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Phase Change memory

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Phase Change memory

  1. 1. 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 />
  2. 2. 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 />
  3. 3. 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 />
  4. 4. 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 />
  5. 5. 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
  6. 6. Tertiary</li></li></ul><li>Memory basics(contd)<br />4 groups<br /><ul><li>You turn computer ON
  7. 7. CPU derives data from ROM &performs POST
  8. 8. Loads BIOS from ROM
  9. 9. Loads OS from HDD into RAM
  10. 10. 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 />
  11. 11. 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 />
  12. 12. Current NVM<br />performance is stagnating!<br />forever!<br />density is improving!<br />for how long?<br />Answer is PCM !!<br />
  13. 13. PCM to the rescue !<br />Contributes attributes of NOR,NAND & RAM<br /><ul><li>Byte alterable
  14. 14. Faster writes
  15. 15. Faster execution
  16. 16. Non volatile
  17. 17. Can store both code and data</li></ul>PCM performance<br /><ul><li>Fast (~50 ns)
  18. 18. Low voltage (0.4-2 V)
  19. 19. Scaling: good
  20. 20. Medium endurance (109-1013)</li></ul>NVM/Flash performance<br /><ul><li>Slow (s-ms)
  21. 21. High voltage (10-15 V)
  22. 22. Scaling: bad
  23. 23. 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 />
  24. 24. 2.PCM-Technology concept<br />
  25. 25. Technology concept(contd)<br />
  26. 26. 3.PCM-Basic structure<br />
  27. 27. 4.PCM-Cell element characteristics<br />Figure 2 – Entire cell diagram<br />Basic Device Operation<br />
  28. 28. 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.
  29. 29. PCM devices are programmed by electrically altering the structure (amorphous or crystalline)of a small volume of chalcogenide alloy
  30. 30. The programming pulse drives the memory cell into a high or low resistance state(phase transition process), depending on current magnitude.
  31. 31. 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)
  32. 32. High switching speed (~ns)
  33. 33. 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
  34. 34. 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 />
  35. 35.
  36. 36. V-I characteristics<br /><ul><li>.
  37. 37. At low voltages, the device exhibits either a low resistance (~1k) or high resistance (>100k), depending on its programmed state : READ region
  38. 38. For a reset device : V >Vth to program
  39. 39. 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.
  40. 40. 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.
  41. 41. 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>
  42. 42. About Chalcogenide alloy<br /><ul><li>Two types : Nucleation dominant material & fast growth material
  43. 43. Chalcogenide or phase change alloys is a ternary system of Gallium, Antimony and Tellurium. Chemically it is Ge2Sb2Te5.
  44. 44. 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 />
  45. 45. PCM Today<br /><ul><li>2004 :Samsung Prototyped a 512 MB module
  46. 46. 2006 :Intel created a mass producible 128 module
  47. 47. 2008: Intel discovered 2 additional states effectively doubling the Capacity
  48. 48. 2008 End: Intel begins shipping beta version called Alverstone
  49. 49. 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 />
  50. 50. Emerging technologies & PCM<br />
  51. 51. Conclusion<br /><ul><li>Near ideal memory qualities
  52. 52. Broadens system applications
  53. 53. – Embedded, System-On-a-Chip (SOC), other products
  54. 54. Highly scalable
  55. 55. Risk factors have been identified
  56. 56. Time to productize
  57. 57. More companies looking forward to OUM
  58. 58. That might completely replace FLASH.</li></li></ul><li>References <br /><ul><li>www.ovonyx.com
  59. 59. www.ieee.org
  60. 60. http://www.xbitlabs.com/news/memory/display/20050912212649.html
  61. 61. http://www.theinquirer.net/default.aspx?article=36841
  62. 62. http://www.tgdaily.com/2008/09/13/bitmicro_rolls_out_155_gig_solid/
  63. 63. http://www.storagesearch.com/semico-art1.html
  64. 64. http://www.samsung.com/he/presscenter/pressrelease/pressrelease_20060524_0000257996.asp
  65. 65. http://www.intel.com/pressroom/archive/releases/20070530corp.htm
  66. 66. 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 />
  67. 67. THANK YOU !!<br />
  68. 68. Any Questions ?!<br />

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