FRAM 2012-13 CHAPTER 1 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 polarization 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. 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. Dept.of ECE AIT Page 1
Transcript
FRAM 2012-13
CHAPTER 1
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
polarization 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.
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 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 2012-13
CHAPTER 2
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, interconnects 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.
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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 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 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.
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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 non ferroelectric 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,
Always 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.
Fig 2.1: Ferroelectric Crystal
2.1 MEMORY BASIC
FRAM offers a unique set of features relative to other memory technologies.
Traditional mainstream semiconductor memories can be divided into two primary
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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 large amounts of power to write.
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Fig2.2: Memory cell Fig2.3: FRAM
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.
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Fig 2.4: FRAM MODULE
CHAPTER 3
3.1 FRAM SPECIFATIONS
4MB FRAM Nonvolatile Memory Module
Features:
Organization: 4 banks >< 32k >< 32 bits
Highest density: Ferroelectric Memory over 22.4kb/mm
10 year data retention at 85̊ C
Unlimited read /write cycles.
Advanced high reliability ferroelectric process
SRAM & DRAM Compatible
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70ns Access time
130ns Cycle time.
Equal access & cycle time for Read and Writes.
3.2 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 millimeter
die.
4 MB FRAM MODULES
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Fig 4.1: FRAM 4 MB MODULE
Fig 4.2: FRAM CHIP
CHAPTER 4
APPLICATIONS
FRAM is faster than flash memory because it is a fast memory with a very low power
requirement and is expected to have many applications in small consumer devices
such as personal digital assistants (PDAs), handheld phones, power meters, smart
cards 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 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
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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 very 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 cyber cash)
Buy gasoline at a gas stations.
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
nonvolatile buffer
SRAM replacement & Expansion
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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.
CONFIGURATION STORAGE
FRAM helps system designers overcome the woes of 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.
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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 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 in to 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.
4.1 FUTURE APPLICATIONS
APPLICATIONS OF FRAM IN 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
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to sixty. Recent introductions include improved ABS systems with traction control,
continuously variable transmissions, electronic shift, dynamic stability control, and
digital radio platforms. In the coming years, new electronic applications will include
adaptive cruise control with collision avoidance, DVD players with car navigation
and control by wire (x-by-wire) and crash recording (black-box) technology.
Additional sophisticated network technologies will continue to improve behind the
scenes the automation and performance.
The challenge of handling and storing data is a pervasive theme in the proliferation of
automotive electronics. One implication is that increased data handling results in an
increase in the frequency of data updates. Existing memory choices are often
inadequate in managing the frequent updates. FRAM, with fast write and effectively
unlimited endurance offers unique benefits for data handling and storage intensive
applications. Consequently it is expected to be widely adopted in automotive
applications.
THE REAL OPPORTUNITIES FOR FRAM
Below are some of the applications for which automotive development engineers are
evaluating or designing with FRAM products today.
AIRBAG
A principle feature of airbag and restraint systems in the near future will be crash
recorders commonly known as black-boxes. The automotive black box will be
integrated into the airbag or restraint system, it is unlikely to be a separate assembly
such as the aircraft black box. This architecture is attractive because the sensor data
that is critical for a crash recorder is largely available to the controller or can be
accessed via busses already in place such as CAN.
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A crash recorder is a data logger. It may be called on to collect data frequently over a
long period of time in a circular buffer or to respond very quickly based on sensor
readings. Ideally the crash recorder would offer both capabilities. In this rugged
environment the data must be stored in a true nonvolatile memory as any form of
battery backup will present crash survivability challenges. Technologies such as Flash
face performance problems as they provide write endurance which is limited when it
comes to long term data collection and they are far too slow to store data in the
moment of impact. Crash statistics show high percentages of serious crashes result in
a power outage during the crash, therefore data must be stored instantly and in a non-
volatile state, before power leaves the vehicle and data is lost. Ramtron is a member
of the IEEE P1616 committee to define a standard for Motor Vehicle Event Data
Recorders (MEVDR). As a result we have gained valuable insight into data recorder
requirements. Today crash recorders are being designed with FRAM products from
16Kb to 64Kb, typically with a SPI interface such as the FM25640.
TELEMATICS/NAVIGATION
Telematic functions are increasingly part of a high end vehicle electronics package.
These systems provide dynamic maps that allow routing to be adjusted based on
traffic patterns or other criteria. FRAM memories are used today in such system to
store navigation waypoints, bookmarks etc. 16Kb memories are commonly used in
this application. Last year Matsushita selected Ramtron's FRAM for its in-car
navigation system. The 16K FM25C160’s fast read/write and high-endurance features
provide Matsushita mobile automotive devices with a distinctive resume play
function. The FM25C160 stores scene changes and unique user data upon power
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