Monthly Archives: September 2018

Monatomic PCMs: A New Direction

Ron NealeUntil now designers of PCM devices have tried to make PCM meet their expectations by experimenting with an almost infinite number of possible multi-element glass compositions, in order to tinker with or emphasise a particular composition-related device characteristic. The apparent advantage of this great variety of materials comes with the baggage of reliability and performance-compromising element separation, driven by the forces of electro-migration, electrostatic effects and phase separation.

Is it possible to cast aside the problems of the multi-element PCM compositions and look at the possibility of monatomic PCMs?  For a team at IBM, Zurich and Aachen University the answer is an unequivocal “Yes!” and recently they have published details of the remarkable progress they have made with amorphous antimony (Sb), as an initial candidate element. This research was published in a June 2018 paper in Nature Materials Letters titled: Monatomic phase change memory, by Martin Salinga et al, IBM and Aachen University).

A difficulty faces those venturing in this new direction: While it is possible to bring many elements to the amorphous state, they very quickly crystallize at room temperature and higher.  The IBM researchers used simulations to find that the keys to obtaining a stable amorphous state is to control the quenching rate and the volume of the sample. That part of the antimony research is underpinned by some very impressive simulations that use only about 200 atoms.

Here’s the issue that this approach Continue reading

Why are NAND Flash Fabs so Huge?

NAND Economies of ScaleMany readers have probably wondered why NAND flash fabs are so enormous.  Although DRAM fabs used to be the largest, running around 60,000 wafers per month, NAND flash fabs now put that number to shame, running anywhere from 100,000-300,000 wafers per month.  Why are they so huge?

The reason is that you need to run that many wafers to reach the optimum equipment balance.  The equipment must be balanced or some of it will be sitting idle, and with some tools costing $50 million (immersion scanners) you want to minimize their idle time to the smallest possible number.  I am sure that this is a tough problem, although I have never had to solve it myself.

The most important reason that so much attention is focused on this is that the cost of the wafer depends on the efficiency of the fab.  If you built a $13 billion NAND flash fab that produced 90,000 wafers per month instead of 100,000 wafers per month, then the amount of investment per wafer would be 10% higher.  That can make a significant difference to Continue reading