Gourt Home::Gourt :: Ferroelectric RAM
- This article is about an information storage medium. For other uses, see Fram (disambiguation).
Ferroelectric RAM (FRAM or FeRAM) is a type of non-volatile computer memory, similar to EEPROM but based on electric field orientation and with near-unlimited number (exceeding 1010 for 5V devices and even more for 3.3V ones) of write-erase cycles. FRAM is currently one of two "advanced" memory technologies attempting to come to market, the other being MRAM which appears to be more successful at this time. The main vendor of FRAM chips is Ramtron International.
Conventional DRAM consists of a grid of small capacitors and their associated wiring and signalling transistors. Each storage element, a "cell", consists of one capacitor and one transistor, and is thus generally twice as large as the basic circuit element 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.
Constructionally, FRAM is very similar to DRAM, with the exception that the dielectric layer in the memory capacitors replaced with a thin ferroelectric film, typically made of lead zirconate titanate (PZT). The resulting cell is electrically similar to the capacitors used in a conventional DRAM cell, but the ferroelectric film will retain an electric field even after the charge in the capacitor (quickly) drains. Depending on the direction of the current flow when the cell is charged, the film will be polarized into one of two directions.
The bit is read by applying an electric field across the capacitor. If the cell is currently in a "1" state the electric field will resist the voltage, and will not allow current to flow until it reaches a critical point at a somewhat higher voltage. Measuring this voltage allows the memory device to read the current state of the cell, revealing it to be "1". If the cell had been in the "0" state, the voltage would remain lower, revealing the "0". However reading the cell also flips the polarity of the electric field, meaning that all reads must be followed by a write to restore the contents, just as on DRAM.
Generally the operation of FRAM is similar to ferrite core memory, one of the primary forms of computer memory in the 1960s. In comparison, FRAM requires far less power to flip the state of the film's polarity, and does so much faster. The requirement for a write cycle for each read cycle, together with the high but not infinite write cycle limit, poses a potential problem for some special applications.
Physically FRAM is almost identical to current DRAMs, with the additional ferroelectic layer. Since the charge quickly drains from the capacitor, DRAMs must be continually "refreshed" with additional current. Unlike DRAM, the charge stored in the capactor does not form the memory in FRAM cells. Although the power needed to read or write an FRAM cell is slightly higher than in a DRAM, there is no need for power when idle, meaning that overall power use is dramatically reduced. Additionally FRAM will retain its contents with no power at all.
It is possible to make FRAM cells using two additional masking steps during normal semiconductor manufacture, leading to the possibility of full integration of FRAM into microcontrollers and other chips. Flash typically requires nine masks. This makes FRAM particularly attractive as an embedded non-volatile memory on microcontrollers, where the simpler process can reduce costs. However, the materials used to make FRAMs are not commonly used elsewhere in integrated circuit manufacturing. Both the PZT ferroelectric layer and the noble metals used for electrodes raise process compatibility and contamination issues.
FRAM does not yet offer the bit density of DRAM and SRAM, but is non-volatile, is faster than Flash/EEPROM memory (write times under 100 nanoseconds, roughly as fast as reading), and has very low power requirements, as unlike the EEPROMs they do not require a charge pump. It is expected to replace EEPROM chips in applications where very many write cycles are required. The very low current consumption makes them suitable for contactless chip cards.
In theory, however, FRAM can be scaled to much smaller cell sizes than currently being utilized. Bulk ferroelectric materials can be self-organized into small magnetic domains only a few nanometers across, known as quantum dots. If a manufacturing technique evolves that allows circuits to be able to interact with these dots, FRAM will have cell sizes on the order of 5 nm, in comparison with Flash which is currently having real difficulty scaling below 65 nm.
Even without this sort of miniaturization, FRAM uses less components per cell (bit) than Flash. Each Flash cell consists of several transistors and a single high-quality insulator. FRAM is much simpler, consisting of a single transistor and the capacitor/magnet. Using the same production lines as modern DRAM devices, even in its current form FRAM should be much less expensive and higher density than Flash. However, Flash production lines have been scaled up to meet massive demand, while DRAM lines are already at capacity. FRAM may not become common until the market demands higher density than Flash can provide.
See also
"Ferroelectric RAM"