Suppression of Parasitic Substrate Modes in Multilayer Integrated Circuits

Integrated circuits (ICs) with multilayer backend process and a large front-side ground plane support the propagation of parasitic substrate modes. These modes resonate at frequencies that typically are within the bandwidth of circuits operating close to and in the submillimeter-wave range, i.e, beyond 300 GHz. The resonances cause unwanted coupling and feedback, which result in circuit instability and degraded performance for circuits operating in the range of these resonances. A common method to suppress these modes from propagating is to use numerous through-wafer vias distributed over the entire circuit. In this letter, we present a study of substrate modes in multilayer ICs with thin-film microstrip interconnects at 125-330 GHz. We show that a doped Si carrier underneath the circuit effectively eliminates the effect of substrate modes on the circuit functionality. This method requires no backside processed through-wafer vias and no backside metallization.