A bulk micromachined tunable microwave lowpass filter for 10–15GHz wireless/satellite communication

This paper reports a bulk micromachined microwave tunable lowpass filter with its resonator tuned by butterfly-like loading bridges on Pyrex glass. Effectively, the filter is tuned by shifting the first transmission zero, and is based on 7th order ellipse architecture, whose use in RF MEMS filters has not been reported as far as the authors know. Perturbation method is used to anticipate analytically the microwave resonant frequency. Based on the analysis, the tunable filter is designed, with CPW filtering sections. The end-loading bridges distributed over resonant CPW branches (DMTL), is used for the variable capacitive loading and filter characteristics tuning. The methodology and the analytical results can also be adapted to bandpass and highpass filters. The bulk microfabrication is shown to be much simpler than surface micromachining conventionally used for RF MEMS switches. Since the bridge skeleton is Si, thermal mismatch between all-metal bridge and dielectric substrate is avoided, and reliability at elevated temperature is guaranteed or at least greatly improved. Besides, the p++ doped Si line patterned simultaneously with the microbridges, can also can be used as a DC biasing resistor, which can eliminate the additional steps to fabricate the resistive alloy line. Analytical and finite element full-wave analysis results show satisfying RF performance, with almost invariable insertion loss of better than 3 dB for each tuned states, and a 3 dB cutoff frequency T_c tuning range of 5 GHz (50%) with 5 bridges. Thus the device maybe a good candidate for novel Ku-band inter-scientific satellite communication or wireless networking.