Temperature compensated surface acoustic wave device having mass loading strip with buffer layer

Abstract

An acoustic wave device and a method of forming the same is disclosed. The acoustic wave device can include a piezoelectric layer, an interdigital transducer electrode formed with the piezoelectric layer, and a temperature compensation layer over the interdigital transducer electrode. The interdigital transducer electrode includes a bus bar and fingers that extend from the bus bar. The fingers each includes an edge portion and a body portion. The acoustic wave device can include a mass loading strip that overlaps the edge portions of the fingers. A portion of the temperature compensation layer is positioned between the mass loading strip and the piezoelectric layer. The acoustic wave device can include a buffer layer that is disposed at least partially between the mass loading strip and the temperature compensation layer. The buffer layer includes a material different from materials of the temperature compensation layer and the mass loading strip.

What is claimed is: 
     
         1 . An acoustic wave device comprising:
 a piezoelectric layer;   an interdigital transducer electrode formed with the piezoelectric layer, the interdigital transducer electrode including a bus bar and fingers extending from the bus bar, the fingers each including an edge portion and a body portion;   a temperature compensation layer over the interdigital transducer electrode;   a mass loading strip overlapping the edge portions of the fingers, a portion of the temperature compensation layer being positioned between the mass loading strip and the piezoelectric layer; and   a buffer layer disposed at least partially between the mass loading strip and the temperature compensation layer, the buffer layer including a material different from materials of the temperature compensation layer and the mass loading strip.   
     
     
         2 . The acoustic wave device of  claim 1  wherein the mass loading strip includes a layer having a density that is at least as high as a most dense layer of a material of the interdigital transducer electrode. 
     
     
         3 . The acoustic wave device of  claim 2  wherein the mass loading strip includes a second layer. 
     
     
         4 . The acoustic wave device of  claim 1  wherein the mass loading strip is embedded in the temperature compensation layer. 
     
     
         5 . The acoustic wave device of  claim 1  wherein the temperature compensation layer is a silicon dioxide layer, and the mass loading strip is a molybdenum layer. 
     
     
         6 . The acoustic wave device of  claim 1  wherein the buffer layer has a coefficient of thermal expansion greater than a coefficient of thermal expansion of the temperature compensation layer and less than a coefficient of thermal expansion of the mass loading strip. 
     
     
         7 . The acoustic wave device of  claim 1  wherein the mass loading strip has a bottom side, an top side, and a sidewall extending between the bottom side and the top side, the buffer layer is disposed between the top side and the sidewall of the mass loading strip and the temperature compensation layer. 
     
     
         8 . The acoustic wave device of  claim 1  wherein a difference between the coefficient of thermal expansion of the temperature compensation layer and the coefficient of thermal expansion of the metal strip is ΔCTE, the coefficient of thermal expansion of the buffer layer is 0.2ΔCTE to 0.75ΔCTE greater than the coefficient of thermal expansion of the temperature compensation layer. 
     
     
         9 . The acoustic wave device of  claim 8  wherein the coefficient of thermal expansion of the buffer layer is 0.3ΔCTE to 0.75ΔCTE greater than the coefficient of thermal expansion of the temperature compensation layer. 
     
     
         10 . A method of forming an acoustic wave device, the method comprising:
 forming an interdigital transducer electrode formed with a piezoelectric layer, the interdigital transducer electrode including a bus bar and fingers extending from the bus bar, the fingers each including an edge portion and a body portion;   forming a temperature compensation layer over the interdigital transducer electrode;   forming a mass loading strip overlapping the edge portions of the fingers, a portion of the temperature compensation layer being positioned between the mass loading strip and the piezoelectric layer; and   forming a buffer layer disposed at least partially between the mass loading strip and the temperature compensation layer, the forming the buffer layer including depositing the mass loading strip.   
     
     
         11 . The method of  claim 10  wherein the mass loading strip includes a layer having a density that is at least as high as a most dense layer of a material of the interdigital transducer electrode, and the mass loading strip is embedded in the temperature compensation layer. 
     
     
         12 . The method of  claim 10  wherein the temperature compensation layer is a silicon dioxide layer, and the mass loading strip is a molybdenum layer. 
     
     
         13 . The method of  claim 10  wherein a thickness of the buffer layer is at least one forth a thickness of the mass loading strip. 
     
     
         14 . The method of  claim 10  wherein the buffer layer has a coefficient of thermal expansion greater than a coefficient of thermal expansion of the temperature compensation layer and less than a coefficient of thermal expansion of the mass loading strip. 
     
     
         15 . The method of  claim 10  wherein the mass loading strip has a bottom side, an top side, and a sidewall extending between the bottom side and the top side, the buffer layer is disposed between the top side and the sidewall of the mass loading strip and the temperature compensation layer. 
     
     
         16 . The method of  claim 15  wherein the buffer layer is further disposed between the bottom side of the mass loading strip and the temperature compensation layer. 
     
     
         17 . The method of  claim 10  wherein a difference between the coefficient of thermal expansion of the temperature compensation layer and the coefficient of thermal expansion of the metal strip is ΔCTE, the coefficient of thermal expansion of the buffer layer is 0.2ΔCTE to 0.75ΔCTE greater than the coefficient of thermal expansion of the temperature compensation layer. 
     
     
         18 . The method of  claim 17  wherein the coefficient of thermal expansion of the buffer layer is 0.3ΔCTE to 0.75ΔCTE greater than the coefficient of thermal expansion of the temperature compensation layer. 
     
     
         19 . The method of  claim 10  wherein the forming the buffer layer includes reacting the mass loading strip and the temperature compensation layer. 
     
     
         20 . The method of  claim 19  wherein the forming the temperature compensation layer includes sputtering the temperature compensation layer at a sputtering temperature in a range between 200° C. and 300° C.