NROM non-volatile memory and mode of operation
Operating NVM memory cell such as an NROM cell by using a combination of Fowler-Nordheim tunneling (FNT), hot hole injection (HHI), and channel hot electron (CHE) injection. In the FNT erase step, only a few cells may be verified, and in the CHE second programming step, the threshold voltage of those cells which were not fully erased in the FNT erase step is increased to a high threshold voltage level (ERS state).
1. A method for operating a plurality of NVM memory cells comprising:
performing an erase step using Fowler-Nordheim tunneling (FNT) to establish a high initial threshold voltage (Vt) for the plurality of memory cells; performing a first programming step using hot hole injection (HHI) to reduce the threshold voltage (Vt) of selected ones of the memory cells; and performing a second programming step using channel hot electron (CHE) injection to increase the threshold voltage (Vt) of bits of memory cells that are unnecessarily programmed in the first programming step.
2. The method of claim 1 , wherein the N-VM cells are NROM cells.
3. The method of claim 1 , wherein: each memory cell has two charge-trapping areas; and each charge-trapping area of a memory cell is capable of storing at least one bit.
4. The method of claim 1 , wherein: in the FNT erase step, at least one cell is verified; and in the CHE second programming step, the threshold voltage of those cells which were not fully erased in the FNT erase step is increased to the high initial threshold voltage level.
5. The method of claim 1 , wherein, for operating on a given bit of an NROM cell comprising a first (source) terminal, a gate, and a second (drain) terminal: in the erase step: the first terminal is set to 0 volts (0 v);the gate is set high, such as 10-14 volts; and the second terminal is set to 0 volts (0 v);in the first programming step: the first terminal is left floating or set to shunt; the gate is set to −5 to −8 volts; the second terminal is set to 4-7 volts; in the second programming step: the first terminal is set to ground, the gate is set to 5-8 volts; the second terminal is set to 4-5 volts.
6. The method of claim 5 , wherein, in all of the three (erase, first programming, second programming) steps: a substrate voltage (Vb) is maintained at 0 volts (0 v).
7. The method of claim 1 , wherein: the FNT erase step is performed using pulses separated by reads; the HHI programming step is performed using pulses separated by reads; and the CHE programming step is performed with pulses separated by reads.
8. The method of claim 1 , wherein: the plurality of memory cells comprise a sector; and the sector size is at least 128 kilobytes (KB) to at least 1 megabyte (MB).
9. A method for operating a plurality of NVM memory cells comprising:
performing an erase step using Fowler-Nordheim tunneling (FNT) to establish a high initial threshold voltage (Vt) for the plurality of memory cells; verifying only a few of the plurality of cells; programming selected ones of the plurality of cells to a low threshold voltage; and using channel hot electron (CHE) injection to raise the threshold voltage of selected ones of those cells which were not fully erased in the FNT erase step, and which are desired to be at the high threshold voltage.