Some of my readers have asked: “How is 3D NAND programmed and erased? Is it any different from planar NAND?”
In a word: No.
(Before I get too far into this allow me to admit that The Memory Guy doesn’t understand quantum physics, so I will be presenting this only to the depth that I understand it. There will be no band-gap diagrams or equations to wrestle with.)
Both 3D NAND and planar NAND use Fowler Nordheim Tunneling (FN) to both program and erase. This differs from NOR flash which programs bits using Continue reading “How Do You Erase and Program 3D NAND?”
A prior post in this series (3D NAND: Making a Vertical String) discussed the difficulties of successfully manufacturing a charge trap flash bit. Still, Spansion, and now other flash makers, have determined to take this route. Why is that?
In Spansion’s case, a charge trap was a means of doubling the bit capacity of its products. It was an inexpensive alternative to standard MLC flash. To date this strategy has worked very well.
As mentioned in that earlier post, 3D NAND uses a charge trap because it’s extremely difficult to create features, like a floating gate, sideways – lithography works from the top down. A charge trap, when used to replace a floating gate, doesn’t need to be patterned, since the Continue reading “3D NAND: Benefits of Charge Traps over Floating Gates”
One of the thornier problems in making 3D NAND is the job of connecting the peripheral logic (the row decoders) to all of those control gates that are on layers buried somewhere within the bit array. Remember that the control gates are the conductive sheets of polysilicon or tantalum nitride at various depths in the chip.
The problem boils down to this: You can’t run connections from each layer up or down the side of the chip to get to the CMOS circuits below. Instead you have to create a terrace structure to expose and connect to each layer.
These connections are made by etching a stair-step pattern into the layers and sinking Continue reading “3D NAND: How do You Access the Control Gates?”
My prior 3D NAND post explained how Toshiba’s BiCS cell works, using a silicon nitride charge trap to substitute for a floating gate. This post will look at an alternative technology used by Samsung and Hynix which is illustrated in the first graphic, a diagram Samsung presented at a technical conference. This cell also uses a charge trap.
Let The Memory Guy warn you, if the process in my prior post seemed tricky, this one promises to put that one to shame!
Part of this stems from the use of a different kind of NAND bit cell. You can shrink flash cells smaller if you use a high-k gate dielectric (one with a high dielectric constant “k”) since it Continue reading “An Alternative Kind of Vertical 3D NAND String”
Let’s look at how one form of 3D NAND is manufactured. For this post we will explore the original design suggested by Toshiba at the IEEE’s International Electron Device Meeting (IEDM) in 2007. It’s shown in the first graphic of this post. (Click on any of the graphics for a better view.)
Toshiba calls this technology “BiCS” for “Bit Cost Scaling.” The technique doesn’t scale the process the way the world of semiconductors has always done to date – it scales the cost without shrinking the length and width of the memory cell. It accomplishes this by going vertically, as is shown in this post’s first graphic.
This takes a special effort. This is where the real Continue reading “3D NAND: Making a Vertical String”
In the prior post we discussed the need to go vertically into the body of the die, since NAND flash can not be scaled much farther in length and width on the die’s surface. Toshiba invented a 3D NAND which has been adopted and refined by all flash makers. The idea is simple: Rather than shrink the cell’s length and width, why not turn the NAND string so that it’s standing on its end?
This concept is illustrated by this post’s first graphic, which was provided by Applied Materials. (Click on the graphic to see the whole thing at a larger size.) A standard NAND string that normally runs longitudinally is turned on its end to become a vertical string. Not only that, but it makes things easier if the string is split into two sections and Continue reading “What is a 3D NAND?”
A memory chip of a certain area costs about the same amount to produce, no matter how many bits it holds. Naturally, the more bits you can cram onto this chip, the cheaper the price per bit will be. Low cost is of the utmost importance in the world of memory.
Memory chip makers have shrunk the cost of a bit some nine orders of magnitude since the 1960s largely by shrinking the process, or “scaling” to increasingly tighter process geometries.
Flash has always been expected to reach a scaling limit. Over the past few generations technologists have developed Continue reading “Why Do We Need 3D NAND?”
In August 2013 Samsung announced its V-NAND, the first production 3D NAND, kicking off a big change in the way that NAND flash will be manufactured. This new technology raises a number of important questions:
- What exactly is a 3D NAND?
- Why does the industry need to go to a 3D topology?
- How the heck do they make such a product?
To answer these questions I assembled a series of articles posted as weekly segments on The Memory Guy blog during the fourth quarter of 2013. The different sections are listed below, with hot links to each section.
Each of these is a topic that is complex enough to warrant its own post, so for the nine Fridays I published a post to explain each one in depth. I hope you find it engaging and informative.
At the Flash Memory Summit yesterday ES Jung, PhD, EVP & GM for the Samsung R&D Center, explained the inner workings of Samsung’s new V-NAND vertical NAND flash technology. I will shortly be writing a series to explain what a 3D NAND is since there is little on the web that gives clear details about the technology.
One key attribute of most 3D NAND approaches is the use of a charge trapping layer. This has to do with the difficulty of manufacturing sideways floating gates.
Dr Jung delighted the show’s audience by explaining that a standard floating gate is like Continue reading “Samsung’s View on Charge Trap Flash”