si oxidation and dielectrics
DESCRIPTION
Si Oxidation and Dielectrics. Topics: Capacitors & Dielectrics Piezoelectrics Oxide films on Silicon. Capacitance. A parallel plate capacitor when in a vacuum (above) and when a dielectric material is present (below. D o ≡ charge density (C/m 2 ) - PowerPoint PPT PresentationTRANSCRIPT
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Si Oxidation and Dielectrics
Topics:
Capacitors & Dielectrics
Piezoelectrics
Oxide films on Silicon
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CapacitanceA parallel plate capacitor
when in a vacuum (above) and when a dielectric
material is present (below
Do ≡ charge density (C/m2)
o = 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 faradsCapacitance can also b e
expressed as
C = oA/l
Where o is permittivity of free space, A is the area of the plate and l is the plate
separation distance
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If a dielectric is placed between the plates, the capacitance is:
C = A/l
is the permittivity of the dielectric. The relative permittivity, r, called the material dielectric constant, is typically used:
r = o
Thus
C = ro 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
Dξ + P
And P may be written as
P = o(r-1)ξ
Thus
D =orξ
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Effect of frequency and temperature on the
permittivity of soda-lime-silica glass
Relative permittivity of nitrobenzene as a
function of temperature
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Variation of dielectric constant with frequency of an alternating electric field. Electronic, ionic and
orientation polarization contributions to the dielectric constant are indicated
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
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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 ntotal CCCC 1....11
21
• In a parallel circuit, circuit, capacitance is
Ctotal = C1 + C2 +…Cn : C1C2
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Piezoelectrics
In a piezoelectric material, e.g. barium titanate, the Ti4+
and O2- ions are offset as shown
Figure (a) shows the electric dipoles in a piezoelectric material
When the material is compressed (b) the central Ti4+ is displaced,
creating a voltageApplying a voltage (c) reverses this
affect, causing the ions to move farther apart
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PiezoelectricsField produced by stress:
gStrain produced by field:
d
Elastic modulus: gd
E 1
= 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 propertiesVirtually 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
Oxides are thermally grown on wafers by heating in the presence of O2 or H2O
In ambient conditions, an oxide ~1 nm thick forms
After several hours, its final
thickness is 1-2 nm
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Basic Concepts
New interface moves downward into Si
Because SiO2 is less dense than Si, it expands.
This places the Si substrate in tension, and
compresses the SiO2, forcing it upward
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SiO2 layers are amorphous. The bridging oxygen bonds can rotate, randomly accommodating
SiO2 tetrahedra
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Charges associated with the SiO2/Si system
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.
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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Charges associated with the SiO2/Si system
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.
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 MethodsOptical
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. is the angle of incident light, is the angle of reflecting light at the bottom of the interface
1
1
1minmax,
sinsin
cos2
nn
wheremxn
o
o
m = 1,2,3… for maxima and ½, 3/2,
5/2… for minima
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Measurement Methods: the MOS Capacitor
accumulation
depletion
If a DC voltage +VG is applied to the gate, negative charges will be attracted across the oxide layer, producing a capacitance Cox
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
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 = s/xD
s = permittivity of Si
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Measurement Methods: the MOS Capacitor
inversion
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
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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