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Friday, December 7, 2007

F-RAM Technology Basics

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 F-RAM memory needs no periodic refresh and when power
fails, F-RAM memory retains its data. It's fast, and doesn't wear

F-RAM memory technology is compatible with industry standard CMOS
manufacturing processes. The ferroelectric thin film is placed over
CMOS base layers and sandwiched between two electrodes. Metal interconnect
and passivation complete the process.

Ramtron's F-RAM memory technology has matured significantly since
its inception. Initial F-RAM memory architectures required a two-transistor/two-capacitor
(2T/2C) memory architecture, which resulted in relatively large
cell sizes. Recent advances in ferroelectric materials and processing
have eliminated the need for an internal reference capacitor within
every cell in the ferroelectric memory array. Ramtron's new one-transistor/one-capacitor
cell architecture operates like a DRAM using a single capacitor
as a common reference for each column in the memory array, effectively
cutting the required cell area in half compared to existing 2T/2C
architectures. The new architecture significantly improves the die
leverage and reduces manufacturing costs for resulting F-RAM memory

Ramtron has also migrated to smaller technology nodes to increase
the cost effectiveness of F-RAM memory cells. A recent move to a
0.35-micron manufacturing process reduces the operating power and
increases the die leverage per wafer compared to earlier generations
of Ramtron’s F-RAM products built on the company’s existing
0.5-micron manufacturing line.

All of these exciting developments in F-RAM memory technology are
finding their way into a host of applications that people use everyday.
From office copiers and high-end servers to automotive airbags and
entertainment systems, F-RAM memory is improving an array of products
and applications worldwide.

How F-RAM differs from floating gate technology

F-RAM memory has several advantages over products that use floating
gate storage technology such as EEPROM or Flash. Floating gate devices
have polysilicon gates isolated from the channel by a thin oxide
layer (see item 1 in figure below).

To program the device, high voltage is generated on the control
gate to accelerate the electrons (N-channel device) toward the source
(see item 2 in figure below). As a result, the electrons gain sufficient
kinetic energy to penetrate the insulating layer and are trapped
in the polysilicon material (see item 3 in figure below).

The programming process for floating gate technology takes several
milliseconds, which is an inordinately long time for high-performance
applications. F-RAM can write in billionths of a second, compared
to millionths of a second with floating gate technologies. The programming
process is also destructive to the insulating layer. As a result
EEPROM and Flash devices have a limited write endurance of typically
100,000 to 1,000,000 writes compared to 1,000,000,000,000 or more
for F-RAM.

Any finally, high voltages are required to program floating gate
technologies whereas F-RAM can operate with a relatively low 3-volt
power supply.

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