JP2004356212A - Ferroelectric thin film device, its manufacturing method, and ferroelectric memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferroelectric thin film device which is capable of improving the degree of crystal orientation of an unpolarized ferroelectric thin film, and to provide its manufacturing method and a ferroelectric memory. <P>SOLUTION: A lower electrode layer 2, an oxide crystalline nucleus layer 3, a ferroelectric thin film layer 4, and an upper electrode layer 5, are successively laminated on a base substrate 1 for the formation of the ferroelectric thin film device. The ferroelectric thin film layer 4 has a crystal orientation rate of 60% or above when it is unpolarized. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ferroelectric thin film element, a method for producing the same, and a ferroelectric memory, and in particular, a lower electrode layer, a crystal nucleus layer made of an oxide, a ferroelectric thin film layer, and an upper electrode layer sequentially laminated on a base substrate. The present invention relates to a ferroelectric thin film element suitably used for a memory or the like, a method of manufacturing the same, and a ferroelectric memory.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, many semiconductor memories such as DRAM, SRAM, flash memory, and EEROM have been used as memory components.
[0003]
However, in the DRAM, the lowering of the voltage is limited from the lower limit of the critical charge of the memory cell, and in the flash memory, a high voltage generating circuit is required, and it is essentially difficult to lower the voltage in order to prevent disturbance.
[0004]
On the other hand, a ferroelectric memory (FeRAM) is a nonvolatile memory using spontaneous polarization peculiar to a ferroelectric, and operates at a low voltage in response to high-speed writing / reading and thinning of a ferroelectric thin film. It is possible to replace not only the conventional nonvolatile memory but also almost all memories such as SRAM and DRAM because of the feature that it can greatly contribute to lower power consumption.
[0005]
In a ferroelectric memory (FeRAM), a voltage is selectively applied by a switching transistor to a ferroelectric thin film element on which individual electrode cells are formed, and the direction of spontaneous polarization of the ferroelectric is changed according to the direction of an electric field. . This corresponds to writing "1" or "0" of the stored information. Even when the electric field is removed, the spontaneous polarization itself does not disappear, so that a charge corresponding to the polarization remains on the electrode.
[0006]
When reading out the stored information, if a voltage is applied again to the electrode formed on the ferroelectric thin film, the amount of charge flowing out to the wiring differs depending on the amount of charge on the electrode. The content of the stored information, that is, “1” or “0” is determined based on the resulting potential difference of the wiring. In this way, a function as a memory is given to the ferroelectric thin film element.
[0007]
Conventionally, a ferroelectric thin film element having such characteristics is formed by, for example, forming a metal film of a lower electrode on a silicon substrate, and sequentially stacking a ferroelectric thin film layer and a metal film of an upper electrode thereon. Is done. The method of forming the ferroelectric thin film layer includes a sol-gel method, a MOD method, a MOCVD method, a laser ablation method, a CVD method, a sputtering method, and the like. Used.
[0008]
For example, in the case of the sol-gel method, a sol liquid in which the metal element ratio is adjusted so as to correspond to a ferroelectric crystal is prepared, and this solution is applied onto a lower electrode provided on a base substrate by a method such as spin coating. Then, the substrate is heated to produce a gelled film. Thereafter, annealing is performed in an oxygen-containing atmosphere to obtain a crystalline ferroelectric thin film. Thereafter, a ferroelectric thin film element can be obtained by forming an upper electrode on the ferroelectric thin film (for example, see Patent Document 1).
[0009]
[Patent Document 1]
JP-A-11-318514
[Problems to be solved by the invention]
However, in the conventional ferroelectric thin film layer manufactured by the sol-gel method as described above, the polarization direction is usually random immediately after the formation of the ferroelectric thin film layer, and the polarization direction is aligned by a polarization process of applying a DC voltage. Due to the orientation, a residual stress is generated inside the ferroelectric thin film layer, and there is a problem that polarization is deteriorated due to repeated fatigue when a voltage is repeatedly applied for reading or writing.
[0011]
In addition, since the crystal grains are coarse in the ferroelectric thin film layer manufactured by the sol-gel method, the orientation of each crystal grain of the ferroelectric thin film layer varies, and the ferroelectric thin film layer is formed on the ferroelectric thin film layer. A large variation occurs in the electrical characteristics of each electrode cell, causing a malfunction.
[0012]
Furthermore, since the ferroelectric thin film layer produced by the sol-gel method has coarse crystal grains, the surface of the ferroelectric thin film has large irregularities, making it impossible to miniaturize the wiring of electrodes formed on the ferroelectric thin film layer. When used as a memory (FeRAM), it has been difficult to achieve high integration of the FeRAM.
[0013]
Further, the ferroelectric thin film layer manufactured by the sol-gel method has a problem that voids exist due to coarse crystal grains, which causes insulation deterioration and lowers reliability. Further, there is a problem that reproducibility at the time of manufacturing the ferroelectric thin film element is low.
[0014]
The present invention provides a ferroelectric thin film element capable of improving the crystal orientation of a ferroelectric thin film immediately after an annealing treatment for crystallizing a ferroelectric thin film, that is, in an unpolarized state, a method for producing the same, and a ferroelectric memory. The purpose is to do.
[0015]
Furthermore, by increasing the uniformity of the orientation of each crystal grain and forming a ferroelectric thin film composed of microcrystals, high integration and high reliability of the memory function can be achieved. It is another object of the present invention to provide a ferroelectric thin film element capable of improving reproducibility at the time, a method of manufacturing the same, and a ferroelectric memory.
[0016]
[Means for Solving the Problems]
The ferroelectric thin film element of the present invention is a ferroelectric thin film element in which a lower electrode layer, a crystal nucleus layer made of an oxide, a ferroelectric thin film layer, and an upper electrode layer are sequentially stacked on a base substrate, The ferroelectric thin film layer has a crystal orientation ratio of 60% or more when not polarized.
[0017]
In the ferroelectric thin film element of the present invention, since the crystal orientation ratio when the ferroelectric thin film layer is not polarized is 60% or more, the residual generated inside the ferroelectric thin film layer by the polarization process of applying a DC voltage. Stress can be reduced, and polarization degradation due to repeated fatigue when a continuous voltage is applied can be significantly reduced.
[0018]
The ferroelectric thin film element of the present invention is characterized in that an insulating film made of SiO ₂ is formed between the base substrate and the lower electrode layer. In such a ferroelectric thin film element, the insulating film made of SiO ₂ can be used as a protective film for an integrated circuit such as a switching transistor formed on a Si single crystal substrate.
[0019]
Furthermore, the ferroelectric thin film element of the present invention is characterized in that the base substrate is any one of Si single crystal, sapphire single crystal, MgO, and SrTiO ₃ . Such a ferroelectric thin film element can function as a ferroelectric memory by applying a single crystal of Si or a single crystal of sapphire, MgO, or SrTiO _3, and can be used as a BAW filter using a ferroelectric thin film. A SAW resonator or a SAW filter can be manufactured.
[0020]
The method for producing a ferroelectric thin-film element of the present invention is a method for producing a ferroelectric thin-film element in which a lower electrode layer, a crystal nucleus layer composed of an oxide, a ferroelectric thin film layer, and an upper electrode layer are sequentially laminated on a base substrate. A lower electrode layer and a crystal nucleus layer are sequentially formed on a base substrate, and then an amorphous ferroelectric thin film layer is formed on the crystal nucleus layer, containing oxygen at a pressure of 1.4 to 180 MPa. After performing isotropic pressure annealing (HIP) treatment in a temperature range of 400 to 600 ° C. in an inert gas to crystallize the ferroelectric thin film layer, an upper electrode layer is formed on the ferroelectric thin film layer. It is characterized by forming.
[0021]
In the method of manufacturing a ferroelectric thin film element of the present invention, when the amorphous ferroelectric thin film layer is crystallized, it grows along the crystal lattice of the crystal nucleus layer made of oxide, so that the orientation becomes high. Furthermore, since the sintering is performed at a low temperature by the HIP process, fine and highly uniform particles can be obtained, the unevenness is reduced, the surface flatness is increased, and the ferroelectric thin film element having high reproducibility during manufacturing is manufactured. Obtainable.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic cross-sectional view of a ferroelectric thin film element of the present invention. Reference numeral 1 denotes a base substrate, 2 denotes a lower electrode layer, 3 denotes a crystal nucleus layer made of oxide, 4 denotes a ferroelectric thin film layer, 4a denotes crystal grains constituting the ferroelectric thin film layer 4, 5 denotes an upper electrode layer, Reference numeral 6 denotes an insulating film formed between the base substrate 1 and the lower electrode 2.
[0023]
The base substrate 1 is made of a single crystal of Si, a single crystal of sapphire, MgO, or SrTiO ₃ , and the lower electrode 2 is made of a single crystal of Si because it is difficult for lattice distortion to remain in the interface layer and reliability in continuous operation is improved. Is desirably formed on the (110) plane.
[0024]
The lower electrode layer 2 is formed by depositing a noble metal such as Pt, Ir, and Pd by a sputtering method or an evaporation method. Further, on the upper surface of the lower electrode layer 2, a crystal nucleation layer 3 of an oxide having a perovskite structure such as PbTiO ₃ , LiTaO ₃ , LiNbO ₃ or a bismuth layer structure such as SrBi ₂ Ta ₂ O ₉ is formed by a high-temperature sputtering method or the like. Is formed. A ferroelectric thin film layer 4 is formed on the upper surface of the crystal nucleus layer 3 made of an oxide by a sputtering method, a sol-gel method, or the like. The ferroelectric thin film layer 4 has a crystal orientation ratio of 60% or more when not polarized. Have been. It is particularly desirable that the orientation ratio is 70% or more.
[0025]
The ferroelectric thin film layer 4 is composed of fine crystal grains 4a of 0.01 to 0.1 μm. The electrode layers 2 and 5 have a thickness of 100 to 300 nm, the ferroelectric thin film layer 4 has a thickness of 50 to 400 nm, and the insulating film 6 has a thickness of 50 to 500 nm.
[0026]
A method for producing the ferroelectric thin film element of the present invention will be described. First, an insulating film 6 is formed on a base substrate 1 of Si single crystal or the like by a sputtering method or an evaporation method, and a lower electrode layer 2 is formed on the insulating film 6 by a sputtering method or an evaporation method. A crystal nucleus layer 3 is formed on the crystal nucleus 2 by a high temperature sputtering method or the like, and a ferroelectric thin film layer 4 is formed on the crystal nucleus layer 3 in an amorphous state by a sputtering method or the like so as to have a desired composition. .
[0027]
Thereafter, the ferroelectric thin film layer 4 is formed in an atmosphere of, for example, an inert gas containing about 1 to 20% oxygen, such as Ar gas or N ₂ gas, at a pressure of 1.4 to 180 MPa. The isotropic pressure annealing (HIP) treatment is performed in the temperature range of 400 to 600 ° C. while the isotropic pressure is applied.
[0028]
As a result, the amorphous ferroelectric thin film layer is crystallized, but grows along the crystal lattice of the crystal nucleus layer 3, so that an oriented film is formed. Here, since the ferroelectric thin film layer 4 has a large polarization moment Pr and needs to invert the polarization at a low voltage, a material having a low coercive electric field Ec, for example, Pb (Zr, Ti) O ₃ is used as a main component. And a bismuth layered structure containing SrBi ₂ Ta ₂ O ₉ or the like as a main component.
[0029]
In addition, the reason why the pressing force was set in the range of 1.4 to 180 MPa under the condition of isotropic pressure annealing (HIP) is that when the pressure is smaller than 1.4 MPa, the crystal orientation does not progress and the orientation ratio α is increased. This is because it does not exceed 60%. When a pressure higher than 180 MPa is further applied, an amorphous layer is again formed at the interface between the ferroelectric thin film once crystallized and the crystal nucleus layer formed of oxide as a crystal nucleus, and the crystal orientation This is because, as well as the area of loss of function as a ferroelectric substance becomes large, the application to a memory or the like becomes impossible. It is particularly desirable that the applied pressure is 1.7 to 20 MPa.
[0030]
The reason why the annealing temperature is set to 400 to 600 ° C. is that when the temperature is lower than 400 ° C., crystallization can be achieved, but a dense thin film cannot be obtained, and when the temperature is higher than 600 ° C., crystal grains become coarse. It is. In particular, the temperature is desirably 450 to 520 ° C.
[0031]
Thereafter, an upper electrode layer 5 is formed on the upper surface of the ferroelectric thin film layer 4 by sputtering or vapor deposition, and a part of the electrode is removed by etching or the like to form a wiring pattern, thereby giving a function as a memory. It is.
[0032]
According to such a method of manufacturing a ferroelectric thin film element, crystallization can be performed at a low temperature due to the HIP treatment, whereby fine particles and uniform and dense crystals can be obtained, and the surface roughness Rms of the ferroelectric thin film 4 can be reduced to about 10%. The surface irregularities can be extremely reduced to less than nm, and a ferroelectric thin film having high surface flatness can be obtained. For this reason, it is possible to miniaturize the wiring pattern formed on the ferroelectric thin film 4 and to achieve high integration.
[0033]
In the ferroelectric thin film element configured as described above, the crystal is oriented with a high degree of orientation in the C-axis direction of the ferroelectric thin film in a non-polarized state. Polarization degradation can be significantly reduced. Furthermore, since the ferroelectric thin film layer is formed densely, voids are not continuously present, so that a ferroelectric thin film element having improved insulation and improved reliability can be obtained. Furthermore, since the low-temperature and isotropic pressure annealing treatment enables high crystallinity to be obtained in the form of fine particles, a stable ferroelectric thin-film element having high repetition reproducibility at the time of production and suppressing variations among substrates is obtained. be able to.
[0034]
In a ferroelectric memory using a ferroelectric thin film element, a circuit such as a transistor is integrated on a base substrate made of Si single crystal, an insulating film made of SiO ₂ is formed thereon, and then a lower electrode layer, It is manufactured by sequentially laminating a crystal nucleus layer made of an oxide, a ferroelectric thin film layer, and an upper electrode layer.
[0035]
【Example】
On a Si (100) surface of a base substrate made of Si single crystal, an SiO ₂ film is formed to a thickness of 400 nm by a sputtering method, Pt is formed to a thickness of 200 nm by a sputtering method, and PbTiO serving as a crystal nucleus is formed on the upper surface. Layer _{3 was formed to} a thickness of 30 nm by a high-temperature sputtering method at 500 ° C.
[0036]
After that, an amorphous thickness 270 nm of a composition of 0.384 PbZrO ₃ -0.376 PbTiO ₃ -0.24 Pb (Zn _1/3 Nb _2/3 ) O ₃ is formed by rf magnetron sputtering using a ceramic target. A ferroelectric thin film layer made of a tetragonal crystal was formed.
[0037]
Thereafter, isotropic pressure annealing (HIP) is performed at a pressure of 1 to 180 MPa and a temperature range of 500 ° C. using an Ar-mixed inert gas containing 20% oxygen, thereby performing crystallization and porcelain. Was performed. Here, since the ferroelectric thin film layer is tetragonal, the orientation α of the (001) plane was determined.
[0038]
FIG. 2 shows the crystallinity and orientation when the treatment temperature was fixed at 500 ° C. and the pressurization condition with an Ar mixed inert gas containing 20% oxygen was changed in isotropic pressure annealing (HIP). The X-ray diffraction diagram showing the properties is described.
[0039]
From FIG. 2, (a) is an X-ray diffraction diagram when the pressing condition is 1 MPa, the orientation α is 2% and non-oriented, and (b) is the pressing condition is 1.3 MPa. FIG. 6 is an X-ray diffraction diagram at the time, where the orientation α is 51%, which is still insufficient.
[0040]
On the other hand, FIG. 2 (c) is an X-ray diffraction diagram when the pressure condition is 1.8 MPa, and the orientation α was 95%, showing a remarkably high orientation. FIG. 2D is an X-ray diffraction diagram when the pressure condition is 8.8 MPa, and the orientation α was as high as 78%. FIG. 2E is an X-ray diffraction diagram when the pressurizing condition is 17.7 MPa, and the orientation α was as high as 77%. FIG. 2 (f) is an X-ray diffraction diagram when the pressing condition is 176.5 MPa. The orientation α is 60%, and although the orientation gradually decreases as the pressure is increased, the orientation α is relatively high. Showed sex.
[0041]
The orientation α was 85% when pressed at a pressure of 1.4 MPa other than that shown in FIG. 2, and the orientation α was 60% when pressed at a pressure of 180 MPa.
[0042]
FIG. 2G shows an X-ray diffraction diagram before isotropic pressure annealing (HIP). From FIG. 2 (g), it can be seen that the crystallinity is not confirmed before the isotropic pressure annealing (HIP).
[0043]
When annealing was performed at a pressure of 1.8 MPa and a temperature of 400 ° C., the orientation α was 93%, and when annealing was performed at 600 ° C., the orientation α was 98%.
[0044]
Further, when the pressure condition of the isotropic pressure annealing (HIP) is increased to be higher than 17.7 MPa, the orientation of the crystal layer containing Pb (Zr, Ti) O ₃ as a main component is maintained in a high state. However, the X-ray intensity of the perovskite crystal phase decreases. This is considered to be due to the fact that the amorphous layer was formed again at the interface between the PbTiO _{3 formed} as a crystal nucleus and the ferroelectric thin film layer by cross-sectional observation with a transmission electron microscope and electron beam diffraction, and residual stress. Here, the thickness generated as amorphous was about 10 nm at 176.5 MPa when observed by a transmission electron microscope.
[0045]
In FIG. 2, Pero indicates the peak of perovskite, Pt indicates the peak of the platinum electrode, and the numbers in parentheses added to Pero and Pt indicate the plane index of the crystal.
[0046]
FIG. 3 (a) shows the polarization when the treatment temperature was fixed at 500 ° C. and the pressurizing condition of the Ar mixed inert gas containing 20% oxygen was changed in the isotropic press annealing (HIP). The relationship between the moment P and the electric field E is shown. FIG. 3A shows a PE hysteresis loop when the pressure is changed from 1.8 MPa to 176.5 MPa under a pressurized condition, and a ferroelectric loop is obtained under any of the conditions.
[0047]
On the other hand, FIG. 3 (b) shown as a comparative example is a conventional PE hysteresis loop when crystallized by annealing at 700 ° C. for 2 hours in an oxidizing atmosphere. Usually, the polarization moment Pr (the value of the polarization P at an electric field of 0) is required to be about 10 μC / cm ² or more. However, when the isotropic pressure annealing (HIP) treatment is performed under the conditions shown in FIG. While the desired characteristics of １４14 μC / cm ² were obtained, the atmospheric pressure firing shown in FIG. 3B shown in the comparative example was as low as about 4 μC / cm ^2, which is not suitable for memory applications.
[0048]
Further, in order to perform the polarization inversion at a low voltage, it is better that the value of the coercive electric field Ec (the value of the electric field E at a polarization moment of 0) is small. However, in the embodiment shown in FIG. Despite the high value of Pr, the coercive electric field Ec is 75 to 90 kV / cm, which is sufficiently smaller than 120 kV / cm that can operate at a low voltage, and has characteristics suitable for memory applications.
[0049]
Furthermore, the conditions of isotropic pressure annealing (HIP) were set to 1.8 MPa and 500 ° C., and the treatment was performed for 1 hour in an Ar mixed inert gas containing 20% oxygen. It is a fine crystal with a thickness of 80 nm. The surface roughness Rms of the ferroelectric thin film layer is 3 nm, and it has excellent surface flatness. The fineness and adhesion of the electrode wiring formed on the upper surface of the ferroelectric thin film are greatly improved. It can be seen that it improves.
[0050]
Further, in the insulation between the upper and lower electrodes of the ferroelectric thin film element shown in the embodiment of the present invention, the leakage current at 100 kV / cm is 1 × 10 ⁻⁷ A / cm or less, and sufficient insulation is maintained. I understood that. Further, when a repeated fatigue test in which a voltage was continuously applied was performed, no deterioration of polarization was observed even after 1 × 10 ¹⁰ times.
[0051]
Furthermore, as a result of changing the isotropic pressure annealing (HIP) processing time from 1 minute to 3 hours, a desired ferroelectric thin film element could be obtained in any case.
[0052]
On the other hand, in the conventional case where the ferroelectric thin film was crystallized by annealing at 700 ° C. for 2 hours in an oxidizing atmosphere, the average crystal grain was 3 μm, the surface roughness Rms of the ferroelectric thin film layer was about 200 nm, and the ferroelectric thin film element was The leakage current at 100 kV / cm between the upper and lower electrodes was 3 × 10 ⁻⁵ A / cm. When a repeated fatigue test in which a voltage was continuously applied was performed, polarization was deteriorated after 1 × 10 ¹⁰ times.
[0053]
Similar effects could be obtained even when the base substrate was a sapphire single crystal. As described above, according to the present invention, it is possible to obtain a ferroelectric thin film element particularly suitable for a memory application and a method for manufacturing the same. Further, a BAW filter, a SAW resonator, or a SAW filter using a ferroelectric thin film can be obtained.
[0054]
【The invention's effect】
According to the ferroelectric thin-film element of the present invention, when the amorphous ferroelectric thin-film layer is crystallized, the isotropic pressure annealing (HIP) treatment is carried out, so that the crystal of the ferroelectric thin film can be unpolarized. Since the orientation ratio is increased, polarization deterioration due to repeated fatigue during continuous voltage application can be significantly reduced. Further, since the dense ferroelectric thin film layer is formed, voids are not continuously present, and a ferroelectric thin film element having improved insulation and improved reliability can be obtained. Further, it is possible to obtain a stable ferroelectric thin-film element having high repetition reproducibility at the time of manufacturing and suppressing variation between substrates.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view of a ferroelectric thin film element of the present invention.
FIG. 2 is an X-ray diffraction diagram in isotropic pressure annealing.
FIG. 3 (a) shows a hysteresis loop of the polarization P and electric field E of the ferroelectric thin film of the present invention, and FIG. 3 (b) shows the polarization P and electric field E of a conventional ferroelectric thin film crystallized in an oxidizing atmosphere. FIG. 4 is a diagram showing a hysteresis loop of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Base substrate 2 ... Lower electrode layer 3 ... Crystal nucleus layer 4 made of an oxide 4 ... Ferroelectric thin film layer 5 ... Upper electrode layer 6 ... Base substrate and lower electrode Insulation film formed between

Claims (5)

A ferroelectric thin-film device comprising a base electrode, a lower electrode layer, a crystal nucleus layer made of an oxide, a ferroelectric thin film layer, and an upper electrode layer, which are sequentially laminated, wherein the ferroelectric thin film layer is not polarized. 3. The ferroelectric thin film device according to claim 1, wherein the crystal orientation ratio is 60% or more. _{2. The} ferroelectric thin-film element according to claim 1, wherein an insulating film made of SiO ₂ is formed between the base substrate and the lower electrode layer. _3. The ferroelectric thin film element according to claim 1, wherein the base substrate is one of Si single crystal, sapphire single crystal, MgO, and SrTiO3. A method of manufacturing a ferroelectric thin-film element in which a lower electrode layer, a crystal nucleus layer made of an oxide, a ferroelectric thin film layer, and an upper electrode layer are sequentially laminated on a base substrate, wherein the lower electrode layer is formed on the base substrate. After forming a crystal nucleus layer sequentially, an amorphous ferroelectric thin film layer is formed on the crystal nucleus layer, and a temperature of 400 to 600 ° C. in an inert gas containing oxygen at a pressure of 1.4 to 180 MPa. A ferroelectric thin-film element comprising: crystallizing the ferroelectric thin-film layer by performing isotropic pressure annealing (HIP) in a range, and then forming an upper electrode layer on the ferroelectric thin-film layer. Recipe. A ferroelectric memory using the ferroelectric thin-film element according to claim 1.

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