Surface acoustic wave device

Abstract

A surface acoustic wave device includes a piezoelectric substrate made of LiTaO 3 or LiNbO 3 having an electromechanical coefficient of about 15% or more, at least one electrode which is disposed on the piezoelectric substrate and which is a laminate film having a metal layer defining a primary metal layer primarily composed of a metal having a density higher than that of Al or an alloy of the metal and a metal layer which is laminated on the primary metal layer and which is composed of another metal, and a first SiO 2 layer which is disposed in a remaining area other than that at which the at least one electrode is located and which has a thickness approximately equivalent to that of the electrode. In the surface acoustic wave device described above, the density of the electrode is at least about 1.5 times that of the first SiO 2 layer. In addition, a second SiO 2 layer disposed so as to cover the electrode and the first SiO 2 layer and a silicon nitride compound layer disposed on the second SiO 2 layer are further provided.

1 . A method of manufacturing a surface acoustic wave device comprising the step of:
 providing a piezoelectric substrate of made one of LiTaO 3  or LiNbO 3  and having an electromechanical coefficient of at least about 15%;   forming at least one electrode having a density greater than that of Al on the piezoelectric substrate;   forming a first SiO 2  layer on a remaining area other than that at which said at least one electrode is formed, the first SiO 2  layer having a thickness approximately equal to that of the at least one electrode;   forming a second SiO 2  layer so as to cover the at least one electrode and the first SiO 2  layer; and   forming a silicon nitride compound layer on the second SiO 2  layer; wherein   the density of the at least one electrode is at least about 1.5 times that of the first SiO 2  layer.   
     
     
         2 . The method of manufacturing a surface acoustic wave device according to  claim 1 , wherein when the thickness of the second SiO 2  film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.08≦h/λ≦0.5 is satisfied. 
     
     
         3 . The method of manufacturing a surface acoustic wave device, according to  claim 1 , wherein the silicon nitride compound layer is formed of an SiN layer, and when the thickness of the SiN layer is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0<h/λ≦0.1 is maintained. 
     
     
         4 . The method of manufacturing a surface acoustic wave device, according to  claim 1 , further comprising the step of:
 forming a diffusion inhibition film made of SiN between the second SiO 2  layer and the at least one electrode; wherein   when the thickness of the diffusion inhibition film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.005≦h/λ≦0.05 is satisfied.   
     
     
         5 . The method of manufacturing a surface acoustic wave device, according to  claim 1 , wherein the at least one electrode is formed of one of Cu, a Cu alloy, and a laminate film comprising a metal layer primarily composed of Cu or a Cu alloy. 
     
     
         6 . The method of manufacturing a surface acoustic wave device, according to  claim 1 , wherein the piezoelectric substrate is made of one of rotated Y-plate X-propagation LiTaO 3  or LiNbO 3 , and when the thickness of the second SiO 2  layer is represented by h 1 , the thickness of the silicon nitride compound layer formed on the second SiO 2  layer is represented by h 2 , the wavelength of a surface acoustic wave is represented by λ, and the following equations are satisfied:
   coefficient A 1 =−190.48;     coefficient A 2 =76.19;     coefficient A 3 =−120.00;     coefficient A 4 =−47.30;     coefficient A 5 =55.25;       H   1   =h   1   /λ,H   2   =h   2 /λ, and θ=( A   1   H   1   2   +A   2   H   1   +A   3 ) H   2   +A   4   H   1   +A   5 ,   
       a Y-X cut angle of the rotated Y-plate X-propagation piezoelectric substrate is in the range of θ±5°. 
     
     
         7 . The method of manufacturing a surface acoustic wave device according to  claim 1 , wherein the density of the at least one electrode is at least about 2.5 times that of the first SiO 2  layer. 
     
     
         8 . The method of manufacturing a surface acoustic wave device according to  claim 1 , wherein the step of forming the at least one electrode includes the steps of:
 laminating a metal layer as a primary metal layer made of a metal having a density greater than that of Al or an alloy primarily composed of the metal; and   laminating a metal layer on the primary metal layer and which is made of another metal.   
     
     
         9 . The method of manufacturing a surface acoustic wave device according to  claim 1 , further comprising the step of:
 planarizing an upper surface of each of the at least one electrode and the first SiO 2  layer before the step of forming the second SiO 2  layer.   
     
     
         10 . The method of manufacturing a surface acoustic wave device according to  claim 1 , further comprising the step of:
 planarizing an upper surface of the second SiO 2  layer before the step of forming the silicon nitride compound layer.