JP2006073275A - Bismuth laminar compound dielectric thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bismuth laminar compound dielectric thin film of which the maximum peak dielectric constant temperature is made to reduce. <P>SOLUTION: The dielectric thin film includes bismuth laminar compound comprising a crystal structure in which a perovskite block layer and a bismuth oxide layer are laminated on each other is represented by the composition formula: (Bi<SB>2</SB>O<SB>2</SB>)<SP>2+</SP>(A<SB>x</SB>B<SB>m</SB>O<SB>3m+1</SB>)<SP>2-</SP>, where the symbol A is Na, K, Pb, Ba, Sr, Ca or Bi, the symbol B is Fe, Co, Cr, Ga, Ti, Nb, Ta, Sb, V, Mo or W, the symbol m is a positive number of not smaller than 2 and x is a positive number of smaller than m-1. The maximum peak dielectric constant temperature T<SB>m</SB>of the requiring bismuth laminar compound is smaller than that obtained from the above composition formula in setting x=m-1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bismuth layered compound dielectric thin film having a reduced maximum dielectric constant temperature in order to use a ferroelectric bismuth layered compound as a paraelectric.

  With the increase in density, integration, and miniaturization of electronic circuits, it is desired to further increase the capacity of capacitors. In order to increase the capacity while reducing the size of the capacitor, it is important to further increase the dielectric constant of the dielectric material.

  Conventionally, a simple perovskite oxide has been known as a dielectric material having a high dielectric constant and enabling a thin film on the order of nanometers (nm). Furthermore, it is known that the use of a bismuth layered compound group having a layered structure of a perovskite layer and a bismuth oxide layer can eliminate the “size effect” in which the dielectric constant decreases as the film thickness decreases (Y. Takeshima et al. al., Jpn. J. Appl. Phys., 39, 5389, 2000). As described above, the bismuth layered compound group is expected to be used as a size effect-free high dielectric material.

On the other hand, when the bismuth layered compound group is used at a high frequency, there is a concern that the dielectric loss increases and the dielectric constant decreases. This is because the bismuth layered compound group is a ferroelectric substance. Ferroelectrics transition to the paraelectric phase at temperatures above the so-called Curie temperature. By using a paraelectric material, suppression of dielectric loss and improvement of the dielectric constant can be expected in the use of high frequency. However, since the bismuth layered compound has a Curie temperature or maximum dielectric constant temperature of 100 ° C. or higher even in the case of the lowest BaBi ₂ Ta ₂ O ₉ , the bismuth layered compound remains at a general use temperature (particularly room temperature) as it is. Cannot be used as a paraelectric material.

A method for lowering the maximum dielectric constant temperature of a bismuth layered compound is described in H. Matsuda et al., Jpn. J. Appl. Phys., 42 (2003) L949. In this method, a lanthanoid such as La is added to a Bi ₄ Ti ₃ O ₁₂ group substance, or another element is added to the bismuth layered compound, such as replacing Sr or Nb. However, when another element is added, the temperature dependence increases due to the broadening of the Curie temperature. In this publication, the maximum dielectric constant temperature of a bismuth layered compound is lowered by eliminating a so-called A-site element in the perovskite block layer without adding another element, particularly to lower to room temperature or lower. Has no description or suggestion.

Y. Takeshima et al., Jpn. J. Appl. Phys., 39, 5389, (2000) H. Matsuda et al., Jpn. J. Appl. Phys., 42 (2003) L949

  Accordingly, an object of the present invention is to provide a bismuth layered compound dielectric thin film in which the maximum dielectric constant temperature is reduced without adding another element in order to expand the range in which the bismuth layered compound can be used as a paraelectric. is there. It is a further object of the present invention to provide a bismuth layered compound dielectric thin film that minimizes dielectric loss when the bismuth layered compound is used at room temperature, particularly at high frequencies.

According to the present invention, there is provided a dielectric thin film comprising a bismuth layered compound having a crystal structure in which a perovskite block layer and a bismuth oxide layer are laminated with each other, wherein the bismuth layered compound has the composition formula:
(Bi ₂ O ₂ ) ²⁺ (A _x B _m O _{3m + 1} ) ²⁻
In the above composition formula, A represents Na, K, Pb, Ba, Sr, Ca or Bi, and B represents Fe, Co, Cr, Ga, Ti, Nb, Ta, Sb, V, Mo or W. M is a positive number of 2 or more, x is a positive number smaller than m−1, and the maximum dielectric constant temperature T _m of the compound is x in the above composition formula. A dielectric thin film is provided that is lower than the maximum dielectric constant temperature of the corresponding bismuth layered compound.

  According to the present invention, the maximum dielectric constant temperature can be lowered by deleting the A site element of the perovskite block layer without adding another element to the bismuth layered compound. In particular, the maximum dielectric constant temperature can be lowered to room temperature or lower by selecting the type of bismuth layered compound and the amount of deficiency of the A-site element. Since the temperature range in which the bismuth layered compound, which is a ferroelectric material, can be used as a paraelectric material is widened, the bismuth layered compound can be advantageously used particularly in high frequency applications where dielectric loss and dielectric constant reduction are problematic.

According to the present invention, a dielectric thin film comprising a bismuth layered compound having a crystal structure in which a perovskite block layer and a bismuth oxide layer are laminated with each other, the bismuth layered compound has a composition formula:
(Bi ₂ O ₂ ) ²⁺ (A _x B _m O _{3m + 1} ) ²⁻
In the above composition formula, A represents Na, K, Pb, Ba, Sr, Ca or Bi, and B represents Fe, Co, Cr, Ga, Ti, Nb, Ta, Sb, V, Mo or W. M is a positive number of 2 or more, x is a positive number smaller than m−1, and the maximum dielectric constant temperature T _m of the compound is x in the above composition formula. It is characterized by being lower than the maximum dielectric constant temperature of the corresponding bismuth layered compound. Since x is a positive number smaller than m-1, the bismuth layered compound according to the present invention is deficient in the A site element. For example, when m is 2, x is a positive number smaller than 1, preferably 0.8 or less, more preferably 0.6 or less. The lower limit value of x is determined within a range in which the desired perovskite structure is not collapsed, and is approximately 0.3 (Ba / Ta ratio is 0.15) although it varies depending on the specific composition.

  m may also be an odd number such as 3, 5, 7, etc. Normally, in the case of a ferroelectric, when m is an odd number, it cannot be used as a dielectric because it has ferroelectricity in the c-axis direction. However, according to the present invention, even if m is an odd number, the c-axis direction of the (001) -oriented thin film is also a paraelectric material, so that it can be advantageously used as a dielectric as in the case where m is an even number. .

The bismuth layered compound in which x is m-1 in the above composition formula has a specific maximum dielectric constant temperature (Curie temperature) corresponding to each specific combination of the A site element and the B site element. The low temperature side becomes the ferroelectric phase and the high temperature side becomes the paraelectric phase at the temperature at which the dielectric constant becomes maximum. By lowering the maximum dielectric constant temperature, the use of a ferroelectric bismuth layered compound as a paraelectric can be expanded, and in particular, suppression of dielectric loss and improvement of dielectric constant can be expected when used at high frequencies. The intrinsic maximum dielectric constant temperature of the bismuth layered compound in which x is m−1 is about 110 ° C. in the case of the lowest BaBi ₂ Ta ₂ O ₉ . In order to lower the maximum dielectric constant temperature to room temperature or lower, it is advantageous to use a compound having a low intrinsic maximum dielectric constant temperature as a starting material when the A site element is lost. From this point of view, BaBi ₂ Ta ₂ O _{9 in} which the A site is barium (Ba) and the B site is tantalum (Ta) is adopted, and the Ba is deficient with respect to Ta, that is, Ba / Ta. By reducing the ratio, the temperature range in which the bismuth layered compound can be used as a paraelectric can be extended to the maximum. In particular, as demonstrated in Examples described later, in Ba _x Bi ₂ Ta ₂ O ₉ , by setting x to 0.8 or less, that is, Ba / Ta ratio to 0.4 or less, the maximum dielectric constant temperature T _m is reduced. It becomes room temperature (300K) or less. Furthermore, by setting x to 0.6 or less, that is, Ba / Ta ratio to 0.3 or less, the temperature T _{tan δ at} which the dielectric loss can be reduced becomes room temperature (300 K) or less. By adopting such a composition, dielectric loss and dielectric constant reduction can be most effectively suppressed, and the range of high frequency applications is widened. As described above, the dielectric thin film according to the present invention reduces the maximum dielectric constant temperature only by losing the A site element in the perovskite block layer (without adding another element), and thus the paraelectric of the bismuth layered compound. It expands the range of use as a body.

  The bismuth layered compound dielectric thin film according to the present invention has a so-called size effect free characteristic in which the dielectric constant does not show film thickness dependence up to a thickness of about 10 nm. On the other hand, the upper limit of the film thickness is desired to be 200 nm or less, more preferably 100 nm or less, considering the increase in capacity as a dielectric.

  The dielectric thin film according to the present invention is formed on a substrate made of a material having a good lattice matching, such as a substrate oriented in the (001) orientation such as cubic, tetragonal, orthorhombic, and monoclinic. Various thin film forming methods such as a vapor deposition method, a high frequency sputtering method, a pulse laser deposition method (PLD), a metal organic chemical vapor deposition method (MOCVD), and a sol-gel method can be employed. As a method of losing the A site element, the composition ratio of the A site element to the B site element in the target of the raw material or the reaction gas may be appropriately reduced in the employed thin film forming method. For example, a method of reducing the A-site element / B-site element ratio using MOCVD is described in H. Funakubo et al., Jpn. J. Appl. Phys. 38 (2B), L199 (1999). .

The dielectric thin film according to the present invention comprises a bismuth layered compound represented by the composition formula: (Bi ₂ O ₂ ) ²⁺ (A _x B _m O _{3m + 1} ) ²⁻ . In general, a bismuth layered compound has a layered structure in which layered perovskite layers and bismuth oxide layers in which perovskite lattices composed of (m−1) ABO ₃ are connected are alternately stacked. In the present invention, the c-axis orientation degree of the bismuth layered compound, that is, the orientation to the (001) direction is particularly preferably 100%, but the c-axis orientation degree is not necessarily 100%. Preferably 80% or more, more preferably 90% or more, and still more preferably 95% or more of the compound should be c-axis oriented. Here, the degree of c-axis orientation (F) of the bismuth layered compound means that the c-axis X-ray diffraction intensity of the polycrystal having a completely random orientation is P0, and the actual c-axis X-ray diffraction intensity. Where P is determined by the formula: F (%) = (P−P0) / (1−P0) × 100. P in the formula is the ratio of the total ΣI (001) of the reflection intensity I (001) from the (001) plane and the total ΣI (hkl) of the reflection intensity I (hkl) from each crystal plane (hkl) ( {ΣI (001) / ΣI (hkl)}), and the same applies to P0. However, in the above formula, the X-ray diffraction intensity P when the orientation is 100% in the c-axis direction is 1. From the above formula, when completely random orientation (P = P0), F = 0%, and when completely oriented in the c-axis direction (P = 1) F = 100%. The c-axis of the bismuth layered compound means a direction connecting a pair of (Bi ₂ O ₂ ) ²⁺ layers, that is, a (001) orientation.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Example 1: Production of dielectric thin film of composition formula (Bi ₂ O ₂ ) ²⁺ (Ba _x Ta ₂ O ₇ ) ^2- SrTiO ₃ single crystal substrate obtained by epitaxially growing SrRuO ₃ as a lower electrode thin film in the (001) direction {(001) SrRuO ₃ /// (001) SrTiO ₃ } was prepared. On the surface of the SrRuO ₃ lower electrode thin film, Bi (CH ₃ ) ₃ and Ba [Ta (OC ₂ H ₅ ) ₅ (OC ₂ H ₄ OCH ₃ )] ₂ are used as raw materials, and a thickness of 100 nm ( Bi ₂ O ₂ ) ²⁺ (Ba _x Ta ₂ O ₇ ) ²⁻ thin film (dielectric thin film) becomes x = 0.5 and 0.8 (Ba / Ta ratio = 0.25 and 0.4) A plurality were formed. Control of Ba / Ta ratio, that is, control of x was performed by changing the flow rate at the time of supplying Ba [Ta (OC ₂ H ₅ ) ₅ (OC ₂ H ₄ OCH ₃ )] ₂ raw material by the bubbling method. .

  When the crystal structure of the obtained dielectric thin film was measured by X-ray diffraction (XRD), it was confirmed that it was oriented in the (001) direction, that is, c-axis oriented perpendicular to the substrate surface. .

Example 2
A thin film capacitor sample was prepared by forming a Pt upper electrode thin film with a diameter of 100 μmφ on the surfaces of the two samples (Ba / Ta = 0.40, 0.25) prepared in Example 1 by vacuum deposition. The temperature dependence of the dielectric constant was evaluated for each capacitor sample.
The dielectric constant is measured with respect to the capacitor sample by using an impedance analyzer (HP4194A) within a temperature range of 79 to 600 K under the condition of a measurement frequency of 100 kHz (AC 20 mV), the electrode size of the capacitor sample, and It calculated from the distance between electrodes. Tan δ was measured under the same conditions as those for measuring the capacitance, and the loss Q value was calculated accordingly. These results are summarized in FIG.

As is apparent from FIG. 1, the temperature dependence of dielectric constant and dielectric loss was observed in the two dielectric thin film samples. Here, the temperature at which the dielectric constant reaches a maximum value is defined as a maximum dielectric constant temperature T _m, and the temperature at which the dielectric loss can be decreased is defined as T _{tan δ} . _{The T m} of a dielectric thin film is a Ba / Ta = 0.25 is 200K, the _{T m} of the dielectric thin film is a Ba / Ta = 0.40 was 250K. The graph plotted against Ba / Ta ratio of these maximum dielectric constant temperature T _m shown in FIG. In FIG. 2, the bulk sample has Ba / Ta = 0.5, and is produced by a normal ceramic production technique. As the Ba / Ta ratio decreases, _Tm also decreases, indicating that both of these two dielectric thin film samples are paraelectric at room temperature. Further, T _{tan [delta} of the dielectric thin film is a Ba / Ta = 0.25 is about 250K, T _{tan [delta} of the dielectric thin film is a Ba / Ta = 0.40 was approximately 350K. A graph in which these T _{tan δ} are plotted against the Ba / Ta ratio is shown in FIG. In FIG. 3, the bulk sample has Ba / Ta = 0.5, and is produced by a normal ceramic production technique. Although T _{tan δ} also decreases as the Ba / Ta ratio decreases, it can be seen that when Ba / Ta = 0.40, T _{tan δ} is higher than room temperature even though it is a paraelectric material, and there is room for suppressing dielectric loss. From FIG. 3, in the case of the dielectric thin film having the composition formula (Bi ₂ O ₂ ) ²⁺ (Ba _x Ta ₂ O ₇ ) ²⁻ , T _{tan δ} is increased to room temperature by setting the Ba / Ta ratio to about 0.3 or less. It can be seen that the merit of the paraelectric material can be maximized.

Example 3
A thin film capacitor sample was prepared by forming a Pt upper electrode thin film with a diameter of 100 μmφ on the surfaces of the two samples (Ba / Ta = 0.40, 0.25) prepared in Example 1 by vacuum deposition. The voltage dependence of dielectric constant was evaluated for each capacitor sample.
The voltage dependence is that for a capacitor sample, the measurement voltage (applied voltage) under a specific frequency (100 kHz) is changed from electric field intensity −1 MV / cm to electric field intensity 1 MV / cm, and the capacitance under the specific voltage is changed. The results of measurement at various temperatures and calculation of the dielectric constant are shown in FIG. An impedance analyzer (HP4194A) was used for the measurement of capacitance. As shown in FIG. 4, it was confirmed that the value of the dielectric constant did not change even when the measurement voltage under a specific frequency was changed to an electric field strength of 1 MV / cm. In particular, it was confirmed that the paraelectric sample of Ba / Ta = 0.25 measured at 300K and 400K has excellent voltage characteristics.

Example 4
A thin film capacitor sample was prepared by forming a Pt upper electrode thin film with a diameter of 100 μmφ on the surfaces of the two samples (Ba / Ta = 0.40, 0.25) prepared in Example 1 by vacuum deposition. For each capacitor sample, the Ba / Ta composition ratio dependence of dielectric loss at room temperature was evaluated.
The dielectric constant is the capacitance measured for the capacitor sample using an impedance analyzer (HP4194A) at room temperature (25 ° C.) and a measurement frequency of 100 kHz (AC 20 mV), the electrode dimensions of the capacitor sample, and the distance between the electrodes. And calculated from Tan δ was measured under the same conditions as those for measuring the capacitance, and the loss Q value was calculated accordingly. These results are shown in FIG. In the figure, the bulk sample has Ba / Ta = 0.5, and is manufactured by a normal ceramic manufacturing technique. As is clear from FIG. 5, the dielectric loss tan δ at room temperature decreases as the Ba / Ta ratio decreases to 0.50, 0.40, and 0.25.

It is a graph which shows the temperature dependence of the dielectric constant and dielectric loss of the capacitor | condenser sample of Example 2. FIG. 6 is a graph showing the Ba / Ta ratio dependency of _Tm of the capacitor sample of Example 2. 6 is a graph showing the Ba / Ta ratio dependence of T _{tan δ} of the capacitor sample of Example 2. It is a graph which shows the voltage dependence of the dielectric constant of the capacitor | condenser sample of Example 3. It is a graph which shows the composition dependence of the dielectric loss at the room temperature of the capacitor sample of Example 4.

Claims (8)

A dielectric thin film comprising a bismuth layered compound having a crystal structure in which a perovskite block layer and a bismuth oxide layer are laminated to each other, wherein the bismuth layered compound has a composition formula:
(Bi ₂ O ₂ ) ²⁺ (A _x B _m O _{3m + 1} ) ²⁻
In the above composition formula, A represents Na, K, Pb, Ba, Sr, Ca or Bi, and B represents Fe, Co, Cr, Ga, Ti, Nb, Ta, Sb, V, Mo or W. M is a positive number of 2 or more, x is a positive number smaller than m−1, and the maximum dielectric constant temperature T _m of the compound is x in the above composition formula. A dielectric thin film characterized by being lower than the maximum dielectric constant temperature of the corresponding bismuth layered compound.   The dielectric thin film according to claim 1, wherein m is an odd number in the composition formula.   The dielectric thin film according to claim 1, wherein m is an even number in the composition formula.   The dielectric thin film according to claim 3, wherein m is 2 in the composition formula.   The dielectric thin film according to claim 4, wherein x is 0.8 or less in the composition formula.   The dielectric thin film according to claim 4, wherein x is 0.6 or less in the composition formula.   The dielectric thin film according to any one of claims 1 to 6, wherein A is Ba and B is Ta in the composition formula. The dielectric thin film according to claim 7, wherein the maximum dielectric constant temperature _Tm is 300K or less.

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