Numerical and experimental analysis of high-aspect-ratio micro-tool electrodes fabrication using a controlled electrochemical machining (ECM) technique is reported. The evolution of the tool electrode shape was first predicted using a finite element method-based numerical simulation and then validated with the detailed experiments. The effects of machining voltages and the machining durations on the change in the tool electrode profile were investigated. High-carbon steel was chosen as an electrode material due to its lower cost, good electrical conductivity, and easy availability. A 5% (i.e., 5 gm of NaCl in 100 ml of deionized water) NaCl solution was used as the electrolyte. The tool profiles predicted using the FEM-based numerical model showed an excellent matching trend with the ECM experimental results. The role of the black surface film formed on the electrode surface was found to be significant. The tool electrodes having an average diameter of 60 mu m and an aspect ratio of more than 75 were fabricated at the optimized machining parameters. These single-tip micro-tool electrodes were used to create through-hole in a 400-mu m-thick glass substrate by electrochemical discharge machining. The obtained ECM process parameters were used to create multiple electrodes having a tip size of 130 mu m and having a smooth surface. These through-holes were filled with copper to form 3D interconnects, i.e., through-glass vias, which are required in the radio-frequency MEMS applications and 3D packaging. Graphic abstract