film resonator including a temperature compensation film inserted into a multilayered film to suppress a frequency change due to temperature, where the temperature compensation film is an insulating film such as a silicon oxide film having a temperature coefficient opposite to those of the piezoelectric film, lower electrode and upper electrode, as disclosed in Japanese Patent Application Publication No. 1-48694 and Proc. IEEE Ultrasonics Symposium 2009, pp 859-862.
There has been known a structure in which electrodes mutually short-circuited are located on upper and lower surfaces of a temperature compensation film to suppress a reduction in excitation efficiency caused by a concentration of electric field in the temperature compensation film when the temperature compensation film is used, as disclosed in Japanese Patent Application Publication No. 60-16010. There has beenknown a piezoelectric thin film resonator including a temperature compensation film embedded in the lower electrode, as disclosed in U.S. Patent Application Publication No. 2011/0266925.
However, the above described techniques improve temperature characteristics, including a resonance frequency, of the piezoelectric thin film resonator but degrade resonance characteristics including an electromechanical coupling coefficient. The temperature characteristics have a trade-off relationship with the resonance characteristics. This makes piezoelectric thin film resonators included in a single chip to have uniform temperature characteristics and resonance characteristics. Therefore, design flexibility is limited. For example, when resonators are formed so as to have insulating films of different thicknesses, the resonators can have different temperature characteristics and resonance characteristics. However, this increases the number of fabrication steps.
In addition, the effective electromechanical coupling coefficient decreases in the