Transcription

Oxidation of Silicon components of silicon-based transistors, including our MOS devices:
Oxidation of Silicon
“Oxide” is the term used to describe silicon dioxide, one of the main material
components of silicon-based transistors, including our MOS devices:
• Gate oxides - isolate the gate from the source and drain of the transistor. This
oxide needs to be very high quality, with highly controllable thickness
• Field oxides / device isolation - keeps components from experiencing cross
talk from nearby components - does not have to be as high of quality but
needs to be much thicker than gate oxides
• Intermetal dielectrics - used to isolate the connecting wires between devices.
Again, lower quality, but much thicker than gates. This material is not
directly grown from substrate material
Ways to grow an oxide:
• Thermally grow oxide from the silicon substrate (Si, oxygen, heat)
• Chemically grow oxide by exposing any material to a stoichiometric combination
of Si and 2O in a chemical vapor deposition process
Also used for many other devices and process steps not used in this class. Refer to figure
8-1 in your textbook.
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Thermal Oxidation of Silicon - MatE 129, Integrated Circuit Manufacturing
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Oxide Properties
The Silicon Dioxide Molecule and it’s material structure
• Tetrahedral structure with Si in the center and O at the corners
• Crystalline quartz - aligns tetrahedra in a simple crystalline structure
• Fused silica - tetrahedral bound together in a random pattern - this is the type of
oxide grown on silicon through thermal processes. Fused silica is an amorphous
material
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Fused silica properties
• Thermodynamically unstable below quartz melting point (~1710 C) - tries to
become crystalline - devitrification - occurs above 1000 C (this is a big problem
for furnace tubes - causes particles)
• Tetrahedra joined by bridging oxygen. Nonbridging oxygen do not connect to
other tetrahedral, like crystalline quartz
o Makes fused silica less dense (2.2 g/cm3 compared to 2.65 g/cm3)
o Allows for much easier interstitial and substitiutional diffusion
§ Substitutional impurities replace silicon in the structure - B and P also called network formers. These also have one extra or less
electron - effect the number of bridging oxygen atoms (B increase P - decrease (make oxide better)
Called a network former because the components can be a basis for
a glassy structure (B2O3, P2O5)
This creates an electrical imbalance, fixed by either creating or
destroying a non-bridging oxygen site.
• B3+ à creates non-bridging oxygens - degrades the quality
of the oxide
• P5+ à destroys non-bridging oxygens - can improve the
quality of the oxide
§ Interstitial impurities - also called network modifiers - Oxides of
certain metals: Na, K, Pb, Ba - oxygen from metal oxide replaces
oxygen in the system - produce more non-bridging oxygens
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Thermal Oxidation of Silicon - MatE 129, Integrated Circuit Manufacturing
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§ Hydroxyl group - replace oxygen in the system:
H20 + Si-O-Si à Si-OH + OH-Si - decreases oxide quality but
increases oxidation rate if constantly present (wet oxidation)
Growing a thermal oxide - the Deal Grove model (linear-parabolic model)
• Two oxidants used - dry oxygen and water (dry oxidation and wet oxidation
respectively)
Dry oxidation:
Wet oxidation:
Sisolid + O2vapor à SiO2solid
Sisolid + H2Ovapor à SiO2solid + 2H2gas
• Reactions occur on the Si/SiO2 surface - oxide grows into the silicon and
consumes the substrate - for every 1000nm of oxide grown, 440nm of Si is
consumed
• Deal-Grove model operating conditions
o Oxide thicknesses between 30-500nm
o Temperatures 700C - 1300C
o Oxidant partial pressures 0.2atm - 25atm
• Deal-Grove assumptions
o Process occurs due to two fluxes that sequentially transport oxidizers from
the ambient gas to the Si/Oxide interface
o A third flux represents the consumption of the oxidant
o Flux = # of particles/cm2/sec
o Three fluxes (again)
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1. the flux of the oxidizing species as moving from the bulk of the
gas phase to the gas/oxide interface (F1)
2. the flux of the oxidizing species as it diffuses through the existing
oxide to the Si/SiO2 interface (F2)
3. the flux of the oxidizing species as it is consumed by reaction at
the Si/SiO2 interface (F3)
o Steady state condition: F1 = F2 = F3
• F1 - caused by the mass transport created by oxidant density gradients at the
wafer surface
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• F1 α Cg - Cs à Need a proportionality constant à use the mass transfer
coefficient hg:
F 1 = h g (C g − C s )
(1)
hg is related to the diffusivity of the oxidizing species into the oxide layer (D) and
the thickness of the boundary layer
Boundary layer à discussed on page 158 of textbook. Is the transition region
between solid surface (gas velocity = 0) and the bulk gas region, where vg > 0
• Need to equate F1, F2, and F3 à Need like variables à use Henry’s Law
Henry’s Law à the concentration of a species dissolved into a solid is
proportional to the partial pressure of the species in the gas phase at the surface
C 0 = HPs
(2)
C * = HPg
(3)
C0 is the equilibrium concentration of the oxidizing species at the surface
Ps is the partial pressure of the oxidizing species at the surface
C* is the equilibrium concentration of the oxidizing species in the oxide bulk
Pg is the partial pressure of oxidizing species in the bulk of the gas phase
• Assume the ideal gas law (PV=kT)
Cg =
Pg
(4)
kT
P
Cs = s
kT
(5)
• Combine equations 1-5
F1 = h(C* - C0)
(6)
h à new mass transfer coefficient related to hg by:
h=
hg
HkT
(7)
• Now we move into the oxide layer itself, look at F2 using Fick’s Law of
Diffusion:
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Thermal Oxidation of Silicon - MatE 129, Integrated Circuit Manufacturing
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F2 = − D
(C − Ci )
dC
=− D 0
dxox
xox
(8)
D is the diffusion coefficient of oxidation species in the oxide layer
Ci is the interface concentration
xox is the oxide thickness
• The flux at the oxide/silicon interface, F3, is determined by the reaction rate
coefficient, and is what ties in oxide growth to the fluxes of the oxidant at the
wafer surface
F3 = ksCi
(9)
ks is the first order reaction rate coefficient
• Recall that F1 = F2 = F3. This sets up a system of equations from which we can
solve for unknowns:
Ci =
C*
k
k x
1 + s + s ox
h
D
 k x 
C * 1 + s ox 
D 
C0 = 
k
k x
1 + s + s ox
h
D
(10)
(11)
• Look at limiting cases
o D is small à flux of oxidant through oxide is small compared to reaction
flux at interface
§ Oxidant consumed as quick as it can get there
§ “Diffusion limited case”
§ Ci = 0, C0 = C*
o D is big à reaction rate limited
Ci = C0 =
C*
k
1+ s
h
(12)
• From these relationships, you can determine the growth rate
o Use two solution cases
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o Include # of oxidant molecules incorporated per unit volume of oxide (Ni)
Typical oxide:
2.2 x 1022 [SiO2]cm-3
2 O’s per molecule
For O2
2.2 x 1022 /cm3
For H2O
4.4 x 1022 /cm3
o Combine equations 9 and 10, recall that flux is density x velocity
Ni
dx ox
=
dt
k sC *
k
k x
1 + s + s ox
h
D
(13)
Solving this differential equation:
Set Boundary Conditions xox = xi at t = 0 (note that xi can include oxide
already on the surface)
xox2 + Axox = B(t + τ )
(14)
τ is the time displacement to account for the initial oxide on the system
 1 1
A = 2 D
k + h 

 s

(15)
2 DC *
B=
Ni
(16)
τ=
xi2 + Axi
B
(17)
• From this equation, we can see the linear and parabolic regimes:
o t >> τ, Diffusion Controlled, xox2 = Bt
B à Depends on Diffusion, Parabolic, “Parabolic Rate Constant”
o
A2
B
, Linear Regime, xox = (t + τ )
4B
A
B/A is the linear rate constant, independent of diffusion
(t + τ )<<
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k h C * 
B

= s 

A ks + h 
 N1 
(18)
Note that h >> ks, therefore 18 can be simplified:
C * 
B
= ks 

N 
A
 1
(19)
• Of course, these values can either be calculated or looked up in a table. For
calculations, refer to Figures 8-4 to 8-6 in Wolf and Tauber
Factors that can contribute to oxidation growth NOT covered by the Deal-Grove model:
• Crystal Orientation - depending on the Si orientation, the surface density of Si
atoms can differ, impacting the rate constant for oxidation.
This only serious impacts the linear, reaction rate limited case
Refer to Table 8-6 for modified DG constants
• Dopant effects - tend to increase oxidation rates
Boron will incorporate into the oxide, increase NBO concentration, providing
easier oxidant diffusion to the interface.
Phosphorus will not incorporate into the surface, but increases the vacancy
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density in the Si, providing enhanced oxidation by increasing the number density
of available sites.
• Water in a dry oxidation process à 25ppm of water can measurably increase both
linear and parabolic oxidation
• There are many other things which impact oxidation rates which are beyond the
context of this class. Please refer to Chapter 8.5 of Wolf and Tauber for a more
detailed discussion.
Optical measurements of oxide thickness
• Interference patterns - oxides are typically grown to thicknesses on the same order
of magnitude of the wavelength of visible light.
• This produces constructive and destructive interference patterns
• Plotting the intensity of reflected light vs. wavelength can provide a thickness
measurement
• Ellipsometry uses polarized light and the differences in interference for tangential
and parallel polarized light, incident at some angle, to make an even more detailed
measurement of thickness
• Spectral ellipsometry uses both phenomena to provide an optical measurement of
both thickness and dielectric constant.
Electrical measurements - measure the capacitance of the oxide layer, treat as a parallel
plate capacitor:
C=
ε0ε ox Area
xox
(20)
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