Permalloy incorporated RF spiral inductors are fabricated on standard Si-substrates with a CMOS compatible process. Various patterns of permalloy thin-film stripe are designed to investigate their influences on the inductor performance with respect to different magnetization orientations, ferromagnetic resonance(FMR), induced eddy current, parasitic capacitance and substrate loss. The BH loop for the array of rectangular permalloy stripes with an aspect ratio of 30 shows good soft magnetic properties, such as a magnetic saturation AπMs of 25KG, a dc coercivity of 5Oe, and an anisotropy field of 8Oe. It is well known that decreasing the pattern size or increasing the aspect ratio can enlarge the anisotropy field and shift FMR to a higher frequency, whereas the initial permeability significantly decreases. And since the direction of magnetic flux generated from the spiral current path keeps rotating, the pattern also needs to follow this trend to ensure a symmetric magnetization. With design optimizations, an inductance improvement up to 50% is achieved without the quality factor degradation. Furthermore, for some special permalloy patterns, a unique step function of effective inductance is observed across a wide frequency range. In this configuration, there exists a critical frequency, fc. Before fc, due to the advantage of the high permeability, large effective inductance is shown. After fc, the effective inductance only degrades to the value of the air-core inductor with the same geometry, and it remains at that level until the self-resonance occurs. This step effective inductance feature is very interesting and attractive in the application of soft magnetic material and in the general RF circuits.
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