LIGA micromachined planar transmission lines and filters

This paper introduces a new class of three-dimensional (3-D) micromachined microwave and millimeter-wave planar transmission lines and filters. The LIGA process allows tall (10 μm-1 mm), high-aspect ratio metal structures to be very accurately patterned and is compatible with integrated circuit-fabrication processes. The tall metal transmission lines will enable the development of high-power monolithic circuits as well as couplers and filters that require very high coupling. Using conductor thickness as a new variable in filter design permits the fabrication of elements requiring a wider than usual range of even- and odd-mode characteristic impedances by lowering the attainable odd-mode impedance without greatly influencing even-mode impedance. Bandpass and low-pass filters fabricated using 200-μm tall nickel microstrip lines are demonstrated at X-band. Insertion losses of the network testing setup and waveguides were calibrated out using the thru-reflection-line (TRL) calibration method via LIGA-fabricated calibration standards. The high aspect ratio and slope that the LIGA process offers will enable the design of end-coupled narrow-band bandpass filters and planar side-coupled 3-dB couplers. Filter structures were fabricated possessing coupling gaps with aspect ratios of better than 6.75 and conductor sidewall slope >89.9°, figures that are easily obtainable with the LIGA process. Additionally, W-band 3-dB coplanar waveguide-coupler LIGA geometries suitable for implementation on gallium arsenide or membrane (i.e., air dielectric) substrates are presented. A thin film-to-LIGA tapered waveguide transition is presented which will allow integration of conventional planar transmission lines with these LIGA devices