Method for erase-verifying a non-volatile memory capable of identifying over-erased and under-erased memory cells
View Patent ↗A memory device verify system determines a state of memory cells in a memory. The memory includes a memory array having a plurality of memory cells coupled to bit lines. A verify circuit is coupled to the bit lines to determine if memory cells have a erase level that is within predetermined upper and lower limits. The verify circuit can include first and second comparators. In one embodiment, the first comparator is used to compare a bit line current with an upper first reference current. The second comparator is used to compare a bit line current with a lower second reference current. The comparator circuit is not limited to reference currents, but can use reference voltages to compare to a bit line voltage. The verify circuit, therefore, eliminates the need for separate bit line leakage testing to identify over-erased memory cells.
1. A method for erase verifying a single level, non-volatile memory cell, the method comprising:
generating a first reference current as an input to a first comparator circuit;
generating a second reference current as an input to a second comparator circuit; and
comparing a bit line current from a column coupled to the non-volatile memory cell substantially simultaneously with the first reference current and the second reference current.
2. The method of claim 1 wherein the non-volatile memory cell is a flash memory cell.
3. The method of claim 1 wherein comparing the bit line current comprises selectively comparing by selecting one of a plurality of bit lines coupled to other memory cells.
4. The method of claim 1 and further including converting the first reference current to a first reference voltage, the second reference current to a second reference voltage, and the bit line current to a bit line voltage prior to comparing.
5. The method of claim 4 wherein the first and second reference currents are converted with a resistor coupled to an activation circuit that regulates a current flow through the resistor.
6. The method of claim 5 wherein the current flow is generated by a current source circuit.
7. A method for erase verifying a single level, non-volatile memory cell, the method comprising:
generating a first reference voltage as an input to a first comparator circuit;
generating a second reference voltage as an input to a second comparator circuit; and
comparing, substantially simultaneously, a bit line voltage from a colunm coupled to the non-volatile memory cell with the first reference voltage and the second reference voltage.
8. The method of claim 7 wherein the non-volatile memory cell is part of a flash memory array having a plurality of memory cells arranged in a row and colunm format.
9. The method of claim 8 and further comprising selectively coupling remaining columns of the memory array for comparing their respective bit line voltages to the first and second reference voltages.
10. A method for erase verifying a single level, non-volatile memory cell, the method comprising:
receiving a first reference current;
converting the first reference current to a first reference voltage;
receiving a second reference current;
converting the second reference current to a second reference voltage;
converting a bit line current, from the memory cell, to a bit line voltage;
comparing, substantially simultaneously, the bit line voltage with the first reference voltage in a first comparator circuit and the second reference voltage in a second comparator circuit; and
identifying if the memory cell is over-erased, under-erased or erased in response to the comparison.
11. The method of claim 10 wherein the memory cell is over-erased if the bit line voltage is greater than the first reference voltage, the memory cell is under-erased if the bit line voltage is less than the second reference voltage, and the memory cell is erased if the bit line voltage is less than the first reference voltage and greater than the second reference voltage.
12. The method of claim 10 and further including performing further erase operations if the cell is under-erased.
13. The method of claim 10 and further including performing a healing operation if the cell is over-erased.
14. A method for erase verifying a single level, non-volatile memory cell, the method comprising:
receiving a reference current;
converting the reference current to a first reference voltage and a second reference voltage;
converting a bit line current, from the memory cell, to a bit line voltage;
comparing, substantially simultaneously, the bit line voltage with the first reference voltage in a first comparator circuit and the second reference voltage in a second comparator circuit; and
identifying if the memory cell is over-erased, under-erased or erased in response to the comparison.
15. A method for erase verifying a single level, non-volatile memory cell, the method comprising:
generating a first reference current;
generating a second reference current;
simultaneously comparing a bit line current from a colun-m coupled to the non-volatile memory cell with the first reference current in a first comparator circuit and the second reference current in a second comparator circuit;
identifying if the memory cell is over-erased, under-erased or erased; and
correcting the memory cell if it is over-erased or under-erased.
16. A method for erase verifying a single level, non-volatile memory cell comprising:
generating a first reference voltage;
generating a second reference voltage;
simultaneously comparing a bit line voltage from a column coupled to the non-volatile memory cell with the first reference voltage in a first comparator circuit and the second reference voltage in a second comparator circuit;
identifying if the memory cell is over-erased, under erased, or erased; and
correcting the memory cell if it is over-erased or under-erased.