Design and fabrication of micromachined passive microwave filtering elements in CMOS technology

This paper describes a novel implementation of micromachined microwave resonators and filters, coupled with low-loss transmission lines, fabricated in standard CMOS technology. Selective etching of the Si substrate beneath the microwave elements results in significantly lowered transmission loss, improved quality factor and operating frequency range. This allows operation at gigahertz frequencies. Resonators with Q-factors of 20 and resonant frequencies of 2 GHz and 21 GHz are reported which are significant results for standard CMOS technology. A low-pass filter is also reported with a 2.5 GHz cutoff frequency. The resonant and cutoff frequencies are in general adjustable within the 1 to 40 GHz band. These devices are useful for integration with microwave power sensors, and have application in wireless communication systems where low-loss operation above 1 GHz is essential.