JP6604238B2 - Elastic wave sensor - Google Patents

Description

The present invention relates to an acoustic wave sensor, and more particularly, to an acoustic wave sensor that detects an object to be detected based on a change in conductivity or a change in mass load applied to the surface acoustic wave device by the object to be detected.

Conventionally, in various fields such as environment, food, and medicine, research has been conducted on sensors using acoustic wave devices that are simple in structure and can be miniaturized to detect various substances and measure the amount and concentration of the substances. Has been.

Among acoustic wave sensors, a sensor using a surface acoustic wave (hereinafter abbreviated as SAW) device is used as an acoustic wave propagating means, for example, on a piezoelectric substrate 2 as shown in FIG. 5 on a piezoelectric substrate 2 (Interdigital Transducer). The surface detection input electrode 3 and the surface detection output electrode 4 are formed. When an electric signal is applied to the surface detection input electrode 3 of the IDT, an electric field is generated between the electrodes, the SAW is excited by the piezoelectric effect, and when the SAW reaches the surface detection output electrode 4, the electric signal again. Is converted to

In a sensor using a surface acoustic wave device, a reaction film layer 5 is formed in a region between the surface detection input electrode 3 and the surface detection output electrode 4, and the reaction film layer of the detection target substance having a large mass is formed. Since the mass increases due to the adsorption of the SAW and the SAW resonance frequency decreases accordingly, the mass of the substance to be detected can be quantitatively measured as the frequency change of the SAW.

Furthermore, it is possible to quantitatively measure not only the mass change of the detection target substance but also the change in conductivity of the reaction film layer due to the adsorption of the detection target substance to the reaction film layer as a change in the frequency of the SAW.

Here, the reaction film layer is a layer coated with a substance near the surface on which specific gas molecules are likely to cause physical adsorption or chemical adsorption.

Many of the detection sensors using the surface acoustic wave device detect the mass of the detection target substance by using the surface acoustic wave (Rayleigh wave) propagating on the piezoelectric substrate.

Further, a sensor using a surface acoustic wave device has two pairs of surface detection input electrodes 3, a surface detection output electrode 4, a surface correction input electrode 8, and a surface correction for the surface of the piezoelectric substrate 2 as shown in FIG. Surface acoustic wave devices including output electrodes 9 are arranged side by side. In one surface acoustic wave device, a reactive substance layer 5 is disposed between the surface detection input electrode 3 and the surface detection output electrode 4, and the other is disposed as a compensation surface acoustic wave device.

The mass change of the reactant layer 5 is connected to detect the difference between the frequency change of the surface acoustic wave propagating on the substrate surface and the frequency change of the surface acoustic wave for compensation, thereby improving the measurement accuracy.
The sensor using the surface acoustic wave device is disclosed in the following prior art documents.

JP-A-2005-331326 WO2010 / 073484 Special table 2008-518201

Sensors using surface acoustic wave devices are widely studied such as chemical sensors and biosensors, but miniaturization is important for practical use. In particular, it is necessary to reduce the occupied volume in order to be mounted on a mobile phone, but there is a trade-off relationship that the sensitivity of the sensor decreases as the sensor becomes smaller.

Furthermore, surface acoustic wave devices are easily affected by ambient temperature, humidity, and atmosphere, and a surface acoustic wave device for compensation is required to correct these effects. As shown in FIG. 6, the occupied area is nearly doubled and it is difficult to reduce the size.

In order to solve the above-described problems, the present invention provides an elastic wave sensor configured as follows.
In order to achieve the above object, an elastic wave sensor of the present invention is disposed on an elastic wave propagation means for exciting an elastic wave and propagating the elastic wave to the substrate surface and the vicinity of the surface, and on the propagation path of the elastic wave, An elastic wave sensor having a reactive substance layer that reacts with a specific substance to be detected, and detecting the specific substance by a change in propagation characteristics of the elastic wave caused by the change of the reactive substance layer by the specific substance The elastic wave propagation means is disposed on both the front and back surfaces of the substrate.

  In the elastic wave sensor of the present invention, the elastic wave propagation means disposed on both the front and back surfaces of the elastic wave sensor excites in the same phase.

The elastic wave propagation means includes a comb-tooth electrode, and the comb-tooth electrode has an overlapping portion when viewed from either the front or the back.

  In the elastic wave sensor according to the present invention, the elastic wave is a transverse elastic wave mainly including displacement in a direction parallel to the substrate surface and orthogonal to the propagation direction of the elastic wave.

Here, the reactant layer includes one or more of graphene oxide, graphene, and graphite.

The reactive substance layer is disposed on either the front or back surface of the substrate.

The substrate is made of any one of LiTaO ₃ , LiNbO ₃ , and quartz.

In the elastic wave sensor of the present invention, the elastic wave propagation means is arranged on both the front and back surfaces of the elastic wave sensor, and one of them is a surface acoustic wave device for compensation, so the influence of ambient temperature, humidity and atmosphere is corrected. In addition, it is possible to reduce the area occupied by the device and to reduce the size.

In addition, since transverse elastic waves are used, the propagation loss due to the formed reaction film layer is small, and surface waves can be efficiently propagated and detected. In addition, the transverse elastic wave is excellent in frequency temperature characteristics and has a high propagation speed, and is therefore suitable for high frequency. Therefore, it is possible to achieve downsizing and high sensitivity of the sensor, which has been difficult to realize with a conventional sensor using a crystal resonator or a surface acoustic wave device.

Further, in the case of a transverse acoustic wave, since there is no excitation in the thickness direction like a surface acoustic wave (Rayleigh wave), it is possible to use an extremely thin piezoelectric substrate.

  In addition, by arranging a reaction film layer on the surface of the excitation electrode, mass change (mass increase) and conductivity change (surface resistance increase) of the reaction film layer that occurs when this reaction film layer reacts with a specific detection substance. Since the frequency change rate of the elastic wave accompanying this is improved (increased), it is possible to achieve higher sensitivity of the sensor.

FIG. 1 is a schematic perspective view of an elastic wave sensor according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing a transverse elastic wave (leaky wave) of the elastic wave sensor according to the embodiment of the present invention. FIG. 3 is a conceptual diagram showing surface acoustic waves (Rayleigh waves). FIG. 4 is a diagram illustrating an example of an oscillation circuit using the elastic wave sensor according to the first embodiment of the present invention. FIG. 5 is a schematic perspective view of a conventional acoustic wave sensor. FIG. 6 is a schematic perspective view of a conventional elastic wave sensor.

Next, an embodiment according to the present invention will be given and described in detail with reference to the drawings. First, the configuration of the elastic wave sensor according to the present embodiment will be described with reference to FIGS. 1 to 4.

As shown in FIG. 1, the acoustic wave sensor 1 according to the present embodiment includes a substrate 2, a surface detection input electrode 3, a surface detection output electrode 4, a reaction film layer 5, and a back surface correction input electrode 6. And a back surface correcting output electrode 7.

The substrate 2 may be any material that can propagate a transverse acoustic wave having a displacement in a direction parallel to the substrate surface and perpendicular to the propagation direction. For example, lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO) ₃ ) There are crystals, etc. For lithium tantalate (LiTaO ₃ ), the cut angle is 36 ° rotated Y-cut, for lithium niobate (LiNbO ₃ ), the cut angle is 64 ° rotated Y-cut, and for quartz, the cut angle is −7.5 ° rotated. In the Y cut, the elastic wave propagation direction is a transverse elastic wave whose direction is perpendicular to the X direction.

In the present embodiment, lithium tantalate (LiTaO ₃ ) is selected as the substrate material because the frequency temperature characteristics of the propagating elastic wave are excellent. In addition, the lithium tantalate (LiTaO ₃ ) substrate is selected because it has a high mechanical coupling coefficient and a high propagation speed of the transverse acoustic wave.

The surface detection input electrode 3 uses a comb electrode as an electrode for exciting an elastic wave, and is arranged on the surface of the substrate 2 so that the propagation direction of the elastic wave is perpendicular to the X direction. The surface detection output electrode 4 is a comb-shaped electrode that receives the elastic wave excited and propagated by the surface detection input electrode 3.

In addition, the back surface correction input electrode 6 uses a comb electrode as an electrode for exciting the elastic wave on the back surface of the substrate, and the substrate is arranged so that the propagation direction of the same elastic wave as the front surface is perpendicular to the X direction. 2 is arranged on the back surface. The back surface correction output electrode 7 is a comb electrode that receives the elastic wave excited and propagated by the back surface correction input electrode 6.

In the present embodiment, each electrode is made of Au. Further, a Ti film may be formed between the Au electrode film and the substrate 2 in order to enhance the adhesion between the Au electrode film and the substrate 2.

As described above, the excitation condition of the surface acoustic wave on the surface of the substrate 2 is set, and the surface detection input electrode 3 and the surface detection are matched with the excitation condition of the surface acoustic wave on the surface of the substrate 2. Since the output electrode 4 for backside and the input electrode 6 for backside correction and the output electrode 7 for backside correction are arranged, a transverse acoustic wave having a displacement parallel to the front and back surfaces of the substrate 2 and perpendicular to the propagation direction is generated. Then, the light propagates from the front surface detection input electrode 3 and the back surface correction input electrode 6 toward the front surface detection output electrode 4 and the back surface correction output electrode 7.

Since the elastic wave propagation means arranged on the front and back sides are arranged to excite in the same phase, the front and back elastic wave propagation means should excite in the same phase even when the ambient temperature, humidity and atmosphere change. Can do.

The elastic wave is a transverse elastic wave whose displacement is mainly parallel to the substrate surface and orthogonal to the propagation direction of the elastic wave. FIG. 2 is a conceptual diagram showing a transverse elastic wave (leaky wave) of the elastic wave sensor according to the embodiment of the present invention. As shown in FIG. 2, the transverse elastic wave is an excitation of only a horizontal wave (SH wave). FIG. 3 is a conceptual diagram showing surface acoustic waves (Rayleigh waves). A surface acoustic wave (Rayleigh wave) is an excitation wave composed of a vertical wave (SV wave) and a traveling wave (P wave). Accordingly, the transverse acoustic wave (Leaky wave) is not excited in the thickness direction of the substrate like the surface acoustic wave (Rayleigh wave). Therefore, the elastic wave propagation means arranged on the front and back sides can be excited in the same phase. .
Furthermore, when a transverse elastic wave (leaky wave) is used, the elastic wave propagation means can be arranged even in an extremely thin substrate without considering the influence of the propagation loss of excitation on the front and back sides.

Further, the reaction film layer 5 has a property of reacting with a specific substance to be detected, and is on a propagation path of a transverse acoustic wave between the surface detection input electrode 3 and the surface detection output electrode 4. Be placed. The reaction film layer 5 includes one or more of graphene oxide, graphene, and graphite.

Then, for example, as shown in FIG. 4, the elastic wave propagation means and the amplifier 15 on the surface of the elastic wave sensor 1 having the above configuration and the elastic wave propagation means and the amplifier 15 on the back surface of the elastic wave sensor 1 having the above configuration are mixed. 16, a feedback type oscillation circuit that outputs to a frequency counter (not shown) that measures the sum frequency of the propagating wave is configured, and when a specific substance is detected, a specific substance to be detected and Since the propagation speed of the transverse elastic wave propagating on the substrate surface changes with the mass change (mass increase) and the conductivity change (surface resistance increase) of the reaction film 5 due to the reaction of the above, the mass change and conductivity due to the above reaction change. The change is output as a frequency change based on a change in the propagation speed of the transverse elastic wave. Therefore, according to the acoustic wave sensor 1, it is possible to measure a change in mass or a change in conductivity as a change in frequency. Based on the measurement result, the presence / absence, mass, concentration, or the like of the specific substance to be detected Can be detected.

Specifically, the mass of the detection target substance having a large mass, for example, the mass due to adsorption of CO and CO _{2 to} the reaction membrane layer increases, and the resonance frequency of the SAW decreases accordingly. It becomes possible to measure quantitatively as the SAW frequency change.
On the other hand, the resonance frequency of the SAW increases due to the decrease in the conductivity of the reaction layer due to the reaction with the reaction film layer, rather than the increase in the mass due to the adsorption of the detection target substance having a small mass, for example, H _{2 to} the reaction film layer. Since the influence of the action is larger, the resonance frequency increases, and the concentration of the detection target substance can be quantitatively measured as an increase in the SAW frequency.

In addition, since the elastic wave sensor 1 according to the present embodiment uses a transverse elastic wave, even if an antibody film or the like is present on the surface of the substrate 2, the propagation loss is reduced, and the surface wave is efficiently propagated and detected. can do. Furthermore, since the transverse acoustic wave is excellent in frequency temperature characteristics and has a high propagation speed, it is suitable for high frequency, so that high sensitivity of the sensor can be achieved.

As described above, the acoustic wave sensor according to the present invention is useful for biosensors and chemical sensors in various other applications including gas sensors.

DESCRIPTION OF SYMBOLS 1 Surface acoustic wave device 2 Piezoelectric substrate 3 Surface detection input electrode 4 Surface detection output electrode 5 Reaction film layer 6 Back surface correction input electrode 7 Back surface correction output electrode 8 Surface correction input electrode 9 Surface correction output electrode 10 Transverse wave Piezoelectric substrate 11 with conceptual diagram of elastic wave (leaky wave) SH wave (leaky wave)
12 Surface acoustic wave (Rayleigh wave) conceptual diagram of piezoelectric substrate 13 SV wave (Rayleigh wave)
14 P wave (Rayleigh wave)
15 Amplifier 16 Mixer

Claims (7)

An elastic wave propagating means for exciting an elastic wave and propagating the elastic wave to and near the substrate surface, and a reactive substance layer disposed on the elastic wave propagation path and reacting with a specific substance to be detected are provided. An elastic wave sensor for detecting the specific material by a change in propagation characteristics of the elastic wave caused by a change in the reactive material layer due to the specific material, wherein the elastic wave propagation means is provided on the front and back sides of the substrate. An elastic wave sensor characterized by being arranged on both sides. The elastic wave sensor according to claim 1, wherein the elastic wave propagation means excites in the same phase. The elastic wave sensor according to claim 1 or 2, wherein the elastic wave propagation means includes a comb-like electrode, and the comb-like electrode has an overlapping portion when viewed from either the front or back side. . 4. The elastic wave according to claim 1, wherein the elastic wave is a transverse elastic wave that is mainly displaced in a direction parallel to the surface of the substrate and perpendicular to the propagation direction of the elastic wave. 5. Elastic wave sensor. 5. The acoustic wave sensor according to claim 1, wherein the reactant layer includes one or more of graphene oxide, graphene, and graphite. 6. 6. The acoustic wave sensor according to claim 1, wherein the reactive substance layer is disposed on one of the front and back surfaces of the substrate. The elastic substrate according to claim 1, wherein the substrate is one of LiTaO ₃ , LiNbO ₃ , and quartz.

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