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 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 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
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 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 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.
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 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 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.
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.
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
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.
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 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)
· 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:
v Data collection and logging
v configuration storage
v nonvolatile buffer
v SRAM replacement & Expansion
v 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.
v 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
End system applications: laser printers and copiers, industrial process control, networking, cable modems and set top boxes, and white goods
v 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 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.
v 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.
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 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 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 in the coming years.
The real opportunities for FRAM:
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 know 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.
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 down, enabling the user to continue where they left off when the unit is powered back up.
Entertainment
Digital car radios are gaining in popularity. Such radios can download station information and store it in nonvolatile memory. The uncertainty of changes in this data makes it risky to use a limited endurance memory such as EEPROM. A common work-around is to maintain such download data in RAM and write it when power is turned off. This requires the use of a large capacitor which can maintain power on the EEPROM while it is written. While inexpensive, these capacitors are physically bulky and undesirable in ever shrinking electronic radios. Matsushita designed a 16K FRAM into their in-car entertainment systems. The FM25C160 saves system board space by eliminating components and allowing a reduced capacitor size, which would not be possible with alternative memory solutions.
Instrument Cluster
Instrument clusters provide varying capabilities. The presence of a low density nonvolatile memory is common, and tracking elapsed miles often leads to frequent writes. The problem of intermittent data errors is frequently experienced by users in this application, possibly associated with electrical noise interfering with slower writing nonvolatile memories. A 4Kb FRAM such as the FM24C04 has been used in such instrumentation with great success and provides robust operating and data integrity in a noisy environment.
Tire Pressure
Automobiles are adopting tire pressure sensing technology in order to mitigate the risks associated with driving with under-inflated tires. Today this technology is implemented by sensing rotational differences between tires and inferring tire pressure. Future systems will likely involve direct sensor technology that can measure tire pressure. A natural extension of this data generation is logging. A historical record of tire pressures could present compelling documentation in determining liability should tire pressure contribute to an accident. Tire pressure logs might be implemented in the car and also in the tire, and FRAM is an ideal solution for this application given its unlimited ability to write in low power environments, such as that of a tire-based historical logger.
ABS - Stability Control
ABS has evolved from its basic form to include traction control and more recently to include stability control. Traction control uses the wheel slip information already produced by ABS sensors to regulate power to prevent spinning tires due to slippery conditions. Stability control is a more sophisticated variety where power is regulated to each wheel depending on driving conditions. Based on speed, turn radius and road conditions the rotation of individual wheels is managed. Such systems are very sophisticated and involve learning algorithms. To use a FRAM for example in this application would be more suitable for users since FRAM allows for unrestricted updates of system data. Currently temperatures for FRAM are specified to 85C and ABS system electronics must normally operate at 125C, however the road map for FRAM products includes meeting these temperature requirements.
Power Train
Like stability control, power train management systems are ever more adaptive and can benefit from a nonvolatile memory that can be updated quickly and often. Also like ABS, these systems operate at 125C and will depend on a future generation of FRAM products, most likely 256Kb parts rated at 125C or higher.
ADVANTAGES:
1. FRAM allows systems to retain information even when power is lost, without resorting to batteries, EEPROM, or flash.
2. Access times are the same as for standard SRAM, so there's no delay-at-write access as there is for EEPROM or flash.
3. Low power consumption , low voltage operation and high write endurance make it superior than other non-volatile memories like EEPROM & FLASH
4. It is less expensive than magnetic memories which require 4 extra mask
DISADVANTAGE:
1. Present high cost .
2. Low density compared to DRAM & SRAM.
FUTURE OF FRAM:
Development of FRAM in full range of densities and operating temperatures to support automotive data handling and storage applications will find a wide variety of applications as said above.
In addition, the FRAM technology can easily be combined with logic and mixed signal technologies to offer more cost effective integrated solutions in the future.
CONCLUSION
The biggest obstacle to large memories is their large power consumption, particularly for wireless applications. But FRAM’s advantage is the low power consumption compared to other new memory technologies , and hence economic. The wide range of applications it has in case of SMART cards and data storage applications, together with the future automotive applications make it one of the best memories among the new memory technologies among ferromagnetic and ovonic memories.
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