3D Micromachined Subsystems Enable mm-Wave Components

With the capacity to integrate many advanced solutions in tight packages, much interest has been given to three-dimensional (3D) microfabrication techniques for terahertz applications. Using millimeter-wave coaxial designs, 3D microfabricated packages enable ultra-low loss, dispersion, and crosstalk to be achieved in transmission networks. According to Nathan Jastram and Dejan Filipovic at the University of Colorado Boulder, prototyping can be facilitated by using systems with millimeter-wave coaxial transmission lines made of stacked printed-circuit boards (PCBs). The result is a low-cost and fast turnaround solution.

To demonstrate this process, the pair constructed a two-element antenna array with an integrated air-dielectric rectangular-coaxial feed and a 90-deg. hybrid made of stacked PCBs. PCBs are a much less expensive and time-consuming method of prototyping millimeter-wave stacked devices. Yet routing bit size, panel size, substrate type, copper thickness, minimum slot width, and the roughness of the plated sidewalls all demand additional consideration.

The designers performed circuit simulations with Ansys HFSS to tune the multisection, 90-deg. hybrid. In doing so, they ensured tuned construction while considering parasitics in line junctions. Each slot in the two-element, tapered slot antenna array was tuned using a full-wave simulator for optimal gain and impedance performance. Corrugations were added to increase gain and decrease the beamwidth by up to 3 dB.

The PCB components were assembled using dowel pins for alignment and screws for prototyping ease and cost efficiency. The structures were thoroughly characterized from 4 to 8 GHz and 4 to 18 GHz. They performed with a reflection coefficient and port-to-port coupling below -10 dB. See “PCB-Based Prototyping of 3-D Micromachined RF Subsystems,” IEEE Transaction on Antennas and Propagation, Jan. 2014, p. 420.