Lecture 34

Lecture 34 - EEE
352—Lecture
34
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Unformatted text preview: EEE
352—Lecture
34
 Dielectric
Materials
  
Dielectric
Materials
  
Polariza7ons
–
electronic
and
molecular
  
Piezoelectric
effect
  
Scanning
tunneling
microscope
  
Transducers
  
Microelectromechanical
systems
 Peter
Debye
 Nobel
prize
in
chemistry,
1936
 “for
his
contribu7ons
to
our
knowledge
of
molecular
structure
 through
his
inves7ga7ons
on
dipole
moments
and
on
the
 diffrac7on
of
X‐rays
and
electrons
in
gases”
 Dielectric
Effects
 Metal
plates
 What
makes
ε
different
from
ε0?
 Dielectric
 POLARIZATION
 In
electrosta7cs,
the
CONSTITUITIVE
RELATION
is
 Polariza9on
 Suscep9bility
 Dielectric
Effects
 POLARIZATION
arises
from
charge
shiYs
in
the
material—there
is
a
 macroscopic
separa7on
of
posi7ve
charge
(e.g.,
the
ions)
and
nega7ve
 charge
(e.g.,
the
BONDING
ELECTRONS).
 Induced
DIPOLE
MOMENT
 Amount
of
charge
shi@
 POLARIZATION
is
then
 There
are
many
sources
of
dipoles.
 When
we
put
an
electric
field
across
a
material,
other
than
vacuum,
the
 electric
field
induces
changes
in
the
structure:
 Field
 nucleus
 nucleus
 electrons
 electrons
 No
field
 In
a
field
 The
electrons
and
the
nucleus
shiY
 in
the
field
and
separate— produces
an
atomic
polariza9on.
 Field
E0
 nucleus
 d
 The
charge
separa7on
d
produces
an
 induced
field
Eind
which
opposes
the
 applied
electric
field.
 Polariza7on
results
from
the
 separa7on
d:
 electrons
 P = nqd Induced
electric
field
E1
 Metal
plates
 € Dielectric
 For
a
given
voltage,
the
charge
is
 increased
(to
keep
the
same
the
 internal
field),
and
therefore
the
 capacitance
is
more.

The
higher
 capacitance
means
that
the
dielectric
 constant
must
have
increased.
 The
total
charge
which
produces
the
internal
total
field
E
is
given
as
 D+ P and
we
define
the
polariza7on
in
terms
of
the
internal
field
as
 P = ε0 χE where
χ
is
the
suscep9bility.

But,
D
has
the
reverse
sign
of
P
(the
posi7ve
 charge
is
at
the
top
for
D),
so
that
we
must
write
 € € D− P =ε E 0 D = ε0 E + P = ε0 E + ε0 χE = ε0 (1 + χ ) E ≡ εE € Piezoelectric
Effect
 In
materials
with
NO
REFLECTION
SYMMETRY
(like
GaAs
or
many
 molecular
species)
the
applied
electric
field
produces
a
DISTORTION
OF
 THE
LATTICE
(size
change)
and
vice
versa.
 FORCE
 ELECTRIC
FIELD
 A
common
piezoelectric
is
Poly‐Vinylidene
Flouride,
which
is
used
in
a
 variety
of
stereo
headsets.

The
most
common
is
crystalline
quartz
used
as
 frequency
control
crystals—pressure
applied
to
the
quartz
has
a
 resonance
which
can
be
used
in
a
feedback
loop
to
create
a
highly‐stable
 oscillator—the
quartz
crystal
oscillator.
 Scanning
Tunneling
Microscope
 Piezoelectric
transducers
 are
used
to
give
 displacement
in
each
of
 the
3
direc7ons
 Typical
size
of
the
 ac7ve
parts
 Scanning
Tunneling
Microscope
 Raster
scanning
of
the
tunneling
head
is
used
to
generate
a
3D
map
of
the
surface.

 Then
the
power
of
the
computer
is
used
to
create
the
impressive
graphics
displays
 that
result.
 Gerd
Binnig
 Nobel
prize
in
physics,
1986
 Heinrich
Rohrer

 Nobel
prize
in
physics,
1986

 “for
their
design
of
the
scanning
tunneling
microscope”

 Scanning
Tunneling
Microscope
 Cs
atoms
on
GaAs
(110)
 surface
 Cu
(111)
surface
steps
 Fe
atoms
on
a
Cu
(111)
surface
 Piezoelectric
Effect
 surface
acous7c
wave
device
 ~
 Oscilla9ng
electric
field
 Surface
acous9c
wave

 Propaga7on
of
the
 surface
wave
can
be
 used
by
a
second
 TRANSDUCER
to
create
 a
signal
processing
chip.
 Piezoelectric
material

 Pumping cells with surface acoustic waves Piezoelectric
Effect
 surface
acous7c
wave
device
 RF
Resonator
 Piezoelectric
Effect
 surface
acous7c
wave
device
 Wafer
with
sets
of
SAW
devices
 Tire
pressure
sensor
using
a
SAW
device‐‐changes
 in
pressure
move
the
resonant
frequency
of
the
 resonator
 Micro‐Electro‐Mechanical
Systems
(MEMs)
 
*
Micromachining—lithography
and
etching
 
*
Piezoelectric
Can7levers
and
sensors
 
*
Resonators
 
*
Other
neat
things
 MEMs
accelerometer

 Microfabrica7on
 Photolithography,
deposi7on,
and
etching
are
the
standard
tools
of
the
 semiconductor
VLSI
industry.

They
can
also
be
used
to
create
novel
micro‐ electro‐mechanical
systems
as
well.

To
a
large
extent,
these
system
used
the
 microfabrica7on
and
piezoelectric
proper7es
of
materials
to
achieve
new
 func7onality.
 New
material
grown/deposited
upon
the
substrate
 semiconductor
substrate
 Processing
begins
with
the
growth
of
an
“ac7ve”
layer
on
the
 substrate.

There
may
well
be
other
“ac7ve”
layers
within
the
 substrate
(as
will
be
seen
below).

However,
this
ac7ve
layer
is
 one
of
the
key
layers.
 Microfabrica7on
 Photoresist
is
deposited
and
exposed
by
an
 appropriate
light
source.
 The
exposed
area
is
then
etched
by
an
appropriate
liquid
or
 gaseous
etchant.
 Microfabrica7on
 A
common
approach
is
to
first
PREFERENTIALLY
etch
the
 TOP
layer,
then
PREFERENTIALLY
etch
the
second
layer
to
 a
prescribed
depth.
 Etching
can
be
stopped
by
9ming
the
etch
or
by
the
use
of
 an
ETCH
STOP
LAYER.
 Silicon
nitride
layer
on
silicon
(the
bending
 arises
from
the
stress
in
the
deposited
layer—once
the
 underlying
layer
is
removed,
the
stress
induces
the
 bending.
 Small
gears
etched
in
the
top
 layer.
 MEMs
Actuators
 FORCE
 Electric
field
 weight

 The
piezoelectric
effect
can
be
used
to
 reverse
the
transducer
property
to
make
 an
ACTUATOR—an
electrical
signal
 creates
a
displacement.
 The
TI
ELP
technique
used
in
video
 projectors.
 An
electrosta9c
relay.
 Ferroelectric
Materials
 Certain
crystals
are
PERMANENTLY
POLARIZED,
so
that

P
≠
0
even
when
 E
=
0.

These
crystals
are
called
FERROELECTRICS.
Semiconductor
 examples
are
GaN
and
AlN
in
the
wurtzite
crystal
structure.
 P
 Residual
polariza7on
exists
even
 the
electric
field
has
been
 removed.
 Polariza7on
usually
SATURATES
at
a
 rela7vely
high
value.
 Polarizaton
can
be
reversed
by
 applica7on
of
a
HIGH
electric
field.

 Polariza7on
remains
reversed
 aYer
field
is
removed.
 E
 Ferroelectric
Materials
 Polariza7on
usually
results
from
a
distor7on
in
the
crystal
structure.
 Ba2+
 Ti4+
 Ba2+
 Ti4+
 δ O2‐
 O2‐
 Above
Tc
(the
Curie
temperature,
 about
130
C
for
BaTiO3),
the
structure
 is
basically
cubic.
 Since
it
cannot
decide
which
to
be,
 body‐centered
orface‐centered,
it
is
 paranoid.
 Below
Tc,
the
crystal
is
STRETCHED
along
 the
z‐axis—tetragonal
distor7on.

The
Ti
 atom
is
not
moved,
however,
which
now
 gives
a
dipole
moment
in
the
crystal.
 Ferroelectric
Materials
 The
LOCAL
electric
field
can
be
wrimen
as
 Now,
we
can
rewrite
the
suscep7bility
as
 Ferroelectric
Materials
 Consider
a
special
case
in
which
the
dipoles
are
randomly
oriented:
 Take
r
→
z:
 Average
over
the
angle
leads
to
a
factor
of
1/3
 Ferroelectric
Materials
 We
now
get
the
CLAUSIUS‐MOSOTTI
rela7on:
 This
can
be
rearranged
to
give
 If
the
polarizability
is
sufficiently
large,
the
denominator
can
go
to
zero,
for
 which
a
PERMANENT
DIPOLE
can
exist.

Hence,
FERROELECTRICS
are
usually
 large
dielectric
constant
materials!
 Ferroelectric
Materials
 Examples
of
materials
are
BaTiO3,
KH2PO4,
SrTiO3,
KNbO3,

 polymers
like
PVDF
(poly‐vinylidene
flouride)
 These
have
high
dielectric
constants,
oYen
>
103.
 Generally,
the
dielectric
constant
is
a
func7on
of
the
direc7on
of
 propaga7on
through
the
crystal—hence,
these
materials
are
good
for
 nonlinear
op7cs,
as
they
oYen
have
a
large
nonlinear
dependence
of
 the
dielectric
constant.
 All
ferroelectrics
are
also
piezoelectric
(the
reverse
is
NOT
true).
 Most
are
also
pyroelectric.
 Ferroelectric
Materials
 We
saw
earlier
that
there
was
a
problem
with
the
small
thickness
of
the
SiO2
layer
 in
the
next
genera7on
MOSFET.

We
can
alleviate
this
if
we
use
a
high
dielectric
 material,
like
a
ferroelectric.
 SiO2
 tox
 hi‐K
material
 thi‐K
 We
want
Cox
(capacitance
per
unit
area
to
be
the
same):
 Then
we
say
that
the
new
gate
oxide
has
the
same
“effec7ve
thickness”
as
the
 desired
SiO2
stack.
 However,
we
cannot
use
materials
like
BaTiO3
or
BaSrO3,
etc.,
which
have
 dielectric
constants
of
several
thousands.

They
have
low
energy
op7cal
phonon
 modes,
which
will
interact
with
the
carriers
in
the
inversion
layer
and
cause
VERY
 LOW
mobility!
 We
have
to
use
materials
with
moderate
dielectric
constants,
so
that
the
phonon
 energies
are
higher,
and
do
not
affect
the
mobility
quite
so
much.

Materials
like
 HfO2
and
ZrO2
are
being
“looked
at”
with
favor
by
the
industry.

In
addi7on,
using
 a
metal
gate
screens
out
some
of
the
phonon
interac7on
and
improves
the
 mobility.
 Pyroelectric
Materials
 Temperature
causes
a
change
in
the
polariza7on,
and
therefore
in
the
 dielectric
constant.

The
pyroelectric
coefficient
is
 EXAMPLE:
We
consider
a
material
like
PZT
(lead‐zirconium‐7tanate).
For
a
ΔT
 of
10‐3
K,
and
a
ppyro
=
3.8
×
10‐4,
we
find
that
ΔP
=
3.8
×
10‐7.


With
εr
=
290,
 this
gives
an
induced
field
of
149
V/m.

If
the
film
is
0.1
mm
thick,
this
gives
 an
induced
voltage
of
~
15
mV,
easily
detected.
 hmp://www.msm.cam.ac.uk/doitpoms/tlplib/ferroelectrics/pyroelectrics.php
 Pyroelectric
Detectors
 chopped
light

 absorber
contact
 L
 back‐contact

 signal

 amplifier

 reflec9ng
surface
 Pyroelectric
Detectors
 Army
head‐mounted
night
vision
device
 ...
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