High resistivity Si as a microwave substrate

Silicon has many advantages as a system substrate material including low cost and a mature technology. However, Si has not been demonstrated as a good microwave substrate compared to semi-insulating GaAs or quartz. The aim of this paper is to evaluate the potential of using high-resistivity silicon as a low-cost low-loss microwave substrate through an experimental comparative study. Coplanar waveguides fabricated on Si, GaAs and quartz substrates are tested and their characteristics are compared. Microwave spiral inductors and meander lines are also fabricated on various substrates, and their performance is also analyzed. The results demonstrate that the losses of a coplanar transmission line (CPW) realized on high-resistivity (3 k to 7 k Ω-cm) silicon substrates are comparable to the losses of a CPW realized on a GaAs substrate covered with insulator. Furthermore, measured unloaded Q´s of microwave inductive structures on high-resistivity silicon substrates are comparable to the measured unloaded Q´s of the same structures on GaAs and on quartz. The measured results are explained using both microwave and semiconductor physics theory. This paper demonstrates that high-resistivity Si can be used as a microwave substrate