JP2007101222A - Pressure sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a capacitance type pressure sensor having high measurement accuracy. <P>SOLUTION: The pressure sensor 1 includes a capacitor constituted of the first electrode 5 formed on a lower side substrate 2 and the second electrode 10 formed on the under surface of a diaphragm 9 on an upper side substrate 3, and a vacuum cavity 18 is demarcated between the first and second electrodes. A dielectric film 8 having a section shape inclined moderately upward from a center part 8a toward the outside and furthermore inclined moderately downward toward the outside over a top part 8b is laminated on the second electrode 10. Though the contact area between the second electrode and the dielectric film is proportional to the square of the distance (radius) from the center part to the fringe of the contact face, since the film thickness of the dielectric film is increased moderately from the center part toward the top part and then decreased moderately over the top part, linearity of a capacitance of the capacitor with respect to an external pressure can be secured. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a capacitance-type pressure sensor that measures pressure by using a change in capacitance corresponding to the amount of diaphragm deflection caused by pressure.

  2. Description of the Related Art Conventionally, a capacitance type pressure sensor is known in which a diaphragm deformed by an applied pressure and a fixed electrode are opposed to each other with a gap, and a pressure is measured from a change in capacitance between the diaphragm and the electrode. . In particular, the capacitive pressure sensor called the touch mode type exhibits excellent linearity in the output characteristics of the capacitance with respect to the applied pressure and high pressure resistance. Use for is proposed.

  Generally, a touch mode type pressure sensor is formed by forming a diaphragm on a silicon substrate and doping it with impurities such as boron to form a movable electrode, and forming a fixed electrode on the glass substrate on the glass substrate, and a dielectric film thereon And the gap between the diaphragm and the dielectric film is sealed in a vacuum (see, for example, Patent Documents 1 and 2). FIG. 7 shows changes in capacitance with applied pressure in a conventional touch mode type pressure sensor. In the non-contact area A before the diaphragm contacts the dielectric film, the capacitance hardly changes. In the initial contact region B where the diaphragm starts to contact the dielectric film at the pressure P0, the capacitance increases rapidly. Thereafter, in the region C between the pressures P1 and P2, the capacitance changes approximately linearly with respect to the pressure. When the pressure P2 is exceeded, the capacitance saturates and does not increase any further. In general, the linear region C is used as a measurable pressure range. The linearity of the output characteristics is related to the shape of the diaphragm, and it is said that the thickness of the diaphragm and the width of the gap need to be optimized (see, for example, Non-Patent Document 1).

  In addition, in order to widen the linear region of the capacitance with respect to pressure and widen the measurement range, the electrode formed on the glass substrate has a shape in which the longitudinal dimension gradually increases from the center in the width direction to the width direction, and the diaphragm There has been proposed a pressure sensor in which the rate of increase of the area in contact with the electrode is constant with respect to the pressure increase (see, for example, Patent Document 3). In addition, a pressure sensor (see, for example, Patent Document 4) that realizes high sensitivity with low capacitance by providing a notch in the portion of the electrode facing the initial contact region of the diaphragm to reduce stray capacitance that does not contribute to measurement. A pressure sensor (see, for example, Patent Document 5) is known in which a divided region is provided in an electrode facing a diaphragm to change the electrode area of a capacitor so that the sensor output can be adjusted.

  In addition, in order to guarantee the long-term stability of the measurement accuracy, a groove is formed in the glass substrate and a conductive material is buried in this, and the lower electrode is formed at the same height as the surface of the glass substrate. There is known a pressure sensor that can perform anodic bonding with no gap (see, for example, Patent Document 6).

  Furthermore, a recess is formed on the surface of the base made of a conductive metal material so that the gap with the diaphragm is reduced outward from the center, and an electrode portion and an insulating layer are formed on the recess. As a result, when the diaphragm bends, the gap between the electrode and the electrode section will change stably at a constant rate of change, so that the capacitance will also change stably and stably, and the electrode contact area will be devised. However, pressure sensors that can eliminate the saturation region and widen the dynamic range have been developed (see, for example, Patent Document 7).

  In addition, a projecting portion toward the diaphragm is formed on the insulating layer covering the fixed electrode to increase the distance between the fixed electrode and the portion in contact with the insulating layer of the diaphragm, or a notch in the central portion of the fixed electrode A pressure sensor has been proposed in which the opposed area of the fixed electrode facing the diaphragm is reduced, the capacitance with respect to the measurement start pressure is lowered, and the amount of change within the measurement range is increased (for example, (See Patent Document 8).

Toshigai Yamamoto, “Touch mode capacitive pressure sensor”, Fujikura technique, October 2001, No. 101, p. 71-74 Japanese National Patent Publication No. 10-509241 Japanese Patent Laid-Open No. 2002-214058 JP 2002-195903 A JP 2002-221460 A JP 2002-221461 A JP 2004-191137 A Japanese Patent Laying-Open No. 2005-83801 JP 2005-233877 A

  However, the above-described conventional touch mode capacitive pressure sensor has the following problems. As is well known, the capacitance C between the movable electrode on the diaphragm side and the fixed electrode on the glass substrate side is such that the film thickness of the dielectric film is d, the dielectric constant is ε, the movable electrode and the dielectric film are When the contact area of S is S, it is expressed by C = ε · S / d. Although the film thickness d of the dielectric film is constant, the contact area S is proportional to the square of the distance (radius) from the center of the diaphragm that bends downward to the outer edge of the contact surface. For this reason, the pressure sensor described in Patent Document 1 and the like has a high measurement accuracy because the change in capacitance in the region C of FIG. 7 is not a complete straight line, but rather changes to draw an upwardly convex parabola. It is difficult to secure. In order to solve this problem, it is necessary to devise some means such as connecting a compensation circuit to the output of the pressure sensor.

  Further, as described in Patent Document 3 and the like, when the fixed-side electrode is formed in a deformed shape, it is difficult to align the bending position of the diaphragm and the electrode position. For this reason, the measurement accuracy may vary, and the yield may be reduced.

  In the pressure sensor described in Patent Document 7, since the base itself is a fixed electrode, it is difficult to ensure electrical insulation from the diaphragm, and thus a complicated structure may be required. . Further, the measurement pressure range can be expanded by eliminating the saturation region on the high pressure side, but the measurement pressure range on the low pressure side cannot be expanded. On the low pressure side, the thickness of the insulating layer in the initial contact state is thick, so there is a possibility that the measurement sensitivity is rather lowered. Similarly, since the pressure sensor described in Patent Document 8 reduces the capacitance change on the low pressure side, it is difficult to widen the measurement pressure range on the low pressure side.

  Further, in the conventional pressure sensor in which the diaphragm itself is a movable electrode, it is necessary to ensure electrical insulation at the joint between the electrode film wired on the surface of the glass substrate and the silicon substrate. Therefore, it is difficult to completely and airtightly join the glass substrate and the silicon substrate, and a structural device as described in Patent Document 6 is required. For this reason, the number of man-hours increases, the process becomes complicated, and the productivity may be reduced to increase the manufacturing cost.

  Therefore, the present invention has been made in view of the above-described conventional problems, and its object is to improve the linearity in the output characteristics of the capacitance related to pressure and to enable higher measurement accuracy. The object is to provide a capacitive pressure sensor. In particular, it is an object of the present invention to improve the linearity of capacitance on the low-pressure side and to set a wider pressure range that can be measured.

  In addition, the present invention can reliably seal the chamber defined between the diaphragm and the dielectric film in a vacuum without complicating the structure or process or increasing the number of steps, and can ensure stable measurement accuracy. It is to provide a touch mode type capacitive pressure sensor.

  According to the present invention, in order to achieve the above object, the lower substrate of the insulating material having the first electrode and the dielectric film laminated thereon on the upper surface, the diaphragm and the second electrode formed on the lower surface thereof are provided. A lower substrate and an upper substrate so that the dielectric film and the second electrode are opposed to each other with a slight gap and the chamber defined therebetween is sealed in a vacuum. Applied to the diaphragm by measuring the electrostatic capacity between the second electrode and the first electrode that are in contact with the dielectric film when the substrate is integrally and airtightly bonded and the diaphragm is bent under pressure. A cross section in which the dielectric film is gradually inclined upward from the center corresponding to the center position of the diaphragm deflection, and gradually inclined downward further beyond the apex. Shape A pressure sensor for are provided.

  As described above, the film thickness of the dielectric film gradually increases with respect to the distance (radius) from the center portion to the outer edge of the contact surface and gradually decreases beyond the apex portion, whereby the contact between the second electrode and the dielectric film is achieved. The area is proportional to the square of the distance (radius) from the center of the dielectric film to the outer edge of the contact surface, but it is effective to increase the capacitance with increasing pressure, compared to the conventional output characteristics that draw a parabola. And can be adjusted to output characteristics with high linearity.

  In one embodiment, the lower substrate and the upper substrate are formed of quartz, and the two substrates are joined together integrally and in an airtight manner along the entire periphery, for example using anodic bonding or metal bonding. Has been. There is no need to provide an insulating film between the lower and upper substrates as in the prior art, and the chamber between the diaphragm and the dielectric film can be surely sealed in a vacuum with a simple structure, so that high measurement accuracy is stably maintained. be able to.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
1A and 1B schematically show the configuration of a preferred embodiment of a pressure sensor according to the present invention. The pressure sensor 1 of the present embodiment includes a lower substrate 2 and an upper substrate 3 each made of a rectangular quartz thin plate. As shown in FIG. 2, the lower substrate 2 has a substantially square shallow recess 4 formed on the surface facing the upper substrate 3 by, for example, wet etching or machining. A substantially square first electrode 5 is formed on the flat bottom of the recess 4 and is connected to the extraction electrode 7 on the upper surface of the lower substrate 2 through a lead 6 drawn out from the side on the center side of the recess. Has been. On the first electrode 5, a dielectric film 8 having a cross-sectional shape to be described later is laminated.

  As shown in FIGS. 1A and 1B, the upper substrate 3 has a diaphragm 9 whose upper surface is thinned by forming a recess by etching or machining, for example. The diaphragm 9 can be formed thin or thick according to the required pressure sensitivity. In the case of the crystal of this embodiment, the diaphragm 9 can be thinned to a thickness of 10 μm or less, for example. As shown in FIG. 3, a second electrode 10 having a substantially square shape is formed on the lower surface of the diaphragm 9. The second electrode 10 and the first electrode 5 are disposed at opposing positions corresponding to each other with the dielectric film 8 interposed therebetween, and constitute a capacitor. The second electrode 10 is connected to the extraction electrode 12 via a lead 11 that is drawn with the lower surface of the upper substrate 3 directed toward the opposite end in the longitudinal direction.

  The first electrode 5 and the second electrode 10 are formed by depositing and patterning a conductive metal material such as an Al film or an Al alloy, for example, by vapor deposition or sputtering. The leads 6 and 11 and the extraction electrodes 7 and 12 are formed of, for example, a Cr / Au film or a Cr / Ni / Au film. Similarly, these conductive metal materials are formed by vapor deposition or sputtering and patterned. Is formed.

  Further, the upper substrate 3 has through holes 13 and 14 at positions corresponding to the extraction electrode 12 of the second electrode 10 of the upper substrate and positions corresponding to the extraction electrode 7 of the first electrode 5 of the lower substrate 2, respectively. Is provided. The through holes 13 and 14 of this embodiment are formed in a tapered shape from the upper surface and the lower surface toward the back by processing the upper substrate 3 made of crystal by wet etching. On the upper surface of the upper substrate 3, external electrodes 15 and 16 are formed on the peripheral edges of the through holes 13 and 14, respectively.

As shown in FIG. 4, the dielectric film 8 has a cross section that gradually inclines outward from the center 8a corresponding to the center position of the diaphragm 9 and gradually inclines further outward beyond the apex 8b. Has a shape. The central portion 8a is gently curved downward and the apex portion 8b is gently curved upward. Accordingly, the film thickness of the dielectric film 8 gradually increases from the central portion 8a toward the vertex portion 8b and gradually decreases beyond the vertex portion 8b. The dielectric film 8 can be formed, for example, by sputtering a glass material. In addition to glass, for example, an insulating material such as SiO ₂ or ceramics can be used for the dielectric film.

  The lower substrate 2 and the upper substrate 3 are integrally and airtightly bonded by the metal bonding portion 17 along the entire periphery. By providing metal joints with a predetermined width along the entire periphery on the opposing surfaces of the lower substrate and the upper substrate, and by thermocompression bonding or eutectic bonding, they can be easily and integrally joined together. it can. As a result, a cavity 18 is defined inside the pressure sensor 1 between the diaphragm 9 and the recess 4 of the lower substrate 2.

  The through holes 13 and 14 are filled with sealing materials 19 and 20 made of a conductive material, respectively. The sealing material is filled so that the through hole is hermetically sealed in a vacuum atmosphere after the lower substrate 2 and the upper substrate 3 are joined. Thereby, the cavity 18 is sealed in a vacuum. At the same time, the extraction electrodes 7, 12, that is, the first and second electrodes 5, 10 and the corresponding external electrodes 15, 16 are electrically connected to each other by the sealing materials 19, 20. As the sealing material, for example, AuSn, AuGe, a solder material, high-temperature solder, or the like can be used. Further, it is advantageous that the inner peripheral surface of each through-hole is previously coated with a metal film because the introduction of the sealing material is facilitated.

  5A to 5C show, in a stepwise manner, when the pressure sensor 1 is used, the contact surface between the second electrode 10 and the dielectric film 8 expands in response to the deflection of the diaphragm 9. . FIG. 5A shows an initial state in which the diaphragm 9 is bent by the external pressure and the second electrode 10 starts to contact the dielectric film 8. Since the dielectric film 8 has a concave shape from the apex portion 8b toward the center portion 8a as described above, the contact area between the second electrode 10 and the dielectric film 8 in this initial contact state is flat as in the prior art. And larger than the case of a dielectric film having a constant film thickness.

  When the external pressure is increased and the diaphragm 9 is further bent, the contact between the second electrode 10 and the dielectric film 8 along the upward inclined surface from the central portion 8a to the apex portion 8b as shown in FIG. 5B. Increases area. Since the apex portion 8b is curved upward, the increase in the contact area decreases when approaching the vicinity thereof. As shown in FIG. 5C, when the diaphragm 9 is further bent beyond the apex portion 8b, the dielectric film 8 is inclined downward to the outside, so that the increase in the contact area is further suppressed, and the limit is reached. Reach.

  FIG. 6 shows the relationship between the pressure and the capacitance in the pressure sensor 1 according to this embodiment with a solid line. In the figure, for comparison, the relationship between pressure and capacitance in a pressure sensor having a dielectric film with a flat and constant film thickness according to the prior art is indicated by a broken line. Similar to FIG. 7 described above in connection with the prior art, in FIG. 6, the pressure P0 is the pressure when the second electrode 10 starts to contact the dielectric film 8, and the pressure P1 is linear from the initial contact state. The pressure at which the relationship starts is the pressure P2 is the pressure at which the linear relationship ends and the saturation state begins. A region B between the pressures P0 P1 is an initial contact state, a region C between the pressures P1 P2 is a region corresponding to the linear relationship of the prior art, and a region D above the pressure P2 is a saturation region.

  The capacitance C of the capacitor formed by the first electrode 5 and the second electrode 10 is such that the film thickness of the dielectric film 8 is d, the dielectric constant of the dielectric film 8 is ε, and the second electrode 10 and the dielectric film. When the contact area with 8 is S, C = ε · S / d. The contact area S is proportional to the square of the distance (radius) from the central portion 8a to the outer edge of the contact surface, but the film thickness d of the dielectric film 8 is related to the distance (radius) from the central portion 8a to the outer edge of the contact surface. It gradually increases and gradually decreases beyond the apex 8b. As a result, in the region C of FIG. 6, the increase in the capacitance with respect to the increase in pressure is suppressed from the conventional output characteristic in which the parabola is drawn as shown by the broken line, and the output characteristic is adjusted to have high linearity as shown by the solid line. can do. For example, if it is assumed that the amount of deflection of the diaphragm 9 increases at a constant rate with respect to an increase in pressure, and the contact area between the second electrode 10 and the dielectric film 8 increases at a constant rate accordingly, The film thickness d of the dielectric film 8 may be determined so that the capacitance change amount ΔC = ε · ΔS / Δd is constant.

  Furthermore, in the present embodiment, the contact area between the second electrode 10 and the dielectric film 8 in the initial contact region B is larger than that in the prior art. Therefore, the capacitance in the initial contact region B, particularly the capacitance at the pressure P0 can be increased as compared with the conventional case. As a result, the contact initial region B can be adjusted to a linear shape continuous to the region C, and the pressure range that can be measured with high accuracy can be expanded to the low pressure side.

  Although the preferred embodiments of the present invention have been described in detail above, the present invention can be implemented by adding various modifications and changes to the above embodiments. For example, both or one of the upper and lower substrates of the above embodiment can be formed of glass materials such as Pyrex glass and soda glass, ceramic materials, and various other known insulating materials in addition to quartz. Moreover, when forming an upper board | substrate and a lower board | substrate with a different material, it is preferable to select so that those thermal expansion coefficients may mutually be equivalent or approximate.

(A) The figure is a top view which shows the Example of the pressure sensor by this invention, (B) The figure is sectional drawing in the II line | wire. The top view which shows the lower board | substrate of the pressure sensor of FIG. The bottom view which shows the upper side board | substrate of the pressure sensor of FIG. Sectional drawing which shows the fixed electrode and dielectric material film of a lower board | substrate. As for the operating state of the pressure sensor of this embodiment, (A) is an initial contact state, (B) is a state where the contact area is increased, and (C) is a cross-sectional view showing a saturated state. The diagram which shows the relationship between the pressure in the pressure sensor of a present Example, and an electrostatic capacitance. The diagram which shows the relationship between the pressure and the electrostatic capacitance in the conventional electrostatic capacitance type pressure sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Pressure sensor, 2 ... Lower side board, 3 ... Upper side board, 4 ... Recessed part, 5 ... 1st electrode, 6, 11 ... Lead, 7, 12 ... Extraction electrode, 8 ... Dielectric film, 8a ... Center part 8b ... apex, 9 ... diaphragm, 10 ... second electrode, 13, 14 ... through-hole, 15, 16 ... external electrode, 17 ... metal joint, 18 ... chamber, 19, 20 ... sealing material.

Claims (2)

A dielectric substrate comprising: a lower substrate of an insulating material having a first electrode and a dielectric film laminated thereon; and an upper substrate of an insulating material having a diaphragm and a second electrode formed on the lower surface thereof. And the second electrode are opposed to each other with a slight gap, and the lower substrate and the upper substrate are integrally and hermetically bonded so that a chamber defined between them is sealed in a vacuum. A pressure sensor for measuring the pressure by measuring a capacitance between the second electrode and the first electrode contacting the dielectric film when the diaphragm is bent under pressure. And
The pressure is characterized in that the dielectric film has a cross-sectional shape that gradually inclines outward from the center corresponding to the center position of the diaphragm deflection, and gradually inclines outward further beyond the apex. Sensor.   2. The pressure sensor according to claim 1, wherein the lower substrate and the upper substrate are formed of quartz, and the two substrates are integrally and airtightly bonded together by a metal bonding portion along the entire periphery.

Images