Ferroelectric Applications

By

Johari AdnanSchool of MicroelectronicUniversiti Malaysia Perlis

SHORT COURSE ON FERROELECTRIC AND OPTOELECTRONIC MATERIALS

FOR MICROELECTRONIC APPLICATIONS 7 & 8 May 2007

Talk outlines

1. Brief overview of ferroelectric

2. BST ferroelectric thinfilms

3. BST sensor applications

Earliest prototype

Objectives

1. Understand ferroelectric phenomena

2. Determine transduction properties of ferroelectric thinfilms

3. Recognize ferroelectric thinfilm sensors potentials

4. Look at implementation problems

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

Magnetic domain Remanence magnetic field

An overview of ferroelectric: continued

Magnetic remanence is due to magnetic domains which are aligned.

Under certain conditions certain materials exhibit electric hysteresis..

An overview of ferroelectric: continued

PR

EC

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

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.

The direction of polarization may be switched by strong external electric field.

Over simplified view of an electric domain

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.

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)

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?

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

BST thinfilm structure

p Silicon

BST

Aluminium

Indium/Silver paste

Fine wire

pros

ess

BST thinfilm

10mm x 10mmBST thinfilm

BST thinfilm

Simple BST thinfilm model from applications point of view

Totransduction

circuit

BST thinfilm as photodiode

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

Vol

tage

(m

V)

acro

ss 1

MΩ

res

isto

r

Light intensity (Lux)

Plot of voltage drop across the 1MΩ resistor versus light intensity (Lux)

BST thinfilm as a variable resistor

Lux

M

Plot of resistance across BST thinfilm versus light intensity (tungsten light source)

A light sensitive switch

BST

BST thinfilm as a capacitor

Simple experimental setup to determinecapacitance of BST thinfilm

CBST 0.5 nF

BST thinfilm as temperature/heat sensor

Peltier

Tovoltmeter

To DCsupply

Type T thermocouple

BST thinfilm as temperature/heat sensor

Plot of output voltage (mV) versus temperature (oC)

Temperature

volta

ge

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

Fabrication technique: version 1

stripboard

glass slide

glue drop

electrode/wire

Fabrication technique: version 2

Fabrication technique: version 3 The idea here is to focus light onto the active BST element

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