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 ill 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] * cm) silicon substrate by flip-chip bonding. Individual and 2 x 2 arrays of meander and spiral inductor designs have been successfully fabricated with air gap heights ranging from 15 to 31 [micro]m. Maximum Q factors of ~15 and ~13 at ~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 ~15 [micro]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. Index Terms--Flip-chip assembly, high Q, microelectromechanical systems (MEMS), modeling, passive inductor, polyimide film, silicon RF integrated circuit (RFIC).