US Patent 7579238 - Multibit nanocrystal nonvolatile memory

http://www.freepatentsonline.com/7579238.html

Due to the discrete nature of nanocrystals they can be useful to facilitate the storage of multiple bits in semiconductor memory cells. This patent from Freescale Semiconductor teaches a variation of this technique which allows for a larger range of applied voltages making bit erasure possible without damage to the memory cell. Claim 1 reads:

1. A method for forming a multi-bit memory cell using a semiconductor substrate, comprising:

forming a first insulating layer over the semiconductor substrate;

forming a second insulating layer over the first insulating layer;

forming a layer of gate material over the second insulating layer;

patterning the gate material to leave a gate portion of the layer of gate material;

etching the second insulating layer to undercut the gate portion and leave a portion of the second insulating layer between the first insulating layer and the gate portion; and

forming nanocrystals on the first insulating layer wherein a first portion of the nanocrystals is under the gate portion on a first side of the portion of the second insulating layer and a second portion of the nanocrystals is under the gate portion on a second side of the portion of the second insulating layer, whereby the first portion of the nanocrystals are for storing a logic state of a first bit and the second portion is for storing a logic state of a second bit.