A CAD Model for Creep Behavior of RF-MEMS Varactors and Circuits

In this paper, we propose a compact computer-aided design (CAD) model that may be utilized to simulate the creep behavior of RF microelectromechanical systems (RF-MEMS) varactors in RF circuits and subsystems. This model is capable of calculating the long-term response of RF-MEMS devices to an arbitrary input waveform. It is implemented using Agilent´s Advanced Design System (ADS). The presented CAD model employs the generalized Voigt-Kelvin model to capture the long-term behavior of RF-MEMS devices. It is experimentally validated with measurements of Ni varactors that extend up to 760 h of constant loading. Its effectiveness is demonstrated with a tun able RF-MEMS resonator and an RF-MEMS phase shifter. The tunable resonator that consists of one λ/2 coplanar waveguide resonator and two nanocrystalline-Ni RF-MEMS varactors is fabricated and measured. S-parameters of this tunable resonator have been recorded for 80 h under a bi-state bias condition of 0 and 40 V. It is shown that the resonant frequency is shifted by 90 MHz and the varactor deformed by 0.12 μm over the 80-h period. Good agreement between the CAD model and the measurements is obtained. The impact of the duty factor of the bias signal is also discussed. The model´s capability of handling arbitrary input is demonstrated on an RF-MEMS phase shifter operated with a sawtooth waveform.