New architectures for micromechanical coupled beam array filters
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TECHNICAL PAPER
New architectures for micromechanical coupled beam array filters A. Bouchaala1
•
R. R. A. Syms1
Received: 6 November 2020 / Accepted: 13 November 2020 The Author(s) 2020
Abstract Coupled resonator filters implemented as microelectromechanical systems (MEMS) offer performance advantages as bandpass filters at MHz frequencies. Here new designs based on resonant cavities for acoustic slow waves are developed to allow alternative frequency responses. Derivation of the lumped element model for coupled beam systems with in-plane motion from Rayleigh–Ritz perturbation theory is first reviewed. Departures from ideal behaviour caused by mechanical and electrostatic detuning are resolved. Slow wave theory is then used to develop linear array topologies with novel responses including band-stop and comb filtering with controlled filter roll-off. A systematic procedure is developed to allow rapid identification of design parameters without the need for lengthy numerical simulation, using the lumped element, stiffness matrix and finite element methods to investigate the layout parameters of initial design concepts, detailed mechanical effects and detailed electrostatic effects, respectively. High performance is demonstrated, with good agreement between the models.
1 Introduction Microelectromechanical systems (MEMS) have had significant economic impact, with widespread adoption in many industries (Elwenspoek and Jansen 1999; Madou 2011; Beeby et al. 2004; Uttamchandani 2013). Applications exploit the high performance, reliability and repeatability and small size of MEMS to enable new, highvalue systems. There is now an increasing drive towards nanoelectromechanical systems (NEMS) (Lyshevski 2002). This paper is concerned with MEMS filters containing nanostructured parts. Band-pass filters have been used in radio receivers since the invention of the super-heterodyne receiver (Armstrong 1921). The super-het uses an intermediate frequency (IF) and requires IF filters with flat passband and small fractional bandwidth. Due to their high Q-factors, coupled mechanical resonators have long been used for filtering (Hathaway and Babcock 1957; Johnson et al. 1971), and electrostatically driven resonators were among the first MEMS (Tang et al. 1989). Low-frequency filters were
& A. Bouchaala [email protected] 1
Optical and Semiconductor Devices Group, EEE Department, Imperial College London, Exhibition Road, London SW7 2AZ, UK
constructed from comb-drive actuators with folded springs (Lin et al. 1998; Wang and Nguyen 1999), and high-frequency filters from parallel plate actuators and clamped beams (Bannon et al. 2000). In most cases, coupling is between adjacent beams, with their number determining the filter order, but non-adjacent coupling has been used to increase roll-off (Li et al. 2004). Gas damping, thermoelastic damping and support losses all reduce Q-factor (Zhang and Tang 1994; Yang et al. 2002; Srikar and Senturia 2002), but the first two can be sma
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