JPH08249876A - Ferroelectric device - Google Patents

Abstract

PURPOSE: To obtain a ferroelectric memory having a high polarization quantity. CONSTITUTION: A ferroelectric memory cell has constitution in which ferroelectric films 35 are held by a pair of electrodes 31, 32. The ferroelectric films 35 have crystals having two nearly equiv. first and second crystal axes ((a) axis and (b) axis) and a third crystal axis (c axis) having the symmetry different from the symmetry having the (a) axis and the (b) axis as symmetrical axes in the direction different from the direction of the (a) axis and (b) axis. The (c) axis is oriented in a direction approximately perpendicular to the electric field impressed by a pair of the electrodes 31, 32.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic device having a ferroelectric thin film such as a ferroelectric memory or a ferroelectric capacitor.

[0002]

2. Description of the Related Art Conventionally, ferroelectric compounds have been applied to many fields by utilizing their unique electrical characteristics. For example, it has been applied to piezoelectric filters and ultrasonic transducers that utilize piezoelectricity, infrared sensors that utilize pyroelectricity, and various fields such as optical modulators and optical shutters that utilize the electro-optic effect. Furthermore, electronic devices using thin films of these materials have been devised, and studies on thinning have been vigorously made. In particular, the non-volatile memory device equipped with a ferroelectric thin film capacitor utilizing the stability of remanent polarization is the field that has received the most attention due to the recent trend toward higher storage capacity and higher integration. In particular, in the fields of computers and image processing devices, high-density and high-performance recording devices have been demanded, and up to now, external recording devices such as magnetic tapes, floppy disks, and magneto-optical disks have been met. , Or semiconductor memory,
That is, DRAM, SRAM, EPROM, EEPROM, flash memory and the like. However, in the near future, due to the fusion of multimedia and computers, higher performance such as 1) non-volatile, 2) high-speed low-voltage drive, and 3) solid-state memory without mechanical drive mechanism. Therefore, a compact memory is needed, but the current memory technology is not enough.

As a memory to meet this demand, for example, USP
4,873,664 (S. Sheffield Eaton Jr., Colorado Springs,
There is a ferroelectric memory as disclosed in CO). This ferroelectric memory has a structure as shown in FIG.
That is, the ferroelectric thin film capacitor 71 in the memory cell 70 is driven by the FET 72, and the conventional DRAM type storage capacitor is replaced with the ferroelectric capacitor. One end of the memory cell 70 is connected to the plate line 74, the gate of the FET 72 is connected to the word line 73, and the source or drain of the memory cell 70 is connected to the bit line 76, and its reading is performed by the sense amplifier 75. In addition, Matubara et al. (USP 5,122,9
23) is a ferroelectric memory that uses a platinum group oxide as the lower electrode, so that even if the film thickness of BaTiO ₃ of high dielectric is reduced to 100 nm, the dielectric constant does not decrease and the dielectric strength is high. Is disclosed. Furthermore, Smol
Among the Bi layered compounds discovered by enskii, B
There are various ferroelectric materials including i ₄ Ti ₃ O ₁₂ .

[0004]

[Problems to be Solved by the Invention] However, USP
In the configuration disclosed in 4,873,664, the device is silicon
Since it is formed on the device, the integration, cost and
In the same way as semiconductor memory DRAM and FLASH memory
It On the other hand, the ferroelectric material used for the ferroelectric memory is
In many cases, PZT (Pb_{1 + x}(Zr_yTi _1-y) O₃)But
It has been used, but it is one of the ferroelectric materials.
By the way, PZT has a high Curie temperature and Zr / Ti ratio.
The dielectric constant and polarizability can be controlled by changing
And the crystal anisotropy is not so strong, and film formation is easy.
This is because until now, except for PZT, destructive ferroelectric
Little has been studied as a material for body memory. Was
Bi Bi is the simplest constitution of layered compound, Bi_FourTi₃
O₁₂Regarding MOCVD and sol-gel method,
However, the crystal anisotropy is too strong, and it is a ferroelectric substance that can be put to practical use.
The capacity could not be formed and configured. In addition, the inventors
The materials centered on ZT have the following problems.
Was found experimentally. (1) The PZT thin film volatilizes
It contains pan Pb, which creates Pb vacancies and oxygen
It becomes space charge and causes various problems including fatigue.
Wake up. (2) This space charge has a low dielectric constant on the surface and electrode interface.
Forming a surface layer with a high rate and thinning it increases the coercive voltage.
Or the polarizability deteriorates. (3) Deterioration of retention characteristics is space
It is thought that this is due to an increase in the internal electric field due to electric charges,
In PZT, the retention characteristics deteriorated. (4) Special imprint
The property is that the internal electric field increases due to space charge
It is said that it is due to the capture / release of ar
The authors found that in PZT, single non-inversion at high temperature
Degradation occurs due to asymmetry in the hysteresis characteristics due to the pulse.
To live. (5) Leakage current is used in electronic devices and memories
Carriers are trapped / released in space charge
Is caused by.

Further, the disclosure of Matubara et al. Does not describe the case of a ferroelectric material having a more sensitive structure, and the inventors made additional tests. However, PZT on RuO ₂ or ITO has a good breakdown voltage. However, the leak current increased by two orders of magnitude. Further, although BaTiO ₃ is well known, it cannot be used at a Curie temperature around room temperature. Also, Smolenskii
The material discovered by J.K. is considered to be difficult to make into a thin film. This is because there is a method of polishing the bulk ceramics from the surface for thinning, but polishing defects of several μm are introduced from the surface.

Recently, the simplest structure, Bi
₄ Ti ₃ O ₁₂ is deposited by MOCVD or spin coating method, but it has not been successful. This is because the Bi layered compound has a strong anisotropy, and the C axis having almost no spontaneous polarization faces the direction perpendicular to the substrate. In view of the above problems, it is an object of the present invention to provide a ferroelectric device having a high polarization amount.

[0007]

In order to achieve the above object, the present invention provides a ferroelectric device having a ferroelectric thin film sandwiched by a pair of electrodes, wherein the ferroelectric thin films are two substantially equivalent. The first and second crystal axes, and the first
And a third symmetry having a symmetry different from that of the first and second crystal axes in directions different from the first and second crystal axes.
A ferroelectric device in which the third crystal axis is oriented substantially perpendicular to the electric field applied by the pair of electrodes. provide.

[0008]

[Operation] An electric field is applied to the ferroelectric thin film via the pair of electrodes. Here, since the third crystal axis, which has a symmetry different from the symmetry with the two substantially equivalent crystal axes as the symmetry axes, is substantially orthogonal to the direction of this electric field, a large number of The domain polarization is directed in the direction of the applied electric field, and a high amount of polarization is obtained.

[0009]

【Example】

(Embodiment 1) A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows the structure of an electronic device having a ferroelectric thin film. As shown in FIG. 1, a ferroelectric thin film 10 sandwiched between a lower electrode 12 made of platinum or the like and an upper electrode electrode 11 also made of platinum or the like is provided on a substrate 13. In such a configuration, when a voltage is applied between the upper electrode 11 and the lower electrode 12, the polarization amount of the ferroelectric thin film 10 changes non-linearly with respect to the applied voltage, as shown in FIG. It has such hysteresis characteristics. In FIG. 2A, the applied voltage is plotted on the horizontal axis,
6 is a graph showing the relationship between the applied voltage and the amount of polarization on the vertical axis. The hysteresis characteristic is usually measured using a continuous sine wave of about 1 kHz or a triangular wave. Where P
r + and Pr- are called remanent polarization amounts, and Ps + and Ps- are called saturated polarization amounts. FIG. 2B shows the relationship between the applied voltage and the charge amount for Ps and Pr. The difference between Ps and Pr is called back switching. V'c + and V'c- are coercive voltages obtained from the hysteresis characteristics of FIG. 2 (a), and Vc is FIG. 2 (b).
It is a coercive voltage obtained from the Q-V characteristic of Pr of 1 and does not always match. Here, for example, in order to function as a memory, information to be written is associated with two polarization states. If the first state is downward (writing by applying a positive voltage to the lower electrode) Pr +, the second state is upward (writing by applying a negative voltage to the lower electrode) Pr-. . When such a device cell is actually used as a memory, as shown in FIG. 3A, a ferroelectric memory cell 22 having the structure shown in FIG. 1 is formed at the intersection of the stripe electrodes 20 and 21 intersecting each other. It may be a simple matrix type arranged, or FIG.
As shown in (b), the ferroelectric memory cell 22 and the pass gate transistor 23 are connected to each other, and the stripe electrode 20,
It may be a DRO type which is arranged at the intersection of 21 and connects another stripe electrode 24 and the other end of the ferroelectric memory cell 22.

As explained above, the ferroelectric memory,
An electronic device having a ferroelectric thin film such as a ferroelectric capacitor is used with a structure in which a thinned ferroelectric material is sandwiched between a lower electrode and an upper electrode. Any material can be used for the lower electrode and the upper electrode as long as they can withstand an oxidizing heat treatment atmosphere, but usually platinum (Pt) or platinum group elements, alloys, oxides thereof, and combinations thereof are used. It may be combined with a diffusion barrier film or an adhesive layer. Here, the method of forming the film of the ferroelectric material is not particularly limited, and spin coating methods such as the sol-gel method and the organometallic thermal decomposition method, mist film formation in which these solutions are vapor-phase coated in the mist state, MOCVD or Fl are used.
Ash type CVD, sputtering, MBE, reactive sputtering, etc. are used.

Next, the material of the ferroelectric thin film will be described. A layered compound is used as the material of the ferroelectric thin film.
Layered refers to a structure in which layers composed of unit crystal cells having an asymmetric structure are stacked. When the unit crystal cells have one kind of asymmetric structure, or the unit crystal cells include two or more kinds of structures. However, as a result of stacking these two or more types of structures, a case where the whole unit crystal cell has an asymmetric structure is included. To explain the structure of the unit cell in more detail, two substantially equivalent crystal axes can be defined in the unit cell, and symmetry in which the two crystal axes are different from each other in the directions different from each other. Has a third crystallographic axis with a symmetry different from. In the following description, the above-mentioned two substantially equivalent crystal axes are referred to as a-axis and b-axis, respectively, and the third crystal axis is referred to as c-axis. The ferroelectric thin film is made of a crystal having a structure in which such unit crystal cells are stacked in the c-axis direction.

Regarding the layered structure, the present inventors
SrBi found to be suitable for child devices₂T
a₂O₉Will be described in more detail. SrBi₂
Ta₂O₉Of the above two types of structures,
It has two types of structures, and these two types of structures are stacked.
It has a structured structure. As shown in FIG. 4, (Bi₂O₂) ²⁺
Bismuth layer consisting of SrTaO₃A pseudo-perov consisting of
It has a two-layer structure of a skyte layer, and these layers alternate.
Are stacked on.

The perovskite structure is a structure described by the general formula ABO ₃ , where A is a cation called A site ion, B is an anion called B site ion, and O is oxygen. The perovskite structure forms a regular octahedron with oxygen at the apex, the B site ion is located at the center of the regular octahedron, and the A site ion is arranged between the regular octahedrons. Basically, such a structure is used in SrTa of FIG.
Although it has a layer made of O ₃ , it has a structure in which the regular octahedron is divided into two parts vertically, and the top and bottom are swapped so that the apex of the tip of the square cone is shared. Will be referred to as a pseudo perovskite structure. In the layered structure as shown in FIG. 4, the c-axis is the stacking direction of the bismuth layer and the pseudo-perovskite layer, and the a-axis and the b-axis are mutually equivalent and are directions orthogonal to the c-axis. From the crystal analysis, the lattice constant is a = 5.5306 angstrom in the a-axis direction, b = 5.5344 angstrom in the b-axis direction, and c = 24.9839 angstrom in the c-axis direction. In such a structure, (Bi ₂ O ₂ ) ²⁺
The layer is outside the quasi-perovskite structure SrTaO ₃ that exhibits the properties of a ferroelectric substance, and the spontaneous polarization of the quasi-perovskite structure is in the a-axis or b-axis direction. Ferroelectrics having such a layered structure are arranged as shown in FIG. 5 to form a memory cell. A lower electrode 31 is formed on a substrate 30, and a bismuth layer 33 ((Bi ₂ O ₂ ) ²⁺ ) and a pseudo perovskite layer 34 are formed on the lower electrode 31.
A ferroelectric thin film 35 made of (SrTaO ₃ ) is arranged so that the c axis is parallel to the substrate 30 and the lower electrode 31. Further, the upper electrode 32 is formed thereon. Here, when a voltage is applied between the upper electrode 32 and the lower electrode 31, a polarization P is generated in the ferroelectric thin film 35 in a direction along an electric field generated by the applied voltage.

FIG. 6 shows the structure of the ferroelectric thin film in more detail. FIG. 6A is a sectional view of the ferroelectric thin film, and its structure is schematically shown. The ferroelectric thin film 35 is composed of a plurality of crystal grains 36, and the c-axis of each crystal grain is oriented in the plane direction of the ferroelectric thin film.
FIG. 6B is a top view of the ferroelectric thin film 35. The plurality of crystal grains 36 in the ferroelectric thin film 35 are randomly oriented within the plane in which the thin film extends. In such a structure, the direction of the c-axis of the ferroelectric thin film as a whole obtained by combining the directions of the c-axes of the respective crystal grains is the direction in which the thin film extends,
That is, since the polarization vector is oriented in the direction perpendicular to the applied electric field, the polarization vector does not cross the bismuth layer, and the high spontaneous polarization amount can be obtained without being affected by the space charge due to the lack of bismuth in the bismuth layer. That is, in this structure, volatile bismuth is included, and the volatilization of bismuth does not affect the oxygen octahedron expressing ferroelectricity, but the bismuth layer ((Bi ₂ O ₂ ) ²⁺ ). This is because the bismuth layer creates space charges due to the lack of bismuth. In order to eliminate the influence of this space charge, it is necessary to prevent the polarization vector from crossing the bismuth layer. The polarization vector of the whole thin film is the vector sum of the polarization of each crystal grain, and for this condition to be satisfied, the whole thin film, or a considerable part of the crystal grains, is oriented so that the polarization vector does not cross the bismuth layer. There is a need. This mechanism is shown in FIG. Here, FIG. 7A shows the case where the polarization vector ΣP of the thin film as a whole crosses the bismuth layer 33. In this case, the space charge 40 in the bismuth layer 33 interacts with the polarization vector ΣP. FIG. 7B shows the case where the polarization vector ΣP of the entire thin film does not cross the bismuth layer 33. In this case, the space charge 40 in the bismuth layer 33 does not interact with the polarization vector ΣP.

Further, since the crystal grains are oriented so that the c-axis is oriented in a certain direction perpendicular to the applied electric field, the coercive electric field can be reduced and kept constant. Further, when the c-axis is oriented in the direction perpendicular to the applied electric field (in this case, the direction in which the ferroelectric thin film extends), the domain is constituted only by the a-axis and the b-axis in the film thickness direction. That is, they are a 180-degree domain and a 90-degree domain composed of the a-axis and the b-axis. 90
Since the degree domain wall has almost no difference in lattice constant between the a-axis and the b-axis, reversal without stress concentration is possible.
This will be described with reference to FIG. FIG. 8A shows the orientation of the crystal axis. In crystal grains (I) and (II), the a axis and the b axis form an angle of 45 degrees with respect to the film thickness direction. In III), the a-axis is the film thickness direction and the b-axis is a
The direction is perpendicular to the axis. 8 (b) is shown in FIG. 8 (a).
The domains corresponding to the crystal grains (I), (II), and (III) are shown. In the crystal grain (I), the polarization is formed in the intermediate direction between the a-axis and the b-axis and constitutes a 180 degree domain.
In the crystal grain (II), the polarization is along the directions of the a-axis and the b-axis, and constitutes a 180-degree domain and a 90-degree domain wall. In this case, the c-axis is oriented in the direction perpendicular to the paper surface, and stress is small. Therefore, polarization reversal without stress concentration becomes possible. In the crystal grain (III), polarization is formed in the directions along the a-axis and the b-axis and constitutes a 180-degree domain. Therefore, by orienting the c-axis perpendicularly to the applied electric field and by polarizing so as to form the 180-degree domain and the 90-degree domain wall along the a-axis and the b-axis, polarization reversal without stress concentration can be achieved. This makes it possible to improve the number of polarization inversions.

Note the ferroelectric thin film constructed in this way
When it is configured as a recell and the characteristics are measured, (1) polarization reversal
Number of turns: 10¹³Times, (2) polarization retention characteristics: 732 (5
(Calculated value at 0 ° C.) was obtained. These are conventional
Measured value 10⁸It is more than 10 years. Also,
(3) Dielectric constant is as small as 25% of PZT.
The S / N ratio of was improved 2.5 times at low voltage, and (4) 2Pr
20 to 35 μC / cm ²Becomes larger, and high integration is possible
The ability was obtained. Furthermore, (5) film thickness dependence of leakage current
Memory and temperature dependence have been reduced, so a large capacity non-volatile memory
The possibility of Mori was obtained, and (6) was a problem in the past.
Implementation caused by repeated writing and reading
Reliability has been improved. In addition, 2Pr
Since it is improved, it is possible to reduce the area of the capacitor.
Therefore, high integration is possible even as a ferroelectric capacitor.
It

As described above, since the characteristics are not deteriorated even if the ferroelectric material is thinned, the conventional 5V operation is 0.5V.
It became possible to operate. (Embodiment 2) A second embodiment of the present invention will be described with reference to FIG. FIG. 9A shows a cross section of the ferroelectric device. A lower electrode 51 is formed on the substrate 50,
On top of that, a ferroelectric thin film 5 made of SrBi ₂ Ta ₂ O ₉
2. Further, the upper electrode 53 is formed thereon. Here, the c axis of the ferroelectric thin film 52 is plus or minus 30 degrees with respect to the direction perpendicular to the electric field applied by the upper electrode 53 and the lower electrode 51 (in this case, the direction in which the ferroelectric thin film extends). Oriented within. FIG. 9B is a diagram showing the cross-sectional structure of the ferroelectric thin film 52 in more detail. The ferroelectric thin film 52 has a plurality of crystal grains 54. The angles θ 1 and θ formed by the c-axis of each crystal grain 54 with respect to the direction perpendicular to the electric field applied by the upper electrode 53 and the lower electrode 52.
2 is oriented so as to form an angle within plus 30 degrees. FIG. 9C is a top view of the ferroelectric thin film 52. The plurality of crystal grains 5 in the ferroelectric thin film 52 are shown in FIG.
4 is oriented in a random direction. At this time, the c-axis of all the crystal grains is within plus or minus 30 with respect to the direction perpendicular to the applied electric field in a plane including the applied electric field direction.
It is not required to make an angle within degrees. That is, 6
It is sufficient that 0% or more of the crystal grains are oriented within plus or minus 30 degrees. FIG. 10 shows the distribution function g (θ) of the angle θ formed by the c-axis of the crystal grain and the plane perpendicular to the applied electric field. Here, the ratio of crystal grains oriented to form an angle within plus or minus 30 degrees is 60% or more.

In the case of such a configuration, when configured as a memory, the following data was obtained. That is,
(1) Number of polarization reversals: 10 ¹³ times, (2) polarization retention characteristics:
732 (calculated value at 50 ° C.) was obtained. These exceed the conventional measured values of 10 ⁸ times and 10 years, respectively. Further, (3) the dielectric constant is as small as 25% of PZT, and the S / N ratio as a memory is improved twice at low voltage.
Pr increased from 15 to 35 μC / cm ² and a possibility of high integration was obtained. Further, (5) the film thickness dependence and temperature dependence of the leak current are reduced, so that the possibility of a large-capacity non-volatile memory is obtained, and (6) repetitive writing and reading, which has been a problem in the past. The imprint deterioration caused by is reduced and the reliability is improved. Also, 2
Since Pr is improved, high integration is possible even when used as a capacitor.

As described above, since the characteristics are not deteriorated even when the ferroelectric material is thinned, the conventional 5V operation can be changed to 1V operation. (Third Embodiment) Next, a third embodiment will be described with reference to FIG. FIG. 11A is a perspective view of the ferroelectric thin film 60 made of SrBi ₂ Ta ₂ O ₉ , and shows the orientation direction of crystal grains of the ferroelectric thin film. That is, in the plane perpendicular to the applied electric field, the angle ψ formed by the c-axis of the crystal grain with respect to a certain predetermined direction is within ± 30 degrees, and within the plane including the above predetermined direction and the applied electric field direction. Then, the angle θ formed by the above predetermined direction and the c-axis of the crystal grain is within ± 30 degrees. FIG. 11B shows a ferroelectric thin film 60.
Is a schematic representation of the structure of a plurality of crystal grains 61
It consists of The c-axis of each crystal grain 61 is plus or minus 3 with respect to a certain direction in a plane perpendicular to the direction of the applied electric field (thickness direction of the ferroelectric thin film in FIG. 11B).
It is oriented so as to form an angle within 0 degree. Figure 11
(C) is a top view of the ferroelectric thin film 60. The crystal grains 61 of the ferroelectric thin film 60 are in the plane where the c-axis of each crystal grain is perpendicular to the applied electric field with respect to the above direction. And is oriented in a direction that forms an angle within 30 degrees of plus plus. At this time, the c-axis of all the crystal grains is oriented within ± 30 degrees within a plane including the electric field and the c-axis with respect to the direction perpendicular to the applied electric field, and within the plane perpendicular to the electric field. It is not required that the predetermined direction and the direction of the projection on the plane perpendicular to the electric field of the c-axis form an angle within plus or minus 30 degrees. That is, 60%
It is only necessary that the above crystal grains satisfy the above conditions. FIG.
2 shows a distribution chart of the orientation of crystal grains. FIG. 12A shows a distribution function g (θ) of the number of crystal grains with respect to an angle θ formed by the c-axis of crystal grains and a plane perpendicular to the applied electric field, and FIG.
Indicates a distribution function f (ψ) of the number of crystal grains with respect to an angle ψ formed by a predetermined direction in a plane perpendicular to the electric field and a direction of projection of the c-axis onto the plane perpendicular to the electric field. Here, the proportion of crystal grains oriented so as to form an angle within ± 30 degrees for each distribution function is 60% or more.

The following data was obtained when a memory was constructed using the ferroelectric thin film having such a configuration. That is, (1) the number of polarization reversals: 10 ¹⁵ times, and (2) the polarization retention characteristics: 732 (calculated value at 50 ° C.) were obtained. These exceed the conventional measured values of 10 ⁸ times and 10 years, respectively. Further, (3) the dielectric constant is as small as 25% of PZT, and the S / N ratio as a memory is improved three times at a low voltage,
(4) 2Pr increased from 20 to 35 μC / cm ² and a possibility of high integration was obtained. Furthermore, (5) the dependence of the leak current on the film thickness and temperature is reduced, so that the possibility of a large-capacity nonvolatile memory can be obtained.
The imprint deterioration caused by the repeated writing and reading, which had been a problem, was reduced, and the reliability was improved. Further, since 2Pr is improved, high integration is possible even when used as a capacitor.

As described above, the characteristics are not deteriorated even if the ferroelectric material is thinned.
It became possible to operate. In the above examples, (Bi ₂ O ₂ ) ²⁺ was used as one layer of the layered compound, but the present invention is not limited to this, and bismuth (Bi)
Instead of scandium (Sc), lanthanum (La),
Antimony (Sb), Yttrium (Y), Chromium (C
r) and thorium (Th) may be used. In the above example, strontium (Sr) was used as the A site ion and tantalum (Ta) as the B site ion of the pseudo perovskite structure, but the present invention is not limited to this.
As site ions, bismuth (Bi), lead (Pb),
At least one element selected from strontium (Sr), barium (Ba), calcium (Ca), and cadmium (Cd), and titanium (Ti), tantalum (Ta), niobium (Nb), and hafnium (H) as B site ions.
f), tungsten (W), Nb, zirconium (Z
At least one element of r) may be used.

Further, by adding impurities to the above layered compound, bismuth deficiency can be compensated for, leak current can be reduced, and impurities can be deposited between crystal grains to improve the breakdown voltage. You can In this case, the impurities include at least one of the group IIIA elements yttrium (Y) and lanthanum (La), and the group IVA elements titanium (Ti), zirconium (Zr), and hafnium (Hf). At least one element selected from the group consisting of vanadium (V), niobium (Nb) and tantalum (Ta) which are group VA elements, and chromium (Cr), molybdenum (Mo) and tungsten (VIA group elements). At least one element of W), at least one element of VIIA group elements such as manganese (Mn), technetium (Tc), and rhenium (Re), and group VIII elements of iron (Fe) and cobalt (Co). ), Nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (O
s) at least one element, IB group element copper (Cu), silver (Ag) at least one element, IIB group element zinc (Zn), cadmium (C)
d) at least one element, IIIB group element gallium (Ga), indium (In) at least one element, IVB group element germanium (G
e), at least one element of tin (Sn), V
Arsenic (As) which is a B group element, selenium (Se) which is a VIB group element may be used, or a combination thereof may be used.

When impurities are doped, it is necessary to dope a charge compensating material to compensate for electrical neutrality. As the charge compensating material, lithium (Li) and sodium ( Na), potassium (K), rubidium (Rb), cesium (Cs), at least one element, Group IIA elements beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), At least one element of barium (Ba) may be used, or a combination thereof may be used.

In the ferroelectric thin film having the above structure, no parasitic dielectric layer is formed on the electrode interface layer.
The characteristics did not deteriorate even if the thickness was reduced to 0 nm. Therefore, even if the film thickness is reduced to 30 nm, it is practically possible, and the film thickness up to 1 μm is suitable for driving at 3 to 5 V. Further, when the size of the device is about 0.1 μm ² , it is preferable that the size of the crystal grain is small in order to suppress the variation between the devices. However, when the size is less than 10 μm, the surface energy increases and the ferroelectricity is lost. Therefore, the size of the crystal grain is 10 nm to 300 nm
It is preferable that it is between.

[0025]

According to the present invention, in a crystal in a ferroelectric thin film, crystal axes having different symmetries in different directions from two substantially equivalent crystal axes are oriented so as to be substantially perpendicular to an applied electric field. As a result, it is possible to obtain a ferroelectric memory with a high polarization amount.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a ferroelectric device according to a first example.

2A is a diagram showing a hysteresis characteristic of the ferroelectric thin film according to the first embodiment, and FIG. 2B is a diagram showing an applied voltage and a charge amount of the ferroelectric thin film according to the first embodiment. Is.

FIG. 3A is a diagram showing a simple matrix type memory configuration using the ferroelectric device according to the first embodiment;
FIG. 2B is a diagram showing a DRO type memory configuration using the ferroelectric device according to the first embodiment.

FIG. 4 is a diagram showing a structure of SrBi ₂ Ta ₂ O ₉ .

FIG. 5 is a diagram showing a configuration of an electronic device using the ferroelectric thin film according to the first example.

6A is a diagram showing a cross-sectional structure of a ferroelectric thin film according to the first embodiment, and FIG. 6B is a diagram of the ferroelectric thin film according to the first embodiment as viewed from above.

FIG. 7 is a diagram showing a relationship between a polarization vector and a thin film.

FIG. 8A is a diagram illustrating the orientation of crystal axes in crystal grains, and FIG. 8B is a diagram illustrating domains formed in crystal grains.

9A is a diagram showing a cross section of an electronic device using a ferroelectric thin film according to a second embodiment, and FIG. 9B is a diagram showing a cross section of a ferroelectric thin film according to the second embodiment. , (C) is the second
FIG. 3 is a diagram of the ferroelectric thin film according to the example of FIG.

FIG. 10 is a diagram showing the distribution of the orientation of crystal grains in the ferroelectric thin film according to the second example.

11A is a diagram showing an orientation direction of crystal grains of a ferroelectric thin film according to a third embodiment, FIG. 11B is a diagram showing a ferroelectric thin film according to the third embodiment, and FIG. 8A is a view of the ferroelectric thin film according to the third embodiment as viewed from above. FIG.

FIG. 12 is a diagram showing a distribution of crystal grain orientations of a ferroelectric thin film according to a third example.

FIG. 13 is a diagram showing a conventional technique.

[Explanation of symbols]

 10, 35, 52, 60 Ferroelectric thin film 12, 31, 51 Lower electrode 11, 32, 53 Upper electrode 36, 54, 61 Crystal grain

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H01L 21/8247 29/788 29/792

Claims (46)

[Claims] 1. A ferroelectric device having a memory cell comprising a ferroelectric thin film sandwiched by a pair of electrodes, wherein the ferroelectric thin film comprises two substantially equivalent first and second crystal axes, and The first and second crystal axes are different from each other in a direction different from each other.
And a third crystal axis having a symmetry different from the symmetry having the second crystal axis as a symmetry axis, the third crystal axis being the pair of electrodes. Ferroelectric device characterized by being oriented in a direction substantially perpendicular to an electric field applied by. 2. The ferroelectric device according to claim 1, wherein the ferroelectric thin film has a plurality of the crystals as crystal grains. 3. The ferroelectric device according to claim 2, wherein the crystal grains have a size of 10 nm to 300 nm. 4. A ferroelectric device having a ferroelectric thin film sandwiched by a pair of electrodes, wherein the ferroelectric thin film comprises two substantially equivalent first and second crystal axes, and the first and second crystal axes. A crystal having a third crystal axis having a symmetry different from that of the first and second crystal axes in a direction different from the second crystal axis. , The third
Is oriented within plus or minus 30 degrees within a plane including the electric field and the third crystal axis with respect to a direction perpendicular to the electric field applied by the pair of electrodes. Ferroelectric device. 5. The ferroelectric device according to claim 4, wherein the ferroelectric thin film has a plurality of the crystals as crystal grains. 6. A ratio of the crystal grains in which the third crystal axis is oriented within ± 30 degrees in a plane including the electric field and the third crystal axis with respect to a direction perpendicular to the electric field. Is 60% or more, The ferroelectric device according to claim 5. 7. The ferroelectric device according to claim 5, wherein the crystal grains have a size of 10 nm to 300 nm. 8. A ferroelectric device having a ferroelectric thin film sandwiched by a pair of electrodes, wherein the ferroelectric thin film comprises two substantially equivalent first and second crystal axes and the first and second crystal axes. A crystal having a third crystal axis having a symmetry different from that of the first and second crystal axes in a direction different from the second crystal axis. , The third
Has a crystal axis oriented within ± 30 degrees in a plane including the electric field and the third crystal axis with respect to a direction perpendicular to the electric field applied by the pair of electrodes, and is perpendicular to the electric field. A ferroelectric device characterized in that a predetermined direction in a plane and a direction of projection of the third crystal axis onto a plane perpendicular to the electric field form an angle within plus or minus 30 degrees. 9. The ferroelectric device according to claim 8, wherein the ferroelectric thin film has a plurality of the crystals as crystal grains. 10. The third crystal axis is oriented within ± 30 degrees within a plane including the electric field and the third crystal axis with respect to a direction perpendicular to the electric field, and is perpendicular to the electric field. Plus / minus 3 between the predetermined direction in the plane and the direction of the projection of the third crystal axis on the plane perpendicular to the electric field.
The ferroelectric device according to claim 9, wherein a ratio of the crystal grains forming an angle of 0 degree or more is 60% or more. 11. The crystal grains are 10 nm to 300 nm.
The ferroelectric device according to claim 9, wherein the ferroelectric device has a size of. 12. The ferroelectric device according to claim 1, wherein a film thickness of the ferroelectric thin film is in a range of 30 nm to 1 μm. 13. The ferroelectric device according to claim 1, wherein the crystal has a structure in which a plurality of different layered structures are stacked. 14. The one of the plurality of different layered structures is a layer having bismuth.
3. The ferroelectric device according to item 3. 15. One of said plurality of different layered structures is bismuth (Bi), scandium (Sc), lanthanum (La), antimony (Sb), yttrium (Y),
Any one of chromium (Cr) and thorium (Th)
15. A layer comprising one element.
7. The ferroelectric device according to. 16. One of the plurality of different layered structures has a pseudo perovskite structure, and A site ions of the pseudo perovskite structure are bismuth (Bi) and lead (P).
b), strontium (Sr), barium (Ba), calcium (Ca), and cadmium (Cd), and the B site ion is titanium (T).
i), tantalum (Ta), niobium (Nb), hafnium (Hf), tungsten (W), zirconium (Zr)
16. The ferroelectric device according to claim 13, wherein the ferroelectric device is at least one of the elements. 17. The crystal has the composition formula SrBi ₂ Ta ₂ O ₉
The ferroelectric device according to any one of claims 1 to 16, wherein the ferroelectric device is a bismuth layer compound. 18. The ferroelectric device according to claim 1, wherein the crystal is SrBi ₄ Ti ₄ O ₁₅ or SrBi ₂ Nb ₂ O ₉ . 19. The crystal is SrBi ₂ Ta ₂ O ₉ and Sr.
The ferroelectric device according to any one of claims 1 to 16, which is a compound of at least two materials of Bi ₄ Ti ₄ O ₁₅ and SrBi ₂ Nb ₂ O _9. . 20. The ferroelectric device according to claim 1, wherein a Group IIIA element is doped as an impurity. 21. The ferroelectric device according to claim 20, wherein the group IIIA element is at least one of yttrium (Y) and lanthanum (La). 22. The ferroelectric device according to claim 1, wherein a Group IVA element is doped as an impurity. 23. The ferroelectric device according to claim 22, wherein the Group IVA element is at least one of titanium (Ti), zirconium (Zr), and hafnium (Hf). 24. The ferroelectric device according to claim 1, wherein a Group VA element is doped as an impurity. 25. The group VA element is vanadium (V),
The ferroelectric device according to claim 24, which is at least one of niobium (Nb) and tantalum (Ta). 26. The ferroelectric device according to claim 1, wherein a Group VIA element is doped as an impurity. 27. The group VIA element is chromium (Cr),
27. The ferroelectric device according to claim 26, which is at least one of molybdenum (Mo) and tungsten (W). 28. The ferroelectric device according to claim 1, wherein a Group VIIA element is doped as an impurity. 29. The Group VIIA element is manganese (M
29. The ferroelectric device according to claim 28, wherein the ferroelectric device is at least one of n), technetium (Tc), and rhenium (Re). 30. The ferroelectric device according to claim 1, wherein a Group VIII element is doped as an impurity. 31. The Group VIII element is iron (Fe), cobalt (Co), nickel (Ni), ruthenium (R).
31. The ferroelectric device according to claim 30, which is at least one of u), rhodium (Rh), palladium (Pd), and osmium (Os). 32. The ferroelectric device according to claim 1, wherein a Group IB element is doped as an impurity. 33. The group IB element is copper (Cu) or silver (A).
33. The ferroelectric device of claim 32, which is at least one of g). 34. The ferroelectric device according to claim 1, wherein a Group IIB element is doped as an impurity. 35. The ferroelectric device according to claim 34, wherein the Group IIB element is at least one of zinc (Zn) and cadmium (Cd). 36. The ferroelectric device according to claim 1, wherein a Group IIIB element is doped as an impurity. 37. The group IIIB element is gallium (G
37. The ferroelectric device according to claim 36, which is at least one of a) and indium (In). 38. The ferroelectric device according to claim 1, wherein a Group IVB element is doped as an impurity. 39. The Group IVB element is germanium (G
39. The ferroelectric device according to claim 38, wherein the ferroelectric device is at least one of e) and tin (Sn). 40. The ferroelectric device according to claim 1, wherein a Group VB element is doped as an impurity. 41. The ferroelectric device according to claim 40, wherein the VB group element is arsenic (As). 42. The ferroelectric device according to claim 1, wherein a VIB group element is doped as an impurity. 43. The ferroelectric device according to claim 42, wherein the VIB group element is selenium (Se). 44. A Group IA element and // as a charge compensation material
Or a Group IIA element is added.
The ferroelectric device according to any one of claims 0 to 43. 45. The Group IA element is lithium (Li),
Sodium (Na), potassium (K), rubidium (R
The ferroelectric device according to claim 44, wherein the ferroelectric device is at least one of b) and cesium (Cs). 46. The group IIA element is beryllium (B
The ferroelectric device according to claim 44, which is at least one of e), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).