JP2001077437A - Variable capacitance element and voltage controlled oscillator - Google Patents

Abstract

[PROBLEMS] To provide a voltage controlled oscillator having a sufficient frequency variable width, which is easy to manufacture, can operate practically in a center frequency band of 2 GHz, and a variable capacitance element used for the same. SOLUTION: On a ferroelectric thin film or an antiferroelectric thin film, a comb-shaped electrode comprising a pair of electrode fingers facing each other is provided,
A variable capacitance element using a change in reactance of the comb-shaped electrode, which is generated by applying a variable control voltage between opposing electrode fingers of the comb-shaped electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a variable capacitance element used in a phase locked loop of mobile communication and the like, and a voltage controlled oscillator using the variable capacitance element.

[0002]

2. Description of the Related Art In recent years, mobile phones and digital cordless phones using the 800 MHz band to the 1.9 GHz band have rapidly spread, and in order to cope with an increase in the number of subscribers and to realize a worldwide common telephone, IMT2000 has been developed. The introduction of a system and the like is under consideration. Phase 1 of this IMT2000 system
Therefore, introduction of a system with a maximum bit rate of up to 2 Mbps is being studied. This system has a bandwidth of 5MHz
A wideband CDMA system expanded to / 10 MHz / 20 MHz and a TDMA system incorporating frequency hopping were studied.
Since the former uses a direct spreading code, the currently used bandwidth is 5 MHz to 1 due to interference of multiple radio wave propagation paths.
0 MHz is said to be the limit, and the development of a new interference cancellation technology is indispensable. The latter requires a high-speed synthesizer for frequency hopping and has a center frequency of 2 GHz.
A voltage-controlled oscillation circuit that operates in the z band and has a frequency variable width of 5 MHz to 20 MHz (normalized frequency variable width of 0.25% to 1%) is required.

[0003] Piezoelectric bulk vibration elements and surface acoustic wave resonators are used as voltage-controlled oscillators in combination with varactor diodes.
At a frequency in the 2 GHz band, such as the T2000 system, the resistance component increases, causing a significant decrease in the Q value, and lowering the C / N, which is not practical.

By the way, in order to improve convenience, it is strongly required that these systems be smaller than before, and miniaturization of portable equipment requires miniaturization of components used. From the conventional discrete components, bare chip mounting and further monolithicization are considered effective. Further, in order to be small in size and used for a long time, it is preferable to set the consumption voltage to about 3 to 5 V. Japanese Patent No. 2,872,054 describes a thin-film voltage-tunable semiconductor bulk acoustic resonator. In this resonator, a via hole is formed from the back surface of the substrate, a first electrode is formed on the surface of the substrate, a thin-film resonator is formed thereon, and a second electrode is formed thereon. . In this resonator, a DC bias voltage is applied between a first electrode and a second electrode via a choke coil from a variable voltage source, and oscillation is performed at a resonance frequency different from the resonance frequency when no voltage is applied. However, the frequency variable width of this resonator is about 1000 ppm even when the DC bias voltage is changed between -30 V and 30 V, which is insufficient for application to the above application.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a voltage controlled oscillator which is easy to manufacture, can operate practically in a center frequency band of 2 GHz, and has a sufficient frequency variable width. That is, to realize a variable capacitance element.

[0006]

The above object is achieved by the following (1).
This is achieved by any one of the above configurations (6) to (6). (1) A comb-shaped electrode comprising a pair of opposing electrode fingers is provided on a ferroelectric thin film or an antiferroelectric thin film, and a variable control voltage is applied between the opposing electrode fingers of the comb-shaped electrode. A variable capacitance element utilizing a change in reactance of the comb-shaped electrode caused by the above operation. (2) After applying a voltage equal to or higher than the saturation voltage in the hysteresis curve of the accumulated charge amount of the ferroelectric thin film or the antiferroelectric thin film between the opposing electrode fingers of the comb-shaped electrode, the variable control voltage is reduced. The variable capacitance element according to (1), wherein the reactance is changed by applying the control voltage or applying a control voltage having a polarity opposite to the saturation voltage in a state where the saturation voltage is applied. (3) The variable capacitance element according to the above (1) or (2), wherein the ferroelectric thin film contains a Pb-containing perovskite crystal or BaTiO ₃ crystal, and the antiferroelectric thin film contains a PbZrO ₃ crystal. (4) a series connection element in which the variable capacitance element according to any one of (1) to (3) and an oscillation element including a piezoelectric bulk vibration element or a surface acoustic wave resonator are connected in series;
A voltage-controlled oscillation device in which impedance characteristics at both ends of the series-connected element are controlled by applying the variable control voltage to the variable capacitance element. (5) The voltage controlled oscillator according to (4), wherein the oscillation element shares a ferroelectric thin film or an anti-ferroelectric thin film provided in the variable capacitance element. (6) When used as a voltage-controlled oscillator in a phase-locked loop, an input set frequency when operating the phase-locked loop matches an oscillation frequency when a saturation voltage is applied to the variable capacitance element, or The voltage controlled oscillator according to any one of the above (1) to (5), which is used in a phase locked loop after a voltage equal to or higher than the saturation voltage is applied to the variable capacitance element.

[0007]

FIG. 1 shows an example of the configuration of a voltage controlled oscillator according to the present invention. This device has a Si substrate 11 on which a diaphragm 10 is formed at a predetermined position by etching from the back surface, and as a buffer layer 12 thereon,
It has a silicon oxide / zirconia oxide layer / barium titanate layer, has a base electrode 13 made of platinum on it, and has a thickness of 0 formed by MBE, multi-source evaporation, RF magnetron sputtering, etc. It has a ferroelectric thin film 14 made of a 0.5 μm PZT epitaxial thin film. This PZT epitaxial thin film has a Zr: Ti ratio of 25:
75. Furthermore, on the ferroelectric thin film 14, an upper electrode 15 is formed in a region corresponding to the diaphragm 10, and a comb-shaped electrode 16 is formed in a region outside the diaphragm 10. These electrodes are formed using a photolithography technique, and are all made of Al.

The upper electrode 15 forms a piezoelectric bulk vibration element together with the ferroelectric thin film 14 and the base electrode 13, and the comb-shaped electrode 16 forms a variable capacitance element together with the ferroelectric thin film 14.

One terminal of the variable capacitance element and one terminal of the piezoelectric bulk vibration element are connected in series using a lead-out wiring 17, and a connection point is a terminal 18a. The other terminal 18b of the variable capacitance element and the other terminal 18c of the piezoelectric bulk vibration element are used as terminals for connection to external terminals, respectively.

FIG. 3 is a sectional view of the variable capacitance element having the comb-shaped electrode 16 shown in FIG. 1 as viewed from the direction of the piezoelectric bulk vibration element 15 in FIG. The reference numerals of the components are the same as those in FIG. Here, a value obtained by dividing the line width 30 of the comb-shaped electrode by the pitch 31 of the comb-shaped electrode is defined as a metallization ratio. Also,
The thickness of the electrode is indicated at 32. In this variable capacitance element, the resonance frequency changes by changing the line width 30 of the comb-shaped electrode, the metallization ratio, and the electrode film thickness 32. Therefore, the film thickness of the ferroelectric thin film 14 is changed in order to change the resonance frequency of the variable capacitance element. There is no need to control the thickness. The capacitance of the comb-shaped electrode 16 can be freely set by changing the number N of comb-shaped electrodes, the opening length W, and the metallization ratio. The capacitance when the metallization ratio is set to 0.5 is given by WNε _eff . Here, ε _eff is the effective permittivity. Since the variable control voltage is applied between the opposing electrode fingers of the comb-shaped electrode, when the interval between the comb-shaped electrode fingers is reduced, the electric field 33 applied to the thin film increases, and the ferroelectric thin film 14 is applied by a relatively small applied voltage.
Is obtained. Conversely, if the distance between the comb-shaped electrode fingers is widened, the saturation capacity of the thin film will not be reached unless a large voltage is applied. Therefore, the distance between the electrode fingers may be set in consideration of the voltage value that can be used in the oscillation circuit. The voltage applied to the comb-shaped electrode is set to such an extent that discharge breakdown does not occur.

As will be described later, it is necessary to shift the resonance frequency of the variable capacitance element from the resonance frequency of the oscillation element. The resonance frequency of the variable capacitance element is controlled to a desired value by controlling the distance between the fingers of the comb-shaped electrode. Can be controlled.

When a high-frequency power is applied from the terminals 18a and 18c, the piezoelectric bulk vibration element constituting the oscillation element resonates at a frequency inversely proportional to the thickness of the ferroelectric thin film 14. Then, in the variable capacitance element connected in series to the oscillation element, when the capacitance value increases, the resonance frequency moves to the high frequency side, while the anti-resonance frequency does not change.

In the voltage controlled oscillator shown in FIG. 1, when a variable control voltage is applied to the variable capacitance element to change the capacitance, the impedance characteristic at both ends of the voltage controlled oscillator changes as shown in FIG. I do. FIG. 2 shows that after applying the same voltage as the saturation voltage of the ferroelectric thin film as a control voltage,
The case where the applied voltage is promptly reduced is shown. When the capacitance value of the variable capacitance element is reduced by changing the control voltage in this manner, as a result, the resonance frequency moves to a lower frequency side, while the anti-resonance frequency does not change. Specifically, by changing this variable control voltage from 4V → 2V → 0V,
The absolute value of the impedance is 20 → 21 → 2 in FIG.
2 and the imaginary part of the impedance is 23 → 24 → 2
It changes to 5.

FIG. 4 shows a configuration example of an oscillation circuit using the voltage controlled oscillation device of the present invention. This oscillation circuit oscillates at a frequency where the reactance of the active circuit side of the oscillation circuit and the reactance of the voltage controlled oscillator of the present invention match.

In the above description, a DC voltage is used as the variable control voltage. However, a low-frequency variable control voltage having the same effect may be used. Generally, the low-frequency variable control voltage is preferably set to an audio frequency.

Although an example in which a piezoelectric bulk vibration element is used as the oscillation element has been described here, a surface acoustic wave resonator having similar impedance characteristics may be used as the oscillation element. In that case, it is preferable to use a variable capacitance element having an appropriate variable capacitance value according to the surface acoustic wave resonator used so that the frequency variable width can be widened. The variable capacitance value can be changed by the number of electrode fingers of the variable capacitance element, the opening length, and the like.

In the variable capacitance element according to the present invention, the impedance becomes inductive between the resonance frequency and the antiresonance frequency.
Therefore, the voltage controlled oscillator using this variable capacitance element is not used in this frequency range. Therefore, it is necessary to shift the resonance frequency of the variable capacitance element from the resonance frequency of the oscillation element. In the present invention, in order to change the resonance frequency of the variable capacitance element, it is only necessary to change the parameters related to the comb-shaped electrodes, and it is not necessary to change the thickness of the ferroelectric thin film or the antiferroelectric thin film. Therefore, it is advantageous when the ferroelectric thin film or the antiferroelectric thin film is shared with the oscillation element as in the example shown in FIG. For example, in the case of the configuration shown in FIG. 1, even if it is necessary to control the resonance frequency of the variable capacitance element, the thickness of the ferroelectric thin film 14 is different between the vicinity of the upper electrode 15 and the vicinity of the comb-shaped electrode 16. There is no need to change the parameters of the comb-shaped electrode.
Therefore, steps such as thin film etching are not required, and the number of manufacturing steps is reduced. Even when the thin films are separated from each other on the variable capacitance element side and the oscillation element side, the same effect is realized when they are formed simultaneously.

As described above, the present invention has a structure in which a piezoelectric bulk vibration element or a surface acoustic wave resonator is provided as an oscillation element, and a variable capacitance element formed of a comb-shaped electrode is connected to this oscillation element. Since a VCO (Voltage Controlled Oscillator) is used, the thickness of the ferroelectric thin film or antiferroelectric thin film as a component of the oscillation element and the thickness of the ferroelectric thin film or antiferroelectric thin film as a component of the variable capacitor The thickness can be made the same.

Further, the voltage controlled oscillator according to the present invention can operate practically in the center frequency band of 2 GHz, and can realize a sufficiently wide frequency variable width.

[0020]

FIG. 5 shows a first embodiment of the voltage controlled oscillator. This voltage controlled oscillation device utilizes a piezoelectric bulk vibration element as an oscillation element.

This device was manufactured in the following procedure. First, the diaphragm 10 having a thickness of about 1 μm was formed by etching the back surface of the Si substrate 11. Next, a silicon oxide / zirconia oxide / barium titanate layer is formed as a buffer layer 12 on the Si substrate 11, a base electrode 13 made of platinum is formed thereon, and a thickness of 0 is formed by a multi-source evaporation method. A ferroelectric thin film 14 composed of a PZT epitaxial thin film of 0.5 μm was formed. This PZT epitaxial thin film has a Zr: Ti ratio of 2
5:75. It was confirmed by RHEED that this thin film was an epitaxial film. Next, the upper electrode 15 was formed in a region on the ferroelectric thin film 14 corresponding to the diaphragm 10, and the comb-shaped electrode 16 was formed in a region outside the diaphragm 10. These electrodes were formed using a photolithography technique, and were all made of Al.

The upper electrode 15 constitutes a piezoelectric bulk vibration element together with the ferroelectric thin film 14 and the base electrode 13, and forms the oscillation element 5. The comb-shaped electrode 16 is made of the ferroelectric thin film 1.
4 together with the variable capacitance element 6.

The size of the upper electrode 15 was 8 μm square.
When the dielectric constant of the oscillation element 5 was measured with a wafer prober, about 240 εo was obtained. Here, εo is the dielectric constant of a vacuum. In the variable capacitance element 6, the number of pairs of comb-shaped electrodes is 10, the opening length is 5 μm, and the line width of the comb-shaped electrode fingers is 0.5 μm.
m and the metallization ratio were 0.5. Further, the position of the variable capacitance element 6 is provided at a distance of about 200 μm from its end so as not to affect the impedance characteristics of the oscillation element 5. Further, one terminal of the upper electrode 15 and one terminal of the comb-shaped electrode 16 were connected by using the lead-out wiring 17.
Finally, the chip is divided into chips by a dicing device, mounted on a package 41 using a die bonding agent 40, and then wired using a wire 42 so that the variable capacitance element 6 and the oscillation element 5 are connected in series, and a lid is provided. The device was sealed at 43 to complete the device.

When a low-frequency voltage having a maximum amplitude of about 5 V is applied between opposing electrode fingers of the comb-shaped electrode 16 in the variable capacitance element 6 using an external terminal of the package 41, the history of the accumulated charge amount is obtained. A curve was obtained. The saturation voltage in this case was about 4.5V. From the slope of this hysteresis curve, the applied voltage dependence 50 of the capacitance was obtained as shown in FIG. In FIG. 6, the vertical axis represents the normalized capacity normalized by the capacity when the applied voltage is zero. This voltage-controlled oscillating device showed little change with time in the hysteresis curve and the capacitance after the application of the low-frequency voltage or the DC voltage, and was found to be sufficiently practical.

When this voltage-controlled oscillator is inserted into the oscillator circuit shown in FIG. 4 and a control voltage is applied from 3 V to 0 V, the oscillation frequency changes from 2.390 GHz to 2.365 GHz and the standardized frequency variable width Was found to be about 1%. The real part of the impedance of the variable capacitance element 6 was several ohms, and the Q value was large.

Next, the operation when the voltage controlled oscillator is inserted into the phase locked loop will be considered. FIG. 7 shows a conceptual diagram of the phase locked loop. This phase locked loop includes a phase detector (phase comparator) 100, a loop filter 101, an amplifier 102, and a VCO 103. When this phase-locked loop is operated, first, the input voltage of the VCO 103 oscillates at the free-running frequency set to zero, and thereafter is pulled to the set frequency of the input signal. When the VCO composed of the voltage controlled oscillator of this embodiment is used as the VCO 103 shown in FIG. 7 and the phase locked loop is operated while applying a DC voltage of 5 V to both ends of the variable capacitance element 6, the VCO fed back from the amplifier 102 When the input voltage to the VCO started to oscillate at zero, the output frequency of the VCO became 2.390 GHz and was automatically pulled into the input frequency set to a frequency lower than this frequency. Here, a DC voltage is applied to the variable capacitance element, but a low-frequency voltage having a similar effect may be applied.

FIG. 8 shows a second embodiment of the voltage controlled oscillator. This voltage controlled oscillator uses a surface acoustic wave resonator as an oscillation element.

This device was manufactured in the following procedure. First, a silicon oxide / zirconia oxide layer / barium titanate layer is formed as a buffer layer 12 on a Si substrate 60, and a 0.5 μm thick P is formed thereon by multi-source evaporation.
A ferroelectric thin film 14 made of a ZT epitaxial thin film was formed. This PZT epitaxial thin film is made of Zr and Ti
Was 25:75. It was confirmed by RHEED that this thin film was an epitaxial film. Next, a comb-shaped electrode 15A and a strip reflector (not shown) were formed on the ferroelectric thin film 14, and a comb-shaped electrode 16 was formed. The comb-shaped electrode and the strip reflector were formed by using a photolithography technique, and both were made of Al.

The comb-shaped electrode 15A and the reflector together with the ferroelectric thin film 14 constitute a surface acoustic wave resonator. The comb-shaped electrode 16 forms the variable capacitance element 6 together with the ferroelectric thin film 14.

In the oscillation element 5, the line width of the comb-shaped electrode 15A and the strip reflector is 0.6 μm, the metallization ratio is 0.5, the number of pairs of comb-shaped electrodes is 20, the number of strip reflectors is 200, and the opening length is Was 20 μm, and the distance between the comb-shaped electrode and the strip reflector was 1.2 μm as the center-to-center distance.
On the other hand, in the comb-shaped electrode 16 of the variable capacitance element 6, the number of electrode pairs was 20, the opening length was 50 μm, the line width was 0.4 μm, and the metallization ratio was 0.5. The position of the variable capacitance element 6 was separated by 200 μm in a direction perpendicular to the propagation direction of the surface acoustic wave so as not to affect the impedance characteristics of the oscillation element 5. Further, the variable capacitance element 6 was connected to one of the comb-shaped electrodes 15A of the oscillation element 5 by using the extraction wiring 17.
Finally, the chip is divided into chips by a dicing device, mounted on a package 41 using a die bonding agent 40, and then wired using a wire 42 so that the variable capacitance element 6 and the oscillation element 5 are connected in series, and a lid is provided. The device was sealed at 43 to complete the device.

Although the variable capacitance element 6 has a different electrode area from the variable capacitance element 6 in the first embodiment because of its different electrode area, it has the same hysteresis curve of accumulated charges and the applied voltage of the capacitance as the device of the first embodiment. Showed dependence. Then, after applying a saturation voltage of 5 V to the variable capacitance element 6, the applied voltage becomes 4
When the oscillation frequency was measured at V and 0 V, it was found that the oscillation frequency changed from 2.197 GHz to 2.191 GHz, and the normalized frequency variable width was about 0.3%. In this device, the normalized frequency variable width did not reach 1%, because the inductive region of the impedance of the surface acoustic wave resonator used as the oscillation element was narrow. If a surface acoustic wave resonator having a wider inductive region is used, it is possible to increase the frequency variable width. The real part of the impedance of the variable capacitance element 6 was several ohms, and the Q value was large.

Although a ferroelectric thin film is used in the above embodiment, an antiferroelectric thin film (Pb
Even if a ZrO ₃ epitaxial thin film is formed, the same operation as in the above embodiment can be obtained if the film thickness and applied voltage are set appropriately. Further, even if a BaTiO ₃ epitaxial thin film which is a ferroelectric material having a double hysteresis curve is used,
A similar function is obtained. In addition to the above ferroelectric thin film composed of a PZT epitaxial thin film, for example,
A c-axis oriented Pb-containing perovskite ferroelectric epitaxial thin film described in JP-A-321362 can also be used.

In the first and second embodiments, a similar voltage controlled oscillator can be obtained even if the variable capacitance element and the oscillation element are formed on separate Si substrates.

The present invention is not limited to the structure in which the base electrode is provided on the substrate, and a conductive substrate may be used as the base electrode.

[0035]

According to the present invention, a variable capacitance element is formed by forming a comb-shaped electrode on a ferroelectric thin film or an antiferroelectric thin film whose accumulated charge shows a history with respect to an applied voltage. By connecting an oscillating element (piezoelectric bulk vibration element or surface acoustic wave resonator) in series with the capacitive element, practical operation in the center frequency band of 2 GHz is possible and voltage control with a sufficiently wide frequency variable width An oscillation device can be realized. In addition, in this voltage controlled oscillator, the thickness of the ferroelectric thin film or the antiferroelectric thin film can be made the same between the variable capacitance element and the oscillation element. Therefore, according to the present invention, it is possible to make the semiconductor element monolithic. The voltage controlled oscillator can be manufactured without complicating the manufacturing process.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a voltage controlled oscillator according to the present invention.

FIG. 2 is a graph showing impedance characteristics of the voltage controlled oscillator of the present invention.

FIG. 3 is a schematic view of a variable capacitance element according to the present invention.

FIG. 4 is a diagram showing an oscillation circuit for explaining the operation of the present invention.

FIG. 5 is a diagram showing a voltage controlled oscillator according to a first embodiment of the present invention.

FIG. 6 is a diagram showing the applied voltage dependence of the capacitance in the first embodiment of the present invention.

FIG. 7 is a conceptual diagram of a conventional phase locked loop.

FIG. 8 is a diagram showing a voltage controlled oscillator according to a second embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshihiko Yano 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDK Corporation (72) Inventor Takao Noguchi 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDK F term (reference) 5J079 AA03 AA06 BA17 BA47 DA14 FA02 FA14 FA15 FA21 FB25 FB29 GA02 KA05 5J097 AA13 AA19 AA32 BB01 KK03 5J106 AA04 BB01 CC01 CC21 CC38 CC41 JJ01 KK08 KK37 LL01

Claims (6)

[Claims] 1. A comb-shaped electrode comprising a pair of opposed electrode fingers on a ferroelectric thin film or an antiferroelectric thin film,
A variable capacitance element using a change in reactance of the comb-shaped electrode, which is generated by applying a variable control voltage between opposing electrode fingers of the comb-shaped electrode. 2. A method according to claim 1, wherein the opposing electrode fingers of the comb-shaped electrode are
After applying a voltage equal to or higher than the saturation voltage in the hysteresis curve of the accumulated charge amount of the ferroelectric thin film or antiferroelectric thin film, apply the variable control voltage, or in the state where the saturation voltage is applied, The variable capacitance element according to claim 1, wherein the reactance is changed by applying a control voltage having a polarity opposite to a saturation voltage. 3. The variable capacitance element according to claim 1, wherein said ferroelectric thin film includes a Pb-containing perovskite crystal or BaTiO ₃ crystal, and said antiferroelectric thin film includes a PbZrO ₃ crystal. 4. A series connection element in which the variable capacitance element according to claim 1 and an oscillation element including a piezoelectric bulk vibration element or a surface acoustic wave resonator are connected in series,
A voltage-controlled oscillation device in which impedance characteristics at both ends of the series-connected element are controlled by applying the variable control voltage to the variable capacitance element. 5. The voltage controlled oscillator according to claim 4, wherein said oscillation element shares a ferroelectric thin film or an antiferroelectric thin film included in said variable capacitance element. 6. When used as a voltage-controlled oscillator in a phase-locked loop, an input set frequency when operating the phase-locked loop matches an oscillation frequency when a saturation voltage is applied to the variable capacitance element, 6. The voltage controlled oscillator according to claim 1, wherein the voltage controlled oscillator is used in a phase locked loop after a voltage equal to or higher than the saturation voltage is applied to the variable capacitance element.