JP2011211029A - Variable capacitive element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of a variable capacitive element using a ferroelectric substance that can attain expansion of a variable region of electrostatic capacity, while at the same time, maintaining a high Q-value.SOLUTION: The variable capacitive element using a ferroelectric substance is provided with a ferroelectric layer 21, connected between first and second electrodes 22, 24; the ferroelectric layer 21 has a Perovskite structure that is phase-transformed from cubic crystal to tetragonal phase by compression stress Y that the ferroelectric layer has inside; and by using the ferroelectric layer, the variable capacitive element can be reduced in size.

Description

  The present invention relates to a variable capacitance element using a ferroelectric material applied to a tunable filter, a high frequency capacitor device, and the like.

  In recent years, in electronic parts such as a tunable filter, a variable capacitance element having a wide variable range of capacitance with respect to an applied voltage has been desired.

In response to such a demand, it has been conventionally studied to increase the variable range of the capacitance of the variable capacitance element by changing the composition of the ferroelectric layer between the electrodes. For example, in a capacitor using (Ba, Sr) TiO ₃ (hereinafter referred to as “BST”) for the ferroelectric layer, the composition ratio of Ba to Sr is set to a certain value (for example, Sr / Ba = 1/9), the perovskite structure, which is the crystal structure of BST, becomes tetragonal and can be largely polarized, so the variable range of the capacitance of the capacitor becomes large.

  Unfortunately, however, a capacitor using a BST ferroelectric layer having a tetragonal crystal structure in this way has a wide capacitance variable range, but on the other hand, the Q value (in the variable capacitance element) As a result, the electrical energy loss (reciprocal value of tan δ) is lowered, which makes it difficult to apply to various electronic components. For example, when used for a tunable filter, a frequency other than the frequency to be selected is detected as a signal and cannot be applied.

  For this reason, there is a demand for a variable capacitance element using a ferroelectric material to realize an expansion of the variable range of capacitance while maintaining a high Q value.

  As one method for realizing this, there is known a variable capacitance element provided with means for applying an external stress to the first dielectric layer 101 as shown in FIG. The variable capacitance element includes a first capacitor as a means for applying external stress in a capacitor in which a first electrode layer 102, a first dielectric layer 101, and a second electrode layer 103 are sequentially stacked on a substrate. A structure in which a piezoelectric element 107 including a third electrode layer 105, a second dielectric layer 104, and a fourth electrode layer 106 is provided under the electrode layer 102 (see, for example, Patent Document 1).

JP 2008-85081 A

  However, as described above, in the variable capacitance element, the first means as means for applying external stress to the first dielectric layer 101 in order to realize expansion of the variable range of the capacitance while maintaining a high Q value. In the case where the piezoelectric element 107 is provided under the electrode layer 102, another layer constituting the piezoelectric element 107 is required in addition to the layers (101 to 103) constituting the variable capacitance element. For this reason, the size of the variable capacitance element has increased. When the size of the variable capacitance element becomes large, it becomes undesirable when used in a portable electronic device or the like that is required to be small and light.

  Accordingly, an object of the present invention is to reduce the size of a variable capacitor using a ferroelectric.

  In order to achieve the above object, the present invention provides a variable capacitance element using a ferroelectric, comprising a ferroelectric layer connected between the first and second electrodes, wherein the ferroelectric layer is formed by itself. The intended purpose is achieved by using a variable capacitance element characterized by having a perovskite type structure transformed from a cubic crystal to a tetragonal crystal by an internal compressive stress.

  As described above, the present invention is a variable capacitance element using a ferroelectric, and includes a ferroelectric layer connected between the first and second electrodes, and the ferroelectric layer is held inside itself. By using a variable capacitance element characterized by having a perovskite type structure transformed from a cubic crystal to a tetragonal crystal by compressive stress, the variable capacitance element can be reduced in size.

  In other words, compressive stress is inherent in the ferroelectric layer itself, and the crystal structure of the ferroelectric layer is transformed from cubic to tetragonal by the compressive stress, so that even without using means for applying external stress, The variable range of the capacitance can be expanded while keeping the Q value of the variable capacitance element high.

  More specifically, a variable capacitance element using a paraelectric layer having a cubic perovskite structure originally has a property that the Q value is high but the variable range of the dielectric constant is small. However, the cubic paraelectric layer can be transformed into a tetragonal ferroelectric layer without changing the composition by making compressive stress inherent. As a result, the dielectric constant can be greatly varied while keeping the Q value of the variable capacitance element high. That is, in the variable capacitance element, the high Q value can be maintained and the variable range of the capacitance can be expanded without using an external stress applying means, and thus the variable capacitance element can be reduced in size. This can also be applied to high-frequency components that require a large variable range although the capacitance of the variable capacitance element is small.

  In addition to the effect of miniaturization, the variable capacitance element of the present invention can further increase the Q value for the following two reasons.

  One is that the thin film is formed while applying compressive stress, so that the formed thin film is formed more densely, so that the Q value can be increased. Specifically, in the thin film formation stage, each crystal grain and grain boundary forming the thin film are subjected to compressive stress in a direction perpendicular to the film forming direction, and thus form a thin film while aggregating each other. Therefore, the formed thin film has a structure with few crystal defects and high crystallinity, and as a result, the Q value of the variable capacitance element is improved.

  The other is that since the leakage current generated when an electric field is applied can be suppressed, the Q value of the variable capacitance element can be increased. Specifically, since a ferroelectric layer is polarized and extends in the direction of an electric field when a voltage is applied, when a voltage is applied to the ferroelectric layer, stress is applied to an interface between a portion to which voltage is not applied and a portion to which voltage is not applied. Due to the stress, crystal defects due to state variation occur near the interface. This crystal defect causes a leakage current and lowers the Q value of the variable capacitance element. However, if the compressive stress is held inside, the direction of the compressive stress coincides with the direction of stress applied to the interface, so that the stress applied to the interface can be relaxed. As a result, the leakage current can be suppressed and the Q value of the variable capacitance element can be increased.

  Furthermore, since the present invention changes the stress applying means for the ferroelectric layer from the external means using the piezoelectric element as in the prior art to the means for holding it inside the ferroelectric layer, the following effects can also be expected. .

  First, since the number of film forming steps can be reduced, labor and time can be reduced and costs can be reduced, thereby improving productivity. In addition, since a piezoelectric element that is an external means is not required, a circuit for driving the piezoelectric element is not required, and the circuit structure is simplified. Furthermore, quality problems such as quality deterioration due to a high-temperature treatment process after formation of the ferroelectric layer, Q value reduction caused by delamination between layers due to external stress and generation of crystal defects in the layer are also improved.

Schematic of counter target sputtering apparatus in Embodiment 1 of the present invention Sectional view of FIG. Sectional drawing of the variable capacitance element in Embodiment 1 of this invention Model diagram of crystal structure of ferroelectric layer of variable capacitance element in embodiment 1 of the present invention Hysteresis characteristic graph of variable capacitance element in Embodiment 1 of the present invention (A) Crystal structure analysis diagram by X-ray diffraction method, (B) Multiple peak separation analysis at 0.25 mTorr Schematic of the counter target sputtering apparatus with an ECR radical source in Embodiment 2 of the present invention Sectional view of prior art variable capacitance element

(Embodiment 1)
The variable capacitance element described in this embodiment is formed as a thin film with reference to a method described in Japanese Patent Application Laid-Open No. 2009-30133, but there is a difference that an ECR radical source is not used.

A schematic view of the thin film forming apparatus of the present embodiment is shown in FIG. 1, and a cross-sectional view is shown in FIG. The thin film forming apparatus shown in FIGS. 1 and 2 forms a (Ba _0.4 Sr _0.6 ) TiO ₃ compound film 22 (hereinafter referred to as a BST film) on a base material (silicon substrate 6) by sputtering a target. It is. Hereinafter, description will be given mainly with reference to FIG.

As shown in FIG. 2, the thin film forming apparatus of the present embodiment includes a sputtering chamber 1, two targets 2 that face each other in parallel, a gas introduction unit 3 disposed in the vicinity of the target 2, and a substrate holder 4 are arranged. In this embodiment, (Ba _0.4 Sr _0.6 ) TiO ₃ with Ba / Sr = 4/6 is used as the target 2 and connected to the cathode 5.

  In the present embodiment, the substrate holder 4 is in a floating state or connected to the ground. The substrate holder 4 has a heating function and can heat the silicon substrate 6 as a base material.

  The gas introduction unit 3 is configured to generate argon gas as a sputtering gas and oxygen gas as a reactive gas.

  These gases are ionized to generate radicals, cations and anions. In the present embodiment, the ratio of the introduced oxygen gas to the sputtering gas is 1: 4, and the gas pressure is about 0.25 mTorr.

  In addition, magnets 7 are arranged on the back of each target 2. The magnet 7 arranged on one target 2a is arranged so that the north pole faces inward (opposite space) and the other target 2b faces the south pole inward (opposite space). Therefore, these magnets 7 cause magnetic lines of force to run in the facing direction of the target 2, that is, from one target 2 a to the other target 2 b, and a magnetic field is generated in the facing space of the target 2.

  In this embodiment, the cooling water inlet 8 and the cooling water outlet 9 are also provided in the cathode 5 so that the target 2 can be cooled by the cooling water. Thereby, the thermal stress to the target 2 is reduced.

  In the present embodiment, a short circuit is prevented between the cathode 5 and the sputtering chamber 1 via the matching unit 10 (shown in FIG. 1). Further, only the target 2 is sputtered by the earth shield 11 provided on the outer periphery of the target 2.

  Next, a thin film forming method using the thin film forming apparatus will be described.

First, a silicon substrate 6 serving as a base material is placed on the substrate holder 4 shown in FIG. 2, and the inside of the sputtering chamber 1 is evacuated to 1.0 × 10 ⁻⁶ mTorr or less by the vacuum pump 12. Further, the silicon substrate 6 is heated to a temperature at which the BST film 22 is crystallized using the heating function of the substrate holder 4. Here, the temperature is measured when a thermocouple is attached to another silicon substrate 6 in advance and the temperature when measured is 520 ° C.

Next, in the sputtering chamber 1, argon gas, which is a sputtering gas, and oxygen gas, which is a reactive gas, are introduced from a gas introduction unit 3 through a gas flow rate controller 13 (shown in FIG. 1). When the pressure is adjusted to be the process gas pressure (here, 0.25 mTorr) and a negative voltage is applied to the target 2, the argon gas or oxygen gas is ionized into ions and electrons to form plasma, and mainly argon ions ( Ar ⁺ ) collides with the target and the target particles jump out of the target 2. In the present embodiment, a high frequency power supply 14 having a power density of 2.7 W / cm ² is used as a voltage supply source.

  Then, the target particles before and after the reaction adhere to the surface of the silicon substrate 6 and are gradually deposited, so that the BST film 22 can be formed.

  The effect in this Embodiment is demonstrated below.

The effect of the present embodiment will be described using a variable capacitor formed on the silicon substrate 6 (SiO ₂ (70 nm) / Si) shown in FIG. This is because the lower electrode 23 is made of Pt / Ti (100 nm / 10 nm), the ferroelectric layer 21 is made of BST film 22 (250 nm), the upper electrode 24 is made of Au (300 nm), the lower electrode 23 is made of parallel plate magnetron sputtering, strong The dielectric layer 21 is formed by the thin film forming method of the present embodiment, and the upper electrode 24 is formed by electron beam evaporation.

  When the BST film 22 is formed by the thin film forming method of the present embodiment described above, the BST film 22 contains a compressive stress Y (shown by an arrow in FIG. 3), and the crystal structure is changed from cubic to square by the compressive stress Y. It is formed in a state transformed into a crystal. Therefore, the variable capacitor using the BST film 22 formed in the present embodiment can realize a high Q value and expansion of the variable range of capacitance without using means for applying external stress. The size of the capacitor can be reduced.

  It is formed in the process in which the kinetic energy in the compression direction (arrow direction in FIG. 3) horizontal to the surface of the silicon substrate 6 possessed by the target particles jumping out from the opposing target 2 adheres to the silicon substrate 6 and gradually accumulates. This is because the compressive stress Y remains in the BST film 22.

  Then, as shown in FIG. 4, when the crystal structure of the BST film 22 is formed while being subjected to the compressive stress Y, it undergoes a phase transformation from cubic to tetragonal.

  Originally, the BST film 22 of Ba / Sr = 4/6 has a cubic crystal structure, and a variable capacitor using the BST film 22 has a high Q value but a narrow capacitance variable range. For example, in this embodiment, when the total pressure in the sputtering chamber 1 is 5 mTorr, the BST film 22 has a cubic crystal structure as shown in FIG. 4A, and the hysteresis shown in FIG. As can be seen from the characteristics, the variable range of the dielectric constant was narrow.

  However, by forming the BST film 22 by the thin film formation method of the present embodiment, as shown in FIG. 4B, the crystal structure is transformed into a tetragonal crystal, so that the variable range of the dielectric constant is expanded. .

  As can be seen from the hysteresis characteristics shown in FIG. 5B, as shown in FIG. 4B, the BST film 22 having a tetragonal crystal structure has a cubic crystal structure (FIG. 4A). ), The ferroelectric layer 21 can be largely polarized.

  Therefore, in the variable capacitor using the BST film 22, the variable range of the capacitance can be increased while maintaining a high Q value.

In addition, it can confirm that the compressive stress Y remains in the BST film | membrane 22 formed in this Embodiment by the residual stress analysis (2D method) by X-ray diffraction method. In this analysis, the stress value can be analyzed from the Debye strain. When the residual stress values in the two directions σ11 and σ22 in the in-plane direction of the substrate were measured from the Debye's strain, the BST film formed with the total pressure in the sputter chamber 1 at 5 mTorr showed a residual tensile stress of 160 to 200 MPa. It could be confirmed. On the other hand, in the BST film formed with the total pressure in the sputtering chamber 1 being 0.25 mTorr, a residual compressive stress of 700 to 870 MPa was confirmed. In addition, it can also be confirmed by X-ray diffraction sin ² Ψ measurement.

  Further, it can be confirmed by the crystal structure analysis by the X-ray diffraction method shown in FIG. 6 that the crystal structure of the BST film 22 formed in this embodiment is transformed from cubic to tetragonal. From the analysis result of FIG. 6A in which the BST film 22 formed by changing the total pressure in the sputtering chamber 1 from 5 mTorr to 0.25 mTorr is compared, the peak shape of the BST film 22 at 5 mTorr is symmetrical. A peak can be confirmed at a position that coincides with the interplanar spacing of BST (100). On the other hand, at 0.25 mTorr, the peak top position shifts to the low angle side, and the peak shape is not symmetrical, so it can be seen that the crystal is composed of crystals having a plurality of interplanar spacings. From FIG. 6B, which shows the result of the separation analysis of multiple peaks at 0.25 mTorr, the two peaks at 2θ = 22.04 ° and 2θ = 22.28 ° are fitted with the Pseudo-Voigt function. In addition to the crystal having a face spacing of 3.98Å, the peak of the crystal having a face spacing of 4.03Å can be confirmed. From this, it can be seen that the BST film 22 at 0.25 mTorr has a different a-axis and c-axis length, that is, a tetragonal crystal structure rather than a cubic crystal structure.

  From the above, it can be seen that the BST film 22 formed in the present embodiment is formed while being loaded with the compressive stress Y, so that the crystal structure is transformed from cubic to tetragonal.

  Therefore, the variable capacitance element using the BST film 22 formed in this embodiment can realize a high Q value and expansion of the variable range of the capacitance without using a means for applying external stress. The size of the capacitor can be reduced. The variable capacitance element having this characteristic can be applied to a high frequency component that requires a small capacitance and a large variable range.

  In addition to the effect of downsizing, the variable capacitance element using the BST film 22 formed in the present embodiment can further increase the Q value for the following three reasons.

  First, since the thin film is formed while applying the compressive stress Y, the formed BST film 22 is formed more densely, so that the Q value can be increased. Specifically, in the thin film formation stage, each crystal grain and grain boundary forming the BST film 22 receives a compressive stress Y in a direction perpendicular to the film forming direction, so that the BST film 22 is formed while agglomerating each other. To do. Therefore, the formed BST film 22 has a structure with few crystal defects and high crystallinity, and as a result, the Q value of the variable capacitance element can be increased.

  Second, since the leakage current generated when an electric field is applied can be suppressed, the Q value of the variable capacitance element can be increased. Specifically, since the ferroelectric layer 21 is polarized when a voltage is applied and extends in the direction of the electric field, when a voltage is applied to the ferroelectric layer 21, the interface X between the portion to which the voltage is not applied and the portion to which the voltage is not applied (FIG. 3). In FIG. 2, a stress is applied to the dotted line). Due to the stress, near the interface X, a crystal defect due to state variation occurs. This crystal defect causes a leakage current and lowers the Q value of the variable capacitance element. However, if the compressive stress Y is held inside, the compressive stress Y direction and the stress direction applied to the interface X coincide with each other, so that the stress applied to the interface X can be relaxed. As a result, the leakage current can be suppressed and the Q value of the variable capacitance element can be increased.

  Third, since the roughness of the surface structure of the BST film 22 to be formed can be reduced while maintaining the size of the crystal grains, that is, without making the crystal grains fine, the high dielectric constant is maintained. As a result, the contact resistance with the electrode interface layer can be reduced, and as a result, the Q value can be increased.

  In addition, in the present embodiment, when forming a thin film, a buffer layer or a seed layer is provided between the silicon substrate 6 and the lower electrode 23 or at the interface between the electrode layers 23 and 24 and the ferroelectric layer 21. By forming it, the orientation of the ferroelectric layer 21 and the electrode layers 23 and 24 or the ferroelectric layer 21 can be controlled. Therefore, by orienting the crystals that transform to tetragonal crystals in one direction, the BST film 22 containing the compressive stress Y can be preferentially grown in the c-axis direction (electric field direction), and the hysteresis characteristics are sharp. Can be. That is, since a large polarization can be obtained at a low voltage, the performance as a variable capacitance element is improved and it is suitable for application to a memory or the like.

  Further, in the present embodiment, the means for applying stress to the ferroelectric layer 21 is held inside the ferroelectric layer 21 from the external means using the piezoelectric element 107 as in the prior art shown in FIG. The following effects can also be expected.

  First, since the number of film forming steps can be reduced, labor and time can be reduced and costs can be reduced, thereby improving productivity. Further, since the piezoelectric element 107 which is an external means is not required, a circuit for driving the piezoelectric element is not required, and the circuit structure is simplified. Furthermore, quality problems such as quality deterioration due to a high-temperature treatment process after the formation of the ferroelectric layer 21 and reduction in Q value caused by delamination due to external stress and generation of crystal defects in the layer are also improved.

  As a means for applying external stress, it may be possible to apply external stress to the ferroelectric layer 21 by making the electrode layers 23 and 24 contain stress, but the electrode layers 23 and 24 are made of metal. Since this is a layer, the thermal expansion change is large, and the thermal expansion change of the electrode layers 23 and 24 is not preferable because the stress contained in the electrode layers 23 and 24 is greatly changed.

  In contrast, in the present embodiment, since the compressive stress Y is inherent in the ferroelectric layer 21 itself, the change in the electrode layers 23 and 24 and the influence of the material are small. Rise.

In the present embodiment, the (Ba _x Sr _1-x ) TiO ₃ film is taken as an example of the compound film 21 to be formed, but Ba (Ti _1-x Zr _x ) O ₃ , PbTiO ₃ , Pb (Zr) is used. _It can also be applied to metal oxide films such as _1-x , Ti _x ) O ₃ . In this embodiment, oxygen gas is used as the reactive gas, but nitrogen gas, methane gas, or the like may be used. Thus, this embodiment can be applied to the formation of a metal nitride film or an organometallic compound film. Moreover, although the silicon substrate 6 was mentioned as a base material, if it can endure the high temperature at the time of compound film 21 formation, and can suppress the spreading | diffusion with a substrate structural element, an electrode, and a ferroelectric substance, There is no particular limitation, and the present invention can be applied to SiO ₂ , glass, Al ₂ O ₃ substrates, MgO substrates, and the like.

  Further, in the present embodiment, a gas different from the sputtering gas and the reactive gas is used, but the same gas may be used when the cations of the reactive gas function as the sputtering gas.

(Embodiment 2)
The thin film formation method described in this embodiment is a thin film formation method using an ECR radical source in Embodiment 1, and is performed with reference to the thin film formation method described in JP-A-2009-30133.

  The difference from Embodiment 1 will be described below in detail.

  The thin film forming apparatus includes a sputtering chamber 1 and an ECR (Electron Cyclotron Resonance) apparatus (high density radical source) attached to the sputtering chamber 1.

  As shown in FIG. 7, the thin film forming apparatus includes two targets 2 facing in parallel in the sputtering chamber 1, a gas introduction unit 3 disposed in the vicinity of the target 2, an ECR device 17, and the like. Opposing substrate holders 4 are arranged.

  Further, the ECR device 17 used in the present embodiment is arranged so that the radical emission surface thereof faces the facing space of the target 2 from a direction substantially perpendicular to the facing direction of the target 2. That is, in the present embodiment, the opposing surface of the target 2 and the radical emission surface of the ECR device 17 are in a substantially vertical relationship. A plasma atmosphere is formed by the ECR device 17 to release radicals and the like from a direction substantially perpendicular to the facing direction of the target 2.

  In the present embodiment, the substrate holder 4 and the ECR device 17 are opposed to each other through the facing space.

  Note that the ECR device 17 used in the present embodiment sends a microwave (2.45 GHz) to the plasma generator 18 via the waveguide 19 to cause discharge.

A magnetic field generation unit 20 using a magnetic coil or a permanent magnet capable of forming a magnetic flux density of 875 G is disposed on the outer periphery of the plasma generation unit 18, thereby applying a magnetic field in the axial direction of the plasma generation unit 18. . An electric field rotates around the magnetic field lines in the magnetic field, and electrons are rotated and accelerated by the electric field. Then, the electron rotation frequency and the microwave frequency are made to coincide with each other to resonate, and the energy of the microwave is efficiently absorbed by the electrons. This phenomenon is called ECR (electron cyclotron resonance). When the electrons heated by the ECR collide with the reactive gas (oxygen gas) introduced from the gas introduction unit 3, the oxygen gas is converted into radicals (O ⁺ ), cations (O ⁺ ), anions (O ⁻ ). Is generated.

  Next, a thin film forming method using the thin film forming apparatus will be described.

First, a silicon substrate 6 serving as a base material is placed on the substrate holder 4 shown in FIG. 7, and the inside of the sputtering chamber 1 is evacuated to 1 × 10 ⁻⁶ Torr or less by the vacuum pump 12. Further, the silicon substrate 6 is heated to a temperature at which the BST film 22 is crystallized using the heating function of the substrate holder 4. Here, the temperature is measured when a thermocouple is attached to another silicon substrate 6 in advance and the temperature when measured is 520 ° C.

Next, in the sputtering chamber 1, when argon gas, which is a sputtering gas, is introduced from the gas introduction unit 3 and a negative voltage is applied to the target 2, the argon gas is ionized into ions and electrons to form plasma, and ions (Ar ⁺ ) Collides with the target 2 and the target 2 particles jump out of the target 2. In the present embodiment, a high frequency power supply 14 having a power density of 2.7 W / cm ² is used as a voltage supply source.

  When oxygen radicals are introduced from the ECR device 17 into the sputtering chamber 1, the oxygen radicals react with the aforementioned target 2 particles. In this way, by supplying oxygen radicals from the ECR apparatus 17 and reacting with the target 2 particles, a compound film with few oxygen vacancies can be formed.

  Then, the target 2 particles after the reaction adhere to the surface of the silicon substrate 6 and are gradually deposited, so that the BST film 22 can be formed.

  In addition to the effects described in the first embodiment, this embodiment has the effects described below.

  Since the present embodiment can suppress the collision of negative ions with the compound film 21, the BST film 22 with few crystal defects and high crystallinity can be formed.

  In the present embodiment, two targets 2 are opposed to each other as shown in FIG. 7, and a magnetic field directed from one target 2 to the other target 2 is generated. A magnetic field is formed. Then, negative ions are emitted from a direction substantially perpendicular to the direction of the magnetic field. Therefore, since the negative ions perform a cyclotron motion in the magnetic field, the negative ions are trapped in the magnetic field, thereby suppressing the progress of the negative ions, and as a result, the crystallinity can be improved.

  Furthermore, when the ECR apparatus 17 is used, radicals can be generated with high efficiency, while negative ions are generated more. Further, when radicals, ions, and the like are generated at a high density, it becomes easy to go straight to the sputtering chamber 1 side having a relatively low magnetic flux density.

  Therefore, the present embodiment has a remarkable effect on the formation of the compound film 21 having excellent crystallinity by suppressing the collision of negative ions and reducing the damage to the compound film 21.

  In addition to this, in the present embodiment, active oxygen radicals are efficiently introduced, and collision of oxygen ions is suppressed, so that the loss of atoms in the crystal of the compound film 21 is suppressed. The compound film 21 having high reliability with respect to insulation can be formed.

In the present embodiment, the (Ba _x Sr _1-x ) TiO ₃ film is taken as an example of the compound film 21 to be formed. However, as in the first embodiment, Ba (Ti _1-x Zr _x ) O _{3 is used.} , PbTiO ₃ , Pb (Zr _1-x , Ti _x ) O ₃ and other metal oxide films can also be applied. In this embodiment, oxygen gas is used as the reactive gas, but nitrogen gas, methane gas, or the like may be used. Thus, this embodiment can be applied to the formation of a metal nitride film or an organometallic compound film. Moreover, although the silicon substrate was mentioned as a base material, if it can endure the high temperature at the time of compound film 21 formation and can suppress the spreading | diffusion with a substrate structural element, an electrode, and a ferroelectric, especially. There is no regulation, and it can be applied to SiO ₂ , glass, Al ₂ O ₃ substrate, MgO substrate and the like.

  Further, in the present embodiment, a gas different from the sputtering gas and the reactive gas is used, but the same gas may be used when the cations of the reactive gas function as the sputtering gas. Further, since the high-density radical source such as the ECR apparatus 17 can also generate ions with high density, the sputtering gas may be released from the high-density radical source in the same manner as the reactive gas.

  The variable capacitance element of the present invention can be used for a variable capacitance element applied to a tunable filter, a high frequency capacitor device, or the like.

DESCRIPTION OF SYMBOLS 1 Sputter chamber 2, 2a, 2b Target 3 Gas introduction part 4 Substrate holder 5 Cathode 6 Silicon substrate 7 Magnet 8 Cooling water inlet 9 Cooling water outlet 10 Matching device 11 Earth shield 12 Vacuum pump 13 Gas flow controller 14 High frequency power source 15 Phase Controller 16 Oscilloscope 17 ECR device (High-density radical source)
18 Plasma generator 19 Waveguide 20 Magnetic field generator 21 Ferroelectric layer, compound film 22 BST film 23 Lower electrode 24 Upper electrode 101 First dielectric layer 102 First electrode layer 103 Second electrode layer 104 Second Second dielectric layer 105 Third electrode layer 106 Fourth electrode layer 107 Piezoelectric element

Claims (4)

Comprising a ferroelectric layer connected between the first and second electrodes;
The variable capacitance element, wherein the ferroelectric layer has a perovskite structure that is transformed from a cubic crystal to a tetragonal crystal by a compressive stress contained therein. A substrate, the first electrode stacked on the substrate, the ferroelectric layer stacked on the first electrode, and the second electrode stacked on the ferroelectric layer. The variable capacitance element according to claim 1. The ferroelectric layer is
A sputter chamber;
Two targets opposed to each other in the sputtering chamber;
A substrate holder facing the facing space of the target from a direction substantially perpendicular to the facing direction of these targets,
Magnets are arranged on the back of each target,
With these magnets, a magnetic field is generated in the facing direction of the target,
2. The variable capacitance element according to claim 1, wherein the variable capacitance element is formed by a thin film forming method in which a negative voltage is applied to each of the targets to form a compound film on a base material placed on the substrate holder. The said ferroelectric layer was further formed with the said thin film formation method provided with the high-density radical source so that it might face the opposing space of the said target from a different direction from the said substrate holder. Variable capacitance element.

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