New fabrication method for high-Q on-chip spiral inductor

Abstract

Current silicon on-chip inductors have the problems of low quality factors (Q), low self-resonant frequencies, poor electromagnetic isolation and lack of a good radio-frequency (RF) ground plane. To address these issues, we present a new method to fabricate an on-chip copper spiral inductor. The basic structure of the inductor consists of a spiral polysilicon coil suspended over a cavity etched into the silicon substrate. Copper (Cu) is electrolessly deposited onto the polysilicon spiral in order to obtain high conductivity. The formation of the suspended coil is realized by first creating a silicon oxide block embedded in the silicon substrate, then fabricating on the oxide the coil by polysilicon surface micromachining, and in the end removing the embedded oxide by hydrofluoric acid (HF). The benefit of using a suspended spiral structure is two-folded: first, the electrical and magnetic coupling between the inductor and the substrate is reduced dramatically, thus decreasing the substrate loss, and second, by reducing the parasitic capacitance between the inductor and the substrate, the self-resonance of the inductor at an undesirably low frequency can be avoided. The metallized bottom and side-walls of the cavity under the inductor serve both as an electromagnetic shield for isolation and as an RF ground plane. Initial experimental results show that the maximum Q-factor can be as high as 26 for a 2.14nH inductor. The self-resonant frequency is measured to be 10.3GHz.