Coupled-resonator micromechanical filters with voltage tuneable bandpass characteristic in thick-film polysilicon technology

This paper reports on the design and characterization of 2.4 and 9.2 MHz fourth and sixth order passband microelectromechanical coupled-resonator filters implemented in a 15 μm thick-film epitaxial polysilicon technology. The work uses a novel approach in the building of coupled-resonator electromechanical filter structures using on-line controlled electrostatic coupling. Mechanical springs commonly used to couple micromechanical resonators were replaced by electrostatical links created by pairs of biased electrostatical transducers. To control the coupling strength, an original biasing scheme of electrostatical transducers is proposed. The idea is to maintain a fixed charge on the floating-potential middle electrode of the electrostatical coupler. This electrode does not need any external electrical connection, thus does not suffer from parasitic capacitances.The coupling factor is directly controlled by coupling transducer bias voltages, allowing the control of the filter pole frequencies. A voltage controlled bandpass filter at 9.2 MHz showed a bandwidth with a tuning range of 3.18–47.4 kHz.