Capacitive device and piezoelectric device

A capacitive device includes a unit cell including a CMUT, and a transmission/reception plate for impedance matching which is provided above the unit cell via a connection portion, in which a membrane of the CMUT constituting the unit cell is connected to the transmission/reception plate via the connection portion having an area smaller than that of the transmission/reception plate. The area of the transmission/reception plate is desirably larger than the area of a hollow portion of the CMUT.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capacitive device and a piezoelectric device, and more particularly to a capacitive device and a piezoelectric device which include a transmission/reception plate for impedance matching.

2. Description of the Related Art

An ultrasonic transducer element is incorporated in an ultrasonic probe (probe) of an ultrasonic imaging apparatus, transmitting and receiving ultrasonic waves and used for various purposes, such as diagnosis of a tumor in a human body or non-destructive inspection for cracks generated in a building.

Conventionally, for this type of probes of ultrasonic imaging apparatuses, piezoelectric ceramics represented by lead zirconate titanate (PZT) or the like has been used as an electro-acoustic transducer element, but in recent years, capacitive micromachined ultrasonic transducers (hereinafter abbreviated as CMUTs) having wider bandwidth characteristics than that of the piezoelectric ceramics have attracted attention and studied and developed (JP 2013-146478 A, JP 2009-194934A, and JP 2009-055474 A).

The CMUT is basically structured so that a hollow portion (cavity) is defined in an insulating layer between a lower electrode and an upper electrode arranged above the lower electrode, and the insulating layer and the upper electrode which are positioned above the hollow portion are functioned as a membrane (also referred to as diaphragm). For transmission of ultrasonic waves, direct voltage and alternating voltage are superimposed and applied between the upper electrode and the lower electrode to vibrate the membrane at the frequency of the alternating voltage by an electrostatic force generated between both electrodes at that time. On the other hand, for reception of ultrasonic waves, the pressure of the ultrasonic waves reaching a surface of the membrane vibrates the membrane, and a change in distance between both electrodes generated at that time is electrically detected as a change in capacitance.

SUMMARY OF THE INVENTION

In non-destructive inspection using ultrasonic waves, in order to improve ultrasonic wave propagation efficiency, objects to be inspected are often immersed in water for the inspection.

However, the states of many of the objects to be inspected changes due to swelling in water or water penetration, and thus, the objects to be inspected need to be inspected in the air.

However, the sound propagation efficiency of ultrasonic waves propagating in the air to a reception element is as low as 0.001%, which is an obstacle of high-precision inspection in the air.

The above and other objects and novel characteristics of the present invention will be apparent from the description of the present specification and the accompanying drawings.

The following is a brief description of the outline of a typical embodiment disclosed in the present application.

A capacitive device according to the typical embodiment includes a CMUT, a connection portion formed on a membrane of the CMUT and having a first area, and a transmission/reception plate formed above the CMUT via the connection portion and having a second area larger than the first area.

According to the present invention, it is possible to achieve a capacitive device having improved sound propagation efficiency in the air.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that, in all the drawings for the description of the embodiments, members having the same functions are denoted by the same reference numerals, and the repetitive description thereof will be omitted. In the drawings for the description of the embodiments, hatching may be omitted even in a cross-sectional view, for easy understanding of the configuration.

First Embodiment

FIG. 1is a cross-sectional view of a unit structure of a capacitive device10according to a first embodiment. The capacitive device10includes a unit cell11including a CMUT (ultrasonic transducer), and a transmission/reception plate13for impedance matching which is provided above the unit cell11via a connection portion12.

The unit cell (CMUT)11includes a lower electrode103formed on a substrate101including monocrystalline silicon via an insulating film102, two layers of insulating films104and106formed above the lower electrode103, a hollow portion110defined by a gap formed between the two layers of the insulating films104and106, an upper electrode107formed above the hollow portion110via the insulating film106, and three layers of insulating films108,111, and112formed above the upper electrode107. The insulating films104,106,108,111, and112include a silicon dioxide film.

Here, the insulating films106,108,111, and112and the upper electrode107have a portion positioned above the hollow portion110, and the portion functions as a membrane120which vibrates when transmitting and receiving ultrasonic waves. Furthermore, the insulating films106,108,111, and112has a portion surrounding the region functioning as the membrane120, and the portion functions as a fixing portion supporting the membrane120.

The insulating films104,106,108,111, and112each have an opening to define a connection hole113, and the connection hole113has a bottom portion to expose therefrom a pad115for external connection constituted by a part of the lower electrode103, and the insulating films108,111and112each have an opening to define a connection hole114, and the connection hole114has a bottom portion to expose therefrom a pad116for external connection constituted by a part of the upper electrode107. To the unit cell11, direct voltage and alternating voltage are applied from an external power supply through conductor layers (not illustrated) connected to the pads115and116.

The lower electrode103is formed by depositing a metal film, such as an aluminum alloy film, over the insulating film102. The upper electrode107is formed by depositing a metal film, such as an aluminum alloy film, over the insulating film106and then patterning the metal film by using photolithography and dry etching.

The hollow portion110is formed by forming a dummy pattern (not illustrated) made of a polycrystalline silicon film or metal film, over the insulating film104and then bringing a wet etching solution into contact with the surface of the dummy pattern through an opening109defined in the insulating films106and108to dissolve the dummy pattern. The hollow portion110has, for example, a rectangular planar shape but may have a circular, elliptical, or polygonal planar shape.

The connection portion12positioned on the unit cell11includes, for example, an insulating film (for example, silicon nitride film) having an etching selectivity to the wet etching solution different from that of the insulating film112, and the insulating film112is formed by depositing this insulating film and a thin film constituting the transmission/reception plate13on the insulating film112and performing patterning by using a difference in etching rate between this insulating film and the insulating film112.

The transmission/reception plate13provided above the unit cell11via the connection portion12may be a conductive film including, for example, an aluminum alloy or an insulating film, but a material having a low density and a high Young's modulus is preferably employed. The transmission/reception plate13has, for example, a rectangular planar shape but may have a circular, elliptical, or polygonal planar shape.

In addition, the materials of the electrodes and insulating films which constitute the unit cell11described above are only a preferable example, the materials are not limited to the above description, and various materials used for semiconductor manufacturing processes can be used. In other words, for the lower electrode103, the upper electrode107, and the transmission/reception plate13, a metal material other than the aluminum alloy, such as W, Ti, TiN, Al, Cr, Pt, or Au, polycrystalline silicon, amorphous silicon, or the like heavily doped with an impurity can also be used. Furthermore, instead of the insulating film including a silicon dioxide film, a silicon oxynitride film, a hafnium oxide film, a hafnium oxide film with silicon doped, or the like can be used.

The capacitive device10according to the present first embodiment is characterized in that the membrane120of the CMUT constituting the unit cell11is connected to the transmission/reception plate13via the connection portion12having an area smaller than that of the transmission/reception plate13. Although it is required that the transmission/reception plate13should have an area larger than the area of the connection portion12, the area of the transmission/reception plate13is desirably larger than the area of the hollow portion110, from the viewpoint of improvement in sound propagation efficiency.

Here, when the impedance of air is Z1(=00044 Mrayls) and the impedance of the unit cell11including the CMUT is Z2(a general CMUT=approximately several Mrayls), the acoustic reflectance (Rp) of sound propagating in the air and reflected on a surface of the capacitive device10is expressed by Rp=(Z2−Z1)/(Z1+Z2). In other words, the reflectance (Rp) decreases as Z2−Z1decreases (in other words, as Z1is closer to Z2).

Therefore, the capacitive device10according to the present embodiment is provided with the transmission/reception plate13having a larger area than that of the connection portion12above the unit cell11via the connection portion12, the capacitive device10has a larger apparent density of air, increasing the impedance Z1(=density×speed) of air, reducing the reflectance (Rp) relative to that of a transmission/reception element configured by CMUT (unit cell11) alone, and thus the capacitive device10functions as a transmission/reception element having higher sound propagation efficiency.

The capacitive device10actually has a composite structure in which a large number of unit structures each including a unit cell11, a connection portion12, and a transmission/reception plate13as described above are arranged in one direction or two orthogonal directions of a main surface of the substrate101.

For example, in a capacitive device10having a composite structure illustrated inFIG. 2andFIG. 3(FIG. 3is a cross-sectional view taken along the line III-III ofFIG. 2), a large number of unit cells11are formed on a common substrate101into an array, one sheet of the transmission/reception plate13is provided above the large number of unit cells11, and each unit cell11is connected to the transmission/reception plate13by a connection portion12having an area smaller than that of the unit cell11.

Furthermore, as illustrated inFIG. 4, a capacitive device10can adopt a composite structure in which the unit structures illustrated inFIG. 1are arranged in an array at predetermined intervals. In this case, the array is advantageously formed readily as compared with the composite structure illustrated inFIGS. 2 and 3. Furthermore, since the transmission/reception plate13is reduced in size as compared with the composite structure illustrated inFIGS. 2 and 3, suppression of transverse waves and design of frequency are advantageously facilitated.

Second Embodiment

FIG. 5is a schematic cross-sectional view of a unit structure of a capacitive device20according to a second embodiment. The capacitive device20according to the second embodiment is characterized in that one end of a transmission/reception plate13is supported by a unit cell11via a fixing portion14including an insulating film positioned in the same layer as a connection portion12. Furthermore, a hollow portion110and a membrane120(seeFIG. 1) above the hollow portion110, both of which function as a CMUT, and the connection portion12are arranged at a position closer to the fixing portion14than the center portion of the transmission/reception plate13. In other words, the hollow portion110, the membrane120, and the connection portion12are arranged at a position on a side closer to the fixing portion14(left side inFIG. 5) which is less likely to be deformed compared with the other end (right end inFIG. 5) of the transmission/reception plate13.

The capacitive device20according to the present embodiment configured as described above reduces the displacement of the transmission/reception plate13upon receiving a sound wave, and thus highly sensitive reception is enabled compared with the capacitive device10according to the first embodiment.

Furthermore, as in the unit structure of the capacitive device10according to the first embodiment, in the unit structure of the capacitive device20according to the second embodiment, the membrane120of the CMUT constituting the unit cell11is connected to the transmission/reception plate13via the connection portion12having a smaller area than that of the transmission/reception plate13, and thus the capacitive device20functions as a transmission/reception element having higher sound propagation efficiency.

The transmission/reception plate13has but is not limited to, for example, a rectangular planar shape, and in a case where the transmission/reception plate13has a planar spiral shape as illustrated inFIG. 6, the transmission/reception plate13having a larger area than that of the unit cell11can be achieved.

Furthermore, as in the case of the capacitive device10according to the first embodiment, the capacitive device20actually has a composite structure in which a large number of the fixing portions14and the above-mentioned unit structures each including the unit cell11, the connection portion12, and the transmission/reception plate13are arranged in one direction or two orthogonal directions of a main surface of a substrate101.

In this case as well, various modifications can be made, for example, a plurality of transmission/reception plates13is arranged in a comb-like shape as illustrated inFIG. 7, one sheet of the transmission/reception plate13is arranged in an annular shape as illustrated inFIG. 8, and a plurality of transmission/reception plates13is arranged radially around a common fixing portion14as illustrated inFIG. 9.

As described above, the invention made by the present inventors has been described in detail on the basis of the embodiments, but the present invention is not limited to the above-mentioned embodiments, and various modifications and alterations may be made without departing from the spirit and scope of the invention.

In the capacitive device10according to the first embodiment and the capacitive device20according to the second embodiment, the transmission/reception plate13is provided above the unit cell11including the CMUT, via the connection portion12, but the transmission/reception plate may be provided above a transmission/reception element other than the CMUT, such as a piezoelectric element, via the connection portion.

Although the piezoelectric element has lower sound propagation efficiency than that of the CMUT, in a case where the transmission/reception plate is provided above the piezoelectric element via the connection portion, the sound propagation efficiency can be improved as compared with using only the piezoelectric element.