Post-CMOS Compatible Micromachining Technique for On-Chip Passive RF Filter Circuits

This paper reports on a post-CMOS compatible micromachining technology for passive RF circuit integration. The micromachining technology combines the formation of high-performance microelectromechanical systems solenoid inductors and metal—insulator—metal (MIM) capacitors by using a post-CMOS process on standard CMOS substrate. Utilizing this process, novel on-chip 3-D configured RF filters for 5 GHz band are integrated on-chip. Two types of compact filters are designed and fabricated, with the layout size of the bandpass filter as 0.65 $\,\times\,$0.67 ${\rm mm}^{ 2}$ and that of the low-pass filter as 0.77$\,{ \times }\,$1.25 ${\rm mm}^{ 2}$. From the measurement results, the fifth-order low-pass filter shows less than 1.06 dB insertion loss up to 5 GHz and ${-}{\rm 1.5}~{\rm dB}$ cutoff frequency at 5.3 GHz. The bandpass filter is a second-order coupled-resonator type, with measured 4.3 dB minimum insertion loss and better than 13 dB return loss in the pass band. Both simulation and shock testing results have shown that the filters are almost free of influence from environmental vibration and shock. From the measured results in various temperatures, the bandpass filters were found to show lower loss under low temperatures, while the passband shift is negligible in the various temperatures. Together with the fabricated filters, the developed micromachining technique has demonstrated the potential of on-chip integration and miniaturization of passive RF circuits.