Abstract-A new micromachined planar spiral inductor, with the strips suspended individually, has been fabricated in standard GaAs high electron-mobility transistor monolithic-microwave integrated-circuit technology through maskless front-side bulk micromachining. The electronic compatibility, the use of industrial integrated-circuit production lines, the straightforward and low-cost additional procedure for structure releasing, and the very short etching time required to do such are the principal features related to such a novel inductor structure. Moreover, the air-gap layer created underneath the device and between the strips significantly reduces shunt and fringing parasitic capacitances, consequently increasing the performance and operating frequency range. Experimental measurements, carried out up to 15 GHz, before and after micromachining, showed for a 12-nH inductor an increase of the maximum factor from 5 (at 3 GHz) to about 20 (at 7 GHz), while the self-resonant frequency was shifted from 5 to 13 GHz. Furthermore, a structure with two interleaved spiral inductors, in a 1 : 1 transformer-like configuration, was also fabricated, and its performance was verified as well in order to demonstrate the promising performance improvements provided by the proposed device. Finally, heating and mechanical characteristics associated with freestanding microstructures are briefly evaluated using finite-element method simulations.

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The literature lacks detailed information about the electrical properties of the plastic filaments used in 3D printing. This opens the way for research on characterizing the types of materials used in these filaments. In this work, a method for the extraction of the dielectric constant and loss tangent of materials is described. This method, which is suitable for characterizing any dielectric material, is then used to characterize 3D-printed samples based on different filament materials and infill densities over a very wide frequency range [0.02-10 GHz]. The selected materials are Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS) and a semi-flex filament that combines two important features of flexibility and endurance. These three types are the most commonly used in 3D printing. The two-line technique is applied to extract the complex permittivity of the material under test (MUT) from the propagation constant. This method employs the uncalibrated scattering parameters with different types of transmission line for any characteristic impedance. A rectangular coaxial transmission-line fixture has been used to validate the theoretical work through simulations and measurements involving the 3D filament samples.