JP7569698B2 - Metal oxynitride thin film and capacitor element - Google Patents

Description

The present invention relates to a metal oxynitride thin film and a capacitance element.

As digital devices become more sophisticated, the dielectrics that make up electronic components that utilize dielectric properties are required to have a high Curie temperature (Tc) and a high dielectric constant. However, it is difficult to achieve both at the same time. For example, barium titanate, which exhibits excellent dielectric properties, has a low Tc because it uses a structural phase transition to achieve its high dielectric constant.

Metal oxynitrides having a perovskite structure represented as ABO ₂ N have attracted attention as dielectric materials and exhibit a high Tc due to their crystal structure. However, although various proposals have been made regarding the mechanism by which the dielectric properties of these metal oxynitrides are expressed, the mechanism has not been clarified.

For example, Patent Document 1 describes a tetragonal perovskite-type oxynitride in which nitrogen atoms are oriented in the c-axis direction. Patent Document 1 describes that the orientation of nitrogen atoms in the c-axis direction makes the tetragonal perovskite-type oxynitride ferroelectric and has good piezoelectric properties.

In addition, Non-Patent Document 1 describes a method of epitaxially growing an oxynitride thin film on a substrate by utilizing the lattice mismatch with the substrate, thereby orienting the nitrogen atoms in the c-axis direction.

Patent Document 2 also describes a dielectric ceramic composition whose main composition is a perovskite-type oxynitride and has a polycrystalline body that exhibits ferroelectricity.

JP 2010-143788 A International Publication No. 2017/135298

Daichi Oka, et al., "Possible ferroelectricity in perovskite oxynitride SrTaO2N epitaxial thin films", SCIENTIFIC REPORTS 4, DOI: 10.1038/srep04987

However, in order to orient the nitrogen atoms in the c-axis direction, Patent Document 1 requires an orientation process for orienting the anisotropic-shaped particles. In other words, an extra process is required. In addition, the method described in Non-Patent Document 1 is a special method and is not suitable for industrial use. The oxynitride described in Patent Document 2 is composed of a ferroelectric material, and has the problem of high dielectric loss.

The present invention was made in consideration of these circumstances, and aims to provide a metal oxynitride thin film that has ferroelectricity and low dielectric loss.

The inventors have found that a metal oxynitride having a region that produces centrosymmetric diffraction and a region that produces non-centrosymmetric diffraction can be obtained, which results in a metal oxynitride that has ferroelectricity as a whole and has low dielectric loss due to its paraelectric structure.

In order to achieve the above objectives, the present invention has the following aspects:

[1] A metal oxynitride thin film containing a metal oxynitride having a perovskite structure and represented by the general formula ABO _x N _y ,
A is at least one selected from the group consisting of barium, strontium, and lanthanum, B is at least one selected from the group consisting of tantalum and titanium,
When the ionic valence indicated when A occupies the A site in the perovskite structure is a and the ionic valence indicated when B occupies the B site in the perovskite structure is b, a and b satisfy the relationship 6.7≦a+b≦7.3;
In the general formula, x and y are
2.000≦x≦3.275,
The relationship 0.150≦y≦1.000 is satisfied,
The metal oxynitride is a metal oxynitride thin film that has regions that produce centrosymmetric diffraction and regions that produce non-centrosymmetric diffraction.

[2] In an X-ray diffraction pattern obtained by powder X-ray diffraction using Cu-Kα radiation as an X-ray source or powder X-ray diffraction simulation for a perovskite metal oxynitride consisting only of regions that produce diffraction exhibiting centrosymmetricity and having the same composition as a metal oxynitride thin film, the strongest diffraction peak among the diffraction peaks that appear within a diffraction angle 2θ range of 20° to 25° and that belong to the perovskite structure is defined as P1, the strongest diffraction peak among the diffraction peaks that appear within a diffraction angle 2θ range of 44° to 50° and that belong to the perovskite structure is defined as P2, and the intensity ratio IRr of P1 to the intensity of P2 is defined as _IRr . In an X-ray diffraction pattern for a metal oxynitride thin film obtained by perpendicular X-ray diffraction and in-plane X-ray diffraction using Cu-Kα radiation as an X-ray source, the intensity ratio of P1 to the intensity of P2 is defined as IRr. The metal oxynitride thin film according to [1], wherein a ratio A of IR to _r is 0% or more and 80% or less, and the metal oxynitride thin film has remanent polarization.

[3] A metal oxynitride thin film according to [1] or [2], in which the region that produces centrosymmetric diffraction and the region that produces non-centrosymmetric diffraction have different ranges of anion arrangement order.

[4] The metal oxynitride thin film according to [3], in which the region that produces centrosymmetric diffraction and the region that produces non-centrosymmetric diffraction have the same cations.

[5] x and y are
2.450≦x≦3.275,
The metal oxynitride thin film according to any one of [1] to [4], which satisfies the relationship 0.150≦y≦0.700.

[6] A capacitive element comprising a metal oxynitride thin film according to any one of [1] to [5].

The present invention provides a metal oxynitride thin film that has ferroelectric properties and low dielectric loss.

FIG. 1 is a schematic cross-sectional view of a thin film capacitor as an example of a capacitive element according to this embodiment. FIG. 2 is a perspective view showing the crystal structure of ABO ₂ N. FIG. 3A is a schematic diagram showing a BO ₄ N _dioctahedron in which nitrogen is in the cis configuration in ABO ₂ N. FIG. 3B is a schematic diagram showing a BO ₄ N _dioctahedron in which the nitrogen in ABO ₂ N is in the trans configuration. FIG. 4 is a schematic diagram showing that BO ₄ N _dioctahedra in which nitrogen is in a cis configuration form an octahedral chain in the c-axis direction. FIG. 5 is a schematic diagram showing the polarization treatment pattern performed on the sample of the example.

The present invention will now be described in detail based on specific embodiments in the following order.
1. Thin film capacitor 1.1. Overall configuration of thin film capacitor 1.2. Metal oxynitride thin film 1.2.1. Metal oxynitride 2. Manufacturing method of thin film capacitor 2.1. Manufacturing method of metal oxynitride thin film 3. Summary of this embodiment

(1. Thin Film Capacitor)
In this embodiment, a thin film capacitor having the metal oxynitride thin film according to this embodiment as a dielectric is described as an example of a capacitive element. Examples of capacitive elements other than thin film capacitors include thermistors, filters, diplexers, resonators, oscillators, antennas, piezoelectric elements, transistor gate materials, ferroelectric memories, etc.

(1.1. Overall configuration of thin film capacitor)
As shown in FIG. 1, a thin film capacitor 1 as an example of a capacitive element according to this embodiment has a configuration in which a substrate 51, a first electrode 52, a metal oxynitride thin film 53, and a second electrode 54 are laminated in this order.

The first electrode 52, the metal oxynitride thin film 53, and the second electrode 54 form a capacitor portion. When the first electrode 52 and the second electrode 54 are connected to an external circuit (not shown) and a voltage is applied, the metal oxynitride thin film 53, which is a dielectric, exhibits a predetermined capacitance and can function as a capacitor.

The substrate 51 shown in FIG. 1 is not particularly limited as long as it is made of a material that can suitably form the first electrode 52, the metal oxynitride thin film 53, and the second electrode 54 formed thereon. Examples of such materials include single crystal and amorphous materials, and in this embodiment, it is preferable to use a Si single crystal substrate.

As shown in FIG. 1, the first electrode 52 and the second electrode 54 are formed in the form of thin films, sandwiching the metal oxynitride thin film 53. There are no particular limitations on the material constituting the first electrode 52 and the second electrode 54, so long as it is a conductive material. Examples include metals such as platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir), gold (Au), silver (Ag), copper (Cu), and nickel (Ni), or alloys thereof.

(1.2. Metal oxynitride thin film)
As will be described later, the metal oxynitride thin film according to this embodiment is composed of a metal oxynitride having a predetermined composition and a perovskite structure.

In this embodiment, the metal oxynitride thin film is a thin film formed by a known film formation method. Such thin films are usually formed by depositing atoms on a substrate, so the metal oxynitride thin film is preferably a metal oxynitride deposited film. Therefore, the metal oxynitride thin film according to this embodiment does not include a fired body (obtained by a solid-phase reaction) obtained by firing a compact formed from a dielectric raw material powder.

The thickness of the metal oxynitride thin film is not particularly limited and can be set arbitrarily depending on the desired characteristics, applications, etc. In this embodiment, the thickness is preferably 10 nm or more and 2 μm or less.

(1.2.1. Metal oxynitride)
The composition of the metal oxynitride constituting the metal oxynitride thin film according to this embodiment is represented by the general formula ABO _x N _y , where x represents the proportion of oxygen in the metal oxynitride, y represents the proportion of nitrogen in the metal oxynitride.

In this embodiment, x satisfies the relationship 2.000≦x≦3.275, and y satisfies the relationship 0.150≦y≦1.000. By making x and y satisfy the above ranges, it becomes easy to obtain a metal oxynitride thin film that has ferroelectricity and small dielectric loss.

x is preferably 2.450 or more, and more preferably 2.750 or more. x is preferably 3.200 or less, and more preferably 2.900 or less.

In addition, y is preferably 0.200 or more, and more preferably 0.400 or more. y is preferably 0.700 or less, and more preferably 0.500 or less.

In the general formula, A is at least one selected from the group consisting of barium (Ba), strontium (Sr) and lanthanum (La), and it is preferable that A contains at least one selected from the group consisting of barium and strontium. In the formula, B is at least one selected from the group consisting of tantalum (Ta) and titanium (Ti).

Therefore, the above composition formula ABO _x N _y can be expressed as (Ba _1-p-q Sr _p La _q ) (Ta _1-r Ti _r ) O _x N _y . Here, p and q satisfy the relationships of 0≦p≦1.0, 0≦q≦1.0. Furthermore, r satisfies the relationship of 0≦r≦1.0. Preferably, the composition formula ABO _x N _y can be expressed as (Ba _1-p Sr _p ) _1-q La _q (Ta _1-r Ti _r ) O _x N _y . Here, it is preferable that p and q satisfy the relationships of 0≦p≦1.0, 0≦q≦0.30.

Metal oxynitrides also have a perovskite structure. Figure 2 shows the perovskite structure. As shown in Figure 2, in the perovskite structure, anions (11, 12) occupy six vertices, and octahedra 15 with a B-site atom 13 at the center share their vertices to form a three-dimensional network, with A-site atoms 14 located in the gaps of this network. As described above, the A-site atom is at least one selected from the group consisting of barium, strontium, and lanthanum, and the B-site atom is at least one selected from the group consisting of tantalum and titanium.

In this embodiment, if the ionic valence indicated when A occupies the A site in the perovskite structure is a, and the ionic valence indicated when B occupies the B site in the perovskite structure is b, a and b satisfy the relationship 6.7≦a+b≦7.3.

The ionic valence a and ionic valence b are each expressed as an average valence. The average valence is a value obtained by averaging the valences of ions present in the A site or the B site according to their abundance ratio. For example, a case will be described in which Sr and La are present in the A site at a molar ratio of 4:1. The valence of the Sr ion in the perovskite structure is 2, and the valence of the La ion in the perovskite structure is 3. Therefore, the average valence a of the A site is calculated by the following (Formula 1), and a is 2.2.
(Equation 1)
(Average valence a in the above case)
= 2 (valence of Sr ions) × 4/5 (abundance ratio of Sr ions) + 3 (valence of La ions) × 1/5 (abundance ratio of La ions)
= 8/5 + 3/5
= 11/5
= 2.2… (1)

Similarly, a case will be described where Ta and Ti exist in the B site at a molar ratio of 4:1. The valence of the Ta ion in the perovskite structure is 5, and the valence of the Ti ion in the perovskite structure is 4. Therefore, the average valence b of the B site is calculated by the following (Equation 2), and b is 4.8.
(Equation 2)
(Average valence b in the above case)
= 5 (valence of Ta ions) × 4/5 (abundance ratio of Ta ions) + 4 (valence of Ti ions) × 1/5 (abundance ratio of Ti ions)
= 20/5 + 4/5
=24/5=4.8...(2)

The above average valence is the valence calculated when the molar ratio of the ions present on the A site to the ions present on the B site is 1:1. On the other hand, the average valence may be the valence calculated when the molar ratio of the ions present on the A site to the ions present on the B site is not 1:1.

For example, when the molar ratio of Sr ions present at the A site to Ta ions present at the B site is 1:1.02, using the above formulas (1) and (2), a has a valence of 2 and b has a valence of 5.1.
(Average valence a)
= 2 x 1 = 2
(Average Valence b)
=5×1.02=5.1

When the sum of the average valence a of the A site and the average valence b of the B site satisfies the above range, it becomes easy to obtain a metal oxynitride thin film that has ferroelectricity and small dielectric loss.

On the other hand, the anions are distributed according to the composition ratio of oxygen and nitrogen in the metal oxynitride ABO _x N _y , and when x = 2 and y = 1 in ABO _x N _y , the octahedron is a BO ₄ N ₂ octahedron. That is, as shown in Figure 2, in the BO ₄ N ₂ octahedron, nitrogen ions 12 occupy two of the six vertices occupied by anions.

In this embodiment, the metal oxynitride has ferroelectricity. Ferroelectricity is a property in which the direction of spontaneous polarization is reversed by an external electric field, and the polarization remains even when the external electric field is removed. Spontaneous polarization is a bias in the distribution of charge caused by a relative change in the positions of cations and anions in the crystal.

Therefore, a ferroelectric substance has a structure that causes a bias in the charge distribution. For example, barium titanate (BaTiO ₃ ), known as a perovskite-type ferroelectric, has a tetragonal crystal structure at room temperature, and the lattice length in the c-axis direction is longer than the lattice lengths in the a-axis and b-axis directions. Therefore, spontaneous polarization occurs in the unit lattice due to an arrangement in which the position of the titanium ion is shifted from the center of gravity of the oxygen ion. This arrangement does not have centrosymmetricity. Whether or not such spontaneous polarization occurs can be evaluated, for example, by whether or not the metal oxynitride exhibits a piezoelectric response.

On the other hand, a material that loses its polarization when the external electric field is removed is called a paraelectric. At high temperatures, barium titanate transitions to a cubic crystal structure, and the lattice length in the c-axis direction becomes the same as the lattice length in the a-axis and b-axis directions. As a result, barium titanate loses its ferroelectricity and becomes a paraelectric. In other words, the center of gravity of the oxygen ions and the center of gravity of the titanium ions are aligned, and the spontaneous polarization disappears as the structure transitions to one in which there is no bias in the charge distribution. This type of arrangement is centrally symmetric.

Normally, a material having ferroelectricity has only regions in which an arrangement that does not have centrosymmetrical properties (an arrangement having non-centrosymmetrical properties) is repeated in an orderly manner. However, in this embodiment, the metal oxynitride has regions that produce centrosymmetrical diffraction and regions that produce non-centrosymmetrical diffraction, and can exhibit ferroelectricity due to the regions that produce non-centrosymmetrical diffraction.

In this embodiment, both the region that produces centrosymmetric diffraction and the region that produces non-centrosymmetric diffraction are configured with a non-centrosymmetric arrangement. However, the range in which the arrangement order of anions (oxygen ions and nitrogen ions) is maintained is different between the region that produces centrosymmetric diffraction and the region that produces non-centrosymmetric diffraction.

In the region where diffraction exhibits centrosymmetric properties, the anions are arranged in a non-centrosymmetric arrangement, but the arrangement is random throughout the crystal grain, and the non-centrosymmetric arrangement is on the order of the size of the unit cell, so the range affected by the polarization caused by the non-centrosymmetric properties is small. Therefore, since the polarization direction is omnidirectional in the region, the region as a whole does not have polarization oriented in a specific direction, and can be considered to have centrosymmetric properties. As a result, when the metal oxynitride according to this embodiment is analyzed by a diffraction method, the region is detected as a region where diffraction exhibits centrosymmetric properties, and is not detected as a region where diffraction exhibits non-centrosymmetric properties as a second phase.

On the other hand, in regions where non-centrosymmetric configurations are arranged in an orderly manner, the configuration order is repeated in a specific direction, and the non-centrosymmetric nature is strengthened in that direction. When such regions reach a certain size, the diffraction occurring in these regions can affect the centrosymmetric diffraction. As will be described later, when non-centrosymmetric diffraction occurs, the centrosymmetric diffraction is affected, resulting in a diffraction pattern that differs from the diffraction pattern exhibited by a metal oxynitride composed only of regions where centrosymmetric diffraction occurs.

In metal oxynitrides that produce such diffraction patterns, the regions where anions are ordered are large, and serve as the starting point for the development of ferroelectricity. On the other hand, because metal oxynitrides have regions where diffraction occurs that exhibits central symmetry, the dielectric loss is reduced.

In the BO ₄ N ₂ octahedron, there are two configurations (cis configuration) in which two nitrogen (N) ions 12 are adjacent to each other as shown in Fig. 3A, and there are two configurations (trans configuration) in which two nitrogen ions (N) 12 are not adjacent to each other as shown in Fig. 3B. Both the cis configuration and the trans configuration are non-centrosymmetric configurations because the bond lengths of the oxygen ions and the nitrogen ions are different.

In this embodiment, the non-centrosymmetric configuration is preferably a cis configuration. Therefore, it is preferable that the region that produces diffraction exhibiting centrosymmetricity and the region that produces diffraction exhibiting non-centrosymmetricity are configured in a cis configuration.

In the cis-configured BO ₄ N ₂ octahedron shown in Fig. 3A, nitrogen ions are present in the a-axis and c-axis directions, but the BO ₄ N ₂ octahedron with nitrogen ions present in the a-axis and b-axis directions and the BO ₄ N ₂ octahedron with nitrogen ions present in the b-axis and c-axis directions are also cis-configured. Therefore, these BO ₄ N ₂ octahedrons are randomly present and share vertices, weakening non-centrosymmetric properties and forming regions that produce diffraction exhibiting centrosymmetric properties.

In contrast, a structure in which the cis configuration exists in an orderly manner is shown in FIG. 4. In FIG. 4, in the cis-configuration ordered structure, nitrogen is arranged at one of the sites (4a) in the c-axis direction and one of the sites (8h) on the plane parallel to the a-axis and b-axis among the vertices of the octahedron. As a result, the BO ₄ N ₂ octahedra form an octahedral chain in which the octahedra are linked in a state tilted from the axis connecting B (c-axis in FIG. 3). In this embodiment, such an ordered structure formed by linking octahedra in which nitrogen is in the cis configuration affects the diffraction pattern due to the region in which the BO ₄ N ₂ octahedra exist randomly (the order range of the BO ₄ N ₂ octahedra is narrow).

The types of cations may be the same or different in the region where centrosymmetric diffraction occurs and the region where non-centrosymmetric diffraction occurs. In this embodiment, it is preferable that the types of cations are the same. By reducing the number of types of elements that make up the metal oxynitride, contamination can be suppressed and the load on the manufacturing process can be reduced.

In a metal oxynitride, whether or not there are regions that produce centrosymmetric diffraction and regions that produce non-centrosymmetric diffraction can be evaluated, for example, as follows. Such structures determined by the positions of atoms are often evaluated by XRD diffraction. However, as described above, in the above metal oxynitride, whether a region produces centrosymmetric diffraction or non-centrosymmetric diffraction depends on the difference in the arrangement of anions (oxygen and nitrogen). The intensity of the diffraction peaks reflecting the positions of light elements such as oxygen and nitrogen is weaker than the intensity of the diffraction peaks reflecting the positions of metal elements (A and B), and it is difficult to directly evaluate the diffraction peaks reflecting the positions of light elements.

However, differences in the arrangement of anions can affect the intensity of the diffraction peaks observed by XRD diffraction. Therefore, in this embodiment, based on the ratio of the intensity of the diffraction peak reflected by a specific plane to the intensity of the diffraction peak reflected by another plane, it is evaluated whether or not there are regions in the metal oxynitride that produce centrosymmetric diffraction and regions that produce non-centrosymmetric diffraction.

First, a measurement result or a simulation result of powder XRD diffraction is obtained for a perovskite metal oxynitride composed only of regions that produce diffraction exhibiting centrosymmetricity. When the X-ray source is Cu-Kα radiation, an intensity ratio IR r (intensity of P1/intensity of P2) is calculated from the intensity of the strongest diffraction peak P1 among the diffraction peaks that appear within a diffraction angle 2θ range of 20° to 25° and the intensity of the strongest diffraction peak P2 among the diffraction peaks that appear within a diffraction angle _2θ range of 44° to 50°, and is used as a reference. XRD diffraction simulation can be performed using known software.

Next, an XRD diffraction measurement is performed on a metal oxynitride having the same composition as the reference to obtain a diffraction peak pattern. In the obtained diffraction peak pattern, an intensity ratio IR (intensity of P1/intensity of P2) is calculated from the intensities of diffraction peaks P1 and P2, and a ratio A (IR/IR _r ) is calculated when the intensity ratio of the reference is set to 100%, thereby making it possible to evaluate whether or not a region causing centrosymmetric diffraction and a region causing non-centrosymmetric diffraction are present in the metal oxynitride.

The intensity ratio of the diffraction peaks also changes depending on the orientation, but since the intensity ratio IR is not due to orientation, the intensity ratio IR (P1 intensity/P2 intensity) shows the same relationship for diffraction peaks measured from both the out-of-plane direction and the in-plane direction. For example, P1 intensity/P2 intensity < 1 is maintained. On the other hand, when oriented, the relationship is reversed in the out-of-plane direction and in-plane direction. For example, P1 intensity/P2 intensity > 1 in the out-of-plane direction, whereas P1 intensity/P2 intensity < 1 in the in-plane direction.

In this embodiment, when ratio A is 0% or more and 80% or less, it is determined that the metal oxynitride contains regions that produce diffraction exhibiting centrosymmetric properties and regions that produce diffraction exhibiting non-centrosymmetric properties.

Note that the metal oxynitride thin film according to this embodiment is preferably non-oriented. In this embodiment, whether or not it is non-oriented is determined as follows. First, two orthogonal planes are selected in the metal oxynitride crystal. XRD measurements are performed on the metal oxynitride thin film from both the in-plane and out-of-plane directions, and the peak with the greater intensity in the in-plane XRD measurement is used as the reference, and the ratio B1 of the smaller peak intensity to the reference peak intensity of 100% (<100% since it is the smaller one) is calculated. Next, in the out-of-plane XRD measurement, the ratio B2 of the other peak intensity to the peak intensity of 100% at the position used as the reference in the in-plane XRD measurement is calculated. Then, if the difference between the two (|B1-B2|) is within 80%, the thin film is determined to be non-oriented.

To give a specific example, in an in-plane XRD measurement, if the peak intensity of the (220) plane is 100% and the peak intensity of the (002) plane is 80%, and in an out-of-plane XRD measurement, the peak intensity of the (220) plane is 100% and the peak intensity of the (002) plane is 120%, then the difference in peak intensity of the (002) plane is 40% (120% - 80%). Therefore, in this case, the metal oxynitride thin film is determined to be non-oriented.

In addition, the positions of light elements can be evaluated by using neutron diffraction. In a crystal structure refined by neutron diffraction measurement, when ABO ₂ N is SrTaO ₂ N or (SrLa)TaO ₂ N, it is known that a structure having centrosymmetrical properties belongs to the space group I4/mcm (Table No. 140), and a structure having non-centrosymmetrical properties due to a cis-ordered structure belongs to the space group Pmc2 ₁ (Table No. 26).

When ABO ₂ N is BaTaO ₂ N or (BaLa)TaO ₂ N, it is known that the structure having centrosymmetric property belongs to the space group Pm-3m (Table No. 221), and the structure having non-centrosymmetric property due to the cis-configuration ordered structure belongs to the space group Pmc2 ₁ (Table No. 26).

(2. Manufacturing method of thin film capacitor)
Next, an example of a method for manufacturing the thin film capacitor 1 shown in FIG. 1 will be described below.

First, prepare a substrate 51. When using, for example, a Si single crystal substrate as the substrate 51, an insulating layer (e.g., SiO ₂ ) is formed on one main surface of the substrate. The insulating layer may be formed by a known film formation method such as a thermal oxidation method or a CVD (Chemical Vapor Deposition) method.

Next, a thin film of the material that will form the first electrode is formed on the insulating layer using a known film formation method, such as sputtering, to form the first electrode 52.

Next, a metal oxynitride thin film 53 is formed on the first electrode 52 using a known film formation method.

Examples of known film formation methods include vacuum deposition, sputtering, pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MOCVD), metal organic decomposition (MOD) or the sol-gel method, and chemical solution deposition (CSD).

Examples of film-forming raw materials include various target materials, deposition materials, and organometallic materials. Film-forming raw materials may contain trace amounts of impurities and secondary components, but this is not a problem as long as they do not significantly impair the effects of the present invention.

Among these film formation methods, the well-known vapor phase growth method is preferred, in which the film formation raw material is first separated or excited at the atomic or molecular level, and then deposited on a substrate to form a film. In the following, a method for forming a metal oxynitride thin film using a sputtering method as a vapor phase growth method will be described.

When forming a metal oxynitride thin film using a sputtering method, a target having a specified composition is used as the film formation raw material. Furthermore, if some of the A-site atoms, barium and/or strontium, are replaced with lanthanum, the perovskite structure is likely to be distorted. Therefore, by using a target containing a specified proportion of lanthanum as the A-site atom, it becomes easier to reduce the symmetry in part of the crystal structure and form a region that produces non-centrosymmetric diffraction in a region that produces centrosymmetric diffraction.

During film formation, it is preferable to heat the substrate in order to crystallize the thin film. The substrate temperature during film formation may be determined according to the constituent elements and composition of the thin film, but in this embodiment, in order to keep the intensity ratio within the above range, the substrate temperature is preferably within the range of 500°C to 800°C. If the substrate temperature is too low, the thin film tends not to crystallize, and if the substrate temperature is too high, nitrogen tends to be desorbed.

In order to keep the intensity ratio within the above range, it is preferable that the high frequency power (applied power) is within the range of 50 W to 300 W. If the high frequency power is too small, the film formation rate tends to decrease significantly. On the other hand, if the high frequency power is too large, abnormal grain growth tends to cause the characteristics to decrease.

When an oxide is used as the target, the atmospheric gas during film formation contains at least oxygen gas and nitrogen gas. By using such an atmospheric gas, the insulating properties of the resulting thin film are ensured, and a thin film as a dielectric is obtained. In this embodiment, in order to keep the intensity ratio within the above range, the oxygen gas pressure is preferably in the range of 0.01 Pa or more and 0.1 Pa or less. By keeping the oxygen gas pressure within the above range, a decrease in symmetry is more likely to occur in the crystal, making it easier to keep the intensity ratio within the above range. Similarly, the nitrogen gas pressure is preferably in the range of 0.01 Pa or more and 0.1 Pa or less.

It is also a good idea to introduce an inert gas such as Ar gas separately from the atmospheric gas. Since Ar gas is used to knock atoms out of the target, it is a good idea to introduce the amount necessary to obtain a pressure at which the plasma can be maintained stably. Note that the state at which the plasma can be maintained stably depends on the input voltage and Ar gas pressure, so these can be adjusted according to the size of the chamber and the size of the target.

In addition, in this embodiment, during film formation, the metal oxynitride may be formed directly on the substrate without passing through a metal oxide thin film, or a metal oxide thin film may be formed and nitrogen may be introduced into the crystal structure of the formed metal oxide thin film to obtain a metal oxynitride thin film. In either case, the nitriding process can be performed by introducing nitrogen radicals into the film formation chamber during metal oxide film formation, by using reactive sputtering using nitrogen gas, or by using nitrogen activated by plasma nitridation.

This method is preferable because it allows the deposition of a metal oxynitride thin film without using toxic gases. It is also possible to use partial oxidation treatment of the metal nitride thin film. In this embodiment, it is preferable to obtain a metal oxynitride by introducing nitrogen to be used for nitridation when depositing the film using a metal oxide raw material.

The metal oxynitride thin film obtained in this way is a deposited film that acts as a dielectric.

Next, in this embodiment, a thin film of a material that constitutes the upper electrode is formed on the formed metal oxynitride thin film 53 using a known film formation method to form the second electrode 54.

Through the above steps, a thin-film capacitor 1 is obtained in which a capacitor portion (second electrode 52, metal oxynitride thin film 53, and second electrode 54) is formed on a substrate 51, as shown in FIG. 1.

(3. Summary of the present embodiment)
In this embodiment, the metal oxynitride has a region that generates centrosymmetric diffraction and a region that generates non-centrosymmetric diffraction. The region that generates centrosymmetric diffraction is composed of a non-centrosymmetric arrangement, and when viewed microscopically, spontaneous polarization occurs due to the non-centrosymmetric arrangement, but the non-centrosymmetric arrangement exists in a random direction, so the spontaneous polarization that occurs is canceled. As a result, despite being composed of a non-centrosymmetric arrangement, centrosymmetric diffraction occurs. Therefore, since no spontaneous polarization occurs in the entire region, the region is paraelectric.

On the other hand, there are regions in which the non-centrosymmetric arrangement is ordered and is large enough for spontaneous polarization to occur, and in these regions, spontaneous polarization occurs due to the displacement of two types of anions with different bond lengths. Therefore, these regions are ferroelectric.

As a result, due to the presence of both regions that produce centrosymmetric diffraction and regions that produce non-centrosymmetric diffraction, the metal oxynitride as a whole exhibits ferroelectricity while having small dielectric loss. Therefore, it is suitable for applications that utilize ferroelectricity, while also being able to combine the small dielectric loss that is a characteristic of paraelectrics.

In addition, since the polarization caused by non-centrosymmetrical crystals is realized by the displacement of two types of anions, the distortion of the crystal is smaller than that caused by the displacement of cations. Therefore, it is easy to achieve both ferroelectricity and dielectric loss.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and may be modified in various ways within the scope of the present invention.

The present invention will be described in more detail in the following examples. However, the present invention is not limited to the following examples.

(Experiment 1)
First, strontium carbonate (SrCO ₃ ), barium carbonate (BaCO ₃ ), lanthanum oxide (La ₂ O ₃ ), titanium oxide (TiO ₂ ), and tantalum oxide (Ta ₂ O ₅ ) were prepared as target materials for film formation. The prepared materials were weighed to have the compositions shown in Table 1 (Examples 1 to 14 and Comparative Examples 1 to 5). The weighed materials were mixed with ethanol as a solvent in a wet ball mill for 16 hours. The resulting mixed slurry was dried at 80°C for 12 hours in a thermostatic dryer. The resulting mixture was lightly crushed in a mortar, placed in a ceramic crucible, and heat-treated in an electric furnace at 1000°C in the air for 2 hours to obtain a calcined product.

The calcined product was ground in a wet ball mill with ethanol as a solvent for 16 hours, and the ground slurry was dried in a thermostatic dryer at 80°C for 12 hours to obtain a ground product. A polyvinyl alcohol solution was added as a binder to the ground product in an amount of 0.6% by weight (calculated as the solid matter in the solution) and mixed to obtain a granulated product. The granulated product was molded into a cylindrical shape with a diameter of approximately 23 mm and a height of approximately 9 mm to obtain a molded product. The molded product was fired in an electric furnace in an air atmosphere at 1400°C for 2 hours to obtain a sintered body. The top and bottom surfaces of the obtained sintered body were mirror-polished to obtain a film-forming target with a height of 5 mm.

The film-forming target obtained as described above was placed in a film-forming apparatus, and then a Si substrate having a Pt film as a first electrode on its surface was placed facing the target. A metal oxynitride thin film was formed to a thickness of 500 nm by a sputtering method using nitrogen radicals. Ar, _N2 , and _O2 were used as atmospheric gases.

The deposition conditions were a substrate temperature of 600°C, oxygen gas pressure and nitrogen gas pressure of 0.01 Pa, and plasma applied power of 150 W. For Examples 1 to 14 and Comparative Examples 2 to 5, the total pressure adjusted with Ar was 2 Pa or more, and for Comparative Example 1, the total pressure adjusted with Ar was 1 Pa or less.

Next, a bulk sample was prepared as follows. First, strontium carbonate ( _SrCO3 ) powder and tantalum oxide ( _Ta2O5 ) powder were prepared as raw material _powders . The prepared powders were weighed so as to obtain a metal oxynitride represented by the composition formula _SrTaO2N .

The weighed SrCO ₃ powder and Ta ₂ O ₅ powder were wet mixed with ethanol and calcined twice in air at 1200° C. for 10 hours to obtain an oxide precursor having Sr and Ta by a solid-phase reaction.

Subsequently, the obtained oxide precursor was subjected to nitriding treatment twice to obtain a metal oxynitride powder represented by the composition formula SrTaO ₂ N. In the nitriding treatment, the NH ₃ supply rate was set to 100 ml/min, the heating temperature was set to 1000° C., and the heating time was set to 20 hours.

The obtained metal oxynitride powder was wet-pulverized in a ball mill for 16 hours and dried. 0.6% by mass of polyvinyl alcohol was added as a binder to 100% by mass of the dried metal oxynitride powder, and the mixture was granulated to obtain a granulated powder. The obtained granulated powder was placed in a mold and press-molded to obtain a molded body.

The obtained compact was sintered under _N2 atmosphere to obtain a bulk metal oxynitride sintered body. The sintering conditions were a holding temperature of 1400°C and a holding time of 16 hours.

The obtained metal oxynitride thin film and the fired metal oxynitride body were subjected to XRD measurement by the method described below to obtain X-ray diffraction charts. In the obtained X-ray diffraction charts, the strongest diffraction peak appearing in the range of 2θ from 20° to 25° was the diffraction peak of the (002) plane of ABO _x N _y , and the strongest diffraction peak appearing in the range of 2θ from 44° to 50° was the diffraction peak of the (220) plane of ABO _x N _y . The intensity (intensity ratio) of the diffraction peak of the (002) plane relative to the intensity of the diffraction peak of the (220) plane was calculated.

On the other hand, in ABO _x N _y consisting of only a structure having centrosymmetricity, the intensity (intensity ratio) of the diffraction peak of the (002) plane to the diffraction peak of the (220) plane was 20.4/21.9=0.93, assuming the intensity of the main peak to be 100. Using this value as a reference (100%), the ratio of the intensity ratio in each example and each comparative example was calculated. In this example, when the ratio of the intensity ratio was 80% or less, it was determined that there were regions in ABO _x N _y that caused diffraction exhibiting centrosymmetricity and regions that caused diffraction exhibiting non-centrosymmetricity. The results are shown in Table 1.

The intensity ratio of the bulk sintered metal oxynitride was 100%. In other words, the bulk sintered metal oxynitride was composed only of structures with central symmetry. In addition, sintered metal oxynitride bodies were produced by changing the composition of the metal oxynitride, but the intensity ratio of all samples was 100%.

The dielectric constant and piezoelectric response of the resulting metal oxynitride thin film were then measured using the methods described below.

(Dielectric constant)
The dielectric constant was evaluated as follows. First, Ag was evaporated to a diameter of 100 μm on the metal oxynitride thin film of the obtained sample to form an upper electrode. Next, an AC voltage was applied to the sample on which the upper electrode was formed at a reference temperature of 25° C. and a frequency of 1 kHz so that the electric field strength applied to the sample on which the upper electrode was formed was 0.5 Vrms/μm, and the dielectric constant (unitless) was calculated from the electrostatic capacitance measured and the thickness of the metal oxynitride thin film as a dielectric. In this example, samples with a dielectric constant of 150 or more were judged to be good. The results are shown in Table 1. This is because, for example, when a crystal equivalent to Sr ₂ Ta ₂ O ₇ , which is outside the target composition, is formed in SrTaO ₂ N, the dielectric constant obtained is about 50, and the dielectric constant does not exceed 100 even in consideration of the variation in sample and composition. If the relative dielectric constant is 100 or more, it can be determined that a crystal similar to SrTaO ₂ N has been formed, and if the relative dielectric constant is 150 or more, it can be determined that a particularly desirable crystal structure has been formed.

(Piezoelectric response)
The piezoelectric response was measured using a Piezo-Response Microscope (PRM), which is one of the modes of AFM.

A specific voltage was applied to the surface of the obtained metal oxynitride thin film to polarize the surface, creating the polarization pattern shown in Figure 5. Specifically, a voltage of -1V was applied to X1 = 10 μm square. Then, or simultaneously, a voltage of +1V was applied to the central X2 = 7 μm square. Then, or simultaneously, a voltage of -1V was applied to the central X3 = 5 μm square. Then, or simultaneously, a voltage of +1V was applied to the central X4 = 3 μm square. Then, or simultaneously, a voltage of -1V was applied to the central X5 = 1 μm square.

A voltage was then applied to the polarization pattern shown in Figure 5 at a scanning frequency of 0.5 Hz and an excitation voltage (external AC electric field) of -0.3 V to +0.3 V to evaluate whether or not the displacement due to remnant polarization could be measured by PRM. If the displacement could be measured, the metal oxynitride was responding piezoelectrically. The results are shown in Table 1.

From Table 1, it was confirmed that when there are regions that produce centrosymmetric diffraction and regions that produce non-centrosymmetric diffraction, and the specified composition and average ionic valence are satisfied, the metal oxynitride thin film as a whole has remnant polarization, i.e., it has ferroelectricity.

REFERENCE SIGNS LIST 1 thin film capacitor 51 substrate 52 first electrode 53 metal oxynitride thin film 54 second electrode 15 octahedron 11 oxygen 12 nitrogen 13 B site atom 14 A site atom

Claims (5)

A metal oxynitride thin film comprising a metal oxynitride having a perovskite structure and represented by the general formula ABO _x N _y ,
A is at least one selected from the group consisting of barium, strontium, and lanthanum, and B is at least one selected from the group consisting of tantalum and titanium,
When the ionic valence indicated when A occupies the A site in the perovskite structure is a and the ionic valence indicated when B occupies the B site in the perovskite structure is b, the a and b satisfy the relationship 6.7≦a+b≦7.3,
In the general formula, x and y are
2.000≦x≦3.275,
The relationship 0.150≦y≦1.000 is satisfied,
the metal oxynitride has a region that produces centrosymmetric diffraction and a region that produces non-centrosymmetric diffraction,
a perovskite metal oxynitride thin film which is composed only of regions which produce diffraction exhibiting centrosymmetricity and has the same composition as the metal oxynitride thin film, wherein, in an X-ray diffraction pattern obtained by powder X-ray diffraction using Cu-Kα radiation as an X-ray source or powder X-ray diffraction simulation, among the diffraction peaks which appear within a diffraction angle 2θ range of 20° to 25° and which are attributed to the perovskite structure, the strongest diffraction peak is designated as P1, and among the diffraction peaks which appear within a diffraction angle 2θ range of 44° to 50° and which are attributed to the perovskite structure, the strongest diffraction peak is designated as P2, and an intensity ratio of P1 to the intensity of P2 is designated as IR _r , and in an X-ray diffraction pattern of the metal oxynitride thin film obtained by perpendicular X-ray diffraction and in-plane X-ray diffraction using Cu-Kα radiation as an X-ray source, the intensity ratio of P1 to the intensity of P2 is designated as IR, wherein a ratio A of IR to IR _r is 0% or more and 80% or less, and the metal oxynitride thin film has remanent polarization . 2. The metal oxynitride thin film according to claim 1 , wherein the region producing centrosymmetric diffraction and the region producing non-centrosymmetric diffraction have different ranges of ordered arrangement of anions. 3. The metal oxynitride thin film of claim 2 , wherein the region exhibiting centrosymmetric diffraction and the region exhibiting non-centrosymmetric diffraction have the same cations. The x and y are
2.450≦x≦3.275,
4. The metal oxynitride thin film according to claim 1, which satisfies the relationship: 0.150≦y≦0.700. A capacitive element comprising the metal oxynitride thin film according to claim 1 .

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