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A micromachined structure with reduced parasitics is proposed to enhance the resonant frequency and the quality factor (Q) of a spiral inductor. Inductors with various etch depths have been fabricated on a high resistivity Si substrate. Two-port S-parameters are measured to characterize the performance of the inductors. With an etch depth of 20 /spl mu/m, a fabricated 1.8 nH spiral inductor achieves a maximum resonant frequency of 25.6 GHz and a maximum Q of 20.2 at 14.5 GHz. This technology is compatible with SiGe-Si HBT technology, and the spiral inductors are especially suitable for Si-based monolithic microwave integrated circuit (MMIC) applications.