LIGA-micromachined tight microwave couplers

Abstract

There are a significant number of microwave applications, including active antenna arrays, wireless communication systems, navigational applications, etc., where improvement of such qualities as manufacturing costs, size, weight, power consumption, etc. is still on the agenda of today’s RF design. In order to meet these requirements, new technologies must be actively involved in fabrication of RF components with improved characteristics. One of such fabrication technologies is called LIGA, used before primarily in fluidics, photonics, bioengineering, and micromechanics, and only recently receiving growing attention in RF component fabrication. One of the RF components suffering limitations in performance due to limitations in fabrication capabilities is the compact single metal layer (SML) coupled-line 3-dB coupler, also called “hybrid”, required in some applications thanks to its ability to divide power equally and electrically isolate the output from the input. In today’s practical edge-coupled SML coupler designs, the level of coupling is limited by the capabilities of the photolithographic process to print the coupled lines close enough for tight coupling and it is usually no tighter that 8 dB. A promising way to overcome this limitation is increasing the area of metallic interface of the coupled lines, thus increasing the mutual capacitance of the lines, and inherently the coupling between them. This should be preferably done with keeping the coupler compact with respect to the footprint area, which is attained by making taller conductors, i.e. employing the third dimension. In contrast with previously used RF component fabrication processes, LIGA is the technology that allows the designer to explore the third dimension and build tall conductors while being also able to use small features. When the two-dimensional edge-coupled SML couplers are extended into the three-dimensional structures, they rather become the side-coupled SML couplers. Tall-conductor coupled lines have been characterized in this work to reveal their dependence on their geometry and a 3-dB SML coupler with tall conductors has been developed and fabricated using LIGA at the Institute for Microstructure Technology (IMT), Karlsruhe, Germany. The simulation and measurement results demonstrate the potentially superior performance of LIGA couplers, and the promising capabilities of LIGA for fabrication of RF microstructures.