Theoretical and Experimental Studies of Flip-Chip Assembled High- $Q$ Suspended MEMS Inductors

This paper reports the theoretical and experimental studies of high- $Q$ suspended microinductors produced by flip-chip assembly for multigigahertz RF integrated-circuit applications. The effects of device and substrate parameters on the $Q$ factor of the inductor devices are studied by numerical simulation using Ansoft's High Frequency Structure Simulator electromagnetic field simulation package. Suspended inductor devices are realized using a flip-chip assembly method in which the inductor structures with the supporting pillars are fabricated on a low-cost polyimide thin-film carrier and then assembled onto a low resistivity (3–4 $\ \Omega\,\cdot\,$cm) silicon substrate by flip-chip bonding. Individual and 2 $\,\times\,$2 arrays of meander and spiral inductor designs have been successfully fabricated with air gap heights ranging from 15 to 31 $\mu$m. Maximum $Q$ factors of ${\sim}$ 15 and ${\sim}$13 at ${\sim}$1 GHz have been achieved for meander and spiral suspended inductor devices before pad deembedding. It is shown that the optimal air gap between the inductor and substrate surface is ${\sim}{\hbox{15}}\ \mu$m beyond which no further enhancement in the $Q$ factor can be obtained for devices on low-resistivity substrates. The experimental results are in excellent agreement with that of theoretical simulation. The inductor assembly method requires minimal chip/wafer processing for integration of high-$Q$ inductors.