Dielectric Elements

Abstract

To provide a dielectric element comprising a dielectric film where the rate of electrostatic capacity change in DC bias voltage application is small.SOLUTION: The present invention relates to a dielectric element in which a first electrode, a dielectric film and a second electrode are laminated. The dielectric film includes a columnar crystal extending in a lamination direction. In the dielectric element, when an inverse of a dielectric film thickness is defined as x (1 / μm) and an average crystallite diameter calculated from a Scherrer formula regarding a dielectric material is defined as y (nm), x and y are inside of a rectangle with a point A (0.10, 5), a point B (10, 20), a point C (10, 35) and a point D (0.10, 10) as apexes. In the dielectric element, an average value of columnar crystal widths within a distance range of 1 / 10 or more and 1 / 3 or less of the dielectric film thickness from one electrode to the other electrode in the lamination direction is less than 40 nm and an average value of columnar crystal widths within a distance range of 1 / 10 or more and 1 / 3 or less of the dielectric film thickness from the other electrode to the one electrode is equal to or more than 40 nm.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a dielectric element, and more particularly to a dielectric element such as a thin-film capacitor having a thin dielectric film. [Background technology]

[0002] In recent years, as electronic devices have become smaller and more powerful, electronic circuits have become increasingly dense and highly integrated. Higher performance and smaller dielectric elements are also desired for the large numbers of dielectric elements mounted on these electronic circuits. Thin-film capacitors, one example of such dielectric elements, are desired to have higher capacitance.

[0003] One known method for achieving high capacitance in thin film capacitors is to use columnar crystals as the crystals that make up the dielectric.

[0004] Patent Document 1 discloses a dielectric element having a dielectric film in which a plurality of dielectric layers made of columnar crystals are stacked in a direction perpendicular to the substrate surface, and the columnar crystals contained in adjacent dielectric layers are discontinuous with each other at the stacking interface. Patent Document 1 states that such a dielectric element can achieve improved leakage characteristics and higher capacitance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-212346 Summary of the Invention [Problem to be solved by the invention]

[0006] A constant direct current voltage (DC bias voltage) is applied to electronic components such as ICs mounted on electronic circuits to ensure proper operation of the electronic components. Therefore, the same DC bias voltage is also applied to dielectric elements such as thin-film capacitors mounted on electronic circuits.

[0007] However, due to the composition, structure, etc. of the dielectric material that constitutes the dielectric element, the capacitance of the dielectric element may change when a DC bias voltage is applied. In particular, as the DC bias voltage increases, the capacitance decreases more significantly, and the dielectric element tends to be unable to fully demonstrate its expected performance. Therefore, a dielectric element is required to have a small rate of change in capacitance when a DC bias voltage is applied.

[0008] However, the dielectric element described in Patent Document 1 has a problem in that the capacitance is highly dependent on the DC bias, and the rate of change in capacitance when a DC bias voltage is applied is large.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a dielectric element having a dielectric film that exhibits a small rate of change in capacitance when a DC bias voltage is applied. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention is embodied as follows. [1] A dielectric element having a structure in which a first electrode, a dielectric film, and a second electrode are stacked, the dielectric film includes columnar crystals extending in the stacking direction of the structure, When the reciprocal of the thickness of the dielectric film is x (1 / μm) and the average crystallite diameter of the dielectric material constituting the dielectric film calculated using the Scherrer equation is y (nm), x and y are the dielectric elements on the (x, y) plane that lie on the line connecting point A (0.10, 5), point B (10, 20), point C (10, 35), and point D (0.10, 10) in that order, and within the area surrounded by this line.

[0011] [2] A dielectric element having a structure in which a first electrode, a dielectric film, and a second electrode are laminated, the dielectric film includes columnar crystals extending in the stacking direction of the structure, This dielectric element has a structure in which, in the stacking direction, the average width of the columnar crystals within a distance range from one electrode to the other electrode that is at least one-tenth and at most one-third of the thickness of the dielectric film is less than 40 nm, and the average width of the columnar crystals within a distance range from the other electrode to the one electrode that is at least one-tenth and at most one-third of the thickness of the dielectric film is 40 nm or more.

[0012] [3] The dielectric element according to [2], wherein the thickness of the dielectric film is 0.1 μm or more and 10 μm or less.

[0013] [4] The dielectric film contains a complex oxide having a perovskite structure and represented by the chemical formula ABO3, In the chemical formula, A is at least one selected from the group consisting of barium, calcium, strontium, and lead, and B is at least one selected from the group consisting of titanium, zirconium, tin, and hafnium.

[0023] The dielectric element according to any one of [1] to [3].

[0014] [5] A dielectric element according to any one of [1] to [4], wherein the dielectric film has a void region containing minute voids between at least one of the first electrode and the second electrode and the columnar crystal. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a dielectric element having a dielectric film with a small rate of change in capacitance when a DC bias voltage is applied. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view of a thin film capacitor as a dielectric element according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of a thin film capacitor for explaining columnar crystals contained in the dielectric film. [Figure 3] FIG. 3 is a graph showing the relationship between the reciprocal of the thickness of the dielectric film and the average crystallite diameter of the dielectric material contained in the dielectric film. [Figure 4] FIG. 4 is an electron microscope image of the sample according to the example, in which the width of the columnar crystals was measured. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below based on specific embodiments with reference to the drawings in the following order.

[0018] 1. Thin film capacitors 1.1 Overall structure of a thin film capacitor 1.2 Dielectric film 1.3 First electrode 1.4 Second electrode 2. Manufacturing method of thin film capacitors

[0019] (1. Thin-film capacitors) First, a thin film capacitor having a thin dielectric film will be described as an example of the dielectric element according to this embodiment.

[0020] (1.1 Overall structure of a thin film capacitor) As shown in FIG. 1, a thin-film capacitor 1, which is an example of a dielectric element according to this embodiment, has a configuration in which a first electrode 10, a dielectric film 30, and a second electrode 20 are stacked in this order.

[0021] When the first electrode 10 and the second electrode 20 are connected to an external circuit and a voltage is applied, the dielectric film 30 exhibits a predetermined capacitance and functions as a capacitor. A detailed description of each component will be provided later. To distinguish the top and bottom of the thin-film capacitor, one electrode may be designated as the upper electrode and the other as the lower electrode. For example, in FIG. 1, the first electrode 10 is the lower electrode and the second electrode 20 is the upper electrode.

[0022] There are no particular restrictions on the shape of the thin film capacitor, but it is usually a rectangular parallelepiped. There are also no particular restrictions on its dimensions, and the thickness and length may be set to appropriate dimensions depending on the application.

[0023] (1.2 Dielectric Film) In this embodiment, the dielectric 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 dielectric film is preferably a dielectric deposited film. Therefore, the dielectric film according to this embodiment does not include a sintered body (obtained by a solid-state reaction) obtained by firing a compact formed from a dielectric raw material powder.

[0024] In this embodiment, the dielectric film includes columnar crystals. As shown in FIG. 2, in a configuration in which the first electrode 10, the dielectric film 30, and the second electrode 20 are stacked, the columnar crystals 31 are crystals extending along the stacking direction D1. That is, the columnar crystals 31 are crystal grains whose length along the stacking direction D1 is longer than their length along a direction D2 perpendicular to the stacking direction D1. The aspect ratio of the columnar crystals 31 depends on the thickness of the dielectric film 30, but is, for example, 2 or greater. Furthermore, the columnar crystals 31 do not have to be perpendicular to the first electrode 10 and the second electrode 20, and may be inclined, for example, by approximately ±5°.

[0025] The dielectric film 30 includes the columnar crystals 31, which can increase the capacitance of the dielectric element. In this embodiment, the proportion of the columnar crystals 31 in the dielectric film 30 is preferably 50% or more, and more preferably 90% or more. The proportion of the columnar crystals 31 can be calculated, for example, as the area proportion occupied by the columnar crystals in a cross section of the dielectric film taken along the stacking direction of the dielectric element.

[0026] As mentioned above, the capacitance of a dielectric element can change when a DC bias voltage is applied. This is because the dielectric material that makes up the dielectric film is made of a ferroelectric material that generates spontaneous polarization.

[0027] When a DC bias voltage is applied to a dielectric element, the direction of spontaneous polarization begins to align according to the strength of the DC electric field. As the DC bias voltage increases (as the DC electric field strength increases), the polarization saturates and the spontaneous polarization becomes constrained. In this state, the spontaneous polarization is less likely to reverse even when an AC voltage is applied as a signal to an electronic circuit. In other words, the capacitance decreases.

[0028] If the decrease in capacitance becomes large, the effective capacitance of the dielectric element when in use will decrease relative to the standard value of the capacitance of the dielectric element.

[0029] In order to suppress such a decrease in effective capacitance, in this embodiment, attention is paid to the thickness of the dielectric film and the crystallite diameter of the dielectric material (particularly the columnar crystals) that constitute the dielectric film, and these are controlled within a predetermined range.

[0030] If the reciprocal of the thickness of the dielectric film is x (1 / μm) and the average crystallite diameter of the dielectric material is y (nm), in this embodiment, x and y are on the line connecting point A (0.10, 5), point B (10, 20), point C (10, 35), and point D (0.10, 10) shown in Figure 3 in that order in the (x, y) plane, and are within the area surrounded by this line (the hatched area in Figure 3).

[0031] By setting x and y within the range shown in FIG. 3, the rate of change in capacitance when a DC bias voltage is applied can be reduced.

[0032] Even with the same DC bias voltage, the electric field strength per unit thickness applied to the dielectric varies depending on the thickness of the dielectric film, so the rate of change in capacitance is affected by the thickness of the dielectric film. Therefore, the thickness of the dielectric film is set within the above range.

[0033] The average crystallite size is also considered to correspond to the size of the region (domain) where spontaneous polarization occurs. If the average crystallite size is large, the domain will be large and the capacitance will be high, but the influence of the DC bias voltage will be large (DC bias dependency will be high). Therefore, the average crystallite size is set to be within the above range.

[0034] In this embodiment, the average crystallite diameter D (nm) of the dielectric material is obtained from the Scherrer equation shown below based on XRD measurement. D=Kλ / Bcosθ In the formula, K is the Scherrer constant, λ is the wavelength of the X-ray (nm), B is the full width at half maximum of the diffraction peak, and θ is the Bragg angle (rad). The full width at half maximum of the diffraction peak can be calculated by fitting a predetermined diffraction peak with a known fitting function.

[0035] In this embodiment, the thickness of the dielectric film is preferably 0.1 μm or more, more preferably 0.2 μm or more, while the thickness of the dielectric film is preferably 10.0 μm or less, more preferably 5.0 μm or less.

[0036] In this embodiment, a method different from the above may be adopted to suppress the decrease in effective capacitance. Specifically, the structure of the columnar crystals is controlled to suppress the decrease in effective capacitance.

[0037] In this embodiment, the average width of the columnar crystals within a distance range of one-tenth to one-third of the thickness of the dielectric film from one electrode to the other electrode in the stacking direction of the dielectric element is less than 40 nm. It is preferably 30 nm or less, and more preferably 25 nm or less. Furthermore, the average width of the columnar crystals within a distance range of one-tenth to one-third of the thickness of the dielectric film from the other electrode to the one electrode is 40 nm or more, and more preferably 45 nm or more.

[0038] That is, the length of the columnar crystals in a direction D2 perpendicular to the stacking direction D1 (the width of the columnar crystals) changes in the stacking direction D1. In Fig. 2, the columnar crystals are thinner on the first electrode 10 side and thicker on the second electrode 20 side. By controlling the columnar crystal structure as described above, it is possible to reduce the rate of change in capacitance when a DC bias voltage is applied.

[0039] The average width of the columnar crystals can be calculated as follows. As shown in Figure 2, first, a straight line L1 is drawn in the stacking direction D1 at a location within a distance of at least one-tenth but not more than one-third of the dielectric film thickness from one electrode. Similarly, a straight line L2 is drawn in the stacking direction D1 at a location within a distance of at least one-tenth but not more than one-third of the dielectric film thickness from the other electrode. Next, the number of grain boundaries of the columnar crystals within predetermined lengths L1a and L2a on the lines L1 and L2, whose end points are located on the grain boundaries, is counted. The average width of the columnar crystals can be calculated by dividing the predetermined lengths L1a and L2a by the number of grain boundaries.

[0040] In this case, the thickness of the dielectric film 30 is preferably 0.1 μm or more and 10 μm or less, and more preferably 0.2 μm or more and 5.0 μm or less.

[0041] The thickness of the dielectric film 30 can be measured by processing a thin film capacitor including the dielectric film 30 using a FIB (focused ion beam) processing device and observing the resulting cross section using a scanning electron microscope (SEM), transmission electron microscope (TEM), etc.

[0042] In order to obtain a high capacitance, the dielectric material constituting the dielectric film preferably contains a complex oxide represented by the chemical formula ABO3 and having a perovskite structure as its main component. In this embodiment, the main component is preferably contained in an amount of 80 mol % to 100 mol % of the entire dielectric film (100 mol %). Furthermore, the columnar crystals preferably contain the above-mentioned complex oxide as their main component.

[0043] In the above chemical formula, "A" is preferably at least one selected from the group consisting of barium (Ba), calcium (Ca), strontium (Sr) and lead (Pb), and "B" is preferably at least one selected from the group consisting of titanium (Ti), zirconium (Zr), tin (Sn) and hafnium (Hf).

[0044] The dielectric film may contain a secondary component in addition to the main component (ABO3). The secondary component is not particularly limited as long as it is a component generally contained in a dielectric material having a perovskite structure. In this embodiment, examples of the secondary component include magnesium (Mg), manganese (Mn), vanadium (V), yttrium (Y), and gadolinium (Gd). The content of the secondary component may be a known amount.

[0045] 2, the dielectric film 30 may have a void region 32 containing fine voids. The void region 32 is a region where fine voids are concentrated. The void region 32 is a region where voids where no dielectric material is present are formed, and therefore has a lower apparent dielectric constant than the region where the columnar crystals 31 are formed. Therefore, when the dielectric film 30 includes the void region 32, although the capacitance of the dielectric film is slightly reduced, the DC bias dependency of the capacitance can be alleviated.

[0046] In this embodiment, the void region 32 is a layered region extending along a direction D2 perpendicular to the stacking direction D1. The void region 32 is preferably formed between at least one of the first electrode and the second electrode and the columnar crystals 31, and more preferably between the first electrode 10 or the second electrode 20 and the columnar crystals 31. In FIG. 2, the void region 32 is formed between the first electrode 10 and the columnar crystals 31.

[0047] The proportion of void regions 32 in the dielectric film 30 is preferably 5% or more and 50% or less, and more preferably 5% or more and 30% or less. The proportion of void regions 32 can be calculated, for example, as the area proportion occupied by void regions in a cross section of the dielectric element cut along the stacking direction. The void regions 32 can be identified, for example, by dividing the dielectric film 30 into multiple layers along a plane parallel to a direction D2 perpendicular to the stacking direction D1, and determining whether the proportion of voids in these layers is equal to or greater than a predetermined proportion (for example, 5%).

[0048] (1.4. First Electrode) As shown in FIG. 1, the first electrode 10 sandwiches the dielectric film 30 with the second electrode 20 (described later) to function as a capacitor. The material constituting the first electrode 10 is not particularly limited as long as it is conductive and can withstand heat treatment of the dielectric film. Examples of the material include metals such as gold (Au), platinum (Pt), silver (Ag), iridium (Ir), ruthenium (Ru), cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), and aluminum (Al), as well as alloys thereof; semiconductors such as silicon (Si), GaAs, GaP, InP, and SiC; and conductive metal oxides such as ITO, ZnO, and SnO.

[0049] In this embodiment, the first electrode also serves as a substrate on which the dielectric film is formed, and is preferably in the form of a plate (foil), which not only allows the thin-film capacitor to be made thinner, but also makes it easier to mount the thin-film capacitor on an electronic circuit board.

[0050] The thickness of the first electrode is not particularly limited as long as it is thick enough to function as an electrode. In this embodiment, the thickness is preferably 0.01 μm or more. When the first electrode also serves as a substrate, the thickness is preferably 5 μm or more and 100 μm or less, and more preferably 10 μm or more and 50 μm or less. If the thickness of the first electrode is too small, it tends to be difficult to handle the first electrode during the manufacture of the thin-film capacitor 1.

[0051] (1.5. Second Electrode) 1, a second electrode 20 is formed as a thin film on the surface of the dielectric film 30 as an upper electrode. The second electrode 20, together with the first electrode 10 described above, sandwiches the dielectric film 30 and functions as a capacitor. Therefore, the second electrode 20 exhibits a polarity different from that of the first electrode 10.

[0052] Like the first electrode, the material constituting the second electrode is not particularly limited as long as it is conductive and can withstand heat treatment of the dielectric film, etc. Examples include metals such as gold (Au), platinum (Pt), silver (Ag), iridium (Ir), ruthenium (Ru), cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), and aluminum (Al), or alloys thereof; semiconductors such as silicon (Si), GaAs, GaP, InP, and SiC; and conductive metal oxides such as ITO, ZnO, and SnO2.

[0053] The thickness of the second electrode is not particularly limited as long as it is thick enough to function as an electrode, similar to the first electrode, and in this embodiment, the thickness is preferably 0.01 μm or more.

[0054] (2. Thin-film capacitor manufacturing method) Next, an example of a method for manufacturing the thin film capacitor 1 shown in FIG. 1 will be described below.

[0055] First, prepare the first electrode 10. In this embodiment, a plate (foil) made of the above-mentioned material is prepared as the first electrode 10. Since the dielectric film and the second electrode are formed on the foil, the foil also serves as the lower electrode and substrate.

[0056] Next, a material constituting the dielectric film is deposited on the first electrode 10 to form the dielectric film 30. In this embodiment, the dielectric film 30 is formed using a known film formation method as follows. First, a dielectric film thinner than the desired thickness is formed (dielectric film partial formation step), and then a heat treatment is performed to promote crystallization of the formed dielectric film (heat treatment step). Then, the dielectric film partial formation step and the heat treatment step are repeated a predetermined number of times to form the dielectric film 30 having the desired thickness on the first electrode 10.

[0057] By repeating the partial dielectric film formation step and the heat treatment step a predetermined number of times to form a dielectric film, the reciprocal of the thickness of the dielectric film and the average crystallite diameter tend to fall within the ranges shown in Figure 2. In addition, the width of the formed columnar crystals tends to be larger on one electrode side (the second electrode side) and smaller on the other electrode side (the first electrode side).

[0058] The thickness of the dielectric film formed in one partial dielectric film formation step is about one-tenth to one-half of the thickness of the dielectric film, in other words, the entire dielectric film may be formed by two to ten partial dielectric film formation steps.

[0059] Examples of known film formation methods include vacuum evaporation, sputtering, PLD (pulsed laser deposition), MO-CVD (metal organic chemical vapor deposition), MOD (metal organic decomposition), sol-gel method, and CSD (chemical solution deposition). In this embodiment, sputtering is preferred from the viewpoint of cost, etc.

[0060] Note that the raw materials used during film formation (evaporation materials, various target materials, organometallic materials, etc.) may contain trace amounts of impurities, secondary components, etc., but this does not pose any particular problem as long as the desired dielectric properties are obtained.

[0061] For example, when a sputtering method is used, a target for forming the dielectric film is used to form a dielectric film of a predetermined thickness on a first electrode serving as a substrate. In the partial dielectric film forming step, the film formation conditions are preferably a substrate temperature of 200 to 600°C, a temperature increase rate of 15 to 30°C / min, an atmosphere during film formation preferably containing argon (Ar), oxygen (O2), or the like, and a pressure during film formation preferably of 0.1 to 2.0 Pa. By setting the film formation conditions within the above ranges, the desired columnar crystals are easily obtained. Furthermore, the proportion of void regions can be controlled by controlling the substrate temperature. For example, lowering the substrate temperature within the above range increases the proportion of void regions. After the dielectric film forming step is completed, the temperature is lowered to room temperature. The temperature decrease rate is preferably 10 to 30°C / min.

[0062] When sputtering is used, the heat treatment conditions in the heat treatment step are as follows: the atmosphere is nitrogen (N2), hydrogen (H2), etc., the temperature rise rate is preferably 5 to 20°C / min, the maximum temperature is 400 to 1200°C, and the maximum temperature is maintained for 0.5 to 4 hours. After the heat treatment step is completed, the temperature is lowered to room temperature. The temperature drop rate is preferably 5 to 10°C / min.

[0063] Next, a thin film of a material for forming the second electrode is formed on the formed dielectric film 30 using a known film formation method, thereby forming the second electrode 20.

[0064] Through the above steps, a thin film capacitor 1 is obtained in which the first electrode 10, the dielectric film 30 and the second electrode 20 are formed in this order, as shown in FIG.

[0065] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and may be modified in various ways within the scope of the present invention. [Example]

[0066] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0067] (Experiment 1) The sputtering target required for forming the dielectric film was a sintered body of barium titanate (BaTiO3) obtained by synthesizing barium carbonate (BaCO3) and titanium oxide (TiO2) by a solid phase method.

[0068] Next, a nickel foil with a thickness of 28 μm was prepared as a first electrode. This nickel foil was heated to 400°C. Using the above sputtering target, a dielectric film with a thickness of 150 nm was formed on the nickel foil maintained at 400°C by sputtering (first partial dielectric film formation step). The film formation conditions were as follows: atmosphere: argon (Ar), temperature increase rate: 20°C / min, pressure: 0.4 Pa. The formed dielectric film with a thickness of 150 nm was cooled to room temperature at a temperature decrease rate of 20 to 30°C / min.

[0069] Next, the dielectric film formed on the first electrode was heated to 550°C and held at that temperature for 1 hour (first heat treatment step). The heat treatment conditions were: atmosphere: nitrogen (N2) + hydrogen (H2) concentration 0.3%, temperature rise rate: 10°C / min. After the heat treatment, the dielectric film was cooled to room temperature.

[0070] The above-described dielectric film partial formation step and heat treatment step were repeated three more times to form a dielectric film with a thickness of 600 nm.

[0071] A nickel (Ni) thin film was formed as a second electrode on the resulting 600 nm thick dielectric film by sputtering, thereby obtaining a thin film capacitor sample (sample number 1-1) having the configuration shown in Figure 1.

[0072] For sample No. 1-2, sample No. 1a and sample No. 1b, thin film capacitor samples were obtained in the same manner as for sample No. 1, except that the heat treatment temperatures were set to the temperatures shown in Table 1.

[0073] In addition, thin film capacitor samples (sample numbers 2-1, 2-2, 2a and 2b) were obtained in the same manner as sample numbers 1-1, 1-2, 1a and 1b, except that the process of forming a dielectric film with a thickness of 150 nm was changed to the process of forming a dielectric film with a thickness of 300 nm and the heat treatment temperature was set to the temperature shown in Table 1.

[0074] Similarly, thin-film capacitor samples (sample numbers 3-1, 3-2, 3a, and 3b) were obtained by the same method as sample numbers 1-1, 1-2, 1a, and 1b, except that the step of forming a 150 nm thick dielectric film was changed to the step of forming a 600 nm thick dielectric film and the heat treatment temperature was set to the temperature shown in Table 1. Thin-film capacitor samples (sample numbers 4-1, 4-2, 4a, and 4b) were obtained by the same method as sample numbers 1-1, 1-2, 1a, and 1b, except that the step of forming a 150 nm thick dielectric film was changed to the step of forming a 1200 nm thick dielectric film and the heat treatment temperature was set to the temperature shown in Table 1.

[0075] The thickness of the dielectric film was measured by processing the thin film capacitor with FIB and observing the resulting cross section with a SEM (scanning electron microscope), and the values ​​shown in Table 1 were obtained.

[0076] The obtained thin film capacitor samples were evaluated as follows.

[0077] (crystallite diameter) The obtained thin-film capacitor sample was cut along a plane parallel to the lamination direction, and the cross section of the exposed dielectric film was subjected to XRD measurement using an XRD measurement device (Empyrean, manufactured by Malvern Panalytical) to obtain an X-ray diffraction chart. The diffraction peak of the (001) plane of a composite oxide (BaTiO3) with a perovskite structure was identified as the clearly observed diffraction peak in the obtained X-ray diffraction chart. The Bragg angle (θ) was calculated from the diffraction angle (2θ) of the identified diffraction peak. The full width at half maximum (B) was calculated by fitting the diffraction peak of the (001) plane with a Gaussian function. The calculated Bragg angle and full width at half maximum were substituted into the Scherrer equation below to calculate the average crystallite diameter D (nm). D=Kλ / Bcosθ In the formula, K is the Scherrer constant (0.94 in this example), and λ is the wavelength of the X-ray (nm).

[0078] The XRD measurement conditions were as follows: Cu-Kα1 radiation (wavelength: 1.54060 Å) was used as the X-ray source, the tube voltage was 45 kV, the tube current was 40 mA, and 2θ was in the range of 15 to 85°. The results are shown in Table 1. The relationship between the reciprocal of the thickness of the dielectric film and the average crystallite diameter is plotted in Figure 3.

[0079] (Capacity and rate of change) The capacitance (C) of the thin-film capacitor samples was measured using a digital LCR meter (Keysight E4980A) at a reference temperature of 25°C. A DC bias voltage was applied, and an AC voltage was applied at a frequency of 1 kHz, with an input signal level (measurement voltage) of 1 Vrms. The capacitance per unit area (C / A) was calculated by dividing the measured capacitance by the area (A) of the second electrode. The DC bias voltages applied were 0 V, 1 V, 5 V, 10 V, and 20 V. The results are shown in Table 1.

[0080] The change rate of the capacitance per unit area (C / A) was calculated based on the capacitance per unit area (C / A) when the DC bias voltage was 0 V, i.e., when no DC bias voltage was applied, which was set as the reference (100%). The results are shown in Table 1.

[0081] [Table 1]

[0082] From Table 1 and Figure 3, it was confirmed that when the thickness and crystallite diameter of the dielectric film are within the ranges shown in Figure 3, a dielectric element having a large capacitance and a small rate of change in capacitance when a DC bias voltage is applied can be obtained.

[0083] (Experiment 2) A thin-film capacitor sample (sample number 5) was obtained using the same method as sample number 1-1, except that a sintered (Ba,Ca,Sr)(Ti,Zr)O3 sputtering target was used to form a 100-nm-thick dielectric film instead of a 150-nm-thick one. A thin-film capacitor sample (sample number 6) was obtained using the same method as sample number 1-1, except that a sintered (Ba,Ca,Sr)(Ti,Zr)O3 sputtering target was used to form a 25-nm-thick dielectric film instead of a 150-nm-thick one. The resulting thin-film capacitor samples (sample numbers 5 and 6) were evaluated in the same manner as in Experiment 1. The results are shown in Table 2 and Figure 3. The ratio of Ba to Sr to Ca was Ba:Sr:Ca = 0.90:0.05:0.05, and the ratio of Ti to Zr was Ti:Zr = 0.1:0.9.

[0084] [Table 2]

[0085] From Table 2 and FIG. 3, it was confirmed that the same results as in Experiment 1 were obtained even when the composition of the complex oxide that constitutes the dielectric film was changed.

[0086] (Experiment 3) The thin film capacitor samples of sample numbers 2-1, 2a, and 2b were cut in a plane parallel to the lamination direction, and the cross section of the exposed dielectric film was observed with a scanning transmission electron microscope (STEM) at magnifications of 100,000 to 1,000,000 times to obtain a STEM-HAADF image.

[0087] In the acquired STEM-HAADF image, a line was drawn across the columnar crystals (perpendicular to the stacking direction) at a distance of approximately 300 nm from the first electrode in the stacking direction. Similarly, a line was drawn across the columnar crystals (perpendicular to the stacking direction) at a distance of approximately 200 nm from the second electrode in the stacking direction.

[0088] For each line, the number of grain boundaries included in a specific length, with the end points set at grain boundaries, was calculated, and the specific length was divided by the number of grain boundaries to calculate the columnar crystal width. The results are shown in Table 3. Figure 4 shows STEM-HAADF images in which the columnar crystal widths were calculated on the first electrode side and the second electrode side. The white circles on the white lines in Figure 4 indicate grain boundaries.

[0089] [Table 3]

[0090] From Table 3, it was confirmed that when the columnar crystal width on the first electrode side is 40 nm or less and the columnar crystal width on the second electrode side is more than 40 nm, a dielectric element having a large capacitance and a small rate of change in capacitance when a DC bias voltage is applied can be obtained.

[0091] (Experiment 4) A thin film capacitor sample (sample number 1-3) was obtained in the same manner as sample number 1-2, except that the temperature of the substrate (nickel foil) in the dielectric film partial formation step was set to the temperature shown in Table 1. The obtained thin film capacitor sample (sample number 1-3) was evaluated in the same manner as in Experiment 1. The results are shown in Table 4.

[0092] [Table 4]

[0093] From Table 4, it was confirmed that the DC bias dependency of the dielectric element was slightly alleviated by forming void regions in the dielectric film. [Industrial Applicability]

[0094] The dielectric element according to the present invention has the above-described configuration, and therefore exhibits a small rate of change in capacitance when a DC bias voltage is applied, making it suitable for use as a thin-film capacitor mounted in an electronic circuit, for example. [Explanation of symbols]

[0095] 1. Thin film capacitor 10... First electrode 30... Dielectric film 31...Columnar crystal 32...Void area 20... Second electrode

Claims

1. A dielectric element having a structure in which a first electrode, a dielectric film, and a second electrode are stacked, the dielectric film includes columnar crystals extending in a stacking direction of the structure, When the reciprocal of the thickness of the dielectric film is x (1 / μm) and the average crystallite diameter calculated from the Scherrer formula for the dielectric material constituting the dielectric film is y (nm), x and y are a dielectric element on a line connecting point A (0.10, 5), point B (10, 20), point C (10, 35), and point D (0.10, 10) in this order in the (x, y) plane, and within a region surrounded by the line.

2. A dielectric element having a structure in which a first electrode, a dielectric film, and a second electrode are stacked, the dielectric film includes columnar crystals extending in a stacking direction of the structure, A dielectric element in which, in the stacking direction of the configuration, the average width of the columnar crystals within a distance range from one electrode to the other electrode that is at least one-tenth and at most one-third of the thickness of the dielectric film is less than 40 nm, and the average width of the columnar crystals within a distance range from the other electrode to the one electrode that is at least one-tenth and at most one-third of the thickness of the dielectric film is 40 nm or more.

3. 3. The dielectric element according to claim 2, wherein the thickness of the dielectric film is 0.1 μm or more and 10 μm or less.

4. The dielectric film has the chemical formula ABO 3 and includes a complex oxide having a perovskite structure, 4. A dielectric element according to claim 1, wherein in the chemical formula, A is at least one selected from the group consisting of barium, calcium, strontium, and lead, and B is at least one selected from the group consisting of titanium, zirconium, tin, and hafnium.

5. 5. The dielectric element according to claim 1, wherein the dielectric film has a void region containing minute voids between at least one of the first electrode and the second electrode and the columnar crystal.

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