Thick silicon nitride blocks embedded in silicon wafers were recently proposed as a substrate for RF devices. In this paper we show that deep trenches filled with silicon nitride—having thin slices of monocrystalline silicon in between—already result in a significantly improved RF behavior. Measurement results are presented on RF coplanar waveguides using solid silicon nitride blocks and silicon nitride filled trenches with various dimensions and orientations with respect to the transmission line. A clear difference exists between trenches parallel and perpendicular to the transmission line due to the different associated loss mechanisms. S-parameter measurements on the coplanar waveguides show an improvement of the transmission losses at 4 GHz from 3.5 dB mm−1 on a standard silicon substrate to 0.7 dB mm−1 on silicon nitride filled trenches and 0.2 dB mm−1 on a solid silicon nitride block. In this way, an RF performance very close to dedicated glass substrates such as AF45 is obtained (with transmission losses of 0.1 dB mm−1 at 4 GHz).