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High-aspect ratio (12.5) through silicon vias (TSV) made in a silicon interposer have been electrically characterized in the direct current (dc) and microwave regimes for 3D interconnect applications. The vias were micro-machined in silicon, insulated, and filled with copper employing a bottom-up copper electroplating technique in a “via-first” approach. DC via resistance measurements show good agreement with the theoretical expected value (~ 16 mΩ) . Radio-frequency (RF) measurements up to 50 GHz have been performed on coplanar waveguides located on the back-side of the wafers and connected to the front-side with TSVs. The S-parameters indicate clearly the beneficial impact of double sided ground planes of the RF signals. The via resistance extracted from impedance measurements is in good agreement with dc values, while the inductance (53 pH) and capacitance (2.4 pF) of the TSV are much lower than conventional wire bonding, which makes the use of TSV very promising for 3D integration. An advanced analytical model is proposed for the interconnect system with vias and lines and shows very good agreement with the experimental data with a limited number of fitting parameters. This work gives a proof of concept for high aspect ratio TSV manufacturing and new insights to improve 3D interconnect modeling for systems-in-package applications in the microwave regime.