Odor detection element

Abstract

To provide an odor detection element capable of performing an output in which influence by frequency-temperature characteristics of a sensitive film is suppressed.SOLUTION: An odor detection element according to the present invention includes a piezoelectric vibrator and a sensitive film. The piezoelectric vibrator includes: a piezoelectric layer containing a piezoelectric material as a main component; and a pair of electrodes provided in the piezoelectric layer. The sensitive film includes: a base layer provided on the piezoelectric vibrator and having positive or negative frequency-temperature characteristics; and an adsorption layer provided on the base layer and having frequency-temperature characteristics opposite to those of the base layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a vibrator-type odor detection element. [Background technology]

[0002] Odor detection elements using a vibrator have a sensitive membrane on the vibrator that adsorbs specific odorants, and can detect odorants by the change in resonant frequency that accompanies the adsorption of odorants to the sensitive membrane. However, the sensitive membrane itself may have a frequency-temperature characteristic, i.e., the resonant frequency may change with temperature changes, and this change in resonant frequency due to temperature changes may reduce the accuracy of odorant detection.

[0003] In response to this, for example, Patent Document 1 discloses an elastic wave sensor in which reactive film layers are placed on both sides of the elastic wave sensor, one for detection and the other for correcting the effects of disturbances such as temperature, thereby improving detection accuracy. Also, Patent Document 2 discloses a gas sensor in which a temperature sensor is provided within the measurement device, and measurement errors due to the temperature difference between a standard gas and a gas to be measured are corrected by calculation, thereby improving detection accuracy. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-156253 [Patent Document 2] Japanese Patent Application Publication No. 2018-048930 Summary of the Invention [Problem to be solved by the invention]

[0005] However, providing a temperature correction sensor as described in Patent Document 1, or performing temperature correction calculations as described in Patent Document 2, entails disadvantages such as increased costs and a more complex system. On the other hand, if the odor detection element itself can produce an output that suppresses the effects of the frequency-temperature characteristics of the sensitive film, these disadvantages can be eliminated.

[0006] In view of the above circumstances, an object of the present invention is to provide an odor detection element that can provide an output while suppressing the influence of the frequency temperature characteristics of the sensitive film. [Means for solving the problem]

[0007] In order to achieve the above object, an odor detection element according to one embodiment of the present invention includes a piezoelectric vibrator and a sensitive film. The piezoelectric vibrator includes a piezoelectric layer containing a piezoelectric material as a main component, and a pair of electrodes provided on the piezoelectric layer. The sensitive film includes an underlayer provided on the piezoelectric vibrator and having a positive or negative frequency-temperature characteristic, and an adsorption layer provided on the underlayer and having a frequency-temperature characteristic opposite to that of the underlayer. [Effects of the Invention]

[0008] As described above, according to the present invention, it is possible to provide an odor detection element that can provide an output while suppressing the influence of the frequency temperature characteristics of the sensitive film. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of an odor detection element (QCM element type) according to an embodiment of the present invention. [Figure 2] 10 is a graph showing the positive and negative frequency-temperature characteristics of a sensitive film. [Figure 3] 4 is a graph showing frequency-temperature characteristics of the sensitive film included in the odor detection element. [Figure 4] 10 is a graph showing the frequency-temperature characteristics of the sensitive film of the odor detection element at each ratio. [Figure 5] 1 is a cross-sectional view of an odor detection element (FBAR element type) according to an embodiment of the present invention. [Figure 6] FIG. 1 is a plan view of an odor detection element (SAW element type) according to an embodiment of the present invention. [Figure 7] 1 is a cross-sectional view of an odor detection element (SAW element type) according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an odor detection element according to an embodiment of the present invention will be described with reference to the drawings.

[0011] [Configuration of odor detection element] As shown in FIG. 1, the odor detection element 100 according to this embodiment includes a piezoelectric vibrator 110 and a sensitive film 120. The piezoelectric vibrator 110 includes a piezoelectric layer 111, a first electrode 112, and a second electrode 113. The piezoelectric layer 111 is a layer whose main component is a piezoelectric material. As shown in FIG. 1, the front surface of the piezoelectric layer 111 is referred to as the front surface 111a, and the back surface is referred to as the back surface 111b. The first electrode 112 and the second electrode 113 are provided on the front surface 111a, and the second electrode 113 is provided on the back surface 111b. The first electrode 112 and the second electrode 113 sandwich the piezoelectric layer 111, forming a pair of electrodes. The first electrode 112 and the second electrode 113 are made of a conductive material, such as gold, silver, aluminum, or another metal.

[0012] We will now discuss the adsorption characteristics of sensitive films. Sensitive films are made of resin, polymers made from resin, metal, metal oxide film, ceramics made from sintered inorganic materials, or films made from a mixture of these. These sensitive films adsorb multiple gases and odor components, although the amount of adsorption varies. These components include gases, suspended liquids, and water vapor. If we prepare sensitive film A, it will be rare for it to adsorb only gas A, and it will also adsorb other gases B, C, etc., but it will be the film that adsorbs gas A the most, and this is called a sensitive film that adsorbs a specific odorant. The sensitive film 120 of the present invention is provided on the piezoelectric vibrator 110 and adsorbs specific odorants contained in the gas supplied to the odor detection element 100. The type of odorant adsorbed by the sensitive film 120 depends on the material of the sensitive film 120. The sensitive film 120 includes an underlayer 121 and an adsorption layer 122 located on the front side. The underlayer 121 is provided on the first electrode 112, as shown in Fig. 1, and the adsorption layer 122 is provided on the underlayer 121. The thickness D of the sensitive film 120 is, for example, 500 nm.

[0013] In the odor detection element 100, when a voltage is applied between the first electrode 112 and the second electrode 113, the inverse piezoelectric effect causes the piezoelectric layer 111 to vibrate at a certain resonant frequency. When an odorant is adsorbed onto the sensitive film 120, the weight of the sensitive film 120 increases, and the resonant frequency decreases. On the other hand, when the odorant adsorbed onto the sensitive film 120 is desorbed, the weight of the sensitive film 120 decreases, and the resonant frequency increases. Therefore, the odor detection element 100 can detect the amount of odorant adsorbed onto the sensitive film 120 based on the change in the resonant frequency.

[0014] [Frequency temperature characteristics of the sensitive film] As described above, the sensitive film 120 includes the base layer 121 and the adsorption layer 122. The base layer 121 has a positive or negative frequency-temperature characteristic, and the adsorption layer 122 has a frequency-temperature characteristic opposite to that of the base layer 121. Specifically, the base layer 121 has a negative frequency-temperature characteristic, and the adsorption layer 122 has a positive frequency-temperature characteristic. The positive or negative frequency-temperature characteristic will be described later.

[0015] Figure 2 shows the change in resonant frequency with respect to temperature in an odor detection element equipped with a sensitive membrane. The horizontal axis represents temperature, and the vertical axis represents the amount of change in resonant frequency (Δf). The graph shows the temperature coefficient of frequency (TCF) of the sensitive membrane. The temperature coefficient of frequency is the range of variation in the amount of change in resonant frequency with respect to temperature change, i.e., the slope of the graph shown in Figure 2. The smaller the absolute value of the temperature coefficient of frequency, the more stable the frequency is with respect to temperature changes.

[0016] In Figure 2, the legend "positive temperature characteristics" indicates that the resonant frequency change increases as the temperature rises, and that the sensitive film has a positive frequency-temperature characteristic. On the other hand, the legend "negative temperature characteristics" indicates that the resonant frequency change decreases as the temperature rises, and that the sensitive film has a negative temperature characteristic. Thus, when the sensitive film has a positive or negative frequency-temperature characteristic, the resonant frequency of an odor detection element equipped with a sensitive film changes with temperature. Therefore, the resonant frequency change of an odor detection element equipped with a sensitive film includes changes caused by the frequency-temperature characteristic of the sensitive film as well as by the adsorption or desorption of odorants. Therefore, to improve the measurement accuracy of the odor detection element, it is desirable to make the frequency-temperature characteristic of the sensitive film as close to 0 as possible.

[0017] This will be explained using Figure 3. In this embodiment, the sensitive film 120 is composed of a base layer 121 having a negative frequency-temperature characteristic and an adsorption layer 122 having a positive frequency-temperature characteristic. In Figure 3, the legend "sensitive film" indicates the amount of change in resonant frequency of the odor detection element 100 equipped with the above-mentioned sensitive film 120. 3, the legend "Base layer material" indicates the amount of change in the resonant frequency of the odor detection element when the sensitive film is hypothetically composed only of the base layer material, and the legend "Adsorption layer material" indicates the amount of change in the resonant frequency of the odor detection element when the sensitive film is hypothetically composed only of the adsorption layer material. As shown in the figure, the base layer has a negative frequency-temperature characteristic, and the adsorption layer has a positive frequency-temperature characteristic.

[0018] As can be seen from Fig. 3, the sensitive film 120 has a frequency-temperature characteristic between that of the base layer and that of the adsorption layer. In this case, the frequency-temperature characteristic of the odor detection element 100 including the sensitive film 120 shows that the amount of change in the resonant frequency approaches almost zero. This is because the negative frequency-temperature characteristic of the base layer 121 and the positive frequency-temperature characteristic of the adsorption layer 122 cancel each other out. By using a two-layer structure of the sensitive film 120, consisting of the base layer 121 and the adsorption layer 122, it is possible to suppress changes in the resonant frequency of the odor detection element 100 due to temperature.

[0019] In the above description, the base layer 121 has a negative frequency-temperature characteristic and the adsorption layer 122 has a positive frequency-temperature characteristic, but it is also possible for the base layer 121 to have a positive frequency-temperature characteristic and the adsorption layer 122 to have a negative frequency-temperature characteristic. In this case as well, the positive frequency-temperature characteristic of the base layer 121 and the negative frequency-temperature characteristic of the adsorption layer 122 cancel each other out, making it possible to make the frequency-temperature characteristic of the sensitive film 120 approach nearly zero.

[0020] Furthermore, the sensitive film 120 is not limited to being composed of two layers, the base layer 121 and the adsorption layer 122, but may include layers other than the base layer 121 and the adsorption layer 122. In this case, the sensitive film 120 may include, in addition to the base layer 121 and the adsorption layer 122, at least one layer having a positive frequency-temperature characteristic and one or more layers having a negative frequency-temperature characteristic.

[0021] [Materials for the base layer and adsorption layer] The material of the base layer 121 or the adsorption layer 122 can be a fluorine-containing acrylate ester polymer such as FS-2060 (manufactured by Fluoro Technology) as a material having a negative frequency-temperature characteristic. Any material can be used as long as it has a negative frequency-temperature characteristic. The material of the base layer 121 or the adsorption layer 122 can be an imide-based resin having a hexafluoroisopropyl group as a material having a positive frequency-temperature characteristic. Any material can be used as long as it has a positive frequency-temperature characteristic.

[0022] In addition, the material of the base layer 121 or the adsorption layer 122 may be a filler material such as a polymer, MOF (Metal Organic Frameworks) or zeolite, or a vapor-deposited film such as copper phthalocyanine, as long as the base layer 121 has a positive or negative frequency-temperature characteristic and the adsorption layer 122 has a frequency-temperature characteristic opposite to that of the base layer 121.

[0023] Note that the legend "Base layer material" in Figures 3 and 4 indicates the amount of change in resonant frequency of the odor detection element 100 when the material of the base layer 121 is FS-2060 (manufactured by Fluoro Technology), and the legend "Adsorption layer material" in Figures 3 and 4 indicates the amount of change in resonant frequency of the odor detection element 100 when the material of the adsorption layer 122 is an imide-based resin having a hexafluoroisopropyl group.

[0024] In the sensitive film 120, the adsorption layer 122 is laminated on the base layer 121 as described above, but if the thickness of the adsorption layer 122 is 300 nm or more, almost all of the odorous substances attached to the sensitive film 120 are adsorbed by the adsorption layer 122. In this case, the material of the base layer 121 does not need to be selected in consideration of the odor substance to be adsorbed, and a material capable of canceling the frequency temperature characteristic of the adsorption layer 122 can be selected. Generally, if the thickness of the sensitive film is too large, the oscillation of the odor detection element becomes unstable and measurement becomes impossible. Considering this point of view, the thickness of the adsorption layer 122 is preferably 300 nm or more and 500 nm or less. When the adsorption layer 122 is an imide resin having a hexafluoroisopropyl group, oscillation is less likely to become unstable even if the thickness of the adsorption layer 122 is increased. For this reason, the thickness of the adsorption layer 122 is preferably 300 nm or more and 1000 nm or less.

[0025] In the sensitive film 120, it is preferable that the hydrophobicity of the base layer 121 is greater than that of the adsorption layer 122. The reasons for this will be explained below. Since moisture is often present in the atmosphere at a percentage of several percent or more, it exists at a much higher concentration than odorants, which may exist in the environment at concentrations of several ppm to ppt, for example. For this reason, humidity in the measurement environment can easily become a problem. If the hydrophobicity of the base layer 121 is less than that of the adsorption layer 122, some moisture may pass through the adsorption layer 122 even if the adsorption layer 122 is hydrophobic. In this case, if the hydrophobicity of the base layer 121 is less than that of the adsorption layer 122, the moisture will easily reach the piezoelectric vibrator 110. When the moisture reaches the piezoelectric vibrator 110, the effect of the moisture on the resonant frequency of the odor detection element 100 becomes significant. If the hydrophobicity of the base layer 121 is greater than that of the adsorption layer 122, some of the moisture that has permeated the adsorption layer 122 cannot permeate the base layer 121 and is prevented from reaching the piezoelectric vibrator 110. In this way, it is possible to suppress the effect of humidity on the resonant frequency of the odor detection element 100.

[0026] [Thickness of the base layer and adsorption layer] The thicknesses of the base layer 121 and the adsorption layer 122 that constitute the sensitive film 120 will be described. The thicknesses of the base layer 121 and the adsorption layer 122 are controlled in consideration of the resonant frequency of the piezoelectric vibrator 110 when manufacturing the odor detection element 100. This is because if the thickness of the sensitive film 120 increases when laminating the sensitive film 120, the resonant frequency of the piezoelectric vibrator 110 decreases due to the increased weight.

[0027] In FIG. 3 , the legend "base layer material" indicates the amount of change in resonant frequency of the odor detection element 100 including the sensitive film 120 formed by stacking the material of the base layer 121 until the amount of resonant frequency reduction reaches a predetermined value. The legend "adsorption layer material" indicates the amount of change in resonant frequency of the odor detection element 100 including the sensitive film 120 formed by stacking the material of the adsorption layer 122 until the amount of resonant frequency reduction reaches the predetermined value. The legend "sensitive film" indicates the amount of change in resonant frequency of the odor detection element 100 including the sensitive film 120 formed by stacking the material of the base layer 121 until the amount of resonant frequency reduction reaches 50% of the predetermined value, and then stacking the material of the adsorption layer 122 until the amount of resonant frequency reduction reaches 50% of the predetermined value. If the materials of the base layer 121 and the adsorption layer 122 have different densities, the thicknesses of the base layer 121 and the adsorption layer 122 will differ even if they are stacked until the amount of resonant frequency reduction is the same.

[0028] The thickness ratio of the base layer 121 to the adsorption layer 122 in the sensitive film 120 can be changed. Fig. 4 shows the amount of change in resonant frequency of the odor detection element 100 having a sensitive film 120 with different thickness ratios of the base layer 121 to the adsorption layer 122. The legend "base layer material" indicates the amount of change in resonant frequency of the odor detection element 100 when the sensitive film is made of only the material of the base layer, as in Fig. 3, and has the above-mentioned predetermined thickness. The legend "adsorption layer material" also indicates the amount of change in resonant frequency of the odor detection element when the sensitive film is made of only the material of the adsorption layer, as in Fig. 3, and has the above-mentioned predetermined thickness.

[0029] 4, the legend "75 / 25" indicates the amount of change in resonant frequency of an odor detection element 100 having a sensitive film 120 in which the adsorption layer 122 has a thickness that is 75% of the above-mentioned predetermined value and the base layer 121 has a thickness that is 25% of the above-mentioned predetermined value. The legend "50 / 55" indicates the amount of change in resonant frequency of an odor detection element 100 having a sensitive film 120 in which the adsorption layer 122 has a thickness that is 50% of the above-mentioned predetermined value and the base layer 121 has a thickness that is 50% of the above-mentioned predetermined value. The legend "25 / 75" indicates the amount of change in resonant frequency of an odor detection element 100 having a sensitive film 120 in which the adsorption layer 122 has a thickness that is 25% of the above-mentioned predetermined value and the base layer 121 has a thickness that is 75% of the above-mentioned predetermined value.

[0030] 4, as the thickness ratio of the base layer 121 increases, the frequency temperature coefficient decreases, and the frequency temperature characteristic approaches 0. This is because the frequency temperature coefficient of the base layer 121 is smaller than the frequency temperature coefficient of the adsorption layer 122, and therefore, by increasing the thickness of the base layer 121, the negative frequency temperature characteristic of the base layer 121 becomes equivalent to the positive frequency temperature characteristic of the adsorption layer 122. Specifically, as indicated by the legend "25 / 75," the frequency temperature characteristic of the odor detection element 100 including the sensitive film 120 approaches 0 most when the adsorption layer 122 has a thickness that is 25% of the above-mentioned predetermined value and the base layer 121 has a thickness that is 75% of the above-mentioned predetermined value.

[0031] The thickness ratio at which the frequency-temperature characteristic approaches zero varies depending on the material of the base layer 121 and the material of the adsorption layer 122, and the thickness ratio of the base layer 121 to the adsorption layer 122 can be adjusted between 10% and 90%. Specifically, the thickness ratio of the base layer 121 to the adsorption layer 122 is preferably the inverse of the ratio of the frequency temperature coefficient of the material of the base layer 121 to the frequency temperature coefficient of the material of the adsorption layer 122. In other words, if the absolute value of the frequency temperature coefficient of the material of the base layer 121 is a and the absolute value of the frequency temperature coefficient of the material of the adsorption layer 122 is b, the thickness of the base layer 121 is preferably b / (a+b) and the thickness of the adsorption layer 122 is preferably a / (a+b).

[0032] [About piezoelectric vibrators] The piezoelectric vibrator 110 is an element that generates vibrations in the piezoelectric layer 111 due to the inverse piezoelectric effect when a voltage is applied between the first electrode 112 and the second electrode 113 as described above. Specifically, the piezoelectric vibrator 110 may be a QCM (quartz crystal microbalance) element in which the piezoelectric layer 111 is made of quartz crystal. A QCM element can be realized by using a quartz crystal for the piezoelectric layer 111 in the configuration shown in FIG. 1. As shown by "QCM" in FIG. 4, the QCM element exhibits almost no change in resonant frequency with temperature, exhibiting a frequency-temperature characteristic of approximately zero. Because an AT-cut quartz crystal was used, the frequency-temperature characteristic was approximately zero at least from approximately 33°C to approximately 65°C, as shown in FIGS. 3 and 4. The frequency-temperature characteristic of an AT-cut quartz crystal is generally known, and it can be said that the frequency-temperature characteristic is approximately zero at temperatures between 10°C and 65°C. However, it is clear that the present invention is effective even outside this temperature range. If the frequency temperature characteristic of the piezoelectric vibrator 110 is approximately 0, then canceling out the frequency temperature characteristic with the base layer 121 and the adsorption layer 122 in the sensitive film 120 makes it possible to bring the frequency temperature characteristic of the entire odor detection element 100 close to 0.

[0033] Alternatively, the piezoelectric vibrator 110 may be an FBAR (Film Bulk Acoustic Resonator) element. As shown in FIG. 5, the piezoelectric vibrator 110, which is an FBAR element, is bonded to a substrate 131. The substrate 131 is made of Si or the like, and has a gap 132 provided on the opposite side of the sensitive film 120. The sensitive film 120 has a base layer 121 provided on a first electrode 112. The piezoelectric layer 101 is a thin film made of aluminum nitride (AlN), zinc oxide (ZnO), or the like. In this configuration, when a voltage is applied between the first electrode 112 and the second electrode 113, the piezoelectric layer 111 vibrates in the thickness direction via the gap 132, causing an elastic wave to resonate in the same direction. This resonant frequency varies depending on the weight of the sensitive film 120. In the FBAR element, too, by taking into consideration the frequency-temperature characteristic of the FBAR element and canceling out the frequency-temperature characteristic with the underlayer 121 and the adsorption layer 122, it is possible to make the frequency-temperature characteristic of the sensitive film 120 approach nearly zero.

[0034] Furthermore, the piezoelectric vibrator 110 may be a SAW (Surface Acoustic Wave) element. As shown in Figs. 6 and 7, the piezoelectric vibrator 110, which is a SAW element, has a first electrode 112 and a second electrode 113 provided on a surface 111a of a piezoelectric layer 111. The sensitive film 120 has a base layer 121 provided on the surface 111a, the first electrode 112, and the second electrode 113. The first electrode 112 and the second electrode 113 are IDTs (Inter-Digital Transducers: comb-shaped electrodes). In addition, a pair of reflectors 141 is provided on the surface 111a so as to sandwich the first electrode 112 and the second electrode 113. The piezoelectric layer 101 is made of LiTaO3, LiNbO3, or the like.

[0035] In this configuration, when a voltage is applied between the first electrode 112 and the second electrode 113, a surface acoustic wave is generated on the surface 111a. The surface acoustic wave propagates along the surface 111a and is reflected by the reflector 141, causing resonance. This resonance frequency varies depending on the weight of the sensitive film 120. In the SAW element, too, by taking into account the frequency-temperature characteristic of the SAW element and canceling out the frequency-temperature characteristic with the base layer 121 and the adsorption layer 122, it is possible to make the frequency-temperature characteristic of the sensitive film 120 approach nearly zero.

[0036] [Surface roughness of the lower layer of the sensitive film] The sensitive film 120 is provided on the first electrode 112 when the piezoelectric vibrator 110 is a QCM element or an FBAR element, and on the piezoelectric layer 111, first electrode 112, and second electrode 113 when the piezoelectric vibrator 110 is a SAW element. The lower layers of these sensitive films 120 preferably have a high surface roughness. This is because a high surface roughness of the lower layers also increases the surface roughness of the base layer 121 and the adsorption layer 122, increasing the surface area of ​​the adsorption layer 122 and improving the amount of odor adsorption, i.e., sensitivity. Specifically, the surface roughness of the electrode 112 or the piezoelectric layer 111 is preferably such that the average developed surface ratio (Sdr) is 0.0185 or greater. This value indicates a 1.85% improvement in surface area compared to a flat surface. The developed surface ratio (Sdr) can be measured using a laser microscope VKX-3000 (manufactured by KEYENCE Corporation) with a 150x magnification (WD 0.2 mm) lens. [Explanation of symbols]

[0037] 100...Odor detection element 110...Piezoelectric vibrator 111...Piezoelectric layer 111a…Surface 111b…Back side 112...1st electrode 113…Second electrode 120...Sensitive membrane 121…base layer 122...Adsorption layer

Claims

1. a piezoelectric vibrator including a piezoelectric layer containing a piezoelectric material as a main component and a pair of electrodes provided on the piezoelectric layer; a sensitive film including an underlayer provided on the piezoelectric vibrator and having a positive or negative frequency-temperature characteristic, and an adsorption layer provided on the underlayer and having a frequency-temperature characteristic opposite to that of the underlayer; An odor detection element comprising:

2. The odor detection element according to claim 1, the underlayer has a negative temperature characteristic; The adsorption layer has a positive temperature characteristic Odor detection element.

3. The odor detection element according to claim 1, the underlayer has a positive temperature characteristic; The adsorption layer has a negative temperature characteristic. Odor detection element.

4. The odor detection element according to any one of claims 1 to 3, The thickness of the adsorption layer is 300 nm or more and 500 nm or less. Odor detection element.

5. The odor detection element according to any one of claims 1 to 4, Either the underlayer or the adsorption layer is mainly composed of an imide-based resin having a hexafluoroisopropyl group. Odor detection element.

6. The odor detection element according to any one of claims 1 to 5, Either the underlayer or the adsorption layer is mainly composed of a fluorine-containing acrylate ester polymer. Odor detection element.

7. The odor detection element according to any one of claims 2 and 4 to 6, the adsorption layer is mainly composed of an imide-based resin having a hexafluoroisopropyl group, The thickness of the adsorption layer is 300 nm or more and 1000 nm or less. Odor detection element.

8. The odor detection element according to any one of claims 1 to 7, The average surface roughness of the base layer, defined as the developed area ratio of the lower layer, is 0.0185 or more. Odor detection element.

9. The odor detection element according to any one of claims 1 to 8, The hydrophobicity of the underlayer is higher than the hydrophobicity of the adsorption layer. Odor detection element.

10. The odor detection element according to any one of claims 1 to 9, The ratio of the thickness of the underlayer to the thickness of the adsorption layer is 10% or more and 90% or less. Odor detection element.

11. The odor detection element according to claim 10 The ratio of the thickness of the underlayer to the thickness of the adsorption layer is the inverse ratio of the ratio of the temperature coefficient of frequency of the underlayer to the temperature coefficient of frequency of the adsorption layer. Odor detection element.

12. The odor detection element according to any one of claims 1 to 11, the pair of electrodes includes a first electrode provided on a front surface of the piezoelectric layer and a second electrode provided on a rear surface of the piezoelectric layer, The sensitive film has the base layer provided on the first electrode. Odor detection element.

13. The odor detection element according to claim 12, The piezoelectric vibrator is a QCM (Quartz crystal microbalance) element. Odor detection element.

14. The odor detection element according to claim 12, The piezoelectric vibrator is an FBAR (Film Bulk Acoustic Resonator) element. Odor detection element.

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