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Three-dimensional (3-D) interconnects built upon multiple layers of polyimide are required for constructing 3-D circuits on CMOS (low resistivity) Si wafers, GaAs, and ceramic substrates. Thin-film microstrip lines (TFMS) with finite-width ground planes embedded in the polyimide are often used. However, the closely spaced TFMS fines are susceptible to high levels of coupling, which degrades the circuit performance. In this paper, finite-difference time domain (FDTD) analysis and experimental measurements are used to demonstrate that the ground planes must be connected by via holes to reduce coupling in both the forward and backward directions. Furthermore, it is shown that coupled microstrip lines establish a slotline type mode between the two ground planes and a dielectric waveguide type mode, and that the connected via holes recommended here eliminate these two modes.