Ferroelectric RAM
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Computer memory and Computer data storage types |
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Volatile |
Non-volatile |
Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a
FeRAM's advantages over Flash include: lower power usage, faster write speeds[2] and a much greater maximum read/write endurance (about 1010 to 1015 cycles).[3][4] FeRAMs have data retention times of more than 10 years at +85 °C (up to many decades at lower temperatures). Marked disadvantages of FeRAM are much lower
History
Ferroelectric RAM was proposed by
In 1955,
FeRAM was commercialized in the mid-1990s. In 1994, video game company
A major modern FeRAM manufacturer is
Description
Conventional
The 1T-1C storage cell design in a FeRAM is similar in construction to the storage cell in DRAM, in that both cell types include one capacitor and one access transistor. In a DRAM cell capacitor, a linear dielectric is used, whereas in a FeRAM cell capacitor the dielectric structure includes
A ferroelectric material has a nonlinear relationship between the applied electric field and the apparently stored charge. Specifically, the ferroelectric characteristic has the form of a
In terms of operation, FeRAM is similar to DRAM. 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". Reading, however, is somewhat different than in DRAM. 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.
In general, the operation of FeRAM is similar to
Comparison with other memory types
Density
The main determinant of a memory system's cost is the density of the components used to make it up. Smaller components, and fewer 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.
The lower limit to this scaling process is an important point of comparison. In general, the technology that scales to the smallest cell size will end up being the least expensive per bit. In terms of construction, FeRAM and DRAM are similar, and can in general 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. For DRAM, this appears to be a problem at around 55 nm, at which point the charge stored in the capacitor is too small to be detected. It is not clear whether FeRAM can scale to the same size, as the charge density of the PZT layer may not be the same as the metal plates in a normal capacitor.
An additional limitation on size is that materials tend to stop being ferroelectric when they are too small.[13][14] (This effect is related to the ferroelectric's "depolarization field".) There is ongoing research on addressing the problem of stabilizing ferroelectric materials; one approach, for example, uses molecular adsorbates.[13]
To date, the commercial FeRAM devices have been produced at 350 nm and 130 nm. Early models required two FeRAM cells per bit, leading to very low densities, but this limitation has since been removed.
Power consumption
The key advantage to FeRAM over DRAM is what happens between the read and write cycles. 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 very short 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 (typically 16 times per second[15]) and this requires a continuous supply of power.
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, so it seems reasonable to suggest that the benchmark quoted by STT-MRAM researchers is useful here too, indicating power usage about 99% lower than DRAM. The destructive read aspect of FeRAM may put it at a disadvantage compared to
Another non-volatile memory type is flash, and like FeRAM it does not require a refresh process. Flash works by pushing electrons across a high-quality insulating barrier where they get "stuck" on one terminal of a transistor. This process requires high voltages, which are built up in a charge pump over time. This means that FeRAM could be expected to be lower power than flash, at least for writing, as the write power in FeRAM is only marginally higher than reading. For a "mostly-read" device the difference might be slight, but for devices with more balanced read and write the difference could be expected to be much higher.
Reliability
Data reliability is guaranteed in F-RAM even in a high magnetic field environment compared to MRAM. Cypress Semiconductor's[16] F-RAM devices are immune to the strong magnetic fields and do not show any failures under the maximum available magnetic field strengths (3,700 Gauss for horizontal insertion and 2,000 Gauss for vertical insertion). In addition, the F-RAM devices allow rewriting with a different data pattern after exposure to the magnetic fields.
Speed
DRAM speed is limited by the rate at which the charge stored in the cells can be drained (for reading) or stored (for writing). In general, this ends up being defined by the capability of the control transistors, the capacitance of the lines carrying power to the cells, and the heat that power generates.
FeRAM is based on the physical movement of atoms in response to an external field, which is extremely fast, averaging about 1 ns. In theory, this means that FeRAM could be much faster than DRAM. However, since power has to flow into the cell for reading and writing, the electrical and switching delays would likely be similar to DRAM overall. It does seem reasonable to suggest that FeRAM would require less charge than DRAM, because DRAMs need to hold the charge, whereas FeRAM would have been written to before the charge would have drained. However, there is a delay in writing because the charge has to flow through the control transistor, which limits current somewhat.
In comparison to flash, the advantages are much more obvious. Whereas the read operation is likely to be similar in speed, the charge pump used for writing requires a considerable time to "build up" current, a process that FeRAM does not need. Flash memories commonly need a millisecond or more to complete a write, whereas current FeRAMs may complete a write in less than 150 ns.
On the other hand, FeRAM has its own reliability issues, including imprint and fatigue. Imprint is the preferential polarization state from previous writes to that state, and fatigue is the increase of minimum writing voltage due to loss of polarization after extensive cycling.
The theoretical speed of FeRAM is not entirely clear. Existing 350 nm devices have read times on the order of 50–60 ns. Although slow compared to modern DRAMs, which can be found with times on the order of 2 ns, common 350 nm DRAMs operated with a read time of about 35 ns,[17] so FeRAM speed appears to be comparable given the same fabrication technology.
Additional Metrics
This section needs additional citations for verification. (May 2022) |
Ferroelectric RAM | Magnetoresistive random-access memory
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nvSRAM | BBSRAM | |
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Technique | The basic storage element is a ferroelectric capacitor. The capacitor can be polarized up or down by applying an electric field[18] | Similar to ferroelectric RAM, but the atoms align themselves in the direction of an external magnetic force. This effect is used to store data | Has non-volatile elements along with high speed SRAM | Has a lithium energy source for power when external power is off |
Data retention[19] | 10-160 yrs[20][4] | 20 yrs | 20 yrs | 7 yrs, dependent on battery and ambient temperature
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Endurance | 1010 to 1015[4][21] | 108 [22] | Unlimited | Limited |
Speed (best) | 55 ns | 35 ns | 15–45 ns
|
70–100 ns |
Applications
- Datalogger in Portable/Implantable medical devices, as FRAM consumes less energy[23] compared to other non-volatile memories such as EEPROM
- Event-data-recorder in automotive systems to capture the critical system data even in case of crash or failure
- FRAM is used in Smart meters for its fast write and high endurance
- In Industrial PLCs, FRAM is an ideal replacement for battery-backed SRAM (BBSRAM) and EEPROM to log machine data such as CNC tool machine position
Market
FeRAM remains a relatively small part of the overall semiconductor market. In 2005, worldwide semiconductor sales were US$235 billion (according to the
The density of FeRAM arrays might be increased by improvements in FeRAM foundry process technology and cell structures, such as the development of vertical capacitor structures (in the same way as DRAM) to reduce the area of the cell footprint. However, reducing the cell size may cause the data signal to become too weak to be detectable. In 2005, Ramtron reported significant sales of its FeRAM products in a variety of sectors including (but not limited to)
Capacity timeline
As of 2021 different vendors were selling chips with no more than 16Mb of memory in storage size (density).[26]
See also
- Magnetic-core memory
- MRAM
- nvSRAM
- Phase-change memory
- Programmable metallization cell
- Memristor
- Racetrack memory
- Bubble memory
References
- ^ "FRAM technology". Cypress semiconductos.
- ^ "FeTRAM: memória não-volátil consome 99% menos energia". 29 September 2011.
- ^ https://www.fujitsu.com/us/Images/MB85R4001A-DS501-00005-3v0-E.pdf [bare URL PDF]
- ^ a b c "CY15B116QI Data Sheet". Cypress Semiconductors. p. 19.
- MIT, June 1952.
- ^ Ridenour, Louis N. (June 1955). "Computer Memories". Scientific American: 92. Archived from the original on 2016-08-22. Retrieved 2016-08-22.
- ^ "1970: Semiconductors compete with magnetic cores". Computer History Museum. Retrieved 19 June 2019.
- ^ Optically Addressed Ferroelectric Memory with Non-Destructive Read-Out Archived 2009-04-14 at the Wayback Machine
- ^ "EDN, Volume 39, Issues 5-8". EDN. Vol. 39, no. 5–8. 1994. p. 14.
In the highest-volume usage yet for nonvolatile ferroelectric RAMs (FRAMs), video-game maker Sega has shipped several million copies of its new game, "Sonic the Hedgehog III," which incorporates FRAMS from Ramtron International Corp to save a game between sessions.
- ^ ISBN 9789400710191.
- ^ "History: 1990s". SK Hynix. Archived from the original on 5 February 2021. Retrieved 6 July 2019.
- ^ "Cypress Semiconductor completes Ramtron acquisition – Denver Business Journal". Archived from the original on 2012-11-30.
- ^ a b Ferroelectric Phase Transition in Individual Single-Crystalline BaTiO3 Nanowires Archived 2010-06-15 at the Wayback Machine. See also the associated press release.
- ^ Junquera and Ghosez, Nature, 2003, DOI 10.1038/nature01501
- ^ "TN-47-16: Designing for High-Density DDR2 Memory" (PDF). Archived from the original (PDF) on September 20, 2006.
- ^ "FRAM - Magnetic field Immunity". Cypress Semiconductors.
- S2CID 62372447– via IEEE Xplore.
- ^ "FRAM technology brief". Cypress Semiconductors.
- ^ https://site.ieee.org/pikespeak/files/2020/06/Non-Volatile-RAM-Review-ECEN-5823.pdf [bare URL PDF]
- ^ "FRAM Data sheets". Cypress Semiconductors.
- ^ "FRAM". Cypress Semiconductors.
- ^ "StackPath".
- ^ "Energy comparison between FRAM and EEPROM". Cypress Semiconductors.
- ^ "User Manual: Single phase, single rate, Credit Meter". Ampy Automation Ltd.
The FRAM is guaranteed for a minimum of 10,000,000,000 write cycles.
- ^ "FRAM – Ultra-Low-Power Embedded Memory". Texas Instruments.
- ^ AG, Infineon Technologies. "F-RAM (Ferroelectric RAM) - Infineon Technologies". www.infineon.com. Retrieved 2021-12-18.
External links
- FRAM(FeRAM) [Cypress
- FRAM(FeRAM) Application Community Sponsored by Ramtron[Language: Chinese]
- FRAM overview by Fujitsu
- FeRAM Tutorial by the Department of Electrical and Computer Engineering at the University of Toronto
- FRAM operation and technology tutorial
- IC Chips