Date: Fri, 21 Nov 1997 05:07:51 GMT Server: Stronghold/1.3.4 Ben-SSL/1.3 Apache/1.1.3 Content-type: text/html Content-length: 9615 Last-modified: Mon, 13 Oct 1997 21:33:54 GMT Racom Systems, Inc. Ferroelectric Memory Technology for Smart Cards

Ferroelectric Random Access Memory (FRAM®)

FRAM TECHNOLOGY

Racom contactless smart cards use a patented memory technology that merges ferroelectric material with silicon to create ferroelectric random access memory (FRAM® ) This advanced memory technology has significant benefits compared to other existing memory products. Racom holds an exclusive license from the developer, Ramtron International Corporation of Colorado Springs, Colorado to utilize this technology in contactless smart cards. Ramtron is the first semiconductor company to make the combined breakthroughs necessary in materials, processing, and design to manufacture solid state ferroelectric devices. Racom has access to multiple Ramtron licensed manufacturers to ensure a reliable, consistent supply of FRAM product. These licensees include Rohm Company Limited of Japan, and other high volume semiconductor manufacturers.

FRAM in SMART CARDS

FRAM technology is particularly well suited to smart card applications due to a number of key benefits:

 FRAM TECHNOLOGY

Ferroelectrics have been used for many years as pressure transducers. In this application they were thick ceramics. In the FRAM application, the material thickness is less than 1.0 micron. The name ferroelectric, while no iron is used, was chosen because the material's electrical properties are analogous to the magnetic properties of ferromagnetic materials.

Ferroelectrics exhibit a unique ability to retain an electric polarization in the absence of an applied electric field. This stable polarization results from the alignment of internal dipoles within the ferroelectric material. Application of an electric field controls this alignment, allowing storage of digital information in the material.

A simplified model of a unit ferroelectric crystal is shown in figure 1.

Source: Ramtron International Corporation

An externally applied electric field will move the center atom into one of the two stable positions shown based upon the direction of the field. Once the external field is removed, the atom remains in a stable position. Since no external electric field or current is required for the ferroelectric material to remain polarized in either state, a memory device can be built for storing digital (binary) data that will not require power to retain information stored within it.

Ramtron has developed a complex proprietary thin-film ferroelectric material which is compatible with standard semiconductor fabrication techniques. The nonvolatile storage element in FRAM memories is a capacitor constructed from two metal electrodes and a ferroelectric thin film inserted between the transistor and metallization layers of a CMOS process.

Data stored in a ferroelectric memory cell can be read by applying an electric field to the capacitor. If the applied field is in the direction to switch the internal dipoles, more charge will be moved than if the dipoles are not reversed. Sense amplifiers built into the FRAM chips measure this charge and produce either a zero or one on the output pins. After the read takes place, the chip automatically restores the correct data to the cell.

Another aspect of the ferroelectric material, its very high dielectric constant, permits the very efficient construction of capacitor elements on the chip. For use as both data storage, such as in a DRAM cell, or power storage, such as on a remotely accessed system, this property of the material offers the potential for a wide variety of new devices.

FRAM home page / Racom home page / email: info@racom.com

FRAM ARCHITECTURE

FRAM memories combine the features of several different types of memory to offer a true system memory solution. They integrate the fast reads and writes of SRAM, the non-volatility of EEPROM, and very high read/write endurance onto a single, cost effective chip.

Current FRAM products are built using a dual element differential sense approach, as shown in figure 1.

 

 In this architecture, somewhat like a SRAM cell, two nonvolatile elements are integrated in every memory cell, each polarized in the opposite direction. To read the state of the memory cell, both nonvolatile elements are polarized in the same direction. A differential amplifier (sense amp) connected to the bit lines measures the difference between the amount of charge transferred from the two cells and sets the output accordingly.

Like a DRAM, all FRAM accesses modify the state of the storage element, which is then internally restored by the chip during the precharge portion of the cycle. This operation takes place automatically, without any intervention from the system.

ADVANTAGES of FRAM

Two key aspects of the FRAM technology are superior to EEPROM technologies. First, it employs a polarization technique instead of a charge tunneling mechanism. Second, it permits all internal operations to utilize five volts, instead of the 15 to 18 volts required by conventional EEPROM technologies. These two aspects result in superior performance in the area of speed, power consumption, and reliability.

FRAM Speed

In order to program (write) a state into a FRAM cell, the electric field need be applied for less than 100ns in order to polarize the nonvolatile elements. In a standard EEPROM, it takes a millisecond or more for sufficient charge to travel through the insulating oxides to charge up the gate element. In addition, the high voltage generation circuitry takes some time to stabilize before it can cause this transfer to take place. These differences allow a FRAM memory cycle time of 500ns worst case, compared to 10ms for an EEPROM.

FRAM Power Consumption

High voltage generation in an EEPROM requires an oscillator, charge pump, charge storage capacitor, and regulator circuit on the chip, which takes significant area on an EEPROM. In addition, this added circuitry increases the power consumption of the chip, which can be quite significant for some products. For example, a typical serial 4K FRAM memory uses 10 to 50 times less active power than comparable EEPROM parts.

FRAM Reliability

In an EEPROM, the charge tunneling across the oxide layer degrades the characteristics of the oxide, causing catastrophic breakdown or excessive trapped charge. For these reasons, EEPROM devices are guaranteed for typically only 100,000 write cycles. FRAM memories do not suffer from these same limitations, and so can provide 10 billion (1010) cycles, although both read and write operations must adhere to these limits.

FRAM home page / Racom home page / email: info@racom.com

Copyright (c) Racom Systems, Inc 1997. All rights reserved