Method for reading and writing memory cells in three-dimensional FeRAM
A programming method for a three-dimensional ferroelectric memory device is disclosed. The programming method includes applying a first voltage on a selected word line of a target memory cell. The target memory cell has a first logic state and a second logic state corresponding to a first threshold voltage and a second threshold voltage, respectively. The first and second threshold voltages are determined by two opposite electric polarization directions of a ferroelectric film in the target memory cell. The programming method also includes applying a second voltage on a selected bit line, where a voltage difference between the first and second voltages has a magnitude larger than a coercive voltage of the ferroelectric film such that the target memory cell is switched from the first logic state to the second logic state.
1. A reading method of a three-dimensional ferroelectric memory device, comprising:
applying a read voltage on a selected word line of a target memory cell, wherein:
the target memory cell comprises two logic states corresponding to a higher threshold voltage and a lower threshold voltage that are determined by two opposite electric polarization directions of a ferroelectric film in the target memory cell; and
the read voltage is smaller than the higher threshold voltage and is larger than the lower threshold voltage; and
measuring an electric current that flows through the target memory cell.
2. The reading method of claim 1 , further comprising:
electrically connecting a selected memory string to a bit line and to an array common source, wherein the selected memory string comprises a first plurality of stacked memory cells that comprise the target memory cell; and
measuring the electric current at the bit line.
3. The reading method of claim 1 , further comprising:
applying a pass voltage on an unselected word line.
4. The reading method of claim 3 , wherein the applying the pass voltage on the unselected word line comprises applying the pass voltage with a magnitude larger than the higher threshold voltage to switch on all memory cells on the unselected word line.
5. The reading method of claim 4 , wherein the applying the pass voltage comprises applying the pass voltage with the magnitude smaller than a coercive voltage of the ferroelectric film such that the electric polarization direction is not switched.
6. A programming method of a three-dimensional ferroelectric NAND memory device, wherein the three-dimensional ferroelectric NAND memory device comprises a plurality of memory strings vertically extending through a film stack of alternating conductive and dielectric layers, each memory string comprising a plurality of memory cells vertically stacked and each memory cell comprising a ferroelectric film disposed over a sidewall of a channel layer, the programming method comprising:
applying a first voltage on a selected word line of a target memory cell, wherein
the target memory cell comprises a first logic state and a second logic state corresponding to a first threshold voltage and a second threshold voltage, respectively, the first and second threshold voltages determined by two opposite electric polarization directions of the ferroelectric film in the target memory cell; and
applying a second voltage on a selected bit line connected to a drain terminal of the target memory cell, wherein
a voltage difference between the first voltage and the second voltage comprises a first magnitude larger than a coercive voltage of the ferroelectric film to switch the target memory cell from the first logic state to the second logic state.
7. The programming method of claim 6 , wherein switching the target memory cell from the first logic state to the second logic state comprises switching an electric polarization direction to an opposite electric polarization direction.
8. The programming method of claim 6 , wherein applying the first voltage and the second voltage comprises applying voltages with opposite signs and same magnitudes.
9. The programming method of claim 8 , wherein applying the first voltage comprises applying a positive voltage, and wherein applying the second voltage comprises applying a negative voltage.
10. The programming method of claim 9 , wherein switching the target memory cell from the first logic state to the second logic state comprises switching the target memory cell from the first threshold voltage to the second threshold voltage, the first threshold voltage is higher than the second threshold voltage.
11. The programming method of claim 8 , wherein applying the first voltage comprises applying a negative voltage, and wherein applying the second voltage comprises applying a positive voltage.
12. The programming method of claim 11 , wherein switching the target memory cell from the first logic state to the second logic state comprises switching the target memory cell from the first threshold voltage to the second threshold voltage, the first threshold voltage is lower than the second threshold voltage.
13. The programming method of claim 6 , further comprising:
applying a pass voltage on an unselected word line, wherein the pass voltage is higher than the first and second threshold voltages and has a second magnitude smaller than the coercive voltage of the ferroelectric film.
14. The programming method of claim 13 , further comprising:
applying the pass voltage on an unselected bit line, wherein
a second voltage difference between the first voltage and the pass voltage comprises a third magnitude smaller than the coercive voltage of the ferroelectric film; and
an unselected memory cell is inhibited to switch between the first and second logic states.
15. The programming method of claim 6 , further comprising:
applying the first voltage on an unselected bit line such that an unselected memory cell is inhibited to switch between the first and second logic states.
16. The programming method of claim 6 , wherein the applying the first and second voltages comprises applying voltage pulses with durations in a range between 10 ns to 100 μs.
17. The programming method of claim 6 , wherein the applying the first and second voltages comprises applying voltages with magnitudes in a range between about 1.5V to about 5V.
18. A three-dimensional ferroelectric NAND memory device, comprising:
a plurality of memory strings vertically extending through a film stack of alternating conductive and dielectric layers, each memory string comprising a plurality of memory cells vertically stacked, wherein:
each memory cell comprises:
a ferroelectric film disposed over a sidewall of a channel layer; and
a first logic state and a second logic state corresponding to a first threshold voltage and a second threshold voltage, respectively, the first and second threshold voltages determined by two opposite electric polarization directions of the ferroelectric film; and
each memory cell is configured to be programmed by applying a first voltage on a selected word line and applying a second voltage on a selected bit line, wherein a voltage difference between the first voltage and the second voltage comprises a magnitude larger than a coercive voltage of the ferroelectric film to switch the memory cell from the first logic state to the second logic state.
19. The three-dimensional ferroelectric NAND memory device of claim 18 , wherein
the first voltage comprises a positive voltage;
the second voltage comprises a negative voltage; and
the first threshold voltage is higher than the second threshold voltage.
20. The three-dimensional ferroelectric NAND memory device of claim 18 , wherein
the first voltage comprises a negative voltage;
the second voltage comprises a positive voltage; and
the first threshold voltage is lower than the second threshold voltage.