In a described example, an integrated circuit includes: a semiconductor substrate having a first surface and an opposite second surface; at least one dielectric layer overlying the first surface of the semiconductor substrate; at least one inductor coil in the at least one dielectric layer with a plurality of coil windings separated by coil spaces, the at least one inductor coil lying in a plane oriented in a first direction parallel to the first surface of the semiconductor substrate, the at least one inductor coil electrically isolated from the semiconductor substrate by a portion of the at least one dielectric layer; and trenches extending into the semiconductor substrate in a second direction at an angle with respect to the first direction, the trenches underlying the inductor coil and filled with dielectric replacement material.