Structure having plural conductive regions and process for production thereof

A structure having a plurality of conductive regions insulated electrically from each other comprises a movable piece supported movably above the upper face of the conductive region, the movable piece having an electrode in opposition to the conductive region, the structure being constructed to be capable of emitting and receiving electric signals through the lower face of the conductive region, the plural conductive regions being insulated by sequentially connected oxidized regions formed from an oxide of a material having through-holes or grooves.

TECHNICAL FIELD

The present invention relates to a microstructure or a like structure having a plurality of conductive regions isolated electrically, and to a process for producing the structure.

BACKGROUND ART

Actuators are disclosed which vibrate a thin membrane by an electrostatic force. Sensors are disclosed which detect displacement of a membrane caused by an external force from a change of the electrostatic capacitance. Such a thin membrane is produced by a technique called MEMS (micro electro-mechanical system) employing a semiconductor working technique.

One application of the actuator or the sensor is an ultrasonic transducer, which has both a function of an actuator emitting an ultrasonic wave by vibrating a membrane and a function of a sensor receiving an ultrasonic wave reflected by an object matter by displacement of the membrane in integration. The ultrasonic transducer is constituted of an array of such elements for emitting and receiving signals.

For connecting the electrodes in the arrayed elements with the respective circuits for driving and detection, wiring is formed through the substrate. For example, in one technique, for decreasing the capacitance between the electrode in the elements and the substrate, a reverse bias is applied to a PN junction formed in a lower portion of the electrode (C. H. Cheng, A. S. Ergun, and B. T. Khuri-Yakub, “Electrical through-wafer interconnects with sub-picofarad parasitic capacitance [MEMS packaging]” in Micro-electromechanical Systems Conference, pp. 18-21, 2001). Further, for decreasing the capacitance between the through-wiring and the substrate, a MIS configuration is formed in the wiring portion (see FIGS. 3AA to 3AB of the above-cited reference). Incidentally the capacitance formed between the electrode in the element and the substrate and the capacitance formed between the through-wiring and the substrate are hereinafter referred to as a parasitic capacitance.

In another technique, the parasitic capacitance is decreased by separating spatially the substrate for forming the membrane for the respective elements to utilize the substrate itself as the electrode as well as the through-wiring (X. Zhuang, A. S. Ergun, Y. Huang, I. O. Wygant, O. Oralkan, and B. T. Khuri-Yakub, “Integration of trench-isolated through-wafer interconnects with 2D capacitive micromachined ultrasonic transducer arrays,” Sensors and Actuators A: Physical, vol. 138, pp. 221-229, Jul. 20, 2007, seeFIG. 1.)

DISCLOSURE OF THE INVENTION

The parasitic capacitance can increase the power consumption at the actuator driving, or can lower the detection performance of the sensor. To offset such a disadvantage, in a technique, the parasitic capacitance is decreased by applying a reverse bias to the PN junction (Micro electromechanical Systems Conference, pp. 18-21, 2001, 2001). However, this technique requires application of a relatively high voltage, which needs a high resistance substrate, limiting the specification of the substrate. On the other hand, in the technique of the above-cited reference “Sensors and Actuators A: Physical, vol. 138, pp. 221-229, Jul. 20, 2007”, the substrate is divided by gaps into elements, which decreases the mechanical strength of the substrate.

The present invention is directed to a structure having a plurality of conductive regions insulated electrically from each other, comprisinga movable piece supported movably above the upper face of the conductive region,the movable piece having an electrode in opposition to the conductive region,the structure being constructed to be capable of emitting and receiving electric signals through the lower face of the conductive region,the plural conductive regions being insulated by sequentially connected oxidized regions formed from an oxide of a material having through-holes or grooves.

The pairs of the conductive region and the movable piece can be provided in plurality.

The present invention is directed to an ultrasonic transducer comprising the structure, wherein the movable piece is a membrane, and a closed gap is formed between the conductive region and the membrane.

The present invention is directed to a physical sensor comprising the structure, being capable of taking out, from the lower face of the conductive region, an electric signal of a change in the electrostatic capacitance between the conductive region and the electrode.

The present invention is directed to a process for producing a structure in a shape of a substrate having conductive regions, functioning as electric wirings in a thickness direction and being insulated electrically from each other, comprising steps of:forming through-holes or grooves at intervals through or on a base material of the substrate, andoxidizing thermally the base material at least at an inside face of the through-holes or grooves to form a sequentially connected oxidized regions to form plural conductive regions insulated electrically from each other.

The process comprising further a step of providing a movable piece supported movably on the upper side of at least one of the conductive regions.

The structure of the present invention has a substrate having electrical wirings in the thickness direction, having conductive regions insulated electrically from each other. The process for producing the structure comprises a first step of forming plural through-holes or grooves at intervals through or on the base material of the substrate, and a second step of thermally oxidizing the base material at least at the inside face of the through-holes or the grooves to form a sequentially connected oxidized region containing the through-holes or grooves to form electrically insulated conductive regions in the base material.

Effects of the Invention

According to the present invention, signals can be emitted or received through the lower faces of the conductive regions, the conductive regions being insulated by the above-mentioned oxidized region. Therefore, a voltage for the insulation need not be applied, and a drop of the mechanical strength is prevented, whereby a micro structure or a like structure can be produced which has a decreased parasitic capacitance.

BEST MODE FOR CARRYING OUT THE INVENTION

The basic embodiment of the present invention is a structure which has a plurality of conductive regions or through-electrodes insulated electrically. The structure has a movable piece such as a movably supported membrane and a swing member above the upper face of the conductive region. The movable piece has an electrode facing the conductive region. The structure is constructed to be capable of emitting and receiving electric signals through the lower face of the conductive region reverse to the upper layer containing the movable piece. The plural conductive regions are insulated from each other by sequentially connected oxidized regions composed of an oxide of a material constituting the through-holes or grooves. An array type structure containing plural elements insulated electrically can be readily formed by employing a plurality of pairs of the conductive region and the movable piece. An array type ultrasonic transducer can be constructed by employing a membrane as the movable piece and forming a closed gap between the conductive region and the membrane. A physical quantity sensor such as an acceleration sensor can be constructed for taking out an electric signal indicating a change in the electrostatic capacitance between the conductive region and the electrode from the lower face of the conductive region.

The aforementioned structure which has an electric wiring in the thickness direction through a substrate and has plural conductive regions insulated electrically can be produced through the process shown below. The production process comprises a first step of forming plural through-holes or grooves at intervals through or on the base material of the substrate, and a second step of thermally oxidizing the base material at least at the inside faces of the through-holes or the grooves to form a sequentially connected oxidized regions to form electrically insulated conductive regions in the base material. The process may further comprise a step of providing a movable piece on the upper face side of at least one of the conductive regions.

The embodiments of the present invention are described below in detail with reference to drawings.

First Embodiment

A first embodiment is described with reference toFIG. 1.FIG. 1is a perspective view of conductive regions, namely through-wirings, of a structure of a first embodiment of the present invention. InFIG. 1, the numerals denote the followings:101, a substrate;102, an insulation film (oxide);103, a through-hole;104, a through-wiring (conductive region).

Through-wirings104in substrate101are insulated from each other electrically by insulating film102having through-holes103. Substrate101has a low electric resistance, and plural insulated regions of the substrate serve as through-wirings104in the thickness direction.

FIGS. 2A,2B, and2C are enlarged views of a part of thermally oxidized film102(sequentially connected oxidized regions having through-holes or grooves).FIG. 2Ais a plan view,FIG. 2Bis a sectional view of the substrate taken along line2B-2B, andFIG. 2Cis a sectional view of the substrate taken along line2C-2C. InFIGS. 2A,2B, and2C, the reference numeral111denotes a first region, and the reference numeral112denotes a second region. First region111and second region112function variously depending on the position in thermally oxidized film102. In the drawing, the symbol H denotes a thickness of thermally oxidized film102between adjacent through-holes103.

In the constitution illustrated inFIGS. 2A to 2C, thermally oxidized film102is formed to be continuous to partition first region111and second region112, thereby insulating electrically first region111from second region112. Thermally oxidized film102has plural through-holes103, which enables minimization of the volume or thickness of the insulating material (thermally oxidized film) between first region111and second region112with the necessary mechanical strength retained. Thereby the structure is less liable to cause deformation of the movable piece or the substrate by stress of the thermally oxidized film.

Next, the process for producing the through-wiring of this embodiment is described below. This process comprises a first step of forming through-holes or grooves at intervals through the base material of the substrate, and a second step of thermally oxidizing at least the base material at the inside face of the through-holes or the grooves to form a sequentially connected oxidized regions to form electrically insulated conductive regions in the base material. FIGS.3AA to3EB are sectional views for describing the process for producing the structure of this embodiment. FIGS.3AA,3BA,3CA,3DA, and3EA are sectional views taken along broken line3AA-3AA inFIG. 1, and FIGS.3AB,3BB,3CB,3DB, and3EB are sectional views taken along broken line3AB-3AB.

For ease of viewing, in FIGS.3AA,3BA,3CA,3DA, and3EA, a less number of through-holes are illustrated, and in

FIGS.3AB,3BB,3CB,3DB, and3EB, a less number of conductive regions are illustrated. In FIG.3AA to3EB, the reference numerals denote the followings:201, a silicon substrate as the base material through which the through-wirings are formed;202, a resist;203, a masking material,204, a thermally oxidized film (oxidized region);210a through-hole in the oxidized region.

Firstly, as illustrated in FIGS.3AA and3AB, a film of masking material203is formed on one face of silicon substrate201, resist202is applied thereon, and resist202is patterned as desired. Masking material203is etched off selectively by utilizing the remaining resist pattern. At the portions where the masking material has been etched off, through-holes210are formed in the subsequent dry etching through the silicon. Specifically, portions of through-holes210of the oxidized region, and the portions not necessary for the structure (necessary portions including movable pieces, beams, movable electrodes, fixed electrodes, supporting substrate, etc.) are removed simultaneously. The material useful as masking material203includes metals such as aluminum; and silicon nitride, silicon oxide, polysilicon, and so forth. However the masking material is not limited thereto, insofar as the material is resistant in the subsequent anisotropic etching step. The pattern of resist203may be removed, or may be kept for use as a mask in the subsequent dry etching step. By use of this resist pattern, another masking material need not be used.

Then, as illustrated in FIGS.3BA and3BB, the substrate is anisotropically etched at the unmasked portions to form through-holes210to penetrate substrate201. The anisotropic etching can be conducted by dry etching such as Si-Deep-RIE (silicon deep reactive ion etching).

After the anisotropic etching, as illustrated in FIGS.3CA and3CB, mask material203and resist202are removed and the surface of silicon substrate201is cleaned.

Then the silicon is thermally oxidized from the surface as illustrated in FIGS.3DA and3DB. In this thermal oxidation step, silicon201is kept in an oxygen atmosphere at a temperature of 1000° C. or higher for a long time. Thereby silicon oxide grows on the bared surface as well as into the silicon to expand the oxidized region204. (The ratio of the thickness of the former to the latter is about 55:45.)

The process of the thermal oxidation is described below.FIGS. 4A,4B, and4C are plan views of the substrate showing the progress of the thermal oxidation. InFIGS. 4A to 4C, the reference numerals denote the followings:111, a first region;112, a second region;121, a thermally oxidized film grown on the surface of the silicon;122, a thermally oxidized film grown into the interior of the silicon.FIG. 4Aillustrates the state before the oxidation. In this state, first region111and second region112are not insulated from each other since the partitions between through-holes103are formed by silicon (partition thickness: X).

FIG. 4Billustrates a state of the thermal oxidation for half of an intended oxidation time length. In this state, thermally oxidized film121grows on the surface of through-holes103, and thermally oxidized film122grows into the interior of the silicon. Thereby the thickness X of the silicon partitioning adjacent through-holes103is decreased to cause slight insulation between first region111and second region112.

FIG. 4Cillustrates a state after the thermal oxidation for an intended time. In this state, thermal oxidized films122grown from the adjacent through-holes103into the silicon come into contact with each other to become integrated. Thereby, first region111and second region112are insulated from each other.

The thickness of the thermal oxidized film204is a logarithmic function of the thermal oxidation time. Therefore the thickness of the thermally oxidized film204will become saturated at a certain thickness (1 μm to several 1 μm) with the duration of the oxidation. Therefore, the thickness of the thermal oxidized film can be controlled with a high precision. In consideration of the practical process time, for sure electrical insulation between the conductive regions by complete oxidation of the silicon between through-holes103, the minimum interval between through-holes103(the thickness X inFIG. 4A) is preferably not larger than 2 μm.

On the other hand, for the anisotropic etching of the silicon, the thickness of the substrate (i.e., the depth of the through-holes) is preferably not more than 100 μm.

With the substrate of a thickness of 100 μm or larger, the etching direction can become tilted to cause non-negligible deformation by thermal stress caused by asymmetry between the shapes of the thermally oxidized film and the silicon (non-symmetry to the face at the portion of an intermediate thickness of the substrate).

Through the above steps, electrically insulated plural conductive regions are formed. That is, electrically insulated plural conductive regions can readily be formed by boring through-holes103at a small intervals and thermally oxidizing the inside face of the through-holes.

Thermally oxidized film204can be formed even in a small gap so long as oxygen is fed thereto. Therefore, the interval between the electrically insulated conductive regions can be made smaller to minimize the influence of the stress in the insulating material (thermally oxidized film) of the oxidized region. Further, in this embodiment, the insulation can be made by complete thermal oxidation of the material intervening between plural through-holes103. Therefore the through-holes need not be filled with the insulating material, without causing problems by the internal stress thereof.

Further, in this embodiment, the structure can be prepared only with a single silicon substrate, so that a special substrate such as an SOI substrate (silicon-on-insulator substrate) need not be employed.

Finally after the step shown in FIGS.3DA and3DB, the silicon oxide film on the surface of substrate201may be removed by dry etching or a like method from the both faces of the substrate as illustrated in FIGS.3EA and3EB. The removal of the thermal oxidized film from the both faces of the substrate enables further decrease of the stress caused by the thermal oxidized film. Incidentally, thermal oxidized film204shown in FIG.3EB is not necessary for the function of the structure, but is formed unavoidably in the production process. This film may be kept unremoved except the portion for electric connection.

In another process, a film of a material like a nitride film resistant to the thermal oxidation may be formed preliminarily on the surface of the substrate and patterned in the same shape as the pattern for dry etching of the through-holes. After the thermal oxidation of this substrate, the preliminarily formed film is removed to remove selectively the formed oxidized film. Thereby the stress formation by the thermal oxidation can be decreased.

When the inside width of through-hole103is small, through-holes103can be nearly completely filled with thermal oxidized film121. In the present invention, the sequentially connected oxidized regions include also such types of through-holes and grooves. However, to prevent the adverse effect of the internal stress, the amount of the oxide in oxidized region204is preferably less insofar as the oxide performs the insulation function. Usually, the width of the through-hole103is made to be several times larger the thickness of the oxidized film.

FIGS. 5A to 5Gare drawings for describing the sectional shapes and arrangement of the through-holes in this embodiment. The shape of the through-hole is not limited to that described in the above first embodiment. The through-holes may be in various shapes: circular (FIG. 5A), ellipsoidal, rectangular (FIG. 5B), triangular (FIG. 5C), square, parallelogram (FIG. 5D), polygonal (e.g., octagonal,FIG. 5E), and so forth.

In the example shown inFIG. 5C, the width of the material between the holes is made nearly constant by alternately reversing the direction of the triangles. Otherwise, as shown inFIG. 5F, different shapes of the through-holes may be combined and arranged: in this drawing, triangles and parallelograms are arranged alternately with the direction changed suitably. The arrangement of the through-holes may be linear, curved, stepwise, and so forth. In the example ofFIG. 5G, the through-holes are arranged in plural rows and the positions of the hole intervals are displaced in a zigzag manner. Naturally in any of the examples, width of the material of the interval portions between the holes should be designed to be oxidized surely from the inside face of the holes on the both sides to the intermediate position of the material to make the interval insulative.

The shape of the holes is selected suitably to lessen the adverse effect of the stress in the thermally oxidized film to obtain a sufficient mechanical strength of the insulating portion of the structure like a micro-structure.

According to this embodiment, the conductive region is capable of functioning as a through-electrode, and the plural conductive regions are insulated from each other. Thereby electric signals can be taken out from or applied to the lower face of the conductive regions. Therefore a voltage for the insulation need not be applied, the decrease of the mechanical strength is prevented, and the parasitic capacitance can be suppressed, which are different from conventional techniques mentioned in the articles of related background art.

Second Embodiment

Second embodiment is described with reference toFIGS. 6A and 6B. This second embodiment uses the structure of the first embodiment as an array type ultrasonic transducer.

FIGS. 6A and 6Bare drawings for describing an array type ultrasonic transducer of this second embodiment.FIG. 6Ais a perspective view of a part of the substrate having through-wirings employed for an array type ultrasonic transducer.FIG. 6Bis a sectional view, taken along line6B-6B, of the part of the ultrasonic transducer of this embodiment.

InFIGS. 6A and 6B, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring (conductive region) which is wholly conductive;301, a membrane (thin film) serving as a movable piece;302, a post for supporting movably membrane301;303, a frame;304, a conductive bump;305, a circuit substrate;306, an integrated circuit. This constitution itself does not have a closed gap between through-wiring104and membrane301. The closed gap can be formed by sealing the space between substrate101and circuit substrate305or filling the one side of through-holes103.

In the drawings after this embodiment, the shape of through-hole103or the groove is shown to be tetragonal for simplification of the drawings, but the shape is not limited thereto.

A process for producing an array type ultrasonic transducer of this embodiment is described below. Firstly, according to the process described in the first embodiment, a through-wiring substrate101is prepared which has an array of plural through-wirings104. This through-wiring substrate101has a constitution illustrated inFIG. 6A. Through-wirings104are respectively surrounded by thermally oxidized film102having through-holes103. Between thermally oxidized films102, silicon frame303is placed. The thermally oxidized film on the front face (reverse face) of through-wiring substrate101is removed by etching for use as the wiring (see FIGS.3EA and3EB of the first embodiment).

Next, on one face of through-wiring substrate101, posts302are formed along the pattern of frame303on the same region. Posts302can be formed, for example, by forming a film once and removing the film with specified portions of the film left unremoved.

Then, thin membrane301for vibration is bonded on the portions of posts302. In this embodiment, membrane301is made of a conductive material. Thus an ultrasonic wave can be emitted by vibrating membrane301by application of a voltage between the lower face of the membrane and the upper face of through-wiring104. Otherwise an intensity of an ultrasonic wave received by membrane301can be measured by detecting change in electrostatic capacitance between the lower face of the membrane and the upper face of the through-wiring104. The output characteristics of the actuator and the detection characteristics of the sensor of such an array type ultrasonic transducer can be controlled by adjusting the height of post302(the height of the above closed gap).

In the last step, bumps304are formed on the lower face of through-wiring substrate101, and circuit substrate305and the bumps are bonded. On this circuit substrate305, integrated circuit306is mounted which has a driving circuit for applying a voltage for vibrating membrane301and a detection circuit for detecting a change of the electrostatic capacitance of membrane301. Thereby a driving circuit can be provided for each of the elements to enable emission of an ultrasonic wave (here, one element is constituted of a combination of a through-wiring104and membrane301formed above the through-wiring). A detection circuit can be placed close to each of the elements, which enables detection of the ultrasonic wave received by the respective element separately with high precision.

In this embodiment, frames303contained in through-wiring substrate101prevent thermally oxidized film102from traversing substrate101from side to side, whereby the mechanical strength of through-wire substrate101is increased, and a damage in the bump bonding can be prevented.

Post302can be formed from a conductive material. Frame303can be formed from the same substrate material as that of through-wiring104. Therefore frame303is also constituted of a low-resistance material. In this state, the portion of frame303, the portion of post302, and membrane301are considered to be at the same potential. Therefore, the potential of membrane301can be held at a desired level by bump304formed under frame303.

Post302may be formed from an insulating material. When the membrane is made from an insulating material, after the membrane is bonded, a portion of membrane301above frame303and a portion of post302are bored and wiring is made through the bored portion to connect electrically frame303with membrane301to keep the potential of membrane301at a desired level.

In such a manner, according to this embodiment, the potential of frame303can be kept at the same level as that of membrane301. Therefore, the potential of membrane301can be controlled at a desired level. Thus, in the array type ultrasonic transducer, an optimum potential can be applied to the membrane for the actuator output characteristics and the sensor detection characteristics. Further, the membrane potential in the actuation and the membrane potential in the sensing can be differentiated to achieve a higher performance.

In this embodiment, membrane301is made from a conductive material, but the material is not limited thereto. When membrane301is made from an insulating material, an electrode is formed on the membrane from a conductive material for the same use. With such a construction, even when the upper electrode portion and the lower electrode portion come into contact physically together, the electrodes are insulated from each other not to cause damage in integrated circuit306, preferably.

This conductive electrode on insulating membrane301need not be formed on the entire face of the membrane, but may be formed in a desired pattern in a shape suitable for the driving and detection. Thereby in the array type ultrasonic transducer, the actuator output characteristics and the sensor detection characteristics can be improved further.

Otherwise, an insulating film may be placed on the face of through-wiring104opposing to membrane301. Thereby even when conductive membrane301comes into contact with the underlying electrode, the electrodes are electrically insulated.

Third Embodiment

Third embodiment is described with reference toFIGS. 7A and 7B. This third embodiment is nearly the same as the second embodiment except that the constitution of through-wiring substrate101of the array type ultrasonic transducer is different from that of the second embodiment.

FIGS. 7A and 7Bare drawings for describing an array type ultrasonic transducer of this third embodiment.FIG. 7Ais a perspective view of the through-wiring substrate employed in an array type ultrasonic transducer.FIG. 7Bis a sectional view, taken along line7B-7B, of a part of the ultrasonic transducer of this embodiment. InFIGS. 7A and 7B, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring;301, a membrane,302, a post;304, a bump;305, a circuit substrate;306, an integrated circuit.

In this embodiment, post302is formed in contact with through-wiring104. Thereby the gap between the upper electrode and the lower electrode can be kept constant precisely, even when post302and membrane301are formed prior to formation of through-wiring104.

In more detail, in the constitution of the above second embodiment, in formation of thermally oxidized film102having through-holes103, the gap (space) under membrane301communicates with through-holes103(seeFIGS. 6A and 6B). Therefore, in the case where the membrane is formed firstly with a gap, the silicon of the upper face of through-wiring104can be oxidized to make smaller the gap between the upper electrode and the lower electrode. Thereby the range of movement of the membrane decreased to affect adversely the driving characteristics of the transducer. Thus, in the second embodiment, when the membrane is formed with a gap firstly, it is difficult to keep the gap between the upper and lower electrodes without giving an adverse effect on the driving characteristics.

On the other hand, in this embodiment, posts302are formed on through-wirings104to close the gap under the membrane301. Therefore, even if membrane301is formed with a gap previously, the oxidation will not occur in the gap, so that through-wirings104can be insulated electrically from each other without changing the gap between the upper and lower electrodes.

In this embodiment as described above, a substrate may be employed which has a membrane formed preliminarily with a gap by a bonding process or by a surface process by use of a sacrifice layer. Thereby limitation of the production process can be reduced to enable formation of the membrane having more satisfactory characteristics.

Further, in this embodiment, the substrate need not be worked preliminarily for the formation of through-wiring104, whereby the defective ratio of the products in the process can be lowered. Furthermore frame303need not be formed to enable use of the substrate having membrane301with a gap. Thereby the distance between through-wirings104can be made larger with the size of through-wiring unchanged, whereby the parasitic capacitance between through-wirings104can be made smaller to decrease cross talk of driving signals and detection signals.

In this embodiment, thermal oxidation can occur at a portion of membrane301placed on trough-holes103. However, this caused no problem since this portion does not affect the transducer characteristics.

Fourth Embodiment

Fourth embodiment is described with reference to FIGS.8AA to and8CB. This fourth embodiment is nearly the same as the second embodiment except the construction of thermally oxidized film102of through-wiring substrate101of the array type ultrasonic transducer.

FIGS.8AA to8CB are drawings for describing the fourth embodiment of the present invention. FIGS.8AA and8AB are sectional views corresponding to FIGS.3BA and3BB. In this embodiment, as illustrated in FIGS.8AA and8AB, the portions of the holes are grooves211which do not penetrate the substrate211completely. In this structure, the difference between the thickness of substrate201and the depth of groove211(i.e., the thickness of the material under the bottom of the groove) should be decided suitably in consideration of the thickness of the thermally oxidized film formed in the silicon by the thermal oxidation. In this embodiment, since the thermal oxidation is conducted from the one side only of substrate201, the difference should be not more than the thickness of the thermally oxidized film. The difference is preferably not more than 1 μm by the reason mentioned in Embodiment 1. The depth of groove211is not more than 100 μm by the reason mentioned in Embodiment 1.

FIGS.8BA and8BB are sectional views corresponding to FIGS.3DA and3DB of Embodiment 1. The thermal oxidation is conducted from the upper side of the substrate as illustrated in FIGS.8AA and8AB to obtain an oxidized article as illustrated in FIGS.8BA and8BB. In the thermal oxidation, as illustrated in FIGS.8BA and8BB, the lower face of the substrate is protected, so that the thermally oxidized film is not formed on the lower face of the substrate.

FIGS.8CA and8CB are drawings for describing an array type ultrasonic transducer of this fourth embodiment. FIG.8CA is a perspective view of a part of the through-wiring substrate employed in an array type ultrasonic transducer. FIG.8CB is a sectional view, taken along broken line8CB-8CB, of a part of the ultrasonic transducer of this embodiment. In FIGS.8CA and8CB, the substrate101is placed upside down in comparison with the illustrations in FIGS.8AA and8AB and FIGS.8BA and8BB.

In this embodiment, grooves105only are formed in the portions of the substrate for forming the insulated region. The grooves do not penetrate through the substrate, so that the gap under membrane301is closed. Therefore, even if membrane301is formed previously with the gap, oxidation will not occur during the thermal oxidation. Therefore, in this embodiment, through-wirings104can be electrically isolated from each other without changing the gap between the upper electrode and the lower electrode without limitation of the positions of posts302.

Fifth Embodiment

Fifth embodiment is described with reference toFIGS. 9A to 9C. This fifth embodiment is nearly the same as the second embodiment except the construction of through-wiring substrate101of the array type ultrasonic transducer.

FIGS. 9A to 9Care drawings for describing an array type ultrasonic transducer.FIG. 9Ais a perspective view of a part of the through-wiring substrate for an array type ultrasonic transducer.FIG. 9Bis a sectional view, taken along broken line9B-9B, of a part of the ultrasonic transducer of this embodiment.FIG. 9Cis a sectional view, taken along broken line9C-9C, of a part of the ultrasonic transducer of this embodiment.

InFIGS. 9A to 9C, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring;301, a membrane;302, a post;303, a frame;304, a bump;305, a circuit substrate;306, an integrated circuit;307, an electrode provided on membrane301;308, wiring for electrically connecting electrode307with portions of frame303.

In this embodiment, frame303is divided into portions. The division of the frame enables application of different potentials to respective frame portions303. In this embodiment, the membrane301is formed from an insulating material, and conductive electrodes307are formed in arbitrary forms on membrane301. Electrodes307on membrane301are connected electrically to portions of frame303through-wiring308formed in post302and in detached portions of membrane301. Thereby different voltages can be applied to respective portions of frame303to apply different potentials to connected portions of membrane301.

According to this embodiment, different potentials can be applied to respective membrane of elements to obtain optimum driving and detection characteristics for the positions in the array to provide an array type ultrasonic transducer having a higher performance and higher functionality.

Sixth Embodiment

Sixth embodiment is described with reference toFIGS. 10A and 10B. This sixth embodiment is nearly the same as the second embodiment except the construction of the bump portion of through-wiring substrate101of the array type ultrasonic transducer.

FIGS. 10A and 10Bare drawings for describing an array type ultrasonic transducer of this sixth embodiment.FIG. 10Ais a perspective view of a part of a through-wiring substrate employed in an array type ultrasonic transducer.FIG. 10Bis a sectional view, taken along broken line10B-10B, of a part of the array type ultrasonic transducer of this embodiment. InFIGS. 10A and 10B, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring;301, a membrane;302, a post;303, a frame;305, a circuit substrate;306, an integrated circuit;309, a projection of frame303or of through-wiring104.

In this embodiment, silicon projection309is provided in place of bump304employed in second embodiment, enabling direct junction of silicon instead of the bump junction.

Projections309are formed at lower portions of frame303, and projections309are also formed at portions of the lower parts of through-wirings104. This projections309can be formed by selective etching from one side of substrate101of through-wiring104having thermally oxidized film102having through-holes103.

In the example illustrated inFIGS. 10A and 10B, in the etching, thermal oxidized film102is partly etched simultaneously. Otherwise, the thermal oxidation may be conducted by use of substrate101in which through-holes103have been formed and a part thereof has been dug selectively by etching. (The order of the formation of through-holes103and the digging may be reversed.) In this thermal oxidation, thermal oxidized film can grow on the portion other than the side face of through-holes103. The thermal oxidized film should be removed at the portions in which the junction with substrate305is made, but in other portions, the thermal oxidized portion may be kept unremoved without causing a problem.

An array type ultrasonic transducer of high junction efficiency can be provided by direct junction of the substrate having projections309as illustrated inFIG. 10Bwith circuit substrate305. Further the direct junction improves more the reliability of the transducer by preventing a trouble caused by bump junction, whereby an array type ultrasonic transducer can be provided with high reliability.

Seventh Embodiment

Seventh embodiment is described with reference toFIGS. 11A and 11B, and FIGS.12AA to12EB. This seventh embodiment is nearly the same as the second embodiment except the construction of the post portions on through-wiring substrate101of an array type ultrasonic transducer.

FIGS. 11A and 11Bare drawings for describing an array type ultrasonic transducer of this seventh embodiment.FIG. 11Ais a perspective view of a part of a through-wiring substrate employed in an array type ultrasonic transducer.FIG. 11Bis a sectional view, taken along broken line11B-11B, of a part of the array type ultrasonic transducer of this embodiment. InFIGS. 11A and 11B, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring;301, a membrane;302, a post;304, a bump;305, a circuit substrate;306, an integrated circuit.

In this embodiment, posts302are placed respectively on thermally oxidized film102having through-hole103. InFIG. 11A, posts302are denoted by dotted lines. Such placement of posts302minimizes the ineffective space and enlarges the electrode area to improve the driving efficiency and detection sensitivity of membrane301. Moreover, the distance between through-wirings104can be increased to decrease the parasitic capacitance between through-wirings104to decrease signal cross-talks in driving and detection to obtain better characteristics.

FIGS.12AA to12EB illustrate a process for producing an array type ultrasonic transducer of this seventh embodiment. Firstly, insulation film205is formed on silicon substrate201(lower side in the drawings, FIGS.12AA and12AB). This insulation film205is etched to leave portions thereof unetched along regions for formation of thermally oxidized film102(FIGS.12BA and12BB). The remaining regions of insulation film205functions as posts302. Then membrane301is formed on posts302(remaining insulation film205as supporters), (FIGS.12CA and12CB). Membrane301may be formed by either a junction process or a surface process.

Then silicon substrate is etched anisotropically from the lower face thereof (upper side in the drawings) to form through-holes210(FIGS.12CA and12CB). In this etching, the etching conditions are selected to stop the etching to form through-holes210to reach just insulation film205on the upper face of substrate201.

Then the side faces of through-holes210are thermally oxidized (FIGS.12DA and12DB). For the thermal oxidation, the upper face of membrane301should be formed from an oxidation-resistant material or be coated with an oxidation-resistant material. On the lower face of substrate201(upper side in the drawings), thermally oxidized film204comes to be formed. This film can be etched on taking a contact with bumps. When the formation of the thermally oxidized film on the substrate causes a trouble, the back face of the substrate is preferably masked with a oxidation-resistant material to cause thermal oxidation only on the side face of through-holes210(FIGS.12EA and12EB). Finally, bumps304are formed for bump junction. Through the above production process, even when membrane301with a gap is preliminarily formed, through-wirings104can be formed without causing oxidation in the gap on the thermal oxidation.

In this embodiment, membrane301with a gap is formed before the formation of the through-holes and thermal oxidation of the side face of the through-holes, but is not limited thereto. Membrane301may be formed after the steps of formation of posts302, formation of the through-holes and thermal oxidation of the side faces of the through-holes. Or in place of insulation film205on a substrate, SOI substrate may be employed.

Eighth Embodiment

Eighth embodiment is described with reference toFIGS. 13A and 13B. This eighth embodiment is nearly the same as the second embodiment except the construction of through-wiring substrate101of an array type ultrasonic transducer.

FIGS. 13A and 13Bare drawings for describing an array type ultrasonic transducer of this eighth embodiment.FIG. 13Ais a perspective view of a part of a through-wiring substrate employed in an array type ultrasonic transducer.FIG. 13Bis a sectional view, taken along broken line13B-13B, of a part of the array type ultrasonic transducer of this embodiment. InFIGS. 13A and 13B, the reference numerals denote the followings:101, a substrate;102, a thermally oxidized film;103, a through-hole;104, a through-wiring;301, a membrane;302, a post;306, an integrated circuit.

This embodiment is different from the above embodiments in that the integrated circuit306is mounted on through-wiring substrate101. An SOI substrate is employed as substrate101. Through-wirings104are formed on the thick handle layer side, and the integrated circuits306are formed on the side of thin active layer305. The constitution of the side of through-wiring104is same as that of the second embodiment. On the other hand, on the side of integrated circuit306, a part of active layer305and a part of BOX layer310(an insulation layer between the active layer of the SOI substrate and the handle layer) are bored to connect the substrate of the handle layer with circuit306by wiring311. With this constitution, through-wiring104is connected with integrated circuit306, and frame303(membrane301) is connected with integrated circuit306electrically. Thereby a potential can be applied from integrated circuit306to membrane301, and driving signals and detection signals can be emitted and received through the lower face of through-wiring104.

In this embodiment, the cause of decrease of the reliability of the bump junction can be eliminated to provide an array type ultrasonic transducer with high reliability. Furthermore, integrated circuit306can be combined into one body to improve mass-productivity and to improve the functionality and performance.

Ninth Embodiment

Ninth embodiment is described with reference toFIGS. 14A and 14B. This ninth embodiment is an array type acceleration sensor employing the structure of the first embodiment.

FIG. 14Ais a perspective view of an acceleration sensor of this embodiment.FIG. 14Bis a sectional view taken along broken line14B-14B. InFIGS. 14A and 14B, the reference numerals denote the followings:902, a supporting beam for supporting movably a movable piece904;903, a substrate member counterposed against movable piece904;905, a holder formed from an insulating material for holding the supporting beam902;104, a through-wiring opposing movable piece904with an interval and functioning also as a detection electrode;906a hole bored through holder905and substrate member903;907, a wiring for transmitting a potential of supporting beam902to the side of circuit substrate305;908, a space between substrate member903and circuit substrate305. Substrate member903serves also as electrode-forming substrate on which detection electrode104is formed.

Movable piece904is supported by supporting beam902to be capable of vibration. Supporting beam902is connected to supporter905fixed to substrate member903. In a non-action state, movable piece904is held at a prescribed distance from detection electrode104. Movable piece904can be displaced or vibrated upward and downward inFIG. 14Bby deformation of supporting beam902under an action onto movable piece904.

When an acceleration force is applied vertically to substrate member903of the sensor, movable piece904is displaced upward or downward inFIG. 14B. Thereby the interval between movable piece904and detection electrode104is changed from that at the acceleration-free state. The acceleration can be detected by detecting a change of the interval. That is, the acceleration is detected by detecting the relative position of movable piece904from detection electrode104counterposed at an interval against movable piece904.

The change of the interval between movable piece904and substrate member903or detection electrode104is detected by a change of the capacitance formed therebetween. In this embodiment, detection electrode104is placed in substrate member903, and the change of the capacitance formed between detection electrode104and movable piece904is detected. The thickness of detection electrode104is equal to the thickness of substrate member903. Bumps304are formed on the face of electrode formation substrate903at the side opposite to the movable piece904. Thus, the detection signal is taken out from detection electrode104by utilizing the wiring through the bump304.

With movable piece904kept at a certain potential by a wiring through the bump304, a decrease of the interval between movable piece904and detection electrode104increases the electrostatic capacitance and strengthens the detection signal correspondingly, whereas an increase of the interval between movable piece904and detection electrode104decreases the electrostatic capacitance and weakens the detection signal correspondingly. In such a manner, corresponding to the strength and direction of the acceleration, a force is applied to movable piece904, changing the interval between movable piece904and detection electrode104to emit a signal reflecting the interval change. Therefore, the strength and direction of the acceleration can be detected by circuit306based on the detection signal.

In the sensor of this embodiment, the back face of substrate member903can be connected with circuit substrate305by simple wiring construction, whereby the detection signal can be detected regardless of the shape and size of movable piece904and the relative positions to other parts. Therefore the movable piece904can be made larger relative to the area of the substrate. A larger size (i.e., a larger mass) of movable piece904will be displaced larger by the same acceleration force, which enables increase of the sensor sensitivity. Further, a larger confronting area of movable piece904with detection electrode104will give a larger electrostatic capacitance for the detection, which contributes also the increase of the sensor sensitivity.

In this embodiment, since the wiring for the detection signal is placed outside the portion facing the movable piece904, the detection by signal will not be deteriorated by interaction of movable piece904with the signal detection wiring to enable the detection with less error and higher precision.

Further, since the constitution on the side of movable piece904is simple, the settlement on the side of movable pert904can be made precisely including the interval between movable piece904and detection electrode104. This also contributes the less error and higher precision of the detection.

In particular, in two-dimensional arrangement of plural movable pieces904at intervals on substrate member903, the movable piece904can be made larger in size, and a wiring error component is made less liable to overlap with the detection signal. Further, the wiring is made on the back face side to make it easier to make larger the distance between the wirings in comparison with the wiring on the front face side to prevent cross talk between the wirings. Thus, a sensor can be provided which is capable of detecting information from two-dimensionally arranged detection parts with high sensitivity and high precision. The sensor of this embodiment can be produced in a similar manner as those mentioned in the above embodiments.

In this specification, descriptions are made on array type ultrasonic transducers and an array type acceleration sensor, but the invention is not limited thereto. The structure of the present invention is applicable, regardless of an array type or a single element, to those for detecting a physical quantity by utilizing an electrostatic attraction force or a change of an electrostatic capacitance, including ultrasonic transducers, pressure sensors, acceleration sensors, and gyro sensors.

This application claims the benefit of Japanese Patent Application No. 2008-144166, filed Jun. 2, 2008, which is hereby incorporated by reference herein in its entirety.