What is a “Multilayer Cell”?

MLCFrom time to time I get questions from investors in the memory business asking: “What is a multilayer cell?”

The answer is: “There is no such thing: It’s a misstatement.”

The term “MLC” has, by a number of people, been mistranslated to “multi-layer cell.”  The misunderstanding appears to have originated in the financial community.  People in the flash memory business never use the term at all.

Yes, we talk about MLC, but to us the term means “multilevel cell”.

A multilevel cell is a cell that uses varying voltage levels to represent different states.  With four voltage levels the resulting four states on a single cell can be turned into two memory bits, so NAND flash makers usually take this approach to double the number of bits stored on a cell.  This doubles the number of bits on the memory chip, effectively cutting the cost of the memory in half.

More adventuresome (and less error-sensitive) systems may use eight voltage levels per cell, to get three bits of data storage in a cell that would ordinarily store only one bit of information.  Three bits for the price of one!  Would anyone complain about that?  Not at all!

This is still called MLC by many, since it is still multiple voltage levels.  Technical purists call it “3-bit MLC” and call the more common 2-bit NAND: “2-bit MLC”.  It is more popular, though, to call this eight-level 3-bit NAND “TLC” for “triple-level cell” or “three level cell”, although purists detest that name.

A Google search for “multilayer cell” flash turns up 3,220 results, including hits from companies that should know better, like Hitachi Data Systems and Information Week.

All that The Memory Guy can say is that this is a sad misunderstanding that I hope will fade away before it becomes common nomenclature.  The word “layer” today usually implies a stack of chips, so the term “multilayer cell” can easily be misunderstood to mean a stack of NAND chips.  Stacks of NAND are very costly compared to technologies like MLC and 3D NAND.  (Some folks believe that a 3D NAND chip is “layers” of NAND chips.)

There’s no stacking in either of these technologies.  3D involves using the depth of the chip to produce more bits, and MLC – multiLEVEL flash, stores either 2 or 3 bits in the space of one to create improved economies.

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Jim Handy Objective Analysis Memory Market Research +1 (408) 356-2549 Jim.Handy (at) Objective-Analysis.com

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