JP2006302942A - Varactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a varactor for suppressing deterioration in a Q value due to voltage application at a desired frequency. <P>SOLUTION: The varactor is provided with a pair of electrodes 3, 6, and a dielectric film 4 sandwiched by the pair of electrodes 3, 6 and having piezoelectricity. The varactor changes specific inductive capacity of the dielectric film 4 by applying voltage to the pair of electrodes 3, 6. The vatactor is provided with acoustic reflection constituents 2, 7 on at least the outside of any one of the pair of electrodes 3, 6. Since the vibration from the dielectric film 4 exhibiting the piezoelectricity can be acoustically insulated by the acoustic reflection constituents 2, 7 arranged outside the electrodes 3, 6, and the vibration of the dielectric film 4 can be suppressed; the varactor having a high Q value can be provided irrespective of a condition of voltage application. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a variable capacitance element that controls the voltage of a capacitance by utilizing the applied voltage dependence of the relative dielectric constant of a dielectric film using a high dielectric constant dielectric, and particularly has a dielectric loss in an operation in a high frequency region. The present invention relates to a variable capacitance element that is small and has a small dielectric loss regardless of the presence or absence of voltage application.

  As the frequency of radio communication and electric circuits increases, electronic components used for these electric signals are also required to be compatible with high frequencies. In particular, in order to use a thin film capacitor as a component such as a filter or a resonator in a high frequency circuit, the capacitor needs to have a high Q value. In addition, with the development of wireless communication technology and switching to a new method, there is an increasing demand for communication devices that support a plurality of transmission / reception systems. At the same time, in order to cope with the reduction in the number of parts and the miniaturization, in recent years, variable elements such as a voltage variable filter and a voltage variable capacitance element have been developed (see, for example, Patent Document 1).

The voltage variable capacitance element is a variable capacitance element utilizing a large change in relative dielectric constant with respect to an applied voltage indicated by a dielectric film having a high dielectric constant. For example, the structure basically includes, for example, a substrate, a lower electrode made of a metal thin film formed on the substrate, a high dielectric constant dielectric film formed on the lower electrode, and further on the dielectric film. And formed upper electrodes. Examples of the high dielectric constant dielectric film include strontium titanate (SrTiO ₃ ), barium strontium titanate (Ba _1-x Sr _x TiO ₃ , hereinafter referred to as BST), lead zirconate titanate (Pb ( A thin film made of a dielectric having a perovskite crystal structure such as Zr _x Ti _1-x ) O ₃ ) is used. Perovskite oxides such as SrTiO ₃ and BST exhibit a high relative dielectric constant near room temperature, and have the highest relative dielectric constant at a composition of Sr / (Sr + Ba) = 0.3, which is close to the structure change boundary between tetragonal and cubic crystals. It is known to have. In particular, SrTiO ₃ —BaTiO ₃ solid solution materials having a paraelectric phase at room temperature are suitable for DRAMs that require high dielectric properties and have been actively developed. In addition, since a non-linear change in dielectric constant is observed when a predetermined voltage is applied to the dielectric film, it is used as a variable capacitance element.
Japanese Patent Laid-Open No. 11-260667

In the variable capacitance element using the dielectric film having a high dielectric constant having the perovskite crystal structure as described above, in addition to high tunability and high Q value, several tens of temperature coefficients are required. It is small as ppm / ° C., high power durability, high insulation resistance, low distortion characteristics, no change with time, and the like. The high tunability indicates a 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 ) / It is represented by C ₀ × 100 (%) or the like. Since the tunability increases as the electric field strength of the voltage applied to the dielectric film increases, the tunability tends to increase as the thickness of the dielectric film decreases. Further, the Q value depends on the loss in each component of the capacitor, and the dielectric loss in the dielectric film, the conductor loss in the electrode, and the like are the main causes for lowering the Q value. The capacities and Q values of these thin film capacitors are obtained by impedance measurement.

  When the present inventors measured impedance in a variable capacitance element using BST as a dielectric film, a periodic change with respect to frequency was observed in the phase characteristics after voltage application. This state is shown by a diagram in FIG. In FIG. 4, the horizontal axis represents frequency [freq.] (Unit: MHz), the vertical axis represents impedance [| Z |] (unit: ohm) on the left side, and the phase [Theta] (unit: deg.) On the right side. The upper characteristic curve represents the impedance frequency characteristic, and the lower characteristic curve represents the phase frequency characteristic. As shown in FIG. 4, the impedance indicates the characteristics of the capacitive element that decreases with increasing frequency, and the impedance increases with voltage application. The frequency dependence before and after voltage application is almost the same. On the other hand, the frequency characteristics of the phase differ greatly before and after voltage application. The phase before voltage application (indicated by a thick line) tends to increase monotonously with the increase in frequency, but the phase after voltage application (indicated by a thin line) increases while periodically changing with increasing frequency. There is a tendency to become.

  Further, when this periodic change with respect to the frequency is viewed as a Q value, it is as shown by a diagram in FIG. In FIG. 5, the horizontal axis represents frequency [freq.] (Unit: MHz), the vertical axis represents capacitance [C] (unit: pF), and the right side represents Q value [Q value]. The curve shows the frequency characteristic of the capacitance, and the lower characteristic curve shows the frequency characteristic of the Q value. As shown in FIG. 5, the Q value before voltage application (indicated by a thick line) tends to decrease monotonously with an increase in frequency, but the Q value after voltage application (indicated by a thin line) has a frequency. There is a tendency to decrease while periodically changing with increasing.

  The periodic impedance phase characteristic and Q value change with respect to this frequency are a result of the dielectric film used for the variable capacitance element exhibiting piezoelectricity by voltage application. That is, when piezoelectricity is generated by voltage application, the thickness longitudinal vibration of the dielectric film is excited by the applied high frequency, and a resonance phenomenon based on the fundamental vibration or harmonics of the thickness longitudinal vibration appears. This resonance frequency is determined by the film thickness and the film configuration of the dielectric film, but a resonance point appears periodically with respect to the change in frequency, and the electrical characteristics of the dielectric film are also correspondingly changed. Causes periodic changes.

  Originally, a dielectric film used for a variable capacitance element should not be piezo-electric and does not resonate, its operating temperature range is equal to or higher than the Curie temperature, and a material exhibiting paraelectricity in the operating temperature range should be used. desirable. This is because when the dielectric film exhibits ferroelectricity, the change in capacitance due to voltage application is accompanied by hysteresis, and therefore the capacitance depends on the electric field history, so that a change in capacitance according to the applied voltage is required. This is because it is not suitable as a variable capacitance element. Hysteresis indicates a delay with respect to alternating current of polarization and is not suitable because it corresponds to a large dielectric loss. Therefore, conventionally, as a dielectric film used for a variable capacitance element, a BST material system which exhibits paraelectricity in the operating temperature range (near room temperature) and does not exhibit piezoelectricity has been mainly used. Alternatively, it has been used in a frequency range that does not cause a problem in electrical characteristics even if it exhibits piezoelectricity.

By the way, BST, which is a solid solution system of SrTiO ₃ and BaTiO ₃ , exhibits ferroelectricity and paraelectricity depending on the composition ratio of Ba / Sr and Ti / (Ba + Sr), and the composition ratio is usually at the operating temperature. It is set to be a paraelectric region in the range. However, in the boundary region, the ferroelectricity of BaTiO ₃ may appear to be weakened and may show a slight capacity hysteresis.

  In order to satisfy many of the required characteristics as described above, the dielectric material of such a boundary region may be used for the dielectric film of the variable capacitance element, but the problem here is that of the dielectric film. This is a case where piezoelectricity occurs in the dielectric film due to the voltage applied to change the capacitance, and resonance occurs in the amplitude and phase characteristics of the impedance. In the case of a variable capacitance element composed of a pair of electrodes and a dielectric film sandwiched between the electrodes, this resonance frequency is determined by the film thickness of the dielectric film, the size of the electrode, and the like. It appears periodically when it is changed. At this periodic resonance frequency, the capacitance of the variable capacitance element changes and the Q value decreases, but this is a problem when the influence is observed in the use frequency range of the variable capacitance element, particularly in the high frequency range. That is, in a variable capacitance element in which the phase characteristic of impedance periodically changes due to voltage application, the Q value is greatly reduced compared to that before voltage application because it is affected by resonance in the operating frequency range. When such a variable capacitance element is used as a component such as a filter or a resonator, there arises a problem that loss due to the variable capacitance element increases due to voltage application.

  In the variable capacitance element using the voltage dependence of the relative dielectric constant in the dielectric film having a high dielectric constant, such knowledge by voltage application due to piezoelectricity was confirmed for the first time by the present inventors. The present invention has been made to address the above-described problems based on the knowledge, and an object of the present invention is to provide a variable capacitance element that suppresses a decrease in Q value due to voltage application at a desired frequency. By using such a variable capacitance element, it is possible to provide a variable filter or the like in which an increase in insertion loss and a change in phase characteristics are reduced.

  The variable capacitance element of the present invention includes a pair of electrodes and a dielectric film having piezoelectricity sandwiched between the pair of electrodes, and changes a relative dielectric constant of the dielectric film by applying a voltage to the pair of electrodes. The variable capacitance element is characterized in that an acoustic reflection element is provided outside at least one of the pair of electrodes.

  That is, the variable capacitance element of the present invention is a variable capacitance element that uses a dielectric film having a high dielectric constant and changes the capacitance by applying a voltage to the dielectric film, the dielectric film having piezoelectricity, An acoustic reflection element is provided on at least one outer side (opposite to the dielectric film) of the pair of electrodes in order to suppress a decrease in Q value due to vibration excited piezoelectrically by application. Is.

  Moreover, the variable capacitance element of the present invention is characterized in that, in the above configuration, the acoustic reflection element is an acoustic multilayer film composed of a plurality of layers having different acoustic impedances.

Moreover, the variable capacitance element of the present invention is characterized in that, in the above configuration, the dielectric film is made of Ba _1-x Sr _x TiO ₃ (0 <x <1).

  In order to obtain a variable capacitance element having a high Q value, the dielectric film material, composition, film quality and film thickness, electrode material and film thickness, and the like must be combined in accordance with desired characteristics. Originally, it is desirable that the dielectric used for the dielectric film is a paraelectric and does not exhibit piezoelectricity. However, in order to satisfy various required performances of the variable capacitance element, the dielectric film is made of a paraelectric and a ferroelectric. A dielectric material in the boundary region may be used. However, as a result, when applying an AC voltage as a high frequency signal while applying a DC voltage for capacity control, or when applying a high voltage AC voltage, resonance due to piezoelectricity occurs in the dielectric film, There is a possibility that the Q value after voltage application is greatly reduced due to the influence of the resonance.

  According to the variable capacitance element of the present invention, it is provided with a pair of electrodes and a dielectric film having piezoelectricity sandwiched between the pair of electrodes, and by applying a voltage to the pair of electrodes, the relative dielectric constant of the dielectric film In the variable capacitance element that changes the frequency, the acoustic reflection element is provided outside at least one of the pair of electrodes, so that the vibration is reflected by the acoustic reflection element without being excited by the layer formed on the outside. Therefore, vibration from a dielectric film exhibiting piezoelectricity at a desired frequency can be acoustically insulated at the boundary between the electrode and the acoustic reflecting element by the acoustic reflecting element disposed outside the electrode, thereby Since vibration of the film can be suppressed, a variable capacitance element having a high Q value can be provided regardless of voltage application conditions. Therefore, there is provided a variable capacitance element that can always obtain a high Q value regardless of the condition of voltage application, even when a thin film made of a dielectric material having a high dielectric constant having a slight piezoelectricity is used as the dielectric film. Become.

  According to the variable capacitance element of the present invention, when the acoustic reflection element is an acoustic multilayer film composed of a plurality of layers having different acoustic impedances, the acoustic reflection element can be formed by a series of thin film processes. In addition, since an air layer is not formed in the vicinity of the capacity forming portion, the manufacturing process is easier than that of a via hole described later, and thermal reliability and mechanical reliability can be increased.

Further, according to the variable capacitance element of the present invention, when the dielectric film is made of Ba _1-x Sr _x TiO ₃ (0 <x <1), the dielectric film exhibits a paraelectric property in an operating temperature range. (However, due to the size and distortion of x.) Since the capacitance change due to voltage application is accompanied by hysteresis, the capacitance is less dependent on the electric field history, so the capacitance change according to the applied voltage is required. The variable capacitance element to be used can be appropriate. In addition, since the hysteresis indicates a delay of the polarization with respect to the alternating current and corresponds to a large dielectric loss, this system which does not exhibit such characteristics is preferable.

  By using such a variable capacitance element of the present invention, it is possible to provide a high-frequency circuit with variable characteristics such as a variable filter and a variable resonator with reduced insertion loss, and a variable characteristic high performance using them. Can provide a simple communication device.

  Hereinafter, an example of an embodiment of a variable capacitance element of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view showing an example of an embodiment of a variable capacitance element of the present invention. In FIG. 1, 1 is a support substrate, 2 is a lower acoustic multilayer film, 3 is a lower electrode film, 4 is a dielectric film, 5 is an insulating film, and 6 is an upper electrode film. , 7 is an upper acoustic multilayer film, 8 is a solder diffusion preventing film, 9 is a protective film, and 10 is a solder terminal. Note that the capacitance forming region in this variable capacitance element is an opposing portion of the electrode that sandwiches the dielectric film 4 between the lower electrode film 3 and the upper electrode film 6 that are a pair of electrodes.

The substrate support substrate 1 is an insulating substrate on which a variable capacitor is formed. Examples of the material include Al ₂ O ₃ , SiO ₂ , MgO, LaAlO ₃ , SrTiO _{3, and the} like. Although the material is not particularly limited, the surface of the film formed on the support base 1 is made smooth. It is preferable to have sufficient flatness.

Lower acoustic multilayer film The acoustic reflection element disposed outside at least one of the pair of electrodes in the variable capacitance element of the present invention can be constituted by, for example, an acoustic multilayer film. This acoustic multilayer film can acoustically insulate the piezoelectrically excited vibration in the dielectric film 4 and is configured by alternately laminating several layers having different acoustic impedances. In order to insulate vibration (acoustic waves) more efficiently, the thickness of each film stacked in multiple layers should be about λ / 4 (where λ is the wavelength of vibration (acoustic waves)). If vibration (acoustic waves) is suppressed in the frequency band, the ratio of the acoustic impedances of the two continuous layers of films should be large. Hereinafter, a frequency region in which vibration is suppressed is referred to as a bandwidth. The lower acoustic multilayer film 2 is an acoustic reflection element disposed outside the lower electrode film 3 that is one electrode.

  In addition to such an acoustic multilayer film, the acoustic reflection element can also be produced by a technique of forming a via hole in the substrate. Specifically, for example, by partially removing the support substrate 1 below the capacitance forming portion of the variable capacitance element by etching, a via hole can be formed and an acoustic wave can be reflected at the interface with air. .

  The acoustic reflection element in the variable capacitance element of the present invention is not limited to these acoustic multilayer films and via holes, and the piezoelectric film excited vibration (acoustic wave) in the dielectric film 4 is used in the variable capacitance element. Various configurations can be used as long as they can be attenuated at the frequency, and other configurations can be used by any other means without departing from the gist of the present invention as long as they exhibit similar functions. May be.

In order to suppress vibration excited in the dielectric film 4, several layers of films having different acoustic impedances are alternately stacked to form the lower acoustic multilayer film 2. At this time, for example, tungsten (W), platinum (Pt), gold (Au), molybdenum (Mo), or the like can be used as the material of the film having high acoustic impedance. For example, silicon (Si), silicon oxide (SiO ₂ ), aluminum (Al), or the like can be used as the material of the film having low acoustic impedance. When laminating these films in multiple layers, if the bandwidth is increased to effectively suppress vibration, the ratio of the acoustic impedance of the two consecutive layers of film is increased in order to increase the reflectance to vibration (acoustic waves). Should be large. However, depending on the combination of films, there are materials with poor adhesion and materials with a low melting point that cause problems in the subsequent manufacturing process. Therefore, materials to be combined are appropriately selected. If a low-resistance acoustic impedance material is selected, the lower acoustic multilayer film 2 can be disposed immediately below the dielectric film 4 to replace a part of the lower electrode film 3, and a variable having a high Q value. The capacitor element can be manufactured by a simple process.

  When only the lower acoustic multilayer film 2 is formed as an acoustic reflection element, unnecessary excitation is caused by the layer structure below the dielectric film 4 (for example, the lower electrode film 3, the adhesion layer, the oxide film layer, and the support substrate 1). Since this can be prevented, the wavelength of vibration is shortened (resonance is on the higher frequency side), and the period is also increased, so that there is an advantage that the frequency at which the Q value is reduced by a change in impedance phase becomes wider. In addition, there is an advantage that the number of manufacturing process steps is reduced.

Lower electrode film In order to obtain a variable capacitance element having a high Q value, it is required to dispose a pair of low resistance electrodes above and below the dielectric film 4 and sandwich the dielectric film 4 therebetween. . As a conductor material of the lower electrode film 3, it is necessary to use a metal having a low resistivity in order to obtain a high Q value. Examples of such metals include copper, aluminum, gold, silver, platinum, and palladium. In particular, in the process of forming the dielectric film 4 which is a high dielectric constant thin film thereon, the variable capacitance element is diffused into the dielectric film 4 or reacted with the dielectric film 4 when accompanied by a high temperature thermal history. Therefore, it is desirable to use platinum, palladium, or the like that hardly reacts or diffuses with the dielectric film 4.

  Although the resonance frequency can be shifted to a higher frequency when the thickness of the lower electrode film 3 is reduced, the loss in the lower electrode film 3 and the upper electrode film 6 is increased by a high resistance material at a high frequency. In other words, there is a problem that the Q value is reduced in the first place. Therefore, it is necessary to set the film thickness in consideration of this point.

Examples of the material of the dielectric film dielectric film 4 include a high dielectric constant dielectric material whose relative dielectric constant can be largely changed by an externally applied voltage, such as perovskite oxides such as BaTiO ₃ , SrTiO ₃ , and BST. It is done. These can be 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, a vapor phase synthesis method such as an MBE (Molecular Beam Epitaxy) method. The dielectric film 4 is formed so as to cover the surface of the lower electrode film 3, for example, and then unnecessary portions are removed by wet etching excluding the capacitance forming region. The thinner the dielectric film 4 is, the higher the fundamental frequency of the resonance frequency becomes, and this is effective in eliminating the influence at the operating frequency. In addition, since the electric field strength in the dielectric film 4 increases as the thickness decreases, there is an advantage that high tunability characteristics can be obtained. On the other hand, the leakage current increases or the effective relative permittivity decreases. Problems may occur. Usually, it is preferable to set the film thickness to 100 nm or more from the viewpoint of ensuring sufficient insulation.

In the variable capacitance element of the present invention, the dielectric film 4 is preferably made of Ba _1-x Sr _x TiO ₃ (0 <x <1). It is also desirable to use a dielectric material that exhibits paraelectric properties in the operating temperature range. This is because when the dielectric film 4 exhibits ferroelectricity, the change in capacitance due to voltage application is accompanied by hysteresis, and therefore the capacitance depends on the electric field history, so that a change in capacitance according to the applied voltage is required. This is because it is not suitable as a variable capacitance element. Hysteresis indicates a delay with respect to alternating current of polarization and is not suitable because it corresponds to a large dielectric loss.

The insulating film insulating film 5 is formed on the dielectric film 4 so as to cover the entire structure including the support substrate 1, the lower acoustic multilayer film 2, the lower electrode film 3 and the dielectric film 4. It is used to ensure insulation between the upper electrode film 6 and the lower electrode film 3 and to reduce the parasitic capacitance generated there. Examples of the material for the insulating film 5 include organic materials such as BCB (benzocyclobutene) and polyimide, and inorganic materials such as SiO ₂ and Si ₃ N _{4. In} order to reduce the parasitic capacitance, the insulating film 5 has high insulation properties. A low dielectric constant is desirable. The formation method is not particularly limited, but it may be formed by a CVD method typified by an MOCVD method or the like that can obtain a relatively uniform film thickness even for an underlying structure having a three-dimensional complicated shape. preferable.

  The insulating film 5 may be made of a material that can be patterned by etching using a resist as a mask to form an opening for forming the upper electrode film 6 on the dielectric film 4. Is preferred. At this time, it is desirable that a terminal opening for exposing the lower electrode film 3 to the upper surface of the variable capacitance element for electrical connection with an external circuit is also formed at the same time.

The upper electrode film 6 constitutes a pair of electrodes sandwiching the dielectric film 4 together with the lower electrode film 3, and is formed using Au, Ag, Al, Cu, Pt or the like as a conductor material. As with the lower electrode film 3, the thickness is set in consideration of resistance in order to obtain a high Q value.

  If a low-resistance acoustic impedance material is selected as the conductor material, the upper electrode film 6 can be used as a part of the upper acoustic multilayer film 7, and a variable capacitance element having a high Q value can be manufactured by a simple process. It becomes possible to do.

In order to efficiently suppress vibration caused by piezoelectricity in the upper acoustic multilayer dielectric film 4, it is preferable to dispose an acoustic multilayer film as an acoustic reflection element outside the electrodes on both sides of the dielectric film 4. . Therefore, preferably, several layers having different acoustic impedances are laminated alternately on the outside of the upper electrode film 6, that is, on the side opposite to the dielectric film 4 to form the upper acoustic multilayer film 7. .

  When only the upper acoustic multilayer film 7 is formed as an acoustic reflection element, unnecessary excitation due to the layer structure above the dielectric film 4 (for example, the upper electrode film 6, the adhesion layer, the insulating film, and the protective film 9) is prevented. Therefore, the wavelength of vibration is shortened (resonance is on the higher frequency side) and the period is also increased, so that there is an advantage that the frequency at which the Q value is reduced by a change in impedance phase becomes wider. In addition, there is an advantage that the number of manufacturing process steps is reduced.

  Further, when the upper acoustic multilayer film 7 is formed together with the lower acoustic multilayer film 2, unnecessary excitation due to the upper and lower layer configurations of the dielectric film 4 can be prevented, and vibration is caused only by the dielectric film 4. Since the vibration wavelength is further shortened (resonance is on the higher frequency side) and the period is further increased, the Q value is extremely reduced by a change in impedance phase. In other words, the frequency region in which the Q value is not reduced by a change in impedance phase can be taken in a wider range.

The protective film protective film 9 mechanically protects the variable capacitance element of the present invention configured as described above from the outside, as well as deterioration of element material due to chemical reaction with atmospheric humidity, oxygen, etc., and dust The element is protected by preventing contamination due to adhesion, deterioration due to breakage during mounting, contamination by chemicals, oxidation, and the like. This protective film 9 is formed so that a part of each of the upper and lower electrode films 6 and 3 is exposed in order to form a solder terminal 10 used for connection with an external circuit. A material for the protective film 9 is preferably a material having high heat resistance and excellent coverage with respect to a step of the underlying structure. Specifically, an organic thermosetting resin such as BCB (benzocyclobutene) resin or a photocurable resin can be used.

The solder terminal 8 is a solder diffusion preventing film formed as a base of the solder terminal 10 prior to the formation of the solder terminal 10, and the solder terminal 8 is soldered during reflow when forming the solder terminal 10 or mounting on an external circuit. It is formed in order to prevent diffusion of solder from the terminal 10 to the lower electrode film 3 and the upper electrode film 6. As the material of the solder diffusion preventing layer 8, Ni is suitable if ordinary solder is used. Further, in order to improve the solder wettability of the film 8 on the surface of the solder diffusion preventing film 8, about 0.1 μm of Au, Cu or the like having high solder wettability may be formed.

  After the solder diffusion prevention layer 8 is formed, solder terminals 10 are formed to facilitate mounting on an external circuit. The solder terminal 10 is formed by printing a solder paste on the solder diffusion preventing film 8 formed in the opening of the protective film 9 and performing reflow.

  The variable capacitance element of the present invention configured as described above is 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. Thus, it is suitably used for high-frequency components such as 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. be able to.

  According to the variable capacitance element of the present invention, it was confirmed by simulation that the piezoelectric vibration generated after voltage application to the dielectric film 4 is suppressed in a wide frequency range, and the decrease in the Q value can be suppressed.

  In this example, the configuration, material, and film thickness of each film of the variable capacitance element of the present invention were set as follows.

A sapphire R substrate having a thickness of 0.3 mm is used as the support substrate 1, and the lower acoustic multilayer film 2 and the upper acoustic multilayer film 7 are alternately replaced with W as a high acoustic impedance material and with SiO ₂ as a low acoustic impedance material. In addition, two layers each having a film thickness of 0.67 μm and 0.74 μm were laminated. On the lower acoustic multilayer film 2, Pt has a thickness of 2 μm as the lower electrode film 3, (Ba _0.5 Sr _0.5 ) TiO ₃ has a thickness of 0.3 μm as the dielectric film 4, and dioxide as the insulating film 5. Silicon is formed with a thickness of 1 μm, Pt is formed as the upper electrode film 6 with a thickness of 2 μm, and a protective film 9 is formed with a benzocyclobutene resin with a thickness of 2 μm. evaluated. The simulation result is shown by a diagram in FIG.

  In FIG. 2, the horizontal axis represents frequency [frequency] (unit: GHz), the vertical axis represents impedance [| Z |] (unit: ohm) on the left side, and the phase [phase] (unit: deg.) On the right side. The upper characteristic curve represents the frequency characteristic of the impedance, and the lower characteristic curve represents the frequency characteristic of the phase of the impedance.

  As shown in FIG. 2, according to the variable capacitance element of the present invention, the impedance phase does not fluctuate over a range of several hundreds of MHz around 2 GHz, and the influence of vibration of the dielectric film 4 is suppressed. It can be seen that it has a high Q value.

  For comparison, FIG. 3 is a diagram similar to FIG. 2 showing a simulation result of a variable capacitance element having a conventional configuration in which upper and lower acoustic multilayer films are not formed. From the results shown in FIG. 3, the periodic change of the impedance phase is all due to the piezoelectric vibration of the dielectric film generated by the voltage application as compared with the variable capacitance element of the present invention shown in FIG. It can be seen in the frequency range that the Q value decreases.

  As can be seen from the results of the above embodiments, according to the variable capacitance element of the present invention, by setting the material and film thickness of each layer based on the above description, for example, at the use frequency of 2 GHz, the impedance phase It is possible to obtain the variable capacitance element of the present invention in which there is no change and the use frequency is set between adjacent peaks among the plurality of resonance peaks.

  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 made without departing from the scope of the present invention. . For example, a via hole may be formed outside the lower electrode film, and an acoustic multilayer film may be formed on the upper electrode film side. In this case, the dielectric crystallinity caused by the formation of the lower acoustic multilayer film It will be possible to eliminate the influence on.

  In addition, the piezoelectricity generated by the applied voltage in the dielectric film of the BST material system as described above is basically small, not the large phase change as seen in the piezoelectric thin film resonator, but the phase characteristic change is very The periodic decrease in the Q value can be confirmed when a variable capacitance element having a high Q value is obtained. In particular, in order to achieve a high Q value in a high frequency band such as a few GHz band, it is necessary to reduce the loss due to the electrodes and at the same time reduce the dielectric loss of the dielectric film having a high dielectric constant. Therefore, the periodic decrease of the Q value after voltage application in such a variable capacitance element has been confirmed by the present inventors as a result of intensively increasing the Q at a high frequency, and the present invention is based on this finding. It has been made.

It is sectional drawing which shows an example of embodiment of the variable capacitance element of this invention. It is a diagram which shows the simulation result of the frequency dependence of the impedance and phase of a capacity | capacitance formation area | region about the Example of the variable capacitance element of this invention. As a comparative example of the variable capacitance element of the present invention, it is a diagram showing a simulation result of the frequency dependence of the impedance and phase of a capacitance formation region for a variable capacitance element having a conventional configuration. It is a diagram which shows the frequency dependence of the phase characteristic of the impedance in the conventional variable capacitance element. It is a diagram which shows the frequency dependence of Q value in the conventional variable capacitance element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Support substrate 2 ... Lower acoustic multilayer film 3 ... Lower electrode film 4 ... Dielectric film 5 ... Insulating film 6 ... Upper electrode film 7 ... Upper acoustic multilayer film 8 ... Solder diffusion prevention film 9 ... Protective film
10 ... solder terminal

Claims (3)

In the variable capacitance element that includes a pair of electrodes and a dielectric film having piezoelectricity sandwiched between the pair of electrodes, and changes a relative dielectric constant of the dielectric film by applying a voltage to the pair of electrodes, A variable capacitance element comprising an acoustic reflection element outside at least one of the electrodes. 2. The variable capacitance element according to claim 1, wherein the acoustic reflection element is an acoustic multilayer film composed of a plurality of layers having different acoustic impedances. The variable capacitance element according to claim 1, wherein the dielectric film is made of Ba _1-x Sr _x TiO ₃ (0 <x <1).

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