JP2007294736A - Variable capacitance element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable capacitance element in which a drop is suppressed in Q value due to voltage application at a desired frequency, and also, to provide a variable filter or the like in which insertion losses and phase characteristics are reduced. <P>SOLUTION: The variable capacitance element includes a lower electrode layer, an upper electrode layer, and a dielectric layer that is sandwiched from above and below by the lower/upper electrode layers and has a density larger than that of the upper electrode layer. The phase characteristics of an impedance generated by the voltage application to the lower/upper electrode layers have periodic peaks corresponding to a frequency of a signal to be inputted or outputted. The frequency of the signal is located between the peaks adjacent to each other. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a variable capacitance element that controls the voltage of a capacitance by utilizing the dependency of the dielectric constant of a dielectric layer using a high dielectric constant thin film on the voltage, and in particular, the dielectric loss is small and the voltage is applied in a high frequency operation. Regardless of the variable capacitance element, the dielectric loss is small, that is, the Q value is high.

  As the frequency of radio communication and electrical circuits increases, electronic components used for these electrical signals are also required to be compatible with high frequencies. In particular, thin-film capacitors such as filters and resonators are used in high-frequency circuits. In order to use it as a component, the Q value of the thin film capacitor is required to be high. In addition, with the development of wireless communication technology and switching to a new method, the demand for communication devices compatible with a plurality of transmission / reception systems is increasing. In recent years, development of variable elements such as voltage variable filters and voltage variable capacitance elements (hereinafter simply referred to as variable capacitance elements) has been developed in order to cope with the reduction in the number of parts and miniaturization as well as support for multiple transmission / reception systems. (For example, refer to Patent Document 1).

A variable capacitance element is a thin film capacitor that utilizes a large change in dielectric constant accompanying an applied voltage of a high dielectric constant thin film, and its configuration is, for example, a substrate and a lower electrode made of a metal thin film formed thereon. A high dielectric constant thin film formed thereon, an upper electrode formed thereon, and the like. As the lower electrode, platinum (Pt) is used to improve the crystallinity of the high dielectric constant thin film formed thereon, and as the upper electrode, gold (Au), which is low resistance and hardly oxidizes, is used. . As the high dielectric constant thin film, for example, strontium titanate (SrTiO _3), barium strontium titanate _{(Ba x Sr 1-x Ti} y O 3, less BST), lead zirconate titanate _(Pb (Zr x _{Ti 1-} A dielectric having a perovskite crystal structure such as _x ) O ₃ ) is used. Perovskite oxides such as SrTiO ₃ and BST exhibit a high dielectric constant near room temperature, and SrTiO ₃ —BaTiO ₃ solid solution materials having a paraelectric phase at room temperature are used for DRAMs and ferroelectrics that require high dielectric properties. Therefore, it has been actively developed.
JP-A-11-260667 JP 2005-108887 A

What is required in the variable capacitance element using the high dielectric constant thin film having the perovskite crystal structure as described above is not only high tunability and high Q value, but also low temperature coefficient, high power durability, These include high insulation resistance, low distortion characteristics, and no change over time. The tunability indicates the variable amount of the capacitance of the variable capacitance element. When the capacitance before voltage application is C ₀ and the capacitance after voltage application is C ₁ , tunability = (C ₀ −C ₁ ) / C It is expressed as ₀ × 100 (%). Since the tunability increases as the electric field strength increases, the tunability tends to increase as the thickness of the high dielectric constant thin film decreases. The Q value depends on the loss in each component of the capacitor. The dielectric loss in the high dielectric constant thin film, the conductor loss in the electrode, and the like are the main causes for lowering the Q value. The capacitance and Q value of these variable capacitance elements are obtained by impedance measurement or the like.

  Originally, a high dielectric constant thin film used for a variable capacitance element does not have piezoelectricity, and the use temperature range is desirably equal to or higher than the Curie temperature, and it is desirable to use a material exhibiting paraelectricity in the use temperature range. . This is because in the case of exhibiting ferroelectricity, a change in capacitance due to voltage application is accompanied by hysteresis and the capacitance depends on the electric field history, which is inappropriate as a variable capacitance element. Hysteresis indicates a delay in polarization with respect to alternating current, and is not suitable because it corresponds to a large dielectric loss. Therefore, conventionally, as a high dielectric constant thin film that has been used for a variable capacitance element, a BST material system exhibiting a paraelectric property in an operating temperature range (around room temperature) has been mainly used.

This solid solution system of SrTiO ₃ and BaTiO ₃ exhibits ferroelectricity and paraelectricity due to the composition of Ba / Sr and Ti / (Ba + Sr), and the composition usually becomes a paraelectric phase region in the operating temperature range. It is set as follows. However, in the boundary region, the ferroelectricity of BaTiO ₃ may appear to be weakened and may show a slight capacity hysteresis.

  Here, in the variable capacitance element, in order to satisfy many of the required characteristics as described above, a material whose operating temperature range falls within such a boundary region may be used, but this is a problem here. However, even for piezoelectric or paraelectric materials that come from its ferroelectricity, signals that are input and output while applying an applied voltage (hereinafter sometimes simply referred to as DC voltage) to change the capacitance of the high dielectric constant thin film Piezoelectricity that occurs when a signal (hereinafter, simply referred to as a high-frequency signal or a high-frequency voltage) is applied. Piezoelectricity is not confirmed only in the boundary region, but also appears in the paraelectric region when the high dielectric constant thin film is strained by application of a DC voltage. When the piezoelectricity is strengthened or manifested by the voltage applied to the variable capacitance element in order to change the capacitance, resonance occurs in the amplitude and phase characteristics of the impedance, and such a variable capacitance element is used as a filter or resonator. When used as a component, the loss may increase due to voltage application.

  When the inventors measured impedance in a variable capacitance element using BST for a high dielectric constant thin film, a periodic change with respect to frequency was observed in the phase characteristics after voltage application (see, for example, Patent Document 2). .

  An example of the result is shown in FIG. In FIG. 1, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents impedance | Z | (unit: Ω) on the left side and phase (unit: deg) on the right side. In addition, the characteristic before voltage application is indicated by a thick line, and the characteristic after voltage application is indicated by a thin line. When this is viewed in terms of Q value (= 1 / tan θ, θ: phase), it is as shown in FIG. In FIG. 2, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents capacitance value C (unit: pF) on the left side and Q value (unit: dimensionless) on the right side.

  The Q value before voltage application (thick line) tended to decrease monotonously, but the Q value after voltage application (thin line) tended to periodically decrease with respect to frequency. In the case of a variable capacitance element composed of a pair of electrodes (upper electrode and lower electrode) and a high dielectric constant thin film sandwiched between the electrodes, the resonance frequency is roughly determined by the film thickness, material type, etc. of each layer. At a resonant frequency, the capacitance changes and the Q value decreases. For this reason, it is necessary to set the material system, composition, film quality, and thickness of each layer so that the use frequency of the variable capacitance element is located between adjacent peaks of the phase characteristics. However, in order to increase the original Q value before voltage application, the material system, composition, film quality, and thickness of each layer must be appropriately combined, and the configuration thereof is limited. It was difficult to set the material system, composition, film quality, and thickness.

  The present invention has been devised in view of the above-described circumstances, and an object thereof is to provide a variable capacitance element that suppresses a decrease in Q value due to voltage application at a desired frequency. It is another object of the present invention to provide a variable filter with reduced insertion loss and phase characteristics by using such a variable capacitance element.

  The variable capacitance element of the present invention includes a lower electrode layer, an upper electrode layer, a dielectric layer sandwiched between the lower electrode layer and the upper electrode layer, and having a density higher than that of the upper electrode layer, The phase characteristic of the impedance generated by voltage application to the lower electrode layer and the upper electrode layer has a periodic peak according to the frequency of the input / output signal, and between the adjacent peaks. The frequency of the signal is located.

  Here, the periodic peak of the phase characteristic of the impedance indicates that when the dielectric layer is applied with a high-frequency voltage under the application of a DC voltage, the vibration whose main vibration is the thickness longitudinal vibration due to the voltage application is It occurs due to periodic appearance (resonance characteristics appear). At this time, the resonance frequency is determined by the material, film thickness, and configuration of the vibration part including the dielectric layer and the pair of electrode layers (the upper electrode layer and the lower electrode layer) sandwiching the dielectric layer that vibrate due to the thickness longitudinal vibration. Come. For this reason, the material of the vibration part, the film thickness, and its configuration are adjusted in accordance with the frequency of the signal to be used so that the frequency of the signal is located between the adjacent peaks of the phase characteristics. The density of the dielectric layer is adjusted to be larger than that of the upper electrode layer.

  In the variable capacitance element of the invention, the dielectric layer is barium strontium titanate and the upper electrode layer is aluminum.

  The variable capacitance element of the present invention includes a lower electrode layer, an upper electrode layer, a dielectric layer sandwiched between the lower electrode layer and the upper electrode layer from above and below and having a density larger than that of the upper electrode layer, The phase characteristic of impedance generated by applying voltage to the lower electrode layer and upper electrode layer has a periodic peak corresponding to the frequency of the input / output signal, and the signal frequency is located between the adjacent peaks. To do. Usually, the density of the electrode layer is higher than the density of the dielectric layer. On the other hand, since the variable capacitance element of the present invention is configured such that the density of the dielectric layer is larger than the density of the upper electrode layer, the specific acoustic impedance of the entire vibration part can be reduced. Of the peak frequencies that appear when the signal frequency is increased (hereinafter referred to as peak frequencies), the peak frequency that appears first (hereinafter referred to as the basic peak frequency) can be increased. In addition, since the specific acoustic impedance of the entire vibration part can be reduced, when a voltage is applied to the upper electrode layer and the lower electrode layer, a frequency having a low phase characteristic compared to the phase characteristic when no voltage is applied. The width (hereinafter referred to as the band width) can be increased. For this reason, in the variable capacitance element of the present invention, the Q value can be prevented from lowering by making the basic peak frequency higher than the use signal frequency. Even when the basic peak frequency is smaller than the used signal frequency, the bandwidth is wide, so that it is easy to position the frequency of the signal between the adjacent peaks. As a result, it is possible to provide a variable capacitance element that suppresses a decrease in Q value due to voltage application in a wide frequency range. Further, by using such a variable capacitance element, it can be applied to a variable filter or the like with reduced insertion loss and phase characteristics.

  Although the reason is not yet clear, when the density of the lower electrode layer is made lower than that of the dielectric layer, the effect of increasing the basic peak frequency and the bandwidth are reduced compared to the case where the density of the upper electrode layer is made lower. Both effects of widening were significantly inferior. For this reason, the effect of raising the fundamental peak frequency and the effect of widening the bandwidth can be exhibited only by increasing the density of the dielectric layer compared to the upper electrode layer.

  In summary, according to the variable capacitance element of the present invention, the dielectric layer has a higher density than that of the upper electrode layer, so that the basic peak frequency is high and the bandwidth is wide. Further, it is possible to suppress a decrease in Q value due to voltage application. Hereinafter, the bandwidth between the basic peak frequency and the next peak frequency is referred to as the basic bandwidth.

  Here, the basic peak frequency and the basic bandwidth will be described in more detail with reference to an example of a diagram showing a phase characteristic of impedance at a frequency near 1.7 GHz used in a mobile phone or the like shown in FIG.

  When the piezoelectricity is increased by applying a DC voltage, the applied high-frequency signal excites the thickness longitudinal vibration of the dielectric layer, and resonance characteristics based on the longitudinal vibration or harmonics of the thickness are developed. At this time, the resonance frequency is determined by the material, film thickness, and configuration of the vibration part. These are variable-capacitance element structures as shown in FIG. 4 which are provided on one main surface in the thickness direction of the dielectric layer 3 and the dielectric layer 3 on the support substrate 1. The lower electrode layer 2 and the upper electrode layer 4 provided on the other main surface in the thickness direction of the dielectric layer 3 are assumed to be included.

The vibration mode in this structure is mainly the thickness longitudinal vibration mode, and the fundamental peak frequency f ₁ for this vibration mode is the thickness of the dielectric layer, the thickness of the electrode layers 2 and 4 sandwiching the dielectric layer, and the material constituting each layer. Determined by the speed of sound and the acoustic impedance. When the total thickness constituting the vibrating part including the electrode layers 2 and 4 is t, and the effective sound velocity of the sound wave propagating through the vibrating part is V, f ₁ = V / 2t, and the higher order peak frequency f _n Is roughly determined by f _n = nV / 2t (n = 1, 3, 5...). This effective sound speed refers to the average sound speed when the sound speed that varies depending on the material of each layer is viewed in the entire vibration part. The peak frequency indicates the phase maximum point, and the more the voltage is applied, the greater the capacitance changes, and the phase characteristic approaches zero from the value near −90 °. Further, as shown in FIG. 3, the bandwidth has a phase characteristic (thin line) after voltage application equal to or less than the phase characteristic (thick line) before voltage application at a frequency between the peak and the peak located next to the peak. Refers to an area. The wider the bandwidth, the more the Q value region that is not affected by resonance after voltage application is increased.

  Here, FIGS. 5 and 6 show changes in the basic peak frequency and the basic bandwidth when the density of the upper electrode layer is changed with respect to the density of the dielectric layer, and the dielectric layer is treated as a piezoelectric layer. The result of one-dimensional simulation is shown by a diagram. In FIG. 5, the horizontal axis represents the density ratio of the upper electrode layer to the dielectric layer (unit: dimensionless), and the vertical axis represents the basic peak frequency (unit: GHz). In FIG. 6, the horizontal axis represents the density ratio (unit: dimensionless) of the upper electrode layer to the dielectric layer, and the vertical axis represents the basic bandwidth (unit: GHz).

  As apparent from FIGS. 5 and 6, by reducing the density ratio of the upper electrode layer to the dielectric layer, the fundamental peak frequency can be dramatically increased and the fundamental bandwidth can be greatly increased.

  Thus, by reducing the size of the density of the upper electrode layer relative to the dielectric layer, that is, by making the density of the dielectric layer smaller than the density of the upper electrode layer, it is possible to apply voltage in a wide frequency range. Since it is possible to provide a variable capacitance element that suppresses a decrease in the Q value, the material and film thickness of the dielectric layer can be selected by paying attention to the tunability, the Q value, etc. As a result, a high-performance variable capacitor can be provided.

  Until now, in order to obtain a variable capacitance element having a particularly high Q value, the material system, composition, film quality, thickness, and electrode material and thickness of the dielectric layer must be appropriately combined. Configuration was limited. In addition, when a voltage is applied to change the capacitance, resonance due to piezoelectricity occurs, and the Q value after the voltage application is affected by the resonance, and there is a problem that it is greatly reduced. Further, even if the material, thickness, etc. of each layer are set so that the Q value after voltage application does not decrease, the frequency range of use is limited. However, according to the present invention, it becomes easy to set the use frequency to be located between adjacent peaks, the degree of freedom of design is increased, and a high Q value can be obtained in a wide frequency range regardless of the applied voltage. It has become possible.

  Here, in particular, when the dielectric layer is barium strontium titanate, the dielectric loss is low, so that the Q value is high and the capacitance variable ratio is large. Also, when the upper electrode layer is made of aluminum, the resistance is low and the density is low, so the loss as an electrode is small, so the Q value is high, the basic peak frequency can be increased, and the basic bandwidth can be widened. It becomes.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings.

  FIG. 7 is a cross-sectional view showing an example of an embodiment of the variable capacitance element of the present invention.

  In FIG. 7, 1 is a support substrate, 2 is a lower electrode layer, 3 is a dielectric layer, 4 is an upper electrode layer, and includes an upper electrode A layer 4A and an upper electrode B layer 4B. . 5 is an insulating layer, 6 is a protective layer, 7A and 7B are terminal electrode layers, 8 is a solder diffusion preventing layer, and 9A and 9B are solder terminals. The capacitance forming region is a facing portion where the dielectric layer 3 is sandwiched between the lower electrode layer 2 and the upper electrode layer 4A. The structure of the capacitance forming region using such a high dielectric constant thin film is not limited to a variable capacitance element, but can be applied to a thin film capacitor such as a decoupling capacitor or a DRAM. As a use frequency range, the frequency 1.7-2.1 GHz used with a mobile telephone is mentioned, for example, It is preferable that Q value by voltage application does not fall in this range.

  Here, the phase characteristic of the impedance of the variable capacitance element caused by voltage application to the upper electrode A layer 4A or the upper electrode A layer 4A, the upper electrode B layer 4B, and the lower electrode layer 2 depends on the frequency of the input and output signals. It has a corresponding periodic peak. This peak indicates the phase maximum point, and the larger the applied voltage, the greater the change in capacitance, and the phase characteristic approaches zero from a value in the vicinity of −90 °. The peak period is such that | fa−fb | is approximately 0.1 GHz to 1 GHz when two adjacent peak frequencies are fa and fb, respectively. Note that this periodicity is an approximation obtained from the example, and has been described in order to distinguish it from noise peaks.

  According to the variable capacitance element of the present invention, since the density of the dielectric layer 3 is larger than the density of the upper electrode layer 4, even if the phase characteristic of the impedance has a periodic peak, Since the basic peak frequency can be increased and the bandwidth can be increased, the Q value can be prevented from being deteriorated before and after voltage application without being influenced by the phase characteristics in a wide frequency range.

<Board>
Supporting substrate _{1, Al 2 O 3, MgO,} LaAlO 3, SrTiO 3, the Si substrate obtained by laminating _{SiO 2 (SiO 2 / Si)} , is such as can be used, a dielectric formed on the substrate 1 In order to make the surface of the layer 3 smooth, it is preferable to have sufficient flatness. The material is not particularly limited as long as it has an insulating property, but it is necessary to consider that the thickness longitudinal vibration is not excited by the substrate thickness.

<Lower electrode layer>
A lower electrode layer 2 is formed on the support substrate 1. As a conductor material constituting the lower electrode layer 2, it is necessary to use a metal having a low resistance in order to increase the Q value, and examples thereof include copper, aluminum, gold, and silver. In the process of forming the dielectric layer 3, when a high temperature thermal history is involved, there is a possibility that a leakage current increases due to diffusion or reaction in the high dielectric constant thin film. Therefore, it is desirable to use platinum, palladium, or the like that hardly causes reaction or diffusion. The thinner the film thickness, the higher the basic peak frequency, so it is possible to eliminate the effect at the operating frequency, but it is preferable to reduce the film thickness. Since the problem that the Q value before voltage application is low occurs, it is necessary to set it in consideration of both. These are produced by vapor phase synthesis methods such as sputtering and vacuum deposition, and are processed by dry etching when a photoresist is used as a mask.

  In order to facilitate connection with an external circuit, it is desirable to form the lower electrode layer 2 by extending to the outside of the capacitance forming region where the solder terminals 9A are formed. Further, in order to make the heights of the solder terminals 9A and 9B uniform, a conductor layer insulated from the lower electrode layer 2 with the same thickness using the same material as the lower electrode layer 2 at the position where the solder terminal 9B is formed. It is desirable to form it.

<Dielectric layer>
The material of the dielectric layer 3 is preferably a material having a high dielectric constant, and a dielectric material whose dielectric constant can be largely changed by an applied voltage, for example, BaTiO ₃ , SrTiO ₃ , (Ba, Sr) TiO _3. And perovskite oxides. In particular, use of barium strontium titanate is preferable because the dielectric loss is low and the Q value is high and the capacitance variable ratio is large.

  Here, the dielectric layer 3 may be made of a material that originally has piezoelectricity and whose piezoelectricity is enhanced by application of a DC voltage and a high-frequency signal, or originally has no piezoelectricity but is applied with a DC voltage. Alternatively, a material whose piezoelectricity is manifested by application of a DC voltage and a high-frequency signal may be used as long as the material exhibits piezoelectricity in a state where a DC voltage and a high-frequency signal are applied. Due to this piezoelectricity, the dielectric layer 3 emits an acoustic wave.

  The dielectric layer 3 made of such a material is formed by a thin film manufacturing method such as a solution method such as a sol-gel method, a sputtering method, a CVD method, a laser ablation method, or a vapor phase synthesis method such as an MBE (Molecular Beam Epitaxy) method. Can do. The dielectric layer 3 is formed, for example, so as to cover the surface of the lower electrode layer 2, and then removed by wet etching or dry etching except at least the capacitance forming region. The thinner the dielectric layer 3 is, the higher the fundamental peak frequency is, so that the influence can be eliminated at the operating frequency, and the thinner the dielectric layer 3, the higher the electric field strength. There is an advantage that characteristics can be obtained. On the other hand, when the film thickness is small, problems such as an increase in leakage current and a decrease in effective relative dielectric constant occur, so it is necessary to set the film thickness appropriately.

<Upper electrode A layer>
As a conductive material constituting the upper electrode A layer 4A, the density is required to be smaller than that of the dielectric layer 3. This depends on the density of the dielectric layer 3, and examples thereof include Al, Mg, Ti having a low density. In particular, when Al (aluminum) is used, it is preferable because the resistance is low and the density is low, and the loss as an electrode is small, so that the Q value is high and the fundamental peak frequency can be increased. However, the material is not limited to these materials, and a layer having a low density may be formed by a forming method or the like by using a material having a high original density, for example, by reducing a pressure during film formation. It is sufficient that such a material is used immediately above the dielectric layer 3, and it is possible to sequentially stack other conductive materials in combination with other materials. The thickness is set in the same manner as the lower electrode layer 2 in order to increase the Q, but it may be considered as an integral part with the upper electrode B layer 4B described later.

  Thus, by setting the density of the upper electrode A layer 4A to be lower than the density of the dielectric layer 3, the basic peak frequency can be increased and the basic bandwidth can be increased. For this reason, it becomes easy to make it the frequency range which does not fall in Q value by voltage application in the use frequency range. That is, by reducing the density of the upper electrode A layer 4A relative to the dielectric layer 3, it is possible to provide a variable capacitance element that suppresses a decrease in Q value due to voltage application in a wide frequency range. 3 can be selected by paying attention to tunability, Q value, etc., and the degree of freedom as a material of the dielectric layer 3 is increased. As a result, a high-performance variable capacitance element can be provided. .

  In addition, as shown in FIG. 7, it is preferable that the dielectric layer 3 is disposed in the upper surface of the lower electrode layer 2, and the upper electrode A layer 4 </ b> A is disposed in the upper surface of the dielectric layer 3. In this case, the lower electrode layer 2 including the upper electrode A layer 4A and the dielectric layer 3 can be formed in the same batch without being exposed to the atmosphere, and sequentially patterned from the upper layer to a desired shape. Therefore, it is possible to prevent extraneous adhesion of foreign matters, oils and fats at the interface of each layer, improve adhesion, and prevent moisture from entering. For this reason, moisture resistance can be significantly improved, and stable characteristics can be produced.

<Insulating layer>
The insulating layer 5 is formed so as to cover all of these structures, and is necessary to ensure insulation between the upper electrode B layer 4B and the lower electrode layer 2 and to reduce the parasitic capacitance generated therebetween. The insulating layer 5 is made of an organic material such as BCB (benzocyclobutene) or polyimide, or an inorganic material such as SiO ₂ or Si ₃ N ₄ , and has a low dielectric constant in order to reduce the parasitic capacitance. Is desirable. The film forming method of the insulating layer 5 made of these materials is preferably a CVD (Chemical Vapor Deposition) method that can obtain a relatively uniform film thickness even on a base having a complicated three-dimensional shape. The insulating layer 5 has an opening directly above the upper electrode A layer 4A by dry etching using a photoresist as a mask in order to electrically connect the upper electrode B layer 4B to be formed later to the upper electrode A layer 4A. Is preferably formed. In addition, it is desirable to simultaneously form a part of the lower electrode layer 2 at the position where the solder terminal 9A is formed and an upper surface of the conductive layer where the solder terminal 9B is formed that extend outside the capacitance forming region.

<Upper electrode B layer>
Next, the upper electrode B layer 4B is formed. The upper electrode B layer 4B connects the upper electrode A layer 4A exposed at the opening of the insulating layer 5 and one terminal electrode layer 7B. As a material, the density is preferably smaller than that of the dielectric layer 3. However, in the practice of the present invention, it is sufficient that at least such a material is used for the upper electrode A layer 4A immediately above the dielectric layer 3. The electrode B layer 4B may have a laminated structure combined with other materials. For example, a low-resistance metal such as Au or Cu may be used. In this case, the resistance due to the electrode can be reduced, so that a variable capacitance element with low loss can be provided, which is preferable. In consideration of adhesion to the insulating layer 5, the use of an adhesion layer such as Ti or Ni does not increase the resistance of the upper electrode B layer 4 </ b> B, and may be within a range where the acoustic mass load is small. Absent.

<Terminal electrode layer>
Next, the terminal electrode layers 7A and 7B are formed on the lower electrode layer 2 at the solder terminal 9A forming position and on the conductive layer formed at the solder terminal 9B forming position. The terminal electrode layers 7A and 7B are not particularly limited as long as they are conductive materials. However, the terminal electrode layers 7A and 7B are made of the same material at the same time as the upper electrode B layer 4B so as to be formed simultaneously with the patterning of the upper electrode B layer 4B. It is desirable to form. In this case, a part of the upper electrode B layer 4B has the function of the terminal electrode layer 7B.

<Protective layer>
Next, the protective layer 6 is formed so as to cover the entire structure. The protective layer 6 is formed so as to expose part of the terminal electrode layers 7A and 7B. The protective layer 6 mechanically protects the device from the outside, degrades the material of the device due to chemical reaction with humidity and oxygen, contamination due to foreign matters such as dust, degradation due to breakage during mounting, contamination due to chemicals, etc. Protect from oxidation, etc. As a material, a material having high heat resistance and excellent coverage with respect to a step is good, and it is necessary to consider so that vibration is not unnecessarily excited. Specifically, an organic thermosetting material such as polyimide or BCB or a photocuring material can be used.

<Terminal>
The solder diffusion preventing layer 8 is disposed on the terminal electrode layers 7A and 7B exposed to the protective layer 6, and the electrode layers 2, 4A, 4B, 7A, and the like are formed during reflow and mounting when forming the solder terminals 9A and 9B. It is formed to prevent the diffusion of solder to 7B. Ni is suitable as the material. Further, on the surface of the solder diffusion preventing layer 8, in order to improve the solder wettability, Au, Cu or the like having a high solder wettability may be formed to about 0.1 μm. Finally, solder terminals 9A and 9B are formed thereon. This is provided to facilitate mounting, and is formed by reflowing after printing the solder paste.

<Evaluation>
The actual impedance measurement of the variable capacitance element manufactured in this way is performed by, for example, connecting a test fixture (Agilent, model number 16197A) with a variable capacitance element to be evaluated and an impedance analyzer (Agilent, model number E4991A). Then, measurement may be performed by inputting a high frequency signal to the variable capacitance element. The densities of the dielectric layer 3 and the upper electrode layer 4 may be measured by, for example, Rutherford backscattering analysis (RBS).

<Summary>
The variable capacitance element of the present invention as described above is a dielectric layer sandwiched from above and below by a lower electrode layer, an upper electrode layer, a lower electrode layer and an upper electrode layer, and having a density larger than that of the upper electrode layer. The phase characteristic of the impedance generated by applying voltage to the lower electrode layer and the upper electrode layer has a periodic peak corresponding to the frequency of the input / output signal, and a signal between the adjacent peaks. The frequency is located. With such a configuration, the basic peak frequency of the variable capacitance element can be shifted to the high frequency side, and the basic bandwidth can be widened, so that the Q value resonates even after the DC voltage is applied. The frequency range that is not affected by can be widened. The variable capacitance element having such a configuration can be used as a part of the resonance circuit of the high-frequency component (capacitance component of the LC resonance circuit) or as a capacitance component for coupling the resonance circuit. Further, the present invention can be applied to high-frequency components such as a voltage-controlled high-frequency resonant circuit component, a voltage-controlled high-frequency filter, a voltage-controlled matching circuit element, and a voltage-controlled thin-film antenna duplexer that are composite components of the resonant circuit.

  As a result of confirming the relationship between the density, sound velocity, and acoustic impedance between the lower electrode layer 2 and the dielectric layer 3, the relationship between the density, sound velocity, and acoustic impedance between the upper electrode layer 2 and the dielectric layer 3 is changed. The effect is remarkably inferior compared with the case where it is made to. The cause of this is not yet clear, but it is considered that the lower electrode layer 2 is in contact with the support substrate 1. Therefore, the present invention can obtain a high Q value in a wide frequency range only by changing the relationship among the density, sound velocity, and acoustic impedance among the vibrating portion, in particular, the upper electrode layer 4 and the dielectric layer 4. It becomes possible.

  As an embodiment of the present invention, referring to the variable capacitance element shown in FIG. 7, the layer configuration, material, and film thickness of the variable capacitance element used near the frequency of 1.9 GHz are set as follows, and the frequency is near 1.9 GHz. Thus, it was confirmed that the periodic phase change caused by the resonance can be suppressed. This means that a decrease in the Q value can be suppressed.

On the sapphire substrate as the support substrate 1, a target made of Pt with a thickness of 2 μm as the lower electrode layer 2 and (Ba _0.5 Sr _0.5 ) TiO ₃ as the dielectric layer 3 is used and a thickness of 0.3 μm. Further, as the upper electrode A layer 4A, a conductive material having a density ratio, an acoustic impedance ratio, and a sound velocity ratio of the dielectric layer 3 and the upper electrode A layer 4A of 0.83, 1.1, and 1.4, respectively, is formed. Sputter deposition was sequentially performed to 0.1 μm. Then, the upper electrode A layer 4A, the dielectric layer 3, and the lower electrode layer 2 were sequentially patterned by dry etching and wet etching, respectively, by a photolithography process. When patterning the lower electrode layer 2, the lower electrode layer 2 is disposed so as to extend beyond the capacitance forming region to the position where the solder terminal 9A is formed, and at the position where the solder terminal 9B is formed, A conductive layer was formed apart from the layer 2. Next, an insulating layer 5 made of amorphous silicon dioxide is formed by plasma CVD over the upper electrode A layer 4A, the high dielectric constant dielectric layer 3, the lower electrode layer 2 and the upper surface of the support substrate 1. did. Subsequently, the entire surface is covered with a resist film, and a rectangular hole is opened in the photoresist film by photolithography, and then on the upper electrode A layer 4A and the lower electrode layer 2 at the position where the solder terminal 9A is formed using the photoresist film as a mask. The insulating layer 5 located on the conductive layer at the position where the solder terminal 9B is formed was selectively removed by reactive ion etching. Thus, an opening for taking out the solder terminals 9A and 9B and an opening for connecting the upper electrode B layer 4B to the upper electrode A layer 4A were formed. The shape of the opening can be appropriately set to a rectangular shape, a circular shape, a polygonal shape, an elliptical shape, or the like.

  Next, Al was formed into a thickness of 0.9 μm and patterned as the upper electrode B layer 4B so as to cover all of these structures. Here, patterning is performed so as to connect between the upper electrode A layer 4A exposed at the opening of the insulating layer 5 and the conductive layer exposed at the opening at the position where the solder terminal 9B is formed to form a part of the upper electrode B layer 4B. A function as the terminal electrode layer 7B was provided. At the same time, patterning was performed so as to form the terminal electrode layer 7A having the same material and the same thickness as the upper electrode B layer 4B on the lower electrode layer 2 exposed at the solder terminal 9A formation position.

  Next, as a protective layer 6 so as to cover all of these structures, a photosensitive benzocyclobutene resin is formed to a thickness of 2 μm by spin coating, and openings are formed at positions where the solder terminals 9A and 9B are formed. Patterned to form.

  Next, a two-layer metal film of Ni and Au was formed in this order on the terminal electrode layers 7A and 7B exposed at the openings of the protective layer 6, and then unnecessary portions were removed using a photolithography technique. . Of these two metal films, the layer made of Ni has a function as the solder diffusion preventing layer 8 and is provided to prevent diffusion of solder formed thereon onto the electrode layer. The Au layer is provided to prevent surface oxidation and improve the wettability with the solder.

  Finally, solder terminals 9A and 9B were formed thereon by printing and reflow using lead-free solder to obtain a variable capacitance element.

  In this way, the lower electrode layer 2 and the solder terminal 9A were electrically connected, and the upper electrode layer 4A and the solder terminal 9B were electrically connected.

  FIG. 8 shows the result of one-dimensional simulation of the impedance characteristic and phase characteristic after voltage application of the variable capacitance element based on the configuration of this example. In FIG. 8, the horizontal axis represents frequency (unit: GHz), the vertical axis represents impedance (unit: Ω) on the left side, and the phase (unit: deg) on the right side, the bold line represents impedance characteristics, and the thin line represents Each of the phase characteristics is shown. As is apparent from FIG. 8, in the phase characteristics near the operating frequency of 1.9 GHz of the variable capacitance element, there is no peak frequency, the basic peak frequency is about 1.1 GHz, and the bandwidth is 1.76-2.17 GHz. It can be seen that the frequency range is as wide as about 400 MHz, and the frequency of the high-frequency signal used between adjacent peaks is located and has good characteristics.

  On the other hand, as a comparative example, a variable capacitance element used at the same 1.9 GHz as in the above-described embodiment is a conventional variable capacitance element formed without making the density of the upper electrode A layer 4A smaller than that of the dielectric layer 3. The simulation result is shown in FIG. This configuration is different from the embodiment of the present invention only in the upper electrode layer 4. The upper electrode A layer 4A has a density ratio, acoustic impedance ratio, and sound velocity ratio with the dielectric layer 3 of 3.9, 2,. 91 and 0.75 conductive materials were used, and the film thickness was 0.1 μm. Further, Al was used for the upper electrode B layer 4B in the same manner as in the example, and the film thickness was 1.2 μm. The rest is the same as the embodiment. The reason for changing the film thickness is that the phase characteristics of the variable capacitance element in the vicinity of the operating frequency of 1.9 GHz are set to be within the bandwidth (band frequency range). In FIG. 9, the horizontal axis indicates frequency (unit: GHz), the vertical axis indicates impedance (unit: Ω) on the left side, and the phase (unit: deg) on the right side, the bold line indicates impedance characteristics, and the thin line indicates Each of the phase characteristics is shown. As is clear from FIG. 9, the basic peak frequency is about 1.1 GHz, which is almost the same as FIG. 8, but the interval from the secondary peak frequency is narrow, and the bandwidth is a frequency of 1.92 to 2.02 GHz. It can be seen that the range is narrowed to about 100 MHz and 1/4. Note that the reason why the simulation result does not show a tendency for the phase to approach zero as the frequency increases as shown in FIG. 1, which is the actual measurement result, is that loss is not taken into consideration, that is, the phase is −90. This is because it becomes a degree.

  Thereby, in the variable capacitance element of this invention, it has confirmed that the change of the periodic phase (decrease of Q value) after voltage application was suppressed in the frequency range wider than before. Note that the above are merely examples of the embodiments of the present invention, and the present invention is not limited to these embodiments, and various modifications and improvements may be added without departing from the scope of the present invention. .

  Since the piezoelectricity in such a BST material system is inherently a small piezoelectric constant, and because the capacitive element is not acoustically isolated, the phase change is not a large phase change as seen in a piezoelectric thin film resonator. The change in characteristics is very small. Such a periodic decrease in the Q value accompanying the change in the phase characteristics can be confirmed when a high Q value is achieved. In particular, in order to achieve a high Q value in a high frequency and several GHz band, At the same time, the loss due to high dielectric constant thin film must be reduced. Therefore, the periodic decrease of the Q value after voltage application in such a variable capacitance element has been confirmed by the inventors as a result of earnestly increasing the Q at a high frequency. The invention has been made.

  As described above, according to the variable capacitance element of the present invention, the dielectric layer is sandwiched between the lower electrode layer, the upper electrode layer, the lower electrode layer, and the upper electrode layer from above and below, and has a higher density than the density of the upper electrode layer. The phase characteristic of the impedance generated by applying voltage to the lower electrode layer and the upper electrode layer has a periodic peak corresponding to the frequency of the input / output signal, and a signal between the adjacent peaks. Is located, the Q value is not lowered in a wide frequency range as compared to before the voltage application, and a high Q value can be obtained regardless of the voltage application.

  Further, according to the present invention, in order to satisfy many required performances other than the Q value, a high Q value can be obtained regardless of voltage application even if a high dielectric constant thin film material having a slight piezoelectricity is used. A variable capacitance element can be provided.

It is a characteristic view which shows the frequency dependence of the phase characteristic of the conventional variable capacitance element. It is a characteristic view which shows the frequency dependence of the Q value of the conventional variable capacitance element. It is an enlarged view of a characteristic diagram showing the frequency dependence of the phase characteristic for explaining the peak frequency and the bandwidth. It is sectional drawing for demonstrating the capacity | capacitance formation area | region of the variable capacitance element of this invention typically. It is a diagram which shows the dependence of the density ratio of an upper electrode layer and a dielectric material layer with respect to a basic peak frequency in the variable capacitance element of this invention. It is a diagram which shows the dependence of the density ratio of an upper electrode layer and a dielectric material layer with respect to a basic bandwidth in the variable capacitance element of this invention. It is sectional drawing which shows an example of embodiment of the variable capacitance element of this invention. It is a simulation result which shows the frequency dependence of a phase characteristic in the Example of the variable capacitance element of this invention. It is a simulation result which shows the frequency dependence of a phase characteristic in the comparative example of the conventional variable capacitance element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Support substrate 2 ... Lower electrode layer 3 ... Dielectric layer 4A ... Upper electrode A layer 4B ... Upper electrode B layer 4 ... Upper electrode layer 5 ... Insulating layer 6 ... Protective layer 7A, 7B ... Terminal electrode layer 8 ... Solder diffusion prevention layer 9A, 9B ... Solder terminal

Claims (2)

A lower electrode layer, an upper electrode layer, and a dielectric layer sandwiched between the lower electrode layer and the upper electrode layer and having a density higher than that of the upper electrode layer, and the lower electrode layer, The impedance phase characteristic generated by applying voltage to the upper electrode layer has a periodic peak corresponding to the frequency of the input / output signal, and the frequency of the signal is located between the adjacent peaks. A variable capacitance element characterized by the above. 2. The variable capacitance element according to claim 1, wherein the dielectric layer is barium strontium titanate, and the upper electrode layer is aluminum.

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