Static and dynamic characteristics of a MEMS Varactor with broad analog capacitive tuning range for wideband RF VCO applications

Achieving a broad analog tuning range with MEMS varactors for wideband RF VCO frequency tuning is still a challenge. In this paper the design, fabrication and characterization of a MEMS varactor with broad analog tuning range is described. The MEMS varactor is embedded in the BEOL (back end of line) metallization stack of a state-of-the-art Si/SiGe BiCMOS semiconductor process and shows an overall capacitance ratio of 2.4∶1 including all layout parasitics with de-embedding up to the RF probe pad. Besides the static CV characteristics of a MEMS varactor, its dynamic parameters significantly determine the phase noise performance of MEMS based RF VCO designs. LDV (Laser-Doppler Vibrometer) time domain measurements are performed on the fabricated MEMS varactor to determine the dynamic parameters such as the mechanical resonance frequency, damping ratio and the modal mass coefficient based on an FFT (Fast Fourier Transform) approach.