SSD Memory Technology

Ronald Dries and Jeremy Williams Agenda

• What is a Solid State Drive? • Improvements over HDD • Memory Technology Requirements • Current Technology: NAND Flash • NAND Flash Drawbacks • Ferroelectric RAM • Magnetoresistive RAM • Racetrack Memory • Millipede Memory • Conclusion What is a SSD?

• Data storage device that does not use any moving mechanical components • Solid state means electronics built completely out of semiconductors • SSD's use non-volatile memory microchips to store data • Three major components o PCB o Single plate silicon memory chips o Controller Improvements Over HDD

• Faster • Less susceptible to physical shock • Significantly quieter • Higher reliability • Less power consumption Memory Technology Requirements

• Non-volatile • Cheap • Use less electricity • High capacity • Large number of program/erase cycles • Small size • Low read/write latency • Fast read/write speeds Current Technology: Flash Cell • Memory value held in floating-gate transistor

• The charge present on the floating gate determines the state of the cell • The floating gate can be charged by applying a large positive voltage to the control gate, and discharged by applying a large negative voltage. • Floating gate can hold charge for many years NAND vs. NOR Flash

• NAND o High density o Medium read speed o High write and erase speeds o Indirect or I/O like access • NOR o Lower density o High read speed o Slow write and erase speeds o Random access interface Current Technology: NAND Flash Layout

• A single cell is read by selecting a group, then pulling all but one of the control gates high. If the cell being read has a charged floating gate, the bit line will be pulled to zero. Therefore, a charged gate is logic zero. • NAND is the choice for most flash memory because it is higher density and inherently uses bit-level addressing, making it a good replacement for hard drives. SLC vs. MLC • SLC - Single layer chip o 1 bit per cell o 100,000 cycles erase/write • MLC - Multiple layer chip o 2 bits per cell o 10,000 erase/write cycles SLC MLC High Density X Low Cost per Bit X Endurance X Operating Temperature Range X Low Power Consumption X Write/Erase Speeds X NAND Flash Drawbacks

• Block Erasure • Limited number of Program/Erase cycles • Read disturb • The manner in which the floating gate is charged and discharged limits the minimum feature size that can be produced without large error rates Ferroelectric RAM

• Structured similar to DRAM, but uses a ferroelectric instead of a dielectric material to create a non-volatile "capacitor". • Has all the advantages and disadvantages of DRAM (high speed, low density, etc.) but the added benefit of being non-volatile and extremely low-power (1% of DRAM power consumption) • Could be a universal replacement for various flash technologies. • Has not been manufactured at a sufficiently low feature size (currently at 130 nm). There is also research ongoing to make materials that stay ferroelectric at such small sizes.

Magnetoresistive RAM

• Like FeRAM, a candidate for a universal memory • The current area of research is spin-transfer torque MRAM • Uses spin-polarized current to change the polarity a ferromagnetic plate • The resistance of that plate is measured to read the cell • Nearly as fast as 6T SRAM, with much smaller density - could be used as high up the hierarchy as L2/L3 cache

Racetrack Memory

• Spintronic technology • Domain walls store data on nanowire • Apply tiny pulse of spin polarised current to move domain walls • Possible higher storage density than flash • Higher read/write performance

Millipede Memory

• Bits stored as nanoscopic pits burned into the surface of a thin polymer layer • Atomic force probes read the bits by moving the sled until the bits are under the probe • The cooling of the probe determines its electrical resistance • Data density of more than 1 terabit per square inch (about the limit of perpendicular recording HDD) • Prototype contains 4000 tips in 7 square mm area

Technology Comparison

Memory Read/Write Energy Density Durability Technology Latency consumed on write

MLC NAND 40 us/1 ms 2.5 mJ/KB 750 10,000 Mb/mm2 writes Ferroelectric 10ns/100n 7 uJ/KB 1014 writes RAM s

Magnetoelectric 3/11ns 40 uJ/KB RAM Racetrack 20-32 ns 300 times less 100,000 Goal: to a times Millions of few mW more writes

Millipede 100 mW 1 terabit / square inch Conclusion

• Current SSD's used for storage implement it with NAND flash • NAND flash has a limited program/erase life • NAND flash is reaching density/capacity limits • NAND requires high voltage levels • Alternative flash memories are faster, consume less power, and have higher data densities