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Computer memory types
Volatile
Non-volatile
For other uses, see CRAM. DDR SDRAM ( double data rate synchronous dynamic random access memory) is a class of memory Integrated circuit used in Computers It achieves nearly twice Static random access memory (SRAM is a type of Semiconductor memory where the word static indicates that unlike ''dynamic'' RAM (DRAM, it does not Z-RAM, short for " zero capacitor RAM " is a new type of Computer memory in development by Innovative Silicon Inc Twin Transistor RAM ( TTRAM) is a new type of Computer memory in development by Renesas The Williams tube or the Williams-Kilburn tube (after inventors Freddie Williams and Tom Kilburn) developed about 1946 or 1947 Genesis in radar The basic concept of the delay line originated with World War II Radar research as a system to reduce clutter from reflections from the ground Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is Computer memory that can retain the stored information A programmable read-only memory ( PROM) or field programmable read-only memory ( FPROM) is a form of digital memory where the setting of each bit is EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable An EPROM, or E rasable P rogrammable '''''R'''ead-'''O'''nly '''M'''emory'', is a type of memory chip that retains its EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead Magnetoresistive Random Access Memory ( MRAM) is a non-volatile computer memory ( NVRAM) technology which has been under development since The programmable metallization cell, or PMC, is a new form of non-volatile Computer memory being developed at Arizona State University and This article is about the music device manufacturer For the computer memory system see SONOS. Resistive random-access memory ( RRAM) is a new Non-volatile memory type being developed by Fujitsu, Sharp, Samsung, Micron IBM Racetrack Memory is an experimental Non-volatile memory device under development at IBM 's Almaden Research Center by a team led by Stuart Nano-RAM is a proprietary Computer memory technology from the company Nantero. Drum memory is a magnetic Data storage device and was an early form of Computer memory widely used in the 1950s and into the 1960s invented by Gustav Tauschek Magnetic core memory, or ferrite-core memory, is an early form of Random access Computer memory. Prehistory twistor memory Bubble memory is largely the brainchild of a single person Andrew Bobeck. Twistor is a form of Computer memory, similar to Core memory, formed by wrapping or closing Magnetic tape around a current-carrying wire

Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory and Chalcogenide RAM C-RAM) is a type of non-volatile computer memory. Non-volatile Random access memory ( NVRAM) is the general name used to describe any type of random access memory which does not lose its information PRAM uses the unique behavior of chalcogenide glass, which can be "switched" between two states, crystalline and amorphous, with the application of heat. A chalcogenide glass (hard "ch" as in "chemistry" is a Glass containing one or more Chalcogenide element (Group VI in the Periodic In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating An amorphous solid is a Solid in which there is no Long-range order of the positions of the Atoms (Solids in which there is long-range atomic order are 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, which has a number of practical problems these replacements hope to address. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed

Contents

Background

The properties of chalcogenide glasses were first explored as a potential memory technology by Stanford Ovshinsky of Energy Conversion Devices in the 1960s. Stanford R Ovshinsky (1922-) is an American engineer inventor and physicist In the September 1970 issue of Electronics, Gordon Moore—co-founder of Intel—published an article on the technology. Electronics was an American Trade journal that covered the radio industry and its later spin-offs in the mid-late 1900s Gordon Earle Moore (born 3 January 1929 in San Francisco, California) is the co-founder and Chairman Emeritus of Intel Corporation and the author 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. Lithography is a method for Printing using a plate or stone with a completely smooth surface

The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity, and this forms the basis by which data are stored. Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of Electric current. The amorphous, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. Chalcogenide is the same material used in re-writable optical media (such as CD-RW and DVD-RW). Compact Disc ReWritable (CD-RW is a rewritable Optical disc format A DVD-RW disc is a rewritable Optical disc with equal storage capacity to a DVD-R, typically 4 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. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium

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. A chalcogenide is a chemical compound consisting of at least one Chalcogen ion and at least one more Electropositive element Germanium (dʒɚˈmeɪniəm is a Chemical element with the symbol Ge and Atomic number 32 Antimony (IPA (Received Pronunciation, /ˈæntɪmoʊni/ (US is a Chemical element with the symbol Sb (stibium meaning "mark" and Tellurium (tɪˈlʊəriəm/ /tɛl- is a Chemical element that has the symbol Te and Atomic number 52 GeSbTe, or Germanium-Antimony-Tellurium, also known as GST, is a Phase change material from the group of Chalcogenide glasses used in rewritable It is heated to a high temperature (over 600°C), at which point the chalcogenide becomes a liquid. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of Once cooled, it is frozen into an amorphic glass-like state and its electrical resistance is high. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is 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. Crystallization is the (natural or artificial process of formation of solid Crystals precipitating from a homogeneous --> identical Solution The melting point of a solid is the temperature range at which it changes state from solid to Liquid. This phase transition process can be completed in as quickly as five nanoseconds, according to a January 2006 Samsung Electronics patent application concerning the technology. This is comparable to conventional volatile memory devices, for instance, modern DRAM cells have a switching time on the order of two nanoseconds.

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. STMicroelectronics (,)is an franco-italian Electronics and Semiconductor manufacturer headquartered in in Geneva, Switzerland. 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. [1]

A cross-section of two PRAM memory cells. One cell is in low resistance crystalline state, the other in high resistance amorphous state.
A cross-section of two PRAM memory cells. One cell is in low resistance crystalline state, the other in high resistance amorphous state.

PRAM vs. Flash

It is the switching time that makes PRAM, and other replacements for flash memory, most interesting. 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 electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals The gate is constructed with a special "stack" designed to trap charges (either on a floating gate or in insulator "traps"). Charge Trap Flash (Often abbreviated to CTF is a new technology to fabricate a NAND flash device invented by Samsung Electronics in 2006. The presence of charge within the gate shifts the transistor's threshold voltage, \,V_{th} higher or lower, corresponding to a 1 to 0, for instance. The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide and the substrate 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. A charge pump is an Electronic circuit that uses Capacitors as energy storage elements to create either a higher or lower Voltage power source General write times for common Flash devices are on the order of one ms (for a block of data), about 100,000 times the typical 10 ns read time, for SRAM for example (for a byte).

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 speed, thousands of times quicker than conventional hard drives, make it particularly interesting in nonvolatile memory roles that are currently performance-limited by memory access speed.

Additionally, 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 only rated for something on the order of 10,000 to 100,000 writes per sector, and many flash controllers perform wear leveling to spread writes across many physical sectors. Wear levelling (also written wear leveling) is a technique for prolonging the Service life of some kinds of erasable Computer storage media such as

PRAM devices also degrade with use, for different reasons than Flash, but degrade much more slowly. A PRAM device may endure around 100 million write cycles. [2] PRAM lifetime is limited by mechanisms such as degradation due to GST thermal expansion during programming, metal (and other material) migration, and other mechanisms still unknown.

Flash parts can be programmed before being soldered on to a board, or even purchased pre-programmed. A printed circuit board, or PCB, is used to mechanically support and electrically connect Electronic components using conductive pathways or traces 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). Reflow soldering is a process in which a Solder paste (a sticky mixture of powdered solder and flux) is used to temporarily hold the components to their attachment Wave soldering is a large-scale Soldering process by which Electronic components are soldered to a Printed circuit board (PCB to form an electronic assembly This is made worse by the recent drive to lead-free manufacturing. The Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment 2002/95/EC ( / commonly referred to as the Restriction of Hazardous The manufacturer using PRAM parts must provide a mechanism to program the PRAM "in-system" after it has been soldered in place.

The special gates used in Flash memory "leak" charge (electrons) over time, causing corruption and loss of data. The resistivity of the memory element in PCM is more stable, allowing data retention for years or a decade, even at elevated temperatures[2] (which is the worst-case scenario).

By carefully modulating the amount of charge stored on the gate, Flash devices can store multiple (usually two) bits in each physical cell. This effectively 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.

PRAM cell selectors can use various devices: diodes, BJTs and MOSFETs. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes A bipolar (junction transistor ( BJT) is a type of Transistor. The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals 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. The chalcogenide resistance being a necessarily larger resistance than the diode entails that the operating voltage must exceed 1 V by a wide margin to guarantee adequate forward bias current from the diode. Perhaps the most severe consequence of using a diode-selected array, particularly 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. A further concern with scaling below 40 nm is the effect of discrete dopants as the p-n junction width scales down.

2000 and later

In August of 2004, Nanochip licensed PRAM technology for use in MEMS (micro-electric-mechanical-systems) probe storage devices. Microelectromechanical systems ( MEMS) is the technology of the very small and merges at the nano-scale into Nanoelectromechanical systems (NEMS and Nanotechnology 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 which can read and write the chalcogenide. Hewlett-Packard's micro-mover technology can accurately position the platter to 3 nanometers so densities of more than 1 terabit per square inch will be possible if the technology can be perfected. A terabit is a unit of Information or Computer storage, abbreviated Tbit (or Tb) 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.

In September 2006, Samsung announced a prototype 512 Mb (64 MB) device using diode switches[3]. The Samsung Group ( Korean:, Samsung Guerup) is South Korea 's largest company or Chaebol and the world's largest conglomerate The announcement was something of a surprise, and it was especially notable for its fairly high density. The prototype features a cell size of only 46. 7 nm, which is even better than commercial Flash devices currently available. Although Flash devices of even higher capacities were already available (64Gb, or 8 GB, was just coming to market for instance) other Flash competitors are generally much lower density. The only production MRAM and FeRAM devices are only 4 Mb, for example. Magnetoresistive Random Access Memory ( MRAM) is a non-volatile computer memory ( NVRAM) technology which has been under development since Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead The high density of this prototype PRAM device suggests that it could be a real Flash competitor, and not limited to niche roles as these other devices have been. This is especially true in the case of NOR Flash, which allows per-bit addressing (the more common NAND flash can only be accessed in "banks" of many bytes at a time), which has generally lagged NAND densities and appears to be about the same density as this PRAM device. Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value

Samsung's announcement was followed by one from Intel and STMicroelectronics, who demonstrated their own PCM devices at the 2006 Intel Developer Forum in October[4]. STMicroelectronics (,)is an franco-italian Electronics and Semiconductor manufacturer headquartered in in Geneva, Switzerland. Intel Developer Forum (IDF is a gathering of technologists to discuss Intel products and products based around Intel products They showed a 128 Mb part that had very recently started fabbing at STMicroelectronics's line in Italy. Intel stated that the devices were strictly proof-of-concept, but they expect to start sampling within months, and have widespread commercial production within a few years. Intel is already the leading NOR Flash producer, and appears to be aiming their PCM at the same market as Samsung.

PCM 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. PCM memory devices have been introduced by BAE Systems, referred to as C-RAM, claiming excellent radiation tolerance (rad-hard) and latchup immunity. BAE Systems plc (BAE is a British defence and aerospace company headquartered in Farnborough, England, that has global interests particularly Radiation hardening is a method of designing and testing electronic components and systems to make them resistant to damage or malfunctions caused by high-energy Subatomic particles A latchup is the inadvertent creation of a low- impedance path between the power supply rails of an electronic component triggering a Parasitic structure, which then Additionally, BAE claims a write cycle endurance of 108, which will allow it to be a contender for replacing PROMs and EEPROMs in space systems. A programmable read-only memory ( PROM) or field programmable read-only memory ( FPROM) is a form of digital memory where the setting of each bit is EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable

In February 2008, Intel engineers, in cooperation with STMicroelectronics, revealed the first four-state PCM array prototype (effectively 256 Mb on a 128 Mb physical array). This means that instead of the normal two states- fully amorphous and fully crystalline- there are an additional two distinct intermediate states representing different degrees of partial crystallization, allowing for essentially double the memory. Even larger bit densities are possible if tip array technologies such as the HP Micromover are perfected. [1]

Challenges

The greatest challenge for phase-change memory has been the requirement of high programming current density (>107 A/cm2) in the active volume. This has led to active areas which are much smaller than the driving transistor area. The discrepancy has forced phase-change memory structures to package the heater and sometimes the phase-change material itself in sublithographic dimensions. This is a process cost disadvantage compared to Flash.

The contact between the hot phase-change region and the adjacent dielectric is another fundamental concern. 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 is susceptible to a fundamental tradeoff of unintended vs. intended phase-change. This stems primarily from the fact that phase-change is a thermally driven process rather than an electronic process. Thermal conditions which allow for fast crystallization should not be too similar to standby conditions, e. g. room temperature. Otherwise data retention cannot be sustained. With the proper activation energy for crystallization it is possible to have high speed crystallization at programming conditions while having very slow crystallization at normal conditions.

Timeline

References

  1. ^ a b c A Memory Breakthrough, Kate Greene, Technology Review, 04-Feb-2008
  2. ^ a b Intel to Sample Phase Change Memory This Year
  3. ^ SAMSUNG Introduces the Next Generation of Nonvolatile Memory - PRAM
  4. ^ Intel Previews Potential Replacement for Flash
  5. ^ Phase Change to Replace Flash?
  6. ^ Techworld.com - Intel set for first public demo of PRAM

External links

Qimonda AG ( (pronounced "key-MON-duh" is a Memory company split out of Infineon Technologies AG on May 1 2006 to form the second
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