JP7804473B2 - Detection Device - Google Patents

Description

The present invention relates to a detection device, for example, a detection device having a resonator with a sensitive membrane.

Environmental sensors are known that detect environmental changes by detecting changes in the mass of a sensitive film. A detection device is known in which a sensitive film is provided on the upper electrode of a piezoelectric thin-film resonator, and environmental changes are detected based on the resonant frequency caused by changes in the mass of the sensitive film (see, for example, Patent Document 1). When sensing in liquid, it is known that the Q value decreases when a sensitive film is grown on the resonator (see, for example, Patent Document 2).

Special Publication No. 2005-533265 Special Publication No. 2020-519886

When detecting environmental changes within a gas, a thin sensitive film is preferable to avoid degrading the resonance characteristics of the resonator. However, making the sensitive film thinner reduces the surface area of the sensitive film, reducing its sensitivity to environmental changes.

The present invention was developed in consideration of the above-mentioned problems, and aims to improve detection sensitivity while suppressing deterioration of the resonance characteristics of the resonator.

The present invention provides a detection device comprising: a piezoelectric section including a piezoelectric layer and first and second electrodes sandwiching at least a portion of the piezoelectric layer; a sensitive membrane provided on the opposite side of the piezoelectric layer from the first electrode, the sensitive membrane having a mass that changes in response to environmental changes; a resonator that responds to the mass change, and when the frequency at which electrical excitation is possible in the piezoelectric section without the sensitive membrane is defined as an odd mode of elastic waves, after the sensitive membrane is provided on the piezoelectric section, at least one of the resonant frequency and anti-resonant frequency in an even mode, which is a vibration mode of elastic waves that can be electrically excited differently from the odd mode, changes; and a detector that detects the change in the environment based on the change in at least one of the frequencies.

In the above configuration, the even-order mode can be a second-order mode.

In the above configuration, the thickness of the sensitive film can be configured to be 0.2 times or more and 1.0 times or less the thickness of the piezoelectric layer.

In the above configuration, the elastic stiffness _C33 of the sensitive film may be 0.06 to 0.62 times the elastic stiffness _C33 of the piezoelectric layer.

In the above configuration, the density of the sensitive film can be 0.35 to 0.54 times the density of the piezoelectric layer.

In the above configuration, the piezoelectric layer may be an aluminum nitride layer, the Young's modulus of the sensitive film may be 20 GPa or more and 80 GPa or less, and the density of the sensitive film may be 1.2 g/cm ³ or more and 1.7 g/cm ³ or less.

In the above configuration, the piezoelectric layer may be an aluminum nitride layer, and the sensitive film may be composed mainly of metal phthalocyanine .

In the above configuration, the resonant frequency of the resonator in the even-order mode can be 0.85 to 1.15 times the resonant frequency in the first-order mode of the resonator from which the sensitive film has been removed.

The objective of the present invention is to improve detection sensitivity.

FIG. 1 is a block diagram illustrating a detection device according to a first embodiment. FIG. 2A is a plan view of a resonator 40 according to the first embodiment, and FIG. 2B is a cross-sectional view taken along line AA of FIG. 2A. 3A and 3B are diagrams showing the magnitude |Z| of impedance and the Q value with respect to frequency in Simulation 1. FIG. 4(a) to 4(c) are diagrams showing impedance |Z| versus frequency in the experiment. FIG. 5 is a diagram showing impedance |Z| versus frequency in the experiment. 6(a) and 6(b) are diagrams showing the Q value versus frequency in the experiment. FIG. 7(a) is a graph showing impedance |Z| versus frequency for a sample with T=450 nm in the experiment, and FIG. 7(b) is an enlarged view of FIG. 7(a). 8A to 8C are diagrams showing displacement relative to position in the Z direction in simulation 2. FIG.

The following describes the examples with reference to the drawings.

Figure 1 is a block diagram showing a detection device according to a first embodiment. The detection device 100 includes an oscillator circuit 42 and a detector 45. The oscillator circuit 42 includes a resonator 40 and an amplifier 41, and oscillates at the resonant frequency or anti-resonant frequency of the even mode of the resonator 40 to output an oscillation signal. A measuring device 44 measures the frequency of the oscillation signal. A calculator 46 detects environmental changes based on changes in the frequency of the oscillation signal measured by the measuring device 44. Examples of environmental changes include changes in the amount of specific atoms or molecules or other substances in the gas, changes in the temperature of the environment, and changes in humidity. When a specific substance in the gas is the source of an odor, the detection device 100 functions as an odor sensor.

Figure 2(a) is a plan view of the resonator 40 according to the first embodiment, and Figure 2(b) is a cross-sectional view taken along the line A-A in Figure 2(a). The normal direction to the top surface of the substrate 10 is the Z direction, the direction in which the lower electrode 12 is drawn out from the resonance region 50 is the X direction, and the planar direction perpendicular to the X direction is the Y direction.

The structure of the resonator 40 is described below, as shown in Figures 2(a) and 2(b). The substrate 10 has a thickness and is rectangular in plan view. A lower electrode 12 extends from the center of the substrate's 10 top surface to one end of the substrate 10 in the longitudinal direction (X direction). A piezoelectric layer 14 and an upper electrode 16 formed on the piezoelectric layer 14 extend from the center of the substrate's top surface to the other end of the substrate in the longitudinal direction. The resonance region 50 is defined by the region where the lower electrode 12 and upper electrode 16 face each other, sandwiching at least a portion of the piezoelectric layer 14. The elliptical portion in Figure 2(a) is the resonance region 50, and the piezoelectric layer 14 corresponding to this resonance region 50 is sandwiched between the lower electrode 12 and upper electrode 16. A dome-shaped void 22 is formed between the substrate 10 and the lower electrode 12, corresponding to the resonance region 50. Applying a voltage to the lower electrode 12 and upper electrode 16 causes the piezoelectric layer 14 to vibrate, and the provision of this gap 22 makes this vibration easier.

A protective film 17 is provided on the entire surface of the upper electrode 16. A sensitive film 18 is provided on at least the protective film 17 corresponding to the resonance region 50. The protective film 17 does not have to be provided. The sensitive film 18 has a thickness T, which will be described later. The sensitive film 18 is only required to be provided so as to include the resonance region 50 in a planar view. The sensitive film 18 may cover the resonance region 50 or may extend beyond the periphery of the resonance region 50. The lower electrode 12, piezoelectric layer 14, upper electrode 16, protective film 17, and sensitive film 18 in the resonance region 50 form a laminated film 20. Before the sensitive film 18 is provided, the lower electrode 12, piezoelectric layer 14, upper electrode 16, and protective film 17 form a piezoelectric portion 21. The laminated film 20 in the resonance region 50 resonates with elastic waves such as a thickness-extensional vibration mode or a thickness-shear vibration mode.

As shown in Figure 2(a), the planar shape of the resonance region 50 is, for example, elliptical. The hatched area in a grid pattern represents the planar shape of the resonance region 50. The planar shape of the resonance region 50 may also be a polygonal shape, such as a pentagon. In this embodiment, the resonator 40 is described as an FBAR (Film Bulk Acoustic Resonator) in which a gap 22 is formed between the substrate 10 and the lower electrode 12 in the resonance region 50.

When substances such as atoms or molecules in the gas are adsorbed onto the sensitive film 18, the mass of the sensitive film 18 increases. As the humidity around the sensitive film 18 increases, moisture is adsorbed onto the sensitive film 18, increasing the mass of the sensitive film 18. Changes in temperature cause changes in the adsorption characteristics of the film, resulting in a change in the mass of the sensitive film 18. Thus, the mass of the sensitive film 18 changes due to changes in the concentration, humidity, and temperature of specific substances in the gas surrounding the sensitive film 18. When the mass of the sensitive film 18 changes, the resonant frequency and anti-resonant frequency of the piezoelectric thin film resonator change. For example, when specific substances are adsorbed onto the sensitive film 18, the mass of the sensitive film 18 increases. Examples of specific substances include acetone, ethanol, and toluene. This lowers the resonant frequency and anti-resonant frequency of the resonator 40. When specific substances are desorbed from the sensitive film 18, the mass of the sensitive film 18 decreases. This increases the resonant frequency and anti-resonant frequency of the resonator 40.

The substrate 10 is, for example, a silicon substrate, sapphire substrate, quartz substrate, glass substrate, ceramic substrate, or GaAs substrate. The lower electrode 12 and upper electrode 16 are, for example, single-layer films of ruthenium (Ru), chromium (Cr), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Ir), or a laminate film made of multiple films selected from these. In this embodiment, both the lower electrode 12 and upper electrode 16 are ruthenium films.

The piezoelectric layer 14 may be, for example, an aluminum nitride (AlN) film, a zinc oxide (ZnO) film, a gallium nitride (GaN) film, a lead zirconate titanate (PZT) film, a lead titanate (PbTiO ₃ ) film, a lithium tantalate (LiTaO ₃ ) film, or a lithium niobate (LiNbO ₃ ) film. The piezoelectric layer 14 is primarily composed of aluminum nitride (AlN) with its principal axis in the (002) direction (Z-axis direction), and may contain other elements to improve resonance characteristics or piezoelectricity. For example, the piezoelectricity of the piezoelectric layer 14 can be improved by adding scandium (Sc), two elements consisting of a Group 2 element or a Group 12 element and a Group 4 element, or two elements consisting of a Group 2 element or a Group 12 element and a Group 5 element, as additive elements. This improves the electromechanical coupling coefficient of the piezoelectric thin film resonator. Examples of Group 2 elements include calcium (Ca), magnesium (Mg), and strontium (Sr), and examples of Group 12 elements include zinc (Zn). Examples of Group 4 elements include titanium, zirconium (Zr), and hafnium (Hf). Examples of Group 5 elements include tantalum, niobium (Nb), and vanadium (V). Furthermore, the piezoelectric layer 14 may be primarily composed of aluminum nitride and may also contain boron (B).

An insertion film may be provided on the piezoelectric layer 14 within the resonance region 50. For example, in FIG. 2(b), a lower piezoelectric layer having half the thickness of the piezoelectric layer 14 is provided on the substrate 10. The insertion film is provided on the lower piezoelectric layer. An upper piezoelectric layer having half the thickness of the piezoelectric layer 14 is further provided on the insertion film. A resonator with this structure can suppress the temperature dependence of the resonance frequency. The insertion film is, for example, a silicon oxide film. The insertion film is, for example, a film for suppressing the temperature dependence of the resonance frequency or a film for improving the Q value (Quality Factor) of the resonator.

The protective film 17 is a film that protects the surface of the upper electrode 16, and is, for example, an insulating film such as a silicon oxide film, a silicon nitride film, or a silicon nitride oxide film, or a metal film.

The sensitive film 18 may be, for example, an organic polymer film, an organic low-molecular-weight film, or an inorganic film. Examples of organic polymer materials that can be used include homopolymers with a single structure, such as polystyrene, polymethyl methacrylate, 6-nylon, cellulose acetate, poly-9,9-dioctylenefluorene, polyvinyl alcohol, polyvinylcarbazole, polyethylene oxide, polyvinyl chloride, poly-p-phenylene ether sulfone, poly-1-butene, polybutadiene, polyphenylmethylsilane, polycaprolactone, polybisphenoxyphosphazene, and polypropylene; copolymers, which are copolymers of two or more homopolymers; and blend polymers, which are mixtures of multiple types of homopolymers.

For example, the organic low-molecular-weight material may be at least one of tris(8-quinolinolato)aluminum (Alq3), naphthyldiamine (α-NPD), BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), CBP (4,4'-N,N'-dicarbazole-biphenyl), copper phthalocyanine, fullerene, pentacene, anthracene, thiophene, Ir(ppy(2-phenylpyridinato)) ₃ , triazine thiol derivatives, dioctylfluorene derivatives, tetratetracontane, parylene, and the like.

For example, the inorganic material may be at least one of alumina, titania, vanadium pentoxide, tungsten oxide, lithium fluoride, magnesium fluoride, aluminum, gold, silver, tin, indium tin oxide (ITO), carbon nanotubes, sodium chloride, magnesium chloride, etc.

The sensitive film 18 may be made of a metal phthalocyanine , such as copper phthalocyanine (CuPc), copper phthalocyanine fluoride ( _CuPcF8 , _CuPcF16 ), cobalt phthalocyanine (CoPc), manganese phthalocyanine (MnPc), iron phthalocyanine (FePc), or nickel phthalocyanine (NiPc).

[Simulation 1]
The response of the resonator 40 was simulated. The simulation was performed using a model in which the laminated film 20 was in the shape of a strip. The simulation conditions are as follows:
Lower electrode 12: A chromium layer with a thickness of about 70 nm is provided on the substrate 10 side, and a ruthenium layer with a thickness of 166 nm is laminated thereon.
Piezoelectric layer 14: an aluminum nitride layer with a thickness of 996 nm. Upper electrode 16: a ruthenium layer with a thickness of 173 nm is provided on the piezoelectric layer 14 side, and a chromium layer with a thickness of 55 nm is laminated thereon.
Protective film 17: The upper surface of the upper electrode 16 is covered with a silicon oxide layer having a thickness of 70 nm.
Sensitive film 18: Copper phthalocyanine layer with varying thickness T

3(a) and 3(b) are graphs showing the magnitude |Z| of impedance and the Q value versus frequency in Simulation 1. The horizontal axis of FIGS. 3(a) and 3(b) represents frequency, the vertical axis of FIG. 3(a) represents the absolute value of impedance, and the vertical axis of FIG. 3(b) represents the Q value. The thickness T of the sensitive film 18 was varied in the range of 75 nm to 700 nm. As shown in FIG. 3(a), as the thickness T of the sensitive film 18 increases, the resonant frequency f _r1 and the antiresonant frequency f _a1 decrease. As the thickness T of the sensitive film 18 increases, the difference between the peaks of the resonant frequency f _r1 and the antiresonant frequency f _a1 decreases. The resonant frequency f _r1 and the antiresonant frequency f _a1 are the resonant frequency and the antiresonant frequency when the elastic wave resonates in the first mode in the laminated film 20. As shown in FIG. 3(b), the Q value reaches its maximum between the resonant frequency f _r1 and the antiresonant frequency f _a1 . As the thickness T of the sensitive film 18 increases, the Q value decreases.

As shown in Figures 3(a) and 3(b), as the thickness T1 of the sensitive film 18 increases, the resonance in the first mode decreases and the Q value decreases. This makes it harder for the resonator 40 to resonate. On the other hand, as the sensitive film 18 becomes thinner, the number of sites in the sensitive film 18 that adsorb specific substances decreases. In addition, the surface roughness of the sensitive film 18 becomes smaller. As the sensitive film 18 becomes thinner, the number of sites in the sensitive film 18 that adsorb specific substances decreases, so changes in the mass of the sensitive film 18 due to environmental changes become smaller. This reduces the detection sensitivity of the detection device 100 to environmental changes. The above-mentioned issues arise when attempting to oscillate and measure the resonator using the first mode.

[experiment]
A resonator 40 was fabricated and the resonance response of the resonator 40 was measured. The fabrication conditions for the resonator 40, except for the sensitive film 18, were essentially the same as those in Simulation 1, although there were some manufacturing errors. Samples were fabricated with the thickness T of the sensitive film 18 set to 0 nm, 130 nm, 297 nm, 324 nm, and 506 nm. The thickness T was measured using a step gauge when the sensitive film 18 was fabricated.

Figures 4(a) to 4(c) show the impedance |Z| versus frequency in the experiment. The horizontal axis is frequency, and the vertical axis is the absolute value of the impedance |Z|. In Figures 4(a) to 4(c), the thickness T of the sensitive film 18 was measured using a step gauge and was found to be 297 nm, 324 nm, and 506 nm, respectively. T = 0 nm in each figure represents the result measured before the sensitive film 18 was formed on the resonator.

As shown in Figure 4(a), before the sensitive film 18 is formed, a peak having a resonant frequency f _r1 and an anti-resonant frequency f _a1 is observed. After the sensitive film 18 with a thickness T = 297 nm is formed, a peak having a resonant frequency f _r1 and an anti-resonant frequency f _a1 , and a peak having a resonant frequency f _r2 and an anti-resonant frequency f _a2 are observed. The resonant frequency f _r1 and the anti-resonant frequency f _a1 are lower than the resonant frequency f _r1 and the anti-resonant frequency f _a1 at T = 0 nm. The resonant frequency f _r2 and the anti-resonant frequency f _a2 are higher than the resonant frequency f _r1 and the anti-resonant frequency f _a1 at T = 0 nm.

As shown in Fig. 4(b), the resonant frequency f _r1 and antiresonant frequency f _a1 of the sample having a sensitive film 18 with a thickness T = 324 nm are shifted to lower frequencies than the resonant frequency f _r1 and antiresonant frequency f _a1 of the sample having a thickness T = 297 nm in Fig. 4(a). The resonant frequency f _r2 and antiresonant frequency f _a2 at T = 324 nm are shifted to lower frequencies than the resonant frequency f _r2 and antiresonant frequency f _a2 at T = 297 nm, and approach the resonant frequency f _r1 and antiresonant frequency f _a1 before the sensitive film 18 was formed. The Z ratio and Q value of |Z| of the antiresonant frequency f _{a2 to |Z| of the resonant frequency f r2 at} T = 324 nm are higher than the Z ratio and Q value at T = 297 nm.

As shown in FIG. 4C, the resonant frequency f _r2 and anti-resonant frequency f _a2 of the sample having the sensitive film 18 with a thickness T=506 nm are approximately the same as the resonant frequency f _r1 and anti-resonant frequency f _a1 before the sensitive film 18 is formed (T=0 nm).

4( a) to 4(c), in the samples with T=297 nm and 324 nm, as the thickness T of the sensitive film 18 increases, the peaks of the resonant frequency f _r1 and the antiresonant frequency f _a1 become smaller and shift to lower frequencies. At T=297 nm, the peaks of the resonant frequency f _r2 and the antiresonant frequency f _a2 are observed near 2600 MHz to 2700 MHz. As in the samples with T=324 nm and 506 nm, as the thickness T of the sensitive film 18 increases, the peaks of the resonant frequency f _r2 and the antiresonant frequency f _a2 become larger and shift to lower frequencies, approaching the designed frequencies (i.e., the resonant frequency f _r1 and the antiresonant frequency f _a1 ) before the sensitive film 18 was formed.

FIG. 5 is a graph showing the impedance |Z| versus frequency in the experiment. The horizontal axis represents frequency, and the vertical axis represents the absolute value of the impedance |Z|. As shown in FIG. 5, in the sample with T=130 nm, large peaks are observed at the resonant frequency f _r1 and the antiresonant frequency f _a1 . No peaks are observed at the resonant frequency f _r2 and the antiresonant frequency f _a2 below 2900 MHz.

6(a) and 6(b) show the Q-factor versus frequency in the experiment. FIG. 6(b) is an enlarged view of FIG. 6(a). The horizontal axis represents frequency, and the vertical axis represents the Q-factor. As shown in FIGS. 6(a) and 6(b), a large Q-factor peak is observed between the resonant frequency f _r1 and the antiresonant frequency f _a1 in the T=130 nm sample. In the T=297 nm sample, a Q-factor peak is observed between the resonant frequency f _r2 and the antiresonant frequency f _a2 . The Q-factor peak for the T=297 nm sample is much smaller than that for the T=130 nm sample. In the T=324 nm sample, the Q-factor peak between the resonant frequency f _r2 and the antiresonant frequency f _a2 is larger than that for the T=297 nm sample. In the T=506 nm sample, the Q-factor peak between the resonant frequency f _r2 and the antiresonant frequency f _a2 is even larger than that for the T=324 nm sample. Thus, the Q value in the second-order mode increases as the sensitive film 18 becomes thicker.

FIG. 7(a) shows the impedance |Z| versus frequency for a T=450 nm sample in an experiment, and FIG. 7(b) is an enlarged view of FIG. 7(a). The horizontal axis represents frequency, and the vertical axis represents the absolute value of impedance |Z|. FIG. 7(a) shows a wider frequency range than FIG. 5. FIG. 7(b) shows an enlarged view of a frequency near 1400 MHz. T=0 nm represents the characteristics before the formation of the sensitive film 18. As shown in FIGS. 7(a) and 7(b), at T=450 nm, there is a small peak at 1400 MHz, which is the peak of the first-order mode resonant frequency f _r1 and antiresonant frequency f _a1 . The resonant frequency f _r2 and antiresonant frequency f _a2 , which have approximately the same frequencies as the peaks of the first-order mode resonant frequency f _r1 and antiresonant frequency f a1 at T=0 nm _{, are considered to be the peaks of the second-order mode resonant frequency f r2 and} antiresonant frequency f _a2 .

[Simulation 2]
To investigate why the Q value of the second-order mode increases as the sensitive film 18 becomes thicker, the displacement in the laminated film 20 in the first and second modes was simulated. The simulation conditions were the same as in Simulation 1. The thickness of the sensitive film 18 was kept constant, and the Young's modulus of the sensitive film 18 was changed to reproduce a state in which the thickness T of the sensitive film 18 was artificially changed.

Figures 8(a) to 8(c) show displacement relative to position in the Z direction in Simulation 2. The vertical axis represents position in the Z direction, with the bottom surface of the lower electrode 12 set at Z=0, and the horizontal axis represents displacement due to elastic waves. The circles + and - written on the lower electrode 12 and under the upper electrode 16 represent the positive and negative potentials generated by displacement of the elastic waves.

In Figure 8(a), the sensitive film 18 is not provided. As shown in Figure 8(a), in the primary mode (fundamental wave mode), the thickness of the laminated film 20 corresponds to half the wavelength λ of the elastic wave. In the primary mode resonance state, over time, a state in which the lower electrode 12 is at a negative potential and the upper electrode 16 is at a positive potential, and a state in which the lower electrode 12 is at a positive potential and the upper electrode 16 is at a negative potential, are repeated. Figure 8(a) shows the state when the lower electrode 12 is at a negative potential and the upper electrode 16 is at a positive potential. This enables electrical excitation in the primary mode.

In the second-order mode (second-order harmonic mode), the thickness of the laminated film 20 corresponds to the wavelength λ of the elastic wave. When the lower electrode 12 is at a positive potential, the upper electrode 16 is also at a positive potential. Thus, in the second-order mode, the lower electrode 12 and upper electrode 16 are at approximately the same potential, making electrical excitation impossible.

In Figures 8(b) and 8(c), a sensitive film 18 is provided on the upper electrode 16. The Young's modulus of the sensitive film 18 is smaller than that of the lower electrode 12, piezoelectric layer 14, and upper electrode 16. As shown in Figure 8(b), when the sensitive film 18 is provided, elastic wave displacement enters the sensitive film 18. Therefore, the thickness of the laminated film 20 including the sensitive film 18 corresponds to λ/2 in the primary mode and λ in the secondary mode. If the Young's modulus of the sensitive film 18 is smaller than that of the lower electrode 12, piezoelectric layer 14, and upper electrode 16, the displacement of the elastic wave in the sensitive film 18 becomes larger, and the potential also changes significantly within the sensitive film 18. In Figure 8(b), the Young's modulus of the sensitive film 18 is not so low, so the primary and secondary mode potentials in the lower electrode 12, piezoelectric layer 14, and upper electrode 16 are almost the same as those in Figure 8(a). This allows electrical excitation in the primary mode, but not in the secondary mode.

In Figure 8(c), the Young's modulus of the sensitive membrane 18 is made smaller than in Figure 8(b), and the sensitive membrane 18 is artificially thicker than the thickness T of the sensitive membrane 18 in Figure 8(b). Compared to Figure 8(b), elastic wave displacement enters the sensitive membrane 18 even more. In the primary mode, a displacement of λ/4 or more of the λ/2 displacement occurs within the sensitive membrane 18. Therefore, the potential difference between the lower electrode 12 and the upper electrode 16 in the primary mode is smaller than in Figure 8(b). As a result, it is thought that the resonant response in the primary mode is smaller than in Figure 8(b). In the secondary mode, a displacement of λ or more of the λ displacement occurs within the sensitive membrane 18. Therefore, the lower electrode 12 has a positive potential and the upper electrode 16 has a negative potential. This enables electrical excitation in the secondary mode.

As described above, it is believed that increasing the thickness of the sensitive film 18 reduces the resonant response of the first mode and increases the resonant response of the second mode.

According to the first embodiment, the resonator 40 includes a piezoelectric portion including an upper electrode 16 (first electrode) and a lower electrode 12 (second electrode) sandwiching at least a portion of the piezoelectric layer 14, and a sensitive film 18 provided on the upper electrode 16 opposite the piezoelectric layer 14. The resonator 40 changes at least one of the resonant frequency f _r2 and the anti-resonant frequency f _a2 in the second mode in response to changes in the mass of the sensitive film 18. The detector 45 detects environmental changes based on the changes in at least one of the resonant frequency f _r2 and the anti-resonant frequency f _a2 . This allows a resonant response with a high Q factor to be obtained even when the sensitive film 18 is thick. Because the sensitive film 18 can be thickened, the change in the mass of the sensitive film 18 corresponding to environmental changes can be increased. These features improve the detection sensitivity of the detection device 100 to environmental changes.

In the first embodiment, the resonant frequency f _r2 and the anti-resonant frequency f _a2 in the second mode were described as an example. However, in the even modes, if the sensitive film 18 is not provided, the potentials of the lower electrode 12 and the upper electrode 16 will be approximately the same, and electrical excitation will hardly occur. Increasing the thickness T of the sensitive film 18 increases the potential difference between the lower electrode 12 and the upper electrode 16, making electrical excitation in the even modes possible. In the fourth mode, which is an even mode, the thickness of the laminated film 20 is approximately 2λ, and in the sixth mode, the thickness of the laminated film 20 is approximately 3λ. Thus, in the even modes, in the nth mode, the thickness of the laminated film 20 is approximately n×λ/2.

Furthermore, although the resonant frequency f _r1 and anti-resonant frequency f _a1 in the first mode have been described as an example, excitation of odd modes is possible if the sensitive film 18 is not provided. In the third mode, which is an odd mode, the thickness of the laminated film 20 is approximately 3λ/2, and in the fifth mode, the thickness of the laminated film 20 is approximately 5λ/2. Thus, in the nth mode, the thickness of the laminated film 20 is approximately n×λ/2 in the odd mode. In the resonator 40, if the frequency at which electrical excitation is possible in the piezoelectric portion 21 before the sensitive film 18 is provided is defined as the odd mode of elastic waves, then after the sensitive film 18 is provided on the piezoelectric portion 21, an even mode, which is a vibration mode of elastic waves that can be electrically excited, different from the odd mode, is provided.

8(a) to 8(d), as the thickness T of the sensitive film 18 increases, the resonator 40 becomes electrically excitable in the second-order mode. From FIG. 5, it appears that the resonator 40 resonates in the second-order mode when the thickness T of the sensitive film 18 is 200 nm or greater. The piezoelectric layer 14 is the thickest in the laminated film 20. Furthermore, even if resonators 40 with various resonant frequencies are designed, the ratio of the thickness of the lower electrode 12 and upper electrode 16 to the thickness of the piezoelectric layer 14 does not change significantly. Therefore, when the thickness T of the sensitive film 18 is normalized by the thickness of the piezoelectric layer 14, the thickness T of the sensitive film 18 is 200 nm, which is 0.2 times the thickness of the piezoelectric layer 14. Therefore, the thickness T of the sensitive film 18 is preferably 0.2 times or greater than the thickness of the piezoelectric layer 14. The thickness T of the sensitive film 18 is more preferably 0.3 times or greater than the thickness of the piezoelectric layer 14, and even more preferably 0.4 times or greater.

When the sensitive film 18 is thick, most of the displacement of the elastic wave is within the sensitive film 18, resulting in a smaller resonant response. For example, if, when T = 506 nm, a displacement equivalent to λ/2 of the λ standing wave in the second-order mode is displaced within the sensitive film 18, then when T = 1000 nm, a displacement equivalent to 2λ/3 is displaced within the sensitive film 18, reducing the potential difference between the lower electrode 12 and the upper electrode 16. This is thought to result in a smaller resonant response. From this perspective, the thickness T of the sensitive film 18 is preferably no more than 1 time the thickness of the piezoelectric layer 14, more preferably no more than 0.7 times, and even more preferably no more than 0.6 times.

As the Young's modulus of the sensitive film 18 decreases, the displacement of the elastic wave becomes more likely to penetrate into the sensitive film 18. The Young's modulus of the copper phthalocyanine used as the sensitive film 18 is 55 GPa and the Poisson's ratio is 0.35. Therefore, we believe that the experimental results can be generalized if the sensitive film 18 has a modulus of 20 GPa to 80 GPa and a Poisson's ratio of 0.2 to 0.4. Because the aluminum nitride layer of the piezoelectric layer 14 is not isotropic, we will use elastic stiffness for our analysis. The piezoelectric layer 14 is a polycrystal oriented so that the Z-axis direction of the crystal orientation is the Z direction in Figures 2(a) and 2(b). Therefore, we will consider the elastic stiffness _C33 . Note that the subscript 3 in elastic stiffness _C33 indicates the Z-axis direction of the aluminum nitride. The _C33 of the aluminum nitride of the piezoelectric layer 14 is 280 GPa to 340 GPa, while the Young's modulus of the sensitive film 18 is 20 GPa to 80 GPa and the Poisson's ratio is 0.2 to 0.4. In this case, it is believed that the experimental results can be generalized if the elastic stiffness _C33 of the sensitive film 18 is 0.06 to 0.62 times the elastic stiffness _C33 of the piezoelectric layer 14. The _C33 of the sensitive film 18 is preferably 0.1 to 0.5 times the _C33 of the piezoelectric layer 14, and more preferably 0.2 to 0.4 times.

The density of the sensitive film 18 also relates to the displacement of the elastic wave. The density of the copper phthalocyanine used as the sensitive film 18 is 1.5 g/cm ^3. Therefore, it is believed that the experimental results can be generalized if the density of the sensitive film 18 is 1.2 g/cm ³ to 1.7 g/cm ^3. The density of the aluminum nitride layer of the piezoelectric layer 14 is 3.2 g/cm ³ to 3.4 g/cm ^3. Therefore, it is believed that the experimental results can be generalized if the density of the sensitive film 18 is 0.35 to 0.54 times the density of the piezoelectric layer 14. It is preferable that the density of the sensitive film 18 is 0.4 to 0.5 times the density of the piezoelectric layer 14.

When the piezoelectric layer 14 is an aluminum nitride layer, the Young's modulus of the sensitive film 18 is preferably 20 GPa to 80 GPa, more preferably 30 GPa to 70 GPa, and even more preferably 50 GPa to 60 GPa. The density of the sensitive film 18 is preferably 1.2 g/ ^cm3 to 1.7 g/ ^cm3 , preferably 1.3 g/ ^cm3 to 1.65 g/ ^cm3 , and more preferably 1.4 g/ ^cm3 to 1.6 g/ ^cm3 .

The piezoelectric layer 14 is an aluminum nitride layer, and the sensitive film 18 is primarily composed of metal phthalocyanine . This gives the sensitive film 18 a Young's modulus and density comparable to those of copper phthalocyanine. This allows the experimental results to be more generalized. Note that the sensitive film 18 being primarily composed of metal phthalocyanine may also contain impurities other than metal phthalocyanine , either intentionally or unintentionally. The proportion of metal phthalocyanine in the sensitive film 18 is, for example, 50% by weight or more, or 80% by weight or more.

In the detection device 100 of FIG. 1, the detector 45 is adjusted using the resonance response in the first mode of the resonator 40 before the sensitive film 18 is provided. If the resonance frequency or anti-resonance frequency of the resonator 40 with the sensitive film 18 provided is significantly different from the resonance frequency or anti-resonance frequency of the resonator before the sensitive film 18 is provided, the detector 45 will need to be adjusted again. From this perspective, the resonance frequency in the even mode of the resonator 40 is preferably 0.85 to 1.15 times the resonance frequency in the first mode of the resonator 40 after the sensitive film 18 has been removed, more preferably 0.9 to 1.1 times, and even more preferably 0.95 to 1.05 times.

Until now, the resonant frequency or anti-resonant frequency of the piezoelectric portion 21 of the resonator 40 has changed when the sensitive film 18 is formed. As a result, the oscillator circuit 42 needed to be adjusted to oscillate at this changed resonant frequency. In this embodiment, a sensitive film 18 is formed on the piezoelectric portion 21 and oscillates in an even-order mode, so the resonant frequency or anti-resonant frequency of the piezoelectric portion 21 can be brought closer to that before the sensitive film 18 was formed. This eliminates the need to adjust the oscillator circuit 42. Furthermore, the sensitive film 18 can be formed thicker, thereby improving the sensitivity of the detection device 100.

Although the present invention has been described in detail above with reference to specific embodiments, the present invention is not limited to these specific embodiments, and various modifications and variations are possible within the scope of the invention as set forth in the claims.

REFERENCE SIGNS LIST 10 Substrate 12 Lower electrode 14 Piezoelectric layer 16 Upper electrode 17 Protective film 18 Sensitive film 20 Laminated film 21 Piezoelectric portion 22 Gap 40 Resonator 42 Oscillator circuit 44 Measuring device 45 Detector 46 Calculator 50 Resonance region T Thickness

Claims (8)

a piezoelectric portion including a piezoelectric layer and a first electrode and a second electrode sandwiching at least a portion of the piezoelectric layer;
a sensitive film that is provided on the opposite side of the piezoelectric layer from the first electrode and whose mass changes in response to a change in the environment;
a resonator in which, when a frequency at which electrical excitation is possible in the piezoelectric portion where the sensitive film is not provided is defined as an odd-order mode of an elastic wave, at least one of a resonance frequency and an anti-resonance frequency in an even-order mode, which is a vibration mode of an elastic wave that can be electrically excited different from the odd-order mode, changes in response to the change in mass, after the sensitive film is provided on the piezoelectric portion;
a detector that detects a change in the environment based on a change in the at least one frequency;
A detection device comprising: The detection device of claim 1, wherein the even-order mode is a second-order mode. The detection device described in claim 2, wherein the thickness of the sensitive film is 0.2 times or more and 1.0 times or less the thickness of the piezoelectric layer. 4. The detection device according to claim 3, wherein the elastic stiffness _C33 of the sensitive film is 0.06 to 0.62 times the elastic stiffness _C33 of the piezoelectric layer. The detection device described in claim 3 or 4, wherein the density of the sensitive film is 0.35 to 0.54 times the density of the piezoelectric layer. 4. The detection device according to claim 3, wherein the piezoelectric layer is an aluminum nitride layer, the Young's modulus of the sensitive film is 20 GPa or more and 80 GPa or less, and the density of the sensitive film is 1.2 g/cm ^<3> or more and 1.7 g/cm ^<3> or less. 4. The detection device according to claim 3, wherein the piezoelectric layer is an aluminum nitride layer, and the sensitive film is mainly composed of metal phthalocyanine . A detection device according to any one of claims 1 to 7, wherein the resonant frequency of the resonator in the even-order mode is 0.85 to 1.15 times the resonant frequency of the resonator in the first mode when the sensitive film is removed.