Why it didn’t become practical then ?because the existing memories were far more economical.PCM was costly then.but this gap will soon close since PCM will become less costly than Dram in a few years. PCM in 1970 article required 25 V at 200 mA to drive statechange during a write.Todays PCM require power similar to that of Nand and Nor chips of today.Why its attractive?there will come a time when flash memory can no longer be shrunk..all developers agree…btitsnt until a few years..
Dynamic memory : should be periodically refreshed or read or written into..otherwise contents would vanish..Nvrameg:CMOS..CMOS is an on-board semiconductor chippowered by a CMOS battery inside computers that stores information such as the system time and system settings foryour computer.
1 GB SDRAM in a computer primarySecondary :HDD 40 GBTertiary :160 GB tape catridge
Whether it comes from input or harddisk it all goes to RAM first..Memory is part of a team …SATA and PATA
Simple :NOR flash and SRAM…. Complx : NAND flash & DRAM
The chalcogenic compound is surrounded by two electrodes along with a wire to the compound. This wire serves to heat the compound thus changing its state and resistance. The wire is heated through Joule heating whereby due to high resistance of the heating element, when current passes through it, it heats up a high temperature very quickly. 0 to 1 == high to low res.== amorphous to crystal == small current 1 to 0 == low to high res. == crystal to amorphous == larger curentRead takes very small current A layout for a whole memory element can be seen in figure 2. When both the bit line and word line are high, a current goes through the device. There are two different currents used to write to the device. A small current used to change the device from a 0 to a 1. The small current raises the temperature above the crystallization and lets it slowly to keep it in that state.A larger current is used to change the state from a 1 to a 0. This is done by melting the crystalline state and quickly cooling it to leave it in the amorphous state.
When melted it loses all crystalline structure, and rapid cooling below glass transition temperature causes the material to be locked into its amorphous phase. This phase is very stable near room temperature, but the rate of nucleation and growth of crystallites increases exponentially as the melting temperature is approached. To switch the memory element back to its conductive state, the material is heated to a temperature between the glass transition temperature and the melting temperature, causing nucleation and crystal growth to rapidly occur over a period of several nanoseconds.
The figure above shows I‐V characteristics of the OUM device.At low voltages, the device exhibits either a low resistance (~1k) or highresistance (>100k), depending on its programmed state. This is the readregion of operation. To program the device, a pulse of sufficient voltageis applied to drive the device into a high conduction “dynamic onstate”. For a reset device, this requires a voltage greater than Vth.Vth is the device design parameter and for current memoryapplication is chosen to be in the range of 0.5 to 0.9 V. to avoid readdisturb, the device read region as shown in the figure, is well below Vthand also below the reset regime.The device is programmed while it is in the dynamic on state. The finalprogrammed state of the device is determined by the current amplitudeand the pulse duration in the dynamic on state. The reciprocal slope ofthe I‐V curve in the dynamic on state is the series device resistance.
The above figure shows the device read resistance resultingfrom application of the programming current pulse amplitude. Startingin the set condition, moving from left to right, the device continues toremain in SET state as the amplitude is increased. Further increase inthe pulse amplitude begins to reset the device with still further increaseresetting the device to a standard amorphous resistance. Beginningagain with a device initially in the RESET state, low amplitude pulses atvoltages less than Vth do not set the device. Once Vth is surpassed, thedevice switches to the dynamic on state and programmed resistance isdramatically reduced as crystallization of the material is achieved.Further increase in programming current further crystallizes thematerial, which drops the resistance to a minimum value. As theprogramming pulse amplitude is increased further, resetting again isexhibited as in the case above. Devices can be safely reset above thesaturation point for margin. Importantly, the right side of the curveexhibits direct overwrite capability, where a particular resistance valuecan be obtained from a programming pulse, irrespective of the priorstate of the material. The slope of the right side of the curve is the devicedesign parameter and can be adjusted to enable a multi‐ state memorycell.
First implemented in international space station (ISS) by European space agency.Will appear in Chandrayaan II
First implemented in international space station (ISS) by European space agency.Will appear in Chandrayaan II
Phase Change memory
Phase Change Memory(PCM)<br />“No one will need more than 637KB of memory for a personal computer. 640KB ought to be enough for anybody,” :Bill Gates (1981)<br />Jibin George Mathews,<br />06142,<br />S7 EA, <br />Department of Electronics and Communication<br />
History<br />Dr. Ovshinsky -1960s<br />He formed his company ECD(Energy conversion devices)<br />Article in 1970 September 28 edition of Electronics magazine by him & Gordon Moore titled “non volatile & reprogrammable ”<br />2000 – STMicroelectronics & ovonyx<br />What is Phase Change Memory ?<br />”PCM/PRAMuses the unique behavior of chalcogenide glass, which can be "switched" between two states, crystalline and amorphous, with the application of heat.”<br />
Technology to produce high purity thin films<br /> Cost <br />Numerous breakthroughs in chalcogenide materials.<br />Scaling - Less material to heat –less energy reqd.<br />Flash memory will soon reach its scaling limit.<br />Why PCM is becoming attractive now ??<br />
A review of memory basics !<br />What is a computer memory ?<br /> Hard disk ?? No,Its simply a type of storage(permanent)<br />“Out of memory” message on computer indicates RAM.<br /> Memory is commonly known as RAM.<br />Two types: volatile & nonvolatile(NVRAM)<br />Volatile : Static & Dynamic<br />Historically RAM and Hard disk were called primary and secondary storages respectively<br /><ul><li> NVRAM also called CMOS RAM</li></ul>on-board semiconductor chip<br />powered by a CMOS battery inside computers that stores information such as the system time and system settings for<br />your computer.<br />
Memory basics(contd)<br />Hierarchy of computer storage<br /><ul><li> Primary, </li></ul>Why we use volatile RAM as primary ?<br /><ul><li> Secondary
Tertiary</li></li></ul><li>Memory basics(contd)<br />4 groups<br /><ul><li>You turn computer ON
Opening applications will load them into RAM.Saving them causes them to be written to the storage device and file deleted from RAM</li></ul>Why does a computer need so many memory systems ?<br />
Flash memory<br /><ul><li>Non volatile </li></ul>Problems related to flash<br />Nand & Nor flash technologies reaching scaling limit<br />If size of code/data increased by a byte that space has to be doubled<br />It can be written to in bytes only ie can be overwritten only if an entire block is erased. So cant be used for small random writes of processor.<br />So a complement of NVRAM & RAM had to be used.<br />Only good for 100k-1M writes<br />Answer is PCM !!<br />No longer should the code and data be separately stored in NVM and RAM<br />
Current NVM<br />performance is stagnating!<br />forever!<br />density is improving!<br />for how long?<br />Answer is PCM !!<br />
PCM to the rescue !<br />Contributes attributes of NOR,NAND & RAM<br /><ul><li>Byte alterable
Short endurance (105-106)</li></li></ul><li>1.PCM - Introduction<br />Physical characteristics<br />Chemical formula: GexSbyTez<br />Uses chalcogenide glass<br />Varies between two states:<br />Crystalline – low resistance, represents binary 0<br />Amorphous – high resistance, represents binary 1<br />Can switch on the order of nanoseconds<br />1.PCM uses a reversible structural phase-change<br /> (between amorphous phase & crystalline phase) <br /> 2.The small volume of active media in each memory cell acts<br /> as a fast programmable resistor.<br />
5.PCM-operating principle<br /><ul><li>The PCM cell is programmed by application of a current pulse at a voltage above the switching threshold.
PCM devices are programmed by electrically altering the structure (amorphous or crystalline)of a small volume of chalcogenide alloy
The programming pulse drives the memory cell into a high or low resistance state(phase transition process), depending on current magnitude.
Phase transition process can be completed in as quickly as 5 nanoseconds. Information stored in the cell is read out by measurement of the cell’s resistance.</li></li></ul><li>A simple scalable device:<br /><ul><li> An access transistor and a programmable element (PE)
Read/write endurance: >1012 (Flash: 106)</li></ul> PE<br />word-lines<br />bit-lines<br />PCM-operating principle(contd)<br />Memory array with NMOS transistors:<br /><ul><li> PE based on a switching resistance
Phase-change materials amorphous phase: ‘high’-Ohmic</li></ul> crystalline phase: ‘low’-Ohmic<br /><ul><li> Fast switching between amorphous and crystalline phase</li></li></ul><li>Switching<br />Electric pulses induce Joule heating<br />RESET pulse:<br />- T > Tmelt<br />- Rapid cooling down amorphization<br /><ul><li>.</li></ul>SET pulse:<br />- T > Tcryst<br />- Longer pulse crystallization<br />17<br />
The reciprocal slope of I-V curve in the dynamic on state is the series device resistance</li></li></ul><li>R-I Characteristics<br /><ul><li>shows the device read resistance resulting from application of the programming current pulse amplitude.
low amplitude pulses at voltages less than Vth do not set the device. Once Vth is surpassed, the device switches to the dynamic on state and programmed resistance is dramatically reduced as crystallization of the material is achieved.
The slope of the right side of the curve is the device design parameter and can be adjusted to enable a multi‐ state memory cell.</li></li></ul><li>
About Chalcogenide alloy<br /><ul><li>Two types : Nucleation dominant material & fast growth material
Chalcogenide or phase change alloys is a ternary system of Gallium, Antimony and Tellurium. Chemically it is Ge2Sb2Te5.
Production Process: Powders for the phase change targets are produced by state‐of –the art alloying through melting of the raw material and subsequent milling. This achieves the defined particle size distribution. Then powders are processed to discs through Hot Isotactic Pressing</li></li></ul><li>PCM - Advantages<br /> PCM uses a reversible structural phase-changescaled device has been demonstrated<br />Cost/Bit Reduction<br />Small active storage medium<br />Small cell size – small die size<br />Simple manufacturing process – low step count<br />Simple planar device structure<br />Low voltage – single supply<br />Reduced assembly and test costs<br /> Highly Scalable<br />Performance improves with scaling<br />Only lithography limited<br />Low voltage operation<br />Multi-state demonstrated<br />
PCM Today<br /><ul><li>2004 :Samsung Prototyped a 512 MB module
2006 :Intel created a mass producible 128 module
2008: Intel discovered 2 additional states effectively doubling the Capacity
2008 End: Intel begins shipping beta version called Alverstone
Multilevel recording on optical CDs</li></li></ul><li>Challenges<br />Challenges include :Management of proximity heating with declining cell space.<br />Increased set/reset resistance and decreased read current/set current margin with scaling.<br />
Objective analysis pcm white paper August 2009/li></ul> PCM – a 180 nm non volatile memory cell element technology forstand alone and embedded applications – Stefan Lai and TylerLowrey<br /> Current status of Phase change memory – Stefan Lai<br />