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3D NAND: How do You Access the Control 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?”

An Alternative Kind of Vertical 3D NAND String

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”

3D NAND: Making a Vertical 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”

Micron NAND Reaches 16nm

Micron has announced that it is sampling a new 128Gb NAND flash chip based upon a 16nm process, with production slated for the fourth quarter. To The Memory Guy’s knowledge this is the tightest process available.

The company, with its partner Intel, gained a lead with its 20nm process generation through its use of a Hi-k tunnel dielectric, a new material that replaces more conventional silicon dioxide layer with a new material (Micron won’t say what) that yields the same capacitance with a thinner layer. This has become very important with today’s tight processes because of issues of inter-cell interference.

Other NAND makers are migrating to Continue reading “Micron NAND Reaches 16nm”