Surface acoustic wave device and method of manufacturing the same

Abstract

An elastic surface wave device includes a piezoelectric substrate and a metal pattern having an IDT formed on the piezoelectric substrate. The IDT has a pair of comb electrodes of a plurality of electrode fingers interdigitated with each other, and the metal pattern includes a first metal and a second metal covered by the first metal. Thus, an elastic surface wave device with high reliability is provided, and generation of small protrusions can be sufficiently suppressed and sufficient power resistance can be ensured.

Description

Technical Field

[0001] This invention relates to an elastic surface acoustic wave (SAW) element and its manufacturing method. Background Technology

[0002] The functions of representative mobile communication terminals such as smartphones have been developing towards higher levels of functionality year by year. As a result, the number of electronic components used is on the rise.

[0003] As described in many specifications, the basic structure of a surface wave element is to form an interdigital transducer (IDT) that excites an elastic surface wave on a piezoelectric substrate such as lithium tantalate or lithium niobate, thereby creating a resonator. The IDT can be formed by forming a conductive film such as aluminum (Al) and then etching it using photoresist as a mask. Sometimes, multiple resonators are appropriately formed, employing a dual-mode surface wave (DMS) design or a trapezoidal design to obtain the desired bandpass filter characteristics.

[0004] Furthermore, high electrical conductivity is sometimes required for elastic surface wave elements. When the IDT is made of aluminum, as was conventionally used, migration can occur due to displacement caused by applied power. As a result, the IDT may crack, leading to a problem of low electrical conductivity.

[0005] For example, according to Patent Document 1 (International Publication No. 2009 / 150786), in order to provide an elastic surface wave element that achieves miniaturization, improved electrical resistance and is less prone to protrusions such as hillocks, an aluminum-copper alloy film with a specified amount of copper is sometimes used to form an IDT. Summary of the Invention

[0006] [The problem the invention aims to solve]

[0007] This invention addresses the main problem it aims to solve. When pure aluminum is used as the metal material for the patterning of an elastic surface wave element or an integrated circuit (IDT), small bumps form in the patterning or IDT, resulting in poor electrical withstand capability. These small bumps are caused by film stress due to thermal processes or by the application of high voltages. These bumps reduce the electrical withstand capability of the IDT.

[0008] Therefore, according to Patent Document 1, the following technology is disclosed: In the past, copper was added in order to suppress the generation of small bumps and improve electrical resistance.

[0009] However, due to the increased resistance or the impact of anti-oxidation measures on transmission characteristics, it is difficult to meet the recent requirements for miniaturization and high-frequency operation of surface wave elements, thus failing to provide sufficient dielectric strength. Furthermore, it is also impossible to adequately suppress the formation of small protrusions.

[0010] The present invention was made in view of the aforementioned problems, and aims to provide a highly reliable elastic surface wave element that effectively suppresses the generation of small protrusions and ensures sufficient electrical resistance.

[0011] [Methods used to solve problems]

[0012] To address the aforementioned problem, the present invention provides an elastic surface wave element, comprising:

[0013] piezoelectric substrate; and

[0014] A metallic pattern is formed on the piezoelectric substrate and has an IDT, the IDT having a pair of comb-shaped electrodes with multiple intersecting electrode fingers;

[0015] The metal pattern comprises a first metal and a second metal.

[0016] The second metal is covered by the first metal.

[0017] One form of the elastic surface wave element of the present invention has the characteristic that the coefficient of linear expansion of the first metal is smaller than that of the second metal.

[0018] One form of the elastic surface wave element of the present invention, wherein the first metal is titanium or a titanium alloy.

[0019] One embodiment of the elastic surface wave element of the present invention, wherein the second metal is aluminum, copper, or an alloy comprising any of the foregoing.

[0020] One form of the elastic surface wave element of the present invention, wherein the piezoelectric substrate is a substrate containing lithium tantalate or lithium niobate.

[0021] In one embodiment of the elastic surface wave element of the present invention, the piezoelectric substrate is bonded to a substrate comprising sapphire, silicon, alumina, spinel, crystal or glass on a main surface opposite to the surface on which the metal pattern is formed.

[0022] One embodiment of the elastic surface wave element of the present invention includes a metal pattern having bump pads and pattern wiring that electrically connects the IDT to the bump pads.

[0023] Furthermore, another aspect of the present invention is a method for manufacturing an elastic surface wave element, comprising:

[0024] Step (a): On a piezoelectric substrate, a metal pattern with a recessed shape of an IDT is formed using a first metal, wherein the IDT has a pair of comb-shaped electrodes with multiple intersecting electrode fingers.

[0025] Step (b): Fill the recessed shape formed by the first metal with the second metal; and

[0026] Step (c): Form the first metal on the second metal.

[0027] In one embodiment of the method for manufacturing the elastic surface wave element of the present invention, step (a) comprises the following steps:

[0028] A sacrificial layer is formed on the piezoelectric substrate; and

[0029] The sacrificial layer is patterned to partially expose the piezoelectric substrate.

[0030] In one embodiment of the method for manufacturing the elastic surface wave element of the present invention, in step (a), the bottom and sidewall portions of the recessed shape are formed together by the first metal.

[0031] One embodiment of the method for manufacturing the elastic surface wave element of the present invention includes, after step (b), a step of grinding the second metal until the sacrificial layer is exposed.

[0032] [Invention Effects]

[0033] According to the present invention, a highly reliable elastic surface wave element can be provided that effectively suppresses the generation of small protrusions and ensures sufficient electrical resistance. Attached Figure Description

[0034] Figure 1 This is a cross-sectional view of the elastic surface wave element in this embodiment.

[0035] Figure 2 This is a top view used to illustrate the structure of a resonator containing an IDT.

[0036] Figure 3 It is a top view showing a general layout of a piezoelectric substrate containing patterned wiring.

[0037] Figure 4 (a)- Figure 4 (c) is a diagram illustrating the manufacturing method of the elastic surface wave element of the present invention.

[0038] Figure 5 (a)- Figure 5 (b) is a cross-sectional view of the IDT using the present invention and the IDT as a comparative example.

[0039] Figure 6This is a diagram showing the stress generated in the IDT of the present invention.

[0040] Figure 7 This is a graph showing the stress generated in the IDT of the comparative example. Detailed Implementation

[0041] The present invention will now be explained by referring to the accompanying drawings and specific embodiments thereof.

[0042] (Example)

[0043] Figure 1 This is a cross-sectional view of the elastic surface wave element 1 in this embodiment. (See image below.) Figure 1 As shown, the elastic surface wave element 1 in this embodiment has a piezoelectric substrate 3.

[0044] In this embodiment, the piezoelectric substrate 3 comprises a lithium tantalate single crystal. The thickness of the piezoelectric substrate 3 may be, for example, 20 μm.

[0045] The piezoelectric substrate 3 can also be formed using other piezoelectric single crystals, such as lithium niobate or crystal, or piezoelectric ceramics.

[0046] On a main surface of the piezoelectric substrate 3 of the elastic surface wave element 1 in this embodiment, a metal pattern is formed. The metal pattern has an IDT5, which is a pair of comb-shaped electrodes with multiple intersecting electrode fingers.

[0047] Figure 2 This is a top view used to illustrate the structure of a resonator containing IDT5.

[0048] like Figure 2 As shown, an IDT5 and a reflector 5a are formed on a piezoelectric substrate 3. The IDT5 has a pair of comb-shaped electrodes 5b facing each other. The comb-shaped electrodes 5b have multiple electrode fingers 5c and a bus bar 5d connecting the multiple electrode fingers 5c. The reflector 5a is disposed on both sides of the IDT5.

[0049] like Figure 1 As shown, IDT5 comprises a first metal 51 and a second metal 52. The second metal 52 is covered by the first metal 51. In this embodiment, titanium (Ti) is used as the first metal and aluminum (Al) is used as the second metal. IDT5 is a thin film with a thickness of, for example, 150 nm to 400 nm.

[0050] The coefficient of linear expansion of titanium (Ti) is approximately 8.5 × 10⁻⁶. -6 / K. The coefficient of linear expansion of aluminum (Al) is approximately 23.9 × 10⁻⁶. -6 / K. Therefore, the coefficient of linear expansion of titanium (Ti) is smaller than that of aluminum (Al).

[0051] The first metal 51 may include, for example, suitable metals (including semi-metals) or alloys thereof, such as silver, copper, tungsten, molybdenum, chromium, zirconium, iridium, and antimony, in addition to titanium alloys and iron-nickel alloys (including invar alloys), and may also be formed from alloys thereof.

[0052] The second metal 52, for example, may include, in addition to aluminum alloys, suitable metals such as silver, copper, iron, nickel, or alloys thereof, or may be formed from alloys thereof.

[0053] On one main surface of the piezoelectric substrate 3 of the elastic surface wave element 1 in this embodiment, a pattern wiring 7 containing a metal pattern is formed. Figure 3 This is a top view showing a general layout of the piezoelectric substrate 3 containing the patterned wiring 7.

[0054] like Figure 3 As shown, a pattern wiring 7, an IDT 5, and a reflector 5a, which are contained in a metal pattern, are formed on the piezoelectric substrate 3.

[0055] The pattern wiring 7, IDT5 and reflector 5a formed on the piezoelectric substrate 3 can be appropriately designed using DMS design or trapezoidal design to obtain the desired characteristics of the bandpass filter.

[0056] like Figure 3 As shown, pattern wiring 7 includes input pad In, output pad Out, and ground pad GND. Additionally, pattern wiring 7 is electrically connected to IDT5.

[0057] like Figure 1 As shown, in this embodiment, the pattern wiring 7 includes a first metal 71 and a second metal 72. The second metal 72 is covered by the first metal 71. In this embodiment, titanium (Ti) is used as the first metal and aluminum (Al) is used as the second metal in the pattern wiring 7. The pattern wiring 7 is a thin film with a thickness of, for example, 150 nm to 400 nm.

[0058] The first metal 71 may include, for example, suitable metals (including semi-metals) or alloys thereof, such as silver, copper, tungsten, molybdenum, chromium, zirconium, iridium, and antimony, in addition to titanium alloys and iron-nickel alloys (including invar alloys), or may be formed from alloys thereof.

[0059] The second metal 72, for example, may include, in addition to aluminum alloys, suitable metals such as silver, copper, iron, nickel, or alloys thereof, or may be formed from alloys thereof.

[0060] like Figure 1 As shown, bumps 9 are respectively bonded to the input pad In, output pad Out, and ground pad GND of the pattern wiring 7 of the elastic surface wave element 1 in this embodiment.

[0061] In this embodiment, the bump 9 contains gold (Au). The height of the bump 9 is, for example, 20 μm to 50 μm.

[0062] like Figure 1 As shown, the elastic surface wave element 1 in this embodiment has a wiring substrate 11. The wiring substrate 11 is an insulating substrate, such as a ceramic substrate or resin substrate such as HTCC (High Temperature Co-Fired Ceramic) or LTCC (Low Temperature Co-Fired Ceramic).

[0063] The wiring substrate 11 has multiple land pads 11a on one main surface and multiple external connection terminals 11b on the other main surface. The piezoelectric substrate 3 is mounted on the wiring substrate 11 via bumps 9. The piezoelectric substrate 3 is electrically connected to the external connection terminals 11b via bumps 9 and land pads 11a.

[0064] like Figure 1 As shown, the elastic surface wave element 1 of this embodiment has a sealing portion 13 for sealing the space between the piezoelectric substrate 3 and the wiring substrate 11 to make it a sealed hollow space. The sealing portion 13 is disposed on the wiring substrate 11 in a manner that surrounds the piezoelectric substrate 3.

[0065] The sealing part 13 may be formed of an insulator such as synthetic resin, or it may be made of metal. For example, a brazing material such as tin-silver solder or gold-tin solder may be used.

[0066] Synthetic resins such as epoxy resin and polyimide can be used, but are not limited to these. Epoxy resin is preferred, and the sealing part 13 is formed using a low-temperature curing process.

[0067] like Figure 1 As shown, in this embodiment, the piezoelectric substrate 3 of the elastic surface wave element 1 is bonded to the support substrate 15 on its main surface opposite to the surface where the metal pattern is formed. In this embodiment, the support substrate 15 is a substrate containing sapphire. The support substrate 15 may also be formed using, for example, silicon, polycrystalline alumina, polycrystalline spinel, crystal, or glass.

[0068] Next, a method for manufacturing the elastic surface wave element 1, which is another aspect of the present invention, will be described.

[0069] (Manufacturing Method 1)

[0070] Figure 4 (a) to Figure 4 (c) is a diagram illustrating a method for manufacturing the elastic surface wave element of the present invention. Additionally, Figure 4 (a) to Figure 4 (c) represents a cross-section along the direction of travel of the elastic surface wave.

[0071] Figure 4 (a) to Figure 4 (c) is a diagram illustrating the manufacturing method of the elastic surface wave element 1 in this embodiment.

[0072] The following uses Figure 4 (a) to Figure 4 (c) The manufacturing method of the elastic surface wave element 1 in this embodiment is explained. In addition, a sapphire substrate that has been activated by FAB (Fast Atomic Beam) or the like is bonded to the piezoelectric substrate 3, but it is not shown.

[0073] like Figure 4 As shown in (a), a patterned sacrificial layer S is formed on the piezoelectric substrate 3, thereby partially exposing the piezoelectric substrate 3. Next, a first metal M1 film is formed on the piezoelectric substrate 3 and the sacrificial layer S.

[0074] The sacrificial layer S can be, for example, silicon. The sacrificial layer S is patterned, for example, by etching the area where the metal pattern is formed using photolithography.

[0075] The first metal M1 can be, for example, titanium (Ti). The first metal M1 is deposited and formed into a film, for example, by sputtering. The first metal M1 is continuously formed on the piezoelectric substrate 3 and on the upper surface and side surface of the sacrificial layer S. That is, the first metal M1 forms a concave bottom and sidewall portion together. The first metal M1 can be set to a thickness of, for example, 20 nm to 30 nm.

[0076] like Figure 4 As shown in (b), the second metal M2 is filled into the recessed shape of the first metal M1. Then, the second metal M2 and the first metal M1 are ground until the sacrificial layer S is exposed. The grinding can be performed, for example, using CMP (Chemical Mechanical Polishing).

[0077] The second metal M2 can be, for example, aluminum (Al). The second metal M2 is filled into the recessed shape portion of the first metal M1, for example, by plating.

[0078] Here, while filling the concave shape portion of the first metal M1 with the second metal M2, a film of the second metal M2 is also formed on the first metal M1 that is formed on the upper surface of the sacrificial layer S. The first metal M1 and the second metal M2 are polished until the upper surface of the sacrificial layer S is exposed.

[0079] The second metal M2, which fills the concave shape portion of the first metal M1, can be set to a thickness of, for example, 300 nm to 400 nm.

[0080] like Figure 4 As shown in (c), a first metal M1 is formed on the second metal M2. The first metal M1 is deposited by sputtering, for example, by vapor deposition. The thickness of the first metal M1 can be set to, for example, 20 nm to 30 nm.

[0081] Next, as Figure 4 As shown in (c), in order to remove the first metal M1 formed on the area other than the second metal M2, the photoresist PR is patterned on the metal pattern.

[0082] Next, the recessed portion of the first metal M1 and the area outside the second metal M2 are removed by etching. Then, by removing the photoresist PR, a metal pattern in which the second metal M2 is covered by the first metal M1 can be formed.

[0083] In addition, using Figure 4 (c) The steps described can also be performed as follows: patterning photoresist on an area outside the metal pattern, and then forming the first metal M1 on the recessed shape portion of the first metal M1 and the second metal M2. In this case, after forming the first metal M1 on the recessed shape portion of the first metal M1 and the second metal M2, removing the photoresist and peeling off the excess first metal M1 allows for the formation and use of [the metal]. Figure 4 (c) The metal pattern in which the second metal M2 is covered by the first metal M1, with the same steps as described.

[0084] Subsequently, gold bumps are joined on the solder pads of the patterned wiring using a bonding device.

[0085] Then, the piezoelectric substrate 3 is monolithically formed and flip-chip mounted on the wiring substrate 11 array. A sealing material is filled and hardened, and then monolithically formed to obtain the elastic surface wave element 1.

[0086] Next, the effects of the present invention will be explained.

[0087] Figure 5 These are cross-sectional views of the IDT using the present invention and an IDT as a comparative example. Figure 5 (a) is a cross-sectional view of the IDT using the present invention. Figure 5 (b) is a cross-sectional view of the IDT of the comparative example used for comparison with the effects of the present invention. The width of both the IDT of the present invention and the IDT of the comparative example is 500 nm.

[0088] Furthermore, both the IDT of the present invention and the IDT of the comparative example are formed on the piezoelectric substrate 3. In addition, both the IDT of the present invention and the IDT of the comparative example have a titanium (Ti) film with a thickness of 25 nm formed in the upper and lower portions of the cross-section in the direction of travel of the elastic surface wave, and an aluminum (Al) film with a thickness of 350 nm formed in the central portion.

[0089] like Figure 5 As shown in (a), the IDT structure of the present invention has a titanium (Ti) film with a thickness of 25 nm formed on both side walls, and the aluminum (Al) in the central part is completely covered by the titanium (Ti) film.

[0090] Figure 6 It means to use Figure 5 (a) A diagram of the stress generated in the IDT of the present invention as illustrated.

[0091] Figure 7 It means to use Figure 5 (b) A diagram of the stress generated in the IDT of the comparative example described.

[0092] Power was applied to both the IDT of the present invention and the IDT of the comparative example in the same manner, and the stress was measured. The results are as follows: Figure 6 and Figure 7 As shown, the maximum stress was generated at the lower end of the IDT in both structures.

[0093] like Figure 6 As shown, the maximum stress generated in the IDT using this invention is 1.1 x 10⁻⁶. 9 [N / m] 2 On the other hand, such as Figure 7 As shown, the maximum stress generated in the comparative example's IDT was 1.32 x 10⁻⁶. 9 [N / m] 2 That is, compared with the IDT of the comparative example, the IDT of the present invention can significantly suppress stress.

[0094] Under the influence of stress applied to the IDT, pores or small protrusions may form and migrate, leading to IDT rupture. That is, by suppressing the stress applied to the IDT, the electrical resistance of the elastic surface wave element can be improved.

[0095] Based on the structure of the present invention described above, a highly reliable elastic surface wave element can be provided that effectively suppresses the generation of small protrusions and ensures sufficient electrical resistance.

[0096] Furthermore, the present invention is not limited to the embodiments described above, but includes embodiments that can achieve the objectives of the present invention.

[0097] Furthermore, while at least one embodiment has been described above, it should be understood that various changes, modifications, or improvements will readily conceive of by those skilled in the art. These changes, modifications, or improvements are also part of this disclosure and fall within the scope of the invention. It should be understood that the embodiments of the methods or apparatus described herein are not limited to the architecture and arrangement of the constituent elements described above or illustrated in the accompanying drawings. Methods and apparatus can be installed or implemented in other embodiments. The embodiments described are for illustrative purposes only and are not intended to be limiting. Moreover, the descriptions or terms used herein are for illustrative purposes only and are not intended to be limiting. The use of "comprising," "possessing," "having," "including," and variations thereof herein means to include the items listed below, their equivalents, and additional items. The term "or," or any term used in the description of "or," can be interpreted as meaning one, more than one, or all of the descriptive terms. References to front, back, left, right, top, bottom, upper, lower, and horizontal and vertical are for convenience of description and are not intended to limit the position and spatial configuration of any constituent element in the invention. Therefore, the above description and accompanying drawings are merely illustrative.

Claims

1. An elastic surface wave element, comprising: Piezoelectric substrate; and A metallic pattern is formed on the piezoelectric substrate and has an IDT, the IDT having a pair of comb-shaped electrodes with multiple intersecting electrode fingers; The metal pattern comprises a first metal and a second metal. The second metal is disposed inside the first metal and is completely covered by the first metal. The thickness of the first metal is 20nm-30nm, and the thickness of the second metal is 300nm-400nm.

2. The elastic surface wave element according to claim 1, characterized in that: The elastic surface wave element has the characteristic that the coefficient of linear expansion of the first metal is smaller than that of the second metal.

3. The elastic surface wave element according to claim 1, characterized in that: The first metal is titanium or a titanium alloy.

4. The elastic surface wave element according to claim 1, characterized in that: The second metal is aluminum, copper, or an alloy containing any of the foregoing.

5. The elastic surface wave element according to claim 1, characterized in that: The piezoelectric substrate is a substrate containing lithium tantalate or lithium niobate.

6. The elastic surface wave element according to claim 1, characterized in that: The piezoelectric substrate is bonded to a substrate containing sapphire, silicon, aluminum oxide, spinel, crystal, or glass on its main surface opposite to the surface where the metal pattern is formed.

7. The elastic surface wave element according to claim 1, characterized in that: The metal pattern has bump pads and pattern wiring that electrically connects the IDT to the bump pads.

8. A method for manufacturing an elastic surface wave element, comprising: Step (a): On a piezoelectric substrate, a metal pattern with a recessed shape of an IDT is formed using a first metal, the first metal covering the recess, the IDT having a pair of comb-shaped electrodes with multiple intersecting electrode fingers, and the thickness of the first metal being 20nm-30nm. Step (b): Fill the recessed shape formed by the first metal with a second metal, the thickness of the second metal being 300nm-400nm; and Step (c): Form the first metal on the second metal, wherein the thickness of the first metal is 20 nm-30 nm.

9. The method for manufacturing an elastic surface wave element according to claim 8, characterized in that: Step (a) includes the following steps: A sacrificial layer is formed on the piezoelectric substrate; and The sacrificial layer is patterned to partially expose the piezoelectric substrate.

10. The method for manufacturing an elastic surface wave element according to claim 8, characterized in that: In step (a), the bottom and sidewall portions of the recess shape are formed together by the first metal.

11. The method for manufacturing an elastic surface wave element according to claim 9, characterized in that: Following step (b), a step of grinding the second metal until the sacrificial layer is exposed is also included.

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