Integrated solenoid-type inductors for high frequency applications and their characteristics

New solenoid-type integrated inductors for high frequency applications have been realized using a surface micromachining technique and a polymer sacrificial layer, and their geometrical characteristics have been investigated. In general, integrated inductors can suffer from low Q factors and/or self-resonant frequencies when compared to their discrete counterparts. A spiral-type inductor, one of the dominant choices as an integrated inductor, requires relatively large two-dimensional spaces. In addition, the direction of flux of the spiral type inductor is perpendicular to the substrate, which can cause more interference with underlying circuitry or other integrated passives in a vertically stacked multi-chip module (MCM). The proposed inductor in this research has an air core to reduce unwanted stray capacitance that can be added due to a magnetic core, and electroplated copper coil to reduce the series resistance. An important feature of the proposed inductor geometry is introducing 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. Inductors with different geometrical aspects, such as air gap height, core size, and number of turns, have been designed and fabricated on ceramic substrates. A variational study of these inductors has been performed to assess the impact of the geometrical aspects on the inductor performance at high frequency. The measured inductance of these inductors varies from 2nH to 20 nH, and maximum Q-factor 10-60.