JP6196144B2 - Sensor element and pressure sensor - Google Patents

Description

  The present invention relates to a sensor element that detects a change in external force.

  A so-called resonance type sensor that outputs a signal representing a change in the resonance frequency of a vibrator is known. Patent Document 1 discloses a vibration detector including a mass portion supported by a support beam as a vibrator. In the vibration detector of Patent Document 1, the mass portion is vibrated by using electrostatic force between two comb electrodes arranged so as to be engaged with each other while being separated from each other, and the change in the amplitude of the mass portion is detected by the comb electrode. It detects based on the change of the electrostatic capacity between.

JP 2000-55670 A

  In the resonance type sensor, it is desirable to improve the vibration characteristics of the vibrator and improve its detection accuracy. For example, if the limit of the amplitude at which the vibrator can vibrate stably can be expanded, the magnitude of the output signal can be increased and the quality of the output signal can be improved. Further, if the linearity of the change in the resonance frequency of the vibrator with respect to the change in the external force is increased, the resolution of the resonance sensor can be improved. In the above Patent Document 1, it is disclosed that the amplitude of the vibrator reaches the limit by adjusting the voltage applied to the comb-teeth electrode and the vibration characteristics of the vibrator are improved. Not. As described above, in the resonance type sensor, sufficient improvement has not been made so far to improve the vibration characteristics of the vibrator and improve the detection accuracy.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms.

[1] According to one aspect of the present invention, a sensor element including a vibrator whose resonance frequency changes according to an applied external force is provided. The vibrator of the sensor element is coupled to the beam portion that receives the force according to the external force along the extending direction and flexibly vibrates; and is displaced by receiving the electrostatic force, thereby An electrode that flexures and vibrates the beam. The electrode bends and vibrates together with the beam when the sensor element is in a resonance state. According to the sensor element of this form, the frequency characteristics of the vibrator can be improved by bending and vibrating the electrode, and the external force detection accuracy by the sensor element can be improved.

[2] In the sensor element of the above aspect, the natural frequency fd of the vibrator is expressed as fd <(k / m) ^0.5 /, where k is the spring constant of the beam and m is the mass of the electrode. It may be 2π. According to the sensor element of this form, since the electrode bends and vibrates together with the beam portion, the frequency characteristics of the vibrator can be improved.

[3] In the sensor element of the above aspect, an electrode resonance ratio that is a value of a ratio of a resonance frequency of the electrode unit to a resonance frequency of the vibrator when the external force is not applied is 1.5 or more. Also good. According to this type of sensor element, noise in the output signal of the sensor element can be reduced, and the sensitivity of the sensor element can be ensured.

[4] In the sensor element of the above aspect, the electrode resonance ratio may be 2.0 or more. According to the sensor element of this embodiment, it is possible to achieve both low noise and sensitivity in the sensor element at a high level.

[5] In the sensor element of the above aspect, the electrode resonance ratio may be 6.0 or less. According to the sensor element of this embodiment, low noise and sensitivity in the sensor element are ensured at a higher level.

[6] In the sensor element of the above aspect, the electrode has an extending portion extending in parallel with the beam portion; and the vibrator has a maximum amplitude in the extending portion of the electrode when in the resonance state. It may be twice or more the maximum amplitude of the part. According to this form of the sensor element, the frequency characteristics of the vibrator are further improved.

[7] In the sensor element of the above aspect, when the vibrator is in a resonance state, the maximum amplitude of the extension portion of the electrode may be three times or more the maximum amplitude of the beam portion. According to this form of the sensor element, the frequency characteristics of the vibrator are further improved.

[8] The sensor element of the above aspect may have a sealed space for accommodating the beam portion and the electrode. According to the sensor element of this form, the vibrator is protected from foreign matters such as dust, so that the durability and environmental performance of the sensor element are improved.

[9] In the sensor element of the above aspect, the sealed space may be depressurized from atmospheric pressure. According to this type of sensor element, resistance to vibration caused by air is reduced, and the vibration efficiency of the vibrator is improved.

[10] In the sensor element of the above aspect, the electrode and the beam portion of the vibrator may be formed by etching an SOI substrate. According to the sensor element of this embodiment, a vibrator as a micromachine formed on the SOI substrate can be obtained. In addition, the mechanical Q value of the vibrator can be increased.

[11] According to another aspect of the present invention, a pressure sensor is provided. The pressure sensor includes: a pressure receiving portion that deforms according to the pressure to be measured; and the sensor element of the above-described form; and the sensor element receives an external force according to the deformation of the pressure receiving portion. Further, it may be arranged in the pressure receiving part. According to the pressure sensor of this embodiment, the pressure detection accuracy is improved.

[12] The pressure sensor according to the above aspect includes a shaft member in which the pressure receiving portion extends in an axial shape; the shaft member is deformed in an axial direction when a pressure to be measured is transmitted to an end surface; It may be arranged on the side surface of the shaft member so that the extending direction of the beam portion coincides with the axial direction of the shaft member, and an external force corresponding to deformation in the axial direction of the shaft member may be detected. According to the pressure sensor of this aspect, since the pressure receiving portion is configured by the shaft member that expands and contracts in the axial direction according to the pressure, the pressure sensor can be reduced in diameter, and the mountability and installation property of the pressure sensor can be reduced. Can be improved.

[13] In the pressure sensor of the above aspect, the sensor element may include first and second sensor elements arranged so as to sandwich the shaft member on a side surface of the shaft member. According to the pressure sensor of this aspect, it is possible to improve the detection accuracy of the pressure sensor in which the pressure receiving portion is constituted by the shaft member.

[14] The pressure sensor according to the above aspect further includes a cylindrical accommodating portion that accommodates at least the sensor element, and the pressure receiving portion closes the opening of the accommodating portion, according to the pressure to be measured. A curved diaphragm may be included. In this configuration, the sensor element is disposed on the surface of the diaphragm such that the extending direction of the beam portion coincides with the direction along the surface of the diaphragm, and an external force corresponding to the degree of curvature of the diaphragm is applied. It may be detected. According to the pressure sensor of this aspect, since a larger external force is applied to the sensor element by the diaphragm that is deformed according to the pressure, the pressure detection accuracy is improved.

[15] In the pressure sensor of the above aspect, the sensor element outputs a pulse signal output at a period corresponding to a resonance frequency of the vibrator; the pressure sensor further includes a reference clock having a predetermined period. A clock generation unit that generates the signal, and a signal detection unit that detects a change amount of the period of the resonance frequency based on the period of the reference clock. According to the pressure sensor of this embodiment, the amount of change in the resonance frequency in the vibrator can be detected with a resolution corresponding to the frequency of the reference clock generated by the clock generation unit, so that the pressure detection accuracy can be improved. it can.

  The present invention can also be realized in various forms other than the sensor element. For example, various types of resonant sensors, control methods or manufacturing methods of sensor elements used in those resonant sensors, computer programs that implement the control methods or manufacturing methods, and non-temporary recordings that record the computer programs It can be realized in the form of a medium or the like.

The schematic perspective view which shows the structure of a sensor element. The schematic perspective view which shows the structure of the vibrator | oscillator with which a sensor element is provided. The schematic sectional drawing which shows the cross-sectional structure for every site | part of a sensor element. The schematic diagram for demonstrating generation | occurrence | production of the vibration in an electrode part. Schematic which shows the structure of the excitation circuit for driving a sensor element. The schematic diagram for demonstrating the resonance state of a vibrator | oscillator. The schematic diagram for demonstrating the change of the resonant frequency of the sensor element by external force. Explanatory drawing for demonstrating the frequency characteristic of a sensor element. Schematic which shows the vibrator | oscillator which comprises the sensor element of a comparative example. Explanatory drawing for demonstrating the frequency characteristic of the sensor element of a comparative example. The schematic diagram for demonstrating a hard spring effect. The schematic diagram which shows notionally the bending vibration of a movable electrode when a vibrator | oscillator exists in a resonance state. FIG. 3 is a schematic diagram illustrating a configuration of a first vibrator model in the first embodiment. FIG. 3 is a schematic diagram illustrating a configuration of a second vibrator model in the first embodiment. FIG. 5 is a schematic diagram illustrating a configuration of a third vibrator model in the first embodiment. Explanatory drawing for demonstrating the improvement of the output characteristic of the sensor element according to the amplitude ratio of each vibrator | oscillator model. Explanatory drawing for demonstrating the acquisition method of a nonlinear index | exponent. FIG. 6 is an explanatory diagram showing a table in which the configurations of the vibrators included in the sample of Example 2 are summarized. Explanatory drawing which shows the evaluation result about the drive stability of each sample. Explanatory drawing for demonstrating the prescription | regulation reason of the limit value of a nonlinear index | exponent. Explanatory drawing which shows the evaluation result about the sensitivity of each sample. Explanatory drawing which shows the evaluation result of compatibility of the drive stability and sensitivity in each sample. Explanatory drawing which shows the relationship between an electrode resonance ratio and a S / N index | exponent. The schematic diagram which shows the manufacturing process of a sensor element in process order. The schematic diagram which shows the manufacturing process of a sensor element in process order. Schematic which shows the structure of the pressure sensor as 2nd Embodiment. Schematic for demonstrating the detection of the variation | change_quantity of the resonant frequency by a signal detection part. Schematic for demonstrating the other example of the detection method of the variation | change_quantity of the resonant frequency by a signal detection part. Schematic which shows the structure of the pressure sensor as 3rd Embodiment. Schematic which shows the structure of the pressure sensor as 4th Embodiment. Schematic which shows the structure of the pressure sensor as 5th Embodiment.

A. First embodiment:
[Configuration of sensor element]
FIG. 1 is a schematic perspective view showing a sensor element 10 as an embodiment of the present invention. The sensor element 10 is used as a sensitive part in a resonance type sensor. In the sensor element 10, a vibrator 100 is formed on an SOI substrate 11 which is a main body substrate. Here, “SOI” is an abbreviation for “Silicon on Insulator”. The resonance frequency of the vibrator 100 changes according to an external force applied in a predetermined direction along the surface of the SOI substrate 11 of the sensor element 10. The sensor element 10 outputs a signal representing the resonance frequency of the vibrator 100.

  FIG. 2 is a schematic perspective view showing the configuration of the vibrator 100 included in the sensor element 10. FIG. 2 illustrates arrows X and Y that are parallel to the surface of the SOI substrate 11 of the sensor element 10 and are orthogonal to each other. An arrow X indicates a direction parallel to the extending direction of the beam unit 111 in the main body vibration unit 110, and a direction indicated by an arrow Y orthogonal to the arrow X coincides with the vibration direction of the beam unit 111. Hereinafter, a direction parallel to the direction indicated by the arrow X is referred to as “X-axis direction”, and a direction parallel to the direction indicated by the arrow Y is referred to as “Y-axis direction”. In FIG. 2, for the sake of convenience, the outer fixed electrode 130 and the inner fixed electrode 140 included in the electrode unit 150 illustrated on the front side of the drawing are omitted.

  The vibrator 100 includes a main body vibration unit 110 and two electrode units 150. The main body vibration unit 110 is configured by a tuning fork vibrator having a frame shape, and includes two beam portions 111, two connection portions 112, and two fixing portions 113. The two beam portions 111 are columnar portions extending substantially linearly and are arranged in parallel to each other. The two connecting portions 112 connect the ends of the beam portions 111 to each other. The two fixing portions 113 are substantially plate-like portions arranged on both end sides of each beam portion 111, and are connected to the center of each connecting portion 112 via a connecting shaft 113s. The main body vibration unit 110 corresponds to a structure in which two substantially U-shaped tuning forks are coupled to each other at the open end.

  The two electrode parts 150 are arranged symmetrically about the main body vibration part 110. Each electrode unit 150 includes a movable electrode 120, an outer fixed electrode 130, and an inner fixed electrode 140. The movable electrode 120 has a central base portion 121, an extension portion 122, and comb teeth portions 123a and 123b. The central base portion 121 is connected to the central portion of the beam portion 111 via a connecting shaft 111s.

  The extending portion 122 is a columnar portion that extends symmetrically in the X-axis direction around the central base portion 121 and extends in parallel with the beam portion 111 of the main body vibrating portion 110. The comb teeth 123a and 123b are configured by a plurality of (for example, several tens or more) protrusions extending in parallel with each other in the Y-axis direction. The protrusions of the comb teeth 123a and 123b are arranged at equal intervals. The comb-tooth portions 123a and 123b are provided on the surface of the extending portion 122 facing the main body vibration portion 110 and on the opposite surface.

  The outer fixed electrode 130 is a drive electrode to which a voltage is applied from an excitation circuit (described later), and is disposed outside the movable electrode 120. Here, “outside of the movable electrode 120” means a side opposite to the side where the main body vibration unit 110 is disposed with respect to the movable electrode 120. Each outer fixed electrode 130 has an extending portion 131 and a comb tooth portion 132. The extending part 131 of the outer fixed electrode 130 extends in parallel with the extending part 122 of the movable electrode 120.

  The comb teeth 132 of the outer fixed electrode 130 are formed on the side surface of the extending portion 131 as a plurality of comb-shaped parallel protrusions. The comb teeth 132 of the outer fixed electrode 130 are arranged so as to mesh with each other with respect to the comb teeth 123 a on the outer side of the movable electrode 120. That is, the comb-tooth portions 132 of the outer fixed electrode 130 and the outer comb-tooth portions 123a of the movable electrode 120 are arranged so as to be alternately arranged in a state where the respective protrusions are separated from each other.

  The inner fixed electrode 140 is a detection electrode that outputs a charge amount that fluctuates with the fluctuation of the capacitance with the excitation movable electrode 120 to the excitation circuit, and is disposed inside the movable electrode 120. Here, “inside the movable electrode 120” means the side on which the main body vibration unit 110 is disposed with respect to the movable electrode 120. Each inner fixed electrode 140 has an extending portion 141 and a comb tooth portion 142 that are separated into two. The two extending portions 141 of the inner fixed electrode 140 are provided symmetrically to each other in the X-axis direction with the connecting shaft 111s between the main body vibrating portion 110 and the movable electrode 120 interposed therebetween, and the extending portion 122 of the movable electrode 120 is provided. And extends in parallel.

  The comb teeth 142 of the inner fixed electrode 140 are formed as a plurality of comb-shaped parallel protrusions on the side surface of each extending portion 141 on the movable electrode 120 side. The comb teeth 142 of the inner fixed electrode 140 are arranged so as to mesh with each other with respect to the comb teeth 123 b inside the movable electrode 120. That is, the comb-tooth portions 142 of the inner fixed electrode 140 and the comb-tooth portions 123b on the inner side of the movable electrode 120 are arranged so as to be alternately arranged in a state where the respective protrusions are separated from each other.

  FIG. 3 is a schematic cross-sectional view showing a cross-sectional configuration of each part in the SOI substrate 11 of the sensor element 10. In the (A) column of FIG. 3, a schematic cross-sectional view of a portion where the main body vibration unit 110 is formed (a portion corresponding to the AA cut in FIG. 2) is illustrated. In the column (B) of FIG. 3, the protrusions constituting the comb teeth 123 a of the movable electrode 120 and the protrusions constituting the comb teeth 132 of the outer fixed electrode 130 are alternately arranged ( A schematic sectional view of a portion corresponding to the BB cut in FIG. 2 is shown. In the column (C) of FIG. 3, the protrusions forming the comb teeth 123 b of the movable electrode 120 and the protrusions forming the comb teeth 142 of the inner fixed electrode 140 are alternately arranged ( A schematic cross-sectional view of a portion corresponding to CC cutting in FIG. 2 is illustrated.

The SOI substrate 11 of the sensor element 10 has a laminated structure in which an intermediate oxide film layer 11m, which is a silicon dioxide (SiO ₂ ) thin film layer, and a surface silicon layer 11s are laminated on a silicon substrate 11b. Yes. The vibrator 100 is formed by etching the surface silicon layer 11s and the intermediate oxide film layer 11m of the SOI substrate 11 (described later). In the sensor element 10, a glass plate is disposed on the surface silicon layer 11s (described later), but the illustration thereof is omitted here for convenience.

  The position of the fixing portion 113 of the main body vibration portion 110 is fixed by being disposed on the silicon substrate 11b with the intermediate oxide film layer 11m interposed therebetween (column (A) in FIG. 3). On the other hand, the portions other than the fixed portion 113 (the beam portion 111, the connecting portion 112, the connecting shafts 111s and 113s) of the main body vibrating portion 110 are removed by removing the intermediate oxide film layer 11m immediately below the silicon substrate. It is held by the fixing portion 113 in a state of floating away from 11b.

  The positions of the outer fixed electrode 130 and the inner fixed electrode 140 of the electrode portion 150 are fixed by being disposed on the silicon substrate 11b with the intermediate oxide film layer 11m interposed therebetween (see the (B) column and (C) in FIG. ) Column). In contrast, the entire movable electrode 120 of the electrode unit 150 is in a state of floating away from the silicon substrate 11b due to the removal of the intermediate oxide film layer 11m immediately below, via the connecting shaft 111s. Are held by the beam unit 111 of the main body vibration unit 110.

[Drive sensor element]
FIG. 4 is a schematic diagram for explaining generation of vibration in the electrode unit 150. FIG. 4 schematically shows a part of the electrode portion 150 (a portion where the comb teeth 123a of the movable electrode 120 and the comb teeth 132 of the outer fixed electrode 130 mesh with each other) in a perspective view. When the sensor element 10 is driven, a voltage is applied between the movable electrode 120 and the outer fixed electrode 130, and an electrostatic force is generated between the comb tooth portions 123a and 132 facing each other.

  As described above, the entire movable electrode 120 is held in a state of floating away from the silicon substrate 11b (FIG. 3), and therefore, the outer fixed electrode 130 and the outer fixed electrode 130 are in accordance with the periodic fluctuation of the electrostatic force. The distance between fluctuates and vibrates. Due to the vibration of the movable electrode 120, the distance between the movable electrode 120 and the inner fixed electrode 140 varies, and the capacitance between the electrodes 120 and 140 varies, so the amount of charge in the inner fixed electrode 140 varies.

  FIG. 5 is a schematic diagram showing the configuration of the excitation circuit 30 for driving the sensor element 10. The sensor element 10 constitutes a positive feedback oscillation circuit together with the excitation circuit 30 and self-oscillates at a specific resonance frequency. The excitation circuit 30 includes a charge amplifier unit 31, a filter unit 32, and a comparator unit 33.

  The charge amplifier unit 31 is configured as an inverting amplifier circuit including an operational amplifier 31a, a feedback resistor 31r, and a feedback capacitor 31c. The negative input terminal of the operational amplifier 31a is connected to the sensor element 10 via the detection terminal 16a. The charge amplifier unit 31 converts a change in the amount of charge in the inner fixed electrode 140 of the vibrator 100 into a voltage signal and outputs the voltage signal to the filter unit 32.

  The filter unit 32 is configured as an inverting amplifier circuit including a coupling capacitor 32c, an input resistor 32ra, an operational amplifier 32a, and a feedback resistor 31rb. The coupling capacitor 32 c performs coupling with the charge amplifier unit 31. The filter unit 32 functions as a filter that amplifies a signal having a predetermined frequency or higher that has passed through the coupling capacitor 32c.

  The comparator unit 33 includes an operational amplifier 33 a and is configured as a non-inverting amplifier circuit that compares voltages of two input terminals, and shapes the output of the filter unit 32. The operational amplifier 33a of the comparator unit 33 has two complementary outputs. The positive output of the operational amplifier 33a is connected to the negative input terminal to which the capacitor 33c is connected via the feedback resistor 33r. The negative output of the operational amplifier 33a is connected to the output terminal 33o, and outputs a signal indicating the resonance frequency of the circuit including the sensor element 10 to an external signal detector (not shown).

Since the excitation circuit 30 functions as a positive feedback circuit with a delay, the sensor element 10 vibrates in a resonance state corresponding to the natural frequency of the vibrator 100. The excitation circuit for driving the sensor element 10 is not limited to the excitation circuit 30 shown in FIG. 5, and various types of excitation circuits can be used as long as they are positive feedback circuits.

  FIG. 6 is a schematic diagram for explaining the resonance state of the vibrator 100. 6A shows a schematic front view of the vibrator 100 in a stationary state, and FIG. 6B shows a schematic front view of the vibrator 100 in a resonance state. . In FIG. 6, illustration of the outer fixed electrode 130 and the inner fixed electrode 140 is omitted. Further, in FIG. 6, the virtual central axis CL of the vibrator 100 is illustrated by a one-dot chain line. In the sensor element 10, the vibrator 100 is in a resonance state when driven.

  Here, the “resonance state” of the vibrator 100 is a state in which the two beam portions 111 of the main body vibration unit 110 are flexibly oscillating in a bilaterally symmetrical manner about the virtual center axis CL. In the sensor element 10 of the present embodiment, when in the resonance state, the two beam portions 111 of the main body vibration unit 110 are flexibly vibrated in the opposite directions with the same period and amplitude due to the displacement of the movable electrode 120.

  In the sensor element 10 of the present embodiment, the two movable electrodes 120 are flexibly vibrated together with the two beam portions 111 of the main body vibration unit 110. The movable electrode 120 can be flexibly vibrated in a resonance state by adjusting the length, thickness, density, and rigidity of the movable electrode 120 according to the shape of the main body vibration unit 110. In the vibrator 100 of the present embodiment, the vibration characteristics of the vibrator 100 are improved by the flexural vibration of the movable electrode 120, details of which will be described later.

  FIG. 7 is a schematic diagram for explaining a change in the resonance frequency of the sensor element 10 due to an external force. In FIG. 7, a tuning fork portion of the main body vibration unit 110 in the vibrator 100 is extracted and schematically illustrated. 7 is a schematic perspective view of the tuning fork portion of the main body vibration unit 110 in a stationary state, and the lower step of FIG. 7 is a schematic front view of the tuning fork portion of the main body vibration unit 110 in a resonance state. It is shown. As described above, the tuning fork portion of the main body vibration unit 110 includes the two parallel beam portions 111 and the connecting portions 112 that connect both ends of each beam portion 111.

  The tuning fork part of the main body vibration unit 110 enters a resonance state when an excitation force F in the Y-axis direction is applied from the outside to each of the two beam units 111 at the same timing. When an external force P is applied to the two beam portions 111 in a direction orthogonal to the excitation force F (X-axis direction), stress is generated in the two beam portions 111 by the external force P. As a result, the ease of bending of each beam part 111 changes, and the resonance frequency of the tuning fork part of the main body vibration part 110 changes. In the sensor element 10 of the present embodiment, the excitation force F is applied from the movable electrode 120, and the external force P is applied via the fixing portion 113 of the main body vibration unit 110 fixed to the silicon substrate 11b (FIG. 3). Is done.

  FIG. 8 is an explanatory diagram for explaining the frequency characteristics of the sensor element 10 of the present embodiment. FIG. 8 is a graph showing the frequency characteristics of the sensor element 10 with the horizontal axis representing frequency and the vertical axis representing amplitude. In FIG. 8, an example of a graph showing the frequency characteristics of the sensor element 10 when no external force is applied is shown by a solid line, and an example of a graph showing the frequency characteristics of the sensor element 10 when an external force is applied is a broken line. It is shown by.

  This graph showing the frequency characteristics shows the amplitude of the signal output from the charge amplifier connected to the inner fixed electrode 140 when the frequency of the AC voltage applied to the outer fixed electrode 130 of the sensor element 10 is changed from the outside. It shows a change. Hereinafter, a graph showing the frequency characteristics like the graph of FIG. 8 is referred to as a “frequency characteristic graph”. In addition, all the frequency characteristic graphs used for description in this specification are obtained by the same method.

  The resonance frequency fr of the sensor element 10 at the resonance peak in the frequency characteristic graph shifts in the horizontal axis direction according to the external force applied to the sensor element 10. Therefore, the external force applied to the sensor element 10 can be detected by detecting the amount of change Δfr in the resonance frequency from the resonance frequency fr of the vibrator 100 in the initial state.

  As described above, the sensor element 10 of the present embodiment is configured such that the movable electrode 120 bends and vibrates when the vibrator 100 is in a resonance state (FIG. 6). The characteristics are improved and its linearity is enhanced. Below, the reason is demonstrated using a comparative example.

[Improvement of vibration characteristics]
FIG. 9 is a schematic view showing a vibrator 100a constituting a sensor element as a comparative example of the present invention. FIG. 9A shows a schematic front view of the vibrator 100a of the comparative example in the stationary state, and FIG. 9B shows a schematic front view of the vibrator 100a in the resonance state. It is shown. In FIG. 9, the same components as those of the vibrator 100 of the present embodiment are denoted by the same reference numerals as those of the vibrator 100 of the present embodiment. In FIG. 9, illustration of the outer fixed electrode 130 and the inner fixed electrode 140 is omitted. The sensor element of the comparative example has substantially the same configuration as the sensor element 10 of the present embodiment, except that the vibrator 100a is provided. The vibrator 100a of the comparative example has a configuration in which the movable electrode 120 only repeats reciprocating movement when in a resonance state, and hardly bends and vibrates.

  FIG. 10 is an explanatory diagram for explaining the frequency characteristics of the sensor element of the comparative example. FIG. 10 shows an example of a frequency characteristic graph of the sensor element of the comparative example. In FIG. 10, an ideal frequency characteristic graph required for the resonance type sensor element is indicated by a broken line. In addition, a frequency characteristic graph when the applied voltage to the sensor element is small is indicated by a solid line, and a frequency characteristic graph when the applied voltage is large is indicated by a one-dot chain line.

  In the sensor element of the comparative example, when the applied voltage is low and the amplitude of the output signal is low, stable vibration of the vibrator 100a is ensured (solid line graph). However, when the applied voltage is increased and the amplitude of the output signal is increased, the vibration of the vibrator 100a becomes unstable, and the symmetry about the resonance peak in the frequency characteristic graph is broken (the one-dot chain line). Graph). This is due to the influence of the hard spring effect in the vibrator 100a described below.

  FIG. 11 is a schematic diagram for explaining the hard spring effect in the vibrator 100a of the comparative example. FIG. 11 conceptually illustrates the hard spring effect generated in the beam unit 111 in the vibrator 100a of the comparative example. In the vibrator 100a of the comparative example, the beam portion 111 bends and vibrates in a state where both ends are fixed to the connecting portion 112. When the amplitude of the beam part 111 in the bending direction increases to some extent, the rigidity of the beam part 111 in the extending direction acts in a direction to suppress the extension of the beam part 111 in the extending direction. For this reason, the spring constant in the extending direction of the beam portion 111 increases, which acts in a direction that suppresses the bending vibration of the beam portion 111. In general, the increase in the spring constant of the beam portion 111 accompanying the increase in the amplitude in the bending direction is called a “hard spring effect”. This hard spring effect prevents stable vibration when the amplitude of the vibrator 100a of the comparative example is increased.

  FIG. 12 is a schematic diagram conceptually showing the flexural vibration of the movable electrode 120 when the vibrator 100 of the present embodiment is in a resonance state. Since the root of the movable electrode 120 is fixed to the central base portion 121 (FIG. 6), the flexural vibration of the movable electrode 120 can be regarded as a vibration model of a cantilever with the root fixed as shown in FIG. it can.

  In the vibrator 100 of this embodiment, when the amplitude is increased, the movable electrode 120 can bend and vibrate without the influence of the hard spring effect. Therefore, when the vibrator 100 is in the resonance state, the influence of the vibration component of the flexural vibration in the movable electrode 120 increases as the amplitude of the beam unit 110 increases. Therefore, when viewed from the entire vibrator 100, the influence of the hard spring effect when the amplitude is large is alleviated, and the linear frequency characteristics of the vibrator 100 are improved. As described above, when the vibrator 100 is in the resonance state, the vibration component of the flexural vibration in the movable electrode 120 affects the vibration characteristics of the whole vibrator 100.

Further, as will be described below, the natural frequency fd of the vibrator 100 of the present embodiment is smaller than the natural frequency fs of the vibrator 100a of the comparative example due to the vibration component of the flexural vibration of the movable electrode 120. Here, the spring constant of the beam portion 111 in the vibrator 100a (FIG. 9) of the comparative example and the vibrator 100 (FIG. 2) of the present embodiment is k, and the mass of the movable electrode 120 is m. At this time, the vibrator 100a of the comparative example in the resonance state is represented as a model in which masses of mass m are attached to both ends of a coiled spring having a spring constant k. Accordingly, the natural frequency fs of the vibrator 100a of the comparative example is expressed as the following formula (1).
fs = (k / m) ^0.5 / 2π (1)

On the other hand, the vibrator 100 according to the present embodiment in the resonance state has springs of mass m at both ends of a string spring having a spring constant k corresponding to the main body vibration unit 110 in consideration of flexural vibration of the movable electrode 120. It is represented as a model with a _{string spring of} constant km. The spring constant k _{imp at} the natural frequency fd of this model corresponds to a value obtained by serially combining the spring constant k _m of the coiled spring corresponding to the movable electrode 120 with the spring constant k of the beam unit 111 of the vibrator 100 ( The following formula (2)).
_{k imp = k · k m /} (k + k m) ... (2)

From the above equation (2), it can be seen that in the resonator 100 of this embodiment in the resonance state, the spring constant k _{imp of the} entire resonator 100 is smaller than the spring constant k in the main body vibration unit 110 (k> k _imp ). From the relationship between the spring constants k and k _imp , the following inequality (3) exists between the natural frequency fs of the vibrator 100a of the comparative example and the natural frequency fd of the vibrator 100 of the present embodiment. The relationship is established.
fd <fs (3)

  As described above, in the vibrator 100 according to the present embodiment, the influence of the hard spring effect on the vibration characteristics is mitigated by the flexural vibration of the movable electrode 120 as compared with the vibrator 100a of the comparative example, and linear vibration characteristics are obtained. Improved. Therefore, with the vibrator 100 of the present embodiment, the detection accuracy of the sensor element 10 can be improved as compared with the vibrator 100a of the comparative example.

[Example 1]
FIGS. 13 to 15 are schematic views showing the configurations of three transducer models MD1 to MD3, respectively. FIGS. 13 to 15 schematically show the stationary state and the resonance state of each of the transducer models MD1 to MD3 in the (A) column and the (B) column, respectively. 13 to 15, illustration of the outer fixed electrode 130 and the inner fixed electrode 140 on both sides of the movable electrode 120 is omitted. In the first embodiment, the amplitude ratio between the main body vibration unit 110 and the movable electrode 120 that ensures the stability of vibration of the vibrator 100 when the amplitude in the resonance state is increased will be described.

  Each transducer model MD <b> 1 to MD <b> 3 includes a main body vibration unit 110 and two electrode units 150, similar to the transducer 100 described with reference to FIG. 2. Each transducer model MD <b> 1 to MD <b> 3 changes the amplitude of the main body vibration unit 110 and the movable electrode 120 by changing the ratio of the length of the beam unit 111 of the main body vibration unit 110 and the length of the movable electrode 120 in the electrode unit 150. The amplitude ratio has been changed.

  Here, in this specification, the ratio of the maximum amplitude am (the amplitude of the central portion of the beam portion 111) of the beam portion 111 of the main body vibration unit 110 to the maximum amplitude as (the amplitude of the tip portion) of the movable electrode 120 in the resonance state. The value (am / as) is called “amplitude ratio”. Each transducer model MD1 to MD3 has an amplitude ratio of 1, 0.5, and 0.3, respectively.

  The first vibrator model MD1 (FIG. 13) is a model in which the maximum amplitude as of the movable electrode 120 in the resonance state and the maximum amplitude am of the beam unit 111 are equal (am = as; amplitude ratio 1.0). . In the first vibrator model MD1, the movable electrode 120 hardly bends and vibrates, and the tip portion of the movable electrode 120 vibrates with almost the same amplitude as the central portion of the beam portion 111 of the main body vibration unit 110.

  In the second vibrator model MD2 (FIG. 14), the maximum amplitude as of the movable electrode 120 in the resonance state is approximately twice the maximum amplitude am of the beam unit 111 (as = 2 · am; amplitude ratio 0). .5). In the second vibrator model MD2, the movable electrode 120 bends and vibrates in the resonance state, so that the amplitude of the tip of the movable electrode 120 is double that of the central portion of the beam unit 111 of the main body vibration unit 110. .

  In the third vibrator model MD3 (FIG. 15), the maximum amplitude as of the movable electrode 120 in the resonance state is almost three times as large as the maximum amplitude am of the beam unit 111 (as = 3 · am; amplitude ratio is about 0.3). In the third vibrator model MD3, the degree of flexural vibration of the movable electrode 120 in the resonance state is larger than that of the second vibrator model MD2, and the amplitude of the tip of the movable electrode 120 is that of the beam part 111 of the main body vibration part 110. Three times the central part.

  FIG. 16 is an explanatory diagram for explaining the improvement of the output characteristics of the sensor element 10 in accordance with the amplitude ratio of each of the transducer models MD1 to MD3. FIG. 16 shows a graph in which the horizontal axis is the amplitude ratio and the vertical axis is the nonlinear index. Here, the “nonlinear index” is an index indicating the asymmetry of the frequency characteristics of the resonance sensor, and is a value obtained as follows.

FIG. 17 is an explanatory diagram for explaining a method of acquiring a nonlinear index. FIG. 17 shows an example of a frequency characteristic graph of the sensor element of the resonance type sensor. The non-linear index is acquired from the frequency characteristic graph of the sensor element as follows.
[1] In the frequency characteristic graph of the sensor element, the maximum value α of the resonance peak and the frequency f _{α at} the maximum value α of the resonance peak are acquired.
[2] A frequency range f _{1 to} f ₂ (f ₁ <f ₂ ) in which the amplitude is α / √2 or more is obtained.
[3] The frequency range f _{1 to} f ₂ is scaled with its center at 0, f ₁ at −1, and f ₂ at 1, and a value nl corresponding to the frequency f _α is obtained on the scale. To do. This value nl is the nonlinear exponent of the sensor element.

  The nonlinear index obtained by the above steps [1] to [3] indicates the degree to which the peak of the resonance peak is off the center in the frequency characteristic graph of the resonance sensor. It shows the degree of symmetry breaking. The nonlinear index indicates that the smaller the absolute value, the more symmetrical the output characteristic graph of the resonant sensor, and the more stable the vibrator of the resonant sensor vibrates.

  The nonlinear index in the graph of FIG. 16 is a value obtained when the capacitance is changed by the same amount in each transducer model MD1 to MD3 for the sensor element including each transducer model MD1 to MD3. As shown in this graph, in the sensor element using each transducer model MD1 to MD3, the non-linear exponent becomes almost linearly smaller than the amplitude ratio of each transducer model MD1 to MD3. Specifically, in the vibrator model MD2 having an amplitude ratio of 0.5, the nonlinear index is smaller than 0.40, and in the vibrator model MD3 having an amplitude ratio of 0.3, the nonlinear index is smaller than 0.20. It was.

  From this result, when the vibrator 100 is in a resonance state, the maximum amplitude as of the movable electrode 120 of the main body vibration unit 110 is preferably at least twice as large as the maximum amplitude am of the beam unit 111, and is at least three times. It turns out that it is more preferable. In this manner, in the vibrator 100, if the amplitude of the movable electrode 120 in the resonance state is increased with respect to the amplitude of the main body vibration unit 110 in the resonance state, stable vibration is ensured, and the linearity of the sensor element 10 is ensured. Frequency characteristics are improved.

[Example 2]
FIG. 18 is an explanatory diagram illustrating a table in which the configurations of the vibrators 100 included in the samples S1 to S4 of the four sensor elements 10 are summarized. The table of FIG. 18 summarizes a schematic configuration diagram of the vibrator 100 included in the samples S1 to S4 of the sensor element 10 and parameters related to the resonance frequency. In the schematic configuration diagram of the vibrator 100 in the table, the electrode unit 150 other than the movable electrode 120 is not shown. In the second embodiment, the definition of the resonance frequency of the vibrator 100 that can ensure the driving stability and sensitivity of the sensor element 10 will be described.

  The vibrator 100 included in each of the samples S1 to S4 includes the main body vibration unit 110 and the electrode unit 150 described with reference to FIG. The vibrators 100 of the samples S1 to S4 have substantially the same configuration except for the following points. In the vibrator 100 of each of the samples S1 to S4, the relationship between the length of the beam unit 111 in the main body vibration unit 110 and the length of the movable electrode 120 in the electrode unit 150 is different as follows.

  The lengths La1 to La3 of the beam unit 111 in the vibrator 100 of the samples S1 to S3 are smaller in the order of the samples S1 to S3 (La1> La2> La3). In the samples S1 to S3, the lengths of the movable electrodes 120 in the vibrator 100 are equal (Lb1 = Lb2 = Lb3). In the sample S4, the length La4 of the beam unit 111 is smaller than the lengths La1 to La3 of the beam unit 111 of the samples S1 to S3 (La4 <La1, La2, La3). In the sample S4, the length Lb4 of the movable electrode 120 is shorter than the lengths Lb1 to Lb3 of the movable electrode 120 of the samples S1 to S3 (Lb4 <Lb1 = Lb2 = Lb3).

  Due to the difference in configuration as described above, in each of the samples S1 to S4, the value of the ratio of the resonance frequency (electrode part resonance frequency fe) of only the movable electrode 120 to the resonance frequency of the whole vibrator 100 (overall resonance frequency fw) (electrode) Resonance ratio Rf; Rf = fe / fw) is different. Here, the “total resonance frequency fw” corresponds to the resonance frequency of the sensor element 10 when no external force is applied. Further, the “electrode part resonance frequency fe” corresponds to a resonance frequency of the movable electrode 120 alone when only the movable electrode 120 is extracted from the vibrator 100 and is flexibly vibrated.

  The “electrode resonance ratio Rf” is a value representing the magnitude of the resonance frequency of the movable electrode 120 on the basis of the resonance frequency of the entire vibrator 100. As the electrode resonance ratio Rf is closer to 1, the influence of the flexural vibration of the movable electrode 120 on the overall vibration of the vibrator 100 increases, and the vibration of the vibrator 100 in the resonance state is further stabilized. That is, the electrode resonance ratio Rf can be interpreted as a value indicating the degree of contribution of the flexural vibration of the movable electrode 120 to the stabilization of the vibration of the vibrator 100.

  The electrode resonance ratio Rf of each sample S1 to S4 is as follows. The electrode resonance ratio Rf of sample S1 was 4.7. The electrode resonance ratio Rf of sample S2 was 2.7. The electrode resonance ratio Rf of sample S3 was 1.5. The electrode resonance ratio Rf of sample S4 was 1.3. As described above, the electrode resonance ratio Rf of each of the samples S1 to S4 was close to 1 in the order of the samples S4, S3, S2, and S1.

  FIG. 19 is an explanatory diagram showing the evaluation results for the driving stability of the samples S1 to S4. A graph showing the relationship between the capacitance variation ΔC and the nonlinear index in each of the samples S1 to S4 is shown in the upper part of the sheet of FIG. 19, and each of the samples S1 to S4 obtained from the graph is shown in the lower part of the sheet. A table summarizing the N index for each is shown.

  Here, the “capacitance change width ΔC” is the electrostatic capacitance between the electrodes 120, 130, and 140 of the electrode unit 150 when an AC voltage is applied to the sensor element 10 from the outside and the drive frequency is inserted. This is the width that the capacity has changed. The change width ΔC of the capacitance corresponds to the amplitude of the vibrator 100 and changes according to the driving voltage of the sensor element 10.

  The “nonlinear index” is a parameter described with reference to FIGS. 16 and 17, and the smaller the absolute value, the more stably the vibrator of the resonance sensor vibrates. In the graph of FIG. 19, the relationship in which the gradient of the change of the nonlinear index with respect to the change width ΔC of the capacitance becomes gentler as the electrode resonance ratio Rf is closer to 1. Thus, the closer the electrode resonance ratio Rf is to 1, the higher the vibration stability of the vibrator 100 in the resonance state.

  The “N index” in the table is a value defined by the inventor of the present invention based on the graph on the upper side of FIG. 19 as an index indicating the noise level (low noise property) in the output signal of the sensor element 10. It is. Specifically, the N index was determined as follows. The non-linear exponent 0.3 is defined as a limit value that allows the sensor element 10 to be driven stably (the reason will be described later), and the capacitance change width ΔC of each sample S1 to S4 at this limit value is shown in FIG. It was obtained from 19 graphs. And the reciprocal number which remove | divided the value (20, 30, 55, 85) of capacitance variation width (DELTA) C obtained about each sample S1-S4 by the smallest value (20) was made into the "N index".

  The N index indicates that the smaller the value, the larger the capacitance change width ΔC at the limit at which the sensor element 10 can be stably driven. That is, the sensor element 10 having a smaller N index has higher vibration stability of the vibrator 100 at the limit that can be stably driven, and noise in the output signal is smaller. The N index of each sample S1 to S4 was 1 (reference), 0.67, 0.36, and 0.24. As described above, the low noise property was evaluated in the order of the samples S4, S3, S2, and S1.

  FIG. 20 is an explanatory diagram for explaining the reason why the limit value of the nonlinear index that allows the sensor element 10 to be stably driven is defined as 0.3. 20A to 20D illustrate examples of frequency characteristic graphs of the sensor element 10 when the nonlinear index is 0.40, 0.36, 0.26, and 0.17, respectively. is there.

  The driving state of the sensor element 10 was hardly stabilized when the nonlinear index was 0.40 (column (A)), and was slightly stabilized when the nonlinear index was 0.36 (column (B)). Further, when the nonlinear index is 0.26, acceptable stability is secured ((C) column), and when the nonlinear index is 0.17, sufficiently high stability is secured ((D) column). Based on this experimental knowledge, the inventor of the present invention has defined the limit value of the non-linear exponent for obtaining driving stability in the sensor element 10 as 0.3.

  FIG. 21 is an explanatory diagram showing evaluation results for the sensitivities of the samples S1 to S4. 21 is a bar graph showing the measured sensitivity values of the samples S1 to S4. The lower chart of FIG. 21 shows the S obtained for the measured sensitivity values of the samples S1 to S4. A table summarizing the indices is shown. Here, the measured value of sensitivity is a value acquired as the amount of change in frequency when a strain of 1 με is applied to each of the samples S1 to S4.

  For each sample S1 to S4, the sensitivity was measured in the driving state when the nonlinear index was the above limit value of 0.3. As a result, 25.0 ppm / με, 19.0 ppm / με, 4.2 ppm / με, 1 0.8 ppm / με. As described above, the sensitivity of the sample whose electrode resonance ratio Rf is a value far from 1 is higher. This is because the bending vibration of the movable electrode 120 is hardly influenced by the external force applied to the sensor element 10, and therefore, when the influence of the bending vibration of the movable electrode 120 on the resonance of the sensor element 10 is large, the change in the resonance frequency with respect to the external force. It is guessed that this is because of the loosening.

  Here, the value obtained by dividing the measured values 25.0, 19.0, 4.2, and 1.8 of the sensitivity obtained for each of the samples S1 to S4 by the largest measured value 25 was defined as “S index”. The S index indicates that high sensitivity is secured even at the limit where the sensor element 10 having a larger value can be stably driven. The S index of each sample S1 to S4 was 1 (reference), 0.75, 0.17, and 0.07. As described above, it was shown that the samples S1 to S4 have the sensitivity at the limit at which the samples can be stably driven in this order.

  FIG. 22 is an explanatory diagram showing evaluation results of compatibility between low noise and sensitivity in each of the samples S1 to S4. The table in FIG. 22 shows a table summarizing the N index obtained in FIG. 19, the S index obtained in FIG. 21, and the S / N index obtained based on them. In addition, a bar graph of the S / N index for each of the samples S1 to S4 is shown in the lower part of the drawing.

  As described so far, in the sensor element 10, the greater the influence of the flexural vibration of the movable electrode 120 in the resonance of the vibrator 100, the more noise can be reduced in the output signal, but the sensitivity to external force is reduced. In the sensor element 10, it is more desirable that the low noise property in the output signal and the sensitivity to external force are compatible at a high level.

  The S / N index is a value obtained by dividing the S index related to the sensitivity of the sensor element 10 by the N index related to the low noise property of the sensor element 10. In other words, the S / N index is an index indicating the high balance between the low noise property and the sensitivity of the sensor element 10, and indicates that the larger the value, the better the balance is obtained. The S / N index of each sample S1 to S4 was 1 (reference), 1.13, 0.47, 0.29. Thus, in the order of samples S2, S1, S3, and S4, low noise and sensitivity are compatible at a high level.

  FIG. 23 is an explanatory diagram showing the relationship between the electrode resonance ratio Rf and the S / N index index obtained in samples S1 to S4. FIG. 23 shows a graph in which the horizontal axis is the electrode resonance ratio Rf and the vertical axis is the S / N index.

  If the electrode resonance ratio Rf is 1.5 or more, an S / N index of 0.4 or more is secured, and if the electrode resonance ratio Rf is 2.0 or more, an S / N index of 0.7 or more is secured. It was done. Therefore, the electrode resonance ratio Rf is preferably 1.5 or more, and more preferably 2.0 or more. When the electrode resonance ratio Rf was 2.5 or more, an S / N index of 1.0 or more was secured. Therefore, the electrode resonance ratio Rf is more preferably 2.5 or more.

  On the other hand, when the electrode resonance ratio Rf was larger than 2.7, the S / N index showed a gradual decreasing tendency. The electrode resonance ratio Rf is preferably at least 6.0 or less, and more preferably 4.0 or less, in order to ensure an S / N index of 0.8 or more. The electrode resonance ratio Rf is more preferably 3.0 or less.

  As described above, since the electrode resonance ratio Rf in the vibrator 100 is defined in the appropriate range as described above, the low noise property and sensitivity of the sensor element 10 are compatible at a high level, which is more desirable. .

[Manufacturing process of sensor element]
24 and 25 are schematic views for explaining the manufacturing process of the sensor element 10. In FIGS. 24A to 24D and FIGS. 25E to 25H, schematic diagrams showing the contents of the manufacturing process of the sensor element 10 are shown in the order of the processes. In the first step, an SOI substrate 11 in which a surface silicon layer 11s, an intermediate oxide film layer 11m, and a silicon substrate 11b are stacked is prepared (column (A) in FIG. 24).

  In the second step, a silicon dioxide thin film is formed on the surface layer of the surface silicon layer 11s, and reactive ion etching (RIE; Reactive Ion Etching) is performed on the thin film (column (B) in FIG. 24). As a result, an etching mask 18 simulating the vibrator 100 and the electrode pad is formed on the surface layer of the surface silicon layer 11s. In the third step, the surface silicon layer 11s is etched by deep RIE (DEEP-RIE), and the outer peripheral shape of the vibrator 100 and the electrode pad is formed on the surface silicon layer 11s (column (C) in FIG. 24).

  In the fourth step, the intermediate oxide film layer 11m is etched with buffered hydrofluoric acid (BHF) (column (D) in FIG. 24). As a result, a space is formed below the beam portion 111 and the movable electrode 120 of the vibrator 100, and the beam portion 111 and the vibrating comb electrode 120 are brought into a floating state. In this step, the etching mask 18 on the surface layer of the surface silicon layer 11s is removed.

  In the fifth step, a glass plate 15 is prepared (column (E) in FIG. 25). In the sixth step, a cavity 15c for forming a housing space for the vibrator 100 is formed on one surface of the glass plate 15 by wet etching (column (F) in FIG. 25). In the seventh step, through holes 15h are formed in the glass plate 15 by sandblasting, electrode materials are embedded in the through holes 15h, and terminal portions 16 (detection terminals 16a and drive terminals 16b) are formed (FIG. 25). (G) column).

  In the eighth step, the glass plate 15 is laminated on the SOI substrate 11 on which the vibrator 100 is formed, and the SOI substrate 11 and the glass plate 15 are joined by anodic bonding (column (H) in FIG. 25). On the surface of the glass plate 15 facing the vibrator 100, a cavity 15c for forming a housing space for the vibrator 100 is formed as a recess. By performing the eighth step in a vacuum environment, the atmospheric pressure in the accommodation space of the vibrator 100 formed by the cavity 15c becomes lower than the atmospheric pressure. The sensor element 10 is completed through these series of steps.

  As described above, in the sensor element 10 of the present embodiment, the vibrator 100 is formed of the silicon of the surface silicon layer 11 s in the SOI substrate 11. Therefore, the mechanical Q value of the vibrator 100 is improved, and the vibration stability of the vibrator 100 is improved. Further, the variation in the resonance frequency of the vibrator 100 is reduced, and the detection accuracy of the resonance type sensor including the sensor element 10 is improved.

  In the sensor element 10 of this embodiment, the surface layer of the SOI substrate 11 is covered with the glass plate 15, and the vibrator 100 is accommodated in a sealed space formed by the cavity 15 c of the glass plate 15. Therefore, foreign matter such as fine dust is prevented from being mixed between the comb teeth 123a, 123b, 132, 142 of the vibrator 100, and the protection of the vibrator 100 is improved.

  In addition, in the sensor element 10 of the present embodiment, the sealed space formed by the cavity 15c of the glass plate 15 is in a reduced pressure state lower than the atmospheric pressure. Accordingly, since the vibration of the vibrator 100 is suppressed from being inhibited by air, the vibration efficiency and vibration stability of the vibrator 100 are improved.

  As described above, with the sensor element 10 of the present embodiment, the vibration characteristics of the vibrator 100 in the resonance state are improved by the flexural vibration of the movable electrode 120, and therefore the frequency characteristics thereof are improved. In addition, by appropriately defining the relationship between the resonance frequency of the main body vibration unit 110 and the resonance frequency of the movable electrode 120, it is possible to achieve both vibration stability in the vibrator 100 and sensitivity of the sensor element 10.

B. Second embodiment:
FIG. 26 is a schematic diagram showing a configuration of a pressure sensor 300 according to the second embodiment of the present invention. In FIG. 26, the central axis of the pressure sensor 300 is illustrated by a one-dot chain line. In FIG. 26, arrows X and Y indicating the arrangement direction of the vibrator 100 are illustrated so as to correspond to FIG. The pressure sensor 300 is a cylindrical sensor having a pressure receiving portion that receives a pressure to be measured at the tip, and measures, for example, an in-cylinder pressure (combustion pressure) in a combustion chamber of an internal combustion engine.

  The pressure sensor 300 includes the sensor element 10 described in the first embodiment and the excitation circuit 30. In addition, the pressure sensor 300 includes a pressure receiving shaft 310, a cap member 321, a bearing portion 322, an inner casing 323, an outer casing 331, an open end member 332, and a signal detection unit 40. .

  The pressure receiving shaft 310 is constituted by a shaft-like member, and when an external force corresponding to the pressure to be measured is received on the end surface, the pressure receiving shaft 310 is contracted and deformed along the axial direction according to the external force. The sensor element 10 is disposed on the side surface of the pressure receiving shaft 310. The sensor element 10 is disposed so that the extending direction (X-axis direction) of the beam unit 111 of the vibrator 100 matches the axial direction of the pressure receiving shaft 210. Thereby, the sensor element 10 receives an external force corresponding to the contraction deformation in the axial direction from the pressure receiving shaft 310. The pressure receiving shaft 310 is preferably formed of a member having a thermal expansion coefficient of 5.0 ppm / ° C. or less. Thereby, the deformation of the pressure receiving shaft 310 due to the change in the environmental temperature is suppressed, and the detection error of the pressure sensor 300 is reduced.

  The cap member 321 is a bottomed cylindrical member having one end opened. The cap member 321 accommodates a portion on the distal end side of the pressure receiving shaft 310, and the end surface on the distal end side of the pressure receiving shaft 310 is in surface contact with the center of the bottom surface thereof. The bearing portion 322 is a columnar member that holds the pressure receiving shaft 310, and the end surface on the rear end side of the pressure receiving shaft 310 is joined to the center of the bottom surface thereof.

  The inner casing 323 is a cylindrical member that houses the sensor element 10, the pressure receiving shaft 310, the cap member 321, and the bearing portion 322. A cap member 321 is slidably fitted in the opening end portion on the front end side of the inner casing 323, and a bearing portion 322 is fixedly fitted in the opening end portion on the rear end side of the inner casing 323. Yes. As a result, the pressure receiving shaft 310 is disposed on the central axis of the inner casing 323.

  The outer casing 331 is a cylindrical member that is open at both ends. The outer casing 331 accommodates and holds the inner casing 323 so that the pressure receiving shaft 310 is disposed on the central axis thereof. The open end member 332 is a cylindrical member whose tip side is enlarged in an umbrella shape. The opening end member 332 is attached to the opening end on the front end side of the outer casing 331 so that the opening is accommodated in the opening of the outer casing 331. A cap member 321 is slidably fitted into the opening of the opening end member 332.

  Here, as described with reference to FIG. 5, the sensor element 10 is connected to the excitation circuit 30, whereby the vibrator 100 enters a resonance state. The excitation circuit 30 outputs a voltage that varies according to the resonance of the vibrator 100 to the signal detection unit 40 as an output signal. The signal detection unit 40 includes a clock generation unit 41. As described below, the signal detection unit 40 detects the amount of change in the resonance frequency of the sensor element 10 using the clock signal generated by the clock generation unit 41.

  FIG. 27 is a schematic diagram for explaining a method of detecting a change amount of the resonance frequency by the signal detection unit 40. In FIG. 27, the output signal Ss of the sensor element 10 in the initial state before the external force is applied, the output signal Sd of the sensor element 10 after the external force is applied, and the clock generation unit 41 of the signal detection unit 40. The generated clock signals Sc are shown in parallel so as to correspond to each other. The signal detection unit 40 is a clock that the clock generation unit 41 generates a phase difference between the output signal Ss before the external force is applied to the sensor element 10 and the output signal Sd after the external force is applied to the sensor element 10. Obtained based on the period of the signal Sc. Based on the acquired phase difference, the amount of change in the resonance frequency of the sensor element 10 is detected.

  FIG. 28 is a schematic diagram for explaining another example of a method for detecting the amount of change in the resonance frequency by the signal detection unit 40. FIG. 28 is substantially the same as FIG. 27 except that frequency-divided signals DSs and Dsd are added between the output signals Ss and Sd of the sensor element 10 and the clock signal Sc. In this example, the output signals Ss and Sd of the sensor element 10 are frequency-divided at a predetermined frequency dividing ratio by a frequency divider (not shown) included in the signal detection unit 40. The signal detection unit 40 acquires the phase difference between the divided frequency-divided signals DSs and Dsd with reference to the cycle of the clock signal Sc. As described above, in the signal detection unit 40 of the present embodiment, the amount of change in the resonance frequency of the sensor element 10 can be detected with a resolution corresponding to the period of the clock signal Sc, and the pressure detection accuracy by the pressure sensor 300 is improved. Can be made.

  As described above, the pressure sensor 300 according to the second embodiment uses the sensor element 10 configured as a micromachine (so-called MEMS) by the SOI substrate 11, and thus can be reduced in size and weight. The mountability has been improved. In the pressure sensor 300 of the second embodiment, the pressure receiving shaft 310 that is a pressure receiving portion that transmits pressure to the sensor element 10 is a shaft-like member, and thus the diameter of the pressure sensor 300 can be reduced.

  In addition, the vibrator 100 of the sensor element 10 has high heat resistance of the component member itself, and its vibration also uses electrostatic force, so that it can vibrate stably even in a high temperature environment. Therefore, the pressure sensor 300 according to the second embodiment is a type of pressure sensor that suppresses a decrease in detection accuracy in a high-temperature environment, includes a magnetic material with low heat resistance, and detects pressure using magnetic force. Higher heat resistance can be obtained. With the pressure sensor 300 according to the second embodiment, the pressure can be measured with high accuracy in a narrow space having a high temperature of, for example, 100 ° C. or higher, such as a combustion chamber of an internal combustion engine of a vehicle.

C. Third embodiment:
FIG. 29 is a schematic diagram showing a configuration of a pressure sensor 300A as a third embodiment of the present invention. The pressure sensor 300A according to the third embodiment is substantially the same as the configuration of the pressure sensor 300 according to the second embodiment described with reference to FIG. 28 except that the two sensor elements 10 are provided. In FIG. 29, the same components as those described in FIG. 28 are denoted by the same reference numerals. In the pressure sensor 300A of the third embodiment, the excitation circuit 30 and the signal detection unit 40 having the same functions as those described in the pressure sensor 300 of the second embodiment are connected to the two sensor elements 10, respectively. However, illustration and description thereof are omitted.

  In the pressure sensor 300A of the third embodiment, the two sensor elements 10 are arranged at positions opposite to each other with the pressure receiving shaft 310 interposed therebetween, and distortion of the pressure receiving shaft 310 can be detected at two locations. Therefore, the distortion of the pressure receiving shaft 310 can be detected with higher accuracy, and the detection accuracy of the pressure sensor 300 is improved. In the pressure sensor 300A of the third embodiment, one or more sensor elements 10 may be further added to other parts of the pressure receiving shaft 310.

D. Fourth embodiment:
FIG. 30 is a schematic diagram showing a configuration of a pressure sensor 400 as the fourth embodiment of the present invention. In FIG. 30, the central axis of the pressure sensor 400 is illustrated by a one-dot chain line, and arrows X and Y indicating the arrangement direction of the vibrator 100 included in the sensor element 10 are illustrated so as to correspond to FIG. 2. Similar to the pressure sensor 300 of the second embodiment, the pressure sensor 400 includes the sensor element 10 described in the first embodiment, and measures, for example, the in-cylinder pressure in the combustion chamber of the internal combustion engine.

  The pressure sensor 400 is a cylindrical sensor and includes a casing 410, an open end member 411, a pressure receiving plate 420, a pressure transmission rod 421, a diaphragm 430, and a ceramic substrate 440. The pressure sensor 400 according to the third embodiment includes the excitation circuit 30 and the signal detection unit 40 connected to the sensor element 10 as in the pressure sensor 300 according to the second embodiment. Omitted.

  The casing 410 is a cylindrical member that is open at both ends, and accommodates the pressure transmission rod 421, the diaphragm 430, and the ceramic substrate 440. The opening end member 411 is an annular member having substantially the same outer diameter and inner diameter as the casing 410, and is attached to the opening end portion on the front end side of the casing 410. The open end member 411 sandwiches the outer peripheral edge portion of the pressure receiving plate 420 between the end surface on the rear end side thereof and the end surface on the front end side of the casing 410.

  The pressure receiving plate 420 is a disk-shaped member and is disposed so as to seal the opening on the front end side of the casing 410. The pressure receiving plate 420 is exposed to a gas whose pressure is to be measured, and bends in the thickness direction according to the pressure. The pressure transmission rod 421 is a cylindrical member and is disposed on the central axis of the pressure sensor 400 and connects the pressure receiving plate 420 and the diaphragm 430. The pressure transmission rod 421 transmits the force generated in the central axis direction due to the curved deformation of the pressure receiving plate 420 to the diaphragm 430.

  The diaphragm 430 includes a flat membrane portion 431 and a connecting shaft portion 432. The flat membrane portion 431 has a substantially circular thin-film portion that is elastically curved in the thickness direction. The flat membrane portion 431 is disposed so that its membrane surface is orthogonal to the central axis of the pressure sensor 400, and its outer peripheral edge portion is fixed to the inner wall surface of the casing 410 over the entire circumference. The connecting shaft portion 432 connects the central portion of the membrane surface of the flat membrane portion 431 and the pressure transmission rod 421 and bends the membrane surface of the flat membrane portion 431 according to the force transmitted from the pressure transmission rod 421.

  Here, the sensor element 10 is disposed at the center of the film surface of the flat film portion 431 opposite to the connecting shaft portion 432. The sensor element 10 receives an external force corresponding to the distortion of the film surface of the flat film portion 431, and outputs a signal having a frequency corresponding to the external force. The ceramic substrate 440 is disposed on the rear end side of the diaphragm 430. Although not shown, the ceramic substrate 440 is electrically connected to the sensor element 10. On the ceramic substrate 440, a conductive pattern for electrically connecting the sensor element 10 and a circuit arranged outside the casing 410 is formed.

  In the pressure sensor 400 of the fourth embodiment, a force corresponding to the pressure received by the pressure receiving plate 420 is transmitted to the diaphragm 430 through the pressure transmission rod 421, and the flat membrane portion 431 of the diaphragm 430 is curved. The sensor element 10 receives an external force corresponding to the curvature of the flat membrane portion 431 and outputs a signal having a frequency corresponding to the external force. Thus, in the case of the pressure sensor 400 of the fourth embodiment, a large external force corresponding to the pressure received by the pressure receiving plate 420 can be applied to the sensor element 10 by using the diaphragm 430. Therefore, the detection accuracy of the pressure sensor 400 can be improved.

E. Fifth embodiment:
FIG. 31 is a schematic diagram showing a configuration of a pressure sensor 400A as a fifth embodiment of the present invention. The pressure sensor 400A according to the fifth embodiment includes a casing 410A instead of the casing 410, and a configuration of the pressure sensor 400 according to the fourth embodiment (FIG. 30) except that a diaphragm 430A is provided instead of the diaphragm 430. It is almost the same.

  The casing 410A of the fifth embodiment is separated into a casing portion 412 on the front end side and a casing portion 413 on the rear end side. In the diaphragm 430A of the fifth embodiment, the flat membrane portion 431 has the same diameter as the outer diameter of the casing 410A. Diaphragm 430 </ b> A is fixed in casing 410 </ b> A by holding the outer peripheral edge portion of flat membrane portion 431 between the end faces of these two casing portions 412 and 413.

  Thus, with the pressure sensor 400A of the fifth embodiment, the area of the membrane surface of the diaphragm 430A can be increased, and the deformation amount of the membrane surface of the diaphragm 430A can be increased according to the pressure. Therefore, since the external force that the sensor element 10 receives from the film surface of the flat film portion 431 can be increased, the detection accuracy of the pressure sensor 400A can be improved.

F. Variations:
F1. Modification 1:
In the electrode unit 150 of each of the above embodiments, both sides of the movable electrode 120 are disposed between the outer fixed electrode 130 and the inner fixed electrode 130. On the other hand, in the electrode part 150, one of the outer fixed electrode 130 and the inner fixed electrode 130 may be omitted. Moreover, in the electrode part 150 of each said embodiment, each electrode 120,130,140 provides the comb-tooth part 123a, 123b, 132,142 in which the protrusion part was arranged at equal intervals on the mutually opposing surface. It was done. On the other hand, the comb teeth 123a, 123b, 132, 142 of the electrodes 120, 130, 140 may have other configurations. In the comb-tooth portions 123a, 123b, 132, and 142 of the electrodes 120, 130, and 140, the arrangement intervals of the protrusions may not be all equal, and some or all of the protrusions may be arranged at different intervals. . Further, the comb teeth 123a, 123b, 132, 142 themselves may be omitted from the electrodes 120, 130, 140. The electrode unit 150 may be configured so that the movable electrode 120 can vibrate by an electrostatic force between the electrodes.

F2. Modification 2:
In the sensor element 10 of each of the embodiments described above, the two beam portions 111 of the main body vibration unit 110 are flexed and vibrated in opposite directions when in the resonance state. However, in the resonance state of the sensor element 10, it is sufficient that the two beam portions 111 of the main body vibration unit 110 bend and vibrate symmetrically about the virtual center axis CL. That is, the two beam portions 111 of the main body vibration unit 110 may be flexibly vibrated with the same amplitude in the same direction with the same period.

F3. Modification 3:
In the above embodiment, the vibrator 100 is accommodated in the sealed space formed by the cavity 15 c of the glass plate 15. On the other hand, the vibrator 100 may not be accommodated in the sealed space formed by the cavity 15 c of the glass plate 15. The vibrator 100 may be accommodated in a sealed space formed by a member other than the glass plate 15, or may be disposed exposed to the outside air without being accommodated in the sealed space.

F4. Modification 4:
In the second to fifth embodiments, the pressure sensor is configured using the sensor element 10. On the other hand, the sensor element 10 may be used for sensors other than the pressure sensor. For example, the sensor element 10 may be used for a strain gauge. The sensor element 10 may be attached to a shaft-like heater portion provided in the glow plug and used integrally with the glow plug.

F5. Modification 5:
In each of the above embodiments, the vibrator 100 is formed on the SOI substrate 11. On the other hand, the vibrator 100 may not be formed on the SOI substrate 11, and may be formed by a method other than etching by a member other than silicon.

  The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized with various configurations without departing from the spirit thereof. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each form described in the summary column of the invention are the above-described problems, downsizing of the resonance sensor, and improvement in usability. In order to solve part or all of the problems such as ease of production, reduction of production cost, resource saving, etc., or to achieve part or all of the above-mentioned effects, replacement or combination as appropriate Can be done. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

DESCRIPTION OF SYMBOLS 10 ... Sensor element 11 ... SOI substrate 11b ... Silicon substrate 11m ... Intermediate oxide film layer 11s ... Surface silicon layer 15 ... Glass plate 15c ... Cavity 15h ... Through-hole 16 ... Terminal part 16a ... Detection terminal 16b ... Drive terminal 18 ... Etching mask DESCRIPTION OF SYMBOLS 20 ... DC power supply part 21 ... Power supply line 22 ... Ground line 30 ... Excitation circuit 31 ... Charge amplifier part 31a ... Operational amplifier 31c ... Feedback capacitor 31r ... Feedback resistor 32 ... Filter part 32a ... Operational amplifier 32c ... Coupling capacitor 32ra ... Input resistance Unit 32rb ... Feedback resistor 33 ... Comparator unit 33a ... Operational amplifier 33c ... Capacitor 33o ... Output terminal 33r ... Feedback resistor 40 ... Signal detection unit 41 ... Clock generation unit 100 ... Vibrator 100a ... Vibrator (comparative example)
DESCRIPTION OF SYMBOLS 110 ... Main-body vibration part 111 ... Beam part 111s ... Connection shaft 112 ... Connection part 113 ... Fixed part 113s ... Connection shaft 120 ... Movable electrode 121 ... Center base part 122 ... Extension part 123a, 120b ... Comb tooth part 130 ... Outer fixed electrode DESCRIPTION OF SYMBOLS 131 ... Extending part 132 ... Comb tooth part 140 ... Inner fixed electrode 141 ... Extending part 142 ... Comb tooth part 150 ... Electrode part 300,300A ... Pressure sensor 310 ... Pressure receiving shaft 321 ... Cap member 322 ... Bearing part 323 ... Inner casing 331 ... External casing 332 ... Opening end member 400, 400A ... Pressure sensor 410, 410A ... Casing 412 and 413 ... Casing part 411 ... Opening end member 420 ... Pressure receiving plate 421 ... Pressure transmission rod 430, 430A ... Diaphragm 431 ... Flat plate part 432 ... Connecting shaft portion 440 ... Ceramic substrate MD 1 to MD3 ... vibrator model

Claims (15)

In a sensor element including a vibrator whose resonance frequency changes according to an applied external force,
The vibrator is
A beam portion that flexures and vibrates while receiving a force according to the external force along the extending direction;
An electrode that is connected to the beam portion and flexes and vibrates the beam portion by being displaced by receiving an electrostatic force;
With
The electrode is a sensor element that bends and vibrates together with the beam when the sensor element is in a resonance state. The sensor element according to claim 1,
The natural frequency fd of the vibrator is a sensor element in which fd <(k / m) ^0.5 / 2π, where k is a spring constant of the beam portion and m is a mass of the electrode. The sensor element according to claim 1 or 2,
A sensor element, wherein an electrode resonance ratio which is a value of a ratio of a resonance frequency of the single electrode to a resonance frequency of the vibrator when the external force is not applied is 1.5 or more. The sensor element according to claim 3,
The sensor element whose electrode resonance ratio is 2.0 or more. The sensor element according to claim 3 or 4, wherein
The sensor element whose electrode resonance ratio is 6.0 or less. The sensor element according to claim 1 or 2,
The electrode has an extending portion extending in parallel with the beam portion,
When the vibrator is in a resonance state, the maximum amplitude in the extended portion of the electrode is twice or more the maximum amplitude of the beam portion. The sensor element according to claim 6,
When the vibrator is in a resonance state, the maximum amplitude of the extension portion of the electrode is three times or more the maximum amplitude of the beam portion. The sensor element according to any one of claims 1 to 7,
A sensor element having a sealed space for accommodating the beam portion and the electrode. The sensor element according to claim 8,
The sensor element, wherein the sealed space is depressurized from atmospheric pressure. The sensor element according to any one of claims 1 to 9,
The sensor element, wherein the electrode and the beam portion of the vibrator are formed by etching an SOI substrate. A pressure sensor,
A pressure receiving portion that deforms according to the pressure to be measured;
The sensor element according to any one of claims 1 to 10,
With
The sensor element is a pressure sensor arranged in the pressure receiving portion so that the vibrator receives an external force according to deformation of the pressure receiving portion. The pressure sensor according to claim 11,
The pressure receiving portion includes a shaft member extending in a shaft shape,
The shaft member is deformed in the axial direction when the pressure to be measured is transmitted to the end face,
The sensor element is disposed on a side surface of the shaft member so that an extending direction of the beam portion coincides with an axial direction of the shaft member, and detects an external force corresponding to deformation in the axial direction of the shaft member. Sensor. The pressure sensor according to claim 12,
The said sensor element is a pressure sensor containing the 1st and 2nd sensor element arrange | positioned so that the said shaft member may be pinched | interposed on the side surface of the said shaft member. The pressure sensor according to claim 11, further comprising:
A cylindrical housing portion that houses at least the sensor element;
The pressure receiving portion includes a diaphragm that closes the opening of the housing portion and curves according to the pressure to be measured,
The sensor element is disposed on the surface of the diaphragm such that the extending direction of the beam portion coincides with the direction along the surface of the diaphragm, and detects an external force corresponding to the degree of curvature of the diaphragm. Sensor. The pressure sensor according to any one of claims 11 to 14,
The sensor element outputs a pulse signal output at a cycle according to the resonance frequency of the vibrator,
The pressure sensor further includes:
A clock generator for generating a reference clock having a predetermined period;
A signal detection unit that detects a change amount of the period of the resonance frequency based on the period of the reference clock;
A pressure sensor.

Images