JP2014071106A - Flow velocity sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flow velocity sensor which is excellent in time resolution at low costs.SOLUTION: The flow velocity sensor includes: a piezoelectric thin film element 20 configured such that a lower electrode 21 and a piezoelectric thin film 22 made of non-lead materials are laminated in this order on a substrate 11, and an upper electrode 23 is formed at a predetermined position on the piezoelectric thin film, and which oscillates according as an AC voltage is applied between the lower electrode 21 and the upper electrode 23; a columnar structure 11b formed so as to be protruded from the plane of oscillation of the piezoelectric thin film element, which adds a stress to the plane of oscillation by interfering with fluid; and a detection electrode 30 formed on the piezoelectric thin film 22, to which detection means for detecting piezoelectric effects to be generated in the piezoelectric thin film when the plane of oscillation receives the stress is connected.

Description

  The present invention relates to a flow rate sensor, and more particularly to a flow rate sensor using a piezoelectric thin film element.

  In recent years, as an energy saving measure, for example, in order to optimize air conditioning in a data center such as various computers and data communication, monitoring of room temperature and airflow in the center is required. In addition, the importance of various environmental monitoring such as temperature and wind speed has been pointed out as a countermeasure for the heat island phenomenon. In such cases, there is a need for inexpensive sensors. In this regard, a low-cost semiconductor temperature sensor or the like is known for measuring temperature.

  On the other hand, for measuring wind speed, etc., high cost such as differential pressure sensor using orifice, thermal sensor using heat transfer (heat transport) using heat source, mechanical sensor using mechanical displacement, etc. The flow rate sensor was the mainstream. Recently, instead of these, a low-cost flow rate sensor such as a thermal type or a mechanical type has been developed using a micro electro mechanical system (MEMS) technology that is easy to integrate (for example, patents). Reference 1).

JP 2005-121631 A

  However, the thermal flow rate sensor as described above has a very high resolution with respect to the flow rate, but is inferior in time resolution. In addition, it has been difficult to adopt a multi-axis detection type capable of measuring the flow direction (azimuth) of a fluid such as air. A low-cost multi-axis detection type flow rate sensor is not known as a mechanical type flow rate sensor that provides a certain degree of resolution with respect to the flow rate and is excellent in time resolution.

  An object of the present invention is to provide a flow rate sensor that is low in cost and excellent in time resolution.

According to a first aspect of the invention,
A lower electrode and a piezoelectric thin film made of a non-lead material are laminated in this order on the substrate, an upper electrode is formed at a predetermined position on the piezoelectric thin film, and an AC voltage is applied between the lower electrode and the upper electrode. Piezoelectric thin film element that vibrates by being
A columnar structure that is formed so as to protrude from the vibration surface of the piezoelectric thin film element and applies stress to the vibration surface by interfering with a fluid;
There is provided a flow rate sensor comprising a detection electrode formed on the piezoelectric thin film and connected to a detection means for detecting a piezoelectric effect generated in the piezoelectric thin film when the vibration surface receives stress.

According to a second aspect of the invention,
When the columnar structure and the fluid interfere with each other and the vibration frequency at which the amplitude of the voltage detected through the detection electrode becomes a predetermined magnitude when the columnar structure and the fluid do not interfere with each other A first configuration configured to detect a flow velocity of a fluid interfering with the columnar structure by measuring a difference between a vibration frequency at which an amplitude of a voltage detected via the detection electrode becomes a predetermined magnitude. A flow rate sensor according to an aspect is provided.

According to a third aspect of the invention,
When the columnar structure and the fluid interfere with each other at the predetermined frequency of the AC voltage, the amplitude of the voltage detected through the detection electrode when the columnar structure and the fluid do not interfere with each other. The flow rate sensor according to the first aspect is configured to detect a flow rate of a fluid that interferes with the columnar structure by measuring a difference between the amplitude of the voltage detected via the detection electrode. Provided.

According to a fourth aspect of the invention,
A plurality of the detection electrodes are formed on the piezoelectric thin film,
The positive and negative voltages detected through the plurality of detection electrodes when the columnar structure and the fluid interfere with each other are determined, and the magnitude relationship between the plurality of voltages and the positional relationship between the plurality of detection electrodes The flow rate sensor according to any one of the first to third aspects configured to detect a flow direction of a fluid that interferes with the columnar structure is provided.

According to a fifth aspect of the present invention,
The flow rate sensor according to a fourth aspect is provided, wherein two or more detection electrodes are formed apart from the upper electrode so as to be symmetric with respect to an axis of symmetry passing through the upper electrode.

According to a sixth aspect of the present invention,
The flow rate sensor according to a fourth aspect, in which the detection electrodes are formed in two or more pairs apart from the upper electrode so as to be symmetric with respect to the upper electrode as a center of symmetry.

According to a seventh aspect of the present invention,
The flow rate sensor according to any one of the first to sixth aspects is provided, wherein the columnar structure has a rotational symmetry of two or more times when the columnar structure is rotated around the longitudinal direction as a rotation axis.

According to an eighth aspect of the present invention,
The flow rate sensor according to any one of the first to seventh aspects, wherein a diaphragm whose periphery is supported by the frame-shaped substrate with a hollowed central portion is formed on the vibration surface of the piezoelectric thin film element. Provided.

According to a ninth aspect of the present invention,
The flow rate sensor according to any one of the first to eighth aspects, in which a resistance coefficient of the columnar structure with respect to a fluid is 0.63 or more and 2.01 or less.

According to a tenth aspect of the present invention,
The flow rate sensor according to any one of the first to ninth aspects, in which the piezoelectric thin film has an alkali niobium oxide-based perovskite structure.

According to an eleventh aspect of the present invention,
The flow rate sensor according to the tenth aspect, wherein the alkali niobium oxide-based perovskite structure is pseudo-cubic.

According to a twelfth aspect of the present invention,
The flow rate sensor according to the tenth or eleventh aspect is provided, wherein the main surface of the piezoelectric thin film is preferentially oriented in the (001) plane.

According to a thirteenth aspect of the present invention,
The piezoelectric thin film, composition formula _{(K 1-X Na X)} NbO 3 ( where, 0.400 ≦ X ≦ 0.730) any of the first to twelfth aspects comprising a potassium sodium niobate represented by Is provided.

According to a fourteenth aspect of the present invention,
The flow velocity sensor according to any one of the first to thirteenth aspects, wherein the lower electrode is made of a Pt film whose main surface is preferentially oriented in the (111) plane.

  ADVANTAGE OF THE INVENTION According to this invention, the flow velocity sensor which is low cost and excellent in time resolution is provided.

(A) is a top view of the flow velocity sensor which concerns on one Embodiment of this invention, (b) is AA sectional drawing of (a). It is operation | movement explanatory drawing of the flow rate sensor package by which the flow rate sensor which concerns on one Embodiment of this invention is mounted in the printed circuit board, Comprising: (a) is sectional drawing of the flow rate sensor package in the state which has not interfered with the fluid. FIG. 6B is a cross-sectional view of the flow rate sensor package in a state where interference with the fluid occurs. It is explanatory drawing of the piezoelectric effect which generate | occur | produces in the piezoelectric thin film element of the flow rate sensor which concerns on one Embodiment of this invention, Comprising: (a) is a figure which shows the piezoelectric effect in the flow rate sensor in the state which has not interfered with the fluid. FIG. 6B is a diagram showing the piezoelectric effect in the flow velocity sensor in a state where interference with the fluid occurs, and FIG. 5C shows the amplitude of the voltage of the piezoelectric thin film element that changes according to the frequency of the AC voltage. It is a graph. It is process drawing (the 1) which shows the manufacturing method of the flow rate sensor which concerns on one Embodiment of this invention with sectional drawing. It is process drawing (the 2) which shows the manufacturing method of the flow rate sensor which concerns on one Embodiment of this invention with sectional drawing. It is explanatory drawing of the piezoelectric effect which generate | occur | produces in the piezoelectric thin film element of the flow velocity sensor which concerns on the modification of one Embodiment of this invention, Comprising: (a) is a piezoelectric effect in the flow velocity sensor in the state which has not interfered with the fluid. (B) is a figure which shows the piezoelectric effect in the flow velocity sensor in the state which has interfered with the fluid, (c) is a figure of the voltage of the piezoelectric thin film element which changes according to the frequency of an alternating voltage. It is a graph which shows an amplitude.

<One Embodiment of the Present Invention>
(1) Structure of Flow Rate Sensor A flow rate sensor according to an embodiment of the present invention is a mechanical flow rate sensor configured as, for example, a micro electro mechanical system (MEMS) using a piezoelectric thin film element. The flow rate sensor according to the present embodiment will be described with reference to FIG. FIG. 1A is a plan view of a flow velocity sensor 1 according to the present embodiment, and FIG. 1B is a cross-sectional view taken along line AA of FIG.

  As shown in FIG. 1, the flow rate sensor 1 includes a lower electrode 21 and a piezoelectric thin film 22 made of a non-lead material on a substrate 11 such as a silicon (Si) substrate formed in a rectangular shape, via a diaphragm 10d. A piezoelectric thin film element 20 is provided which is laminated in this order and has an upper electrode 23 formed at a predetermined position on the piezoelectric thin film 22. The piezoelectric thin film element 20 is configured to vibrate when an AC voltage such as a high frequency voltage is applied between the lower electrode 21 and the upper electrode 23.

  The flow rate sensor 1 includes a columnar structure 11b formed so as to protrude from the vibration surface 20s of the piezoelectric thin film element 20 through the diaphragm 10d. The columnar structure 11b is configured to apply stress to the vibration surface 20s of the piezoelectric thin film element 20 by interfering with a fluid such as gas or liquid.

  The flow velocity sensor 1 includes a detection electrode 30 formed on the piezoelectric thin film 22. The sensing electrode 30 receives a stress from the vibrating surface 20s of the piezoelectric thin film element 20 via the diaphragm 10d from the columnar structure 11b, thereby detecting a piezoelectric effect described later that detects the piezoelectric effect generated in the piezoelectric thin film 22. 2) is connected.

(Piezoelectric thin film element)
The substrate 11 on which the piezoelectric thin film element 20 is formed is made of, for example, Si, and has a frame shape with a central portion cut out in a circular shape. On the substrate 11, for example, a silicon oxide (SiO ₂ ) film 12 and a silicon (Si) film 13 are stacked in this order.

The lower electrode 21 provided in the piezoelectric thin film element 20 is made of, for example, a platinum (Pt) film formed on the substrate 11 via the SiO ₂ film 12 and the Si film 13. The upper surface as the main surface of the Pt film, that is, the surface on which the piezoelectric thin film 22 is formed is preferentially oriented to the (111) plane. The lower electrode 21 may include an adhesion layer (not shown) between the lower electrode 21 and the silicon (Si) film 13. The adhesion layer is made of, for example, titanium (Ti), tantalum (Ta), or the like.

The piezoelectric thin film 22 included in the piezoelectric thin film element 20 is, for example, a lead-free piezoelectric thin film. The lead-free piezoelectric thin film 22 has, for example, an alkali niobium oxide-based perovskite structure, and the structure is, for example, a pseudo cubic crystal. In this case, the upper surface as the main surface of the piezoelectric thin film 22, that is, the surface on which the upper electrode 23 is formed is preferentially oriented to, for example, the (001) plane. Alternatively, the piezoelectric thin film 22 is made of, for example, potassium sodium niobate (KNN) represented by a composition formula of (K _1-X Na _X ) NbO ₃ (where 0.400 ≦ X ≦ 0.730). For example, in one corner of the piezoelectric thin film 22 formed in a rectangular shape, an opening 22h is formed through which the lower electrode 21 serving as a base is exposed.

  The upper electrode 23 provided in the piezoelectric thin film element 20 is formed at a predetermined position on the piezoelectric thin film 22, for example, at the center. That is, the upper electrode 23 is located on the hollowed central portion of the frame-like substrate 11. In addition, the upper electrode 23 is connected to a lead-out wiring 23 w that extends on a diagonal line of the piezoelectric thin film 22 formed in a rectangular shape, for example, onto the frame of the substrate 11. The lead wiring 23 w is connected to an electrode pad 23 p formed on the frame of the substrate 11. The upper electrode 23, the lead wiring 23w, and the electrode pad 23p are made of, for example, a Pt film. These may be formed through an adhesion layer made of Ti, Ta, or the like.

  The lower surface side of the piezoelectric thin film element 20, that is, the surface opposite to the side on which the piezoelectric thin film 22 of the lower electrode 21 is formed is a vibration surface 20s of the piezoelectric thin film element 20 that vibrates when applied with an AC voltage or the like. .

On the vibration surface 20 s of the piezoelectric thin film element 20, for example, a diaphragm 10 d made of the above-described SiO ₂ film 12 and Si film 13 stacked on the substrate 11 is formed. The diaphragm 10d is supported by a frame-shaped substrate 11 and has a predetermined resonance frequency in a steady state without stress, for example.

(Columnar structure)
The columnar structure 11b is connected substantially perpendicularly to, for example, the central portion of the diaphragm 10d that is supported by the substrate 11 around the columnar structure 11b, and the surface opposite to the surface in contact with the vibration surface 20s of the diaphragm 10d. That is, the columnar structure 11 b is provided below the upper electrode 23.

  In addition, the columnar structure 11b has a cross-sectional shape having a rotational symmetry of two or more times, for example, when the longitudinal direction is taken as a rotation axis. Specifically, the cross-sectional shape of the columnar structure 11b is a circle, a square, a regular hexagon, or the like.

  The columnar structure 11b is made of, for example, Si and has a resistance coefficient with respect to fluid of 0.63 or more and 2.01 or less. When the cross-sectional shape of the columnar structure 11b is circular, the resistance coefficient is about 0.63 or more and 1.20 or less, and when the cross-sectional shape of the columnar structure 11b is square, the resistance coefficient is 1.12 or more. It is about 01 or less. If the ratio of the height to the diameter of the columnar structure 11b, that is, the aspect ratio is 1 or more, the resistance coefficient of the columnar structure 11b is within the above range and does not change so much.

(Detection electrode)
The detection electrode 30 is arranged, for example, at a position above the diaphragm 10d, and includes a flow velocity detection electrode 31 related to detection of the flow velocity of the fluid and azimuth detection electrodes 32x and 32y related to detection of the direction (direction) of the fluid flow.

  The flow velocity detection electrode 31 is formed at a predetermined position on the piezoelectric thin film 22, for example, at the center portion on the diaphragm 10d. That is, in the present embodiment, the upper electrode 23 located on the central portion of the piezoelectric thin film element 22 and on the central portion of the frame-shaped substrate 11 is also used as the flow velocity detection electrode 31. In order to realize the functions of both the upper electrode 23 and the flow velocity detection electrode 31 on the disk-shaped electrode located at the central portion on the piezoelectric thin film 22, the electrode is divided into, for example, two electrically separated electrodes. It can be divided into Or it is good also as a time-division type electrode which divided | segmented the function of the upper electrode 23 and the function of the flow velocity detection electrode 31 for every predetermined time. Similarly, the lead-out wiring 23w and the electrode pad 23p included in the upper electrode 23 can also be used as the lead-out wiring 31w and the electrode pad 31p of the flow velocity detection electrode 31.

  A plurality of, for example, a pair of azimuth detection electrodes 32x and 32y are formed at positions separated from the upper electrode 23 (flow velocity detection electrode 31) on the piezoelectric thin film 22.

  The direction detection electrode 32x is, for example, a diaphragm sandwiching the upper electrode 23 when the direction parallel to one side of the flow velocity sensor 1 formed in a rectangular shape is the x-axis direction and the direction parallel to the other side is the y-axis direction. 10d are formed one by one at equal distances in the x-axis direction. Therefore, the azimuth detection electrode 32x is symmetric with respect to an axis of symmetry passing through the upper electrode 23 and parallel to the y axis, that is, line symmetric. Similarly, the azimuth detection electrodes 32y are formed one by one on the diaphragm 10d with the upper electrode 23 sandwiched therebetween at equal distances in the y-axis direction. Therefore, the azimuth detection electrode 32y is symmetric with respect to an axis of symmetry passing through the upper electrode 23 and parallel to the x axis, that is, line symmetric.

  In other words, in the present embodiment, two pairs of detection electrodes 30 (32x, 32y) relating to fluid orientation detection are formed symmetrically with respect to the upper electrode 23 as a center of symmetry, that is, in point symmetry. Thus, the flow velocity sensor 1 is configured as a multi-axis detection sensor (here, a biaxial detection sensor).

  The pair of azimuth detection electrodes 32x includes an extraction wiring 32w extending on the frame of the substrate 11 in the x-axis direction and an electrode pad 32p formed on the frame of the substrate 11 and connected to the extraction wiring 32w. Prepare. The pair of azimuth detection electrodes 32y includes a lead wiring 32w extending on the frame of the substrate 11 in the y-axis direction and an electrode pad 32p formed on the frame of the substrate 11 and connected to the lead wiring 32w. Prepare.

  The direction detection electrodes 32x and 32y, the lead wiring 32w, and the electrode pad 32p are made of, for example, a Pt film.

(2) Operation of Flow Rate Sensor Subsequently, the operation of the flow rate sensor 1 according to the present embodiment will be described with reference to FIGS. FIG. 2 is an operation explanatory diagram of the flow rate sensor package 2 in which the flow rate sensor 1 according to the present embodiment is mounted on the printed circuit board 51, and (a) is a flow rate sensor package in a state where no interference with the fluid occurs. FIG. 2B is a cross-sectional view of the flow velocity sensor package 2 in a state where interference with a fluid occurs. FIG. 3 is an explanatory diagram of the piezoelectric effect generated in the piezoelectric thin film element 20 of the flow rate sensor 1 according to the present embodiment. FIG. 3A is a piezoelectric effect in the flow rate sensor 1 in a state where no interference with the fluid occurs. (B) is a figure which shows the piezoelectric effect in the flow velocity sensor 1 in the state which has interfered with the fluid, (c) is the piezoelectric thin film element 20 which changes according to the frequency of an alternating voltage. It is a graph which shows the amplitude of voltage.

  As shown in FIG. 2, the flow rate sensor 1 is used in a state of a flow rate sensor package 2 in which the flow rate sensor 1 is mounted on a printed circuit board 51, for example. When mounted on the printed circuit board 51, the surface on which the upper electrode 23 and the like of the flow rate sensor 1 are formed faces the printed circuit board 51 side. The electrode pads 23p, 31p, 32p provided in the upper electrode 23, the flow velocity detection electrode 31, the direction detection electrodes 32x, 32y of the flow velocity sensor 1 and the lower electrode 21 exposed in the opening 22h of the piezoelectric thin film 22 are, for example, The solder balls 52 are connected to predetermined electrodes or the like (not shown) on the printed circuit board 51.

  The electrode on the printed circuit board 51 to which the electrode pad 23p of the upper electrode 23 provided in the flow velocity sensor 1 is connected is electrically connected to an AC power source 53 that applies an AC voltage such as a high frequency voltage to the piezoelectric thin film element 20, for example. . Further, the electrodes on the printed circuit board 51 to which the electrode pads 31p, 32p of the detection electrode 30 provided in the flow velocity sensor 1 are connected are electrically connected to, for example, detection means 54 that detects the piezoelectric effect generated in the piezoelectric thin film element 20. ing. The detection means 54 includes a voltage detection circuit, a voltage signal processing circuit, and the like, and is further connected to a control unit (not shown) including a computer that analyzes the detected voltage signal and the like. Moreover, the electrode on the printed circuit board 51 to which the lower electrode 21 of the flow velocity sensor 1 is connected is grounded, for example.

  The flow rate sensor package 2 configured as described above is installed in a state in which the columnar structure 11b included in the flow rate sensor 1 is protruded from the measurement fluid. At this time, as shown in FIG. 2, the flow rate sensor package 2 may be left at a predetermined position with the columnar structure 11b facing upward, or the columnar structure 11b is directed horizontally. May be hung from the wall or the like, or suspended from the ceiling or the like with the columnar structure 11b facing downward.

  As described below, the columnar structure 11b placed in the fluid is tilted in the direction in which the fluid flows due to the interference of the fluid. As a result, the diaphragm 10d is bent. Moreover, the diaphragm 10d in which the bending has occurred changes its resonance frequency. The piezoelectric thin film element 20 detects the fluid flow velocity from the change in the resonance frequency of the entire diaphragm 10d. In addition, the piezoelectric thin film element 20 detects the direction of fluid flow from the stress state at each point due to the deflection of the diaphragm 10d.

(Interference between columnar structure and fluid)
First, a state in which the columnar structure 11b that receives fluid interference is tilted will be described.

  As shown in FIG. 2A, when there is no movement in the fluid in which the flow rate sensor package 2 is installed, that is, when the fluid is stagnating in a “no wind” state or the like, the flow rate sensor 1 is provided. There is no interference between the columnar structure 11b and the fluid. Moreover, the stress from the columnar structure 11b is hardly applied to the diaphragm 10d to which the columnar structure 11b is connected.

  As shown in FIG. 2B, when a flow is generated in the fluid in which the flow rate sensor package 2 is installed, interference occurs between the columnar structure 11b included in the flow rate sensor 1 and the fluid. For example, when the fluid flows from the left to the right of the sheet of FIG. 2B along the x-axis direction of the flow velocity sensor 1 (the white arrow in the figure), the columnar structure 11b It causes bending deformation due to interference and tilts to the right of the page.

  When the columnar structure 11b is tilted in this way, the diaphragm 10d to which the columnar structure 11b is connected is bent. Therefore, on the upstream side (the left side of the drawing) of the fluid across the columnar structure 11b, the diaphragm 10d is pulled toward the downstream side (the right side of the drawing) of the fluid and receives tensile stress. Further, on the downstream side of the fluid across the columnar structure 11b, the diaphragm 10d is compressed from the upstream side of the fluid and receives a compressive stress.

(Detection of fluid flow rate)
FIG. 3 shows a state in which a state in which no stress is applied to the diaphragm 10d and a state in which the diaphragm 10d is applied are detected as a piezoelectric effect generated in the piezoelectric thin film element 20. As described above, the state of the diaphragm 10d determined from the piezoelectric effect includes a change in the resonance frequency of the diaphragm 10d and a stress state at each point due to the deflection of the diaphragm 10d.

  First, a case where a change in the resonance frequency of the diaphragm 10d is detected by the piezoelectric effect will be described with reference to FIGS.

As described above, the diaphragm 10d has a predetermined resonant frequency f _R in a state where stress is not applied. Further, when the tensile stress and the compressive stress as described above are applied by the interference between the columnar structure 11b and the fluid, the diaphragm 10d has the predetermined resonance frequency f _R according to the state of the stress of the diaphragm 10d as a whole. And have a different resonance frequency f _R ′.

Flow sensor 1 according to this embodiment, by measuring the resonance frequency f _R when no stress is applied to the diaphragm 10d, the resonance frequency f _{R 'when} stress is applied, the difference Delta] f _R, It is configured to detect the flow rate of the fluid.

  Specifically, when an AC voltage is applied from the AC power source 53 to the piezoelectric thin film element 20 via the upper electrode 23 and the frequency is changed, the piezoelectric thin film element 20 corresponds to the frequency of the applied AC voltage. Vibrates itself at the vibration frequency. Thereby, the piezoelectric thin film element 20 vibrates the diaphragm 10d.

  The diaphragm 10d changes the magnitude of its own vibration, that is, the amplitude, according to the vibration frequency of the piezoelectric thin film element 20. When the vibration frequency of the piezoelectric thin film element 20 becomes equal to the resonance frequency of the diaphragm 10d, the amplitude becomes maximum.

  This time, the amplitude of the diaphragm 10 d is transmitted to the piezoelectric thin film element 20. That is, the vibration effect of the diaphragm 10d causes the vibration surface 20s of the piezoelectric thin film element 20 to receive a stress having a magnitude corresponding to the amplitude of the vibration, thereby generating a piezoelectric effect in the piezoelectric thin film element 20. This is detected by the detection means 54 as the amplitude of the voltage corresponding to the amplitude of vibration through the flow velocity detection electrode 31 provided in the piezoelectric thin film element 20.

  FIG. 3C shows a graph in which the amplitude of the voltage generated in the piezoelectric thin film element 20 changes according to the frequency of the AC voltage applied to the piezoelectric thin film element 20. The horizontal axis of the graph is the frequency of the alternating voltage applied to the piezoelectric thin film element 20, and the vertical axis is the amplitude of the voltage generated in the piezoelectric thin film element 20. A change in voltage amplitude when no stress is applied to the diaphragm 10d is indicated by a solid line, and a change in voltage amplitude when a stress is applied is indicated by a broken line.

As shown in FIG. 3C, the amplitude of the voltage generated in the piezoelectric thin film element 20 changes according to the frequency of the alternating voltage applied to the piezoelectric thin film element 20. The vibration frequency at which the amplitude of the voltage is maximum is the resonance frequencies f _R and f _R ′ of the diaphragm 10d in each stress state.

That is, as shown in FIG. 3A, when the fluid stagnates, the columnar structure 11b does not interfere with the fluid, and no stress is applied to the diaphragm 10d, the flow velocity detection electrode 31 of the piezoelectric thin film element 20 the amplitude of the voltage sensed via the vibration frequency becomes maximum, it is identified as the resonance frequency f _R.

Further, as shown in FIG. 3B, when a flow occurs in the fluid, the columnar structure 11b interferes with the fluid, and stress is applied to the diaphragm 10d, the flow rate detection electrode 31 of the piezoelectric thin film element 20 is interposed. The vibration frequency at which the amplitude of the detected voltage is maximum, that is, the resonance frequency f _R ′ is specified.

The magnitude of the stress applied to the diaphragm 10d is calculated by measuring the difference Δf _R between the resonance frequencies f _R and f _R ′ thus specified. Further, the inclination of the columnar structure 11b that has received the fluid interference can be determined from the calculated magnitude of the stress of the diaphragm 10d, and the flow velocity of the fluid that interferes with the columnar structure 11b can be detected.

Note that the resonance frequency f _R when no stress is applied to the diaphragm 10d and the relationship between the frequency change and the voltage amplitude indicated by the solid line in FIG. 3C are connected to the detection means 54 in advance as known values. May be stored in a control unit or the like. When measuring the flow velocity, the flow velocity at that time can be calculated by comparing the known value with an actual measurement value.

(Detection of fluid direction)
Next, the case where the stress state at each point due to the deflection of the diaphragm 10d is detected by the piezoelectric effect will be described with reference to FIGS.

  As described above, in the diaphragm 10d in which the columnar structure 11b is tilted due to interference with the fluid and is bent, the upstream side of the fluid is pulled to the downstream side across the columnar structure 11b, and the downstream side is compressed from the upstream side. It is in the state. This is transmitted to the piezoelectric thin film element 20 through the vibration surface 20s, and the piezoelectric thin film 22 near the orientation detection electrode 32x placed on the upstream side of the fluid receives tensile stress. Further, the piezoelectric thin film 22 in the vicinity of the orientation detection electrode 32x placed on the downstream side of the fluid receives compressive stress.

  The flow velocity sensor 1 according to the present embodiment detects the direction in which the fluid flows by discriminating between positive and negative voltages detected via the direction detection electrodes 32x and 32y as the piezoelectric effect generated in the piezoelectric thin film element 20. Composed. The detection of the flow rate of the fluid and the detection of the direction of flow can be performed separately at different times. Or you may carry out in parallel.

  As shown in FIG. 3A, the piezoelectric thin film 22 constituting the piezoelectric thin film element 20 does not generate a voltage when it is not subjected to stress. Therefore, the voltage detected from the direction detection electrode 32x is the zero voltage Vx (0). Alternatively, the voltage difference between the orientation detection electrodes 32x is zero (0). Further, when detecting the direction in which the fluid flows in parallel with the detection of the flow velocity, since the diaphragm 10d itself vibrates, the direction detection electrode 32x strictly applies a positive or negative voltage corresponding to the vibration. Occur. However, in a state where the stress due to the fluid is not received, since the symmetry of the vibration of the diaphragm 10d is maintained in various places such as on the x axis and the y axis, the voltage difference between the paired electrodes is zero (0 )

  On the other hand, as shown in FIG. 3B, when the piezoelectric thin film 22 receives a predetermined stress, the symmetry on the x-axis is lost. That is, the piezoelectric thin film 22 is configured to generate, for example, a negative voltage Vx (−) due to the piezoelectric effect when it receives a tensile stress, and to be detected from the orientation detection electrode 32x. Further, when a compressive stress is applied, for example, a positive voltage Vx (+) is generated due to the piezoelectric effect and is detected from the direction detection electrode 32x. The voltage generated by the tensile stress and the compressive stress may be configured so that the positive and negative are reversed.

  As in the above example, if the direction of fluid flow is parallel to the x-axis of the flow velocity sensor 1, symmetry on the y-axis is maintained. That is, voltage values having the same positive and negative signs and the same magnitude are detected from a pair of azimuth detection electrodes 32y formed across the upper electrode 23 symmetrically with respect to the symmetry axis parallel to the x axis. The

  From the above, it is detected that the fluid flows from the left to the right of the paper surface of FIG. 3B along the flow direction of the fluid, that is, in this case, along the x-axis direction of the flow velocity sensor 1. The Even when the direction in which the fluid flows is not parallel to the x-axis or y-axis, the magnitude relationship between the voltages detected via the direction detection electrodes 32x and 32y is in light of the positional relationship between the direction detection electrodes 32x and 32y. By performing vector synthesis etc., the direction of fluid flow can be detected. Further, when the direction in which the fluid flows is not horizontal with respect to the installation surface of the flow velocity sensor package 2 (when it has a z component), that is, when the fluid does not hit the longitudinal direction of the columnar structure, the orientation detection electrode 32x , 32y is compared with the estimated value of the flow velocity obtained from the vector synthesis of the voltage detected via 32y and the more accurate value of the flow velocity obtained from the voltage detected via the flow velocity detection electrode 31 described above. Thus, the z component in the direction in which the fluid flows can be determined. Thereby, the direction in which the fluid flows can be detected.

(3) Manufacturing method of flow velocity sensor Next, the manufacturing method of the flow velocity sensor 1 which concerns on this embodiment is demonstrated using FIG. 4 and 5 are process diagrams (part 1 and part 2) illustrating the method of manufacturing the flow velocity sensor 1 according to the present embodiment in cross-sectional views.

(Preparation process of substrate)
First, as shown in FIG. 4A, a substrate 11 made of, for example, Si and having a SiO ₂ film 12 and a Si film 13 laminated in this order is prepared. At this time, the substrate 11 including the SiO ₂ film 12 and the Si film 13 includes, for example, a silicon thermal oxide film as an insulating film on a Si substrate, and a single crystal Si film whose upper surface is a (001) plane or the like. Alternatively, an SOI (Silicon On Insulator) wafer may be used.

(Laminated film formation process)
As shown in FIG. 4B, the lower substrate 21 and the piezoelectric thin film 22 are laminated in this order on the prepared substrate 11 to manufacture the laminated substrate 3.

  Specifically, the lower electrode 21 made of a Pt film or the like is formed on the Si film 13 of the substrate 11 by, for example, an RF magnetron sputtering method in which plasma is excited using a radio frequency (RF) and a magnet. At this time, an adhesion layer made of Ti, Ta, or the like may be formed on the Si film 13 in advance by a sputtering method or the like.

  For example, by forming the Si film 13 from a single crystal Si film whose upper surface is the (001) plane and forming a Pt film thereon, the upper surface is preferentially oriented to the (111) plane due to the self-orientation property of Pt. The lower electrode 21 is easily obtained. Further, by forming an adhesion layer in advance, the adhesion between the Si film 13 and the lower electrode 21 can be enhanced.

On the lower electrode 21 thus formed, a piezoelectric thin film 22 made of, for example, (K _1-X Na _X ) NbO ₃ or the like is formed by an RF magnetron sputtering method or the like.

  At this time, for example, the substrate temperature, the discharge power due to the high frequency, the type and mixing ratio of the sputtering gas, the inside of the sputtering chamber, so as to obtain the piezoelectric thin film 22 having a perovskite structure and the upper surface preferentially oriented to the (001) plane. Adjust the pressure and other conditions. In the present embodiment, since the underlying lower electrode 21 is composed of a Pt film whose upper surface is preferentially oriented in the (111) plane, the piezoelectric thin film 22 having a desired crystal structure is more easily obtained.

  Next, an opening 22 h is provided in the piezoelectric thin film 22 to expose the lower electrode 21.

  Specifically, a resist pattern is formed on the piezoelectric thin film 22, and a mask pattern 41m made of chromium (Cr) or the like is formed by sputtering, lift-off, or the like as shown in FIG.

  Subsequently, as shown in FIG. 4D, an opening 22h where the lower electrode 21 is exposed is formed in the piezoelectric thin film 22 by wet etching or the like using the mask pattern 41m as a mask. After the opening 22h is formed, the mask pattern 41m is removed.

(Upper electrode and detection electrode formation process)
Next, the upper electrode 23 (flow velocity detection electrode 31) and the orientation detection electrodes 32x and 32y are formed at predetermined positions on the piezoelectric thin film 22, respectively.

  Specifically, as shown in FIG. 4E, a resist pattern 42r is formed on the piezoelectric thin film 22 provided with the opening 22h so as to cover the opening 22h.

  As shown in FIG. 4 (f), the upper electrode 23 (flow velocity detection electrode 31) and the orientation detection electrodes 32x and 32y are connected to these by sputtering and lift-off of Pt or the like using the resist pattern 42r as a mask. Lead wires 23w, 31w, 32w and electrode pads 23p, 31p, 32p are formed respectively (only a part is shown).

  Thus, the upper electrode 23 (flow velocity detection electrode 31) and the orientation detection electrodes 32x and 32y are formed, and the piezoelectric thin film element 20 is formed.

  Prior to the formation process of the upper electrode 23 and the like, an adhesion layer made of Ti or Ta may be formed.

  Further, in FIG. 4F, the electrodes 23 (31), 32x, 32y and the electrode pads 23p, 31p, 32p are shown to be higher than the lead wires 23w, 31w, 32w in order to clearly show them. 23 (31), 32p, and 32w only), all of these configurations may be substantially the same height. Alternatively, sputtering or plating is further performed only on the electrodes 23, 31, 32x, 32y and the electrode pads 23p, 31p, 32p, and the lead wires 23w, 31w, 32w are used as shown in FIG. It may be projected to further improve the electrical connection. For example, when general wire bonding or the like is used for connection between the piezoelectric element 20 and the printed circuit board 51, gold (Au) or the like is deposited on the electrode pads 23p, 31p, and 32p according to a general example. .

(Columnar structure forming process)
Subsequently, the substrate 11 is cut out in a frame shape to form a columnar structure 11 b protruding from the vibration surface 22 s of the piezoelectric thin film element 20. At this time, a diaphragm 10d whose periphery is supported by the frame-shaped substrate 11 is also formed. Such a process is performed using a bulk micromachining technique such as deep Si etching.

  First, mask patterns 43m and 46m are formed in order to finish the columnar structure 11b and the frame-shaped substrate 11 into a predetermined shape.

Specifically, as shown in FIG. 5A, the SiO ₂ film 43 is formed on the lower surface of the substrate 11, that is, on the surface opposite to the surface on which the piezoelectric thin film element 20 is formed, by plasma CVD (Chemical Vapor Deposition) or the like. Further, a resist pattern 44 r is formed on the SiO ₂ film 43.

Subsequently, as shown in FIG. 5B, the SiO ₂ film 43 is etched by wet etching or the like using the resist pattern 44r as a mask to form a mask pattern 43m. Thereafter, the resist pattern 44r is removed.

  Next, as shown in FIG. 5C, a resist pattern 45r is formed on the lower surface side of the substrate 11, and the exposure on the mask pattern 43m and the substrate 11 is performed by sputtering, liftoff, or the like using the resist pattern 45r as a mask. A mask pattern 46m made of Cr or the like is formed on the lower surface. FIG. 5D shows a state after the mask pattern 46m is formed and the resist pattern 45r is removed (after lift-off).

  Subsequently, the columnar structure 11b and the frame-shaped substrate 11 are formed using the mask patterns 43m and 46m thus formed.

  Specifically, as shown in FIG. 5E, the central portion of the substrate 11 is thickened while protecting the planned formation portion of the columnar structure 11b by deep etching of Si using the mask pattern 46m as a mask. Etch halfway in the direction.

Here, as shown in FIG. 5 (f), after removing the mask pattern 46m, the substrate 11 is continuously removed by Si deep etching using the mask pattern 43m as a mask, except for the portions where the columnar structures 11b are to be formed. Etch the entire lower surface of. At this time, the SiO ₂ film 12 formed on the upper surface of the substrate 11 is used as a base etch stop film, and etching is continued until the SiO ₂ film 12 at the center of the substrate 11 is exposed.

As a result, as shown in FIG. 5G, the substrate 11 has a frame shape with the central portion cut out. A diaphragm 10 d composed of the SiO ₂ film 12 and the Si film 13 remaining without being etched is formed at the center of the substrate 11. Moreover, the columnar structure 11b which protrudes from the height of the outer peripheral part of the board | substrate 11 is formed by etching the outer peripheral part of the board | substrate 11 protected by the mask pattern 46m to the middle.

  As described above, the flow rate sensor 1 according to the present embodiment is manufactured.

(4) Modified example of the present embodiment In the flow rate sensor according to the modified example of the present embodiment, when detecting the flow rate of the fluid, not the change in the resonance frequency of the diaphragm but the change in the amplitude of the diaphragm at the predetermined frequency of the alternating voltage. The point which detects the flow velocity of is different from the above-mentioned embodiment.

  The operation of the flow velocity sensor according to this modification will be described with reference to FIG. FIG. 6 is an explanatory diagram of the piezoelectric effect generated in the piezoelectric thin film element of the flow rate sensor according to this modification, and FIG. 6A is a diagram showing the piezoelectric effect in the flow rate sensor in a state where no interference with the fluid occurs. (B) is a diagram showing the piezoelectric effect in the flow velocity sensor in a state where interference with the fluid occurs, and (c) shows the amplitude of the voltage of the piezoelectric thin film element that changes according to the frequency of the AC voltage. It is a graph to show.

Also in this modification, the flow velocity of the fluid can be detected using the flow velocity sensor 1 and the flow velocity sensor package 2 having the same configuration as that of the above-described embodiment. Specifically, the frequency of the voltage corresponding to the resonance frequency f _R of the diaphragm 10d of the absence stress, applied to the piezoelectric thin film element 20 from the AC power supply 53 via an upper electrode 23. A piezoelectric effect corresponding to the frequency, that is, a voltage amplitude is generated in the piezoelectric thin film element 20, and this is detected by the detection means 54 via the flow velocity detection electrode 31.

As described above, when the stress is applied to the diaphragm 10d, having different resonant frequencies f _{R 'is} a resonant frequency f _R of the diaphragm 10d of the absence stress. Accordingly, as shown in FIG. 6 (c), even by applying a voltage having a frequency corresponding to the resonance frequency f _R, the voltage of the piezoelectric thin-film element 20 is not the maximum amplitude A _R of the absence stress To a predetermined amplitude A _R ′.

That is, as shown in FIG. 6 (a), does not interfere with columnar structure 11b and the fluid fluid is stagnant, when no stress is applied to the diaphragm 10d, a voltage of the resonance frequency f _R is applied voltage sensed through the flow velocity sensing electrodes 31 of the piezoelectric thin-film element 20 exhibits a maximum amplitude a _R.

Further, as shown in FIG. 6 (b), interfere with columnar structure 11b and fluid flow occurs in the fluid, when the stress is applied to the diaphragm 10d, a piezoelectric voltage of the resonance frequency f _R is applied voltage sensed through the flow velocity sensing electrodes 31 of the thin film element 20 shows the amplitude a _{R 'with} a reduced predetermined amount from the maximum amplitude a _R.

By measuring the difference .DELTA.A _R such that the amplitude A _R of the sensed _voltage, A R _', and calculates the magnitude of the stress applied to the diaphragm 10d, for detecting the flow rate of the interfering fluid columnar structure 11b be able to.

(5) Effects according to the present embodiment According to the present embodiment and its modifications, the following one or more effects are achieved.

(A) That is, in the present embodiment, the piezoelectric thin film element 20, the columnar structure 11b, and the detection electrode 30 constitute a mechanical flow velocity sensor 1 using the MEMS technology. Thereby, the flow velocity sensor 1 excellent in time resolution at low cost is provided. Therefore, it is suitably used in environmental monitoring applications where time resolution is important.

(B) Further, in the present embodiment, by measuring the resonance frequency f _R when no stress is applied to the diaphragm 10d, the resonance frequency f _{R 'when} stress is applied, the difference Delta] f _R, Detect the fluid flow rate. Thereby, the flow velocity of the fluid can be detected with high accuracy.

(C) Further, in the present embodiment, the amplitude _A R of the voltage at the resonance frequency _{f R} of the diaphragm 10d, by measuring the difference .DELTA.A _R of _{A R} ', which detects the flow velocity of the fluid. Thereby, the flow velocity of the fluid can be detected with high accuracy. Further, unlike the case where the resonance frequencies f _R and f _R ′ are calculated and the difference Δf _R is measured, a predetermined voltage such as the resonance frequency f _R may be applied, and the configuration of the AC power supply 53 and the like can be simplified and the measurement can be performed. Measurement time can be increased by shortening the time.

(D) As described above, in the present embodiment, the flow velocity of the fluid is detected by using the change in the resonance frequencies f _R and f _R ′ at which the voltage amplitude becomes maximum. Thereby, a sufficiently strong voltage signal can be obtained, and the measurement accuracy can be improved.

(E) In the present embodiment, the positive and negative voltages detected through the plurality of detection electrodes 30 are discriminated, and the flow of the fluid is determined based on the magnitude relationship between the plurality of voltages and the positional relationship between the plurality of detection electrodes 30. Detect orientation. Thereby, the flow velocity sensor 1 which can detect the flow direction of the fluid with high accuracy is provided.

(F) In this embodiment, two or more detection electrodes 30 are formed apart from the upper electrode 23 so as to be symmetric with respect to an axis of symmetry passing through the upper electrode 23. Alternatively, two or more pairs of detection electrodes 30 are formed apart from the upper electrode 23 so as to be symmetric with respect to the upper electrode 23 as a center of symmetry. Thereby, the multi-axis detection type flow velocity sensor 1 which is low cost and excellent in time resolution is provided.

(G) Further, in the present embodiment, the columnar structure 11b has two or more rotational symmetries when the longitudinal direction is taken as one rotation around the rotation axis. Thus, the sensitivity of the predetermined direction with respect to the direction of fluid flow is substantially reduced, for example, the columnar structure 11b has two orientations if it is a square cross section, three orientations if it is a regular hexagon, and all orientations if it is a circle. Can be equal.

(H) In this embodiment, the diaphragm 10d is supported on the vibration surface 20s of the piezoelectric thin film element 20 with the periphery supported by the frame-shaped substrate 11 with the center portion hollowed out. Thus, the sensitivity of the flow rate sensor 1 can be further improved by adopting a diaphragm structure that is susceptible to stress due to the inclination of the columnar structure 11b and vibration of the piezoelectric thin film element 20.

(I) Moreover, in this embodiment, the columnar structure 11b is formed in the center part of the diaphragm 10d so as to protrude from the height of the substrate 11. Thereby, the stress due to the interference with the fluid can be transmitted to the diaphragm 10d and the piezoelectric thin film element 20 more equally and with high sensitivity.

(J) In the present embodiment, the upper electrode 23 is formed at the center of the diaphragm 10d. Thereby, the vibration of the piezoelectric thin film element 20 can be transmitted to the diaphragm 10d substantially evenly. Further, since the flow velocity detection electrode 31 is formed at the central portion on the diaphragm 10d, the vibration of the entire diaphragm 10d can be detected in a state with little bias.

(K) In the present embodiment, the detection electrode 30 is formed on the diaphragm 10d. Thereby, the bending and vibration transmitted from the diaphragm 10d can be detected with high sensitivity.

(L) In the present embodiment, other configurations such as the electrode pads 23p, 31p, 32p and the opening 22h of the piezoelectric thin film 22 are formed on the frame of the substrate 11. Thereby, it can suppress that the movement by the bending and vibration of the diaphragm 10d will be prevented.

(M) In this embodiment, the piezoelectric thin film 22 is a lead-free piezoelectric thin film. Thus, even when used in flow sensor 1 is, for example, environmental monitoring applications, etc., for example, lead zirconate titanate _{(PZT: Pb (Zr X Ti} 1-X) O 3) constructed from other materials such as The influence on the environment can be reduced as compared with the piezoelectric thin film. However, the distinction between lead-free and lead-based does not affect the characteristics as a flow rate sensor such as detection accuracy, sensitivity, and resolution.

(N) In the present embodiment, the piezoelectric thin film 22 has an alkali niobium oxide perovskite structure. Such a structure is pseudo cubic. Further, the upper surface of the piezoelectric thin film 22 is preferentially oriented in the (001) plane. Also, composition formula _{(K 1-X Na X)} NbO 3 ( where, 0.400 ≦ X ≦ 0.730) consisting of potassium sodium niobate represented by. Thereby, the piezoelectric thin film 22 having excellent crystallinity and high piezoelectric effect can be obtained.

(O) In the present embodiment, the lower electrode 21 is made of a Pt film whose upper surface is preferentially oriented in the (111) plane. As a result, it is possible to obtain the piezoelectric thin film 22 which is further excellent in crystallinity and has a high piezoelectric effect.

<Other Embodiments of the Present Invention>
As mentioned above, although embodiment of this invention was described concretely, this invention is not limited to the above-mentioned embodiment, It can change variously in the range which does not deviate from the summary.

  For example, in the above-described embodiment, the upper electrode 23 and the flow velocity detection electrode 31 are configured as an electrically separated electrode or a time-division type electrode. May be physically separated and may be arranged at spaced positions on the piezoelectric thin film. Alternatively, at least one of the orientation detection electrodes may have a function as a flow velocity detection electrode.

  The arrangement of the electrodes shown in FIG. 1A and the like is also arbitrary and can be variously changed. For example, each electrode does not necessarily have to be formed on the diaphragm. Further, for example, two or more azimuth detection electrodes are sufficient, and the line-symmetrical or point-symmetrical arrangement is not necessary.

  For example, in the above-described embodiment, the columnar structure 11b has two or more rotational symmetry. However, the present invention is not limited to this, and the cross-sectional shape of the columnar structure may be, for example, an ellipse, streamline, or triangle. Other shapes such as a rectangle and a rhombus may be used. The columnar structure may be formed at a position other than the central portion of the diaphragm.

  In this way, by changing the arrangement of the orientation detection electrodes and the columnar structures, the cross-sectional shape of the columnar structures, and the like, it is possible to impart directivity to the sensitivity in a predetermined direction with respect to the direction in which the fluid flows.

  In the above-described embodiment, the upper electrode 23 and the detection electrode 30 are made of a Pt film, but these electrodes do not directly affect the crystallinity of the piezoelectric thin film. Therefore, various other materials can be used. Other than Pt, for example, tungsten (W), Ta, Au, silver (Ag), aluminum (Al), nickel (Ni), Ti, copper (Cu), cobalt (Co), indium (In), magnesium (Mg) ) Etc.

In the above-described embodiment, the lower electrode 21 is made of a Pt film. However, an alloy containing Pt, a metal such as Au, Ru, Ir, tin (Sn), In, or strontium ruthenate. Metal oxides such as (SrRuO ₃ ) and lanthanum nickelate (LaNiO ₃ ) can also be used.

  Further, the upper electrode and the lower electrode may not have an adhesion layer.

  In the above embodiment, the piezoelectric thin film 22 is potassium sodium niobate, but other elements may be added thereto. As other elements, for example, lithium (Li), Ta, antimony (Sb), calcium (Ca), Cu, barium (Ba) Ti, or the like can be added at 5% or less.

In the above-described embodiment, the diaphragm 10d is composed of the SiO ₂ film 12 and the Si film 13. However, other materials and film configurations may be used, for example, a silicon nitride (SiN) film or a silicon carbide. (SiC) films or the like can be used in appropriate combination.

In the above-described embodiment, the substrate 11 having the SiO ₂ film 12 and the Si film 13 is made of an SOI wafer. However, the present invention is not limited to this. A SiO ₂ film, Si film, or the like may be formed on a substrate such as Si by sputtering or CVD. At this time, a quartz glass substrate, a gallium arsenide (GaAs) substrate, a sapphire (Al ₂ O ₃ ) substrate, a metal substrate such as stainless steel, a magnesium oxide (MgO) substrate, a strontium titanate (SrTiO ₃ ) substrate, or the like is used. Also good. A substrate formed by anodic bonding or mechanical bonding may be used.

In the above-described embodiment, the resonance frequencies f _R and f _R ′ of the diaphragm 10d at which the amplitude of the voltage of the piezoelectric thin film element 20 is maximized are detected. The vibration frequency may be detected. In the above-described modification, the voltage amplitudes A _R and A _R ′ of the piezoelectric thin film element 20 at the resonance frequency f _R of the diaphragm 10d are detected, but the amplitudes of the voltages at other predetermined frequencies are detected. Also good.

  In the above-described embodiment, the upper electrode 23, the flow velocity detection electrode 31, and the direction detection electrodes 32x and 32y are provided. However, the direction detection electrodes 32x and 32y are eliminated and only the upper electrode and the flow velocity detection electrode are provided. It is good also as detecting only the flow velocity of fluid mainly.

  Alternatively, the upper electrode 23 and the flow velocity detection electrode 31 may be eliminated so that only the direction detection electrode is provided, and only the direction in which the fluid flows is mainly detected. Even in this case, it is possible to estimate the flow velocity of the fluid by vector synthesis of voltages obtained from a plurality of orientation detection electrodes.

  Next, examples according to the present invention will be described below.

(Production of flow rate sensor)
First, a flow rate sensor according to this example was manufactured as follows.

  A silicon thermal oxide film having a thickness of 2 μm on a Si substrate having a thickness of 500 μm, and a single crystal Si film having a thickness of 10 μm having a (001) plane and a thermal oxide film having a thickness of 205 nm on the upper surface. A stacked 4-inch SOI wafer was prepared. The thickness of the SOI wafer is a standard value of 500 μm ± 25 μm.

  A lower electrode made of Pt and having a thickness of 212 nm was formed on the SOI wafer by RF magnetron sputtering. At this time, a 2.3 nm-thick adhesion layer made of Ti was previously formed by RF magnetron sputtering.

  The deposition conditions for the adhesion layer and the lower electrode were a substrate temperature of 100 ° C. to 350 ° C., a discharge power of 200 W, an argon (Ar) gas atmosphere as a sputtering gas, and a pressure in the sputtering chamber of 2.5 Pa. The deposition time for the adhesion layer was 1 to 3 minutes, and the deposition time for the lower electrode was 10 minutes.

A piezoelectric thin film having a thickness of about 2 μm made of (K _0.45 Na _0.55 ) NbO ₃ was formed on the lower electrode by RF magnetron sputtering. A target material made of a (K _0.35 Na _0.65 ) NbO ₃ sintered body was used for forming the piezoelectric thin film. The film forming conditions were as follows: the substrate temperature was 520 ° C., the discharge power was 700 W, and the pressure in the sputtering chamber was 1.3 Pa in an oxygen (O ₂ ) / Ar mixed gas atmosphere having a mixing ratio of 0.005 as a sputtering gas. The film formation time was set to a time when the thickness of the piezoelectric thin film was about 2 μm.

  In the step of providing an opening in the piezoelectric thin film, the resist used for the resist pattern was ZPN-1150 manufactured by Nippon Zeon Co., Ltd., and the thickness of the mask pattern made of Cr was 400 nm. For wet etching of the piezoelectric thin film, hydrofluoric acid (HF aqueous solution) having a concentration of 49% by volume was used, and the etching time was about 10 to 20 minutes.

  In the step of forming the upper electrode and the detection electrode, the resist used for the resist pattern was OFPR-800 manufactured by Tokyo Ohka Kogyo Co., Ltd. A Pt film having a thickness of 100 nm was formed on a 2 nm-thick adhesion layer made of Ti to obtain an upper electrode and a detection electrode.

In the step of forming the columnar structure mask pattern, an SiO ₂ film having a thickness of about 2.5 μm serving as one mask pattern was formed by plasma CVD. The film forming conditions are as follows: substrate temperature is 300 ° C., silane (SiH ₄ ) / nitrogen (N ₂ ) mixed gas and dinitrogen monoxide (N ₂ O) gas are used as source gases, and the pressure in the plasma CVD chamber is 0.35 Torr. (46.6628 Pa). An ammonium fluoride (NH ₄ F) aqueous solution was used for wet etching of the SiO ₂ film. Cr was used for the other mask pattern.

  In the step of forming a columnar structure or the like, as in the above-described embodiment, two-stage etching is performed by switching between two mask patterns, the substrate is etched by about 300 μm in the depth direction in the first half process, and the remaining in the second half process. About 200 μm was etched. For the etching, a so-called Bosch process was used.

The Bosch process is widely used for deep etching of Si and the like, and includes a step of etching Si in the depth direction using sulfur hexafluoride (SF ₆ ) gas, and a perfluorocyclobutane (C ₄ F ₈ ) gas. By alternately repeating the step of forming a protective film on the sidewall etched with Si, an anisotropic etching shape can be obtained. Note that a plurality of horizontal streaks corresponding to the switching timing of the two steps are engraved on the etching side wall, and a so-called scallop shape having a waveform with a predetermined period of about 3 μm in the depth direction is formed.

  A plurality of flow velocity sensors having different columnar structures and diaphragm diameters were obtained by varying the sizes of the two mask patterns. Specifically, the cross-section of the columnar structure was circular, and the diameters were 50 μm, 100 μm, and 250 μm, respectively. The shape of the diaphragm at the center of the substrate was circular, and the diameters were 0.4 mm, 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.8 mm, respectively.

(Evaluation of flow velocity sensor)
The flow velocity sensor manufactured as described above was mounted on a printed board using a solder ball, and the following evaluation was performed.

  As described above, the diaphragm of the present example includes a Si film having a thickness of 10 μm, and has a diameter of 0.4 mm to 1.8 mm. When the resonance frequency of the diaphragm was evaluated in a steady state without stress, it was within a range of about 20 kHz to 170 kHz.

The columnar structure of this example has a height corresponding to the initial substrate thickness, that is, a height of about 500 μm, and a diameter of 50 μm to 250 μm. Thus, even if the aspect (height / diameter) ratio changes, the resistance coefficient of the columnar structure against air as a fluid only changes within the range of about 0.63 to 1.20. Also showed good operating characteristics. In addition, since highly viscous air is used as the fluid, the Reynolds number is about 10 ⁻² or more and 10 ⁻⁴ or less in a columnar structure having a height of about 500 μm. Therefore, the columnar structure is considered to be placed in a laminar flow with high viscosity. For this reason, it is considered that the influence of the scalop shape remaining on the side surface by the Bosch process is not so great.

  In the flow velocity sensor of this example, the sensitivity to the flow velocity of the fluid was several tens Hz per 10 m / s, and a good result was obtained.

DESCRIPTION OF SYMBOLS 1 Flow velocity sensor 2 Flow velocity sensor package 10d Diaphragm 11 Substrate 11b Columnar structure 12 SiO ₂ film 13 Si film 20 Piezoelectric thin film element 21 Lower electrode 22 Piezoelectric thin film 23 Upper electrode 30 Detection electrode 31 Flow velocity detection electrode 32x, 32y Orientation detection electrode 51 Print Substrate 52 Solder ball 53 AC power supply 54 Detection means

Claims (14)

A lower electrode and a piezoelectric thin film made of a non-lead material are laminated in this order on the substrate, an upper electrode is formed at a predetermined position on the piezoelectric thin film, and an AC voltage is applied between the lower electrode and the upper electrode. Piezoelectric thin film element that vibrates by being
A columnar structure that is formed so as to protrude from the vibration surface of the piezoelectric thin film element and applies stress to the vibration surface by interfering with a fluid;
A flow rate sensor comprising: a detection electrode formed on the piezoelectric thin film and connected to detection means for detecting a piezoelectric effect generated in the piezoelectric thin film when the vibration surface receives stress. When the columnar structure and the fluid interfere with each other and the vibration frequency at which the amplitude of the voltage detected through the detection electrode becomes a predetermined magnitude when the columnar structure and the fluid do not interfere with each other The flow velocity of the fluid that interferes with the columnar structure is detected by measuring the difference between the vibration frequency at which the amplitude of the voltage detected through the detection electrode becomes a predetermined magnitude. The flow rate sensor according to claim 1. When the columnar structure and the fluid interfere with each other at the predetermined frequency of the AC voltage, the amplitude of the voltage detected through the detection electrode when the columnar structure and the fluid do not interfere with each other. 2. The apparatus according to claim 1, wherein the flow velocity of the fluid that interferes with the columnar structure is detected by measuring a difference between the amplitude of the voltage detected through the detection electrode and the amplitude of the voltage. Flow rate sensor. A plurality of the detection electrodes are formed on the piezoelectric thin film,
The positive and negative voltages detected through the plurality of detection electrodes when the columnar structure and the fluid interfere with each other are determined, and the magnitude relationship between the plurality of voltages and the positional relationship between the plurality of detection electrodes The flow rate sensor according to any one of claims 1 to 3, wherein a flow direction of a fluid that interferes with the columnar structure is detected. 5. The flow rate sensor according to claim 4, wherein two or more detection electrodes are formed apart from the upper electrode so as to be symmetric with respect to an axis of symmetry passing through the upper electrode. 5. The flow rate sensor according to claim 4, wherein two or more pairs of the detection electrodes are separated from the upper electrode so as to be symmetric with respect to the upper electrode as a center of symmetry. The flow rate sensor according to any one of claims 1 to 6, wherein the columnar structure has a rotational symmetry of two or more times when it makes one turn around the longitudinal direction as a rotation axis. 8. The diaphragm of which the periphery is supported by the frame-shaped substrate having a hollowed central portion is formed on the vibration surface of the piezoelectric thin film element. 9. Flow rate sensor. The flow rate sensor according to any one of claims 1 to 8, wherein a resistance coefficient of the columnar structure to a fluid is 0.63 or more and 2.01 or less. The flow rate sensor according to claim 1, wherein the piezoelectric thin film has an alkali niobium oxide-based perovskite structure. The flow rate sensor according to claim 10, wherein the alkali niobium oxide-based perovskite structure is a pseudo cubic crystal. The flow rate sensor according to claim 10 or 11, wherein the main surface of the piezoelectric thin film is preferentially oriented in the (001) plane. The piezoelectric thin film according to claim, characterized in that it consists of potassium sodium niobate represented by the composition formula _{(K 1-X Na X)} NbO 3 ( where, 0.400 ≦ X ≦ 0.730) 1~12 The flow rate sensor according to any one of the above. The flow rate sensor according to claim 1, wherein the lower electrode is made of a Pt film having a main surface preferentially oriented in a (111) plane.

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