Coarse and fine programming in a solid state memory
Memory devices adapted to receive and transmit analog data signals representative of bit patterns of two or more bits facilitate increases in data transfer rates relative to devices communicating data signals indicative of individual bits. Programming of such memory devices includes initially programming a cell with a coarse programming pulse to move its threshold voltage in a large step close to the programmed state. The neighboring cells are then programmed using coarse programming. The algorithm then returns to the initially programmed cells that are then programmed with one or more fine pulses that slowly move the threshold voltage in smaller steps to the final programmed state threshold voltage.
1. A method for programming a first memory cell in a solid state memory comprising an array of memory cells, the method comprising:
biasing the first memory cell with an initial programming voltage, the initial programming voltage determined in response to a state to be programmed in the first memory cell and respective states to be programmed in adjacent memory cells to the first memory cell along a word line of the array of memory cells; and
biasing the first memory cell with at least one subsequent programming voltage until the first memory cell reaches the state to be programmed.
2. The method of claim 1 wherein the initial programming voltage is one of a first set of incrementing voltages and the at least one subsequent programming voltage is one of a second set of incrementing voltages.
3. The method of claim 1 wherein the first memory cell starts in an erased state.
4. The method of claim 1 wherein the first memory cell starts at a negative threshold voltage prior to the first programming voltage.
5. The method of claim 1 wherein the first memory cell has a positive threshold voltage after the first programming voltage.
6. The method of claim 1 wherein each of the subsequent programming voltages are substantially equal.
7. The method of claim 1 wherein the state to be programmed is a particular threshold voltage.
8. The method of claim 7 wherein the initial programming voltage is a voltage that programs the first memory cell to within a particular percentage of the particular threshold voltage.
9. The method of claim 1 wherein the state to be programmed is a digital representation of a threshold voltage.
10. The method of claim 1 wherein the initial programming voltage is further determined in response to states to be programmed in memory cells adjacent to the first memory cell in a bit line direction.
11. The method of claim 1 wherein the initial programming voltage increases a threshold voltage of the first memory cell at a faster rate than the at least one subsequent programming voltage.
12. The method of claim 1 wherein the at least one subsequent programming voltage comprises a plurality of subsequent programming voltages.
13. The method of claim 12 wherein the plurality of subsequent programming voltages are a plurality of different voltages.
14. The method of claim 1 and further including inhibiting programming of the first memory cell after the first memory cell reaches the state to be programmed.
15. The method of claim 14 wherein inhibiting the programming of the first memory cell comprises biasing a bit line coupled to the first memory cell with an inhibit voltage.