3D NAND

# The ONO Film Stack: Oxide, Nitride, Oxide

## 6. 6. The ONO Film Stack Oxide Nitride Oxide

The charge-trap layer never sits by itself against the channel; it is sandwiched between two oxide layers in what the industry calls an ONO stack — oxide, nitride, oxide. The inner oxide, nearest the silicon channel, plays the same role the tunnel oxide played in planar NAND: a thin barrier that electrons must tunnel through to reach the storage layer during a program or erase operation, while being thick enough to block unwanted leakage the rest of the time. The silicon nitride in the middle is the charge-trap layer described in the previous step, the material actually holding the stored bit. The outer oxide, between the nitride and the surrounding control-gate material, is a blocking layer: it prevents the trapped charge from leaking out toward the control gate on the opposite side from the channel, and prevents charge from the control gate leaking back in. All three films together form the complete electrical barrier structure of the cell, and the entire ONO stack is deposited as a thin, continuous film running the length of the vertical channel, the same way the nitride layer alone was described in the previous step — one deposition sequence coats every memory layer in the stack at once.

The ONO Stack: Three Thin Films, Three Different Jobs 6. The ONO Film Stack Oxide Nitride Oxide INNER OXIDE (TUNNEL) Nearest the channel, electrons tunnel through during program/erase Thin enough to tunnel, thick enough to block idle leakage NITRIDE (CHARGE TRAP) The actual storage layer, holds the trapped electrons Same continuous film described in the previous step OUTER OXIDE (BLOCKING) Nearest the control gate, blocks charge leaking either way Keeps trapped charge from draining toward the gate side channel side to gate side: tunnel oxide, nitride trap, blocking oxide
One Deposition Sequence Coats Every Layer of the Stack at Once 6. The ONO Film Stack Oxide Nitride Oxide THE GEOMETRY ONO stack lines the wall of one vertical channel hole Runs continuously past every word-line layer it crosses WHY THIS MATTERS No separate ONO formation step needed per memory layer Layer count grows without multiplying this process step the ONO stack scales with the channel, not with the layer count

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