Nonvolatile memory design magnetic resistive and phase change pdf
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- Nonvolatile Memory Design: Magnetic, Resistive, and Phase Change
- Phase-change memory
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- Recipe for ultrafast and persistent phase-change memory materials
PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of titanium nitride was used to either quickly heat and quench the glass, making it amorphous , or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies most notably flash memory with the same capability.
Nonvolatile Memory Design: Magnetic, Resistive, and Phase Change
PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of titanium nitride was used to either quickly heat and quench the glass, making it amorphous , or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state.
PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies most notably flash memory with the same capability. Newer PCM technology has been trending in two different directions.
One group has been directing a lot of research towards attempting to find viable material alternatives to Ge 2 Sb 2 Te 5 GST , with mixed success. Another group has developed the use of a GeTe—Sb 2 Te 3 superlattice to achieve non-thermal phase changes by simply changing the co-ordination state of the Germanium atoms with a laser pulse. Leon Chua has argued that all two-terminal non-volatile-memory devices, including PCM, should be considered memristors. In the s, Stanford R.
Ovshinsky of Energy Conversion Devices first explored the properties of chalcogenide glasses as a potential memory technology. In , Charles Sie published a dissertation,   at Iowa State University that both described and demonstrated the feasibility of a phase-change-memory device by integrating chalcogenide film with a diode array.
A cinematographic study in established that the phase-change-memory mechanism in chalcogenide glass involves electric-field-induced crystalline filament growth. However, material quality and power consumption issues prevented commercialization of the technology.
More recently, interest and research have resumed as flash and DRAM memory technologies are expected to encounter scaling difficulties as chip lithography shrinks. The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity values. The amorphous, high resistance state represents a binary 0, while the crystalline, low resistance state represents a 1. In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's refractive index also changes with the state of the material.
Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium , antimony and tellurium GeSbTe called GST. The stoichiometry or Ge:Sb:Te element ratio is Once cooled, it is frozen into an amorphous glass-like state  and its electrical resistance is high.
By heating the chalcogenide to a temperature above its crystallization point , but below the melting point , it will transform into a crystalline state with a much lower resistance. The time to complete this phase transition is temperature-dependent. Cooler portions of the chalcogenide take longer to crystallize, and overheated portions may be remelted.
A more recent advance pioneered by Intel and ST Microelectronics allows the material state to be more carefully controlled, allowing it to be transformed into one of four distinct states; the previous amorphic or crystalline states, along with two new partially crystalline ones.
Each of these states has different electrical properties that can be measured during reads, allowing a single cell to represent two bits, doubling memory density. PRAM's switching time and inherent scalability  make it most appealing. PRAM's temperature sensitivity is perhaps its most notable drawback, one that may require changes in the production process of manufacturers incorporating the technology. Flash memory works by modulating charge electrons stored within the gate of a MOS transistor.
The gate is constructed with a special "stack" designed to trap charges either on a floating gate or in insulator "traps". Changing the bit's state requires removing the accumulated charge, which demands a relatively large voltage to "suck" the electrons off the floating gate.
This burst of voltage is provided by a charge pump , which takes some time to build up power. PRAM can offer much higher performance in applications where writing quickly is important, both because the memory element can be switched more quickly, and also because single bits may be changed to either 1 or 0 without needing to first erase an entire block of cells.
PRAM's high performance, thousands of times faster than conventional hard drives, makes it particularly interesting in nonvolatile memory roles that are currently performance-limited by memory access timing. In addition, with Flash, each burst of voltage across the cell causes degradation. As the size of the cells decreases, damage from programming grows worse because the voltage necessary to program the device does not scale with the lithography.
Most flash devices are rated for, currently, only 5, writes per sector, and many flash controllers perform wear leveling to spread writes across many physical sectors. PRAM devices also degrade with use, for different reasons than Flash, but degrade much more slowly.
A PRAM device may endure around million write cycles. Apart from limiting the lifetime, the limited write endurance also makes PRAM vulnerable to write attack, since an adversary can repeatedly write to a cell to make it fail. Flash parts can be programmed before being soldered on to a board , or even purchased pre-programmed.
The contents of a PRAM, however, are lost because of the high temperatures needed to solder the device to a board see reflow soldering or wave soldering. This is made worse by the recent drive to lead-free manufacturing requiring higher soldering temperatures. The special gates used in Flash memory "leak" charge electrons over time, causing corruption and loss of data.
By carefully modulating the amount of charge stored on the gate, Flash devices can store multiple usually two bits in each physical cell. In effect, this doubles the memory density, reducing cost. PRAM devices originally stored only a single bit in each cell, but Intel's recent advances have removed this problem.
Because Flash devices trap electrons to store information, they are susceptible to data corruption from radiation, making them unsuitable for many space and military applications. PRAM exhibits higher resistance to radiation. Using a diode or a BJT provides the greatest amount of current for a given cell size. However, the concern with using a diode stems from parasitic currents to neighboring cells, as well as a higher voltage requirement, resulting in higher power consumption.
Perhaps the most severe consequence of using a diode-selected array, in particular for large arrays, is the total reverse bias leakage current from the unselected bit lines.
In transistor-selected arrays, only the selected bit lines contribute reverse bias leakage current. The difference in leakage current is several orders of magnitude. Chalcogenide-based threshold switch has been demonstrated as a viable selector for high density PCM arrays .
These devices are not solid state. Instead, a very small platter coated in chalcogenide is dragged beneath many thousands or even millions of electrical probes that can read and write the chalcogenide. The basic idea is to reduce the amount of wiring needed on-chip; instead of wiring every cell, the cells are placed closer together and read by current passing through the MEMS probes, acting like wires. This approach bears much resemblance to IBM's Millipede technology. The prototype featured a cell size of only The high density of Samsung's prototype PRAM device suggested it could be a viable Flash competitor, and not limited to niche roles as other devices have been.
Intel stated that the devices were strictly proof-of-concept. PRAM is also a promising technology in the military and aerospace industries where radiation effects make the use of standard non-volatile memories such as Flash impractical.
This means that instead of the normal two states—fully amorphous and fully crystalline—an additional two distinct intermediate states represent different degrees of partial crystallization, allowing for twice as many bits to be stored in the same physical area.
In June , Samsung and Numonyx B. Phase-change memory devices based on germanium, antimony and tellurium present manufacturing challenges, since etching and polishing of the material with chalcogens can change the material's composition. The dielectric may begin to leak current at higher temperature, or may lose adhesion when expanding at a different rate from the phase-change material.
Phase-change memory has high write latency and energy, which present challenge in its use, although recently, many techniques have been proposed to address this issue. Phase-change memory is susceptible to a fundamental tradeoff of unintended vs. This stems primarily from the fact that phase-change is a thermally driven process rather than an electronic process. Thermal conditions that allow for fast crystallization should not be too similar to standby conditions, e.
Otherwise data retention cannot be sustained. With the proper activation energy for crystallization it is possible to have fast crystallization at programming conditions while having very slow crystallization at normal conditions.
Probably the biggest challenge for phase-change memory is its long-term resistance and threshold voltage drift. This severely limits the ability for multilevel operation a lower intermediate state would be confused with a higher intermediate state at a later time and could also jeopardize standard two-state operation if the threshold voltage increases beyond the design value. This is likely due to the use of highly temperature sensitive p—n junctions to provide the high currents needed for programming.
From Wikipedia, the free encyclopedia. Novel computer memory type. Fons; A. Kolobov; T. July Nature Nanotechnology. Bibcode : NatNa Bibcode : Nanot.. Sie, A. Pohm, P. Uttecht, A. Kao and R. Sie, R. Uttecht, H. Stevenson, J. Griener and K. Retrieved Applied Physics Letters. Bibcode : ApPhL.. Horii et al. Nano Letters. Bibcode : NanoL.. Archived from the original on Redaelli, A. Pellizzer, F.
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Nonvolatile Memory Design: Magnetic, Resistive, and Phase Change The manufacture of flash memory, which is the dominant nonvolatile memory technology.
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Continuous dimensional scaling of the CMOS technology, along with its cost reduction, has rendered Flash memory as one of the most promising nonvolatile memory candidates during the last decade. These devices have indeed exhibited better scaling capability than Flash memory while also facing their respective physical drawbacks. The consequent tradeoffs therefore drive the information storage device technology towards further advancement; as a result, new types of nonvolatile memories, including carbon memory, Mott memory, macromolecular memory, and molecular memory have been proposed. In this paper, the nanomaterials used for these four emerging types of memories and the physical principles behind the writing and reading methods in each case are discussed, along with their respective merits and drawbacks when compared with conventional nonvolatile memories. The potential applications of each technology are also briefly assessed.
Recipe for ultrafast and persistent phase-change memory materials
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PDF | The memory and storage system, including processor caches, main memory, and storage, MRAM), phase-change memory (PCM), and resistive memory tion while the other one (free layer) can change its magnetic.