The majority of uniplanar circuits are based on transmission lines of two basic topologies: Microstrip and Coplanar Waveguide (CPW). Microstrip offers low loss, while suffering from large circuit size, high dispersion, poor design flexibility, and the necessity of large via holes. CPW improves upon microstrip by bringing the ground to the same level as the signal, therefore providing low dispersion, smaller circuit size, high design flexibility and no via holes. However, the fields in CPW concentrate on the thin edges and are therefore less confined. This translates into higher conductor loss, cross-talk and poor power handling capability.
We present a new topology for planar microwave circuitry: bulk titanium waveguide. The waveguide is formed by coplanar titanium segments separated by low-loss dielectric that is planarized to the same level as the titanium. Due to the high aspect ratio of the dielectric cross section, the electric field is confined primarily within the dielectric, therefore reducing losses due to radiation and parasitic coupling. This geometry distributes the surface current across the entire height of the structure, reducing conductor losses while enhancing power handling capabilities. The high aspect ratio in-plane electrical isolation also serves as through-wafer interconnects with a high packing density. When integrated with a bulk titanium package, this waveguide provides a robust, compact, packaging solution for microwave subsystems as well as other microcomponents such as MEMS. We have successfully designed, modeled, fabricated and characterized packaged waveguides which measured, at 40GHz, ~0.7dB/mm insertion loss and the impact due to package <0.1dB.
Titanium has one of the highest strength to weight ratio among metals. It is naturally resistant to corrosive environments and is a widely used as implants due to its bio-compatibility. Titanium is also one of the few materials with an endurance limit, which means that it can be deformed repeatedly without breaking. However, the current manufacturing processes for titanium sheets are not tailored for microfabrication. We developed a suite of techniques that enable the use of bulk titanium for microfabrication by overcoming issues such as residual stress, thickness variation, embedded defects and surface roughness.