coplanar waveguides having a deep trench between a signal line and a ground plane and methods of their fabrication are disclosed . an oxide layer is provided over a silicon substrate and a photoresist is applied and patterned to define areas where the signal line and the ground plane will be formed . a barrier layer is provided over the oxide layer in the defined areas . a metal layer is then deposited over the barrier layer . an etch mask is deposited over the metal layer for the subsequent trench formation . the photoresist and the underlying portion of the oxide and barrier layers are removed and a deep trench is formed in the substrate between the signal line and the ground plane using etching through the mask .