MEMS based bulk acoustic wave resonators for mobile applications

A silicon based film bulk acoustic wave resonator (FBAR) composed with a filter and a duplexer is fabricated using the bulk micro-machining process. It has a simple MIM (metal-insulator-metal) membrane structure using molybdenum (Mo) for the top and bottom electrodes. The bulk acoustic wave resonances are generated by the piezoelectricity of aluminum nitride (AlN) with an air gap cavity fabricated below the membrane by silicon deep-etch process to reduce acoustic loss of FBAR. The fabricated FBAR is measured with HP 8510C vector network analyzer in wide (0.5 similar to 10.5 GHz) and narrow (1.7 similar to 2.1 GHz) frequency range. The measured series and parallel resonance frequencies are 1856 MHz and 1907 MHz, respectively. The minimum insertion losses are less than 0.07 dB at the series resonance frequency. With the increase of the membrane area, insertion loss decreases and effective electromechanical coefficient increases. The measured effective electromechanical coefficients are higher than 6.4%. The circuit modeling of FBAR is preformed based on the MBVD (modified Butterworth Van-Dyke) model. The above results demonstrate that the fabricated FBAR has sufficient performance to be the building block of RF filters for mobile applications.