# Patent

## Title
Memory access method and flash memory using the same

## Abstract

A memory access method is applied in a memory controller for accessing a memory array, including a number of respective select switches globally controlled with a string select signal. The memory access method includes: enabling the string select signal and disabling the string select signal before a read phase.

## Background
This application is a continuation application of U.S. patent application Ser. No. 13/298,443 filed on Nov. 17, 2011, now U.S. Pat. No. 8,542,532, the content of which is incorporated here by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a memory access method, and more particularly to a memory access method applied on a flash memory array.
2. Description of the Related Art
In the present age, non-volatile memory devices have become more popular for use in various electronic devices. For example, flash memory is one of the most popular non-volatile semiconductor memories applied in the present age. Generally, flash memory includes flash memory cells with programmable threshold voltages, so that each of the flash memory cells can be programmed to indicate at least one bit of data.
With the demands for flash memories with higher storage volume, flash memory arrays with three-dimensional structures have been developed. However, conventional memory access method generally surfers from access inaccuracy issues. Consequently, how to provide a memory access method capable of remedy the inaccuracy issues has became still a prominent object for the industries.
SUMMARY OF THE INVENTION
The invention is directed to memory access method, applied in a memory controller for accessing a memory array, in which memory cells are arranged into a number of streams globally controlled with a string select signal. The memory access method directed to by the invention firstly determines a selected cell of a selected stream by means of supplying stream bias signal and selected word line signal in a setup phase, wherein the memory access method further has rest of memory cells of the memory array biased and turned on as pass transistors in the setup phase. The memory access method directed to by the invention then provides a discharge path connected to the streams to accordingly eliminate coupling charge presented on at least an unselected stream among the streams before the memory access method actually reads the selected cell. Thus, in comparison to conventional memory access method, the memory access method directed to by the invention is advantageously capable of eliminating the coupling charge on the at least a unselected stream, preventing the coupling charge from influencing the access operation of the selected cell, and achieving memory access with higher accuracy.
According to a first embodiment of the present invention, a memory access method, applied in a memory controller for accessing a memory array, in which memory cells are arranged into a number of streams globally controlled with a string select signal, is provided. The memory access method includes: enabling the string select signal and disabling the string select signal before a read phase.
According to a second embodiment of the invention, a flash memory is provided. The flash memory includes a sense unit, a memory array and a memory controller. The memory array includes a number of memory cells, wherein the memory cells are arranged into a number of streams, globally controlled with a string select signal. The memory controller is coupled to the memory array for determining a setup phase and a read phase, which does not overlap with the setup phase. The memory controller enables the string select signal and disabled the string select signal before the read phase.
The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a flash memory according to an embodiment of the invention.
FIG. 2 is a circuit diagram of a subarea of a memory array in FIG. 1.
FIG. 3 is a timing diagram of the related signals shown in FIG. 2.
FIG. 4 is another timing diagram of the related signals shown in FIG. 2.
FIG. 5 is still another timing diagram of the related signals shown in FIG. 2.



DETAILED DESCRIPTION OF THE INVENTION
The memory access method according to the present embodiment of the invention provides a discharge path for eliminating coupling charge presented on the unselected streams, and accordingly achieving access operation of a selected cell, situated on a selected stream, with higher accuracy.
Referring to FIG. 1, a circuit diagram of a flash memory according to an embodiment of the invention is shown. The flash memory 1 includes a memory array 10, a memory accessing circuit, and a memory controller 12. For example, the memory array 10 is a three-dimensional NAND memory array, which includes a number of memory cells, wherein parts of the memory cells are arranged into a number of streams, globally connected to a same metal bit line through respective a same number of string select switches.
Referring to FIG. 2, a circuit diagram of a subarea of the memory array 10 is shown. For example, the memory array 10 includes a metal bit line MBL, which is employed for accessing N streams S—0, S—1, S—2, . . . and S_N−1, wherein N is a natural number greater than 1. Since the N streams S—1 to S_N−1 have substantially the same circuit structure, the ith stream S_i is employed as an example for having the N streams S—0 to S_N−1 illustrated more specifically, wherein i is an integer greater than or equal to 0 and smaller than N.
The streams S_i includes M memory cells MC(i,0), MC(i,1), MC(i,2), . . . , and MC(i,M−1), a string select switch SW_i, a ground select switch SWG_i, a PN diode D_i, and a node N_i, wherein the string select switch SW_i and the ground select switch SWG_i are implemented as metal oxide semiconductor (MOS) transistors, and the memory cells MC(i,0) to MC(i,M−1) are implemented as MOS transistors with programmable threshold voltage. The source side and the drain side of the string select switch SW_i are respectively connected to the metal bit line MBL and the source side of the memory cell MC(i,M−1). The P end and the N end of the PN diode D_i are respectively connected to the node N_i and the drain side of the ground select switch SWG_i. The source side of the ground select switch SWG_i is connected to the drain side of the memory cell MC(i,0). The memory cells MC(i,1) to MC(i,M−2) are serially connected between the source end of the memory cell MC(i,0) and the drain end of the memory cells MC(i,M−1).
The control ends of the string select switches SW—0 to SW_N−1, respectively corresponding to the streams S—0 to S_N−1, receive a string select signal SSL, which is employed for globally controlling the operations of the string select switches SW—0 to SW_N−1. The control ends of the ground select switches SWG—0 to SWG_N−1 receive a ground select signal GSL, which are employed for globally controlling the operations of the ground select switches SWG—0 to SWG_N−1. The control ends of memory cells corresponding to the same row position receive a same word line signal, and are thus globally controlled with the same word line signal. For example, the memory cells MC(0,0), MC(1,0), MC(2,0), . . . , and MC(N−1,0) correspond to the row position of 0, and are globally controlled with the word line signal SWL—0.
Referring back to FIG. 1. The accessing circuit is connected to the memory array 10 and the memory controller 12, and is controlled by the memory controller 12 to access the memory array 10. For example, the accessing circuit includes a drain side bias circuit 14a, Y-multiplexers 14b1, 14b2, sense unit 14c, and x-decoder 14d. The drain side bias circuit 14a provides a stream bias voltage VB to the Y-multiplexer 14b1. For example, the memory array 10 is operated in a reverse read scheme, and the stream bias voltage VB is a high level voltage, e.g. greater than a ground voltage GND. The Y-multiplexer 14b1 is controlled by the memory controller 12 to provide stream bias signals CSL—0 to CSL_N−1, wherein one the stream bias signals CSL—0 to CSL_N−1 corresponds to the stream bias voltage VB for accordingly having the corresponding one of the corresponding streams S—0 to S_N−1 chosen as a selected stream, while the other N−1 stream bias signals CSL—0 to CSL_N−1 correspond the ground voltage GND. In an example, the stream bias signal CSL—0 corresponds to the stream bias voltage VB, while the other stream bias signals CSL—1 to CSL_N−1 correspond to the ground voltage GND. Thus, the stream S—0 is accordingly chosen as the selected stream.
The X-decoder 14d is controlled by the memory controller 12 to provide the string select signal SSL, the ground select signal GSL, and the word line signals SWL—0 to SWL_M−1. For example, one of the word line signals SWL—0 to SWL_M−1 corresponds to a selected word line voltage VS, for reading a selected cell on the selected stream, while the other word line signals SWL—0 to SWL_M−1 correspond to a pass word line voltage VP, for having the rest memory cells of the memory array 10 biased as pass transistors. In an example, the word line signal SWL—0 corresponds to the selected word line voltage VS, while the other word line signals SWL—1 to SWL_M−1 correspond to the pass word line voltage VP. In other words, the memory cell, within the selected stream S—0 and further corresponding to the row position of 0, is chosen as the selected cell.
For example, the level of the selected word line voltage VS is determined with respect to the threshold voltage of the memory cells MC(0,0) to MC(N−1,M−1) of the memory array 10. In an embodiment, the threshold voltages of the memory cells MC(0,0) to MC(N−1,M−1) are greater than the ground voltage GND, and the selected word line voltage VS is accordingly set with a level greater than the ground voltage GND. In other embodiment, the threshold voltage of the memory cells MC(0,0) to MC(N−1,M−1) are substantially equal to the ground voltage GND, and the selected word line voltage VS is accordingly set to the ground voltage GND. In the present embodiment, only the situation that the threshold voltages of the memory cells MC(0,0) to MC(N−1,M−1) are greater than the ground voltage GND, i.e. the selected word line voltage VS is greater than the ground voltage GND, is illustrated, while the situation that threshold voltages of the memory cells MC(0,0) to MC(N−1,M−1) substantially equal the ground voltage GND can be inferred accordingly.
The Y-multiplexer 14b2 is controlled by the memory controller 12 to provide the stream current C—0, e.g. the cell current of the selected cell MC(0,0), flowed on the metal bit line MBL to the sense unit 14c, so that the data value corresponding to the selected cell can be accordingly obtained. For example, the sense unit 14c employs a voltage sensing scheme to have the data value of the selected cell obtained.
The memory controller 12 is coupled to the memory array 10 via the accessing circuit, and accordingly achieves the access operation of the selected cell on the selected stream. In an example, the memory controller 12 is implemented as a state machine within the flash memory 1. In the following paragraphs, examples for accessing the selected cell, e.g. the memory cell MC(0,0), are cited to more specifically illustrate the timing control operations of the memory controller 12.
Referring to FIG. 3, a timing diagram of the related signals shown in FIG. 2. The memory controller 12 determines a setup phase Tsetup and a read phase Tdevelop, wherein the setup phase Tsetup and the read phase Tdevelop do not overlap with each other.
In the setup phase Tsetup, the memory controller 12 drives the Y-multiplexer 14b1 to provides the stream bias signals CSL—0 to CSL_N−1, wherein the stream bias signal CSL—0 corresponds to the stream bias voltage VB, while the stream bias signals CSL—1 to CSL_N−1 correspond to the ground voltage GND. Besides, the memory controller 12 further drives the x-decoder 14d to have the word line signal SWL—0 biased to the selected word line voltage VS, and have the word line signals SWL—1 to SWL_N−1 biased to the pass word line voltage VP. Thus, the selected cell, e.g. the memory cell MC(0,0), on the selected stream S—0 is accordingly determined, and the rest of memory cells are turned on as pass transistors.
Furthermore, the string select signal SSL and the ground select signal GSL are also raised to a high voltage HV, e.g. higher than the ground voltage GND, in the setup phase Tsetup. Thus, the selected stream S—0 is substantially biased as a pass transistor string in the setup phase Tsetup.
On the contrary, the PN diodes D—1 to D_N−1 on the unselected streams S—1 to S_N−1 are turned off in the setup phase Tsetup due to the grounded stream bias signals CLS—1 to CSL_N−1. Thus, before the string select signal SSL is raised high enough to turn on the string select switches SW—1 to SW_N−1, the unselected streams S—1 to S_N−1 are substantially floating, wherein each of the unselected streams S—1 to S_N−1 substantially forms a huge capacitor, with one side of it receiving the word line signals SWL—1 to SWL_M−1 and the other side of it floating. What is even worse is that the word line signals SWL—0 to SWL_M−1 are simultaneously boosted up to the stream bias voltage VB or the pass word line voltage VP in the setup phase Tsetup. Consequently, the steam voltages V(S—0) to V(S_N−1), e.g. the voltages at source or drain of any of the memory cells in the floating unselected streams S—1 to S_N−1 and the voltage at source or drain of the memory cells in the selected stream S—0, are also boosted up due to the capacitance coupling effect.
In order to eliminate the unwanted voltage boost of the stream voltages V(S—0) to V(S_N−1) due to the capacitance coupling effect, the string select signal SSL is also raised to the high voltage HV in the setup phase Tsetup, to have the string select switches SW—1 to SW_N−1 turned on. Thus, a discharge path, formed with the metal bit line MBL and the sense unit 14c, connected to the streams S—0 to S_N−1 is provided to accordingly have the stream voltages V(S—0) to V(S_N−1) pulled down to the ground voltage GND, and eliminate the coupling charge presented thereon in the setup phase Tsetup.
In the read phase Tdevelop, the metal bit line MBL is floating, and the memory controller 12 keeps providing the enabled string select signal SSL, so that the cell current of the selected cell MC(0,0) can be provided to have capacitance presented on the metal bit line MBL charged, and the voltage of the metal bit line MBL V(MBL) rises accordingly. The memory controller 12 further drives the sense unit 14c to sense the voltage V(MBL), and accordingly achieves the access operation of the selected cell MC(0,0).
Based on the above, the memory access method according to the present embodiment is able to eliminate the unwanted charge, presented in the unselected streams S—1 to S_N−1, via the discharge path in the setup phase Tsetup, so that the cell current of the selected cell MC(0,0) obtained in the read phase Tdevelop is substantially free from the interferences caused by the unselected streams S—1 to S_N−1. Consequently, the cell current can be employed to accurately determine the data value stored in the selected cell MC(0,0).
Though only the situation that the memory controller 12 determines two phases, e.g. the setup phase Tsetup and the read phase Tdevelop, and accordingly operates therein has been illustrated in the present embodiment, the memory controller 12 is not limited thereto. In other example, the memory controller 12 further determines a second setup phase Tsetup′, in which page buffer within the sense unit 14c is set and the metal bit line MBL is biased, as depicted in FIG. 4. For example, the string select signal SSL is pulled to the ground voltage GND before the second setup phase Tsetup′, so that the string select switches SW—0 to SW_N−1 are turned off and the metal bit line MBL and the page buffer can be biased and set.
In still another example, the memory controller 12 further determines a first sub-phase Tsetup—1 and a second sub-phase Tsetup—2 within the setup phase Tsetup, as depicted in FIG. 5. In the first sub-phase Tsetup—1, the memory controller 12 provides the stream bias signals CSL—0 to CSL_N−1 and the word line signals SWL—0 to SWL_N−1. In the second sub-phase Tsetup—2, the memory controller 12 provides the discharge path by means of enabling the string select signal SSL. In other words, the memory controller 12 executes the operation of eliminating the unwanted charge presented on the unselected streams S—1 to S_N−1 after the operations of providing the stream bias signals CSL—0 to CSL_N−1 and the word line signals SWL—0 to SWL_N−1.
The memory access method and the memory controller according to the present embodiment of the invention are employed for accessing an NAND memory array, in which memory cells are arranged into a number of streams, including a number of respective select switches globally controlled with a string select signal. The memory access method according to the present embodiment firstly determines a selected cell of a selected stream by means of supplying stream bias signal and selected word line signal in a setup phase, wherein the memory access method further has rest of memory cells of the NAND memory array biased and turned on as pass transistors in the setup phase. The memory access method according to the present embodiment then provides a discharge path connected to the number of streams to accordingly eliminate coupling charge presented on at least an unselected stream among the number of streams before the memory access method actually reads the selected cell. Thus, in comparison to conventional memory access method, the memory access method and the memory controller according to the present embodiment are advantageously capable of eliminating the coupling charge on the at least a unselected stream, preventing the coupling charge from influencing the access operation of the selected cell, and achieving memory access with higher accuracy.
While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.

## Claims

1. A memory access method, applied in a memory controller for accessing a memory array, in which memory cells are arranged into a plurality of streams, which are globally controlled with a string select signal, the memory access method comprising:
enabling the string select signal and disabling the string select signal before a read phase.

2. The memory access method according to claim 1, further comprising:
providing a stream bias signal on a selected stream among the plurality of streams and providing a selected word line signal on a selected cell of the selected stream, in a setup phase;
providing a plurality of unselected word line signals on the rest of memory cells, to have the rest of memory cells turned on as pass transistors, in the setup phase;
providing a discharge path connected to the plurality of streams to accordingly eliminate coupling charge presented on at least an unselected stream among the plurality of streams, in the setup phase; and
enabling the string select signal to have the selected stream connected to a sense unit via a metal bit line and according read the selected cell in a voltage sensing scheme, in the read phase, which does not overlap with the setup phase.

3. The memory access method according to claim 2, wherein step of providing the discharge path further comprises:
in response to the enabled string select signal in the setup phase, having the plurality of streams globally connected to the metal bit line and having the at least an unselected stream discharged via the metal bit line.

4. The memory access method according to claim 2, further comprising:
determining a second setup phase following the setup phase;
wherein the string select signal is disabled in the second setup phase.

5. The memory access method according to claim 2, further comprising:
determining a second setup phase following the setup phase;
determining a first sub-phase within the setup phase,
wherein,
the step of providing the stream bias signal and the selected word line signal and the step of providing the plurality of unselected word line signals are both executed in the first sub-phase.

6. The memory access method according to claim 5, further comprising:
determining a second sub-phase within the setup phase, wherein,
the step of providing the discharge path is executed in the second sub-phase; and
the string select signal is disabled in the first sub-phase, enabled in the second sub-phase and disabled in the second setup phase.

7. A flash memory, comprising:
a sense unit;
a memory array, comprising a plurality of memory cells, wherein the memory cells are arranged into a plurality of streams, which are globally controlled with a string select signal; and
a memory controller, coupled to the memory array, the memory controller further determining a setup phase and a read phase, which does not overlap with the setup phase, wherein,
the memory controller enables the string select signal and disables the string select signal before the read phase.

8. The flash memory according to claim 7, wherein
the memory controller provides a stream bias signal on a selected stream among the plurality of streams and provides a selected word line signal on a selected cell of the selected stream, in the setup phase;
the memory controller further provides a plurality of unselected word line signals on the rest of memory cells, to have the rest of memory cells turned on as pass transistors, in the setup phase;
the memory controller provides a discharge path connected to the plurality of streams to accordingly eliminate coupling charge presented on at least an unselected stream among the plurality of streams, in the setup phase; and
the memory controller further enables the string select signal to have the selected stream connected to the sense unit via a metal bit line, so that the sense unit accordingly reads the selected cell in a voltage sensing scheme, in the read phase.

9. The flash memory according to claim 8, wherein by enabling the string select signal in the setup phase, the memory controller has the plurality of streams globally connected to the metal bit line, and has the at least an unselected stream discharged via the metal bit line, to accordingly provide the discharge path.

10. The flash memory according to claim 8, wherein
the memory controller determines a second setup phase following the setup phase; and
the memory controller disables the string select signal in the second setup phase.

11. The flash memory according to claim 8, wherein the memory controller further determines a second setup phase following the setup phase and determines a first sub-phase within the setup phase, and the memory controller provides the stream bias signal, the selected word line signal, and the plurality of unselected word line signals in the first sub-phase.

12. The flash memory according to claim 11, wherein
the memory controller further determines a second sub-phase within the setup phase;
the memory controller provides the discharge path in the second sub-phase; and
the memory controller disables the string select signal in the first sub-phase, enables the string select signal in the second sub-phase and disables the string select signal in the second setup phase.