Desgign and fabrication of low phase noise VCOs utilizing MEMS structures

Abstract

Voltage controlled oscillators (VCOs) are essential component in RF communication systems. Stringent phase noise specifications and high frequency operation for high data rate communication have become challenging research topics. For low frequency applications below ten gigahertz, LC type VCOs have been frequently used due to their superior phase noise performance. As the operating frequency increases, distributed oscillators, which use phase delay, have been reported recently. High-frequency circuits have been implemented in compound semiconductor technologies such as GaAs and InP. And CMOS has been avoided in RF systems due to its high noise figure, low transconductance, and low cutoff frequency. But, during last decade, dramatic development in CMOS technology made it possible for CMOS to be a strong candidate for RF systems from the motivation of system integration on a single chip. Nonetheless this motivation, CMOS is still tough for RF system applications. One of the reasons comes from low performance of CMOS passives. The lossy passives cause signal loss and noise generation, eventually degrade signal to noise ratio. Inductor is one of most important components in RF system. It has robust thermal characteristics and excellent noise performance so that inductor has been widely used as load of amplifier and mixer, and resonator of oscillator. Unfortunately, it is not easy to acquire high quality inductor in CMOS process. CMOS usually low resistivity silicon substrate to block latch-up. This causes significant signal loss through silicon. Another reason for low Q of inductor comes from limited metal thickness of CMOS inductor. These reasons live with CMOS itself. There is similar phenomenon in CMOS transmission lines. As an alternative technology, MEMS technology has become a strong candidate to breakthrough the limits of CMOS. Thick metal and low substrate loss can be easily acquired from MEMS process by employing electroplating, and substrate etching p...