GB2367424A

GB2367424A - Non volatile ferroelectric memory device

Info

Publication number: GB2367424A
Application number: GB0023792A
Authority: GB
Inventors: Piero Migliorato; Satoshi Inoue; Ichio Yudasaka
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2000-09-29
Filing date: 2000-09-29
Publication date: 2002-04-03
Anticipated expiration: 2020-09-29
Also published as: GB2367424B; GB0023792D0

Abstract

The ferroelectric memory device comprises a thin film transistor (TFT) 20 stacked over a ferroelectric capacitor 30. A plate line connection is provided to the ferroelectric capacitor plate 35 and separate bit line connections BL1 and BL2 are provided to the source and drain regions 22,23 of the TFT. Binary data is stored by applying appropriate voltages to the first and second bitlines, the gate electrode, and the capacitor plate electrode. Data is read from the device by utilizing the electrostatic effects on the channel region of the charge stored in the ferroelectric capacitor structure. When the data is read the ferroelectric capacitor is not discharged.

Description

SEMICONDUCTOR MEMORY DEVICE The present invention generally relates to a semiconductor memory device and, more particularly, to a non-volatile semiconductor memory device.

Among various kinds of semiconductor memory devices, a FeRAM (a ferroelectric memory) has memory cells, in each of which a gate electrode 21A and a source region 22A of, for example, an N-channel type MIS transistor are electrically connected to a word line WL and a bit line BL, respectively, and a ferroelectric capacitor 30A is electrically connected between a drain region 23A and a plate line PL of this MIS transistor, as is seen from FIG. 10 (A) showing an equivalent circuit thereof.

In the case of such a kind of a semiconductor memory device 1A, when one of binary data"1"and"0", for instance, data"1"is written to a memory cell 10A, the word line WL is set at a high level (H) in a state in which the bit line BL is maintained at the high level (H), while the plate line PL is maintained at a low level (L).

Consequently, the MIS transistor 10A is put into an onstate. Thus, in the ferroelectric capacitor 30A, an electrode 31A electrically connected to the drain region 23A of the MIS transistor 10A is set at the high level (H), while an electrode 32A electrically connected to the plate line PL thereof is set at the low level (L), as illustrated in FIG. 10 (B). This results in occurrence of polarization in a ferroelectric layer of the ferroelectric capacitor 30A.

In contrast with this, when data"0"is written to the memory cell 10A (that is, when deleting data written thereto), the word line WL is set at the high level (H) in a state in which the bit line BL is maintained at the low level (L) and the plate line PL is set at the high level (H), as illustrated in FIG. 11 (A). Consequently, the MIS transistor 10A is brought into an on-state. In the ferroelectric capacitor 30A, an electrode 31A electrically connected to the drain region 23A of the MIS transistor 10A is set at the low level (L), while an electrode 32A electrically connected to the plate line PL is set at the high level (H), as illustrated in FIG. 11 (B). Thus, the ferroelectric layer of the ferroelectric capacitor 30A is polarized in a direction opposite to the direction of polarization in the case of writing the data"1"thereto.

Next, an operation of reading information is described

hereinbelow. First, the bit line BL is recharged at ground potential. Thereafter, the bit line BL is put in a high impedance state. Subsequently, the electric potential of the plate line PL is fixed at ground potential. At that time, the ferroelectric capacitor 30A is maintained in the polarized state as before. Then, the plate line PL is set at the high level (H). At that time, charges are discharged from the ferroelectric capacitor 30A. The amount of the discharged charge varies with a direction in which the polarization is previously caused. Moreover, the discharged electric charge appears as a voltage of the bit line BL.

Thus, it is determined by amplifying this voltage by means of a sense amplifier which of"1"and"0"the data represents.

However, in the case of the conventional FeRAM, when information is read, the ferroelectric capacitor 30A discharges. Thus, it is necessary for holding the data to write the data thereto again. That is, in the case of the conventional FeRAM, a destructive read operation is performed. Meanwhile, generally, in the case of FeRAMs, the number of times of writing data is limited. Therefore, in the case of the destructive read operation, the writing of data is needed every time information is read.

Consequently, the scope of applications of the conventional FeRAM is extremely limited.

Thus, problems to be solved by the present invention reside in constituting a memory cell from one transistor and one capacitor and in providing a non-volatile semiconductor memory device that can read data nondestructively.

To solve the foregoing problems, according to the present invention, there is provided a semiconductor memory device, which first, comprises at least a plurality of memory cells each formed by stacking a plate electrode, a ferroelectric layer, an insulating film, a channel region of a MIS (Metal Insulator Semiconductor) transistor, a gate insulating film of the MIS, and a gate electrode of the MIS transistor in this order. A word line is electrically connected to the gate electrode of each of the plurality of memory cells. First and second bit lines are electrically connected to a source region and a drain region of the MIS transistor, respectively. A plate line is electrically connected to the plate electrode.

In the specification of the present application, the term"MIS"is strictly for the purpose of representing a structure and does not mean that the gate electrode is limited to a metallic one. The term"MIS"implies that, for example, a doped silicon film may be used as the gate electrode.

According to the present invention, the MIS transistor is, for instance, a thin film transistor.

In the semiconductor memory device of the present invention, when data is stored in the memory cell, a voltage of a polarity corresponding to the data is applied between the plate line and each of the first and second bit lines.

Moreover, a gate voltage for turning on the MIS transistor is applied to the gate electrode from the word line.

For example, when one kind of binary data is stored in the memory cell, a voltage of a polarity corresponding to the one kind of binary data is applied between the plate line and each of the first and second bit lines. Moreover, a gate voltage for turning on the MIS transistor is applied to the gate electrode from the word line. In contrast, when the other kind of binary data is stored in the memory cell, a voltage of a polarity opposite to the polarity of the voltage applied in the case of storing the one kind of data is applied between the plate line and each of the first and second bit lines. Moreover, a gate voltage for turning on the MIS transistor is applied to the gate electrode from the word line.

When data is written thereto in this manner, electric charges corresponding to the polarity of the voltage applied between the plate line and each of the first and second bits are stored in the ferroelectric layer.

In the semiconductor memory device constructed in this way, the channel region of the MIS transistor is affected in a manner varying according to the polarity of the polarized ferroelectric layer by the charges stored in the ferroelectric layer. This results in change in the sourcedrain-current-gate-voltage characteristic of the MIS transistor. Thus, let"a first gate voltage"represent a gate voltage, at which the source-drain current reaches a predetermined level according to the source-drain-currentgate-voltage characteristic of the MIS transistor, when one kind of binary data"1"and"0"is written thereto.

Further, let"a second gate voltage,, represent a gate voltage, at which the source-drain current reaches a predetermined value, when the other kind of binary data"1" and"0"is written thereto. Moreover, let"a data reading gate voltage"represent an electric potential level between the first and second gate voltages. When a reading voltage is applied between the source and the drain of the MIS transistor while the data reading gate voltage is applied to the gate electrode from the word line, it is detected from the first bit line or the second bit line whether or not a source-drain current flows. Consequently, it is decided which kind of the data"1"and"0"is written thereto.

When data is read in this way, the charges stored in the ferroelectric layer merely electrostatically affect the channel region and are not discharged in the semiconductor memory device according to the present invention.

Therefore, even when the data written to the memory cells are read, the electric charges having been stored in the ferroelectric layer remain still stored therein.

Consequently, the data is not destroyed.

Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which: FIG. 1 is a circuit diagram showing an equivalent circuit for illustrating the configuration of each of a plurality of memory cells formed in a matrix-like manner in a semiconductor memory device, which is a first embodiment of the present invention; FIG. 2 is a plan diagram showing the configuration of the memory cells formed in the semiconductor memory device illustrated in FIG. 1; FIG. 3 is a sectional view taken on line II-II'of FIG.

2; FIGS. 4 (A) and 4 (B) are a diagram illustrating a manner, in which one kind"1"of binary data is written to the memory cell in the semiconductor memory device shown in FIG. 1, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the data"1"is written thereto, respectively; FIGS. 5 (A) and 5 (B) are a diagram illustrating a manner, in which the other kind"0"of binary data is written to the memory cell in the semiconductor memory device shown in FIG. 1, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the latter data"0"is written thereto, respectively; FIGS. 6 (A) and 6 (B) are a diagram illustrating the principle of reading data written in the memory cells in the semiconductor memory device shown in FIG. 1, and a graph illustrating the relation between the source-drain current and the gate voltage in the MIS transistor, which corroborates this principle; FIG. 7 is a diagram illustrating the manner of reading data written to the memory cells in the semiconductor memory device shown in FIG. 1; FIG. 8 is a plan diagram showing the configuration of the memory cells formed in a semiconductor memory device, which is a second embodiment of the present invention ; FIG. 9 is a sectional view taken on line VIII-VIII'of FIG. 8; FIGS. 10 (A) and 10 (B) are a diagram illustrating a manner, in which one kind"1"of binary data is written to the memory cell of the conventional FeRAM, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the data"1"is written thereto, respectively; and FIGS. 11 (A) and 11 (B) are a diagram illustrating a manner, in which the other kind"0"of binary data is written to the memory cell of the conventional FeRAM, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the latter data "0"is written thereto, respectively.

Hereinafter, the preferred embodiments of the present invention will be described by referring to the accompanying drawings.

[First Embodiment (Overall Configuration) FIG. 1 is a circuit diagram showing an equivalent circuit for illustrating the configuration of memory cells formed in a matrix-like manner in a semiconductor memory device, to which the present invention is applied.

As viewed in FIG. 1, in the semiconductor memory device 1 according to this embodiment, a plurality of first bit lines BLla, BLlb, Bic,... (hereunder referred to as"first bit lines BL1"), a plurality of second bit lines BL2a, BL2b, BL2c,... (hereunder referred to as"second bit lines BL2") of the number equal to that of the first bit lines, and a plurality of plate lines PLa, PLb, PL2c,... (hereunder referred to as"plate lines PL") extend in the longitudinal direction. Further, as viewed in this figure, a plurality of word lines WLa, WLb, WLc.,... (hereunder referred to as word lines WL) extend in the transversal direction. These word lines WL intersect with the first bit lines BL1, the second bit lines BL2 and the plate lines PL.

Further, in the semiconductor memory device 1, memory cells 10aa, 10ab, 10ac,..., 10ba, 10bb, l0bc,..., 10ca, lOcb, 10cc,... (hereunder referred to as the memory cells 10) are placed in a matrix-like manner correspondingly to the intersections among these signal lines.

In the case of the semiconductor memory device 1 shown in this figure, one MIS transistor 20, one ferroelectric capacitor 30 are formed in each of the memory cells 10. In the case of this embodiment, an N-channel type thin film transistor is used as the MIS transistor 20.

This embodiment is similar to the conventional FeRAM described with reference to FIGS. 10 (A) and 10 (B) and FIGS.

11 (A) ad 11 (B) in that the gate electrodes 21 and the source regions 22 are electrically connected to the word lines WL and the first bit lines BL1, respectively.

Incidentally, in the case of the semiconductor memory device 1 according to this embodiment, the drain region 23 of the MIS transistor 10 is electrically connected to the second bit line BL2 in each of the memory cells 10.

Further, the ferroelectric capacitor 30 is provided between the channel region 24 of the MIS transistor 20 and the plate line PL in each of the memory cells 10.

In this embodiment, such a configuration of the memory cell 10 is realized by the structures illustrated in FIGS. 2 and 3.

FIG. 2 is a plan diagram showing the configuration of each of the memory cells. FIG. 3 is a sectional view taken on line II-II'of FIG. 2.

As shown in FIGS. 2 and 3, a plate electrode 35, a ferroelectric layer 36 and an insulating film 37 are stacked in a direction from the bottom layer to the top layer in this order on the top surface of an insulating substrate 2 serving as a base of the semiconductor memory device 1. The ferroelectric capacitor 30 is formed from these layers.

Incidentally, the ferroelectric capacitor 30 may employ lead-zirconate-titanate (PZT), barium-strontium-titanate (BST) and strontium-bismuth-niobium-tantalate (Yl system) as the material of the ferroelectric layer 36. This embodiment employs PZT. Additionally, the ferroelectric capacitor 30 uses the plate electrode 35 as one of two electrodes, and the channel region 24 of the MIS transistor 20 (to be described later) as the other electrode.

A silicon film 200 serving as an active layer of the MIS transistor 20 is formed on the top surface of the insulating film 37 of the ferroelectric capacitor 30. A gate insulating film 25 constituted by a silicon oxide film is formed on the surface of this silicon film 200. Further, a gate electrode 21 constituted by a metallic film or by a doped silicon film is formed on the surface of the gate insulating film 25. In this embodiment, the source region 22 and the drain region 23 are formed on the silicon film 200 in such a way as to self-align with the gate electrode 21. The channel region 24 facing the gate electrode 21 through the gate insulating film 25 is provided between the source region 22 and the drain region 23.

In the case of the example illustrated in this figure, the gate electrode 21 is a part of the word line WL.

Moreover, in the case of the example shown in this figure, an inter-layer insulating film 26 is formed on the top surface of the gate electrode 21. The first bit line BL1, the second bit line BL2, and the plate line PL are electrically connected to the source region 22, the drain region 23, and the plate electrode 35 through contact holes penetrating this inter-layer insulating film 26, the gate insulating film 25 or the insulating film 37.

Additionally, the channel region 24 has nearly the same length as the gate electrode 21 in the direction of length of the channel. Also, the ferroelectric layer 36 is of nearly the same length as the gate electrode 21 in the direction of length of the channel.

(Data Write Operation) FIGS. 4 (A) and 4 (B) are a diagram illustrating a manner, in which one kind"1"of binary data is written to this memory cell, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the data"1"is written thereto, respectively. FIGS. 5 (A) and 5 (B) are a diagram illustrating a manner, in which the other kind"0"of binary data is written to this memory cell, and a diagram illustrating a manner, in which electric charges are stored in the ferroelectric capacitor when the latter data"0"is written thereto, respectively In the case of this kind of the semiconductor memory device 1, when one of the binary data"1"and"0", for instance, the data"1"is written to the memory cell 10, the first bit line BL1 and the second bit line BL2 are set at the high level (H), while the plate line PL is fixedly set at the low level (L), as shown in FIG. 4 (A). In this state, the word line WL is set at a higher level (HH) of a voltage, which is higher than the level (H) of the first bit line BL1 and the second bit line BL2. Consequently, as illustrated in FIG. 4 (B), the MIS transistor 20 is turned on, so that the channel region 24 is at the high level of a voltage, which is higher than the level of the plate line PL. Thus, polarization occurs in the ferroelectric capacitor 30, correspondingly to the applied electric field, as illustrated in FIG. 4 (B).

In contrast, when the data"0"is written to the memory cell 10, the first bit line BL1 and the second bit line BL2 are set at the low level (L), while the plate line is fixedly set at the high level (H), as shown in FIG. 5 (A).

In this state, the word line WL is set at a higher level (HH) of a voltage, which is higher than the level (H) of the plate line PL. Consequently, as illustrated in FIG. 5 (B), the MIS transistor 20 is turned on, so that the channel region 24 is at an electric potential level, which is lower than the level of the plate line PL. Thus, polarization occurs in the ferroelectric capacitor 30, correspondingly to the applied electric field, as illustrated in FIG. 5 (B).

(Data Read Operation) FIGS. 6 (A) and 6 (B) are a diagram illustrating the principle of reading data written in this memory cell 10, and a graph illustrating the relation between the sourcedrain current and the gate voltage in the MIS transistor, which corroborates this principle. FIGS. 7 (A) and 7 (B) are diagrams illustrating the manner of reading data written to this memory cell.

When data is written to the memory cell 10, electric charges corresponding to the polarity of a voltage applied between the plate line PL and each of the first bit line BL1 and the second bit line BL2 are stored in the ferroelectric layer 36.

Effects of the charges, which are stored in the ferroelectric layer 36 of the ferroelectric capacitor 30, on the characteristic of the MIS transistor 20 will be described hereinbelow with reference to FIG. 6.

FIG. 6 (B) illustrates the source-drain current characteristic at the time of changing a voltage Vsub to be applied to a counter electrode (namely, a plate electrode 35) placed opposite to the gate electrode 21 with respect to the channel region 24 in the MIS transistor 20 illustrated in FIG. 3. As shown in this graph, the source-drain current characteristic of the MIS transistor 20 vary as indicated by solid lines LI, one-dot chain lines L2, and dotted lines L3, respectively corresponding to the cases that the voltage Vsub has a positive value, a negative value, and zero.

That is, in the case that the voltage Vsub is 0 in the MIS transistor 20, the source-drain current has a minimum value when the gate voltage has a value of Vgs (Int), as indicated by the dotted lines L3. At that time, the sourcedrain voltage is constant. Further, if the gate voltage is raised from such a value, an"on-current"flows through the MIS transistor 20. Moreover, the source-drain current increases to a certain level with a rise in the gate voltage. Furthermore, when the gate voltage is lowered, an off-leak current (namely, a source-drain current) flows through the MIS transistor 20. Moreover, this source-drain current increases with a rise in the gate voltage.

Incidentally, a threshold value is defined herein as a gate voltage at the time when electric current having a certain constant level flows therethrough. The threshold value in this case is denoted by"Vth (Int)".

Such relation between the source-drain current and the gate voltage similarly holds for the cases of positive and negative values of the voltage Vsub. However, the gate voltage, at which the source-drain current has a minimum value, is shifted in a direction, in which the value of this voltage increases or decreases, due to the electrostatic effects of this voltage Vsub on the channel region 24. For instance, in the case that the voltage Vsub has a positive value, the source-drain current has a minimum value when the gate voltage is equal to a value VgsO, as indicated by solid lines LI. Further, the threshold value (namely, the first gate voltage) changes to a voltage value Vth (O).

In contrast, in the case that the voltage Vsub has a negative value, the source-drain current has a minimum value when the gate voltage is a value Vgsl, as indicated by the one-dot chain lines L2. Moreover, the threshold value (namely, a second threshold gate voltage) changes to a voltage value Vth (l).

Incidentally, when one of the binary data"1"and"O", namely, the data"1"is written to the memory cell 10, the ferroelectric layer 36 is polarized in the semiconductor memory device 1 according to this embodiment, as illustrated in FIG. 4 (B). This state corresponds to a state, in which the voltage Vsub has a negative value in the MIS transistor 20. Therefore, the MIS transistor 20 exhibits the sourcedrain current characteristic indicated by the one-dot chain lines L2 in FIG. 6 (B).

In contrast, when the data"0"is written to the memory cell 10, the ferroelectric layer 36 polarizes, as shown in FIG. 5 (B). This state corresponds to a state, in which the voltage Vsub has a positive value in the MIS transistor 20.

Therefore, the MIS transistor 20 shows the source-drain current characteristic indicated by the solid lines LI in FIG. 6 (B).

Thus, in the case of the semiconductor memory device 1 according to this embodiment, when data is read from the memory cell 10, an intermediate voltage Vg (namely, a reading gate voltage) between the first voltage Vth (O) and the second voltage Vth (l) is applied from the word line WL to the MIS transistor 20, as illustrated in FIG. 7.

Moreover, a voltage, which is nearly equal to this reading gate voltage, is applied to the plate line PL as reading plate potential Vplate.

Further, a reading voltage VDD (which is a positive voltage) is applied to the first bit line BL1, while a ground voltage (namely, zero voltage 0) is applied to the second bit line BL2.

Consequently, when the data"1"is written to the memory cell 10, source-drain current hardly flows through the MIS transistor 20. Conversely, when the data"0"is written to the memory cell 10, a large source-drain current flows therethrough. Therefore, when this source-drain current is detected on the first bit line BL1 or the second bit line BL2, if the level of the source-drain current is not more than a certain level, it is decided that the data "1"is written to the memory cell 10. Conversely, if the level of the source-drain current is not less than the certain level, it is decided that the data"0"is written to the memory cell 10.

Incidentally, in the case that the memory cells 10 are placed in a matrix-like manner, it is necessary for enabling the selective reading of data that the voltage Vth (O) is a positive voltage. That is, the following inequality should be satisfied: 0 < Vth (O) < Vg < Vth (l) The threshold value is controlled so that the relation represented by such an inequality holds In this embodiment utilizing such a principle, the reading voltage VDD (which is a positive voltage) is applied to the first bit line BL1 (for example, the first bit line BLlb) corresponding to the memory cell 10 (for instance, the memory cell 10bb), from which the data should be read.

Furthermore, the ground potential (0) is applied to the second bit line BL2 (for example, the second BL2b) corresponding to the memory cell 10, from which data should be read. In such a state, the reading gate voltage Vg and the plate potential Vplate (= the gate voltage Vg) are respectively applied to the word line WL (for instance, the word line WLb) and the plate line PL (for example, the plate line PLb), which correspond to the memory cell 10, from which data should be read. During that, zero voltage 0 is supplied to other signal lines.

Therefore, if a change in current at the time of applying the reading gate voltage Vg on the word line WL corresponding to the memory cell 10, from which data should be read, is detected from the first bit line BL1 and the second bit line BL2 by sensing up, it can be detected which

of binary data"1"and"0"is the data stored in the memory cell, which is selected by the word line WL.

(Effects of This Embodiment) Thus, the semiconductor memory device 1 according to this embodiment determines data written to the memory cell 10 by utilizing the fact that when the channel region 24 of the MIS transistor 20 is electrostatically affected by electric charges stored in the ferroelectric layer 36, the source-drain-current-gate-voltage characteristic of the MIS transistor 20 changes.

Moreover, the electric charges stored in the ferroelectric layer 36 at the time of reading data merely electrostatically affects the channel region 24 and thus are not discharged. Thus, even when data written to the memory cell 10 is read, the charges stored in the ferroelectric layer 36 remain stored in the ferroelectric capacitor 30.

(Manufacturing Method) The semiconductor memory device 1 constructed in this manner can be manufactured by combining well known semiconductor processes, especially, low temperature polysilicon TFT (Thin-Film Transistor) processes.

Therefore, a method of manufacturing this semiconductor memory device 1 will be described hereunder by referring to FIGS. 2 and 3.

First, a clean insulating substrate 2 is prepared.

Subsequently, a conductive film constituted by a metallic film or by a doped silicon film is formed on the surface of the insulating substrate 2. Thereafter, the patterning of this conductive film is performed by using photolithography techniques. Thus, a plate electrode 35 is formed.

Next, a thin film made of a ferroelectric material is formed on approximately the entire surface of the substrate 2 by a film forming method, such as a CVD method, a sol-gel method using a metal alkoxide solution, and a sputtering method. Then, the patterning of this thin film is performed by using the photolithography techniques, so that a ferroelectric layer 36 is formed.

Subsequently, an insulating film 37 constituted by a silicon oxide film is formed on nearly the entire substrate 2 by various kinds of film forming methods, such as a plasma CVD method.

Next, a silicon film is formed on the insulating substrate 2 by various kinds of film forming methods, such as the plasma CVD method. Thereafter, the patterning of the silicon film is performed by utilizing the photolithography techniques, so that an island-like silicon film 200 is formed. Incidentally, a material obtained by forming an amorphous silicon film in a low temperature process and then crystallizing the amorphous silicon film by a laser anneal method may be used to form the silicon film.

Subsequently, a gate insulating film 25 constituted by a silicon oxide film is formed on nearly the entire surface of the insulating substrate 2 by various kinds of film forming methods, such as the plasma CVD method.

Then, a conductive film constituted by a metallic film or by a doped silicon film is formed on the top surface of the gate insulating film 25. Thereafter, the patterning of this conductive film is performed by using the photolithography techniques, so that the gate electrode 21 is formed.

Next, N-type impurities are introduced into the silicon film 200 by using the gate electrode 21 as a mask.

Consequently, the source region 22 and the drain region 23 are formed on the silicon film 200 in such a way as to selfalign with the gate electrode 21.

Subsequently, the inter-layer insulating film 26 constituted by a silicon oxide film is formed on nearly the entire surface of the insulating substrate 2 by various kinds of film forming methods, such as the plasma CVD method. Thereafter, contact holes are formed in this interlayer insulating film 26.

Next, a conductive film, for example, a metallic film, such as an aluminum film, or a doped silicon film is formed.

Subsequently, the patterning of this conductive film is performed by using the photolithography techniques, so that the first bit line BL1, the second bit line BL2, and the word line WL are formed.

[Second Embodiment FIG. 8 is a plan diagram showing the configuration of the memory cells formed in another semiconductor memory device, which is a second embodiment of the present invention. FIG. 9 is a sectional view taken on line VIII VIII'of FIG. 8. Incidentally, the basic structure of the semiconductor memory device according to this embodiment is similar to that of the semiconductor memory device according to the first embodiment, which has been described by referring to FIGS. 2 and 3. Thus, in FIGS. 8 and 9, like characters designate common constituent components of the first embodiment. Therefore, the description of such components is omitted herein. Further, data write and read operations of the semiconductor memory device 1 according to the second embodiment are similar to those of the semiconductor memory device according to the first embodiment. Thus, the description of these operations of the second embodiment is omitted herein.

In the case of the semiconductor memory device 1 described by referring to FIGS. 2 and 3, the first bit lines BL1 and the second bit lines BL2 are formed by using conductive films formed on the surface of the inter-layer insulating film 26. In contrast, in the case of the semiconductor memory device 1 illustrated in FIGS. 8 and 9, wiring parts obtained by directly extending the conductive film of the plate electrode 35 in each of the memory cells 10 are used as the plate lines PL. Further, in each of the memory cells 10, wiring parts obtained by being extended from both end portions of a region, which is formed as the active layer, in the silicon film 200 constituting the active layer of the MIS transistor 20 are used as the first bit line BL1 and the second bit line BL2. That is, each of the first bit lines BL1 is a conductive region that is integral with the source region 22 of the MIS transistor 20, while each of the second bit lines BL2 is a conductive region that is integral with the drain region 23 of the MIS transistor 20. Incidentally, each of the word lines WL is integral with the gate electrode 21 of the MIS transistor 20, similarly as in the case of the first embodiment.

In the case of forming the semiconductor memory device in this way, when the source region 22 and the drain region 23 are formed, the first bit lines BL1 and the second bit lines BL2 can be simultaneously formed. Further, when the plate electrode 35 is formed, the plate line PL can be formed. Therefore, the second embodiment has an advantage in that the number of processes in the case of manufacturing the semiconductor memory apparatus 1 is small.

[Other Embodiments In any of the aforementioned embodiments, the top gate type TFT is formed, as the MIS transistor 20 to be formed in the memory cell 10. However, the bottom gate of TFT may be used. Further, an N-channel type thin film transistor is formed as the MIS transistor 20. Alternatively, a P-channel type thin film transistor may be used as the MIS transistors 20.

Claims

CLAIMS:

1. A semiconductor memory device comprising a plurality of memory cells each formed by stacking a plate electrode, a ferroelectric layer, an insulating film, a channel region of a MIS transistor, a gate insulating film of said MIS, and a gate electrode of said MIS transistor in said order, wherein a word line is electrically connected to said gate electrode of each of the plurality of memory cells, wherein first and second bit lines are electrically connected to a source region and a drain region of said MIS transistor, respectively, and wherein a plate line is electrically connected to said plate electrode.

2. The semiconductor memory device according to claim 1, wherein said MIS transistor is a thin film transistor.

3. The semiconductor memory device according to claim 1 or 2, wherein an active layer of said thin film transistor is a low temperature polysilicon film.

4. The semiconductor memory device according to one of claims 1 to 3, wherein when data is stored in said memory cell, a voltage of a polarity corresponding to the data is applied between said plate line and each of said first and second bit lines, and wherein a gate voltage for turning on said MIS transistor is applied to said gate electrode from said word line.

5. The semiconductor memory device according to one of claims 1 to 3, wherein when one kind of binary data is stored in said memory cell, a voltage of a polarity corresponding to the one kind of binary data is applied between said plate line and each of said first and second bit lines, and a gate voltage for turning on said MIS transistor is applied to said gate electrode from said word line, wherein when the other kind of binary data is stored in said memory cell, a voltage of a polarity opposite to the polarity of the voltage applied in a case of storing the one kind of data is applied between said plate line and each of said first and second bit lines, and wherein a gate voltage for turning on said MIS transistor is applied to said gate electrode from said word line.

6. The semiconductor memory device according to claim 4 or 5, wherein when data is read from said memory cell, electric potential between a first gate voltage and a second gate voltage is applied as a reading gate voltage to said gate electrode from said word line, the first gate voltage being a gate voltage, at which a source-drain current reaches a predetermined level according to a source-draincurrent-gate-voltage characteristic of said MIS transistor, when one kind of binary data written thereto, the second gate voltage being a gate voltage, at which the source-drain current reaches a predetermined value according to the source-drain-current-gate-voltage characteristic of said MIS transistor, when the other kind of binary data is written thereto, and wherein a source-drain current is detected from said first bit line or said second bit line when a reading voltage is applied between said source and said drain of said MIS transistor while a voltage being nearly equal to the reading gate voltage is applied to said gate electrode from said word line.