JPH04362550A - Ferroelectric substance memory and driving method thereof - Google Patents

Abstract

PURPOSE:To inexpensively improve an information density by forming an information recording layer consisting of the thin film of an amorphous ferroelectric substance which is formed with electrodes on both surfaces and has a hysteresis characteristic on a light transmissive substrate having a specific thickness. CONSTITUTION:The electrode 3 consisting of a light transmissive conductive thin film is formed on the light transmissive substrate 2 having >=0.1mm thickness. The thin film 1 of the amorphous ferroelectric substance, such as thin film of an Fe2O3-Bi2O-PhTiO3 system, is formed by an RF sputtering method, etc., on this film 3. Further, the electrode 4 is formed of the thin film of Al on the film 1. The writing of the information is executed by impressing a forward electric field between the electrodes 3 and 4 to unify and saturate the polarization in the same direction, then removing the impressed electric field. A digital value '0' is then written as a residual polarization. A digital value '1' is written by impressing the electric field of an opposite direction at the electric field intensity at which the polarization inverts or below and irradiating one point of the electrode 3 with a laser beam to change the state of a hysteresis loop. The information is stored at the high density by this constitution.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric memory and a method for driving the same.

0002

2. Description of the Related Art Ferroelectric memories, which are based on the DE hysteresis characteristics of ferroelectric materials, have recently attracted attention as information storage devices for computers and the like. That is, in a ferroelectric material having a square DE hysteresis curve as shown in FIG.
correspond to When a read electric field E in the positive direction is applied to a ferroelectric material, if the digital storage value of the ferroelectric material is 1 in this state, the electric polarization changes from state Ar to state Br on the D-E hysteresis curve. After that, it changes in the direction of Cr. The slope of the hysteresis loop in this part is steep, and the capacitance changes rapidly. Therefore, by extracting this change in capacitance as a change in current, the memory of the digital value 1 can be read out. on the other hand,
When the digital storage value of the ferroelectric is 0, the electric polarization changes from the state Dr through the state Br' in the direction of Cr on the DE hysteresis curve with respect to the read electric field E in the positive direction. The slope of the hysteresis loop in this part is gentle, and the change in capacitance is small, so
Only small changes in current can be extracted. Therefore, the digital value 0 can be read as distinct from the digital value 1. The advantages of this ferroelectric memory are that, unlike hard disks and floppy disks, which are typical conventional information storage media, it is not affected by magnetic influences from the outside world, and the power consumption associated with storing and reading information is small. Furthermore, it has advantages such as a short memory access time of 1 microsecond or less and a small storage area per bit, and has the potential to provide a compact, large-capacity, and highly reliable data storage device. It is expected that However, the ferroelectric memory that has been studied so far is
As stated in Publication No. 229472, one storage unit is composed of a MOS-FET and a capacitor using a ferroelectric material for storing information, and this one structural unit can only store one bit of information. In terms of improving data storage density, it could not be said to be sufficient.

[0003] In order to solve such problems,
In Publication No. 7186, one side of a BaTiO3 polycrystalline ferroelectric thin film having hysteresis characteristics is coated with a transparent conductive thin film, and a conductive thin film is coated on the other side of the BaTiO3 polycrystalline ferroelectric thin film through an insulating film. A memory equipped with a substrate has been proposed. In this memory configuration, a laser beam focused to a very fine beam diameter is irradiated from the transparent conductive thin film side, and at the same time, a voltage is applied between the transparent conductive thin film and the conductive substrate. , an electric field smaller than the coercive electric field of the BaTiO3 polycrystalline ferroelectric thin film is applied in advance. The region irradiated with the laser beam is heated and the coercive electric field is lowered, so that the polarization direction is reversed along with the direction of the applied electric field. Furthermore, in reading, it is possible to determine the digital information value written by the magnitude of the inversion current. It is stated that by this method, it is possible to construct a nonvolatile ferroelectric memory that has a large storage capacity, can be accessed at high speed, and is also nonvolatile. However, the polycrystalline ferroelectric thin film described in JP-A-57-117186 is formed by the manufacturing method described in JP-A-55-56621. According to the publication, it is stated that in order to obtain a ferroelectric material having hysteresis made of BaTiO3, a heat treatment temperature as high as 1300 to 1400 DEG C. is required. Therefore, since the formation of a ferroelectric thin film involves such a high-temperature process, it is necessary to use a conductive substrate for the polycrystalline thin film that is heat resistant and does not diffuse at high temperatures, such as a Pt substrate. , expensive substrate materials must be used. Further, an insulating layer is required to ensure insulation, and forming the ferroelectric memory requires cost and time, which poses a problem in supplying an inexpensive memory. Furthermore, since this memory only has a light-transmitting conductive thin film formed on the surface of the ferroelectric memory layer, the laser light is scattered by micron-meter-sized foreign particles attached to the memory surface, and digital values cannot be written. , it is thought that many errors occur during read operations.

0004

OBJECTS OF THE INVENTION The present invention solves the above-mentioned problems and provides a ferroelectric thin film having ferroelectric hysteresis characteristics, which becomes an information storage layer, at a substrate temperature at the time of film formation or at a heat treatment temperature after film formation of 500%. It can be produced at low temperatures below ℃, and
It is an object of the present invention to provide a ferroelectric memory that is inexpensive, has a high information storage density, and whose information write/read operations are not disturbed even by foreign matter adhering to the surface of the memory, and a method for driving the same.

[0005]

[Means for Solving the Problems] The present invention provides an information storage layer made of an amorphous ferroelectric thin film having hysteresis characteristics in electric field dependence of electric polarization, electrodes formed on both sides of the information storage layer, Thickness in contact with one side of the electrode is 0
．． The present invention relates to a ferroelectric memory characterized by comprising a transparent substrate having a thickness of 1 mm or more. The present invention also provides an information storage layer made of an amorphous ferroelectric thin film having hysteresis characteristics in electric field dependence of electric polarization, an electrode formed on one side of the information storage layer, and an electrode formed on the other side. The present invention relates to a ferroelectric memory comprising a conductive and transparent substrate having a thickness of 0.1 mm or more. In the present invention, an amorphous ferroelectric thin film having dielectric hysteresis characteristics is used as a ferroelectric layer for storing information. This amorphous ferroelectric thin film is produced as an amorphous thin film on a substrate using a film forming method such as vacuum evaporation or sputtering while maintaining the substrate temperature at 500°C or less, and is left in the same state as it was produced. Alternatively, it can be obtained by heat treatment at a temperature of 500° C. or lower. The amorphous ferroelectric thin film thus obtained has hysteresis characteristics in the electric field dependence of electric polarization. The amorphous ferroelectric thin film is not particularly limited as long as it can be obtained by the above manufacturing method, but for example, Fe2
Examples include ternary oxide thin films containing O3-Bi2O3-ABO3 (perovskite compound) as a main component. AB
O3 is PbTiO3, PbZrO3, BaTiO3
Ferroelectric and antiferroelectric materials such as ferroelectric and antiferroelectric materials.

FIG. 2 shows the structure of the ferroelectric memory of the present invention. 2 is a transparent substrate material. The thickness of this substrate is 0.
It is desirable that the thickness is 1 mm or more. There are no restrictions on the substrate material as long as it has a high transmittance to the laser beam, but the substrate is selected depending on the substrate temperature during formation of the amorphous ferroelectric thin film and the heat treatment temperature of the thin film after the thin film is formed. Base temperature during formation of amorphous ferroelectric thin film is 120℃
In the following description, a polycarbonate resin substrate is suitable as the substrate material when heat treatment is not required after forming the thin film. On the other hand, the base substrate temperature and the heat treatment temperature after film formation were 120
If the temperature exceeds ℃, glass is suitable as the substrate material. 3 and 4 are conductive electrode thin films for applying an electric field to the amorphous ferroelectric thin film. The electrode 3 does not particularly need to be translucent, but if it is not translucent, it is desirable that it has a high light absorption rate for laser light. Also, board 2
When a conductive material is used, the electrode 3 is not particularly required. The electrode 4 is made of a conductive material, and there are no particular restrictions on the material as long as the adhesive strength to the amorphous ferroelectric thin film is ensured. Further, the electrode 4 does not need to be a thin film. 5
is a laser beam, which is focused by the lens 6 into a beam with a diameter of about 1 μm near the amorphous ferroelectric thin film 1 and the thin film electrode 3. Here, in order to keep the area in which information is written into the amorphous ferroelectric thin film by the laser in a narrow area, the total thickness of each of the conductive thin film and the amorphous ferroelectric thin film must be less than or equal to the diameter of the focused beam of the laser. desirable. That is, in this case, the total film thickness is 1 μm.
The following is desirable. Also, when the laser beam is incident on a transparent substrate, the beam diameter of the laser beam on the surface of the transparent substrate on the laser beam incident side can be changed by making the transparent substrate thicker, for example, by choosing a thickness of about 1 mm. The beam diameter of the laser beam on the substrate surface is about 0.5 mm, so that writing and reading of information will not be disturbed by minute foreign matter adhering to the substrate surface. Furthermore, it is possible to write digital information over a wide area by relatively moving the substrate and the laser light source 7. For example, high-density recording of information is possible by rotating a memory substrate and recording information on the same circumference, as in a magneto-optical disk.

[0007]

[Examples] The present invention will be explained in detail with reference to Examples below. The configuration of the memory is shown in Figure 2. A polycarbonate substrate with a thickness of 1 mm was used as the substrate. Furthermore, a conductive thin film layer 3
Transparent I made by sputtering deposition method
A TO film was formed. Here, the thickness of the ITO film was approximately 100 nm. Furthermore, a Fe2O3-Bi2O3-PbTiO3 thin film or the like was formed on the ITO thin film by RF sputtering or the like. This amorphous ferroelectric thin film is Fe2O3-Bi2O3-PbTi
Not only O3-based thin films but also PbZr instead of PbTiO3
Various ferroelectric and antiferroelectric perovskite materials such as O3, BaTiO3, etc. may be used. The amorphous ferroelectric thin film cited in this example was produced by the method described below. That is, an RF magnetron sputtering device was used to fabricate the thin film, and the sputtering gas was a mixed gas of Ar:O2=7:3. During sputtering, the copper anode fixing the polycarbonate substrate on which the ITO film was formed was cooled with water, and the substrate temperature during film formation was maintained at 20 to 25°C. The thin film thus obtained became an amorphous ferroelectric film having hysteresis characteristics in the electric field dependence of electric polarization without performing any heat treatment after film formation. Since this amorphous ferroelectric thin film has high electrical insulation, it is not necessary to form an insulating layer, which is required in the case of a polycrystalline ferroelectric film. Furthermore, it is possible to form a ferroelectric memory that can stably retain information for a long time without leakage of surface charges generated along grain boundaries in a polycrystalline ferroelectric thin film. Furthermore, an Al thin film having a thickness of 200 nm was formed as an electrode 4 on the surface of the amorphous ferroelectric thin film thus formed by vacuum evaporation. Furthermore, the wavelength of the laser light source 7 is 830 nm.
, a semiconductor laser with an output of 10 mW was used.

Next, information writing and reading operations in the ferroelectric memory having the above structure will be explained. First, writing information will be explained. The direction of the electric field from the light-transmitting electrode 3 to the electrode 4 is defined as the forward direction, and this direction is
It is assumed that this corresponds to the graph axis of the hysteresis curve in FIG. When a forward electric field is applied between the light-transmitting electrode 3 and the electrode 4 to align the polarization in the same direction and saturate it, and then remove the applied electric field, the hysteresis loop in FIG. 1 becomes the state Dr, and the digital value becomes 0. It is written as residual polarization Pr. To write a digital value of 1, when the electric field strength at which the polarization is reversed is Ec, an electric field -Em in the opposite direction with an absolute value of less than Ec is applied, and a laser beam is irradiated to one point on the light-transmitting electrode 3. . As a result, the polarization at one point irradiated with the laser beam turns in the direction of the applied electric field due to the temperature rise caused by the laser beam, and the hysteresis loop becomes the state Dr' (
Corresponds to the electric field -Em. ) to state Er. Here, when the electric field -Em and the laser beam irradiation disappear, the hysteresis loop changes from the state Er to the state Ar, so a digital value of 1 is written as the residual polarization (-Pr). Next, when reading information, a forward electric field Em smaller than the coercive electric field Ec is applied between the light-transmitting electrode 3 and the electrode 4, and the light is focused on the part where the information to be read is written. irradiate the laser beam. In this case, if the digital memory value of the laser beam irradiation point is 0,
The DE hysteresis loop changes from state Dr to state Cr. The slope of the hysteresis loop in this part is gentle and the change in capacitance is very small, so only a small change in current can be extracted. On the other hand, when the digital storage value of the laser beam irradiation point is 1, the D-E hysteresis loop passes from state Ar to state Br to state Cr.
Changes to Since this state change brings about a reversal of polarization, the capacitance changes rapidly, and a large current change can be extracted from the light-transmitting electrode 3 and the electrode 4. Therefore, by detecting this current change, the digital storage value 1 can be read separately from the digital storage value 0. However, in this case, since the stored value is inverted from 1 to 0 when the information is read, it is necessary to rewrite the information using the method of writing the digital value 1 described above. In this case, by moving a laser beam on the light-transmitting electrode 3, for example within the plane of the light-transmitting substrate, very high-density information can be written and read in one ferroelectric memory. Therefore, it is possible to realize a ferroelectric memory that is small, thin, and has a very large storage capacity. Furthermore, since the electric fields Em, -Em applied between the light-transmitting electrode 3 and the electrode 4 are smaller than the coercive electric field Ec, a high-frequency electric field can be used, and the operation speed can be increased. For example, if the electric field Em, -Em is set to 1MHz
In the case of a high frequency electric field of , the operating time is 1 microsecond.

[0009]

As described above, according to the present invention, an amorphous ferroelectric material having ferroelectric hysteresis characteristics at room temperature or a low temperature of 500° C. or lower is used for the information storage layer of a ferroelectric memory, and By using a light-transmitting substrate instead of the conventional conductive light-transmitting thin film layer, it is inexpensive, has a high information storage density, and can speed up the writing and reading of information. It is also possible to provide a ferroelectric memory in which information writing and reading operations are not disturbed.

[Brief explanation of the drawing]

[Fig. 1] Fig. 1 shows how information is written in a ferroelectric memory.
FIG. 3 is a diagram showing a ferroelectric hysteresis loop for explaining a reading method.

FIG. 2 is a configuration diagram of a ferroelectric memory illustrating an embodiment of the present invention.

[Explanation of symbols]

1 Amorphous ferroelectric thin film 2
Transparent substrates 3, 4 Conductive electrode 5 Laser light 6 Lens 7 Laser light source

Claims (5)

[Claims] Claims: 1. An information storage layer made of an amorphous ferroelectric thin film having hysteresis characteristics in electric field dependence of electric polarization, electrodes formed on both sides of the information storage layer, and a 0.0mm thick layer in contact with one side of the electrode. A ferroelectric memory comprising a transparent substrate of 1 mm or more. 2. An information storage layer made of an amorphous ferroelectric thin film having hysteresis characteristics in electric field dependence of electric polarization, an electrode formed on one side of the information storage layer, and a thickness formed on the other side. A ferroelectric memory comprising a conductive, transparent substrate with a diameter of 0.1 mm or more. 3. An amorphous ferroelectric thin film is produced as an amorphous thin film on a substrate using a film forming method while maintaining the substrate temperature at 500° C. or less, and is in the state as it is produced, or Claim 1 characterized in that the product is obtained by heat treatment at a temperature of ℃ or less.
Or a ferroelectric memory according to claim 2. 4. The amorphous ferroelectric thin film is made of Fe2
3. The ferroelectric memory according to claim 1, wherein the ferroelectric memory is made of a ternary oxide thin film whose main component is O3-Bi2O3-ABO3 (perovskite compound). 5. Information is written and read by irradiating laser light onto the amorphous ferroelectric thin film while applying an electric field smaller in absolute value than the coercive electric field to the amorphous ferroelectric thin film. A method for driving a ferroelectric memory according to claim 1 or claim 2, characterized in that: