| Summary: | Film bulk acoustic resonators (FBARs) with a desired effective electromechanical coupling coefficient (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mrow><mi>e</mi><mi>f</mi><mi>f</mi></mrow><mn>2</mn></msubsup></mrow></semantics></math></inline-formula>) are essential for designing filter devices. Using AlN/AlScN composite film with the adjustable thickness ratio can be a feasible approach to obtain the required <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mrow><mi>e</mi><mi>f</mi><mi>f</mi></mrow><mn>2</mn></msubsup></mrow></semantics></math></inline-formula>. In this work, we research the resonant characteristics of FBARs based on AlN/AlScN composite films with different thickness ratios by finite element method and fabricate FBAR devices in a micro-electromechanical systems process. Benefiting from the large piezoelectric constants, with a 1 μm-thick Al<sub>0.8</sub>Sc<sub>0.2</sub>N film, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mrow><mi>e</mi><mi>f</mi><mi>f</mi></mrow><mn>2</mn></msubsup></mrow></semantics></math></inline-formula> can be twice compared with that of FBAR based on pure AlN films. For the composite films with different thickness ratios, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mrow><mi>e</mi><mi>f</mi><mi>f</mi></mrow><mn>2</mn></msubsup></mrow></semantics></math></inline-formula> can be adjusted in a relatively wide range. In this case, a filter with the specific N77 sub-band is demonstrated using AlN/Al<sub>0.8</sub>Sc<sub>0.2</sub>N composite film, which verifies the enormous potential for AlN/AlScN composite film in design filters.
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