Heating NAND brings it back to life

Read an article today in ARS Technica titled NAND flash gets baked, lives longer that researchers at Macronix have come up with a technique that rejuvenates NAND bit cells by heating them.  The process releases the bit cells captive electrons and returns it back to a fresh NAND cell.

As discussed previously in this blog (e.g., see The End of NAND is near, maybe… and What eMLC and eSLC do for SSD longevity) as NAND technology shrinks to smaller transistor diameters, their longevity and durability decreases proportionally. Which means that with denser NAND chips coming out over the coming years, they will become increasingly short lived.

With this new approach and an awful lot of engineering to zap a NAND bit cell with intense heat (800C) can take dead NAND cells and bring them back to life. Apparently this rejuvenation process has been known for some time and had been in use for phase change memory but had not been applied to NAND cells in memory.  They were heating batches of NAND cells for hours at 200C but this wouldn’t be very practical in production.

The new NAND memory cells are designed with a resistive heating element on top of them, which when enabled can heat the NAND bit cells beneath them. According to some news reports I’ve read this enables the NAND cell to go from 10K P/E cycles to 100K P/E cycles.  And the heat only needs to be applied in occasional pulses to keep cells operating within parameters.   As such, it can be used sparingly and not cost too much energy in the process.

Another side effect of heating is that erase cycles operate faster than at normal temperatures, which now adds the possibility of heat assisted NAND cells.   Erasure being one of the key bottlenecks to NAND write performance anything that can speed this up would help.

Hot NAND may have some life in them after all.



Image: Blow Torch by xlibber

Graphene Flash Memory

Model of graphene structure by CORE-Materials (cc) (from Flickr)
Model of graphene structure by CORE-Materials (cc) (from Flickr)

I have been thinking about writing a post on “Is Flash Dead?” for a while now.  Well at least since talking with IBM research a couple of weeks ago on their new memory technologies that they have been working on.

But then this new Technology Review article came out  discussing recent research on Graphene Flash Memory.

Problems with NAND Flash

As we have discussed before, NAND flash memory has some serious limitations as it’s shrunk below 11nm or so. For instance, write endurance plummets, memory retention times are reduced and cell-to-cell interactions increase significantly.

These issues are not that much of a problem with today’s flash at 20nm or so. But to continue to follow Moore’s law and drop the price of NAND flash on a $/Gb basis, it will need to shrink below 16nm.  At that point or soon thereafter, current NAND flash technology will no longer be viable.

Other non-NAND based non-volatile memories

That’s why IBM and others are working on different types of non-volatile storage such as PCM (phase change memory), MRAM (magnetic RAM) , FeRAM (Ferroelectric RAM) and others.  All these have the potential to improve general reliability characteristics beyond where NAND Flash is today and where it will be tomorrow as chip geometries shrink even more.

IBM seems to be betting on MRAM or racetrack memory technology because it has near DRAM performance, extremely low power and can store far more data in the same amount of space. It sort of reminds me of delay line memory where bits were stored on a wire line and read out as they passed across a read/write circuit. Only in the case of racetrack memory, the delay line is etched in a silicon circuit indentation with the read/write head implemented at the bottom of the cleft.

Graphene as the solution

Then along comes Graphene based Flash Memory.  Graphene can apparently be used as a substitute for the storage layer in a flash memory cell.  According to the report, the graphene stores data using less power and with better stability over time.  Both crucial problems with NAND flash memory as it’s shrunk below today’s geometries.  The research is being done at UCLA and is supported by Samsung, a significant manufacturer of NAND flash memory today.

Current demonstration chips are much larger than would be useful.  However, given graphene’s material characteristics, the researchers believe there should be no problem scaling it down below where NAND Flash would start exhibiting problems.  The next iteration of research will be to see if their scaling assumptions can hold when device geometry is shrunk.

The other problem is getting graphene, a new material, into current chip production.  Current materials used in chip manufacturing lines are very tightly controlled and  building hybrid graphene devices to the same level of manufacturing tolerances and control will take some effort.

So don’t look for Graphene Flash Memory to show up anytime soon. But given that 16nm chip geometries are only a couple of years out and 11nm, a couple of years beyond that, it wouldn’t surprise me to see Graphene based Flash Memory introduced in about 4 years or so.  Then again, I am no materials expert, so don’t hold me to this timeline.