Transcript FERROELECTRIC RAM

FERROELECTRIC RAM
INTRODUCTION
OBJECTIVE OF THE PAPER
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Ferroelectric RAM (FeRAM or FRAM) is a random access memory similar in
construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to
achieve non-volatility.
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FeRAM is one of a growing number of alternative non-volatile memory technologies
that offer the same functionality as Flash memory.
SCOPE OF THE TOPIC
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FeRAM is competitive in niche applications where its operating characteristics give it
an advantage over Flash.
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The advanced smart card is a promising technology to extend information-providing
services from terminal (PC and mobile phone)
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FeRAM can improve the transaction time in transportation applications through its
fast programming time.
STUDY OF THE TOPIC
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Development of FeRAM began in the late 1980s. Work was done in 1991 at NASA's
Jet Propulsion Laboratory on improving methods of read out, including a novel
method of non-destructive readout using pulses of UV radiation.
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Much of the current FeRAM technology was developed by Ramtron a semiconductor
company.
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FeRAM research projects have also been reported at Samsung, Matsushita, Oki,
Toshiba, Infineon, Hynix, Symetrix, Cambridge University, University of Toronto and
the Interuniversity Microelectronics Centre (IMEC, Belgium).
DESCRIPTION
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Conventional DRAM consists of a grid of small capacitors
and their associated wiring and signaling transistors.
Each storage element, a cell, consists of one capacitor
and one transistor, a so-called "1T-1C" device.
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DRAM cells scale directly with the size of the
semiconductor fabrication process being used to make it.
For instance, on the 90 nm process used by most
memory providers to make DDR2 DRAM, the cell size is
0.22 μm², which includes the capacitor, transistor, wiring,
and some amount of "blank space" between the various
parts – it appears 35% utilization is typical
1T/1C-Cell
•In a DRAM cell capacitor a linear dielectric is
used whereas in a FeRAM cell capacitor the
dielectric structure includes ferroelectric
material, typically lead zirconate titanate (PZT).
•A ferroelectric material has a nonlinear
relationship between the applied electric field
and the apparent stored charge.
Lead Zirconate Titanate (PZT)
•The dielectric constant of a ferroelectric is typically much higher than that of a linear
dielectric because of the effects of semi-permanent electric dipoles formed in the
crystal structure of the ferroelectric material. When an external electric field is
applied across a dielectric, the dipoles tend to align themselves with the field
direction, produced by small shifts in the positions of atoms and shifts in the
distributions of electronic charge in the crystal structure.
In the figure a "1" is encoded using the negative remnant polarization "-Pr", and
a "0" is encoded using the positive remnant polarization "+Pr".
WORKING
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Writing is accomplished by applying a field across the ferroelectric layer by charging
the plates on either side of it, forcing the atoms inside into the "up" or "down"
orientation (depending on the polarity of the charge), thereby storing a "1" or "0".
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Reading the transistor forces the cell into a particular state, say "0". If the cell
already held a "0", nothing will happen in the output lines. If the cell held a "1", the
re-orientation of the atoms in the film will cause a brief pulse of current in the output
as they push electrons out of the metal on the "down" side. The presence of this
pulse means the cell held a "1". Since this process overwrites the cell, reading
FeRAM is a destructive process, and requires the cell to be re-written if it was
changed.
STRUCTURE OF A FERAM CELL
COMPARISON WITH DRAM SYSTEMS
DENSITY
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Smaller components, and less of them, means that more cells can be packed
onto a single chip, which in turn means more can be produced at once from a
single silicon wafer. This improves yield, which is directly related to cost.
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The lower limit to this scaling process is an important point of comparison,
generally the technology that scales to the smallest cell size will end up being
the least expensive per bit. FeRAM and DRAM are constructionally similar, and
can generally be built on similar lines at similar sizes. In both cases the lower
limit seems to be defined by the amount of charge needed to trigger the sense
amplifiers.
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An additional limitation on size is that materials tend to stop being ferroelectric
when they are too small.
POWER CONSUMPTION
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In DRAM, the charge deposited on the metal plates leaks across the insulating
layer and the control transistor, and disappears. In order for a DRAM to store
data for anything other than a microscopic time, every cell must be periodically
read and then re-written, a process known as refresh. Each cell must be
refreshed many times every second (~65 ms) and this requires a continuous
supply of power.
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In contrast, FeRAM only requires power when actually reading or writing a cell.
The vast majority of power used in DRAM is used for refresh, indicating power
usage about 99% lower than DRAM.
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Another non-volatile memory type is Flash RAM, and like FeRAM it does not
require a refresh process. Flash works by pushing electrons across a highquality insulating barrier where they get "stuck" on one terminal of a transistor.
SPEED
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DRAM speed is limited by the speed at which the current stored in the cells can
be drained (for reading) or stored (for writing).
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FeRAM is based on the physical movement of atoms in response to an external
field, which happens to be extremely fast, settling in about 1ns. In theory, this
means that FeRAM could be much faster than DRAM.
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Flash memories commonly need about 1ms to write a bit, whereas even current
FeRAMs are at least 100 times that speed.
FAILURE MECHANISMS
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A decrease of the remanant polarization reduces the difference between
switching- and non-switching charge
 Polarization fatigue (after repeated read-write cycles)
 Retention loss (with time)
Imprint
 shift of the hysteresis loop leads to preference of one polarization state
(write failure; only critical at low voltage) or loss of polarization (read failure)
Increase of temperature leads to worse material properties (i.e. defect
distribution)
TECHNOLOGY USED
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Ferroelectricity is a physical property of a material whereby it exhibits a
spontaneous electric polarization, the direction of which can be switched
between equivalent states by the application of an external electric field.
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Piezoelectricity is the ability of some materials (notably crystals and certain
ceramics, including bone) to generate an electric potential in response to
applied mechanical stress. This may take the form of a separation of electric
charge across the crystal lattice. The word is derived from the Greek piezein,
which means to squeeze or press.
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Pyroelectricity is the ability of certain materials to generate an electrical
potential when they are heated or cooled. As a result of this change in
temperature, positive and negative charges move to opposite ends through
migration (i.e. the material becomes polarized) and hence, an electrical
potential is established.
FUTURE ENHANCEMENT
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Magneto resistive RAM
Magneto resistive Random Access Memory (MRAM) is a non-volatile
computer memory (NVRAM) technology, which has been under development
since the 1990s. The advantages are so overwhelming that MRAM will
eventually become dominant for all types of memory, becoming a true "universal
memory". In MRAM data is not stored as electric charge or current flows, but by
magnetic storage elements.
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Phase Change RAM
Phase-change memory (also known as PCM, PRAM, PCRAM, and
Chalcogenide RAM C-RAM) is a type of non-volatile computer memory. PRAM
uses the unique behavior of chalcogenide glass, which can be "switched"
between two states, crystalline and amorphous, with the application of heat.
Recent versions can achieve two additional distinct states, effectively doubling
its storage capacity. PRAM is one of a number of new memory technologies that
are attempting to compete in the non-volatile role with the almost universal
Flash memory.
FUTURE ENHANCEMENT
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Programmable metallization cell
The programmable metallization cell, or PMC, is a new form of non-volatile
computer memory being developed at Arizona State University and its spinoff, Axon
Technologies. PMC is one of a number of technologies that are being developed to
replace the widely used flash memory, providing a combination of longer lifetimes,
lower power, and better memory density.
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Silicon-Oxide-Nitride-Oxide-Silicon(SONOS)
SONOS, short for Silicon-Oxide-Nitride-Oxide-Silicon, is a type of highperformance non-volatile computer memory. It is similar to the widely used Flash
RAM, but offers lower power usage and a somewhat longer lifetime. SONOS is being
developed as one of a number of potential Flash replacements, and is currently used
in Cypress Semiconductor's PsoC line of products.
FUTURE ENHANCEMENT
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Resistive random-access memory
Resistive random-access memory (RRAM) is a new non-volatile memory type
being developed by Fujitsu, Sharp, Samsung, Micron Technology, Spansion,
Macronix, Winbond, Unity Semiconductor, and other companies.
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Racetrack Memory
IBM Racetrack Memory is an experimental non-volatile memory device under
development at IBM's Almaden Research Center by a team led by Stuart Parkin. In
early 2008 a 3-bit version was successfully demonstrated. Developed successfully,
racetrack would offer storage density higher than comparable solid-state memory
devices like Flash RAM and similar to conventional disk drives, but with much higher
read/write performance. It is one of a number of new technologies vying to become
a "universal memory" in the future.
FUTURE ENHANCEMENT
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Nano-RAM
Nano-RAM is a proprietary computer memory technology from the company
Nantero. It is a type of nonvolatile random access memory based on the mechanical
position of carbon nanotubes deposited on a chip-like substrate. In theory the small
size of the nanotubes allows for very high density memories. Nantero also refers to it
as NRAM in short.
ADVANTAGES & DISADVANTAGES
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FeRAM advantages include: lower power usage, faster write speed and a much
greater maximum number (exceeding 1016 for 3.3 V devices) of write-erase cycles.
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FeRAM disadvantages are: much lower storage densities than Flash devices, storage
capacity limitations and higher cost.
CONCLUSION
FeRAM remains a relatively small part of the overall semiconductor
market. Flash memory is produced using semiconductor line widths of
30 nm at Samsung (2007) while FeRAMs are produced in line widths of
350 nm at Fujitsu and 130 nm at Texas Instruments (2007). The areal
bit densities of flash memory are consequently much higher than
FeRAM, and thus the cost per bit of flash memory is orders of
magnitude cheaper than FeRAM.