Gather round, kids, and let's try to understand the science of 3D NAND

Part 1: Start with the flat 2D planar stuff

Backgrounder The world of flash is moving to 3D NAND, chips with 36, 48, 72 or 64 layers of flash cells, and 96-layer chips being prototyped. How has it developed from 2D NAND?

To say 2D or planar NAND is a single layer of flash cells is true, as is saying that 3D NAND is layered, but 3D NAND is not actually manufactured from layers of 2D NAND stacked one on top of the other.

It's actually made from flipping the orientation of 2D NAND cells from horizontal to perpendicular and then having strings or pillars of flash cells, one above the other. This has consequences for how the die is constructed.

To understand these consequences we need to have a grasp on how 2D NAND is laid out.

2D planar NAND

A NAND cell consists of the cell itself, wordlines and bitlines. The cell is a transistor, a floating-gate MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which stores an electrical charge. It is composed of a control gate above and separated from a floating gate by insulating material or dielectric, such as SiO2, which also separates the floating gate from an underlying substrate.

It signals binary 1 or 0 status by whether there are electrons in the floating gate component or not. When there are, no current flows through the transistor and the logic or binary value is 0.

When electrons are taken away from the floating gate, the transistor conducts electricity and that signals a binary 1. The cell is operated by pumping electrons into or taking electrons away from the floating gate.


Flash cell diagram

A wordline is a horizontal strip of polysilicon, a hyper-pure form of silicon, and it connects the to the transistor's (cell's) control gate.

A bitline is connected to a cell's drain. Different voltage combinations applied to the wordline and bitline define a read, erase or write (program) operation on the cell. The charge or absence thereof in the floating gate affects the threshold at which current will flow across the control+floating gate combination.

An erased NAND cell has a negative threshold voltage while a programed cell has a positive threshold voltage.

Reading a cell involves applying a current to the control gate and then seeing if current flows from the cell's source to its drain. If current flow is absent then the floating gate is charged with electrons (binary 0). A binary 1 is when current does flow from the source to the drain, with the floating gate not being charged.

To write or program a cell, a higher voltage is applied to the control gate, which causes electrons to move from the underlying substrate across the insulating layer into the floating gate (charging).

Erasing a cell involves applying a high voltage to the substrate so that electrons flow from the floating gate across the insulating layer into the substrate.

NAND cells are laid out in rows or strings by being connected end-to-end:

NAND String

NAND cell string diagram. Click to enlarge

Strings typically have 32 or 64 cells in them. A string is connected at one end to a source line and at the other end to a bitline. A string is the minimum read unit. The string (bitline) and ground select transistors are control mechanisms at the ends of a bitline.

A page is a row of cells sharing a wordline, and has thousands of cells in it. A 4K page has 4,096 bytes, meaning 32,768 bits, with each bit being a NAND cell.

A block is a 2D matrix or array comprising pages (rows) and strings (columns).

A flash die has rows and columns of flash cells with the bitline providing horizontal connections to the cells and the wordlines having a vertical connection.

The diagram below shows a schematically simplified array of NAND cells with one horizontal row and a single horizontal column. The bitline and wordline connections are shown. The bitline and wordline do not come into contact with each other.

Let's now add in the other rows and columns to depict a 2D or planar array:


Schematic diagram of a 2D NAND array

Moving onto 3D NAND means taking the individual NAND cells and flipping then into a perpendicular, or upright, orientation. In effect a bitline string is thus flipped upright, and that means the bitline has to run perpendicularly through the cell as well.

Devising that required a touch of genius from a Toshiba researcher, but we'll look at that in part 2. ®

Other stories you might like

  • Yet again, Cream Finance skimmed by crooks: $130m in crypto assets stolen

    Third time's the unlucky charm for loan outfit

    Decentralized finance biz Cream Finance became further decentralized on Wednesday with the theft of $130m worth of crypto assets from its Ethereum lending protocol.

    Cream ( and not reported the loss via Twitter, the third such incident for the loan platform this year.

    "Our Ethereum C.R.E.A.M. v1 lending markets were exploited and liquidity was removed on October 27, 1354 UTC," the Taiwan-based biz said. "The attacker removed a total of ~$130m USD worth of tokens from these markets, using this address. No other markets were impacted."

    Continue reading
  • OpenID-based security features added to GitHub Actions as usage doubles

    Single-use tokens and reusable workflows explained at Universe event

    GitHub Universe GitHub Actions have new security based on OpenID, along with the ability to create reusable workflows, while usage has nearly doubled year on year, according to presentations at the Universe event.

    The Actions service was previewed three years ago at Universe 2018, and made generally available a year later. It was a huge feature, building automation into the GitHub platform for the first time (though rival GitLab already offered DevOps automation).

    It require compute resources, called runners, which can be GitHub-hosted or self-hosted. Actions are commands that execute on runners. Jobs are a sequence of steps that can be Actions or shell commands. Workflows are a set of jobs which can run in parallel or sequentially, with dependencies. For example, that deployment cannot take place unless build and test is successful. Actions make it relatively easy to set up continuous integration or continuous delivery, particularly since they are cloud-hosted and even a free plan offers 2,000 automation minutes per month, and more than that for public repositories.

    Continue reading
  • REvil gang member identified living luxury lifestyle in Russia, says German media

    Die Zeit: He's got a Beemer, a Bitcoin watch and a swimming pool

    German news outlets claim to have identified a member of the infamous REvil ransomware gang – who reportedly lives the life of Riley off his ill-gotten gains.

    The gang member, nicknamed Nikolay K by Die Zeit newspaper and the Bayerische Rundfunk radio station, reportedly owns a €70,000 watch with a Bitcoin address engraved on its face and rents yachts for €1,300 a day whenever he goes on holiday.

    "He seems to prefer T-shirts from Gucci, luxurious BMW sportscars and large sunglasses," reported Die Zeit, which partly identified him through social media videos posted by his wife.

    Continue reading

Biting the hand that feeds IT © 1998–2021