JP2010278971A - Elastic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an elastic wave device in which a hollow space has high liquid hermeticity by preventing flux from flowing into the inside of the hollow space when solder bumps are used for mounting. <P>SOLUTION: This elastic wave device is provided with: a substrate 11; a vibrating section 14 formed on one main surface 11a of the substrate 11; a pad 13 electrically connected to electrodes 12 of the vibrating section 14; a supporting layer 20 provided so as to surround the vibrating section 14; a sheet-like cover layer 22 provided so as to cover the vibrating section 14, having a hollow space 19 formed around the vibrating section 14, containing at least a synthetic rubber and a resin and having 10-25 wt.% of weight ratio to the entire of the synthetic rubber; a protective layer 24 made of resin having flux resistance; a via conductor 16 penetrating through the protective layer 24, the cover layer 22 and the supporting layer 20 and connected to the pad 13; and external electrodes 18 provided on the end of protective layer 24 side of the via conductor 16 and made of a solder bump. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an elastic wave device, and more particularly, to an elastic wave device in which a vibrating part such as a resonator or a filter is formed on a substrate.

  Conventionally, an elastic wave device configured to cover a vibrating portion formed on a substrate with a cover layer has been proposed, for example, as in Patent Document 1.

  FIG. 7 shows a configuration example of the elastic wave device. 7A is a cross-sectional view, FIG. 7B is a cross-sectional view of the upper side along line XX in FIG. 7A, and FIG. 7C is a line X- in FIG. 7A. FIG. 6 is a cross-sectional view of the lower side along X. In the acoustic wave device 110 of FIG. 7, a conductive pattern including an IDT electrode 112, a pad 113, and a wiring line 118 is formed on one main surface 111 a of the piezoelectric substrate 111. Further, the vicinity of the portion of the piezoelectric substrate 111 where the IDT electrode 112 is formed constitutes a vibrating portion. A frame-like support layer 116 is formed of resin so that a hollow space 119 is formed around the vibration part. An insulating sheet cover layer 115 is formed on the support layer 116. An external electrode 117 is provided on the cover layer 115, and the external electrode 117 and the pad 113 are electrically connected by a via conductor 114 that penetrates the cover layer 115 and the support layer 116.

JP 2002-261582 A

  In the acoustic wave device having such a configuration, when solder bumps are provided on external electrodes and mounted on another circuit board or the like, flux is applied to the solder bumps to improve solder wettability. However, there is a problem that the flux permeates the cover layer and flows into the hollow space. In addition, depending on the material of the cover layer, there is a problem that the liquid tightness of the hollow space cannot be secured, for example, the flux cleaning agent used when forming the solder bumps penetrates into the hollow space or the characteristics deteriorate after the pressure cooker test. It was.

  In view of such circumstances, the present invention intends to provide an elastic wave device that suppresses the inflow of flux into the hollow space when mounted using solder bumps and has high liquid tightness in the hollow space. .

  The present invention provides an elastic wave device configured as follows.

  The acoustic wave device includes a substrate, a vibrating portion formed on one main surface of the substrate, and a pad formed on the one main surface of the substrate and electrically connected to the electrode of the vibrating portion. A support layer that is provided on the one main surface of the substrate so as to surround the periphery of the vibration part, and has a thickness larger than the thickness of the vibration part, and on the support layer so as to cover the vibration part A sheet-like cover layer that has a hollow space around the vibration part, includes at least a synthetic rubber and a resin, and a weight ratio of the synthetic rubber to the whole is 10 wt% to 25 wt%; A protective layer made of a resin having flux resistance, and a via conductor that penetrates the protective layer, the cover layer, and the support layer and is connected to the pad. And in front of the via conductor Provided at an end portion of the protective layer side, and comprising: an external electrode made of solder bumps, the.

  In the present invention, a protective layer made of a resin having flux resistance is provided on a sheet-like cover layer made of a resin containing a synthetic rubber. Since the flux does not pass through the protective layer, the flux can be prevented from flowing into the hollow space. In the present invention, the weight ratio of the synthetic rubber in the cover layer is 10 wt% to 25 wt% of the entire cover layer. Thereby, the penetration | invasion to the inside of the hollow space of the flux cleaning agent used at the time of solder bump formation can be prevented.

  In the present invention, the weight ratio of the synthetic rubber to the entire cover layer is 10 wt% to 15 wt%.

  In this case, it is possible to prevent the organic substance from flowing into the hollow space after the pressure cooker test.

  In the present invention, the substrate is a piezoelectric substrate, and the vibration part includes an IDT electrode.

  In this case, the acoustic wave device is a surface wave device.

  In the present invention, the substrate is an insulating substrate, and the vibration part includes a piezoelectric thin film having electrodes formed on both sides.

  In this case, the acoustic wave device is a bulk acoustic wave device such as a bulk acoustic wave resonator (BAW resonator).

  In the present invention, the protective layer is made of the same material as the support layer.

  In this case, since the protective layer and the support layer are made of the same material, the types of materials can be reduced, and the manufacturing process can be simplified.

  In the present invention, the protective layer is made of a polyimide resin.

  In this case, the difference in luminance profile between the solder bump and the protective layer becomes large, and it becomes easy to recognize the solder bump. Therefore, the elastic wave device can be mounted with higher accuracy than when the outer shape of the elastic wave device is recognized.

  In the present invention, the cover layer is made of a non-photosensitive epoxy resin.

  In this case, a low temperature curing process is possible.

  The present invention is further characterized by further comprising a nitride film or an oxide film interposed in at least a part between the substrate and the support layer.

  In this case, since the nitride film or the oxide film has a rougher surface than the piezoelectric substrate, the adhesion is improved by the anchor effect. Thereby, in the process step after forming the hollow space, it is possible to prevent a problem that the plating solution enters the hollow space and causes a characteristic defect.

  In the elastic wave device according to the present invention, a protective layer made of a resin having flux resistance is provided on the cover layer. Since the flux does not pass through the protective layer, the flux can be prevented from flowing into the hollow space.

  In addition, the flux cleaning agent can be prevented from entering the hollow space by setting the weight ratio of the synthetic rubber in the cover layer to 10 wt% to 25 wt% of the entire cover layer.

It is sectional drawing of a surface wave apparatus. (Embodiment 1) It is sectional drawing of the electronic component by which the surface wave apparatus was mounted. (Embodiment 1) It is sectional drawing of a surface wave apparatus. (Embodiment 2) It is sectional drawing of the electronic component by which the bulk acoustic wave apparatus was mounted. (Embodiment 3) It is principal part sectional drawing of a bulk acoustic wave apparatus. (Embodiment 3) It is an example of the waveform of a surface wave device. (Experimental example) It is sectional drawing of an elastic wave apparatus. (Conventional example)

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.
(Embodiment 1)
The surface acoustic wave device 10 and the electronic component 30 according to the first embodiment will be described with reference to FIGS.

  FIG. 1 is a cross-sectional view of the surface acoustic wave device 10. An element part, for example, a SAW (Surface Acoustic Wave) filter, is formed on the piezoelectric substrate 11. That is, an IDT (Inter Digital Transducer) electrode 12 that is a comb-like electrode of the vibration unit 14, a pad 13, an IDT electrode 12, and a pad 13 are connected to the upper surface 11 a that is one main surface of the piezoelectric substrate 11. A conductive pattern including a wiring (not shown) is formed. A support layer 20 is formed in a frame shape around the vibrating portion 14 on which the IDT electrode 12 is formed. The thickness of the support layer 20 is larger than the thickness of the conductive pattern such as the IDT electrode 12 of the vibration unit 14. The support layer 20 is also formed on the pad 13.

  A cover layer 22 is disposed on the support layer 20, and the periphery of the vibration part 14 formed on the piezoelectric substrate 11 is covered with the support layer 20 and the cover layer 22, which are insulating members, and a hollow space 19 is formed. Has been. A surface acoustic wave freely propagates in a portion of the upper surface 11 a of the piezoelectric substrate 11 in the hollow space 19. A protective layer 24 is formed on the cover layer 22.

  The hollow space 19 is sealed with a cover layer 22 and a protective layer 24. Sealing by the cover layer 22 and the protective layer 24 is not necessarily airtight, and it is sufficient that liquid tightness is ensured. The size of the hollow space needs to be about 250 μm × 1000 to 500 μm × 1400 μm in consideration of the frequency band in which a general SAW filter is used.

  The cover layer 22 includes a synthetic rubber, a resin, and a filler. Examples of the material of the resin include a non-photosensitive epoxy resin sheet material. When a non-photosensitive epoxy resin is used, a low temperature curing process of the resin is possible. The cover layer 22 has a sticky shape due to the addition of synthetic rubber, and is difficult to crack. In order to maintain the strength as a sheet, it is desirable that 10 wt% or more of synthetic rubber is added. Further, when the synthetic rubber is 48 wt% or more, the flux cleaning agent used at the time of forming the solder bumps enters the hollow space and the liquid tightness is impaired. Further, when the synthetic rubber is 27.5 wt% or more, the filter characteristics after the pressure cooker test are deteriorated. Therefore, the weight ratio of the synthetic rubber to the entire cover layer is preferably 10 wt% to 25 wt%.

  In order to prevent the inflow of organic matter after the pressure cooker test, the weight ratio of the synthetic rubber to the entire cover layer is more preferably 10 wt% to 15 wt%.

  The protective layer 24 is made of a resin having flux resistance. When the solder bump is formed, the flux does not pass through the protective layer 24 made of a resin having flux resistance. Therefore, the flux can be prevented from flowing into the hollow space.

  The protective layer 24 can be formed after the cover layer 22 is overlaid on the support layer 20. Further, by providing a sheet-like composite material in which a sheet material to be the cover layer 22 and a sheet material to be the protective layer 24 are laminated in advance on the support layer 20, the cover layer 22 and the protective layer 24 are simultaneously formed. Also good.

  The materials of the protective layer 24 and the support layer 20 may be different, but the same material can be used. In such a case, since the types of materials do not increase, the manufacturing process can be simplified, which is preferable.

  In the protective layer 24, the cover layer 22, and the support layer 20, via holes (through holes) 15 reaching the pads 13 formed on the upper surface 11 a of the piezoelectric substrate 11 are formed. The via hole 15 is filled with under bump metal as the via conductor 16. On the under bump metal, an external electrode 18 made of a solder bump exposed to the outside is formed.

  In the process of mounting the surface acoustic wave device 10, the positional accuracy during mounting can be improved by recognizing and mounting the external electrode 18 rather than recognizing and mounting the external shape of the surface acoustic wave device 10. This is because, when recognizing the outer shape of the surface acoustic wave device 10, the positional accuracy during mounting depends on the dicing accuracy.

  When the external electrode 18 is recognized and mounted, the protective layer 24 is preferably made of a polyimide resin. This is because the difference in luminance profile between the external electrode 18 and the protective layer 24 becomes large and it becomes easy to recognize the external electrode 18.

  FIG. 2 is a cross-sectional view of the electronic component 30 on which the surface acoustic wave device 10 of FIG. 1 is mounted.

  In the electronic component 30, two surface wave devices 10 are mounted on the side 40 a that is one main surface of the common substrate 40. That is, the land 42 formed on the upper surface 40 a of the common substrate 40 and the surface acoustic wave device 10 are electrically connected via the external electrode 18. A resin 32 is disposed around the surface wave device 10, and the surface wave device 10 is covered with the resin 32. The resin 32 ensures the airtightness of the hollow space 19.

  An external connection electrode 44 for mounting the electronic component 30 on another circuit board or the like is exposed on the lower surface 40b side which is the other main surface of the common substrate 40. A substrate via conductor 46 and an internal wiring pattern 48 that electrically connect the land 42 and the external connection electrode 44 are formed inside the common substrate 40.

  For example, the common board 40 is a printed board. The resin 32 is formed by embedding in a pressurized state of 2 to 5 Pa by lamination or a resin mold method.

For example, the electronic component 30 is a duplexer, and surface acoustic wave filter elements for transmission and reception are mounted on the common substrate 40 as the surface acoustic wave device 10.
(Embodiment 2)
The surface acoustic wave device 10a of Embodiment 2 will be described with reference to FIG.

  In the surface acoustic wave device 10a of the second embodiment, the following configuration is added to the same configuration as the surface acoustic wave device 10 of the first embodiment.

That is, as shown in the cross-sectional view of FIG. 3, the intermediate layer 26 is formed between the piezoelectric substrate 11 and the support layer 20. The intermediate layer 26 is formed in a region other than the vibration part 14a including the IDT electrode 12 and the pad 13, and is interposed between the piezoelectric substrate 11 and the support layer 20 so as to surround the periphery of the vibration part 14a. Examples of the material of the intermediate layer include a SiO ₂ film and a SiN film. Since the intermediate layer 26 of the SiO ₂ film or SiN film has a rougher surface than the piezoelectric substrate 11, the adhesion strength is improved by the anchor effect. These films can be formed by a sputtering method or a CVD (Chemical Vapor Deposition) method.

  By providing the intermediate layer 26 between the piezoelectric substrate 11 and the support layer 20, the liquid tightness of the hollow space 19 can be further ensured. Thereby, in the process after forming the hollow space, for example, in the process of filling the via hole with the electrode by plating, it is possible to prevent a problem that the plating solution enters the hollow space 19 to cause a characteristic defect.

A part of the support layer 20 may be formed on the piezoelectric substrate 11 around the pad 13. In this case, the intermediate layer 26 is interposed between the other part of the support layer 20 and the piezoelectric substrate 11, and the intermediate layer is interposed between the piezoelectric substrate 11 and the support layer 20 on the IDT electrode 12 side than the pad 13. The layer 26 is formed so as to surround the periphery of the vibration part 14a.
(Embodiment 3)
The electronic component 30x according to the third embodiment will be described with reference to FIGS. Below, it demonstrates centering around difference with Embodiment 1, and uses the same code | symbol for the same component.

  FIG. 4 is a cross-sectional view of the electronic component 30x. Unlike the first embodiment, the electronic component 30x includes a surface wave device 10 and a bulk acoustic wave device 10x. That is, the land 42 formed on the upper surface 40 a of the common substrate 40 and the surface wave device 10 and the bulk acoustic wave device 10 x are electrically connected via the external electrode 18. A resin 32 is disposed around the surface acoustic wave device 10 and the bulk acoustic wave device 10 x, and the surface acoustic wave device 10 and the bulk acoustic wave device 10 x are covered with the resin 32. An external connection electrode 44 for mounting the electronic component 30x on another circuit board or the like is exposed on the lower surface 40b side which is the other main surface of the common substrate 40. Inside the common substrate 40, a substrate via conductor 46 and an internal electrode pattern 48 that electrically connect the land 42 and the external connection electrode 44 are formed.

  For example, the electronic component 30x is a duplexer, the surface acoustic wave device 10 is a surface acoustic wave filter element, and the bulk acoustic wave device 10x is a bulk acoustic wave filter element. One is for transmission and the other is for reception.

  The bulk acoustic wave device 10x is configured in substantially the same manner as the surface acoustic wave device 10 of the first embodiment, except that a vibrating portion 14x is formed on an insulating substrate 11x such as Si.

  The bulk acoustic wave device 10 x has a package structure similar to that of the surface acoustic wave device 10. In the bulk acoustic wave device 10x, the vibration part 14x is formed on one main surface side of the substrate 11x, and the support layer 20 is formed in a frame shape around the vibration part 14x.

  The thickness of the support layer 20 is larger than the thickness of the vibration part 14x. The support layer 20 is also formed on the pad 13. A cover layer 22 is disposed on the support layer 20, and the periphery of the vibrating portion 14 x is covered with the support layer 20 and the cover layer 22, which are insulating members, and a hollow space 19 is formed. A protective layer 24 is formed on the cover layer 22. Via holes (through holes) 15 that reach the pads 13 are formed in the protective layer 24, the cover layer 22, and the support layer 20. The via hole 15 is filled with an under bump metal as a via conductor 16, and an external electrode 18 exposed to the outside is formed on the under bump metal.

  Such a package structure is not limited to the SMR (Solidly Mounted Resonator) type bulk acoustic wave device according to the third embodiment, and a type in which a vibration part is disposed on a cavity formed in a substrate and a sacrificial layer are removed. The present invention can also be applied to a bulk acoustic wave device that is supported in a state where the vibration part is lifted from the substrate.

  FIG. 5 shows a cross-sectional view of the main part of the bulk acoustic wave device. Unlike the surface acoustic wave device of the first embodiment, the bulk acoustic wave device of the third embodiment includes a vibrating part 14x in which a piezoelectric thin film 12s is sandwiched between an upper electrode 12a and a lower electrode 12b. The vibrating portion 14x is acoustically separated from the insulating substrate 11x via the acoustic reflector 17.

  In the acoustic reflector 17, the low acoustic impedance layer 17s having a relatively low acoustic impedance and the high acoustic impedance layer 17t having a relatively high acoustic impedance are alternately stacked on the insulating substrate 11x. The upper electrode 12a and the lower electrode 12b are electrically connected to a pad 13 (not shown in FIG. 5).

The pad 13 may be formed directly on the main surface of the insulating substrate 11x or may be formed on the main surface of the insulating substrate 11x via another layer (for example, the low acoustic impedance layer 17s).
(Experimental example)
(Experimental example 1)
The surface wave device of Embodiment 1 was produced as follows, and the liquid tightness of the hollow space with respect to the flux cleaning agent was evaluated when the synthetic rubber amount, the resin amount, and the filler amount of the cover layer were changed.

  A conductive layer including an IDT electrode, a pad, and a wiring connecting the IDT electrode and the pad is formed on one main surface of the piezoelectric substrate, and then a support layer is formed around the vibrating portion where the IDT electrode is formed. Formed. A photosensitive polyimide resin was applied to the entire main surface of the piezoelectric substrate on the support layer, and the periphery of the vibrating portion was opened (removed) by photolithography.

  Next, the cover layer and the protective layer were simultaneously formed by providing a sheet-like composite material in which the sheet material to be the cover layer and the sheet material to be the protective layer were previously laminated on the support layer by lamination. For the cover layer, a non-photosensitive epoxy film capable of a low temperature curing process was used. Further, a polyimide resin was used for the protective layer.

  Next, a via hole that penetrates the protective layer, the cover layer, and the support layer and exposes the pad at the bottom was formed by laser processing. Then, as a via conductor filled in the via hole, an under bump metal is formed by electrolytic plating of Cu and Ni, and Au having a thickness of about 0.05 to 0.1 μm is formed on the surface of the under bump metal to prevent oxidation. did. Then, a solder paste such as Sn-Ag-Cu is printed directly on the under bump metal via a metal mask, and the solder is fixed to the under bump metal by heating at 260 ° C. where the solder paste dissolves, The flux was removed with a flux cleaning agent to form spherical solder bumps. Thereafter, the chip was cut out by dicing or the like to produce a surface wave device of 1.2 mm × 1.6 mm × 0.12 mm. The height of the support layer was 14 μm, the thickness of the cover layer was 12.5 μm, and the thickness of the protective layer was 17.5 μm.

  The liquid tightness of the hollow space with respect to the flux cleaning agent was evaluated when the amount of synthetic rubber, the amount of resin, and the amount of filler in the cover layer were changed. The flux cleaning agent contains glycol ether, water, a surfactant, and an additive. The washing time was 40 minutes, and the presence or absence of moisture inside the hollow space after washing was confirmed. The number of samples is 10. Table 1 shows the composition of the synthetic rubber amount, the resin amount, and the filler amount of the cover layer, and the experimental results.

As is apparent from Table 1, in the surface wave devices under conditions 5 and 6 in which the synthetic rubber was 48 wt% or more, the flux cleaning agent entered the hollow space after cleaning. On the other hand, in the conditions 1 to 4 where the synthetic rubber was 25 wt% or less, the flux cleaning agent did not enter.
(Experimental example 2)
A pressure cooker test was performed on the surface acoustic wave device manufactured in the same manner as in Experimental Example 1 in order to evaluate moisture resistance. The temperature was 121 ° C., the humidity was 100%, the atmospheric pressure was 2 atm, and the time was 192 hours. After the pressure cooker test, the hollow space was opened and the hollow space was visually confirmed. The number of samples is 10. Table 2 shows the composition of the synthetic rubber amount, the resin amount, and the filler amount of the cover layer, and the experimental results. ◎ is the level at which inflow of organic matter is not seen. ○ is a level at which some organic inflow is observed but the characteristics are not degraded. X is the level at which the inflow of organic matter is observed and the characteristics deteriorate.

As is apparent from Table 2, when the amount of synthetic rubber was 27.5 wt% or more, organic substances flowed into the hollow space, resulting in deterioration of characteristics. This organic matter
It is estimated that the resin component in the cover layer was hydrolyzed during the pressure cooker test. When the amount of the synthetic rubber was 20 wt% to 25 wt%, the organic substance flowed into a part of the hollow space, but the characteristics of the surface wave device were not deteriorated. When the amount of the synthetic rubber was 10 wt% to 15 wt%, no inflow of organic matter into the hollow space was observed.

  FIG. 6 shows an example of the filter waveform of the surface acoustic wave device. FIG. 6A is an example of a waveform when the synthetic rubber amount under condition 12 is 15 wt%, and FIG. 6B is an example of a waveform when the synthetic rubber amount under condition 18 is 48 wt%. In both cases, the filter waveforms before and after the pressure cooker test are superimposed and displayed. In the waveform before peak expansion, one scale in the figure corresponds to 10 dB. In the waveform after peak enlargement, one scale in the figure corresponds to 1 dB.

  In FIG. 6A, the filter waveforms before and after the test are almost overlapped. On the other hand, in FIG. 6B, it can be seen that there is a difference in the filter waveform before and after the pressure cooker test, and the characteristics are changed.

  Table 3 shows the amount of change in the minimum value (peak value) of insertion loss under conditions 12 and 18 before and after the pressure cooker test. The number of samples is 20.

  From Table 3, the change amount of the minimum value of the insertion loss under the condition 12 was −0.13 dB, whereas the change amount of the insertion loss under the condition 18 was −0.41 dB. In manufacturing, the allowable value of the amount of change before and after the pressure cooker test is within −0.2 dB, and it was revealed that the condition 18 in which the organic substance is flowing in has a remarkable characteristic deterioration.

10, 10a: Surface wave device 10x: Bulk acoustic wave device 11: Piezoelectric substrate 11x: Insulating substrate 12: IDT electrode 12a: Upper electrode 12b: Lower electrode 12s: Piezoelectric thin film 13: Pad 14, 14a, 14x: Vibrating part 15: Via hole 16: Via conductor 17: Acoustic reflector 17s: Low acoustic impedance layer 17t: High acoustic impedance layer 18: External electrode 19: Hollow space 20: Support layer 22: Cover layer 24: Protective layer 26: Intermediate layer 30, 30x: Electronic component 32: Resin 40: Common substrate 42: Land 44: External connection electrode 46: Substrate via conductor 48: Internal wiring pattern 110: Elastic wave device 111: Piezoelectric substrate 112: IDT electrode 113: Pad 114: Via conductor 115: Cover Layer 116: Support layer 117: External electrode 118: Wiring line 119: Hollow space

Claims (8)

A substrate,
A vibrating portion formed on one main surface of the substrate;
A pad formed on the one main surface of the substrate and electrically connected to the electrode of the vibrating portion;
A support layer provided on the one main surface of the substrate so as to surround the vibration part, and having a thickness larger than the thickness of the vibration part;
It is provided on the support layer so as to cover the vibration part, forms a hollow space around the vibration part, includes at least a synthetic rubber and a resin, and a weight ratio with respect to the whole synthetic rubber is 10 wt%. A sheet-like cover layer of ˜25 wt%;
A protective layer made of a resin having a flux resistance, provided on the opposite side of the cover layer from the support layer;
A via conductor penetrating the protective layer, the cover layer and the support layer and connected to the pad;
Provided at the end of the via conductor on the protective layer side, an external electrode made of a solder bump,
An elastic wave device comprising: The elastic wave device according to claim 1, wherein a weight ratio of the synthetic rubber to the entire cover layer is 10 wt% to 15 wt%. The elastic wave device according to claim 1, wherein the substrate is a piezoelectric substrate, and the vibration unit includes an IDT electrode. The acoustic wave device according to claim 1, wherein the substrate is an insulating substrate, and the vibration unit includes a piezoelectric thin film having electrodes formed on both surfaces. The elastic wave device according to claim 1, wherein the protective layer is made of the same material as the support layer. The elastic wave device according to claim 1, wherein the protective layer is made of a polyimide resin. The elastic wave device according to claim 1, wherein the cover layer is made of a non-photosensitive epoxy resin. The acoustic wave device according to any one of claims 1 to 7, further comprising a nitride film or an oxide film interposed in at least a part between the substrate and the support layer.