Just how close to the end of NAND Flash are we?

One of the surprises that came from the many excellent presentations given at GlobalFoundrries’ Global Technology Conference (GTC) on September 1 at the Santa Clara Convention Center was a down-and-dirty discussion of the limits of current optical lithography by Senior VP of Technology Gregg Bartlett. Currently, we’re using immersion lithography with 193nm light sources to produce features as small as the mid-20nm range. In fact, a big part of the GTC event was GlobalFoundries’ rollout of its 28nm process technology. To get to 20nm, said Bartlett, you need to use double patterning, which splits a mask into two masks--each with half the required resolution--and then you expose the wafer twice for each mask step using the exposure system to offset the two masks by half the bigger resolution. This procedure doubles mask resolution but it also doubles the cost of each mask step. As a result, wafer-exposure costs jump--quite a lot. In fact they nearly double, which is enough to make alternative lithographic approaches such as EUV (extreme-ultraviolet) and multi-E-beam direct-write lithographies start to look economically viable said Bartlett.

So what’s that have to do with NAND Flash? The top NAND Flash manufacturers are already using mid-20nm lithographies for the current generations of Flash devices. To get to the next process step, the next bump in Moore’s Law, it appears that lithography costs are about to make a big jump in the wrong direction. Worse, that jump may well be the end of the line for optical lithography. To make matters even worse, the constant feature shrinkage has now brought inter-transistor electric field up to the point where disturb problems are apparently becoming significant. All of these factors put pressure on the cost of NAND Flash.

These issues consequently hasten the development of alternative NAND Flash cell designs that are more resistant to field-excited disturb problems and they also hasten the search for commercially viable alternatives to nonvolatile NAND Flash memory. Some of the alternatives include PCM, memristors, and MRAM. However, for the immediate future, NAND Flash continues to reign supreme, unchallenged as long as these competing memory-cell technologies continue to elude commercial production.

But do not be fooled. Every memory technology eventually reaches end of life. Otherwise we’d still be using Williams tubes, mercury delay lines, magnetoresistive wire memories, and magnetic cores. When do you think the end will come for NAND Flash?