Elastic wave device, communication device, and manufacturing method

Abstract

IDT electrodes are fine and it is, therefore, necessary to improve the power tolerance to migration. This elastic wave device comprises: a piezoelectric layer; an electrode that is provided on the piezoelectric layer; and a cover part that covers at least some of the side surfaces of the electrode and that includes elements constituting the piezoelectric layer, said elements being other than oxygen and nitrogen.

Description

Acoustic wave device, communication device, and manufacturing method

[0001] The present disclosure relates to acoustic wave devices.

[0002] In an acoustic wave device, an IDT electrode is formed on a piezoelectric substrate using an aluminum film.

[0003] Japanese Patent Application Publication No. 2003-078384

[0004] According to one aspect of the present disclosure, a piezoelectric element includes a piezoelectric layer, an electrode provided on the piezoelectric layer, and a cover portion covering at least a portion of a side surface of the electrode, and the cover portion contains elements other than oxygen and nitrogen that constitute the piezoelectric layer.

[0005] According to another aspect of the present disclosure, the method includes the steps of forming a piezoelectric layer, forming an electrode on the piezoelectric layer, and dry etching the piezoelectric layer and the electrode.

[0006] 5 is a schematic diagram illustrating the cross-sectional structure of an elastic wave device according to embodiment 1. FIG. 6 is a diagram illustrating a comparison of elastic wave devices according to a comparative example and embodiment 1 side by side. FIG. 7 is a diagram illustrating a case in which a protective film is formed on the elastic wave device shown in FIG. 1. FIG. 8 is an example cross-sectional structure of an elastic wave device according to embodiment 1. FIG. 9 is a photograph illustrating an enlarged view of a portion of FIG. 4. FIG. 10 is a graph illustrating the composition amount of tantalum on a specific line segment in FIG. 15. FIG. 11 is a result of comparing power durability when dry etching is performed and when not. FIG. 12 is a schematic diagram illustrating the cross-sectional structure of an elastic wave device according to embodiment 2. FIG. 13 is a diagram illustrating the general configuration of a communication device according to embodiment 3. FIG. 14 is a schematic diagram illustrating the cross-sectional structure of an elastic wave device.

[0007] The IDT electrodes are so fine that they have a problem of low power resistance due to electromigration. Also, if a process for forming a cover that covers the side surfaces of the IDT electrodes is added to improve power resistance due to electromigration, multiple processes must be added to the manufacturing process, which increases manufacturing costs.

[0008] [Embodiment 1] (Plate Waves) The elastic wave device excites, for example, plate waves. Alternatively, the elastic wave device may excite bulk waves. Plate waves are classified into Lamb waves, whose vibration plane is perpendicular to the plate surface, and SH waves, whose vibration plane is parallel to the plate surface. Lamb waves are classified into S mode, which is a symmetric mode, and A mode, which is an antisymmetric mode. A1 mode corresponds to the first-order antisymmetric mode. This embodiment describes an elastic wave device using A1 mode Lamb waves, but is not limited to this. For example, an elastic wave device using bulk waves in thickness shear mode may also be used. Among various types of waves, plate waves exhibit significant changes in resonance characteristics when the thickness of the piezoelectric layer 3, described below, changes. Bulk waves also exhibit significant changes in resonance characteristics when the thickness of the piezoelectric layer 3, described below, changes.

[0009] An example of the cross-sectional structure of an acoustic wave device will now be described. Fig. 9 is a schematic diagram showing the cross-sectional structure of an acoustic wave device 100. The acoustic wave device 100 is configured by stacking a support substrate 2, a multilayer film 6, a piezoelectric layer 3, and an electrode 4 in this order.

[0010] The support substrate 2 is a substrate that supports the piezoelectric layer 3 and the electrodes 4. The support substrate 2 may be made of silicon, or various materials such as sapphire or glass may be used.

[0011] The multilayer film 6 is an acoustic reflection film in which a large number of low acoustic impedance layers 6a and high acoustic impedance layers 6b are alternately stacked. In FIG. 9, four low acoustic impedance layers 6a and four high acoustic impedance layers 6b are alternately stacked. That is, the multilayer film 6 reflects Lamb waves propagating from above the acoustic wave device 100 at the interfaces between the low acoustic impedance layers 6a and the high acoustic impedance layers 6b. The low acoustic impedance layers 6a may contain SiO2. The high acoustic impedance layers 6b may contain HfO ２ , Ta ２ O ５ , and ZrO ２ may contain at least one of, for example, HfO ２ The multilayer film 6 may be omitted.

[0012] The piezoelectric layer 3 is formed relatively thin. The thickness W of the piezoelectric layer 3 is determined by the pitch of the electrodes 4, which will be described later, and may be equal to or less than one wavelength of the plate wave excited by the electrodes 4. The piezoelectric layer 3 is made of lithium tantalate, lithium niobate, or quartz crystal. However, the piezoelectric layer 3 is not limited to these, and various piezoelectric materials may be used. In the following description, the piezoelectric layer 3 will be described as being made of lithium tantalate.

[0013] The electrode 4 is a comb-shaped IDT (interdigital transducer) electrode, i.e., has many electrode fingers. The piezoelectric layer 3 is exposed in the areas where the electrode 4 is not formed. The wavelength of the plate wave excited by the piezoelectric layer 3 is determined by the pitch of the IDT electrode. The electrode 4 is made of aluminum, copper, or an alloy using these materials.

[0014] The piezoelectric layer 3 is also configured so that plate waves are excited in the piezoelectric layer 3 when an alternating current is applied to the electrodes 4 .

[0015] 9 , in the elastic wave device 100 according to the comparative example, the surface of the piezoelectric layer 3 and the interface between the piezoelectric layer 3 and the electrode 4 are substantially aligned. In other words, the electrode 4 is formed on the piezoelectric layer 3, and no subsequent etching or other processes are performed.

[0016] (Overview of Method for Adjusting Frequency Characteristics) In the elastic wave device 100 of the comparative example, the thickness of the piezoelectric layer 3 and / or the electrode 4 varies depending on the position on the support substrate 2. The support substrate 2 is, for example, a silicon wafer. Therefore, the frequency characteristics of the elastic wave device 100 vary depending on the position on the support substrate 2.

[0017] In order to adjust the frequency characteristics of acoustic wave device 100, the thickness of piezoelectric layer 3 and electrode 4 can be adjusted by etching or the like.

[0018] (Overview of Elastic Wave Device 1) In elastic wave device 1 according to embodiment 1, each element or each portion of an element in each elastic wave device is dry etched to a different thickness based on a thickness measured in advance. Fig. 1 is a schematic diagram showing the cross-sectional structure of elastic wave device 1 according to embodiment 1. Fig. 2 is a side-by-side comparison of elastic wave devices 100 and 1 according to embodiment 1 and a comparative example.

[0019] As shown in Fig. 2, in the elastic wave device 1, the piezoelectric layer 3 and the electrode 4 are thinner than those in the elastic wave device 100. Therefore, the elastic wave device 1 is adjusted to obtain desired frequency characteristics, thereby improving the frequency characteristics. Fig. 3 shows an example cross-sectional structure of the elastic wave device 1 according to the first embodiment. In Fig. 3, a scanning electron microscope (SEM) was used as the imaging method.

[0020] In particular, the region where the surface of the piezoelectric layer 3 is exposed is thinner than the region where the electrode 4 is located at the interface between the piezoelectric layer 3 and the electrode 4, and the difference in thickness is 50 nm or less. That is, the thickness of the region of the electrode 4 in the piezoelectric layer 3 is greater than the thickness of the region of the piezoelectric layer 3 where the surface is exposed, and the difference in thickness between the region of the electrode 4 and the region of the piezoelectric layer 3 where the surface is exposed is 50 nm or less.

[0021] 2, the edges of the ends of the electrode 4 are scraped off by dry etching, resulting in beveling at reference numeral 43. This provides the effect of reducing stress migration due to vibration of the electrode 4.

[0022] (Formation of Cover Portion 5) The elastic wave device 1 is also provided with a cover portion 5 that covers at least a portion of the side surface of the electrode 4. This cover portion 5 is not present in the elastic wave device 100. Here, the support substrate 2 supports the piezoelectric layer 3, the electrode 4, and the cover portion 5.

[0023] The thickness of the thickest part of the cover part 5 is 3 nm or more. The cover part 5 has a first part 51 and a second part that is located above the electrode 4 relative to the first part 51 and is thinner than the first part. That is, the cover part 5 does not have a uniform thickness, but the first part 51 and the second part 52 of the cover part 5 have different thicknesses. The cover part 5 may also become thinner as it goes higher above the electrode 4.

[0024] The side surface of the electrode 4 has a first surface 41 that is not perpendicular to the surface of the piezoelectric layer 3. The first surface is located above the electrode 4. The cover portion 5 is also in contact with the first surface.

[0025] Furthermore, the side surface of the electrode 4 has a second surface 42 near the surface of the piezoelectric layer 3, which is closer to perpendicular to the surface of the piezoelectric layer 3 than the first surface 41. The first surface and the second surface are continuous. The cover portion also contacts the second surface 42.

[0026] Here, the first surface 41 includes the second portion 52, and the second surface 42 includes the first portion 51. In other words, the angle between the second surface 42 and the direction perpendicular to the surface of the piezoelectric layer 3 is smaller than the angle between the first surface 41 and the direction perpendicular to the surface of the piezoelectric layer 3. Therefore, the thickness of the cover portion 5 is likely to increase at the second surface 42.

[0027] Because the first surface 41 is inclined with respect to the surface of the piezoelectric layer 3, the contact area between the electrode 4 and the cover portion 5 is increased. This reduces the formation of hillocks (protrusions) on the surface of the electrode 4, thereby improving power resistance against electromigration. Furthermore, because the second surface 42 is nearly perpendicular to the surface of the piezoelectric layer 3, the amount of deposition of the cover portion 5 increases. Therefore, when a protective film is formed to cover the piezoelectric layer 3 and the electrode 4, the coverage of the protective film near the second surface 42 is improved, thereby reducing electrolytic corrosion.

[0028] 3 is a diagram showing the elastic wave device 1 shown in FIG. 1 with a protective film 7 formed thereon. The protective film 7 covers the piezoelectric layer 3 and at least a portion of the electrode 4. Specifically, the protective film 7 covers the lower side of the electrode 4, i.e., the portion of the electrode 4 that is close to the piezoelectric layer 3, via the first portion 51. The protective film 7 is made of SiO ２ , SiN, SiON, AlN, ZrO ２ , Ta ２ O ５ , ZnO, Al ２ O ３ , and HfO ２ Various materials such as SiO ２ , SiN, AlN, Ta ２ O ５ etc.

[0029] (Composition of Cover Portion 5) As shown in the SEM image in Fig. 3, it is highly likely that the piezoelectric layer 3 and the cover portion 5 have similar compositions. This is thought to be due to redeposition of the portion removed by dry etching. In other words, it is thought that the electrode 4 is covered by the cover portion 5.

[0030] Next, in order to analyze the composition of the cover portion 5 in detail, Fig. 4 is an enlarged view of a part (reference numeral 8) of the photograph in Fig. 3. Fig. 5 shows the measured composition amount of tantalum on the line segment indicated by reference numeral 9 in Fig. 4.

[0031] As shown in the lower part of Figure 5, the measured value of the tantalum content increases between 60 nm and 80 nm. That is, it is clear that elemental tantalum is contained in this section of the cover portion 5. In fact, the shading changes between this section and the corresponding section in the upper part of Figure 5, which suggests that the composition is changing, and therefore it is believed that elemental tantalum is being measured.

[0032] The electrode 4 is considered to be covered by the composition of the piezoelectric layer 3. When the piezoelectric layer 3 is lithium tantalate, the elements constituting the cover portion 5 may include lithium or tantalum. When the piezoelectric layer 3 is lithium niobate, the elements constituting the cover portion 5 may include lithium or niobium. When the piezoelectric layer 3 is quartz, the elements constituting the cover portion 5 may include silicon. When the piezoelectric layer 3 is made of another piezoelectric material, the cover portion 5 may include various elements contained in the piezoelectric material. The various elements are, for example, metal elements. In other words, the cover portion 5 includes elements constituting the piezoelectric layer 3 other than oxygen and nitrogen. This description is sufficient as long as the cover portion 5 includes elements other than oxygen and nitrogen from the elements constituting the piezoelectric layer 3, and does not deny that the cover portion 5 may include oxygen and nitrogen.

[0033] When the piezoelectric layer 3 is made of lithium tantalate, the cover 5 may be made of lithium tantalate, i.e., tantalum oxide. When the piezoelectric layer 3 is made of lithium niobate, the cover 5 may be made of lithium niobate, i.e., niobium oxide.

[0034] 6 shows the results of comparing the power durability between cases where dry etching was performed and cases where dry etching was not performed. That is, the case where dry etching was performed was the case where acoustic wave device 1 was tested, and the case where dry etching was not performed was the case where acoustic wave device 100 was tested. Two types of tests were performed: measuring the breakdown power of the elements of acoustic wave device 1 or 100, and measuring the migration power at which electromigration begins to occur in electrode 4.

[0035] 6, the breakdown power and migration power increase as a result of dry etching, which indicates that the power resistance is improved by dry etching.

[0036] This is thought to be the effect of the cover portion 5 covering the electrode 4. In other words, the electrode 4 is covered with tantalum or niobium, which are elements with a higher density than the aluminum that constitutes the electrode 4, making it difficult for electromigration to occur.

[0037] In addition, the insulating distance is increased by covering the electrode 4 with the cover portion 5. Furthermore, the possibility of hillock formation is reduced by covering the electrode 4 with the cover portion 5.

[0038] Furthermore, by varying the inclination angle of the first surface 41 and the second surface 42 relative to the piezoelectric layer 3, it is possible to increase the deposition amount of the cover portion 5 or the area in contact with the cover portion 5. As a result, electromigration becomes less likely to occur.

[0039] (Manufacturing Method) The acoustic wave device 1 is manufactured by the following steps: First, a piezoelectric layer 3 is formed on a support substrate 2. Then, an aluminum film is formed on the piezoelectric layer 3, and the aluminum film is etched using a photoresist mask to form electrodes 4.

[0040] Thereafter, the thicknesses of the piezoelectric layer 3 and the electrodes 4 are measured using various thickness measurement methods in order to calculate the degree of etching required to obtain an acoustic wave device 1 whose frequency characteristics are adjusted to desired values.

[0041] The etching amount required to achieve the desired frequency characteristics is determined from the measured thickness, and the piezoelectric layer 3 and electrode 4 are dry-etched using an etching amount that varies for each element, thereby obtaining the acoustic wave device 1. The material removed from the piezoelectric layer 3 by dry etching is then attached to at least a portion of the side surface of the electrode 4, thereby forming the cover portion 5. In other words, dry etching adjusts the thickness of the piezoelectric layer 3 to achieve the desired frequency characteristics, while simultaneously forming the cover portion 5 that improves power durability. Therefore, this process is a minimum of steps that are originally required to achieve the desired frequency characteristics, and does not require multiple additional steps to be added to the manufacturing process.

[0042] 7 is a schematic diagram showing the cross-sectional structure of an elastic wave device 1a according to embodiment 2. Unlike elastic wave device 1, elastic wave device 1a has a portion of electrode 4 embedded in piezoelectric layer 3. Then, dry etching is performed in the same manner as embodiment 1, resulting in the formation of cover portion 5.

[0043] Therefore, the electrode 4 is covered by the piezoelectric layer 3 in the embedded portion in addition to the cover portion 5. That is, the electrode 4 is covered by tantalum or niobium, which are elements with a higher density than aluminum. As a result, the electrode 4 is less susceptible to electromigration, and its power durability is improved.

[0044] Third Embodiment FIG. 8 illustrates a schematic configuration of a communication device 151 according to a third embodiment. The communication device 151 is an application example of an elastic wave device according to an aspect of the present disclosure and performs wireless communication using radio waves. The communication device 151 may include one duplexer 101 as a transmit filter 109 and another duplexer 101 as a receive filter 111. Each of the two duplexers 101 may include an elastic wave device according to an aspect of the present disclosure. The elastic wave device may be the elastic wave device 1 or the elastic wave device 1a. In this manner, the communication device 151 may include an elastic wave device according to an aspect of the present disclosure.

[0045] In the communication device 151, a transmission information signal TIS containing information to be transmitted may be modulated and frequency-raised by an RF-IC (Radio Frequency-Integrated Circuit) 153 and converted into a transmission signal TS. The modulation and frequency-raising may be conversion into a high-frequency signal having a carrier frequency. A bandpass filter 155 may remove unnecessary components from the TS outside the transmission passband. Next, the TS after the unnecessary components have been removed may be amplified by an amplifier 157 and input to the transmission filter 109.

[0046] The transmission filter 109 may remove unnecessary components outside the transmission passband from the input transmission signal TS. The transmission filter 109 may output the TS after removing the unnecessary components to the antenna 159 via an antenna terminal. The antenna terminal may be, for example, the above-mentioned TCin. The antenna 159 may convert the TS, which is an electrical signal input thereto, into radio waves as a wireless signal and transmit the radio waves to the outside of the communication device 151.

[0047] Furthermore, the antenna 159 may convert the received external radio waves into a received signal RS, which is an electrical signal, and input the RS to the receiving filter 111 via the antenna terminal. The receiving filter 111 may remove unnecessary components outside the receiving passband from the input RS. The receiving filter 111 may output the received signal RS after the unnecessary components have been removed to the amplifier 161. The output RS may be amplified by the amplifier 161. The bandpass filter 163 may remove unnecessary components outside the receiving passband from the amplified RS. The RS after the unnecessary components have been removed may be frequency-downgraded and demodulated by the RF-IC 153, and converted into a received information signal RIS.

[0048] The TIS and RIS may be low-frequency signals (baseband signals) containing appropriate information. For example, the TIS and RIS may be analog audio signals or digitized audio signals. The passband of the wireless signals may be set appropriately and may comply with various known standards.

[0049] [Summary] The elastic wave device according to aspect 1 of the present disclosure includes a piezoelectric layer, an electrode provided on the piezoelectric layer, and a cover portion that covers at least a portion of a side surface of the electrode and contains elements that constitute the piezoelectric layer other than oxygen and nitrogen.

[0050] According to the above configuration, the cover can reduce the risk of electromigration of the electrodes, thereby improving the power durability of the acoustic wave device.

[0051] According to a second aspect of the present disclosure, in the elastic wave device of the first aspect, the piezoelectric layer may contain lithium tantalate, and the element may be lithium or tantalum.

[0052] According to a third aspect of the present disclosure, in the elastic wave device of the first aspect, the piezoelectric layer may contain lithium niobate, and the element may be lithium or niobium.

[0053] An elastic wave device according to a fourth aspect of the present disclosure is the same as the first aspect, wherein the piezoelectric layer includes quartz crystal and the element is silicon.

[0054] According to the above configuration, the cover portion contains some of the elements that constitute the piezoelectric layer, and therefore the elements that constitute the piezoelectric layer that are redeposited by dry etching cover the electrodes.

[0055] According to a fifth aspect of the present disclosure, in the elastic wave device of the first or second aspect, the piezoelectric layer may be made of lithium tantalate, and the cover may include tantalum oxide.

[0056] According to a sixth aspect of the present disclosure, in the elastic wave device of the first or third aspect, the piezoelectric layer may be made of lithium niobate, and the cover portion may include niobium oxide.

[0057] According to the above configuration, the cover portion contains a compound of the elements that constitute the piezoelectric layer, and therefore the compound related to the piezoelectric layer redeposited by dry etching covers the electrode.

[0058] According to a seventh aspect of the present disclosure, in the elastic wave device of any one of the first to sixth aspects, the density of the cover portion may be greater than the density of the electrodes.

[0059] According to the above configuration, the electrode is covered by the cover portion made of an element having a higher density than the electrode, thereby making it possible to prevent electromigration of the electrode.

[0060] An elastic wave device according to an eighth aspect of the present disclosure is any one of the first to seventh aspects, wherein the thickness of the thickest portion of the cover is 3 nm or more.

[0061] With this configuration, the cover can be made 3 nm or thicker, which is sufficient to cover the electrodes, thereby improving the power durability of the acoustic wave device.

[0062] An elastic wave device according to aspect 9 of the present disclosure may be any one of aspects 1 to 8, wherein the cover portion has a first portion and a second portion that is located above the electrode and is thinner than the first portion.

[0063] According to the above configuration, the cover portion can be made thicker on the piezoelectric layer side.

[0064] An elastic wave device according to aspect 10 of the present disclosure is any one of aspects 1 to 9, and has a protective film that covers the piezoelectric layer and at least a portion of the electrode, and the protective film may cover a portion of the electrode that is close to the piezoelectric layer via a first portion.

[0065] According to the above-mentioned configuration, the coverage of the protective film in the vicinity of the lower side of the electrode is improved, and the effect of reducing electrolytic corrosion is obtained.

[0066] An elastic wave device according to aspect 11 of the present disclosure is any one of aspects 1 to 10, wherein the side of the electrode has a first surface that is non-perpendicular to the surface of the piezoelectric layer, and the cover portion is in contact with the first surface.

[0067] According to the above configuration, the side surface of the electrode can have a first surface that is non-perpendicular to the surface of the piezoelectric layer, thereby increasing the contact area between the electrode and the cover.

[0068] An elastic wave device according to aspect 12 of the present disclosure is the same as aspect 11, except that the side of the electrode is located near the surface of the piezoelectric layer and further has a second surface continuous with the first surface, the angle between the direction perpendicular to the surface of the piezoelectric layer and the second surface is smaller than the angle between the perpendicular direction and the first surface, and the cover portion may be in contact with the second surface.

[0069] According to the above configuration, the side surface of the electrode can have a second surface that is closer to perpendicular than the first surface near the surface of the piezoelectric layer, thereby increasing the deposition amount of the cover portion on the second surface.

[0070] The elastic wave device according to aspect 13 of the present disclosure is any one of aspects 1 to 12, wherein the thickness of the electrode region in the piezoelectric layer may be greater than the thickness of the region in the piezoelectric layer where the surface is exposed.

[0071] According to the above configuration, the thickness of the piezoelectric layer can be made thinner in the region where the surface of the piezoelectric layer is exposed than in the region where the electrode overlaps, which means that the piezoelectric layer can be dry-etched together with the electrode, making processing easier.

[0072] The elastic wave device according to aspect 14 of the present disclosure is any one of aspects 1 to 13, wherein the difference between the thickness of the electrode region and the thickness of the region where the surface of the piezoelectric layer is exposed may be 50 nm or less.

[0073] According to the above configuration, the difference in thickness between the piezoelectric layer in the region where the electrode overlaps and the piezoelectric layer in the region where the surface of the piezoelectric layer is exposed can be set to 50 nm or less.

[0074] An elastic wave device according to aspect 15 of the present disclosure may be any one of aspects 1 to 14, further comprising a support substrate supporting the piezoelectric layer, the electrode, and the cover portion, the electrode being an electrode finger, the thickness of the piezoelectric layer being equal to or less than one wavelength determined by the pitch of the electrode fingers, and the piezoelectric layer being configured to excite a plate wave.

[0075] According to the above configuration, the elastic wave device can be configured to function as an elastic wave device. The elastic wave device according to Aspect 16 of the present disclosure may excite waves in any one of Aspects 1 to 14. The elastic wave device according to Aspect 17 of the present disclosure may excite bulk waves in any one of Aspects 1 to 14.

[0076] A communication device according to an eighteenth aspect of the present disclosure may include the acoustic wave device according to any one of the first to seventeenth aspects.

[0077] A manufacturing method according to aspect 19 of the present disclosure includes the steps of forming a piezoelectric layer, forming an electrode on the piezoelectric layer, dry etching the piezoelectric layer and the electrode, and attaching material removed from the piezoelectric layer by the dry etching to at least a portion of a side surface of the electrode.

[0078] [Additional Notes] The invention according to the present disclosure has been described above based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. In other words, the invention according to the present disclosure can be modified in various ways within the scope of the present disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that a person skilled in the art could easily make various modifications or corrections based on the present disclosure. It should also be noted that these modifications or corrections are included in the scope of the present disclosure.

[0079] REFERENCE SIGNS LIST 1, 100 Acoustic wave device 2 Support substrate 3 Piezoelectric layer 4 Electrode 5 Cover portion 6 Multilayer film 6a Low acoustic impedance layer 6b High acoustic impedance layer 7 Protective film 41 First surface 42 Second surface 51 First portion 52 Second portion

Claims

1. A piezoelectric layer; an electrode provided on the piezoelectric layer; a cover portion covering at least a portion of a side surface of the electrode and containing an element constituting the piezoelectric layer other than oxygen and nitrogen.

2. The acoustic wave device according to claim 1 , wherein the piezoelectric layer contains lithium tantalate, and the element is lithium or tantalum.

3. The acoustic wave device according to claim 1 , wherein the piezoelectric layer contains lithium niobate, and the element is lithium or niobium.

4. The acoustic wave device according to claim 1 , wherein the piezoelectric layer contains quartz crystal and the element is silicon.

5. The acoustic wave device according to claim 1 , wherein the piezoelectric layer is made of lithium tantalate, and the cover portion contains tantalum oxide.

6. The acoustic wave device according to claim 1 , wherein the piezoelectric layer is made of lithium niobate, and the cover portion contains niobium oxide.

7. The acoustic wave device according to claim 1 , wherein a density of the cover portion is greater than a density of the electrodes.

8. The acoustic wave device according to claim 1 , wherein the cover has a thickness of 3 nm or greater at its thickest portion.

9. The acoustic wave device according to claim 1 , wherein the cover portion has a first portion and a second portion that is located above the electrode relative to the first portion and is thinner than the first portion.

10. a protective film that covers the piezoelectric layer and at least a part of the electrode; The acoustic wave device according to claim 1 , wherein the protective film covers a portion of the electrode close to the piezoelectric layer via a first portion.

11. a side surface of the electrode having a first surface that is non-perpendicular to a surface of the piezoelectric layer; The acoustic wave device according to claim 1 , wherein the cover is in contact with the first surface.

12. a side surface of the electrode is located near a surface of the piezoelectric layer and further has a second surface continuous with the first surface; an angle between a direction perpendicular to a surface of the piezoelectric layer and the second surface is smaller than an angle between the perpendicular direction and the first surface; The acoustic wave device according to claim 11 , wherein the cover portion is in contact with the second surface.

13. The acoustic wave device according to claim 1 , wherein a thickness of the piezoelectric layer in a region where the electrodes are provided is greater than a thickness of a region where the surface of the piezoelectric layer is exposed.

14. The acoustic wave device according to claim 1 , wherein a difference between a thickness of the electrode region and a thickness of a region where the surface of the piezoelectric layer is exposed is 50 nm or less.

15. a support substrate that supports the piezoelectric layer, the electrode, and the cover portion, the electrodes are electrode fingers, a thickness of the piezoelectric layer is equal to or smaller than one wavelength determined by the pitch of the electrode fingers; The acoustic wave device according to claim 1 , wherein the piezoelectric layer is configured to excite a plate wave.

16. The acoustic wave device according to claim 1 , which excites a wave.

17. The acoustic wave device according to claim 1 , which excites bulk waves.

18. A communication device comprising the acoustic wave device according to claim 1 .

19. forming a piezoelectric layer; forming an electrode on the piezoelectric layer; dry etching the piezoelectric layer and the electrode, and adhering material removed from the piezoelectric layer by the dry etching to at least a portion of a side surface of the electrode.