JP2011101163A - Capacitance type electromechanical conversion device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a capacitance type electromechanical conversion device capable of improving sensitivity. <P>SOLUTION: The capacitance type electromechanical conversion device includes a cell having: a first electrode 2; a second electrode 4 disposed, by facing the first electrode, with an air gap 3 between the first electrode and the second electrode; and a support for supporting the second electrode in a movable manner. The support includes: a first support 6 for supporting the outer circumferential edge part of the second electrode 4; and a second support 7 for supporting a part inside the outer circumferential edge part of the second electrode 4. A part corresponding to the second support 7 in the direction in which the first and second electrodes face each other is located in a region where at least one of the first electrode 2 and the second electrode 4 does not exist. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a capacitive electromechanical transducer such as a capacitive ultrasonic transducer.

In recent years, capacitive electromechanical transducers using a micromachining process have been actively studied. A typical capacitive electromechanical transducer includes a cell having a lower electrode, a vibrating membrane supported at a predetermined interval, and an upper electrode disposed on the surface of the vibrating membrane. This is used, for example, as a capacitive ultrasonic transducer (CMUT: Capacitive-Micromachined-Ultrasonic-Transducer). Such a conversion device performs at least one of the conversion from an electric signal to an ultrasonic wave and the conversion from an ultrasonic wave to an electric signal by using a lightweight vibrating membrane, and is excellent in liquid and air. Those with wide band characteristics can be easily obtained. When this conversion device is used, ultrasonic diagnostics with higher accuracy than conventional medical diagnostic modalities are possible, and therefore, it is attracting attention as a promising technology. The principle of operation of this converter is as follows. When transmitting ultrasonic waves, a voltage obtained by superimposing a minute AC voltage on a DC voltage is applied between the lower electrode and the upper electrode. As a result, the vibrating membrane vibrates and ultrasonic waves are generated. When receiving the ultrasonic wave, the vibration film is deformed by the ultrasonic wave, and thus a signal is detected by a change in capacitance between the lower electrode and the upper electrode accompanying the deformation. In a normal converter, a plurality of elements (elements) in which a plurality of cells are electrically connected in parallel are arranged.

In such a capacitive ultrasonic transducer, it is desirable to reduce the distance between the electrodes in order to improve the electromechanical conversion performance. For such a purpose, in order to reduce the distance between the electrodes, a method has been proposed in which struts are provided in the gaps between the electrodes and the distance between the electrodes is reduced by bringing the vibrating membrane into contact with the struts (see Patent Document 1). .

US-A1-20060004289 Specification

As described above, the electromechanical conversion performance or sensitivity is improved by providing the support in the gap between the electrodes and reducing the distance between the electrodes. However, a further increase in sensitivity is required for the capacitive electromechanical transducer. In the configuration in which the support is provided in the gap between the electrodes and the vibration film is in contact with the support, the vibration film in the portion adjacent to the support does not vibrate or vibrates slightly with respect to the input wave. For this reason, the electrostatic capacitance in that portion may become a parasitic capacitance, which may limit sensitivity improvement.

In view of the above problems, a capacitance-type electromechanical transducer according to the present invention includes a first electrode, a second electrode that faces the first electrode and is spaced from the first electrode, and the second electrode. And a cell having a support portion for movably supporting the electrode. The support portion includes a first support portion for supporting the outer peripheral edge portion of the second electrode, and a second support portion for supporting an inner portion of the outer peripheral edge portion of the second electrode. including. A portion corresponding to the second support portion in a direction in which the first and second electrodes face each other is a region lacking at least one of the first and second electrodes.

In the capacitive electromechanical transducer of the present invention, since the second support portion is provided in the gap between the electrodes, the interval between the electrodes can be reduced, and the capacitance change between the electrodes with respect to the input wave is increased. It becomes possible to do. In addition, since the portion corresponding to the second support portion is a region lacking at least one of the first and second electrodes, the portion that does not vibrate or slightly vibrates with respect to the input wave. Parasitic capacitance can be reduced and the sensitivity of the device can be further improved.

The figure which shows the cross section and upper surface of the capacitive electromechanical transducer of embodiment of this invention. The cross section and AA cross section of the capacitive electromechanical transducer of other embodiment of this invention are shown.

Hereinafter, embodiments of the present invention will be described. What is important in the capacitive electromechanical transducer of the present invention is that the portion corresponding to the second support portion in the direction in which the first and second electrodes oppose each other is the first and second electrodes. It is an area lacking at least one of them. Based on this concept, the basic form of the capacitive electromechanical transducer of the present invention has the configuration as described above. On the basis of this basic form, the following embodiments are possible.

For example, a vibrating membrane that supports a second electrode (upper electrode described later) is provided, and the first supporting portion supports the outer peripheral edge of the second electrode by supporting the outer peripheral edge of the vibrating membrane, The second support portion can support the inner portion of the outer peripheral edge portion of the second electrode by supporting the inner portion of the outer peripheral edge portion of the vibration film. In this case, the vibration film is formed of an insulating material. The vibration film may also serve as the second electrode. In this case, the first support portion directly supports the outer peripheral edge portion of the second electrode, and the second support portion directly supports a portion inside the outer peripheral edge portion of the second electrode. In this configuration, since the second electrode always exists in the portion corresponding to the second support portion, the first electrode (lower electrode described later) in this portion is necessarily omitted.

Alternatively, the first electrode may be arranged on the substrate. In this case, the substrate is usually made of an insulating material. The substrate may be formed of a conductor material so that it also serves as the first electrode. In this configuration, since the first electrode always exists in the portion corresponding to the second support portion, the second electrode in this portion is necessarily omitted. In addition, the region lacking at least one of the first and second electrodes may be provided only in a portion in contact with the second support portion, but may be provided in the vicinity including the portion. What is important is to avoid a configuration in which the first and second electrodes oppose each other and generate parasitic capacitance in the region of the second support portion that is almost stationary. The form of the region lacking at least one of the second electrodes is not limited. Further, the vibration part may be continuously formed over a plurality of cells, the movable part may be a vibration film, and the non-moving part may be a first support part. At this time, the second support portion can also be formed continuously with the vibrating membrane. Such a configuration can be easily manufactured by a manufacturing method using surface micromachining.

From the viewpoint of improving sensitivity, it is preferable that the second electrode or the vibrating membrane be bent in the direction toward the first electrode to be concave. In such a configuration, the pressure of the gap between the first and second electrodes is reduced to atmospheric pressure or less, or the voltage application unit applies a voltage between the first and second electrodes and electrostatically attracts the second electrode or This is achieved by adjusting the shape of the diaphragm. It is preferable that the voltage application unit can adjust the voltage so that the lowermost part of the second electrode or the vibration film is lower than the second support unit.

As described above, the capacitive electromechanical transducer usually has a plurality of elements having a configuration in which a plurality of the cells are electrically connected. Specifically, the second electrodes of all the elements are connected to the electric circuit by connection wiring, but the first electrode is connected to the electric circuit independently for each element, so that the element An output signal that is electrically separated for each time can be obtained. However, the configuration is not limited to this, and any configuration may be used as long as an output signal can be obtained for each element. With such a configuration, an elastic wave such as a sound wave, an ultrasonic wave, an acoustic wave, or a photoacoustic wave can be detected by a change in capacitance between the first and second electrodes. Further, an elastic wave such as an ultrasonic wave can be generated by applying a modulation voltage between the first and second electrodes to vibrate the second electrode or the vibrating membrane.

The second electrode used in the capacitive electromechanical transducer of the present invention can be formed of the following material. That is, a conductor selected from Al, Cr, Ti, Au, Pt, Cu, Ag, W, Mo, Ta, Ni, etc., a semiconductor such as Si, AlSi, AlCu, AlTi, MoW, AlCr, TiN, AlSiCu, etc. It can be formed of at least one material selected from the group consisting of alloys. Further, the second electrode is provided in at least one of the top surface, the back surface, and the inside of the vibration film, or when the vibration film is formed of a conductor or a semiconductor as described above, the vibration film is the second film. A structure that also serves as an electrode may be employed. The first electrode used in the present invention can also be formed using the same conductor, semiconductor, or the like as the second electrode. The materials of the first electrode and the second electrode may be different. As described above, when the substrate is a semiconductor substrate such as silicon, the substrate can also serve as the first electrode.

Hereinafter, an embodiment of a capacitive electromechanical transducer of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 shows a capacitive electromechanical transducer according to a first embodiment. As shown in FIG. 1, the cell of the capacitive electromechanical transducer of this embodiment includes a lower electrode 2 disposed on a substrate 1 and an upper electrode disposed opposite to the lower electrode 2 with a gap 3 therebetween. 4, a vibration film 5 that supports the upper electrode 4, and a first support portion 6 that supports the outer peripheral edge of the vibration film 5. In the present embodiment, the vibration film 5 is an insulating material. The vibration film 5 is made of SiN, but other insulating materials may be used. The first support 6 is made of SiN, but other insulating materials may be used. As shown in FIG. 1B, the vibrating membrane 5 is movably supported in a circular shape by the first support portion 6, but may be an elliptical shape or a polygonal shape. The diameter of the vibrating membrane 5 supported in a circular shape by the first support portion 6 is preferably in the range of 10 μm to 100 μm, and the thickness of the vibrating membrane 5 is preferably in the range of 100 nm to 800 nm. The height of the first support 6 is preferably in the range of 50 nm to 300 nm. In the capacitive electromechanical transducer, since the capacitance change due to the change of the vibration film 5 with respect to the input wave is increased by narrowing the gap of the gap 3 between the electrodes 2 and 4, high sensitivity can be realized.

In the capacitive electromechanical transducer of this embodiment, the columnar second support portion 7 is disposed at the center of the cell on the substrate 1, and the second support portion 7 is the first support portion 6. It is set lower than the height. The diameter of the second support portion 7 is preferably 1 μm to 5 μm, but may be sufficiently small with respect to the diameter of the vibrating membrane 5. The height of the second support portion 7 is preferably about ½ or less of the height of the first support portion 6. Moreover, although the 2nd support part 7 is also formed of SiN, another insulating material may be sufficient. The second support portion 7 is preferably cylindrical, but may be any shape as long as it can hold a part of the inside of the outer peripheral edge portion of the vibration film 5. The second support portion 7 is most preferably arranged at the center of the cell on the substrate 1, but it has an effect at other positions.

The air gap 3 is in a reduced pressure state as compared with the upper environment of the vibration film 5, and a force that makes the vibration film 5 concave in the direction of the lower electrode 2 is applied. Furthermore, a voltage application unit (not shown) applies a voltage between the lower electrode 2 and the upper electrode 4 so that the vibrating membrane 5 becomes concave in the direction of the lower electrode 2 due to electrostatic attraction between the electrodes 2 and 4. Apply power. The vibration film 5 is deformed in the direction of the lower electrode 2 by these two forces. However, since the central portion of the vibration film 5 is in contact with the second support portion 7 disposed in the central portion of the lower electrode 2, the shape of the vibration film 5 is reduced. As shown in FIG. 1A, the lowermost portion of the vibration film 5 is lower than the highest portion of the second support portion 7. Here, the lowest part of the vibration film 5 is a part where the vibration film 5 and the substrate 1 are closest, and the highest part of the second support part 7 is the second support part 7 farthest from the substrate 1. Part.

By adjusting the reduced pressure state of the gap 3 and the applied voltage of the voltage application unit, the lowermost part of the vibration film 5 is close to the lower electrode 2 and the distance between the upper electrode 4 and the lower electrode 2 on the vibration film 5 is narrowed. can do. In the conventional capacitive electromechanical transducer, the region where the distance between the electrodes is the shortest is one point at the center of the diaphragm. On the other hand, in the present embodiment, since the region where the distance between the electrodes is minimum is wide compared to the conventional one, the capacitance change due to the change of the vibration film 5 with respect to the input wave becomes large, and high sensitivity can be realized.

In addition, the capacitive electromechanical transducer can achieve higher sensitivity when the parasitic capacitance is smaller. The parasitic capacitance is a capacitance component that does not change with respect to the input pressure. When the parasitic capacitance increases, the sensitivity of the signal decreases due to an increase in signal noise. When a support part is provided inside the gap and brought into contact with the vibration film, the vibration film in the vicinity of the support part does not vibrate with respect to the input wave or vibrates little. Therefore, the capacitance between the upper electrode and the lower electrode in the vicinity of the support portion becomes a parasitic capacitance. In the capacitive electromechanical transducer of this embodiment, the vibration film 5 with little vibration in the vicinity of the second support portion 7 is provided with a region without the upper electrode 4, and the upper electrode 4 is near the lowermost part of the vibration film 5. Placed in. Thus, as shown in FIG. 1B, the upper electrode 4 as the second electrode is provided only near the lowermost part, and has a donut shape. Further, the lower electrode 2 is not formed in a region corresponding to the second support portion 7. Therefore, the parasitic capacitance in the region of the vibration film 5 that does not vibrate or has little vibration can be reduced.

Here, the region without the upper electrode 4 is a region having an inner diameter of the donut-shaped electrode. This region is preferably 10% to 50% of the in-plane direction distance between the lowest part of the vibration film 5 and the highest part of the second support part 7 around the contact point of the vibration film 5 and the second support part 7. % Is the region where the inner diameter radius of the donut is taken. More preferably, the length of 30% of the distance between the lowest part of the vibrating membrane and the highest part of the second support part is the donut-shaped inner radius. For example, when the diameter of the second support portion 7 is 5 μm, the height is 50 nm, the film thickness of the vibration film 5 is 100 nm, and the relative dielectric constant between the vibration film and the second support portion is 6, the second support portion 7. The capacitance C between the upper and lower electrodes related to the parasitic capacitance is about 0.007 pF from the following equation 1.
C = ε ₀ ε _r S / d Equation 1
Here, ε ₀ is the dielectric constant of vacuum, ε _r is the relative dielectric constant, S is the electrode area, and d is the distance between the electrodes. The capacity between the upper and lower electrodes in the second support portion 7 is as large as 10% or more with respect to the entire capacity. By eliminating the upper and lower electrodes in the second support portion 7 as in the present embodiment, this increase in parasitic capacitance can be prevented.

Thus, by not providing the second support part and the upper and lower electrodes in the vicinity of the second support part, it is possible to reduce the parasitic capacitance in the vibration film region with little change, and to reduce the capacitance only in the vibration film region with a large change. Can be bigger. Thereby, a highly sensitive capacitive electromechanical transducer can be realized. In the present embodiment, one second support portion 7 is arranged on the substrate 1, but the same effect can be obtained even if a plurality of second support portions 7 are arranged. In this case, it is preferable that the portion corresponding to all of the plurality of second support portions be a region in which any of the electrodes as described above is eliminated, but the portion corresponding to at least one second support portion. Only one of the above-described regions may be eliminated. Further, although the upper and lower electrodes in the region of the second support portion 7 are eliminated, the same effect can be obtained even if one of the electrodes is eliminated.

(Second Embodiment)
A capacitive electromechanical transducer according to the second embodiment will be described with reference to FIG. As shown in FIG. 2, the cell of the capacitive electromechanical transducer of this embodiment is provided with a lower electrode 2 disposed on a substrate 1 made of an insulating material, and opposed to the lower electrode 2 with a gap 3 therebetween. The vibration film 5 and the first support portion 6 that supports the vibration film 5 are provided. A circular upper electrode 4 is disposed on the vibration film 5. The vibration film 5 may be formed of a conductor so that the vibration film 5 also serves as an upper electrode. In this case, the upper electrode 4 can be omitted. Further, the shape of the upper electrode 4 on the vibration film 5 may be a donut shape according to the shape of the lower electrode 2 described below.

In the case where the vibration film 5 also serves as the upper electrode, the vibration film 5 can be formed of Si or the like, but other conductor materials may be used. The first support portion 6 can also be formed of low resistance Si or the like, but may be other conductive material. The support form of the vibration film 5 by the first support part 6, the diameter and film thickness of the vibration film 5, the height of the first support part 6, and the like are as described in the first embodiment.

In the present embodiment, the second support portion 7 is formed of Si0, but other insulating materials may be used. A region without the lower electrode 2 is provided in the vicinity of the second support portion 7, and the lower electrode 2 is disposed near the lowermost portion of the vibrating membrane 5, and has a donut shape as shown in FIG. 2 (b). Yes. That is, the lower electrode 2 as the first electrode is present only in the portion facing the vicinity of the lowermost portion of the upper electrode 4 as the second electrode. Therefore, it is possible to reduce the parasitic capacitance of the vibration film 5 in a region where the change is small. The region having no lower electrode is a region having an inner diameter of the doughnut-shaped electrode 2, and preferably the lowermost portion of the vibrating membrane 5 and the second support portion 7 around the contact point between the substrate 1 and the second support portion 7. 10 to 50% of the distance in the in-plane direction with the highest part of the doughnut-shaped inner radius. More preferably, the length of 30% of the distance between the lowest part of the vibrating membrane and the highest part of the second support part is the donut-shaped inner radius. For example, when the diameter of the second support portion 7 is 5 μm, the height is 50 nm, and the relative permittivity of the second support portion 7 is 3.7, the capacitance C between the upper and lower electrodes of the second support portion 7 is the above formula. 1 is about 0.013 pF. Again, the capacity between the upper and lower electrodes of the second support portion 7 is as large as 10% or more with respect to the total capacity. By eliminating the lower electrode in the vicinity of the second support portion 7 as shown in the present embodiment, this increase in parasitic capacitance can be prevented.

As described above, also in this embodiment, by not providing the lower electrode in the vicinity of the second support portion, the parasitic capacitance in the vibration film region with little change can be reduced, and the capacitance of only the vibration film region with large change can be reduced. Can be bigger. In this way, a highly sensitive capacitive electromechanical transducer can be realized.

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Lower electrode (1st electrode), 3 ... Space | gap, 4 ... Upper electrode (2nd electrode), 5 ... Vibration film, 6 ... 1st support part, 7 ... 2nd support part

Claims (6)

Capacitive type comprising a cell having a first electrode, a second electrode opposed to the first electrode and disposed with a gap, and a support part for movably supporting the second electrode An electromechanical converter,
A first support for supporting the outer peripheral edge of the second electrode; and a second support for supporting an inner portion of the outer peripheral edge of the second electrode. Including
A portion corresponding to the second support portion in a direction in which the first and second electrodes face each other is a region lacking at least one of the first and second electrodes. Capacitive electromechanical converter. A vibrating membrane supporting the second electrode is provided;
The said 1st support part supports the outer-periphery edge part of the said diaphragm, The said 2nd support part supports the part inside the outer-periphery part of the said diaphragm. Capacitance type electromechanical transducer. 3. The capacitive electromechanical transducer according to claim 1, wherein a height of the second support portion is lower than a height of the first support portion. The lowermost part of the second electrode which is concave in the direction toward the first electrode is lower than the highest part of the second support part. The capacitance-type electromechanical transducer according to Item. The capacitive electromechanical transducer according to claim 4, wherein the second electrode is provided only in the vicinity of the lowermost part. 6. The capacitive electromechanical transducer according to claim 4, wherein the first electrode is provided only in a portion facing the vicinity of the lowermost portion of the second electrode.

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