Seminar Report ’03 FRAM INTRODUCTION: A ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its construction is similar to the storage cell of a DRAM. The difference is in the dielectric properties of the material between the capacitor's electrodes. This material has a high dielectric constant and can be polarized by an electric field. The polarisation remains until it gets reversed by an opposite electrical field. This makes the memory non-volatile. Note that ferroelectric material, despite its name, does not necessarily contain iron. The most well-known ferroelectric substance is BaTiO3. A Ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its Dept. of CSE MESCE, Kuttippuram 1
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Seminar Report ’03 FRAM
INTRODUCTION:
A ferroelectric memory cell consists of a ferroelectric
capacitor and a MOS transistor. Its construction is similar to
the storage cell of a DRAM. The difference is in the
dielectric properties of the material between the capacitor's
electrodes. This material has a high dielectric constant and
can be polarized by an electric field. The polarisation
remains until it gets reversed by an opposite electrical field.
This makes the memory non-volatile. Note that ferroelectric
material, despite its name, does not necessarily contain
iron. The most well-known ferroelectric substance is
BaTiO3.
A Ferroelectric memory cell consists of a ferroelectric
capacitor and a MOS transistor. Its construction is similar to
the storage cell of a DRAM. The difference is in the
dielectric properties of the material between the capacitor's
electrodes. This material has a high dielectric constant and
can be polarized by an electric field. The polarisation
Dept. of CSE MESCE, Kuttippuram1
Seminar Report ’03 FRAM
remains until it gets reversed by an opposite electrical field.
This makes the memory non-volatile.
Data is read by applying an electric field to the
capacitor. If this switches the cell into the opposite state
(flipping over the electrical dipoles in the ferroelectric
material) then more charge is moved than if the cell was
not flipped. This can be detected and amplified by sense
amplifiers. Reading destroys the contents of a cell which
must therefore be written back after a read. This is similar
to the precharge operation in DRAM, though it only needs
to be done after a read rather than periodically as with
DRAM refresh.
FRAM is found mainly in consumer devices and
because of its low power requirements, could also be used
in devices that only need to activate for brief periods. FRAM
allows systems to retain information even when power is
lost, without resorting to batteries, EEPROM, or flash.
Access times are the same as for standard SRAM, so there's
no delay-at-write access as there is for EEPROM or flash. In
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addition, the number of write cycles supported by the FRAM
components is nearly unlimited—up to 10 billion
read/writes. FRAM combines the advantages of SRAM -
writing is roughly as fast as reading, and EPROM - non-
volatility and in-circuit programmability
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FRAM Technology
When an electric field is applied to a ferroelectric
crystal, the central atom moves in the direction of the field.
As the atom moves within the crystal, it passes through an
energy barrier, causing a charge spike. Internal circuits
sense the charge spike and set the memory. If the electric
field is removed from the crystal, the central atom stays in
position, preserving the state of the memory. Therefore, the
FRAM memory needs no periodic refresh and when power
fails FRAM memory retains its data. It's fast, and doesn't
wear out!
To increase the memory capacity, the cell size must
always be reduced, and the design, process, and materials
have been improved aggressively for this purpose.
ferroelectric RAM products (FRAMs) are the most advanced
of the flash challengers. The pioneer, Ramtron International
Corp. (Colorado Springs, Colo.), has been selling FRAM
chips since 1992. Their memory capacities are low,
however, the largest being 256Kb—still a small fraction of
the multimegabit chips offered by the major flash memory
makers. In current commercial FRAMs, the interconnects
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that link individual transistors into circuits are 0.5 µm wide
and operate at 3 V. Narrower interconnects are desirable so
that memory cells may be made smaller and be packed in
greater numbers onto an IC. Ramtron's FRAMs are made by
Fujitsu Ltd., Tokyo, which also sells its own FRAM products,
mostly as embedded memory in microcontrollers and smart
cards.
The biggest hurdle for FRAM developers is to advance
the manufacturing technology to smaller geometries and
lower voltages. R&D at Ramtron is aiming at 0.35-µm
interconnect widths and 1.8-V operation. And last
November, Texas Instruments Inc. (Dallas) announced that
it had built a 64Mb FRAM in a standard 0.13-µm CMOS
process, using technology licensed from Ramtron.
At the core of an FRAM cell is a capacitor filled with a
ferroelectric crystalline material, usually a lead-zirconium-
titanate (PZT) compound .Each unit cell (a crystal's basic
building block) of a ferroelectric material has a permanent
electric field around it. That's because the geometric center
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of all the electrons in the unit cell is at a different spot from
the geometric center of all the protons. It's as though two
small particles with equal and opposite charges are
separated from each other by a short distance—in short, it
is an electric dipole.
Many materials form electric dipoles. But what sets
ferroelectric materials apart from other dipolar materials is
that millions of dipoles, in a region called a domain, line up
to point in the same direction. When an electric field is
applied in the opposite direction, the dipoles flip over so
that they again point in the direction of the electric field.
Each unit cell of PZT is shaped like an elongated cube.
At each of the cube's eight corners is an atom of lead; in
the center of each cube face is an oxygen atom; and in the
interior of the cube is an atom of either zirconium or
titanium. This last has two stable positions, explains Mike
Alwais, Ramtron's vice president of FRAM products: "One is
near the cube's top face and the other is near the bottom."
Apply an electric field and the atoms in the interiors of
all the unit cells in the ferroelectric material move in the
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field's direction. Remove the field and the atoms stay put.
The positions of the atoms in the cubes store the bit of
data, a binary 1 or 0.
To read a bit, an electric field is applied. If the atoms
are near the cube "floors" and the electric field pushes
them to the top, the cell gives off a current pulse. This
pulse, representing a stored 1 or 0, is detected by a sense
amplifier. Contributing to pulse amplitude are the
movements of the interior atoms in the crystals of the
ferroelectric material and the capacitor itself. If the atoms
are already near their cubes' "ceilings," they don't budge
when the field is applied and the cell gives off a smaller
pulse, due only to the electric charges stored on the cell
capacitor.
Reading an FRAM cell destroys the data stored in its
capacitor. So after the bit is read, the sense amplifier writes
the data back into the cell, just as in a DRAM.
The FRAM in fact is like the DRAM in every way but
one: the DRAM cell's capacitor is of a nonferroelectric
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material, usually silicon dioxide. When data is stored as
charge on the DRAM cell's capacitor, the charge leaks away
into the silicon substrate almost immediately—unless it is
rewritten several times a second. That requirement drives
up power consumption, and of course when the power is
turned off, the charge stored in the capacitors quickly
disappears.
Because the basic operation and structure of the FRAM
and the DRAM are so similar, Alwais expects that FRAMs will
eventually run as fast as DRAMs with the
same memory capacity and cell size. Texas
Instruments is interested in FRAMs for
embedded applications—for example, for on-
chip storage of the operating instructions for
digital signal processors and microcontrollers.
Memory Basic
FRAM offers a unique set of features relative to other
memory technologies. Traditional mainstream
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semiconductor memories can be divided into two primary
categories -- volatile and nonvolatile. Volatile memories
include SRAM (static random access memory) and DRAM
(dynamic random access memory). They share the property
that they lose their contents after power is removed from
the electronic system. RAM type devices are very easy to
use, and are high performing, but they share the annoying
quirk of losing their mind when the lights go out.
Nonvolatile memories do not lose their contents when
power is removed. However all of the mainstream
nonvolatile memories share a common ancestry that
derives from ROM (read only memory) technology. The
disadvantage is that read only memory is not easy to write
it's impossible. All of its descendants make it very difficult
to write new information into them. They include
technologies called EPROM (almost obsolete now), EEPROM,
and Flash. ROM based technologies are very slow to
write.Another disadvantage is that ROM based memories
wear out after being written a small number of times, and
use a large amounts of power to write.
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FRAM offers features consistent with a RAM
technology, but is nonvolatile like a ROM technology. FRAM
bridges the gap between the two categories and creates
something completely new -- a nonvolatile RAM.
FRAM SPECIFATION :
4MB FRAM Nonvolatile Memory Module
Features:
Organization:4 banks >< 32k >< 32 bits
Highest density: Ferroelectric Memory over 22.4kb/mm
10 year data retension at 85o C
Unlimited read /write cycles.
Advanced high reliability ferroelectric process
SRAM & DRAM Compatible
70ns Access time
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130ns Cycle time.
Equal access & cycle time for Read and Writes.
LOW POWER OPERATION:
2.7V to 3.6V operation .
15mA Active Current.
15microA stand by Current.
The latest 32-Mbit ferroelectric RAM highest density
RAM reported has been developed by Toshiba Corp. This
FRAM uses a new chain cell structure that links together
eight memory cells .Each cell has a ferroelectric capacitor
and field effect transistor in parallel and not in series.The
32Mbit FRAM is made on0.2micron processing ,which
provides 1.875square micron cell size on a 96 square
millimetre die.
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APPLICATIONS:
FRAM is faster than flash memory,because it is fast
memory with a very low power requirement, it is expected
to have many applications in small consumer devices such
as personal digital assistants (PDAs), handheld phones,
power meters, and smart card, and in security systems.
A smart card is a plastic card about the size of a credit
card, with an embedded microchip that can be loaded with
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data, used for telephone calling, electronic cash payments,
and other applications, and then periodically refreshed for
additional use. . A Smart Card is an IC card that contains a
microcomputer, storage circuit, and RF circuit. The
ferroelectric RAM (FRAM) has been developed as a
nonvolatile memory that satisfies the above requirements.
An FRAM embedded in an LSI must operate as a low-voltage
peripheral logic IC. We have developed a new FRAM sensing
scheme that can read bit-line potentials close to the GND
potential.
Currently or soon, you may be able to use a smart card to:
Dial a connection on a mobile telephone and be charged on
a per-call basis
Establish your identity when logging on to an Internet
access provider or to an online bank
Pay for parking at parking meters or to get on subways,
trains, or buses
Give hospitals or doctors personal data without filling out a
form
Make small purchases at electronic stores on the Web (a
kind of cybercash)
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Buy gasoline at a gasoline station
Fujitsu has developed Smart Cards and other high-
security devices that use secure ferroelectric RAM (FRAM)
memory. This type of memory has an anti-tampering
function and is used to keep the keys and parameters
needed for encryption/decryption algorithms, modify the
keys and parameters for application services, store a high-
speed calculation table for encryption/decryption systems,
and support a firewall between applications.
Contactless Smart Cards in particular have rapidly
come into wide use because they are easy to use, can
perform high-speed processing, and can be used in a wide
variety of applications. In keeping with this trend, Fujitsu
has produced various FRAM-embedded (ferroelectric-RAM-
embedded) LSIs for Contactless Smart Cards.
Current applications for FRAM memory products can be
divided into the following four categories:
Data collection and logging
configuration storage
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nonvolatile buffer
SRAM replacement & Expansion
Data Collection & Logging
FRAM allows system designers to write data to
nonvolatile memory faster and more often -- a luxury not
afforded to users of EEPROMs.
Data collection consists of the acquisition and storage
of data, which must be retained in the absence of power
(not temporary or scratchpad in nature). These are
systems, or subsystems that have the primary function of
collecting data that varies over time. In most cases, a
history of the changes is important.
End system applications: metering (electric, gas, water,
flow), RF/ID, instrumentation, and certain automotive
application such as airbag controllers.
Configuration Storage
FRAM helps system designers overcome the woes of
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sudden power loss by providing the flexibility to store
configuration information in real time -- not just on power
down.
Configuration storage deals with the tracking of a
system as it changes over time. The goal is either to restore
its state on power up, or to identify the cause of an error. In
general, data collection is often the function of a system or
subsystem, where as configuration storage is a low level
engineering function regardless of the system type.
End system applications: laser printers and copiers,
industrial process control, networking, cable modems and
set top boxes, and white goods
Nonvolatile Buffer
FRAM can store operating data quickly, before transmitting
or storing in other nonvolatile media.
In this case, information is being sent from one
subsystem to another, this information is critical and should
not be lost if power fails. In some cases, the target system
is a larger storage device. FRAM, with its fast write and high
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endurance allows the user to store data before it is sent on
to another system.
End system applications: industrial systems and in banking
systems such as ATM machines, future applications will
include hard disk drives with nonvolatile caching.
SRAM Replacement & Expansion
FRAM's fast write and nonvolatile features allow
system designers to combine SRAM and EEPROM into one
device, or simply expand SRAM.
In many cases, a system uses multiple memory types.
FRAM offers the ability to perform ROM, RAM, and EEPROM
functions with one device, saving space, power and
sometimes cost. The most common example is an
embedded microcontroller with external serial EEPROM.
FRAM can replace the EEPROM, and offer additional SRAM
functionality to the micro as well.
End system applications: all-in-one memories tend to occur
in portable applications, and in any system using low-end
(resource poor) microcontrollers.
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FUTURE APPLICATIONS:
APPLICATIONS OF FRAM ON AUTOMOTIVE
APPLICATIONS:-
Today's passenger automobiles and trucks offer
increased electronic content and this trend is expected to
accelerate. With some 55 million passenger vehicles sold
worldwide in 2002 and numerous applications that can
benefit from FRAM technology, the automotive market is
certainly very attractive for FRAM. The average low end
auto has five to ten electronic control units while a luxury
car may have fifty to sixty. Recent introductions include
improved ABS systems with traction control, continuously