Download Si oxidation and dielectrics

Si Oxidation and
Dielectrics
Topics:
Capacitors & Dielectrics
Piezoelectrics
Oxide films on Silicon
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Capacitance
A parallel plate capacitor
when in a vacuum (above)
and when a dielectric
material is present (below
Do ≡ charge density (C/m2)
eo = permittivity of free space =
8.85 x 10-12 F/m
ξ ≡ electric field strength = V/l
P ≡ polarization, or increase in
charge density due to presence of
a dielectric
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The capacitance, C is related to
the quantity of charge stored on
either plate Q by :
C = Q/V
Where V = applied voltage.
Units for C are coulombs per
volt, or farads
Capacitance can also b e
expressed as
C = eoA/l
Where eo is permittivity of
free space, A is the area of
the plate and l is the plate
separation distance MSE630
If a dielectric is placed between the plates, the capacitance is:
C = e A/l
e is the permittivity of the dielectric. The relative permittivity, er, called the
material dielectric constant, is typically used:
er = e/eo
Thus
C = ereo A/l
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The applied electric field
aligns the poles of the
molecules in the dielectric.
This is the source of
polarization.
The charge density, D, is
D = eoξ + P
And P may be written as
P = eo(er-1)ξ
Thus
D =eoerξ
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Relative permittivity of
nitrobenzene as a
function of temperature
Effect of frequency and
temperature on the
permittivity of soda-limesilica glass
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Electronic polarization:
fluctuations in the electron
cloud
Ionic polarization:
displacement of ions in an
ionic compound
Orientation polarization:
rotation of permanent
dipole moments in
presence of an applied
field
Variation of dielectric constant with frequency of
an alternating electric field. Electronic, ionic and
orientation polarization contributions to the
dielectric constant are indicated
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Dielectric Breakdown
Dielectric strength of various solids,
gases and vacuum in uniform fields.
Breakdown voltage versus dielectric
thickness is plotted
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• In a series circuit,
capacitance is :
C1
C2
• In a parallel circuit,
circuit, capacitance is
Ctotal = C1 + C2 +…Cn :
1
1
1
Ctotal =

 ....
C1 C2
Cn
C1
C2
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Piezoelectrics
Figure (a) shows the electric
dipoles in a piezoelectric material
In a piezoelectric material,
e.g. barium titanate, the Ti4+
and O2- ions are offset as
shown
When the material is compressed
(b) the central Ti4+ is displaced,
creating a voltage
Applying a voltage (c) reverses this
affect, causing the ions to move
farther apart
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Piezoelectrics
Field
produced
by stress:
 = g
Strain
produced
by field:
e = d
Elastic
modulus:
1
E=
gd
 = electric field
 = applied
stress
E=Elastic
modulus
d = piezoelectric
constant
g = constant
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Thermal Oxidation and the Si/SiO2 Interface
Oxides play an important part in semiconductor
fabrication. They are:
•Easily grown or deposited on many substrates
•Adhere well
•Block diffusion of dopants and other unwanted impurities
•Resistant to most processing chemicals
•Easily patterned and etched with plasmas or specific chemicals
•Excellent insulators
•Have stable and reproducible properties
Virtually all other semiconductor/insulator combinations
suffer from one or more problems that significantly limit their
applicability
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The most critical application of insulators in CMOS devices is as gate
insulators. As seen above, these need to be <1 nm thick within the next 4
years
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• SiO2 layers in CMOS
are used as:
– Gate dielectric layers
– A mask against
implantation
– An isolation region
laterally between
devices
– An insulator between
metal layers
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Oxide Growth
In ambient
conditions, an
oxide ~1 nm
thick forms
After several
hours, its final
thickness is 1-2
nm
Oxides are thermally grown on
wafers by heating in the
presence of O2 or H2O
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Basic Concepts
Because SiO2 is less
dense than Si, it expands.
This places the Si
substrate in tension, and
compresses the SiO2,
forcing it upward
New interface moves
downward into Si
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SiO2 layers are
amorphous. The bridging
oxygen bonds can rotate,
randomly accommodating
SiO2 tetrahedra
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There are four basic
types of defects or
charges at the interface:
1. Qf, the fixed oxide charge. It has
magnitude of 109 – 1011 cm-2 very
close to the interface. Results from
incompletely oxidized Si atoms with
a net positive charge. Qf is stable.
Charges associated with the SiO2/Si system
2. Qit, the interface trapped charge.
Similar to Qf, with dangling bonds
located in oxide, very close to
interface. Charge on Qit may be
positive, negative or neutral and can
change during operation. Density is
about the same as Qf.
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There are four basic
types of defects or
charges at the interface:
3. Qm, mobile charges. These are
often processing artifacts causing
erratic gate threshold voltages.
These problems have been largely
eradicated with proper cleanroom
techniques.
Charges associated with the SiO2/Si system
4. Qot, the oxide trapped charge.
Occurring anywhere in the oxide,
these result from broken Si-O
bonds, away from the interface.
Usually caused by processing
damage, they can often be removed
by high-temperature annealing.
All types of charged defects have a
negative effect on device performance
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Oxide Measurement Methods
Optical
Light reflection from a sample with a
transparent thin film on its surface.
no is the index of refraction in air
(1.0), ni is that of the film and n2 is
that of the substrate. f is the angle
of incident light, b is the angle of
reflecting light at the bottom of the
interface
max, min =
2n1 xo cos b
m
where
 no sin f 
b = sin 

n
1


1
m = 1,2,3… for
maxima and ½, 3/2,
5/2… for minima
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Measurement Methods: the MOS Capacitor
If a DC voltage +VG is applied to the gate,
negative charges will be attracted across
the oxide layer, producing a capacitance
Cox
accumulation
If a negative voltage -VG is applied to the gate,
negative charges will be repelled across the
oxide layer, producing depletion region with its
own capacitance, CD, in series with the oxide
capacitance Cox
|QG| = |QD| = ND/xD
depletion
Where QG, QD are units of number
of charges per cm2, ND is the
doping in the substrate (assumed
uniform). xD is the depth of the
depletion region.
CD = es/xD
es = permittivity of Si
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Measurement Methods: the MOS Capacitor
If a large enough –VG is applied, it will
attract minority carrier holes in the
substrate to the surface and form an
inversion layer (in this case, of P-type
carriers.
The gate voltage at which this occurs is called
the threshold voltage, Vth. At this point, xD
stops expanding at xDMax
inversion
For all regions of the capacitor, the gate
charge must be balanced by the charge on the
substrate, or:
QG = NDxD + QI,
Where QI is the charge density on the
inversion layer. Since xD is maximum, the CV
curve reaches a minimum as shown above.
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The End
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