Performance Improvement of a 0.18- $\mu{\rm m}$ CMOS Microwave Amplifier Using Micromachined Suspended Inductors: Theory and Experiment

This paper presents the quality factor improvement of the micromachined suspended inductors and their usage in a fully integrated 0.18- $\mu{\rm m}$ CMOS microwave amplifier. Removing the lossy silicon substrate underneath the spiral inductors by adopting post-CMOS compatible microelectromechanical system (MEMS) technology allows construction of suspended structures with a high $Q$ -factor. Although micromachining technology has had a remarkable impact on industries in recent years, it is still challenging to design fully integrated CMOS-MEMS active circuits at microwave frequencies. To evaluate the performance improvement of 0.18- $\mu{\rm m}$ CMOS microwave amplifiers by using micromachined suspended inductors, two amplifiers in the 24-GHz industrial, scientific, and medical band were designed and fabricated with and without the MEMS process for comparison. At 0.85-V low supply voltage, the measured peak gain of the microwave amplifier can be significantly increased from 12.3 to 13.7 dB due to the high $Q$ -factor suspended inductors. In addition, the measured noise figure of the 24-GHz microwave amplifier is effectively improved from 5.8 to 5.0 dB. Theories for predicting the gain and noise figure improvement are also presented, and the mechanisms are validated by experiments. Also, a proposed simplified double- $\pi$ micromachined inductor model considering the skin effect is introduced and characterized.