F-RAM Technology Brief

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Overview
Established semiconductor memory technologies are
divided into two categories:
1. RAMs are Random Access Memories, which
simply means that the access time for reads and
writes are symmetric.
2. Nonvolatile memories have traditionally been
ROM (Read Only Memory) until the advent of
floating gate technology, which produced electrically
erasable memories such as Flash and EEPROM.
These products allow for in-system programming but
read and write access times are dissimilar. In fact, the
write access times can be several orders of magnitude
greater than the read access times.
Ferroelectric Random Access Memory or F-RAM
has attributes that make it the ideal nonvolatile
memory. It is a true nonvolatile RAM. The write
advantages and non-volatility make it quite suitable
for storing data in the absence of power.

Ferroelectric Property
The ferroelectric property is a phenomena observed
in a class of materials known as Perovskites. Figure 1
shows a Perovskite crystal. The atom in the center
has two equal and stable low energy states. These
states determine the position of the atom. If a field is
applied in the proper plane, the atom will move in the
direction of the field.


Read Operation
The static state of the cell is Bit Line low, Plate Line
low, Word Line low (Figure 6A). The access begins
by applying voltage to the Word Line and the Plate
Line (6B). This applies a field across the ferroelectric
capacitor and the ferroelectric capacitor switches
(6C). The induced charge (Qs) is shared with the
parasitic Bit Line capacitance (Cbit) and the switched
ferroelectric capacitor (Cs). The voltage on the bit
line, therefore, is proportional to the ratio of the
capacitances Cs/Cbit (Cs includes a small
contribution from transistor and interconnect
parasitics).


F-RAM Benefits
Traditional writable nonvolatile memories derived
from floating gate technology use charge pumps to
develop high voltage on-chip (10V or more) to force
carriers through the gate oxide. Therefore, there are
long write delays, high write power, and the write
operation is actually destructive to the memory cell.
Floating gate devices are incapable of supporting
writes that exceed 106 accesses. To put this in
perspective, a data recorder using EEPROM that was
recording data at 1 sample/s would wear out in less
than 12 days. In comparison, the 3V F-RAM
products offer virtually unlimited endurance (1015
accesses).



Applications
1. Power failure
Any nonvolatile memory can retain a configuration.
However, if the configuration changes and power
failure is a possibility, the higher write endurance of
F-RAM allows changes to be recorded without
restriction. Any time the system state changes, it
writes the new state. This avoids writing to memory
on power down when the available time is short and
power is disappearing.
2. RF/ID
In the area of contactless memory, F-RAM provides
an ideal solution. Since RF/ID memory is powered
by an RF field, and the available energy in the field
declines exponentially with distance, low energy
access is critical. The tag must be close enough to the
field to induce the minimum amount of energy to
write and it must extract the energy while in the field.
Applications that require writes (e.g. debit cards, tags
used in manufacturing processes) benefit from
improved write distance, lower sensitivity to motion
(time in the field), and lower RF power required for
the transmitter /receiver.