JPH0955337A - Variable capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable capacitor which allows large variable rate of electrostatic capacity. SOLUTION: A driving electrode 10 and support parts 16, 17 are fixed and formed on a board 3 and a movable electrode 6 is connected to an upper part of the support part 16 through a beam 15. The movable electrode 6 has an electrode part 19 at the side of a board which faces the driving electrode 10 with a clearance between and a comb-like electrode part 18 which is connected to the electrode part 19 and is expanded and formed in a horizontal direction along a board surface with a clearance to a board surface. A reference electrode 4 is expanded and formed in a direction from the support part 17 toward the movable electrode 6, an electrode surface 4a of the reference electrode 4 and an electrode surface 18a of the electrode part 18 of the movable electrode 6 face each other with a clearance between in a horizontal direction and a capacity capacitor is formed of the electrodes. When a bias voltage is applied between the movable electrode 6 and the driving electrode 10 by a bias voltage application means 12, the movable electrode 6 is displaced to the side of the board 3 according to a size of a voltage, an overlap area of an electrode surface of the movable electrode 6 and the reference electrode 4 changes and electrostatic capacity of the capacitor becomes variable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable capacitance capacitor capable of variably controlling electrostatic capacitance used in various electronic circuits.

[0002]

2. Description of the Related Art FIG. 3 shows an essential structure of a variable capacitor 1 proposed in Japanese Patent Laid-Open No. 5-74655 as an example of the variable capacitor. It is formed using micromachining technology. In the figure, a concave portion 8 is formed in a substrate 3 made of silicon, and a reference electrode 4 formed in a thin film body by vapor deposition of aluminum or the like is fixed to the substrate 3 in a central region of a bottom surface 5 of the concave portion 8. Are arranged. A movable electrode 6 is formed at the upper end of the recess 8 so as to extend over the recess opening and face the electrode surface 4a of the reference electrode 4 fixed at both ends of the recess opening. The reference electrode 4 forms a capacitance capacitor. Like the reference electrode 4, the movable electrode 6 is also formed into a thin film body by vapor deposition of aluminum or the like.

Bias voltage applying means 12 for applying a bias voltage to the capacitance capacitor is connected to the capacitance capacitor formed by the reference electrode 4 and the fixed electrode 6, and each of the movable electrode 6 and the reference electrode 4 is connected. By applying a DC bias voltage from the bias voltage applying means 12 between terminal portions (not shown) formed to be drawn out from one end side, a potential difference is applied to the reference electrode 4 and the movable electrode 6. .

In the variable capacitor having the above structure, when the bias voltage applying means 12 applies an external bias voltage between the reference electrode 4 and the movable electrode 6 to give a potential difference to the reference electrode 4 and the movable electrode 6. , The movable electrode 6 is bent and deformed toward the reference electrode 4 side by the action of the Coulomb force (electrostatic force action), and the state shown by the chain line in FIG. 3 is obtained, whereby the gap between the movable electrode 6 and the reference electrode 4 is formed. That is, the distance between the electrodes changes. Then, the electrostatic capacitances of the movable electrode 6 and the reference electrode 4 change in accordance with the external bias voltage applied between both electrodes, and the capacitance works as a variable capacitor controlled by the applied external bias voltage. .

The proposed variable capacitor is composed of a single element as described above, and does not require a complicated mechanism such as a rotating mechanism unlike the conventionally used variable air capacitor (varicon). In addition, it has an advantage that it is easy to manufacture and can be miniaturized. Further, it has a low withstand voltage like a varactor diode, and if the internal resistance is increased to increase the withstand voltage, the Q value is lowered. However, it is attracting attention as an excellent variable capacitor having a high withstand voltage and a high Q value.

[0006]

However, in the above-mentioned proposed variable capacitor, the Coulomb force applied to the movable electrode 6 and the movable electrode 6 flexibly deformed by the action of the Coulomb force are returned to the original position before the deformation. There is a problem in that the amount of displacement of the movable electrode 6 is limited due to the relationship with the spring force to be tried, and therefore the rate of change in capacitance obtained by the bending deformation of the movable electrode 6 cannot be easily increased.

This is because the amount of deformation of the movable electrode 6 is
When the distance becomes larger than 1/3 of the distance between the reference electrode 4 and the movable electrode 6, it is difficult to balance the spring force and the Coulomb force from the relationship described below. You cannot do it.

The relationship between the amount of change in the movable electrode 6 and the Coulomb force and spring force applied to the movable electrode 6 at that time will be described below. The movable electrode 6 has a Coulomb force applied to the movable electrode 6 due to a potential difference between the movable electrode 6 and the reference electrode 4,
Since the movable electrode 6 is bent and deformed by the action of the Coulomb force, the movable electrode 6 is fixed at a position balanced with the spring force that tries to return to the original position (the position when it is not deformed). At this time, it can be seen that the relationship of the following expression (1) is established.

F = kx = (1/2) · εS {V / (x ₀ −x)} ² (1)

In the equation (1), k is the movable electrode 6
Spring constant, S is the area of the movable electrode 6 facing the reference electrode 4, ε is the dielectric constant, V is the potential difference between the electrodes 4 and 6, and x ₀
Is the distance between the movable electrode 6 and the reference electrode 4, and x is the amount of displacement of the movable electrode 6. Here, u = x / x ₀ , K = εS /
Rearranging the above formula (1) as 2kX ₀ ^3, the following equation (2)
Becomes

U (1-u) ² = KV ² (2)

From this equation (2), u (1-u) ² = f
(U) leads to the relationship shown in FIG. 4, and the function f
(U) is a cubic function having a peak at KV ² of about 0.15 when u = 1/3. V becomes large from this figure,
When u exceeds 1/3, it is found that the spring force and the Coulomb force cannot be balanced, and then the movable electrode 6 comes into contact with the reference electrode 4. Also, u is 1/3
It is possible to balance the spring force and the Coulomb force in a state of exceeding the above condition, but in this case, it is necessary to control the bias voltage V according to the spring force by some control.

Therefore, the displacement amount of the movable electrode 6 is limited to 1/3 of the distance between the movable electrode 6 and the reference electrode 4, and the capacity change rate of this variable capacitor is maximum.
It was 50%, and it was not possible to obtain a larger variable rate.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a variable capacitor capable of achieving a large variable ratio (capacity change ratio).

[0015]

In order to achieve the above object, the present invention takes the following means. That is, according to the present invention, a supporting portion for the movable electrode is provided on the substrate, the base end side of the movable electrode is connected to the supporting portion, and the tip end side of the movable electrode is substantially horizontal along the substrate surface with a gap from the substrate surface. The reference electrode is extended from the opposite side of the movable electrode facing the extension tip side of the movable electrode to the movable electrode side, and the extension tip side of the movable electrode and the reference electrode is the extension direction of both electrodes. The movable electrode is vertically displaced by an electrostatic force at a position facing the movable electrode with an interval between the movable electrode on one side above and below the movable electrode, and the movable electrode and the reference electrode are separated from each other. The means for solving the above-mentioned problem is provided with a structure in which drive electrodes for variably adjusting the overlap area are provided.

In the present invention having the above structure, by applying a voltage between the drive electrode and the movable electrode, the movable electrode is attracted to the drive electrode by an electrostatic force (Coulomb force), and the movable electrode has a magnitude of the applied voltage. Accordingly, it is displaced in the direction toward the drive electrode. As a result, the overlap area between the movable electrode and the reference electrode is changed according to the displacement amount of the movable electrode, and the electrostatic capacitance between the movable electrode and the reference electrode (capacitor) is changed.

In the present invention, since the tip end side of the movable electrode is a free end, for example, the tip end of the movable electrode is more flexible than the flexural displacement of the central portion of the movable electrode of the conventional example in which both ends are fixed. The side can be greatly displaced. That is, it is possible to greatly change the overlap area between the movable electrode and the reference electrode, and it is possible to take a large variable rate of the electrostatic capacitance between the movable electrode and the reference electrode. Solves the problem of small.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. In the following description of the embodiments, the same reference numerals are given to the same parts as those in the conventional example, and the overlapping description will be omitted.

FIG. 1 shows an embodiment of a variable capacitor according to the present invention. This variable capacitor 1 comprises a drive electrode 10, a reference electrode 4, a movable electrode 6 and a beam 15 formed on a substrate 3 by using a surface micromachining technique.
Bias voltage applying means 12 for variably controlling the electrostatic capacitance is connected to the variable capacitor.

In FIG. 2, the substrate 3 is shown on the substrate (b) in FIG.
The drive electrode 10 is arranged as shown in FIG. 3, and the supporting portions 16A, 16B, 17 are fixed to the substrate 3 near the drive electrode 10 with a space therebetween as shown in FIG. The movable electrode 6 is formed above the supporting portions 16A and 16B through the beam 15.
, And the reference electrode 4 is connected to the upper portion of the support portion 17.

The movable electrode 6 comprises a reference electrode counter electrode portion 18 and a drive electrode counter electrode portion 19, and the reference electrode counter electrode portion 18 on the front end side and the drive electrode counter electrode portion on the base end side. And 19 are integrally formed. Drive electrode Counter electrode part
19 is formed to face the drive electrode 10 with a gap, and is connected to the support portion 16 via the beam 15 and is supported in a doubly supported beam shape. The reference electrode counter electrode portion 18 connected to the drive electrode counter electrode portion 19 has a comb-teeth shape and is formed to extend in the horizontal direction along the substrate surface with a gap from the substrate surface, and the extended tip side is a free end. The beam 15 is supported in a cantilever shape via the drive electrode counter electrode portion 19.

The comb-teeth-shaped reference electrode 4 connected to the supporting portion 17 is extended and formed so as to mesh with the reference electrode facing electrode portion 18 from the side opposite to the extending tip side of the reference electrode facing electrode portion 18. The electrode surface 4a of the reference electrode 4 and the electrode surface 18a of the reference electrode counter electrode portion 18 are arranged to face each other with a gap in the horizontal direction intersecting the extending direction of the electrodes 4 and 18 (orthogonal in the example shown in the drawing). . The reference electrode 4 and the reference electrode counter electrode portion 18 form a capacitor.

In the variable capacitor 1 having the above structure, as shown in FIG.
Bias voltage applying means 12 for applying a DC bias voltage is connected to the drive electrode 10 and the drive electrode 10. By applying a DC bias voltage between the drive electrode counter electrode section 19 and the drive electrode 10 by the bias voltage applying means 12 to give a potential difference between the drive electrode counter electrode section 19 and the drive electrode 10, the Coulomb force (static Drive electrode counter electrode portion 19 is displaced to the drive electrode 10 side (substrate 3 side) according to the magnitude of the bias voltage by the electric power),
Due to this displacement, the reference electrode counter electrode portion 18 is displaced toward the substrate 3 side as shown by the dotted line in FIG. When the reference electrode counter electrode portion 18 is displaced downward in this manner, the overlap area between the reference electrode counter electrode portion 18 and the reference electrode 4 changes, and the capacitance capacitor of the reference electrode counter electrode portion 18 and the reference electrode 4 changes. The capacitance changes. That is, the capacitance of the capacitance capacitor changes according to the magnitude of the bias voltage, and the capacitance of the capacitance capacitor is variably controlled by the bias voltage.

In the variable capacitor 1 shown in FIG. 1, the larger the bias voltage becomes,
The overlap area between the reference electrode counter electrode portion 18 and the reference electrode 4 is reduced, that is, the electrostatic capacitance of the capacitance capacitor is reduced.

According to this embodiment, the drive electrode counter electrode portion 19 of the movable electrode 6 is supported in a double-supported beam shape, and the reference electrode counter electrode portion is formed so as to be connected to the drive electrode counter electrode portion 19 and extend. Since the extended tip side of 18 is a free end (the reference electrode counter electrode portion 18 is supported by the beam 15 in a cantilever shape via the drive electrode counter electrode portion 19), the bias voltage applying means 12 is used. When a bias voltage is applied between the drive electrode counter electrode portion 19 and the drive electrode 10, the drive electrode counter electrode portion 19 supported by the double-supported beam is flexibly displaced downward in conjunction with the deflection of the beam 15. Further, the reference electrode counter electrode portion 18 supported by the cantilever is flexibly displaced with the drive electrode counter electrode portion 19 as a base end, and the drive electrode counter electrode portion 19 and the reference electrode counter electrode portion.
The displacement of 18 acts synergistically and the reference electrode counter electrode portion 18 can be largely displaced with respect to the reference electrode 4. Therefore, in the configuration shown in FIG. 1, the reference electrode counter electrode portion 18 of the movable electrode 6 should be displaced by a larger amount than the bending displacement of the central portion of the movable electrode 6 in which both ends of the movable electrode 6 are fixed as shown in the conventional example. That is, the reference electrode counter electrode portion 18
Therefore, the overlap area of the reference electrode 4 can be greatly changed, and the variable rate of the electrostatic capacitance of the capacitance capacitor formed of the reference electrode counter electrode portion 18 and the reference electrode 4 can be increased.

Further, as described above, the reference electrode counter electrode portion 18
Is displaced more than the drive electrode counter electrode portion 19, so that, for example, the electrostatic capacity of the capacitive capacitor is changed by the variable rate X.
The magnitude of the applied bias voltage when the variable capacitance is varied may be smaller than the bias voltage required to vary the capacitance of the conventional variable capacitance capacitor by the variable rate X. The power consumption required for variable control of the capacity can be reduced.

The present invention is not limited to the above embodiment, but various embodiments can be adopted. For example, in the above-described embodiment, as shown in FIG. 1B, the reference electrode counter electrode portion 18 (movable electrode 6) and the reference electrode 4 are provided.
In the above description, the heights of the substrate 3 and the substrate surface of the substrate 3 are matched, but as shown in FIG. 2, the movable electrode 6 may be formed so as to be displaced above the reference electrode 4. In the variable capacitor 1 shown in FIG. 2, the drive electrode counter electrode portion 19 and the drive electrode 10 are
As the bias voltage applied between the reference electrode counter electrode portion 18 and the reference electrode 4 increases, the capacitance of the capacitance capacitor composed of the reference electrode counter electrode portion 18 and the reference electrode 4 increases. It is variably controlled in the direction.

Further, in the above embodiment, the movable electrode 6 is used.
Although the drive electrode counter electrode portion 19 of is supported by the beam 15 supported at both ends, it may be supported by the beam supported at one end. Furthermore, in the above-described embodiment, the reference electrode counter electrode portion 18 and the reference electrode 4 are provided.
Has a comb tooth shape, but it does not necessarily have to be a comb tooth.
However, the electrode surface 18a of the reference electrode facing electrode portion 18 and the electrode surface 4a of the reference electrode 4 are, of course, at least partially overlapped (driven) with a gap in the horizontal direction intersecting the extending direction of both electrodes 18, 4. Even if they do not overlap when they are not in operation, at least some of them overlap when they are driven). However, as shown in the above embodiment, when the comb-shaped structure is used, the variable rate of the capacitance of the capacitance capacitor can be increased.

Furthermore, in the above embodiment, the drive electrode 10 is arranged below the drive electrode counter electrode portion 19 of the movable electrode 6, that is, on the substrate 3 side. The position where the drive electrode 10 is disposed is movable by vertically displacing the movable electrode 6 by electrostatic force and variably adjusting the overlap area between the movable electrode 6 and the reference electrode 4, as shown by the chain line. It may be any position where the capacitance of the capacitor formed by the electrode 6 and the reference electrode 4 can be changed.

[0030]

According to the present invention, since the base end side of the movable electrode is connected to the support portion and the extending front end side of the movable electrode is a free end, both ends of the conventional movable electrode are fixed. The tip end side of the movable electrode according to the present invention can be displaced more than the one in which the central portion of the movable electrode is flexibly displaced, that is, the overlap area between the movable electrode and the reference electrode can be largely changed. This makes it possible to increase the variable rate of the electrostatic capacity of the capacitive capacitor including the movable electrode and the reference electrode as compared with the conventional one.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of an embodiment of a variable capacitor according to the present invention.

FIG. 2 is an explanatory diagram showing another embodiment example.

FIG. 3 is an explanatory diagram showing a conventional example.

4 is a graph showing a relationship between a displacement rate of a movable electrode and a spring force in the variable capacitance capacitor shown in FIG.

[Explanation of symbols]

 1 Variable Capacitor 3 Substrate 4 Reference Electrode 6 Movable Electrode 10 Drive Electrode 16 Support

Claims (1)

[Claims] 1. A support portion for a movable electrode is provided on a substrate,
The base end side of the movable electrode is connected to the support portion, and the tip end side of the movable electrode is extended and formed in a substantially horizontal direction along the substrate surface with a gap from the substrate surface and faces the extended tip end side of the movable electrode. From the opposite side, the reference electrode is extendedly formed on the movable electrode side, and the extension tip side of the movable electrode and the reference electrode is provided with a horizontal gap that intersects the extension direction of both electrodes,
A drive electrode for displacing the movable electrode in the vertical direction by electrostatic force and variably adjusting the overlap area of the movable electrode and the reference electrode is arranged at a position facing the movable electrode with a space on one side above and below. A variable capacitor that is installed.