Contactless four-terminal MEMS variable capacitor for capacitive adiabatic logic

This paper reports the design, energy recovery and logical functionality modeling of four-terminal microelectromechanical (MEMS) comb-drive devices for capacitive adiabatic logic (CAL). The proposed electromechanical element consists of the moving mass with two insulated electrodes and two fixed electrodes. The two pairs of fixed and moving electrodes form an input and an output comb-drive capacitive transducer. The voltage across the input port allows us to control the capacitance of the output port. The developed contactless four-terminal design is simulated in Coventor MEMS+ (R) software. In order to speed up transient simulation of numerous devices in an electronic Simulation Program with Integrated Circuit Emphasis (SPICE) simulator, the obtained electrical and mechanical characteristics are used to fit our Verilog-A analytical compact model. SPICE simulation results demonstrate CAL logical functionalities using a cascadable power clock scheme, i.e. logic state differentiation and cascadability. We also show that MEMS-based calculation is energy efficient: for example, in a chain of four buffers, 99.1% of the energy transferred to the device is recovered for later use when devices operate at 25 Hz. The non-recoverable energy is mainly dissipated by mechanical damping during the logic state transition from high to low level and can be removed by using retractable power clocks. For this mm-scale device the energy dissipated per operation is of the order of 1 pJ. This is still far from the energy dissipated by a nm-scale field-effect transistor (FET), which is of the order of tens of attojoules. However, for the contactless design constant electric field scaling is possible and the energy dissipation decreases proportionally to the cube of the size. Finally, the limitations of irreversible MEMS-based CAL are discussed.