Surface acoustic wave device and method of fabricating the same

Abstract

A surface acoustic wave device includes a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first film are provided. The first film is located so as to surround the comb-like electrodes. A base substrate has a second surface on which second pads joined to the first pads and a second film joined to the first film are provided. The first and second films joined by a surface activation process define a cavity in which the comb-like electrodes and the first and second pads are hermetically sealed.

What is claimed is: 
     
       1. A surface acoustic wave device comprising:
 a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first film are provided, the first film being located so as to surround the comb-like electrodes;   a base substrate having a second surface on which second pads joined to the first pads and a second film joined to the first film are provided;   an electronic element provided on an area of the second surface facing the first surface; and   a ceramic substrate supporting the base substrate, a chip electronically coupled to the second pads being mounted on the ceramic substrate, the base substrate having a plate shape, and the first and second films joined by a surface activation process defining a cavity in which the comb-like electrodes, the first and second pads, and the electronic element are hermetically sealed.   
     
     
       2. The surface acoustic wave device as claimed in  claim 1 , wherein each of the first and second films contains a metal as a major component. 
     
     
       3. The surface acoustic wave device as claimed in  claim 1 , wherein the first and second films are joined via joining surfaces that contain gold. 
     
     
       4. The surface acoustic wave device as claimed in  claim 1 , wherein the base substrate is one of a semiconductor substrate and an insulator substrate. 
     
     
       5. The surface acoustic wave device as claimed in  claim 1 , wherein the base substrate is made of silicon. 
     
     
       6. The surface acoustic wave device as claimed in  claim 1 , wherein the first film is provided on peripheral portions on the first surface of the piezoelectric substrate, and the second film is provided on peripheral portions on the second surface of the base substrate. 
     
     
       7. The surface acoustic wave device as claimed in  claim 1 , further comprising an impedance matching circuit on the second surface of the base substrate, the impedance matching circuit being coupled to at least one of the comb-like electrodes. 
     
     
       8. The surface acoustic wave device as claimed in  claim 1 , wherein the comb-like electrodes and the first pads form a transmit filter and a receive filter. 
     
     
       9. The surface acoustic wave device as claimed in  claim 1 , wherein:
 the comb-like electrodes and the first pads form a transmit filter and a receive filter;   the surface acoustic wave device comprises an impedance matching circuit coupled to at least one of the transmit filter and the receive filter, and a common terminal via which an external connection with the impedance matching circuit can be made.   
     
     
       10. The surface acoustic wave device as claimed in  claim 1 , wherein the base substrate has via-wiring lines connected to the second pads, so that electric connections with the first pads can be made on a surface of the base substrate opposite to the second surface. 
     
     
       11. A surface acoustic wave device comprising:
 a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first film are provided, the first film being located so as to surround the comb-like electrodes; and   a base substrate having a second surface on which second pads joined to the first pads and a second film joined to the first film are provided,   the first and second films joined by a surface activation process defining a cavity in which the comb-like electrodes and the first and second pads are hermetically sealed,   wherein the surface acoustic wave device further comprises a support substrate joined to a third surface of the piezoelectric substrate opposite to the first surface;   the piezoelectric substrate and the support substrate have been subjected to the surface activation process; and   the support substrate is one of a silicon substrate and a sapphire substrate.   
     
     
       12. A method of fabricating a surface acoustic wave device comprising the steps of:
 (a) forming a first metal film on a first surface of a piezoelectric substrate on which comb-like electrodes and first pads are provided, wherein the first metal film surrounds the comb-like electrodes and the first pads and is provided along edges of the piezoelectric substrate; 
 (b) forming a second metal film on a second surface of a base substrate other than a piezoelectric substrate, the base substrate comprising second pads, wherein the second metal film is provided along edges of the base substrate, and a position of the second metal film and the second pads corresponds to a position of the first metal film and the first pads respectively; 
 (c) applying a surface activation process to surfaces of the first metal film and surfaces of the second metal film so that residual impurities are removed; and 
 (d) joining the first metal film and the second metal film so as to join bond the activated surfaces thereof directly, wherein the comb-like electrodes, the first pads and the second pads are hermetically sealed in a cavity defined by the first metal film and the second metal film, 
 edges of the first and second metal films being flush with side surfaces of the piezoelectric substrate and those of the base substrate and forming parts of side surfaces of the surface acoustic wave device, 
 the base substrate having a contact hole for making an external connection with the first and second metal films. 
 
     
     
       13. A method of fabricating a surface acoustic wave device comprising the steps of:
 (a) forming a first film on a first surface of a piezoelectric substrate on which comb-like electrodes and first pads are formed so as to be surrounded by the first film;   (b) forming a second film on a second surface of a base substrate on which second pads and an electronic element are formed so as to be surrounded by the second film, the second film and the second pads corresponding to the first film and the first pads in position, the base substrate having a plate shape and is being supported by a ceramic substrate on which a chip electronically coupled to the electronic element is mounted;   (c) subjecting a surface activation process to surfaces of the first and second films; and   (d) joining the first and second films so as to join activated surfaces thereof,   the comb-like electrodes, the first and second pads and the electronic element being hermetically sealed in a cavity defined by the first and second films.   
     
     
       14. The method as claimed in  claim 13 , wherein the steps (a) and/or (b) forms the first and/or second film that contains a metal as a major component. 
     
     
       15. The method as claimed in  claim 13 , further comprising a step of forming an electric element on the second surface. 
     
     
       16. The method as claimed in  claim 13 , further comprising a step of forming via-wiring lines in the base substrate so that the second pads can be extended to a third surface of the base substrate opposite to the second surface. 
     
     
       17. A method of fabricating a surface acoustic wave device comprising the steps of:
 (a) forming a first film on a first surface of a piezoelectric substrate on which comb-like electrodes and first pads are formed so as to be surrounded by the first film;   (b) forming a second film on a second surface of a base substrate, the second film corresponding to the first film in position;   (c) subjecting a surface activation process to surfaces of the first and second films; and   (d) joining the first and second films so as to join activated surfaces thereof,   the comb-like electrodes being hermetically sealed in a cavity defined by the first and second films,   wherein the method further comprises a step of joining a support substrate to a backside of the piezoelectric substrate opposite to the first surface after an interface between the piezoelectric substrate and the support substrate is subjected to the surface activation process,   the support substrate being one of a silicon substrate and a sapphire substrate.   
     
     
       18. A surface acoustic wave device comprising:
 a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first film are provided, the first film surrounding the comb-like electrodes;   a base substrate having a second surface on which second pads joined to the first pads and a second film joined to the first film are provided; and   an electronic element provided on an area of the second surface facing the first surface, wherein the first film and the second film are configured to be joined by a surface activation process and to define a cavity in which the comb-like electrodes, the first pads, the second pads, and the electronic element are hermetically sealed,   the second pads being electrically extended via a through hole penetrating the base substrate, to a third surface of the base substrate opposite to the second surface thereof,   the first and second films being electrically extended, via another through hole penetrating the base substrate, to the third surface,   the piezoelectric substrate and the base substrate having an identical width and length forming flat side surfaces of the surface acoustic wave device.   
     
     
       19. A method of fabricating a surface acoustic wave device, comprising the steps of:
 (a) forming a first film on a first surface of a piezoelectric substrate comprising comb-like electrodes and first pads, wherein the first film surrounds the comb-like electrodes and the first pads;   (b) forming a second film on a second surface of a base substrate comprising second pads and an electronic element, wherein the second film surrounds the second pads and the electronic element, and a position of the second film and the second pads corresponds to a position of the first film and the first pads, respectively;   (c) applying a surface activation process to surfaces of the first film and surfaces of the second film; and   (d) joining the first film and the second film so as to join the activated surfaces thereof, wherein the comb-like electrodes, the first pads, the second pads, and the electronic element are hermetically sealed in a cavity defined by the first film and the second film,   the second pads being electrically extended, via a through hole penetrating the base substrate, to a third surface of the base substrate opposite to the second surface thereof,   the first and second films being electrically extended, via another through hole penetrating the base substrate, to the third surface,   the piezoelectric substrate and the base substrate having an identical width and length forming flat side surfaces of the surface acoustic wave device.   
     
     
       20. A surface acoustic wave device comprising:
 a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first metal film are provided, the first metal film being provided along edges of the piezoelectric substrate and being located so as to surround the comb like electrodes; and 
 a base substrate other than a piezoelectric substrate, the base substrate having a second surface on which second pads joined to the first pads and a second metal film joined to the first metal film are provided, the second metal film being provided along edges of the base substrate, 
 wherein the first and second metal films are configured to be joined by a surface activation process directly bonded and define a cavity in which the comb-like electrodes and the first and second pads are hermetically sealed, 
 edges of the first and second metal films being flush with side surfaces of the piezoelectric substrate and those of the base substrate and forming parts of side surfaces of the surface acoustic wave device, 
 the base substrate having a contact hole for making an external connection with the first and second metal films. 
 
     
     
       21. The surface acoustic wave device of claim 20, wherein the first and second metal films each comprise at least one metal selected from a group consisting of Au, Al, Cu, Ti, Cr, and Ta.  
     
     
       22. The surface acoustic wave device of claim 20, wherein the base substrate is one of a semiconductor substrate and an insulator substrate.  
     
     
       23. The surface acoustic wave device of claim 20, wherein the comb-like electrodes and the first pads form a transmit filter or a receive filter.  
     
     
       24. The surface acoustic wave device of claim 20, wherein the piezoelectric substrate and the base substrate have an identical width and length forming flat side surfaces of the acoustic wave device.  
     
     
       25. An acoustic wave device comprising:
 a first substrate having a first surface on which electrodes of an acoustic wave filter are provided, first pads connected thereto, and a first film made of a metal are provided, the first film located so as to surround the electrodes; and   a base substrate other than the first substrate, the base substrate having a second surface on which second pads joined to the first pads and a second film made of a metal joined to the first film are provided,   wherein the first and second films are directly bonded without solder and define a cavity in which the electrodes and the first and second pads are hermetically sealed,   edges of the first and second films being flush with side surfaces of the first substrate and those of the base substrate and forming parts of side surfaces of the acoustic wave device,   the base substrate having a contact hole for making an external connection with the first and second films.    
     
     
       26. The acoustic wave device of claim 25, wherein the first and second films each comprise at least one metal selected from a group consisting of Au, Al, Cu, Ti, Cr, and Ta.  
     
     
       27. The acoustic wave device of claim 25, wherein the base substrate is one of a semiconductor substrate and an insulator substrate.  
     
     
       28. The acoustic wave device of claim 25, wherein the first film is provided on peripheral portions on the first surface of the first substrate, and the second film is provided on peripheral portions on the second surface of the base substrate.  
     
     
       29. The acoustic wave device of claim 25, wherein the electrodes and the first pads form a transmit filter and a receive filter.  
     
     
       30. The acoustic wave device of claim 25, wherein the first substrate and the base substrate have an identical width and length forming flat side surfaces of the acoustic wave device.  
     
     
       31. The method of claim 12, wherein the first and second metal films each comprise at least one metal selected from a group consisting of Au, Al, Cu, Ti, Cr, and Ta.  
     
     
       32. The method of claim 12, wherein the base substrate is one of a semiconductor substrate and an insulator substrate.  
     
     
       33. The method of claim 12, wherein the comb-like electrodes and the first pads form a transmit filter or a receive filter.  
     
     
       34. The method of claim 12, wherein the piezoelectric substrate and the base substrate have an identical width and length forming flat side surfaces of the acoustic wave device.  
     
     
       35. A method of fabricating an acoustic wave device comprising the steps of:
 (a) forming a first film made of a metal on a first surface of a first substrate on which electrodes of an acoustic wave filter and first pads are provided, wherein the first film surrounds the electrodes and the first pads;   (b) forming a second film made of a metal on a second surface of a base substrate other than the first substrate, the base substrate comprising second pads, wherein a position of the second film and the second pads corresponds to a position of the first film and the first pads respectively;   (c) applying a surface activation process to surfaces of the first film and surfaces of the second film so that residual impurities are removed; and   (d) joining the first film and the second film so as to bond the activated surfaces thereof directly, wherein the electrodes, the first pads and the second pads are hermetically sealed in a cavity defined by the first metal film and the second metal film,   edges of the first and second metal films being flush with side surfaces of the first substrate and those of the base substrate and forming parts of side surfaces of the acoustic wave device,   the base substrate having a contact hole for making an external connection with the first and second films.    
     
     
       36. The method of claim 35, wherein the first and second films each comprise at least one metal selected from a group consisting of Au, Al, Cu, Ti, Cr, and Ta.  
     
     
       37. The method of claim 35, wherein the base substrate is one of a semiconductor substrate and an insulator substrate.  
     
     
       38. The method of claim 35, wherein the first film is provided on peripheral portions on the first surface of the first substrate, and the second film is provided on peripheral portions on the second surface of the base substrate.  
     
     
       39. The method of claim 35, wherein the electrodes and the first pads form a transmit filter and a receive filter.  
     
     
       40. The method of claim 35, wherein the first substrate and the base substrate have an identical width and length forming flat side surfaces of the acoustic wave device.