CN101620879A - Operating method for realizing multi-bit data storage of ferroelectric memory - Google Patents

Abstract

The invention proposes an operation method for realizing multi-bit data storage of ferroelectric memory, which belongs to the field of microelectronics technology. In the storage unit of ferroelectric memory, different write pulse voltages are applied to make the ferroelectric film correspondingly produce residuals of different sizes. Polarization intensity, and correspondingly defined as different storage states, and then read the corresponding storage states through a fixed external positive read voltage, so as to realize multi-bit storage function in a single memory cell. By using the operation method of the multi-bit data storage of the present invention, the multi-bit storage device can greatly increase the storage density and greatly reduce the production cost.

Description

Operation method of multi-bit data storage in ferroelectric memory

technical field

The invention provides an operation method for realizing multi-bit data storage of a ferroelectric memory, which belongs to the technical field of microelectronics.

Background technique

In the semiconductor market, memory occupies an extremely important position. With the gradual popularization of portable electronic devices, the market of non-volatile memory is also increasing. At present, Flash accounts for 90% of the market of non-volatile memory. However, with the advancement of semiconductor technology, Flash has encountered more and more technical bottlenecks, and the tunnel oxide layer cannot be thinned indefinitely with the development of integrated circuit technology. The idea to solve this problem is to develop a new generation of non-volatile memory, mainly including magnetic memory (MRAM), phase change memory (PCM), resistive memory (ReRAM) and ferroelectric memory (FeRAM), among which ferroelectric memory is the first to be commercialized , with strong commercial potential.

The key to ferroelectric memory lies in its ferroelectric thin film capacitor unit. It mainly uses the spontaneous polarization characteristic of ferroelectric materials, that is, the property that the direction of spontaneous polarization can be reversed under the action of a reverse external electric field to realize the storage function. Generally, ferroelectric memories use perovskite structure series materials, such as lead zirconate titanate (PZT). The main feature of this material is that it has ferroelectric properties, that is, there is an electric hysteresis loop relationship between the polarization direction and the external electric field. This feature makes it very suitable for memory, the positive and negative states of the remnant polarization correspond to the "0" and "1" states of the memory, and the storage state can be changed by changing the direction of the external electric field or through the peripheral circuit to sense its polarization state and read the information.

Traditional ferroelectric memory cells generally only use a pair of positive and negative pulse voltages with the same absolute value to generate a pair of positive and negative polarization values to define the "0" and "1" states. Obtain a single state value, that is, only one bit of data storage can be realized, the resource utilization rate is low, and the storage efficiency is low.

Contents of the invention

The object of the present invention is to provide an operation method for realizing multi-bit data storage of ferroelectric memory, improve storage density and reduce production cost.

The technical solution of the present invention is: an operation method for realizing multi-bit data storage of a ferroelectric memory, on the ferroelectric thin film capacitor storage unit of the ferroelectric memory, by adding n pairs of positive and negative write pulse voltages with different voltage values ±V _i , i=1, 2, ..., n, get 2n remanent polarization values, and define 2n storage states accordingly, and then read 2n storage states by adding a fixed positive read voltage; its read and write operations Methods as below:

Write operation: take n=2 as an example, and so on when n>2; 2 pairs of positive and negative write pulse voltages ±V ₁ and ±V ₂ with different voltage values, definition: V ₂ >V ₁ >0, can be Generate 4 different remanent polarization values P _rj , j=0, 1, 2, 3, assign and define the 4 remanent polarization values P _rj , the specific steps are as follows:

Step 1: After applying a large positive write pulse voltage V ₂ to the ferroelectric film capacitor, the remanent polarization value of the ferroelectric film capacitor is P _r0 , which is defined as the storage state "00";

Step 2: After applying a small positive write pulse voltage V ₁ to the ferroelectric film capacitor, the remanent polarization value of the ferroelectric film capacitor is P _r1 , which is defined as the storage state "01";

Step 3: After applying a small negative write pulse voltage -V ₁ to the ferroelectric film capacitor, the remanent polarization value of the ferroelectric film capacitor is P _r2 , which is defined as the storage state "11";

Step 4: after a large negative write pulse voltage -V ₂ is applied to the ferroelectric film capacitor, the remanent polarization value of the ferroelectric film capacitor is P _r3 , which is defined as the storage state "10";

Read operation: By applying a fixed positive read voltage, the remanent polarization values P _r of the ferroelectric thin films in different storage states are polarized to the saturation polarization value P _sat corresponding to the fixed positive read voltage, thereby correspondingly generating Different difference values P _sat -P _r are converted into specific charges and read by the read operating system; corresponding to the example of n=2 in the above write operation, the read signals of four different storage states are as follows:

The read signal difference corresponding to the storage state "00" is P _sat -P _r0 ;

The read signal difference corresponding to the storage state "01" is P _sat -P _r1 ;

The read signal difference corresponding to the storage state "11" is P _sat -P _r2 ;

The read signal difference corresponding to the storage state "10" is P _sat -P _r3 .

In the write operation, when the external write pulse voltage continues, the ferroelectric film produces a saturation polarization value; after the external write pulse voltage is eliminated, the ferroelectric film will change from the saturated polarization value to the corresponding residual polarization value _Pr . The saturation polarization value and the remnant polarization value P _r of the ferroelectric thin film material are related to the applied write pulse voltage value.

In a read operation, the applied fixed positive read voltage value is equal to the applied maximum positive write pulse voltage value. When the external fixed positive read voltage is continued, a positive saturation polarization value P _sat is correspondingly generated; and various storage states are distinguished by the difference P _sat -P _r ; only a fixed read voltage can be added to read different storage states. The amount of charge caused by the difference P _sat -P _r is determined by the size of the ferroelectric film capacitor.

The number 2n of the storage states depends on the ability of the read operating system to distinguish the charge signal.

The advantage of the operation method of the multi-bit data storage of the ferroelectric memory of the present invention is: by adding many pairs of positive and negative pulse voltages with equal absolute values to generate many pairs of positive and negative polarization strength values, thereby obtaining multi-state values on a ferroelectric memory unit , can realize multi-bit data storage, and at the same time, only one external read voltage is used to read the corresponding storage state. The design is simple and easy, thereby simplifying the logic design cost of the external read circuit. The invention greatly improves the storage density of the ferroelectric memory and reduces the production cost at the same time.

Description of drawings

Fig. 1 is a hysteresis loop diagram measured under an applied voltage for a ferroelectric film capacitor;

Fig. 2 is a ferroelectric thin film capacitance under different positive and negative voltages, the graph of the corresponding remanent polarization value that measures;

3 is a schematic diagram of a ferroelectric memory cell with a 1T1C structure used in an embodiment of the operation method for multi-bit data storage of the present invention;

Fig. 4 is the hysteresis loop of the ferroelectric material obtained by adding the writing pulse voltage and four storage state diagrams correspondingly defined;

5A-5D are respectively timing diagrams of four storage state write operations defined in the embodiment;

FIG. 6 is a timing diagram of an embodiment read operation;

Fig. 7 is a schematic diagram of an electric hysteresis loop for reading the "01" state in the embodiment;

Fig. 8 is a statistics table of reading information operation of the embodiment.

Explanation of symbols in the figure:

00: Saturation polarization value when a positive voltage lasts;

01: Remanent polarization value after a positive voltage;

02: Remanent polarization value after a negative voltage of the same size;

001: an ordinary capacitor C _B ;

002: ferroelectric film capacitor C _FE ;

003: The upper electrode of the ferroelectric film capacitor C _FE ;

004: The lower electrode of the ferroelectric film capacitor C _FE ;

005: An NMOS transistor (the gate is connected to the word line WL, the drain is connected to the bit line BL, and the source is connected to the upper electrode 003 of the ferroelectric film capacitor),

010: plate line PL;

020: word line WL;

030: bit line BL;

101, 102, 103, 104, 105, 201, 202, 203, 204, 205: time period;

10: Difference of P _sat -P _r .

Detailed ways

The operation method of the multi-bit data storage of the ferroelectric memory of the present invention is specifically described below in conjunction with the accompanying drawings:

First, test the hysteresis loop of the ferroelectric film capacitor with an applied voltage. When the applied voltage continues, there will be a saturation polarization P _sat 00; when the applied positive and negative voltages are eliminated, two remanent polarizations can be obtained respectively. Intensity values 01, 02, as shown in Figure 1.

In the same ferroelectric film capacitor, different values of remanent polarization can be obtained under different positive and negative voltages; in addition, when the absolute values of positive and negative pulse voltages are equal, the obtained remanent polarization values are almost equal, And as shown in Figure 2.

The ferroelectric memory cell with 1T1C structure adopted in the embodiment of the present invention (2-bit, 4-state data storage), as shown in FIG. 3 . During the write operation, the word line WL020 of the cell must be set at a high level, and the gate tube 005 is turned on, so that the ferroelectric film capacitor C _FE 002 is connected to the bit line BL030. In addition, since the gate tube 005 has a threshold loss, the bit line 030 should be raised to V+V _TN when the level is high (V is the required write voltage on the ferroelectric film capacitor 002, and V _TN is the threshold voltage of the gate tube 005 ), so that the voltage applied to the ferroelectric film capacitor C _FE 002 is the required writing voltage.

In the embodiment, by applying two pairs of positive and negative write pulse voltages, the hysteresis loop of the ferroelectric material and four correspondingly defined storage states (shown in FIG. 4 ) can be obtained. The 4 remanent polarization values under the applied voltage of 4V, in addition to the saturation polarization value under the voltage of 8V, and their specific values are shown in a table with reference to Figure 4.

In the write operation, in order to avoid the interference of the original storage state on the newly written storage state, each time the write pulse voltage is applied, a pulse voltage with the same voltage and opposite polarity is applied in advance to eliminate and reset.

Write "00" state: 101 period, the bit line 030 is 0 level, the word line 020 starts to appear high level, the plate line is 0 level, and the gate tube 005 is opened accordingly, so that the bit line 030 and the ferroelectric film The capacitor lower electrode 004 is connected. During period 102, the bit line 030 is at a positive level of 8V+V _TN , the word line 020 maintains a high level, and the plate line is at a 0 level, so that the electrode 004 under the ferroelectric film capacitor is equivalent to adding a voltage of 8V. The entire ferroelectric film capacitor 002 is equivalent to a pre-reverse polarization voltage of -8V, so as to avoid the original storage state of this cell from interfering with the storage state to be written; the bit line 030 is at 0 level during the 103 period, and the word line 020 remains High level, add a positive pulse voltage of 8V to the plate line 010, thereby equivalently adding a positive pulse voltage of 8V to the ferroelectric film capacitor 002, and polarize the ferroelectric film in a positive saturation polarization state at 8V; During period 104, the bit line 030 is at 0 level, the word line 020 is at high level, and the plate line is at 0 level, so that the ferroelectric film is reduced from the positive saturation polarization state at 8V to the remanent polarization at 8V State P _r0 , which realizes writing "00"state; 105 period, bit line 030 is 0 level, word line 020 is 0 level, plate line is 0 level, gate 005 is closed, one write operation cycle end, as shown in Figure 5A.

Write "01" state: 101 period, the bit line 030 is 0 level, the word line 020 starts to appear high level, and the plate line is 0 level, so that the gate tube 005 is opened, so that the bit line 030 and the ferroelectric film capacitor The lower electrode 004 is connected. During period 102, the bit line 030 is a positive level of 4V+V _TN , the word line 020 maintains a high level, and the plate line is at a 0 level, so that the electrode 004 under the ferroelectric film capacitor is equivalent to adding a voltage of 4V. The entire ferroelectric film capacitor 002 is equivalent to a pre-reverse polarization voltage of -4V, so as to avoid the original storage state of this cell from interfering with the storage state to be written; the bit line 030 is at 0 level during the 103 period, and the word line 020 remains High level, a 4V positive pulse voltage is added to the plate line 010, thereby equivalently adding a 4V positive pulse voltage to the ferroelectric film capacitor 002, and the ferroelectric film is polarized in a positive saturation polarization state at 4V; During period 104, the bit line 030 is at 0 level, the word line 020 is at high level, and the plate line is at 0 level, so that the ferroelectric film is reduced from the positive saturation polarization state at 4V to the remanent polarization at 4V State Pr ₁ , that is, write "01"state; 105 period, bit line 030 is 0 level, word line 020 is 0 level, plate line is 0 level, gate 005 is closed, one write operation cycle end, as shown in Figure 5B.

Write "11" state: 101 period, the bit line 030 is 0 level, the word line 020 starts to appear high level, and the plate line is 0 level, so that the gate tube 005 is opened, so that the bit line 030 and the ferroelectric film capacitor The lower electrode 004 is connected. During period 102, the bit line 030 is at 0 level, the word line 020 is at high level, and the plate line is at 4V level, so that the electrode 003 on the ferroelectric film capacitor is equivalent to adding a 4V voltage, and the entire ferroelectric film capacitor 002 Equivalently add a 4V pre-reverse polarization voltage, so as to avoid the original storage state of this cell interfering with the storage state to be written; 103 period bit line 030 is at 4V+V _TN level, word line 020 maintains high level, The plate line 010 is at 0 level, so that a negative pulse voltage of 4V is equivalently added to the ferroelectric film capacitor 002, and the ferroelectric film is polarized in a negative saturation polarization state under -4V; 104 periods, The bit line 030 is at 0 level, the word line 020 is kept at high level, and the plate line is at 0 level, so that the ferroelectric film is reduced from the negative saturation polarization state at -4V to the residual polarization state at -4V Pr ₂ , that is, write "11"state; 105 period, bit line 030 is 0 level, word line 020 is 0 level, plate line is 0 level, gate 005 is closed, and a write operation cycle ends , as shown in Figure 5C.

Write "10" state: 101 period, the bit line 030 is 0 level, the word line 020 starts to appear high level, and the plate line is 0 level, so that the gate tube 005 is opened, so that the bit line 030 and the ferroelectric film capacitor The lower electrode 004 is connected. During period 102, the bit line 030 is at 0 level, the word line 020 is at high level, and the plate line is at 8V level, so that the electrode 003 on the ferroelectric film capacitor is equivalent to adding a voltage of 8V, and the entire ferroelectric film capacitor 002 Equivalently add a pre-reverse polarization voltage of 8V, so as to prevent the original storage state of this cell from interfering with the storage state to be written; the bit line 030 is at 8V+V _TN level in 103 periods, and the word line 020 remains at a high level, The plate line 010 is at 0 level, so that a negative pulse voltage of 8V is equivalently added to the ferroelectric film capacitor 002, and the ferroelectric film is polarized in a negative saturation polarization state at -8V; 104 periods, The bit line 030 is at 0 level, the word line 020 is kept at high level, and the plate line is at 0 level, so that the ferroelectric film is reduced from the negative saturation polarization state at -8V to the residual polarization state at -8V Pr ₃ , that is to realize writing "10"state; 105 period, bit line 030 is 0 level, word line 020 is 0 level, plate line is 0 level, gate 005 is closed, and a write operation cycle ends , as shown in Figure 5D.

Read operation: during period 201, the bit line 030 is at 0 level, the word line 020 starts to appear high level, and the plate line is at 0 level, so that the gate tube 005 is turned on, so that the bit line 030 and the lower electrode of the ferroelectric film capacitor 004 , the ordinary capacitor 001 are connected, and the other end of the ordinary capacitor 001 is grounded. During period 202, the word line 020 maintains a high level, the plate line 010 is 8V, and the bit line 030 passes through the ferroelectric film capacitor 002 with different capacitance values in different storage states and can read different voltage values in series with an ordinary capacitor 001; During period 203, word line 020 maintains a high level, plate line 010 is 8V, and different voltage values of bit line 030 can be amplified into different voltage values with more obvious differences through an external sensitive amplifier; during period 204, word line 020 maintains a high level , plate line 010 is at 0 level, bit line 030 maintains different voltage values amplified by the external sense amplifier; at period 205, word line 020 maintains high level, plate line 010 is at 0 level, and bit line 030 has been The circuit reads back to 0 level; this read is a destructive read, so the original state must be rewritten after the read operation. The rewrite operation has been described in detail in reference to Figures 5A-5D. Such a The read operation ends, as shown in Figure 6.

In order to facilitate the understanding of the read operation, a schematic diagram of the corresponding electric hysteresis loop when reading the "01" state is given (Fig. 7). The difference between the saturation polarization and the remanent polarization of the "01" state is 10. It can be clearly seen that when an external read voltage of 8V is applied, the polarization state of the ferroelectric film changes, which in turn produces a corresponding charge change, and finally triggers the bit line The voltage value on the 030 changes.

Finally, make a statistical table for the read operation information, as shown in Figure 8. The table shows in detail the defined storage states under different write voltages, and the charge changes that occur under the 8V read voltage. Through different charge changes, the same ferroelectric film capacitor produces different capacitances, which are finally reflected as bit lines. Different potentials on 030.

What set forth above is the read and write operation method of n=2 (2, 4 states) ferroelectric memory multi-bit data storage,

When n>2, and so on. For example: when n=3 write operations, 3 pairs of positive and negative write pulse voltages ±V ₁ , ±V ₂ and ±V ₃ with different voltage values, V ₃ >V ₂ >V ₁ >0, can generate 6 Different remanent polarization values P _rj , j=0, 1, 2, 3, 4, 5, the assignment of the 6 remanent polarization values P _rj is defined as follows:

V ₃ corresponds to the remanent polarization value P _r0 , which is defined as the storage state "000";

V ₂ corresponds to the remanent polarization value P _r1 , which is defined as the storage state "001";

V ₁ corresponds to the remanent polarization value P _r2 , which is defined as the storage state "010";

-V ₁ corresponds to the remanent polarization value P _r3 , which is defined as the storage state "011";

-V ₂ corresponds to the remanent polarization value P _r4 , which is defined as the storage state "100";

-V ₃ corresponds to the remanent polarization value P _r5 , which is defined as the storage state “101”.

Corresponding to the above example of n=3 in the write operation, by applying a fixed positive read voltage V ₃ , six read signals of different storage states can be obtained as follows:

The read signal difference corresponding to the storage state "000" is P _sat -P _r0 ;

The read signal difference corresponding to the storage state "001" is P _sat -P _r1 ;

The read signal difference corresponding to the storage state "010" is P _sat -P _r2 ;

The read signal difference corresponding to the storage state "011" is P _sat -P _r3 ;

The read signal difference corresponding to the storage state "100" is P _sat -P _r4 ;

The read signal difference corresponding to the storage state "101" is P _sat -P _r5 .

Claims (7)

1, a kind of method of operating that realizes the storage of ferroelectric memory long numeric data is characterized in that: on the ferroelectric capacitor storage unit of ferroelectric memory, by adding positive and negative write pulse voltage ± V that n does not wait magnitude of voltage _i, i=1,2 ..., n, obtain 2n remanent polarization value, and correspondingly define 2n storage attitude, read 2n and store attitude by adding the fixing voltage of just reading again; Its read-write operation method is as follows:

Write operation: with n=2 is example, and o'clock the rest may be inferred in n＞2; The 2 pairs of magnitudes of voltage do not wait adds positive and negative write pulse voltage ± V ₁With ± V ₂, V ₂＞V ₁＞0, can produce 4 different remanent polarization value P _Rj, j=0,1,2,3 is to 4 remanent polarization value P _RjThe assignment definition, concrete steps are as follows:

Step 1: on ferroelectric capacitor, add a big positive write pulse voltage V ₂After, making ferroelectric capacitor remanent polarization value is P _R0, be defined as storage attitude " 00 ";

Step 2: on ferroelectric capacitor, add a little positive write pulse voltage V ₁After, making ferroelectric capacitor remanent polarization value is P _R1, be defined as storage attitude " 01 ";

Step 3: on ferroelectric capacitor, add a little negative write pulse voltage-V ₁After, making ferroelectric capacitor remanent polarization value is P _R2, be defined as storage attitude " 11 ";

Step 4: on ferroelectric capacitor, add a big negative write pulse voltage-V ₂After, making ferroelectric capacitor remanent polarization value is P _R3, be defined as storage attitude " 10 ";

Read operation: by adding the fixing voltage of just reading, the remanent polarization value P of the ferroelectric thin film of difference storage attitude _rAll be polarised to the described fixing saturated polarization value P that is just reading the voltage correspondence _SatThereby, the difference P that corresponding generation is different _Sat-P _r, convert the concrete quantity of electric charge to and read by the read operation system; Corresponding to the example of n=2 in the above-mentioned write operation, it is as follows that four different storage attitudes read signal:

The corresponding signal difference that reads of storage attitude " 00 " is P _Sat-P _R0

The corresponding signal difference that reads of storage attitude " 01 " is P _Sat-P _R1

The corresponding signal difference that reads of storage attitude " 11 " is P _Sat-P _R2

The corresponding signal difference that reads of storage attitude " 10 " is P _Sat-P _R3

2, the method for operating of long numeric data according to claim 1 storage is characterized in that: in write operation, described add write pulse voltage when continuing ferroelectric thin film produce the saturated polarization value; The described write pulse voltage elimination back ferroelectric thin film that adds can become corresponding remanent polarization value P by the saturated polarization value _r

3, the method for operating of long numeric data storage according to claim 2 is characterized in that: the saturated polarization value of described ferroelectric thin-flim materials and remanent polarization value P _rRelevant with the described write pulse magnitude of voltage that adds.

4, the method for operating of long numeric data according to claim 1 storage is characterized in that: in read operation, described adding fixingly just read magnitude of voltage and added maximum just write pulse magnitude of voltage and equate.

5, the method for operating of long numeric data according to claim 4 storage is characterized in that: describedly add fixing corresponding positive saturated polarization value P of generation when just reading voltage and continuing _SatAnd by difference P _Sat-P _rDifference distinguish various storage attitudes; Only need add one fixedly reads voltage and just can read different storage attitudes.

6, the method for operating of long numeric data storage according to claim 5 is characterized in that: described difference P _Sat-P _rThe quantity of electric charge size that causes is by the size decision of ferroelectric capacitor.

7, the method for operating of long numeric data storage according to claim 1, it is characterized in that: the quantity 2n of described storage attitude depends on the resolution characteristic of read operation system to charge signal.

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