Acoustic wave device and method for producing same

An acoustic wave device comprises a substrate and an acoustic wave element on one main surface of the substrate. Side surfaces of the substrate comprises a protruding portion which protrudes out at a side of an another main surface closer than a side with the one main surface side.

TECHNICAL FIELD

The present invention relates to an acoustic wave device such as a surface acoustic wave (SAW) device or a film bulk acoustic resonator (FBAR) or the like and a method of producing the same.

BACKGROUND ART

An acoustic wave device having a substrate and an acoustic wave element provided on a main surface of the substrate is known. Patent literature 1 discloses an acoustic wave device improved in shock resistance by covering side surfaces or a back surface (main surface on the side opposite to the main surface having an acoustic wave element provided thereon) of the substrate by a resin.

An acoustic wave device is sometimes impacted from a side direction of the substrate at the time of transport during the period from manufacture to mounting etc. Note that, Patent Literature 1 refers to the shock resistance, but does not particularly take note of impact from a side direction of the substrate. As a result, the acoustic wave device of Patent Literature 1 is not a particularly preferred aspect against impact from the side direction of a substrate.

Accordingly, it is preferable that an acoustic wave device capable of improving the shock resistance from a side direction of a substrate and a method of production of the same be provided.Patent literature 1: Japanese Patent Publication (A) No. 2008-5464

SUMMARY OF INVENTION

An acoustic wave device according to an embodiment of the present invention has a substrate and an acoustic wave element on one main surface of the substrate. On the side surface of the substrate, a protruding portion is provided which protrudes out from the side surfaces at a side of the other main surface compared with a side of the one main surface.

A method of production of an acoustic wave device according to an embodiment of the present invention has a first cutting step of cutting a wafer, on one main surface of which a plurality of acoustic wave elements are provided, at the part of that one main surface side by a first blade so as to form groove portions which partition a plurality of acoustic wave devices and a second cutting step of cutting the wafer at the part of the other main surface side along the groove portions by a second blade having a thinner blade thickness than the first blade so as to separate the wafer.

According to the above constitution and procedure, when shock is applied to a side surface of an acoustic wave device at the time of transport etc. of the acoustic wave device, the protruding portion is more easily impacted. On the other hand, for maintaining the performance of the acoustic wave device, rather than maintaining the shape of the part at the other main surface side, maintaining the shape of the part at the one main surface side on which the acoustic wave element is provided is more important. Accordingly, this means that the acoustic wave device is improved in shock resistance from the side direction of the substrate as a whole. Therefore, the maintenance of shape of the acoustic wave element and maintenance of adhesion between the cover and the substrate are improved.

DESCRIPTION OF EMBODIMENTS

Below, a SAW device according to an embodiment of the present invention is explained with reference to the drawings. Note that, the drawings used in the following explanation are schematic. Dimensions and ratios etc. on the drawings do not always coincide with the actual ones.

(Constitution of SAW Device)

FIG. 1is a perspective view of the appearance of a SAW device1according to an embodiment of the present invention.

The SAW device1is constituted by a so-called wafer level package (WLP) type SAW device. The SAW device1is formed in a general block shape as a whole. At one surface of the block, a plurality of terminals3are exposed. The number and arrangement positions of the plurality of terminals3are suitably set in accordance with the configuration of the electronic circuit inside the SAW device1. The present embodiment illustrates a case where six terminals3are arranged along an outer edge of one surface.

The SAW device1receives as input a signal through any of the plurality of terminals3. The input signal is filtered by the SAW device1. Then, the SAW device1outputs the filtered signal through any of the plurality of terminals3. The SAW device1is for example mounted on the mounting surface of a not shown circuit board or the like with the surface at which the plurality of terminals3are exposed made to face that mounting surface and is sealed by a resin in that state. Due to this, it is mounted in a state with the terminals3connected to the terminals on the mounting surface.

FIG. 2is a perspective view showing the SAW device1partially cut away. Further,FIG. 3is a cross-sectional view taken along the line III-III inFIG. 1.

The SAW device1has a substrate5and SAW elements7provided on the substrate5. Further, for the purpose of protection etc. of the SAW elements7, the SAW device1has a cover9covering the SAW elements7, a resin film11covering side surfaces of the substrate5, a back surface electrode13provided on the substrate5at the side opposite to the SAW element7side, and a resin layer15laminated on the back surface electrode13.

The substrate5is constituted by a piezoelectric substrate. Specifically, for example, the substrate5is a single crystal substrate having a piezoelectric property such as a lithium tantalite single crystal, lithium niobate single crystal, or the like. The substrate5is generally formed in a thin block shape and has a first main surface5a, a second main surface5b(FIG. 3) on the back surface side of the same, and side surfaces5cfacing the sides (outer circumference side) of the first main surface5aand second main surface5b. On the side surfaces5c, a protruding portion5dprotruding outward from the side surfaces5cis formed.

The protruding portion5dis provided along the outer circumference of the second main surface5bover the entire outer circumference. In other words, it is also possible to grasp that the protruding portion5dis formed by the side surfaces5cbeing expanded at the parts at the second main surface5bsides and possible to grasp that it is formed by parts which remain without being cut away when cutting away the piezoelectric substrate3at the first main surface5aside over the entire outer circumference. The cross-sectional shape (cross-sectional shape shown inFIG. 3) of the protruding portion5dis generally rectangular. Further, the protruding portion5dis provided in a “second region” when equally dividing the side surfaces5cin the vertical direction into two into a first region (region on first main surface5aside) and a second region (region on second main surface5bside).

The length of one side of the substrate5is for example 0.5 mm to 2 mm. The thickness of the substrate5is for example 0.2 mm to 0.5 mm. The amount of protrusion of the protruding portion5dis for example 5 to 10 μm. The thickness of the protruding portion5dis for example 25 to 50 μm.

Each SAW element7is an element for filtering a signal which is input to the SAW device1. The SAW element7is provided on the first main surface5a. The SAW element7has a pair of comb-shaped electrodes (IDT electrodes)17. Each comb-shaped electrode17has a bus bar17a(FIG. 2) extending in the propagation direction (X-direction) of the SAW in the substrate5and a plurality of electrode fingers17bextending from the bus bar17ain a direction (Y-direction) perpendicular to the above propagation direction. The comb-shaped electrodes17are provided so that their electrode fingers17bmesh with each other.

Note that,FIG. 2andFIG. 3are schematic views, so show one pair of comb-shaped electrodes17each having several electrode fingers17b. In actuality, two or more pairs of comb-shaped electrodes each having a number of electrode fingers larger than this may be provided. Further, a ladder type SAW filter, double mode SAW resonator filter, or the like may be constituted by a plurality of SAW elements7connected in serial connection, parallel connection, or other method. The SAW elements7are formed by for example Al alloy such as Al—Cu alloy or the like.

The cover9has a frame19surrounding the SAW elements7in a plan view of the first main surface5aand has a lid21closing the opening of the frame19. Further, the spaces surrounded by the first main surface5a(strictly speaking, a protective film29which is explained later), frame19, and lid21form vibration spaces S for facilitating the propagation of the SAW. Note that, the vibration spaces S may be provided in suitable numbers and shapes. The present application illustrates a case where two vibration spaces S are provided.

The frame19is comprised of a layer having a generally constant thickness in which one or more openings which become vibration spaces S are formed. In the present embodiment, two vibration spaces S are provided. The thickness of the frame19(height of the vibration spaces S) is for example several μm to 30 μm. The lid21is constituted by a layer having a generally constant thickness which is laminated on the frame19. The thickness of the lid21is for example several μm to 30 μm.

The planar shape of the cover9is similar to the planar shape of the substrate5and is rectangular in the present embodiment. The cover9has for example a generally equivalent area with the first main surface5aand covers generally the entire surface of the first main surface5a. However, the cover9is a bit smaller than the first main surface5a, so steps is formed between the side surfaces5cand the side surfaces9cof the cover9. The steps is formed over the entire circumference of the first main surface5a. The size of the steps is for example 5 to 20 μm.

The frame19and lid21are formed by for example a photosensitive resin. The photosensitive resin is for example a urethane acrylate-based, polyester acrylate-based, or epoxy acrylate-based resin which is cured by radical polymerization of acryl groups, methacryl groups, or the like.

The frame19and lid21may be formed by the same material or may be formed by materials different from each other. In the present example, for convenience of explanation, the borderline between the frame19and the lid21is clearly indicated. However, in an actual product, the frame19and lid21may be formed by the same material and formed integrally as well.

The resin film11covers the side surfaces5cat the parts at the first main surface5aside other than the protruding portion5dand the side surfaces9cof the cover9. The resin film11is provided so as to bury the steps caused by the protruding portion5dand the step between the substrate5and the cover9so that the outer shape of the SAW device1constituted by the substrate5, cover9, and resin film11becomes a block shape. That is, the resin film11covers the entire surfaces of the side surfaces5cat the parts at the first main surface5aside other than the protruding portion5dand the entire surfaces of the side surfaces9c. Further, outer circumferential surface11eof the resin film11is flush with the top face5eof the protruding portion5d, and end surface11aof the resin film11on the first main surface5aside is flush with the top face9aof the cover9.

The resin film11is formed by for example a novolac-based resin, epoxy resin, Biphenol resin, or polyimide resin. The resin film11has a lower Young's modulus than the substrate5. That is, the resin film11is softer than the substrate5and easily absorbs shock.

The back surface electrode13covers the entire surface of the second main surface5b. The thickness of the back surface electrode13is for example 1 μm to several μm. The back surface electrode13is formed by for example an Al alloy such as an Al—Cu alloy or the like. The charges formed in the substrate5due to temperature change etc. flow to the back surface electrode13, whereby pyroelectric breakdown of the SAW elements7is suppressed.

The resin layer15covers the entire surface of the second main surface5b(back surface electrode13). The thickness of the resin layer15is for example 25 μm to 50 μm. The resin layer15is formed by for example a thermosetting resin such as an epoxy resin or the like. The resin layer15has a lower Young's modulus than the substrate5in the same way as the resin film11.

The terminals3are formed standing at the first main surface5aand are exposed at the upper surface9aof the cover9through holes9hformed in the cover9. Specifically, the holes9hpenetrate through the frame19and lid21in directions facing the first main surface5aat the outsides of the vibration spaces S.

The first main surface5ais provided with lines23(FIG. 2) connected to the SAW elements7and a plurality of pads25connected to the lines23. The terminals3are connected to the SAW elements7by being provided on the pads25.

As shown inFIG. 3, on the first main surface5a, a conductive layer27and a protective film29covering the conductive layer27are provided.

The conductive layer27forms the SAW elements7, at least a part of the lines23(FIG. 2), and at least a part of the pads25. The conductive layer27is formed by for example an Al alloy such as an Al—Cu alloy or the like. Its thickness is for example 100 to 300 nm.

The protective film29contributes to prevention of oxidation etc. of the conductive layer27. The protective film29is formed by for example a material which has an insulating property and has a mass light enough so as not to influence the propagation of the SAW. For example, the protective film29is formed by silicon oxide (SiO2etc.), silicon nitride, silicon or the like. The thickness of the protective film29is for example about 1/10 (10 nm to 30 mm) of the thickness of the conductive layer27or equal to or more than the thickness of the conductive layer27(100 nm to 300 nm).

The protective film29is for example provided over generally the entire first main surface5a, while the cover9is laminated over the protective film29. Further, also the part of the resin film11which is on the first main surface5ais laminated over the protective film29. On the other hand, at the positions of arrangement of the terminals3, the protective film29is removed so that the pads25are exposed from the protective film29.

Note that, strictly speaking, the cover9is not directly provided on the first main surface5a, but is provided on the protective film29or the like. In the present example, even in a case where predetermined members, layers, etc. are indirectly provided on the main surface of the substrate5in this way and are not directly provided on the main surface of the substrate5, it is sometimes expressed so that these predetermined members, layers, etc. are provided on the main surface of the substrate5. This is true for the word “laminate” as well.

On the first main surface5a, other than this, an insulation film which is laminated on the conductive layer27or protective film29, another conductive layer which is laminated on the conductive layer27with the insulation film interposed therebetween and forms a part of the lines23, a connection strengthening layer which forms upper layer portions of the pads25and strengthens the connection between the pads25and the terminals3, and so on may be provided as well.

(Method of Production of SAW Device)

FIG. 4AtoFIG. 6Dare cross-sectional views for explaining the method of production of the SAW device1. The steps are advanced in order fromFIG. 4AtoFIG. 6D.

The steps explained below are realized in a so-called “wafer process”. That is, a mother board (wafer31) which is later divided to form the substrates5is formed with a thin film, processed by photolithography, etc., then is diced to form a large number of SAW devices in parallel.

Note, inFIG. 4AtoFIG. 5C, only parts corresponding to one SAW device1are shown. Further, inFIG. 6AtoFIG. 6D, only parts corresponding to three SAW devices1are shown. The conductive layer, insulation layer, etc. change in shapes along with the progress in the process. However, common notations are used before and after the changes. In the same way, notations of the first main surface5aand second main surface5bof the substrate5are assigned to the first main surface and second main surface of the wafer31.

As shown inFIG. 4A, first, on the first main surface5aof the substrate5, a conductive layer27is formed. Specifically, first, the thin film forming method such as the sputtering method, vapor deposition method, CVD (chemical vapor deposition) method, or the like is used to form a metal layer which becomes the conductive layer27on the first main surface5a. Next, the metal layer is patterned by photolithography etc. using a reduced protrusion exposure machine (stepper) and RIE (reactive ion etching) device. Therefore, a conductive layer27including the SAW elements7, at least a part of the lines23, and at least a part of the pads25is formed.

Next, as shown inFIG. 4B, the protective film29is formed. Specifically, first, a thin film which becomes the protective film29is formed by the thin film forming method such as the CVD method or vapor deposition method or the like. Next, parts of the thin film are removed by the photolithography method so that parts of the conductive layer27which constitute the pads25are exposed. Accordingly, the protective film29is formed.

After the protective film29is formed, as shown inFIG. 4C, a thin film which becomes the frame19is formed. The thin film is formed by for example adhesion of a film formed by a photosensitive resin or a thin film forming method the same as that for the protective film29etc.

After the thin film which becomes the frame19is formed, as shown inFIG. 4D, the photolithography method is used to remove parts of the thin film and form openings which becomes the vibration spaces S and lower side portions of the holes9h. Further, groove portions are formed along the dicing lines, and side surfaces of the frame19are formed as well. That is, the frame19is formed from the thin film. Note that, the photolithography may be either of the positive type or the negative type.

After the frame19is formed, as shown inFIG. 5A, the lid21is formed by the same method as the method of formation of the frame19. Specifically, first, a thin film which becomes the lid21is formed. The thin film is formed by for example adhesion of a film formed by a photosensitive resin. By laminating the thin film on the frame19, the openings of the frame19are closed, and the vibration spaces S are constituted.

Next, by the photolithography method, parts of the thin film are removed, and upper side portions of the holes9hare formed. Further, groove portions are formed along the dicing lines, and side surfaces of the frame19are formed. That is, the lid21is formed from the thin film. Note that, the photolithography may be either of the positive type or negative type.

After the lid21is formed, as shown inFIG. 5D, terminals3are formed. Specifically, first, a base layer33is formed over the upper face9aof the cover9and the inside of the holes9h. The base layer33is a metal layer and is formed by for example the sputtering method.

Next, on the base layer33, a resist layer37is formed. The resist layer37is for example formed by having a thin film formed on the substrate by a spin coating method or other technique and having that thin film patterned by the photolithography method. By removal of parts of the thin film by patterning, the base layer33is exposed at the holes9hand their peripheral parts.

After that, the electroplating method is used to cause a metal to deposit on the exposed parts of the base layer33. Accordingly, solid parts35are formed.

After the solid parts35are formed, as shown inFIG. 5C, the parts of the base layer33covered by the resist layer37and the resist layer37are removed. Therefore, the terminals3are formed. That is, the surface parts of the terminals3are constituted by the base layer33, and internal parts (majority) of the terminals3are constituted by the solid parts35. Note that, inFIG. 3, illustration of the base layer33is omitted.

After that, on the second main surface5b, the back surface electrode13and resin layer15are sequentially formed (FIG. 5C). Specifically, the back surface electrode13is formed by the thin film forming method such as the sputtering method, vapor deposition method, CVD method, or the like. The resin layer15is formed by for example adhering a resin sheet to the back surface electrode13, and then thermosetting it. Note that, the resin layer15may be formed by a potting method or printing method as well.

After the resin layer15is formed, as shown inFIG. 6A, the resin layer15of the wafer state SAW devices1and a dicing tape39are bonded.

Next, as shown inFIG. 6D, a first blade41is used to the portions of the wafer31on the first main surface5aside along the dicing lines. Accordingly, groove portions31apartitioning a plurality of SAW devices1are formed.

After the groove portions31aare formed, as shown inFIG. 6C, resin constituting the resin film11is filled in the groove portions31a. The resin is filled by using for example a dispenser43. Further, the filled resin is cured by heating.

After the resin is filled and hardened, as shown inFIG. 6D, a second blade45having a thinner blade thickness than the first blade41is used to cut the wafer-state SAW devices1along the groove portions31afrom the first main surface5aside. Specifically, the resin filled in the groove portions31a, parts of the wafer31at the second main surface5bside, the back surface electrode13, and the resin layer15are cut at schematically the center of the groove portions31a.

Therefore, the plurality of SAW devices1are separated from each other. Further, due to the difference of blade thickness between the first blade41and the second blade45, the protruding portions5dare formed. For example, when the blade thickness of the first blade41is 50 μm and the blade thickness of the second blade is 40 μm, the amount of protrusion of the protruding portions5dis (50−40)/2=5 μm.

Note that, the first blade41is for example a fixed abrasive type blade and has a plurality of fixed abrasive grains41aand a connecting material41bholding the plurality of fixed abrasive grains41a. In the same way, the second blade45has a plurality of fixed abrasive grains45aand a connecting material45bholding the plurality of fixed abrasive grains45a.

The material of the abrasive grains, particle size, the density of the abrasive grains, the material of the connecting material, and blade thickness may be suitably selected. Part of the conditions other than the blade thickness may be shared between the first blade41and the second blade45. For example, between the first blade41and the second blade45, the material of the abrasive grains, particle size, and the density of abrasive grains are shared, but the type of the connecting material is different. As the connecting material41b, one suitable for cutting a piezoelectric substrate (substrate5) is selected, while as the connecting material45b, one suitable for cutting a resin (at least one of the resin film11and resin layer15) is selected.

According to the above embodiment, each SAW device1has a substrate5and SAW elements7on the first main surface5aof the substrate5. On the side surfaces5cof the substrate5, a protruding portion5dis provided at the second main surface5bside compared with the first main surface5a.

Accordingly, at the time of transport etc. of the SAW device1, when shock is applied to a side surface5c, the protruding portion5dis more easily impacted than the part of the side surface5cat the first main surface5aside. On the other hand, for maintaining the performance of the SAW device1, rather than maintaining the shape of the part at the second main surface5bside, maintaining the shape of the part at the first main surface5aside is more important. Accordingly, this means that the SAW device1is improved in shock resistance from the side direction of the substrate5as a whole. Specifically, by strengthening of protection of the substrate5at the part at the first main surface5aside against shock from the side, maintenance of the shape of the SAW elements7(maintenance of filter precision) and maintenance of adhesion between the cover9and the substrate5(protective film29) (antioxidation effect of conductive layer) are improved. Note that, Patent Literature 1 does not disclose the idea of protecting the first main surface5aside against the impact from the side surface with priority over the second main surface5bside.

The protruding portion5dis provided along the outer circumference of the second main surface5b. Accordingly, it is possible to strengthen the protection of the first main surface5aside against impact from various directions parallel to the first main surface5a. Further, as explained with reference toFIG. 6BandFIG. 6D, the protruding portion5dcan be formed by a simple and convenient method of changing the cutting width at the time of dicing, for example, the protruding portion5dcan be formed by using the first blade41and second blade45having different blade thicknesses.

The SAW device1has the resin layer11which covers the side surfaces5cat the parts of the first main surface5aside other than the protruding portion5d, abuts against the protruding portion5dfrom the first main surface5aside, and is softer than the substrate5. Accordingly, due to the resin film11, shock with respect to a side surface5ccan be absorbed, and protection of the parts of the SAW device1at the first main surface5aside can be strengthened. Note that, a “resin film11which is softer than the substrate5” means “softer” when compared in Young's modulus. That is, the resin film11has a smaller Young's modulus than the substrate5. Further, the protruding portion5dfunctions as a stopper which limits movement of the resin film11to the second main surface5bside, whereby peeling of the resin film11from the side surfaces5cis suppressed.

The resin film11may also be formed so that its side surfaces are located at the inner side compared with the protruding portion5d. In other words, the protruding portion5dmay be formed so that it protrudes outward compared with the side surfaces of the resin film11. Due to this, when an object having a surface parallel to a side surface strikes a SAW device1from the side direction of the substrate5, that object strikes the protruding portion5d, so propagation of a large impact to the first main surface5aof the SAW device1can be suppressed. On the other hand, when a pointed object that does not contact the protruding portion5dstrikes the SAW device1from the side direction of the substrate5, the shock at the time of impact is eased by the resin film11, so propagation of a large shock to the first main surface5aof the SAW device1can again be suppressed.

The SAW device1has the cover9which is provided on the first main surface5aand seals the SAW elements7. The side surfaces9cof the cover9form steps by being positioned at the inner sides from the side surfaces5cof the substrate5. The resin film11is provided so as to straddle the side surfaces9cof the cover9from the side surfaces5cof the substrate5and abuts against the steps from the first main surface5aside. Accordingly, by covering of mating parts of the cover9and substrate5(strictly speaking, protective film29) by the resin film11, invasion of moisture from the mating parts and peeling of the cover9from the substrate5are suppressed. Further, the steps formed by the cover9and substrate5function as stoppers which limit movement of the resin film11to the second main surface5bside, so peeling of the resin film11from the side surface9cand side surface5cis suppressed.

Further, the method of production of the SAW device1has a first cutting step (FIG. 6B) of cutting the wafer31, on the first main surface5aof which a plurality of SAW elements7are provided, at the parts of the first main surface5asides by the first blade41so as to form groove portions31afor partitioning the plurality of SAW devices1. Further, this method of production has a second cutting step (FIG. 6D) of cutting the wafer31at parts of the second main surface5bsides along the groove portions31aby the second blade45having a thinner blade thickness than the first blade41so as to separate the wafer31.

Accordingly, the protruding portions5dcan be simply formed. Further, blades which are different from each other are used in the two steps, therefore blades suitable for the steps can be used. For example, in the first cutting step (FIG. 6B), cutting is carried out at a high speed by cutting by the first blade41having the large particle size, while in the second cutting step (FIG. 6D), chipping, which easily occurs when a blade passes through a wafer, can be suppressed by cutting by the second blade45having a small particle size.

The method of production of the SAW device1further has a step (FIG. 6C) of filling resin in the groove portions31aafter the first cutting step (FIG. 6B) and before the second cutting step (FIG. 6D). Accordingly, the above-mentioned resin film11which covers the side surfaces5cat the first main surface5aside other than the protruding portion5dand adheres to the protruding portion5dcan be simply constituted.

The method of production of the SAW device1further has a step (FIG. 5C) of forming the resin layer15on the second main surface5bof the wafer31before the first cutting step (FIG. 6B). The resin layer15is not cut in the first cutting step (FIG. 6B), but the resin layer15is cut in the second cutting step (FIG. 6D). Accordingly, by selecting as the first blade45one suitable for cutting the piezoelectric substrate and as the second blade45selecting one suitable for cutting the resin layer15, chipping and cracks in the second main surface5bcan be suppressed.

FIG. 7AtoFIG. 7Care cross-sectional views showing first to third modifications of the protruding portion5d. The protruding portions5dof the first to third modifications are formed tapered so as to spread further outward toward the second main surface5bside. In other words, the protruding portions5dof the first to third modifications protrude out further the more toward the second main surface5bsides (the closer in position to the second main surfaces5b).

More specifically, in the first modification shown inFIG. 7A, the protruding portion5dis formed so that its cross-sectional shape becomes a generally right triangle. In the second modification shown inFIG. 7B, the protruding portion5dis formed so that its cross-sectional shape becomes generally trapezoidal. In the third modification shown inFIG. 7C, the protruding portion5dis formed so that the cross-sectional shape of the part at the first main surface5aside is generally rectangular (square) and the cross-sectional shape of the part at the second main surface5bside is generally trapezoidal. The angle of inclination of the tapered surface (5e) of the protruding portion5drelative to the side surface5c(defined as 0° when the tapered surface is parallel to the side surface5c) is for example 5° to 40°.

Note that, in the first to third modifications, the protruding portion5dmay be provided along the outer circumference of the second main surface5b, the resin film11which abuts against the protruding portion5dfrom the first main surface5aside may be provided, and so on in the same way as the above embodiment.

The protruding portion5din the first modification can relieve stress concentration at the base of the protruding portion5dat the first main surface5aside compared with the protruding portion5din the above embodiment, while can cause the positions where impact occurs to concentrate at the second main surface5bside. Further, compared with the protruding portion5din the above embodiment, the protruding portions5din the second and third modifications can cause the positions where impact occurs to concentrate at the second main surface5bside. The protruding portions5din the first to third modifications can be formed by for example forming the entire second blade45or its outer circumferential edge to be tapered.

EXAMPLES

For the substrates5in the above embodiment and first modification, concrete dimensions etc. were set and simulations were performed concerning impact. Specifically, this was as follows:

Simulation ConditionsBasic dimensions of substrate5Lx (see FIG.2)=0.6 mmLy (see FIG.2)=0.8 mmLz (see FIG.2)=0.2 mmDimensions of protruding portion5dd1(seeFIG. 2and FIG.7A)=0.0075 mmd2(seeFIG. 2and FIG.7A)=0.037 mmYoung's modulus of substrate5: 230 GPa (assuming LiTaO3)Poisson's ratio of substrate5: 0.3 (assuming LiTaO3)Density of substrate5: 7450 kg/m3(assuming LiTaO3)Assumed situation: A situation was assumed where the substrate5was dropped in a Y-direction (seeFIG. 2) and struck an XZ surface (surface perpendicular to Y-direction). A velocity at the time of impact of 3.2 m/s and an acceleration at the time of impact of 9.8 m/s2(gravitational acceleration) were assumed.

Computation Method

The finite element method was used to calculate the stress distribution in the substrate5in time sequence.

That is, when dropping the substrate5in the Y-direction, the protruding portion5dimpacts the XZ surface whereby stress is generated at the protruding portion5d. This stress is propagated to the entire substrate5along with the elapse of time. This situation was reproduced and the stress of each part of the substrate5at each point of time was examined.

Evaluation Method

The maximum stress generated at the first main surface5awas extracted.

Reason: This is because, as explained above, for maintenance of performance of the SAW device1, it is considered that maintaining the shape of the part at the first main surface5aside is more important than maintaining the shape of the part at the second main surface5bside. Further, it is considered that the influence of the maximum value is greater than the mean value of stress.

Note that, the position of generation and point of time of generation in the first main surface5aof the maximum stress which is generated in the first main surface5adiffer according to the simulation conditions.

Simulation Results

Case of the protruding portion5din the above embodimentMaximum stress in first main surface5a:2.8×108Pa

Case of protruding portion5din the first modificationMaximum stress in first main surface5a:2.5×108Pa

It was confirmed from the simulation results that the maximum value of stress generated in the first main surface5awas smaller in the case where the protruding portion5dwas tapered so as to further protrude outward toward the second main surface5bside than the case where the protruding portion5dwas rectangular.

The present invention is not limited to the above embodiment and modifications and may be executed in various ways.

The acoustic wave device is not limited to a SAW device. For example, the acoustic wave device may be a film bulk acoustic resonator. Further, the acoustic wave device may be a boundary acoustic wave device utilizing a boundary acoustic wave.

In the acoustic wave device, the resin film (11), back surface electrode (13), resin layer (15), and protective film (29) may be omitted. Conversely, other suitable layers etc. may be formed. Further, in the case of the boundary acoustic wave device, an acoustic wave device can be prepared without provision of vibration spaces S.

Further, the shape of the protruding portion5dis not limited to the above explained ones. For example, the protruding portion5dmay be formed so as to gradually become broader from the first main surface3aof the piezoelectric substrate3toward the second main surface3b. In other words, the protruding portion5dmay be provided so that the shape of the piezoelectric substrate3becomes generally trapezoidal when viewing it from the side surface.

The cutting of the wafer is not limited to cutting carried out by using a blade. For example, the cutting may be carried out by using a laser. Further, a plurality of methods may be combined. For example, the first cutting step may be carried out by using a blade and the second cutting step may be carried out by using a laser.

Note that, in the SAW device, whether a side surface (5c) at the one main surface side (5a) other than the protruding portion (5d) and the surface (5e) of the protruding portion facing the side direction of the substrate are formed by surfaces cut by a blade can be identified by for example observation of the surfaces by an SEM (scanning electron microscope). For example, when a wafer is cut by a blade, straight (strictly speaking, arc-shaped) grooves which are formed by cutting by abrasive grains and are parallel to the main surface is formed in the side surface, therefore these grooves can be observed by the SEM.

The cutting in the second cutting step (FIG. 6D) need not be carried out from one main surface5aside (side cut in the first cutting step) or may be carried out from the other main surface5bside.

In the filling step (FIG. 6C) of resin which forms the resin film11, the resin need not be filled up to the upper surface9aof the cover9. For example, the resin may be filled up to one main surface5aof the substrate5or a part lower than it, or may be filled up to the upper face of the cover or a part lower than it.

REFERENCE SIGNS LIST