JP3545850B2 - Ferroelectric thin film element - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a structure of a ferroelectric thin film element, and more particularly, to a ferroelectric thin film capacitor used for a ferroelectric nonvolatile memory element, a pyroelectric infrared sensor element, a piezoelectric element, and the like, and a method of manufacturing the same.
[0002]
[Prior art]
Ferroelectric crystals have functions such as spontaneous polarization, high dielectric constant, pyroelectric effect, piezoelectric effect, and electro-optic effect. Conventionally, capacitors, infrared sensors, ultrasonic oscillators, pressure sensors, frequency filters, optical switches, etc. Has been applied to the development of many devices.
[0003]
Recently, with the development of thin film technology for ferroelectric materials, high quality ferroelectric thin films can be formed on various substrates. By applying this ferroelectric thin film to a semiconductor device, it is possible to improve its performance and to develop a new device which has not existed before. For example, by applying a high dielectric constant material to a capacitor of a DRAM, planar type high integration can be realized, and the manufacturing process can be simplified and the cost can be reduced. Further, a non-volatile memory (FRAM) utilizing spontaneous polarization of the ferroelectric capacitor has been developed, and a new memory device having a non-volatile operation added to a DRAM has been realized. To develop such a device, the residual spontaneous polarization (Pr) is large, the coercive electric field (Ec) is small, the leakage current is low, and the film thickness is 200 nm or less for reducing the driving voltage and matching with the semiconductor process. Requires a high quality thin film. In addition, for the development of a device using spontaneous polarization, it is essential to develop a highly reliable material with less deterioration (fatigue) of ferroelectric characteristics due to repeated polarization inversion.
[0004]
At present, a series of Bi-based layered perovskite ferroelectrics has attracted attention as a ferroelectric material with little film property fatigue. These materials, chemical formula _{Bi 2 A m B m O 3m} + 3 ( however, A is ^{Na 1+, K 1+, Pb 2+} , Ca 2+, Sr 2+, any one selected from ^{Ba 2+} and ^{Bi 3+,} B is Any one selected from Fe ³⁺ , Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , W ⁶⁺ , and Mo ⁶⁺ , and m is a natural number of 1 or more).
[0005]
Among these Bi-based ferroelectric materials, Bi ₄ Ti ₃ O ₁₂ has a layered perovskite structure belonging to an orthorhombic system (lattice constant: a = 5.4100, b = 5.4489, c = 32.815 Å). The spontaneous polarization is such that Pr in the a-axis direction is about 50 μC / cm ² , coercive electric field Ec is about 50 kV / cm, Pr in the c-axis direction is about 4 μC / cm ² , and Ec is about 4 kV / cm ² . cm and excellent properties.
[0006]
In order to realize these excellent material characteristics with a thin film, it is important to consider an electrode material for forming a capacitor structure. In other words, when the work function difference between the ferroelectric and the electrode is large, carriers due to oxygen defects and space charges in the ferroelectric thin film are trapped at the electrode interface, and an anti-electric field is generated. Since the electric field intensity applied to the ferroelectric thin film decreases, repeated polarization reversal causes fatigue of film characteristics. Furthermore, a residual reaction occurs in the ferroelectric thin film due to a difference in expansion coefficient between the ferroelectric and the electrode, which causes film leakage due to leak current or insufficient adhesion.
[0007]
Conventionally, Pt having excellent heat resistance and oxidation resistance when forming a ferroelectric thin film has been used as an electrode for a ferroelectric capacitor used in a DRAM or FRAM. However, considering the difference between the work function and the expansion coefficient at the interface with the ferroelectric, the oxide electrode material similar to the ferroelectric material is desirable as the electrode material. In fact, in the case of ferroelectric PZT (Pb (Zr, Ti) O ₃ ), it has been reported that fatigue of film characteristics is reduced by using RuO ₂ or IrO ₂ instead of the Pt electrode. (C. Kwok et al., 4th International Symposium on Integrated Ferroelectrics, Proceedings (1992) 421).
[0008]
[Problems to be solved by the invention]
As described above, it is important to select an electrode material that matches each ferroelectric material in order to bring out sufficient ferroelectric capacitor characteristics and make it applicable to DRAMs and FRAMs. That is, it is necessary to develop an electrode material having the smallest possible difference between the work function and the expansion coefficient at the interface with the ferroelectric.
[0009]
The present invention provides an electrode material which can be applied to a ferroelectric capacitor using a ferroelectric material having a Bi-based layered perovskite structure, has less fatigue in film characteristics and less leakage current, and has good adhesion to the ferroelectric material. The purpose is to do.
[0010]
[Means for Solving the Problems]
The above-mentioned object is to provide a ferroelectric thin film element having a lower electrode layer, a ferroelectric thin film, and an upper electrode layer in this order on a substrate, wherein the ferroelectric thin film ^converts A into Na ¹⁺ , K ¹⁺ , Pb ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ and Bi ³⁺ , B is Fe ³⁺ , Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , W ⁶⁺ and Mo ⁶⁺ , m is a natural number of 1 or more upon a has a Bi-based layered perovskite crystal structure expressed by a chemical formula _{Bi 2 a m-1 B m} O 3m + 3, at least one of the lower electrode layer and the upper electrode layer, Ru, and Rh and Ir The ferroelectric thin film element according to claim 1, wherein the ferroelectric thin film element is an oxide conductor containing any one of them and Bi .
[0013]
The object described above is a method of manufacturing a ferroelectric thin film element in which a lower electrode layer, a ferroelectric thin film, and an upper electrode layer are sequentially formed on a substrate, wherein the ferroelectric thin film is formed by converting A into Na ^{1 +} , K ^{1 +} , Pb ^{2 +} , Ca ^{2 +} , Sr ^{2 +} , Ba ^{2 +,} or Bi ^{3 +} , B is Fe ^{3 +} , Ti ^{4 +} , Nb ^{5 +} , Ta ^{5 +} , W ^{6 +} and one of ^{Mo 6 +,} upon one or more natural number m, have a Bi-based layered perovskite crystal structure expressed by a chemical formula _{Bi 2 a m-1 B m} O 3m + 3, the 3. The ferroelectric thin film according to claim 2 , wherein a metal oxide film containing any one of Ru, Rh and Ir is formed on the ferroelectric thin film, and the surface of the ferroelectric film is made conductive by a solid-phase reaction. This is achieved by a method of manufacturing a dielectric thin film element.
[0014]
[Action]
In the present invention, attention has been paid to the fact that all ferroelectrics having a Bi-based layered perovskite structure contain Bi as a constituent element thereof. That is, by using the oxide electrode containing Bi, the difference between the work function and the expansion coefficient at the interface between the ferroelectric and the electrode can be reduced. In the present invention, Bi ₂ Ru ₂ O _{7 -X} , Bi ₂ Rh ₂ O _{7 -X} , and Bi ₂ Ir ₂ O _{7 -X} were used as the oxide conductor containing Bi. That is, by using an oxide electrode containing a constituent element of the ferroelectric material, the introduction of impurities into the ferroelectric material is suppressed as much as possible, and by lattice matching with the ferroelectric material, The residual stress can be reduced. The crystal structure of these conductive ceramics is a pyrochlore type (cubic crystal system), and the resistivity is 7 × 10 ⁻⁴ Ωcm for Bi ₂ Ru ₂ O _{7-X and} 3 × for Bi ₂ Rh ₂ O _7-X. 10 ⁻³ Ωcm and Bi ₂ Ir ₂ O _7-X are 2 × 10 ⁻³ Ωcm.
[0015]
Since the temperature characteristics of these oxide conductor materials behave metallically, they can be sufficiently used as electrodes. Furthermore, when used as a lower electrode, the nucleation density of an oxide ferroelectric is higher on an oxide thin film of the same type than on a general metal thin film electrode, and the ferroelectric thin film itself can be densified. .
[0016]
The substrate used in the present invention is a single-crystal silicon substrate whose surface is covered with a SiO ₂ insulating film. Of course, as a silicon single crystal substrate, an element such as a transistor may be formed on the surface thereof. The above oxide conductor film is formed as a lower electrode on this substrate. Various methods such as a sputtering method, a CVD method, a laser ablation method, and a reactive vapor deposition method are possible as the forming method. When used as an electrode, the thickness is preferably 100 to 500 nm.
[0017]
On the other hand, when used as the upper electrode, RuO ₂ , RhO ₂ , and IrO ₂ were formed on the ferroelectric in addition to directly forming the above oxide conductor film on the Bi-based ferroelectric thin film. After that, there is a method of imparting conductivity to the ferroelectric surface by performing a heat treatment.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of a ferroelectric thin film capacitor and a method of manufacturing the same according to the present invention will be described with reference to the drawings.
[0019]
First, a first embodiment of the ferroelectric capacitor of the present invention will be described.
[0020]
FIG. 1 is a diagram showing a ferroelectric capacitor according to a first embodiment of the present invention. The ferroelectric capacitor shown in FIG. 1 has a 200 nm thick SiO ₂ layer 2 formed on the surface of a silicon wafer 1 by thermal oxidation, and a Pt electrode layer 3 formed thereon with a 100 nm thickness as a substrate. Used.
[0021]
On this substrate, a Bi ₂ Ru ₂ O _{7 -X} thin film is formed as the lower electrode 4, and a Bi ₄ Ti ₃ O ₁₂ film 5 is formed as a ferroelectric substance by MOCVD. Ru (C ₁₁ H ₁₉ O ₂ ) _{3 is used} as a ruthenium raw material, Bi (o-C ₇ H ₇ ) ₃ is used as a bismuth raw material, and Ti (i-OC ₃ H ₇ ) ₄ is used as a titanium raw material.
[0022]
The lower electrode 4, the ruthenium source to 140 ° C., also bismuth raw material respectively heated and vaporized in 160 ℃, Bi ₂ Ru ₂ O by being supplied onto the substrate maintained at 500 ° C. with the reaction gas oxygen and argon gas as a carrier Form a _7-X thin film. Here, the flow rate of the ruthenium raw material carrier gas is 150 sccm, the flow rate of the bismuth raw material carrier gas is 100 sccm, the flow rate of the oxygen gas is 500 sccm, and the reaction pressure is 5 Torr. 1 thickness 100nm in film formation hours _Bi 2 _Ru 2 _{O 7-X} thin film 4 is obtained.
[0023]
The film 5 of the ferroelectric Bi ₄ Ti ₃ O ₁₂ stops supplying the ruthenium raw material, heats and vaporizes the titanium raw material to 50 ° C., bubbling with an argon carrier gas at a gas flow rate of 50 sccm, and Bi ₂ Ru with the bismuth raw material and oxygen gas. _It is supplied on a ₂ O _7-X thin film electrode. The film formation temperature at this time is 600 ° C. Thickness 200nm _Bi 4 _Ti 3 _{O 12} film 5 can be obtained by deposition of 1 hour. A 100 nm-thick Pt film is deposited as the upper electrode 6 on the ferroelectric thin film thus manufactured to form a capacitor structure.
[0024]
The leakage current of the formed capacitor was 3 × 10 ⁻⁷ A / cm ² when 3 V was applied. FIG. 2 is a graph showing the result of the hysteresis measurement of this capacitor. Regarding remanent polarization, Pr = 4.3 μC / cm ² was obtained. FIG. 3 is a graph showing the fatigue characteristics of the remanent polarization Pr. After 10 ¹⁰ polarization inversions, Pr was 4.1 μC / cm ² , which indicates that it was slightly reduced. As a comparative example, when only Pt was used as the lower electrode, the leak current density was 7 × 10 ⁻⁷ A / cm ² , Pr was 5.3 μC / cm ² , and 2 after 10 ¹⁰ polarization inversions. 0.6 μC / cm ² .
[0025]
_Thus, by using a Bi ₂ Ru _{2 O 7-X} electrode, the initial value of Pr decreases slightly, but the film fatigue resistance it can be seen that greatly improved. When only Pt was used as the lower electrode, film peeling was observed on a part of the Bi ₄ Ti ₃ O ₁₂ film-formed wafer, but in the case of the Bi ₂ Ru ₂ O _7-X electrode, the film was peeled off. A uniform ferroelectric thin film was obtained on the entire surface.
[0026]
Next, a second embodiment of the ferroelectric capacitor of the present invention will be described.
[0027]
Like the first embodiment described above, after forming the lower electrode _Bi 2 _Ru 2 _{O 7-X} and the ferroelectric _Bi 4 _Ti 3 _{O 12} by MOCVD, the upper electrode in a subsequently the same conditions as the lower electrode the Bi ₂ Ru ₂ O _7-X is formed to a thickness of 50nm. Further, a Pt electrode is deposited on the upper electrode in the same manner as in the first embodiment, to produce a ferroelectric capacitor structure.
[0028]
The leakage current of the formed capacitor was as low as 4 × 10 ⁻⁸ A / cm ² . This is because, in the first embodiment, since the ferroelectric and the Pt electrode were in direct contact with each other, residual stress was generated in the ferroelectric thin film due to the difference in expansion coefficient between Bi ₄ Ti ₃ O ₁₂ and Pt. In the second embodiment, an oxide electrode effective for stress relaxation is interposed between the ferroelectric and the Pt electrode, while the leak current is increased. Therefore, the leak current can be reduced. It is considered that it became. FIG. 4 is a graph showing a result of the hysteresis measurement of the capacitor, and FIG. 5 is a graph showing a fatigue characteristic of the residual polarization Pr. Pr was 5.3 μC / cm ² when 3 V was applied, and was 5.3 μC / cm ² even after 10 ¹⁰ polarization inversions, indicating no fatigue. From now on, by sandwiching the upper and lower portions of the ferroelectric thin film between the oxide electrodes, the problems of the conventional metal electrode such as Pt (film fatigue due to a difference in work function, increase in leakage current due to residual stress in the film) ) Can be solved.
[0029]
Next, a third embodiment of the ferroelectric capacitor of the present invention will be described.
[0030]
This is an attempt to make the surface of the ferroelectric Bi ₄ Ti ₃ O ₁₂ conductive as a method of forming the upper Bi ₂ Ru ₂ O _{7 -X} electrode of the _second embodiment. That is, after forming the Bi ₄ Ti ₃ O ₁₂ thin film of the first embodiment, a ruthenium raw material and oxygen gas are supplied for a short time (5 minutes).
[0031]
A Pt electrode was formed on the thin film thus obtained, and the electrical characteristics were measured. As a result, the leak current was 7 × 10 ⁻⁸ A / cm ² . As a result of the hysteresis measurement, Pr = 5.0 μC / cm ² , which was slightly inferior to the second embodiment, but improved compared to the first embodiment. This is because, in the third embodiment, Bi in the film reacts with Ru on the surface of Bi ₄ Ti ₃ O _{12 to make} the surface of the ferroelectric thin film conductive, and as a result, similar to the second embodiment. It is considered that a significant effect was obtained.
[0032]
In the above embodiment, Bi ₄ Ti ₃ O ₁₂ was used as the ferroelectric and Bi ₂ Ru ₂ O _7-X was used as the oxide electrode. However, other Bi-based layers such as SrBi ₂ Ta ₂ O ₉ were used. The same effect can be obtained by using a perovskite ferroelectric and Bi ₂ Rh ₂ O _{7 -X} or Bi ₂ Ir ₂ O _{7 -X} electrodes. Further, it goes without saying that a method other than the MOCVD method, that is, other film forming methods such as a sputtering method, a laser ablation method, and a sol-gel method can be used as the film forming method.
[0033]
【The invention's effect】
According to the present invention, in a capacitor using a Bi-based layered perovskite ferroelectric, an oxide conductor Bi ₂ Ru ₂ O _{7 -X} , Bi ₂ Rh ₂ O _{7 -X} , Bi ₂ Ir ₂ containing Bi as an electrode is used. By using O _{7 -X} , problems such as film fatigue and leak current when a conventional Pt electrode is used can be greatly improved. Further, even in the fine processing required for manufacturing a device, the oxide electrode can be easily etched using O ₂ gas as compared with Pt which is difficult to dry-etch, and there is no film peeling during the process. Therefore, by using the present invention, the excellent characteristics of the Bi-based ferroelectric can be effectively brought out, and the process of manufacturing a device such as an FRAM can be simplified.
[Brief description of the drawings]
FIG. 1 is a diagram showing a sectional structure of a ferroelectric capacitor of the present invention.
FIG. 2 is a graph showing hysteresis characteristics of the capacitor according to the first embodiment of the present invention.
FIG. 3 is a graph showing fatigue characteristics of the capacitor according to the first embodiment of the present invention.
FIG. 4 is a graph showing hysteresis characteristics of the capacitor according to the second embodiment of the present invention.
FIG. 5 is a graph showing fatigue characteristics of the capacitor according to the second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Silicon wafer 2 Insulating film 3 Electrode layer 4 Lower electrode 5 Ferroelectric thin film 6 Upper electrode

Claims (2)

What is claimed is: 1. A ferroelectric thin-film device comprising a substrate in which a lower electrode layer, a ferroelectric thin film, and an upper electrode layer are provided in this order, wherein the ferroelectric thin film is composed of Na ¹⁺ , K ¹⁺ , Pb ²⁺ , Ca ²⁺ , When any one of Sr ²⁺ , Ba ²⁺ and Bi ³⁺ , B is any one of Fe ³⁺ , Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , W ⁶⁺ and Mo ⁶⁺ and m is a natural number of 1 or more, has a Bi-based layered perovskite crystal structure expressed by a chemical formula _{Bi 2 a m-1 B m} O 3m + 3, at least one of the lower electrode layer and the upper electrode layer, Ru, or one of Bi Rh and Ir A ferroelectric thin film element characterized by being an oxide conductor containing: A method of manufacturing a ferroelectric thin film element in which a lower electrode layer, a ferroelectric thin film, and an upper electrode layer are sequentially formed on a substrate, wherein the ferroelectric thin film ^converts A into Na ¹⁺ , K ¹⁺ , Pb ^2+. , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ and Bi ³⁺ , B is Fe ³⁺ , Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , W ⁶⁺ and Mo ⁶⁺ , m is a natural number of 1 or more. metal when the have a Bi-based layered perovskite crystal structure expressed by a chemical formula _{Bi 2 a m-1 B m} O 3m + 3, comprising the ferroelectric thin film, Ru, one of Rh and Ir A method for manufacturing a ferroelectric thin film element, comprising forming an oxide film and making the surface of the ferroelectric film conductive by a solid phase reaction.

Images