JP2005181305A - Pressure sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure sensor which is miniaturized and which can obtain high sensor sensitivity. <P>SOLUTION: The thickness of a piezoelectric substrate 10 positioned immediately below a reflector 22 is made to be approximately equal to that of a region immediately below an inter digital transducer 21, near the inter digital transducer 21, and is made gradually thick as it is apart from the inter digital transducer 21. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a pressure sensor used in, for example, a tire condition monitoring device (TPMS).

  Surface acoustic wave devices using surface acoustic wave propagation characteristics such as surface acoustic wave resonators and surface acoustic wave filters are widely used in various wireless communication devices, in-vehicle devices, medical devices, etc. that use the microwave band. ing. Further, as another application example using the propagation characteristics of surface acoustic waves, a surface acoustic wave sensor that detects pressure, temperature, and the like based on a difference in propagation time of surface acoustic waves has been proposed (see, for example, Patent Document 1). .)

  In this surface acoustic wave sensor, a pair of reflectors are arranged at a predetermined interval on a piezoelectric substrate made of quartz or the like having a certain thickness, and a pair of comb-like electrodes are disposed between the two reflectors. It has a structure in which interdigital electrodes combined with interdigital are arranged.

According to such a pressure sensor, when a stress is applied to the piezoelectric substrate, a change in the propagation speed of the surface acoustic wave accompanying a change in the elastic coefficient of the substrate and a change in the arrangement pitch of the electrode fingers of the interdigital electrodes occur. The resonance frequency changes, and the change in pressure can be measured by measuring this change.
JP 59-17127 A

  However, in the above-described conventional pressure sensor, the elastic coefficient of the piezoelectric substrate is changed by the applied stress to change the propagation speed of the surface acoustic wave. However, since the crystal or the like used as the piezoelectric substrate has high hardness, The rate of change of the elastic modulus is small, and the sensitivity as a sensor is low. In addition, for example, when measuring the air pressure of a running tire, there is a large noise generated while the car is running, and there is a risk of malfunction due to this noise, and a sensor with higher sensitivity is required. ing.

  Therefore, by making the thickness of the piezoelectric substrate located immediately below the interdigital electrode thinner than the thickness of the piezoelectric substrate located directly below the reflector, the amount of deflection when pressure is applied to the piezoelectric substrate is increased. At the same time, it has been studied to increase the sensitivity of the pressure sensor by increasing the amount of change in the resonance frequency.

  FIG. 7 is a diagram schematically showing such an improved pressure sensor.

  The pressure sensor 100 mainly includes a piezoelectric substrate 110 and an interdigital transducer 121 formed on the upper surface of the piezoelectric substrate 110, a reflector 122, and a pressure detection resonator 120 including a pad electrode 123. A recess 140 is formed, and the piezoelectric substrate 110 located immediately below the interdigital transducer 121 has a structure that is thinner than other regions.

  According to this pressure sensor 100, since the thickness of the piezoelectric substrate 110 located immediately below the interdigital transducer 121 is thinner than other regions, the piezoelectric substrate 110 immediately below the interdigital transducer 121 is changed according to the change in pressure. Since the amount of change in the resonance frequency is increased, the pressure detection sensitivity can be increased.

  However, the above-described pressure sensor 100 tends to deteriorate the resonance characteristics due to a change in pressure. In order to detect an accurate pressure, it is necessary to improve the deterioration of the resonance characteristics.

  The present invention has been devised in view of the above-described problems, and an object thereof is to provide a pressure sensor in which resonance characteristics are less likely to deteriorate due to a change in pressure.

  In the pressure sensor of the present invention, an interdigital transducer is formed on the upper surface of a piezoelectric substrate, and reflectors are arranged on both sides of the surface acoustic wave propagation direction with respect to the interdigital transducer, immediately below the interdigital transducer. A pressure sensor configured to detect pressure fluctuations by deformation of a piezoelectric substrate positioned, wherein the thickness of the piezoelectric substrate positioned immediately below the reflector is approximately equal to a region directly below the interdigital transducer in the vicinity of the interdigital transducer. It is not equal and gradually increases in thickness as the distance from the interdigital transducer increases.

  Further, a concave portion is formed on the bottom surface of the piezoelectric substrate directly below the interdigital transducer, and a curved surface is formed at a corner portion between the bottom surface and the inner wall surface of the concave portion.

  According to the pressure sensor of the present invention, the thickness of the piezoelectric substrate located immediately below the reflector is substantially equal to the region immediately below the interdigital transducer in the vicinity of the interdigital transducer, and gradually increases with increasing distance from the interdigital transducer. Therefore, the energy confinement state of the surface acoustic wave formed by the interdigital transducer and the reflector does not fluctuate greatly due to pressure fluctuation, and the resonance characteristics of the resonator do not fluctuate greatly due to pressure fluctuation and are stable. Resonance characteristics can be obtained.

  In addition, in this case, since the reflector gradually becomes thicker as it moves away from the interdigital transducer, only the interdigital transducer that detects pressure fluctuation is thinned, so that it is possible to reduce the size and obtain a large sensor sensitivity. it can.

  Hereinafter, a pressure sensor of the present invention will be described in detail with reference to the accompanying drawings.

  1A and 1B are views showing a pressure sensor according to an embodiment of the present invention, in which FIG. 1A is a top view and FIG. 1B is a cross-sectional view.

  The pressure sensor 1 shown in FIG. 1 generally includes a piezoelectric substrate 10, a pressure detection resonator 20 including an interdigital transducer 21 and a reflector 22 formed on the upper surface, an interdigital transducer 31 and a reflector 32. And a local oscillation resonator 30 comprising:

  Further, in the pressure sensor of the present embodiment, a recess 40 is formed on the lower surface of the piezoelectric substrate 10 located immediately below the interdigital transducer 21, and the thickness of this portion is thinner than other regions, and the reflection is reflected. The piezoelectric substrate 10 located immediately below the vessel 22 has a structure in which the thickness is gradually increased as the distance from the interdigital transducer 21 increases.

  Such a piezoelectric substrate 10 is, for example, a piezoelectric single crystal such as crystal, lithium tantalate single crystal, lithium niobate single crystal, lithium tetraborate single crystal, or piezoelectric ceramics such as lead titanate or lead zirconate. When electric power is applied to the piezoelectric substrate 10 through the pressure detecting resonator 20, the surface of the piezoelectric substrate 10 generates a predetermined surface acoustic wave.

  The pressure detecting resonator 20 is made of, for example, a metal material such as aluminum or an alloy containing aluminum as a main component, and includes an interdigital transducer 21 that excites a surface acoustic wave, and an interdigital along the propagation direction of the surface acoustic wave. A reflector 22 disposed on both sides of the transducer 21, an external connection pad electrode 23 electrically connected to the interdigital transducer 21, and the like.

  The interdigital transducer 21 constituting the pressure detecting resonator 20 includes a pair of comb-like electrodes 21a each formed of a strip-shaped common electrode and a plurality of electrode fingers extending in a direction perpendicular to the common electrode. 21b is a state in which the common electrodes are arranged in parallel to each other, and the electrode fingers of the comb-like electrodes 21a and 21b are alternately arranged in the propagation direction of the surface acoustic wave in the facing region of the two common electrodes. It is configured to be opposed to each other.

  When a predetermined electric power is applied from the outside, the interdigital transducer 21 has a predetermined surface acoustic wave corresponding to the arrangement pitch of the electrode fingers on the upper surface of the piezoelectric substrate 10, specifically, the arrangement pitch of the electrode fingers. The surface acoustic wave having a wavelength of 1/2 is generated.

  On the other hand, the reflector 22 functions to effectively generate a standing wave by confining the surface acoustic wave energy generated in the formation region of the interdigital transducer 21 between the pair of reflectors 22a and 22b.

  The pair of reflectors 22a and 22b and the interdigital transducer 21 arranged therebetween constitute a one-terminal pair resonator.

  Also, a pad electrode 23 is generally formed on the surface of the piezoelectric substrate 10 so as to be electrically connected to the interdigital transducer 21, and this pad electrode 23 is a metal that is electrically connected to the outside. Fine wires and bumps are joined, and the interdigital transducer 21 has a function of applying predetermined external power.

  Furthermore, an annular electrode 51 is formed on the surface of the piezoelectric substrate 10 so as to surround the pressure detection resonator 20 and the local oscillation resonator 30, and is formed on the substrate on which the pressure sensor 1 is mounted. By connecting to the annular conductor via solder or the like, the vibration space is formed.

  The local oscillation resonator 30 has the same configuration as the pressure detection resonator 20 described above.

  Such a pressure sensor 1 is manufactured by the method described below.

  First, a single crystal mother substrate (wafer) made of quartz or the like is prepared, and an electrode film is formed on the wafer by a well-known vapor deposition method or sputtering method. Next, a resist is spin-coated on the electrode film, and after exposure and development using a stepper device or the like, etching is performed using an RIE device or the like to form an electrode pattern such as an interdigital transducer or a reflector on the wafer. Form. As a result, a large number of pressure detecting resonators 20 and local oscillation resonators 30 are formed vertically and horizontally on the surface of the wafer.

  Next, the electrode formation surface of the pressure detection resonator 20 and the like of this wafer is attached to a dicing tape, and in this state, the thickness of the piezoelectric substrate 10 positioned immediately below the interdigital transducer 21 is changed by etching or sandblasting. Processing is performed so as to be thinner than the region. Thereafter, the wafer is diced to complete individual pressure sensor pieces.

  The chip size of the pressure sensor 1 varies depending on the resonance frequency of the resonator used, but when used at a resonance frequency of about 300 MHz, the chip size (length × width × thickness) is about 10 mm × 5 mm × 0.3 mm.

  The thickness of the piezoelectric substrate 10 immediately below the interdigital transducer 21 is preferably set in the range of 10 μm to 100 μm depending on the relationship between the desired sensor sensitivity and the resonance frequency of the pressure detection resonator 20. When the thickness is 10 μm or less, the strength of the piezoelectric substrate 10 becomes brittle, and there is a tendency that problems such as cracking from the portion occur. On the other hand, if the thickness is 100 μm or more, the amount of deflection due to pressure decreases, and the sensitivity tends to deteriorate.

  Next, the operation and actual pressure measuring method of the pressure detecting resonator 20 for detecting the pressure of the pressure sensor 1 of the present invention will be described. In the present invention, the pressure is detected by the change in the resonance frequency before and after the pressure fluctuation in the pressure detecting resonator 20. Further, the local oscillation resonator 30 is disposed at a distance from the pressure detection resonator 20 so that there is no fluctuation in the resonance frequency before and after the pressure fluctuation. Here, in the local oscillation resonator 30, an interdigital transducer 31 and a reflector 32 are formed on the piezoelectric substrate 10 so that the propagation direction of the surface acoustic wave is the same as that of the pressure detection resonator 20.

  FIG. 2 is a cross-sectional view showing a state in which the pressure sensor 1 of the present invention is mounted on the substrate 60.

  In FIG. 2, an annular electrode 51 formed on the surface of the piezoelectric substrate 10 and an annular conductor 61 formed on the substrate 60 are connected via solder 62, thereby forming a vibration space 63 having airtightness. Further, the outside is covered with a resin 50.

  Although not shown, the pad electrodes 23 and 33 are electrically connected to connection electrodes formed on the substrate 60 by solder bumps.

  FIG. 3 is a diagram schematically showing an actual pressure measurement system. In this figure, the pressure sensor 1 is pressurized by nitrogen injected from a nitrogen cylinder 91 in a gas chamber 90. Then, the network analyzer 93 measures the resonance frequency of the pressure detecting resonator 20 while checking the pressure rise with the pressure gauge 92. In the pressure sensor 1 shown in FIGS. 2 and 3, the piezoelectric substrate 10 and the substrate 60 are held at an atmospheric pressure and hermetically sealed, and a differential pressure between the atmospheric pressure and the pressurized pressure is measured. The pressure is measured.

  Next, in the pressure sensor 1 of the present invention, an enlarged view of the pressure detecting resonator 20 formed on the upper surface of the piezoelectric substrate 10 and resonance characteristics before and after the pressure change (atmospheric pressure state) and after the pressure change (pressurized state) are shown. As shown in FIG. 4A is an enlarged view of a cross section of the pressure detecting resonator 20 which is a main part of the present invention, and FIG. 4B shows resonance characteristics of the pressure detecting resonator 20 before and after pressure fluctuation. In FIG. 4B, the vertical axis represents insertion loss (dB), and the horizontal axis represents frequency (MHz).

  In this experiment, a surface acoustic wave resonator (resonance frequency: 315.0 MHz) for RKE (Remote Keyless Entry Security) was used as the resonator.

  Here, the resonance characteristic indicated by the dotted line shown in FIG. 4B is before the pressure fluctuation (atmospheric pressure state), and the resonance characteristic indicated by the solid line is after the pressure fluctuation (pressurized state). As can be seen from FIG. 4B, when the resonance characteristics before and after pressure fluctuation are compared, the resonance frequency after pressure fluctuation (pressurized state) is shifted to the low frequency side. This is because the piezoelectric substrate 10 located immediately below the interdigital transducer 21 of the pressure detecting resonator 20 is deformed into a convex shape due to pressure fluctuation. That is, the propagation speed of the surface acoustic wave in the portion where the piezoelectric substrate 10 is distorted changes, and the arrangement pitch of the electrode fingers of the interdigital transducer 21 changes, and the resonance frequency is shifted to the low frequency side by these two actions. It will be done.

  Here, by obtaining the relationship between the amount of change in the resonance frequency and the pressure in advance, the change in the resonance frequency can be measured and converted into pressure.

  Here, a characteristic point of the pressure sensor according to the present invention is that the insertion loss of the resonance frequency in the resonance characteristics after pressure fluctuation (pressurized state) of the pressure detection resonator 20 indicated by a solid line in FIG. There is no deterioration.

  For comparison, in the pressure sensor 100 shown in FIG. 7, an enlarged view of the pressure detecting resonator 120 formed on the upper surface of the piezoelectric substrate 110, and before (after atmospheric pressure) and (after pressurized) this pressure fluctuation. The resonance characteristics are shown in FIG.

  8A is an enlarged view of the cross section of the pressure detecting resonator 120 of the pressure sensor 100 shown in FIG. 7, and FIG. 8B shows the resonance characteristics of the pressure detecting resonator 120 before and after pressure fluctuation. ing. The resonance characteristics indicated by the dotted line in FIG. 8B are before the pressure change (atmospheric pressure state), and the resonance characteristics indicated by the solid line are after the pressure change (pressurized state).

  In FIG. 8B, the resonance characteristic after pressure fluctuation (pressurized state) indicated by a solid line is the same as the resonance characteristic after pressure fluctuation (pressurized state) of FIG. Although shifted to the low frequency side, in FIG. 8B, it can be seen that the insertion loss after pressure fluctuation (pressurized state) is degraded. This is thought to be because the energy confinement effect of the pressure detection resonator 20 is not sufficient because the reflectors 22 on both sides thereof are not interlocked with the deformation of the interdigital transducer 21 due to pressure fluctuation, and the reflection efficiency of the reflectors 22 is reduced. It is done.

  As is clear from the above experimental results, in the pressure sensor of the present invention, the thickness of the piezoelectric substrate 10 immediately below the reflector 22 of the pressure detecting resonator 20 is set to be close to the interdigital transducer 21 and immediately below the interdigital transducer 21. Since the thickness is gradually increased as the distance from the interdigital transducer 21 is increased, the reflectors 22 on both sides follow the deformation of the interdigital transducer 21 due to the pressure fluctuation, so that the reflection efficiency of the reflector 22 decreases due to the pressure fluctuation. Therefore, it is possible to obtain a resonance characteristic with a small loss and a large energy confinement effect of the surface acoustic wave in the pressure detection resonator 20.

  The pressure sensor of the present invention can be used for a tire condition monitoring device (TPMS) or the like. FIG. 5 shows a block diagram of the transmission system in that case. FIG. 5 shows an example in which the pressure sensor of the present invention is applied to a pressure detection circuit 70 and a local oscillation / reference circuit 80. That is, the pressure detection resonator 20 is used as a resonator of the pressure detection circuit 70, and the local oscillation resonator 30 is used as a resonator of the local oscillation / reference circuit 80.

  Here, as shown in FIG. 1A, the local oscillation resonator 30 includes a piezoelectric substrate 10, an interdigital transducer 31, a reflector 32, and a pad electrode 33 formed on the upper surface. Further, in addition to the function as a local oscillation resonator, it is also possible to function as a reference for correcting the influence due to the temperature change of the pressure detection resonator.

  Next, a pressure sensor according to another embodiment of the present invention will be described with reference to FIG. In the present embodiment, only differences from the above-described embodiment will be described, and the same components will be denoted by the same reference numerals, and redundant description will be omitted.

  FIG. 6 is a cross-sectional view schematically showing the pressure sensor 2 of the present embodiment. The difference between the pressure sensor 2 of the present embodiment and the pressure sensor 1 described above is that a curved surface (concave surface) 41 is formed at the corner where the inner wall surface and the bottom surface of the recess 40 formed in the piezoelectric substrate 10 intersect. It is that. As a result, no corner is formed at the connection portion between the inner wall surface and the bottom surface of the recess 40, and the curved surface 41 is formed. Therefore, when pressure is applied to the recess 40, the connection to the connection portion between the inner wall surface and the bottom surface is reduced. Stress concentration can be prevented and mechanical strength can be improved. In addition, since the curved surface is formed at the connection portion between the inner wall surface and the bottom surface of the recess 40, the reflection direction of the bulk wave by the portion is dispersed. In an interdigital transducer, a bulk wave is generated in addition to a surface acoustic wave. When the bulk wave is reflected on the bottom surface of the piezoelectric substrate and reaches the interdigital transducer again, it becomes a bulk wave spurious and deteriorates electrical characteristics. By dispersing the reflection direction, an effect of suppressing deterioration of electrical characteristics can be achieved.

  In addition, this invention is not limited to the above-mentioned embodiment, A various change, improvement, etc. are possible in the range which does not deviate from the summary of this invention.

  For example, in the above-described embodiment, the example in which the pressure detection resonator 20 and the local oscillation resonator 30 are formed on the piezoelectric substrate 10 has been shown, but only the pressure detection resonator 20 is formed. It doesn't matter.

  In the embodiment described above, the recess 40 is formed only in the vicinity of the formation region of the interdigital transducer 21, but the formation region of the recess 40 may be extended in a direction perpendicular to the propagation direction of the surface acoustic wave. The end of the piezoelectric substrate 10 may be reached in that direction. As a result, the amount of deformation due to pressure in the region immediately below the interdigital transducer 21 of the piezoelectric substrate 10 can be increased, and the sensitivity to pressure can be increased.

  Furthermore, in the above-described embodiment, the vibration space 63 is hermetically sealed by the annular electrode 51, the annular conductor 61, and the solder 62 that connects the two, but these include a dam that prevents inflow of resin. Only the above function may be provided, and the resin 50 may be hermetically sealed.

  Furthermore, in the above-described embodiment, an example in which the present invention is applied to a one-terminal-pair resonator has been described. However, as in the present invention, a resonator using a design method using the energy confinement effect of a surface acoustic wave is used. Applicable to filters and the like.

  The present invention is also applicable to a two-terminal pair resonator, a multimode filter, and the like.

(A) is a top view of a pressure sensor according to an embodiment of the present invention, and (b) is a cross-sectional view of the pressure sensor of FIG. It is sectional drawing which shows the state which mounted the pressure sensor concerning one Embodiment of this invention on the board | substrate. It is a figure which shows the pressure measurement system of the pressure sensor concerning one Embodiment of this invention. (A) is sectional drawing of the resonator for pressure detection used for the principal part of the pressure sensor concerning one Embodiment of this invention, (b) is a figure which shows the resonance characteristic of the resonator for pressure detection of (a). . It is a block diagram of the transmission system which shows embodiment at the time of applying the pressure sensor of this invention to a tire condition monitoring apparatus (TPMS). It is sectional drawing which shows typically the pressure sensor concerning other embodiment of this invention. (A) is a top view which shows typically the pressure sensor considered in order to solve the subject of the conventional pressure sensor, (b) is sectional drawing of the pressure sensor of (a). (A) is sectional drawing of the resonator for pressure detection used for the pressure sensor currently examined to solve the subject of the conventional pressure sensor, (b) is resonance of the resonator for pressure detection of the pressure sensor of (a). It is a figure which shows a characteristic.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... Pressure sensor 10 ... Piezoelectric substrate 20 ... Pressure detection resonator 21 ... Interdigital transducer 21a, 21b ... Comb-like electrode 22 ... Reflector 23 ... Pad electrode 30 ... Local oscillation resonator 31 ... Interdigital transducer 32 ... Reflector 33 ... Pad electrode 40 ... Recess 50 ... Resin 51 ... Annular electrode 60 ... Substrate 61 ... annular conductor 62 ... solder 63 ... vibration space 70 ... pressure detection circuit 80 ... local oscillation / reference circuit 90 ... gas chamber 91 ... nitrogen cylinder 92 ...・ Pressure gauge 93 ・ ・ ・ Network analyzer

Claims (2)

An interdigital transducer is formed on the upper surface of the piezoelectric substrate, and reflectors are arranged on both sides of the surface acoustic wave in the propagation direction with respect to the interdigital transducer, and the piezoelectric substrate located immediately below the interdigital transducer is deformed. A pressure sensor for detecting pressure fluctuations,
The pressure is characterized in that the thickness of the piezoelectric substrate located immediately below the reflector is substantially equal to the area immediately below the interdigital transducer in the vicinity of the interdigital transducer, and gradually increases with increasing distance from the interdigital transducer. Sensor.   2. The pressure sensor according to claim 1, wherein a concave portion is formed on the bottom surface of the piezoelectric substrate immediately below the interdigital transducer, and a curved surface is formed at a corner portion between the bottom surface and the inner wall surface of the concave portion. .

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