A novel technological method to improve the quality factor (Q) of RF integrated inductors for wireless applications is presented in this article. A serious reduction of substrate losses caused by capacitive coupling is provided. This is realised by removing the oxide layers below the coils with optimized under-etching techniques. This special etching procedure is used to establish an environment in the inductor substructure with very low permittivity. A set of solid oxide-metal-columns placed below the metal windings stabilizes the coil and prevents the hollowed out structure from mechanical collapse. The oxide capacitance is lowered significantly by the reduction of the permittivity r from values around 4 to nearly 1. Capacitive coupling losses into substrate are decreasing in the same ratio. The resulting maximum Q-factors of the new designs are up to 100% higher compared to the same devices including the oxide layers, but shifted significantly to higher frequencies. Improvements of Q from 10 up to 15 have been obtained at a frequency of 3 GHz for a 2.2 nH inductor with an outer diameter of 213 µm. Coils with different layouts and values for inductance (L) were verified and showed similar results.
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