Ferroelectric Field Effect Transistor The Memory Technology of Tomorrow

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Ferroelectric Field Effect Transistor-The
Memory Technology of Tomorrow?
Mini-Project - Smart Electronic Materials 2006/P1

• Antoine Brugere
• Arndt von Bieren
• Florent Boyer Chammard
• Michael Kallenberg

2006-10-17
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Introduction on FeFET

• Combination of (Si MOSFET) transistor technology
  and ferroelectric materials
• Like a conventional transistor, but it can
  remember its state
• Provides wide spectrum of possible applications,
  e.g.

RFID tag
non volatile memory
artificial neural network
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Contents

• Ferroelectrics
• Principles of FeFET
• Problems and Improvements
• Conclusion

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Contents

• Ferroelectrics
• Basic Properties
• Ferroelectric Domains Hysteresis
• Important Ferroelectric Materials
• Principles of FeFET
• Problems and Improvements
• Conclusion

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1. Ferroelectrics Basic Properties

• Ferroelectrics
• dielectric, ionic crystals, which exhibit
  spontaneous polarization
• defined states depending on structure, switchable
  by external electric fields
• occurs only below material-specific
  Curie-temperature

• Polarization is related to surface charge density
  and temperature
• ferroelectric materials show piezoelectric and
  pyroelectric effects

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1. FerroelectricsOrigin of Spontaneous
Polarization

• splitting of optical vibration modes in ionic
  crystals
• softening of TO mode due to partial force
  compensation (elastic electrostatic)

• soft phonons can condense out at low
  temperatures
• spontaneous polarization
• increase of e due to LST

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1. Ferroelectrics Ferroelectric Domains
Hysteresis

• domains are regions with uniform direction of
  spontaneous polarization
• separated by domain walls (1-10 a thick) which
  appear along specific crystal planes

• domain formation mechanically or electrically
  driven
• i.e. avoidance of depolarizing field

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1. Ferroelectrics Ferroelectric Domains
Hysteresis

• Hysteresis is caused by irreversible polarization
  processes
• pinning of domain walls at lattice defects
• newly created domains do not disappear after
  removal of field
• small displacements in weak fields are reversible
• wall movement can be described by a potential

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1. Ferroelectrics Important Ferroelectric
Materials

• PZT - Pb(ZrxTi1-x)O3
• large available spontaneous polarization, high
  piezoelectric coefficients
• high transition temperature (370C)
• SBT SrBi2Ta2O9
• only few allowed directions of spontaneous
  polarization
• low remanent polarization
• very high transition temperature (570C)

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Contents

• Ferroelectrics
• Principles of FeFET
• Problems and Improvements
• Conclusion

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Contents

• Ferroelectrics
• Principles of FeFET
• Function and Properties
• Non-Volatile Writing Process
• Non-Destructive Reading Process
• Requirements
• Problems and Improvements
• Conclusion

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2. Principles of FeFET Function and Properties

• Function information storage quantified by the
  two-state polarization of a ferroelectric layer
  in the gate of a FET.

Pr " 1 "
P
-Pr " 0 "
Case of MFS structure nMOS

• Properties
• - Non-volatile, due to the remanent
  polarization Pr
• - Non-destructive reading, as it is a measure
  of resistance
• - Fast (20 ns, compared to flash memory 100
  µs), due to the physical process of polarization
  switching.

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2. Principles of FeFET Non-Volatile Writing
Process

• Data stored in the orientation of the
  polarization P
• By applying an electric field (gate voltage)
  higher than the coercitive field Ec (V gt Vc)
  .

E
P
Example 1 writting
Case of MFS structure nMOS
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2. Principles of FeFET Non-Volatile Writing
Process

• Data stored in the orientation of the
  polarization P
• By applying an electric field (gate voltage)
  higher than the coercitive field Ec (V gt Vc)
  .
• After turning off the power, P becomes equals to
  Pr

E
Pr
Example 1 writting
Case of MFS structure nMOS
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2. Principles of FeFET Non-Destructive Reading
Process

• Due to the polarization, charges appear at the
  interface F/Si

Accumulation of electrons
Accumulation of holes
Case of MFS structure nMOS
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2. Principles of FeFET Non-Destructive Reading
Process

• Due to the polarization, charges appear at the
  interface F/Si
• Those charges influence the resistivity of the
  FET channel.

Vg gt VT
Vg gt 0
-
creation of a channel
Case of MFS structure nMOS
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2. Principles of FeFET Non-Destructive Reading
Process

• Due to the polarization, charges appear at the
  interface F/Si
• Those charges influence the resistivity of the
  FET channel.
• The reading is processed by measuring this
  resistivity

Low resistivity of the channel
High resistivity of the channel
Case of MFS structure nMOS
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2. Principles of FeFET Requirements

• Compatibility with CMOS technology
• -gt Integration of the material without change of
  ferroelectric properties
• No retention loss
• -gt conservation of the polarization Pr (more
  than 10 years)
• Easily switchable
• -gt switch must be fast and not need much power.
• High cycle endurance
• -gt more than 1015 writing processes

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Contents

• Ferroelectrics
• Principles of FeFET
• Problems and Improvements
• Conclusion

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Contents

• Ferroelectrics
• Principles of FeFET
• Problems and Improvements
• Interface Issues
• Threshold Voltage
• Retention Time
• Fatigue Effect
• Conclusion

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3. Problems and Improvements Interface Issues
To guaranty the properties of the ferroelectric
material, there must not be any (chemical)
interaction with the substrate.

• Problems
• Interdiffusion between the ferroelectric layer
  and Si during the deposition process.
• Charge injection from Si to the ferroelectric
  during the switching of P.

• Solutions proposed
• New deposition technique (e.g. molecular beam
  epitaxy).
• To isolate the ferroelectric material MFIS and
  MFMIS structures

MFMIS
MFIS
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3. Problems and Improvements Threshold Voltage
The threshold voltage corresponds to the minimum
voltage necessary to apply on the gate to switch
the polarization (in a MFS structure, VVc).

• Problems
• For MFIS and MFMIS (only), the system is
  equivalent to two serial capacitors (voltage
  divider).

VGVFVI, but VIgtgtVF

• Solutions proposed
• The voltage drop is reduced using high e
  insulator such as SrTa2O6

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3. Problems and Improvements Retention Time
The polarization should stay oriented in the same
direction

• Problems
• For MFIS and MFMIS (only), apparition of an
  electric field opposed to the polarization
• Ferroelectrics materials with a low Vc shows a
  unstable polarization

• Solutions proposed
• 1. Buffer layer high capacitance (high e
  material or low thickness)
• 2. Ferroelectrics low remanent polarization Pr
• 3. (MFMIS) M/F layer surface smaller than M/I
• Better stability with large thickness
  ferroelectric layer

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3. Problems and Improvements Fatigue Effect
After several millions of switching, the remanent
polarization must be unchanged

• Problems
• Pr decreases with increasing number of cycles
• (reduction of 50 after 1012 cycles)
• no distinction between on and off state

• Solutions proposed
• SBT instead of PZT
• perfect interface
• new composition of materials

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Contents

• Ferroelectrics
• Principles of FeFET
• Problems and Improvements
• Conclusion

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4. Conclusion

• Big potential for memory application
  non-volatile data storage, non-destructive
  readout...
• Solutions proposed to solve the different problems

MFIS
MFMIS
Buffer layers high ? SBT low Pr Retention
time 160 hours
High writing speed (20 ns) Retention time 16
days

• No perfect solutions (e.g retention time)
  necessity of another approaches

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4. Conclusion

• Novel approach FeFET based on organic materials
  

- Very good retention behaviour -
But long writing time (0.5 ms)

• Considerable efforts are made, but a commercial
  product using FeFET is not available yet

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The End

• Thank you
• for your attention
• References can be found in our report