Charge cycling by equalizing the source and bit line levels between pulses during no-verify write operations for NAND flash memory
In non-volatile memory devices, a write typically consists of an alternating set of pulse and verify operations. At the end of a pulse, the device must be biased properly for an accurate verify, after which the device is re-biased for the next pulse. In some cases a non-volatile memory is programmed by an alternating set of pulses, but, for at least some pulses without any intervening verify operations. After a one pulse, but before biasing the memory for the next pulse without an intervening very, the source and bit line levels can be left to float.
1. In a non-volatile memory circuit having non-volatile memory cells formed along a plurality of bit lines and a plurality of word lines according to a NAND type architecture, a method of programming the memory cells along a selected word line comprising:
individually biasing the bit lines to one of a plurality of values, including a program inhibit level and a program enable level;
biasing a common source line for the bit lines to a first non-zero voltage level; and
applying a series of a plurality of programming pulses to the selected word line while the bit lines and common source line are so biased, wherein the series of programming pulses are applied without intervening verify operations, wherein the common source line is maintained at the first non-zero voltage level between the individual pulses of the series of programming pulses, and wherein bit lines biased at the program inhibit level are maintained at the program inhibit level between the individual pulses of the series of programming pulses.
2. The method of claim 1 , further comprising:
subsequent to a first pulse of the series and prior to the pulse subsequent to the first pulse, changing the biasing on one or more of the bit lines from the program enable level to the program inhibit level.
3. The method of claim 2 , wherein which of the one or more bit lines have the bias level changed from the program enable level to the program inhibit level and after which pulse the respective one or more bias levels are so changed, is determined prior to the first pulse of the series being applied to the selected word line.
4. The method of claim 1 , wherein the memory cells are formed upon a well structure and the well structure is biased to ground while applying said series of a plurality of programming pulses.
5. The method of claim 1 , wherein subsequent to applying said series of a plurality of programming pulses without intervening verify operations, applying one or more programming pulses to the selected word line with intervening verify operations being performed.
6. A non-volatile memory circuit comprising:
an array of non-volatile memory cells formed along a plurality of bit lines and a plurality of word lines according to a NAND type architecture;
biasing circuitry connectable to the bit lines to individually bias the bit lines to one of a plurality of values, including a program inhibit level and a program enable level and connectable to a common source line for the bit lines to bias the common source line to a first non-zero voltage level; and
programming circuitry connectable to a selected word line to apply a series of a plurality of programming pulses to the selected word line while the bit lines are individually bias the bit lines to a selected one of a plurality of values and the common source line is biased to the first non-zero voltage level,
wherein the series of programming pulses are applied without intervening verify operations, wherein the common source line is maintained at the first non-zero voltage level between the individual pulses of the series of programming pulses, and wherein bit lines biased at the program inhibit level are maintained at the program inhibit level between the individual pulses of the series of programming pulses.
7. The non-volatile memory circuit of claim 6 , wherein subsequent to a first pulse of the series and prior to the pulse subsequent to the first pulse, the biasing on one or more of the bit lines is changed from the program enable level to the program inhibit level.
8. The non-volatile memory circuit of claim 7 , wherein which of the one or more bit lines have the bias level changed from the program enable level to the program inhibit level and after which pulse the respective one or more bias levels are so changed, is determined prior to the first pulse of the series being applied to the selected word line.
9. The non-volatile memory circuit of claim 6 , wherein the memory cells are formed upon a well structure and the well structure is biased to ground while applying said series of a plurality of programming pulses.
10. The non-volatile memory circuit of claim 6 , wherein subsequent to applying said series of a plurality of programming pulses without intervening verify operations, applying one or more programming pulses to the selected word line with intervening verify operations being performed using sensing circuitry of the non-volatile memory circuit.