JP2012135041A - Acoustic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an acoustic sensor capable of effectively reducing the phenomena that vibration of a vibration electrode plate is prevented by the vibration electrode plate being fixed to a counter electrode plate.SOLUTION: A vibration electrode plate 24 sensitive to acoustic pressure opposes a counter electrode plate 25 to constitute a capacitive acoustic sensor. The counter electrode plate 25 has an acoustic hole 31 which allows vibrations to pass therethrough and the plurality of protrusions 36 extended on a face opposing to the vibration electrode plate 24. A distance between the protrusions 36 neighboring in an opposing area of the counter electrode plate 25 which is opposite to a highly flexible area of the vibration electrode plate 24 is narrower than that of the protrusions 36 neighboring in the opposing area of the counter electrode plate 25 which is opposite to a low flexible area of the vibration electrode plate 24.

Description

  The present invention relates to an acoustic sensor, and more particularly to an acoustic sensor for detecting sound pressure propagating in a gas or liquid, that is, acoustic vibration.

  As an acoustic sensor, there is one disclosed in JP 2006-157863 A (Patent Document 1).

  This acoustic sensor has a structure in which a vibrating electrode plate (movable electrode) and a counter electrode plate (fixed electrode) are opposed to each other with a minute gap (gap). Since this vibrating electrode plate is formed of a thin film having a film thickness of about 1 μm, it vibrates minutely in response to the vibration when it receives sound pressure. When the vibrating electrode plate vibrates, the gap distance between the vibrating electrode plate and the counter electrode plate changes, so that the acoustic vibration is detected by detecting the change in capacitance between the vibrating electrode plate and the counter electrode plate at that time. The

  In addition, this acoustic sensor is manufactured using a micromachining (semiconductor micromachining) technique, and has a very small size with a side of several millimeters in plan view while being highly sensitive.

  However, in such an acoustic sensor, the vibration electrode plate 12 may be fixed to the counter electrode plate 13 as shown in FIG. The state where the whole is fixed to the counter electrode plate and the gap disappears, or the phenomenon is called a stick). When the vibrating electrode plate 12 sticks to the counter electrode plate 13 in this way, the vibration of the vibrating electrode plate 12 is hindered, and therefore the acoustic sensor 11 cannot detect the acoustic vibration.

  2 (a) and 2 (b) are schematic diagrams for explaining the cause of the occurrence of a stick in the acoustic sensor 11, and an enlarged view corresponding to the portion X in FIG. is there. Since the acoustic sensor 11 is manufactured using a micromachining technique, for example, moisture 14 permeates between the vibrating electrode plate 12 and the counter electrode plate 13 in a cleaning process after etching. Further, even during use of the acoustic sensor 11, moisture may accumulate between the vibrating electrode plate 12 and the counter electrode plate 13 or the acoustic sensor 11 may get wet with water.

  On the other hand, since the acoustic sensor 11 has a minute size, the gap distance between the vibrating electrode plate 12 and the counter electrode plate 13 is only a few μm. Moreover, in order to increase the sensitivity of the acoustic sensor 11, the thickness of the vibrating electrode plate 12 is as thin as about 1 μm, and the spring property of the vibrating electrode plate 12 is weakened.

  Therefore, in such an acoustic sensor 11, a stick may occur through a two-step process as described below, for example. In the first stage, as shown in FIG. 2A, when moisture 14 enters between the vibrating electrode plate 12 and the counter electrode plate 13, the vibrating electrode plate is caused by the capillary force P1 or surface tension caused by the moisture. 12 is attracted to the counter electrode plate 13.

  In the second stage, after the moisture 14 between the vibrating electrode plate 12 and the counter electrode plate 13 evaporates, the vibrating electrode plate 12 adheres to the counter electrode plate 13 and the state is maintained. As the force P2 for fixing the vibrating electrode plate 12 to the counter electrode plate 13 and holding it after the moisture 14 evaporates, the intermolecular force and inter-surface force acting between the surface of the vibrating electrode plate 12 and the surface of the counter electrode plate 13 are used. There is static electricity. As a result, the vibration electrode plate 12 is held in a state of being attached to the counter electrode plate 13, and the acoustic sensor 11 does not function.

  Here, the case where the vibrating electrode plate 12 sticks to the counter electrode plate 13 in the first stage due to the capillary force of the water that has entered has been described. In addition, the vibrating electrode plate may stick to the counter electrode plate. Further, the first stage process may occur when the vibrating electrode plate is statically attached to the counter electrode plate. However, in the following description, it is assumed that the vibration electrode plate sticks to the counter electrode plate due to moisture.

  As a method of reducing the stick as described above, the elastic restoring force Q of the vibrating electrode plate 12 is increased, and the elastic restoring force Q overcomes the capillary force P1 of the moisture 14 in the first stage and the holding force P2 in the second stage. Thus, the vibrating electrode plate 12 may be returned to the original state. In order to increase the elastic restoring force Q of the vibration electrode plate 12, the film thickness of the vibration electrode plate 12 may be increased to increase the spring property. However, when the elastic restoring force Q of the vibration electrode plate 12 is increased, the vibration electrode plate 12 is less likely to vibrate, so that the sensitivity of the acoustic sensor 11 is deteriorated.

  Alternatively, the stick can be reduced even if the capillary force P1 is smaller than the elastic restoring force Q of the vibrating electrode plate 12 in the first stage. Since the capillary force P1 increases as the gap distance between the vibrating electrode plate 12 and the counter electrode plate 13 decreases, the capillary distance P1 can be reduced by increasing the gap distance. However, when the gap distance between the vibrating electrode plate 12 and the counter electrode plate 13 is increased, the thickness of the acoustic sensor 11 is increased, and miniaturization of the acoustic sensor 11 is prevented. Moreover, the sensitivity of the acoustic sensor 11 is also reduced.

  In view of such a situation, in the acoustic sensor disclosed in Patent Document 1, as shown in FIG. 3, the vibration electrode is provided by providing a large number of protrusions 15 on the surface of the counter electrode plate 13 facing the vibration electrode plate 12. The stick between the plate 12 and the counter electrode plate 13 is reduced. The protrusions are generally arranged at equal intervals over the entire counter electrode plate. It is known that the holding force P2 between the vibrating electrode plate 12 and the counter electrode plate 13 has a correlation with the contact area of both the electrode plates 12 and 13, and the holding force P2 is reduced when the contact area is small. . Therefore, if the counter electrode plate 13 is provided with the protrusion 15 and the protrusion 15 is made as thin as possible, the contact area between the vibration electrode plate 12 and the counter electrode plate 13 (protrusion 15) is reduced, and the holding force P2 is also weakened. The stick of the electrode plate 12 hardly occurs.

  In Non-Patent Document 2, since the ratio of the surface area to the mass is increased in the microstructure, the inter-surface force acting between the member surfaces plays an important role, particularly in a micro device having a diaphragm. It is described that there is a case where the diaphragm and the counter substrate are not attached to each other due to the force and may not operate. Non-Patent Document 2 describes that cantilever sticking can be reduced by providing a protrusion (stopper) on the cantilever.

JP 2006-157863 A

Shigeki Tsuchiya, 5 others, “Measurement of inter-surface force and reduction of inter-surface force in micro structure” Transactions of the Society of Instrument and Control Engineers, Japan, Society of Instrument and Control Engineers, 1994, Vol. 30, No. 2 136-142

  However, in the acoustic sensor, as a result of repeating the experiment by changing the interval between the protrusions provided on the vibration electrode plates in various ways, if it is attempted to prevent the vibration electrode plate from sticking by providing the protrusions, It has been found that the interval must be adjusted to an appropriate value.

  FIGS. 4A to 4C are diagrams schematically showing the state of the vibrating electrode plate 12 when the interval between the protrusions 15 is too large, when it is appropriate, and when it is too small. . FIG. 4B shows a case where the distance d between the protrusions 15 is appropriate. In this case, even if the vibration electrode plate 12 adheres to the counter electrode plate 13 with moisture, the contact area between the protrusion 15 and the vibration electrode plate 12 is small as shown by a two-dot chain line in FIG. When the moisture evaporates, the holding force P2 is smaller than the elastic restoring force Q of the vibrating electrode plate 12. Therefore, as shown by a solid line in FIG. 4B, the vibrating electrode plate 12 returns to its original state by its own elastic restoring force Q.

  On the other hand, as shown in FIG. 4A, when the distance d between the protrusions 15 is smaller than an appropriate distance, even if the protrusion 15 is thin and the tip area is small, the tip surface of the protrusion 15 is miniaturized. Since there is a limit, the total value of the area of the tip surface of the protrusion 51 as a whole is large. Therefore, in this case, the vibration electrode plate 12 sticks to the tip surface of the protrusion 15 over substantially the whole or a wide area, and the vibration electrode plate 12 sticks to the protrusion 15. Note that the state in which the vibrating electrode plate 12 is attached to the tip surfaces of the many protrusions 15 as shown in FIG.

  Further, as shown in FIG. 4C, when the distance d between the protrusions 15 is larger than the appropriate distance, even if the vibration electrode plate 12 abuts the protrusion 15, the vibration electrode plate is between the adjacent protrusions 15. A part of 12 falls and contacts the counter electrode plate 13. Thus, in the state where the vibration electrode plate 12 is attached to the counter electrode plate 13, the contact area is considerably larger than the tip end area of the protrusion 15 even if there is only one contact location. 13 will be fixed. A state in which a part of the vibrating electrode plate 12 sticks to the counter electrode plate 13 between the protrusions 15 as shown in FIG.

  When the whole stick and the local stick are compared, generally, the whole stick is more likely to occur than the local stick. Therefore, when determining the spacing between the projections at the design stage, it is desirable to keep the spacing between the projections wide even if there is a risk of a local stick. However, in the capacitive acoustic sensor, the vibrating electrode plate and the counter electrode plate are opposed to each other with a small gap of about several μm, so that a small force exceeding the sound pressure is applied to the vibrating electrode plate. The vibrating electrode plate comes into contact with the counter electrode plate. Further, since the vibrating electrode plate is so soft and soft that it is deformed by sound pressure, the restoring force is weak when it adheres to the counter electrode plate. For this reason, when the interval between the protrusions becomes wide, a structure in which a local stick is likely to occur is obtained.

  As a result, in the conventional acoustic sensor, even if the interval between the protrusions is too large or too small, sticks are easily generated, and it is difficult to provide the protrusions at an appropriate interval. Also, even if the projections are provided at appropriate intervals assuming values such as the spring property of the vibrating electrode plate, the tip tip area, the capillary force of the liquid, and the inter-surface force, values such as the spring property of the vibrating electrode plate There was a risk that either stick would occur.

  It should be noted that, if the projection is provided on the vibration electrode plate, the rigidity is increased and the vibration electrode plate is less likely to vibrate with sound pressure. When the protrusions are provided on the vibrating electrode plate, the entire stick has a large number of protrusions on the vibrating electrode plate attached to almost the entire counter electrode plate, and the local stick has a protrusion on the vibrating electrode plate that touches the counter electrode plate. The portion between the protrusions of the vibrating electrode plate is deformed and is attached to the counter electrode plate.

  In addition, when the protrusions are provided at almost equal intervals on the vibration electrode plate or the counter electrode plate, a large number of protrusions are provided even in an area where the density of the protrusions does not need to be increased so much. The total number increases. When the protrusions increase, it becomes difficult for the air between the vibrating electrode plate and the counter electrode plate to be discharged outside when the vibrating electrode plate approaches the counter electrode plate side, and when the vibrating electrode plate moves away from the counter electrode plate side. Air hardly flows between the vibration electrode plate and the counter electrode plate. As a result, the air resistance when the vibrating electrode plate vibrates increases, the vibration of the vibrating electrode plate is suppressed by air damping, and the frequency characteristics (especially on the high frequency side) of the acoustic sensor deteriorate.

  The present invention has been made in view of the technical problems as described above, and an object thereof is to effectively prevent a phenomenon in which the vibration electrode plate is fixed to the counter electrode plate and the vibration of the vibration electrode plate is hindered. An object of the present invention is to provide an acoustic sensor that can be reduced.

  An acoustic sensor according to the present invention includes a vibration electrode plate fixed to a substrate and sensitive to sound pressure, and a counter electrode plate fixed to the substrate and opposed to the vibration electrode plate through a gap. In the sensor, a plurality of protrusions are provided on the surface on the gap side of the vibration electrode plate or the counter electrode plate, and an interval between adjacent protrusions is changed according to a protrusion formation region in the vibration electrode plate or the counter electrode plate. In the electrode plate on the side where the protrusion is provided in the vibrating electrode plate or the counter electrode plate, a highly flexible region of the vibrating electrode plate or a counter region of the counter electrode plate facing the highly flexible region The distance between adjacent protrusions in the region is less than the distance between adjacent protrusions in the opposing region of the counter electrode plate facing the low flexibility region or the low flexibility region of the vibrating electrode plate It is characterized by small.

  In the acoustic sensor according to the present invention, since the plurality of protrusions are provided on the surface of the vibration electrode plate or the counter electrode plate on the gap side, when the vibration electrode plate is deformed and contacts the counter electrode plate Means that the vibrating electrode plate and the counter electrode plate are in contact with each other with the protrusion interposed therebetween. As a result, the substantial contact area between the vibration electrode plate and the counter electrode plate can be reduced, and the stick of the vibration electrode plate can be reduced.

  Moreover, in the acoustic sensor of the present invention, since the interval between the adjacent protrusions is changed according to the protrusion formation region, the local stick in which the vibration electrode plate is fixed to the counter electrode plate between the adjacent protrusions. In addition, it is possible to reduce the entire stick in which the vibrating electrode plate or the counter electrode plate is fixed to a large number of protrusions over a wide area.

  Further, in the method of making the interval between adjacent protrusions constant throughout the vibration electrode plate or the counter electrode plate, and adjusting the interval between adjacent protrusions to an appropriate value, the spring property of the vibration electrode plate varies, If the capillary force of water that has entered between the vibrating electrode plate and the counter electrode plate is different, a local stick or a whole stick may occur. On the other hand, in the acoustic sensor of the present invention, since the stick of the vibration electrode plate is reduced by changing the interval between adjacent protrusions according to the protrusion formation region, the spring property of the vibration electrode plate varies, Even if the capillary force of moisture entering between the vibrating electrode plate and the counter electrode plate is different, the local stick and the whole stick are less likely to occur. Therefore, the allowable range of the design value of the interval between adjacent protrusions is widened, the characteristics of the acoustic sensor are stabilized, and the design and manufacture of the acoustic sensor are facilitated.

  Since the local stick is likely to occur in a highly flexible area of the vibrating electrode plate, the acoustic sensor of the present invention can reduce the local stick by relatively reducing the interval between adjacent protrusions in the area. Further, by increasing the interval between the adjacent protrusions in the low flexibility region of the vibration electrode plate, the entire stick can be reduced while suppressing the local stick. Further, in the acoustic sensor of the present invention, the interval between the projections is reduced only in the region where the vibration electrode plate is highly flexible, so that the number of projections can be reduced as a whole. If the number of protrusions is reduced, the air flow between the vibrating electrode plate and the counter electrode plate is less likely to be obstructed, so that air damping is reduced and the frequency characteristics (especially the characteristics on the high frequency side) of the acoustic sensor are flattened. Thus, the frequency band is widened.

  In one embodiment of the acoustic sensor according to the present invention, the vibrating electrode plate is fixed to the substrate along the outer peripheral edge of the movable part, and the central part of the movable part or the central part of the counter electrode plate is fixed. The interval between the adjacent projections in the region facing each other is smaller than the interval between the adjacent projections in the outer peripheral portion of the movable part or the region facing the outer peripheral portion of the counter electrode plate.

  In the acoustic sensor (the above embodiment) in which the vibration electrode plate is fixed to the substrate along the outer peripheral edge of the movable part, a local stick is likely to occur at the center of the vibration electrode plate. A local stick can be reduced by making the space | interval of adjacent protrusions in the area | region facing the said center part of a board comparatively small. Further, by relatively increasing the interval between adjacent protrusions in the outer peripheral portion of the movable portion or the region facing the outer peripheral portion of the counter electrode plate, the entire stick can be reduced while suppressing local sticks. .

  In another embodiment of the acoustic sensor according to the present invention, the movable part of the vibration electrode plate is formed in a disc shape, and when the radius of the movable part is R, the center of the vibration electrode plate, or An interval between adjacent projections in a region having a radius of R / 8 or more and R / 2 or less centered on a position facing the center of the counter electrode plate is a distance between adjacent projections in a region outside the region. It is smaller than the interval. In the region where the radius r is (1/2) R or more from the center of the vibrating electrode plate, the symmetry of the elastic deflection of the vibrating electrode plate is broken. Risk of sticking. In addition, since the symmetry of the elastic deflection of the vibrating electrode plate is maintained even in the region where the radius r is outside (1/8) R from the center, the radius r is more than (1/8) R from the center. This is because if the distance between the protrusions is shortened only on the inner side, a local stick may be formed on the outer side.

  In still another embodiment of the acoustic sensor according to the present invention, the outer peripheral portion of the movable portion of the vibration electrode plate is partially fixed to the substrate at a plurality of locations, and the fixed portions of the vibration electrode plate are opposed to each other or opposed to each other. An interval between adjacent projections in a region located in the middle of the portions facing the fixed portion of the electrode plate is smaller than an interval between adjacent projections in other projection forming regions.

  In still another embodiment of the acoustic sensor according to the present invention, a local stick is likely to occur in a region located between the fixed portions of the vibration electrode plate or between the portions facing the fixed portion of the counter electrode plate. The local stick can be reduced by relatively reducing the interval between adjacent protrusions in the region. In addition, in other projection formation regions where local sticks are unlikely to occur, the entire stick of the vibrating electrode plate can be reduced by increasing the interval between adjacent projections.

  In still another embodiment of the acoustic sensor according to the present invention, the protrusions are arranged along a plurality of concentric circles or a plurality of polygons having different sizes. Since the distribution of the deflection of the vibrating electrode plate is often concentric or concentric polygon, avoiding sticking of the vibrating electrode plate evenly and efficiently by arranging the protrusions along the concentric circle or polygon. can do.

  In another embodiment of the acoustic sensor according to the present invention, the counter electrode plate has a plurality of acoustic holes for allowing sound pressure to pass therethrough, and each of the protrusions is formed at a central portion of the region surrounded by the acoustic holes. Is arranged. According to such an embodiment, the acoustic hole and the protrusion can be separated as much as possible, so that the acoustic hole and the protrusion can be easily manufactured.

  In another embodiment of the acoustic sensor according to the present invention, the counter electrode plate has a plurality of acoustic holes for allowing sound pressure to pass therethrough, and each of the protrusions is separated from the center of the region surrounded by the acoustic holes. It is arranged at the position. According to this embodiment, since the projection is provided at a position close to the acoustic hole, moisture that has entered between the vibrating electrode plate and the counter electrode plate is less likely to remain at the position of the projection. Therefore, the vibrating electrode plate is less likely to stick to the counter electrode plate due to the capillary force of moisture, and the stick of the vibrating electrode plate is reduced.

  The means for solving the above-described problems in the present invention has a feature in which the above-described constituent elements are appropriately combined, and the present invention enables many variations by combining such constituent elements. .

FIG. 1 is a schematic cross-sectional view showing a state in which a vibrating electrode plate sticks to a counter electrode plate in a conventional acoustic sensor. FIG. 2A and FIG. 2B are diagrams for explaining the cause of the occurrence of a stick in a conventional acoustic sensor. FIG. 3 is a schematic cross-sectional view showing a counter electrode plate and a vibrating electrode plate provided with protrusions for stick prevention. 4A shows a case where the interval between the projections is too short, FIG. 4B shows a case where the interval between the projections is appropriate, and FIG. 4C shows the interval between the projections. It is a figure showing the case where it is too long. FIG. 5 is a perspective view showing the acoustic sensor according to the first embodiment of the present invention. FIG. 6 is an exploded perspective view of the acoustic sensor according to the first embodiment. FIG. 7 is a cross-sectional view taken along line YY of FIG. FIG. 8 is a diagram illustrating a positional relationship between the vibration electrode plate, the acoustic hole, and the protrusion when viewed from a direction perpendicular to the vibration electrode plate. FIG. 9 is a diagram showing the distribution of the degree of flexibility of the vibrating electrode plate in which the fixing portions at the four corners are fixed to the silicon substrate. FIG. 10 is an explanatory diagram illustrating the operation of the acoustic sensor according to the first embodiment, and illustrates a vertical cross section in the diagonal direction of the vibrating electrode plate. FIG. 11 is a view showing a vibrating electrode plate and a counter electrode plate for comparison, and shows a vertical cross section in the diagonal direction of the vibrating electrode plate. 12A is a schematic cross-sectional view showing a state where a local stick is raised at the center of the vibrating electrode plate, and FIG. 12B is a schematic cross-sectional view showing a state where the local stick is raised at the end of the vibrating electrode plate. It is. FIG. 13 is a diagram for explaining how to determine the interval between the protrusions at the center of the vibration electrode plate in the acoustic sensor and how to determine the interval between the protrusions in a region other than the central portion of the vibration electrode plate. FIG. 14 shows the shape of the vibration electrode plate used in the acoustic sensor according to the second embodiment of the present invention, and the positions of the vibration electrode plate, the acoustic holes, and the protrusions when viewed from the direction perpendicular to the vibration electrode plate. It is a figure showing the relationship. FIG. 15 is a diagram showing the distribution of flexibility of the disc-shaped vibrating electrode plate whose outer peripheral portion is fixed to the silicon substrate. FIG. 16 is a diagram illustrating another positional relationship between the vibrating electrode plate, the acoustic hole, and the protrusion in the second embodiment. FIG. 17 is a diagram illustrating still another positional relationship between the vibrating electrode plate, the acoustic hole, and the protrusion in the second embodiment. FIG. 18 is a diagram illustrating the positional relationship between the vibration electrode plate according to the third embodiment, the acoustic holes, and the protrusions when viewed from a direction perpendicular to the vibration electrode plate. FIG. 19 is an enlarged view showing a part of the counter electrode plate in the third embodiment. FIG. 20 is a partially enlarged cross-sectional view showing a state in which the moisture that has entered the minute gap has partially evaporated in the acoustic sensor of the third embodiment. FIG. 21 is an enlarged view showing a part of the counter electrode plate in the first and second embodiments. FIG. 22 is a partially enlarged cross-sectional view showing a state in which the moisture that has entered the minute gap has partially evaporated in the acoustic sensors of the first and second embodiments. FIG. 23 is an enlarged view showing a part of the counter electrode plate in a modification of the third embodiment. FIG. 24 is a partially enlarged cross-sectional view showing a state in which the moisture that has entered the minute gap has partially evaporated in the acoustic sensor according to the modification of the third embodiment. FIG. 25 is a schematic cross-sectional view showing still another embodiment of the acoustic sensor of the present invention. FIG. 26 is a schematic cross-sectional view showing still another embodiment of the acoustic sensor of the present invention. FIG. 27 is a schematic cross-sectional view showing still another embodiment of the acoustic sensor of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments, and various design changes can be made without departing from the gist of the present invention. In particular, the numerical values described below represent rough numerical values such as the dimensions of each member, and the acoustic sensor of the present invention is not limited to these numerical values.

(Embodiment 1)
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 5 is a perspective view showing the acoustic sensor 21 according to the first embodiment, and FIG. 6 is an exploded perspective view thereof. FIG. 7 is a sectional view taken along line YY in FIG.

  The acoustic sensor 21 is a capacitance type sensor, and a vibration electrode plate 24 is provided on the upper surface of a silicon substrate 22 via an insulating film 23, and a counter electrode plate 25 is provided thereon via a minute gap (gap). It is a thing.

  The silicon substrate 22 is provided with a prismatic through hole 26 or a truncated pyramid-shaped recess. In the figure, a prismatic through hole 26 is shown. The size of the silicon substrate 22 is 1 to 1.5 mm square (can be made smaller than this) in plan view, and the thickness of the silicon substrate 22 is about 400 to 500 μm. An insulating film 23 made of an oxide film or the like is formed on the upper surface of the silicon substrate 22.

  The vibrating electrode plate 24 is formed of a polysilicon thin film having a thickness of about 1 μm. The vibrating electrode plate 24 is a substantially rectangular thin film, and fixed portions 27 extend outward in the diagonal direction at the four corners. The vibrating electrode plate 24 is disposed on the upper surface of the silicon substrate 22 so as to cover the upper surface opening of the through hole 26 or the recess, and each fixing portion 27 is fixed on the insulating coating 23. A portion of the vibrating electrode plate 24 supported in the air above the through-hole 26 or the recess (in this embodiment, a portion other than the fixed portion 27) is a diaphragm 28 (movable portion), and is sensitive to sound pressure. Then vibrate.

  The counter electrode plate 25 is provided with a fixed electrode 30 made of a metal thin film on the upper surface of an insulating support layer 29 made of a nitride film. The counter electrode plate 25 is disposed on the vibration electrode plate 24 and is fixed to the upper surface of the silicon substrate 22 with an insulating film 33 made of an oxide film or the like outside the region facing the diaphragm 28. The counter electrode plate 25 covers the diaphragm 28 with a minute gap of about 3 μm in a region facing the diaphragm 28. The fixed electrode 30 and the support layer 29 have a plurality of acoustic holes (acoustic holes) 31 through which sound pressure (vibration) passes so as to penetrate from the upper surface to the lower surface. At the end of the counter electrode plate 25, an electrode pad 32 that is electrically connected to the fixed electrode 30 is provided. Since the vibrating electrode plate 24 vibrates in resonance with the sound pressure, it is a thin film of about 1 μm. However, since the counter electrode plate 25 is an electrode that is not excited by the sound pressure, its thickness is, for example, It is as thick as 2 μm or more.

  In a region of the counter electrode plate 25 facing the vibration electrode plate 24, a plurality of protrusions 36 are provided so as to prevent the vibration electrode plate 24 from coming into close contact with the counter electrode plate 25. The protrusion 36 is desirably as thin as possible and has a small tip area, and preferably has a diameter of 10 μm or less. However, in order to make the protrusion 36 thin, there is a manufacturing limit. Therefore, it is desirable to provide the protrusion 36 having a protrusion length of about 1 μm and a diameter of about 4 μm.

  An extended portion 27 a extending from the fixed portion 27 is exposed from the opening 34 formed in the support layer 29, and the electrode pad 35 provided on the upper surface of the end portion of the support layer 29 has an opening. 34 is connected to the extending portion 27a. Therefore, the vibrating electrode plate 24 and the counter electrode plate 25 are electrically insulated, and the vibrating electrode plate 24 and the fixed electrode 30 constitute a capacitor.

  Therefore, in the acoustic sensor 21 of the first embodiment, when acoustic vibration (air density wave) enters from the upper surface side, the acoustic vibration passes through the acoustic hole 31 of the counter electrode plate 25 and the diaphragm 28. The diaphragm 28 is vibrated. When the diaphragm 28 vibrates, the distance between the diaphragm 28 and the counter electrode plate 25 changes, whereby the capacitance between the diaphragm 28 and the fixed electrode 30 changes. Therefore, if a DC voltage is applied between the electrode pads 32 and 35 and the change in capacitance is extracted as an electrical signal, the vibration of the sound is converted into an electrical signal and detected. Can do.

  The acoustic sensor 21 is manufactured using a micromachining (semiconductor microfabrication) technique, but the manufacturing method is a known technique, and thus the description thereof is omitted.

  Next, the arrangement of the protrusions 36 provided on the counter electrode plate 25 will be described. FIG. 8 is a view showing the positional relationship between the vibration electrode plate 24, the acoustic hole 31 and the protrusion 36 when viewed from the direction perpendicular to the vibration electrode plate 24. The acoustic hole 31 is represented by a white circle, and the protrusion 36 is shown in FIG. Is represented by a black circle. The acoustic holes 31 are arranged in a lattice pattern at equal intervals throughout.

  On the other hand, the protrusions 36 are arrayed along polygons having similar shapes that are arranged concentrically from the central portion to the outer side (in FIG. 8, an octagon indicated by a broken line). Is arranged in the center of the region surrounded by the four acoustic holes 31.

  Further, the interval between the protrusions 36 is relatively short in a region facing the central portion a surrounded by the one-dot chain line of the diaphragm 28 and the central portion b of each side, and is relatively long in other regions. Yes. For example, in the example shown in FIG. 8, the length L of one side of the diaphragm 28 is 800 μm, and the interval between the projections 36 is 50 μm in the region facing the central part a of the diaphragm 28 and the central part b of each side. In the area c, it is 100 μm.

  FIG. 9 is a diagram showing, in a divided manner, the magnitude of bending when a uniform pressure is applied to the entire diaphragm 28 in the rectangular vibrating electrode plate 24 in which the four fixing portions 27 are fixed to the silicon substrate 22. In this case, the larger the hatched dot density, the larger the deflection, and the smaller the dot density, the smaller the deflection. As can be seen from FIG. 9, the vibrating electrode plate 24 becomes less flexible and less bent as it goes from the center to the outside, and the deflection is smaller than the surroundings at the center a and the center b of each side. It is getting bigger.

  Therefore, in this acoustic sensor 21, as schematically shown in FIG. 10, the distance between the protrusions 36 is large in the region where the vibrating electrode plate 24 is flexible and has a large deflection center portion a or the central portion b of each side. In a region where the vibrating electrode plate 24 is opposed to the region c where the vibration electrode plate 24 has relatively high rigidity and small deflection, the interval between the protrusions 36 is large. As a result, it is possible to reduce the local stick and the whole stick described in the prior art. The reason for this will be described below.

  As described in the conventional example, in the flexible portion (center portion) of the vibration electrode plate, the vibration electrode plate is likely to fall between the protrusions and cause a local stick that contacts the counter electrode plate. On the other hand, in this acoustic sensor 21, since the space | interval of protrusion 36 is made small in the area | region which opposes the center part a of the diaphragm 28, and the center part b of the side, it becomes difficult to raise | generate a local stick. In addition, when the distance between the protrusions is uniform as a whole, if the distance between the protrusions is small, the vibrating electrode plate is likely to stick to almost the entire protrusion and easily cause the entire stick. On the other hand, in this acoustic sensor 21, since the interval between the protrusions 36 is increased except in a region where local sticking is likely to occur, the number of protrusions 36 (that is, the total area of the end surfaces of the protrusions 36) can be reduced, The stick can be reduced. Therefore, the local stick and the whole stick can be effectively reduced.

  More specifically, the distance between the protrusions 36 in the region facing the central part a of the vibrating electrode plate 24 and the central part b of each side is smaller than a limit value D3 at which a local stick occurs in the softest part of the protrusion 36. ing. However, if the distance between the protrusions 36 in the region facing the central portions a and b is too small, the vibrating electrode plate 24 sticks to the protrusions 36 in the entire central portions a and b as shown in FIG. Therefore, the interval between the protrusions 36 in the region facing the central part a of the vibrating electrode plate 24 and the central part b of each side is smaller than the limit value D3 at which the vibrating electrode plate 24 locally sticks at the central part a or b. In addition, it must be larger than a limit value D1 at which the vibrating electrode plate 24 sticks to the entire protrusion 36 in a region facing the central portion a or b.

  Further, the distance between the protrusions 36 in the region facing the region c other than the central portions a and b of the vibration electrode plate 24 is larger than the limit value D2 at which the vibration electrode plate 24 causes the entire stick. However, if the distance between the protrusions 36 in the region facing the region c other than the central portions a and b is too large, the vibrating electrode plate 24 is moved in the region c other than the central portions a and b as shown in FIG. It falls between the protrusions 36 to cause a local stick. Therefore, the interval between the protrusions 36 in the region facing the region c other than the central portions a and b of the vibration electrode plate 24 is larger than the limit value D2 at which the vibration electrode plate 24 causes the entire stick, and the vibration electrode plate 24 must be smaller than a limit value D4 at which the vibrating electrode plate 24 causes a local stick in a region facing the region c other than the central portions a and b.

  Here, the limit value D3 at which the central portions a and b of the vibrating electrode plate 24 cause a local stick as shown in FIG. 12A and the region other than the central portions a and b as shown in FIG. The limit value D4 for causing the stick is compared. As shown in FIG. 9, the central portions a and b are portions where the vibrating electrode plate 24 is soft and easily deformed. Therefore, the central portions a and b are more likely to be deformed than the local sticks at the central portions a and b as shown in FIG. The local stick in the region c as shown in (b) is less likely to occur. Therefore, generally, the vibrating electrode plate 24 locally sticks in the region c other than the central portions a and b than the limit value D3 of the distance between the protrusions 36 when the vibrating electrode plate 24 locally sticks at the central portion a or b. The limit value D4 of the interval between the 36 times becomes larger.

  The limit value D1 of the interval between the projections 36 when the vibrating electrode plate 24 sticks only to the entire projection 36 in the region facing the central portions a and b is the distance between the projections 36 when the entire projection 36 is stuck. It becomes smaller than the limit value D2 of the interval.

  Therefore, the magnitudes of the four limit values are D1 <D2 <D3 <D4, and the distribution of the intervals between the protrusions 36 in the acoustic sensor 21 is as shown in FIG.

  When the intervals between the projections are made uniform as in the conventional acoustic sensor, the interval between the projections must be adjusted to be an appropriate interval that is larger than D2 and smaller than D3. Since the adjustment range is narrow, it is difficult to manufacture an acoustic sensor. On the other hand, in the case of the acoustic sensor 21 of the first embodiment, the distance between the protrusions 36 is larger than D1 and larger than D3 in the region facing the central portions a and b of the vibrating electrode plate 24. Just make it smaller. Moreover, what is necessary is just to make the distance of protrusion 36 larger than D2 and smaller than D4 in the area | region which opposes area | region c other than center part a and b. Therefore, the allowable range is wide in any of the central portions a and b and the region c.

  Therefore, according to the acoustic sensor 21 of the first embodiment, the stick of the vibrating electrode plate 24 can be easily reduced, and the acoustic sensor 21 can be easily manufactured. Further, the acoustic sensor 21 can suppress the local stick and the whole stick even if the spring property of the vibrating electrode plate 24 is varied, the capillary force of the infiltrated water is different, or the force between the surfaces is varied. The reliability of the sensor 21 is improved.

  Further, since the distribution of the deflection of the vibrating electrode plate 24 is often a concentric circle or a concentric polygon, if the protrusions 36 are arranged along the concentric polygon as described above (see FIG. 8), The stick of the vibrating electrode plate 24 can be avoided evenly and efficiently.

  Furthermore, according to the acoustic sensor 21, the number of the protrusions 36 can be reduced as compared with the case where the distance between the protrusions is made uniform. Therefore, the air flow in the minute gap between the vibrating electrode plate 24 and the counter electrode plate 25 is not easily obstructed by the protrusions 36, and air damping of the vibrating electrode plate 24 is reduced. As a result, the frequency characteristics (particularly, characteristics on the high frequency side) of the acoustic sensor 21 are flattened, and the frequency band is widened.

(Embodiment 2)
Next, the acoustic sensor according to the second embodiment will be described with reference to FIGS. Since the structure of the acoustic sensor according to the second embodiment is substantially the same as the structure of the acoustic sensor 21 according to the first embodiment, the entire structure and description are omitted.

  The main differences between the acoustic sensor of the second embodiment and the first acoustic sensor are the shape of the vibrating electrode plate 24 and the arrangement of the protrusions 36. Therefore, these differences will be described.

  FIG. 14 is a diagram showing the positional relationship between the vibrating electrode plate 24, the acoustic hole 31, and the protrusion 36 according to the second embodiment when viewed from a direction perpendicular to the vibrating electrode plate 24. The vibration electrode plate 24 has a disc shape, and the silicon substrate 22 is provided with a cylindrical through hole or a truncated cone-shaped recess according to the shape of the vibration electrode plate 24. The vibration electrode plate 24 is disposed so as to cover the through hole of the silicon substrate 22 or the upper surface opening of the recess, and substantially the entire outer peripheral portion is fixed to the silicon substrate 22 by the fixing portion 27.

  In the counter electrode plate 25 facing the vibration electrode plate 24, the acoustic holes 31 are arranged in a triangular shape or a hexagonal shape at regular intervals. In addition, a plurality of protrusions 36 protrude from the surface of the counter electrode plate 25 facing the vibration electrode plate 24 substantially at the center of the region surrounded by the acoustic holes 31. The protrusions 36 have a relatively small interval between the protrusions 36 in the circular central portion a concentric with the outer peripheral edge of the vibrating electrode plate 24, and the interval between the protrusions 36 in the region c outside the central portion a. Is relatively large.

Here, the radius r of the circular region a in which the interval between the protrusions 36 is reduced is set so that the radius of the vibrating electrode plate 24 is R.
(1/8) R ≦ r ≦ (1/2) R
It is said. FIG. 15 is a diagram showing, in a sectioned manner, the magnitude of bending when a uniform pressure is applied to the entire diaphragm 28 in the circular vibrating electrode plate 24. As can be seen from FIG. 15, the vibration electrode plate 24 is less flexible and less bent as it goes from the center to the outer side, and the deflection is greatest at the central portion a. In the region where the radius r is equal to or greater than (1/2) R from the center, the symmetry of the elastic deformation of the diaphragm 28 is broken, so that a local stick is unlikely to occur. Then there is a risk of causing the whole stick. In addition, since the symmetry of elastic deformation of the diaphragm 28 is maintained even in a region where the radius r is outside (1/8) R from the center, the radius r is inside from (1/8) R from the center. However, if the distance between the protrusions 36 is not reduced, a local stick may be formed just outside the protrusion 36. Therefore, the region where the interval between the protrusions 36 is kept small is
(1/8) R ≦ r ≦ (1/2) R
It is desirable to set the region a within a circle having a radius r as shown in FIG.

  In the acoustic sensor of the second embodiment, for example, the vibration electrode plate 24 has a film thickness of 1 μm, the counter electrode plate 25 has a thickness of 2 μm, the minute gap between the vibration electrode plate 24 and the counter electrode plate 25 is 3 μm, and the projection The height of 36 is 1 μm. The protrusion 36 has a diameter of 10 μm or less and is preferably as thin as possible. However, since there are limitations in the manufacturing process, the diameter is preferably about 4 μm. Further, if the radius R of the vibrating electrode plate 24 is 500 μm, the interval between the projections 36 is 50 μm inside the circular central portion “a”, and the interval between the projections 36 is 100 μm in the other region “c”.

  In the second embodiment, since the vibrating electrode plate 24 has a circular shape, there is no region corresponding to the central portion b of the first embodiment, but the interval between the protrusions 36 is reduced at the central portion a, In the other region c, the same effect as that of the first embodiment can be obtained by increasing the interval between the protrusions 36. That is, also in the second embodiment, the local stick and the whole stick can be further reduced, and the reliability of the acoustic sensor can be improved. In addition, the distance between the protrusions 36 is reduced in the highly flexible region of the vibration electrode plate 24, and the distance between the protrusions 36 is increased in the low flexibility region of the vibration electrode plate 24. It is possible to widen an appropriate range of the interval between the two (see FIG. 13), and it is possible to easily design and manufacture the acoustic sensor. Furthermore, even if the spring property of the vibrating electrode plate 24 varies or the capillary force of the infiltrated liquid is different, local sticks and whole sticks hardly occur, and the reliability of the acoustic sensor can be further improved. Further, since the number of protrusions 36 can be reduced, air damping of the vibrating electrode plate 24 can be reduced, the frequency characteristics (particularly, characteristics on the high frequency side) of the acoustic sensor 21 can be flattened, and the frequency band can be widened.

  Note that the protrusions 36 may be arranged along a circle concentric with the vibrating electrode plate 24 as shown in FIG. Or as shown in FIG. 17, you may arrange | position to the vertex of the equilateral triangle arranged without a gap. Since the distribution of the deflection of the vibrating electrode plate 24 is often concentric or polygonal, if the protrusions 36 are arranged concentrically or equilaterally, the stick of the vibrating electrode plate 24 can be avoided evenly and efficiently. can do.

(Third embodiment)
FIG. 18 is a diagram showing the positional relationship between the vibration electrode plate 24 according to the third embodiment, the acoustic holes 31 and the protrusions 36 when viewed from the direction perpendicular to the vibration electrode plate 24. FIG. 19 is an enlarged view showing a part of the counter electrode plate 25 in the third embodiment. In this embodiment, the protrusion 36 is provided close to or in contact with the acoustic hole 31.

  In the first and second embodiments, as shown in FIG. 21, a protrusion 36 is provided in the center of the region surrounded by the acoustic hole 31. Therefore, the protrusion 36 is at a position far from any acoustic hole 31. Therefore, when the moisture 37 that has entered the minute gap between the vibrating electrode plate 24 and the counter electrode plate 25 evaporates and exits from the acoustic hole 31, the moisture 37 finally reaches the position of the protrusion 36 as shown in FIG. It remains until. Since the distance between the gaps between the counter electrode plate 25 and the vibration electrode plate 24 is the shortest at the position of the protrusion 36, if moisture 37 remains, the distance between the vibration electrode plate 24 and the counter electrode plate 25 is left. A large capillary force works until the end, and the vibrating electrode plate 24 becomes difficult to be separated from the counter electrode plate 25.

  On the other hand, as shown in FIGS. 18 and 19, when the protrusion 36 is removed from the center of the region surrounded by the acoustic hole 31 and is located near the acoustic hole 31, the moisture 37 that has entered the minute gap is absorbed by the acoustic hole. When evaporating from 31, the moisture 37 evaporates earliest at the position of the protrusion 36, as shown in FIG. 20. Therefore, there is no protrusion 36 at the place where the moisture 37 is finally dried, and the capillary force acting between the vibrating electrode plate 24 and the counter electrode plate 25 is reduced early, and the vibrating electrode plate 24 is easily separated from the counter electrode plate 25. Become.

  FIG. 23 and FIG. 24 show a modification of the third embodiment, in which the protrusion 36 overlaps the position of the acoustic hole 31. If the position of the projection 36 and the position of the acoustic hole 31 overlap, if the acoustic hole 31 is provided in the counter electrode plate 25 after the formation of the projection 36, the acoustic hole 31 is opened by etching and at the same time the projection 36 is formed. Some are also etched away. Therefore, the area of the end face of the protrusion 36 can be made smaller than the processing limit of the protrusion 36, and the effect of reducing the local stick and the entire stick becomes higher.

(Other forms)
FIG. 25 is a schematic sectional view showing still another embodiment of the acoustic sensor of the present invention. In the first to third embodiments, the protrusions 36 are provided on the counter electrode plate 25, but in this embodiment, the protrusions 36 are provided on the vibration electrode plate 24. According to such an embodiment, a local stick in which the central portion of the vibrating electrode plate 24 is bent and a portion between the protrusions 36 sticks to the counter electrode plate 25, or an entire stick in which almost the whole of the protrusion 36 sticks to the counter electrode plate 25 Can be prevented.

  In the first to third embodiments, the vibration electrode plate 24 is provided on the silicon substrate 22 and the upper portion thereof is covered with the counter electrode plate 25. However, as shown in FIGS. The counter electrode plate 25 may be provided on the upper side, and the vibration electrode plate 24 may be provided thereon. In FIG. 26, the protrusions 36 are provided on the counter electrode plate 25, and in FIG. 27, the protrusions 36 are provided on the vibration electrode plate 24.

DESCRIPTION OF SYMBOLS 21 Acoustic sensor 22 Silicon substrate 23 Insulation film 24 Vibrating electrode plate 25 Counter electrode plate 26 Through-hole 27 Fixing part 28 Diaphragm 31 Acoustic hole 36 Protrusion 37 Water | moisture content

Claims (7)

A vibrating electrode plate fixed to the substrate and sensitive to sound pressure;
A counter electrode plate fixed to the substrate and opposed to the vibrating electrode plate via a gap;
In an acoustic sensor having
A plurality of protrusions are provided on the surface of the vibration electrode plate or the counter electrode plate on the gap side,
Change the interval between adjacent protrusions according to the protrusion formation region in the vibrating electrode plate or the counter electrode plate,
In the vibration electrode plate or the electrode plate on the side of the counter electrode plate on which the protrusion is provided, in the highly flexible region of the vibration electrode plate or in the counter region of the counter electrode plate facing the highly flexible region An interval between adjacent protrusions is smaller than an interval between adjacent protrusions in a region where the vibration electrode plate is low in flexibility or in a counter region of the counter electrode plate facing the low flexibility region. Acoustic sensor. Fixing the vibrating electrode plate to the substrate along the outer periphery of the movable part;
A region in which the interval between adjacent projections in the central part of the movable part or in the region facing the central part of the counter electrode plate faces the outer peripheral part of the movable part or the outer peripheral part of the counter electrode plate The acoustic sensor according to claim 1, wherein the acoustic sensor is smaller than an interval between adjacent protrusions in the case. The movable part of the vibrating electrode plate is formed in a disc shape,
When the radius of the movable part is R, the adjacent protrusions in the region of radius R / 8 or more and R / 2 or less centered on the center of the vibrating electrode plate or the position facing the center of the counter electrode plate The acoustic sensor according to claim 2, wherein an interval between the projections is smaller than an interval between the adjacent protrusions in a region outside the region. The outer peripheral part of the movable part of the vibrating electrode plate is partially fixed to the substrate at a plurality of locations,
The interval between the adjacent projections in the region located between the fixed portions of the vibrating electrode plate or in the region between the portions facing the fixed portion of the counter electrode plate is equal to the adjacent projections in the other projection forming regions. The acoustic sensor according to claim 1, wherein the acoustic sensor is smaller than the interval of.   The acoustic sensor according to claim 1, wherein the protrusions are arranged along a plurality of concentric circles or a plurality of polygons. The counter electrode plate has a plurality of acoustic holes for passing sound pressure,
The acoustic sensor according to claim 1, wherein each of the protrusions is disposed at a central portion of a region surrounded by the acoustic hole. The counter electrode plate has a plurality of acoustic holes for passing sound pressure,
2. The acoustic sensor according to claim 1, wherein each of the protrusions is disposed at a position deviated from a center of a region surrounded by the acoustic hole.

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