Abstract:
An ultrasonic sensor includes a plurality of converters and a protection component. The plurality of converters convert one of a received ultrasonic wave into an electric signal and an electric signal into an ultrasonic wave for transmission. The plurality of converters are juxtaposed. The protection component protects each of the converters.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No.: 11/586,561filed on Oct. 26, 2006 now U.S. Pat. No. 7,329,975 which is a continuation of application Ser. No. 11/208,724 filed on Aug. 23, 2005 now abandoned. This application is also based upon and claims the benefit of priority of Japanese Patent Application No. 2004-245541, filed on Aug. 25, 2004 and Japanese Patent Application No. 2005-42449, filed on Feb. 18, 2005, the contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an ultrasonic sensor and, more particularly, to an ultrasonic sensor for converting a received ultrasonic wave into an electric signal or an electric signal into an ultrasonic wave so as to transmit it. 
   BACKGROUND 
   Recently, a technique of monitoring the vicinity of a vehicle for the purpose of driving safety has been developed. According to this technique, an ultrasonic sensor is mounted in the vehicle, which may include an automobile. The ultrasonic sensor receives a reflected wave of an ultrasonic wave harmless to a human body, which is transmitted from the ultrasonic sensor, so as to measure the position of or a distance from an object present in the vicinity of the automobile, a two-dimensional shape, or a three-dimensional shape of the object and the like. 
   For example, the following automatic parking system has been put into practical use. An ultrasonic sensor is mounted in a rear part of an automobile. A device, generally called “a back sonar,” is used while reversing the automobile into a parking space to avoid the collision with an object. The “back sonar” is for detecting the object, which may include a human or another obstacle, present behind the automobile. 
   As an ultrasonic sensor used for the above-described usage, a piezoelectric or a capacitive (condenser) ultrasonic sensor fabricated by employing a Micro Electro Mechanical Systems (MEMS) technique is known. 
   For example, a technique of juxtaposing a plurality of ultrasonic sensor elements has been disclosed as a piezoelectric ultrasonic sensor employing the MEMS technique. Each of the ultrasonic sensor elements is composed of a piezoelectric sensor, which includes a ferroelectric member interposed between a pair of electrodes. The piezoelectric sensor has a predetermined resonance frequency to detect an ultrasonic wave. Such a device is disclosed in Japanese Patent Laid-Open Publication No. 2003-284182. 
   The ultrasonic sensor disclosed in the above publication includes a piezoelectric element, which serves as a piezoelectric sensor, formed on a semiconductor chip having a “Silicon On Insulator” (SOI) structure. The piezoelectric element includes a thin film made of a PZT (lead zirconate titanate) ceramic corresponding to a ferroelectric material interposed between two thin electrode layers including an upper electrode layer and a lower electrode layer. 
   Therefore, each of the electrode layers and the PZT ceramic thin film have a low mechanical strength. As a result, there arises a problem that each of the electrode layers or the PZT ceramic thin film is vulnerable to damage upon application of an external force to the upper electrode layer so that the piezoelectric element is likely to be broken. 
   On the other hand, the capacitive ultrasonic sensor using the MEMS technique includes: a fixed electrode layer formed on a semiconductor chip; and a thin movable electrode layer provided on the fixed electrode layer through a gap. The fixed electrode layer and the movable electrode layer form a capacitive element. 
   With the above structure, the movable electrode layer has a low mechanical strength. Therefore, there arises a problem that the movable electrode layer is vulnerable to damage upon application of an external force to the movable electrode layer so that the capacitive electrode is likely to be broken. 
   As described above, the conventional piezoelectric or capacitive ultrasonic sensors fabricated by employing the MEMS technique are likely to be damaged under the application of an external force. Therefore, it is difficult to mount the conventional ultrasonic sensor in an automobile as external equipment. 
   SUMMARY 
   The present invention was devised to solve the above-described and other problems and to provide a robust ultrasonic sensor that that can withstand the application of an external force. 
   An ultrasonic sensor according to a first aspect of the present invention includes a plurality of conversion means and a protection means. The plurality of conversion means are for converting one of a received ultrasonic wave and an electric signal to the other of the electric signal and the ultrasonic wave for transmission. The plurality of conversion means are juxtaposed. The protection means is for protecting each of the conversion means. 
   An ultrasonic sensor according to a second aspect of the present invention is characterized in that the protection means includes a protective film provided in front of each of the plurality of conversion means and a first gap is provided between the protective film and the conversion means. 
   An ultrasonic sensor according to a third aspect of the present invention is characterized in that the first gap is filled with a filler selected from a liquid, a sol, and a gel. 
   An ultrasonic sensor according to a fourth aspect of the present invention is characterized in that the ultrasonic sensor includes a vent hole for bringing the first gap and exterior into communication with each other. 
   An ultrasonic sensor according to a fifth aspect of the present invention includes separation means for separating the conversion means and the first gap located in front of the conversion means and the protective film for each of the conversion means. 
   An ultrasonic sensor according to a sixth aspect of the present invention includes a housing member for housing each of the plurality of the conversion means therein; a second gap surrounded by the housing member and the conversion means; and a vent hole for bringing the second gap and exterior into communication with each other. 
   An ultrasonic sensor according to a seventh aspect of the present invention is characterized in that the conversion means is a transmission element for converting an electric signal into an ultrasonic wave for transmission. 
   An ultrasonic sensor according to an eighth aspect of the present invention includes a housing member for housing each of the plurality of conversion means therein; and a second gap corresponding to a sealed space surrounded by the housing member and the conversion means. 
   An ultrasonic sensor according to a ninth aspect of the present invention is characterized in that the second gap is filled with a filler selected from a liquid, a sol, and a gel. 
   An ultrasonic sensor according to a tenth aspect of the present invention is characterized in that each of the conversion means is a receiving element for converting a received ultrasonic wave into an electric signal. 
   An ultrasonic sensor according to an eleventh aspect of the present invention includes a transfer member for independently connecting each of the conversion means and the protective film with each other for each of the conversion means. 
   An ultrasonic sensor according to a twelfth aspect of the present invention includes a protective member attached to and fixed in front of each of the plurality of conversion means, the protective member being provided for each of the conversion means, a clearance being provided between the protective members, and the clearance serving to separate the protective members from each other for each of the conversion means. 
   An ultrasonic sensor according to a thirteenth aspect of the present invention includes an acoustic horn provided in front of each of the plurality of conversion means, wherein the acoustic horn is provided for each of the conversion means so as to have a gradually increasing sectional area from a throat provided in front of each of the conversion means toward an opening. 
   An ultrasonic sensor according to a fourteenth aspect of the present invention is characterized in that each of the conversion means is formed on a surface of a semiconductor substrate, the surface side of the semiconductor substrate being regarded as the front side of each of the conversion means so as to serve as any one of a receiving surface and a transmission surface of an ultrasonic wave, a bonding wire is connected to the surface side of the semiconductor substrate, and each of the conversion means is surface-mounted on a sensor substrate by a wire bonding method through the bonding wire. 
   An ultrasonic sensor according to a fifteenth aspect of the present invention is characterized in that each of the conversion means is formed on a surface of a semiconductor substrate, a bottom side of the semiconductor substrate being regarded as the front side of each the conversion means so as to serve as any one of a receiving surface and a transmission surface of an ultrasonic wave, a bump is connected to the surface side of the semiconductor substrate, and each of the conversion means is surface-mounted on a sensor substrate by flip-chip connection through the bump. 
   An ultrasonic sensor according to a sixteenth aspect of the present invention is characterized in that each of the conversion means is any one of a piezoelectric conversion type and a capacitive conversion type. 
   According to the first aspect of the present invention, the conversion means is composed of the receiving element for converting the received ultrasonic wave into the electric signal or the electric signal into the ultrasonic wave so as to transmit it. The plurality of conversion means are juxtaposed. 
   Moreover, according to the first aspect of the present invention, the protection means for protecting each of the conversion means is provided. Therefore, even if each of the conversion means has a low mechanical strength, it becomes possible to prevent the conversion means from being damaged so as to be hardly broken. As a result, a robust ultrasonic sensor can be obtained. 
   According to the second aspect of the present invention, the protective film is provided in front of the plurality of the conversion means, and the first gap is provided between the protective film and the conversion means. Therefore, even if an external force is applied to the ultrasonic sensor, the external force is applied only to the protective film but not directly to each of the conversion means. 
   Thus, according to the second aspect of the present invention, even if each of the conversion means has a low mechanical strength, it is possible to prevent the conversion means from being damaged so that the conversion means is hardly broken, thereby obtaining a robust ultrasonic sensor. 
   Therefore, the ultrasonic sensor according to the second aspect of the present invention can be mounted in an automobile as external equipment without any modification. If the ultrasonic sensor is to be mounted in an automobile as external equipment of an automobile, it is necessary to use a highly weather-resistant material for the protective film. Examples of such a material include various metals (such as an aluminum alloy), various synthetic resins, glasses, rubbers, and the like. 
   According to the third aspect of the present invention, an acoustic impedance of the filler selected from a liquid, a sol, and a gel filling the first gap is brought close to that of the protective film. As a result, the propagation of oscillation of the protective film to each of the conversion means through the filler can be ensured so as to enhance receiving sensitivity in the case where each of the conversion means is used as a receiving element. 
   The acoustic impedance of a material corresponds to a product of a density of the material and a propagation acoustic speed. Then, as a difference in acoustic impedance between materials becomes larger, the propagation characteristic of an acoustic wave is degraded. Specifically, as a difference in acoustic impedance between the filler in the first gap and the protective film becomes greater, an ultrasonic wave is reflected by the protective film so as to be less likely to propagate to the filler. 
   Therefore, if a synthetic resin film is used as the protective film, a sol obtained by dispersing fine particles of the synthetic resin in a liquid or a polymer gel made of the synthetic resin is used as the filler. Moreover, the filler is required not to affect the conversion means. Examples of the filler meeting such a requirement include a silicon gel, a fluorine gel, and the like. 
   As an example, if the first gap is filled with one of various gases (air, an inert gas, and the like), the oscillation of the protective film does not satisfactorily propagate to each of the conversion means because the gas has an acoustic impedance extremely smaller than that of the protective film. Accordingly, there is a possibility that receiving sensitivity is lowered when each of the conversion means is used as a receiving element. 
   If air remains in the first gap, the oscillation of the protective film is less likely to propagate to each of the conversion means. Therefore, it is desirable to completely remove air from the first gap so as to fill the first gap with the filler. 
   If each of the conversion means is used as a transmission element, the acoustic impedance of the filler selected from a liquid, a sol, and a gel filling the first gap is brought close to that of the protective film. As a result, the propagation of oscillation of the transmission element through the filler to the protective film can be ensured, thereby enhancing a transmission output of the transmission element. 
   Moreover, if the first gap is filled with one of various gases, the oscillation of the transmission element does not satisfactorily propagate to the protective film because the acoustic impedance of the gas is extremely smaller than that of the protective film. As a result, there is a possibility that a transmission output of the transmission element becomes low. 
   Moreover, if the first gap is filled with the filler such as a liquid, a sol, or a gel, it is desirable to completely remove air from the first gap so as to fill the first gap with the filler because the oscillation of the transmission element is less likely to propagate to the protective film if air remains in the first gap. 
   According to the fourth aspect of the present invention, when the filler in the first gap contains air bubbles, it is possible to remove the air bubbles from the first gap through the vent hole to the exterior. 
   Specifically, if the filler in the first gap contains air bubbles, the air bubbles make it hard to propagate the oscillation of the protective film to each of the conversion means. 
   On the other hand, according to the fourth aspect of the present invention, since the air bubbles are removed through the vent hole, it becomes possible to completely fill the first gap with the filler. Therefore, if each of the conversion means is used as a receiving element, the receiving sensitivity can be prevented from being lowered by the presence of air bubbles contained in the filler in the first gap. 
   If each of the conversion means is used as a transmission element, it becomes possible to completely fill the first gap with the filler because the air bubbles contained in the filler in the first gap are removed through the vent hole. In this manner, the propagation of oscillation of the transmission element through the filler to the protective film can be ensured to prevent the transmission output of the transmission element from being lowered. 
   According to the fifth aspect of the present invention, the oscillation of a single protective film separated by the separation means propagates only to the conversion means through the first gap situated below the protective film but not to the other conversion means. 
   Therefore, according to the fifth aspect of the present invention, the propagation of an ultrasonic wave to each of the conversion means can be performed in a completely separate manner. Therefore, a crosstalk characteristic of each of the conversion means can be prevented from being degraded. Alternatively, a plurality of adjacent conversion means can be grouped into one. Separation means may be provided for each group of the conversion means so as to separate the corresponding group from the other groups. 
   The separation means has to surely block the oscillation of the protective film, the first gap, and the conversion means, which are vertically provided so as to be grouped into one, so that the oscillation does not propagate to members of the other adjacent groups. 
   For this reason, a material having a high oscillation blocking property is required to be used for the separation means. Examples of the material include rubbers. 
   According to the sixth aspect of the present invention, since the oscillation of each of the conversion means is not inhibited, receiving sensitivity when each of the conversion means is used as a receiving element can be prevented from being lowered. 
   Specifically, if a vent hole is not provided for the second gap, the second gap forms a sealed space. Air filling the sealed space acts as a spring so as to apply a damping force due to air onto the back face side of each of the conversion means. As a result, the free oscillation of each of the conversion means is inhibited. 
   On the other hand, according to the sixth aspect of the present invention, air passes through the vent hole. Accordingly, no damping force due to air is applied onto the back face side of each of the conversion means. As a result, each of the conversion means is capable of freely oscillating. 
   If each of the conversion means is used as a transmission element, air passes through the vent hole of the second gap. Therefore, no damping force due to air is applied to the back face side of a transmission surface of the transmission element for transmitting an ultrasonic wave. As a result, the transmission surface can freely oscillate without inhibiting the oscillation. Therefore, the transmission output of the transmission element can be increased. 
   To obtain satisfactory functions and effects described above, the number, the position, the shape, and the size of the vent hole may be set by experimentally finding their optimal values in a cut-and-try method. 
   When air passes through the vent hole of the second gap, no damping force due to air is applied onto the back face side of the transmission element so as not to inhibit the free oscillation of the transmission surface. Accordingly, a resonance value Q of the transmission element (a diaphragm of the conversion means) is increased. 
   The resonance value Q of the transmission element and the transmission output are positively correlated with each other. Thus, as the resonance value Q increases, the transmission output becomes greater. 
   The transmission element including a piezoelectric element or a capacitive element fabricated by employing the MEMS technique is not suitable for the transmission element because of its small transmission output of an ultrasonic wave. Therefore, such a transmission element is required to increase the transmission output as much as possible. 
   Thus, the seventh aspect of the present invention can demonstrate the functions and effects of the sixth aspect particularly when the aspect is embodied as the transmission element fabricated by employing the MEMS technique. 
   According to the eighth aspect of the present invention, air filling the second gap corresponding to the sealed space acts as a spring so as to apply a damping force due to air onto the back face side of each of the conversion means. As a result, since the free oscillation of each of the conversion means is inhibited, the resonance value Q of the diaphragm of the conversion mean is reduced. 
   Moreover, according to the sixth aspect of the present invention, if each of the conversion means is used as a receiving element, the receiving sensitivity is lowered because the oscillation of each of the conversion means is inhibited. 
   According to the ninth aspect of the present invention, by filling the second gap with a material for suppressing the oscillation of the diaphragm of the conversion means (for example, a liquid, a sol, a gel, or the like), the diaphragm of the conversion means can be prevented from excessively oscillating to be broken. 
   The resonance value Q of the receiving element and the receiving sensitivity are positively correlated with each other. Thus, as the resonance value Q increases, the receiving sensitivity becomes greater. 
   Herein, a plurality of receiving elements have a fluctuation in primary resonance frequency due to a fabrication process. 
   If the resonance value Q of the receiving element is increased, the receiving sensitivity is increased. However, since the receiving sensitivity exhibits a steep characteristic with respect to a change in frequency, the receiving sensitivity suddenly drops at a frequency offset from the primary resonance frequency even if the offset is slight. 
   On the contrary if the resonance value Q of the receiving element is set small, the receiving sensitivity become correspondingly low. However, since the receiving sensitivity exhibits a gentle characteristic with respect to a change in frequency, the receiving sensitivity does not greatly drop even at a frequency far from the primary resonance frequency. 
   The receiving element comprising a piezoelectric element or a capacitive element fabricated by employing the MEMS technique is suitable for a receiving element because of its high receiving sensitivity of an ultrasonic wave. Therefore, it is necessary to increase the receiving sensitivity over a broad frequency range as much as possible rather than to increase the receiving sensitivity at the primary resonance frequency. 
   Therefore, the tenth aspect of the present invention can demonstrate the functions and effects of the eighth aspect particularly when the aspect is embodied as a receiving element fabricated by employing the MEMS technique. 
   According to the tenth aspect of the present invention, by filling the second gap with a material for suppressing the oscillation of the diaphragm of the conversion means, the resonance value Q of the diaphragm of the conversion means can be reduced as compared with the case where the second gap is filled with air. 
   Accordingly, if the filler in the second gap is appropriately selected, a desired resonance characteristic can be obtained without changing the structure of the receiving element. 
   According to the eleventh aspect of the present invention, when an ultrasonic wave oscillates the protective film, the oscillation of the protective film propagates to each of the conversion means through each of the transfer members. 
   Herein, since the transfer member is provided for each of the conversion means, the oscillation of an arbitrary transfer member never propagates to the other transfer members. As a result, since the reception or transmission of an ultrasonic wave can be performed in a separated manner for each of the conversion means, a crosstalk characteristic of each of the conversion means can be prevented from being degraded. 
   Moreover, the acoustic impedance of each of the transfer members is brought close to that of the protective film. As a result, the propagation of oscillation of the protective film to each of the conversion means can be ensured, thereby enhancing the receiving sensitivity in the case where each of the conversion means is used as a receiving element. 
   Furthermore, the acoustic impedance of each of the transfer members is brought close to that of the conversion means. As a result, the propagation of oscillation of each of the transfer members to each of the conversion means can be ensured, thereby enhancing the receiving sensitivity in the case where each of the conversion means is used as a receiving element. 
   Therefore, it is desirable that the transfer member be made of the same material as that of the protective film or the conversion means. 
   If each of the conversion means is used as a transmission element, the propagation of oscillation of the transfer member to the protective film can be ensured by bringing the acoustic impedance of the transfer member close to that of the protective film. As a result, the transmission output of the transmission element can be increased. 
   Moreover, if each of the conversion means is used as a transmission element, the propagation of oscillation of the transmission element to the transfer member can be ensured by bringing the acoustic impedance of the transfer member close to that of the transmission element. As a result, the transmission output of the transmission element can be increased. 
   According to the twelfth aspect of the present invention, if each of the conversion means is used as a receiving element, the oscillation of the protective member propagates to the receiving element when an ultrasonic wave oscillates the protective member because the protective film is attached and fixed in front of the receiving element. 
   On the other hand, according to the twelfth aspect of the present invention, if each of the conversion means is used as a transmission element, the oscillation of the transmission element propagates to the protective member when the transmission element oscillates because the protective member is attached and fixed in front of the transmission element. As a result, the protective member oscillates to transmit an ultrasonic wave. 
   Herein, since each of the conversion means is reinforced by the protective member, each of the conversion means can be prevented from being damaged so as to be hardly broken even if an external force is applied to the ultrasonic sensor. As a result, a robust ultrasonic sensor can be obtained. 
   Therefore, the ultrasonic sensor according to the twelfth aspect of the present invention can be mounted as external equipment of an automobile without any modification. If the ultrasonic sensor is mounted as external equipment of an automobile, it is necessary to use a highly weather-resistant material for the protective member. Examples of the material include various metals (such as an aluminum alloy), various synthetic resins, glasses, rubbers, and the like. 
   As a method of attaching and fixing the protective member to each of the conversion means, any method (for example, thermal welding, ultrasonic welding, bonding with an adhesive, and the like) may be used. 
   According to the thirteenth aspect of the present invention, an acoustic horn is provided for each of the conversion means. As a result, each of the conversion means can be imparted with directivity of a receiving direction or a transmitting direction of an ultrasonic wave. 
   Specifically, each of the acoustic horns has acute directivity on its horn axis. Therefore, by forming the acoustic horns to have the same size and shape, the directivity of each of the conversion means can be the same if the horn axes of the acoustic horns are set in the same direction. Moreover, in the case where the horn axes of the acoustic hones are set to be in arbitrary different directions by changing the size and shape of each of the acoustic horns, the directivity of each of the conversion means can also be set in an arbitrary direction. 
   A horn wall member of each of the acoustic horns is required to be formed of a material having enough strength to hardly cause oscillation by an ultrasonic wave. Examples of the material include various metals, various synthetic resins, and the like. 
   According to the fourteenth aspect of the present invention, the ultrasonic sensor formed by surface-mounting each of the conversion means on a sensor substrate by using wire bonding can be obtained. 
   According to the fifteenth aspect of the present invention, each of the conversion means and the sensor substrate are connected and fixed to each other through a bump. Therefore, since it can be ensured to keep the electric connection between each of the conversion means and the sensor substrate, the reliability of the ultrasonic sensor can be enhanced while extending a lifetime of the ultrasonic sensor. 
   Moreover, by employing flip-chip connection, the fabrication cost for surface-mounting each of the conversion means onto the sensor substrate can be reduced as compared with the case where the wire bonding is employed. 
   In the case where each of the conversion means is used as a receiving element, a bonding wire is not provided above the receiving surface of an ultrasonic wave and therefore no obstacle is present in front of the receiving surface. Therefore, the ultrasonic wave is not inhibited from getting to the receiving surface, thereby enhancing the receiving sensitivity of the receiving element. Moreover, since a bonding wire is not provided above the receiving surface of the receiving element, the bonding wire is not cut by an ultrasonic wave received by the receiving element. 
   Moreover, in the case where each of the conversion means is used as a transmission element, a bonding wire is not provided above the transmission surface of the transmission element and therefore no obstacle is present in front of the transmission surface. Therefore, the ultrasonic wave is not inhibited from being transmitted from the transmission surface, thereby enhancing the transmission output of the transmission element. Moreover, since a bonding wire is not provided above the transmission surface of the receiving element, the bonding wire is not cut by an ultrasonic wave transmitted from the transmission element. 
   Furthermore, since an inductance of the bump is reduced as compared with that of the bonding wire, a transfer rate of an electric signal in each of the conversion means can be increased. 
   Moreover, it is no longer necessary to provide an electrode pad to which the bonding wire is connected. Since the sensor substrate is reduced by an area occupied by the electrode pad, the ultrasonic sensor can be reduced in size as well as in weight. 
   In addition, according to the fifteenth aspect of the present invention, if a thickness of the diaphragm of each of the conversion means is reduced by forming a concave portion on the bottom face side of a semiconductor substrate so as to facilitate the oscillation of each of the conversion means, the functions and effects of the thirteenth aspect of the invention can be easily obtained without providing the acoustic horn as an independent member. 
   Moreover, since the acoustic horn is not required to be provided as an independent member, the fabrication cost can be reduced. At the same time, the ultrasonic sensor can be reduced in size as well as in weight. 
   According to the sixteenth aspect of the present invention, a piezoelectric or capacitive ultrasonic sensor can be obtained. 
   Other features and advantages of the present invention will be appreciated, as well as methods of operation and the function of the related parts from a study of the following detailed description, appended claims, and drawings, all of which form a part of this application. In the drawings: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a first embodiment of the present invention; 
       FIG. 2  is an enlarged cross-sectional side view of a piezoelectric receiving element of the receiving section of  FIG. 1 ; 
       FIG. 3  is a perspective view of a first ultrasonic sensor according to the principles of the present invention; 
       FIG. 4  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a second embodiment of the present invention; 
       FIG. 5  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a third embodiment of the present invention; 
       FIG. 6  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a fourth embodiment of the present invention; 
       FIG. 7  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a fifth embodiment of the present invention; 
       FIG. 8  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a sixth embodiment of the present invention; 
       FIGS. 9A and 9B  are cross-sectional side views of a receiving section of an ultrasonic sensor according to a seventh embodiment of the present invention; 
       FIG. 10  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to an eighth embodiment of the present invention; 
       FIG. 11  is an enlarged cross-sectional side view of a capacitive receiving element adapted for use in any one of the receiving sections in the first through eighth embodiments of the present invention; 
       FIG. 12  is a perspective view of a second ultrasonic sensor according to the principles of the present invention; 
       FIG. 13  is a cross-sectional side view of the receiving section of  FIG. 4  and a transmission section of the ultrasonic sensor of  FIG. 12 ; 
       FIG. 14  is a cross-sectional side view of the receiving section of  FIG. 5  and a transmission section of the ultrasonic sensor of  FIG. 12 ; 
       FIG. 15  is a cross-sectional side view of the receiving section of  FIG. 8  and a transmission section of the ultrasonic sensor of  FIG. 12 ; 
       FIGS. 16A and 16B  are cross-sectional side views of the receiving section of  FIGS. 9A and 9B  and a transmission section of the ultrasonic sensor of  FIG. 12 ; 
       FIG. 17  is a cross-sectional side view of the receiving section of  FIG. 10  and a transmission section of the ultrasonic sensor of  FIG. 12 ; 
       FIG. 18  is a cross-sectional side view of a receiving section of an ultrasonic sensor according to a ninth embodiment of the present invention; 
       FIG. 19  is an enlarged cross-sectional side view of a piezoelectric receiving element of the receiving section of  FIG. 18 ; 
       FIG. 20  is a perspective view of a third ultrasonic sensor according to the principles of the present invention; 
       FIG. 21  is a cross-sectional side view of a receiving section according to a tenth embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 22  is a cross-sectional side view of a receiving section according to an eleventh embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 23  is a perspective view of a fourth ultrasonic sensor according to the principles of the present invention; 
       FIGS. 24A and 24B  are graphs showing a resonance characteristic corresponding to the relation between a resonance value of a diaphragm and a frequency according to the principles of the present invention; 
       FIG. 25  is a cross-sectional side view of a receiving section according to an twelfth embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 26  is a cross-sectional side view of a first alternative receiving section of the twelfth embodiment of the present invention; 
       FIG. 27  is a cross-sectional side view of a second alternative receiving section and a transmission section of the twelfth embodiment of the present invention; 
       FIG. 28  is a cross-sectional side view of a receiving section according to a thirteenth embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 29  is a cross-sectional side view of a first alternative receiving section of the thirteenth embodiment; 
       FIG. 30  is a cross-sectional side view of a second alternative receiving section according to the thirteenth embodiment and the transmission section adapted for use in the ultrasonic sensor of  FIG. 23 ; 
       FIG. 31  is a cross-sectional side view of a receiving section according to a fourteenth embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 32  is a cross-sectional side view of a first alternative receiving section of the fourteenth embodiment of the present invention adapted for use in the ultrasonic sensor of  FIG. 20 ; 
       FIG. 33  is a cross-sectional side view of a second alternative receiving section of the fourteenth embodiment and a transmission section adapted for use in the ultrasonic sensor of  FIG. 23 ; 
       FIG. 34  is an enlarged cross-sectional side view of a capacitive receiving element adapted for use in any one of the receiving sections of the ninth to the fourteenth embodiments including the alternatives to these embodiments; 
       FIG. 35  is a cross-sectional side view of the receiving section and the transmission section of the ninth embodiment adapted to the ultrasonic sensor of  FIG. 23 ; 
       FIG. 36  is a cross-sectional side view of the receiving section and the transmission section of the twelfth embodiment adapted to the ultrasonic sensor of  FIG. 23 ; 
       FIG. 37  is a cross-sectional side view of the receiving section and the transmission section of the thirteenth embodiment adapted to the ultrasonic sensor of  FIG. 23 ; and 
       FIG. 38  is a cross-sectional side view of the receiving section and the transmission section of the fourteenth embodiment adapted to the ultrasonic sensor of  FIG. 23 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments where the present invention is embodied will be described with reference to the accompanying drawings. In the embodiments, the same components are denoted by the same reference numerals, and the description of the same contents is herein omitted. 
   Embodiment 1 
     FIG. 1  is a cross-sectional side view showing a receiving section  10  in an ultrasonic sensor M according to Embodiment 1. 
   The receiving section  10  includes a plurality of piezoelectric receiving elements  11  arranged in an array. In an example shown in  FIG. 1 , a cross-sectional side view of three receiving elements  11  is shown. 
   Each of the receiving elements  11  is formed on a single-crystalline silicon substrate (a single-crystalline silicon chip)  12  having an SOI structure. 
   The substrate  12  is housed within a housing member  13  having a rectangular parallelepiped box shape with an upper open end. Moreover, an outer circumferential end of the substrate  12  is attached and fixed to an inner wall of the housing member  13  by an appropriate method (for example, thermal welding, ultrasonic welding, bonding with an adhesive or the like) so as to air-seal a connection part between the outer circumferential end of the substrate  12  and the housing member  13 . 
   Each of the receiving elements  11  is located so that a receiving surface  11   a  for receiving an ultrasonic wave is oriented toward an opening  13   a  of the housing member  13 . 
   A protective film  14  for closing the opening  13   a  is attached over the opening  13   a  of the housing  13 . Specifically, the protective film  14  is provided in front of the receiving elements  11 . 
   An outer circumferential end of the protective film  14  is attached and fixed to an inner circumferential end of the opening  13   a  of the housing member  13  by the above-described appropriate method so as to air-seal a connection part between the outer circumferential end of the protective film  14  and the housing member  13 . 
   The protective film  14  is a thin film made of a material that is likely to be oscillated by an ultrasonic wave. Although the material of the protective film  14  transmits an ultrasonic wave without refraction, it does not transmit air, dust, water and the like. 
   A gap S is provided between the protective film  14  and the substrate  12 . The gap S is filled with a gas, a liquid, a sol, a gel or the like. 
   A gap R surrounded by the back face side (the bottom face side) of the substrate  12  and the housing member  13  is filled with air. 
     FIG. 2  is an enlarged cross-section side view showing one piezoelectric receiving element  11 . 
   A through hole  12   a  passing through the substrate  12  is formed in the substrate  12 . 
   On a surface of the substrate  12 , an insulating layer  21 , a silicon active layer  22 , and an insulating layer  23  are formed in this order. Each of the layers  22  and  23  is formed to close the upper opening of the through hole  12   a.    
   On a surface of the insulating layer  23  situated above (in front of) the through hole  12   a , a lower electrode layer  24 , a thin film layer  25  made of a ferroelectric (for example, PZT or the like), and an upper electrode layer  26  are formed in this order. 
   An insulating layer  27  is formed around the layers  24  to  26 . Surfaces of the insulating layer  27  and the upper electrode layer  26  (a device surface) are evened. 
   A bonding wire (a lead wire)  28  is connected to the lower electrode layer  24 , whereas a bonding layer  29  is connected to the upper electrode layer  26 . 
   In the above-described manner, a piezoelectric element (a piezoelectric sensor) E having a structure in which the ferroelectric thin film layer  25  is sandwiched between the two thin electrode layers  24  and  26  is formed. The receiving element  11  includes the piezoelectric element E fabricated by employing the MEMS technique. 
   Moreover, a receiving surface  11   a  of the receiving element  11  is formed by the surface of the upper electrode layer  26 . 
   When the thin film layer  25  is oscillated by an ultrasonic wave, an electric signal is generated by a piezoelectric effect. The thus generated electric signal is output from each of the electrodes  24  and  26  through the bonding wires  28  and  29 . 
   The through hole  12   a  is provided so as to facilitate the oscillation of a diaphragm composed of the layers  22  to  26 . 
     FIG. 3  is a perspective view showing the ultrasonic sensor M. 
   The ultrasonic sensor M is composed of a hybrid IC (Integrated Circuit) including: a receiving section  10 ; a transmission section  31 ; a sensor substrate  32 ; and electrode pads  33 . 
   The sensor substrate  32  is a printed wiring board. A plurality of electrode pads  33  are formed on a surface of the sensor substrate  32  formed of an insulating plate material while the receiving section  10  and the transmission section  31  corresponding to chip parts are attached and fixed thereto. 
   A tip of each of the bonding wires  28  and  29  led from each of the receiving elements  11  in the receiving section  10  is connected to each of the electrode pads  33 . 
   In the example shown in  FIG. 3 , the receiving section  10  is constituted by nine receiving elements  11  arranged 3 by 3. 
   The transmission section  31  has the same structure as that of the receiving section  10 . The transmission section  31  includes a single piezoelectric transmission element having the same structure as that of the receiving element  11 . The thin film layer  25  oscillates due to the piezoelectric effect to produce an ultrasonic wave in accordance with input signals applied from the electrode layers  24  and  26  to the thin film layer  25  made of a ferroelectric. In this case, the receiving surface  11   a  of the receiving element  11  acts as a transmission surface for transmitting the ultrasonic wave from the transmission element. 
   Specifically, the transmission element of the transmission section  31  converts an electric signal into an ultrasonic wave so as to transmit it. 
   Then, the transmission section  31  transmits an ultrasonic wave in accordance with an input signal from the exterior. A reflection sound generated by the ultrasonic wave reflected by an object to be detected is received by each of the receiving elements  11  in the receiving section  10 . 
   Specifically, each of the receiving elements  11  in the receiving section  10  converts the received ultrasonic wave into an electric signal. 
   The ultrasonic wave-transmitted from the transmission section  31  and the ultrasonic wave received by each of the receiving elements  11  in the receiving section  10  are compared with each other so as to obtain an acoustic pressure difference, a time difference, and a phase difference between them. As a result, the position of the object to be detected, a distance between the ultrasonic sensor M and the object to be detected, a two-dimensional or three-dimensional shape of the object to be detected and the like can be measured based on the obtained differences. 
   Functions and Effects of Embodiment 1 
   According to Embodiment 1, the following functions and effects can be obtained. 
   [1-1] 
   The protective film  14  is provided in front of the substrate  12  on which the receiving elements  11  are formed. The gap S is provided between the protective film  14  and the substrate  12 . 
   Therefore, even if an external force is applied to the receiving section  10  of the ultrasonic sensor M, the external force is applied merely to the protective film  14  but not directly to each of the thin layers  22  to  26  formed on the substrate  12 . 
   Therefore, according to Embodiment 1, even if each of the thin layers  22  to  26  has a low mechanical strength, each of the layers  22  to  26  can be prevented from being damaged so as to be unlikely to break the receiving section  10 . As a result, the robust receiving section  10  can be obtained. 
   Moreover, since the transmission element of the transmission section  31  has the same structure as that of the receiving element  11 , each of the layers  22  to  26  can be prevented from being damaged so that the transmission section  31  is hardly broken. As a result, the robust transmission section  31  can be obtained. 
   Therefore, the ultrasonic sensor M including the receiving section  10  and the transmission section  31  can be mounted as external equipment of an automobile without any modification. If the ultrasonic sensor M is to be mounted as external equipment of an automobile, it is necessary to use a highly weather-resistant material for the protective film  14 . Examples of the material include various metals (such as an aluminum alloy), various synthetic resins, glasses, rubbers, and the like. 
   [1-2] 
   In the case where the gap S between the protective film  14  and the substrate  12  is filled with a filler selected from a liquid, a sol and a gel, an acoustic impedance of the filler is brought close to that of the protective film  14 , so that it becomes possible to propagate the oscillation of the protective film  14  through the filler to each of the receiving elements  11 . As a result, the receiving sensitivity of each of the receiving elements  11  can be enhanced. 
   The acoustic impedance of a material is a product of a density of the material and a propagation acoustic speed. Then, as a difference in acoustic impedance between materials becomes larger, the propagation characteristic of an acoustic wave is degraded. Specifically, as a difference in acoustic impedance between the filler in the gap S and the protective film  14  becomes greater, an ultrasonic wave is reflected by the protective film  14  so as be less likely to propagate to the filler. 
   Therefore, if a synthetic resin film is used as the protective film  14 , a sol obtained by dispersing fine particles of the synthetic resin in a liquid or a polymer gel made of the synthetic resin is used as the filler. Moreover, the filler is required not to affect the receiving elements  11 . Examples of the filler meeting such a requirement include a silicon gel, a fluorine gel, and the like. 
   In order to fill the gap S with the filler, after the attachment of the protective film  13  onto the housing member  13 , the filler is injected into the gap S while removing air from the gap S. 
   Alternatively, after the substrate  12  is housed within the housing member  13  and the filler is then poured on the substrate  12  through the upper opening of the housing member  13 , the protective film  14  may be attached onto the housing member  13 . 
   Further alternatively, after the filler is dropped onto the substrate  12 , the substrate  12  is rotated so as to form a thin film made of the filler on the surface of the substrate  12  by spin coating. Subsequently, the substrate  12  may be housed within the housing member  13 . 
   As an example, if the gap S is filled with one of various gases (air, an inert gas and the like), the oscillation of the protective film  14  does not satisfactorily propagate to the receiving elements  11  because the gas has an acoustic impedance extremely smaller than that of the protective film  14 . Accordingly, there is a possibility that the receiving sensitivity of each of the receiving elements  11  is lowered. 
   In the case where the gap S is filled with the filler such as a liquid, a sol, and a gel, the oscillation of the protective film  14  is less likely to propagate to the receiving elements  11  if air remains in the gap S. Therefore, it is desirable to completely remove air from the gap S so as to fill the gap S with the filler. 
   If the gap S between the protective film  14  and the substrate  12  is filled with the filler selected from a liquid, a sol and a gel, the propagation of oscillation of the transmission element through the filler to the protective film  14  can be ensured by bringing the acoustic impedance of the filler close to that of the protective film  14  because the transmission element in the transmission section  31  has the same structure as that of the receiving element  11 . As a result, a transmission output of the transmission element can be enhanced. 
   Moreover, if the gap S is filled with one of various gases, the oscillation of the transmission element does not satisfactorily propagate to the protective film  14  because the acoustic impedance of the gas is extremely smaller than that of the protective film  14 . Therefore, there is a possibility that a transmission output of the transmission element becomes low. 
   Moreover, in the case where the gap S is filled with the filler such as a liquid, a sol and a gel, it is desirable to completely remove air from the gap S so as to fill the gap S with the filler because the oscillation of the transmission element is less likely to propagate to the protective film  14  if air remains in the gap S. 
   [1-3] 
   In the example shown in  FIG. 3 , the ultrasonic sensor includes the receiving section  10  including nine receiving elements  11  (piezoelectric elements E). However, the number of the receiving elements  11  constituting the receiving section  10  affects the accuracy of the measurement (the measurement of position, a distance and a shape) of the object to be detected; as the number of the receiving elements  11  is increased, the accuracy can be enhanced. 
   An interval between the receiving elements  11  is required to be set shorter than a wavelength of the ultrasonic wave transmitted from the transmission section  31 . The interval between the receiving elements  11  also affects the measurement accuracy. 
   Therefore, the number of and the interval between the receiving elements  11  can be set by experimentally finding their optimal values in a cut-and-try method in accordance with the required measurement accuracy. 
   For example, if only the directional position of the object to be detected with respect to the ultrasonic sensor M is to be measured, several receiving elements  11  are satisfactory. However, if a precise two-dimensional shape of the object to be detected is measured, it is necessary to provide several tens to several hundreds of the receiving elements  11 . Furthermore, if a precise three-dimensional shape of the object to be detected is measured, a larger number of the receiving elements  11  than the number needed for two-dimensional shape measurement are required. 
   [1-4] 
   In the example shown in  FIG. 3 , the arrangement of the transmission elements constituting the transmission section  31  is appropriately determined so as to adjust the directivity of a transmission direction of an ultrasonic wave. 
   Therefore, the number and the arrangement of the transmission elements constituting the transmission section  31  can be set by experimentally fining their optimal values in a cut-and-try method in accordance with the required transmission output and directivity. 
   Embodiment 2 
     FIG. 4  is a cross-sectional side view showing a receiving section  40  in the ultrasonic sensor M according to Embodiment 2. 
   The receiving section  40  according to Embodiment 2 differs from the receiving section  10  in Embodiment 1 only in that the protective film  14  is replaced by a thin-plate like protective member  41  attached and fixed onto the receiving surface  11   a  of each of the receiving elements  11 . 
   Specifically, in the receiving section  40 , the protective member  41  is attached to the front side of each of the receiving elements  11 . A clearance K is provided between the protective members  41  of the respective receiving elements  11 . The clearance K separates the protective members  41  for each of the receiving elements  11 . 
   The structure of the ultrasonic sensor M according to Embodiment 2 is obtained by replacing the receiving section  10  in the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  40 . 
   Functions and Effects of Embodiment 2 
   According to Embodiment 2, the following functions and effects can be obtained in addition to the same functions and effects described in [1-3] and [1-4] of Embodiment 1 above. 
   [2-1] 
   The thin plate-like protective member  41  is attached and fixed to the receiving surface  11   a  of each of the receiving elements  11 . Therefore, when the protective member  41  is oscillated by an ultrasonic wave, the oscillation of the protective member  41  propagates to each of the layers  22  to  26  (not shown in  FIG. 4 ; see  FIG. 2 ) on the receiving surface  11   a . As a result, the thin film layer  25  made of a ferroelectric oscillates to generate an electric signal due to a piezoelectric effect. 
   Herein, since the layers  22  to  26  on the receiving surface  11   a  are reinforced with the protective member  41 , each of the thin layers  22  to  26  can be prevented from being damaged so as to be unlikely to break the receiving section  40  even if an external force is applied to the receiving section  40  of the ultrasonic sensor M. As a result, the robust receiving section  40  can be obtained. 
   Moreover, in the case where the transmission section  31  is made to have the same structure as that of the receiving section  40  so that the thin plate-like protective member  41  is attached and fixed to the transmission surface of the transmission element, the oscillation of the thin film layer  25  propagates to the protective member  41  when the thin film layer  25  is oscillated by the piezoelectric effect. Then, the protective member  41  is oscillated in turn to transmit the ultrasonic wave. 
   In this case, since the layers  22  to  26  on the transmission surface of the transmission element are reinforced by the protective member  41 , each of the thin layers  22  to  26  can be prevented from being damaged so as to be unlikely to break the transmission section  31  even if an external force is applied to the transmission section  31  of the ultrasonic sensor M. As a result, the robust transmission section  31  can be obtained. 
   [2-2] 
   Since the receiving section  40  and the transmission section  31  are robust, the ultrasonic sensor M including the receiving section  40  and the transmission section  31  can be mounted as external equipment of an automobile without any modification. If the ultrasonic sensor M is to be mounted as external equipment of an automobile, it is necessary to use a highly weather-resistant material as a material of the protective member  41 . Examples of such a material include various metals (such as an aluminum alloy), various synthetic resins, glasses, rubbers, and the like. 
   As a method of attaching and fixing the protective member  41  onto the receiving surface  11   a  of each of the receiving elements  11  (the transmission surface of the transmission element), any method (for example, thermal welding, ultrasonic welding, bonding with an adhesive and the like) may be used. 
   [2-3] 
   The clearance K is provided between the protective members  41  of the respective receiving elements  11 . The clearance K serves to separate the protective members  41  for each of the receiving elements  11 . Therefore, the oscillation of one protective member  41  propagates only to the receiving element  11  to which the protective member  41  is attached and fixed but not to the other receiving elements  11  through the adjacent protective member  41 . 
   Thus, according to Embodiment 2, since an ultrasonic wave can be received by each of the receiving elements  11  in a completely separate manner, a crosstalk characteristic of each of the receiving elements  11  can be prevented from being degraded. 
   Embodiment 3 
     FIG. 5  is a cross-sectional side view showing a receiving section  50  in the ultrasonic sensor M according to Embodiment 3. 
   The receiving section  50  according to Embodiment 3 differs from the receiving section  10  according to Embodiment 1 merely in that separation members  51  and separation grooves  52  are provided. 
   A lower end of each of the separation members  51  is embedded in each of the separation grooves  52  formed in the substrate  12  between the receiving elements  11 . On the other hand, an upper end of each of the separation members  51  separates the gaps S and the protective films  14  for each of the receiving elements. 
   Specifically, in the example shown in  FIG. 5 , the lower end of each of the separation members  51  is embedded into each of the separation grooves  52  formed in the substrate  12  between three receiving elements  11 A to  11 C. As a result, the receiving elements  11 A to  11 C are separated from each other by the separation members  51  and the separation grooves  52 . 
   The gaps SA to SC and the protective films  14 A to  14 C situated above (in front of) the respective receiving elements  11 A to  11 C are also separated from each other by the separation members  51  for each of the receiving elements  11 A to  11 C. 
   The structure of the ultrasonic sensor M according to Embodiment 3 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  50 . 
   Functions and Effects of Embodiment 3 
   According to Embodiment 3, the following functions and effects can be obtained in addition to the above-described functions and effects according to Embodiment 1. 
   [3-1] 
   The receiving elements  11 , and the gaps S and the protective films  14  situated above (in front of) the respective receiving elements  11  are separated by the separation members  51  and the separation grooves  52  for each of the receiving elements  11 . Therefore, the oscillation of one protective film  14 A obtained by the separation propagates only to the receiving element  11 A through the gap SA situated below the protective film  14 A but not to the other receiving elements  11 B and  11 C. 
   Therefore, according to Embodiment 3, an ultrasonic wave can be received by each of the receiving elements  11 A to  11 C in a completely separate manner. Accordingly, a crosstalk characteristic of each of the receiving elements  11 A to  11 C can be prevented from being degraded. 
   Alternatively, a plurality of the receiving elements  11  adjacent to each other may be grouped into one. The separation member  51  and the separation groove  52  may be provided for each group so as to separate the group from the other groups. 
   [3-2] 
   Each of the separation members  51  is required to surely block the oscillation of the protective film  14 A, the gap SA and the receiving element  11 A, which are vertically arranged to be grouped into one, so that the oscillation does not propagate to the members of the other adjacent groups (the protective films  14 B and  14 C, the gaps SB and SC, and the receiving elements  11 B and  11 C). 
   For this reason, a material having a high oscillation blocking property is required to be used for each of the separation members  51 . Examples of the material include rubbers. 
   Embodiment 4 
     FIG. 6  is a cross-sectional side view showing a receiving section  60  in the ultrasonic sensor M according to Embodiment 4. 
   The receiving section  60  according to Embodiment 4 differs from the receiving section  50  according to Embodiment 3 only in that a vent hole  61  for bringing the gap R and the exterior of the housing member  13  into communication with each other is formed in the bottom face of the housing member  13  below each of the receiving elements  11 . 
   The structure of the ultrasonic sensor M according to Embodiment 4 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  60 . 
   However, it is necessary to provide a clearance between the receiving section  60  and the sensor substrate  32  so that each of the vent holes  61  is not closed when the receiving section  60  is attached and fixed onto the sensor substrate  32 . Specifically, a spacer may be provided between the receiving section  60  and the sensor substrate  32 , or a groove or a vent hole may be provided in the sensor substrate  32  at a position corresponding to each of the vent holes  61 . 
   Functions and Effects of Embodiment 4 
   According to Embodiment 4, the receiving sensitivity of each of the receiving elements  11  can be prevented from being lowered in addition to the above-described functions and effects according to Embodiment 3 because the oscillation of each of the layers  22  to  26  (not shown in  FIG. 6 ; see  FIG. 2 ) on the receiving surface  11   a  of each of the receiving elements  11  is not inhibited. 
   Specifically, in the case where the vent holes  61  are not provided for the housing member  13 , the gap R surrounded by the substrate  12  and the housing member  13  becomes a sealed space. In this manner, air filling the sealed space acts as a spring so as to apply a damping force due to air to the back face side of the receiving surface  11   a  of each of the receiving elements  11 . Therefore, there is a possibility that free oscillation of each of the layers  22  to  26  on the receiving surface  11   a  is inhibited so as to lower the receiving sensitivity of each of the receiving elements  11 . 
   On the other hand, in Embodiment 4, air passes through the vent holes  61 . Therefore, a damping force due to air is not applied to the back face side of the receiving surface  11   a  of each of the receiving elements  11 . As a result, each of the layers  22  to  26  on the receiving surface  11   a  is capable of freely oscillating. 
   The number, the arrangement, and the size and shape of the vent holes  61  can be determined by experimentally finding their optimal values in a cut-and-try method so as to obtain satisfactory functions and effects described above. 
   In the case where the transmission section  31  is made to have the same structure as that of the receiving section  60  and the vent holes  61  are provided for the housing member  13  of the transmission section  31 , air passes through the vent holes  61 . Therefore, a damping force due to air is not applied onto the back face side of the transmission surface of the transmission element. As a result, since the oscillation is not inhibited so that each of the layers  22  to  26  on the transmission surface can freely oscillate, the transmission output of the transmission element can be enhanced. 
   Embodiment 5 
     FIG. 7  is a cross-sectional side view showing a receiving section  70  in the ultrasonic sensor M according to Embodiment 5. 
   The receiving section  70  according to Embodiment 5 differs from the receiving section  10  according to Embodiment 1 only in that a vent hole  71  for bringing the gap S and the exterior of the housing member  13  into communication with each other is provided through a side wall of the housing member  13 . 
   The structure of the ultrasonic sensor M according to Embodiment 5 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  70 . 
   However, it is necessary to attach and fix a side wall of the housing member  13 , which faces the side wall carrying the vent hole  71 , onto the sensor substrate  32  so that an opening of the vent hole  71  is oriented upward when the receiving section  70  is attached and fixed onto the sensor substrate  32 . 
   Functions and Effects of Embodiment 5 
   According to Embodiment 5, in addition to the above-described functions and effects of Embodiment 1, if a filler such as a liquid, a sol or a gel contains air bubbles when the gap S is filled with the filler, the air bubbles can be removed through the vent hole  71  to the exterior of the gap S. 
   Specifically, if the filler filling the gap S contains air bubbles, the air bubbles make it difficult to propagate the oscillation of the protective film  14  to the receiving elements  11 . 
   On the other hand, according to Embodiment 5, the air bubbles are removed through the vent hole  71 . Therefore, the gap S can be completely filled with the filler so as to prevent the receiving sensitivity of each of the receiving elements  11  from being lowered by the air bubbles contained in the filler filling the gap S. 
   In the case where the transmission section  31  is made to have the same structure as that of the receiving section  70  and the vent hole  71  is provided for the housing member  13  of the transmission section  31 , the air bubbles contained in the filler of the gap S are removed through the vent hole  71 . As a result, the gap S can be completely filled with the filler. In this manner, the oscillation of the transmission element is allowed to surely propagate through the filler to the protective film  13  so as to prevent the transmission output of the transmission element from being lowered. 
   Embodiment 6 
     FIG. 8  is a cross-sectional side view showing a receiving section  80  in the ultrasonic sensor M according to Embodiment 6. 
   The receiving section  80  according to Embodiment 6 differs from the receiving section  10  in Embodiment 1 only in that column-like transfer members  81  for independently connecting the receiving surfaces  11   a  of the respective receiving elements  11  and the protective film  14  with each other for each of the receiving elements  11  are provided in the gap S. 
   The structure of the ultrasonic sensor M according to Embodiment 6 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  80 . 
   Functions and Effects of Embodiment 6 
   According to Embodiment 6, the following functions and effects can be obtained in addition to the above-described functions and effects [1-1], [1-3], and [1-4] according to Embodiment 1. 
   [6-1] 
   When the protective film  14  is oscillated by an ultrasonic wave, the oscillation of the protective film  14  propagates through each of the transfer members  81  to each of the receiving elements  11 . 
   In this case, since the transfer member  81  is provided for each of the receiving elements  11 , the oscillation of an arbitrary one of the transfer members  81  does not propagate to the other transfer members  81 . Therefore, an ultrasonic wave can be received by each of the receiving elements  11  in a separate manner. As a result, a crosstalk characteristic of each of the receiving elements  11  can be prevented from being degraded. 
   Moreover, an acoustic impedance of each of the transfer member  81  is brought close to that of the protective film  14  so as to surely propagate the oscillation of the protective film  14  to each of the transfer members  81 . As a result, the receiving sensitivity of each of the receiving elements  11  can be enhanced. 
   Furthermore, by bringing the acoustic impedance of each of the transfer members  81  close to that of the upper electrode layer  26  (not shown in  FIG. 8 ; see  FIG. 2 ), it becomes possible to surely propagate the oscillation of each of the transfer members  81  to the upper electrode layer  26  so as to increase the receiving sensitivity of each of the receiving elements  11 . 
   Therefore, it is desirable to form the transfer member  81  of the same material as that of the protective film  14  or the upper electrode layer  26 . 
   In the case where the transmission section  31  is made to have the same structure as that of the receiving section  80  and the transfer member  81  for connecting the transmission surface of the transmission element and the protective film  14  with each other is provided, it becomes possible to surely propagate the oscillation of the transfer member  81  to the protective film  14  by bringing the acoustic impedance of the transfer member  81  close to that of the protective film  14 . As a result, the transmission output of the transmission element can be increased. 
   Moreover, by bringing the acoustic impedance of the transfer member  81  close to that of the upper electrode layer  26  of the transmission element, it becomes possible to surely propagate the oscillation of the upper electrode layer  26  of the transmission element to the transfer member  81 . As a result, the transmission output of the transmission element can be increased. 
   [6-2] 
   In order to prevent the crosstalk characteristic of each of the receiving elements  11  from being degraded, it is necessary to prevent the oscillation of an arbitrary one of the transfer members  81  from propagating to the other transfer members  81  through the filler in the gap S. 
   Therefore, it is the most desirable in Embodiment 6 to put the gap S in a vacuum state. 
   In the case where the gap S is filled with the filler in Embodiment 6, a gas having a small acoustic impedance or a highly vibration absorbent material (for example, a gel having a high viscosity or the like) is used as the filler. 
   Embodiment 7 
     FIGS. 9A and 9B  are cross-sectional side views, each showing a receiving section  90  in the ultrasonic sensor M according to Embodiment 7. 
   The receiving section  90  according to Embodiment 7 differs from the receiving section  50  in Embodiment 3 only in that acoustic horns  91 A,  91 B, and  91 C are provided on the outer side of the protective film. For ease of disclosure, the acoustic horns  91 A,  91 B, and  91 C may be referred to collectively as the acoustic horns  91 . 
   Each of the acoustic horns  91  is formed so as to have a gradually increasing sectional area from a throat  91   a  toward an opening  91   b.    
   The acoustic horn  91  is provided for each of the receiving elements  11 . 
   The throat  91   a  of each of the acoustic horns  91  is located on the protective film  14  situated above (in front of) each of the receiving elements  11 . Specifically, the throat  91   a  of each of the acoustic horns  91  is provided in front of each of the receiving elements  11 . 
   In each of the acoustic horns  91 , a horn wall member  91   c  on an outer circumference of the throat  91   a  is attached and fixed to an upper end of each of the separation members  51 . 
   Specifically, in the example shown in  FIGS. 9A and 9B , the acoustic horns  91 A to  91 C are provided for three receiving elements  11 A to  11 C, respectively. The throats  91   a  of the respective acoustic horns  91 A to  91 C are provided on the protective films  14 A to  14 C situated above (in front of) the receiving elements  11 A to  11 C, respectively. 
   The structure of the ultrasonic sensor M in Embodiment 7 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  90 . 
   Functions and Effects of Embodiment 7 
   According to Embodiment 7, the following functions and effects can be obtained in addition to the above-described functions and effects of Embodiment 3. 
   [7-1] 
   By providing the acoustic horn  91  for each of the receiving elements  11 , the directivity of a receiving direction of an ultrasonic wave can be provided for each of the receiving elements  11 . 
   Specifically, the acoustic horns  91 A,  91 B, and  91 C have acute directivity on their horn axes α, β, and γ, respectively. 
   Therefore, the acoustic horns  91 A to  91 C are formed to have the same size and shape as shown in  FIG. 9A , so that the directivity of the receiving elements  11 A,  11 B, and  11 C can be set in the same direction in the horn axes α, β, and γ of the respective acoustic horns  91 A,  91 B, and  91 C are set in the same direction. 
   Moreover, as shown in  FIG. 9B , if the horn axes α, β, and γ of the respective acoustic horns  91 A,  91 B, and  91 C are set in arbitrary different directions by changing the size and shape of each of the acoustic horns  91 A,  91 B, and  91 C, the directivity of each of the receiving elements  11 A,  11 B, and  11 C can be set in an arbitrary direction. 
   If the transmission section  31  is made to have the same structure as that of the receiving section  90  and the acoustic horns  91  are provided on the outer side of the protective film  14 , the directivity of a transmission direction of an ultrasonic wave can be imparted to the transmission element. 
   [7-2] 
   In each of the acoustic horns  91 , the horn wall member  91   c  on the outer circumference of the throat  91   a  is attached and fixed to the upper end of each of the separation members  51 . 
   Therefore, the oscillation of each of the protective films  14 A to  14 C is not inhibited even if the acoustic horns  91  are provided, the receiving sensitivity of each of the receiving elements  11  can be prevented from being degraded. 
   The horn wall member  91   c  of the acoustic horn  91  is required to be formed of a material having enough strength to be hardly oscillated by an ultrasonic wave. Examples of the material include various metals, various synthetic resins, and the like. 
   If the transmission section  31  is made to have the same structure as that of the receiving section  90  and the acoustic horns  91  are provided on the outer side of the protective film  14 , the oscillation of the protective film  14  is not inhibited even if the acoustic horns  91  are provided because the horn wall member  91   c  on the outer circumference of the throat  91   a  is attached and fixed to the upper end of each of the separation members  51 . Accordingly, the transmission output of the transmission element can be prevented from being lowered. 
   Embodiment 8 
     FIG. 10  is a cross-sectional side view showing a receiving section  100  in the ultrasonic sensor M according to Embodiment 8. 
   The receiving section  100  according to Embodiment 8 differs from the receiving section  80  according to Embodiment 6 only in that the acoustic horns  91  are provided on the outer side of the protective film  14  as in the receiving section  90  according to Embodiment 7. 
   In Embodiment 8, however, the horn wall member  91   c  on the outer circumference of the throat  91   a  in each of the acoustic horns  91  is attached and fixed to the protective film  14 . 
   The structure of the ultrasonic sensor M according to Embodiment 8 is obtained by replacing the receiving section  10  of the ultrasonic sensor M shown in  FIG. 3  according to Embodiment 1 with the receiving section  100 . 
   Therefore, according to Embodiment 8, the functions and effects described in [7-1] above in Embodiment 7 can be obtained in addition to the above-described functions and effects of Embodiment 6. 
   Exemplary Variations of Embodiments 1 to 8 
   Embodiments 1 to 8 may be changed as follows. Even in such a case, the functions and effects equivalent to or superior to those of each of the embodiments described above can be obtained. 
   [1] 
   Each of the receiving sections  10  to  100  in Embodiments 1 to 8 includes the plurality of piezoelectric receiving elements  11 . 
   However, the piezoelectric receiving elements  11  may be replaced by capacitive receiving elements  111  so that the plurality of capacitive receiving elements  111  constitute each of the receiving sections  10  to  100 . 
     FIG. 11  is an enlarged cross-sectional side view showing one capacitive receiving element  111 . 
   An insulating layer  112  is formed on the surface of the substrate  12 . A fixed electrode layer  113  is formed on a surface of the insulating layer  112 . A movable electrode layer  114  is formed on a surface of the fixed electrode layer  113  through a clearance P. 
   An insulating layer  115  is formed around the electrode layers  113  and  114 . Surfaces of the insulating layer  115  and the movable electrode layer  114  (a device surface) are evened. 
   The bonding wires  28  and  29  are connected to the electrode layers  113  and  114 , respectively. 
   In this manner, a capacitive element F is formed to have a structure in which the two electrodes  113  and  114  are provided so as to be opposed to each other through the clearance P. The receiving element  111  includes the capacitive element F fabricated by employing the MEMS technique. 
   The surface of the movable electrode layer  114  forms the receiving surface  111   a  of the receiving element  111 . 
   When the movable electrode layer  114  is oscillated by an ultrasonic wave, a distance between the electrode layers  113  and  114  changes so as to change a capacitance. Therefore, a conversion circuit (not shown) connected to the bonding wires  28  and  29  is used so as to convert a change in capacitance between the electrode layers  113  and  114  into an electric signal. 
   As described above, even in each of the receiving elements  10  to  100  is formed to include the plurality of capacitive receiving elements  111 , the movable electrode layer  114  can be prevented from being damaged so as to hardly break the receiving sections  10  to  100  even if the thin movable electrode layer  114  has a low mechanical strength as in the case where each of the receiving sections  10  to  100  is formed with the piezoelectric receiving elements  11 . As a result, the robust receiving sections  10  to  100  can be obtained. 
   [2] 
   The transmission section  31  in Embodiments 1 to 8 is formed with the piezoelectric transmission element having the same structure as that of the piezoelectric receiving element  11 . 
   However, the transmission section  31  may also be formed with a capacitive transmission element having the same structure as that of the capacitive receiving element  111  shown in  FIG. 10 . In such a case, electrostatic attraction is generated between the electrode layers  113  and  114  in accordance with input signals applied to the electrode layers  113  and  114 . The electrostatic attraction oscillates the movable electrode layer  114  to generate an ultrasonic wave. 
   In this case, the receiving face  111   a  of the receiving element  111  acts as the transmission face of the transmission element for transmitting an ultrasonic wave. 
   [3] 
   In Embodiments 1 to 8, the gap R surrounded by the substrate  12  and the housing member  13  is filled with air. 
   However, if the gap R is filled with a material (for example, a liquid, a sol, a gel or the like) for suppressing excessive oscillation of the layers  22  to  26 , each of the layers  22  to  26  can be prevented from being excessively oscillated to be damaged. 
   [4] 
   The ultrasonic sensor M according to Embodiments 1 to 8 is composed of a hybrid IC in which any one of the receiving sections  10  to  100  and the transmission section  31  corresponding to chip parts are attached and fixed onto the sensor substrate  32  made of an insulating plate material. 
   Alternatively, the ultrasonic sensor M may also be composed of a monolithic IC in which any one of the receiving sections  10  to  100  and the transmission section  31  are formed on the single substrate  12 . In this manner, the ultrasonic sensor M can be further reduced in size as well as in weight. 
   In this case, any one of or a plurality of the receiving elements  11  arranged on the substrate  12  may be made to act as a transmission element(s) of the transmission section  31 . 
     FIG. 12  is a schematic perspective view showing an ultrasonic sensor T. 
   The ultrasonic sensor T includes: a monolithic IC in which receiving section  10 ,  40 ,  50 ,  80 ,  90  or  100  and a transmission section U are formed on the single substrate  12  (not shown in  FIG. 12 ; see  FIGS. 13 to 17 ); the bonding wires  28  and  29 ; the sensor substrate  32 ; and electrode pads  33 . 
   With this structure, the ultrasonic sensor T is further reduced in size as well as in weight as compared with the ultrasonic sensor M. 
   The transmission section U is composed of a single transmission element W. The transmission element W has the same structure as that of each of the receiving elements  11  constituting the receiving section  10 ,  40 ,  50 ,  80 ,  90 , or  100 . 
   A tip of each of the bonding wires  28  and  29  led from the transmission element W is connected to each of the electrode pads  33  as in the case of the receiving element  11 . 
   Moreover, the transmission element W having the same structure as that of the receiving element  11  transmits an ultrasonic wave from a transmission surface Wa (not shown) corresponding to the receiving surface  11   a  of the receiving element  11  (not shown in  FIG. 12 ; see  FIGS. 13 to 17 ). 
   In the example shown in  FIG. 12 , among nine elements having the same structure arranged 3 by 3 on the single substrate  12  (not shown), one element arranged at the corner is made to act as the transmission element W, whereas the other eight elements are made to act as the receiving elements  11 . 
   However, a plurality of arbitrary elements may be made to act as the transmission elements W among a plurality of elements having the same structure arranged on the substrate  12 . 
     FIG. 13  is a cross-sectional side view showing an example in which Embodiment 2 is applied to the ultrasonic sensor T, illustrating the receiving section  40  and the transmission section U of the ultrasonic sensor T. 
     FIG. 14  is a cross-sectional side view showing an example in which Embodiment 3 is applied to the ultrasonic sensor T, illustrating the receiving section  50  and the transmission section U of the ultrasonic sensor T. 
     FIG. 15  is a cross-sectional side view showing an example in which Embodiment 6 is applied to the ultrasonic sensor T, illustrating the receiving section  80  and the transmission section U of the ultrasonic sensor T. 
     FIGS. 16A  and B are cross-sectional side views showing an example in which Embodiment 7 is applied to the ultrasonic sensor T, illustrating the receiving section  90  and the transmission section U of the ultrasonic sensor T. 
     FIG. 17  is a cross-sectional side view showing an example in which Embodiment 8 is applied to the ultrasonic sensor T, illustrating the receiving section  100  and the transmission section U of the ultrasonic sensor T. 
   Embodiment 9 
     FIG. 18  is a cross-sectional side view showing a receiving section  200  in an ultrasonic sensor N according to Embodiment 9. 
   The receiving section  200  includes a plurality of piezoelectric receiving elements  201  arranged in an array. The example shown in  FIG. 18  illustrates a cross-sectional side view of three receiving elements  201 . 
   The receiving elements  201  are formed on a single-crystalline silicon substrate (a single-crystalline silicon chip)  202  having an SOI structure. 
   The substrate  202  is provided on the sensor substrate  32 . The substrate  202  is surrounded by a rectangular frame member  203 . An outer circumference of the substrate  202  is attached and fixed to an inner wall of the frame member  203  by an appropriate method (for example, thermal welding, ultrasonic welding, bonding with an adhesive and the like) so as to air-seal a connection part between the outer circumference of the substrate  202  and the frame member  203 . 
   A lower end of the frame member  203  is attached and fixed to the sensor substrate  32  by the above-mentioned appropriate method so as to air-seal a connection part between the lower end of the frame member  203  and the sensor substrate  32 . 
   The frame member  203  and the sensor substrate  32  form a housing member  204  having a rectangular parallelepiped box shape with an upper open end. 
   Specifically, the substrate  202  is housed within the housing member  204  having a rectangular parallelepiped box shape with an upper open end. 
   Each of the receiving elements  201  is arranged so that a receiving surface  201   a  for receiving an ultrasonic wave is oriented toward an opening  204   a  of the housing member  204 . Moreover, the receiving surfaces  201   a  of the respective receiving elements  201  are arranged so as to be flush with each other. 
   The protective film  14  for closing the opening  204   a  is attached over the opening  204   a  of the housing member  204 . Specifically, the protective film  14  is provided in front of the receiving elements  201 . 
   An outer circumference of the protective film  14  is attached and fixed to an inner circumference of the frame member  203  (an inner circumference of the opening  204   a  of the housing member  204 ) so as to air-seal a connection part between the outer circumference of the protective film  14  and the frame member  203 . 
   The gap S, which is provided between the protective film  14  and the substrate  202 , is filled with a gas, a liquid, a sol, a gel or the like. 
   The gap R surrounded by the back face side (the bottom face side) of the substrate  202  and the housing member  204  (the frame member  203  and the sensor substrate  32 ) is filled with air. 
     FIG. 19  is an enlarged cross-sectional side view showing one piezoelectric receiving element  201 . 
   A through hole  202   a  penetrating through the substrate  202  is formed in the substrate  202 . 
   On the back face side of the substrate  202 , the insulating layer  21 , the silicon active layer  22 , and the insulating layer  23  are formed on the surface of the substrate  202  in this order. The layers  22  and  23  are formed to close a lower end of the through hole  202   a.    
   On the back face side, the lower electrode layer  24 , the thin film layer  25  made of a ferroelectric (for example, PZT or the like), and the upper electrode layer  26  are formed in this order on the surface of the insulating layer  23  situated below (behind) the through hole  202   a.    
   The sensor substrate  32  is a printed wiring board. Wiring layers  205  and  206  are formed on a surface of the sensor substrate  32 . 
   The lower electrode layer  24  and the wiring layer  205  are connected to each other through a bump  207 , whereas the upper electrode layer  26  and the wiring layer  206  are connected to each other through a bump  208 . 
   The bumps  207  and  208  may be formed by an appropriate method (plating, a stud method or the like) using various conductive materials (metals such as a solder, gold, copper and nickel, a conductive adhesive or the like). 
   In this manner, a piezoelectric element (the piezoelectric sensor) E is formed to have a structure in which the thin film layer  25  made of a ferroelectric is sandwiched between the two thin electrode layers  24  and  26 . The piezoelectric element E fabricated by employing the MEMS technique constitutes the receiving element  201 . 
   A surface of the silicon active layer  22  exposed through the through hole  202   a  forms the receiving surface  201   a  of the receiving element  201 . 
   When the thin film layer  25  is oscillated by an ultrasonic wave, an electric signal is generated by a piezoelectric effect. The thus generated electric signal is output from the electrode layers  24  and  26  through the bumps  207  and  208  and the wiring layers  205  and  206 , respectively. 
   The through hole  202   a  is provided so that a diaphragm composed of the layers  22  and  26  is more likely to be oscillated. 
     FIG. 20  is a schematic perspective view showing the ultrasonic sensor N. 
   The ultrasonic sensor N is composed of a hybrid IC including the receiving section  200 , a transmission section  209 , and the sensor substrate  32 . 
   The receiving section  200  and the transmission section  209  corresponding to chip parts are attached and fixed to the surface of the sensor substrate  32 . 
   In the example shown in  FIG. 20 , the receiving section  200  includes nine receiving elements  201  arranged 3 by 3. 
   The transmission section  209  has the same structure as that of any one of the receiving sections  10  to  100  and  200 . The transmission section  209  includes one piezoelectric transmission element having the same structure as that of the receiving element  11  or  201 . The thin film layer  25  is oscillated by a piezoelectric effect in accordance with an input signal applied to the thin film  25  made of a ferroelectric from the electrode layers  24  and  26 , thereby generating an ultrasonic wave. 
   In the case where the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , the receiving surface  201   a  of the receiving element  201  serves as a transmission surface for transmitting an ultrasonic wave. 
   Specifically, the transmission element of the transmission section  209  converts an electric signal into an ultrasonic wave so as to transmit it. 
   Then, the transmission section  209  transmits an ultrasonic wave in accordance with an input signal from the exterior. A reflection sound generated by the ultrasonic wave reflected by an object to be detected is received by each of the receiving elements  201  of the receiving section  200 . 
   Specifically, each of the receiving elements  201  of the receiving section  200  converts the received ultrasonic wave into an electric signal. 
   Then, the ultrasonic wave transmitted from the transmission section  209  and the ultrasonic wave received by each of the receiving elements  201  of the receiving section  200  are compared with each other so as to obtain a sound pressure difference, a time difference, and a phase difference between them. As a result, the position of an object to be detected, a distance between the ultrasonic sensor N and the object to be detected, a two-dimensional shape or a three-dimensional shape of the object to be detected and the like can be measured based on the thus obtained differences. 
   Functions and Effects of Embodiment 9 
   According to Embodiment 9, the following functions and effects can be obtained in addition to the same functions and effects as [1-1] to [1-4] described above in Embodiment 1. 
   [9-1] 
   In each of the receiving elements  11  shown in  FIGS. 1 to 3  in Embodiment 1, the surface of the upper electrode layer  26  serves as the receiving surface  11   a.    
   On the other hand, in each of the receiving elements  201  in Embodiment 9, the surface of the silicon active layer  22  exposed through the bottom face of the through hole  202   a  serves as the receiving surface  202   a.    
   Specifically, the receiving element  201  according to Embodiment  201  corresponds to a reversed version of the receiving element  11  according to Embodiment 1 for use. 
   Moreover, in Embodiment 1, the packaged receiving section  10  including the substrate  12  housed within the housing member  13  is attached and fixed onto the sensor substrate  32 . Then, the electrode layers  24  and  26  of each of the receiving elements  11  constituting the receiving section  10  and each of the electrode pads  33  are connected to each other through the bonding wires  28  and  29 , respectively. 
   Specifically, since the receiving section  10  (each of the receiving elements  11 ) is surface-mounted on the sensor substrate  32  by using a wire bonding technique in Embodiment 1, the ultrasonic sensor according to Embodiment 1 has the following problems [A] to [E]. 
   [A] There is a possibility that each of the bonding wires  28  and  29  is cut by the oscillation. In the case where the ultrasonic sensor M is to be mounted on an automobile, in particular, each of the bonding wires  28  and  29  is more likely to be cut because the oscillation of an engine or the oscillation propagating from a road surface is applied to the ultrasonic sensor M. 
   [B] The fabrication cost for surface-mounting the receiving section  10  on the sensor substrate  32  is high. 
   If the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  11 , the fabrication cost for surface-mounting the transmission section  209  on the sensor substrate  32  is high. 
   [C] Since the bonding wires  28  and  29  are provided above the receiving surface  11   a  of each of the receiving elements  11 , there is a possibility that the bonding wires  28  and  29  become obstacles to inhibit an ultrasonic wave from getting to the receiving surface  11   a , resulting in a lowered receiving sensitivity of each of the receiving elements  11 . 
   Since the bonding wires  28  and  29  are provided above the receiving surface  11   a  of each of the receiving elements  11 , the bonding wires  28  and  29  are likely to be cut by an ultrasonic wave received by each of the receiving elements  11 . 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  11 , the bonding wires  28  and  29  become obstacles to inhibit an ultrasonic wave from being transmitted from the transmission surface of the transmission element. As a result, there is a possibility that the transmission output is lowered. 
   Moreover, since the bonding wires  28  and  29  are provided above the transmission surface of the transmission element, the bonding wires  28  and  29  are likely to be cut by an ultrasonic wave transmitted from the transmission element. 
   [D] Since an inductance of a signal wiring in the receiving section  10  is increased by a length of each of the bonding wires  28  and  29 , a transfer rate of an electric signal generated from the receiving section  10  is lowered. 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  11 , an inductance of a signal wiring in the transmission section  209  is increased by a length of each of the bonding wires  28  and  29 . Therefore, a transfer rate of an input signal to the transmission section  209  is lowered to lower an operation speed of the transmission section  209 . 
   [E] Since the sensor substrate  32  is increased in size by an area occupied by the electrode pads  33  (a layout area) formed on the sensor substrate  32 , the ultrasonic sensor M is disadvantageously increased in size. 
   On the other hand, in each of the receiving elements  201  according to Embodiment 9, the unpackaged substrate  202  corresponding to a bare chip (die) is directly mounted facedown on the sensor substrate  32 . The electrode layers  24  and  26  of each of the receiving elements  201  formed on the substrate  202  and the wiring layers  205  and  206  on the sensor substrate  32  are connected through the bumps  207  and  208 , respectively. 
   Specifically, since the receiving elements  201  are surface-mounted on the sensor substrate  32  by flip-chip connection in Embodiment 9, the above-described problems [A] to [E] can be solved to obtain the following effects [F] to [J]. 
   [F] Since the receiving section  200  (the receiving elements  201 ) and the sensor substrate  32  are connected and fixed to each other through the bumps  207  and  208 , it can be ensured that the electrical connection between each of the receiving elements  201  and the substrate  32  is kept. As a result, the reliability of the ultrasonic sensor N can be enhanced with an extended lifetime. 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , it can be ensured that the electrical connection between the transmission element and the sensor substrate  32  is kept. 
   [G] The fabrication cost for surface-mounting the receiving section  200  on the sensor substrate  32  can be lowered. 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , the fabrication cost for surface-mounting the transmission section  209  on the sensor substrate  32  can be lowered. 
   [H] Since the bonding wire is not provided above the receiving surface  201   a  of each of the receiving elements  201  and therefore no obstacle is present in front of the receiving surface  201   a , an ultrasonic wave is not inhibited from getting to the receiving surface  201   a . Therefore, the receiving sensitivity of each of the receiving surface  11  can be increased. 
   Moreover, since the bonding wire is no longer provided above the receiving surface  11   a  of each of the receiving elements  11 , the bonding wire is never cut by the ultrasonic wave received by each of the receiving elements  11 . 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , the transmission of an ultrasonic wave from the transmission surface is not inhibited because the bonding wire is not provided above the transmission surface of the transmission element and therefore no obstacle is present in front of the transmission surface. As a result, the transmission output of the transmission element can be enhanced. 
   Moreover, since the bonding wire is not provided above the transmission surface of the transmission element, the bonding wire is never cut by the ultrasonic wave transmitted from the transmission element. 
   [I] Since an inductance of each of the bumps  207  and  208  is smaller than that of each of the bonding wires  28  and  29 , an inductance of the signal wiring of the receiving section  200  is reduced to allow a transfer rate of the electric signal generated from the receiving section  200  to be increased. 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , the inductance of the signal wiring of the transmission section  209  becomes smaller. Accordingly, the transfer rate of an input signal to the transmission section  209  becomes higher to increase the operation speed of the transmission section  209 . 
   [J] It is no longer necessary to provide the electrode pads  33  on the sensor substrate  32 . As a result, since the sensor substrate  32  can be reduced in size by the area which was otherwise occupied by the electrode pads  33 , the ultrasonic sensor N can be reduced in size as well as in weight. 
   [9-2] 
   If the through hole  202   a  is formed in the substrate  202  so that its sectional area gradually increases from the bottom of the through hole  202   a  closed by the silicon active layer  22  toward the opening, the through hole  202   a  can be made to act as the same acoustic horn as the acoustic horn  91  in Embodiment 7. In this case, the bottom of the through hole  202   a  corresponds to the throat  91   a  of the acoustic horn  91 . 
   In this manner, the acoustic horn formed by the through hole  202   a  can be provided for each of the receiving elements  201 . As a result, each of the receiving elements  201  can be provided with the directivity of a receiving direction of an ultrasonic wave as in the above-described [7-1] in Embodiment 7. 
   Moreover, if the transmission element of the transmission section  209  is made to have the same structure as that of the receiving element  201 , the transmission element can be provided with the directivity of a transmission direction of an ultrasonic wave. 
   Moreover, according to Embodiment 9, the through hole  202   a  can be made to act as an acoustic horn simply by appropriately shaping the through hole  202   a . Since it is no longer necessary to provide the acoustic horn  91  as an independent member as in Embodiment 7, the fabrication cost of the receiving section  200  and the transmission section  209  can be lowered as compared with the receiving section  90  and the transmission section  31  in Embodiment 7. At the same time, the receiving section  200  and the transmission section  209  can be reduced in size as well as in weight. 
   Embodiment 10 
     FIG. 21  is a cross-sectional side view showing a receiving section  220  in the ultrasonic sensor N according to Embodiment 10. 
   The receiving section  220  according to Embodiment 10 differs from the receiving section  200  according to Embodiment 9 only in that at least one (three in the illustrated example) vent hole  221  for bringing the gap R and the exterior of the housing member  204  into communication with each other is formed at a position of the sensor substrate  32  below each of the receiving elements  201 . 
   The structure of the ultrasonic sensor N according to Embodiment 10 is obtained by replacing the receiving section  200  in the ultrasonic sensor N shown in  FIG. 20  according to Embodiment 20 with the receiving section  220 . 
   Functions and Effects of Embodiment 10 
   According to Embodiment 10, in addition to the above-described functions and effects of Embodiment 9, the receiving sensitivity of each of the receiving elements  201  can be prevented from being lowered because the oscillation of the layers  22  to  26  on the receiving surface  201   a  of each of the receiving elements  201  is not inhibited. 
   Specifically, in the case where the vent holes  221  are not provided in the sensor substrate  32 , the gap R surrounded by the substrate  202  and the housing member  204  (the frame member  203  and the sensor substrate  32 ) forms a sealed space. Air filling the sealed space acts as a spring so as to apply a damping force due to air on the back face side of the receiving surface  201   a  of each of the receiving elements  201 . As a result, there is a possibility that the free oscillation of the layers  22  to  26  on the receiving surface  201   a  is inhibited to lower the receiving sensitivity of each of the receiving elements  201 . 
   On the other hand, in Embodiment 10, since air passes through the vent holes  221 , a damping force due to air is not applied to the back face side of the receiving surface  201   a  of each of the receiving elements  201 . As a result, each of the layers  22  to  26  on the receiving surface  201   a  is capable of freely oscillating. 
   The number, the position of arrangement, and the size and shape of the vent hole  221  can be determined by experimentally finding their optimal values by a cut-and-try method so as to obtain satisfactory functions and effects described above. 
   Moreover, a filter material (for example, a mesh filter or the like), which does not suppress the air permeability of the vent hole  221 , may be attached and fixed. 
   If the transmission section  209  is made to have the same structure as that of the receiving section  220  and the vent hole  221  is provided at a position of the sensor substrate  32  below each of the transmission elements in the transmission section  209 , a damping force due to air is not applied to the back face side of the transmission surface of the transmission element because air passes through the vent holes  221 . Accordingly, the layers  22  to  26  on the transmission surface can freely oscillate so as not to suppress the oscillation, thereby increasing the transmission output of the transmission element. 
   Embodiment 11 
     FIG. 22  is a cross-sectional side view showing a receiving section  230  and a transmission section  231  in an ultrasonic sensor L according to Embodiment 11. 
     FIG. 23  is a schematic perspective view showing the ultrasonic sensor L. 
   The ultrasonic sensor L according to Embodiment 11 differs from the ultrasonic sensor N according to Embodiment 9 only in the following points. 
   [a] 
   The ultrasonic sensor L is composed of the monolithic IC including the receiving section  200  and the transmission section  231  formed on the single substrate  202  so as to be further reduced in size and weight as compared with the ultrasonic sensor N. 
   The transmission section  231  is composed of one transmission element  232 . The transmission element  232  has the same structure as that of each of the receiving elements  201  constituting the receiving section  200 . 
   The transmission element  232  having the same structure as that of the receiving element  201  transmits an ultrasonic wave from a transmission surface  232   a  corresponding to the receiving surface  201   a  of the receiving element  201 . 
   In the example shown in  FIG. 23 , among nine elements having the same structure arranged 3 by 3, one element arranged at the corner is made to act as the transmission element  232 , whereas the other eight elements are made to act as the receiving elements  201 . 
   However, among a plurality of elements having the same structure arranged on the substrate  12 , an arbitrary number of elements can be made to act as the transmission elements  232 . 
   [b] 
   Partition members  233  are provided in the gap R surrounded by the substrate  202  and the housing member  204  (the frame member  203  and the sensor substrate  32 ). 
   A lower end of each of the partition members  233  is attached and fixed to an upper surface of the sensor substrate  32  by an appropriate method (for example, thermal welding, ultrasonic welding, bonding with an adhesive or the like) so as to air-seal a connection part between the lower end of each of the partition members  233  and the sensor substrate  32 . An upper end of each of the partition members  233  is attached and fixed to a lower face of the insulating layer  23  on the substrate  202  by the above-mentioned appropriate method so as to air-seal a connection part between the upper end of each of the partition members  233  and the substrate  202 . 
   The partition members  233  partition the gap R for each of the elements  201  and  232  in an air-tight manner. 
   [c] 
   At least one (three in the illustrated example) vent hole  221  for bringing the gap R and the exterior of the housing member  204  into communication with each other is formed at a position of the sensor substrate  32  below each of the transmission elements  232 . 
   The vent hole  221  is not formed at a position of the sensor substrate  32  below each of the receiving elements  201 . 
   Functions and Effects of Embodiment 11 
   According to Embodiment 11, the following functions and effects can be obtained in addition to the above-described functions and effects of Embodiment 9. 
   [11-1] 
   When the gap R surrounded by the substrate  202  and the housing member  204  (the frame member  203  and the sensor substrate  32 ) is formed as a sealed space, air filling the sealed space acts as a spring so as to apply a damping force due to air on the back face side of each of the faces (the receiving surface and the transmission surface)  201   a  and  232   a  of the respective elements  201  and  232 . Since the free oscillation of the layers  22  to  26  on each of the faces  201   a  and  232   a  is inhibited, a resonance value Q of the diaphragm composed of the layers  22  to  26  is reduced. 
   On the other hand, in the case where the vent holes  221  are provided in the sensor substrate  32 , air passes through the vent holes  221 . Therefore, a damping force due to air is not applied on the back face side of each of the faces  201   a  and  232   a  of the respective elements  201  and  232 , the free oscillation of the layers  22  to  26  on each of the faces  201   a  and  232   a  is not inhibited. Accordingly, the resonance value Q of the diaphragm composed of the layers  22  to  26  becomes large. 
     FIGS. 24A and 24B  are characteristic views, each showing a resonance characteristic corresponding to the relation between the resonance value Q of the diaphragm and a frequency f. 
   As shown in  FIG. 24A , if a peak value Qa of the resonance value Q of the diaphragm is large, the resonance value Q demonstrates a steep change with respect to a change in frequency f corresponding to the peak value Qa mainly at primary resonance frequencies fa and fb. 
   As shown in  FIG. 24B , if a peak value Qb of the resonance value Q of the diaphragm is small, the resonance value Q demonstrates a gentle change with respect to a change in frequency f corresponding to the peak value Qb mainly at the primary resonance frequencies fa and fb. 
   The resonance value Q of the diaphragm and the transmission output of the transmission element  232  are positively correlated with each other; as the resonance value Q becomes larger, the transmission output becomes greater. 
   The piezoelectric element or the capacitive element fabricated by employing the MEMS technique is not suitable as a transmission element because it has a small transmission output of an ultrasonic wave. 
   Thus, the piezoelectric transmission element  232  fabricated by employing the MEMS technique is required to increase its transmission output as much as possible so as to have the resonance characteristic shown in  FIG. 24A . 
   Therefore, according to Embodiment 11, since air passes through the vent holes  221  formed in the sensor substrate  32 , a damping force due to air is not applied to the back face side of the transmission surface  232   a  of each of the transmission elements  232 . As a result, since the layers  24  to  26  on the transmission surface  232   a  can freely oscillate so as not to be inhibited from oscillating, each of the transmission elements  232  can be provided with the resonance characteristic shown in  FIG. 24A  so as to increase the transmission output. 
   [11-2] 
   The resonance value Q of the diaphragm and the receiving sensitivity of the receiving element  201  are positively correlated with each other; as the resonance value Q becomes larger, the receiving sensitivity becomes greater. 
   Herein, each of the receiving elements  201  has a fluctuation in primary resonance frequency due to a fabrication process. 
   For example, if two receiving elements  201  have the resonance characteristic shown in  FIG. 24A  so that one of the receiving elements  201  has the primary resonance frequency fa whereas the other receiving element  201  has the primary resonance frequency fb, the receiving sensitivity at the frequencies fa and fb becomes extremely high. However, the receiving sensitivity at a frequency fc between the frequencies fa and fb becomes extremely low. 
   On the other hand, if two receiving elements  201  have the resonance characteristic shown in  FIG. 24B  so that one of the receiving elements  201  has the primary resonance frequency fa whereas the other receiving element  201  has the primary resonance frequency fb, the receiving sensitivity at the frequencies fa and fb is lower than that in  FIG. 24A . However, the receiving sensitivity at the frequency fc becomes higher than that in  FIG. 24A . 
   Specifically, if the resonance value Q of the receiving element  201  is increased, the receiving sensitivity demonstrates a steep characteristic with respect to a change in frequency although the receiving sensitivity becomes high. Therefore, the receiving sensitivity at a frequency offset from the primary resonance frequency is suddenly lowered even if the offset is slight. 
   On the contrary, if the resonance value Q of the receiving element  201  is reduced, the receiving sensitivity demonstrates a gentle characteristic with respect to a change in frequency although the receiving sensitivity is lowered. Therefore, the receiving sensitivity at a frequency offset from the primary resonance frequency is not greatly lowered. 
   Since the piezoelectric element or the capacitive element fabricated by employing the MEMS technique has a high receiving sensitivity of an ultrasonic wave, it is suitable as the receiving element. 
   Therefore, the piezoelectric receiving element  201  fabricated by employing the MEMS technique is required to have a high receiving sensitivity over a broad frequency range as much as possible rather than to have a high receiving sensitivity at the primary resonance frequency. Therefore, the piezoelectric receiving element  201  is required to have the resonance characteristic shown in  FIG. 24B . 
   Thus, according to Embodiment 11, since the vent hole  221  is not formed at a position of the sensor substrate  32  below each of the receiving elements  201 , a damping force due to air is applied to the back face side of the receiving surface  201   a  of each of the receiving elements  201 . As a result, the oscillation of the layers  24  to  26  on the receiving surface  201   a  is inhibited. Accordingly, each of the receiving elements  201  is provided with the resonance characteristic shown in  FIG. 24B  to increase receiving sensitivity over a broad frequency range as much as possible. 
   [11-3] 
   If the gap R situated below each of the receiving elements  201  is filled with a material for suppressing the oscillation of the layers  22  to  26  (for example, a liquid, a sol, a gel or the like), the resonance value Q of the diaphragm composed of the layers  22  to  26  can be reduced as compared with the case where the gap R is filled with air. 
   Therefore, if the material for filling the gap R situated below each of the receiving elements  201  is appropriately selected, a desired resonance characteristic can be obtained without altering the structure of each of the receiving elements  201 . 
   Moreover, if the gap R is filled with a material for preventing the layers  22  to  26  from being excessively oscillated, the layers  22  and  26  can be prevented from being excessively oscillated to be broken. 
   As the filler in the gap R situated blow each of the receiving elements  201 , an optimal material can be experimentally found by a cut-and-try method so as to obtain satisfactory functions and effects described above in [11-2]. 
   Even in Embodiments 1, 3 and 5 to 8, a desired resonance characteristic can be obtained without altering the structure of each of the receiving elements  11 . 
   Embodiment 12 
     FIG. 25  is a cross-sectional side view showing a receiving section  240  in the ultrasonic sensor N according to Embodiment 12. 
   The receiving section  240  according to Embodiment 12 differs from the receiving section  200  according to Embodiment 9 only in that separation members  241  are provided. 
   A lower end of each of the separation members  241  is attached and fixed to the substrate  202  between the receiving elements  201  by an appropriate method (for example, thermal welding, ultrasonic welding, bonding with an adhesive or the like) so as to air-seal a connection part of the lower end of each of the separation members  241  and the substrate  202 . 
   The upper end of each of the separation members  241  separates the gap S and the protective film  14  for each receiving element. 
   Specifically, in the example shown in  FIG. 25 , the lower ends of the separation members  241  are attached and fixed to the substrate  202  between the receiving elements  201 A and  202 B, and  202 B and  202 C, respectively. 
   Then, the gaps SA to SC and the protective films  14 A to  14 C situated above (in front of) the receiving elements  201 A to  201 C are separated by the separation members  241  for the receiving elements  201 A to  201 C, respectively. 
   The structure of the ultrasonic sensor N according to Embodiment 12 is obtained by replacing the receiving section  200  of the ultrasonic sensor N shown in  FIG. 20  according to Embodiment 9 with the receiving section  240 . 
   Functions and Effects of Embodiment 12 
   According to Embodiment 12, the following functions and effects can be obtained in addition to the above-described functions and effects of Embodiment 9. 
   [12-1] 
   The gaps SA to SC and the protective films  14 A to  14 C situated above (in front of) the receiving elements  201 A to  201 C are separated by the separation members  241  for each of the receiving elements  201 A to  201 C, respectively. Therefore, the oscillation of the single protective film  14 A obtained by the separation propagates only to the receiving element  201 A through the gap SA situated below the protective film  14 A but not to the other receiving elements  201 B and  201 C. 
   Therefore, according to Embodiment 12, an ultrasonic wave can be received by each of the receiving elements  201 A to  201 C in a separate manner. Accordingly, a crosstalk characteristic of each of the receiving elements  201 A to  201 C can be prevented from being degraded. 
   Alternatively, a plurality of the adjacent receiving elements  201  may be grouped into one. The separation member  241  may be provided for each group so as to separate the group from the other groups. 
   [12-2] 
   The separation members  241  have to surely block the oscillation of the protective film  14 A and the gap SA vertically arranged to be grouped into one so that the oscillation does not propagate to the members of the other adjacent groups (the protective films  14 B and  14 C and the gaps SB and SC). 
   For this reason, a material having a high oscillation blocking property is required to be used for the separation members  241 . Examples of the material include rubbers. 
   [12-3] 
     FIG. 26  is a cross-sectional side view showing the receiving section  240  in the ultrasonic sensor N according to a first variation of Embodiment 12. 
   The first variation shown in  FIG. 26  differs from Embodiment 12 shown in  FIG. 25  only in that the vent holes  221  for bringing the gap R and the exterior of the housing member  204  into communication with each other are formed at a position of the sensor substrate  32  below each of the receiving elements  201 . 
   Specifically, the first variation of Embodiment 12 corresponds to the combination of Embodiments 12 and 10. Accordingly, the functions and effects of Embodiment 10 can be obtained in addition to the functions and effects of Embodiment 12. 
   [12-4] 
     FIG. 27  is a cross-sectional side view showing the receiving section  240  and the transmission section  231  in the ultrasonic sensor L according to a second variation of Embodiment 12. 
   The second variation shown in  FIG. 27  differs from Embodiment 12 shown in  FIG. 25  only in that the same points as described in [a] to [c] of Embodiment 12. 
   Specifically, the second variation of Embodiment 12 corresponds to the combination of Embodiments 12 and 11. Therefore, the above-described functions and effects of Embodiment 11 can be obtained in addition to the functions and effects of Embodiment 12. 
   Embodiment 13 
     FIG. 28  is a cross-sectional side view showing a receiving section  250  in the ultrasonic sensor N according to Embodiment 13. 
   The receiving section  250  of Embodiment 13 differs from the receiving section  200  of Embodiment 9 only in that separation members  251  are provided. 
   A lower end of each of the separation members  251  is attached and fixed to an upper surface of the sensor substrate  32  by an appropriate method (for example, thermal welding, ultrasonic welding, bonding with an adhesive and the like) so as to air-seal a connection part between the lower end of each of the separation members  251  and the sensor substrate  32 . 
   The upper end of each of the separation members  251  separates the space S and the protective film  14  for each of the receiving elements  201 . 
   Specifically, in the example illustrated in  FIG. 28 , the lower ends of the separation members  251  are attached and fixed to the upper surface of the sensor substrate  32  between the receiving elements  201 A and  201 B, and  201 B and  201 C, respectively, whereby the separation members  251  separate the receiving elements  201 A to  201 C from each other. 
   The gaps SA to SC and the protective films  14 A to  14 C respectively situated above (in front of) the receiving elements  201 A to  201 C are separated by the separation members  251  for each of the receiving elements  201 A to  201 C, respectively. 
   Specifically, the receiving section  200  according to Embodiment 9 is composed of a monolithic IC including the receiving elements  201  of the receiving section  230  formed on the single substrate  202 . 
   On the other hand, the receiving section  250  of Embodiment 13 is composed of a hybrid IC including the receiving elements  201  corresponding to chip parts attached and fixed on the sensor substrate  32 . 
   The structure of the ultrasonic sensor N according to Embodiment 13 is obtained by replacing the receiving section  200  of the ultrasonic sensor N shown in  FIG. 20  according to Embodiment 9 with the receiving section  250 . 
   Functions and Effects of Embodiment 13 
   According to Embodiment 13, the following functions and effects can be obtained in addition to the functions and effects of Embodiment 9. 
   [13-1] 
   The receiving elements  201 A to  201 C, and the gaps SA to SC and the protective films  14 A to  14 C situated above (in front of) the receiving elements  201 A to  201 C are separated by the separation members  251  for each of the receiving elements  201 . Therefore, the oscillation of one protective film  14 A obtained by the separation propagates only to the receiving element  201 A through the gap SA situated below the protective film  14 A but not to the other receiving elements  201 B and  201 C at all. 
   Thus, according to Embodiment 13, an ultrasonic wave can be received by each of the receiving elements  201 A to  201 C in a completely separate manner, so that a crosstalk characteristic of each of the receiving elements  201 A to  201 C can be prevented from being degraded. 
   A plurality of adjacent receiving elements  201  may be grouped into one. The separation member  251  may be provided for each of the groups so as to separate the group from the other groups. 
   [13-2] 
   The separation members  251  have to surely block the oscillation of the protective film  14 A, the gap SA and the receiving element  201 A, which are vertically arranged so as to be grouped into one, so that the oscillation does not propagate to the members of the other adjacent groups (the protective films  14 B and  14 C, the gaps SB and SC, and the receiving elements  201 B and  201 C). 
   For this reason, a material having a high oscillation blocking property is required to be used for the separation member  251 . Examples of the material include rubbers. 
   [13-3] 
     FIG. 29  is a cross-sectional side view showing the receiving section  250  in the ultrasonic sensor N according to a first variation of Embodiment 13. 
   The first variation shown in  FIG. 29  differs from Embodiment 13 shown in  FIG. 28  only in that the vent holes  221  for bringing the gap R and the exterior of the housing member  204  into communication with each other are formed at a position of the sensor substrate  32  below each of the receiving elements  201 . 
   Specifically, the first variation of Embodiment 13 corresponds to the combination of Embodiments 13 and 10. Therefore, in addition to the functions and effects of Embodiment 13, the functions and effects of Embodiment 10 can be obtained. 
   [13-4] 
     FIG. 30  is a cross-sectional side view showing the receiving section  250  and the transmission section  231  in the ultrasonic sensor L according to a second variation of Embodiment 13. 
   The second variation shown in  FIG. 30  differs from Embodiment 13 shown in  FIG. 28  only in that one of the receiving elements  201  (the receiving element  201 A) constituting the receiving section  250  is made to act as the transmission element  232  constituting the transmission section  231  as in the ultrasonic sensor L and the same points as the above-described [b] and [c] in Embodiment 11. The separation members  251  function as the partition members  233  of Embodiment 11. 
   Specifically, the second variation of Embodiment 13 corresponds to the combination of Embodiments 13 and 11. Therefore, in addition to the functions and effects of Embodiment 13, the functions and effects of Embodiment 11 can be obtained. 
   Embodiment 14 
     FIG. 31  is a cross-sectional side view showing a receiving section  260  in the ultrasonic sensor N in Embodiment 14. 
   The receiving section  260  of Embodiment 14 differs from the receiving section  200  of Embodiment 9 only in that a column-like transfer member  261  for connecting the receiving surface  201   a  of each of the receiving elements  201  and the protective film  14  with each other independently for each of the receiving elements  201  is provided in the gap S. 
   The structure of the ultrasonic sensor N according to Embodiment 14 is obtained by replacing the receiving section  200  of the ultrasonic sensor N shown in  FIG. 20  according to Embodiment 9 with the receiving section  260 . 
   Functions and Effects of Embodiment 14 
   According to Embodiment 14, the following functions and effects can be obtained in addition to the functions and effects described above in [9-1] of Embodiment 9. 
   [14-1] 
   When the protective film  14  is oscillated by an ultrasonic wave, the oscillation of the protective film  14  propagates to each of the receiving elements  201  through each of the transfer members  261 . 
   Herein, since the transfer member  261  is provided for each of the receiving elements  201 , the oscillation of arbitrary one of the transfer members  261  does not propagate to the other transfer members  261 . Therefore, an ultrasonic wave can be received by each of the receiving elements  201  in a separate manner, thereby preventing a crosstalk characteristic of each of the receiving elements  201  from being degraded. 
   The propagation of oscillation of the protective film  14  to each of the transfer members  261  can be ensured by bringing an acoustic impedance of each of the transfer members  261  close to that of the protective film  14 . As a result, the receiving sensitivity of each of the receiving elements  201  can be enhanced. 
   Moreover, the propagation of oscillation of each of the transfer members  261  to the silicon active layer  22  can be ensured by bringing an acoustic impedance of each of the transfer members  261  close to that of the silicon active layer  22  of each of the receiving elements  201 . As a result, the receiving sensitivity of each of the receiving elements  201  can be enhanced. 
   Therefore, it is desirable to form the transfer members  261  of the same material as that of the protective film  14  or the silicon active layer  22 . 
   If the transmission section  209  is made to have the same structure as that of the receiving section  260  and the transfer member  261  for bringing the transmission surface of the transmission element and the protective film  14  into communication with each other is provided, the propagation of oscillation of the transfer member  261  to the protective film  14  can be ensured by bringing the acoustic impedance of the transfer member  261  close to that of the protective film  14 . As a result, the transmission output of the transmission element can be enhanced. 
   Moreover, the propagation of oscillation of the silicon active layer  22  of the transmission element to the transfer member  261  can be ensured by bringing the acoustic impedance of the transfer member  261  close to that of the silicon active film  22 . As a result, the transmission output of the transmission element can be enhanced. 
   More specifically, according to Embodiment 14, the same functions and effects as those in [6-1] of Embodiment 6 described above can be obtained. 
   [14-2] 
   In order to prevent the crosstalk characteristic of each of the receiving elements  201  from being degraded, it is necessary to prevent the oscillation of arbitrary one of the transfer members  261  from propagating to the other transfer members  261  through the filler in the gap S. 
   Therefore, in Embodiment 14, it is the most desirable that the gap S be in a vacuum state. 
   If the gap S is filled with a filler in Embodiment 14, a gas with a small acoustic impedance or a material having a high oscillation absorbance (for example, a highly viscous gel or the like) is used as the filler. 
   Specifically, according to Embodiment 14, the same functions and effects as those in [6-2] in Embodiment 6 above can be obtained. 
   [14-3] 
     FIG. 32  is a cross-sectional side view showing a receiving section  260  in the ultrasonic sensor N according to a first variation of Embodiment 14. 
   The first variation shown in  FIG. 32  differs from Embodiment 14 shown in  FIG. 31  only in that the vent holes  221  for bringing the gap R and the exterior of the housing member  204  into communication with each other are formed at a position of the sensor substrate  32  below each of the receiving elements  201 . 
   Specifically, the first variation of Embodiment 14 corresponds to the combination of Embodiments 14 and 10. Therefore, the functions and effects of Embodiment 10 described above can be obtained in addition to the above-described functions and effects of Embodiment 14. 
   [14-4] 
     FIG. 33  is a cross-sectional side view showing the receiving section  260  and the transmission section  231  in the ultrasonic sensor N according to a second variation of Embodiment 14. 
   The second variation shown in  FIG. 33  differs from Embodiment 14 shown in  FIG. 31  only in the same points as [a] to [c] of Embodiment 11 described above. 
   Specifically, the second variation of Embodiment 14 corresponds to the combination of Embodiments 14 and 11. Therefore, the functions and effects of Embodiment 11 described above can be obtained in addition to the above-described functions and effects of Embodiment 14. 
   Exemplary variations of Embodiments 9 to 14 
   Each of the receiving sections  200  to  260  according to Embodiments 9 to 14 is constituted by the plurality of piezoelectric receiving elements  201 . 
   However, each of the piezoelectric receiving elements  201  may be replaced by a capacitive receiving element  271  so that each of the receiving sections  200  to  260  is constituted by the plurality of capacitive receiving elements  271 . 
     FIG. 34  is an enlarged cross-sectional side view showing one capacitive receiving element  271 . 
   The through hole  202   a  penetrating through the substrate  202  is formed in the substrate  202 . 
   On the back face side of the substrate  202 , an insulating layer  272  is formed on the surface of the substrate  202  so as to close the lower end of the through hole  202   a.    
   On the back face side of the substrate  202 , a fixed electrode layer  273  is formed on a surface of the insulating layer  272  situated below (behind) the through hole  202   a . On a surface of the fixed electrode layer  273 , a movable electrode layer  274  is formed on a surface of the fixed electrode layer  273  through a clearance P. Spacers  275  are provided between the electrode layers  273  and  274  in their circumferential area. The electrode layers  273  and  274  are connected and fixed to each other through the spacers  275 . 
   The wiring layers  205  and  206  are formed on the surface of the sensor substrate  32 . 
   The fixed electrode layer  273  and the wiring layer  205  are connected with each other through the bump  207 , whereas the movable electrode layer  204  and the wiring layer  206  are connected with each other through the bump  208 . 
   In this manner, a capacitive element F having a structure in which the electrodes  273  and  274  are opposed to each other through the clearance P is formed. The receiving element  271  includes the capacitive element F fabricated by employing the MEMS technique. 
   The surface of the insulating layer  272  exposed through the bottom of the through hole  202   a  forms a receiving surface  271   a  of the receiving element  271 . 
   When the movable electrode layer  274  is oscillated by an ultrasonic wave, a distance between the electrode layers  273  and  274  changes so as to change a capacitance. Then, a converting circuit (not shown) connected to the wiring layers  205  and  206  is used to convert a change in capacitance between the electrode layers  273  and  274  into an electric signal. 
   As described above, even if a plurality of the capacitive receiving elements  271  constitute each of the receiving sections  200  to  260 , the movable electrode layer  274  is prevented from being damaged so as to be unlikely to break each of the receiving sections  200  to  260  even if the thin movable electrode layer  274  has a low mechanical strength as in the case where each of the receiving sections  200  to  260  includes the piezoelectric receiving elements  201 . As a result, the robust receiving sections  200  to  260  can be obtained. 
   [2] 
   Each of the transmission sections  209  and  231  according to Embodiments 9 to 14 includes the piezoelectric transmission elements having the same structure as that of the piezoelectric receiving element  201 . 
   However, each of the transmission sections  209  and  231  may be composed of a capacitive transmission element having the same structure as that of the capacitive receiving element  271  shown in  FIG. 34 . In such a case, electrostatic attraction is generated between the electrode layers  273  and  274  in accordance with an input signal applied to each of the electrode layers  273  and  274 . The electrostatic attraction causes the oscillation of the movable electrode layer  274  to generate an ultrasonic wave. 
   In this case, the receiving surface  271   a  of the receiving element  271  acts as a transmission surface of the transmission element for transmitting an ultrasonic wave. 
   [3] 
   In Embodiment 14, the protective film  14  may be omitted while the transfer member  261  may be replaced by the same protective member as the protective member  41  in Embodiment 2. 
   In this manner, the same functions and effects as those of Embodiment 2 can be obtained. 
   [4] 
   The ultrasonic sensor N according to Embodiments 9, 10, 12 and 14 and the first variations of Embodiments 12 and 14 is composed of a hybrid IC in which the receiving section  200 ,  220 ,  240  or  260  and the transmission section  209  corresponding to chip parts are attached and fixed onto the sensor substrate  32  made of an insulating plate material. 
   However, the ultrasonic sensor N according to Embodiments 9, 10, 12 and 14 and the first variations of Embodiments 12 and 14 may be composed of a monolithic IC in which the receiving section  200 ,  220 ,  240  or  260  and the transmission section  231  are formed on the single substrate  202  as in the case of the ultrasonic sensor L shown in  FIG. 23 . 
   As in the case of the ultrasonic sensor L according to the second variation of Embodiment 13, at least arbitrary one of the receiving elements  201  constituting the receiving section  250  may be made to act as the transmission element  232  constituting the transmission section  231  in the ultrasonic sensor N according to Embodiment 13 and the first variation of Embodiment 13. 
     FIG. 35  is a cross-sectional side view showing an example in which Embodiment 9 is applied to the ultrasonic sensor L, illustrating the receiving section  200  and the transmission section  231  of the ultrasonic sensor L. 
   This example differs from Embodiment 9 only in the above-described point [a] in Embodiment 11. 
     FIG. 36  is a cross-sectional side view showing an example in which Embodiment 12 is applied to the ultrasonic sensor L, illustrating the receiving section  240  and the transmission section  231  of the ultrasonic sensor L. 
   This example differs from Embodiment 12 only in the above-described point [a] in Embodiment 11. 
     FIG. 37  is a cross-sectional side view showing an example in which Embodiment 13 is applied to the ultrasonic sensor L, illustrating the receiving section  250  and the transmission section  231  of the ultrasonic sensor L. 
   This example differs from Embodiment 13 only in that one (the receiving element  201 A) of the receiving elements  201  constituting the receiving section  250  is made to act as the transmission element  232  constituting the transmission section  231 . 
     FIG. 38  is a cross-sectional side view showing an example in which Embodiment 14 is applied to the ultrasonic sensor L, illustrating the receiving section  260  and the transmission section  231  of the ultrasonic sensor L. 
   This example differs from Embodiment 14 only in the above-described point [a] in Embodiment 11. 
   The Other Embodiments 
   The present invention is not limited to the above-described embodiments, but can also be embodied as follows. In such a case, the functions and effects equivalent to or higher than those of each of the embodiments described above can be obtained. 
   [1] 
   For each of the transmission sections  31 ,  209 ,  231 , an existing small ultrasonic sensor may be used. 
   Although a piezoelectric element or a capacitive element fabricated by employing the MEMS technique is suitable for a receiving element for its high receiving sensitivity of an ultrasonic wave, it is not suitable for a transmission element for its small transmission output of an ultrasonic wave. 
   Therefore, optimal one of the transmission sections  31  and  209  and  231  may be selected for use in accordance with the field of use of the ultrasonic sensor M. 
   [2] 
   The above-described embodiments may be carried out in appropriate combination. In such a case, the effects of each of the above embodiments can be further enhanced by the synergistic effect of the combination.