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Ferroelectric Applications
By
Johari AdnanSchool of MicroelectronicUniversiti Malaysia Perlis
SHORT COURSE ON FERROELECTRIC AND OPTOELECTRONIC MATERIALS
FOR MICROELECTRONIC APPLICATIONS 7 & 8 May 2007
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Talk outlines
1. Brief overview of ferroelectric
2. BST ferroelectric thinfilms
3. BST sensor applications
Earliest prototype
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Objectives
1. Understand ferroelectric phenomena
2. Determine transduction properties of ferroelectric thinfilms
3. Recognize ferroelectric thinfilm sensors potentials
4. Look at implementation problems
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An overview of ferroelectric
The word ‘ferro’ is derived from the word ferrum (iron).Ferromagnetic materials exhibit magnetic hysteresis loop.Magnetization persists even when there is no magnetic field present.
MR
Hc
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Magnetic domain Remanence magnetic field
An overview of ferroelectric: continued
Magnetic remanence is due to magnetic domains which are aligned.
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Under certain conditions certain materials exhibit electric hysteresis..
An overview of ferroelectric: continued
PR
EC
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Net spontaneous polarization
No spontaneous polarization
In zero electric field, they possesses spontaneous polarization.These materials are called ferroelectric materials
An overview of ferroelectric: continued
Reason why certainsamples do notexhibit ferroelectricproperties
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How does spontaneous polarization occur in ferroelectric?
The diagram below shows perovskite structure of BaTiO3 (BST).Cubic at high temperature and non-cubic below Curie temperature.In non-cubic phase Ti atom is shifted relative to the O atom and resultedin a net polarized state.
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The direction of polarization may be switched by strong external electric field.
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Over simplified view of an electric domain
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Note: after fabrication, ferroelectric material may not have net polarization characteristic since the domains may be randomized. Application of strong electric field helps align the electric domains giving the sample net spontaneous polarization.
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1. Pyroelectricity is a migration of positive and negative charge (and therefore establishment of electric polarization) to opposite ends of a crystal's polar axis as a result of a change in temperature.
2. The property of some materials to store a permanent electric field, by analogy with the storage of a magnetic field by ferromagnetic materials. The BaTiO3, in a perovskite structure, is used to create ferroelectrics in the lab under the imposition of a strong electric field which permanently creates electric dipoles.
3. Certain crystals are called piezoelectric when they exhibit a relationship between mechanical strain (tension or compression) and voltage across their surfaces. Specifically, when compressed or pulled, a piezoelectric crystal will build up alternate charges on opposite faces, thus acting like a capacitor with an applied voltage. A current, called piezoelectricity, can then be generated between the faces. On the other hand, when subjected to an external voltage, the crystal will expand or contract accordingly.
Some definitions (related to presence of domains)
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Ferroelectric materials when subjected to temperature variation, T, electrical field, E, and stress, , will develop electrical charges, q
q k1T + k2E + k3 + k4I
pyroelectric
ferroelectric
piezoelectric
Features central to ferroelectric sensors
photoelectric?
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Our research is on ferroelectric materials specifically BST. BST is just a small subset of ferroelectric materials. The rest of talk is specific to BST thinfilm.
Three reasons why we choose BST thinfilms
1.Researchers who are knowledgeable and interested in BST
2.Availability of fabrication facilities
3.Potential of BST thinfilms as sensing/storage elements
Focus area
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BST thinfilm structure
p Silicon
BST
Aluminium
Indium/Silver paste
Fine wire
pros
ess
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BST thinfilm
10mm x 10mmBST thinfilm
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BST thinfilm
Simple BST thinfilm model from applications point of view
Totransduction
circuit
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BST thinfilm as photodiode
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The dimension of the BST thinfilm is about 8mm x 8mmAt 550 Lux, the voltage across the 1M resistor is 70mVCurrent through the 1M is 0.07uA
Irzaman et. al. determined from I-V measurementsthat BST thinfilm has diode properties
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Vol
tage
(mV
) acr
oss
1MΩ
resi
stor
Light intensity (Lux)
Plot of voltage drop across the 1MΩ resistor versus light intensity (Lux)
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BST thinfilm as a variable resistor
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Lux
M
Plot of resistance across BST thinfilm versus light intensity (tungsten light source)
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A light sensitive switch
BST
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BST thinfilm as a capacitor
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Simple experimental setup to determinecapacitance of BST thinfilm
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CBST 0.5 nF
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BST thinfilm as temperature/heat sensor
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Peltier
Tovoltmeter
To DCsupply
Type T thermocouple
BST thinfilm as temperature/heat sensor
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Plot of output voltage (mV) versus temperature (oC)
Temperature
volta
ge
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Challenges in sensors development
Ferroelectric materials when subjected to temperature variation, T, electrical field, E, and stress, , will develop electrical charges q
q k1T + k2E + k3
Some problems associated with sensor development
1. Mask unwanted contributions/modes2. Suitable packaging3. Right recipe tailored for a specific sensing characteristic
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Fabrication technique: version 1
stripboard
glass slide
glue drop
electrode/wire
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Fabrication technique: version 2
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Fabrication technique: version 3 The idea here is to focus light onto the active BST element
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Immediate future plans include development of:
1. Gas sensors
2. Ultrasonic sensors
3. Electro-optic devices
Other future plans
1. Integrating transduction circuits on the same sensor substrate
2. Active BST configurations, i.e. in the form of BJT and MOSFET
3. Hybrid sensor (one sensor which is capable of measuring multiple parameters)
1. Other issues such as miniaturization, low power, smart, etc