Flash memory cell and method to achieve multiple bits per cell
AssigneeInfineon Technologies AG
InventorsDanny Shum, Georg Tempel, Christoph Ludwig
Life of this patent
Estimating the term…
Abstract
A method of flash memory cell programming is provided which uses a uniform electric potential across tunnel oxide. The tight Vt distribution and very stable Vt shift over program/erase cycling allows for a multi-level cell capable of having more than 2 bits per cell.
Claims12
A flash memory comprising:
- a plurality of multi-level cells, each said cell including a floating gate transistor formed in a triple well structure including
- a first well comprising the source and drain of the transistor, a second well of a polarity opposite to the first well and containing the first well, and a substrate of the same polarity at the first well and containing the first and second wells,
- a channel extending between the source and drain and covered with a tunnel insulator and a floating gate,
- a control gate over the floating gate and insulated therefrom, and
- means for applying substantially uniform electric potential to the channel for tunneling charge carriers between the channel and the floating gate and through the tunnel insulator.
A method of programming a flash memory array comprising a plurality of floating gate transistor in a substrate having first and second wells, with the first well inside the second well and the second well is of opposite polarity compared to the substrate and the first well, said first well comprising source and drain regions with a channel disposed between said source and drain regions, a floating gate insulating layer over the channel, a floating gate electrode on the floating gate insulator, an inter-gate insulator on the floating gate, and a control gate electrode on the inter-gate insulator, comprising the steps of:
- applying a substantially uniform electric field over the channel;
- holding the first well at a fixed potential;
- allowing the source and drain regions to float to a potential determined by the potentials applied to the channel and the first well;
- and
- adjusting the intensity of the substantially uniform field an amount sufficient to tunnel electrical charges through the gate oxide for the storage between the control gate and the floating gate or away from the floating gate and into the channel.
Dependencies are inferred from the claim text.