Surface micromachined solenoid inductors for high frequency applications

As operation frequencies and performance requirements of wireless devices increase, the resultant demands on the performance of passive components also increase. Miniaturization of inductive components for high frequency has been a key research area to address this issue; however, in general, miniaturized integrated inductors can suffer from low Q factors and/or self-resonant frequencies when compared to their discrete counterparts. In this research, a modified geometry of a solenoid type inductor using a surface micromachining technique is proposed. This inductor has an air core and an electroplated copper coil to reduce the series resistance, and its low temperature process is suitable for various packaging applications. An important feature of the proposed inductor geometry is the introduction of an air gap between the substrate and the conductor coil in order to reduce the effects of the substrate dielectric constant. This air gap can be realized using a polyimide sacrificial layer and a surface micromachining technique. Therefore, the resulting inductor can have less substrate-dependent magnetic properties, less stray capacitance, and higher Q-factor. The measurement result shows that this inductor has high Q-factor and stable inductance over a wide range of operating frequency. Also, various effects of geometrical factors have been investigated. Various inductors with the inductance varying from 1 to 20 nH and maximum Q-factor from 7 to 60 have been fabricated and measured.