Patent Publication Number: US-10777272-B2

Title: Semiconductor memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japan application serial no. 2018-024250, filed on Feb. 14, 2018. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to a semiconductor memory device, and more particularly relates to a reading method for a variable resistance random access memory that uses a variable resistance element. 
     Description of Related Art 
     A variable resistance memory stores data by applying a pulse voltage to a variable resistance element and reversibly and non-volatilely setting the variable resistance element to a high resistance state or a low resistance state. The variable resistance element is composed of a thin film of a metal oxide, such as hafnium oxide (HfOx), and enters the low resistance state or the high resistance state according to the value and polarity of the applied pulse voltage, for example (Japanese Patent No. 5748877). For example, writing the variable resistance element to the low resistance state is called SET, whereas writing the variable resistance element to the high resistance state is called RESET (and vice versa). There are two types of variable resistance memories. i.e., the unipolar type and the bipolar type. For the unipolar type, the write voltages applied to the variable resistance element during set and reset have the same polarity, and the set or reset is performed by changing the write voltage. For the bipolar type, the polarities of the write voltages applied to the variable resistance element during set and reset are reversed. That is, the write voltages are applied to the variable resistance element in two directions. 
       FIG. 1  shows a schematic configuration of the variable resistance memory. A variable resistance element and an access transistor are connected in series with one source line SL 1 . The memory cell MC can be selected in the unit of bits. At the time of writing, for example, the access transistors in the row direction are selected via the word line WL 1  and a write pulse voltage is applied between the bit line BL 1  and the source line SL 1 , so as to set or reset the variable resistance element. At the time of reading, the access transistors in the row direction are selected via the word line WL 1  and a reading voltage is applied to the bit line BL 1  and the source line SL 1 , and a current or voltage corresponding to set or reset, which flows through the variable resistance element, is detected by a sense amplifier. Similarly, the above description with respect to the source line SL 1  also applies to a source line SL 2 , a source line SL 3 , or a source line SL 4 , and correspondingly the above description with respect to the bit line BL 1  also applies to a bit line BL 2 , a bit line BL 3 , or a bit line BL 4 . Further, the above description with respect to the word line WL 1  also applies to a word line WL 2 . 
     In addition, when a thin film of a metal oxide, such as hafnium oxide (HfO x ) and titanium oxide (TiO x ), is used as the material of the variable resistance element, the metal oxide is applied with a forming operation as the initial setting. Generally, the forming operation puts the variable resistance element, for example, in the low resistance state, i.e., a state close to set, by applying a voltage somewhat greater than the voltage for writing the variable resistance element to the thin film. 
     There is a variable resistance memory that stores complementary data in a pair of memory cells by two variable resistance elements and two access transistors, that is, so-called 2T×2R configuration.  FIG. 2  shows an outline of a 2T×2R memory array. The memory cell MC 11  includes a variable resistance element and an access transistor between the source line SL 1  and the bit line BL 1  and writes true data to the variable resistance element. The memory cell/MC 11  includes a variable resistance element and an access transistor between the source line/SL 1  and the bit line/BL 1  and writes complementary data to the variable resistance element. Similarly, the above description with respect to the source line SL 1  also applies to the source line/SL 1 , the source line SL 2 , or a source line/SL 2 , and the above description with respect to the bit line BL 1  also applies to the bit line/BL 1 , a bit line BL 2 , or a bit line/BL 2 . In addition, the above description with respect to the word line WL 1  also applies to the word line WL 2 , and the above description with respect to the memory cell MC 11  and the memory cell/MC 11  also applies to a memory cell MC 12 , a memory cell/MC 12 , a memory cell MC 21 , a memory cell/MC 21 , or a memory cell/MC 22 . 
     Reading data from the 2T×2R memory cells is to detect a differential signal between the current flowing through the variable resistance element that stores the true data and the current flowing through the variable resistance element that stores the complementary data.  FIG. 3  shows an example of reading data from a pair of memory cells MC 11  and/MC 11 . The word line WL 1  is selected by a row decoder (not shown), the bit lines BL 1  and/BL 1  and the source lines SL 1  and/SL 1  are selected by a column decoder, and a reading voltage is applied to the selected bit lines and source lines. The bit line BL 1  of the memory cell MC 11  that stores the true data is coupled to one input of the sense amplifier  10 , and the bit line/BL 1  of the memory cell/MC 11  that stores the complementary data is coupled to the other input of the sense amplifier  10 . The source lines SL 1  and/SL 1  are coupled to the ground voltage (GND). 
     It is assumed that the memory cell MC 11  is set (data “1”) and the memory cell/MC 11  is reset (data “0”), for example. In this case, the variable resistance element of the memory cell MC 11  is in the low resistance state, and a relatively large current flows to the source line SL 1  from the bit line BL 1 . Meanwhile, the variable resistance element of the memory cell/MC 11  is in the high resistance state, and a relatively small current flows to the source line/SL 1  from the bit line/BL 1 . The sense amplifier  10  is activated by an enable signal SAE, detects the differential signal between the current flowing through the memory cell MC 11  and the current flowing through the memory cell/MC 11 , and outputs data “1” according to the detection result. By reading such a differential signal, it is possible to perform reading with higher reliability than reading of a single bit with 1T×1R, and to achieve high-speed access. 
       FIG. 4  shows an exemplary distribution of the currents flowing through the variable resistance element during set/reset. In the initial state (or a normal state), a relatively large read margin is present between the current distribution HRS_initial that flows during set and the current distribution LRS_initial that flows during reset, and the sense amplifier  10  can correctly read the differential signal of a pair of memory cells. However, as the number of times of writing to the variable resistance element increases and the retention or endurance characteristic of the data deteriorates, that is, the filamentous current path formed between the electrodes of the variable resistance element deteriorates, resulting in tail bit shifts, as indicated by the current distribution HRS_drift and the current distribution LRS_drift. When the tail bit shifts occur, the read margin between the upper limit value of the current distribution HRS_drift and the lower limit value of the current distribution LRS_drift becomes narrow, which may cause a data reading error. 
     For example, it is assumed that the upper limit value of the current distribution HRS_initial in the initial state is 3 μA and the lower limit value of the current distribution LRS_initial is 16 μA. 
     (1) When data “0” is stored: 
     3 μA flows through the memory cell MC 11  that stores the true data and 16 μA flows through the memory cell/MC 11  that stores the complementary data. The current difference between them is 13 μA, and the sense amplifier  10  outputs data “0” according to a good detection result of the differential signal.
 
(2) When data “1” is stored:
 
16 μA flows through the memory cell MC 11  that stores the true data and 3 μA flows through the memory cell/MC 11  that stores the complementary data. The current difference between them is 13 μA. and the sense amplifier  10  outputs data “1” according to a good detection result of the differential signal.
 
     Here, it is assumed that a tail bit shift occurs in the memory cell MC 11 , and the upper limit value of the current distribution HRS_drift is 6 μA (a shift of +3 μA) and the lower limit value of the current distribution LRS_drift is 10 μA (a shift of −6 μA). 
     (1) When data “0” is stored: 
     6 μA flows through the memory cell MC 11  and  161 A flows through the memory cell/MC 11 . The current difference between them is 10 μA. and the sense amplifier  10  outputs data “0” according to a good detection result of the differential signal. 
     (2) When data “1” is stored: 
     10 μA flows through the memory cell MC 11  and 3 μA flows through the memory cell/MC 11 . The current difference between them is 7 μA. When the margin becomes small, the sense amplifier  10  may not be able to correctly detect the differential signal, that is, the sense amplifier  10  may not be able to correctly output the data “1”. 
     For the above reason, the variable resistance memory is provided with an ECC circuit for error detection/correction. However, execution of the ECC processing will cause the reading speed to drop, and the ECC circuit needs to occupy a certain area on the chip. In particular, such problems become noticeable when there is a higher requirement for the capability (bit number) of detecting and correcting errors. 
     SUMMARY 
     The disclosure provides a semiconductor memory device that improves the reliability of data reading and achieves good area efficiency. 
     The semiconductor memory device according to the disclosure includes: a memory array including a plurality of memory cells; a writing part writing identical data to a pair of memory cells selected from the plurality of memory cells; and a reading part reading the data stored in the pair of memory cells selected from the plurality of memory cells, wherein the reading part includes a sense amplifier that compares a total of currents respectively flowing through the pair of memory cells with a reference value and outputs the data based on a comparison result. 
     In an embodiment, the writing part writes the identical data to a pair of memory cells adjacent in a row direction. In an embodiment, each of the memory cells includes a reversible and non-volatile variable resistance element and an access transistor connected to the variable resistance element, and the writing part sets or resets each variable resistance element of the pair of memory cells. In an embodiment, the memory array includes a dummy memory cell for generating the reference value. In an embodiment, the dummy memory cell is set to generate a current between a current flowing through a set variable resistance element and a current flowing through a reset variable resistance element. In an embodiment, the semiconductor memory device further includes a setting part that sets the reference value according to a retention characteristic or an endurance characteristic. In an embodiment, the semiconductor memory device includes a row selection part selecting the memory cells in a row direction based on address information and a column selection part selecting the memory cells in a column direction based on the address information, wherein the writing part writes identical data to a pair of memory cells selected by the row selection part and the column selection part, and the reading part reads data stored in a pair of memory cells selected by the row selection part and the column selection part. 
     A reading method for reading data of a semiconductor memory device according to the disclosure includes: writing identical data to a pair of memory cells selected from a plurality of memory cells included in a memory cell array; and comparing a total of currents respectively flowing through the pair of memory cells with a reference value and outputting data based on a comparison result when reading the data from the pair of memory cells. 
     In an embodiment, each of the memory cells includes a reversible and non-volatile variable resistance element and an access transistor connected to the variable resistance element. In an embodiment, each of the memory cells includes a variable resistance element, and the reference value is set to a current between a current flowing through a set variable resistance element and a current flowing through a reset variable resistance element. In an embodiment, the reference value is varied according to a retention characteristic or an endurance characteristic. 
     According to the disclosure, identical data is written to a selected pair of memory cells, and when the data is read from the selected pair of memory cells, the total of the currents respectively flowing through the pair of memory cells is compared with the reference value, and the data is outputted based on the comparison result. Therefore, compared with the conventional reading method, the disclosure can improve the reliability of data reading. As a result, it is possible to reduce the error detection/correction function for the read data and reduce the area it occupies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the conventional variable resistance random access memory. 
         FIG. 2  is a diagram illustrating a 2T×2R configuration of the conventional variable resistance random access memory for writing complementary data. 
         FIG. 3  is a diagram illustrating the reading method for data of the conventional 2T×2R. 
         FIG. 4  is a diagram showing an example that a tail bit shift occurs in a current distribution during set/reset. 
         FIG. 5  is a diagram showing the configuration of a variable resistance random access memory according to an embodiment of the disclosure. 
         FIG. 6  is a diagram illustrating a reading method according to an embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the disclosure will be described hereinafter. A variable resistance random access memory is illustrated as an embodiment of the disclosure. 
       FIG. 5  is a block diagram showing a schematic configuration of the variable resistance random access memory according to an embodiment of the disclosure. The variable resistance memory  100  of this embodiment includes a memory array  110 , a row decoder and drive circuit (X-DEC)  120 , a column decoder (Y-DEC)  130 , a control circuit  150 , a sense amplifier  160 , and a write driver/read bias circuit  170 . In the memory array  110 , a plurality of memory cells, including variable resistance elements and access transistors, are arranged in rows and columns. The row decoder and drive circuit  120  selects and drives a word line WL based on a row address Ax. The column decoder  130  selects a bit line BL and a source line SL based on a column address Ay. The control circuit  150  controls each part based on a command, an address, data, etc. received from outside via an I/O buffer  140 . The sense amplifier  160  senses data read from the memory cell via the bit line BL and the source line SL. The write driver/read bias circuit  170  applies a bias voltage to the bit line BL or the source line SL in a read operation, applies a pulse voltage corresponding to write data in a write operation to the bit line BL or the source line SL, and applies a voltage for the read operation in the read operation to the bit line BL or the source line SL. 
     In this embodiment, as shown in  FIG. 2 , data is written and read by using a pair of memory cells of 2T×2R. However, unlike the related art that stores complementary data in a pair of memory cells, in this embodiment, identical data is stored in a pair of memory cells. For example, according to the example of  FIG. 2 , the memory cell MC 11  and the memory cell/MC 11  are set or reset to the same state, that is, identical data “1” or data “0” is written. 
     The sense amplifier  160  is connected to the control circuit  150  via an internal data bus DO, and the result sensed by sense amplifier  160  is outputted to the control circuit  150  via the internal data bus DO. Details of the sense amplifier  160  will be described later. 
     The write driver/read bias circuit  170  is connected to the control circuit  150  via an internal data bus DI and receives the write data via the internal data bus DI. When setting a selected pair of memory cells, for example, the write driver/read bias circuit  170  applies a positive voltage to a pair of bit lines BL, applies the ground voltage to a pair of source lines SL, and sets the variable resistance element of each of the pair of memory cells to a low resistance state. When resetting a selected pair of memory cells, the ground voltage is applied to the bit lines BL, a positive voltage is applied to the source lines SL, a current having a polarity opposite to that of set is applied to the variable resistance element, and the variable resistance element of each of the pair of memory cells is reset to a high resistance state. 
     The control circuit  150  performs control of reading and writing based on a command inputted from the outside via the I/O buffer  140 . When a write operation is performed, the row decoder  120  selects the word line WL based on the inputted row address Ax. and the column decoder  130  selects a pair of bit lines BL and a pair of source lines SL based on the inputted column address Ay, thereby selecting a pair of memory cells. For example, according to the example of  FIG. 2 , the word line WL 1  is selected based on the row address, and a pair of bit lines BL 1  and/BL 1  and a pair of source lines SL 1  and/SL 1  are selected based on the column address, which makes it possible to access the pair of memory cells MC 11  and/MC 11 . In addition, the write driver/read bias circuit  170  applies a bias voltage to the selected bit line and source line via a global bit line GBL and a global source line GSL based on inputted write data, and writes identical data “0” or identical data “1” to the selected pair of memory cells. 
     When a read operation is performed, the row decoder  120  selects the word line WL based on the inputted row address Ax, and the column decoder  130  selects a pair of bit lines BL and a pair of source lines SL based on the inputted column address Ay, thereby selecting a pair of memory cells. In addition, the write driver/read bias circuit  170  applies a reading bias voltage to the selected bit line and source line via the global bit line GBL and the global source line GSL. The sense amplifier  160  senses the data read from the selected pair of memory cells and outputs a result of the sensing to the control circuit  150 . 
     Next, a data reading method according to this embodiment is described with reference to  FIG. 6 . In this example, the word line WL 1  is selected by the row decoder  120 , and a group of bit line BL 1  and source line SL 1  and another group of bit line/BL 1  and source line/SL 1  are selected by the column decoder  130 , and a pair of memory cells MC_A and MC_B is selected. 
     As described above, identical data is written to the pair of memory cells MC_A and MC_B. In the read operation, the write driver/read bias circuit  170  controls the transistors Q 1  to Q 8  according to an instruction from the control circuit  150 , and applies a reading bias voltage to the selected bit line BL 1  and source line SL 1  and the selected bit line/BL 1  and source line/SL 1 . 
     The selected bit line BL 1  and bit line/BL 1  are coupled to one input of the sense amplifier  160 , and a reference generation part  162  for generating a reference current Iref is coupled to the other input. The sense amplifier  160  is activated in response to a sense enable signal SAE from the control circuit  150 , to compare voltages corresponding to the currents that flow to the respective inputs, and outputs a comparison result thereof. 
     The access transistor is turned on via the word line WL 1 , the reading voltage is applied to the memory cells MC_A and MC_B via the bit line, and the currents I_A and I_B corresponding to set or reset of the variable resistance elements flow through the memory cells MC_A and MC_B from the bit line toward the source line. If the memory cells MC_A and MC_B are in the normal state, that is, before a tail bit shift occurs, ideally I_A is equal to I_B. At one input of the sense amplifier  160 , a total current of the current I_A and the current I_B or a voltage representing the total current thereof is generated. 
     In this embodiment, the reference current Iref may be set between the upper limit value of the current distribution HRS_initial and the lower limit value of the current distribution LRS_initial, as shown in  FIG. 4 . The reference generation part  162  can be of any configuration and is configured, for example, by using a replica (dummy) of the memory cell, a current source circuit, a resistor, a transistor, etc. for generating the reference current Iref. Moreover, in an embodiment, the reference generation part  162  may vary the reference current Iref according to an instruction from the control circuit  150 . For example, the control circuit  150  can set the reference current Iref according to a retention or endurance characteristic of the variable resistance element. For example, the control circuit  150  can count the number of times of rewriting the variable resistance memory  100 , retains the count result in a non-volatile storage area, and varies the reference current Iref according to the number of times of rewriting. When the reference generation circuit  162  is configured by using a dummy memory cell, the write driver/read bias circuit  170  may program the dummy memory cell, so that the resistance of the variable resistance element is variable. Alternatively, the reference current Iref may be varied by controlling an operating voltage (e.g., gate voltage) of the access transistor of the dummy memory cell. In another embodiment, the control circuit  150  may change the setting value of the reference current Iref according to a user command from the outside. 
     The conventional sense amplifier  10  compares the current flowing through the memory cell that stores the true data and the current flowing through the memory cell that stores the complementary data, so as to sense the data stored in a pair of memory cells. In the disclosure, however, the sense amplifier  160  compares the total of the current I_A flowing through the memory cell MC_A and the current I_B flowing through the memory cell MC_B with the reference current Iref, so as to sense the data stored in the pair of memory cells MC_A and MC_B, and outputs the result of the sensing. 
     Here, an example of performing reading on the variable resistance random access memory under the same current distribution condition as that exemplified in  FIG. 4  is described below. It is assumed that, in the initial state, the upper limit value of the current distribution HRS_initial is 3 μA and the lower limit value of the current distribution LRS_initial is 16 μA. 
     (1) When data “0” is stored: 
     3 μA flows through the memory cell MC_A and 3 μA flows through the memory cell MC_B, and the total current thereof is 6 μA. 
     (2) When data “1” is stored: 
     16 μA flows through the memory cell MC_A and 16 μA flows through the memory cell MC_B. and the total current thereof is 32 μA. 
     At this time, the window width for data “0” and data “1” is 26 μA (32 μA−6 μA), and the reference current Iref is set in this range. 
     It is assumed that a tail bit shift occurs in the memory cell MC_A, and the upper limit value of the current distribution HRS_drift is 6 μA (a shift of +3 μA) and the lower limit value of the current distribution LRS_drift is 10 μA (a shift of −6 μA). 
     (1) When data “0” is stored: 
     6 μA flows through the memory cell MC_A and 3 μA flows through the memory cell MC_B, and the total current thereof is 9 μA. 
     (2) When data “1” is stored: 
     10 μA flows through the memory cell MC_A and 16 μA flows through the memory cell MC_B, and the total current thereof is 26 μA. 
     At this time, even though a tail bit shift occurs in one memory cell MC_A, the window width for data “0” and data “1” is 17 μA (26 μA−9 μA), and the reference current Iref may be set to 17.5 μA (9 μA+8.5 μA) (½ window width). It can be seen that this read margin is larger than that of the conventional reading method. 
     According to the embodiment of the disclosure, when the retention characteristic or endurance characteristic of one memory cell deteriorates and a tail bit shift occurs, the data stored therein can be determined by comparing the total of the currents respectively flowing through the pair of memory cells with a reference value. Therefore, compared with the conventional variable resistance random access memory, the read margin can be made larger and the reliability of data reading can be improved. As a result, the requirement for the ECC circuit can be reduced, and the size of the ECC circuit and the area it occupies can be reduced to increase the integration of the memory cells. Drop of the access speed during reading, caused by the ECC processing, can be suppressed. 
     The above embodiment illustrates a variable resistance memory, in which the memory cells are formed in a two-dimensional array. Nevertheless, the reading method of the disclosure may also be applied to a variable resistance memory with memory cells formed in a three-dimensional structure. Furthermore, the reading method of the disclosure may also be applied to semiconductor memory devices other than the variable resistance memory, such as a NOR type flash memory capable of random access. 
     Although exemplary embodiments of the disclosure have been described in detail above, the disclosure is not limited to specific embodiments, and various modifications and changes may be made within the scope of the disclosure defined in the claims.