Patent Description:
Non-volatile memory devices, such as flash memory, have become the storage of choice in various electrical products, such as personal computers, flash drives, digital cameras, and mobile phones. Flash memory devices have undergone rapid development. The flash memory can store data for a considerably long time without powering, and have advantages such as high integration level, fast access, easy erasing, and rewriting. To further improve the bit density and reduce cost of the flash memory device, a three-dimensional (3D) NAND flash memory has been developed. A 3D NAND memory architecture stacks memory cells vertically in multiple layers, achieving a higher density than traditional NAND memory. As more layers are added, the bit density increases, thus increasing more storage capacity. With the layer increases, the program disturb becomes worse. A pre-pulse signal may be applied to an unselected bit line connect to an unselected memory string so as to remove boosting charges (electrons) of the unselected memory string. However, as the layer increases, the channel length increases accordingly. The effect of bit line pre-charge for electrons remained in the bottom channel will be reduced because of the increased channel length. A traditional method for improving the bit line pre-charge effect is to extend the bit line pre-pulse time, but this would sacrifice and influence the data programming time. Another traditional method for improving the bit line pre-charge effect is to increase the voltage level of the bit line pre-pulse, but this would increase the risk of the breakdown phenomenon of the bit line transistor. Thus, there is a need for improvement. Document <CIT> discloses a semiconductor memory device with a memory cell array, a peripheral circuit for programming memory cells in the memory cell array and a control logic to control the peripheral circuit and the memory cell array such that, during a programming of the memory cells, pre-bias voltages are applied to multiple word lines coupled to the memory cells to pre-charge channel regions, wherein different pre-bias voltages are applied to different word lines depending on the relative positions of the word lines.

In accordance with the present invention non-volatile memory device, as set forth in claim <NUM>, and a control method of non-volatile memory device, as set forth in claim <NUM> are provided. Preferred embodiments of the invention are claimed in the dependent claims.

It is therefore an objective of the present invention to provide a non-volatile memory device and a control method capable of applying word line pre-pulse signals with different voltage levels and reducing programming disturb.

An embodiment provides a non-volatile memory device. The non-volatile memory device includes a memory array including a plurality of memory strings, each memory string including a select gate transistor and a plurality of memory cells connected in series with the select gate transistor; a bit line connected to a first memory string of the plurality of memory strings; a select gate line connected to the select gate transistor of the first memory string of the plurality of memory strings; a plurality of word lines connected to the plurality of memory cells of the first memory string of the plurality of memory strings, each word line connected to a respective memory cell of the first memory string; a first control circuit configured to apply a bit line pre-pulse signal to the bit line during a pre-charge period; and a second control circuit configured to apply a word line signal to a selected word line of the plurality of word lines and apply a plurality of word line pre-pulse signals to word lines disposed between the select gate line and the selected word line during the pre-charge period, wherein voltage levels of the plurality of word line pre-pulse signals are incremental.

Another embodiment provides a control method of non-volatile memory device. The non-volatile memory device includes a memory array including a plurality of memory strings and each memory string includes a select gate transistor and a plurality of memory cells connected in series with the select gate transistor. The control method includes applying a bit line pre-pulse signal to a bit line connected to a first memory string of the plurality of memory strings during a pre-charge period; applying a word line signal to a selected word line connected to a selected memory cell of the plurality of memory cells of the first memory string during the pre-charge period; and applying a plurality of word line pre-pulse signals to a plurality of word lines connected to the plurality of memory cells of the first memory string and disposed between the select gate line and the selected word line during the pre-charge period, wherein voltage levels of the plurality of word line pre-pulse signals are different.

Please refer to <FIG>, which is a schematic diagram of a non-volatile memory device <NUM> according to an embodiment of the present invention. The non-volatile memory device <NUM> may be an NAND flash memory. For example, the non-volatile memory device <NUM> may be a three-dimensional (3D) NAND flash memory. The non-volatile memory device <NUM> includes a memory array <NUM> and control circuits <NUM> and <NUM>. The memory array <NUM> includes a plurality of memory strings. Each memory string includes a plurality of memory cells. The memory cells of each string are connected together in series. The intersection of a word line and a semiconductor channel forms a memory cell. Top select gate lines TSGs, word lines WLs, top dummy word lines TDMYs, bottom dummy word lines BDMYs and bottom gate lines BSGs are connected between the memory array <NUM> and the control circuit <NUM>. Bit lines BLs are connected between the memory array <NUM> and the control circuit <NUM>.

<FIG> is a schematic diagram illustrating a memory string and related connection lines of the non-volatile memory device <NUM> shown in <FIG> according to an embodiment of the present invention. The memory string of the memory array <NUM> includes, but not limited thereto, a top select gate transistor, at least one top dummy memory cell, a plurality of memory cells, at least one bottom dummy memory cell and a bottom select gate transistor. A bit line BL is coupled to the memory string. A top select gate line TSG is connected to the top select gate transistor of the memory string. The at least one top dummy memory cell is connected in series with the top select gate transistor. At least one top dummy word line TDMY is connected to the at least one top dummy memory cell of the memory string. Each top dummy word line is separately connected to a top dummy memory cell. The plurality of memory cells can be configured to store data. The plurality of memory cells can be connected in series with the at least one top dummy memory cell. Word lines WL are connected to the memory cells of the memory string. Each word line WL is separately connected to a memory cell. Further, the memory cells of the memory string of the memory array <NUM> are disposed sequentially along a first direction between the top dummy memory cell and the bottom dummy memory cell and accordingly the word lines WL are disposed sequentially along the first direction between the top dummy word line TDMY and the bottom dummy word line BDMY.

Moreover, the at least one bottom dummy memory cell is connected in series with the plurality of memory cells. At least one bottom dummy word line BDMY is connected to the at least one bottom dummy memory cell of the memory string. Each bottom dummy word line BDMY is separately connected to a bottom dummy memory cell. The bottom select gate transistor is connected in series with the at least one bottom dummy memory cell. A bottom select gate line BSG is connected to the bottom select gate transistor of the memory string. Writing and erasing data in the memory cells can be controlled from the control circuits and external circuits through the connection lines of the non-volatile memory device <NUM>.

During a pre-charge period (before programming), the control circuit <NUM> is configured to apply a bit line pre-pulse signal to unselected bit lines BL of unselected memory strings of the memory array <NUM>. For example, for each unselected memory string, the control circuit <NUM> is configured to apply a bit line pre-pulse signal to an unselected bit line BL of the each unselected memory string during the pre-charge period. The control circuit <NUM> is configured to apply a top select gate pre-pulse signal to the top select gate line TSG and apply a bottom select gate pre-pulse signal to the bottom select gate line BSG. Moreover, the control circuit <NUM> is configured to apply a word line signal to a selected word line of the unselected memory string. The control circuit <NUM> is configured to apply a plurality of word line pre-pulse signals to word lines which are disposed between the selected word line and the top dummy word line TDMY (or the top select gate TSG). The control circuit <NUM> is also configured to apply a top dummy word line pre-pulse signal to the top dummy word lines TDMY disposed between the word lines and the top select gate TSG and apply a bottom dummy word line pre-pulse signal to the bottom dummy word lines BDMY disposed between the word lines and the bottom select gate BSG.

Moreover, voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY may be different. For example, the voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY may be incremental. For example, the voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY are incremental sequentially from a word line disposed adjacent to the selected word line. For example, a first word line pre-pulse signal of the plurality of word line pre-pulse signals can be applied to a first word line of the plurality of word lines, and the first word line is disposed adjacent to the selected word line and between the selected word line and the top dummy word line TDMY. A second word line pre-pulse signal of the plurality of word line pre-pulse signals can be applied to a second word line of the plurality of word lines, and the second word line is disposed adjacent to the first word line and between the first word line and the top dummy word line TDMY. The voltage level of the second word line pre-pulse signal is greater than the voltage level of the first word line pre-pulse signal.

A third word line pre-pulse signal of the plurality of word line pre-pulse signals can be applied to a third word line of the plurality of word lines, and the third word line is disposed adjacent to the second word line and between the second word line and the top dummy word line TDMY. The voltage level of the third word line pre-pulse signal is greater than the voltage level of the second word line pre-pulse signal. A fourth word line pre-pulse signal of the plurality of word line pre-pulse signals can be applied to a fourth word line, and the fourth word line is disposed adjacent to the third word line and between the third word line and the top dummy word line TDMY, and so on. In an embodiment, the voltage level of the fourth word line pre-pulse signal is greater than the voltage level of the third word line pre-pulse signal, and so on.

In other words, the farer away from the selected word line, the larger voltage level of the word line pre-pulse signal can be applied since the word line pre-pulse signals are applied to word lines disposed between the top select gate TSG and the selected word line. The voltage level of the word line pre-pulse signal applied to the word line located farthest from the selected word line may be the largest among the voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY. The voltage level of the word line pre-pulse signal applied to the word line located closet from the selected word line may be the smallest among the voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY. Since the word line pre-pulse signals with different voltage levels are applied to the word lines between the top select gate line and selected word line, the channel potential gradient is therefore enhances, and thus enhancing the pre-charge effect of the unselected bit line and reducing programming disturb.

In addition, a voltage level of the top dummy word line pre-pulse signal applied to the top dummy word lines TDMY is greater than the voltage levels of the plurality of word line pre-pulse signals applied to the word lines disposed between the selected word line and the top dummy word line TDMY.

Moreover, the word lines disposed between the selected word line and the top select gate TSG can be divided into multiple groups of the word lines. Each divided group of word lines may include at least one word line. Note that, the amount of the word lines of each group of the word lines is not limited, and may be varied and designed in accordance with practical system demands and requirements. Each group of the word lines may include at least one word line. For example, please refer to <FIG> is a schematic diagram illustrating an unselected memory string and related connection lines of the non-volatile memory device <NUM> shown in <FIG> according to an embodiment of the present invention. A top select gate transistor TT, top dummy memory cells TDMY, memory cells MC0 to MCn, bottom dummy memory cells BDMC and a bottom select gate transistor BT are connected in series. <FIG> shows the unselected bit line BL, the top select gate line TSG, the top dummy word lines TDMY, the word lines WL0 to WLn, the bottom dummy word lines BDMY and the bottom select gate line BSG. As shown in <FIG>, suppose the word line WL0 is the selected word line, the word lines WL1 to WLn are divided into a first group of word lines (Bottom WL), a second group of word lines (Middle WL) and a third group of word lines (Top WL). From the bottom to top, the first group of word lines (Bottom WL) includes the word lines WL1 to WLp. The first group of word lines (Bottom WL) is between the selected word line (word line WL0) and the second group of word lines (Middle WL). The second group of word lines (Middle WL) includes the word lines WL(p+<NUM>) to WLq. The second group of word lines (Middle WL) is between the first group of word lines (Bottom WL) and the third group of word lines (Top WL). The third group of word lines (Top WL) includes the word lines WL(q+<NUM>) to WLn. The third group of word lines (Top WL) is between the second group of word lines (Middle WL) and the top dummy word lines TDMY.

Please further refer to <FIG> and <FIG> is a signal timing diagram of the memory string shown in <FIG> according to an embodiment of the present invention. Sequentially from the top of <FIG>, the signal waveforms in a pre-charge period are: a bit line pre-pulse signal VP_BL, a top select gate pre-pulse signal VP_TSG, a top dummy word line pre-pulse signal VP_TDMY, word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL and a word line signal V_SELWL. During a pre-charge period, the bit line pre-pulse signal VP_BL is applied to the unselected bit line BL of the unselected memory string of the memory array <NUM>. The top select gate pre-pulse signal VP_TSG is applied to the top select gate line TSG. The top dummy word line pre-pulse signal VP_TDMY is applied to the top dummy word lines TDMY. The word line pre-pulse signals VP_TOPWL is applied to the third group of word lines (Top WL). The word line pre-pulse signals VP_MIDDLEWL is applied to the second group of word lines (Middle WL). The word line pre-pulse signals VP_BOTTOMWL is applied to the first group of word lines (Bottom WL). The bit line pre-pulse signal VP_BL applied to the unselected bit line BL may be a first power supply voltage Vdd. The top select gate pre-pulse signal VP_TSG applied to the top select gate line TSG may be a second power supply voltage Vcc. The top dummy word line pre-pulse signal VP_TDMY applied to the top dummy word lines TDMY may also be the first power supply voltage Vdd. The word line signal V_SELWL applied to the selected word line (WL0) can be a programming voltage.

In an embodiment, as shown in <FIG>, the voltage level (<NUM> volts) of the word line pre-pulse signal VP_MIDDLEWL is greater than the voltage level (<NUM> volts) of the word line pre-pulse signal VP_BOTTOMWL. The voltage level (<NUM> volts) of the word line pre-pulse signal VP_ TOPWL is greater than the voltage level (<NUM> volts) of the word line pre-pulse signal VP_MIDDLEWL. The voltage level (Vdd) of top dummy word line pre-pulse signal VP_TDMY is greater than the voltage levels of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL. In another embodiment, Please further refer to <FIG> is a signal timing diagram of the memory string shown in <FIG> according to an alternative embodiment of the present invention. Sequentially from the top of <FIG>, the signal waveforms in a pre-charge period are: a bit line pre-pulse signal VP_BL, a top select gate pre-pulse signal VP_TSG, a top dummy word line pre-pulse signal VP_TDMY, word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL and a word line signal V_SELWL. The word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL and the word line signal V_SELWL are negative pulse signals. As shown in <FIG>, the voltage level (-<NUM> volts) of the word line pre-pulse signal VP_MIDDLEWL is greater than the voltage level (-<NUM> volts) of the word line pre-pulse signal VP_BOTTOMWL. The voltage level (-<NUM> volts) of the word line pre-pulse signal VP_ TOPWL is greater than the voltage level (-<NUM> volts) of the word line pre-pulse signal VP_MIDDLEWL. The voltage level (Vdd) of top dummy word line pre-pulse signal VP_TDMY is greater than the voltage levels of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL.

In an embodiment, please further refer to <FIG>. During the pre-charge period, the ending of the word line pre-pulse signal VP_MIDDLEWL applied to the second group of word lines (Middle WL) occurs after the ending of the word line pre-pulse signal VP_BOTTOMWL applied to the first group of word lines (Bottom WL). The ending of the word line pre-pulse signal VP_TOPWL applied to the third group of word lines (Top WL) occurs after the ending of the word line pre-pulse signal VP _MIDDLEWL applied to the second group of word lines (Middle WL). The ending of the top dummy word line pre-pulse signal VP_TDMY applied to the top dummy word lines TDMY occurs after the endings of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL. As shown in <FIG> and <FIG>, the end point of the word line pre-pulse signal VP_MIDDLEWL applied to the second group of word lines (Middle WL) is after the end point of the word line pre-pulse signal VP_ BOTTOMWL applied to the first group of word lines (Bottom WL) during the pre-charge period. The end point of the word line pre-pulse signal VP_TOPWL applied to the third group of word lines (Top WL) is after the end point of the word line pre-pulse signal VP_MIDDLEWL applied to the second group of word lines (Middle WL) during the pre-charge period. The end point of the top dummy word line pre-pulse signal VP_TDMY applied to the top dummy word lines TDMY is after the end points of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL. Since the word line pre-pulse signals with different ending timing are applied to the word lines between the top select gate line and selected word line, the whole programming speed can be improved effectively.

Moreover, as shown in <FIG>, since the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL are positive pulse signals, the falling edge of the word line pre-pulse signal VP _MIDDLEWL is after the falling edge of the word line pre-pulse signal VP_BOTTOMWL during the pre-charge period. The falling edge of the word line pre-pulse signal VP_TOPWL is after the falling edge of the word line pre-pulse signal VP_MIDDLEWL during the pre-charge period. The falling edge of the top dummy word line pre-pulse signal VP_TDMY applied to the top dummy word lines TDMY is after the falling edges of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL. As shown in <FIG>, since the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL are negative pulse signals, the rising edge of the word line pre-pulse signal VP_MIDDLEWL is after the rising edge of the word line pre-pulse signal VP_ BOTTOMWL during the pre-charge period. The rising edge of the word line pre-pulse signal VP_TOPWL is after the rising edge of the word line pre-pulse signal VP _MIDDLEWL during the pre-charge period. The rising edge of the top dummy word line pre-pulse signal VP_TDMY is after the rising edges of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL.

In an embodiment, please further refer to <FIG>. During the pre-charge period, the pulse durations (signal lengths) of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL are incremental. The pulse duration of the word line pre-pulse signal VP _MIDDLEWL applied to the second group of word lines (Middle WL) is greater than the pulse duration of the word line pre-pulse signal VP_ BOTTOMWL applied to the first group of word lines (Bottom WL). The pulse duration of the word line pre-pulse signal VP_TOPWL applied to the third group of word lines (Top WL) is greater than the ending of the word line pre-pulse signal VP _MIDDLEWL applied to the second group of word lines (Middle WL). The pulse duration of the top dummy word line pre-pulse signal VP_TDMY applied to the top dummy word lines TDMY is greater than the pulse durations of the word line pre-pulse signals VP_TOPWL, VP_MIDDLEWL and VP_BOTTOMWL.

In summary, the embodiments of the present invention provide word line pre-pulse signals with different to drive the word lines between the top select gate line and selected word line so as to enhance channel potential gradient, and thus enhancing the pre-charge effect of the unselected bit line and reducing programming disturb. Moreover, the embodiments of the present invention provide word line pre-pulse signals with different ending timing to the word lines between the top select gate line and selected word line, and thus improving the whole programming speed effectively.

Claim 1:
A non-volatile memory device (<NUM>), comprising:
a memory array (<NUM>) comprising memory strings, each comprises a select gate transistor and memory cells connected in series with the select gate transistor;
a select gate line connected to the select gate transistor;
word lines (WL), each word line (WL) connected to a respective memory cell; and
a control circuit (<NUM>, <NUM>) coupled to the select gate line and the word lines (WL), and configured to:
apply a first word line pre-pulse signal to a first group of the word lines (Bottom WL) during a pre-charge period;
apply a second word line pre-pulse signal to a second group of the word lines (Middle WL) during the pre-charge period;
apply a third word line pre-pulse signal to a third group of the word lines (Top WL) during the pre-charge period; and
apply a word line signal to a selected word line of the word lines (WL) during the pre-charge period;
characterized in that:
the first group of the word lines (Bottom WL), the second group of the word lines (Middle WL), and the third group (Top WL) of the word lines (WL) are disposed on the same side with respect to the selected word line;
a voltage level of the second word line pre-pulse signal is greater than a voltage level of the first word line pre-pulse signal, a voltage level of the third word line pre-pulse signal is greater than the voltage level of the second word line pre-pulse signal, and the voltage level of the second word line pre-pulse signal and the voltage level of the third word line pre-pulse signal are greater than 0V; and
the first group of the word lines (Bottom WL), the second group of the word lines (Middle WL), and the third group of the word lines (Top WL) are disposed in an order of distance to the select gate line, wherein the voltage level of a word line signal applied to the selected word line is smaller than the voltage levels of at least two word line pre-pulse signals applied to the first, second and third groups of word lines (Bottom WL, Middle WL, Top WL), wherein the voltage level of the word line signal is different from the voltage level of the word line pre-pulse signal that is applied to the group of the word lines (Bottom WL) adjacent to the selected word line.