Method of compensating variations along a word line in a non-volatile memory
Variation in programming efficacy due to variation in time constants along a word line that spans across a memory plane is compensated by adjusting the bit line voltages across the plane to modify the programming rates. In this way, the variation in programming efficacy is substantially reduced during programming of a group of memory cells coupled to the word line. This will allow uniform optimization of programming across the group of memory cells and reduce the number of programming pulses required to program the group of memory cells, thereby improving performance. In one embodiment, during programming, the bit lines in a first half of the memory plane closer to a source of word line voltage is set to a first voltage by a first voltage shifter and the bit lines in a second half of the memory plane further from the source of word line voltage is set to a second voltage by a second voltage shifter.
1. In a non-volatile memory including a group of memory cells coupled to a word line and each memory cell of the group accessible by a bit line, a method of programming the group of memory cells in parallel, comprising:
providing an access node to the word line for applying a programming voltage thereto, with each memory cell of the group being at a corresponding distance of the word line relative to the access node;
setting the bit line voltage of each memory cell of the group as a function of the corresponding distance thereof; and
programming the group of memory cells in parallel by applying the programming voltage to the access node.
2. The method as in claim 1 , wherein the bit line voltage of each memory cell of the group decreases as a monotonic function of the corresponding distance of each memory cell.
3. The method as in claim 1 , wherein the programming voltage comprises a series of voltage pulses.
4. The method as in claim 3 , wherein the voltage pulses increase in amplitude with every pulse.
5. The method as in claim 3 , wherein:
each memory cell of the group receives the programming voltage from the access node through a corresponding RC network defined by the corresponding distance of the word line relative to the access node.
6. The method as in claim 5 , wherein:
the programming voltage received by each memory cell has a programming efficacy modified by finite charging and discharging time due to the corresponding RC network; and
said setting the bit line voltage for the memory cell of the group as a function of the corresponding distance compensates for the modified programming efficacy.
7. The method as in claim 1 , wherein the word line is divided into two halves with a first half closer to the access node and a second half further from the access node; and
the function is such that it produces a first bit line voltage for memory cells coupled to the first half and a second bit line voltage for memory cells coupled to the second half of the word line.
8. The method as in claim 7 , wherein the second bit line voltage is at a ground potential.
9. The method as in claim 1 , wherein the word line is divided into multiple portions; and
the function is such that it produces a distinct bit line voltage for memory cells coupled to each portion of the word line.
10. The method as in claim 1 , wherein the non-volatile memory is a flash EEPROM.
11. The method as in claim 1 , wherein the non-volatile memory has a NAND structure.
12. The method as in claim 1 , wherein the non-volatile memory is on a removable memory card.
13. The method as in claim 1 , wherein the non-volatile memory has memory cells with a floating gate structure.
14. The method as in claim 1 , wherein the non-volatile memory comprises memory cells with a dielectric layer structure.
15. The method as in any one of claims 1 - 14 , wherein the non-volatile memory has memory cells that individually store one bit of data.
16. The method as in any one of claims 1 - 14 , wherein the non-volatile memory has memory cells that individually store more than one bit of data.