Ladder type filter

Abstract

A ladder filter includes a series resonator having a first film laminate in which an upper electrode and a lower electrode face each other across a piezoelectric film, and a first film provided on the first film laminate, and a parallel resonator having a second film laminate having a structure similar to that of the first film laminate, a second film provided on the second film laminate, and another first film identical to the first film.

1 . A ladder filter comprising:
 a series resonator having a first film laminate in which an upper electrode and a lower electrode face each other across a piezoelectric film, and a first film provided on the first film laminate; and   a parallel resonator having a second film laminate having a structure similar to that of the first film laminate, a second film provided on the second film laminate, and another first film identical to the first film.   
   
   
       2 . The ladder filter as claimed in  claim 1 , wherein the second film comprises an electrically conductive film. 
   
   
       3 . The ladder filter as claimed in  claim 1 , further comprising a third film provided on the first film of the series resonator, and another third film provided on the another first film of the parallel resonator, the another third film being identical to the third film. 
   
   
       4 . The ladder filter as claimed in  claim 3 , wherein the third film and the another third film are insulation films. 
   
   
       5 . The ladder filer as claimed in  claim 1 , wherein a region in which the second film is provided includes another region in which the upper electrode and the lower electrode face each other across the piezoelectric film. 
   
   
       6 . The ladder filter as claimed in  claim 1 , wherein the first film of the series resonator has a thickness equal to that of the another first film. 
   
   
       7 . The ladder filter as claimed in  claim 1 , wherein the upper electrode comprises a ruthenium film, the first film and the another first film comprise a chromium film, and the second film and the another second film comprise a titanium film. 
   
   
       8 . The ladder filter as claimed in  claim 1 , wherein a region in which the upper electrode faces the lower electrode across the piezoelectric film has an oval shape. 
   
   
       9 . The ladder filter as claimed in  claim 1 , wherein a region in which the upper electrode faces the lower electrode across the piezoelectric film has a polygonal shape composed of anti-parallel sides. 
   
   
       10 . The ladder filter as claimed in  claim 1 , wherein the lower electrode faces a hollow space in a region in which the upper electrode faces the lower electrode across the piezoelectric film, the hollow space being provided on or in a substrate. 
   
   
       11 . The ladder filter as claimed in  claim 1 , wherein the film laminate is formed so as to have hollow space having a dome shape defined on a substrate. 
   
   
       12 . The ladder filter as claimed in  claim 10 , wherein the region is included in the hollow space. 
   
   
       13 . The ladder filter as claimed in  claim 11 , wherein the region is included in the hollow space. 
   
   
       14 . The ladder filter as claimed in  claim 11 , wherein the film laminate has compressive stress. 
   
   
       15 . The ladder filter as claimed in  claim 1 , wherein the piezoelectric film is made of aluminum nitride or zinc oxide having an orientation having a main axis in a (002) direction. 
   
   
       16 . The ladder filter as claimed in  claim 2 , further comprising a third film provided on the first film of the series resonator, and another third film provided on the another first film of the parallel resonator, the another third film being identical to the third film.