Elastic wave device and method of manufacturing the device

An elastic wave device includes a piezoelectric substrate, a comb-shaped electrode above an upper surface of the piezoelectric substrate, a wiring connected to the comb-shaped electrode, an element cover above the upper surface of the piezoelectric substrate for covering the comb-shaped electrode across a space, a first electrode above an upper surface of the element cover, a sealing resin above the upper surface of the piezoelectric substrate for covering the element cover and the first electrode, a terminal electrode above an upper surface of the sealing resin, and a second electrode passing through the sealing resin for electrically connecting the first electrode with the terminal electrode. The first and second electrodes are made of films produced by electro-plating. The diameter of plating particles of the first electrode may be larger than that of plating particles of the second electrode.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2013/000802, filed on Feb. 14, 2013, which in turn claims the benefit of Japanese Application No. 2012-041644, filed on Feb. 28, 2012, the disclosures of which Applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to an elastic wave device and a method of manufacturing the device.

BACKGROUND ART

FIG. 5is a cross sectional view of a conventional elastic wave device1. The elastic wave device1includes piezoelectric substrate2, comb-shaped electrode3disposed on piezoelectric substrate2, wiring4disposed on piezoelectric substrate2, side wall5surrounding comb-shaped electrode3, covering part7disposed on an upper surface of side wall5, electrode8disposed on covering part7, sealing resin9sealing cover part7and electrode8, electrode10passing through sealing resin9, and terminal electrode11formed on an upper surface of electrode10. Side wall5surrounds space6in which comb-shaped electrode3is excited. Covering part7covers space6from above the space.

A conventional elastic wave device similar to elastic wave device1is described in Patent Literature 1.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

An elastic wave device includes a piezoelectric substrate, a comb-shaped electrode provided above an upper surface of the piezoelectric substrate, a wiring connected to the comb-shaped electrode, an element cover provided above the upper surface of the piezoelectric substrate for covering the comb-shaped electrode across a space, a first electrode provided above an upper surface of the element cover, a sealing resin provided above the upper surface of the piezoelectric substrate for covering the element cover and the first electrode, a terminal electrode provided above an upper surface of the sealing resin, and a second electrode passing through the sealing resin for electrically connecting the first electrode with the terminal electrode.

In this elastic wave device, the first electrode and the second electrode may be made of a film produced by electro-plating. A diameter of plating particles of the first electrode may be larger than a diameter of plating particles of the second electrode.

The Young's modulus of the first electrode may be smaller than the Young's modulus of the second electrode.

The first electrode may be made of a film produced by matte copper plating, and the second electrode may be made of a film produced by bright copper plating.

An internal stress of the first electrode may be a tensile stress. An internal stress of the second electrode may be a compressive stress. The internal stress of the first electrode may be smaller than the internal stress of the second electrode.

The density of the first electrode is lower than the density of the second electrode.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1is a cross sectional view of elastic wave device21according to an exemplary embodiment.

As shown inFIG. 1, elastic wave device21includes comb-shaped electrode23provided on and above upper surface22A of piezoelectric substrate22, wiring24provided on and above upper surface22A of piezoelectric substrate22, side wall25provided on upper surface22A of piezoelectric substrate22, covering part27provided on and above upper surface25A of side wall25, electrode29provided from upper surface24A of wiring24to upper surface27A of covering part27, sealing resin30sealing covering part27and electrode29from above covering part27and electrode29, electrode31passing through sealing resin30, and terminal electrode32provided on and above upper surface31A of electrode31. Side wall25surrounds comb-shaped electrode23and constitutes space26in which comb-shaped electrode23is excited.

Piezoelectric substrate22is made of rotated Y-cut X-propagation single crystal lithium tantalite, having a thickness ranging from about 100 to 350 μm.

Comb-shaped electrode23is made of metal mainly containing aluminum and is provided on upper surface22A of piezoelectric substrate22. Upon a voltage being applied, comb-shaped electrode23A excites a surface acoustic wave at upper surface22A of piezoelectric substrate22. A protective film made of dielectric material, such as silicon oxide, may be provided on a surface of comb-shaped electrode23if necessary.

Wiring24is a conductor provided on upper surface22A of piezoelectric substrate22, and is electrically connected to comb-shaped electrode23.

Side wall25is formed by exposing and developing polyimide-based light curing resin.

Space26is provided above comb-shaped electrode23allowing a surface acoustic wave to be excited therein.

Covering part27covers space26from above space26for sealing space26. Covering part27is formed by exposing and developing a polyimide-based light curing resin sheet attached onto upper surface25A of side wall25. Side wall25and covering part27constitute element cover28sealing comb-shaped electrode23and space26.

Electrode29is made of a conductive film formed by matt electrolytic copper plating and provided from upper surface24A of wiring24to upper surface27A of covering part27through an outer side surface of side wall25. The film constituting electrode29is formed by the electro-plating using a plating solution composed of 150 to 200 g/L of copper sulfate pentahydrate, 50 to 90 g/L of sulfuric acid, an appropriate amount of chloride ion, and an additive mainly made of a surface active agent. The diameter of plating particles ranges from 3 to 10 μm. The Young's modulus of the plating film ranges from 8 to 17 GPa. An internal stress of the plating film is a tensile stress ranging from about 10 to 20 MPa. Electrode29covers most of element cover28to provide a mechanical strength of element cover28, effectively shielding comb-shaped electrode23and wiring24, and thus restraining water to enter into space26. Electrode29has a larger particle size, a rougher plating surface, a smaller Young's modulus, and a softer plating surface than electrode31formed by bright copper electrolytic plating. The internal stress of electrode29is a tensile stress smaller than an internal stress of electrode31. The density of electrode29is lower than electrode31.

Sealing resin30is provided on and above upper surface22A of piezoelectric substrate22made of hardened epoxy-based resin containing filler, such as silica, and seals element cover28and electrode29.

Electrode31is a via-electrode passing through sealing resin30and connecting electrode29with terminal electrode32, and is made of a film formed by bright copper electrolytic plating. The bright electrolytic copper plating film constituting electrode31is obtained by the electroplating using a copper sulfate plating solution composed of 150 to 250 g/L of copper sulfate pentahydrate, 50 to 120 g/L sulfuric acid, an appropriate amount of chloride ion, an additive, such as polyethylene glycol, and a surface-active agent. Polyethylene glycol functions as a suppressive component in plating deposition, and prevents local concentration of a current, making deposited particle in copper plated film small. The surface-active agent reduces a surface tension of the plating solution, increasing wettability between an object to be plated and plating solution. The surface-active agent weakly controls plating and hardly controls the diameter of a deposited particle effectively. However, upon being used with polyethylene glycol, the surface-active agent can control the diameter of the particle while maintaining the wettability of the plating solution. The bright electrolytic copper plating film constituting electrode31has a compressive stress ranging from about 5 to 10 MPa as an internal stress. Electrode31has a smaller particle size, a brighter plating surface, larger Young's modulus, and a harder plating surface than electrode29formed by matte electrolytic copper plating. The internal stress of electrode31is a relatively higher compressive stress. Electrode31has a higher density and larger conductivity.

Terminal electrode32is provided on upper surface31A of electrode31and on upper surface30A of sealing resin30which surrounds the electrode for electrically connecting elastic wave device21to an external electronic circuit.

Elastic wave device21is called a wafer-level CSP, having a significantly small size identical to that of piezoelectric substrate22having the elastic wave element provided thereon.

In the conventional elastic wave device1shown inFIG. 5, piezoelectric substrate2may warp and break during a manufacturing process with an internal stress of electrode8, reducing manufacturing yield.

In elastic wave device21according to the embodiment, a diameter of plating particles of electrode29is larger than a diameter of plating particles of electrode31, allowing the plating film of electrode29to be softer than the plating film of electrode31. The warping of piezoelectric substrate22and the breaking of piezoelectric substrate22caused by the warping are consequently reduced, improving manufacturing yield of elastic wave device21.

As the diameter of the plating particle of electrode29increases, the surface of electrode29can be rougher. Resultantly, without specifically applying a roughing process to the surface of electrode29, bonding strength is secured between electrode29and sealing resin electrode30, reducing manufacturing processes while ensuring mechanical strength of elastic wave device21.

In elastic wave device21, the Young's modulus of electrode29is smaller than the Young's modulus of electrode31, allowing the plating film of electrode29to be softer than the plating film of electrode31. Warping of piezoelectric substrate22and breaking of piezoelectric substrate22caused by the warping of piezoelectric substrate22are therefore reduced, improving manufacturing yield of elastic wave device21.

Further, in elastic wave device21, an internal stress of electrode29is a tensile stress while an internal stress of electrode31is a reversely exerted compressive stress, hence cancelling the stresses each other and reducing a total stress applied to piezoelectric substrate22. Resultantly, the warping caused with piezoelectric substrate22and the breaking caused by the warping are reduced, improving manufacturing yield of elastic wave device21. Furthermore, since the internal stress of larger dimension electrode29which greatly affects the warping of piezoelectric substrate22is lower than an internal stress of small electrode31which less affects warping of piezoelectric substrate22, hence reducing total stress applied to piezoelectric substrate22and further reducing the warping of piezoelectric substrate22.

Further, in elastic wave device21, the density of electrode29is small, and allows the film of electrode29to be soft. The stress of electrode29is therefore small and reduces warping of piezoelectric substrate22, thus allowing the surface of electrode29to be roughened.

In elastic wave device21, electrode31has a high density and is a fine film, and reduces a conductive resistance of the electrode. This reduces an influence of electrode31on electric performance of elastic wave device21, and ensures a mechanical strength and connecting reliability as the via electrode. Moreover, electrode31generates a relatively strong compressive stress in a direction reverse to the stress of electrode29, reducing the total stress applied to piezoelectric substrate22.

In elastic wave device21, electrode29is formed by matte electrolytic copper plating. The diameter of the plating particles of electrode29is large. The surface of electrode29is rough and the density of the film is low. Therefore, the Young's modulus of electrode29is small. The internal stress is low and is a tensile stress.

In elastic wave device21, electrode31is formed by bright copper electrolytic plating. The particle size of the plating of electrode31is small and the density of the electrode is high. Thus, the surface of electrode31is bright. The Young's modulus of electrode31is large. An internal stress is relatively large and is compressive stress.

A method of manufacturing elastic wave device21will be described below.FIGS. 2A to 2C, 3A to 3C, and 4A to 4Care cross sectional views of elastic wave device21for schematically illustrating a method of manufacturing elastic wave device21.

First, a metal thin film is formed on upper surface22A of piezoelectric substrate22, and is etched by a photolithographic technology, thereby forming plural comb-shaped electrode23and wiring24, as shown inFIG. 2A.

Then, a sheet made of polyimide-based light curing resin is placed on upper surface22A of piezoelectric substrate22. The sheet is exposed, developed, and hardened, thereby forming side wall25which surrounds comb-shaped electrode23and which constitutes space26, as shown inFIG. 2B.

Then, a sheet made of polyimide-based light curing resin is placed on upper surface25A of outside wall25. The sheet is then exposed, developed and hardened, thereby forming covering part27covering space26from above in which comb-shaped electrode23is excited. Side wall25and covering part27constitute element cover28, as shown inFIG. 2C.

Next, a feeding conductor for electrolytic copper plating is formed. Plating resist33is formed on upper surface22A of piezoelectric substrate22. Plating resist133is formed on upper surface27A of covering part27. Then, electrode29covering upper surface24A of wiring24through side wall25to upper surface27A of covering part27is formed by matte electrolytic copper plating, as shown inFIG. 3A. Plating resist133prevents wiring24to short-circuit through electrode29.

Then, plating resists33and133and the feeding conductor used for the electro-plating are removed, as shown inFIG. 3B.

Then, a sheet made of epoxy-based resin containing a filler is placed and hardened to form sealing resin30covering element cover28and electrode29, as shown inFIG. 3C.

Next, as shown inFIG. 4A, opening34passing through sealing resin30to expose a part of upper surface29A of electrode29is formed by laser cutting. Then, an inside surface of opening34and upper surface30A of sealing resin30are desmeared and roughened with permanganic acid.

Then, a feeding conductor for electrolytic copper plating and plating resist35are formed on upper surface30A of sealing resin30. Electrode31and terminal electrode32filling opening34are formed by bright electrolytic copper plating, as shown inFIG. 4B.

Finally, plating resist35and the feeding conductor for electrolytic copper plating are removed to provide elastic wave device21, as shown inFIG. 4C.

As explained, in elastic wave device21, electrode29is produced by matte electrolytic copper plating while electrode31is produced by bright copper electrolytic plating, thereby reducing warping and breaking of piezoelectric substrate22.

According to the embodiment, terms, such as “upper surface” and “above”, indicating directions indicate a relative direction depending on a relative positional relationship of constituent components, such as the piezoelectric substrate and the comb-shaped electrode, of the elastic wave device, and do not indicate absolute directions, such as a vertical direction.

INDUSTRIAL APPLICABILITY

An elastic wave device according to the present invention is useful for a high frequency filter, a splitter, a duplexer or the like to be used for a mobile communication device.

REFERENCE MARKS IN THE DRAWINGS