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We present our detailed study of a micromachined substrate approach for a wide-band lithium-niobate modulator and other ultra-wideband RF interconnection applications. Resonant substrate mode coupling often results in large RF loss that affects the frequency performance in LiNbO3 modulators. Using the micromachining approach, we demonstrated significantly reduced resonant mode coupling loss in LiNbO3 modulator electrodes. In this paper, the effects of substrate geometry and material combination on microwave transmission are experimentally investigated. Our investigation showed that the RF launch of the coplanar-waveguide circuit is a major contributor to the resonant mode coupling. Micromachining the launch area allows us not only to suppress resonant couplings, but also to retain substrate mechanical strength for high-yield production. Two micromachined modulator die configurations were proposed and experimentally demonstrated.