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.