THB-filled monolithic rectangular waveguides for millimeter wave applications

A monolithic wafer-level dielectric-filled rectangular waveguide for millimeter-wave applications is designed and fabricated. Unlike the existing wafer-level rectangular waveguides that are based on hybrid integration techniques to assemble the waveguide, the three-step fabrication process of the proposed waveguide enables the simultaneous monolithic fabrication of the dielectric-filled waveguide as well as planar circuitry on one substrate. The maximum temperature during the entire fabrication process stays lower than 200 °C which is desirable for integrated circuit (IC) fabrication. Besides, the RF performance of the presented waveguide only depends on the RF properties of the filling dielectric not the carrier substrate (as in the case of Substrate Integrated Waveguides), which makes it ideal for silicon micro-fabrication technology. A negative-tone thick photoresist called THB N151 which can reach a thickness of 90 μm by single spinning is used and characterised. The results show a relative permittivity of ε_r´ = 3 and a loss tangent of tanδ = 0.018 for THB 151 which surpasses the RF performance of the highly used SU8 photoresist. An average insertion loss of 0.21 dB/mm is measured for the THB-filled waveguide for the frequency range of 35-50 GHz. To perform the wafer-level measurements, the waveguide is integrated with CPW to rectangular transition. Furthermore, it has been illustrated that other monolithic waveguide structures such as Turn or Y-junction can be potentially developed by using the proposed solution.