The present paper deals with the analysis of the squeeze film effects in circular capacitive micromachined ultrasound transducers (CMUT) operating in air, with emphasis on improved bandwidth. Firstly, a 1D analytical approach based on parallel plate approximation is recalled. The opposing influences of the electrostatic softening and the squeeze film stiffening make the resonant frequency dependent on the bias voltage with respect to air spring constant to mechanical spring constant ratio. In a second part, FEM models using COMSOL Multiphysics ® are built to analyze the influences of the plate flexibility and the lateral venting boundary conditions on the squeeze film effects. The associated numerical results show that viscous losses are involved in sealed air-filled cavities. Moreover, the dimensionless elastic and viscous damping forces do not depend on the lateral venting boundary conditions for high squeeze number range, usual for CMUT operation. Finally, 2D full coupled simulations of flexible CMUT are compared with experimental data. Thus, the squeeze film damping increases bandwidth of aircoupled CMUTs with both sealed and laterally vented cavities.

This paper presents a novel four-terminal variable capacitor (FTVC) dedicated to the recent concept of low power capacitive adiabatic logic (CAL). This FTVC is based on silicon nano/micro technologies and is intended to achieve adiabatic logic functions with a better efficiency that by using field effect transistor (FET). The proposed FTVC consists of two capacitors mechanically coupled and electrically isolated, where a comb-drive input capacitor controls a gap-closing capacitor at the output. To fully implement the adiabatic combinational logic, we propose two types of variable capacitors: a positive variable capacitor (PVC) where the output capacitance value increases with the input voltage, and a negative variable capacitance (NVC) where the output capacitance value decreases when the input voltage increases. A compact and accurate electromechanical model has been developed. The electromechanical simulations demonstrate the ability of the proposed FTVC devices for CAL, with improved features such as high logic states differentiation larger than 50% of the full-scale input signal and cascability of both buffers and inverters. Based on the presented analysis, 89% of the total injected energy in the device can be recovered, the remaining energy being dissipated through mechanical damping. During one cycle of operation, a buffer gate of 10x2.5 µm 2 dissipates only 0.9 fJ.

This paper reports the design, energy recovery and logical functionality modelling of four-terminal 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 transducers. 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+® software. In order to speed-up transient simulation of numerous devices in an electrical 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 cascadable power clock scheme, i.e. logic states differentiation and cascadability. Also we 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 in the order of one pJ. This is still far from the energy dissipated by a nm-scale FET transistor, which is of the order of 10's aJ. 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 difference between the signal energy and the distinguish energy in MEMS-based adiabatic logic is discussed.