LIGA cavity resonators and filters for microwave and millimetre-wave applications

High performance microwave cavities for various circuits in the front-end of transceivers such as filters, diplexers, and oscillators have conventionally been built with rectangular or cylindrical metallic waveguides, which typically have low loss, high quality (Q) factor, and higher power handling capability. However such waveguide cavity based circuits made by traditional metal machining techniques tend to be costly, particularly for complex multiple cavity based circuits, and not well suited to high volume commercial applications and integration with planar microwave integrated circuits. As commercial transceiver applications progress toward higher microwave and millimetre-wave frequencies, the use of waveguide based circuits for compact, highly integrated transceivers is becoming feasible, along with an increasing need for cost effective batch fabrication processes for realizing complex metallic cavity circuits without sacrificing structural quality and performance. It is expected that significant advancements in both microwave performance and integration will be achieved through the development of novel technologies for realizing vertically oriented three-dimensional (3-D) structures.Although improvement has been made on increasing the resonator Q factor by exploiting silicon micromachining and low-temperature cofired ceramics (LTCC) techniques, there are some drawbacks inherent to silicon cavity micromachining and LTCC technology, including non-vertical sidewalls, depth limitations, and surface roughness for the silicon resonator, and dielectric and radiation loss for LTCC resonator.Polymer-based fabrication is a promising alternative to silicon etching and LTCC technologies for the batch fabrication of ultra-deep microwave cavity structures. In particular, deep X-ray lithography (XRL), as part of the LIGA process, is a microfabrication technology for precisely structuring polymers, and is increasingly being applied to RF/microwave microstructures. In addition to precise patterning capabilities, deep XRL is able to structure ultra-deep cavities due to the penetration ability of hard X-rays. Cavities of several millimetres are possible in a single lithographic exposure, and with excellent sidewall quality, including verticality near 90 degrees and surface roughness on the order of tens of nanometres. These structured polymers are subsequently used as electroforming templates for fabricating metal structures with correspondingly good sidewall quality.This thesis investigates the possibility of realizing high-Q cavity resonators and filters at microwave frequencies using the LIGA microfabrication process. Finite element method (FEM) electromagnetic simulation results based on the cavity models representing different fabrication conditions show that smooth LIGA cavity structures result in promising Q improvement over silicon and LTCC structures. And the potential advantages of LIGA resonators are more dramatic with cavity height and increasing operating frequency. Deep polymer cavity structures (1.8 mm) fabricated using deep XRL demonstrate excellent sidewall verticality in the PMMA structure, with only slight shrinkage at the top surface of 8.5 2.5 mm in either lateral dimensions. This corresponds to sidewalls with verticality between 89.82o and 89.9o. The structure polymers are subsequently used as templates for metal electroforming to produce cavity resonators. The performance of the resonator is measured in a planar environment. A RT/duroid6010 soft substrate patterned with coupling structures forms the sixth side, and thus completes the cavity. Despite the rather crude test assembly for the sixth side made by clamping, the measured resonator has a high unloaded Q of 2122.2 85 at the resonant frequency of 24 GHz, indicating that LIGA cavities are especially promising for high performance applications. The relatively simple, single-step lithographic exposure also facilitates extension to more structurally complicated waveguide and multiple cavity-based circuits. This research work also proposes a high performance ``split-post' 3-pole cylindrical post coupled Chebyshev bandpass filter suitable for LIGA fabrication. In addition to potentially batch fabricating such a filter lithographically by exposing the entire waveguide depth in a single exposure, the filter structures composed of three cavities with metallic multi-post coupling would be extremely difficult to fabricate using traditional machining techniques, due to the extremely fine post structure and high vertical aspect ratio required. However, these types of structures could be ideal for LIGA fabrication, which offers sub-micron features, aspect ratios of 100:1 or higher, resist thicknesses of up to 3 mm, and almost vertical and optically smooth sidewalls. Also, representative LIGA sidewall roughness is used to predict very low loss and high performance, suggesting that complicated structures with multiple resonator circuits and high internal components with high aspect ratios are possible.

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