JP5000473B2 - Spherical surface acoustic wave device - Google Patents

Description

  The present invention relates to a spherical surface acoustic wave element, and more particularly to a spherical surface acoustic wave element used for a sensor.

When a surface acoustic wave (SAW) propagates on the surface of a sphere made of a single crystal of a piezoelectric material such as quartz, langasite, LiNbO ₃ , LiTaO ₃ , the propagation speed changes depending on the surrounding hydrogen concentration. A hydrogen sensor (ball SAW sensor) utilizing this is known (see Patent Documents 1 and 2). When a surface acoustic wave is excited on the surface of the sphere, the surface acoustic wave does not spread like a normal wave, but almost attenuates an annular region with a finite width along the great circle of the sphere around a predetermined optical axis. Rotate many times without doing. Since the change in the propagation velocity is amplified in proportion to the number of times that the surface acoustic wave goes around the sphere, the ball SAW sensor is a very sensitive hydrogen sensor.

  FIG. 3 simply shows the configuration of the surface acoustic wave device. A comb-shaped electrode 12, a sensitive film 13, an input / output electrode 14 and an external terminal 15 are formed on a spherical base 11 made of a single crystal of a piezoelectric material. The sensitive film 13 is made of Pd, Ni, Pd—Ni alloy or the like that occludes hydrogen. Since the sensitive film 13 that has absorbed hydrogen becomes hard and the propagation speed of the surface acoustic wave is increased on the sensitive film 13, it can be used as a hydrogen sensor.

Here, each of the comb-shaped electrode 12, the sensitive film 13, the input / output electrode 14, and the external terminal 15 needs to be formed at predetermined positions on the substrate 11. Specifically, as shown in FIG. 8, when the optical axis 16 passing through the center of the substrate 11 is the ground axis, the comb-shaped electrode 12 and the sensitive film 13 are formed on the equator. In addition, input / output electrodes 14 are formed at both pole points of the optical axis 16. The external terminals 15 are formed on the input / output electrodes 14 at the positions of both polar points of the optical axis 16. The name of the optical axis varies depending on the type of piezoelectric crystal. For example, in quartz and Langaside, it is called the Z axis and is one (uniaxial crystal). LiNbO ₃ is known to have a plurality of optical axes (biaxial crystal). Since the sensitivity of the ball SAW sensor is drastically reduced when the formation positions of the respective constituent elements are shifted, it is required to form them with high accuracy.
JP 2003-115743 A JP 2005-291955 A

  For example, the pin-shaped external terminal 15 having a diameter of 0.3 mm needs to be accurately bonded to the two extreme points of the spherical substrate 11 having a diameter of 1 mm. This bonding operation had to be aligned while observing, and took a long time. In particular, the bonding of the second external terminal 15 is extremely difficult because it is performed with the external terminal 15 on one side bonded. That is, the conventional spherical surface acoustic wave device is inferior in productivity.

  An object of the present invention is to provide a spherical surface acoustic wave device having excellent productivity.

  A spherical surface acoustic wave element according to a first aspect of the present invention includes a spherical base, a pair of comb-shaped electrodes that excite a surface acoustic wave that circulates around the surface of the base and receives the surface acoustic wave that has circulated. Formed on the first input / output electrode and the first and second input / output electrodes formed near both pole points of the optical axis passing through the center of the substrate and connected to each of the pair of comb electrodes. An external terminal and an external electrode formed in a non-contact manner facing the second input / output electrode.

  The spherical surface acoustic wave device according to aspect 2 of the present invention is the first aspect, in which a flexible conductive material is filled between the second input / output electrode and the external electrode. It is a feature.

  The spherical surface acoustic wave device according to aspect 3 of the present invention is characterized in that, in the aspect of the invention described above, the base is made of a piezoelectric material having an electromechanical coupling coefficient larger than that of quartz.

  An environmental difference detection apparatus according to aspect 4 of the present invention includes the spherical surface acoustic wave element according to the aspect of the invention described above.

  The environmental difference detection apparatus according to aspect 5 of the present invention is characterized in that, in the fourth aspect, a plurality of the spherical surface acoustic wave elements are provided, and the external electrodes of the plurality of spherical surface acoustic wave elements are common. Is.

  ADVANTAGE OF THE INVENTION According to this invention, the spherical surface acoustic wave element excellent in productivity can be provided.

  Embodiments of the present invention will be described below. However, the present invention is not limited to the following embodiment. Further, in order to clarify the explanation, the following description and drawings are appropriately omitted and simplified.

Embodiment 1
A hydrogen sensor according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically illustrating a configuration of a hydrogen sensor according to an embodiment. As shown in FIG. 1, the hydrogen sensor 100 according to the embodiment includes a spherical surface acoustic wave element 110, a transmitter 120, an impedance matching circuit 130, a high-frequency amplifier 140, a digital oscilloscope 150, and a concentration signal generator 160. The spherical surface acoustic wave device 110 includes a base 111, a comb electrode 112, a sensitive film 113, an input / output electrode 114, an external terminal 115, and an external electrode 116.

The base 111 is made of a single crystal of a piezoelectric material such as quartz, LiNbO ₃ , LiTaO ₃ , and is formed in a spherical shape. The diameter of the sphere is preferably about 1 mm. When a surface acoustic wave is excited on the surface of the substrate 111, the surface acoustic wave propagates along a great circle of a sphere around a predetermined optical axis. The surface acoustic wave of a sphere does not spread like a normal wave, and circulates many times in the annular region of a finite width along the great circle with almost no attenuation. Here, as a material constituting the substrate 111, a material having a larger electromechanical coupling coefficient is preferable because a surface acoustic wave to be transmitted has a higher intensity.

  The comb-shaped electrode 112 is formed on the base 111 and is made of a metal such as Au, Al, Cu, or Cr, for example. It is a pair of electrodes formed so that the comb teeth face each other and are staggered. The comb electrode 112 can be formed, for example, by forming a metal film on the substrate 111 by vacuum deposition, sputtering, or the like, and then patterning by photolithography. When an electric signal is input to the comb electrode 112, a surface acoustic wave is excited. The comb electrode 112 receives a surface acoustic wave that has circulated around the surface of the substrate 111 and generates a signal. That is, the comb-shaped electrode 112 corresponds to the surface acoustic wave excitation unit / reception unit in the hydrogen sensor according to the present invention.

  The sensitive film 113 is formed on at least a part of the circulation path of the surface acoustic wave on the substrate 111. The sensitive film 113 is made of a Pd, Ni or Pd—Ni alloy layer that occludes hydrogen. The sensitive film 113 can be formed on the substrate 111, for example, by performing vacuum deposition, sputtering or the like in a state where a portion other than the sensitive film 113 forming portion is masked. Water vapor is decomposed into oxygen and hydrogen on the sensitive film 113, and this hydrogen is absorbed into the sensitive film 113. Since the sensitive film 113 that has absorbed hydrogen becomes hard, the propagation speed of the surface acoustic wave is increased on the sensitive film 113. Therefore, the higher the hydrogen concentration, the faster the surface acoustic wave propagation speed, while the lower the hydrogen concentration, the slower the surface acoustic wave propagation speed. The sensitive film 113 reversibly absorbs and releases hydrogen according to the hydrogen concentration. Therefore, the hydrogen concentration can be measured by measuring the propagation speed of the surface acoustic wave.

  Similarly to the comb-shaped electrode 112, the input / output electrode 114 is also formed on the base 111, and is made of a metal such as Au, Al, Cu, or Cr, for example. The input / output electrode 114 can also be formed, for example, by forming a metal film on the substrate 111 by vacuum deposition, sputtering, or the like, and then patterning by photolithography. Two input / output electrodes 114 are formed, and each input / output electrode 114 is connected to one of the comb-shaped electrodes 112 formed of a pair of electrodes. An external terminal 115 is formed on one input / output electrode 114. On the other input / output electrode 114, no external terminal is formed, and a non-contact external electrode 116 is formed instead.

  The external terminal 115 is formed on one input / output electrode 114 and in contact with the pole of the optical axis of the substrate 111. The external terminal 115 is a pin-shaped terminal having a diameter of 0.3 mm, for example. The external terminal 115 is made of a metal such as Au, Al, Cu, or Cr, for example. The external terminal 115 is accurately bonded in the vicinity of the pole of the optical axis of the base 111 using a conductive adhesive.

External electrodes 116, rather than one of the input and output electrodes 114 that the external terminal 115 is formed, an electrode made of a non-contact so as to face the other of the input and output electrodes 114. The external electrode 116 is a metal plate made of, for example, Au, Al, Cu, Cr or the like. That is, the external electrode 116 and the input / output electrode 114 form a capacitor. For example, in the case where a crystal ball having a diameter of 1 mm is used as the base 111, the distance between the external electrode 116 and the input / output electrode 114 is set to about 10 μm, so that about 30% of the conventional element in which the external terminals 115 are formed at both pole points. It is possible to excite a surface acoustic wave having a high intensity. If a strength of about 30% can be obtained, it can be sufficiently used as a sensor.

Here, in the case of a parallel plate capacitor, the capacitance C of the capacitor is expressed by C = ε ₀ · εr · S / d (where ε ₀ is the dielectric constant of vacuum, εr is the relative dielectric constant of the substance, and S is the electrode area) , D is expressed by a distance between electrodes). Since the relative dielectric constant of air is about 1.0, even if the input / output electrode 114 and the external electrode 116 are not in contact with each other, elastic waves can be excited through the air.

  Further, as described above, the constant (electromechanical coupling coefficient) representing the efficiency of converting electrical energy applied between the electrodes into mechanical energy differs depending on the type of piezoelectric material. Therefore, if a material having an electromechanical coupling coefficient larger than that of quartz is selected, a surface acoustic wave can be generated even with a small potential difference. That is, if the condition is that the surface acoustic wave intensity is comparable to that of quartz, the inter-electrode distance d can be made larger than that of quartz.

  Further, a flexible conductive material may be filled between the external electrode 116 and the input / output electrode 114. Since the capacitance C of the capacitor is proportional to the relative dielectric constant εr of the interelectrode material from the above formula, a higher-intensity elastic wave can be excited by filling a material having a relative dielectric constant larger than air. As the conductive substance, propylene carbonate, silicone rubber, urethane rubber, or the like can be used.

  As described above, in the spherical surface acoustic wave device according to the present invention, it is not necessary to form the external terminal 115 on one of the input / output electrodes 114. Therefore, the manufacturing process can be simplified, and a spherical surface acoustic wave element excellent in productivity can be provided. In the conventional spherical surface acoustic wave element, it is very difficult to form the second external terminal 115. Therefore, productivity was inferior.

  Next, the operation of the hydrogen sensor according to the embodiment of the present invention will be described with reference to FIG. First, a signal output from the transmitter 120 is input to the comb electrode 112 through the impedance matching circuit 130, the external terminal 115, the external electrode 116, and the input / output electrode 114. A surface acoustic wave is excited on the surface of the substrate 111 by a signal input to the comb electrode 112. As described above, the surface acoustic wave circulates many times on the circular path along the great circle of the sphere around the predetermined optical axis with almost no attenuation.

  The circulating surface acoustic wave is received by the comb electrode 112, and a signal generated thereby is input to the high frequency amplifier 140 via the input / output electrode 114 and the impedance matching circuit 130. The signal amplified by the high frequency amplifier 140 is input to the digital oscilloscope 150. A sensitive film 114 is formed on the circumferential path of the surface acoustic wave. As described above, the hardness of the sensitive film 114 varies depending on the surrounding hydrogen concentration. Therefore, the propagation speed of the surface acoustic wave changes. This change in propagation speed can be known as a phase shift of the signal output from the digital oscilloscope 150, that is, a delay time. The signal output from the digital oscilloscope 150 is input to a concentration signal generation unit 160 that generates a hydrogen concentration signal based on the signal. Finally, a hydrogen concentration signal is output from the concentration signal generator 160.

Embodiment 2
Next, another embodiment of the present invention will be described. In the present embodiment, as shown in FIG. 2A, a plurality of bases 111 are arranged on the same plane. As shown in FIG. 2B, the external electrode 116 is common. Further, the external terminals 115 of the respective elements are arranged so as to be parallel to each other. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  When the spherical surface acoustic wave element is applied to an odor sensor or an electric nose, it is necessary to mount a large number of spherical surface acoustic wave elements. Here, when a plurality of external electrodes are connected to both sides, not only the degree of freedom of the structure is hindered due to the influence of wiring, but also the inclination and length of the plurality of electrodes need to be strictly adjusted. Such a problem can be solved by using the spherical surface acoustic wave device according to the present invention. A large number of spherical surface acoustic wave elements can be mounted at high density. It can also be used as an environmental difference detection device such as a pressure sensor and a dew point sensor, including a hydrogen sensor and an odor sensor.

2 is a schematic diagram illustrating a configuration of a hydrogen sensor according to Embodiment 1. FIG. 6 is a schematic diagram showing a configuration of a hydrogen sensor according to Embodiment 2. FIG. It is a schematic diagram which shows the structure of the conventional hydrogen sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Hydrogen sensor 110 Surface acoustic wave element 111 Base | substrate 112 Comb electrode 113 Sensing film | membrane 114 Input / output electrode 115 External terminal 116 External electrode 120 Transmitter 130 Impedance matching circuit 140 High frequency amplifier 150 Digital oscilloscope 160 Concentration signal generation part

Claims (5)

A spherical substrate;
A pair of comb electrodes for exciting the surface acoustic waves that circulate around the surface of the substrate and receiving the surface acoustic waves that have circulated;
First and second input / output electrodes formed near both pole points of the optical axis passing through the center of the substrate and connected to each of the pair of comb electrodes;
An external terminal formed on the first input / output electrode;
A spherical surface acoustic wave device comprising: an external electrode formed in a non-contact manner facing the second input / output electrode.   2. The spherical surface acoustic wave device according to claim 1, wherein a conductive material having flexibility is filled between the second input / output electrode and the external electrode.   The spherical surface acoustic wave element according to claim 1, wherein the base is made of a piezoelectric material having an electromechanical coupling coefficient larger than that of quartz.   An environmental difference detection apparatus comprising the spherical surface acoustic wave element according to claim 1.   The environmental difference detection device according to claim 4, wherein a plurality of the spherical surface acoustic wave elements are provided, and the external electrodes of the plurality of spherical surface acoustic wave elements are common.

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