Reservoir element

Abstract

A reservoir element (10) is provided with a substrate (11) having a main surface (12a), and a laminate (20) formed on the main surface (12a). A plurality of holes (13) are formed in the main surface (12a). The laminate (20) includes a piezoelectric film (24), a first electrode (23), and a plurality of second electrodes (25a, 25b, 25c, 25d). The piezoelectric film (24) is arranged between the first electrode (23) and the plurality of second electrodes (25a, 25b, 25c, 25d), and has a polycrystalline structure. Portions of the laminate (20) facing the plurality of holes (13) is a plurality of diaphragms (29).

Description

Reservoir element

[0001] The present disclosure relates to a reservoir element.

[0002] Artificial intelligence (AI) using recursive neural networks (RNN) is known. Although RNN has high performance, it requires a very large amount of computational resources and power consumption for machine learning of RNN. Therefore, reservoir computing, which enables machine learning with fewer computational resources and lower power consumption, has attracted attention. Japanese Unexamined Patent Publication No. 2022-182198 (Patent Document 1) discloses a reservoir used in a reservoir computing apparatus. This reservoir includes a plurality of electrodes and an ion member in contact with the plurality of electrodes. The ion member is an ionic liquid or a porous member impregnated with an ionic liquid.

[0003] Japanese Unexamined Patent Publication No. 2022-182198

[0004] The reservoir disclosed in Patent Document 1 includes an ionic liquid or a porous member impregnated with an ionic liquid, and thus is not suitable for mass production. The present disclosure has been made in view of the above problem, and an object of the present disclosure is to provide a reservoir element more suitable for mass production.

[0005] The reservoir element of the present disclosure includes a substrate having a main surface, and a stacked body formed on the main surface. A plurality of holes are formed in the main surface. The stacked body includes a piezoelectric film, a first electrode, and a plurality of second electrodes. The piezoelectric film is disposed between the first electrode and the plurality of second electrodes, and has a polycrystalline structure. Portions of the stacked body that face the plurality of holes are a plurality of diaphragms.

[0006] The reservoir element of the present disclosure is more suitable for mass production.

[0007] This is a schematic diagram of a reservoir computing device. This is a schematic plan view of the reservoir element of Embodiment 1. This is a schematic cross-sectional view of the reservoir element of Embodiment 1 along the cross-sectional line III-III shown in Figure 2. This is a schematic enlarged view of a piezoelectric film. This is a schematic cross-sectional view showing one step in the manufacturing method of the reservoir element of Embodiment 1. This is a schematic cross-sectional view showing the next step after the step shown in Figure 5 in the manufacturing method of the reservoir element of Embodiment 1. This is a schematic cross-sectional view showing the next step after the step shown in Figure 6 in the manufacturing method of the reservoir element of Embodiment 1. This is a schematic cross-sectional view showing the next step after the step shown in Figure 7 in the manufacturing method of the reservoir element of Embodiment 1. This is a schematic cross-sectional view showing the next step after the step shown in Figure 8 in the manufacturing method of the reservoir element of Embodiment 1. This is a diagram showing an example of the waveform of the input voltage signal input to the reservoir element. This is a schematic enlarged cross-sectional view of a reservoir element. This is a diagram showing the relationship between the frequency of vibration of a diaphragm and the amplitude of vibration of a diaphragm. This is a diagram showing the relationship between the frequency of vibration of another diaphragm and the amplitude of vibration of another diaphragm. This is a diagram showing an example of the waveform of the first output voltage signal output from the reservoir element. This figure shows an example of the waveform of the second output voltage signal output from the reservoir element. This figure shows an example of the waveform of the third output voltage signal output from the reservoir element. This figure shows the relationship between the voltage input to the piezoelectric film and the displacement of the piezoelectric film. This is a schematic partially enlarged cross-sectional view of the first modified reservoir element of Embodiment 1. This is a schematic partially enlarged cross-sectional view of the second modified reservoir element of Embodiment 1. This is a schematic plan view of the reservoir element of Embodiment 2. This is a schematic partially enlarged cross-sectional view of the reservoir element of Embodiment 2 along the cross-sectional line XVIII-XVIII shown in Figure 17. This is a schematic partially enlarged cross-sectional view of the reservoir element of Embodiment 3. This is a schematic cross-sectional view of the reservoir element of Embodiment 4. This is a schematic cross-sectional view of the reservoir element of Embodiment 5. This is a schematic cross-sectional view of the first modified reservoir element of Embodiment 5. This is a schematic cross-sectional view of the second modified reservoir element of Embodiment 5.

[0008] Embodiments of the present disclosure will be described below. The same components will be given the same reference numerals, and their descriptions will not be repeated.

[0009] Embodiment 1. Referring to Figure 1, a reservoir computing device 1 to which the reservoir element 10 of Embodiment 1 (see Figures 2 and 3) is applied will be described.

[0010] The reservoir computing device 1 comprises an input layer 2, a reservoir layer 3, and a readout layer 4. A time-series input signal is input to the input layer 2. The reservoir layer 3 converts the time-series input signal into a spatiotemporal pattern and outputs multiple output signals. The readout layer 4 is a linear model and performs linear classification or linear regression processing on the multiple output signals output from the reservoir layer 3. In this way, the reservoir computing device 1 can analyze the time-series input signal.

[0011] Reservoir layer 3 possesses nonlinearity, high dimensionality, and short-term memory. The nonlinearity of reservoir layer 3 is its function of nonlinearly transforming time-series input signals. The high dimensionality of reservoir layer 3 is its function of mapping time-series input signals to a high-dimensional feature space. The short-term memory of reservoir layer 3 is its function of remembering past input signals in order to classify or regress the input signals. Therefore, a linear model can be used as the readout layer 4. The connection weights between input layer 2 and reservoir layer 3, and the feedback weights within reservoir layer 3 are fixed. Only the connection weights between reservoir layer 3 and readout layer 4 are optimized using a linear learner. In this way, readout layer 4 can be machine-learned with fewer computational resources and less power consumption than deep neural networks (DNNs) such as recurrent neural networks (RNNs).

[0012] The reservoir element 10 of Embodiment 1 will be described with reference to Figures 2 and 3. The reservoir element 10 is a physical reservoir applied to the reservoir layer 3 (see Figure 1). The reservoir element 10 comprises a substrate 11 and a laminate 20.

[0013] The substrate 11 may be, for example, a semiconductor substrate such as a silicon (Si) substrate or a silicon carbide (SiC) substrate, an insulating substrate such as a glass substrate, a sapphire substrate or a resin substrate, or a metal substrate such as an aluminum (Al) substrate or a stainless steel substrate. The substrate 11 has a main surface 12a and a main surface 12b opposite to the main surface 12a. The substrate 11 has a thickness of, for example, 100 μm or more and 1 mm or less. The thickness of the substrate 11 is the distance between the main surface 12a and the main surface 12b.

[0014] Multiple holes 13 are formed in the main surface 12a. The multiple holes 13 have a depth of, for example, 0.1 μm or more and less than 1 mm. The multiple holes 13 may have the same depth as each other. The multiple holes 13 are separated from the main surface 12b and do not have to penetrate the substrate 11 in the thickness direction of the substrate 11. In a plan view of the main surface 12a, each of the multiple holes 13 may have a circular shape as shown in Figure 1, or it may have other shapes such as an ellipse, triangle, square or hexagon. In a plan view of the main surface 12a, the multiple holes 13 may have different sizes (e.g., diameters) as each other. In a plan view of the main surface 12a, the multiple holes 13 may be arranged randomly.

[0015] The laminate 20 is formed on the main surface 12a of the substrate 11. The laminate 20 covers the multiple holes 13 and closes the multiple holes 13. The portion of the laminate 20 facing the multiple holes 13 is a multiple diaphragm 29. In a plan view of the main surface 12a, the multiple diaphragms 29 may be arranged randomly.

[0016] The laminate 20 includes a piezoelectric film 24, a first electrode 23, and a plurality of second electrodes 25a, 25b, 25c, 25d. The laminate 20 may further include a base layer 21 and a first insulating layer 22. The laminate 20 may further include a first pad 26e, second pads 26a, 26b, 26c, 26d, first wiring 27e, second wiring 27a, 27b, 27c, 27d, and a second insulating layer 28.

[0017] The base layer 21 is formed on the main surface 12a of the substrate 11. The base layer 21 covers and closes the plurality of holes 13. The base layer 21 is, for example, a Si layer. The substrate 11, the plurality of holes 13, and the base layer 21 may form a C-SOI (Cavity Silicon on Insulator) structure. The base layer 21 may be, for example, a semiconductor layer such as a SiC layer, an insulating layer such as a glass layer, a sapphire layer, or a resin layer, or a metal layer such as an Al layer or a stainless steel layer. The base layer 21 has a thickness of, for example, 1 μm or more and 100 μm or less.

[0018] The first insulating layer 22 is formed on the base layer 21. The first insulating layer 22 electrically insulates the first electrode 23 from the substrate 11. The first insulating layer 22 is, for example, a silicon nitride film, a silicon oxide film, or a silicon oxynitride film. The insulating film has a thickness of, for example, 0.01 μm or more and 1.0 μm or less.

[0019] The first electrode 23 is formed on the first insulating layer 22. In a plan view of the main surface 12a, the first electrode 23 may overlap all of the multiple holes 13. In a plan view of the main surface 12a, the first electrode 23 may be a single electrode formed across all of the multiple diaphragms 29. That is, the first electrode 23 may be a single electrode formed in common across the multiple diaphragms 29. The first electrode 23 is made of a conductive material such as platinum (Pt), silver (Ag), or nickel (Ni) alloy. The first electrode 23 has a thickness of, for example, 0.01 μm or more and 1.0 μm or less. To improve the adhesion between the first electrode 23 and the first insulating layer 22, an underlayer (not shown), such as a titanium (Ti) layer, may be formed between the first electrode 23 and the first insulating layer 22.

[0020] The piezoelectric film 24 is formed on the first electrode 23. The piezoelectric film 24 is positioned between the first electrode 23 and the plurality of second electrodes 25a, 25b, 25c, and 25d. In a plan view of the main surface 12a, the piezoelectric film 24 may overlap all of the plurality of holes 13. In a plan view of the main surface 12a, the piezoelectric film 24 may be a single piezoelectric film formed over all of the plurality of diaphragms 29. That is, the piezoelectric film 24 may be a single piezoelectric film formed in common over the plurality of diaphragms 29.

[0021] As shown in Figure 4, the piezoelectric film 24 has a polycrystalline structure with multiple domains. The piezoelectric film 24 has been subjected to polarization treatment. Therefore, the polarization directions of the multiple domains contained in the piezoelectric film 24 are not completely random, but they are not perfectly aligned in one direction either. The piezoelectric film 24 is made of, for example, lead zirconate titanate (PZT), barium titanate (BaTiO2), etc. ３ ), lead titanate (PbTiO ３ It is formed of a piezoelectric material such as potassium sodium niobate (KNN) or zinc oxide (ZnO). The piezoelectric film 24 has a thickness of, for example, 0.1 μm or more and 10 μm or less.

[0022] Multiple second electrodes 25a, 25b, 25c, 25d are formed on the piezoelectric film 24. The multiple second electrodes 25a, 25b, 25c, 25d are separated from each other and are not electrically connected to each other. In a plan view of the main surface 12a, the shapes or sizes of the multiple second electrodes 25a, 25b, 25c, 25d may be the same or different from each other. In a plan view of the main surface 12a, the entirety of the multiple second electrodes 25a, 25b, 25c, 25d overlaps all of the multiple holes 13. In a plan view of the main surface 12a, each of the second electrodes 25a, 25b, 25c, 25d overlaps a portion of the multiple holes 13. Each of the multiple second electrodes 25a, 25b, 25c, 25d is provided for at least one corresponding diaphragm 29 among the multiple diaphragms 29. In a plan view of the main surface 12a, each of the second electrodes 25a, 25b, 25c, and 25d is formed on a portion of the plurality of diaphragms 29.

[0023] The second electrodes 25a, 25b, 25c, and 25d are formed of a conductive material such as a Pt, Ag alloy, or Ni alloy. The second electrodes 25a, 25b, 25c, and 25d have a thickness of, for example, 0.01 μm or more and 1.0 μm or less. The number of the multiple second electrodes 25a, 25b, 25c, and 25d is not limited to four, but can be two or more. In order to improve the adhesion between the second electrodes 25a, 25b, 25c, and 25d and the piezoelectric film 24, an underlayer (not shown) such as a Ti layer may be formed between the second electrodes 25a, 25b, 25c, and 25d and the piezoelectric film 24.

[0024] A conductive wire (not shown) is bonded to the first pad 26e. The first wiring 27e is connected to the first pad 26e and the first electrode 23. The first pad 26e and the first wiring 27e are made of a conductive material. The first pad 26e and the first wiring 27e may be made of the same material as the first electrode 23.

[0025] Conductive wires (not shown) are bonded to the second pads 26a, 26b, 26c, and 26d. The second wiring 27a is connected to the second pad 26a and the second electrode 25a. The second wiring 27b is connected to the second pad 26b and the second electrode 25b. The second wiring 27c is connected to the second pad 26c and the second electrode 25c. The second wiring 27d is connected to the second pad 26d and the second electrode 25d. The second pads 26a, 26b, 26c, and 26d and the second wirings 27a, 27b, 27c, and 27d are formed of a conductive material. The second pads 26a, 26b, 26c, and 26d and the second wirings 27a, 27b, 27c, and 27d may be formed of the same material as the second electrodes 25a, 25b, 25c, and 25d.

[0026] The second insulating layer 28 is positioned between the second wirings 27a, 27b, 27c, and 27d and the first electrode 23, electrically insulating the second wirings 27a, 27b, 27c, and 27d from the first electrode 23. The second insulating layer 28 may also be positioned between the second wirings 27a, 27b, 27c, and 27d and the first insulating layer 22, between the second pads 26a, 26b, 26c, and 26d and the first insulating layer 22, between the first wiring 27e and the first insulating layer 22, and between the first pad 26e and the first insulating layer 22. The second insulating layer 28 may be formed from the same material as the piezoelectric film 24, or from a different insulating material (for example, a silicon nitride film, a silicon oxide film, or a silicon oxynitride film).

[0027] The laminate 20 may further include a protective layer (not shown). The protective layer is arranged on a plurality of second electrodes 25a, 25b, 25c, 25d, portions of the piezoelectric film 24 exposed from the plurality of second electrodes 25a, 25b, 25c, 25d, portions of the first electrode 23 exposed from the second electrodes 25a, 25b, 25c, 25d and the piezoelectric film 24, and first wiring 27e, second wiring 27a, 27b, 27c, 27d. The protective layer is an insulating film such as a silicon nitride film, a silicon oxide film, or a silicon oxynitride film. The first pad 26e and the second pads 26a, 26b, 26c, 26d are exposed from the protective layer.

[0028] An example of a manufacturing method for the reservoir element 10 of this embodiment will be described with reference to Figures 5 to 9.

[0029] Referring to Figure 5, multiple holes 13 are formed in the main surface 12a of the substrate 11 by etching the main surface 12a of the substrate 11.

[0030] In the first example, a photoresist (not shown) is formed on the main surface 12a. The photoresist is exposed to light and patterned. The patterned photoresist is used as an etching mask to etch the main surface 12a by reactive ion etching (RIE) or the like. In this way, a plurality of holes 13 are formed on the main surface 12a. In a plan view of the main surface 12a, the plurality of holes 13 may be formed randomly.

[0031] In the second example, a negative-type photoresist is formed on the main surface 12a. Multiple microbeads (not shown) are scattered on the negative-type photoresist. The multiple microbeads have random sizes. The multiple microbeads are made of, for example, silicon, glass, or plastic. The negative-type photoresist coated with multiple microbeads is exposed to light. The light irradiated onto the negative-type photoresist to expose it does not penetrate the multiple microbeads. The portion of the negative-type photoresist below the microbeads is removed to pattern the negative-type photoresist. The patterned photoresist is used as an etching mask to etch the main surface 12a using RIE or the like. In this way, multiple holes 13 are formed on the main surface 12a. According to the second example, in a plan view of the main surface 12a, the multiple holes 13 are further formed randomly.

[0032] Referring to Figure 6, an SOI (Silicon on Insulator) substrate 30 is bonded to the main surface 12a of the substrate 11. Specifically, the SOI substrate 30 includes a silicon substrate 31, an insulating layer 32, and an active layer 33. The insulating layer 32 is disposed between the silicon substrate 31 and the active layer 33, and is, for example, a silicon oxide layer. The active layer 33 is, for example, a Si layer. The SOI substrate 30 is bonded to the main surface 12a by bringing the active layer 33 into contact with the main surface 12a. The SOI substrate 30 may be bonded to the substrate 11 at a high temperature of 1000 degrees Celsius or higher, or it may be bonded to the substrate 11 at a temperature of less than 1000 degrees Celsius by plasma activated bonding.

[0033] Refer to Figure 7 to remove the silicon substrate 31. For example, the silicon substrate 31 may be polished, or it may be dissolved using a solution. If the silicon substrate 31 is dissolved, the substrate 11 and the active layer 33 are protected beforehand with a protective film (for example, an oxide film) made of a material that does not dissolve in the solution.

[0034] Referring to Figure 8, the insulating layer 32 is removed by dry etching such as RIE or wet etching using hydrofluoric acid or a buffered hydrofluoric acid solution. The active layer 33 becomes the base layer 21. In this way, a C-SOI structure is formed, which includes the substrate 11, a plurality of holes 13, and the base layer 21.

[0035] Referring to Figure 9, a first insulating layer 22 is formed on the base layer 21. The first insulating layer 22 is formed, for example, by chemical vapor deposition (CVD).

[0036] A first electrode 23, a piezoelectric film 24, and second electrodes 25a, 25b, 25c, and 25d are formed on the first insulating layer 22.

[0037] The first electrode 23 and the second electrodes 25a, 25b, 25c, and 25d are formed by a process of depositing a conductive layer by sputtering or vapor deposition, and a process of etching the conductive layer by RIE or the like. The etching of the conductive layer may be wet etching, or Cl ２ Dry etching using an Ar-based etching gas is also acceptable.

[0038] The piezoelectric film 24 is formed by, for example, a step of depositing a piezoelectric layer by sputtering or sol-gel method, a step of etching the piezoelectric layer by RIE or the like, and a step of polarization treatment of the piezoelectric layer. The etching of the piezoelectric layer is performed by, for example, Cl ２ / BCl ３ / CH ４ This is a dry etching process using an etching gas. The polarization treatment process of the piezoelectric layer causes the polarization directions of the multiple domains contained in the piezoelectric layer to be aligned to some extent, as shown in Figure 4. The polarization directions of the multiple domains contained in the piezoelectric film 24 are not completely random, but they are not perfectly aligned in one direction either. If the second insulating layer 28 is formed of the same material as the piezoelectric film 24, the second insulating layer 28 is formed together with the piezoelectric film 24.

[0039] Thus, the reservoir element 10 shown in Figures 2 and 3 is obtained using a photolithography process.

[0040] An example of the operation of the reservoir element 10 according to the present embodiment will be described with reference to FIGS. 2, 3 and 10 to 13C.

[0041] The first electrode 23 is connected to a ground potential. An input voltage signal shown in FIG. 10 as a time-series input signal is input to the reservoir element 10 via at least one of the second pads 26a, 26b, 26c, and 26d. An input voltage signal is applied to at least one of the second electrodes 25a, 25b, 25c, and 25d. The piezoelectric film 24 formed on at least one of the second electrodes 25a, 25b, 25c, and 25d to which the input voltage signal is applied vibrates. As shown in FIG. 11, the diaphragm 29 including this piezoelectric film 24 vibrates. The piezoelectric film 24 is a single piezoelectric film formed commonly for the plurality of diaphragms 29. Therefore, the vibration of at least one diaphragm 29 among the plurality of diaphragms 29 is transmitted to other diaphragms 29 via the piezoelectric film 24. In this way, the plurality of diaphragms 29 form a network that influences each other.

[0042] The plurality of diaphragms 29 have vibration characteristics different from each other. For example, when the plurality of holes 13 have sizes different from each other in a plan view of the main surface 12a, the plurality of diaphragms 29 can have vibration characteristics different from each other. For example, the diaphragm 29 on the left side of FIG. 11 has the vibration characteristic shown in FIG. 12A, and the diaphragm 29 on the right side of FIG. 11 has the vibration characteristic shown in FIG. 12B. Therefore, the plurality of diaphragms 29 have vibration characteristics different from each other.

[0043] The piezoelectric film 24 converts the vibrations of the plurality of diaphragms 29 into voltage signals. Output voltage signals are output from at least two of the second electrodes 25a, 25b, 25c, and 25d. For example, a first output voltage signal (see FIG. 13A) is output from the second electrode 25a, a second output voltage signal (see FIG. 13B) is output from the second electrode 25b, and a third output voltage signal (see FIG. 13C) is output from the second electrode 25c.

[0044] As shown in FIG. 14, the displacement of the piezoelectric film 24 has hysteresis with respect to the voltage input to the piezoelectric film 24. Therefore, the vibration of the piezoelectric film 24 with respect to an input signal has nonlinearity. The reservoir element 10 including the piezoelectric film 24 has nonlinearity. As shown in FIG. 4, the piezoelectric film 24 has a polycrystalline structure having a plurality of domains. Therefore, in the piezoelectric film 24, it can be considered that a large number of nodes are randomly connected. The reservoir element 10 including the piezoelectric film 24 has high-dimensionality. The amplitude of the vibration of the plurality of diaphragms 29 gradually decreases as time passes. Therefore, the plurality of diaphragms 29 have short-term memory. Since the reservoir element 10 has nonlinearity, high-dimensionality and short-term memory as described above, it can be used as the reservoir layer 3 of the reservoir computing apparatus 1 shown in FIG. 1.

[0045] Referring to FIG. 15, in a first modification of the present embodiment, the input signal to the reservoir element 10 may be a sound wave 40. When the sound wave 40 is input to the reservoir element 10, the piezoelectric film 24 vibrates, and at least one diaphragm 29 among the plurality of diaphragms 29 vibrates. The vibration of the at least one diaphragm 29 is transmitted to other diaphragms 29 via the piezoelectric film 24. The piezoelectric film 24 converts the vibrations of the plurality of diaphragms 29 into voltage signals. Output voltage signals are output from at least two of the second electrodes 25a, 25b, 25c, 25d.

[0046] Referring to Figure 16, in a second modification of this embodiment, the passage 14 may be formed in the substrate 11. The passage 14 communicates with and is connected to at least two of the plurality of holes 13. The depth of the passage 14 may be less than the depth of each of the at least two holes 13 connected to the passage 14. The width of the passage 14 may be less than the width of each of the at least two holes 13 connected to the passage 14. The vibration of at least one of the plurality of diaphragms 29 is transmitted to the other diaphragms 29 not only by the piezoelectric film 24 but also by fluctuations in the air pressure in the holes 13 through the passage 14. In Figure 16, the passage 14 is formed to be narrow in the thickness direction of the element, but the passage 14 may be formed to be narrow in the in-plane direction of the element.

[0047] The effects of the reservoir element 10 of this embodiment will now be explained. The reservoir element 10 of this embodiment comprises a substrate 11 having a main surface 12a and a laminate 20 formed on the main surface 12a. A plurality of holes 13 are formed in the main surface 12a. The laminate 20 includes a piezoelectric film 24, a first electrode 23, and a plurality of second electrodes 25a, 25b, 25c, and 25d. The piezoelectric film 24 is arranged between the first electrode 23 and the plurality of second electrodes 25a, 25b, 25c, and 25d, and has a polycrystalline structure. The portion of the laminate 20 facing the plurality of holes 13 is a plurality of diaphragms 29.

[0048] The reservoir element 10 of this embodiment does not contain a liquid such as an ionic liquid and is a solid-state reservoir element that can be formed by methods such as photolithography. Therefore, the reservoir element 10 is more suitable for mass production.

[0049] In the reservoir element 10 of this embodiment, the piezoelectric film 24 is a single piezoelectric film that is commonly formed on multiple diaphragms 29.

[0050] The vibration of at least one of the multiple diaphragms 29 is transmitted to the other diaphragms 29 via the piezoelectric film 24. The multiple diaphragms 29 form a network that influences each other. As a result, the reservoir element 10 has improved nonlinearity and improved high-dimensionality.

[0051] In the reservoir element 10 of this embodiment, a passage 14 is formed in the substrate 11. The passage 14 communicates with at least two of the plurality of holes 13.

[0052] Therefore, vibrations of at least one of the multiple diaphragms 29 are transmitted to the other diaphragms 29 not only by the piezoelectric film 24 but also by pressure fluctuations in the air through the passage 14. This can improve the dimensionality of the reservoir element 10.

[0053] In the reservoir element 10 of this embodiment, the multiple holes 13 have different sizes when viewed in plan on the main surface 12a.

[0054] Therefore, the multiple diaphragms 29 have different vibration characteristics. The reservoir element 10 has improved nonlinearity and improved short-term memory performance.

[0055] In the reservoir element 10 of this embodiment, the multiple diaphragms 29 and the multiple holes 13 are arranged randomly in a plan view of the main surface 12a.

[0056] Therefore, the variation in the effect that the vibration of one diaphragm 29 has on the vibration of other diaphragms 29 increases. The reservoir element 10 has improved nonlinearity.

[0057] Embodiment 2. The reservoir element 10 of Embodiment 2 will be described with reference to Figures 17 and 18. The reservoir element 10 of this embodiment has the same configuration as the reservoir element 10 of Embodiment 1, but differs mainly in the following points.

[0058] In this embodiment, the piezoelectric film 24 includes a plurality of island portions 24i that are separated from each other. Each of the plurality of island portions 24i is provided with respect to a corresponding diaphragm 29 among a plurality of diaphragms 29. In a plan view of the main surface 12a, the size of each of the plurality of island portions 24i is larger than the size of the corresponding hole 13 among a plurality of holes 13. In a plan view of the main surface 12a, the shapes or sizes of the plurality of island portions 24i may differ from each other. The number of diaphragms 29 corresponding to each of the plurality of island portions 24i is less than the number of diaphragms 29 corresponding to the piezoelectric film 24 in Embodiment 1. The number of the plurality of island portions 24i may be equal to the number of diaphragms 29. In a plan view of the main surface 12a, the plurality of island portions 24i may be arranged randomly.

[0059] The reservoir element 10 of this embodiment may include a plurality of first connection portions 24j. Each of the plurality of first connection portions 24j connects at least two of the plurality of island portions 24i. In a plan view of the main surface 12a, the width of the first connection portion 24j is smaller than the width of each of the at least two island portions 24i connected to the first connection portion 24j. The first connection portion 24j may be formed of the same material as the plurality of island portions 24i, and the piezoelectric film 24 may include the plurality of island portions 24i and the first connection portion 24j. The first connection portion 24j may be formed of a different insulating material (for example, silicon oxide or silicon nitride) than the plurality of island portions 24i.

[0060] The multiple second electrodes 25i are configured similarly to the multiple second electrodes 25a, 25b, 25c, and 25d of Embodiment 1, but differ from the multiple second electrodes 25a, 25b, 25c, and 25d of Embodiment 1 mainly in the following points.

[0061] Each of the multiple second electrodes 25i is provided on at least one corresponding diaphragm 29 among the multiple diaphragms 29. The multiple second electrodes 25i are arranged on corresponding island portions 24i among the multiple island portions 24i. In a plan view of the main surface 12a, the size of each of the multiple second electrodes 25i is smaller than the size of each of the multiple second electrodes 25a, 25b, 25c, and 25d of Embodiment 1. In a plan view of the main surface 12a, the size of each of the multiple second electrodes 25i may be larger than the size of the corresponding hole 13 among the multiple holes 13. The shapes or sizes of the multiple second electrodes 25i are different from each other.

[0062] The number of multiple second electrodes 25i is greater than the number of multiple second electrodes 25a, 25b, 25c, and 25d in Embodiment 1. The number of diaphragms 29 corresponding to each of the multiple second electrodes 25i is less than the number of diaphragms 29 corresponding to each of the multiple second electrodes 25a, 25b, 25c, and 25d in Embodiment 1. The number of multiple second electrodes 25i may be equal to the number of multiple island portions 24i, or it may be equal to the number of multiple diaphragms 29. In a plan view of the main surface 12a, the multiple second electrodes 25i may be arranged randomly.

[0063] The reservoir element 10 of this embodiment may include a plurality of second connection portions 25j. Each of the plurality of second connection portions 25j connects at least two of the plurality of second electrodes 25i. Each of the plurality of second connection portions 25j electrically connects at least two of the plurality of second electrodes 25i. In a plan view of the main surface 12a, the width of the second connection portion 25j is smaller than the width of each of the at least two second electrodes 25i connected to the second connection portion 25j. The plurality of second connection portions 25j are formed of a conductive material. The plurality of second connection portions 25j may be formed of the same material as the plurality of second electrodes 25i.

[0064] Multiple second electrodes 25i may be directly connected to second pads 26a, 26b, 26c, and 26d, or they may be connected to second pads 26a, 26b, 26c, and 26d via multiple second connection parts 25j.

[0065] The reservoir element 10 may have at least one of a plurality of first connection portions 24j or a plurality of second connection portions 25j, or it may not have both a plurality of first connection portions 24j and a plurality of second connection portions 25j.

[0066] The reservoir element 10 of this embodiment provides the following effects in addition to the effects of the reservoir element 10 of Embodiment 1.

[0067] In the reservoir element 10 of this embodiment, the piezoelectric film 24 includes a plurality of island portions 24i that are separated from each other. Each of the plurality of island portions 24i is provided for at least one corresponding diaphragm 29 among the plurality of diaphragms 29. The plurality of second electrodes 25i are arranged on the corresponding island portions 24i among the plurality of island portions 24i.

[0068] Therefore, the way vibrations are transmitted between the multiple diaphragms 29 can be changed. Depending on the type of input signal, a reservoir element 10 suitable for processing the input signal may be provided.

[0069] The reservoir element 10 of this embodiment further comprises at least one of a first connection portion 24j or a second connection portion 25j. The first connection portion 24j connects at least two of the plurality of island portions 24i. In a plan view of the main surface 12a, the width of the first connection portion 24j is smaller than the width of each of the at least two island portions 24i connected to the first connection portion 24j. The second connection portion 25j connects at least two of the plurality of second electrodes 25i. In a plan view of the main surface 12a, the width of the second connection portion 25j is smaller than the width of each of the at least two second electrodes 25i connected to the second connection portion 25j.

[0070] Therefore, the way vibrations are transmitted between the multiple diaphragms 29 can be changed. Depending on the type of input signal, a reservoir element 10 suitable for processing the input signal may be provided.

[0071] Embodiment 3. The reservoir element 10 of Embodiment 3 will be described with reference to Figure 19. The reservoir element 10 of this embodiment has the same configuration as the reservoir element 10 of Embodiment 1, but differs mainly in the following points.

[0072] In this embodiment, the multiple diaphragms 29 are deformed when no input signal is input to the reservoir element 10. Since the multiple diaphragms 29 have an initial deformation in this way, the vibration characteristics of the multiple diaphragms 29 can be changed. The initial deformation of the multiple diaphragms 29 may be an upward convex deformation as shown in Figure 19, or a downward convex deformation.

[0073] In one example, initial deformation can be imparted to multiple diaphragms 29 by generating internal stress in the first insulating layer 22. The internal stress may be compressive stress or tensile stress. By adjusting the parameters (e.g., temperature) of at least one of the deposition processes of the first insulating layer 22, the first electrode 23, the piezoelectric film 24, or the second electrodes 25a-25d, internal stress can be generated in at least one of the first insulating layer 22, the first electrode 23, the piezoelectric film 24, or the second electrodes 25a-25d. The internal stress may also be generated in the protective layer (not shown) described in Embodiment 1.

[0074] In another example, an offset voltage may be applied between the first electrode 23 and the second electrodes 25a-25d to impart initial deformation to the multiple diaphragms 29. This initial deformation of the multiple diaphragms 29 generates internal stress in the films constituting the multiple diaphragms 29 (at least one of the first insulating layer 22, the first electrode 23, the piezoelectric film 24, or the second electrodes 25a-25d).

[0075] When the internal stress of the film constituting the multiple diaphragms 29 (at least one of the first insulating layer 22, the first electrode 23, the piezoelectric film 24, or the second electrodes 25a-25d) is compressive stress, the resonant frequency of the multiple diaphragms 29 becomes lower. Therefore, a soft spring effect is more likely to occur in the multiple diaphragms 29. In this specification, the soft damping effect means that as the amplitude of the diaphragm 29 increases, the resonant frequency of the diaphragm 29 decreases. Conversely, when the internal stress of the film constituting the multiple diaphragms 29 is tensile stress, a hard spring effect is more likely to occur. In this specification, the soft damping effect means that as the amplitude of the diaphragm 29 increases, the resonant frequency of the diaphragm 29 increases. Therefore, the nonlinearity of the vibration of the piezoelectric film 24 in response to the input signal can be increased. The nonlinearity of the reservoir element 10 can be further increased.

[0076] The reservoir element 10 of this embodiment provides the following effects in addition to the effects of the reservoir element 10 of Embodiment 1.

[0077] In the reservoir element 10 of this embodiment, the multiple diaphragms 29 are deformed when no input signal is input to the reservoir element 10.

[0078] Therefore, the nonlinearity of the vibration of the piezoelectric film 24 in response to the input signal can be increased. The nonlinearity of the reservoir element 10 can be further enhanced.

[0079] Embodiment 4. The reservoir element 10 of Embodiment 4 will be described with reference to Figure 20. The reservoir element 10 of this embodiment has the same configuration as the reservoir element 10 of Embodiment 1, but differs mainly in the following points.

[0080] In this embodiment, when no input signal is input to the reservoir element 10, the pressures in the multiple holes 13, 13a, and 13b are different from each other.

[0081] In the first example, some of the multiple holes 13, 13a, 13b, and 13c (hole 13a) penetrate the substrate 11. Specifically, hole 13a extends from the main surface 12a to the main surface 12b and is a through-hole that penetrates the substrate 11 in the thickness direction. Since hole 13a is in communication with the atmosphere surrounding the reservoir element 10, the pressure in hole 13a is, for example, atmospheric pressure. In contrast, hole 13 is isolated from the atmosphere surrounding the reservoir element 10 by the substrate 11 and the base layer 21. The SOI substrate 30 is bonded to the main surface 12a of the substrate 11 at a temperature higher than room temperature (see Figure 6). Therefore, the pressure in hole 13 is lower than atmospheric pressure. As a result, the pressure inside hole 13a can be made greater than the pressure inside hole 13. The vibration of the diaphragm 29 on hole 13a is damped faster than the vibration of the diaphragm 29 on hole 13.

[0082] In the second example, the reservoir element 10 further comprises a gas adsorbent 41. The gas adsorbent 41 is placed in a plurality of holes 13, 13a, 13b, and a portion of 13 (hole 13b). Hole 13b may have the same size as hole 13a. The gas adsorbent 41 is made of, for example, a titanium, zirconium, or vanadium-based alloy. The gas adsorbent 41 adsorbs gas in hole 13b. Therefore, the pressure in hole 13b can be made lower than the pressure in hole 13. The vibration of the diaphragm 29 on hole 13b is damped more slowly than the vibration of the diaphragm 29 on hole 13.

[0083] Furthermore, the multiple holes 13, 13a, 13b, and 13c include holes 13 and 13c. The depth of hole 13c is smaller than the depth of hole 13. For example, the depth of hole 13c is 10 μm or less. Therefore, when the diaphragm 29 on hole 13c vibrates, a squeeze film damping effect occurs on the diaphragm 29 on hole 13c. That is, when multiple diaphragms 29 vibrate, the diaphragm 29 on hole 13c approaches the bottom of hole 13c more closely than the diaphragm 29 on hole 13. Therefore, the diaphragm 29 on hole 13c experiences a greater air resistance force than the diaphragm 29 on hole 13. The vibration of the diaphragm 29 on hole 13c is damped faster than the vibration of the diaphragm 29 on hole 13.

[0084] The reservoir element 10 of this embodiment provides the following effects in addition to the effects of the reservoir element 10 of Embodiment 1.

[0085] In the reservoir element 10 of this embodiment, when no input signal is input to the reservoir element 10, the pressures in the multiple holes 13, 13a, and 13b are different from each other.

[0086] Therefore, the damping time of the vibration of the diaphragm 29 can be changed between multiple diaphragms 29. The short-term memory performance of the reservoir element 10 can be adjusted.

[0087] In the reservoir element 10 of this embodiment, some of the multiple holes 13, 13a, 13b, and 13c (hole 13a) penetrate the substrate 11.

[0088] Therefore, the damping time of the vibration of the diaphragm 29 can be changed between multiple diaphragms 29. The short-term memory performance of the reservoir element 10 can be adjusted.

[0089] The reservoir element 10 of this embodiment further comprises a gas adsorbent 41 disposed in a portion (hole 13b) of the plurality of holes 13, 13a, 13b, 13c.

[0090] Therefore, the damping time of the vibration of the diaphragm 29 can be changed between multiple diaphragms 29. The short-term memory performance of the reservoir element 10 can be adjusted.

[0091] In the reservoir element 10 of this embodiment, the multiple holes 13, 13a, 13b, and 13c include a first hole (hole 13) and a second hole (hole 13c). The depth of the second hole is smaller than the depth of the first hole.

[0092] Therefore, the damping time of the vibration of the diaphragm 29 can be changed between multiple diaphragms 29. The short-term memory performance of the reservoir element 10 can be adjusted.

[0093] Embodiment 5. The reservoir element 10 of Embodiment 5 will be described with reference to Figure 21. The reservoir element 10 of this embodiment has the same configuration as the reservoir element 10 of Embodiment 1, but differs mainly in the following points.

[0094] The reservoir element 10 of this embodiment further comprises a lid 45. The lid 45 covers a plurality of diaphragms 29. The lid 45 is made of, for example, glass or Si. The surface of the lid 45 facing the plurality of diaphragms 29 may be flat, curved, or rough. Steps may be formed on the surface of the lid 45 facing the plurality of diaphragms 29. The lid 45 is bonded to the first insulating layer 22, the base layer 21, or the substrate 11 by anode bonding, surface activation direct bonding, or bonding with an adhesive.

[0095] An example of the operation of the reservoir element 10 in this embodiment will be described. An input voltage signal is input to the reservoir element 10 as a time-series input signal. The piezoelectric film 24 vibrates, and at least one of the multiple diaphragms 29 vibrates. When at least one of the multiple diaphragms 29 vibrates, sound waves 42 are generated. The sound waves 42 generated by at least one of the multiple diaphragms 29 are reflected by the lid 45 and input to the other diaphragms 29. The vibration of at least one of the multiple diaphragms 29 is transmitted to the other diaphragms 29 not only by the piezoelectric film 24 but also by the sound waves 42 reflected by the lid 45. The dimensionality of the reservoir element 10 can be improved.

[0096] (Modified Version) Referring to Figure 22, in the first modified version of this embodiment, a through hole 46 may be formed in the lid 45, and the input signal to the reservoir element 10 may be a sound wave 40. The input sound wave signal is input to the reservoir element 10 as a time-series input signal through the through hole 46 in the lid 45. The piezoelectric film 24 vibrates, causing at least one of the multiple diaphragms 29 to vibrate. When at least one of the multiple diaphragms 29 vibrates, a sound wave 42 is generated. The sound wave 42 is reflected by the lid 45 and input to the other diaphragms 29. The vibration of at least one of the multiple diaphragms 29 is transmitted to the other diaphragms 29 not only by the piezoelectric film 24 but also by the sound wave 42 reflected by the lid 45. The piezoelectric film 24 converts the vibrations of the multiple diaphragms 29 into a voltage signal. Output voltage signals are output from at least two of the second electrodes 25a, 25b, 25c, and 25d.

[0097] Referring to Figure 23, in the second modification of this embodiment, the multiple diaphragms 29 are arranged in a sealed space 47 formed by a lid 45. The sealed space 47 is a vacuum. Therefore, it is possible to prevent sound waves 40 (see Figure 22) from being input from outside the reservoir element 10. In addition, it is possible to prevent sound waves 42 (see Figure 22) generated by at least one of the multiple diaphragms 29 from being transmitted to the other diaphragms 29 by reflection at the lid 45. The lower the vacuum level of the sealed space 47, the smaller the impact on the other diaphragms 29 can be, but a vacuum level of 1000 Pa or less is sufficient.

[0098] The reservoir element 10 of this embodiment provides the following effects in addition to the effects of the reservoir element 10 of Embodiment 1.

[0099] The reservoir element 10 of this embodiment further includes a cover 45 that covers a plurality of diaphragms 29.

[0100] When at least one of the multiple diaphragms 29 vibrates, a sound wave 42 is generated. The sound wave 42 generated by at least one of the multiple diaphragms 29 is reflected by the lid 45 and input to the other diaphragms 29. The vibration of at least one of the multiple diaphragms 29 is transmitted to the other diaphragms 29 not only by the piezoelectric film 24 but also by the sound wave 42 reflected by the lid 45. The dimensionality of the reservoir element 10 can be improved.

[0101] In the reservoir element 10 of this embodiment, the multiple diaphragms 29 are arranged within a sealed space 47 formed by a lid 45. The sealed space 47 is a vacuum.

[0102] Therefore, it is possible to prevent sound waves 40 from being input from outside the reservoir element 10. The environmental resistance of the reservoir element 10 is improved.

[0103] Embodiments 1-5 and their variations disclosed herein should be considered in all respects as illustrative and not restrictive. To the extent that they do not contradict each other, at least two of Embodiments 1-5 and their variations disclosed herein can be combined. The scope of this disclosure is indicated by the claims rather than the foregoing description and is intended to include all modifications within the meaning and scope of the claims.

[0104] 1 Reservoir computing device, 2 Input layer, 3 Reservoir layer, 4 Readout layer, 10 Reservoir element, 11 Substrate, 12a, 12b Main surface, 13, 13a, 13b, 13c Hole, 14 Passage, 20 Laminate, 21 Base layer, 22 First insulating layer, 23 First electrode, 24 Piezoelectric film, 24i Island portion, 24j First connection portion, 25a, 25b, 25c, 25d, 25i Second electrode, 25j Second connection portion, 26a, 26b, 26c, 26d Second pad, 26e First pad, 27a, 27b, 27c, 27d Second wiring, 27e First wiring, 28 Second insulating layer, 29 Diaphragm, 30 SOI substrate, 31 Silicon substrate, 32 Insulating layer, 33 Active layer, 40, 42 41 Sound wave, 45 Gas adsorbent, 46 Lid, 46 Through hole, 47 Sealed space.

Claims

1. A reservoir element comprising a substrate having a main surface and a laminate formed on the main surface, wherein a plurality of holes are formed on the main surface, the laminate includes a piezoelectric film, a first electrode, and a plurality of second electrodes, the piezoelectric film is disposed between the first electrode and the plurality of second electrodes and has a polycrystalline structure, and the portion of the laminate facing the plurality of holes is a plurality of diaphragms.

2. The reservoir element according to claim 1, wherein the piezoelectric film is a single piezoelectric film formed in common with the plurality of diaphragms.

3. The reservoir element according to claim 1, wherein the piezoelectric film includes a plurality of island portions that are separated from each other, each of the plurality of island portions is provided for at least one corresponding diaphragm among the plurality of diaphragms, and the plurality of second electrodes are arranged on the corresponding island portions among the plurality of island portions.

4. The reservoir element according to claim 3, further comprising at least one of a first connection portion or a second connection portion, wherein the first connection portion connects at least two of the plurality of island portions, and in a plan view of the main surface, the width of the first connection portion is smaller than the width of each of the at least two island portions connected to the first connection portion, and the second connection portion connects at least two of the plurality of second electrodes, and in a plan view of the main surface, the width of the second connection portion is smaller than the width of each of the at least two second electrodes connected to the second connection portion.

5. The reservoir element according to any one of claims 1 to 4, wherein a passage is formed in the substrate, and the passage communicates with at least two of the plurality of holes.

6. The reservoir element according to any one of claims 1 to 3, wherein, in a plan view of the main surface, the plurality of holes have different sizes from each other.

7. The reservoir element according to any one of claims 1 to 3, wherein, in a plan view of the main surface, the plurality of diaphragms and the plurality of holes are arranged randomly.

8. The reservoir element according to any one of claims 1 to 7, wherein the plurality of diaphragms are deformed when no input signal is input to the reservoir element.

9. The reservoir element according to any one of claims 1 to 7, wherein when no input signal is input to the reservoir element, the pressures in the plurality of holes are different from each other.

10. The reservoir element according to claim 9, wherein some of the plurality of holes penetrate the substrate.

11. The reservoir element according to claim 9, further comprising a gas adsorbent disposed within a portion of the plurality of pores.

12. The reservoir element according to claim 9, wherein the plurality of holes include a first hole and a second hole, and the depth of the second hole is less than the depth of the first hole.

13. The reservoir element according to any one of claims 1 to 12, further comprising a lid that covers the plurality of diaphragms.

14. The reservoir element according to claim 13, wherein the plurality of diaphragms are arranged in a sealed space formed by the lid, and the sealed space is a vacuum.

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