Non-volatile memory having a bias on the source electrode for HCI programming
View Patent ↗Each cell of a memory is programmed by first using a source bias that is typically effective for programming the cells. If a cell is not successfully programmed in the first attempt, that is typically because a number of cells on the same column as that of the cell that did not successfully program have a relatively low threshold voltage, a low enough threshold voltage that these memory cells are biased, even with grounded gates, to be conductive. The vast majority of the cells do not have this problem, but it is common for there to be a few memory cells that do have this low threshold voltage characteristic. To overcome this, a different source bias is applied during subsequent programming attempts. Thus, the vast majority of the cells are programmed at the faster programming condition, and only the few that need it are programmed using the slower approach.
1. A method for programming a non volatile memory cell, the method comprising:
applying a first programming pulse to a drain of a memory cell with a source of the memory cell at a first voltage level;
applying a second programming pulse, after the applying the first programming pulse, to the drain of the memory cell with the source of the memory cell at a second voltage level, the second voltage level different than the first voltage level.
2. The method of claim 1 wherein the second voltage level is greater than the first voltage level.
3. The method of claim 1 wherein:
the applying the first programming pulse with the source of the memory cell at a first voltage level further includes coupling the source to a circuit with a first resistance level;
the applying the second programming pulse with the source of the memory cell at the second voltage level includes coupling the source to a circuit with a second resistance level, the second resistance level is different than the first resistance level.
4. The method of claim 3 wherein the circuit with a first resistance level includes a first resistor circuit, wherein the circuit with the second resistance level includes the first resistor circuit in series with a second resistive circuit.
5. The method of claim 3 wherein the first resistance level is lower than the second resistance level.
6. The method of claim 1 further comprising:
applying a third programming pulse, after the applying the second programming pulse, to the drain of the memory cell with the source of the memory cell at a third voltage level.
7. A method for programming a non-volatile memory cell, the method comprising:
applying a first programming pulse to a drain of a memory cell of a plurality memory cells with a source of the memory cell at a first voltage level if a state of the memory cell is determined not to be at a desired programmed level;
determining whether to apply a second programming pulse to the drain of the memory cell with the source of the memory cell at the second voltage level after the applying the first programming pulse;
applying the second programming pulse, after the applying the first programming pulse, to the drain of the memory cell with the source of the memory cell at the second voltage level if it is determined in the determining to apply a programming pulse to the drain of the memory cell with the source of the memory cell at the second voltage level, wherein the second voltage level is different than the first voltage level.
8. The method of claim 7 wherein the determining whether to apply the second programming pulse to the drain further includes determining whether the state of the memory cell is at a desired programmed level, wherein the second programming pulse is not applied if the state of the memory cell is determined to be at a desired programmed level.
9. The method of claim 7 wherein the determining whether to apply the second programming pulse to the drain further includes determining whether a predetermined number of programming pulses have been applied to the drain with the source of the memory cell at the first voltage level.
10. The method of claim 9 further comprising:
applying a third programming pulse, after the applying the second programming pulse, to the drain of the memory cell with the source of the memory cell at the first voltage level if it is determined in the determining that a predetermined number of programming pulses have not been applied to the drain with the source of the memory cell at the first voltage level and if a state of the memory cell is not at the desired programmed level.
11. The method of claim 7 wherein the determining whether to apply a programming pulse to the drain further includes determining whether a predetermined number of programming pulses have been applied to the drain of the memory cell, wherein the second programming pulse is not applied if the predetermined number of program pulses have been applied to the drain of the memory cell.
12. The method of claim 7 further comprising:
determining whether to apply a third programming pulse to the drain of the memory cell with the source of the memory cell at a third voltage level after the applying the second programming pulse;
applying a third programming pulse, after the applying the second programming pulse, to the drain of the memory cell with the source of the memory cell at the third voltage level if it is determined in the determining to apply the third programming pulse to the drain with the source of the memory cell at the third voltage level, wherein the third voltage level is different than the first voltage level and the second voltage level.
13. The method of claim 7 wherein:
the applying the first programming pulse to the drain of the memory with the source of the memory cell at the first voltage level further includes coupling the source to a circuit with a first resistance level;
the applying the second programming pulse to the drain of the memory with the source of the memory cell at the second voltage level includes coupling the source to a circuit with a second resistance level, the second resistance level is different than the first resistance level.
14. The method of claim 13 wherein the first resistance level is a negligible resistance level.
15. The method of claim 13 wherein the circuit with a first resistance level includes a first resistor circuit, wherein the circuit with the second resistance level includes the first resistor circuit in series with a second resistive circuit.
16. The method of claim 13 wherein the first resistance level is lower than the second resistance level.
17. The method of claim 13 wherein the second resistance level is lower than the first resistance level.
18. The method of claim 7 wherein the first voltage level is greater than the second voltage level.
19. The method of claim 7 wherein the second voltage level is greater than the first voltage level.
20. The method of claim 7 wherein the first programming pulse and the second programming pulse are hot carrier injection programming pulse.
21. The method of claim 7 wherein each memory cell of the plurality is one of a floating gate memory cell and a thin film memory cell.
22. A memory comprising:
a memory array including a plurality of memory cells;
a source bias circuit coupled to sources of the memory cells of the plurality of memory cells, the source bias circuit configured to provide any one of a plurality of source bias voltages to the sources of the memory cells of the plurality of memory cells during a programming pulse applied to a drain of a memory cell of the plurality of memory cells being programmed, the source bias circuit being responsive to at least one voltage control signal to provide a desired source bias voltage of the plurality of source bias voltages, as indicated by the at least one voltage control signal, to the sources of the memory cells of the plurality of memory cells during the programming pulse applied to a drain of a memory cell of the plurality of memory cells;
wherein the source bias circuit is configured to provide a current path with any one of a plurality of resistance levels during the programming pulse applied to a drain of a memory cell of the plurality of memory cells being programmed, wherein the source bias circuit being responsive to the at least one voltage control signal to provide a current path with a desired resistance level, as indicated by the at least one voltage control signal, during a programming pulse applied to a drain of a memory cell of the plurality of memory cells.
23. The memory of claim 22 further comprising:
a first resistive circuit;
a second resistive circuit;
wherein when the source bias circuit provides the current path with a first resistance level of the plurality of resistance levels, the first resistive circuit and the second resistive circuit are par of the current path;
wherein when the source bias circuit provides a current path with a second resistance level of the plurality of resistance levels, the first resistive circuit, but not the second resistive circuit, is part of the circuit path.
24. The memory of claim 23 wherein the first resistive circuit is coupled to the sources of the memory cells of the plurality of memory cells and is in series with the second resistive circuit when the source bias circuit provides the current path with the first resistance level during the programming pulse applied to the drain of a memory cell of the plurality of memory cells.
25. The memory of claim 22 wherein the source bias circuit further includes:
a first switch having a first current terminal coupled to the sources of the memory cells of the plurality of memory cells, the first switch having a control terminal and a second current terminal, wherein the control terminal is responsive to a signal for making the switch conductive to conduct current from the programming pulse during the programming pulse applied to a drain of a memory cell of the plurality of memory cells; and
a least one resistor circuit coupled to the second current terminal.
26. The memory of claim wherein:
the memory array includes a second plurality of memory cells;
wherein the source bias circuit is configured to provide a second current path with any one of a plurality of resistance levels during the programming pulse applied to a drain of a memory cell of the second plurality of memory cells being programmed, wherein the source bias circuit is responsive to provide the second current path with a desired resistance level, as indicated by at least one voltage control signal, during the programming pulse applied to a drain of a memory cell of the second plurality of memory cells.
27. The memory of claim 22 wherein each memory cell of the plurality of memory cells is one of a floating gate memory cell and a thin film memory cell.
28. A memory comprising:
a memory array including a plurality of memory cells;
a source bias circuit coupled to sources of the memory cells of the plurality of memory cells, the source bias circuit configured to provide any one of a plurality of source bias voltages to the sources of the memory cells of the plurality of memory cells during a programming pulse applied to a drain of a memory cell of the plurality of memory cells being programmed, the source bias circuit being responsive to at least one voltage control signal to provide a desired source bias voltage of the plurality of source bias voltages, as indicated by the at least one voltage control signal, to the sources of the memory cells of the plurality of memory cells during a programming pulse applied to a drain of a memory cell of the plurality of memory cells; and
a controller having at least one output to provide the at least one voltage control signal, wherein the controller provides an indication via the at least one voltage control signal to the source bias circuit to provide a different source voltage of the plurality of source voltages during a subsequent programming pulse applied to a memory cell of the plurality of memory cells from a source voltage of the plurality of source voltages that was provided during a previous programming pulse applied to the memory cell.
29. The memory of claim 28 further comprising:
a sense amplifier coupled to the controller;
a data buffer coupled to the controller;
wherein the controller provides the at least one voltage control signal to the source bias circuit to provide the different source voltage of the plurality of source voltages in response to a determination that a state of the memory cell is not at a desired programmed level as read by the sense amplifier.